JP2783100B2 - Manufacturing method of composite structure cold rolled steel sheet with excellent impact resistance - 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 cold rolled steel sheet having a composite structure having excellent impact resistance, and provides a steel sheet which is resistant to impact deformation generated at the time of collision or the like.

[0002]

2. Description of the Related Art In recent years, it has been desired to improve the fuel efficiency of automobiles from the viewpoint of global environmental protection. That is, for this reason, the weight of the vehicle body is reduced by thinning the steel plate used in the automobile, and the decrease in the vehicle body strength accompanying the thinning is compensated for by increasing the strength of the steel plate. The demand for higher ductility is becoming more and more demanding, and materials having both high strength and high ductility are expected.

[0003] In response to such demands, a tensile strength of 8% is obtained by utilizing the work-induced transformation of retained austenite.
A steel sheet having a breaking elongation of 30% or more at 0 kgf / mm ² or more is disclosed in JP-A-60-43430 and JP-A-61-2175.
29 publication. Compared to the conventionally used dual-phase ferritic and martensitic steels (Dual-Phase steels), these steel sheets are superior in press formability even at the same strength level, and the rigid shape makes it difficult to apply high-strength materials. By applying these steel plates to possible members, it is possible to contribute to a reduction in the weight of an automobile body.

[0004]

However, the top priority in the design of an automobile body is to ensure the safety of the occupants, and if the body is fragile against the impact received in the event of an accident. Cannot guarantee the safety of the occupants. Therefore, a vehicle body structure has been considered in which a shock is absorbed by deforming the front and rear portions in the event of an accident. However, members surrounding the occupant must be strong against the shock. To that end, it is essential to improve the impact resistance of the steel sheet used for the vehicle body as well as to optimize the vehicle body structure.

However, regarding the steel sheet containing retained austenite, the impact resistance is mentioned only in the above-mentioned Japanese Patent Application Laid-Open No. 61-217529. It is said that the elongation is increased, and as a result, the impact resistance of the press-formed product is improved, but no data on the actual impact resistance is disclosed. The present inventors have studied the impact resistance of a steel sheet containing retained austenite, and as a result, it has been found that sufficient impact resistance cannot be ensured only by controlling the ferrite phase. That is, it was found that the conventional method could not provide a practically satisfactory impact resistance.

[0006]

Means for Solving the Problems Accordingly, the present inventors have:
As a result of repeated studies on the structural factors that govern the impact resistance of steel sheets containing retained austenite, it was found that the form of retained austenite greatly affected the impact resistance. Then, the present inventors have found a chemical component and a production process capable of obtaining a preferred form of retained austenite with respect to impact resistance, and have reached the present invention. The constitution is as follows.

(1) In wt%, C: 0.05 to 0.25%, Si: 0.
6 to 2.3%, Mn: 0.8 to 2.3%, P: 0.02% or less, S:
A steel slab containing 0.01%, sol. Al: 0.02 to 0.06%, and N: 0.008% or less, with the balance being Fe and unavoidable impurities, has a finishing temperature FT and a winding temperature CT of the following formula (1) and After performing the hot rolling in the range of the formula (2), the steel sheet obtained by performing the cold rolling is heated to a temperature range of A _{C1 to} A _C3 and held for 20 seconds to 3 minutes. ~ 200 ° C
A cold-rolled steel sheet having excellent impact resistance, wherein the steel sheet is cooled to a temperature range of 350 to 480 ° C. at a cooling rate of / sec, kept at this temperature range for 30 seconds to 10 minutes, and then cooled to room temperature. Manufacturing method. FTmin ≤ FT (℃) ≤ FTmax ・ ・ ・ ・ (1) CTmin ≤ CT (℃) ≤ CTmax ・ ・ ・ ・ (2) However, FTmax = 5 ｛-10C (wt%) + Si (wt%)-2Mn (wt
%)｝ + 893 FTmin = 5 ｛-10C (wt%) + Si (wt%)-2Mn (wt%)｝ + 835 CTmax = 4 ｛-15C (wt%) + 3Si (wt%) + Mn (wt %)｝ + 630 CTmin = 4 ｛-15C (wt%) + 3Si (wt%) + Mn (wt%)｝ + 564

(2) In wt%, Ti: 0.005 to 0.05%, Nb:
0.005 to 0.05%, but the sum of Ti and Nb is 0.005 to 0.05%
, B: one or more elements selected from the group consisting of 0.0005 to 0.003%, wherein the cold rolled steel sheet with a composite structure excellent in impact resistance according to the above (1) is characterized by containing one or more elements selected from the group consisting of: Production method.

