JPH10130733A - Manufacturing method of bake hardenable steel sheet with little aging deterioration - Google Patents

Abstract

PROBLEM TO BE SOLVED: To production a steel sheet high in baking hardenability and free from the generation of aging deterioration without causing disadvantages in the points of the uniformity of the material and the stability of the operation. SOLUTION: A slab having a compsn. contg., by weight, <=0.01% C, <=0.10% Si, <=1.5% Mn, <=0.20% P, <=0.010% S, 0.030 to 0.150% Al, <=0.0040% N, and the balance Fe with inevitable impurities is subjected to hot rolling so as to regulate the finish rolling temp. to 800 to 950 deg.C and is coiled at >=600 deg.C, this hot rolled sheet is subjected to pickling and cold rolling, is thereafter subjected to decarburizing annealing by the quantity to be decarburized of >=0.0005wt.% by annealing treatment of holding to the recrystallization temp. or above for >=10sec in an atmosphere in which the gaseous compsn. is composed of >=3% hydrogen, and the balance substantial nitrogen, and the dew point is regulated to >=-20 deg.C and is subjected to secondary cold rolling at a draft of 1 to 5%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing a bake hardenable steel sheet, and particularly to an aging method suitable for use requiring dent resistance after processing, such as automobile panels. The present invention relates to a method for producing a steel sheet having little deterioration and having bake hardenability.

[0002]

2. Description of the Related Art Bake hardenable steel sheets for press working are:
Generally, processed products that are soft at the time of molding, have the property of being hardened by heating (approximately 170 ° C for 20 minutes) in the baking process after molding and painting, and finally have sufficient strength What can be manufactured is ideal. Conventionally, several studies have been conducted to impart such properties. For example, Japanese Patent Publication No. 5-48283 discloses a method for controlling the amount of solute C in steel within an appropriate range by mainly defining the steel component.
Japanese Patent Application Laid-Open No. 57-192225 discloses, as a similar technique, a method of controlling the amount of C in a solid solution state by adding Nb instead of Ti and controlling the dissolution and precipitation behavior of Nb. It has been disclosed. This method involves high-temperature annealing at 850 ° C or higher,
By re-dissolving a part of NbC in the precipitated state and rapidly cooling from that state, re-precipitation is prevented to secure the amount of dissolved C in an appropriate range.

[0003]

However, even these prior arts have not reached the range that satisfies the recent requirements for bake-hardenable steel sheets. That is, the specific properties required are high bake hardenability (40M
Pa or more), good r-value, and low room temperature aging deterioration characteristics. In the above-described conventional technology, it was particularly difficult to achieve both baking hardenability and room temperature aging deterioration characteristics. That is, if the amount of solid solution C is increased in order to secure a large bake hardening amount, the aging characteristics at room temperature are deteriorated, and there is a risk that a stretcher strain is generated at the time of press molding, resulting in poor appearance. There was no production method that satisfied the above.

[0004] Incidentally, as the prior art for obtaining the bake hardenability while suppressing the stretcher strain, a) a method of carburizing by controlling the atmosphere in an annealing step (for example, Japanese Patent Publication No. 8-14019) b) A method of continuously annealing ultra-low carbon steel in the two-phase region (for example,
JP-A-2-232316), but all have problems in terms of uniformity of materials and stability of operation. In particular, in the above-mentioned a), since the atmosphere in the furnace is set to a strong carburizing atmosphere, a problem arises when a material that is not carburized is processed on the same line. Further, in the above b), there is a problem that it is essential to make the composition of the steel sheet a special component system.

Further, according to the investigations by the inventors, according to the empirical rule regarding the equivalence between temperature and time, which is adopted in the above-mentioned prior art, to measure the aging deterioration characteristics at room temperature, 100 ° C. Conventional accelerated tests conducted at moderate temperatures have shown that the actual aging degradation cannot be simulated. As a method of measuring the aging deterioration characteristics at room temperature, which has better simulating properties, the aging temperature is set at 50 in consideration of the most severe conditions that can be achieved under normal storage conditions.
It was also found that it is better to use a high temperature of about ° C (when exporting, etc., it will be stored at this temperature when passing through the equator). For this reason, there has been a demand for a bake-hardenable steel sheet that can withstand the correct evaluation method of the deterioration characteristics at room temperature in accordance with the actual use environment.

