JP2002053935A - High tension cold rolled steel sheet excellent in strain age hardening characteristics and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high tensile strength cold rolled steel sheet excellent in strain age hardening property suitable for an automobile body and a method for producing the same. [Solution] Al: 0.02% or less, N: 0.0050 to 0.0250%
Slab containing N / Al 0.3 or more FDT: 80
After hot rolling at 0 ° C or higher, take up at CT: 750 ° C or lower. Then, after cold rolling, continuous annealing at a temperature not lower than the recrystallization temperature and not higher than 900 ° C, primary cooling rapidly cooling to not higher than 500 ° C, and residence time in a temperature range not lower than the primary cooling stop temperature and not lower than 400 ° C for 300 s.
The following secondary cooling is performed, and the F phase having a particle size of 10 μm or less is formed.
A steel sheet containing 0.0010% or more of N in a solid solution state with a structure containing 50% or more. After continuous annealing, it may be cooled to 600 ° C. or lower at 70 ° C./s or lower, or may be further overaged. Further, the continuous annealing temperature is set to 2 of Ac ₁ to Ac ₃ .
Cooling is performed so that the average cooling rate between 600 and 300 ° C is equal to or higher than CR defined according to the amount of alloying elements.
The structure may include 50% or more and M phase of 3% or more.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-workability, high-tensile cold-rolled steel sheet suitable mainly for use in an automobile body, and particularly to a high-tensile cold-rolled steel sheet having a tensile strength (TS) of 440 MPa or more and excellent strain-age hardening characteristics. The present invention relates to a rolled steel sheet and a method for manufacturing the same. The high-tensile cold-rolled steel sheet of the present invention can be used in a wide range of applications, from those subjected to relatively light working such as forming into pipes by light bending and roll forming to those subjected to relatively severe drawing. It is suitable for. In addition, the steel sheet in the present invention includes a steel sheet and a steel strip.

In the present invention, "excellent in strain aging hardening characteristics" means that after pre-deformation with a tensile strain of 5%, aging treatment is performed at a temperature of 170 ° C. for 20 minutes before and after the aging treatment. Deformation stress increase (BH amount; BH amount = (yield stress after aging treatment)-(pre-deformation stress before aging treatment))
Is 80 MPa or more, and the amount of increase in tensile strength before and after the strain aging treatment (the pre-deformation + the aging treatment) (denoted as ΔTS; Δ
TS = (tensile strength after aging treatment) − (tensile strength before pre-deformation)) is 40 MPa or more.

[0003]

2. Description of the Related Art Reduction of vehicle body weight in automobiles has become an extremely important issue in connection with recent emission regulations due to global environmental problems. In order to reduce the body weight of an automobile, it is effective to increase the strength of a steel plate used in large quantities, that is, to apply a high-tensile steel plate to reduce the thickness of the steel plate used.

[0004] However, even in the case of automobile parts using thin high-strength steel sheets, the performances required according to their roles must be sufficiently exhibited. Such performance includes, for example, static strength against bending and torsion deformation, fatigue resistance, impact resistance, and the like. Therefore, a high-tensile steel sheet applied to an automobile part needs to have excellent properties after forming.

[0005] In the process of manufacturing automobile parts,
Press forming is performed on steel sheets, but if the strength of the steel sheets is too high, press freezing will result in reduced shape freezing properties and reduced ductility, resulting in problems such as cracking and necking during forming. This has prevented the application of high-strength steel sheets to automobile bodies.

[0006] As a method for overcoming this, for example, in the cold rolled steel sheet for the outer panel, ultra low carbon steel is used as a material,
There is known a steel sheet in which the amount of C finally remaining in a solid solution state is controlled to an appropriate range. This type of steel sheet is kept soft during press forming, secures shape freezing properties and ductility, and uses a strain aging hardening phenomenon that occurs in the paint baking process of 170 ° C × 20 min after press forming to reduce the yield stress. Get a rise,
It is intended to ensure dent resistance. In this type of steel sheet, during press forming, C forms a solid solution in the steel and is soft. On the other hand, after press forming, in the coating baking process, solid solution C is fixed to dislocations introduced during press forming, yielding Stress increases.

However, in this type of steel sheet, the yield stress increase due to strain age hardening is suppressed low from the viewpoint of preventing the occurrence of a strainer strain which becomes a surface defect. For this reason, the portion that actually contributes to the weight reduction of components is small. That is, in order to reduce the weight of a component, it is necessary to increase not only the yield stress due to strain aging but also the strength characteristics when the deformation is further advanced. In other words, an increase in tensile strength after strain aging is desired.

On the other hand, for applications in which the appearance does not matter much, baking hardening can be achieved by using a steel sheet in which the amount of baking hardening is further increased using solid solution N, or by forming the structure into a composite structure composed of ferrite and martensite. A steel sheet with further improved properties has been proposed. For example, JP-A-60-52528 discloses that C: 0.02 to 0.15%, Mn: 0.8 to 3.5%, P: 0.02%.
Up to 0.15%, Al: 0.10%, N: 0.005 to 0.025%, hot rolling at a temperature of 550 ° C or less, and ductility and spot weldability, where annealing after cold rolling is controlled cooling heat treatment. A method for producing a high-strength thin steel sheet that is good in both cases is disclosed. The steel sheet manufactured by the technique described in JP-A-60-52528 has a mixed structure composed of a low-temperature transformation product phase mainly composed of ferrite and martensite, has excellent ductility, and is positively added. The purpose of the present invention is to obtain high strength by utilizing strain aging at the time of paint baking with N.

However, in the technique described in JP-A-60-52528, the increase in the yield stress YS due to strain age hardening is large, but the increase in the tensile strength TS is small.
Since there is a large variation in mechanical properties such as a large increase in the yield stress YS and a large variation, it is not possible to expect a steel sheet to be thin enough to contribute to a reduction in the weight of automobile parts currently demanded.

In Japanese Patent Publication No. 5-24979, C: 0.
08-0.20%, Mn: 1.5-3.5%, with the composition of the balance Fe and unavoidable impurities, the structure was composed of uniform bainite with a ferrite content of 5% or less or bainite partially containing martensite. Bake hardenable high tensile strength cold rolled steel sheets are disclosed. The cold-rolled steel sheet described in JP-B-5-24979 has a cooling range of 400 to 20 in the cooling process after continuous annealing.
By rapidly cooling the temperature range of 0 ° C. and gradually cooling it thereafter, the structure is changed to a structure mainly composed of bainite, and an unprecedented high bake hardening amount is to be obtained.

[0011] However, in the steel sheet described in Japanese Patent Publication No. 5-24979, the yield strength increases after paint baking, and a higher bake hardening amount than before can be obtained, but the tensile strength can be increased. However, when applied to a strength member, improvement in fatigue resistance and impact resistance after molding cannot be expected. For this reason, there remains a problem that it cannot be applied to applications that require strong fatigue resistance and impact resistance.

A steel sheet which is subjected to heat treatment after press forming to increase not only the yield stress but also the tensile strength is proposed as a hot rolled steel sheet. For example, in Japanese Patent Publication No. 8-23048, C: 0.02 to 0.13%, Si: 2.
0% or less, Mn: 0.6 to 2.5%, sol.Al: 0.10% or less,
N: steel containing 0.0080 to 0.0250% is reheated to 1100 ° C or higher, subjected to hot rolling to finish rolling at 850 to 900 ° C, and then cooled at a cooling rate of 15 ° C / s or higher to a temperature of less than 150 ° C. A method for producing a hot-rolled steel sheet has been proposed in which the material is cooled and wound to form a composite structure mainly composed of ferrite and martensite. However, in the steel sheet manufactured by the technique described in Japanese Patent Publication No. 8-23048, although the tensile strength increases with the yield stress due to strain aging hardening, the steel sheet is wound at an extremely low winding temperature of less than 150 ° C. However, there is a problem that the fluctuation of the characteristic is large. There is also a problem that the variation in the amount of increase in the yield stress after the press forming-painting bake treatment is large, the hole expansion ratio (λ) is low, the stretch flangeability is reduced, and the press formability is insufficient.

As a high-tensile steel sheet having a relatively high yield stress, there is a so-called precipitation-strengthened steel in which a carbonitride forming element such as Ti, Nb or V is added and strengthened by fine precipitates thereof. However, aside from the hot-rolled steel sheet that undergoes a step of keeping sufficient heat after the hot-rolling winding, in the cold-rolled steel sheet, it is difficult to advance sufficient precipitation in a short-time continuous annealing step, and a high yield ratio ( It was difficult to manufacture a steel sheet having a ratio of yield stress to tensile strength (YS / TS). In particular, when trying to reduce C in consideration of weldability, there is also a problem that it is more difficult to obtain a high yield ratio, probably because the amount of precipitate itself is reduced in a region where the amount of C is low.

Further, although the above-mentioned conventional steel sheet is excellent in strength evaluation after paint baking treatment by a simple tensile test, there is a large variation in strength when plastically deformed according to actual pressing conditions. It was not always enough to be applied to parts that required reliability.

[0015]

SUMMARY OF THE INVENTION The present invention overcomes the limitations of the prior art described above, and has high moldability, or even higher impact resistance and stable quality characteristics.
High strength cold-rolled steel sheet with excellent strain aging hardening properties, which can provide sufficient strength as an automobile part after forming into an automobile part and can sufficiently contribute to weight reduction of an automobile body, and these steel sheets are industrially inexpensive, and An object of the present invention is to provide a manufacturing method capable of manufacturing without disturbing the shape. The strain age hardening characteristics in the present invention are as follows.
Under the aging condition of holding for 20 minutes, the BH amount is 80MPa or more, ΔT
The goal is for S to be at least 40 MPa. Furthermore, in order to increase the yield stress in the original plate state and stabilize the strength of the parts in order to stably maintain the strength of the parts, in order to be able to be applied advantageously to parts with relatively small strains, a high yield ratio mold with a yield ratio of 0.7 or more is used. One of the objects of the present invention is to use a cold-rolled steel sheet.

[0016]

Means for Solving the Problems In order to achieve the above object, the present inventors manufactured steel sheets with various compositions and manufacturing conditions, and conducted many material evaluation experiments. as a result,
In fields where high workability is required, N, which has been rarely used so far in the past, is used as a strengthening element, and by taking advantage of the large strain age hardening phenomenon developed by the action of this strengthening element, formability can be improved. It has been found that it is possible to easily achieve both improvement of the strength and high strength after molding.

Furthermore, the present inventors, in order to advantageously utilize the strain age hardening phenomenon due to N, use the strain age hardening phenomenon due to N under the conditions of baking paint for automobiles or more aggressively with the heat treatment conditions after molding. Must be combined advantageously
For this purpose, it has been found that it is effective to control the microstructure of the steel sheet and the amount of solute N in a certain range by optimizing the conditions of hot rolling, cold rolling and cold rolling annealing. In addition, it has been found that in order to stably develop the strain age hardening phenomenon due to N, it is important in terms of composition to control the Al content particularly in accordance with the N content. Further, the present inventors, the microstructure of the steel plate, the main phase of ferrite, the average grain size is 10μ
By setting m or less, it has been found that N can be fully utilized without the conventional problem of deterioration due to aging at room temperature.

Further, the present inventors have made the microstructure of the steel sheet a ferrite main phase and a second phase having a martensite phase in an area ratio of 3% or more,
A low yield ratio can be achieved, ductility and workability can be improved, and the strength after processing can be increased by utilizing the strain age hardening phenomenon exhibited by N, and the impact resistance as a part property can be improved. I found

That is, the present inventors used N as a strengthening element, controlled the Al content in an appropriate range according to the N content, and optimized the hot rolling conditions, cold rolling, and cold rolling annealing conditions. By optimizing the microstructure and the solute N, the formability of the C-Mn-based steel sheet and the precipitation-strengthened steel sheet of the conventional solid-solution strengthening is much better than that of the conventional steel sheet. It has been found that a steel sheet having no strain-age hardening characteristics and, moreover, excellent impact resistance characteristics as component characteristics can be obtained.

In addition to optimizing the above-mentioned microstructure and solid solution N (N in solid solution state), the present inventors
By optimizing the precipitated Nb (precipitated Nb) by containing, a yield ratio of as high as 0.7 or more can be obtained, together with superior formability compared to conventional steel sheets and strain age hardening characteristics not available in conventional steel sheets I found something. In addition, the steel sheet of the present invention has a higher strength after a paint baking treatment by a simple tensile test than a conventional steel sheet, and furthermore, has a small variation in strength when plastically deformed according to actual pressing conditions, and has a stable component strength. Characteristics are obtained. For example, the part where the plate thickness is reduced due to large strain is larger than the other parts, and the hardening allowance is equalized when evaluated by the load capacity of (plate thickness) x (strength), and the strength as a part is stable. You do it. Conversely, in parts where the strain is small, the strength increase due to strain aging is small, but the required strength can be secured because the original strength is high, so the strength stability of real parts formed with various ranges of strain Contribute to advantage.

The present invention has been completed based on the above findings and further studies. That is, the gist of the high-tensile cold-rolled steel sheet according to the first invention is as follows. (1) In mass%, Al: 0.02% or less, N: 0.0050 to 0.0250
%, N / Al is 0.3 or more, and N in solid solution is 0.00
High-tensile cold with excellent tensile aging hardening characteristics at tensile strength of 440MPa or more, characterized by having a composition containing 10% or more and a structure containing 50% or more area ratio of a ferrite phase with an average crystal grain size of 10μm or less. Rolled steel sheet. (2) The composition is, by mass%, C: 0.15% or less, Si: 2.
0% or less, Mn: 3.0% or less, P: 0.08% or less, S: 0.02
% Or less, Al: 0.02% or less, N: 0.0050 to 0.0250%, N / Al is 0.3 or more, N in solid solution is 0.0010% or more, and the balance is Fe and unavoidable impurities. The high-tensile cold-rolled steel sheet according to (1), wherein (3) in addition to the composition, further, in mass%,
The high-tensile cold-rolled steel sheet according to (2), which includes at least one group of group d or two or more groups.

