JP3818024B2 - Method for producing soft cold-rolled steel sheet with excellent aging resistance - Google Patents

Description

【００３０】
【表２】

【００３１】
［実施例２］
表３に示す成分の鋼を溶製・鋳造した後、表４に示す条件で熱間圧延（巻取温度６４０℃）、酸洗、冷間圧延（冷間圧延率８０％）、連続焼鈍（焼鈍温度７５０℃）し、焼鈍後の急冷における冷却停止温度を変化させて過時効処理を行った後、伸長率１．０％で調質圧延を行い、板厚０．８ｍｍの冷延鋼板を得た。このようにして得られた冷延鋼板の時効指数を測定するとともに、引張試験（ＪＩＳ５号試験片）によりＴＳ、ＥＬを測定し、各冷延鋼板の耐時効性と延性の評価を行った。その結果を表４に示す。
なお、時効指数の測定では、８％予歪み付加後に１００℃で１時間保持することで時効させた。
【００３２】
Ｎｏ．１〜Ｎｏ．２の本発明例は、過時効処理開始温度付近で焼鈍後の急冷を停止したものである。これに対してＮｏ．３〜Ｎｏ．５の本発明例は、過時効処理開始温度よりも３０℃以上過冷した後に過時効処理を開始したものであり、これらは時効指数がＮｏ．１〜Ｎｏ．２よりも低く、過時効処理前に過冷することにより耐時効性がさらに向上することが判る。
【００３３】
【表３】