In view of the above-mentioned problems of the prior art, the main object of the present invention is to have a large bake hardenability and an aging property so that parts baked and painted after press molding have sufficient strength. It is an object of the present invention to provide a technique for producing a steel sheet which does not cause deterioration (stretcher strain generation during press forming) without causing disadvantages in terms of uniformity of materials and stability of operation. A specific object of the present invention is that the baking hardening amount (BH) is 40MP.
a, yield point elongation after aging at 50 ° C for 3 months is 0.2%
Hereinafter, it is an object of the present invention to provide a technique for producing a steel sheet having a material characteristic having an r value (in-plane average) of 1.8 or more without incurring disadvantages in terms of material uniformity and operation stability.

[0007]

Means for Solving the Problems In order to solve the above-mentioned problems, the present inventors have made extensive experiments and studies on steel components, rolling and heat treatment conditions, and as a result, the following findings have been obtained. 1) As the steel component, an ultra-low carbon steel from which a high r value can be easily obtained is suitable. It is also effective from the viewpoint of stabilizing the material to secure the amount of C in a solid solution state by selectively adding a carbonitride forming element. 2) The target material characteristics can be obtained by actively utilizing the decarburization phenomenon in the annealing furnace, which has been endeavored to suppress as much as possible in the conventional continuous annealing process, for the ultra-low carbon steel. That is, in the continuous annealing step, the dew point is increased to -20 ° C. or higher, and decarburization is caused on the steel sheet surface to reduce aging property, while sufficient solid solution C is secured inside the steel sheet to secure baking hardenability. By remaining, both baking hardenability and aging deterioration resistance can be achieved.

[0008] The present invention has been made based on the above findings, and the gist thereof is as follows. 1) C: 0.01 wt% or less, Si: 0.10 wt% or less, Mn: 1.5
wt% or less, P: 0.20 wt% or less, S: 0.010 wt% or less, Al: 0.030 to 0.150 wt%, N: 0.0040 wt% or less, with the balance being Fe and unavoidable impurities. Hot-rolled at a finishing rolling temperature of 800 to 950 ° C, 600
After hot-rolled sheet is pickled and cold-rolled, the gas composition is made up of hydrogen having a gas composition of 3% or more and substantially nitrogen and the dew point is -20 ° C or more. Decarburizing annealing at a decarburizing amount of 0.0005 wt% or more in the atmosphere by annealing at a temperature not lower than the recrystallization temperature for 10 seconds or more, and performing secondary cold rolling at a rolling reduction of 1 to 5% A method for producing a bake-hardenable steel sheet having little aging deterioration.

2) In the above 1), the steel composition further comprises
Nb: 0.003 to 0.040 wt% and Ti: 0.003 to 0.040 wt%
Nb and Ti are each represented by the following formula: {Nb (wt%) / 93} / {C (wt%) / 12} ≤0.8 and {Ti ^* (wt% ) / 48｝ / {C (wt%) / 12｝ ≦ 0.8 where Ti ^* (wt%) = Ti (wt%)-(48/32) × S
A method for producing a bake-hardenable steel sheet with less aging deterioration, wherein the composition satisfies the relationship of (wt%)-(48/14) × N (wt%).

3) The method for producing a bake hardenable steel sheet with little aging deterioration according to 1) or 2) above, wherein the steel composition further contains B: 0.0002 to 0.0020 wt%.

