JP2011149066A - Cold-rolled steel sheet, hot-rolled steel sheet and methods for producing them - Google Patents

Abstract

The present invention provides a cold-rolled steel sheet and a hot-rolled steel sheet having a fine-grained structure, high strength and excellent workability.
SOLUTION: In mass%, C: 0.06 to 0.25%, Si: 0.01 to 2.0%, Mn: 0.5 to 2.0%, and Al: 0.01 to 2.0%, and Ti: 0.20% or less and Nb: Contains one or two of 0.10% or less, has a predetermined amount of Si + Al and Ti + Nb, and contains by volume%, ferrite: 70% or more and residual austenite: 3% or more, with the balance being bainite And the inevitable martensite, the average grain size of ferrite is 3.0 μm or less, the average grain size of retained austenite is 1.0 μm or less, and the proportion of retained austenite with an aspect ratio of 2 or less in the retained austenite is 60% by volume. A cold-rolled steel sheet having a steel structure as described above, and a hot-rolled steel sheet having a solid solution Ti + solid solution N content of 0.003% by mass or more. Manufacturing method of hot-rolled steel sheet and cold-rolled steel sheet.
[Selection figure] None

Description

  The present invention relates to a cold-rolled steel sheet, a hot-rolled steel sheet suitable for the base material of the cold-rolled steel sheet, and a method for producing them. More specifically, the present invention is suitable as a material for structural members of automobiles and industrial equipment, and is suitable for a cold-rolled steel sheet having a fine structure with high strength while being excellent in workability and a base material of the cold-rolled steel sheet. The present invention relates to a hot-rolled steel sheet and a manufacturing method thereof.

  For the purpose of reducing the weight of the vehicle body and improving the safety at the time of collision, the application of high-tensile steel plates to automobile parts is being promoted. Automobile parts are often processed into a predetermined shape by molding such as press processing. Therefore, in order to enable molding into a car part having a complicated shape and to ensure high dimensional accuracy for the car part after molding, it is not necessary to simply have high strength, and it has excellent workability. Is required. However, strength and workability are generally in a trade-off relationship, and when the strength is increased, workability such as ductility is lowered. For this reason, it is generally difficult to achieve both high strength and excellent workability.

  Under such circumstances, as steel sheet having high strength and excellent ductility, so-called “residual austenite”, that is, transformation-induced plasticity (hereinafter also referred to as “TRIP”) of austenite remaining untransformed is used. Steel plates that have been made are known.

For example, in JP-A-4-333524 (Patent Document 1), by weight, C: 0.05 to 0.12%, Si: 0.5 to 3.00%, Mn: 0.5 to 2. The steel material containing 50% and the balance Fe and inevitable impurities is heated to a range of Ac _{1 to} Ae ₃ transformation temperature after cold rolling, and then to a range of 550 to 700 ° C. at a cooling rate of 1 to 10 ° C./sec. After cooling and subsequently cooling to 200 to 450 ° C. at a cooling rate of 10 to 200 ° C./sec, holding at a temperature range of 300 to 450 ° C. for 15 seconds to 20 minutes, and cooling to room temperature, ferrite and bainite And a method for producing a high-strength steel sheet containing residual austenite having a volume fraction of 3 to 10%.

  On the other hand, for strengthening steel, solid solution strengthening, precipitation strengthening, transformation strengthening, fine grain strengthening (strengthening by crystal grain refinement) and the like are known. Among these, the refinement of crystal grains is a strengthening technique that is attracting attention from the viewpoints of recyclability and cost because it makes it possible to increase the strength without depending on the additive elements. However, when the crystal grain size of ferrite is made finer, the strength generally increases due to grain boundary strengthening, while work hardening hardly occurs and plastic instability appears, resulting in deterioration of workability. .

  The present inventors have been able to produce a steel having an excellent “strength-ductility balance” that combines high strength and excellent ductility by combining strengthening by crystal grain refinement with the TRIP phenomenon of retained austenite. The structure and manufacturing method thereof were proposed in Japanese Patent Application Laid-Open No. 2006-348353 (Patent Document 2) and Japanese Patent Application Laid-Open No. 2007-23339 (Patent Document 3). However, the inventions proposed in these publications relate to a hot-rolled steel sheet and a method for producing the same, and a method for combining the refinement of crystal grains and the TRIP phenomenon of retained austenite in a cold-rolled steel sheet has not been elucidated. .

For a method of combining the refinement of crystal grains and the TRIP phenomenon of retained austenite in a cold-rolled steel sheet, for example, in JP-A-2004-204341 (Patent Document 4), C: 0.03 to 0.16% in mass%. Si: 0.2-2.0%, Mn: 1.0-3.0% and / or Ni: 0.5-3.0%, Ti: 0.2% or less and / or Nb: 0.3% 2% or less, Al: 0.01 to 0.1%, P: 0.1% or less, S: 0.02% or less and N: 0.005% or less, and C, Si, Mn, Ni, Ti And Nb in a range satisfying each of the predetermined formulas, and the balance is Fe and unavoidable impurities, the steel material is heated to 1200 ° C. or higher, then hot-rolled, and then cold-rolled, and then the predetermined Recrystallization at temperature A ₃ ° C or higher and (A ₃ +30) ° C or lower determined by the formula Annealed, pickled, or A ₁ ° C. obtained by a predetermined formula, (A 1 ₊₇₀₎ ℃ heat treatment of 5-30 seconds at a temperature range of, subsequently galvanizing treatment, or even alloyed A hot-dip galvanized steel sheet and a method for producing the same are disclosed. According to the above method, a cold-rolled steel sheet having an average crystal grain size of ferrite of 3.5 μm or less and containing 5 vol% or more of retained austenite and having an excellent strength-ductility balance is obtained.

