JP2014051683A - Cold rolled steel sheet and its manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a cold rolled steel sheet satisfying tensile strength of 980 MPa or more, yield strength of 690 MPa to 850 MPa, total elongation of 12% or more and bendability R/t≤1.0, where R is a minimum inside radius having no cracking in a V block method bending test with a bending angle of 90° and t is board thickness.SOLUTION: A cold rolled steel sheet contains, by mass%, C:0.09 to 0.16%, Si:0.05 to 0.60%, Mn:1.95 to 3.00%, Al:0.02 to 0.45%, Ti:0.01 to 0.2%, has a total content of Si and Al of 0.15 to 0.64%, has a chemical composition containing one or more kind selected from Cr:0.02 to 1.0%, Mo:0.01 to 2.0% and B:0.0003 to 0.01%, has a steel structure having, by area%, 60% or more of bainite, 6 to 40% of ferrite, 3% or less of retained austenite, and an average particle diameter of bainite and ferrite of 5 μm or smaller and a maximum particle diameter of 9 μm or less, and has an average Mn concentration at a surface layer part from the surface of the steel sheet to 5 μm depth, or Mn(mass%), of 2.60% or less and satisfying Mn/[Mn]≤0.90.

Description

  The present invention relates to a cold-rolled steel sheet and a manufacturing method thereof. More specifically, the present invention is capable of achieving good formability when performing composite molding that requires high formability while having high tensile strength and appropriate yield strength. The present invention relates to a high-strength cold-rolled steel sheet and a manufacturing method thereof.

  In recent years, weight reduction of automobiles has become very important from the viewpoint of preventing global warming caused by exhaust gas and saving limited resources. In addition, collision safety standards have been tightened since around 1990, and the use ratio of high-strength steel sheets has been rapidly increasing in terms of both collision safety improvement and vehicle weight reduction. Along with this, the application of high strength steel plates having a tensile strength of 980 MPa or more, which has been widely used for collision response members, to vehicle body parts that require high formability, such as sheet parts and pillars, is being expanded. Especially for sheet parts, not only extremely severe bending formability, burring workability and stretch flange formability are required, but also part rigidity with high yield strength is required. However, if the yield strength is too high, the amount of spring back at the time of component molding increases and causes the dimensional accuracy of the component to be poor. Therefore, it is necessary to control the yield strength within an appropriate range.

  Conventionally, various attempts have been made to achieve both high tensile strength and excellent formability as described below.

(1) As a high-strength steel sheet excellent in formability, a composite structure steel sheet having ferrite as a main phase and a low-temperature transformation phase such as martensite or bainite as a second phase has been proposed. For example, Patent Document 1 discloses a hot-dip galvanized steel sheet having a composite structure containing ferrite as a main phase, having a tensile strength of 80 kgf / mm ² or more and a yield ratio of 60% or less. However, a high-strength steel sheet using such a hard low-temperature transformation phase has a low yield ratio because a large number of movable dislocations are introduced into ferrite, and the rigidity required for sheet parts cannot be ensured. Furthermore, since the hardness difference between the hard phase and the soft phase is large, cracks are likely to occur at the interface between the two phases, and cracks are likely to occur during bending or burring.

(2) In order to ensure component rigidity and suppress the occurrence of cracks as described above, it is necessary to have a uniform structure with a small hardness difference. Patent Document 2 discloses a steel sheet that has both a tensile strength of 980 N / mm ² or more and excellent bendability by using a bainite structure as a main component. However, since this steel sheet has a very high yield strength of 900 MPa or more and tends to increase the amount of springback during forming, there is room for improvement from the viewpoint of component dimensional accuracy.

(3) Patent Document 3 discloses a steel sheet having a tensile strength of 780 N / mm ² or more and excellent elongation and hole expansibility. However, for steel sheets having a tensile strength of 980 N / mm ² or more, only an example of reheating after the end point of cooling at the time of continuous annealing is disclosed, and such a manufacturing method has a heavy load.

  (4) Patent Document 4 discloses a steel sheet having a ferrite single-phase structure in which fine precipitates having a particle size of less than 10 nm are dispersed and having a tensile strength of 550 MPa or more, and the steel sheet is a hot-rolled steel sheet. Or cold rolled steel sheet. However, only the hot-rolled steel sheet is specifically disclosed, and no specific disclosure is made about the cold-rolled steel sheet. And since the manufacturing process of a cold-rolled steel plate differs from the manufacturing process of a hot-rolled steel plate, the technical idea of a hot-rolled steel plate cannot be applied to a cold-rolled steel plate simply. That is, in order to obtain a final product by annealing a cold-rolled steel sheet after cold rolling, in the method of adding a large amount of carbonitride-forming elements as disclosed in Patent Document 4, the recrystallization temperature is increased. Since high temperature annealing is required, coarsening of precipitates and coarsening of the cold-rolled annealed plate structure occur, and thus formability deteriorates.

  (5) Patent Document 5 discloses a steel plate having a high uniform deformability and a method for producing the same, using processing-induced transformation of retained austenite. However, there is a problem that after the processing-induced transformation, a structure containing hard martensite is formed, the hardness difference between the structures is large, and the bendability is usually deteriorated.

  (6) Patent Document 6 discloses a steel sheet having excellent bendability by reducing the Mn concentration segregation ratio in the vicinity of the steel sheet surface layer and a manufacturing method thereof. However, a manufacturing method in which a steel material having a predetermined chemical composition is held at 1200 to 1350 ° C. for 5 to 30 hours has a large manufacturing load because the manufacturing cost increases.

  In this way, among the methods that have been used in the past, those that achieve both proper yield strength and excellent total elongation and bendability have a large manufacturing load, and like sheet parts, have high part rigidity and part dimensional accuracy. To obtain a high-strength steel sheet that is suitable as a material for parts that require appropriate yield strength and excellent formability at the same time without going through the tempering process after annealing. Was difficult.

JP-A-4-236671 JP-A-6-65685 JP-A-7-188767 JP 2002-322539 A JP-A 61-157625 JP 2009-221519 A

  In view of the above-described prior art, an object of the present invention is to provide a low-cost high-strength cold-rolled steel sheet having an appropriate yield strength and excellent ductility and bendability, and a method for manufacturing the same. Specifically, the tensile strength is 980 MPa or more, the yield strength is 690 MPa or more and 850 MPa or less, the total elongation is 12% or more, and the bendability is R / t ≦ 1.0 (R is a V block method with a bending angle of 90 °. It is an object of the present invention to provide a high-strength cold-rolled steel sheet excellent in formability and a method for producing the same, satisfying a minimum inner radius that does not cause cracking in a bending test by t, and t is a sheet thickness.

