JP3110624B2 - Method for producing high strength cold rolled steel sheet for deep drawing with excellent dent resistance and no stretcher strain - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high-strength cold-rolled steel sheet for deep drawing having excellent dent resistance.

[0002]

2. Description of the Related Art Conventionally, high-strength cold-rolled steel sheets having excellent dent resistance have been mainly used for outer panels of automobiles. As the prior art, a method for producing a high-strength cold-rolled steel sheet having excellent dent resistance has been disclosed in JP-A-60-17485.
No. 2, JP-A-57-76131, JP-B-60-170
JP-A-58-84928 discloses a method for imparting secondary workability by adding B to JP-A-58-84928. In other words, JP 60-174852 discloses is a method of making a continuous annealing temperature Ac ₃ above 1000 ° C. or less and a composite structure, JP 57-76131 discloses includes a continuous annealing temperature 900 ° C. to Ac ₃ Are characterized by the extremely high annealing temperature of continuous annealing.
In ordinary continuous annealing equipment, when a steel strip having a high temperature exceeding 900 ° C. passes through a hearth roll in a furnace, the steel sheet is too soft and squeezed to cause wrinkles, which makes it impossible to produce a sheet having a good shape. is there.

[0003] Japanese Patent Publication No. Sho 60-17004 discloses that
In cooling after soaking during continuous annealing, at least 650
Temperature range of 10 ° C / s or more (preferably 50 ° C /
s or more) to produce a cold-rolled steel sheet for deep drawing with excellent bake hardening properties, for example, 50 ° C. /
If the cooling after soaking is performed by rapid cooling such as s, it is difficult to make the temperature of the steel strip coincide with the hearth roll temperature when the steel strip passes through the hearth roll because the cooling rate is extremely high. In addition, since the steel strip is rapidly heated or cooled by the hearth roll, the shape of the steel sheet is deteriorated, and there is a problem that it is difficult to stably produce a steel sheet having a good shape.

Further, Japanese Patent Application Laid-Open No. 58-84928 discloses a method for improving the secondary workability and preventing the adverse effect of N.
In this method, B is added in a weight ratio of 0.8 to 1.4, and as shown in Table 2 after correction in the publication, the r value (hereinafter referred to as an index of the deep drawing characteristic here, r) In some examples, the in-plane average value of the values was slightly higher at 2.03, but 1.6.
Includes an example in which only a very low deep drawability, such as 3,1.65, is obtained, and stably manufactures a high-strength steel sheet for a fender of a recent automobile that requires a more severe deep drawability. There is a problem that it is difficult to do.

As described above, a high-strength cold-rolled steel sheet for deep drawing with excellent dent resistance, such as a recent automobile fender, which can withstand more severe deep drawing work, has a sheet shape defect by a continuous annealing method. There is not yet a manufacturing method that can be manufactured without trouble. Therefore, it is strongly desired to provide a method for producing a high-strength cold-rolled steel sheet for deep drawing with excellent dent resistance to solve this problem.

[0006]

The problem to be solved by the present invention is to provide a high-strength cold drawing for deep drawing which does not generate a stretcher strain excellent in dent resistance that can withstand more severe deep drawing processing like a fender. An object of the present invention is to provide a method for manufacturing a rolled steel sheet.

[0007]

Means for Solving the Problems In order to solve the above-mentioned problems, the present inventors have studied (1) the manufacturing conditions under which the steel sheet can be manufactured by the continuous annealing method without any trouble of defective shape of the steel sheet. The present inventors have studied a method for producing a high-strength cold-rolled steel sheet for deep drawing with excellent dent resistance that can withstand more severe deep drawing, and have found the method described in claim 1. Furthermore, for some applications, a production method capable of imparting secondary workability without deteriorating the characteristic value of the steel sheet was studied, and a method according to claim 2 was found.