  As another method of combining the refinement of crystal grains and the TRIP phenomenon of retained austenite in a cold-rolled steel sheet, JP 2006-83403 A (Patent Document 5) includes C: 0.05 to 0.3%, Si : 0.01 to 2.0%, Mn: 1 to 3%, P: 0.001 to 0.05%, S: 0.0001 to 0.01%, Al: more than 0.10% to 2.0 %, N: 0.001 to 0.01%, Si / Al = 0.01 to 10 is satisfied, and a cold-rolled steel sheet having a composition consisting of Fe and inevitable impurities is used as the dew point of the atmospheric gas. : -50 ° C to 0 ° C, hydrogen concentration of atmospheric gas: 1.0 to 100%, annealing temperature: 700 to 900 ° C, holding time: 10 to 1000 seconds, heating, cooling rate: 5 to 150 C./second, cooling stop temperature: 300 to 500 [deg.] C., followed by heat treatment temperature: 3 By performing heat treatment at 0 to 500 ° C. and a heat treatment time of 100 to 1400 seconds, a ferrite phase having an average crystal grain size of 10 μm or less is 40 to 90% in volume fraction and a residual austenite phase is 1.0 to 1.0 in volume fraction. A high-strength cold-rolled steel sheet having a steel structure containing 20% and the balance being a low-temperature transformation phase and having a maximum Si concentration / average Si concentration on the steel sheet surface of 1.1 to 4.0 and a method for producing the same are disclosed. ing.

JP-A-4-333524 JP 2006-348353 A JP 2007-23339 A JP 2004-204341 A JP 2006-83403 A

However, among the conventional cold-rolled steel sheets that combine the refinement of crystal grains and the TRIP phenomenon of retained austenite, the cold-rolled steel sheet disclosed in Patent Document 4 has the following problems.
1) Si and Al are important elements that suppress the precipitation of cementite and promote the concentration of carbon to austenite, so that residual austenite is generated in low-carbon steel. However, in the cold-rolled steel sheet described in Patent Document 4, the value of A ₃ (predicted value of the Ac ₃ transformation point temperature of steel, hereinafter also referred to as “A ₃ point”) obtained by a predetermined formula is 860 (° C. ) Since the following is essential, the amount of addition of Si as a ferrite stabilizing element is limited, or a large amount of austenite stabilizing element needs to be added. For example, the maximum Si content in the examples is 1.30% by mass (steel type C). In this steel type, Mn is 2.7% by mass and Ni is 1.00% by mass. is a ₃ point to 860 ° C. or less by the addition of reduction elements significantly large amount. In addition, Al promotes carbon concentration to austenite and has an effect of stabilizing the retained austenite phase higher than that of Si. From this viewpoint, Al is an element more advantageous than Si. However, in this patent document, the Al content is limited to 0.1% by mass or less.

2) Mn and Ni is an austenite stabilizing element and is an effective element for lowering the _three points A. In Patent Document 4, the total content of Mn and Ni is defined as 1.3 mass% or more by the expression (3). The total content of Mn and Ni in the examples is 1.3% by mass at the minimum (steel type D), but in this steel type, the Si content is limited to a very low value of 0.40% by mass. Si and Al steels C total content as being examples at least 0.8 wt% of, E, there is a M, the total content of Mn and Ni in order to below 860 ° C. The _3-point A at all Is 2.6 mass% or more. When such a large amount of austenite stabilizing element is contained, the formation of ferrite is greatly suppressed, and it becomes difficult to obtain a structure mainly composed of ferrite during annealing. Moreover, the carbon concentration to austenite is delayed and the amount of retained austenite is reduced.

3) Since annealing is performed in two steps, there are problems in both productivity and cost. Furthermore, in the first annealing, temperature control in a very narrow range of A ₃ ° C or more and (A ₃ +30) ° C or less is essential. However, it is normal for industrial production processes to cause some fluctuations in the soaking temperature, so there is a concern about the significant fluctuations in mechanical properties caused by such fluctuations in the soaking temperature, and in terms of material stability. There's a problem.

  On the other hand, in the cold rolled steel sheet disclosed in Patent Document 5, a ferrite average particle size of 1.7 μm is achieved at the minimum in the examples. However, the refinement mechanism has not been elucidated, and the conditions for realizing a fine grain structure are not defined. Of the twelve invention examples, half of the six examples have an average ferrite grain size exceeding 3 μm, and a fine grain structure having an average ferrite grain size of 3 μm or less cannot be obtained with certainty.

  The object of the present invention is a cold-rolled steel sheet having a fine structure excellent in workability and high strength, suitable as a structural member for automobiles and industrial equipment, and a hot-rolled steel sheet as a base material of the cold-rolled steel sheet, As well as methods for their production. A more specific object of the present invention is to provide a fine-grain cold-rolled steel sheet having a ferrite average grain size of 3.0 μm or less in a component range of Mn ≦ 2.0 mass% and Si + Al ≧ 0.8 mass%. .

  The present inventors, for steel containing Ti and / or Nb, the amount of solute Ti and the amount of solute Nb in the steel after hot rolling, as well as the ferrite grain size after cold rolling and annealing. The relationship was studied and the following new knowledge was obtained, and the present invention was completed.

  (A) By optimizing the chemical composition and hot rolling conditions of the steel material, precipitation of Ti and / or Nb at the stage of the hot-rolled steel sheet is suppressed, and a certain amount or more of Ti and / or Nb is supersaturated in the steel. It can exist in a solid solution state. Since these supersaturated solid solution Ti and solid solution Nb are thermodynamically unstable, the driving force of precipitation is large. Therefore, when strain or heat is applied to the material in the processes after cold rolling, precipitates such as TiC and NbC precipitate finely and uniformly in the steel. The pinning effect of the finely and uniformly precipitated precipitates remarkably suppresses the ferrite grain growth during annealing, and the ferrite in the final product stage is refined to an average grain size of 3.0 μm or less.

  On the other hand, if precipitation of Ti and Nb is completed in the hot-rolled steel sheet stage, the precipitates become coarse and the distribution of the precipitates becomes uneven in the cold rolling process and the annealing process. The stopping effect cannot be obtained, the ferrite grain growth proceeds easily, and a fine grain structure cannot be obtained.