In order to solve the above-mentioned problems, the present inventors have conducted intensive studies as follows.
In order to secure an appropriate yield strength and a predetermined total elongation, it is necessary to make the structure mainly composed of bainite capable of improving the appropriate yield strength and including ferrite for improving the ductility.

  However, in the conventional general manufacturing method, it is difficult to ensure excellent bendability because the difference in hardness between the structures of ferrite and bainite and other hard phases is large even if the structure has the above structure. there were.

  That is, the conventional general manufacturing method adopts a chemical composition containing Mn in order to ensure high strength, hot-rolls the slab manufactured by continuous casting in the usual way, pickling and cold After rolling, annealing is performed in a two-phase region in which ferrite and austenite coexist, thereby discharging C from ferrite to concentrate C into austenite, and then rapidly cooling to partially cool austenite at a low temperature transformation phase. It is a thing to transform into.

  In such a manufacturing method, hardness unevenness due to Mn segregation formed on the surface layer of the slab remains until after annealing, so stress is concentrated on a soft part with less Mn during bending, and cracking is likely to occur. Become. Further, since C is concentrated from ferrite to austenite by the two-phase annealing, the difference in inter-structure hardness between ferrite and bainite and other hard phases increases. Due to these reasons, it becomes difficult to ensure excellent bendability.

  In view of this, the inventors have conceived that an excellent bendability can be achieved by securing a microstructure containing bainite as a main component and reducing the influence of the Mn segregation and reducing the stress concentration during bending.

  Then, a specific method for mitigating the influence of the Mn segregation is examined, and first, by increasing the solidification rate of the surface layer part when manufacturing the slab, the dendrite trees are narrowed to suppress the Mn segregation itself. Inspired.

  In addition, by holding the slab in a high temperature range when subjected to hot rolling, it promotes the diffusion of Mn in the slab surface layer part to alleviate the segregation of Mn, and the Mn in the slab surface part is formed on the slab surface. Concentrate and reduce the scale, optimize the hot rolling process and steadily remove the oxidized scale to reduce the Mn concentration of the steel sheet surface layer, thereby reducing the effect of Mn segregation. Inspired.

  Furthermore, by making the highest temperature achieved in the annealing process an austenite single phase region that is higher than the conventional general manufacturing method, the influence of the structure before the hot rolling step is eliminated and two-phase region annealing is performed. It was conceived that the concentration of C from ferrite to austenite occurring in the steel can be prevented, thereby reducing the difference in interstitial hardness between ferrite and bainite and other hard phases.

  Furthermore, the idea is to improve the bend formability by suppressing the stress concentration during bend forming by refining the main structures bainite and ferrite, and as a specific means to realize this, The idea was to optimize the rolling and annealing conditions.

  As a result of further investigation based on these ideas, we have newly found that it is possible to achieve both proper yield stress and excellent total elongation and bendability.

This invention is based on the said new knowledge, The summary is as follows.
(1) By mass%, C: 0.09% to 0.16%, Si: 0.05% to 0.60%, Mn: 1.95% to 3.00%, P: 0.00% Contains 02% or less, S: 0.01% or less, Al: 0.02% or more and 0.45% or less, Ti: 0.01% or more and 0.2% or less, and N: 0.01% or less. From the group satisfying the formula (1), further comprising Cr: 0.02% to 1.0%, Mo: 0.01% to 2.0% and B: 0.0003% to 0.01% It contains one or two or more selected chemical compositions, the balance being Fe and impurities, and in area%, bainite: 60% or more, ferrite: 6% or more and 40% or less, residual austenite: 3% or less In addition, the average particle size of the bainite and ferrite is 5 μm or less. And a steel structure maximum diameter is 9μm or less, Mn ^sur is the average Mn concentration in the surface layer of a steel plate surface to 5μm depth ^(unit: mass%) of the following formula (2) and (3) A cold-rolled steel sheet characterized by satisfying mechanical properties satisfying a tensile strength of 980 MPa or more, a yield strength of 690 MPa or more and 850 MPa or less, a total elongation of 12% or more, and a bendability satisfying the following formula (4):
0.15 ≦ [Si] + [Al] ≦ 0.64 (1)
Mn ^sur ≦ 2.60 (2)
Mn ^sur /[Mn]≦0.90 (3)
R / t ≦ 1.0 (4)
Here, [M] is the content of element M (unit: mass%), Mn ^sur is as described above, and R is not cracked in the bending test by the V-block method with a bending angle of 90 °. The minimum inner radius (unit: mm), t is the plate thickness (unit: mm).

  (2) The chemical composition contains one or two selected from the group consisting of Cu: 1.0% or less and Ni: 1.0% or less in mass% instead of a part of the Fe The cold-rolled steel sheet according to (1) above.

  (3) The chemical composition contains one or two selected from the group consisting of Nb: 0.10% or less and V: 0.10% or less in mass%, instead of a part of the Fe The cold-rolled steel sheet according to (1) or (2) above.

  (4) The chemical composition is selected from the group consisting of REM: 0.10% or less, Mg: 0.01% or less, and Ca: 0.01% or less in mass% instead of part of the Fe. The cold-rolled steel sheet according to any one of the above (1) to (3), which contains one or more kinds.

  (5) The cold-rolled steel sheet according to any one of (1) to (4), wherein the chemical composition contains Bi: 0.05% or less instead of part of the Fe.

(6) A method for producing a cold-rolled steel sheet comprising the following steps (A) to (D):
(A) The molten steel having the chemical composition according to any one of the above (1) to (5) is averaged in the temperature range from the liquidus temperature to the solidus temperature at a position 10 mm deep from the slab surface. A casting process in which the casting rate is 10 ° C./second or more;
(B) The slab obtained by the casting step is held in a temperature range of 1180 ° C. or more and 1280 ° C. or less for 2 hours or more and 5 hours or less, and then subjected to rough hot rolling to form a rough bar having a thickness of 36 mm or more, Descale treatment is performed with the coarse bar at 1100 ° C or higher, and finish hot rolling is performed to complete the rolling at a temperature range of 860 ° C or higher and 950 ° C or lower, and then winding is performed at a temperature range of 420 ° C or higher and 570 ° C or lower. Hot rolling process to produce hot rolled steel sheet;
(C) Pickling / cold rolling step of pickling and cold rolling the hot-rolled steel sheet obtained by the hot rolling step to obtain a cold-rolled steel plate; and (D) The pickling / cold rolling step. To the temperature range of Ac ₃ points to 880 ° C. at an average heating rate of 1.0 ° C./second or more and hold in the temperature range for 10 seconds to 200 seconds, It is cooled to a temperature range of 600 ° C. to 740 ° C. at an average cooling rate of 1 ° C./second to 15 ° C./second, and further, 330 ° C. to 500 ° C. at an average cooling rate of 20 ° C. to 200 ° C./second. An annealing process in which a heat treatment is performed by cooling to the following temperature range and holding in the temperature range for 20 seconds to 500 seconds.