The gist of the present invention is as follows. (1) C: 0.0010 to 0.0030 wt%, Si ≦
0.50 wt%, Mn: 0.02 to 0.23 wt%,
P: 0.035 to 0.150 wt%, S ≦ 0.030 w
t%, sol. Al: 0.025 to 0.100 wt%,
N: 0.0005 to 0.0100 wt%, Nb: 0.0
07-0.020 wt%, and the relationship between the C amount and the Nb amount satisfies the range of the following formula, and the cast slab composed of the inevitable impurities and iron is rolled by finishing rolling with Ar ₃ or more, and the rolled hot rolled steel strip. Cold rolling is performed, and 750 to 8
After heating to 90 ° C. for recrystallization annealing, and then cooling, at least to 730 ° C., at a cooling rate of 2 ° C./s or more and less than 10 ° C./s, to produce an annealed plate,
Thereafter, a method for producing a high-strength cold-rolled steel sheet for deep drawing, which does not generate stretcher strain and has excellent dent resistance, characterized by performing a temper rolling of 0.5 to 3%. Nb: 7.75 × wt% C−0.013 ≦ Nb (wt%) ≦ 7.75 ×
wt% C + 0.007 (2) In the method described in (1), the steel composition
B (wt%) is 0.004 × wt% P or more, 0.001
The present invention is also directed to a manufacturing method for imparting excellent secondary workability, characterized in that the content is 0 wt% or less and B / N ≦ 0.7 (wt% ratio).

Hereinafter, the present invention will be described in detail. The present inventors have solved the problem to be solved by the present invention,
(1) Manufacturing conditions to obtain a steel strip having a good shape stably with ordinary continuous annealing equipment; (2) Relationship between dent properties of outer panel made of extremely low carbon cold rolled steel sheet and steel sheet properties; (3) The present inventors have studied a method for producing a cold-rolled steel sheet having material properties that can provide good dent properties, and have found the method of the present invention. The concept is as outlined below. (1) Manufacturing conditions for obtaining a steel strip having a good shape stably with ordinary continuous annealing equipment A cold rolled steel sheet for an automobile outer panel has a thickness of 0.60 to 0.80.
mm and a sheet width of about 1400 to 1700 mm are mainly used. However, the most hindrance in producing these cold-rolled steel sheets by continuous annealing equipment is when the heating temperature is high or in a high temperature range. When rapid cooling is performed, the plate is narrowed due to the compressive stress in the width direction due to the roll crown (convex) of the hearth roll portion, and the plate shape is defective due to the thermal stress generated due to the mismatch between the plate temperature and the roll temperature. The present inventors have dealt with this matter,
As a result of various studies, in order to stably obtain a steel strip having a good shape that can be applied to an automobile outer panel, at least the annealing temperature is set to 890 ° C or lower, and the cooling rate in the temperature range of 730 ° C or higher is set to at least. It was found that the temperature had to be lower than 20 ° C./s.

(2) Relationship between Dent Property of Outer Panel Made of Ultra-Low Carbon Cold Rolled Steel Sheet and Steel Sheet Properties On the other hand, the present inventors have studied the dent property of an outer panel made of extremely low carbon cold rolled steel sheet. As a result of various studies on the relationship between the properties of the steel sheets, the outer panel made of a completely non-ageable ultra-low carbon cold-rolled steel sheet was subjected to the box annealing method (B
AF method), the dent property is remarkably inferior to that of a completely non-aged low-carbon Al-killed cold-rolled steel sheet. However, even a very low-carbon cold-rolled steel sheet has solid solution C, and is about 1 kgf / mm.
It has been found that a cold-rolled steel sheet provided with a BH property of ² or more has significantly improved dent resistance. That is, in the ultra-low carbon cold rolled steel sheet, unlike the case of the low carbon Al-K steel manufactured by the BAF method, the dent property of the completely non-aging steel sheet is Y. P (0.2
% Proof stress), but the BH amount is 1 kg.
If f / mm ² or more, Y. It has been found that good dent resistance corresponding to P can be ensured, and it is not necessarily 3 kgf / mm as a cold rolled steel sheet for an automobile outer panel.
That may do it by applying a 1 kgf / mm ² or more BH amount is not necessary to impart ^two or more BH revealed. The dent resistance of the ultra-low carbon cold-rolled steel sheet is 0, and the BH amount is 0.
Although it is not necessarily clear that such a large difference between 1 kgf / mm ² and 1 kgf / mm ² , the elastic limit when the BH amount is 0 is Y. It is presumed that it is caused by only about 50 to 60% of P.

(3) A method for producing a cold-rolled steel sheet having a material property capable of obtaining good dent properties without generation of stretch strain, and a high-strength deep-drawing cold-rolling sheet having excellent dent resistance to withstand more severe deep drawing. An important issue in the method for manufacturing a steel sheet is to “obtain an excellent r-value that can withstand stricter deep drawing” and “stable 1 kgf / m”.
(a) obtaining a BH amount of not less than m ^2, that no stretcher strain is generated at all ”, and“ (1) “production conditions for stably obtaining a steel strip having a good shape with ordinary continuous annealing equipment”, ie, at least: That is, it can be manufactured under the condition that the annealing temperature is 890 ° C. or less and the cooling rate in the temperature range of 730 ° C. or more is at least less than 20 ° C./s.