【００３４】
【表４】

【００３５】
【発明の効果】
以上述べたように本発明法によれば、ＢがＮに対して当量以上に添加されたＢ添加低炭素鋼から耐時効性と延性とを兼ね備えた冷延鋼板を製造することができる。
【図面の簡単な説明】
【図１】ＢがＮに対して当量以上に添加されたＢ添加低炭素鋼から得られた冷延鋼板について、その時効指数を過時効処理温度と過時効処理時間との関係で示すグラフ[0001]
[Technical field to which the invention belongs]
The present invention relates to a method for producing a soft cold-rolled steel sheet having excellent aging resistance suitable for automobiles and home appliances.
[0002]
[Prior art]
Steel sheets used for automobiles, home appliances, and the like are required to have high formability. For this reason, increasing the softness and ductility of steel sheets has been energetically promoted. When manufacturing such a high workability cold-rolled steel sheet by continuous annealing, IF steel in which C and N in steel are reduced and carbonitride forming elements are added to completely fix C and N is used. Although mainly used, this IF steel has the disadvantage of high production costs because it reduces C and N in the steel to the limit and adds Ti, Nb, and the like.
[0003]
For this reason, B-added low-carbon steel in which B is added to low-carbon steel and only N is fixed as BN has been studied as a technique for obtaining high workability without reducing C to the limit. However, in this B-added low carbon steel, aging resistance must be considered unlike IF steel. That is, if the amount of B added is equal to or less than the amount of N, there is no problem in aging resistance. However, in normal production, the amount of N varies due to charging, so B is inevitable even if a certain amount of B is added. More than N equivalent is added, and in this case, the aging resistance is inferior.
[0004]
In order to improve the aging resistance of such a B-added low carbon steel, the following method has been proposed.
Japanese Patent Application Laid-Open No. 56-130431 discloses a method in which B is added in an equivalent amount or more with respect to N, and N is completely fixed with B. However, in this method, in order to prevent a rimming action, Al must be reduced to 0.01 mass% or less, and an increase in the amount of oxide that reduces ductility is inevitable.
[0005]
Japanese Patent Application Laid-Open No. 56-158821 discloses a method for improving aging resistance by subjecting a steel having B / N> 0.5 or more to an overaging treatment for a long time exceeding 10 minutes at a low temperature of 300 ° C. or less. Yes. However, the line speed must be extremely slow for overaging for 10 minutes or more as in this method, and there is a risk that the strip breaks in the furnace.
Japanese Patent Application Laid-Open No. 60-165321 discloses a technique for precipitating cementite in ferrite grains with MnS as nuclei by overaging at a low temperature of 200 to 400 ° C. for 2 to 10 minutes. MnS to be obtained is only a fine one, and there is a drawback that a sufficient effect cannot be obtained depending on the slab heating temperature or the effect varies greatly.
[0006]
Japanese Patent Application Laid-Open No. 7-3332 discloses a method of finely precipitating cementite in grains using BN as a core by setting the annealing temperature to an ultrahigh temperature of 930 ° C. or lower and quenching to an overaging temperature of 400 to 300 ° C. It is disclosed. However, in this method, cementite precipitates ultrafinely in the grains, so that the uniform elongation is lowered and the deterioration of EL is inevitable.
Japanese Patent Application Laid-Open No. 9-3550 discloses a method of precipitating cementite with MnS as a nucleus by combining slab thermal history, high temperature annealing, and low temperature overaging treatment at 400 ° C. or lower. However, with this method, although the aging resistance is improved, since a large amount of fine cementite is dispersed in the grains, a decrease in EL is inevitable.
[0007]
[Problems to be solved by the invention]
Thus, various methods for improving the aging resistance of B-added low carbon steel have been proposed in the past, but both aging resistance and ductility can be achieved in steel in which B is added in an amount equal to or more than N. Such technology has not been developed yet.
Accordingly, an object of the present invention is to provide a production method capable of obtaining a cold-rolled steel sheet having excellent aging resistance while maintaining ductility with B-added low carbon steel in which B is added in an amount equal to or more than N. It is in.
[0008]
[Means for Solving the Problems]
In the B-added steel in which B is excessively added to N, solute B is generated and segregates preferentially at the grain boundaries, so that cementite cannot precipitate at the grain boundaries during the overaging treatment after continuous annealing. As a result, BN in the grains is precipitated in the nucleus. However, since there are a lot of BN in the grains, the cementite is finely dispersed in the grains in the normal overaging treatment conditions or in the low temperature overaging treatment in consideration of the aging resistance shown in the prior literature, and the EL It will decline. Therefore, the present inventors changed the way of thinking and intensively studied the possibility of overaging treatment at high temperature and the effect thereof. As a result, it has been found that by performing an overaging treatment at a high temperature contrary to the prior art, the aging resistance can be effectively improved without coarsening the intragranular cementite and lowering the EL, thereby forming the present invention. It came.
[0009]
Hereinafter, the experimental results obtained from the above findings will be described.
C≈0.02 mass%, Si≈0.01 mass%, Mn≈0.2 mass%, S≈0.01 mass%, P≈0.01 mass%, Al≈0.04 mass%, N≈0.002 mass%, B After hot rolling a steel containing about 1.3 at B / N, it was wound at 640 ° C. The hot-rolled sheet was pickled, cold-rolled at a rolling rate of 70%, then continuously annealed at 720 ° C., then cooled to an overaging temperature at a cooling rate of 120 ° C./second, and an overaging treatment was performed. In this overaging treatment, the treatment temperature was changed in the range of 400 to 500 ° C., and the treatment time was changed in the range of 3 to 7 minutes. After this overaging treatment, temper rolling with an elongation of 1% was performed. The final thickness of the cold rolled steel sheet was 1 mm.
[0010]
In order to evaluate the aging resistance by the aging index, a JIS No. 5 test piece was prepared from the obtained steel sheet, and the aging index (held at 100 ° C. for 1 hour after 8% tension) was measured. The result is shown in FIG. In FIG. 1, “x” indicates that the aging index exceeds 40 MPa, and “◯” indicates that the aging index ranges from 20 MPa to 40 MPa, and each aging index is shown in the figure. According to this, in the overaging treatment at 400 ° C. for about 4 minutes, which is a normal condition, the aging index is as high as about 50 MPa, whereas the aging index is higher as the overaging temperature is higher and the treatment time is longer. It turns out that becomes low. Furthermore, there is a relationship between the overaging treatment temperature and the overaging treatment time, and the overaging treatment temperature is equal to or higher than the temperature T (° C) calculated from the overaging treatment time t (minutes) based on the following equation (1). In addition, it can be seen that the aging resistance is good.
T (° C.) = 500− (t−3) × 30 (1)
Where t: overage treatment time (minutes)
[0011]
The reason why such a result can be obtained is considered as follows. That is, even when the overaging temperature is high, solid solution B exists at the grain boundary, so cementite precipitates in the grains, but when the overaging temperature is high, fine cementite does not form and the cementite becomes coarse. To do. Since this coarse cementite does not affect EL, the ductility does not decrease even if coarse cementite is generated in the grains. On the other hand, since solid solution B exists in the grain boundary, the solid solution C remaining in the grains in a non-equilibrium state keeps a high degree of supersaturation which is the driving force for precipitation of cementite. It is thought to decrease.
