JPH08104926A - Production of high strength cold rolled steel sheet for deep drawing excellent in dent resistance and free from generation of stretcher strain - Google Patents

Abstract

PURPOSE: To produce a high strength cold rolled steel sheet capable of producing without troubles such as deterioration in the shape of the steel sheet by a contin uous annealing method and capable of withstanding severer deep drawing such as fender. CONSTITUTION: A slab having a compsn. contg., by weight, 0.0010 to 0.0030% C, <=0.50% Si, 0.02 to 0.80% Mn, 0.035 to 0.150% P, <=0.030% S, 0.025 to 0.100% sol.Al, 0.0005 to 0.0100% N and 0.007 to 0.020% Nb, furthermore satisfying 7.75×C-0.013<=Nb<=7.75×C+0.007, and the balance iron with inevitable impurities is subjected to finish rolling at the Ar3 or above and is coiled to form into a hot rolled steel strip, which is subjected to cold rolling and is subjected to recrystallization annealing under heating to 750 to 890 deg.C by a continuous annealing method. Next, at the time of executing cooling, it is cooled at least to 730 deg.C at 2 to <10 deg.C/s cooling rate to make an annealed sheet, and after that, it is subjected to skin-pass rolling of 0.5 to 3%.

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 which is excellent in 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 panel of automobiles. As the conventional technique, there has been disclosed a method for producing a high strength cold rolled steel sheet having excellent dent resistance, which has been disclosed in JP-A-60-17485.
No. 2, JP-A-57-76131, JP-B-60-170.
No. 04, and JP-A No. 58-84928 discloses a method of adding B to impart secondary workability. That is, JP-A-60-174852 is a method for producing a composite structure with a continuous annealing temperature of Ac _{3 to} 1000 ° C., and JP-A-57-76131 discloses a continuous annealing temperature of 900 ° C. to Ac ₃ . It is a feature that the annealing temperature of continuous annealing is extremely high in both methods.
In a normal continuous annealing facility, when a steel strip having a high temperature exceeding 900 ° C. passes through a hearth roll in the furnace, the steel sheet is too soft and is narrowed down to cause wrinkles, which makes it impossible to produce a well-shaped sheet. is there.

Further, Japanese Patent Publication No. 60-17004 discloses
At least 650 in cooling after soaking during continuous annealing
Temperature range above ℃ 10 ℃ / s or more (preferably 50 ℃ /
It is a method of producing a cold-rolled steel sheet for deep drawing having excellent bake hardenability by cooling by rapid cooling of s or more), for example, 50 ° C. /
If the cooling is performed after soaking by rapid cooling such as s, the cooling rate is extremely high, and it becomes difficult to match the temperature of the steel strip and the hearth roll temperature when the steel strip passes through the hearth roll. However, since the steel strip is rapidly heated or cooled by the hearth roll, the shape of the steel sheet is deteriorated, and it is difficult to stably produce a steel sheet having a good shape.

Further, JP-A-58-84928 discloses that in order to improve the secondary workability and prevent the adverse effect of N,
This is a method of adding 0.8 to 1.4 by weight ratio of B, but as shown in the corrected Table 2 of the publication, the r value (hereinafter, r which is an index of the deep drawing characteristic is used. The average value of the values is 2.03, which is slightly higher than 2.03, but is 1.6.
It also includes examples in which a very low deep drawability such as 31.65 is obtained, and stably manufactures high-strength steel sheets for recent automobile fenders that require 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 having excellent dent resistance, which can withstand even more severe deep drawing work, such as a recent automobile fender, has been produced by the continuous annealing method in the form of a defective steel sheet shape. There is still no manufacturing method that can be manufactured without trouble. Therefore, it has been strongly desired to provide a method for manufacturing a high strength cold rolled steel sheet for deep drawing, which is excellent in dent resistance and solves this problem.

[0006]

SUMMARY OF THE INVENTION The problem to be solved by the present invention is to provide a high-strength cold drawing for deep drawing which does not cause stretcher strain with excellent dent resistance, which can withstand even deeper deep drawing like a fender. It is to provide a method for manufacturing a rolled steel sheet.