  (B) According to thermodynamic equilibrium calculation (for example, Thermo-Calc), precipitation of carbides of Ti and Nb starts in a temperature range of 1000 ° C. or higher. Accordingly, it is theoretically assumed that most of Ti and Nb contained are precipitated in a temperature range of 800 to 900 ° C., which is a general hot rolling completion temperature.

  However, in an actual continuous hot rolling line, it takes a short time until the hot rolling is completed after the steel ingot or steel slab is brought into a high temperature state, and therefore does not reach the equilibrium precipitation state. When the amount of precipitation of Ti and / or Nb was investigated for a hot-rolled steel sheet that had been quenched by water and immediately after completion of hot rolling at 900 ° C., it was less than half of the amount of precipitation determined by equilibrium calculation. Only a large amount of Ti and / or Nb was found to be present in the supersaturated solid solution state in the steel. And since supersaturated solid solution Ti and Nb have high precipitation driving force, precipitation occurs in the cooling and winding process after hot rolling, but after completion of hot rolling, it is cooled to 750 ° C. or less within a certain time, Further, it has been found that by winding at 650 ° C. or lower, the precipitation is significantly suppressed, and a substantial part of Ti and Nb can be present in the hot-rolled steel sheet in a supersaturated solid solution state.

(C) When a cold-rolled steel sheet is manufactured using the hot-rolled steel sheet containing Ti and Nb in the supersaturated solid solution state described in (b) above, no special treatment is performed in cold rolling and subsequent annealing. In both cases, a cold-rolled steel sheet having a fine-grained structure can be obtained by manufacturing conditions applicable to industrial production. In particular, if the annealing temperature is in the two-phase region of ferrite + austenite, for example, even if it is 900 ° C., grain growth is suppressed, and a fine structure is obtained. Get smaller. However, when the annealing temperature is more than A ₃ temperature, immediately grain growth acceleration to proceed, fine tissue can not be obtained.

The present invention completed based on the above knowledge is as follows.
(1) By mass%, C: 0.06% to 0.25%, Si: 0.01% to 2.0%, Mn: 0.5% to 2.0%, and Al: 0 0.01% or more and 2.0% or less, and further containing one or two of Ti: 0.20% or less and Nb: 0.10% or less, and the following formulas (1) and (2) And the balance has a chemical composition consisting of Fe and impurities, and in volume%, contains ferrite: 70% or more and residual austenite: 3% or more, and the balance consists of bainite and inevitable martensite, The average particle diameter Dα (μm) at a depth position of 1/4 of the sheet thickness from the steel sheet surface of ferrite is 3.0 μm or less, and the average particle diameter at a depth position of 1/4 of the sheet thickness from the steel sheet surface of the retained austenite. Dγ (μm) is 1.0 μm or less, and the following formula (3) Satisfying beauty (4), further, cold-rolled steel sheet the ratio of aspect ratio of 2 or less of residual austenite accounts for the residual austenite is characterized by having a steel structure is 60 vol% or more.

0.8 ≦ Si + Al ≦ 3.0 (1)
0.002 ≦ Ti + Nb ≦ 0.20 (2)
1.5 ≦ Dα / Dγ ≦ 15 (3)
3 ≦ (Dα / Dγ) × (Dα + Dγ) ≦ 30 (4)
Each element symbol in the above formula means the content (unit: mass%) of the element.

The chemical composition may contain one or two of Ca: 0.01% or less and Zr: 0.10% or less in mass%, instead of part of Fe.
(2) A hot-rolled steel sheet having the above-described chemical composition, wherein the total content of solute Ti and solute Nb in the steel at room temperature is 0.003 mass% or more.

In a preferred embodiment, the average grain size of ferrite of this hot-rolled steel sheet is 4 μm or less.
(3) The steel ingot or steel slab having the above chemical composition is brought to a temperature of 1150 ° C. or higher and 1300 ° C. or lower, hot rolled to complete the rolling at 800 ° C. or higher, and cooled to 750 ° C. after the hot rolling is completed. A method for producing a hot-rolled steel sheet, characterized in that after cooling for 10 seconds or less, winding is performed in a temperature range of 650 ° C. or less within 30 seconds after completion of the cooling.

  (4) A steel ingot or steel slab having the above chemical composition is brought to a temperature of 1150 ° C. or higher and 1300 ° C. or lower, subjected to hot rolling to complete rolling at 800 ° C. or higher, and cooled from completion of hot rolling to 720 ° C. A method for producing a hot-rolled steel sheet, characterized in that after the cooling is performed for 0.7 seconds or less, winding is performed in a temperature range of 650 ° C. or less within 30 seconds after the completion of the cooling.

  (5) Cold-rolling the hot-rolled steel sheet according to (2) or the hot-rolled steel sheet manufactured by the method according to (3) or (4) above, After holding in the temperature range of 950 ° C. or less for 30 seconds or more and 600 seconds or less, the average cooling rate in the temperature range of 650 ° C. to 550 ° C. is set to 5 ° C./second or more and 200 ° C./second or less and cooled to 500 ° C., 300 The manufacturing method of the cold-rolled steel sheet characterized by hold | maintaining for 30 second or more in the temperature range (degreeC) or more and 500 degrees C or less.

  In the present invention, the “ferrite average particle diameter Dα” is obtained by observing a ¼ depth position of the plate thickness from the surface of the steel plate in the cross section in the plate thickness direction, and the ferrite particle size obtained by the intercept method is 1.128 times (= 2). / √π) (ie equivalent circle diameter). “Residual austenite average particle diameter Dγ” was also determined by observing the above position, and the measurement was evaluated by electron beam backscatter diffraction (EBSP). Since there are granular crystal grains having a small ratio, the particle diameter is not a circle-equivalent diameter but a short diameter (for example, film thickness in the case of film-like crystal grains).

  According to the present invention, a cold-rolled steel sheet having a fine structure suitable for automobiles and industrial equipment, having a high strength and excellent workability, and a hot-rolled steel sheet used as a base material of the cold-rolled steel sheet A method is established, and it is possible to reliably manufacture such cold-rolled steel sheets and hot-rolled steel sheets industrially.

