JP3418999B2 - High-strength cold-rolled steel sheet excellent in deep drawability and secondary work brittle resistance, and method for producing the same - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength steel sheet and a method for producing the same, which are suitable for applications such as automobiles where relatively strict processing is performed. Such a high-strength steel sheet is, for example, as a steel sheet for automobiles, in order to reduce the thickness of the vehicle body while ensuring the required strength and reduce the weight of the vehicle body and the fuel consumption thereof, and further improve the strength of each member. This is useful for improving reliability and safety. Thus, in recent years, the momentum for the preservation of the global environment and the improvement of passive safety has been increasing, and it has received a great deal of attention.

[0002]

2. Description of the Related Art Conventionally, as a cold-rolled steel sheet having excellent formability, as described in, for example, JP-A-56-139654, an ultra-low carbon steel is used as a base to improve workability and aging. In order to achieve this, many steel sheets have been proposed in which a carbonitride forming component such as Ti or Nb is contained, and a reinforcing component such as P is contained within a range that does not impair the workability to enhance the strength. However, the tensile strength (TS) of these steel sheets was at most 40 kgf / mm ² , and there was a limit to the increase in strength. Therefore, in order to further strengthen the steel plate,
For example, Japanese Patent Application Laid-Open No. 59-193221 proposes a cold-rolled steel sheet further containing Si.
It is difficult to avoid another problem resulting from the large content of (i.e., mainly surface quality problems (for example, deterioration of chemical conversion processability and deterioration of plating property), and it cannot be used at all as a target automobile steel sheet. Further, with respect to a steel sheet of a component system containing a large amount of P as a reinforcing component, there is a problem that the secondary work brittleness resistance is deteriorated.

[0003]

According to the present invention, as a high strength steel sheet used for automobiles or the like, particularly, the tensile strength is 40 kg.
The main development purpose is to exceed f / mm ² . In such a high-strength steel sheet, it is, of course, necessary to satisfy not only the workability (mechanical properties) but also all the properties required for its use, such as platability and secondary work brittleness resistance. It is a requirement. Some of these characteristics are in conflict with each other, but the other characteristics are required to be even better without being inferior to the conventional steel sheet.
Among them, the secondary work brittleness resistance is apt to deteriorate when the workability is improved, so that it is required to satisfy extremely severe conditions compared with other properties.

Therefore, an object of the present invention is to propose a high-strength cold-rolled steel sheet excellent in deep drawability and resistance to secondary working brittleness, which satisfies the above conditions, and an advantageous manufacturing method thereof. .

[0005]

Means for Solving the Problems The gist of this invention, to improve the workability, the ultra low carbon steel with an adjusted amount of C in 0.0005～0.0050Wt% as basic components, P, contained in the Si及 beauty Mn , More preferably further increase the strength by the inclusion of Mo,
In order to secure the resistance to secondary working brittleness, by containing a relatively large amount of Ni and B and suppressing the Ti and Al contents to be lower than in the past, it is possible to prevent the inclusion of impurity phases in the steel more than necessary. Therefore, the workability is further improved.

In the manufacturing process, hot rolling, cold rolling and annealing are properly performed on the component steel having the above composition, including the heating and cooling conditions between the respective processes.

That is, the present invention is C: 0.0005 to 0.0050
wt% (hereinafter simply referred to as%), Si: 0.10 to 1.50%, Mn: 1.
00 to 3.50%, Ni: 0.020 to 1.000%, Ti: 0.015 to 0.10
0%, Nb: 0.003 to 0.020%, B: 0.0015 to 0.0050%,
Al: 0.030% or less, P: 0.040 to 0.150%, S: 0.010
% Or less and N: 0.0050% or less, the above Si, Mn, Ni and P
The respective contents of% Si,% Mn,% Ni and% P are

[Formula 5] A = 100 ×% P + 80 ×% Si−60 × {% Mn +% Ni} The content of A satisfies the condition of -50 ≦ A <0, and the balance is Fe and unavoidable. Strength cold-rolled steel sheet with excellent deep drawability and secondary work embrittlement resistance (1st
Invention).