[0012] The present inventors have firstly described "manufacturing conditions for obtaining a steel strip having a good shape stably with ordinary continuous annealing equipment".
That is, at least the annealing temperature is 890 ° C. or lower, and the cooling rate in the temperature range of 730 ° C. or higher is at least 20 ° C.
Under conditions of less than ° C./s, a manufacturing method that can obtain an excellent r-value that can withstand more severe deep drawing was studied. The inventors of the present invention smelted steels of various components using a laboratory vacuum melting furnace, and performed hot rolling, cold rolling, continuous annealing, and temper rolling. As a result, BH properties were obtained and r It has been found that a cold-rolled steel sheet having a high value must be obtained from a steel having the composition shown in claim 1. Among them, the C content and the Nb content greatly affect the r value.

When the C content exceeds 0.0030 wt%, the Nb content is less than 0.007 wt%,
If the content exceeds 0.020 wt%, the r value decreases, and sufficient deep drawability cannot be obtained. In particular, the C content is 0.003.
It has been found that when the content exceeds 0 wt%, the deterioration of the r value is large. Next, the present inventors have studied a method for achieving a stable BH amount of 1 kgf / mm ² or more and no stretcher strain at all within the above-mentioned ranges of C and Nb contents. . In order for this problem to be substantially solved in a steel mill, as a precondition, a manufacturing method must be able to tolerate a variation range of the C content when manufacturing a slab. That is, 1 kgf /
In order to achieve a BH amount of at least ² mm2 and no stretcher strain at all, the C content, which has the greatest effect on these properties, can be achieved with a slab within the control range of the current steelmaking components. This is because an industrially meaningless invention is required unless the production method is appropriate. Therefore, assuming C: ± 10 ppm, which is a range of variation of the C content when producing a cast piece of ultra-low carbon steel in the current steelmaking,
And, the range of the C and Nb contents for securing the r value, that is, C: 0.0010 to 0.0030 wt%, Nb:
The study was conducted on the premise that the steel had a composition of 0.007 to 0.020 wt%.

FIG. 1 shows that the point of the present invention is to achieve both the stable BH amount of 1 kgf / mm ² or more and the absence of stretcher strain at all. C: 0.0030wt
%, Nb: A cold-rolled steel sheet of steel of 0.011 wt% (the component in which YP-El is most likely to be generated in the component range within the range of the present invention) is recrystallized and annealed at 850 ° C. for 60 sec. It is cooled to 650 ° C. at a cooling rate shown as the X axis, and then cooled to room temperature at 30 ° C./s to produce an annealed plate.
0% temper rolling, cold rolled steel sheet, 100 ℃ ×
This is a plot obtained by performing a full aging treatment of 4Hr, performing a tensile test with JIS- # 5 pieces, measuring YP-El, and plotting. (B) shows C: 0.0010 wt%, Nb:
0.014 wt% steel (with a component range within the scope of the present invention,
The BH amount of the cold-rolled steel sheet of the component (the component with the least BH amount) is given (2% prestrain is given, after unloading, aging treatment at 170 ° C. × 20 min)
ΔY when re-tension was performed. P) is plotted.

As is clear from FIGS. 1 (a) and 1 (b), according to the method of the present invention, the range of variation of the C content at the time of producing a slab of ultra-low carbon steel with the current steelmaking is within a range. Some C:
The range of the C and Nb contents for assuming ± 10 ppm and securing the r value, that is, C: 0.0010 to 0.
0030 wt%, Nb: 0.007 to 0.020 wt%
It is clear that the use of a slab premised on steel having the following composition can achieve both a stable BH amount of 1 kgf / mm ² or more and no stretch strain at all. The economic and economic value is great.
In the prior art, the cooling rate after soaking in continuous annealing was set at 10
In the method disclosed in Japanese Patent Publication No. Sho 60-17004, in which the BH property is imparted by increasing the temperature to 650 ° C. at 3 ° C./s (preferably 50 ° C./s). BH (substantially the same annealing conditions as in FIG. 1 of the present invention)
The amount is almost 0 (the BH amount in the publication is represented by WH + BH, so that 3.5 to 4 kgf / mm ² must be subtracted to convert it into a value normally expressed as a BH amount, which is almost 0) This is a result different from the result of FIG. 1 of the present invention. This is because the C content is 0.005wt
%, Nb = 0.03 wt% and Nb content is 0.03 wt%
It is considered that the fact that the percentage and the range significantly exceed the scope of the present invention may have an effect.