Group a: One or more of Cu, Ni, Cr and Mo are combined for a total of 1.
0% or less b group: 0.1% or more of one or more of Nb, Ti and V
Group c: B is 0.0030% or less Group d: One or two of Ca and REM are 0.0010 to 0.01 in total
0% (4) The composition is, by mass%, C: 0.15% or less, Si: 2.
0% or less, Mn: 3.0% or less, P: 0.08% or less, S: 0.02
% Or less, Al: 0.02% or less, N: 0.0050 to 0.0250%, Nb:
0.007 to 0.04%, N / Al is 0.3 or more, N in solid solution is 0.0010% or more, and Nb in a precipitated state is 0.1% or more.
005% or more, the balance being Fe and unavoidable impurities, and the structure has an average crystal grain size of 10 μm
The high-tensile cold-rolled steel sheet according to (1), wherein the following ferrite phase is included in an area ratio of 50% or more, and the remainder has a structure mainly composed of pearlite, and has a yield ratio of 0.7 or more. (5) In addition to the above composition, further, in mass%, the following groups e to
The high-tensile cold-rolled steel sheet according to (4), which includes at least one group of h group or two or more groups.

Group e: one or two or more of Cu, Ni, Cr and Mo are combined for a total of 1.
0% or less f group: 0.1% or less of one or two of Ti and V in total g group: 0.0030% or less of B group: h group: 0.0010 to 0.01 of one or two of Ca and REM in total
0% (6) When the composition is mass%, C: 0.15% or less, Mn: 3.%
0% or less, S: 0.02% or less, Al: 0.02% or less, N: 0.00
50-0.0250%, Mo: 0.05-1.0%, Cr:
Containing one or two of 0.05 to 1.0%, and
/ Al contains 0.3 or more and N in solid solution state 0.0010% or more,
The balance is a composition comprising Fe and unavoidable impurities, and the structure includes a ferrite phase having an average crystal grain size of 10 μm or less in an area ratio of 50% or more, and a martensite phase in an area ratio of 3% or more. The high-tensile cold-rolled steel sheet according to (1), wherein: (7) In addition to the above composition, in mass%,
The high-tensile cold-rolled steel sheet according to (6), which includes one group or two or more groups of m groups.

Note j group: Si: 0.05-1.5%, P: 0.03-0.15%, B: 0.00
One or more kinds of 03 to 0.01% k group: Nb: 0.01 to 0.1%, Ti: 0.01 to 0.2%, V: 0.01
1 or 2 or more of 0.2% 1 group: Cu: 0.05 to 1.5%, Ni: 1 or 2 kinds of 0.05 to 1.5% m group: Ca or REM 1 or 2 kinds in total 0.0010 to 0.01
0% (8) The high-tensile cold-rolled steel sheet according to any one of (1) to (7), wherein the high-tensile cold-rolled steel sheet has a thickness of 3.2 mm or less. (9) A high-tensile cold-rolled steel sheet obtained by electroplating or hot-dip coating the high-tensile cold-rolled steel sheet according to any one of (1) to (8).

The gist of the method for producing a high-tensile cold-rolled steel sheet according to the second invention is as follows. That is, in the second invention, Al: 0.02% or less and N: 0.0050% by mass.
A steel slab having a composition containing 0.00.0250% and N / Al of 0.3 or more is heated to a slab heating temperature of 1000 ° C. or more, rough-rolled into a sheet bar, and a finish-rolling exit temperature is applied to the sheet bar. : Finish rolling at 800 ° C. or higher, hot rolling at a winding temperature of 750 ° C. or lower to form a hot rolled sheet, and pickling and cold rolling of the hot rolled sheet to form a cold rolled sheet A cold rolling step and annealing the cold-rolled sheet at a predetermined temperature for a predetermined time, and then performing a cold-rolled sheet annealing step of cooling at a predetermined cooling rate. This is a method for producing a high-tensile cold-rolled steel sheet having a strain age hardening property of 440 MPa or more.

In the second aspect of the present invention, C:
0.15% or less, Si: 2.0% or less, Mn: 3.0% or less, P: 0.
08% or less, S: 0.02% or less, Al: 0.02% or less, N: 0.00
50 to 0.0250%, and N / Al is 0.3 or more, or the following groups a to d: a group: one or more of Cu, Ni, Cr, and Mo in total of 1.
0% or less b group: 0.1% or more of one or more of Nb, Ti and V
Group c: B is 0.0030% or less Group d: One or two of Ca and REM are 0.0010 to 0.01 in total
A steel slab containing at least one group selected from 0% or more, and preferably having a composition consisting of the balance of Fe and unavoidable impurities, is heated to a slab heating temperature of 1000 ° C. or more, rough-rolled and subjected to sheet rolling. The sheet bar is subjected to finish rolling at a finish-rolling exit temperature of 800 ° C. or higher, and after finish rolling,
Cooling is preferably started within 0.5 seconds, and the cooling rate is 40 ° C /
quenching at a temperature of 750 ° C. or less, preferably at a temperature of 650 ° C. or less, and a hot rolling step of forming a hot rolled sheet, and pickling and cold rolling the hot rolled sheet. A cold rolling step of forming a cold rolled sheet, annealing the cold rolled sheet at a temperature not lower than the recrystallization temperature and not higher than 900 ° C. for a holding time of 10 to 60 s, and then cooling to a temperature range of not higher than 500 ° C. A cold-rolled sheet annealing step of performing primary cooling of cooling at ~ 300 ° C / s, and secondary cooling of a residence time in a temperature range of 400 ° C or higher at a stop temperature of the primary cooling or lower and 300 ° C or lower, It is preferable to sequentially apply, and in the second aspect of the present invention, it is preferable to perform temper rolling or leveler processing at an elongation of 1.0 to 15% following the cold-rolled sheet annealing step. In the second aspect of the present invention, it is preferable that adjacent sheet bars are joined between the rough rolling and the finish rolling, and in the second aspect of the present invention, the rough rolling and the finish rolling are performed. It is preferable to use one or both of a sheet bar edge heater for heating the width end of the sheet bar and a sheet bar heater for heating the length end of the sheet bar.

In the second aspect of the present invention, C:
0.15% or less, Si: 2.0% or less, Mn: 3.0% or less, P: 0.
08% or less, S: 0.02% or less, Al: 0.02% or less, N: 0.00
50 to 0.0250%, Nb: 0.007 to 0.04%, and N / Al
Is 0.3 or more, or the following groups e to h:
0% or less f group: 0.1% or less of one or two of Ti and V in total g group: 0.0030% or less of B group: h group: 0.0010 to 0.01 of one or two of Ca and REM in total
A steel slab containing one or more groups selected from 0%, preferably having a composition consisting of a balance of Fe and unavoidable impurities, is heated to a slab heating temperature of 1100 ° C. or more, and is roughly rolled to obtain a sheet. The sheet bar is subjected to finish rolling in which the rolling reduction in the final pass of the final rolling: 25% or more and the finish-rolling exit temperature: 800 ° C. or more.
Start cooling within seconds and cool at a cooling rate of 40 ° C / s or more.
Winding temperature: 750 ° C. or lower, preferably 650 ° C. or lower, a hot rolling step of forming a rolled hot rolled sheet, and pickling and cold rolling the hot rolled sheet to form a cold rolled sheet. Rolling process;
The cold-rolled sheet is annealed at a temperature not lower than the recrystallization temperature and not higher than 900 ° C. for a holding time of 10 to 90 s, and then cooled to a temperature range of not higher than 600 ° C. at a cooling rate of 70 ° C./s or lower. And the steps are preferably performed sequentially to obtain a cold-rolled steel sheet having a yield ratio of 0.7 or more. In the second aspect of the present invention, after the cold-rolled sheet annealing step, it is preferable to further perform temper rolling or leveler processing at an elongation of 1.0 to 15%, preferably 1.5 to 15%. In the second aspect of the present invention, it is preferable that adjacent sheet bars are joined between the rough rolling and the finish rolling, and in the second aspect of the present invention, the rough rolling and the finish rolling are performed. It is preferable to use one or both of a sheet bar edge heater for heating the width end of the sheet bar and a sheet bar heater for heating the length end of the sheet bar.

In the second aspect of the present invention, C:
0.15% or less, Mn: 3.0% or less, S: 0.02% or less, Al: 0.
02% or less, N: 0.0050 to 0.0250%, Mo:
0.05 to 1.0%, Cr: one or two of 0.05 to 1.0%
Containing a seed and having an N / Al of 0.3 or more, or a further j group to m group: j group: Si: 0.05 to 1.5%, P: 0.03 to 0.15%, B: 0.00
One or more kinds of 03 to 0.01% k group: Nb: 0.01 to 0.1%, Ti: 0.01 to 0.2%, V: 0.01
1 or 2 or more of 0.2% 1 group: Cu: 0.05 to 1.5%, Ni: 1 or 2 kinds of 0.05 to 1.5% m group: Ca or REM 1 or 2 kinds in total 0.0010 to 0.01
A steel slab containing at least one group selected from 0% or more, and preferably having a composition consisting of the balance of Fe and unavoidable impurities, is heated to a slab heating temperature of 1000 ° C. or more, rough-rolled and subjected to sheet rolling. The sheet bar is subjected to finish rolling at a finish-rolling exit temperature of 800 ° C. or higher, and after finish rolling,
Preferably start cooling within 0.5 seconds and cooling rate: 40 ℃ / s
A hot rolling step of rapidly cooling to form a rolled hot rolled sheet at a winding temperature of 750 ° C. or lower; a cold rolling step of pickling and cold rolling the hot rolled sheet to form a cold rolled sheet; (A)
Holding time at a temperature of c ₁ transformation point to (Ac ₃ transformation point): 10 to 1
Annealing is performed for 20 s, and then the average cooling rate between 600 and 300 ° C. is calculated by the following formula (1) or (2). 05Cu + 0.05Ni] +3.95 (1) When B ≧ 0.0003% log CR = -1.73 [Mn + 2.67Mo + 1.3Cr + 0.26Si + 3.5P + 0.05Cu + 0.05Ni] +3.40 ... (2) (here , CR: cooling rate (° C / s), Mn, Mo, Cr, S
i, P, Cu, Ni: a cold-rolled sheet annealing step of cooling at a critical cooling rate CR or more defined by the content of each element (% by mass) or higher in order to obtain a cold work with excellent workability and impact resistance. It is preferable to use a rolled steel sheet. Further, in the second aspect of the present invention, the elongation percentage is 1.0 to 15% following the cold-rolled sheet annealing step.
Is preferably subjected to temper rolling or leveling.
In the second aspect of the present invention, it is preferable that adjacent sheet bars are joined between the rough rolling and the finish rolling, and in the second aspect of the present invention, the rough rolling and the finish rolling are performed. It is preferable to use one or both of a sheet bar edge heater for heating the width end of the sheet bar and a sheet bar heater for heating the length end of the sheet bar.

[0029]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The steel sheet of the present invention has a tensile strength of 440 MPa.
This is a high-tensile cold-rolled steel sheet having excellent strain age hardening characteristics.
First, the reasons for limiting the composition of the steel sheet of the present invention will be described. Hereinafter, the mass% is simply described as%. The steel sheet of the present invention has A
l: 0.02% or less, N: 0.0050 to 0.0250%, and N
/ Al has a composition containing 0.3 or more, and 0.0010% or more of N in a solid solution state.

Al: 0.02% or less Al is an element that acts as a deoxidizing agent and is effective in improving the cleanliness of steel, and is also an element that refines the structure of the steel sheet. In the present invention, Al is 0.001% or more. Is desirable.
On the other hand, excessive Al content deteriorates the surface properties of the steel sheet, further reduces N in the solid solution state, which is an important constituent element of the present invention, and causes shortage of solid solution N contributing to the strain aging hardening phenomenon. When the conditions vary, the strain aging hardening characteristic, which is a feature of the present invention, tends to vary. Therefore, in the present invention, the Al content is limited to as low as 0.02% or less. From the viewpoint of material stability, the content of Al is preferably 0.015% or less.

N: 0.0050 to 0.0250% N is an element that increases the strength of the steel sheet by solid solution strengthening and strain age hardening, and is the most important element in the present invention. In addition, N also has a function of lowering the transformation point of steel, and the inclusion of N is also useful for stabilizing the operation in a situation in which rolling is avoided, which is a thin material and greatly lowers the transformation point. In the present invention,
By containing an appropriate amount of N and controlling the manufacturing conditions,
A necessary and sufficient amount of N in the solid solution state is ensured in the cold rolled product or the plated product, whereby the effect of strengthening the solid solution and increasing the strength (YS, TS) in strain age hardening is sufficiently exhibited, and T
The mechanical property requirements of the steel sheet of the present invention, that is, S440 MPa or more, bake hardening amount (BH amount) 80 MPa or more, and tensile strength increase ΔTS40 MPa or more before and after the strain aging treatment can be stably satisfied.

If N is less than 0.0050%, the above-mentioned effect of increasing the strength is unlikely to be stably exhibited. On the other hand, when N exceeds 0.0250%, the internal defect generation rate of the steel sheet increases, and slab cracks and the like during continuous casting occur frequently. Therefore, N is set in the range of 0.0050 to 0.0250%. Note that N is more preferably in the range of 0.0070 to 0.0170% from the viewpoint of improving the stability and yield of the material in consideration of the entire manufacturing process. If the N content is within the range of the present invention, there is no adverse effect on weldability such as spot welding and arc welding.

N in the solid solution state: 0.0010% or more In order to ensure sufficient strength with a cold-rolled product and to sufficiently exhibit strain aging hardening due to N, it is necessary to use N in the steel in a solid solution state.
(Also referred to as solid solution N) must be present in an amount (concentration) of 0.0010% or more. Here, the amount of solute N is determined by subtracting the amount of precipitated N from the total amount of N in steel. As a method of analyzing the amount of deposited N, according to the results of comparative studies of various analysis methods by the present inventors, it is effective to obtain the amount by the electrolytic extraction analysis method using the potentiostatic electrolysis method. In addition, methods for dissolving ground iron used for extraction analysis include an acid decomposition method, a halogen method, and an electrolytic method. Among these, the electrolysis method can stably dissolve only ground iron without decomposing extremely unstable precipitates such as carbides and nitrides. Electrolysis is performed at a constant potential using an acetylacetone-based electrolyte. In the present invention, the result of measuring the amount of deposited N using the potentiostatic electrolysis method showed the best correspondence with the actual component strength.