4) In any one of the above items 1) to 3), the steel composition further includes Cu: 0.01 to 0.2 wt%, Ni: 0.01
-0.2 wt% Cr: 0.01-0.2 wt%, Mo; 0.01-0.2 wt%
A method for producing a bake-hardenable steel sheet with less aging deterioration, characterized in that the composition contains at least one kind selected from the group consisting of:

5) In the hot-rolled sheet as a cold-rolled sheet, at least 80% of the N content in steel is precipitated as AlN, and the aging deterioration described in any one of the above 1) to 4) is characterized. Method for producing bake-hardenable steel sheet with less heat.

[0013]

Next, the reason why the composition of the components, the production conditions and the like are limited as described above will be described. (1) Steel component C: 0.01 wt% or less C is a harmful element from the viewpoint of workability, and a stable high r value and good ductility can be obtained by reducing C. In order to obtain such an effect, the C content needs to be 0.01 wt% or less, and it is desirable to reduce the C content to 0.004 wt% or less in consideration of the r value. In addition, in order to obtain effective bake hardenability, it is preferable to contain approximately 0.001 wt% or more.

Si: 0.10 wt% or less Si, when added in a large amount, causes deterioration of surface treatment properties and corrosion resistance, etc., so the upper limit is made 0.10 wt%. When particularly excellent corrosion resistance is required, the content is preferably limited to 0.02 wt% or less.

Mn: 1.5 wt% or less Mn is an effective element for preventing hot cracking caused by S, and is preferably added according to the amount of S contained. Further, Mn is an element effective for refining crystal grains and improving the material. In order to exhibit these effects, it is desirable to add 0.1 wt% or more. On the other hand, when Mn is added in a large amount, although high strength of the steel sheet can be achieved, corrosion resistance decreases and flange workability deteriorates. Therefore, the upper limit is set to 1.5 wt%. In applications where better moldability is required, 0.
80 wt% or less is desirable.

P: 0.20 wt% or less P is a useful element for increasing the strength by solid solution strengthening, but when contained in a large amount, it hardens the steel,
While deteriorating the flange workability and neck workability,
In order to reduce the corrosion resistance, the upper limit is set to 0.20 wt%. In the case where emphasis is placed on workability and corrosion resistance, the content is preferably suppressed to 0.01 wt% or less.

S: 0.010 wt% or less S is an element that exists as an inclusion in steel, reduces ductility, and further deteriorates corrosion resistance. These effects become remarkable when the S content exceeds 0.010 wt%,
Limit to 010 wt% or less. In addition, it is desirable to suppress the content to 0.005 wt% or less especially for applications requiring good workability.

Al: 0.030 to 0.150 wt% Al is an element necessary for stably fixing N,
It is necessary to add 0.030 wt% or more, but if it is contained in a large amount, it leads to phenomena such as deterioration of surface properties, increase in anisotropy in the rolling direction, and generation of flange cracks due to softening of the welded part. wt%. In order to further stabilize the material, it is desirable to add it in the range of 0.040 to 0.080 wt%.

N: 0.0040 wt% or less N is an element that increases the aging property, has a high solid solubility in ferrite, and is an element whose control of the amount of solid solution is important.
In the present invention, the bake hardening characteristic is not dependent on N, but is based on C
Only aim to get in. With the above added Al,
Although the amount of solid solution of N can be reduced, the total amount of N in steel is 0.0040wt
%, It becomes difficult to stably fix the solute N. Therefore, the amount of N is 0.0040 wt% or less, preferably 0.1%.
0020 wt% or less.

In addition to the above basic elements, the following elements can be selectively added. Nb: 0.003 to 0.040 wt%, and {Nb (wt%) / 93} /
{C (wt%) / 12} ≦ 0.8 Nb has an effect of reducing solid solution C and N by forming carbonitrides, and has a remarkable effect of refining crystal grains. These effects are exhibited when added at 0.003 wt% or more,
If added in excess of 040 wt%, the steel becomes hard and not only hinders the cold rolling process, but also increases the risk of cracking in the continuous slab casting process. Therefore, the upper limit of the amount of Nb added is set to 0.040 wt%, but is preferably set to 0.02 wt% in order to stably secure the baking hardening amount. Further, the atomic ratio of Nb to C, that is, {Nb (wt%) / 93} / {C (wt
%) / 12% exceeds 0.8, it becomes difficult to obtain a target sufficient amount of bake hardenability. Therefore, the atomic ratio is set to 0.8 or less, preferably 0.75 or less.