  The cold-rolled steel sheet and hot-rolled steel sheet according to the present invention and methods for producing them will be described below. In the following description, “%” indicating chemical composition means “mass%”, and “%” indicating steel structure means “volume%”.

(A) Chemical composition of steel C: 0.06% or more and 0.25% or less C has an action of concentrating in austenite and stabilizing austenite, and is essential for allowing austenite to remain at room temperature. It is an element. If the C content is less than 0.06%, the retained austenite does not reach a sufficient amount, and desired mechanical properties may not be obtained. Therefore, the C content is 0.06% or more, preferably 0.10% or more. On the other hand, when the C content exceeds 0.25%, the formation of pearlite is promoted, and it becomes difficult to secure the intended retained austenite, or the weldability of the cold-rolled steel sheet is significantly deteriorated. Therefore, the C content is 0.25% or less. Preferably it is 0.20% or less.

Si: 0.01% or more and 2.0% or less Si is an important element having an action of promoting the formation of residual austenite phase by promoting the formation of ferrite and further delaying the precipitation of cementite from austenite. It is. Moreover, it has the effect | action which deoxidizes molten steel, and the effect | action which solid-solution strengthens a ferrite phase. If the Si content is less than 0.01%, it is difficult to sufficiently obtain the effects of these actions. Therefore, the Si content is set to 0.01% or more. On the other hand, when the Si content exceeds 2.0%, the leading ductility and weldability occurs, which may make it difficult to stable hot rolling inviting a significant increase in the A ₃ point. Therefore, the Si content is 2.0% or less.

Mn: 0.5% or more and 2.0% or less Mn is an important element that acts to solidify and strengthen the ferrite phase and stabilize the austenite to promote the formation of the retained austenite phase. If the Mn content is less than 0.5%, the above-described effects may not be sufficiently obtained. Therefore, the Mn content is 0.5% or more, preferably 0.8% or more. On the other hand, if the Mn content exceeds 2.0%, austenite is excessively stabilized, and it becomes difficult to obtain a structure mainly composed of ferrite during annealing, and the concentration of carbon to austenite is delayed, so that residual austenite The amount may decrease. Therefore, the Mn content is 2.0% or more. Preferably it is 1.8% or less.

Al: 0.01% or more and 2.0% or less Al is an important element that promotes the formation of residual austenite phase by accelerating the formation of ferrite and further delaying the precipitation of cementite from austenite. is there. Moreover, it has the effect | action which deoxidizes molten steel. If the Al content is less than 0.01%, it is difficult to sufficiently obtain the effects of these actions. Therefore, the Al content is set to 0.01% or more. On the other hand, when the Al content exceeds 2.0%, inviting a significant increase in the same way A ₃ point and Si, which may make it difficult to stable hot rolling. Therefore, the Al content is 2.0% or less.

0.8 ≦ Si + Al ≦ 3.0
If the total content of Si and Al is less than 0.8%, the stability and volume ratio of the retained austenite phase become insufficient, and desired mechanical characteristics may not be obtained. Therefore, the total content of Si and Al is set to 0.8% or more in order to promote the formation of the retained austenite phase. This amount is preferably at least 1.0%, more preferably at least 1.2%. On the other hand, when the total content of Si and Al exceeds 3.0%, the weldability and surface properties of the cold-rolled steel sheet deteriorate significantly. Therefore, the total content of Si and Al is 3.0% or less. Preferably it is 2.5% or less, More preferably, it is 2.0% or less.

Ti: 0.20% or less and / or Nb: 0.10% or less, and 0.002 ≦ Ti + Nb ≦ 0.20
Ti and Nb are important elements in the present invention. By containing one or two of Ti and Nb, fine carbides are formed during annealing after cold rolling, and ferrite is finely grained. In addition to strengthening by refining the steel structure, precipitation strengthening contributes to high strength. When the total content of Ti and Nb is less than 0.002%, it is difficult to sufficiently obtain the above-described effects. Therefore, the total content of Ti and Nb is set to 0.002% or more. This total content is preferably 0.005% or more, more preferably 0.008% or more.

  On the other hand, if the total content of Ti and Nb exceeds 0.20%, the effect of the above action is saturated and the cost increases. Moreover, since the precipitate in steel coarsens, the workability of a cold-rolled steel sheet also deteriorates. Therefore, the total content of Ti and Nb is set to 0.20% or less. This total content is preferably 0.15% or less, more preferably 0.10% or less.

  For each element, even if Ti is contained in an amount exceeding 0.20% and Nb is contained in an amount exceeding 0.10%, the effect by the above action is saturated and the cost is increased. Moreover, since the precipitate in steel coarsens, the workability of a cold-rolled steel sheet also deteriorates. Furthermore, since a large amount of C is consumed for the precipitation of carbide, the amount of retained austenite decreases, and it may not be possible to secure a predetermined amount of retained austenite. Therefore, the contents of Ti and Nb are set to 0.20% or less for Ti and 0.10% or less for Nb, respectively. Preferably, Ti is 0.15% or less, Nb is 0.08% or less, more preferably, Ti is 0.10% or less, and Nb is 0.05% or less.

The balance is Fe and impurities. It is desirable to regulate S, P, and N in the impurities as follows.
S: S is an impurity element that forms sulfide inclusions and reduces workability. For this reason, it is preferable that S content shall be 0.05% or less. In order to secure further excellent workability, the S content is preferably 0.008% or less, and more preferably 0.003% or less.

P: P is an impurity element that adversely affects toughness and ductility. For this reason, the P content is preferably 0.05% or less, and more preferably 0.02% or less.
N: N is an impurity element that reduces workability. For this reason, the N content is preferably 0.01% or less, and more preferably 0.006% or less.