  Here, the “crack” at the time of bending molding refers to a state in which a crack having a depth of 10 μm or more and a width of 15 μm or more has occurred on the surface of the bent portion after bending.

  Hereinafter, the present invention will be described in more detail. In the following description, all percentages relating to the chemical composition of steel are mass%.

1. Chemical composition (C: 0.09% or more and 0.16% or less)
C is an element that has the effect of suppressing the ferrite transformation and promoting the bainite transformation and contributes to obtaining a steel structure to be described later. Moreover, it is an element which has the effect | action which raises the intensity | strength of a steel plate. If the C content is less than 0.09%, it is difficult to ensure predetermined characteristics. Therefore, the C content is 0.09% or more. On the other hand, if the C content exceeds 0.16%, the strength of the nugget portion in resistance welding is increased and the strength of the welded portion is significantly reduced. Therefore, the C content is 0.16% or less. Preferably it is 0.15% or less.

(Si: 0.05% or more and 0.60% or less)
Si is an element that contributes to strength improvement, and has an effect of promoting the formation of ferrite, and is an element that contributes to obtaining a steel structure to be described later. If the Si content is less than 0.05%, not only is it difficult to stably secure a tensile strength of 980 MPa or more, but the amount of ferrite produced may be insufficient. Therefore, the Si content is set to 0.05% or more. Preferably it is 0.2% or more. On the other hand, if the Si content exceeds 0.60%, the ferrite area ratio becomes excessive, and the desired yield strength may not be obtained. Therefore, the Si content is not more than 0.60%.

(Mn: 1.95% to 3.00%)
Mn is an element that has an action of promoting bainite transformation by suppressing ferrite transformation and contributes to obtaining a steel structure to be described later. If the Mn content is less than 1.95%, it is difficult to ensure predetermined characteristics. Therefore, the Mn content is 1.95% or more. On the other hand, if the Mn content is more than 3.00%, the strength of the welded nugget is increased, and the strength of the weld is significantly reduced. Therefore, the Mn content is 3.00% or less.

(P: 0.02% or less)
P is an element contained as an impurity, and has an action of causing segregation in the nugget of resistance welding and reducing the toughness of the nugget portion. When the P content exceeds 0.02%, the toughness of the nugget portion in resistance welding is significantly reduced. Therefore, the P content is 0.02% or less.

(S: 0.01% or less)
S is an element contained as an impurity and has an effect of reducing the toughness of the nugget portion of resistance welding. Moreover, MnS is formed in steel and the workability of a steel plate is reduced. If the S content exceeds 0.01%, the toughness of the nugget portion in resistance welding will be significantly reduced, and the workability of the steel sheet will be significantly reduced. Therefore, the S content is 0.01% or less.

(Al: 0.02% to 0.45%)
Al is an element having an action of deoxidizing steel and refining steel in the steel refining process. Further, like Si, it is an element that promotes ferrite transformation. If the Al content is less than 0.02%, it is difficult to obtain the effect by the above action. Therefore, the Al content is set to 0.02% or more. When the Al content exceeds 0.45%, the deterioration of surface properties and workability due to the increase in oxide inclusions become significant. Therefore, the Al content is 0.45% or less. Preferably it is 0.40% or less. The Al content in the steel in the present invention is acid-soluble Al (sol. Al).

(Ti: 0.01% or more and 0.2% or less)
Ti has the effect | action which improves the workability of a steel plate by forming a fine precipitate in steel and refining the crystal grain of a steel plate. If the Ti content is less than 0.01%, it is difficult to obtain the effect by the above action. Therefore, the Ti content is set to 0.01% or more. Preferably it is 0.02% or more. On the other hand, if the Ti content exceeds 0.20%, the effect of the above action is saturated, which is disadvantageous in terms of cost. Therefore, the Ti content is 0.2% or less. Preferably it is 0.08% or less.

(N: 0.01% or less)
N is an element contained as an impurity, and has the effect of forming coarse nitrides in the steel and reducing the workability of the steel sheet. When the N content is more than 0.01, the workability of the steel sheet is significantly reduced. Therefore, the N content is 0.01% or less.

(Cr: 0.02% to 1.0%, Mo: 0.01% to 2.0% and B: one or more selected from 0.0003% to 0.01%)
These elements are elements that have the effect of promoting the bainite transformation by enhancing the hardenability of the steel sheet and suppressing the ferrite transformation, and contribute to obtaining a steel structure to be described later. When the Cr content is less than 0.02%, the Mo content is less than 0.01%, and the B content is less than 0.0003%, it is difficult to obtain the effect by the above-described action. Therefore, one or more selected from the group consisting of Cr: 0.02% or more, Mo: 0.01% or less, and B: 0.0003% or more are contained. When Cr is contained, the Cr content is preferably 0.1% or more. When Mo is contained, the Mo content is preferably 0.05% or more, and B is contained. In such a case, the B content is preferably set to 0.0005% or more. However, when the Cr content is more than 1.0%, the chemical conversion treatment is significantly deteriorated. Therefore, the Cr content is 1.0% or less. Preferably it is 0.9% or less. Further, even if the Mo content exceeds 2.0% or the B content exceeds 0.01%, the effect by the above action is saturated, and the manufacturing cost is unnecessarily increased. Therefore, the Mo content is 2.0% or less, and the B content is 0.01% or less. The Mo content is preferably 1.6% or less, and the B content is preferably 0.008% or less.

(0.15 ≦ [Si] + [Al] ≦ 0.64)
As described above, Si and Al are elements that promote the formation of ferrite, and have the effect of increasing the ferrite area ratio and increasing the total elongation. When the value of [Si] + [Al], which is the total content of Si and Al, is less than 0.15%, it is difficult to secure a predetermined ferrite area ratio and ensure excellent total elongation. Therefore, the total content of Si and Al is 0.15% or more. Preferably it is 0.30% or more. On the other hand, if the value of [Si] + [Al] exceeds 0.64%, the ferrite area ratio becomes excessive, and it may be difficult to ensure the intended yield strength. Therefore, the total content of Si and Al is 0.64% or less.