Ti: 0.003 to 0.040 wt%, and ΔTi ^* (wt
%) / 48｝ / {C (wt%) / 12｝ ≦ 0.8 where Ti ^* (wt%) = Ti (wt%)-(48/32) × S
(Wt%) − (48/14) × N (wt%) Ti is also an element effective for reducing the amount of solute C and for refining the structure, like Nb. Such effects are 0.00
The effect is exhibited by addition of 3 wt% or more, but when added in excess of 0.04 wt%, the bake hardening amount decreases. Therefore, the addition amount of Ti is desirably 0.003 to 0.040 wt%, and the desirably is in the range of 0.005 to 0.020 wt% in order to stably secure the baking hardening amount. Also, the atomic ratio between Ti and C, that is, {Ti ^* (wt%) / 48} / {C (wt%) / 12} ≦ 0.8
Where Ti ^* (wt%) = Ti (wt%)-(48/32) x S
If (wt%) − (48/14) × N (wt%) exceeds 0.8, it becomes difficult to obtain a target sufficient amount of bake hardenability. Therefore, the atomic ratio is 0.8 or less, preferably 0.1.
75 or less.

B: 0.0002 to 0.0020 wt% B is an additive element effective for improving the aging resistance at room temperature without reducing the bake hardenability of the steel sheet, although the detailed mechanism is not necessarily clear. Such effects are 0.
0002wt% or more is exerted. However, if it exceeds 0.0020wt%, not only this effect is saturated, but also the in-plane anisotropy of the mechanical properties of the steel sheet increases, so that 0.0002 to 0.0002%.
Add in the range of 20 wt%. In addition, stabilization of mechanical properties,
From the viewpoint of homogenization, it is preferable to add in the range of 0.0005 to 0.0010 wt%.

Cu: 0.01 to 0.2 wt%, Ni: 0.01 to 0.2 wt%
%, Cr: 0.01 to 0.2 wt% and Mo; 0.01 to 0.2 wt% Cu, Ni, Cr and Mo have almost the same material improvement effect, and increase the strength of the steel sheet without causing significant ductility deterioration. Can be done. Such an effect is exhibited by addition of 0.01 wt% or more. However, even if it is added in excess of 0.2 wt%, the effect is saturated, and the hot-rolled base plate is hardened and the cold rolling process is performed. The risk of malfunctioning increases.
The effects described above are not canceled out even if these elements are added in combination, and can be obtained by either single addition or multiple addition.

(2) Manufacturing Conditions Hot Rolling The heating of the steel material before hot rolling may be achieved by complete solution heat treatment, and may be performed by heating to _three or more Ac points. Specifically, 1050 ~
1300 ° C is suitable. In the hot rolling following the above-mentioned heating, the finish rolling temperature is important from the viewpoint of securing the r value and ductility of the steel sheet. In order to obtain the target high r value and bake hardening characteristics, it is necessary to set the finish rolling temperature to 800 ° C. or higher. However, when the finish rolling is performed at a temperature higher than 950 ° C., the load on the hot rolling roll increases, and the structure becomes coarse. Therefore, the finishing hot rolling temperature is
The temperature range is 800 to 950 ° C, preferably 840 to 920 ° C.

Winding temperature The winding temperature affects the stable fixing of N by Al. By setting the winding temperature to 600 ° C. or higher, preferably 650 ° C. or higher, precipitation and fixation of N by Al can be achieved over almost the entire length of the hot-rolled coil. Although the upper limit of the winding temperature is not particularly defined, it is preferable to keep the temperature at 780 ° C. or lower in consideration of suppressing the deterioration of the descaling property.