Further, as a result of further studies, it has been found that the above-described effects can be further enhanced by once cooling to a temperature 30 ° C. or more lower than the overaging temperature before the overaging treatment.
[0012]
The present invention has been made based on such findings, and the features thereof are as follows.
[1] Substantially, C: 0.01 to 0.05 mass%, Si: 0.1 mass% or less, Mn: 0.5 mass% or less, S: 0.03 mass% or less, P: 0.03 mass% or less, Al: 0.04 mass% or less, N: 0.004 mass% or less, B containing 1 or more in atomic ratio with respect to N, and the remainder of the steel composed of Fe is wound at 660 ° C. or less after hot rolling. After pickling and pickling, it is cold-rolled at a rolling rate of 50% or higher, then continuously annealed at a temperature of 700 ° C. or higher and 800 ° C. or lower, and cooled at a cooling rate of 100 ° C./second or higher after the annealing. The overaging treatment is started at a temperature of T ° C. or more and 530 ° C. or less represented by the formula (1), and during the overaging treatment, it is gradually cooled at a cooling rate of 10 ° C./min to 30 ° C./min and overaging treatment A method for producing a soft cold-rolled steel sheet having excellent aging resistance, characterized in that the time is 3 minutes or more and 7 minutes or less.
T (° C.) = 500− (t−3) × 30 (1)
Where t: overage treatment time (minutes)
[0013]
[2] Substantially, C: 0.01 to 0.05 mass%, Si: 0.1 mass% or less, Mn: 0.5 mass% or less, S: 0.03 mass% or less, P: 0.03 mass% or less, Al: 0.04 mass% or less, N: 0.004 mass% or less, B containing 1 or more in atomic ratio with respect to N, and the remainder of the steel composed of Fe is wound at 660 ° C. or less after hot rolling. After pickling and pickling, it is cold-rolled at a rolling rate of 50% or higher, then continuously annealed at a temperature of 700 ° C. or higher and 800 ° C. or lower. After the annealing, at a cooling rate of 100 ° C./second or higher, the following (1) After cooling to T-30 ° C. or less with respect to the temperature T (° C.) shown in the formula, overaging treatment is started at a temperature of T ° C. or more and 530 ° C. or less. During the overaging treatment, 10 ° C./min or more and 30 Slow cooling at a cooling rate of ℃ / min or less, and over-aging treatment time of 3 minutes to 7 minutes Excellent production method for a soft cold-rolled steel sheet sexual.
T (° C.) = 500− (t−3) × 30 (1)
Where t: overage treatment time (minutes)
[0014]
DETAILED DESCRIPTION OF THE INVENTION
The details of the present invention and the reasons for limitation will be described below.
First, the reason for limiting the component composition of steel will be described.
C: If C is excessive, a large amount of carbide precipitates and the steel is hardened, so the content was set to 0.05 mass% or less. Moreover, in order to further improve aging resistance, 0.03 mass% or less is desirable. Moreover, when C is less than 0.01 mass%, C does not precipitate in the overaging treatment, so the lower limit was made 0.01 mass%.
[0015]
Si: When Si is added excessively, the strength is increased and the moldability is deteriorated, so the content is set to 0.1 mass% or less (including the case of 0 mass%).
P: P is a solid solution strengthening element, and excessive addition causes hardening of the steel, so it was set to 0.03 mass% or less (including the case of 0 mass%).
Mn: Mn fixes S in the form of MnS and has the function of improving hot ductility. Therefore, it is desirable to add 0.05 mass% or more. However, excessive addition leads to hardening of the steel and formability. In order to deteriorate the amount, it was set to 0.5 mass% or less (including the case of 0 mass%).
[0016]
S: Since S is an element that inhibits hot ductility and formability, it is fixed as MnS. For this reason, it is desirable that the S is low, and when the amount of MnS increases, the stretch flangeability deteriorates.
Al: Since Al is used as a deoxidizer, it is generally contained to some extent in steel. However, when Al is added in excess of 0.04 mass%, fine AlN precipitates and hardens, so it was set to 0.04 mass% or less (including the case of 0 mass%).
[0017]
N: N is completely fixed as BN by B added in an equivalent amount or more. However, if a large amount of nitride is present in the steel, the ductility is lowered and a large amount of B needs to be added, and rolling cannot be stably performed. Therefore, 0.004 mass% or less (including the case of 0 mass%) is included. ).
B: B is added to fix N, but is added so that B / N is 1 or more by atomic ratio in order to completely fix N. However, if the amount of B is large, the rolling load during hot rolling increases and rolling cannot be performed stably, so the upper limit is preferably set to 0.004 mass%.
The composition of the steel is substantially composed of the above components and Fe, and therefore, it is not prevented that other elements such as inevitable impurities are contained in a trace amount as long as the effects of the present invention are not impaired.
[0018]
Next, manufacturing conditions will be described.
In the present invention, the steel having the above component composition is hot-rolled, wound up at 660 ° C. or lower, pickled, cold-rolled at a rolling rate of 50% or higher, and then continuously at a temperature of 700 to 800 ° C. After annealing and cooling at a cooling rate of 100 ° C./second or more after annealing, overaging treatment was started at a temperature of T ° C. or more and 530 ° C. or less represented by the following formula (1). During this overaging treatment, 10 ° C. / While gradually cooling at a cooling rate of at least 30 ° C./minute, the overaging treatment time is at least 3 minutes and at most 7 minutes. Preferably, after annealing, after cooling once to T-30 ° C. or lower with respect to the temperature T (° C.) expressed by the following equation (1) at a cooling rate of 100 ° C./second or higher, the temperature is T ° C. or higher and 530 ° C. or lower. Start overaging at temperature.
T (° C.) = 500− (t−3) × 30 (1)
Where t: overage treatment time (minutes)
[0019]
Winding temperature: When the winding temperature exceeds 660 ° C., cementite precipitates coarsely in the hot-rolled sheet, so that re-solution of cementite does not proceed during continuous annealing, and as a result, the amount of dissolved C during annealing is equalized. To reduce. This decrease in the amount of solute C reduces the driving force for precipitation of cementite during the overaging treatment, resulting in an increase in the amount of solute C in the annealed sheet at room temperature, and deterioration of aging resistance and ductility. For this reason, the upper limit of coiling temperature was 660 degreeC.
Cold rolling rate: When the cold rolling rate is less than 50%, the carbide in the hot-rolled sheet cannot be pulverized by rolling, and the amount of cementite that does not re-dissolve during annealing is increased. Aging and ductility are reduced. For this reason, the lower limit of the rolling rate was set to 50%.
[0020]
Annealing temperature of continuous annealing: The lower limit of the annealing temperature was set to 700 ° C. in order to complete the recrystallization and allow sufficient grain growth. Further, when the annealing temperature exceeds 800 ° C., the annealing structure becomes a two-phase, and during cooling, the ferrite grain boundary bulges and the γ → α transformation occurs, so that the resulting annealed plate structure becomes mixed and the ductility decreases. For this reason, the upper limit of the annealing temperature was set to 800 ° C.