(Cu: one or two selected from 1.0% or less and Ni: 1.0% or less)
These elements are optional elements and have the effect of increasing the strength of the steel sheet by increasing the hardenability of the steel sheet. Therefore, in order to easily ensure a tensile strength of 980 MPa or more, one or more of these elements may be contained. However, even if the Cu content is more than 1.0% or the Ni content is more than 1.0%, the effect by the above action is saturated, and the manufacturing cost is unnecessarily increased. Therefore, the Cu content is 1.0% or less, and the Ni content is 1.0% or less. More preferably, the Cu content is 0.8% or less, and the Ni content is 0.8% or less. In order to more reliably obtain the effect of the above action, it is preferable to contain Cu in an amount of 0.05% or more and Ni in an amount of 0.05% or more.

(Nb: one or two selected from 0.10% or less and V: 0.10% or less)
These elements are optional elements, and have the effect of improving the workability of the steel sheet by forming fine precipitates in the steel and refining the crystal grains of the steel sheet in the same manner as Ti. Therefore, in order to ensure better processability, one or two of these elements may be contained. However, even if the Nb content exceeds 0.10% or the V content exceeds 0.10%, the effect of the above action is saturated, and the cost is unnecessarily increased. Therefore, the Nb content is 0.10% or less, and the V content is 0.10% or less. The Nb content is preferably 0.05% or less, and the V content is preferably 0.08% or less. In order to more reliably obtain the effect of the above action, it is preferable to contain Nb in an amount of 0.02% or more and V in an amount of 0.02% or more.

(REM: 0.10% or less, Mg: 0.01% or less, and Ca: one or more selected from 0.01% or less)
These elements are arbitrary elements, and are elements that can spheroidize inclusions such as sulfides and oxides to make harmless deterioration of formability due to inclusions. Moreover, since it becomes an oxide which becomes a production nucleus of nitrides such as TiN, TiN can be finely dispersed and can be made harmless with respect to deterioration of moldability. Therefore, you may contain 1 type, or 2 or more types of these elements. However, even if the REM content exceeds 0.10%, the Mg content exceeds 0.01%, or the Ca content exceeds 0.01%, the effect by the above action is saturated, Unnecessarily increases the manufacturing cost. Therefore, the REM content is 0.10% or less, the Mg content is 0.01% or less, and the Ca content is 0.01% or less. In order to more reliably obtain the effect of the above action, it is preferable that REM is contained in an amount of 0.0001% or more, Mg is 0.0001% or more, and Ca is 0.0001% or more.

  Here, REM refers to a total of 17 elements of Sc, Y, and lanthanoid. In the case of lanthanoid, it is added industrially in the form of misch metal. In the present invention, the content of REM refers to the total content of these elements.

(Bi: 0.05% or less)
Bi serves as an inoculation nucleus for coagulation, and has the effect of reducing the interval between dendritic arms during coagulation and making the coagulated tissue finer. As a result, segregation of easily segregating elements such as Mn and Si is suppressed, the local strength difference of the steel sheet is improved, and bending workability is improved. Therefore, Bi may be included. However, since Bi forms an oxide in the steel, if the Bi content exceeds 0.05%, the amount of oxide in the steel increases, and the oxide becomes the starting point of cracking, which deteriorates the bending workability. Therefore, the Bi content is set to 0.05% or less. Preferably it is 0.03% or less. In addition, in order to acquire the effect by the said action | operation more reliably, it is preferable to make Bi content 0.0003% or more.

2. Steel structure In this invention, a steel structure means the steel structure in the position of 1/4 of plate | board thickness from the steel plate surface which shows the average steel structure in steel.

(Bainite area ratio: 60% or more, ferrite area ratio: 6% or more and less than 40%, residual austenite volume ratio: 3% or less)
In order to set the yield strength within an appropriate range, the bainite area ratio is set to 60% or more and the ferrite area ratio is set to 6% or more and 40% or less. Here, setting the ferrite area ratio to 6% or more is important in order to ensure excellent total elongation.

  Further, in order to ensure excellent bendability, the retained austenite area ratio is set to 3% or less. This is because the retained austenite undergoes plastic-induced transformation during processing to become hard martensite, resulting in a decrease in bendability. Accordingly, the retained austenite area ratio is preferably as small as possible, and may be 0%.

(Average particle size of bainite and ferrite: 5 μm or less, maximum value of particle size of bainite and ferrite: 9 μm or less)
The grain sizes of bainite and ferrite are very important for ensuring bendability.
By reducing the grain size of bainite and ferrite, the strain during bending is dispersed and excellent bendability can be obtained. Therefore, the average particle size of bainite and ferrite is 5 μm or less, and the maximum value of the particle size of bainite and ferrite is 9 μm or less.

3. Average Mn concentration (Mn ^sur ) in the surface layer from the steel sheet surface to a depth of 5 μm
(Mn ^sur ≦ 2.60 and Mn ^sur /[Mn]≦0.90)
When the Mn concentration in the steel sheet surface layer portion is high, stress concentration is likely to occur during bending forming due to Mn segregation during slab manufacturing, and cracking is likely to occur. When Mn ^sur (unit: mass%), which is the average Mn concentration in the surface layer part from the steel sheet surface to a depth of 5 μm, exceeds 2.60%, the Mn concentration in the steel sheet surface layer part is reduced by the manufacturing method described later. ^{Even if sur} satisfies the following formula (3), it is difficult to suppress a decrease in bendability due to Mn segregation in the steel sheet surface layer portion, and it is difficult to ensure excellent bendability. Therefore, Mn ^sur satisfies the following formula (2).

On the other ^hand, even if ^{Mn sur} satisfies the following formula ^(2), the Mn sur /[Mn]>0.90, it is difficult to suppress a reduction in bending resistance due to the Mn segregation of the steel sheet surface layer portion, excellent It is difficult to ensure high bendability. Therefore, Mn ^sur shall satisfy the following formula (3).

Mn ^sur ≦ 2.60 (2)
Mn ^sur /[Mn]≦0.90 (3)
Here, [Mn] is the Mn content (unit: mass%) in the steel, and Mn ^sur is the average Mn concentration (mass%) in the surface layer part from the steel sheet surface to the depth of 5 μm as described above.

4). Mechanical properties (Tensile strength: 980 MPa or more)
For a cold-rolled steel sheet having a tensile strength of less than 980 MPa, there is little problem of a decrease in formability when the yield strength intended by the present invention is within an appropriate range. Therefore, in order to clarify the object of the present invention, the tensile strength is set to 980 MPa or more.

(Yield strength: 690 MPa or more and 850 MPa or less)
If the yield strength is less than 690 MPa, the rigidity of the parts is insufficient particularly in seat rail applications, and safety at the time of collision cannot be ensured. Therefore, the yield strength is 690 MPa or more. On the other hand, when the yield strength exceeds 850 MPa, the amount of springback increases, and the dimensional accuracy of the part cannot be ensured. Therefore, the yield strength is 850 MPa or less.