[0007]

Means for Solving the Problems Regarding the method for solving the above-mentioned problems, the inventors of the present invention have studied (1) manufacturing conditions by which continuous annealing can be carried out without troubles such as defective steel sheet shape, and Then, a method for producing a high-strength cold-rolled steel sheet for deep drawing, which has excellent dent resistance and can withstand even more severe deep drawing, was studied, and the method according to claim 1 was found. Furthermore, for some uses, a manufacturing method that can impart secondary workability without deteriorating the characteristic values of the steel sheet was examined, and the 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: 0.02-0.80 wt%,
P: 0.035 to 0.150 wt%, S ≦ 0.030w
t%, sol. Al: 0.025 to 0.100 wt%,
N: 0.0005 to 0.0100 wt%, Nb: 0.0
A cast slab containing 07 to 0.020 wt% and having a relationship between the amount of C and the amount of Nb satisfying the range of the following formula and comprising the balance unavoidable impurities and iron is subjected to finish rolling with Ar ₃ or more and rolled into a hot rolled steel strip. And cold rolling is performed, and it is 750-8 by the continuous annealing method.
Recrystallization annealing is performed by heating to 90 ° C., and then cooling is performed at least up to 730 ° C. at a cooling rate of 2 ° C./s or more and less than 10 ° C./s to form an annealed plate,
Then, 0.5-3% temper rolling is performed, and a method for producing a high-strength cold-rolled steel sheet for deep drawing, which is excellent in dent resistance and does not cause stretcher strain. Nb: 7.75 x wt% C-0.013 ≤ Nb (wt%) ≤ 7.75 x
wt% C + 0.007 (2) In the method described in (1), the composition of steel is further
B (wt%) 0.004 x wt% P or more, 0.001
It is a manufacturing method for imparting excellent secondary workability, which is characterized in that it is 0 wt% or less and B / N ≦ 0.7 (wt% ratio).

Hereinafter, the present invention will be described in detail. The present inventors, regarding the problems to be solved by the present invention,
(1) Manufacturing conditions for obtaining a steel strip having a stable and good shape by a normal continuous annealing equipment, (2) Relationship between dent property of outer panel made of ultra low carbon cold rolled steel sheet and steel sheet characteristics, (3) The method of the present invention was found by studying a method for producing a cold-rolled steel sheet having material characteristics capable of obtaining good dent property. The idea is as outlined below. (1) Manufacturing conditions for stably obtaining a good-shaped steel strip by a normal continuous annealing equipment Cold-rolled steel sheets for automobile outer plates have a sheet thickness of 0.60 to 0.80.
Although thin and wide materials with a width of 1 mm and a width of 1400 to 1700 mm are mainly used, the biggest obstacle in producing these cold-rolled steel sheets with continuous annealing equipment is when the heating temperature is high or in a high temperature range. When abrupt cooling is performed, the plate shape is narrowed due to 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 thermal stress caused by the mismatch between the plate temperature and the roll temperature. The present inventors are concerned 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 plate, at least an annealing temperature of 890 ° C. or lower and a cooling rate of at least 730 ° C. in a temperature range of at least It was found that it was necessary to set it to less than 20 ° C / s.

(2) Relationship between Dentness of Outer Panel Made of Ultra-Low Carbon Cold Rolled Steel Sheet and Steel Sheet Properties On the other hand, the present inventors As a result of various studies on the relationship between steel sheet properties, an outer panel made of a completely non-aging ultra-low carbon cold-rolled steel sheet was found to be a completely non-aging low carbon Al-killed cold box produced by the box annealing method (BAF method). Although the dent property is remarkably inferior to that of the rolled steel plate, the cold rolled steel plate with BH property of about 1 kgf / mm ² or more is remarkably dent resistant even if the ultra-low carbon cold rolled steel plate contains solid solution C. Have been found to improve. That is, in the ultra low carbon cold rolled steel sheet, unlike the case of the ultra low carbon Al-K steel manufactured by the BAF method, the dent property of the completely non-aging steel sheet is Y. P. (Powder strength 0.2%) is greatly reduced, but when the amount of BH is 1 kgf / mm ² or more, Y.O. It was found that good dent resistance corresponding to P can be secured, and it is not always necessary to provide BH of 3 kgf / mm ² or more as a cold-rolled steel sheet for automobile outer panels, and BH of 1 kgf / mm ² or more is required. It became clear that the amount should be given. Incidentally, it is not always clear that the dent resistance of the ultra low carbon cold-rolled steel sheet differs greatly between BH amounts of 0 and 1 kgf / mm ² , but it is not clear that the BH amount is 0. The elastic limit of Y. 50 of P
It is presumed that this is due to the fact that it is only about 60%.