The present invention also provides C: 0.0005 to 0.0050%,
Si: 0.10 to 1.50%, Mn: 1.00 to 3.50%, Ni: 0.020 to 1.
000%, Ti: 0.015 to 0.100%, Nb: 0.003 to 0.020
%, B: 0.0015 to 0.0050%, Al: 0.030% or less, P: 0.
040 to 0.150%, S: 0.010% or less, and N: 0.0050% or less with the respective contents of Si, Mn, Ni and P described above in% Si,% Mn,% Ni
And% P is

[Equation 6] A = 100 ×% P + 80 ×% Si −60 × {% Mn +% Ni} The A value calculated contains -50 ≦ A <0, and the balance is Fe and unavoidable impurities. After continuous casting, the steel slab having the composition of No. 1 is heated to 1150 to 1300 ° C without being cooled to below 300 ° C, hot-rolled, and finish rolling is performed in the temperature range of 700 to 1000 ° C. Water cooling is started within 3 seconds after the completion of rolling, followed by cooling at a cooling rate of 30 ° C / s or more, winding at 500 to 680 ° C, pickling, and then cold rolling with a rolling reduction of 70% or more. Deep drawability and secondary work embrittlement resistance, which consist of applying, further annealing at 800 ℃ or more and 850 ℃ or less , and then rapidly cooling to a temperature range of 400 ℃ or less at a cooling rate of 20 ℃ / sec or more. It is an excellent high-strength cold-rolled steel sheet manufacturing method (second invention).

Further, the present invention is one which contains Mo: 0.0150 to 0.5000% in addition to the respective component compositions of the first and second inventions (third and fourth inventions).

[0010]

The function of the present invention will be described. The inventors have set the tensile strength of the steel plate to 45 to 60 kgf / mm ² ,
Various studies have been carried out based on ultra-low carbon steel, and as a result, characteristics such as secondary work brittleness resistance can be improved by containing Ni and B after leaving an appropriate amount of solute C. It was also found that the inclusion of a relatively large amount of P can suppress deterioration of various surface properties due to Si contained as a reinforcing component to a minimum. Further, they have also found that by controlling the combination of the contents of Si, Mn, Ni and P within a predetermined range, an extremely good material can be obtained as compared with the conventional steel types, and the present invention was accomplished. .

The reasons why the component composition range and the manufacturing process are limited in the present invention will be described below.

C: 0.0005 to 0.0050% It is desirable to reduce the C content from the viewpoint of improving the elongation and the r value, but if it is less than 0.0005%, the C content is 2% or more.
Subsequent processing is not preferable because it deteriorates brittleness and reduces the strength of the welded portion (welding heat affected zone). Also, industrially, it is not worth the cost to reduce it to less than 0.0005%. On the other hand, if the C content exceeds 0.0050%, even if the equivalent amounts of Ti and Nb are included, a remarkable effect of improving the material quality (especially ductility) cannot be obtained, and there is a possibility that inconvenience may occur in the hot rolling of the steelmaking process and other manufacturing processes. It is not preferable because it is remarkable. Therefore, the C content is
The range was limited to 0.0005 to 0.0050%.

Si: 0.10 to 1.50% As the Si content, first, 0.10% was made the lower limit as a limit at which a sufficient strengthening effect can be obtained. The Si content may be basically adjusted according to the target level of tensile strength,
When the content of Al exceeds 1.50%, the hot rolled base plate is significantly hardened, so that the cold ductility is deteriorated and the chemical conversion processability is also significantly deteriorated. Further, various internal defects tend to increase, which is not preferable. Therefore, the upper limit of the Si content is set to 1.50%.

Mn: 1.00 to 3.50% Mn, when contained alone, deteriorates the mechanical properties after cold rolling annealing, especially the r value, but it is used in combination with other components to
When it is contained in the range of 00 to 3.50% and satisfying the correlation equation described later, the strength can be increased without causing remarkable deterioration of the material. Mn content here is 1.00%
If it does not meet the requirement, it cannot be strengthened sufficiently, while 3.50
%, The steel sheet is significantly hardened, resulting in great difficulty in the cold rolling process. Therefore, the Mn content is 1.00 to 3.50.
It was limited to the range of%.

Ni: 0.020 to 1.000% Since Ni is an expensive component, it has not been contained in the prior art, but it has an important meaning in the present invention.
In other words, the inclusion of Ni not only improves the secondary work brittleness resistance, but also increases the strength and improves the workability by limiting the content to the prescribed range specified by the relationship with Si, Mn, and P. It is possible to improve the sex. The effect is Ni 0.020%
Since it becomes remarkable when it is contained above, and it tends to be saturated when it exceeds 1.000%, the Ni content is set in the range of 0.020 to 1.000% in consideration of a large increase in cost.