(Total elongation: 12% or more)
If the total elongation is less than 12%, cracks may occur during stretch flange molding or draw molding. Therefore, the total elongation is 12% or more.

(Bendability: R / t ≦ 1.0)
When R / t> 1.0, cracks may occur when severe bending is performed. Therefore, it shall have the bendability which satisfies following formula (4). Preferably, it has bendability that satisfies the following formula (4-1).

R / t ≦ 1.0 (4)
R / t ≦ 0.5 (4-1)
Here, R is the minimum inner radius that does not cause cracks in the bending test by the V-block method with a bending angle of 90 °, t is the plate thickness, and the unit is mm. In addition, as described above, “cracking” refers to a state in which a crack having a depth of 10 μm or more and a width of 15 μm or more has occurred on the surface of the bent portion after bending in the bending test.

5. Plating layer The cold-rolled steel sheet according to the present invention may be a surface-treated steel sheet provided with a plating layer on the surface of the steel sheet for the purpose of improving corrosion resistance. The plating layer may be an electroplating layer or a hot dipping layer. Examples of electroplating include electrogalvanizing and electro-Zn-Ni alloy plating. Examples of the hot dip plating include hot dip galvanizing, alloyed hot dip galvanizing, hot dip aluminum plating, hot dip Zn-Al alloy plating, hot dip Zn-Al-Mg alloy plating, hot dip Zn-Al-Mg-Si alloy plating and the like. . The amount of plating adhesion is not particularly limited, and may be the same as the conventional one. Further, it is possible to further improve the corrosion resistance by performing an appropriate chemical conversion treatment (for example, application and drying of a silicate-based chromium-free chemical conversion treatment solution) after plating.

6). Production Method The cold-rolled steel sheet according to the present invention may be any sheet that satisfies the above chemical composition, steel structure, element concentration of the steel sheet surface layer portion and mechanical properties, and the production method is not particularly limited. It is preferred to manufacture by the method described.

  This manufacturing method includes a casting process, a hot rolling process, a pickling / cold rolling process, and an annealing process, as in a general method for manufacturing a cold-rolled steel sheet.

(Casting process)
In the casting process, in order to suppress Mn segregation in the surface layer portion that occurs in the casting process, the molten steel having the above chemical composition is moved from the liquidus temperature to the solidus temperature at a position 10 mm deep from the slab surface. Casting is performed under the condition that the average cooling rate in the temperature range is 10 ° C./second or more.

  The temperature range from the liquidus temperature to the solidus temperature is the temperature range from the start of solidification to the end of solidification. Therefore, the average cooling rate in this temperature range means the average solidification rate of the slab. The solidification rate at a depth of 10 mm from the surface of the slab is used as an index. Of course, the temperature of the surface of the slab in contact with the cooling medium (usually cooling water) varies greatly and is not suitable as an index for the cooling rate. Because. The average cooling rate within the temperature range from the liquidus temperature to the solidus temperature at a depth of 10 mm from the slab surface is 10 mm from the slab surface by etching the cross section of the obtained slab with picric acid. Measure the dendrite secondary arm interval λ (μm) at the depth of the temperature, and based on the following formula, the average cooling rate A (° C. in the temperature range from the liquidus temperature to the solidus temperature of the molten steel / Sec).

λ = 710 × A ^−0.39
When the average cooling rate in the above temperature range is less than 10 ° C./second, solidification is too slow, so that the slab dendrite trees spread, the segregation in the width direction of Mn expanded in the rolling direction in the final product increases, and bending workability Deteriorate. Therefore, the average cooling rate is set to 10 ° C./second or more. Preferably, it is 15 ° C./second or more. The average cooling rate can be achieved by, for example, increasing the amount of cooling water used for water cooling immediately after casting.

(Hot rolling process)
In the hot rolling process, in order to reduce the Mn concentration in the surface layer portion and to obtain the target steel structure in the final product, the slab obtained by the casting process is set to a temperature range of 1180 ° C. or higher and 1280 ° C. or lower. After holding for at least 5 hours and after rough hot rolling, a rough bar with a thickness of 36 mm or more is formed, the rough bar is subjected to descaling treatment at 1100 ° C. or higher, and a temperature range of 860 ° C. or higher and 950 ° C. or lower Then, finish hot rolling to complete the rolling is performed, and then wound in a temperature range of 420 ° C. or higher and 570 ° C. or lower to obtain a hot rolled steel sheet.

A) Temperature maintenance before rough hot rolling The cast slab used for rough hot rolling concentrates Mn in the surface layer portion in the oxide scale, and removes this scale in the descaling step in the subsequent step. In order to reduce the Mn concentration and to sufficiently disperse Mn in the surface layer portion by diffusing Mn sufficiently, it is kept in a temperature range of 1180 ° C. or more for 2 hours or more. On the other hand, the upper limit of the temperature range is set to 1280 ° C. and the upper limit of the holding time is set to 5 hours in order to avoid a decrease in yield due to overproduction of scale and to reduce heating costs.

B) Rough hot rolling to descaling After the hot slab is subjected to rough hot rolling to obtain a rough bar having a thickness of 36 mm or more, the obtained rough bar is formed on the surface thereof. In order to remove the oxidized scale, descaling is performed at 1100 ° C. or higher. This descaling process is typically performed by high pressure water injection.

  Since firelite is formed in the form of a film at the interface between the coarse bar surface and the oxide scale, the temperature of the coarse bar is set to a temperature near the melting point of the firelite, so that the heat before finish hot rolling and finish heat In the descaling process during the hot rolling, the firelite and the oxide scale are easily removed from the surface of the coarse bar.

  If oxidized scale remains on the rough bar surface, the oxidized scale thickness on the surface of the steel sheet after finish hot rolling increases, and the subsequent oxidation rate of the grain boundaries increases, resulting in cracks on the surface of the steel sheet after annealing. This causes a decrease in bendability. For this reason, before the finish rolling, the oxidized scale of the coarse bar is once removed. At the time of removing the oxide scale, Mn concentrated in the scale is removed, so that a decrease in the Mn concentration in the surface layer portion can be achieved.

  Here, if the thickness of the rough bar is less than 36 mm, the rolling reduction in the finish hot rolling becomes low, so the passing speed of the rough bar to be used for finish hot rolling becomes high, and the reheating before the finish rolling is insufficient. Thus, it may be difficult to secure the temperature of 1100 ° C. or higher necessary for descaling over the entire length of the coil. Accordingly, the thickness of the coarse bar is set to 36 mm or more.