[0009]

The present invention as described above will now be described in detail. The present inventors have stated that, in terms of wt% (hereinafter simply referred to as%), C: 0.
08%, Si: 0.94%, Mn: 1.57%, P: 0.008%, S: 0.
A steel slab containing 004%, sol.Al: 0.035%, N: 0.0032% is melted, and the finishing temperature (FT) and the winding temperature (C
T) was varied to perform hot rolling. Further, a steel sheet having a thickness of 1.6 mm was formed by cold rolling.

[0010] The cold-rolled steel sheet obtained as described above is heated and held at 825 ° C for 1 minute, and then cooled at a cooling rate of 60 ° C / se.
After cooling to 425 ° C. at 5 ° C. and maintaining the temperature isothermally for 5 minutes, continuous annealing was performed to cool to room temperature. The product obtained after 0.5% temper rolling was subjected to an impact test. For this impact test, a Charpy impact tester having a capacity of 5 kgf · m was used, and the test piece was formed at room temperature (20 ° C.) with a total thickness of 2 mm V notch. In addition, in order to investigate the impact resistance after press molding, an impact test piece was sampled after applying a 10% tensile strain.

FIG. 1 shows the results obtained as described above. It can be seen that the impact value is increased only when both the finishing temperature and the winding temperature are within appropriate ranges.
The present inventors have further investigated the reason why the impact value varies depending on the finishing temperature and the winding temperature during hot rolling in this way, the form of the matrix ferrite and bainite, and the form of the retained austenite are strong. Turned out to affect.

By making the carbide in the hot-rolled sheet fine by low-temperature winding, the retained austenite after annealing is also finely dispersed. In the case of such a structure, when deformation is performed at a strain rate on the order of a tensile test, a work-induced transformation is effectively generated, which is preferable in terms of a balance between strength and ductility. However, when subjected to shock deformation, the fine austenite at the crack tip easily transforms into hard martensite, and generates a new crack at the interface with the surrounding matrix, so it is rather brittle against shock. turn into. Here, when the retained austenite is increased to some extent, the impact resistance is improved because the austenite itself absorbs the impact. However, if the winding temperature is too high, the carbides become too coarse and do not completely re-dissolve during continuous annealing, so that the impact resistance is rather deteriorated. On the other hand, the matrix itself is preferably fine for impact resistance. Therefore, the finishing temperature must be set so that the matrix does not become coarse.

The present inventors have further investigated the effects of chemical components. As a result, the optimum ranges of the finishing temperature and the winding temperature for improving the impact resistance are determined by the functions of C, Si and Mn. I found that it was. The steels with varied C content, Si content, and Mn content were subjected to hot rolling at various finishing temperatures, followed by cold rolling, continuous annealing, and temper rolling in the same manner as described above, and subjected to an impact test. The results are shown in FIG.

FIG. 3 shows the results of hot rolling at various winding temperatures. The optimum ranges of the finishing temperature and the winding temperature differ depending on the amounts of C, Si and Mn. ,
For example, even if the winding temperature is constant at 650 ° C. in FIG.
It can be seen that the impact value may be higher or not depending on the amount, Si amount, and Mn amount. Here, C: 0.05 to 0.25
%, Si: 0.6 to 2.3%, and Mn: 0.8 to 2.3%, the value of this function may fall within the range of the present invention.
However, in such a case, the impact value decreases regardless of the finishing temperature or the winding temperature, as described later.

As described above, in order to improve the impact resistance, the morphology of the matrix and the retained austenite is optimized by optimizing the chemical composition and the finishing temperature and the winding temperature during hot rolling. Turned out to be very important. That is, the present inventors also examined other elements and conditions of continuous annealing, and the result obtained is the present invention as described above, and the reason for limiting the chemical components of the present invention. Is as follows.