In addition to the above component elements, the cold-rolled steel plate and hot-rolled steel plate according to the present invention may contain the following elements as necessary.
Ca: 0.01% or less and / or Zr: 0.10% or less Both Ca and Zr have the effect of adjusting the shape of inclusions to improve cold workability. Therefore, you may contain 1 type or 2 types of Ca and Zr. However, if Ca is contained in an amount exceeding 0.01%, if Zr is contained in an amount exceeding 0.10%, the inclusions in the steel become excessive, leading to a decrease in workability. Therefore, the respective contents of Ca and Zr are 0.01% or less for Ca and 0.10% or less for Zr. Preferably, Ca is 0.005% or less and Zr is 0.05% or less. In addition, Ca and Zr may contain only 1 type, and may contain 2 types in combination. In order to more reliably obtain the effect of the above action, it is preferable to contain at least one of 0.0002% or more of Ca and 0.002% or more of Zr. It is more preferable to contain at least one of 0.005% or more of Ca and 0.01% or more of Zr.

(B) Structure of cold-rolled steel sheet The structure of the cold-rolled steel sheet according to the present invention contains ferrite: 70% or more and retained austenite: 3% or more, and the balance is composed of bainite and inevitable martensite. The average particle size is 3.0 μm or less, the average particle size of the retained austenite is 1.0 μm or less, and the proportion of retained austenite having an aspect ratio of 2 or less in the retained austenite is 60% by volume or more. It is attached.

  Since ferrite has a small amount of carbon solid solution, increasing the proportion thereof can promote carbon concentration in austenite and increase the proportion of retained austenite in the steel structure. If the volume fraction of ferrite is less than 70%, carbon concentration in austenite becomes insufficient, and 3% or more of retained austenite may not be ensured. Therefore, although the volume fraction of ferrite is 70% or more, this volume fraction is preferably 80% or more, and more preferably 85% or more.

  Residual austenite has the effect of improving ductility by TRIP. If the retained austenite is less than 3%, the effect of improving ductility by TRIP cannot be obtained sufficiently, and the target ductility may not be ensured. Therefore, the retained austenite is 3% or more. Since the ductility improves as the volume fraction of retained austenite increases, the upper limit is not particularly set, but the upper limit of the retained austenite volume fraction obtained with the C content of the present invention is approximately 30%.

  When the average grain size of ferrite exceeds 3.0 μm, sufficient refinement and strengthening cannot be obtained. Therefore, the average particle diameter of ferrite is 3.0 μm or less, preferably 2.5 μm or less, more preferably 2.0 μm or less. Since ferrite is preferably as fine as possible, the lower limit of the average grain size of ferrite need not be specified.

  When the average grain size of retained austenite exceeds 1.0 μm, hard work-induced martensite generated by the TRIP phenomenon becomes a starting point of fracture, and the workability of the cold-rolled steel sheet deteriorates. Therefore, the average particle size of retained austenite is 1.0 μm or less.

  The proportion of retained austenite having an aspect ratio of 2 or less in the entire retained austenite is 60% by volume or more. When this ratio is less than 60% by volume, the anisotropy of the mechanical properties of the cold-rolled steel sheet increases and the hole expandability deteriorates. Austenite having an aspect ratio of 2 or less generally means the granular austenite described above. The above-described film-like austenite generally has an aspect ratio greatly exceeding 2.

The remaining structure is basically bainite. Although martensite may inevitably be mixed, there is no actual harm if the volume ratio is 5% or less.
In order to obtain more excellent mechanical properties, the ferrite average particle diameter Dα (μm) and the retained austenite average particle diameter Dγ (μm) at a depth position of ¼ of the sheet thickness from the steel sheet surface are expressed by the following formula (3): And satisfies equation (4).

1.5 ≦ Dα / Dγ ≦ 15 (3)
3 ≦ (Dα / Dγ) × (Dα + Dγ) ≦ 30 (4)
The above formulas (3) and (4) define the relationship between the average grain sizes of ferrite and retained austenite. When this range is satisfied, a greater TRIP effect can be obtained.

(C) Hot-rolled steel sheet as a base material for high-tensile cold-rolled steel sheet and manufacturing method thereof The structure control of the hot-rolled steel sheet as a base material for cold rolling obtains the cold-rolled steel sheet having the structure described above according to the present invention. Is an important factor for. The structure and manufacturing method will be described below.

(Solution of Ti and Nb in hot-rolled steel sheet)
The solute Ti and solute Nb contained in the hot-rolled steel sheet are necessary for generating fine ferrite grains in the cold-rolled steel sheet after cold rolling and annealing and achieving fine grain strengthening. . If the total content of solute Ti and solute Nb in the hot-rolled steel sheet at room temperature is less than 0.003 mass%, sufficient fine grain strengthening cannot be obtained. Therefore, the total content of solute Ti and solute Nb in the hot-rolled steel sheet at room temperature is set to 0.003 mass% or more. This content is preferably 0.01% or more, more preferably 0.02% or more. As the total content of solute Ti and solute Nb in the hot-rolled steel sheet at room temperature increases, the steel structure of the cold-rolled steel sheet becomes finer, so the solute Ti and solute Nb in the hot-rolled steel sheet at room temperature There is no particular upper limit on the total content. Therefore, the total content of solute Ti and solute Nb may be equal to the total content of Ti and Nb in the steel composition, but a part of Ti and Nb is precipitated in the hot rolling process. Actually, it is less than the total content of Ti and Nb.

(Average ferrite particle diameter of hot-rolled steel sheet: 4 μm or less)
It is desirable to refine the ferrite grains of the hot-rolled steel sheet to obtain a finer steel structure after cold rolling and annealing. This is because the grain interface that becomes the recrystallization nucleation site of ferrite in the annealing process after cold rolling increases due to the refinement of the hot rolled steel sheet. Therefore, it is desirable that the average diameter of ferrite of the hot-rolled steel sheet is 4 μm or less.