Hereinafter, the chemical composition conditions of steel will be described in detail. C is an important element for securing excellent deep drawability, non-stretch strainability and dent resistance.
If the content exceeds 030 wt%, the r value decreases and it becomes impossible to apply to applications that require stricter deep drawability, so it is necessary to regulate the content to 0.0030 wt% or less. Also, if the content is less than 0.0010 wt%, 1 kgf
/ Mm ² or more, the BH amount cannot be secured.
It must be regulated to 010 wt% or more.

Si, Mn, and P are all elements that increase the strength of the steel sheet, and may be added as needed to increase the strength. However, when the Si content exceeds 0.50 wt%, the coating adhesion is increased. Is deteriorated, so that when Mn exceeds 0.23 wt% , the r value deteriorates, and P becomes 0.15 wt%.
If the content exceeds 0 wt%, the segregation of P in the slab becomes large, and a surface defect called a P band defect occurs, which makes it unusable as an automobile outer plate.
%, 0.80 wt%, and 0.150 wt%. It should be noted that Si does not cause any hindrance at least, so there is no particular need to regulate it. However, Mn is at least 0.02 wt% in order to prevent hot rolling, and P is the strength as a high-strength steel sheet. Since 0.035 wt% is necessary to obtain, the lower limits are 0.02 wt% and 0.035 wt%, respectively.
It is necessary to regulate to 0.035 wt%.

If the content of S exceeds 0.030 wt%, the reversibility of hot rolling deteriorates, so it is necessary to regulate S to 0.030 wt%. It should be noted that the lower limit value does not need to be particularly restricted because it does not cause any trouble even if it is small. sol.
Since it is necessary to fix N as AlN to prevent the occurrence of stretch strain, Al is 0.025 wt%.
The above is necessary. On the other hand, if the content exceeds 0.100 wt%, air oxidation of molten steel tends to occur at the time of casting, the amount of inclusions increases, and the workability and plating quality also deteriorate, so 0.100 wt% was made the upper limit. N is fixed as AlN by Al and detoxified, but 0.01%.
If it exceeds 00 wt%, AlN becomes too large and the r-value deteriorates. Therefore, the upper limit is set to 0.0100 wt%. In addition, it is difficult to reduce the content to less than 0.0005 wt% in ordinary steelmaking improvement, so the lower limit is 0.0005 wt%.
And

If the Nb content is less than 0.007 wt% or more than 0.020 wt%, an excellent r value cannot be obtained, so that 0.007 wt% to 0.0200 w
It is necessary to regulate to t%. If Nb is less than 0.007 wt%, it is not enough to reduce the crystal grain size of the hot-rolled sheet, and C cannot be sufficiently fixed as NbC. As a result, the r-value of the annealed sheet is not sufficient. It will not be higher. On the other hand, if the content exceeds 0.020 wt%, the amount of solute Nb increases, and the growth of crystal grains during annealing is suppressed, so that the r value does not become sufficiently high. Further, the Nb amount is set to Nb (wt%): 7.7 in order to secure the non-stretch strain property.
It is necessary to add 5 × wt% C−0.013 or more. Further, in order to stably secure the BH amount of 1 kgf / mm ² or more, Nb (wt%): 7.75 × wt% C + 0.007
It is necessary to:

The B content need not be added particularly when used in applications where secondary processing resistance is not required. However, when used in applications where secondary processing resistance is required, B (wt.
%): 0.004 × wt% P or more, 0.0010 wt%
It may be added below and in the range of B / N ≦ 0.7 (wt% ratio). It is well known that secondary workability can be improved by adding a small amount of B to a Ti-added ultra-low carbon steel, but it is also well known that the r-value is also reduced at the same time.
The present inventors have studied various methods for achieving both secondary workability and deep drawability, and have found a manufacturing method according to claim (2) of the present invention. When the B content exceeds 0.0010 wt% or the B / N exceeds 0.7 (wt% ratio), BN is generated, and as a result, the r value decreases. However, addition within the range of the present invention hardly causes deterioration of the r value. On the other hand, the secondary workability is B (wt
%): Notably improved by adding 0.004 × wt% P or more.