C) Finish hot rolling Finish hot rolling is performed on the descaled rough bar to obtain a hot rolled steel sheet having a predetermined thickness. The finish hot rolling completion temperature is set to 860 ° C. or higher in order to suppress hunting due to ferrite transformation during hot rolling and to ensure good workability for the steel sheet after cold rolling and annealing. Moreover, in order to suppress excessive grain growth and to obtain the target mechanical characteristics for the steel sheet after cold rolling and annealing, the temperature is set to 950 ° C. or less.

D) From finish hot rolling to winding The conditions from finish hot rolling completion to winding are not specified, but the surface temperature of the hot rolled steel sheet should be 600 ° C or less within 10 seconds after finishing hot rolling is completed. When cooled to a low temperature, the formation of ferrite and pearlite in the structure of the hot-rolled steel sheet is suppressed, and the steel structure is more uniform and refined, making it easy to obtain a suitable steel structure in the final product. This is preferable. Such cooling can be achieved by increasing the amount of cooling water within 10 seconds after completion of finish hot rolling.

E) Winding The winding temperature is very important for obtaining a desired structure by making the structure of the hot-rolled steel sheet uniform and fine and controlling the structure in the subsequent annealing. When the coiling temperature exceeds 570 ° C., ferrite and pearlite are generated in the structure of the hot-rolled steel sheet, making it difficult to make the structure uniform and fine. Moreover, the progress of the overall oxidation and grain boundary oxidation of the steel plate surface layer portion becomes remarkable, and fine cracks are generated on the steel plate surface after pickling and cold rolling, and the bendability may be lowered. Therefore, the coiling temperature is 570 ° C. or less. Preferably it is 520 degrees C or less. On the other hand, when the coiling temperature is less than 420 ° C., martensite is generated in the hot-rolled steel sheet, and the steel sheet is easily flattened or broken in cold rolling. In addition, it becomes difficult to make the microstructure fine after annealing. Accordingly, the coiling temperature is 420 ° C. or higher. Preferably it is 440 degreeC or more.

(Pickling / Cold rolling process)
The hot-rolled steel sheet obtained by the hot rolling process is pickled and cold-rolled to obtain a cold-rolled steel sheet. Pickling and cold rolling may be carried out according to conventional methods. The conditions for the cold rolling need not be specified, but the rolling reduction is preferably 20% or more from the viewpoint of obtaining an appropriate texture in order to achieve workability. The rolling reduction is more preferably 30% or more. The upper limit of the rolling reduction is not particularly specified, but is usually 90% or less.

(Annealing process)
The cold-rolled steel sheet obtained by the pickling / cold rolling step is heated to a temperature range of Ac ₃ points to 880 ° C. at an average heating rate of 1.0 ° C./second or more, and the temperature range is maintained for 10 seconds or more. Hold for 200 seconds or less, then cool to a temperature range of 600 ° C. or more and 740 ° C. or less at an average cooling rate of 1 ° C./second or more and 15 ° C./second or less, and further average 20 ° C./second or more and 200 ° C./second or less A heat treatment is performed by cooling to a temperature range of 330 ° C. or more and 500 ° C. or less at a cooling rate and maintaining the temperature range for 20 seconds or more and 500 seconds or less. This heat treatment is intended for annealing and is generally performed in a continuous annealing facility.

The average heating rate up to a temperature below the range Ac _three or more points 880 ° C. In the (heating also called speed) is less than 1.0 ° C. / sec, precipitation ends up coarsening, Ac ₃ point or more 880 ° C. below the temperature range When retained, the austenite grows coarsely and the moldability of the final product deteriorates. Therefore, the average heating rate up to a temperature range of Ac ₃ points or more and 880 ° C. or less is set to 1.0 ° C./second or more. Preferably it is 5.0 degrees C / sec or more. The upper limit of the average heating rate is not particularly required, but is preferably set to 100 ° C./second or less from the viewpoint of industrial production from the viewpoint of industrial production.

Ac If the maximum temperature in the annealing process is set to the two-phase region temperature without heating to a temperature range of ₃ points or higher, C concentration in the austenite proceeds excessively, and the hardness difference between the structures increases. In addition, since the influence of the cold-rolled structure remains and a band structure is formed, the bendability of the final product is deteriorated. Therefore, the highest temperature reached in the annealing process is set to Ac ₃ points or higher. On the other hand, if the maximum temperature reached in the annealing process exceeds 880 ° C., the crystal grains become coarse, and the desired characteristics cannot be obtained in the final product. Therefore, the highest temperature reached in the annealing process is 880 ° C. or less. Preferably it is 870 degrees C or less.

Therefore, in the annealing process, it is held in a temperature range of Ac ₃ points or more and 880 ° C. or less. However, if the holding time (also referred to as annealing time) at this time is less than 10 seconds, the structure of the hot rolled steel sheet is affected. Since it remains strongly, the structure of the final product becomes non-uniform, and the formability of the final product deteriorates. Therefore, the holding time is 10 seconds or longer. Preferably it is 30 seconds or more. On the other hand, if the holding time exceeds 200 seconds, the crystal grains become coarse, and the desired characteristics cannot be obtained in the final product. Therefore, the holding time is 200 seconds or less.

If the average cooling rate from the temperature range of Ac ₃ points to 880 ° C. to the temperature range of 600 ° C. to 740 ° C. is less than 1 ° C./second, pearlite is generated in the cooling process, and the final product has predetermined characteristics. It may not be obtained. Therefore, the average cooling rate is set to 1 ° C./second or more. On the other hand, when the average cooling rate exceeds 15 ° C./second, the generation of ferrite becomes insufficient, and the yield strength of the steel sheet after continuous annealing may be higher than desired. Therefore, the average cooling rate is set to 15 ° C./second or less.

  If the average cooling rate from the temperature range of 600 ° C. to 740 ° C. to the temperature range of 330 ° C. to 500 ° C. is less than 20 ° C./second, ferrite may be excessively generated during the cooling process. Therefore, the average cooling rate is set to 20 ° C./second or more. On the other hand, when the average cooling rate exceeds 200 ° C./sec, unevenness in the cooling rate is likely to occur in the steel sheet, causing variation in characteristics. Therefore, the average cooling rate is set to 200 ° C./second or less.

  That is, the cooling from the annealing temperature is a slow cooling with an average cooling rate of 1 ° C./second to 15 ° C./second until a temperature range of 600 ° C. to 740 ° C., and then a temperature range of 330 ° C. to 500 ° C. The cooling to is rapid cooling with an average cooling rate of 20 ° C./second or more and 200 ° C./second or less. Generally, slow cooling may be performed by gas jet cooling, and rapid cooling may be performed by air-water cooling or roll cooling. Control of the cooling rate is well known to those skilled in the art.