(Temperature of steel ingot or steel slab used for hot rolling: 1150 ° C or higher and 1300 ° C or lower)
The temperature of the steel ingot or steel slab used for hot rolling is 1150 ° C or higher and 1300 ° C or lower. If the temperature is less than 1150 ° C., it is insufficient to make Ti or Nb into a solid solution state, and sufficient hot-melt Ti or solid solution Nb cannot be secured after hot rolling, and cold rolling is performed. In addition, a fine grain structure cannot be obtained after annealing. Therefore, the temperature of the steel ingot or steel slab used for hot rolling is 1150 ° C. or higher. On the other hand, when the temperature of the steel ingot or steel slab subjected to hot rolling exceeds 1300 ° C., the yield decreases due to scale loss. Therefore, the temperature of the steel ingot or steel slab used for hot rolling is 1300 ° C. or less. In addition, hot rolling is a direct-feed rolling in which a continuously cast steel ingot or a piece-rolled steel slab is heated and charged in a heating furnace without being cooled to room temperature. Either direct rolling, which is rolled immediately after heat retention, or rolling after reheating after cooling the steel material to room temperature may be used.

(Hot rolling completion temperature: 800 ° C or higher)
The rolling completion temperature of hot rolling is 800 ° C. or higher. When the hot rolling completion temperature is less than 800 ° C., most of Ti and Nb in the steel are precipitated as carbides, and a predetermined amount of solid solution Ti and solid solution Nb cannot be secured in the hot rolled steel sheet stage. Therefore, the hot rolling completion temperature is 800 ° C. or higher. The upper limit of the hot rolling completion temperature does not need to be specified in particular, but is usually 1100 ° C. or lower when the temperature of the steel ingot or steel slab used for hot rolling is the above temperature.

(Cooling conditions after completion of hot rolling: cooling time from completion of hot rolling to 750 ° C. within 10 seconds)
The cooling time from completion of hot rolling to 750 ° C. is within 10 seconds. When the cooling time from the completion of hot rolling to 750 ° C. exceeds 10 seconds, most of Ti and Nb in the steel precipitate as carbides in the hot-rolled steel sheet, and a predetermined amount of solute Ti or The solid solution Nb cannot be secured. This cooling is typically performed by water cooling.

(Preferable cooling condition after completion of hot rolling: cooling time from completion of hot rolling to 720 ° C. within 0.7 seconds)
It is desirable to refine the ferrite grains of the hot-rolled steel sheet in order to obtain a finer steel structure after cold rolling and annealing. By setting the cooling time from completion of hot rolling to 720 ° C. within 0.7 seconds, it becomes easy to make the ferrite crystal grains of the hot-rolled steel sheet 4 μm or less. Therefore, it is preferable to set the cooling time from the completion of hot rolling to 720 ° C. within 0.7 seconds. Such cooling can be achieved by water cooling using a high-pressure water injection device attached immediately after the rolling mill exit side.

(Winding condition: Winding in a temperature range of 650 ° C. or less within 30 seconds after completion of the cooling)
It winds up within the temperature range below 650 degreeC within 30 second after completion of the said cooling. When the time until winding after completion of the cooling is more than 30 seconds and / or the winding temperature is more than 650 ° C., most of Ti and Nb are precipitated as carbides in the hot-rolled steel sheet. It is difficult to secure a predetermined total amount of solute Ti and solute Nb. Therefore, it winds in the temperature range below 650 degreeC within 30 second after completion of the said cooling. It is preferable that the time from the completion of the cooling to the winding is within 20 seconds. The coiling temperature is preferably 600 ° C. or less, more preferably 550 ° C. or less. Since the precipitation of Ti and Nb can be suppressed as the time from the completion of the cooling to the winding is shorter, the lower limit of the time is not particularly specified. The lower limit of the coiling temperature is not particularly required for the same reason, but if it is below 300 ° C, hard martensite is generated and subsequent cold rolling is difficult. Above preferred. Any of air cooling, water cooling, and a combination of both may be used for cooling from the completion of cooling to winding.

  As described above, in the present invention, the precipitation of Ti and / or Nb in the hot-rolled steel sheet is suppressed by adjusting the chemical composition of the steel material and optimizing the hot rolling conditions. This makes it possible to precipitate fine carbides and effectively refine the ferrite.

(D) Method for producing cold-rolled steel sheet using hot-rolled steel sheet as a base material A hot-rolled steel sheet having a total amount of solid solution Ti and solid solution Nb obtained through the hot rolling step described above is 0.002% or more. The steel sheet is pickled according to a conventional method and then cold-rolled to obtain a cold-rolled steel sheet. The rolling reduction in the cold rolling is not particularly limited, but is desirably 30% or more in order to make the austenite grain size during annealing finer and further refine the ferrite grain size. In order to avoid an excessive load of 90%, it is preferably 90% or less. The cold-rolled steel sheet is annealed and cooled under the following conditions for strain relief. In order to ensure flatness, the cold-rolled steel sheet after annealing may be appropriately subjected to skin pass or correction according to a conventional method.

(Annealing conditions of cold-rolled steel sheet: maintained in a temperature range of 700 ° C. or more and 950 ° C. or less for 30 seconds or more and 600 seconds or less)
The cold-rolled steel sheet is annealed by holding it in a temperature range of 700 ° C. or more and 950 ° C. or less for 30 seconds or more and 600 seconds or less. When the holding temperature is less than 700 ° C., the processed structure remains and becomes a band-shaped structure, and the workability of the cold-rolled steel sheet is significantly deteriorated. On the other hand, if the holding temperature exceeds 950 ° C., the coarsening of ferrite proceeds rapidly, and a desired fine grain structure cannot be obtained. If the holding time is less than 30 seconds, homogenization of segregation of substitutional elements such as Mn becomes insufficient, the steel structure after annealing becomes non-uniform, and the workability of the cold-rolled steel sheet deteriorates. On the other hand, if the holding time exceeds 600 seconds, the ferrite after annealing becomes coarse and a desired fine grain structure cannot be obtained.