Therefore, in the present invention, the residue extracted by the potentiostatic electrolysis method is subjected to chemical analysis to determine the amount of N in the residue, which is defined as the amount of deposited N. In addition, higher BH
In order to obtain the amount, ΔTS, the amount of solute N should be 0.0020% or more,
In order to obtain an even higher value, the content is preferably 0.0030% or more, and in order to obtain an even higher value, the content is preferably 0.0050% or more.

N / Al (ratio between N content and Al content): 0.3
As described above, in order to stabilize 0.0010% or more of dissolved N in the product state, it is necessary to limit the amount of Al, which is an element that strongly fixes N. N content and Al within the composition range of the present invention
As a result of examining steel sheets with widely varied combinations of content, 0.0010% of solute N in cold rolled products and plated products
In order to achieve the above, it was found that when the amount of Al was limited to as low as 0.02% or less, it was necessary to set N / Al to 0.3 or more. That is, the Al content is limited to (N content) /0.3 or less.

Further, the steel sheet of the present invention has an average crystal grain size of 10 μm.
It has a structure containing 50% or more of an area ratio of a ferrite phase of m or less. The structure of the steel sheet of the present invention will be described. The area ratio of ferrite phase: 50% or more The cold-rolled steel sheet of the present invention is intended for use in steel sheets for automobiles and the like that require high workability. % Or more, with a ferrite phase as the main phase. Ferrite phase area ratio is 50
%, It is difficult to secure the required ductility as a steel sheet for automobiles requiring high workability. In the case where better ductility is required, the area ratio of the ferrite phase is preferably set to 75% or more. The ferrite in the present invention includes not only ferrite (polygonal ferrite) in a normal meaning but also bainitic ferrite and acicular ferrite containing no carbide.

The second phase other than the ferrite phase is not particularly limited, but is preferably a single phase or a mixed phase of bainite and martensite from the viewpoint of increasing strength. Further, as the second phase, a structure mainly composed of pearlite may be used. In addition, when the retained austenite is in the composition range of the steel sheet of the present invention, the area ratio may appear less than approximately 3% in some cases.

Average grain size of ferrite phase: 10 μm or less In the present invention, the grain size is determined from the cross-sectional structure photograph by ASTM.
Whichever is larger, the value calculated by the quadrature method specified in (1) and the nominal particle size obtained from the cross-sectional micrograph by the cutting method specified in ASTM (for example, see Umemoto et al., Heat Treatment, 24 (1984), 334). Is adopted.

The cold-rolled steel sheet of the present invention secures a predetermined amount of solid solution N as a product. However, according to the results of experiments and studies by the present inventors, even if the solid solution N amount is kept constant, It has been found that when the average crystal grain size of the phase exceeds 10 μm, large variations occur in the strain aging hardening characteristics. In addition, the deterioration of the mechanical properties when stored at room temperature becomes remarkable. Although the detailed mechanism is unknown at present, one of the causes of the variation in strain age hardening characteristics is the crystal grain size, and segregation and precipitation of alloy elements at the crystal grain boundaries, and furthermore, processing and heat treatment on these It is presumed to be related to the effect of Therefore, in order to stabilize the strain age hardening characteristics, the average crystal grain size of the ferrite phase needs to be 10 μm or less. In order to stably obtain a further increase in the BH amount and the ΔTS amount, the average crystal grain size of the ferrite is preferably 8 μm or less.

The cold-rolled steel sheet of the present invention having the above-described composition and structure is a cold-rolled steel sheet having a tensile strength TS of 440 MPa or more and excellent strain aging hardening characteristics. A steel sheet having a TS below 440 MPa cannot be widely applied to members having structural elements. To further expand the application range, TS is 50
It is desirable that the pressure be 0 MPa or more.

In the present invention, "excellent in strain age hardening characteristics" means that after pre-deformation of 5% tensile strain,
When subjected to aging treatment at a temperature of 0 ° C for 20 minutes,
The amount of increase in deformation stress before and after this aging treatment (hereinafter referred to as BH amount; B
H content = yield stress after aging treatment-pre-deformation stress before aging treatment) is 80 MPa or more, and the amount of increase in tensile strength before and after strain aging treatment (pre-deformation + aging treatment) (denoted as ΔTS; ΔTS = Tensile strength after aging treatment-tensile strength before pre-deformation) is 40 MPa or more.

When defining the strain age hardening characteristics, the amount of pre-strain (pre-deformation) is an important factor. The present inventors have investigated the effect of the amount of pre-strain on the strain age hardening characteristics, assuming a deformation mode applied to a steel sheet for automobiles. As a result, the deformation stress in the deformation mode is extremely deep drawing. Except in the case of (1), it is possible to arrange by the amount of strain (tensile strain) equivalent to one axis. In actual parts, the amount of strain equivalent to one axis is more than 5%. It has been found that they correspond well to the strengths (YS and TS) obtained later. Based on this finding, in the present invention, the pre-deformation of the strain aging treatment is set to a tensile strain of 5%.

Conventional paint baking conditions are 170 ° C. × 20
min is adopted as a standard. In general, in order to increase the amount of hardening, it is said that it is advantageous to hold at a higher temperature and for a longer time as long as it is not softened by excessive aging. When a strain of 5% or more is applied, curing is achieved even with a milder (lower temperature) treatment, in other words, aging conditions can be broadened.

Specifically, in the steel sheet of the present invention, the lower limit of the heating temperature at which the hardening becomes remarkable after pre-deformation is approximately 100 ° C. On the other hand, if the heating temperature exceeds 300 ° C., the curing hardens, and on the contrary, it tends to soften slightly, and the occurrence of heat distortion and temper color becomes conspicuous. If the holding time is about 30 seconds or more when the heating temperature is about 200 ° C., almost sufficient curing can be achieved. In order to obtain even larger stable curing, the holding time is preferably 60 seconds or more. However, if the holding time exceeds 20 minutes, not only no further curing can be expected, but also the production efficiency is significantly reduced, which is disadvantageous in practical use.

From the above, according to the present invention, the aging treatment condition is a heating temperature of 170 which is the conventional coating baking treatment condition.
It was determined that the evaluation was performed at 20 ° C. and a holding time of 20 min. Low-temperature heating and
Even under the condition of aging treatment for a short time, large hardening is stably achieved in the steel sheet of the present invention. The method of heating is not particularly limited, and in addition to atmospheric heating using a furnace employed for normal coating baking, any of induction heating, heating using a non-oxidizing flame, laser, plasma, or the like can be preferably used.

It is necessary that the strength of automobile parts be able to withstand complicated external stress loads. Therefore, not only the strength characteristics in a small strain range but also the strength characteristics in a large strain range are important for a material steel plate. . In view of this point, the present inventors set the BH amount of the steel sheet of the present invention to be used as a material for automobile parts to be 80 MPa or more and the ΔTS amount to be 40 MPa or more. More preferably, the BH amount is 100 MPa or more and ΔTS50 MPa or more. To increase the BH amount and the ΔTS amount, the heating temperature during the aging treatment may be set to a higher temperature and / or the holding time may be set to a longer time.

In the state where the steel sheet of the present invention is not formed, the aging deterioration (phenomenon in which YS increases and El (elongation) decreases) is extremely unlikely to occur even when left at room temperature for about one year. With unprecedented advantages. By the way, the effect of the present invention can be exerted even when the product sheet thickness is relatively thick, but when the product sheet thickness exceeds 3.2 mm, it is possible to secure a necessary and sufficient cooling rate in the cold rolled sheet annealing step. However, strain aging occurs during continuous annealing, making it difficult to obtain the desired strain aging hardening characteristics as a product. Therefore, the steel sheet of the present invention preferably has a thickness of 3.2 mm or less.

In the present invention, there is no problem even if the surface of the above-mentioned cold rolled steel sheet of the present invention is subjected to electroplating or hot dip plating. These plated steel sheets also exhibit the same amount of TS, BH and ΔTS as before plating. As the type of plating, any of electrogalvanizing, hot-dip galvanizing, alloyed hot-dip galvanizing, electrotin plating, electrochromic plating, and electronickel plating can be preferably applied.

In the steel sheet of the present invention, chemical components other than the above-mentioned Al and N can be appropriately selected according to required characteristics. The steel sheet according to the first preferred embodiment is characterized in that the composition is the above-mentioned Al: 0.02% or less;
N: contains 0.0050 to 0.0250%, and N / Al is 0.3 or more;
In addition to a composition containing 0.0010% or more of N in a solid solution state, C: 0.15% or less, Si: 2.0% or less, Mn: 3.0% or less,
P: 0.08% or less, S: 0.02% or less, or further, the following groups a to d: a group: one or more of Cu, Ni, Cr, Mo in total of 1.
0% or less b group: 0.1% or more of one or more of Nb, Ti and V
Group c: B is 0.0030% or less Group d: One or two of Ca and REM are 0.0010 to 0.01 in total
A steel sheet containing 0% of one or more groups, the balance being Fe and unavoidable impurities, and a structure containing a ferrite phase having an average crystal grain size of 10 μm or less in an area ratio of 50% or more.

The first preferred embodiment of the above-described steel sheet, Al,
The reasons for limiting the composition other than N will be described. The reason for limiting the organization is the same as the reason described above. C: 0.15% or less C is an element that increases the strength of a steel sheet, and from the viewpoint of further securing a desired strength in order to achieve an average ferrite grain size of 10 μm or less, which is an important component of the present invention. , 0.005% or more. In addition, more preferably, it is 0.03% or more. On the other hand, if it exceeds 0.15%, the carbide fraction in the steel sheet becomes excessively large, the ductility is remarkably reduced, the formability is deteriorated, and the spot weldability,
The arc weldability and the like are significantly reduced. From the viewpoints of such formability and weldability, C is limited to 0.15% or less. In addition, it is preferably 0.10% or less, and more preferably 0.08% or less for applications requiring better ductility.

Si: 2.0% or less Si is a useful element capable of increasing the strength of a steel sheet without remarkably reducing the ductility of the steel, and is preferably contained in an amount of 0.1% or more. On the other hand, Si is an element that significantly raises the transformation point during hot rolling, makes it difficult to ensure quality and shape, or has an adverse effect on the beauty of the steel sheet surface such as surface properties and chemical conversion treatment. Then 2.0%
Limited to the following. When the content of Si is 2.0% or less, a remarkable increase in the transformation point can be suppressed by adjusting the amount of Mn added in combination, and good surface properties can be secured. In addition, when it is desired to ensure high ductility in a tensile strength TS500MPa super-class high-tensile steel sheet, from the viewpoint of the balance between strength and ductility, Si is set to 0.
More preferably, the content is 3% or more.

Mn: 3.0% or less Mn is an effective element for preventing hot cracking due to S.
It is preferable to add in accordance with the amount of S contained, and Mn has a great effect on refining crystal grains, which is an important constituent element of the present invention. Is preferred. From the viewpoint of stably fixing S, Mn
Is preferably contained in an amount of 0.2% or more.

Further, Mn is an element for increasing the strength of the steel sheet, and it is preferable that Mn is contained in an amount of 1.2% or more for a strength requirement of more than 500 MPa. The content is more preferably 1.5% or more from the viewpoint of ensuring a stable strength. When the Mn content is increased to this level, the variation in the mechanical properties and strain age hardening characteristics of the steel sheet with respect to variations in manufacturing conditions including hot rolling conditions is reduced, which is effective in stabilizing quality.

Further, Mn has a function of lowering the transformation point during hot rolling, and by containing it together with Si, the rise of the transformation point due to the inclusion of Si can be offset. Especially for products with a small thickness, the quality and shape change sensitively due to the change of the transformation point, so it is important to strictly balance the Mn and Si contents. Therefore, Mn / Si is 3.0
It is more preferable to make the above.

On the other hand, when Mn is contained in a large amount exceeding 3.0%, the hot deformation resistance of the steel sheet tends to increase, and the spot weldability and the formability of the welded portion tend to deteriorate. , The ductility tends to be significantly reduced. Therefore, Mn is 3.0%
Limited to the following. In applications where better corrosion resistance and moldability are required, Mn is desirably 2.5% or less.

P: 0.08% or less P is a useful element as a solid solution strengthening element for steel. From this viewpoint, the content of P is desirably 0.001% or more. Reduces stretch flangeability. In addition, P has a strong tendency to segregate in steel, which results in embrittlement of the weld. Therefore, P is limited to 0.08% or less. When stretch flangeability and weld toughness are particularly important, the content is preferably 0.04% or less. It is more preferably 0.02% or less from the viewpoint of weld toughness.

S: 0.02% or less S is an element that exists as an inclusion in the steel sheet and causes deterioration in ductility and corrosion resistance of the steel sheet. In the present invention, S is limited to 0.02% or less. In addition, especially good workability
(Especially, for applications requiring stretch flangeability and hole expandability), the content is preferably 0.015% or less. Further, when the required level of stretch flangeability is high, S is 0.008.
% Is preferable. Further, in order to stably maintain the strain age hardening characteristics at a high level, although the detailed mechanism is unknown, it is preferable to reduce S to 0.008% or less.

In the first preferred embodiment of the steel sheet of the present invention, in addition to the above-described composition, the following groups a to d: a group: one or more of Cu, Ni, Cr and Mo in total of 1.
0% or less b group: 0.1% or more of one or more of Nb, Ti and V
Group c: B is 0.0030% or less Group d: One or two of Ca and REM are 0.0010 to 0.01 in total
It is preferable to contain 0% of one or more groups.