Ti: 0.015 to 0.100% Ti is an essential component for improving the r value. Ti's
The effect of improving r-value becomes remarkable when the content is 0.015%, but 0.10%
Even if the content exceeds 0%, the effect is saturated, and the deterioration of the surface properties becomes remarkable. Therefore, the lower limit of Ti content is limited to 0.015% and the upper limit is limited to 0.100%. Note that, for the first time in the present invention, it was discovered that the above effect is particularly remarkable in the case of P-containing steel.

Nb: 0.003 to 0.020% By containing 0.003% or more of Nb, a higher r value can be obtained as compared with the case of containing Ti alone. Also,
The inclusion of Nb has the effect of suppressing abnormal grain growth during annealing, and is advantageous for obtaining a uniform and fine steel sheet structure. However, when Nb is contained in excess of 0.020%,
The secondary work brittleness resistance deteriorates, and the workability including the r value tends to deteriorate. If the Nb content is less than 0.003%, the effect cannot be obtained. Therefore, the Nb content is limited to 0.003 to 0.020%.

B: 0.0015 to 0.0050% B is one of the important components in the present invention. According to the conventional publicly known document, it has been reported that the content of B exerts a great effect on the secondary work brittleness of steel, but at the same time, deterioration of the material (mainly r value) is unavoidable. It has been said that the optimum range of the content is 0.0005 to 0.0010%. However, in the Si-, Mn-, and P-containing steels described in the present invention, it has been found that the B content in the above range cannot sufficiently improve the secondary work brittleness. That is, when these alloy elements are added in combination, the conventional knowledge is greatly deviated, and the brittleness is deteriorated.
Therefore, the balance of the content of Si, Mn, P and B to be contained
When the mechanical properties and the secondary processing brittleness resistance were investigated by changing the amount variously, the secondary processing brittleness resistance was improved to a level where there was no practical problem by adding B in an amount of 0.0015% or more. It became clear that it was possible. But this effect
It saturates at 0.0050%, and depending on the annealing conditions, it is rather gratifying to reduce workability. Therefore, the B content is 0.00
Limited to 15-0.0050%.

Al: 0.030% or less Al is also an important component in the present invention. The detailed mechanism is unknown, but the content is lower than that of the conventional one, 0.03%.
When the content is 0% or less, the composition-based steel of the present invention has an effect of improving workability. The material tends to be improved as the amount of Al is reduced, but if the content falls below 0.001% in general, inclusions increase and the workability eventually decreases accordingly. Therefore, the tentative lower limit is 0.00
1%, but if the inclusions are sufficiently removed,
It is presumed that even Al-free steel does not deteriorate in properties.

P: 0.040 to 0.150% The content of P has a very important meaning in the present invention.
That is, in the component steel of the present invention, by including P, the detailed mechanism is unknown, but while increasing the strength,
Further, it was found that the workability (mainly the r value) was significantly improved. This effect is remarkable when the content is 0.040% or more. When P is contained in an amount of more than 0.150%, segregation during solidification becomes extremely strong, resulting in saturation of the increase in strength and deterioration of workability. In terms of properties, it also deteriorates to a level where it is practically unusable. Therefore, the upper limit was set to 0.150%.

S: 0.010% or less S is a component to be reduced as much as possible in the present invention. By reducing the amount of S, it contributes to the reduction of the precipitates in the steel to improve the workability and the effective amount of Ti that fixes C. Such an effect can be obtained by setting the S content to 0.010% or less.

N: 0.0050% or less N is a component to be reduced as much as possible in the present invention. By reducing the N content, the material (especially ductility, r
Value) can be expected to improve. However, if it is reduced to 0.0050% or less, almost satisfactory effect can be obtained.
It was set to 0.0050%.

Mo: 0.0150 to 0.5000% In the third and fourth inventions, Mo is 0.0150 to 0.5000%.
It is contained in the range of. Mo is an effective component for improving the strength, but if its content is less than 0.0150%, it has the disadvantage that the target strength increasing effect cannot be obtained, while if it exceeds 0.5000%, it is As a result of the hardened remarkably hardened base plate, there arises a disadvantage that cold rolling becomes difficult. Therefore, the Mo content is set to the range of 0.0150 to 0.5000%.