Hereinafter, the manufacturing conditions other than the constituent conditions of the steel sheet will be described in detail. The production conditions of the cast slab may be any method as long as the components of the steel of each claim can be obtained, and need not be particularly limited. There is no particular restriction on the hot rolling conditions, and the cast slab may be subjected to finish rolling with Ar ₃ or more to form a rolled hot rolled steel strip. When the N content is large or when the heating temperature is high, a better r-value can be obtained by using a winding temperature of 680 ° C. or higher, and may be adopted as needed. The cold rolling does not need to be particularly restricted, and may be a usual method. The continuous annealing method is 750 to 89
After heating to 0 ° C. to perform recrystallization annealing, and then cooling, it is necessary to cool at least to 730 ° C. at a cooling rate of 2 ° C./s or more and less than 10 ° C./s to produce an annealed plate. . If the annealing temperature is lower than 750 ° C., a sufficient r value cannot be obtained, and if it exceeds 890 ° C., a steel sheet having a good shape cannot be obtained.

The cooling rate after annealing greatly affects the amount of BH and the occurrence of stretch strain, and as shown in FIG. 1, cooling must be performed at a cooling rate of 2 ° C./s or more and less than 10 ° C./s. Also, this effect is at least 730
It is obtained by cooling at a cooling rate of not less than 2 ° C./s and less than 10 ° C./s. Note that there is no particular need to regulate the overaging treatment. If the temper rolling is less than 0.5%, no matter how much an annealed sheet having excellent stretch-strain resistance is produced, it is not possible to prevent the occurrence of stretch-strain at the time of pressing, so 0.5% or more must be applied. . If it exceeds 3%, the ductility is greatly deteriorated, so it is necessary to regulate it to less than 3%.

[0023]

EXAMPLES The effects of the present invention will be described below with reference to examples. A slab of the components shown in Table 1 was produced, and a hot-rolled sheet of 3.8 mm was manufactured under the continuous hot-rolling conditions shown in Table 2, and 0.65 m after pickling.
m, cold-rolled to 850 ° C x 60sec by continuous annealing
And then cooled under the conditions shown in Table 2 after soaking, passed through an overage treatment zone at 400 ° C., and then cooled to room temperature to produce an annealed sheet. , To produce a 35-38 kgf / mm ² class high-strength cold-rolled steel sheet. The materials of the manufactured high-strength cold-rolled steel sheets were investigated and are shown in Table 2. Y. P, T. The S, El, and r values are characteristic values after temper rolling, YP-El is a value after aging treatment at 100 ° C. × 4 hours, and the BH amount is measured under a pre-strain amount of 2%.
These values are obtained when the heat treatment conditions equivalent to paint baking are 170 ° C. × 20 min. The secondary workability was 2.52.
The cup was squeezed at −40 ° C., and the length of the brittle fracture when the cup was crushed was evaluated.
The following items were evaluated as 〇 and more as ×.

[0024]

[Table 1]

[0025]

[Table 2]

The steels A, B, C and D are all steels within the composition range of the present invention, and the steels A, B, C and D according to the present invention do not contain B. The steel added with 6, 6, 9 ppm of not less than 3.3, 3.1, 4.4 ppm of the lower limit of the amount of B added according to claim 2 in which D steel has P of 0.1.
10 wt% added steel to produce 38 kgf / mm ² class high strength cold rolled steel sheet. Steel E has a C content of 0.1.
0040 wt% and Nb content of 0.040 wt% which are higher than the range of the present invention.
4 is an example of a steel component according to Japanese Patent Publication No. 4 (Kokai) No. 4; Steel F is a comparative example in which the Nb content was 0.003 wt%, which was below the range of the steel of the present invention. Steel G has a C content of 0.0040 wt%, B
This is an example in which the content is 0.0045 wt% and B is added in a larger amount than the range of the present invention, and is an example of a steel component according to the conventional method of Japanese Patent Application Laid-Open No. 58-84928. The steel H has a C content of 0.0011 wt% and an Nb content of 0.019 wt%, the upper limit of the Nb content (7.75 × w) in the formula (1).
It is an example of a steel component deviating from t% C (0.0011) = 0.0155%). Steel I has a B content of 0.0038 wt.
2 is an example of a steel component in which% and B are added in a larger amount than the range of the present invention.

Samples 1, 2, 3, and 5 are all examples of the present invention. Sample 4 has a cooling rate of 1 ° C. after soaking during continuous annealing.
/ S, which is lower than the range of the present invention, sample 7 is a comparative example in which the components are out of the present invention, and samples 6 and 8 have annealing conditions of the present invention but steel components are conventional components. It is. Samples 1, 2, 3, and 5 were all YP-El, BH after aging.
The amounts and r values are all excellent values, and it is understood that the method of the present invention can produce a high-strength cold-rolled steel sheet for deep drawing with excellent dent resistance and free of stretcher strain. In addition, all the obtained annealed sheets have a good shape, and the manufacturing method of the present invention in which the cooling rate is less than 10 ° C./s can stably obtain a steel sheet having a good shape. It can be seen that is also excellent.