Elements of group a: Cu, Ni, Cr and Mo are all elements that contribute to the increase in strength of the steel sheet. To obtain this effect, Cu: 0.01% or more, Ni: 0.01% or more, Cr : 0.
The content is preferably set to 01% or more and Mo: 0.01% or more, and may be selected as necessary and contained alone or in combination. However, if the content is too large, the hot deformation resistance increases, or the chemical conversion property and the surface treatment characteristics in a broad sense deteriorate, and the welded part is hardened and the weldability is reduced. For this reason, it is preferable that the total of the elements in group a be 1.0% or less.

Elements of group b: Nb, Ti, and V are elements that contribute to the refinement and uniformization of crystal grains. To obtain this effect, each of Nb: 0.002% or more and Ti: 0.002% or more. , V: preferably 0.002% or more, and may be selected alone or in combination as necessary. However, if the content is too large, the hot deformation resistance increases, and the chemical conversion property and the surface treatment properties in a broad sense deteriorate. For this reason, it is preferable that the total of the elements of group b be 0.1% or less.

Element c group: B is an element having the effect of improving the hardenability of the steel, and is required for the purpose of increasing the fraction of the low-temperature transformation phase other than the ferrite phase and increasing the strength of the steel. It can be contained depending on. In order to obtain this effect, B is preferably contained at 0.0002% or more. However, if the amount is too large, the hot deformability decreases, and the amount of solute N is reduced by generating BN. Therefore, B is preferably set to 0.0030% or less.

Elements of group d: Ca and REM are both elements useful for controlling the form of inclusions, and when stretch flangeability is required, they are preferably contained alone or in combination. In that case, the total of the elements of group d is 0.0010
%, The effect of controlling the morphology of inclusions is insufficient.
If it exceeds 0%, generation of surface defects becomes conspicuous. For this reason, it is preferable to limit the total of the elements of group d to the range of 0.0010 to 0.010%.

The balance other than the above components is Fe and inevitable impurities. O: 0.00 as inevitable impurities
50% or less is acceptable. The cold-rolled steel sheet of the present invention having the above composition and structure has a tensile strength TS of 440 MPa or more,
Cold rolled steel sheet with excellent strain age hardening characteristics. Next, a second preferred embodiment of the steel sheet of the present invention will be described.

A steel sheet according to a second preferred embodiment contains the above-mentioned composition containing Al: 0.02% or less and N: 0.0050 to 0.0250%,
In addition to a composition containing 0.3% or more of N / Al and 0.0010% or more of N in a solid solution state, C: 0.15% or less, Si: 2.0%
Hereinafter, Mn: 3.0% or less, P: 0.08% or less, S: 0.02% or less, Nb: 0.007 to 0.04%,
0.005% or more, or the following groups e to h: group e: one or more of Cu, Ni, Cr and Mo in total of 1.
0% or less f group: 0.1% or less of one or two of Ti and V in total g group: 0.0030% or less of B group: h group: 0.0010 to 0.01 of one or two of Ca and REM in total
0%, one or more groups, the balance being a composition comprising Fe and unavoidable impurities, and the structure containing at least 50% by area of a ferrite phase having an average crystal grain size of 10 μm or less, and the balance being A high-tensile cold-rolled steel sheet having a structure mainly composed of pearlite, having excellent strain aging hardening properties and having a yield ratio of 0.7 or more.

The reason for limiting the composition of the steel sheet of the second preferred embodiment other than Al and N will be described. C: 0.15% or less C is an element that increases the strength of the steel sheet, and furthermore, as in the first preferred embodiment, achieves an average ferrite grain size of 10 μm or less, which is an important constituent element of the present invention. From the viewpoint of securing the strength, the content is preferably 0.005% or more. In addition, it is more preferably at least 0.03%.
On the other hand, if it exceeds 0.15%, the carbide fraction in the steel sheet becomes excessively large, ductility is remarkably reduced, formability is deteriorated, and spot weldability, arc weldability, etc. are further reduced. From the viewpoint of such formability and weldability, C is 0.15
% Or less. In the second preferred embodiment, the content is preferably 0.08% or less, and for applications requiring even better ductility, the content is preferably 0.05% or less.

Si: 2.0% or less Si is a useful element capable of increasing the strength of a steel sheet without significantly lowering the ductility of the steel. In the second preferred embodiment, Si is at least 0.005%, and In order to achieve this, the content is preferably 0.1% or more. On the other hand, Si
It is an element that significantly raises the transformation point during hot rolling and makes it difficult to ensure quality and shape, or has an adverse effect on the beauty of the steel sheet surface such as surface properties and chemical conversion treatment. Limited. When the content of Si is 2.0% or less, a remarkable increase in the transformation point can be suppressed by adjusting the amount of Mn added in combination, and good surface properties can be secured.

Mn: 3.0% or less Mn is an effective element for preventing hot cracking due to S.
As in the first preferred embodiment, it is preferable to add in accordance with the amount of S contained, and Mn has a great effect on the refinement of crystal grains, which is an important component of the present invention. It is preferable to use it for improving the material quality. From the viewpoint of stably fixing S, Mn is preferably contained at 0.2% or more.

Further, Mn is an element for increasing the strength of the steel sheet, and it is preferable that Mn is contained in an amount of 1.2% or more for a strength requirement of more than 500 MPa. The content is more preferably 1.5% or more from the viewpoint of ensuring a stable strength. When the Mn content is increased to this level, the variation in the mechanical properties and strain age hardening characteristics of the steel sheet with respect to variations in manufacturing conditions including hot rolling conditions is reduced, which is effective in stabilizing quality.

On the other hand, when Mn is contained in a large amount exceeding 3.0%, the hot deformation resistance of the steel sheet tends to increase, and the spot weldability and the formability of the welded portion tend to deteriorate. , The ductility tends to be significantly reduced. Therefore, Mn is 3.0%
Limited to the following. Mn is desirably set to 1.5% or less particularly for applications requiring better corrosion resistance and formability.

Further, Mn has a function of lowering the transformation point during hot rolling. By containing Mn together with Si, it is possible to offset an increase in the transformation point due to the inclusion of Si. Especially for products with a small thickness, the quality and shape change sensitively due to the change of the transformation point, so it is important to strictly balance the Mn and Si contents. Therefore, Mn / Si is 3.0
It is more preferable to make the above.

P: 0.08% or less P is a useful element as a solid solution strengthening element for steel. From this viewpoint, the content of P is desirably 0.001% or more. Decreases flange workability. In addition, P has a strong tendency to segregate in steel, which results in embrittlement of the weld. For this reason,
P was limited to 0.08% or less. When stretch flangeability and weld toughness are particularly important, the content is preferably 0.04% or less. It is more preferably 0.02% or less from the viewpoint of weld toughness.

S: 0.02% or less S is an element existing as an inclusion in a steel sheet and causes deterioration of ductility and corrosion resistance of the steel sheet. In the present invention, S is limited to 0.02% or less. In applications where good workability (especially stretch flangeability, hole expandability, etc.) is required, the content is preferably 0.015% or less. Further, when the required level of stretch flangeability is high, S is 0.008.
% Is preferable. Further, in order to stably maintain the strain age hardening characteristics at a high level, although the detailed mechanism is unknown, it is preferable to reduce S to 0.008% or less.

Nb: 0.007 to 0.04% Nb is important for remarkably refining crystal grains, raising YS, improving the yield ratio (YR = YS / TS) to 0.7 or more, and exhibiting large strain age hardening due to N. It is one of the important elements, and it is preferable to contain 0.007% or more in order to obtain this effect. On the other hand, in the present invention, in order to secure the required amount of solid solution N, when considering with other nitride forming elements,
Preferably, the Nb content is limited to 0.04% or less.

Nb in the precipitated state: 0.005% or more Regarding the Nb content in the steel sheet of the second preferred embodiment, the state of Nb present in the steel is also important. That is, it is necessary that a certain amount of Nb (also referred to as precipitated Nb) existing in a precipitated state be present in order to obtain stable strain aging hardening characteristics and to obtain a high yield ratio. If the Nb content is the second preferred embodiment of the present invention, it is desirable that at least 0.005% or more of precipitated Nb be present. For the determination of Nb, it is assumed that the Nb is dissolved and extracted by an electrolytic extraction method using an acetylacetone-based solvent. There are various melting methods, but the reason is that the values obtained by this method showed the best correlation with the strain age hardening characteristics of steel. Within the scope of the second preferred embodiment of the present invention, Nb is presumed to be more associated with C than with N, but details are unknown.

In the steel sheet according to the second preferred embodiment of the present invention, in addition to the above-mentioned chemical components, one or more of the following groups e to h: e, Cu, Ni, Cr and Mo are used in total. 1.
0% or less f group: 0.1% or less of one or two of Ti and V in total g group: 0.0030% or less of B group: h group: 0.0010 to 0.01 of one or two of Ca and REM in total
0% of one or more groups.

Elements of group e: Cu, Ni, Cr, and Mo are all elements that contribute to an increase in strength without a significant decrease in ductility of the steel sheet. In the second preferred embodiment, this effect is achieved by Cu: It is recognized at 0.01% or more, Ni: 0.01% or more, Cr: 0.01% or more, Mo: 0.01% or more, and can be selected as necessary and contained alone or in combination. However, if the content is too large, the hot deformation resistance increases, or the chemical conversion property and the surface treatment characteristics in a broad sense deteriorate, and the welded part is hardened and the weldability is reduced. Therefore, the elements of group e are 1.0 in total.
% Is preferable.

Elements of group f: Ti and V are elements that contribute to the refinement and uniformization of crystal grains, and this effect is observed at Ti: 0.002% or more and V: 0.002% or more, respectively.
It can be selected as necessary and contained alone or in combination. However, if the content is too large, the hot deformation resistance increases, and the chemical conversion property and the surface treatment properties in a broad sense deteriorate. For this reason,
It is preferable that the total of the elements in group f be 0.1% or less.

Elements of group g: B is an element having the effect of improving the hardenability of the steel, and is required for the purpose of increasing the fraction of the low-temperature transformation phase other than the ferrite phase and increasing the strength of the steel. It can be contained depending on. This effect is B: 0.
Appears when 0002% or more is added. However, if the amount is too large, the hot deformability decreases, and the amount of solute N is reduced by generating BN. For this reason, B is preferably set to 0.0030% or less.

Elements of Group h: Ca and REM are both elements that are useful for controlling the form of inclusions, and when stretch flangeability is required, they are preferably contained alone or in combination. In that case, the total of the elements of group h is 0.0010
%, The effect of controlling the morphology of inclusions is insufficient.
If it exceeds 0%, generation of surface defects becomes conspicuous. For this reason, it is preferable that the total of the elements in the h group be in the range of 0.0010 to 0.010%.

The balance other than the above chemical components is Fe and inevitable impurities. Inevitable impurities include O:
0.0050% or less is acceptable. The steel sheet of the second preferred embodiment of the present invention has an average crystal grain size of 10 μm in addition to the composition described above.
The ferrite phase of m or less has an area ratio of 50% or more, and the remainder has a structure mainly composed of pearlite. In addition to having a ferrite phase having the same average crystal grain size and area ratio as the steel sheet of the first preferred embodiment, as a phase other than the ferrite phase (second phase), a structure mainly composed of pearlite, that is, a single pearlite phase It is preferable to use a structure composed of a phase or a structure composed of bainite or martensite at an area ratio of 2% or less and the remainder composed of pearlite. By making the second phase other than the ferrite phase a structure mainly composed of the pearlite phase, the strength and the yield ratio are improved.

The second composition of the present invention having the above composition and structure
The preferred embodiment of the steel sheet is a high yield ratio type high tensile cold rolled steel sheet having a tensile strength TS of 440 MPa or more and a yield ratio of 0.7 or more and excellent strain aging hardening characteristics. Next, a third preferred embodiment of the steel sheet of the present invention will be described. The steel sheet according to a third preferred embodiment is characterized in that the composition is such that the above-mentioned Al: 0.02% or less, and N: 0.0050 to 0.0250%.
And N / Al is not less than 0.3 and N in solid solution is 0.0010
% Or more, C: 0.15% or less,
Mn: 3.0% or less, S: 0.02% or less, and Mo:
0.05 to 1.0%, Cr: one or two of 0.05 to 1.0%
Containing seeds or further groups j to m: j: Si: 0.05-1.5%, P: 0.03-0.15%, B: 0.00
One or more kinds of 03 to 0.01% k group: Nb: 0.01 to 0.1%, Ti: 0.01 to 0.2%, V: 0.01
1 or 2 or more of 0.2% 1 group: Cu: 0.05 to 1.5%, Ni: 1 or 2 kinds of 0.05 to 1.5% m group: Ca or REM 1 or 2 kinds in total 0.0010 to 0.01
0%, one or more groups, the balance being Fe and unavoidable impurities, and the structure contains a ferrite phase having an average crystal grain size of 10 μm or less in an area ratio of 50% or more,
Furthermore, it is a structure containing a martensitic phase in an area ratio of 3% or more, and has excellent strain aging hardening properties, workability,
It is a steel sheet characterized by excellent impact resistance.

The reason for limiting the composition other than Al and N of the steel sheet of the third preferred embodiment will be described. C: 0.15% or less C is an element that increases the strength of the steel sheet. Further, in order to achieve an average ferrite grain size of 10 μm or less, which is an important component of the present invention, a further desired strength is secured. From the viewpoint of forming a martensitic phase as two phases, 0.1
It is preferable to contain 005% or more. In addition, it is more preferably at least 0.03%. On the other hand, if it exceeds 0.15%, the carbide fraction in the steel sheet becomes excessively large, the ductility is remarkably reduced, the formability is deteriorated, the martensitic transformation temperature is lowered, and the spot weldability, arc weldability, etc. are remarkable. To decline. C was limited to 0.15% or less from the viewpoints of such formability, weldability, and microstructure control. Incidentally, preferably 0.
10% or less, 0.08 for applications requiring even better ductility
% Is preferable.

Mn: 3.0% or less Mn is an effective element for preventing hot cracking due to S.
It is preferable to add in accordance with the amount of S contained, and Mn has a great effect on refining crystal grains, which is an important constituent element of the present invention. preferable. Further, Mn is an element for improving the hardenability, and is preferably added positively from the viewpoint of stably forming a martensite phase as the second phase. From the viewpoint of fixing S and forming a martensite phase, it is preferable that Mn is contained in an amount of 0.2% or more.