(3) Method of manufacturing cold-rolled steel sheet having material characteristics capable of obtaining good dentability without occurrence of stretcher strain. High-strength cold-rolling for deep drawing excellent in dent resistance capable of withstanding more severe deep drawing. An important issue in the method of manufacturing a steel sheet is “to obtain an excellent r value that can withstand even deeper deep drawing”, and “stably 1 kgf / m.
"To obtain a BH amount of m ² or more, no stretcher strain is generated at all", "(Conditions for producing a steel strip having a stable and good shape by a normal 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 can be manufactured under the condition of at least less than 20 ° C./s.

The inventors of the present invention firstly referred to “the manufacturing conditions for stably obtaining a steel strip having a good shape with a normal 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.
Under the condition of less than ° C / s, a manufacturing method that can obtain an excellent r value that can withstand more severe deep drawing of was investigated. The inventors of the present invention conducted a study by performing melting of steels of various components using a laboratory vacuum melting furnace and performing hot rolling, cold rolling, continuous annealing and temper rolling, and as a result, BH property was also obtained and r It has been found that the cold rolled steel sheet having a high value must be obtained from the steel having the composition as set forth in claim 1. Among them, the C content and the Nb content have a great influence on the r value.

If the C content exceeds 0.0030 wt% or the Nb content is less than 0.007 wt%,
If it exceeds 0.020 wt%, the r value decreases and sufficient deep drawability cannot be obtained. In particular, the C content is 0.003
It was found that the deterioration of the r value when the content was more than 0 wt% was large. Next, the inventors of the present invention examined a method for achieving both a stable BH amount of 1 kgf / mm ² or more and no stretcher strain at all within the above C and Nb content ranges. . In order for this problem to be practically solved in a steel mill, as a precondition, it must be a manufacturing method that allows a variation range of the C content when manufacturing a cast slab. That is, 1 kgf /
In order to achieve both a BH amount of mm ² or more and no stretcher strain at all, the C content that has the greatest effect on these properties can be achieved with a slab within the current steelmaking composition control range. This is because unless the manufacturing method is a proper manufacturing method, the invention is industrially meaningless. Therefore, assuming that C: ± 10 ppm, which is the variation range of the C content when producing an extremely low carbon steel slab with the current steelmaking,
In addition, the range of 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 of a steel having a composition of 0.007 to 0.020 wt%.

FIG. 1 shows that the point of the present invention is that it is possible to achieve a stable BH amount of 1 kgf / mm ² or more and no stretcher strain at all, and (a) is C: 0.0030 wt
%, Nb: 0.011 wt% steel (in the composition range within the scope of the present invention, the component that is most prone to YP-El) cold-rolled steel sheet is recrystallized and annealed at 850 ° C. for 60 sec. Cool to 650 ° C. at the cooling rate shown as the X-axis, and then cool to room temperature at 30 ° C./s to make an annealed plate.
After subjecting to 0% temper rolling to make a cold rolled steel sheet, 100 ° C x
This is a plot obtained by performing a full aging treatment for 4 hours and performing a tensile test on a JIS- # 5 piece to measure YP-El. Further, (b) shows C: 0.0010 wt%, Nb:
0.014 wt% steel (within the composition range within the scope of the present invention,
BH amount (2% pre-strain is given to the cold rolled steel sheet with the most difficult component to obtain BH amount), after unloading, 170 ° C. × 20 min aging treatment,
ΔY when re-pulling was performed. P) is plotted.