-80% or more of the N content in the steel is precipitated as AlN in the hot rolled sheet. The hot rolled sheet is pickled, cold rolled, and then continuously annealed. During this annealing, some of the solute N precipitates as AlN, but it is difficult to completely precipitate the solute N because of the short time. If N in the solid solution state exists after annealing, aging deterioration at room temperature, particularly recovery of yield point elongation becomes remarkable. For this reason, it is desirable that N be precipitated at 80% or more, or even 85% or more of the N content in the steel at the stage of hot rolling. The N in the precipitation state defined here refers to the amount of N (N as AlN) calculated from the equivalence relationship after analyzing AlN by the usual electrolytic extraction analysis. In order to precipitate and fix N in steel within the above range, it is extremely important to control the above-described winding temperature after hot rolling. In addition to this, it is also effective to heat or gradually cool the wound coil for at least one hour.

Continuous annealing The continuous annealing step is one of the particularly important requirements in the present invention. In an atmosphere in which the gas composition is 3% or more of hydrogen and the balance is substantially nitrogen, and the dew point is -20 ° C or more,
Perform continuous annealing at a temperature higher than the recrystallization temperature for 10 seconds or more, and decarburize (decarbonize) 0.0005 wt% or more during the annealing process.
Cause. By finally decarburizing 0.0005 wt% or more, it is possible to achieve both the sufficient bake hardenability and the excellent room temperature aging resistance, which are the aims of the present invention. Although the detailed mechanism of the decarburization phenomenon is not always clear, it is estimated as follows. Decarburization uses a surface reaction to remove C in the steel from the surface layer of the steel sheet as CO (or CH ₃ or CO ₂ ) by a solid-gas reaction. In this case, a large C concentration gradient is generated from the surface layer toward the inside of the steel sheet. Actually, it is difficult to analyze the concentration distribution of C in the plate thickness direction. However, when the calculation is performed on the assumption that the reaction is the diffusion control of C, the result is as shown in FIG. The reason why this analysis result is appropriate to a certain extent can be said that the C content integrated in the sheet thickness direction well corresponds to the C analysis values before and after annealing. Such a concentration gradient can be maintained at room temperature by rapid cooling (at least 10 ° C./sec or more) after soaking such as continuous annealing.

When the secondary rolling at a rolling reduction of 1 to 5% is performed in a state where the concentration gradient in the thickness direction of C is attained as described above, the movable dislocation causes the extremely C content in the surface layer portion. Will be introduced into the area where the number is small. That is, the movable dislocations introduced into the surface layer portion can easily avoid sticking by C. Thereby, excellent room temperature aging resistance appears. On the other hand, at the stage of using the product, after at least about 5% plastic working strain is given,
Aging treatment for about 20 minutes will be performed. in this case,
Deformation naturally occurs in the entire thickness direction of the steel sheet, and many dislocations are newly introduced. Then, at the time of aging, since a sufficient amount of solute C remains to fix these large amounts of dislocations in the steel, the steel exhibits the desired large bake hardening characteristics.

In order to cause the above-described effective decarburization reaction, continuous annealing capable of rapid heating and rapid cooling is performed, and at that time, the balance contains hydrogen of 3% or more, and the balance substantially consists of nitrogen. It is necessary to make the atmosphere of the annealing different. The dew point is-
Annealing must be performed at a temperature of 20 ° C. or higher, preferably 1-10 ° C. or higher, and at a temperature higher than the recrystallization temperature for about 10 seconds or longer, preferably 20 seconds or longer. The upper limit of the annealing time is not particularly defined, but is preferably less than 40 seconds in consideration of the efficiency of the actual production process. Although the upper limit of the hydrogen concentration is not particularly defined, it is desirable to set the concentration to 10% or less in consideration of the cost of the atmosphere gas and the stability of operation. As the decarburization amount, the C content in the steel before and after annealing is measured by thickness penetration analysis, and if the difference is 0.0005 wt% or more, the desired excellent room temperature aging resistance is obtained. Properties and sufficiently large bake hardenability are obtained. Originally, the profile in the depth direction should be considered, but if the steel composition and manufacturing conditions of the present invention are used, the penetration analysis in the thickness direction can be substituted.