Further, Mn is an element for increasing the strength of the steel sheet, and it is preferable that Mn is contained in an amount of 1.2% or more for a strength requirement of more than 500 MPa. From the viewpoint of obtaining a stable strength, the content is more preferably 1.5% or more. Further, when the Mn content is increased to this level, the variation in the mechanical properties and the strain age hardening characteristics of the steel sheet with respect to the fluctuations in the manufacturing conditions including the hot rolling conditions is reduced, which is effective in stabilizing the quality.

On the other hand, when Mn is contained in a large amount exceeding 3.0%, the hot deformation resistance of the steel sheet tends to increase, and the spot weldability and the formability of the welded portion tend to deteriorate. , The ductility tends to be significantly reduced. Therefore, Mn is 3.0%
Limited to the following. In applications where better corrosion resistance and moldability are required, Mn is desirably 2.5% or less.

S: 0.02% or less S is an element present as an inclusion in the steel sheet and causes deterioration of the ductility and corrosion resistance of the steel sheet. In the present invention, S is limited to 0.02% or less. For applications requiring particularly good workability (especially stretch flangeability and hole expandability), the content is preferably 0.015% or less. If the required level of stretch flangeability is high, S is 0.008%
It is preferable to set the following. Further, in order to stably maintain the strain age hardening characteristics at a high level, although the detailed mechanism is unknown, it is preferable to reduce S to 0.008% or less.

One or two of Mo: 0.05 to 1.0% and Cr: 0.05 to 1.0% Mo and Cr both contribute to an increase in the strength of the steel sheet and further improve the hardenability of the steel. An element that facilitates the formation of a martensite phase as a phase, and is contained alone or in combination. In particular, Mo and Cr have the effect of finely dispersing the martensite phase, and have the effect of reducing the yield strength and easily realizing a low yield ratio. These effects are
Both Mo and Cr are recognized at 0.05% or more. Meanwhile, Mo
If the content exceeds 1.0%, the processability and surface treatment properties are reduced, and the production cost is increased, which is economically disadvantageous. If the content of Cr exceeds 1.0%, the plating wettability decreases. Therefore, Mo is 0.05-1.0% and Cr is 0.05-1.0%
It is preferred to limit to.

In the steel sheet according to the second preferred embodiment of the present invention, in addition to the above-mentioned composition, the following j group to m group: j group: Si: 0.05 to 1.5%, P: 0.03 to 0.15%, B: 0.00
One or more kinds of 03 to 0.01% k group: Nb: 0.01 to 0.1%, Ti: 0.01 to 0.2%, V: 0.01
1 or 2 or more of 0.2% 1 group: Cu: 0.05 to 1.5%, Ni: 1 or 2 kinds of 0.05 to 1.5% m group: Ca or REM 1 or 2 kinds in total 0.0010 to 0.01
It is preferred to contain 0% or two or more groups.

Elements of group j: Si, P, and B are all effective elements for improving hardenability to form a martensite phase, and also improve ductility and r value to improve workability. It is an element that is effective for performing the above-mentioned process, and can be contained alone or in combination as necessary. Such an effect is obtained in the third preferred embodiment in which each of Si is 0.05% or more, P is 0.03% or more,
B is recognized at a content of 0.0003% or more. On the other hand,
When the content of Si exceeds 1.5%, the content of P exceeds 0.15%, and the content of B exceeds 0.01%, toughness, weldability, workability, etc., decrease. For this reason, Si
Is 0.05-1.5%, P is 0.03-0.15%, B is 0.0003-0.01
% Is preferable.

Elements of group k: Nb, Ti, and V are elements that contribute to the refinement and uniformization of crystal grains, and can be selectively used alone or in combination as necessary. In the third preferred embodiment, such an effect is achieved by Nb: 0.01% each.
As described above, it is recognized that the content of Ti: 0.01% or more and the content of V: 0.01% or more.
If the content exceeds 3, the hot deformation resistance is increased, and the chemical conversion property and the surface treatment properties in a broad sense are deteriorated. For this reason, N
b: 0.01-0.1%, Ti: 0.01-0.2%, V: 0.01-0.2
%.

Group 1 elements: Cu and Ni are both elements that improve the hardenability and contribute to the increase in the strength of the steel sheet, and can be optionally selected singly or in combination. Such an effect is, in the third preferred embodiment, each of C
u: 0.05% or more, Ni: 0.05% or more.
However, if the content exceeds 1.5% for Cu and 1.5% for Ni, respectively, the hot deformation resistance increases, or the chemical conversion property and the surface treatment properties in a broad sense deteriorate, and the welded part hardens and the formability of the welded part increases. Deteriorates. Therefore, each Cu: 0.05 ~
It is preferable to limit the content to 1.5% and Ni: 0.05 to 1.5%.

Elements of Group m: Ca and REM are both elements that are useful for controlling the form of inclusions, and when stretch flangeability is required, they are preferably contained alone or in combination. In that case, the total of the elements of group d is 0.0010
%, The effect of controlling the morphology of inclusions is insufficient.
If it exceeds 0%, generation of surface defects becomes conspicuous. For this reason, it is preferable to limit the total number of elements in the m group to the range of 0.0010 to 0.010%.

The balance other than the above components is Fe and inevitable impurities. O: 0.0050 as inevitable impurities
% Or less is acceptable. Next, the structure of the steel sheet according to the third preferred embodiment of the present invention will be described. The structure of the steel sheet according to the third preferred embodiment of the present invention contains a ferrite phase having an average crystal grain size of 10 μm or less as a main phase in an area ratio of 50% or more, and a martensite phase as a second phase in an area ratio of 3% or more. Microscopic tissue.

By containing the ferrite phase in an area ratio of 50% or more, the ductility required for a steel sheet for automobiles requiring high workability can be secured, as in the first preferred embodiment. If better ductility is required,
The area ratio of the ferrite phase is preferably set to 75% or more.
If the area ratio of the ferrite phase exceeds 97%, the effect as a composite structure cannot be expected. The martensite phase as the second phase exists mainly in the grain boundaries of the ferrite phase, which is the main phase. Martensite is a hard phase and has an effect of increasing the strength of a steel sheet by strengthening the structure. Furthermore, since transformation is accompanied by the occurrence of movable dislocations, it has an effect of improving ductility and decreasing the yield ratio of a steel sheet. These effects are remarkable when the martensite phase is present in an area ratio of 3% or more. If the content exceeds 30%, there is a problem that ductility is reduced. For this reason, the area ratio of the martensite phase as the second phase is 3% or more, preferably 30% or less, more preferably 20% or less. In addition, as the second phase, in addition to such an amount of martensite,
There is no problem even if the content of bainite is 10% or less.

The average crystal grain size of the ferrite phase is set to 10 μm or less in order to stabilize the strain aging hardening characteristics as in the first preferred embodiment. The steel sheet of the third preferred embodiment of the present invention having the above-described composition and structure has a tensile strength TS of 440 MPa or more, has excellent strain aging hardening properties, and has excellent workability and impact resistance properties. It becomes a steel plate.

Next, a method for producing the steel sheet of the present invention will be described. The steel sheet of the present invention basically has a composition within the above-mentioned range, that is, Al: 0.02% or less, N: 0.0050-0.
A steel slab containing 0250% and having a composition in which N / Al is 0.3 or more is heated to a slab heating temperature: 1000 ° C. or more,
Rough rolling is performed to form a sheet bar, and the sheet bar is subjected to finish rolling at a finish-rolling exit temperature of 800 ° C. or higher, and a winding temperature is:
A hot rolling step of rolling a hot-rolled sheet at 750 ° C. or lower, a cold rolling step of pickling and cold rolling the hot-rolled sheet to form a cold-rolled sheet, and applying a predetermined temperature to the cold-rolled sheet. This is manufactured by sequentially performing annealing for holding for a predetermined time, and then performing a cold rolled sheet annealing step for cooling at a predetermined cooling rate.

The slab used in the production method of the present invention is desirably manufactured by a continuous casting method in order to prevent macro segregation of components, but may be manufactured by an ingot-making method or a thin slab continuous casting method. In addition to the normal process of cooling the slab to room temperature and heating it again after production, the slab is inserted directly into a heating furnace without cooling and then rolled, or is rolled immediately after a slight heat retention. Energy saving processes such as direct rolling can be applied without any problem. In particular, in order to effectively secure N in a solid solution state, direct rolling in which precipitation of N is delayed is one of useful techniques.

First, the reasons for limiting the hot rolling process conditions will be described. Slab heating temperature: 1000 ° C or higher The slab heating temperature is set as the initial state, ensuring a necessary and sufficient amount of solid solution N, and the target value of solid solution N in the product (0.0010%
In order to satisfy the above conditions, the temperature is preferably set to 1000 ° C. or higher. Note that the slab heating temperature is preferably set to 1280 ° C. or lower from the viewpoint of avoiding an increase in loss due to an increase in oxidation weight.

The slab heated under the conditions described above is turned into a sheet bar by rough rolling. The conditions for the rough rolling need not be particularly defined, but may be performed according to a conventional method. However, from the viewpoint of securing the amount of dissolved N, it is desirable to perform the treatment in as short a time as possible. Next, the sheet bar is finish-rolled to obtain a hot-rolled sheet.

In the present invention, it is preferable that the sheet bars adjacent to each other be joined between the rough rolling and the finish rolling and the finish rolling be performed continuously. As a joining means, a pressure welding method,
It is preferable to use a laser welding method, an electron beam welding method, or the like. As a result, the existence ratio of the unsteady portion (the leading end and the trailing end of the material to be processed), which is likely to cause shape disturbance in the finish rolling and the subsequent cooling, is reduced, and the stable pressure is extended (the continuous length that can be rolled under the same conditions). ) And the stable cooling length (continuous length that can be cooled while applying tension) increase the product shape / dimensional accuracy and yield.

Further, lubricating rolling for thin materials and wide widths, which was difficult to perform in conventional single-rolling for each sheet bar due to problems such as threadability and biting properties, can be easily performed. And the life of the roll is extended. In the present invention, between the rough rolling and the finish rolling, one or both of a sheet bar edge heater for heating the width end of the sheet bar and a sheet bar heater for heating the length end of the sheet bar are used. Thus, it is preferable to uniform the temperature distribution in the width direction and the longitudinal direction of the sheet bar. Thereby, the material variation in the steel plate can be further reduced. It is preferable that the sheet bar edge heater and the sheet bar heater are of an induction heating type.

It is desirable to use the sheet bar edge heater to compensate for the temperature difference in the width direction. The heating amount at this time depends on the steel composition and the like, but is preferably set so that the temperature distribution range in the width direction on the finish-rolling exit side is approximately 20 ° C. or less. Next, the temperature difference in the longitudinal direction is compensated by the sheet bar heater. The heating amount at this time is preferably set such that the temperature at the end of the length is higher by about 20 to 40 ° C. than the temperature at the center.

Finish rolling exit side temperature: 800 ° C. or more The finish rolling exit side temperature FDT is set to 800 ° C. or more in order to make the structure of the steel sheet uniform and fine. If the FDT is lower than 800 ° C., the structure becomes non-uniform, and a partially processed structure remains. Such a residue of the processed structure can be avoided by setting the winding temperature to a high temperature. However, when the winding temperature is set to a high temperature, coarse crystal grains are generated, and the amount of solute N is greatly reduced. In order to further improve the mechanical properties, it is desirable that the FDT be 820 ° C. or higher. Further, from the viewpoint of preventing the occurrence of scale flaws or the like due to an excessively high rolling temperature, the FDT is preferably set to 1000 ° C. or less. After the finish rolling, it is desirable to cool the steel sheet early in order to refine the crystal grains and secure the amount of solute N.

Cooling after finish rolling: 0.5 after finishing rolling
In the present invention, cooling is started within seconds and the cooling rate is 40 ° C./s or more. It is desirable. By satisfying this condition, Al
The high-temperature region where N precipitates can be rapidly cooled, and N in a solid solution state can be effectively secured. If the cooling start time or cooling rate does not satisfy the above conditions, the grain growth proceeds too much to make the crystal grain size finer, and the precipitation of AlN due to the strain energy introduced by rolling is too advanced. As a result, the possibility that the amount of solute N is deficient increases. From the viewpoint of ensuring the uniformity of the material and the shape, it is preferable to suppress the cooling rate to 300 ° C./s or less.

Winding temperature: 750 ° C. or less As the winding temperature CT decreases, the steel sheet strength tends to increase. In order to secure the target tensile strength of TS440 MPa or more, it is preferable that CT is 750 ° C. or less. Note that C
If T is less than 200 ° C., the shape of the steel sheet tends to be disordered, and there is a high risk of causing a problem in actual operation, and the uniformity of the material tends to be reduced. For this reason, CT is desirably 200 ° C. or higher. When more uniform material is required, the CT is preferably set to 300 ° C. or higher. The temperature is more preferably 450 ° C. or higher.

In the present invention, lubricating rolling may be performed in finish rolling to reduce the hot rolling load. By performing the lubrication rolling, there is an effect that the shape and material of the hot rolled sheet are made more uniform. The coefficient of friction during lubricating rolling is preferably in the range of 0.25 to 0.10.
The operation of hot rolling is further stabilized by combining lubricating rolling and continuous rolling.

The hot-rolled sheet that has been subjected to the above-mentioned hot rolling step is then subjected to pickling and cold rolling in a cold-rolling step to become a cold-rolled sheet. The conditions for pickling may be generally known conditions, and are not particularly limited. When the scale of the hot-rolled sheet is extremely thin, cold rolling may be immediately performed without performing pickling.