As is apparent from FIGS. 1 (a) and 1 (b), according to the method of the present invention, in the range of variation in the C content when producing a cast slab of ultra-low carbon steel by the current steelmaking. C:
The range of C and Nb contents for ensuring the r value, that is, C: 0.0010 to 0.
0030 wt%, Nb: 0.007 to 0.020 wt%
It is clear that even when using a cast slab premised on the steel of component (1), it is possible to obtain both a stable BH amount of 1 kgf / mm ² or more, no stretcher strain at all, and the present invention. Has great economic and economic value.
In the prior art, the cooling rate after soaking in continuous annealing was 10
According to the method of Japanese Examined Patent Publication No. 60-17004, which imparts the BH property by setting the temperature to be not less than C / s (preferably 50 [deg.] C / s), in FIG. (Heat conditions similar to those of FIG. 1 of the present invention) BH
The amount is almost 0 (because the BH amount in the same publication is indicated by WH + BH, 3.5 to 4 kgf / mm ² must be subtracted in order to convert it into a value normally expressed as the BH amount, which is almost 0). The result is different from the result of FIG. 1 of the present invention. This has a C content of 0.005 wt.
%, Nb = 0.03 wt% and Nb content is 0.03 wt
%, And it is considered that the influence greatly exceeds the range of the present invention.

The chemical composition conditions of steel will be described in detail below. C is an important element for ensuring excellent deep drawability, non-stretcher strain resistance and dent resistance.
If it exceeds 030 wt%, the r value decreases and it cannot be applied to applications requiring more stringent deep drawability. Therefore, it is necessary to regulate the content to 0.0030 wt% or less. Also, when the amount is less than 0.0010 wt%, it is stable at 1 kgf.
Since it is not possible to secure a BH amount of / mm ² or more, 0.0
It is necessary to regulate it 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 necessary to increase the strength. However, if Si is more than 0.50 wt%, coating adhesion Is deteriorated, the r value is deteriorated when Mn exceeds 0.80 wt%, and P is 0.15
If it exceeds 0 wt%, the segregation of P in the slab becomes large, and surface defects called P band defects occur, and it cannot be used as an automobile outer plate. Therefore, the upper limit of each is 0.50 wt%.
%, 0.80 wt%, 0.150 wt% must be regulated. It should be noted that Si does not need to be restricted because it does not cause any hindrance at least, but Mn is at least 0.02 wt% to prevent ear rolling of hot rolling, and P is strength as a high strength steel plate. 0.035 wt% is necessary to obtain
It is necessary to regulate it to 0.035 wt%.

If the content of S exceeds 0.030 wt%, the ear roll property of hot rolling deteriorates, so it is necessary to regulate S to 0.030 wt%. It should be noted that there is no need to restrict the lower limit value because it does not become an obstacle no matter how small. sol.
For Al, it is necessary to fix N as AlN to prevent the occurrence of stretcher strain, so 0.025 wt%
The above is necessary. On the other hand, if it exceeds 0.100 wt%, air oxidation of the molten steel is likely to occur during casting, the amount of inclusions increases, and the workability and plating quality also deteriorate, so 0.100 wt% was made the upper limit value. N is detoxified by being fixed as AlN by Al, but 0.01
If it exceeds 00 wt%, the amount of AlN becomes too large and the r value deteriorates, so the upper limit value was made 0.0100 wt%. Since it is difficult to reduce the amount to less than 0.0005 wt% by improving ordinary steelmaking, the lower limit is set to 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 0.007 wt% to 0.0200 w
It is necessary to regulate t%. When Nb is less than 0.007 wt%, it becomes insufficient to reduce the crystal grain size of the hot rolled sheet and C cannot be sufficiently fixed as NbC, resulting in a sufficient r value of the annealed sheet. It won't get higher. On the other hand, if it exceeds 0.020 wt%, the amount of solute Nb increases, the growth of crystal grains during annealing is suppressed, and the r value cannot be sufficiently high. Further, the amount of Nb is Nb (wt%): 7.7 in order to secure non-stretchable strain.
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 does not need to be added particularly when it is used in applications where secondary workability is not required, but when it is used in applications where secondary workability is required, it is B (wt).
%): 0.004 x wt% P or more, 0.0010 wt%
It may be added below, within the range of B / N ≦ 0.7 (wt% ratio). It is well known that the secondary workability can be improved by adding a trace amount of B to the Ti-added ultra-low carbon steel, but it is also well known that the r value is lowered at the same time.
The present inventors have conducted various studies on a method for achieving both the secondary workability and the deep drawability, and have found the manufacturing method of claim (2) of the present invention. When the B content exceeds 0.0010 wt% or B / N exceeds 0.7 (wt% ratio), BN starts to be generated, and as a result, the r value decreases, but the B content However, the addition within the range of the present invention causes almost no deterioration of the r value. On the other hand, the secondary workability is B (wt
%): Addition of 0.004 × wt% P or more significantly improves.