  When the holding time in the temperature range of 330 ° C. or more and 500 ° C. or less is less than 20 seconds, the transformation amount of bainite becomes insufficient, martensite or retained austenite is excessively generated, and it is difficult to secure predetermined characteristics. There is a case. Therefore, the holding time is 20 seconds or longer. Preferably it is 40 seconds or more. On the other hand, if the holding time exceeds 500 seconds, energy loss becomes significant and productivity is lowered. Accordingly, the holding time is 500 seconds or less.

  If the holding temperature is less than 330 ° C., the amount of transformation of bainite becomes insufficient, the martensitic transformation proceeds excessively, or the untransformed austenite remains excessively, making it difficult to ensure predetermined characteristics. . Therefore, holding temperature shall be 330 degreeC or more. Preferably it is 340 degreeC or more. On the other hand, when the holding temperature exceeds 500 ° C., upper bainite having very low toughness is generated, and thus predetermined characteristics cannot be obtained. Accordingly, the holding temperature is 500 ° C. or lower. Preferably it is 460 degrees C or less.

  After holding this temperature, it may be cooled to near room temperature and wound up. However, when hot-dip plating is performed as described below, hot-dip plating may be carried out as it is after holding the temperature. In that case, an annealing process can be implemented in the annealing apparatus in a continuous hot dipping plating equipment.

(Plating process-optional)
As described above, the cold-rolled steel sheet according to the present invention may be a surface-treated steel sheet by forming a plating layer on the surface thereof. In that case, the cold-rolled steel sheet produced by the above method is plated according to a conventional method. The plating type is not particularly limited, but is generally zinc-based plating (zinc plating or zinc alloy plating) excellent in corrosion resistance. The plating method is not particularly limited, but is generally hot dipping or electroplating. When plating is hot dipping, hot dipping can be carried out following the annealing step.

  The steel having the chemical composition shown in Table 1 is melted and cast into a slab. The slab is hot-rolled, and the obtained hot-rolled steel sheet is pickled in a conventional manner, followed by cold rolling and continuous Annealing was performed to obtain various cold-rolled steel sheets. Table 2 summarizes the conditions for each process of casting, hot rolling, cold rolling, and continuous annealing.

  In Table 2, the average cooling rate in the temperature range from the liquidus temperature to the solidus temperature at a position 10 mm deep from the slab surface in the casting process is the liquid phase at a position 10 mm deep from the slab surface. The average cooling rate in the temperature range from the line temperature to the solidus temperature is obtained by etching the cross section of the obtained slab with picric acid, and the dendrite secondary arm interval λ () at a depth of 10 mm from the slab surface. μm) was measured, and the average cooling rate A (° C./sec) in the temperature range from the liquidus temperature to the solidus temperature of the molten steel was determined from the value based on the following formula.

λ = 710 × A ^−0.39
The heating temperature of the coarse bar in the hot rolling process is a heating temperature for descaling, and after this heating, descaling was performed by a high-pressure water jet according to a conventional method, and then finish rolling was performed. Cooling after completion of finish rolling was performed by water cooling.

  Steel plate temperature 10 seconds after completion of finish hot rolling (temperature after 10 seconds of rolling in Table 2), data and through plate of radiation thermometer installed at finish rolling exit, ROT (runout table) and winding position Calculated by interpolation of velocity data. As described above, when the steel plate temperature is 600 ° C. or less, the steel structure is made uniform and refined.

  In the continuous annealing step, heating was performed at the indicated temperature increase rate (average heating rate) to the indicated annealing temperature, and this temperature was maintained for the time indicated as the annealing time, and then cooled. Cooling is performed by gas jet cooling at the indicated average cooling rate (1) from the annealing temperature (in the austenite single-phase state in the invention example) to a temperature range of 600 ° C. or higher and 740 ° C. or lower. Cooling was stopped at the indicated average cooling rate (2) until the cooling stop / low temperature holding temperature, and the cooling stop temperature was held for the time indicated as the low temperature holding time, and then cooled to room temperature.

  In Table 2, the “structure during annealing” in the continuous annealing process is the heat treatment of the test pieces collected from each cold-rolled steel sheet at the heating rate, annealing temperature, and annealing time shown in Table 2, and the change in expansion coefficient at that time is shown. The result of analyzing to determine whether it is austenite single phase (γ in the table) or a multiphase structure of austenite and other structures (γ + α in the table is a multiphase structure of austenite and ferrite) Indicates.

  The following test was done about the obtained cold-rolled steel plate. These test results are summarized in Table 3.

(1) Tensile test From each cold-rolled steel sheet, a JIS No. 5 tensile test piece having a direction perpendicular to the rolling direction as the longitudinal direction was sampled and examined for tensile properties (yield strength YP, tensile strength TS, total elongation El).

(2) Observation of steel plate cross section The structure of the steel plate is a cross section in the direction parallel to and perpendicular to the rolling at 2000 times from the steel plate surface at the 1/4 position in the plate width direction at a thickness of 1/4 using a SEM. Fifty visual fields were observed, and the area fraction of each phase and structure and the particle sizes of ferrite and bainite were measured by image analysis. The measurement of the particle diameter was carried out in accordance with the cross line segment method of JISG0552, and expressed as an average value and a maximum value.

  The volume ratio of retained austenite was obtained by reducing the thickness of each steel sheet by 0.3 mm by chemical polishing, subjecting the surface after chemical polishing to X-warp diffraction, and calculating the amount of retained austenite.

(3) Bendability A JIS No. 1 bending test specimen having a longitudinal direction perpendicular to the rolling direction as a longitudinal direction was sampled from each cold-rolled steel sheet, and the bendability was investigated by a V-block method in accordance with JIS Z 2248. Judgment of the crack was carried out according to the above-mentioned criteria by examining the surface and cross section of the bent portion using an optical microscope and SEM.

(4) Average value of Mn concentration from the steel sheet surface to the surface layer depth of 5 μm (Mn ^sur )
Using GDS, the Mn concentration from the steel sheet surface to the surface layer depth of 5 μm was measured (n = 10), and the average value was calculated.

  Steel plates No. 1 and 14 to 22 are examples according to the present invention, the tensile strength TS is 980 MPa or more, the yield strength YP is 690 MPa or more and 850 MPa or less, the total elongation El is 12% or more, and the bendability is R / t ≦ 1.0 (R is the minimum inner radius that does not cause cracks in the bending test by the V-block method with a bending angle of 90 °, and t is the plate thickness) and has excellent formability. It was proved to be a high-strength cold-rolled steel sheet.