[0108] The cold rolling conditions may be those generally known, and are not particularly limited. In addition, it is preferable that the cold rolling reduction is 40% or more from the viewpoint of ensuring the uniformity of the structure. Next, the cold rolled sheet is subjected to a cold rolled sheet annealing step by continuous annealing. After the cold-rolled sheet annealing step, temper rolling or leveler processing with an elongation of 1.0 to 15% may be further performed. By performing temper rolling or leveling after the cold-rolled sheet annealing step, strain age hardening characteristics such as BH amount and ΔTS amount can be stably improved. The total elongation in the temper rolling or leveler processing is preferably 1.0% or more. When the elongation is less than 1.0%, the improvement of the strain age hardening property is small, while when the elongation exceeds 15%,
The ductility of the steel sheet decreases. Although the working modes are different between the temper rolling and the leveler working, the present inventors have confirmed that there is no significant difference in the effect on the strain aging hardening characteristics of the steel sheet.

In a first preferred embodiment of the present invention, the slab composition contains Al: 0.02% or less, N: 0.0050 to 0.0250%, N / Al is 0.3 or more, and C: 0.15% or less, : Not more than 2.0%, Mn: not more than 3.0%, P: not more than 0.08%, S: not more than 0.02%, or further groups a to d:
Group a group: One or more of Cu, Ni, Cr, and Mo are 1.
0% or less b group: 0.1% or more of one or more of Nb, Ti and V
Group c: B is 0.0030% or less Group d: One or two of Ca and REM are 0.0010 to 0.01 in total
0%, preferably using a slab composed of the balance of Fe and unavoidable impurities, and passing through the above-mentioned hot rolling step and the above-mentioned cold rolling step to form a cold-rolled sheet. Hold temperature at recrystallization temperature and 900 ° C or less: 10-60s
The primary cooling is performed at a cooling rate of 10 to 300 ° C./s to a temperature range of 500 ° C. or less, and the residence time in the temperature range of 400 ° C. or more below the primary cooling stop temperature is 300 s. A cold rolled sheet annealing step for performing the following secondary cooling is sequentially performed.

The reason for limiting the slab composition in the first preferred embodiment of the present invention is the same as the reason for limiting the steel sheet composition in the first preferred embodiment of the present invention. In the first preferred embodiment of the present invention, the winding temperature in the hot rolling step is more preferably 650 ° C. or less from the viewpoint of securing strength.
Next, the reason for limiting the cold-rolled sheet annealing step in the first preferred embodiment of the present invention will be described.

Continuous annealing temperature: not lower than recrystallization temperature and not higher than 900 ° C. The annealing temperature in continuous annealing is preferably not lower than the recrystallization temperature. If the continuous annealing temperature is lower than the recrystallization temperature, recrystallization is not completed and the strength satisfies the target, but the ductility is low. In order to further improve the formability, the continuous annealing temperature is preferably set to 700 ° C. or higher. On the other hand, if the continuous annealing temperature exceeds 900 ° C., nitrides such as AlN are precipitated, and the amount of solute N in the product steel plate becomes insufficient. For this reason,
The continuous annealing temperature is preferably not lower than the recrystallization temperature and not higher than 900 ° C.

The holding time at the continuous annealing temperature: 10 to 60 s The holding time at the continuous annealing temperature is preferably as short as possible from the viewpoint of refining the structure and securing the amount of solute N more than desired. However, it is desirable to set it to 10 s or more from the viewpoint of operation stability. If the holding time exceeds 60 s, it will be difficult to refine the structure and secure the amount of dissolved N. For this reason, the holding time at the continuous annealing temperature is preferably in the range of 10 to 60 s.

Primary cooling: Cooling rate to a temperature range of 500 ° C. or less: 10-300 ° C./s Cooling after soaking in continuous annealing is important from the viewpoint of refining the structure and securing the amount of dissolved N. In the present invention, as primary cooling, continuous cooling is performed at a cooling rate of 10 to 300 ° C./s to a temperature range of 500 ° C. or less. When the cooling rate is less than 10 ° C./s, it is difficult to secure a uniform and fine structure and a desired amount of solid solution N or more. On the other hand, if the cooling rate exceeds 300 ° C./s, the uniformity of the material in the width direction of the steel sheet is insufficient. If the cooling stop temperature at the time of cooling at a cooling rate of 10 to 300 ° C./s exceeds 500 ° C., fine structure cannot be achieved.

Secondary cooling condition: Cooling in which the residence time in a temperature range of 400 ° C. or more, which is lower than the cooling stop temperature of the primary cooling, is 300 s or less. Secondary cooling after the primary cooling is important from the viewpoint of strain aging hardening characteristics. Become. Although the detailed mechanism is unknown at present, it is presumed that the amount of dissolved C and N changes depending on the conditions of the secondary cooling, which affects the strain aging characteristics. In the present invention, it is preferable that the cooling is continued after the primary cooling, and the cooling time is set to 300 s or less in a temperature range of 400 ° C. or more which is lower than the primary cooling stop temperature. In the present invention, a so-called overaging treatment may be performed after the continuous annealing, but when the overaging treatment is performed, the strain aging hardening characteristic is deteriorated. Therefore,
In the present invention, when the overaging zone of the continuous annealing furnace is passed through, it is desirable that the temperature of the overaging zone be set to an extremely low temperature. In the first preferred embodiment of the present invention, the above-mentioned temper rolling or leveling may be performed.

Further, in the second preferred embodiment of the present invention, the slab composition is such that Al: 0.02% or less, N: 0.0050 to 0.0250%.
And N / Al is 0.3 or more, and C: 0.15
%, Si: 2.0% or less, Mn: 3.0% or less, P: 0.08%
In the following, S: 0.02% or less, Nb: 0.007 to 0.04%, or further the following groups e to h: e group: one or more of Cu, Ni, Cr and Mo in total of 1.
0% or less f group: 0.1% or less of one or two of Ti and V in total g group: 0.0030% or less of B group: h group: 0.0010 to 0.01 of one or two of Ca and REM in total
0%, a slab containing at least one group selected from the group consisting of Fe and unavoidable impurities is used, and in the above-mentioned hot rolling step, the slab heating temperature is set to 1100 ° C. or more. Further, after a hot rolling step of reducing the final pass of the final pass of the finish rolling applied to the sheet bar to 25% or more, and a cold-rolled sheet through the above-described cold rolling step, a recrystallization temperature Holding time at a temperature not less than 900 ° C and not more than:
Annealing is performed for 10 to 90 s, and then a cold rolled sheet annealing step of cooling at a cooling rate of 70 ° C./s or less to a temperature range of 600 ° C. or less is sequentially performed.

The reason for limiting the slab composition in the second preferred embodiment of the present invention is the same as the reason for limiting the steel sheet in the second preferred embodiment of the present invention. In the second preferred embodiment of the present invention, the slab heating temperature is set so as to secure a necessary and sufficient amount of solid solution N as an initial state and to satisfy a target value (0.0010% or more) of the amount of solid solution N in a product. Preferably, the temperature is 1100 ° C. or higher. Note that the slab heating temperature is preferably set to 1280 ° C. or lower from the viewpoint of avoiding an increase in loss due to an increase in oxidation weight.

[0117] From the viewpoint of refining the microstructure of the steel sheet and improving workability such as hole expandability, it is preferable that the rolling reduction in the final pass of the finish rolling is 25% or more. The final pass of finish rolling is one of the important factors governing the microstructure of the steel sheet. By applying a rolling reduction of 25% or more in this pass, the ferrite can be transformed from the unrecrystallized austenite state in which the strain is sufficiently accumulated, thereby remarkably refining the microstructure of the hot-rolled base plate. Achieved. By performing cold rolling and annealing using this as a material, not only the microstructure after cold rolling annealing but also uniformity can be achieved. That is, the particle size distribution of the ferrite phase does not vary, and the dispersed pearlite phase is also finely divided so as to be present uniformly. This has the advantage that hole expandability and the like are also improved.

Further, in the second preferred embodiment of the present invention, the winding temperature in the hot rolling step is 650 from the viewpoint of securing strength.
It is preferable that the temperature is set to not more than ° C. Next, the reason for limiting the cold-rolled sheet annealing step in the second preferred embodiment of the present invention will be described. Continuous annealing temperature: not less than recrystallization temperature and not more than 900 ° C The annealing temperature in continuous annealing was set to be not less than the recrystallization temperature.

When the continuous annealing temperature is lower than the recrystallization temperature, recrystallization is not completed, and the strength satisfies the target, but the ductility is low. In order to further improve the formability, the continuous annealing temperature is preferably set to 700 ° C. or higher. On the other hand, if the continuous annealing temperature exceeds 900 ° C., nitrides such as AlN are precipitated, and the amount of solute N in the product steel plate becomes insufficient. In addition, the crystal grains are coarsened, and the risk of impairing the surface appearance is increased. Therefore, the continuous annealing temperature is 900
C. or lower is preferred. When a particularly high yield ratio is desired, the annealing temperature is preferably set to 850 ° C. or less from the viewpoints of preventing structure coarsening and reducing solid solution N loss due to the progress of precipitation.

Holding time at the continuous annealing temperature: 10 to 90 s It is preferable that the holding time at the continuous annealing temperature is as short as possible from the viewpoint of refining the structure and securing the amount of solute N more than desired. From the viewpoint of operation stability, it is desirable to set the time to 10 s or more. On the other hand, if the holding time exceeds 90 s, it becomes difficult to refine the structure and secure the amount of dissolved N. For this reason, the holding time at the continuous annealing temperature is preferably in the range of 10 to 90 s.

Cooling after holding at the annealing temperature (primary cooling): 60
Cooling rate of 70 ° C / s or less to a temperature range of 0 ° C or less.
Cool continuously at a cooling rate of 70 ° C / s or less to the following temperature range. When the cooling rate exceeds 70 ° C./s, the yield ratio decreases, and further, the uniformity of the material in the width direction of the steel sheet is insufficient. Preferably, the cooling rate is 5 ° C./s or more to secure TS and YS. If the cooling stop temperature when cooling at such a cooling rate is higher than 600 ° C, bake hardenability decreases,
Not preferred.

After the primary cooling is completed, a so-called overaging treatment for maintaining the temperature in a predetermined temperature range may be performed, or may not be performed. However, from the viewpoint of further improving the material, especially the ductility, it is desirable to reduce the solid solution C as much as possible to reduce the room-temperature aging hardening, and to further make the influence of the solid solution N on the strain aging hardening characteristics more apparent. , 35
It is preferable to carry out an overaging treatment of maintaining the temperature in a temperature range of 0 to 450 ° C. for a time of 120 s or less.

Further, in the second preferred embodiment of the present invention, similarly to the first preferred embodiment, after the cold-rolled sheet annealing step, further temper rolling or leveler processing with an elongation of 1.0 to 15% is performed. You may. In the second preferred embodiment of the present invention,
More preferably, the elongation is 1.5% or more. In a third preferred embodiment of the present invention, the slab composition
Al: 0.02% or less, N: 0.0050 to 0.0250%, and N
/ Al is 0.3 or more, C: 0.15% or less, Mn: 3.
0% or less, S: 0.02% or less, and Mo: 0.05 to
1.0%, Cr: One or two of 0.05 to 1.0%, or the following j group to m group: j group: Si: 0.05 to 1.5%, P: 0.03 to 0.15%, B: 0.00
One or more kinds of 03 to 0.01% k group: Nb: 0.01 to 0.1%, Ti: 0.01 to 0.2%, V: 0.01
1 or 2 or more of 0.2% 1 group: Cu: 0.05 to 1.5%, Ni: 1 or 2 kinds of 0.05 to 1.5% m group: Ca or REM 1 or 2 kinds in total 0.0010 to 0.01
0%, one or more groups selected from the group consisting of 0%, preferably a slab composed of the balance of Fe and unavoidable impurities, and subjected to cold rolling through the above hot rolling step and the above cold rolling step. After forming the sheet, the cold-rolled sheet is added with (Ac ₁ transformation point)
Annealing is performed at a temperature of (Ac ₃ transformation point) at a holding time of 10 to 120 s, and then the average cooling rate between 600 and 300 ° C. is calculated by the following equation (1) or (2). -1.73 [Mn + 2.67Mo + 1.3Cr + 0.26Si + 3.5P + 0.05Cu + 0.05Ni] +3.95 ... (1) When B≥0.0003% log CR = -1.73 [Mn + 2.67Mo + 1.3Cr + 0.26Si + 3.5P + 0.05Cu + 0 .05Ni] +3.40 ... (2) (where, CR: cooling rate (° C / s), Mn, Mo, Cr, S
i, P, Cu, Ni: a cold rolled sheet annealing step of cooling at a critical cooling rate CR or more defined by the content of each element (% by mass).

The reasons for limiting the slab composition in the third preferred embodiment of the present invention are the same as those for limiting the third preferred embodiment of the present invention. The reason for limiting the cold-rolled sheet annealing step in the third preferred embodiment of the present invention will be described. Annealing temperature: (Ac ₁ transformation point) to (Ac ₃ transformation point) Annealing is preferably performed continuously from the viewpoint of productivity. In the annealing treatment, heating is performed to a temperature in a two-phase region of (Ac ₁ transformation point) to (Ac ₃ transformation point). By heating to the two-phase region, the austenite (γ) phase and the ferrite (α) phase
C is concentrated in the γ phase, and the γ phase is transformed into a martensite phase during cooling, forming a second phase to form a composite structure of α + martensite. Thereby, ductility and workability are improved, and a low yield ratio is realized.

[0125] On the other hand, it is less than the annealing temperature Ac ₁ transformation point, become a ferrite + pearlite structure, in more than Ac ₃ transformation point,
The alloy element concentration in the γ phase is insufficient, so that it is difficult to obtain a composite structure of α + martensite, and both have insufficient ductility. Annealing holding time: 10 to 120 s The holding time at the annealing temperature is preferably 10 to 120 s. The holding time at the annealing temperature is as follows:
From the viewpoint of securing the amount, it is preferable to set the time as short as possible, but from the stability of operation, it is preferable to set the time to 10 s or more. If the holding time exceeds 120 s, it becomes difficult to refine the structure and secure the amount of dissolved N.