Secondary Cold Rolling The secondary cold rolling is usually carried out at a light rolling reduction of about 0.8%. However, in the present invention, in order to sufficiently improve the aging resistance at room temperature, a higher value is required. It is necessary to provide rolling at a reduction ratio. In other words, by applying a reduction of 1% or more, the target elongation at yield at 50 ° C. for three months can be suppressed to 0.2% or less by aging. But 5%
When the rolling is performed in excess of the above, the ductility, particularly the uniform elongation, of the steel sheet is reduced, so that there is an increased risk of causing problems such as cracks during press forming. Therefore, the rolling reduction of the secondary cold rolling is set to a rolling reduction of 1 to 5%. When more excellent room-temperature aging resistance is required, it is desirable to apply rolling strain at a rolling reduction of 2 to 3%.

The material properties targeted by the present invention are that the r value (in-plane average) is 1.8 or more, and the bake hardening amount (BH) is
The yield point elongation after aging at 40 MPa or more and 50 ° C for 3 months is 0.2
% Or less. The amount of bake hardening guarantees the strength characteristics of the molded part during use. Although the value varies depending on the thickness of the steel sheet and the strength level, in a part to which the steel sheet having extremely excellent formability as in the present invention is applied, a baking hardening amount of about 40 MPa or more is sufficient for practical use. is there. The room temperature aging characteristics also differ depending on the target component, but generally, it is practically sufficient to withstand aging at 50 ° C. for 3 months. If the value of the yield point elongation recovered under the aging condition is 0.2% or less, the appearance defect due to the occurrence of the stretcher strain does not occur during the press molding.

[0032]

Embodiment 1 Next, an embodiment of the present invention will be described. A steel having the composition shown in Table 1 was smelted in a converter, and the steel slab was subjected to hot rolling, continuous annealing and secondary cold rolling under the conditions shown in Table 2, and a final sheet thickness of 0.7 mm was cold rolled. A steel plate was used. From the steel sheet thus obtained, a JIS No. 5 test piece was を
Samples were taken from the position and the usual mechanical properties and bake hardenability (BH property) were investigated. Regarding the room temperature aging property, the tensile test pieces similarly collected were aged at 50 ° C. for 1 month using a thermostat. Table 3 shows the results of these investigations. The bake hardenability (BH property) was determined as follows. That is, after applying a 2% pre-strain, once unloading, 170 ℃
The aging was performed for 20 minutes at, and tension was again applied, and the difference between the deformation stress before aging and the yield stress after aging was defined as the BH amount. Also,
The r value was determined as an average r value by the following equation. _{r = (r 0 + r 90} + 2r 45) / 4 However, r _0: the rolling direction of the r value r _90: the rolling direction and the direction of the r value r ₄₅ which forms an inclination of 90 degrees: forming a slope in the rolling direction and 45 degrees R value in direction

[0033]

[Table 1]

[0034]

[Table 2]

[0035]

[Table 3]

From Tables 1 to 3, it can be seen that the steel sheet produced by the method of the present invention has a high r-value and elongation (El), a bake hardening characteristic of 40 MPa or more, and a yield point even under severe aging. Since elongation does not occur, poor appearance due to the occurrence of so-called stretcher strain does not occur. This is true for actual press forming.
Increased component strength due to BH properties and suppression of stretcher strain generation were achieved. As in the above experiment, an experiment was conducted on a continuous hot-dip galvanized steel sheet production line while setting the annealing conditions within the range of the present invention. As in the case of the cold-rolled steel sheet described above, excellent formability ( High r value, high El) and high BH properties were obtained, and there was no deterioration due to aging at room temperature. Therefore, it was found that the present invention can be applied not only to cold-rolled steel sheets but also to hot-dip galvanized steel sheets (and alloyed hot-dip galvanized steel sheets). As a matter of course, it can be applied without any problem to the use of electrogalvanizing or the like after the production of a cold-rolled steel sheet.