C: 0.05 to 0.25% C concentrates into austenite in the process of transforming supercooled austenite into bainite, and stabilizes austenite to form retained austenite. Also,
To strengthen bainite and martensite, the impact absorption energy of steel increases. In order to exhibit these effects, 0.05% or more of C must be added. On the other hand, 0.25%
When added in excess of, the amount of retained austenite increases, but the martensite generated by the work-induced transformation becomes hard and brittle, and even with the production method according to the present invention, deterioration of impact resistance is inevitable. Therefore, the upper limit of C is set to 0.25%. FIG. 4 shows the change in the impact value depending on the C content. By setting the C content to be 0.06% or more and less than 0.1%, the impact resistance can be further improved.

Si: 0.6-2.3% Si has an effect of promoting the concentration of C in austenite and facilitating generation of retained austenite, and has an effect of improving the impact resistance by increasing the strength level. Is effective for
It is necessary to add 0.6% or more. However, excessive addition rather causes brittleness of ferrite and deteriorates impact resistance. Therefore, the addition amount is limited to 2.3% or less.
FIG. 5 shows the change in the impact value depending on the Si content.
By setting the content to 8% or more and 1.6% or less, the impact resistance can be further improved.

Mn: 0.8 to 2.3% Mn is an indispensable element for the generation of retained austenite by bainite transformation because the nose of ferrite-pearlite transformation shifts to a longer time side. Moreover, it is necessary to strengthen the steel and obtain excellent impact resistance. Since these effects are not exhibited by addition of less than 0.8%, the lower limit is set to 0.8%. On the other hand, if added excessively, a band-like structure is generated, and the impact resistance is rather deteriorated. Therefore, the upper limit is 2.
3%. FIG. 6 shows the change in the impact value depending on the Mn content. The impact resistance can be further improved by setting the Mn content to 1.0% or more and 1.9% or less.

P: not more than 0.02% If P is added excessively, it causes embrittlement due to grain boundary segregation and degrades impact resistance. Therefore, in the present invention, it is limited to 0.02% or less.

S: 0.01% or less S is an inclusion such as MnS and remarkably deteriorates the impact resistance of steel. Therefore, in the present invention, the content is limited to 0.01% or less.

Sol.Al: 0.02 to 0.06% sol.Al is a deoxidizing element, and needs to be added in an amount of 0.02% or more in order to prevent deterioration of impact resistance due to oxygen in steel. However, if added in excess, the impact resistance is rather deteriorated, so the upper limit is made 0.06%.

N: 0.008% or less N deteriorates the impact resistance of steel, so it is desirable that N is as small as possible. Therefore, in the present invention, the content is limited to 0.008% or less.

The basic elements in the present invention are as described above. By further adding the following elements, the impact resistance can be further improved. That is, Ti, Nb
Are powerful carbonitride forming elements, and the matrix is refined by fine carbonitride, and B is dissolved in steel to refine the matrix, thereby improving the impact resistance. To obtain this effect, for Ti and Nb
0.005% or more of B and 0.0005% or more of B are required. However, if added excessively, a large amount of carbonitrides such as TiC and BN precipitate, and these precipitates deteriorate the impact resistance of the steel. Therefore, for Ti and Nb each 0.05 or less,
However, when combined, the total content is limited to 0.05% or less, and the content of B is limited to 0.003% or less.

Note that Ca and REM are elements that control the shape of inclusions such as MnS, and it is preferable to add them in a total amount of 0.02% or less in order to improve the impact resistance of steel. .

Next, the reasons for the limitation in the production method of the present invention will be described. When hot rolling is performed on a steel slab containing the above chemical components, the finishing temperature FT and the winding temperature CT are determined by the following equations (1) and (1). It is very important in the present invention to make the range of 2). FTmin ≤ FT (℃) ≤ FTmax ・ ・ ・ ・ (1) CTmin ≤ CT (℃) ≤ CTmax ・ ・ ・ ・ (2) However, FTmax = 5 ｛-10C (wt%) + Si (wt%)-2Mn (wt
%)｝ + 893 FTmin = 5 ｛-10C (wt%) + Si (wt%)-2Mn (wt%)｝ + 835 CTmax = 4 ｛-15C (wt%) + 3Si (wt%) + Mn (wt %)｝ + 630 CTmin = 4 ｛-15C (wt%) + 3Si (wt%) + Mn (wt%)｝ + 564

When the finishing temperature exceeds FTmax, the matrix does not become fine and the impact resistance is deteriorated. On the other hand, if the finishing temperature is lower than FTmin, the ferrite may undergo abnormal grain growth during recrystallization, and the matrix becomes coarse, thus deteriorating impact resistance. In addition, from the industrial point of view, the load at the time of rolling becomes too large, and it becomes difficult to secure a target winding temperature. Here, the upper and lower limits of the finishing temperature are affected by the transformation point of the steel, which differs depending on the amounts of C, Si and Mn.