The heating to the soaking temperature for annealing is preferably performed at a heating rate of at least 5 ° C./s between at least 600 ° C. and (Ac ₁ transformation point). If it is less than 5 ° C./s, there is a problem in securing the amount of dissolved N. More preferably 5-30 ° C / s
It is. Cooling after soaking: The average cooling rate between 600 to 300 ° C. is equal to or higher than the critical cooling rate CR. According to the present invention, the average cooling rate between 600 and 300 ° C. is calculated according to the following formula (1) or (2) according to the amount of alloying elements. .26Si + 3.5P + 0.05Cu + 0.05Ni] +3.95 ... (1) When B ≧ 0.0003% log CR = -1.73 [Mn + 2.67Mo + 1.3Cr + 0.26Si + 3.5P + 0.05Cu + 0.05Ni] +3.40 ... ( 2) (where, CR: cooling rate (° C / s), Mn, Mo, Cr, S
Cooling is performed at a critical cooling rate CR or more defined by i, P, Cu, and Ni: content of each element (% by mass). In addition,
In the formulas (1) and (2), elements that are not contained are calculated as 0.

By cooling at an average cooling rate equal to or higher than the critical cooling rate CR in one of the formulas (1) and (2) according to the amount of alloying elements, pearlite precipitation during cooling can be prevented. When cooled at a cooling rate less than CR (° C./s) defined by the above equations, the second phase is transformed into martensite M
(Some may contain bainite B), and the structure of the product plate cannot be a composite structure composed of α + M (+ B). The average cooling rate is 300
If the temperature exceeds ℃ / s, the uniformity of the material in the width direction of the steel sheet is insufficient. For this reason, the cooling rate after annealing is set so that the average cooling rate between 600 and 300 ° C. is equal to or higher than CR defined by the formula (1) or (2), preferably equal to or lower than 300 ° C./s. The average cooling rate in a temperature range lower than 300 ° C. is preferably 5 ° C./s or more.

Further, in the third preferred embodiment of the present invention, similarly to the first or second preferred embodiment, after the cold-rolled sheet annealing step, temper rolling or elongation of 1.0 to 15% is further performed. Leveler processing may be performed.

[0129]

EXAMPLES Example 1 Molten steel having the composition shown in Table 1 was smelted in a converter and made into a slab by a continuous casting method. These slabs were heated under the conditions shown in Table 2, rough-rolled to form sheet bars having the thickness shown in Table 2, and then hot-rolled by a hot rolling step of finish rolling under the conditions shown in Table 2. In addition, lubrication rolling was performed by finish rolling about a part.

These hot-rolled sheets were formed into cold-rolled sheets by a cold rolling process including pickling and cold rolling under the conditions shown in Table 2.
Next, continuous annealing was performed on these cold-rolled sheets in a continuous annealing furnace under the conditions shown in Table 2. Some parts were subjected to temper rolling after the cold-rolled sheet annealing step. The annealing temperatures in the continuous annealing were all higher than the recrystallization temperature. With respect to the obtained cold-rolled annealed sheet, the amount of dissolved N, microstructure, tensile properties, formability, strain age hardening properties, fatigue resistance properties and impact resistance properties were investigated. (1) Investigation of the amount of solute N The amount of solute N was determined by subtracting the amount of precipitated N from the total amount of N in steel determined by chemical analysis. The amount of precipitated N was determined by an analytical method using the above-described potentiostatic electrolysis method. (2) Microstructure A specimen is collected from each cold-rolled annealed plate, and a microstructure is imaged using a light microscope or a scanning electron microscope for a cross section (C cross section) orthogonal to the rolling direction, and an image analyzer is used. Was used to determine the structure fraction of ferrite as the main phase and the type of the second phase or further the structure fraction.

The crystal grain size of ferrite, which is the main phase, can be determined by a value calculated by a quadrature method specified by ASTM from a micrograph of a cross section (C cross section) perpendicular to the rolling direction or by a cutting method specified by ASTM. The larger of the determined nominal particle sizes was adopted. (3) Tensile properties A JIS No. 5 test piece was sampled from each cold-rolled annealed plate in the rolling direction.
A tensile test was performed at a strain rate of 3 × 10 ⁻³ / s in accordance with the provisions of JIS Z 2241, yield strength YS, tensile strength TS, elongation E
1 was determined. (4) Moldability The r value was determined as an index of moldability.

Rolling direction (L direction) of each cold-rolled annealed sheet, rolling
45 ° direction (D direction), 90 ° direction to rolling direction
From the direction (C direction), JIS No. 13B test pieces were collected. this
Specimens when 15% uniaxial tensile prestrain was applied to them
Is obtained, and the ratio of the width distortion to the thickness distortion is given by: r = ln (w / w₀) / Ln (t / t₀(Where w₀, T₀Is the width and thickness of the specimen before the test
And w and t are the width and thickness of the test piece after the test.
You. ) Is determined in each direction from the following equation._mean= (R_L+2 r_D+ R_c) / 4 gives the average r value r_meanI asked. Where r_LRolled
R value in the direction (L direction), and r_DIs the rolling direction (L direction
R) in the 45 ° direction (D direction) with respect to _cIs
The r value in the 90 ° direction (C direction) with respect to the rolling direction (L direction)
is there. (5) Strain aging hardening characteristics A JIS No. 5 test piece was sampled from each cold-rolled annealed sheet in the rolling direction.
As a pre-deformation, a tensile pre-strain of 5% is given here.
A heat treatment equivalent to a paint baking treatment at 170 ° C for 20 minutes was performed.
C, strain rate: 3 × 10^-3/ S tensile test and pre-deformation
-Tensile properties after paint baking (yield stress YS_BH, Tensile strength
Sa TS_BH), And BH amount = YS_BH-YS_Five%, ΔTS =
TS_BH-TS was calculated. YS_Five%Is 5 product plates
% Is the deformation stress when pre-deformed, YS_BH, TS_BHIs
Pre-deformation-the yield stress and tensile strength after paint baking,
TS is the tensile strength of the product plate. (6) Fatigue resistance characteristics Fatigue test specimens were taken from each cold-rolled annealed sheet in the rolling direction and subjected to JIS.
Tensile with minimum stress of 0 MPa according to the rules of Z 2273
Perform a fatigue test and limit the fatigue (number of repetitions: 10⁷Times) σ_FL
I asked. In addition, a 5% tensile prestrain is given as pre-deformation.
Then, heat treatment equivalent to paint baking at 170 ° C for 20 minutes
After performing the same fatigue test, the fatigue limit (σ_FL)
_BHAnd the direction of fatigue resistance characteristics due to pre-deformation-paint baking treatment.
Era ((σ _FL)_BH−σ_FL) Was evaluated. (7) Impact resistance properties An impact test specimen was sampled from each cold-rolled annealed sheet in the rolling direction, and "Jo
urnal of the Society of Materials Science Japan, 10
(1998), p1058 ''
And strain rate: 2 × 10^Three/ S high-speed tensile test
The stress-strain curve was measured. Using the obtained stress-strain curve,
And integrate the stress in the range of strain 0-30% to absorb energy
E was sought. In addition, a 5% tensile prestrain is given as pre-deformation.
Then, heat treatment equivalent to baking at 170 ° C for 20 minutes
After performing the same impact test,
ー E_BHAnd the impact resistance due to pre-deformation-paint baking treatment
Improvement E_BH/ E was evaluated.

[0133] Hot-dip galvanizing was performed on some of the steel sheet surfaces to obtain galvanized steel sheets, and various properties were similarly evaluated. Table 3 shows the results.

[0134]

[Table 1]

[0135]

[Table 2]

[0136]

[Table 3]

The examples of the present invention all have excellent ductility and excellent strain aging hardening characteristics, and have a markedly high BH content and ΔTS
In addition, the improvement in fatigue resistance and impact resistance by strain aging treatment is large. In addition, the characteristics of the galvanized steel sheets obtained by applying the hot-dip galvanizing to the steel sheet surfaces of No. 11 and No. 13 were almost the same as those before the plating.

Further, with respect to the steel sheet No. 1 as an example of the present invention and the steel sheet No. 5 as a comparative example, the aging conditions were variously changed and the strain aging hardening characteristics were examined. Table 4 shows the results. The test method was the same except that only the aging temperature and the aging time were changed.

[0139]

[Table 4]

In the steel sheet No. 1 which is an example of the present invention, the BH amount was 115 MPa by the aging treatment of 170 ° C. × 20 min which is the standard aging condition.
, ΔTS60MPa, but the BH amount was 80MPa or more and ΔTS40
MPa or more could be satisfied. On the other hand, in the comparative example, 10
Even when the aging temperature was changed in the range of 0 to 300 ° C., a large BH amount and ΔTS were not shown as in the examples of the present invention.

That is, the steel sheet of the present invention can secure a high BH amount and ΔTS even under a wide range of aging conditions. (Example 2) A steel having a composition shown in Table 5 was formed into a slab in the same manner as in Example 1, and the slab was heated under the conditions shown in Table 6 and roughly rolled to form a 25 mm thick sheet bar. Subsequently, a hot-rolled sheet was obtained by a hot rolling step of performing finish rolling under the conditions shown in Table 6. In addition, after rough rolling, the sheet bars that were adjacent to each other on the entry side of the finish rolling were joined by a melt pressure welding method and were continuously rolled.
In addition, the sheet bar temperature was adjusted at the width end and the end in the length direction of the sheet bar by using an induction heating type sheet bar edge heater and a sheet bar heater.

These hot-rolled sheets were cold-rolled to a thickness of 1.6 mm by a cold rolling process comprising pickling and cold rolling under the conditions shown in Table 6. Subsequently, continuous annealing was performed on these cold-rolled sheets by a continuous annealing furnace under the conditions shown in Table 6. In addition, the annealing temperature of the continuous annealing was all higher than the recrystallization temperature. With respect to the obtained cold-rolled annealed sheet, the amount of dissolved N, microstructure, tensile properties, moldability, strain aging hardening properties, fatigue resistance properties and impact resistance properties were examined in the same manner as in Example 1.

Table 7 shows the results.

[0144]

[Table 5]

[0145]

[Table 6]

[0146]

[Table 7]

Each of the examples of the present invention has excellent strain aging hardening characteristics, exhibits a significantly high BH amount and ΔTS stably despite the fluctuations in the manufacturing conditions, and exhibits fatigue resistance due to the strain aging treatment. The improvement in the characteristics and impact resistance is also large. Further, in the example of the present invention, the thickness accuracy and the shape of the product steel plate were improved by performing the continuous rolling and the temperature adjustment in the longitudinal direction and the width direction of the sheet bar. (Example 3) Molten steel having the composition shown in Table 8 was melted in a converter and made into a slab by a continuous casting method. These slabs were heated under the conditions shown in Table 9 and roughly rolled to obtain sheet bars having the thickness shown in Table 9, and then hot rolled by a hot rolling step of finish rolling under the conditions shown in Table 9. In addition, lubrication rolling was performed by finish rolling about a part.

These hot-rolled sheets were formed into cold-rolled sheets by a cold rolling process comprising pickling and cold rolling under the conditions shown in Table 9.
Next, continuous annealing was performed on these cold-rolled sheets in a continuous annealing furnace under the conditions shown in Table 9. After the cold-rolled sheet annealing step, temper rolling was performed. The annealing temperatures in the continuous annealing were all higher than the recrystallization temperature. With respect to the obtained cold rolled annealed sheet, the amount of solute N, microstructure, tensile properties, and strain age hardening properties were investigated in the same manner as in Example 1.

For No. 3-G steel (Steel Sheet No. 3-9), a hot-dip galvanized steel sheet was also used to produce a plated steel sheet, and various properties were evaluated in the same manner. The annealing conditions in the continuous plating line were the same as those in the continuous annealing line. Table 10 shows the obtained results.

[0150]

[Table 8]

[0151]

[Table 9]

[0152]

[Table 10]

All of the examples of the present invention had excellent ductility, a high yield ratio, and excellent strain aging hardening characteristics, and exhibited a remarkably high BH amount and ΔTS. No.3-G steel (steel plate No.3
On the other hand, the tensile properties of hot-dip galvanized coated steel sheet tend to decrease TS slightly compared to the tensile properties without galvanizing. However, considering the balance between strength and elongation, almost the same properties can be obtained. Can be

Further, with respect to the steel sheet No. 3-1 as an example of the present invention and the steel sheet No. 3-10 as a comparative example, strain aging hardening characteristics were examined by changing aging conditions variously. Table 11 shows the results. In the test method, only the aging temperature and the aging time among the above conditions were changed.

[0155]

[Table 11]

In the example of the present invention (steel No. 3-1), the aging treatment at 170 ° C. × 20 min, which is the standard aging condition, was carried out at a BH amount of 90 MPa,
A value of ΔTS50MPa was obtained, but even under a wide range of aging treatment conditions as shown in Table 11, the BH amount was 80MPa or more and ΔTS40MPa.
I was able to satisfy the above. On the other hand, the comparative example (steel sheet No. 3
In the case of -10), even when the aging temperature was changed in the range of 100 to 300 ° C., a large BH amount and ΔTS as in the examples of the present invention were not exhibited.

That is, the steel sheet of the present invention can ensure a high BH content and ΔTS even under a wide range of aging conditions. (Example 4) A steel having the composition shown in Table 12 was formed into a slab in the same manner as in Example 3, and the slab was heated under the conditions shown in Table 13 and roughly rolled into a sheet bar having a thickness of 25 mm. Table
A hot rolled sheet was obtained by a hot rolling step of performing finish rolling under the conditions shown in FIG. In addition, after rough rolling, the sheet bars that were adjacent to each other on the entry side of the finish rolling were joined by a melt pressure welding method and were continuously rolled.
In addition, the sheet bar temperature was adjusted at the width end and the end in the length direction of the sheet bar by using an induction heating type sheet bar edge heater and a sheet bar heater.

These hot-rolled sheets were formed into a cold-rolled sheet having a thickness of 1.6 mm by a cold rolling process including pickling and cold rolling under the conditions shown in Table 13. Subsequently, continuous annealing was performed on these cold-rolled sheets in a continuous annealing furnace under the conditions shown in Table 13. In addition, the annealing temperature of the continuous annealing was all higher than the recrystallization temperature. For the obtained cold-rolled annealed sheet, in the same manner as in Example 1, the amount of solute N,
The microstructure, tensile properties and strain age hardening properties were investigated.