(Cooling condition after annealing of cold-rolled steel sheet: cooling to 500 ° C. with an average cooling rate in the temperature range from 650 ° C. to 550 ° C. being 5 ° C./second or more and 200 ° C./second or less)
After the annealing, the average cooling rate in the temperature range from 650 ° C. to 550 ° C. is set to 5 ° C./second or more and 200 ° C./second or less to cool to 500 ° C. When the average cooling rate is less than 5 ° C./second, not only the crystal grains become coarse, but also pearlite and cementite are generated, and a desired retained austenite volume ratio cannot be ensured. When the average cooling rate is more than 200 ° C./second, a non-uniform structure due to uneven cooling may be generated, and the material stability may be lowered. When good material stability is required, the average cooling rate is preferably 5 ° C./second or more and 80 ° C./second or less.

(Holding condition after cooling of cold-rolled steel sheet: Hold for 30 seconds or more in a temperature range of 300 ° C or higher and 500 ° C or lower)
After the annealed cold-rolled steel sheet is cooled to 500 ° C. or lower under the above conditions, it is held in a temperature range of 300 ° C. or more and 500 ° C. or less for 30 seconds or more. When the holding temperature at this time exceeds 500 ° C., the formation of pearlite is promoted, and when the holding temperature is below 300 ° C., the formation of martensite is promoted, both resulting in a decrease in the carbon concentration in the austenite, and the desired volume fraction of retained austenite. May not be obtained. Even when the holding time is less than 30 seconds, carbon cannot be sufficiently concentrated in austenite, and the target volume fraction of retained austenite may not be obtained.

  According to the present invention, by adjusting the chemical composition of the steel and optimizing the hot rolling and annealing conditions after cold rolling, ferrite: 70% or more and retained austenite: 3% or more, with the balance being Residue comprising bainite and inevitable martensite, the ferrite having an average grain size of 3.0 μm or less, the residual austenite having an average grain size of 1.0 μm or less, and an aspect ratio of 2 or less in the residual austenite A high-tensile cold-rolled steel sheet having a steel structure having an austenite ratio of 60% by volume or more and excellent workability can be obtained.

  In addition, it is good also as a surface-treated steel plate by providing a plating layer in the surface of the steel plate mentioned above for the purpose of the improvement of corrosion resistance. The plating layer may be either an electroplating layer or a hot dipping layer. Examples of electroplating include electrogalvanizing and electro-Zn-Ni alloy plating. Examples of the hot dip plating include hot dip galvanizing, alloyed hot dip galvanizing, hot dip aluminum plating, hot dip Zn-Al alloy plating, hot dip Zn-Al-Mg alloy plating, hot dip Zn-Al-Mg-Si alloy plating and the like. . The amount of plating adhesion is not particularly limited, and may be the same as the conventional one. Further, it is possible to further improve the corrosion resistance by performing an appropriate chemical conversion treatment (for example, application and drying of a silicate-based chromium-free chemical conversion treatment solution) after plating.

  Steel ingots were obtained by melting and casting steel having the chemical composition shown in Table 1 in a 150 kg high-frequency vacuum melting furnace. Each obtained steel ingot was hot forged by a conventional method to obtain a steel piece having a width of 200 mm and a thickness of 35 mm. Next, after grinding each steel slab to a thickness of 30 mm, the steel slab is heated to a desired temperature in a heating furnace under the conditions shown in Table 2 and hot rolled in 5 to 8 passes to obtain a thickness of 3 A steel plate of 0.0 mm was finished. After completing the hot rolling, cooling and winding processes were performed under the conditions shown in Table 2. As for cooling, only No. 33 in Table 2 was air-cooled, and the rest was water-cooled.

  The hot-rolled steel sheet thus obtained was pickled and then cold-rolled at the rolling reduction shown in Table 3 to obtain a cold-rolled steel sheet. The obtained cold-rolled steel sheet was heat-treated for annealing under the conditions shown in Table 3. That is, first, the cold-rolled steel sheet is heated at 10 ° C./s to the indicated annealing temperature and held for the indicated annealing time, and then gradually cooled to the indicated cooling start temperature at the indicated slow cooling rate (in a temperature range of 700 ° C. or higher). The holding time was shown in the table as “holding time of 700 ° C. or higher”), followed by gas cooling at the indicated speed and stop temperature, and then gas cooling to room temperature at 10 ° C./s. Table 3 also shows the holding time in the temperature range of 300 to 500 ° C. during cooling. Finally, the obtained cold-rolled steel sheet was subjected to a skin pass at a rolling rate of 0.2%.

Each hot-rolled steel sheet and cold-rolled steel sheet thus obtained were evaluated as follows.
For the hot-rolled steel sheet, the amount of solute Ti and the amount of solute Nb and the average particle diameter of ferrite were measured. The amount of solute Ti and the amount of solute Nb in the hot-rolled steel sheet were measured by collecting a test piece from the central part in the width direction of the central part in the longitudinal direction of the steel sheet and chemically analyzing the electrolytic extraction residue. The average grain size of ferrite was measured in the same manner as in the case of a cold-rolled steel sheet described later. These measurement results are shown in Table 2.

  For cold-rolled steel sheet, identification of phase and structure, ferrite volume fraction (Vα) and average grain size (Dα), volume fraction of retained austenite (Vγ) and mean grain size (Dγ), aspect ratio 2 in retained austenite The volume fraction of retained austenite (Vγ <2) was determined as follows.

The volume ratio of ferrite was determined by observing the cross section in the plate thickness direction with a scanning electron microscope.
For the average grain size of ferrite, the average grain section length at each position was measured by the section method using a scanning electron micrograph taken at a 1/4 depth position from the steel sheet surface in the section in the thickness direction. These arithmetic average values were obtained by multiplying them by 1.128.

The volume fraction of retained austenite was determined by X-ray diffraction.
The residual austenite average particle diameter was determined by evaluating the position of a quarter depth of the plate thickness from the steel plate surface in the plate thickness direction cross section by electron beam backscattering diffraction (EBSP). Here, the particle diameter of the retained austenite was a short diameter (for example, a film thickness in the case of a film).

  The mechanical properties of the cold-rolled steel sheet were obtained by taking a No. 5 tensile test piece specified in JIS Z 2201 (1998) in the rolling direction and conducting a tensile test at room temperature to obtain a tensile strength (TS) and an upper yield strength (YS). And by measuring the total elongation (EL).