The production conditions other than the compositional conditions of the steel sheet will be described in detail below. The production conditions of the cast slab may be any method as long as the components of the steel of each claim are obtained, and it is not particularly limited. The hot rolling conditions do not have to be particularly limited, and the slab may be finished rolled with Ar ₃ or more to obtain a rolled hot rolled steel strip. When the N content is high or the heating temperature is high, a better r value can be obtained by adopting a coiling temperature of 680 ° C. or higher, and it may be adopted if necessary. Cold rolling does not need to be particularly restricted and may be a usual method. The continuous annealing method is 750-89.
When heating to 0 ° C. to perform recrystallization annealing, and then cooling, it is necessary to cool at least 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 has a great influence on the amount of BH and the generation of stretcher strain, and as shown in FIG. 1, the cooling rate must be 2 ° C./s or more and less than 10 ° C./s. Also, this effect is at least 730
It can be obtained by cooling up to ° C at a cooling rate of 2 ° C / s or more and less than 10 ° C / s. Note that there is no particular need to regulate overaging treatment. If the temper rolling is less than 0.5%, no matter how excellent annealed sheet having excellent stretcher strain resistance can be produced, it is not possible to prevent the occurrence of stretcher strain during pressing, so 0.5% or more must be applied. . If it exceeds 3%, the ductility deteriorates greatly, 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 is made, a 3.8 mm hot rolled sheet is manufactured under the continuous hot rolling conditions shown in Table 2, and 0.65 m after pickling
m by cold rolling and continuous annealing at 850 ° C x 60 sec
Recrystallization annealing was performed and the cooling rate after soaking was cooled under the conditions shown in Table 2, then passed through an overaging zone of 400 ° C., and then cooled to room temperature to produce an annealed plate, and then 1.0% Was subjected to temper rolling to produce a high strength cold rolled steel sheet of 35 to 38 kgf / mm ² grade. The materials of the manufactured high-strength cold-rolled steel sheet were investigated and shown in Table 2. Y. P, T. S, El, r values are characteristic values after temper rolling, YP-El is a value after aging treatment at 100 ° C. × 4 Hr, BH amount is measured under a prestrain amount of 2%,
It is a value when the heat treatment condition equivalent to coating baking is 170 ° C. × 20 min. In addition, the secondary workability is a drawing ratio of 2.52.
When the cup was squeezed with -40 ° C. and crushed at -40 ° C., the length of brittle fracture was evaluated.
The following items were evaluated as ◯ and more as ×.

[0024]

[Table 1]

[0025]

[Table 2]

All of the steels A, B, C and D are steels within the composition range of the present invention, and the steels A, B, C and D having the composition of claim 1 in which B is not added are steels of the present invention. 6. The steel containing 6,6,9 ppm of the lower limit of 3.3, 3.1, 4.4 ppm or more of the B addition amount of claim 2, wherein the D steel has a P of 0.1.
It is a component steel that is added with 10 wt% to produce a high strength cold rolled steel sheet of 38 kgf / mm ² grade. Steel E has a C content of 0.
In an example in which the content of 0040 wt% and the content of Nb are 0.040 wt%, which is higher than the range of the present invention, the conventional method of Japanese Patent Publication No. 60-1700
It is an example of the steel composition according to the gazette of No. 4. Steel F is a comparative example in which the Nb content is 0.003 wt%, which is 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 it is an example of a steel component according to the conventional method of JP-A-58-84928. Steel H has a C content of 0.0011 wt% and an Nb content of 0.019 wt%, and the upper limit value (7.75 × w) of the Nb content in the formula (1) of the claim.
It is an example of a steel composition that deviates from t% C (0.0011) = 0.0155%). Steel I has a B content of 0.0038 wt.
% Is an example of a steel component added with B in a larger amount than the range of the present invention.