Cooling rate after continuous annealing: In order to reduce solid solution C effectively during over-aging treatment by sufficiently re-dissolving cementite during annealing and increasing the degree of supersaturation, C re-dissolved during annealing. It is necessary to start the overaging treatment before starting to precipitate. In the present invention, the supersaturation degree during the overaging treatment must be maintained by setting the cooling rate after annealing to 100 ° C./second or more.
[0021]
Overaging treatment start temperature: The overaging treatment start temperature is most important in the present invention. The precipitation of C is promoted by setting the overaging start temperature to T ° C or higher given by the following formula (1). When there is no solid solution B and cementite precipitates at the grain boundary, the diffusion rate of the grain boundary is several orders of magnitude faster than the inside of the grain. The discharge of the solid solution C to the grain boundary is promoted, and the amount of the solid solution C is effectively reduced. However, as in the present invention, cementite in which precipitation at the grain boundary is inhibited by solute B starts to precipitate with BN in the grain as a nucleus. In this case, unless the overaging treatment temperature is sufficiently high, C cannot sufficiently diffuse in the grains, and a large amount of solid solution C remains in the annealed plate, resulting in deterioration of aging resistance and ductility. Since this overaging temperature promotes intragranular diffusion of C, it can be traded off with the overaging treatment time, so the lower limit of the overaging treatment temperature is determined by the overaging treatment time. In the present invention, the lower limit is defined by the following equation (1). However, when the overaging temperature is higher than 530 ° C., sufficient driving force for precipitation of cementite cannot be obtained, and the overaging temperature is increased, so that solid solution C remains. Therefore, the upper limit of the overaging treatment start temperature was set to 530 ° C.
T (° C.) = 500− (t−3) × 30 (1)
t: Overaging treatment time (minutes)
[0022]
Cooling rate during overaging treatment: The cooling rate during overaging treatment is also important in the present invention. As the overaging progresses, the driving force for precipitation of cementite decreases due to a decrease in the amount of dissolved C. For this reason, in order to effectively reduce the solute C, it is necessary to lower the overaging temperature as aging progresses. In the present invention, the lower limit of the cooling rate is 10 ° C./min. Further, when the cooling rate is high, cementite is finely precipitated, so that the aging resistance is improved, but the ductility is lowered. Therefore, in the present invention, the upper limit of the cooling rate during the overaging treatment is set to 30 ° C./min.
[0023]
Overaging treatment time: If the overaging treatment time is less than 3 minutes, cementite cannot be completely precipitated and the aging resistance is not improved, so the lower limit was set to 3 minutes. In addition, when the treatment time exceeds 7 minutes, the line speed becomes slow, and the upper limit is set to 7 minutes because the strip may be necked or broken in the soaking zone maintained at a high temperature.
Rapid quenching stop temperature after annealing and before overaging treatment: The effect of the present invention is promoted by once cooling to T-30 ° C or lower before overaging treatment after annealing and then overaging at T ° C or higher and 530 ° C or lower. . However, cooling to less than 300 ° C. before the overaging treatment after annealing increases the tendency to refine cementite and lowers the ductility. Therefore, the cooling stop temperature is desirably 300 ° C. or higher.
[0024]
In the hot rolling according to the present invention, no problem arises even if rough rolling is continuously performed after rough rolling by joining the rough bars. Further, after rough rolling, there is no problem even if the rough bar is heated for the purpose of temperature adjustment or wound on a coil box. Furthermore, heating of the rough bar and continuous rolling may be combined.
When the slab is hot-rolled, the continuously cast slab may be rolled as it is, or heat retention or heating within 100 minutes may be performed for slab soaking without cooling to room temperature. Furthermore, hot rolling may be performed by omitting rough rolling using a thin slab, and in any case, the effect of the present invention does not change.
[0025]
There is no particular restriction on the conditions for temper rolling, but if the elongation rate exceeds 1.5%, the EL decreases significantly, so that the elongation rate is preferably 1.5% or less.
In addition, the steel used by this invention may use what adjusted the component in any of the converter and the electric furnace, and may use the scrap for the raw material. There is no particular limitation on the impurities mixed when scrap is used.
Further, even if the steel sheet obtained by the present invention is subjected to chemical conversion treatment such as galvanization, tin plating, chromate treatment, zinc phosphate treatment, etc., the effect of the present invention is not affected at all.
[0026]
【Example】
[Example 1]
After melting and casting the steel having the composition shown in Table 1, hot rolling, pickling, cold rolling, continuous annealing and overaging treatment were performed under the conditions shown in Table 2, and then the elongation was 1.0%. Temper rolling was performed. While measuring the aging index of the cold-rolled steel sheet thus obtained, TS and EL were measured by a tensile test (JIS No. 5 test piece), and the aging resistance and ductility of each cold-rolled steel sheet were evaluated. . The results are also shown in Table 2.
In addition, in the measurement of an aging index, it aged by hold | maintaining at 100 degreeC for 1 hour after 8% pre-strain addition. Moreover, the final plate thickness of each cold-rolled steel plate was adjusted by adjusting the hot-rolled plate thickness.
[0027]
In Tables 1 and 2, no. 1-No. No. 5 is an example in which the winding temperature is changed. 1-No. No. 4 (Example of the present invention) has a low aging index and good EL, but the winding temperature is high. In 5 (comparative example), the aging index is high.
No. 6-No. No. 10 is an example in which the annealing temperature was changed. In 6 (comparative example), the aging index is high and the EL is also low. In addition, No. having a high annealing temperature. At 10 (comparative example), the aging index is good, but the EL is remarkably low. In contrast, no. 7-No. 9 (Example of the present invention) is good in both aging index and EL.
[0028]
No. No. 11 (comparative example) has a high aging index because of its low cold rolling rate. No. No. 12 (comparative example) has a high aging index due to a short overaging treatment time, and accordingly, EL is also low. No. In No. 13 (Comparative Example), the overaging treatment time is long and the aging index is high because the cooling rate during the overaging treatment is small. No. 14 (Comparative Example) has a high aging index because the cooling rate after annealing is slow. No. No. 15 (example of the present invention) achieves good EL and aging index at the same time.
No. 16-No. No. 20 is an example in which the overaging treatment temperature was changed. 16 (Comparative Example) has a high aging index and a low EL. No. with high overaging temperature. 20 (comparative example) has a high aging index. In contrast, no. 17-No. 19 (example of the present invention) is good in both aging index and EL.
[0029]
[Table 1]