For the above Si, Mn, Ni and P, the respective contents% Si,% Mn,% Ni and% P are

## EQU00007 ## It is necessary that the A value calculated by A = 100.times.% P + 80.times.% Si-60.times. {% Mn +% Ni} is a content satisfying -50≤A <0. By including each component in the range satisfying this condition, the detailed mechanism is unknown, but the required high strength is obtained, and the r value is hardly deteriorated, and high strength and high r value are obtained. Steel sheets can be manufactured. FIG. 1 is a graph showing the effect of the A value on the average r value of the steel sheet. This steel plate is
Various continuous cast slabs with different Si, Mn, Ni and P contents were heated to 1170 to 1270 ℃ (maintained at 350 ℃ or higher after continuous casting), then finish rolling temperature: 900 ℃ Rolled,
Start quenching within 2 seconds after finish rolling, cooling rate approx. 35 ℃ /
After cooling for 2 seconds and winding at 550 ° C, cold rolling with a reduction rate of 73% to a plate thickness of 0.8 mm was performed, followed by short-time annealing at 850 ° C for about 20 seconds, followed by a cooling rate of 25 ° C / It is manufactured by cooling to 350 ℃ in seconds. As is clear from FIG.
It can be seen that by maintaining the value within the range of −50 ≦ A <0, a steel sheet with a high r value can be manufactured.

The cold-rolled steel sheet according to the present invention having the above composition range has a ferrite single-phase structure.

Next, the reasons for limiting each manufacturing condition in the preferred method for manufacturing a cold rolled steel sheet according to the present invention will be described. -Slab heating temperature: 1150 to 1300 ° C If the heating temperature of the continuous casting slab prior to hot rolling is less than 1150 ° C, it is difficult to secure a sufficiently high hot rolling finishing temperature as described below. However, if this hot rolling temperature is secured, it is advantageous from the viewpoint of the material to further lower the slab heating temperature. However, since the load during hot rolling will also increase, the lower limit is set to 1150 ° C assuming the current equipment. On the other hand, if the slab heating temperature exceeds 1300 ° C, the properties of the steel sheet surface will deteriorate significantly in the end. Therefore, the upper limit was set to 1300 ° C. Further, in the steel of the present invention, since the low temperature toughness of the slab is deteriorated, it is necessary to avoid lowering the temperature to 300 ° C. or lower before charging into the slab heating furnace.

Finishing rolling temperature: 700 to 1000 ° C. The finishing rolling temperature is required to be at least 700 ° C. in order to improve workability represented by r value after cold rolling and annealing.
When hot-rolled at a temperature of less than 700 ° C., the rolling structure in the hot-rolled sheet remarkably remains, and finally a texture undesired for workability is formed, which is not preferable. On the other hand, if the finish rolling temperature exceeds 1000 ° C., the roll damage of the rolling mill becomes large, which is a serious obstacle to actual production. It is also disadvantageous in terms of collective organization. Therefore, the finish rolling temperature of the hot rolling was set in the range of 700 to 1000 ° C.

Cooling condition after hot rolling For cooling after hot rolling, water cooling is started within 3 seconds after finishing rolling, and subsequently at a cooling rate of 30 ° C / s or more, at 500 to 680 ° C described below. It is necessary to cool to the coiling temperature. If air cooling is performed for more than 3 seconds after finishing rolling, Ti
The precipitation of phosphides of the steel markedly progresses, and not only the strength of the steel sheet but also the workability such as r value is deteriorated. If the rapid cooling is not continued at a cooling rate of 30 ° C / s or more, the precipitation of Ti phosphide will proceed remarkably and the transformation mechanism will change. Deteriorates.

Winding temperature: 500 to 680 ° C. If the winding temperature is less than 500 ° C., the unevenness of cooling causes disorder in the plate shape, and the hot rolled mother plate becomes harder, and the next step It interferes with pickling and cold rolling. From the viewpoint of the material, it is considered that the precipitation of TiC is excessively suppressed, but the material is deteriorated. On the other hand, when the winding temperature exceeds 680 ° C, Ti phosphide is formed and not only the deterioration of the material but also the deterioration of the pickling property with the increase of the scale thickness becomes remarkable. Furthermore, various problems due to the surface concentration of Si etc. will become apparent. Therefore, the winding temperature was set in the range of 500 to 680 ° C.

Cold rolling reduction rate: 70% or more The cold rolling reduction rate after pickling is limited to 70% or more. If it is less than 70%, sufficient deep drawability cannot be obtained. 80% is desirable
That is all.