Samples 1, 2, 3, and 5 are all examples of the present invention, and Sample 4 has a cooling rate of 1 ° C. after soaking during continuous annealing.
/ S and a comparative example below the range of the present invention, sample 7 is a comparative example in which the components are outside the present invention, and the annealing conditions of samples 6 and 8 are those of the present invention, but the steel components are comparative examples Is. Samples 1, 2, 3 and 5 are all YP-El and BH after aging.
Since both the amount and the r value are excellent values, it can be seen that the method of the present invention can produce a high strength cold rolled steel sheet for deep drawing which is excellent in dent resistance and in which stretcher strain does not occur. Further, all of the obtained annealed plates have a good shape, and the manufacturing method of the present invention in which the cooling rate is less than 10 ° C./s is that a steel sheet having a good shape can be stably obtained. Also proves to be excellent.

Further, the sample 2, which is the embodiment of claim 2,
Nos. 3 and 5 are also excellent in secondary workability, and B (w of the present invention
t%) is 0.004 x wt% P or more, 0.0010 wt
It can be seen that excellent secondary workability can also be imparted by setting the ratio to be B / N ≦ 0.7 (wt% ratio) in the range of 0.1% or less. On the other hand, the steel 4 of the comparative example has a cooling condition of continuous annealing of 1 ° C./s, which is below the range of the present invention, and the BH amount is as low as 0.5 kgf / mm ^2. In comparison with Sample 3, which is an example of the present invention,
It can be clearly seen whether the manufacturing method of the present invention in which the cooling condition for continuous annealing is 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% and Nb content of 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 amount of 0.019 wt% exceeding the upper limit of the formula described in the claims, and Steel 10 has a B amount of 0.00.
38 wt% and a comparative example out of the steel composition range of the present invention,
Steel 6 has BH content and r value, Steels 7, 8 and 10 have r value, Steel 9
Has an inferior characteristic value in the amount of BH.

As is clear from the results of the above examples,
The high-strength cold-rolled steel sheet for deep drawing, which is excellent in dent resistance and does not generate stretcher strain, has the excellent secondary workability according to the method of claim 2 of the present invention. It can be given that the industrial value is extremely high. The production method of the present invention has the same effect as a production method of a raw plate of various surface-treated steel sheets such as electrogalvanized sheets and also in the production of hot-dip galvanized steel sheets.

[0030]

As described above, the present invention has been described in detail. However, according to the manufacturing method of the present invention, it is possible to manufacture a high strength cold rolled steel sheet for deep drawing which is excellent in dent resistance and does not cause stretcher strain. It can be effective and its industrial value is great.

[Brief description of 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.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Hirokazu Haga 1 Fuji-machi, Hirohata-ku, Himeji-shi, Hyogo Shin Nippon Steel Corp. Hirohata Works

Claims (2)

[Claims] 1. C: 0.0010 to 0.0030 wt
%, Si ≦ 0.50 wt%, Mn: 0.02-0.80
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%, and C amount and N
A slab consisting of the balance unavoidable impurities and iron satisfying the range of the following formula is subjected to finish rolling with Ar ₃ or more to make a hot rolled steel strip, cold rolling, and continuous annealing at 750 ~ 890 ℃ to heat recrystallization annealing, then,
For cooling, at least up to 730 ℃,
A stretcher strain with excellent dent resistance, which is characterized by cooling at a cooling rate of 2 ° C./s or more and less than 10 ° C./s to produce an annealed plate, and then temper rolling 0.5 to 3%. A method for producing a high-strength cold-rolled steel sheet for deep drawing that does not occur. Nb: 7.75 x wt% C-0.013 ≤ Nb (wt%) ≤ 7.75 x
wt% C + 0.007 2. The method according to claim 1, wherein the composition of the steel further comprises B (wt%) of 0.004 × wt% P or more,
0.0010 wt% or less, and B / N ≦ 0.7 (w
(t% ratio). A manufacturing method that also imparts excellent secondary workability.