[0030]
[Table 2]

[0031]
[Example 2]
After melting and casting the steels having the components shown in Table 3, hot rolling (winding temperature 640 ° C.), pickling, cold rolling (cold rolling rate 80%), continuous annealing (conditions shown in Table 4) An annealing temperature of 750 ° C.) and after changing the cooling stop temperature in the rapid cooling after annealing and performing an aging treatment, temper rolling was performed at an elongation rate of 1.0%, and a cold rolled steel sheet having a thickness of 0.8 mm was obtained. Obtained. While measuring the aging index of the cold-rolled steel sheet thus obtained, TS and EL were measured by a tensile test (JIS No. 5 test piece), and the aging resistance and ductility of each cold-rolled steel sheet were evaluated. The results are shown in Table 4.
In addition, in the measurement of an aging index, it aged by hold | maintaining at 100 degreeC for 1 hour after 8% pre-strain addition.
[0032]
No. 1-No. In the present invention example 2, rapid cooling after annealing is stopped in the vicinity of the overaging treatment start temperature. In contrast, no. 3-No. In the inventive example of No. 5, the overaging treatment was started after overcooling by 30 ° C. or more from the overaging treatment start temperature. 1-No. It is lower than 2, and it can be seen that the aging resistance is further improved by overcooling before the overaging treatment.
[0033]
[Table 3]