  Table 4 shows the investigation results of the steel structure and mechanical properties of the cold-rolled steel sheet.

  As is apparent from Table 4, the cold-rolled steel sheets of test numbers 1 to 5, 8, 9, 12, 13, 16 to 18, 21, 23 to 28 according to the present invention are 70% ferrite or more and 3% residual austenite. The volume ratio of ferrite having an average particle diameter of ferrite of 3.0 μm or less and a fine grain structure and having an aspect ratio of 2 or less in the retained austenite is 60% or more. As a result, the product of tensile strength TS (MPa) and total elongation EL (%) had excellent strength and elongation characteristics of 22000 MPa ·% or more. That is, the cold-rolled steel sheet having the chemical composition and the steel structure according to the present invention is excellent in workability while having high strength.

  On the other hand, even in the case of having the chemical composition defined in the present invention, in the test numbers 29 to 34 in which the hot rolling conditions deviate from the regulations defined in the present invention, the cold-rolled structure cannot be refined and yielded. Both the strength and the tensile strength are low, and the product of the tensile strength TS (MPa) and the total elongation EL (%) remains below 21000 MPa ·%.

  Moreover, even if it has a chemical composition defined by this invention, the test numbers 6, 7, 10, 11, 14, 15, 19, 20, 22 in which the annealing conditions after cold rolling deviate from the regulation defined by this invention. Then, a fine grain structure cannot be obtained, or the volume fraction of retained austenite cannot be secured sufficiently, and both are inferior in strength-ductility balance as compared with the inventive examples.

  On the other hand, in test numbers 35 to 45 where the chemical composition deviates from the provisions stipulated in the present invention, the reduction in ductility is particularly noticeable, and the mechanical property is remarkably inferior to that of the inventive examples, with the strength-ductility balance being less than 19000 MPa ·%.

  As can be seen from the above, the high-tensile cold-rolled steel sheet according to the present invention is excellent in that the product TS × El value of the tensile strength TS (MPa) and the total elongation El (%) is 22000 MPa ·% or more. It is suitable as a material for a high-strength structural member used for a vehicle body or a shock absorbing member. This high-tensile cold-rolled steel sheet can be manufactured relatively easily by a general continuous annealing facility by the method of the present invention.

Claims (8)

In mass%, C: 0.06% to 0.25%, Si: 0.01% to 2.0%, Mn: 0.5% to 2.0%, and Al: 0.01% More than 2.0% is contained, and further, one or two of Ti: 0.20% or less and Nb: 0.10% or less is contained, and the following formulas (1) and (2) are satisfied. And the balance has a chemical composition consisting of Fe and impurities, and contains, by volume, ferrite: 70% or more and residual austenite: 3% or more, and the balance is composed of bainite and unavoidable martensite, The average particle diameter Dα (μm) at a depth position of 1/4 of the sheet thickness from the surface is 3.0 μm or less, and the average particle diameter Dγ (μm at a depth position of 1/4 of the sheet thickness from the steel sheet surface of the retained austenite. ) Is 1.0 μm or less, and the following formula (3) and A cold-rolled steel sheet satisfying (4) and having a steel structure in which the ratio of retained austenite having an aspect ratio of 2 or less to the retained austenite is 60% by volume or more.
0.8 ≦ Si + Al ≦ 3.0 (1)
0.002 ≦ Ti + Nb ≦ 0.20 (2)
1.5 ≦ Dα / Dγ ≦ 15 (3)
3 ≦ (Dα / Dγ) × (Dα + Dγ) ≦ 30 (4)
Each element symbol in the above formula means the content (unit: mass%) of the element.   The cold-rolled steel sheet according to claim 1, wherein the chemical composition contains one or two of Ca: 0.01% or less and Zr: 0.10% or less in mass% instead of a part of Fe. .   A hot-rolled steel sheet having the chemical composition according to claim 1 or 2, wherein the total content of solute Ti and solute Nb in the steel at room temperature is 0.003 mass% or more.   The hot rolled steel sheet according to claim 3, wherein the average particle diameter of the ferrite is 4 µm or less.   The steel ingot or steel slab having the chemical composition according to claim 1 or 2 is brought to a temperature of 1150 ° C or higher and 1300 ° C or lower, hot-rolled to complete rolling at 800 ° C or higher, and 750 from completion of hot rolling. A method for producing a hot-rolled steel sheet, comprising performing cooling at a cooling time to 10 ° C. within 10 seconds, and then winding in a temperature range of 650 ° C. or less within 30 seconds after completion of the cooling.   The steel ingot or steel slab having the chemical composition according to claim 1 or 2 is brought to a temperature of 1150 ° C. or higher and 1300 ° C. or lower, hot-rolled to complete rolling at 800 ° C. or higher, and 720 from completion of hot rolling. A method for producing a hot-rolled steel sheet, wherein the steel sheet is cooled in a temperature range of 650 ° C. or less within 30 seconds after completion of this cooling after cooling to 0.7 ° C. within 0.7 seconds.   The hot-rolled steel sheet according to claim 3 or 4 is cold-rolled, and the cold-rolled steel sheet is held in a temperature range of 700 ° C to 950 ° C for 30 seconds to 600 seconds, and then 650 ° C to 550 ° C. An average cooling rate in the temperature range up to 5 ° C./second to 200 ° C./second is cooled to 500 ° C. and kept in a temperature range of 300 ° C. to 500 ° C. for 30 seconds or more. Production method.   After cold rolling the hot-rolled steel sheet produced by the method according to claim 5 or 6 and holding the cold-rolled steel sheet in a temperature range of 700 ° C to 950 ° C for 30 seconds to 600 seconds, The average cooling rate in the temperature range from 650 ° C. to 550 ° C. is set to 5 ° C./second or more and 200 ° C./second or less, cooled to 500 ° C., and maintained in the temperature range of 300 ° C. or more and 500 ° C. or less for 30 seconds or more. A method for producing cold-rolled steel sheet.