Annealing condition Annealing condition is that ferrite single phase (polygon
Ferrite single phase) becomes tissue, 800 ° C. is defined as the minimum temperature at which good material is obtained. On the other hand, when annealing is performed at high temperature , the TS
There the high strength Tokunan Kunar to reduced target, defining the 850 ° C. as acceptable limit. The soaking time in this annealing is not particularly limited, but is 20 to 18
It is about 0 s. If the soaking time is less than about 20 s, the recrystallization may not be completed stably depending on the conditions, while if it exceeds about 180 s, the surface quality may be deteriorated.

Cooling conditions: The cooling rate from the temperature in the above-mentioned annealing to 400 ° C.
20 ℃ / sec or more. . If the cooling rate is less than 20 ° C / sec, the secondary processing brittleness resistance deteriorates. Also, such cooling
Even if it is not rapidly cooled to 400 ° C or less, the secondary working brittleness resistance similarly deteriorates. By performing such rapid cooling,
Some improvement in TS can be achieved without deterioration of ductility and r-value.

[0033]

Example 1 Steels having various compositional compositions shown in Table 1 were melted in a converter, and a cold-rolled steel sheet having a thickness of 0.7 mm was produced under the following conditions, and its mechanical properties were evaluated. investigated. Underlines attached to components and the like indicate outside the scope of the present invention.

[0034]

[Table 1]

Slab heating temperature: 1220 to 1280 ° C. (maintained at 350 ° C. or higher after continuous casting) Finish rolling temperature: 850 to 880 ° C. Cooling condition: Rapid cooling starts within 3 seconds after finishing rolling, cooling at about 40 ° C./second Winding temperature: 520 ° C Cold rolling reduction: 78% Annealing condition: 840 ° C, 30 seconds Soaking cooling condition: Cooling from annealing temperature to 350 ° C at a cooling rate of 25 ° C / sec.

The tensile properties of the obtained cold-rolled steel sheet were evaluated by a usual test method using JIS No. 5 tensile test pieces. Also,
Regarding the secondary processing brittleness, after conical cups drawn with a drawing ratio of 2.0 were flange cut, a 5 kg weight was dropped from a height of 80 cm at various temperatures to give an impact load, and The upper limit temperature at which various cracks occur is evaluated. If this temperature is approximately -45 ° C or lower, it can be determined that there is no problem in a normal use environment. The results thus obtained are shown in Table 2.

[0037]

[Table 2]

As is clear from Table 2, the examples according to the present invention have superior El. (Ductility) and high r as compared with the comparative examples.
It can be seen that the steel plate has a good workability and high tensile strength.
Also, regarding the secondary processing brittleness, the brittleness temperature is as good as -50 ° C or lower. The steel sheets of the examples all had a polygonal ferrite single-phase structure.

Example 2 Steels having the components shown in Table 3 were melted in a converter and manufactured by actual equipment under various manufacturing conditions shown in Table 4 to obtain cold-rolled steel sheets having a thickness of 0.75 mm. Various tests were conducted on these steel sheets to investigate various properties. The results thus obtained are also shown in Table 4.

[0040]

[Table 3]

[0041]

[Table 4]

As is apparent from Table 4, the conforming examples manufactured under the conditions of the present invention have better characteristics than the comparative examples. Further, when the chemical conversion treatment and the plating adhesion of the electroplating were performed on these steel sheets, the levels were almost the same as those of conventional cold-rolled steel sheets for automobiles.

[0043]

INDUSTRIAL APPLICABILITY The cold-rolled steel sheet according to the present invention is particularly useful in applications such as automobiles because it has both high strength and excellent deep drawability and secondary processing brittleness resistance.

[Brief description of drawings]

FIG. 1 is a graph showing the effect of an A value on the average r value of a steel sheet.