If the winding temperature is lower than CTmin, carbides in the hot-rolled sheet are finely dispersed, and after annealing, a structure containing fine residual austenite is formed, which is not preferable from the viewpoint of impact resistance. On the other hand, if the winding temperature exceeds CTmax, the carbides in the hot-rolled sheet become too coarse and do not completely re-dissolve during continuous annealing, so the impact resistance is rather deteriorated. Here, the upper and lower limits of the winding temperature differ depending on the amounts of C, Si and Mn. For example, when the carbon content increases, the carbide in the hot-rolled sheet tends to increase. Therefore, it is necessary to prevent the carbide from becoming coarse by lowering the winding temperature. Also, if the amount of Si increases, C
Becomes slower, and as the amount of Mn increases, the transformation point decreases.
In any case, since carbides are easily made finer, it is necessary to prevent the carbides from becoming too fine by increasing the winding temperature.

The hot-rolled sheet thus obtained is subjected to continuous annealing after being cold-rolled. However, if the conditions for the continuous annealing are not appropriate, the structure will be unfavorable in terms of impact resistance. If the heating temperature during annealing is lower than A _C1 , the reverse transformation to austenite does not occur, it is impossible to obtain retained austenite, bainite, martensite itself, the strength level is greatly reduced, and the impact resistance is reduced. to degrade. On the other hand, when it exceeds A _C3 , the austenite is completely formed, so that the effect of matrix refinement by low-temperature finishing during hot rolling is impaired. Therefore, the heating temperature during annealing is limited to A _{C1 to} A _C3 . If the holding time in the above-mentioned temperature range is shorter than 20 seconds, a homogeneous two-phase structure cannot be obtained. If it is longer than 3 minutes, the matrix becomes coarse. Therefore, the heating holding time during annealing is 20 seconds to 3 seconds.
Minutes.

The temperature range of A _{C1 to} A _C3 is 350 to 480 ° C.
Cooling rate of 5 ℃ / se
If it is less than c, a large amount of pearlite is precipitated. Meanwhile, 2
If the temperature exceeds 00 ° C./sec, the retained austenite becomes unstable with respect to external force. In any case, the cooling rate is limited to 5 to 200 ° C./sec in order to cause deterioration of impact resistance. It should be noted that even if the cooling rate changes in this range, there is no notice.

The above-mentioned rapid cooling needs to be completed at 350 to 480 ° C. and maintained. If this temperature is lower than 350 ° C, C
Is difficult to diffuse, so it is difficult to enrich C in supercooled austenite, and the volume fraction of martensite is too large. On the other hand, if the temperature is higher than 480 ° C., a large amount of carbide will be precipitated, and in any case, the impact resistance will be deteriorated.
If the temperature is in the range of 350 to 480 ° C., there is no problem even if the temperature changes during the holding.

If the holding time at 350 to 480 ° C. is shorter than 30 seconds, the enrichment of C due to bainite transformation is insufficient, and the volume fraction of martensite becomes too large. On the other hand, if it is longer than 10 minutes, most of the supercooled austenite is transformed into bainite, and the strength level is reduced. In any case, since the impact resistance is deteriorated, the holding time is limited to 30 seconds to 10 minutes.

[0032]

【Example】

(Embodiment 1) A specific embodiment of the present invention will be described below. The chemical compositions of typical steels according to the present invention examples and comparative examples specifically adopted by the present inventors are as shown in the following Tables 1, 2 and 3, and steels A1 to R2 are present invention examples. And steels a to l are comparative examples. In this,
For steels A2 to R2, Ti, Nb,
One or more elements of B and Ca are added.