[0034]
[Table 4]

[0035]
【The invention's effect】
As described above, according to the method of the present invention, a cold-rolled steel sheet having both aging resistance and ductility can be produced from B-added low carbon steel in which B is added in an amount equal to or greater than N.
[Brief description of the drawings]
FIG. 1 is a graph showing the aging index of a cold-rolled steel sheet obtained from a B-added low carbon steel in which B is added in an amount equal to or greater than N in relation to the overaging temperature and the overaging time.

Claims (2)

Essentially, C: 0.01-0.05 mass%, Si: 0.1 mass% or less, Mn: 0.5 mass% or less, S: 0.03 mass% or less, P: 0.03 mass% or less, Al: 0 0.04 mass% or less, N: 0.004 mass% or less, B containing 1 or more in atomic ratio with respect to N, and the remainder of the steel composed of Fe is wound up at 660 ° C. or less after hot rolling, and acid After washing, it is cold-rolled at a rolling rate of 50% or higher, then continuously annealed at a temperature of 700 ° C. or higher and 800 ° C. or lower, and after cooling at a cooling rate of 100 ° C./second or higher, the following (1) The overaging treatment is started at a temperature of T ° C. or more and 530 ° C. or less represented by the formula, and during the overaging treatment, it is gradually cooled at a cooling rate of 10 ° C./min to 30 ° C./min, and the overaging treatment time is set to 3 A method for producing a soft cold-rolled steel sheet having excellent aging resistance, characterized by being from 7 minutes to 7 minutes.
T (° C.) = 500− (t−3) × 30 (1)
Where t: overage treatment time (minutes) Essentially, C: 0.01-0.05 mass%, Si: 0.1 mass% or less, Mn: 0.5 mass% or less, S: 0.03 mass% or less, P: 0.03 mass% or less, Al: 0 0.04 mass% or less, N: 0.004 mass% or less, B containing 1 or more in atomic ratio with respect to N, and the remainder of the steel composed of Fe is wound up at 660 ° C. or less after hot rolling, and acid After washing, it is cold-rolled at a rolling rate of 50% or higher, and then continuously annealed at a temperature of 700 ° C. or higher and 800 ° C. or lower. After the annealing, it is expressed by the following formula (1) at a cooling rate of 100 ° C./second or higher. After cooling to T-30 ° C. or less with respect to temperature T (° C.), overaging treatment is started at a temperature of T ° C. or more and 530 ° C. or less. During the overaging treatment, 10 ° C./min to 30 ° C./min Slow cooling at the following cooling rate and overaging treatment time of 3 minutes to 7 minutes A method for producing a soft cold-rolled steel sheet with excellent resistance.
T (° C.) = 500− (t−3) × 30 (1)
Where t: overage treatment time (minutes)

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