─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-6-10095 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C22C 38/00 301 C21D 8/04 C21D 9 / 48 C22C 38/14

Claims (4)

(57) [Claims] 1. C: 0.0005 to 0.0050 wt%, Si: 0.10 to 1.50 wt%, Mn: 1.00 to 3.50 wt%, Ni: 0.020 to 1.000 wt%, Ti: 0.015 to 0.100 wt%, Nb: 0.003 to 0.020. wt%, B: 0.0015 to 0.0050 wt%, Al: 0.030 wt% or less, P: 0.040 to 0.150 wt%, S: 0.010 wt% or less and N: 0.0050 wt% or less of the above Si, Mn, Ni and P. Each content% Si,% Mn,% Ni
And% P are contained under the condition that A = 100 ×% P + 80 ×% Si-60 × {% Mn +% Ni} and the value of -50 ≦ A <0 is satisfied. A high-strength cold-rolled steel sheet that has a composition of Fe and unavoidable impurities in the balance and has a ferrite single-phase structure and is excellent in deep drawability and secondary work embrittlement resistance. 2. C: 0.0005 to 0.0050 wt%, Si: 0.10 to 1.50 wt%, Mn: 1.00 to 3.50 wt%, Ni: 0.020 to 1.000 wt%, Ti: 0.015 to 0.100 wt%, Nb: 0.003 to 0.020. wt%, B: 0.0015 to 0.0050 wt%, Al: 0.030 wt% or less, P: 0.040 to 0.150 wt%, S: 0.010 wt% or less and N: 0.0050 wt% or less of the above Si, Mn, Ni and P. Each content% Si,% Mn,% Ni and% P is -50 ≦ A at the A value calculated by the following formula: A = 100 ×% P + 80 ×% Si −60 × {% Mn +% Ni} A steel slab containing under the condition of <0, with the balance being Fe and unavoidable impurities is used for 1150 to 13
Heat to 00 ℃ and hot roll it, and finish rolling in the temperature range of 700-1000 ℃. Start water cooling within 3 seconds after finishing rolling and continue cooling at a cooling rate of 30 ℃ / s or more. After picking it up at 500-680 ° C, pickling, then cold rolling with a rolling reduction of 70% or more, annealing at 800 ° C or more and 850 ° C or less , and cooling rate of 20 ° C A method for producing a high-strength cold-rolled steel sheet excellent in deep drawing property and secondary work brittleness resistance, which comprises quenching to a temperature range of 400 ° C / sec or more per second or more. 3. C: 0.0005 to 0.0050 wt%, Si: 0.10 to 1.50 wt%, Mn: 1.00 to 3.50 wt%, Ni: 0.020 to 1.000 wt%, Mo: 0.0150 to 0.5000 wt%, Ti: 0.015 to 0.100. wt%, Nb: 0.003 to 0.020 wt%, B: 0.0015 to 0.0050 wt%, Al: 0.030 wt% or less, P: 0.040 to 0.150 wt%, S: 0.010 wt% or less and N: 0.0050 wt% or less Content of Si, Mn, Ni and P% Si,% Mn,% Ni
And% P are contained under the condition that A = 100 ×% P + 80 ×% Si-60 × {% Mn +% Ni} is satisfied and -50 ≦ A <0 is satisfied. A high-strength cold-rolled steel sheet that has a composition of Fe and unavoidable impurities in the balance and has a ferrite single-phase structure and is excellent in deep drawability and secondary work embrittlement resistance. 4. C: 0.0005 to 0.0050 wt%, Si: 0.10 to 1.50 wt%, Mn: 1.00 to 3.50 wt%, Ni: 0.020 to 1.000 wt%, Mo: 0.0150 to 0.5000 wt%, Ti: 0.015 to 0.100. wt%, Nb: 0.003 to 0.020 wt%, B: 0.0015 to 0.0050 wt%, Al: 0.030 wt% or less, P: 0.040 to 0.150 wt%, S: 0.010 wt% or less and N: 0.0050 wt% or less Each content of Si, Mn, Ni and P% Si,% Mn,% Ni and% P is calculated by the following formula [Formula 4] A = 100 ×% P + 80 ×% Si−60 × {% Mn +% Ni} A steel slab containing A on the condition that -50 ≦ A <0 is satisfied and the balance is Fe and unavoidable impurities is used for 1150 ~ 13 without continuous cooling to 300 ℃ or less after continuous casting.
Heat to 00 ℃ and hot roll it, and finish rolling in the temperature range of 700-1000 ℃. Start water cooling within 3 seconds after finishing rolling and continue cooling at a cooling rate of 30 ℃ / s or more. After picking it up at 500-680 ° C, pickling, then cold rolling with a rolling reduction of 70% or more, annealing at 800 ° C or more and 850 ° C or less , and cooling rate of 20 ° C A method for producing a high-strength cold-rolled steel sheet excellent in deep drawing property and secondary work brittleness resistance, which comprises quenching to a temperature range of 400 ° C / sec or more per second or more.