[0033]

[Table 1]

[0034]

[Table 2]

[0035]

[Table 3]

Each of the steel slabs having the compositions shown in Tables 1, 2 and 3 was melted, cast and heated to 1200 ° C., and then the finishing temperature was 860 ° C. and the winding temperature was 620 ° C. Hot rolling was performed to obtain a 3.2 mm thick steel sheet. Note that the above finishing temperature and winding temperature satisfy Expression (1) and Expression (2) for all steels. After pickling, a steel sheet having a thickness of 1.2 mm was formed by cold rolling. The cold-rolled steel sheet thus obtained was heated at 825 ° C for 60 seconds, and then heated at 60 ° C /
Cool to 425 ° C at a cooling rate of sec.
After holding at 5 ° C. for 5 minutes, continuous annealing for cooling to room temperature was performed. The product obtained after the 0.3% temper rolling was subjected to an impact test. 5 kgf ・ m capacity for impact test
The test piece shape is full thickness, 2
The test was performed at room temperature (20 ° C.) as a mmV notch.
In order to investigate the impact resistance after press molding, 10
% Tensile strain was applied, and then an impact test piece was taken.

The impact values obtained by the impact test on the test pieces are shown in Tables 4 and 5 below. According to the results, the samples Nos. 1A to 42A according to the present invention have an impact value of 2
0 kgf · m ² / cm ² or more, which clearly shows excellent impact resistance. In particular, Sample Nos. 25A to 42A
The impact resistance is further improved by adding i, Nb, B, and Ca. For example, comparing sample Nos. 6A and 25A, it can be seen that the impact value is increased by about 4 kgf · m / cm ² due to the addition of Ti. However, sample No. 37A
Since the amount of B added is insufficient, the impact value hardly changes when compared with Sample No. 21A in which the basic components are the same and B is not added. On the other hand, Sample Nos. 1B to 12B, in which the chemical components of the steel are out of the range of the present invention, all have low impact values. It is clear that problems arise.

[0038]

[Table 4]

[0039]

[Table 5]

(Example 2) Using some of the steel slabs of the examples of the present invention in Tables 1, 2 and 3 described above,
After heating to 00 ° C., hot rolling was performed at various finishing temperatures and winding temperatures to obtain a 3.2 mm thick steel sheet. Thereafter, pickling, cold rolling, continuous annealing, and temper rolling were performed under the same conditions as in Example 1, and then an impact test was performed as in Example 1. The test results are shown in Tables 6 and 7 below. Tables 6 and 7 show the finishing temperature FT, the winding temperature CT, the upper and lower limits of the finishing temperature and the upper and lower limits of the winding temperature (FTmax, FTmin, and CTmax, respectively) determined from the chemical components. , CT
min) is also shown.

[0041]

[Table 6]

[0042]

[Table 7]

That is, Sample Nos. 43A to 4 of the present invention
8A, and Sample Nos. 13B to 16B which are comparative examples,
In both cases, the finishing temperature and the winding temperature are changed using steel No. H2 as a material. From this result, if either the finishing temperature or the winding temperature deviates from the equations (1) and (2), It can be seen that the impact value is greatly reduced. Sample Nos. 49A to 51A of the present invention and Sample No. 17B of the comparative example are obtained by hot rolling steel slabs having different chemical components at the same finishing temperature and winding temperature.
For sample Nos. 49A to 51A, the finishing temperature is expressed by equation (1).
The winding temperature satisfies the formula (2), and a good impact value of 20 kgf · m ² / cm ² or more was obtained for each of the samples.

FT> FTmax is satisfied only for sample No. 17B, which does not satisfy the equation (1), indicating that the impact value is reduced. In addition, sample No. 52A
~ 62A and sample Nos. 18B ~ 20B, the C amount, Si amount, Mn even at the same finishing temperature and winding temperature.
Since the upper and lower limits of the finishing temperature or the winding temperature change depending on the amount, there are cases where the formula (1) or formula (2) is satisfied and not satisfied, and when the formula is not satisfied, the impact value may decrease. Recognize. That is, in order to improve the impact resistance, hot rolling is performed at an optimal finishing temperature and an optimal winding temperature according to the chemical composition of the steel so as to satisfy the formulas (1) and (2). It is clear that is very important.