  On the other hand, when the comparative example was seen, bendability was low in steel plate Nos. 2-9. The reason for this is that steel plate No. 2 has an average cooling rate in the temperature range from the liquidus temperature to the solidus temperature at a depth of 10 mm from the surface of the slab, so steel plate No. 3 has a slab heating time. Because the surface layer Mn concentration was short and the reheating temperature after rough rolling was insufficient for steel plate No. 4, steel plate No. 5 had a high coiling temperature and a non-uniform structure at the hot rolling stage. Since coarse crystal grains were produced after annealing, steel sheet No. 6 had a low coiling temperature and became a non-uniform structure in the hot rolling stage, and coarse crystal grains were produced after annealing. Since the temperature rise rate of the steel plate No. 8 was slow and the crystal grains became coarse, the steel plate No. 8 was a two-phase region annealing, so the influence of the cold rolled structure remained and a band structure was formed. The annealing temperature is too high, and the austenite during annealing grows and cools. This is because the crystal grain size of increased.

  In steel plate No. 10, the average cooling rate from the austenite single-phase structure state to the temperature range of 600 to 740 ° C. was too high, so ferrite formation was insufficient, the yield strength of the steel plate after continuous annealing was too high, and the total elongation was Became lower. In steel plate No. 11, the average cooling rate from the temperature range of 600 to 740 ° C. to the temperature range of 330 to 500 ° C. was too low, so ferrite was generated excessively in the cooling process, yield strength was lowered and bendability was also low. It was low. Steel plate No. 12 is too low in cooling stop and low temperature holding temperature, and since steel plate No. 13 is too short in low temperature holding time, both bainite generation is insufficient and the martensitic transformation proceeds excessively. Like No. 11, predetermined characteristics were not obtained.

  Steel plates Nos. 23 to 28 are comparative examples in which the chemical composition of the steel was outside the scope of the present invention. Since No. 23 had a small amount of C, the desired strength could not be obtained. Since No. 24 had a small amount of Si, the desired metal structure and characteristics could not be obtained. No. 25 had too much total amount of Si and Al, and the desired metal structure and characteristics were not obtained. Since No. 26 had a small amount of Mn, the desired strength could not be obtained. No. 27 had a small amount of Ti, so the crystal grains became coarse and desired characteristics could not be obtained. In No. 28, Cr, Mo, and B were not added, so the desired strength could not be obtained.

Claims (7)

In mass%, C: 0.09% to 0.16%, Si: 0.05% to 0.60%, Mn: 1.95% to 3.00%, P: 0.02% or less , S: 0.01% or less, Al: 0.02% or more and 0.45% or less, Ti: 0.01% or more and 0.2% or less, N: 0.01% or less, and the following formula (1 In addition, Cr is selected from the group consisting of 0.02% to 1.0%, Mo: 0.01% to 2.0%, and B: 0.0003% to 0.01%. A chemical composition containing one or more kinds, the balance being Fe and impurities,
In area%, bainite: 60% or more, ferrite: 6% or more and 40% or less, retained austenite: 3% or less, the average particle size of the bainite and ferrite is 5 μm or less, and the maximum particle size of the bainite and ferrite A steel structure having a value of 9 μm or less,
Mn ^sur (unit: mass%) which is the average Mn concentration in the surface layer part from the steel sheet surface to a depth of 5 μm satisfies the following formulas (2) and (3),
A cold-rolled steel sheet having mechanical properties such that a tensile strength is 980 MPa or more, a yield strength is 690 MPa or more and 850 MPa or less, a total elongation is 12% or more, and a bendability satisfies the following formula (4).
0.15 ≦ [Si] + [Al] ≦ 0.64 (1)
Mn ^sur ≦ 2.60 (2)
Mn ^sur /[Mn]≦0.90 (3)
R / t ≦ 1.0 (4)
Here, [M] is the content of element M (unit: mass%), Mn ^sur is as described above, and R is not cracked in the bending test by the V-block method with a bending angle of 90 °. The minimum inner radius (unit: mm), t is the plate thickness (unit: mm).   The chemical composition contains one or two selected from the group consisting of Cu: 1.0% or less and Ni: 1.0% or less in mass%, instead of a part of the Fe. The cold rolled steel sheet according to 1.   The chemical composition contains one or two selected from the group consisting of Nb: 0.10% or less and V: 0.10% or less in mass%, instead of a part of the Fe. The cold-rolled steel sheet according to claim 1 or 2.   The chemical composition is selected from the group consisting of REM: 0.10% or less, Mg: 0.01% or less, and Ca: 0.01% or less in mass%, instead of a part of the Fe. Or the cold-rolled steel plate in any one of Claim 1 to 3 containing 2 or more types.   The cold-rolled steel sheet according to any one of claims 1 to 4, wherein the chemical composition contains Bi: 0.05% or less instead of a part of the Fe. A method for producing a cold-rolled steel sheet comprising the following steps (A) to (D):
(A) An average of the molten steel having the chemical composition according to any one of claims 1 to 5 in a temperature range from a liquidus temperature to a solidus temperature at a depth of 10 mm from a slab surface. A casting process in which the casting rate is 10 ° C./second or more;
(B) The slab obtained by the casting step is held in a temperature range of 1180 ° C. or more and 1280 ° C. or less for 2 hours or more and 5 hours or less, and then subjected to rough hot rolling to form a rough bar having a thickness of 36 mm or more, Descale treatment is performed with the coarse bar at 1100 ° C or higher, and finish hot rolling is performed to complete the rolling at a temperature range of 860 ° C or higher and 950 ° C or lower, and then winding is performed at a temperature range of 420 ° C or higher and 570 ° C or lower. Hot rolling process to produce hot rolled steel sheet;
(C) Pickling / cold rolling step of pickling and cold rolling the hot-rolled steel sheet obtained by the hot rolling step to obtain a cold-rolled steel plate; and (D) The pickling / cold rolling step. To the temperature range of Ac ₃ points to 880 ° C. at an average heating rate of 1.0 ° C./second or more and hold in the temperature range for 10 seconds to 200 seconds, It is cooled to a temperature range of 600 ° C. to 740 ° C. at an average cooling rate of 1 ° C./second to 15 ° C./second, and further, 330 ° C. to 500 ° C. at an average cooling rate of 20 ° C. to 200 ° C./second. An annealing process in which a heat treatment is performed by cooling to the following temperature range and holding in the temperature range for 20 seconds to 500 seconds.   The manufacturing method of the cold rolled sheet steel of Claim 6 which has a plating process after the said annealing process (D).