The sample 2 according to the embodiment of claim 2
Nos. 3 and 5 also have excellent secondary workability, and the B (w) of the present invention
t%) is 0.004 x wt% P or more, 0.0010 wt%
% Or less and B / N ≦ 0.7 (wt% ratio), it can be seen that excellent secondary workability can also be imparted. On the other hand, in the steel 4 of the comparative example, the cooling condition of the continuous annealing was 1 ° C./s, which was lower than the range of the present invention, and the BH amount was as low as 0.5 kgf / mm ^2. In comparison with Sample 3 which is an example of the present invention,
It is clear that the manufacturing method of the present invention in which the cooling conditions for continuous annealing are 2 to 10 ° C./s is excellent. In addition, steel 6,7,
8, 9, and 10 are all comparative examples, and steel 6 has a C content of 0.00.
40 wt%, Nb content is 0.040 wt%, Steel 7 has Nb content of 0.003 wt%, and Steel 8 has B content of 0.004 wt%.
5 wt%, steel 9 has an Nb content of 0.019 wt% exceeding the upper limit of the expression according to the claim, and steel 10 has a B content of 0.00.
38 wt%, a comparative example out of the steel component range of the present invention,
Steel 6 has a BH content and r value, steels 7, 8, and 10 have an r value, and steel 9
Indicates that the BH amount is an inferior characteristic value.

As is clear from the results of the above embodiments,
According to the production method of claim 1 of the present invention, a high-strength cold-rolled steel sheet for deep drawing that does not generate a stretcher strain and has excellent dent resistance can also have characteristics with excellent secondary workability by the method of claim 2. It can be given, and it is understood that the industrial value is extremely high. The same effect can be obtained by the production method of the present invention as a method for producing an original sheet of various surface-treated steel sheets such as electrogalvanizing, and also in producing a hot-dip galvanized steel sheet.

[0030]

As described above, the present invention has been described in detail. According to the manufacturing method of the present invention, it is possible to manufacture a high-strength cold-rolled steel sheet for deep drawing with excellent dent resistance and free of stretcher strain. The effect can be demonstrated and its industrial value is great.

[Brief description of the drawings]

FIG. 1: Cooling rate of continuous annealing and YP-El, B after aging
It is a figure which shows the relationship with H amount.

────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Hirokazu Haga 1-Fuji-cho, Hirohata-ku, Himeji-shi, Hyogo Nippon Steel Corporation Hirohata Works (56) References JP-A-6-108153 (JP, A JP-A-63-241122 (JP, A) JP-A-6-116651 (JP, A) JP-A-2-232316 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C21D 9/46-9/48 C21D 8/02-8/04 C22C 38/00-38/60

Claims (2)

(57) [Claims] 1. C: 0.0010 to 0.0030 wt.
%, Si ≦ 0.50 wt%, Mn: 0.02 to 0.23
wt% , P: 0.035 to 0.150 wt% , S ≦ 0.
030 wt%, sol. Al: 0.025 to 0.100
wt%, N: 0.0005 to 0.0100 wt%, N
b: 0.007 to 0.020 wt%, C content and N
A slab having the relationship of the amount of b satisfying the range of the following formula and the balance consisting of unavoidable impurities and iron is subjected to finish rolling with Ar ₃ or more to form a rolled hot-rolled steel strip, cold-rolled, and 750 by a continuous annealing method. To 890 ° C to perform recrystallization annealing, and then
When performing cooling, at least up to 730 ° C.
A stretcher strain having excellent dent resistance, characterized in that it is cooled at a cooling rate of 2 ° C./s or more and less than 10 ° C./s to produce an annealed plate, and then to perform a temper rolling of 0.5 to 3%. A method for producing high-strength cold-rolled steel sheets for deep drawing that does not occur. Nb: 7.75 × wt% C−0.013 ≦ Nb (wt%) ≦ 7.75 ×
wt% C + 0.007 2. The method according to claim 1, wherein the composition of the steel and the content of B (wt%) are 0.004 × wt% P or more.
0.0010 wt% or less, and B / N ≦ 0.7 (w
(t% ratio) in a range that also provides excellent secondary workability.