(Example 3) Using the steel slab A1 of the example of the present invention in Table 1 described above, after heating to 1200 ° C, hot rolling was performed at a finishing temperature of 860 ° C and a winding temperature of 620 ° C. The steel plate was 3.2 mm thick. In addition, the above finishing temperature,
The winding temperature satisfies the equations (1) and (2). Then, after pickling and cold rolling to make a 1.2mm thick steel sheet,
Continuous annealing was performed under various conditions as shown in Table 8. For example, in the case of annealing No. A, after heating at 700 ° C. for 60 seconds,
This means that the steel sheet was cooled to 425 ° C. at a cooling rate of 60 ° C./sec, subsequently held at 425 ° C. for 5 minutes, and then subjected to continuous annealing for cooling to room temperature. The product obtained after the 0.3% temper rolling was subjected to an impact test in the same manner as in Example 1, and the results are as shown in Table 9 below.

[0046]

[Table 8]

[0047]

[Table 9]

Sample No. subjected to continuous annealing according to the present invention
Each of 63A to 72A has an impact value of 25 kgf · m / cm.
It is clearly ² or more, which is excellent in impact resistance. On the other hand, the sample Nos. 21B to 31B, which are comparative examples, had an impact value of 20 kgf · m / cm ² or less because any one of the conditions of the continuous annealing did not conform to the present invention. There is something wrong.

As shown in Examples 1 to 3 described above, the scope of the present invention is not limited to any one of the chemical composition of steel, the finishing temperature and the winding temperature during hot rolling, and the condition of continuous annealing. It is evident that the impact resistance is degraded when deviated from the above range.

[0050]

According to the present invention as described above, it is possible to manufacture a steel sheet having excellent impact resistance, and therefore, its industrial utility value is very large. It is clear that this is a very useful invention because it can be achieved.

[Brief description of the drawings]

FIG. 1 is a graph showing a relationship between a finishing temperature, a winding temperature, and an impact value during hot rolling.

FIG. 2 is a graph showing a relationship between a C content, a Si content, a Mn content and a finishing temperature range during hot rolling, and a relationship with an impact value.

FIG. 3 is a graph showing a relationship between a C content, a Si content, a Mn content and a winding temperature range during hot rolling, and a relationship with an impact value.

FIG. 4 is a graph showing a relationship between a C amount and an impact value.

FIG. 5 is a graph showing the relationship between the amount of Si and the impact value.

FIG. 6 is a graph showing the relationship between the Mn content and the impact value.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Yuki Osaki 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (72) Inventor Yasuyuki Takada 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Japan (56) References JP-A-62-182225 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C21D 9/46 C21D 8/02

Claims (2)

(57) [Claims] (1) In wt%, C: 0.05 to 0.25%, S
i: 0.6 to 2.3%, Mn: 0.8 to 2.3%, P:
0.02% or less, S: 0.01% or less, sol.Al: 0.02-0.
A steel slab containing 0.6% and N: 0.008% or less, with the balance being Fe and unavoidable impurities, is heated at a finishing temperature FT and a winding temperature CT in the range of the following equations (1) and (2). After performing the cold rolling, the steel sheet obtained by performing the cold rolling is heated to a temperature range of A _{C1 to} A _C3 and held for 20 seconds to 3 minutes.
A cold-rolled steel sheet having excellent impact resistance, wherein the steel sheet is cooled to a temperature range of 350 to 480 ° C. at a cooling rate of / sec, kept at this temperature range for 30 seconds to 10 minutes, and cooled to room temperature Manufacturing method. FTmin ≤ FT (℃) ≤ FTmax ・ ・ ・ ・ (1) CTmin ≤ CT (℃) ≤ CTmax ・ ・ ・ ・ (2) However, FTmax = 5 ｛-10C (wt%) + Si (wt%)-2Mn (wt
%)｝ + 893 FTmin = 5 ｛-10C (wt%) + Si (wt%)-2Mn (wt%)｝ + 835 CTmax = 4 ｛-15C (wt%) + 3Si (wt%) + Mn (wt %)｝ + 630 CTmin = 4 ｛-15C (wt%) + 3Si (wt%) + Mn (wt%)｝ + 564 2. In wt%, Ti: 0.005 to 0.05%, Nb: 0.0
05-0.05%, but the sum of Ti and Nb is 0.005-0.05%,
B: One or two selected from 0.0005 to 0.003%
The method for producing a cold-rolled steel sheet having a composite structure excellent in impact resistance according to claim 1, further comprising at least one kind of element.