Example 2 A cold-rolled steel sheet was manufactured from steel 1 in Table 1 under the manufacturing conditions shown in Table 4, and a Zn-Ni alloy-plated steel sheet was manufactured in a continuous electroplating line. Then, after coating and baking a resin on the surface, mechanical properties, BH properties and aging deterioration characteristics at room temperature were investigated in the same manner as in Example 1. Table 5 shows the results. The resin-coated composite plated steel sheet produced by the method of the present invention has a high r value, elongation, and has bake hardening characteristics of 40 MPa or more, but does not cause yield point elongation even under the severe aging as described above. There is no appearance defect due to the occurrence of the stretcher strain. Also, in actual press forming, the expected increase in component strength due to BH properties and suppression of stretcher strain were achieved.

[0038]

[Table 4]

[0039]

[Table 5]

[0040]

As described above, according to the present invention,
After hot-rolling and cold-rolling a steel material in a specific component range, decarburization is performed in the continuous annealing step to positively form a C concentration distribution in the sheet thickness direction, and further passivate at a relatively high temper. By providing, it is possible to stably satisfy both the sufficiently large baking hardenability of the paint and the excellent aging resistance characteristic, which were conventionally difficult to achieve both.
Therefore, the steel sheet according to the present invention is soft and excellent in formability at the time of press forming and assembling, and when actually used as a product, the strength is greatly increased by baking hardening and shows excellent part strength. It is possible to reduce the gauge of the steel sheet necessary for obtaining strength. As a matter of course, the steel sheet according to the present invention can be used as various types of plating base sheets.

[Brief description of the drawings]

FIG. 1 is a diagram showing a calculated C concentration distribution in a plate thickness direction.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Osamu Furukun 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Inside the Technical Research Institute of Kawasaki Steel Co., Ltd. Kawasaki Steel Corporation Chiba Works

Claims (5)

[Claims] C: 0.01 wt% or less, Si: 0.10 wt% or less, Mn: 1.5 wt% or less, P: 0.20 wt% or less, S: 0.010 wt% or less, Al: 0.030 to 0.150 wt%, N: A slab containing 0.0040 wt% or less, with the balance being Fe and inevitable impurities, was subjected to a finish rolling temperature of 800 to 950.
After hot rolling at 600 ° C., winding at 600 ° C. or higher, and pickling and cold rolling the hot rolled sheet, the gas composition is composed of hydrogen of 3% or more and the balance substantially consisting of nitrogen, and Dew point is-
In an atmosphere of 20 ° C. or more, the decarburization amount is 0.00
A method for producing a bake-hardenable steel sheet with little aging deterioration, comprising decarburizing annealing of at least 05 wt% and performing secondary cold rolling at a rolling reduction of 1 to 5%. 2. The steel composition according to claim 1, further comprising: Nb: 0.003 to 0.040 wt% and Ti: 0.003 to 0.040 wt%.
Nb and Ti are respectively represented by the following formulas: {Nb (wt%) / 93} / {C (wt%) / 12} ≦ 0.8 and {Ti ^* (wt% ) / 48｝ / {C (wt%) / 12｝ ≦ 0.8 where Ti ^* (wt%) = Ti (wt%)-(48/32) × S
A method for producing a bake-hardenable steel sheet with less aging deterioration, wherein the composition satisfies the relationship of (wt%)-(48/14) × N (wt%). 3. The method according to claim 1, wherein the steel composition further contains B: 0.0002 to 0.0020 wt%. 4. The method according to claim 1, wherein
The steel composition further contains one or more selected from Cu: 0.01 to 0.2 wt%, Ni: 0.01 to 0.2 wt%, Cr: 0.01 to 0.2 wt% and Mo; 0.01 to 0.2 wt%. A method for producing a bake-hardenable steel sheet with less aging deterioration, characterized by having a composition. 5. The hot-rolled sheet, wherein 80% or more of the N content in steel is precipitated as AlN.
4. The method for producing a bake-hardenable steel sheet having little aging deterioration according to any one of 4.