Table 14 shows the results.

[0160]

[Table 12]

[0161]

[Table 13]

[0162]

[Table 14]

Each of the examples of the present invention has excellent ductility, a high yield ratio, and excellent strain aging hardening characteristics.
Was presented. Further, in the example of the present invention, the thickness accuracy and the shape of the product steel plate were improved by performing the continuous rolling and the temperature adjustment in the longitudinal direction and the width direction of the sheet bar. (Example 5) Molten steel having the composition shown in Table 15 was melted in a converter and made into a slab by a continuous casting method. These slabs were heated under the conditions shown in Table 16, rough-rolled to obtain sheet bars having the thickness shown in Table 16, and then hot-rolled by a hot rolling step of performing finish rolling under the conditions shown in Table 16. In addition, about a part (steel sheet N
o.5-2, No.5-3), lubrication rolling was performed by finish rolling. In addition, for a part thereof, the sheet bars that are adjacent to each other on the entry side of the finish rolling after the rough rolling were joined by a melt pressure welding method and continuously rolled.
In addition, the sheet bar temperature was adjusted at the width end and the end in the length direction of the sheet bar by using an induction heating type sheet bar edge heater and a sheet bar heater.

[0164] These hot-rolled sheets were formed into cold-rolled sheets by a cold rolling process including pickling and cold rolling under the conditions shown in Table 16.
Next, the cold-rolled sheets were annealed in a continuous annealing furnace (continuous annealing) under the conditions shown in Table 16 and then subjected to a cold-rolled sheet annealing step of cooling under the conditions shown in Table 16 after annealing. Some parts were subjected to temper rolling after the cold-rolled sheet annealing step. About the obtained cold rolled annealed sheet, in the same manner as in Example 1,
The amount of solute N, microstructure, tensile properties, moldability, strain age hardening properties, and impact resistance properties were investigated.

Table 17 shows the results.

[0166]

[Table 15]

[0167]

[Table 16]

[0168]

[Table 17]

All of the examples of the present invention show excellent ductility and a low yield ratio, further have excellent strain aging hardening properties, exhibit a remarkably high BH content and ΔTS, and have an impact resistance property by strain aging treatment. The cost of improvement is large.

[0170]

According to the present invention, the yield stress is 80 MPa or more and the tensile strength is 40 MPa by the pre-deformation-paint baking treatment.
With the above, a high tensile cold-rolled steel sheet having both high strain age hardening characteristics and high formability, or even higher impact resistance characteristics, or a high yield ratio mold having high strain age hardening characteristics and high formability. It is possible to manufacture a cold-rolled tension steel sheet at low cost and without disturbing the shape. Furthermore, when the high-tensile cold-rolled steel sheet of the present invention is applied to an automobile part, it is possible to obtain stable and high part properties by increasing the tensile strength together with the yield stress by paint baking treatment and the like,
The thickness of the steel plate to be used can be reduced by one grade, for example, from 2.0 mm to 1.6 mm, which also has the effect of sufficiently contributing to the weight reduction of the vehicle body.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C22C 38/58 C22C 38/58 (72) Inventor Akio Tosaka 1 Kawasaki-cho, Chuo-ku, Chuo-ku, Chiba-shi, Chiba (72) Inventor Kazunori Osawa 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Inside the Technical Research Institute, Kawasaki Steel Corporation (72) Inventor Yamazaki Saki Takuya Kawasaki, Chuo-ku, Chiba Prefecture No. 1 Kawasaki Steel Engineering Co., Ltd. (72) Inventor Takashi Ishikawa No. 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Chiba Works Chiba Works (72) Inventor Shinjiro Kaneko Kawasaki, Chuo-ku, Chiba Chiba No. 1 town Kawasaki Steel Corp. Technical Research Laboratory F term (reference) 4K037 EA01 EA02 EA04 EA05 EA06 EA09 EA11 EA13 EA15 EA16 EA17 EA18 EA19 EA20 EA23 EA25 E A27 EA28 EA31 EA32 EA36 EB06 EB11 FA02 FA03 FB03 FB08 FB10 FC03 FC04 FD04 FE01 FE02 FE03 FH01 FJ05 FJ06 FK03 FL02 FM02 GA05 JA06 JA07

Claims (17)

[Claims] (1) In terms of mass%, Al: 0.02% or less, N: 0.0050
0.00.0250%, N / Al is 0.3 or more, and N is 0.0010% or more in solid solution, and the average crystal grain size is 10 μm.
A high-tensile cold-rolled steel sheet having a structure containing the following ferrite phase in an area ratio of 50% or more and having excellent tensile strength at 440 MPa and excellent strain aging hardening characteristics. 2. The composition, in mass%, C: 0.15% or less, Si: 2.0% or less, Mn: 3.0% or less, P: 0.08% or less, S: 0.02% or less, Al: 0.02% or less, N : 0.0050-0.0250%, N / Al is 0.3 or more, N in solid solution is 0.0010%
The high tensile strength cold-rolled steel sheet according to claim 1, wherein the high-strength cold-rolled steel sheet has a composition of at least 0.1%, and the balance is Fe and unavoidable impurities. 3. The high-tensile cold-rolled steel sheet according to claim 2, further comprising one or more of the following groups a to d in mass% in addition to the composition. Note Group a: One or more of Cu, Ni, Cr, and Mo are 1.
0% or less b group: 0.1% or more of one or more of Nb, Ti and V
Group c: B is 0.0030% or less Group d: One or two of Ca and REM are 0.0010 to 0.01 in total
0% 4. The composition, in mass%, C: 0.15% or less, Si: 2.0% or less, Mn: 3.0% or less, P: 0.08% or less, S: 0.02% or less, Al: 0.02% or less, N : 0.0050 to 0.0250%, Nb: 0.007 to 0.04%, N / Al is 0.3 or more, and N in solid solution is 0.0010%.
% Or more, and 0.005% or more of Nb in a precipitated state, the balance being Fe and unavoidable impurities, and the structure has a ferrite phase having an average crystal grain size of 10 μm or less in an area ratio of 50%. The high-tensile cold-rolled steel sheet according to claim 1, wherein the above-mentioned and the remainder have a structure mainly composed of pearlite and have a yield ratio of 0.7 or more. 5. The high-tensile cold-rolled steel sheet according to claim 4, further comprising one or more of the following groups e to h in mass% in addition to the composition. Note Group e: One or more of Cu, Ni, Cr and Mo are combined for 1.
0% or less f group: 0.1% or less of one or two of Ti and V in total g group: 0.0030% or less of B group: h group: 0.0010 to 0.01 of one or two of Ca and REM in total
0% 6. The composition contains, by mass%, C: 0.15% or less, Mn: 3.0% or less, S: 0.02% or less, Al: 0.02% or less, N: 0.0050 to 0.0250%, and Mo: 0.05-1.0%, Cr: 0.05-1.0%
One or two of the above, N / Al is not less than 0.3, N in solid solution is not less than 0.0010%, and the balance is Fe and unavoidable impurities. The high-tensile cold-rolled steel sheet according to claim 1, wherein the structure includes a ferrite phase having a crystal grain size of 10 µm or less in an area ratio of 50% or more, and a martensite phase in an area ratio of 3% or more. 7. The high-tensile cold-rolled steel sheet according to claim 6, further comprising one or more of the following groups j to m in mass% in addition to the composition. Note j group: Si: 0.05 to 1.5%, P: 0.03 to 0.15%, B: 0.00
One or more kinds of 03 to 0.01% k group: Nb: 0.01 to 0.1%, Ti: 0.01 to 0.2%, V: 0.01
1 or 2 or more of 0.2% 1 group: Cu: 0.05 to 1.5%, Ni: 1 or 2 kinds of 0.05 to 1.5% m group: Ca or REM 1 or 2 kinds in total 0.0010 to 0.01
0% 8. The high-tensile cold-rolled steel sheet according to claim 1, wherein the high-tensile cold-rolled steel sheet has a thickness of 3.2 mm or less. 9. A high-tensile cold-rolled steel sheet obtained by electroplating or hot-dip coating the high-tensile cold-rolled steel sheet according to claim 1. 10. In mass%, Al: 0.02% or less, N: 0.00
A steel slab containing 50 to 0.0250% and having a composition in which N / Al is not less than 0.3 is heated to a slab heating temperature of not less than 1000 ° C., roughly rolled into a sheet bar, and the sheet bar is subjected to finish rolling. Temperature: 800 ° C. or higher, finish rolling, winding temperature: 750 ° C. or lower, a hot rolling process of forming a hot rolled sheet, and pickling and cold rolling the hot rolled sheet to form a cold rolled sheet. A cold rolling step of performing, and annealing the cold-rolled sheet at a predetermined temperature for a predetermined time, and then performing a cold-rolled sheet annealing step of cooling at a predetermined cooling rate. A method for producing a high-tensile cold-rolled steel sheet with a strength of 440 MPa or more and excellent strain aging hardening characteristics. 11. The steel slab is, by mass%, C: 0.15% or less, Si: 2.0% or less, Mn: 3.0% or less, P: 0.08% or less, S: 0.02% or less, Al: 0.02% or less, N: a steel slab containing 0.0050 to 0.0250% and having a N / Al of 0.3 or more, or a composition containing one or more groups selected from the following groups a to d. In the strip annealing step, the annealing at a predetermined temperature for a predetermined time is carried out at a temperature not lower than a recrystallization temperature and not higher than 900 ° C. for a holding time of 10 to
Annealing is performed for 60 s, and the cooling after annealing is performed at a cooling rate of 10 to 300 ° C./s to a temperature range of 500 ° C. or less, and a temperature range of 400 ° C. or more below the stop temperature of the primary cooling. 11. The method for producing a high-tensile cold-rolled steel sheet according to claim 10, wherein the cooling is a secondary cooling in which the residence time in the steel sheet is 300 s or less. Note Group a: One or more of Cu, Ni, Cr, and Mo are 1.
0% or less b group: 0.1% or more of one or more of Nb, Ti and V
Group c: B is 0.0030% or less Group d: One or two of Ca and REM are 0.0010 to 0.01 in total
0% 12. The steel slab is, by mass%, C: 0.15% or less, Si: 2.0% or less, Mn: 3.0% or less, P: 0.08% or less, S: 0.02% or less, Al: 0.02% or less, N: 0.0050 to 0.0250%, Nb: 0.007 to 0.04%, and N / Al is 0.3 or more, or further contains one or more groups selected from the following groups e to h. Steel slab having, the slab heating temperature is 1100 ° C. or higher, and the rolling reduction of the final rolling final pass in the finish rolling applied to the sheet bar is 25% or more, and the predetermined rate in the cold-rolled sheet annealing step is The annealing maintained at a temperature for a predetermined time, the recrystallization temperature is 900 or more
Holding time at a temperature of not more than 10 ° C .: annealing at 10 to 90 seconds, and cooling after the annealing to a temperature range of not more than 600 ° C.
11. The method for producing a high-tensile cold-rolled steel sheet according to claim 10, wherein cooling is performed at 70 ° C./s or less. Note Group e: One or more of Cu, Ni, Cr and Mo are combined for 1.
0% or less f group: 0.1% or less of one or two of Ti and V in total g group: 0.0030% or less of B group: h group: 0.0010 to 0.01 of one or two of Ca and REM in total
0% 13. The steel slab contains, by mass%, C: 0.15% or less, Mn: 3.0% or less, S: 0.02% or less, Al: 0.02% or less, and N: 0.0050 to 0.0250%. : 0.05-1.0%, Cr: 0.05-1.0%
A steel slab containing one or two of the above, and having a composition in which N / Al is 0.3 or more, or further contains one or two or more groups selected from the following j groups to m groups:
In the cold-rolled sheet annealing step, the annealing, which is performed at a predetermined temperature for a predetermined time, is performed by (Ac ₁ transformation point) to (Ac ₃ transformation point).
And a cooling time of 10 to 120 s, and the cooling after the annealing is performed at an average cooling rate of 600 to 300 ° C. or higher than a critical cooling rate CR defined by the following formula (1) or (2). 11. The method for producing a high-tensile cold-rolled steel sheet according to claim 10, wherein cooling is performed. Note j group: Si: 0.05 to 1.5%, P: 0.03 to 0.15%, B: 0.00
One or more kinds of 03 to 0.01% k group: Nb: 0.01 to 0.1%, Ti: 0.01 to 0.2%, V: 0.01
1 or 2 or more of 0.2% 1 group: Cu: 0.05 to 1.5%, Ni: 1 or 2 kinds of 0.05 to 1.5% m group: Ca or REM 1 or 2 kinds in total 0.0010 to 0.01
0% B <0.0003% log CR = -1.73 [Mn + 2.67Mo + 1.3Cr + 0.26Si + 3.5P + 0.05Cu + 0.05Ni] +3.95 ... (1) When B≥0.0003% log CR = -1.73 [Mn + 2 .67Mo + 1.3Cr + 0.26Si + 3.5P + 0.05Cu + 0.05Ni] +3.40 (2) where, CR: cooling rate (° C / s) Mn, Mo, Cr, Si, P, Cu, Ni: each element Content (% by mass) 14. The method according to claim 10, wherein cooling is started within 0.5 s after the finish rolling, quenched at a cooling rate of 40 ° C./s or more, and the winding is performed. Manufacturing method of high tensile cold rolled steel sheet. 15. Following the cold-rolled sheet annealing step,
15. The method for producing a high-tensile cold-rolled steel sheet according to claim 10, wherein temper rolling or leveling at an elongation of 1.0 to 15% is performed. 16. The method for producing a high-tensile cold-rolled steel sheet according to claim 10, wherein successive sheet bars are joined between the rough rolling and the finish rolling. 17. A sheet bar edge heater for heating a width end of the sheet bar or a sheet bar heater for heating a length end of the sheet bar between the rough rolling and the finish rolling. 17. The method for producing a high-tensile cold-rolled steel sheet according to claim 10, wherein both of them are used.