WO2023037878A1 - Cold-rolled steel sheet and method for manufacturing same - Google Patents

Abstract

The present invention is a cold-rolled steel sheet characterized by having a prescribed chemical composition and being such that: a steel structure is, in terms of areal percentage, 90.0-99.5% martensite, 0-5% ferrite, and 0.5-7.0% residual austenite, the remainder being bainite; the percentage of tempered martensite in the total martensite is 80-100%; the maximum value of a curvature 1/R represented by formula (1) below and obtained by measuring the shape of a region equal to the full width by a length of 300 mm is 0.010 or less; and the tensile strength is 1470 MPa or more.

Description

Cold-rolled steel sheet and manufacturing method thereof

The present disclosure relates to cold-rolled steel sheets and manufacturing methods thereof.

In recent years, from the viewpoint of regulations on greenhouse gas emissions accompanying global warming countermeasures, there is a need to improve the fuel efficiency of automobiles. be. In recent years, in particular, there has been an increasing need for ultra-high-strength steel sheets with a tensile strength of 980 MPa or more and ultra-high-strength steel sheets with even higher tensile strength. In addition, high-strength hot-dip galvanized steel sheets with hot-dip galvanized surfaces are required for parts of vehicle bodies that require rust resistance.

However, when applying ultra-high-strength steel sheets with such high tensile strength as automotive parts, it is necessary to solve the problem of hydrogen embrittlement cracking of steel sheets as well as press formability.

　Hydrogen embrittlement cracking is a phenomenon in which steel members, which are subjected to high stress during use, suddenly break due to hydrogen entering the steel from the environment. This phenomenon is also called delayed fracture from the mode of occurrence of fracture. Generally, it is known that hydrogen embrittlement cracking of steel sheets is more likely to occur as the tensile strength of steel sheets increases. It is believed that this is because the higher the tensile strength of the steel sheet, the greater the stress remaining in the steel sheet after part forming. This susceptibility to hydrogen embrittlement cracking (delayed fracture) is called hydrogen embrittlement resistance.

Various attempts have been made to improve the hydrogen embrittlement resistance of steel sheets.

For example, in Patent Document 1, it has a predetermined chemical composition, and the value of the solid solution B amount solB [mass%] and the prior austenite grain size Dγ [μm] in the steel is expressed by the formula (1): solB Dγ≧0 .0010, and the area ratios are polygonal ferrite of 10% or less, bainite of 30% or less, retained austenite of 6% or less, and tempered martensite of 60% or more. The Fe carbide number density of 1 × 10 ⁶ /mm ² or more, the average dislocation density of the entire steel is 1.0 × 10 ¹⁵ /m ² or more and 2.0 × 10 ¹⁶ /m ² or less, and the effective grain An ultra-high-strength cold-rolled steel sheet having a tensile strength of 1300 MPa or more and excellent hydrogen embrittlement resistance is disclosed, which is characterized by having a steel structure with a diameter of 7.0 μm or less.

Patent Document 2 discloses that tempered martensite and bainite have a predetermined chemical composition, a total area ratio of 95% or more and 100% or less to the entire structure of tempered martensite and bainite, and are distributed in the rolling direction and/or in a dotted pattern. One or more long axes: composed of inclusion particles of 0.3 μm or more, and when the inclusion particles are composed of two or more, the distance between the inclusion particles is 30 μm or less, and the rolling direction Inclusion groups with total length exceeding 120 μm are 0.8/mm ² or less, aspect ratio is 2.5 or less, and major axis is 0.20 μm or more and 2 μm or less, mainly composed of Fe The number of carbides is 3500/mm ² or less, the number of carbides with a diameter of 10 to 50 nm distributed in the tempered martensite and/or the bainite is 0.7×10 ⁷ /mm ² or more, and prior γ grains A cold-rolled steel sheet having an average grain size of 18 μm or less, a thickness of 0.5 to 2.6 mm, and a tensile strength of 1320 MPa or more is disclosed. Further, Patent Document 2 describes that with the above configuration, it is possible to obtain an ultra-high-strength cold-rolled steel sheet having a tensile strength of 1300 MPa or more, which is excellent in resistance to hydrogen embrittlement.

In Patent Document 3, it has a predetermined chemical composition, and has a structure consisting of martensite: 90% or more and retained austenite: 0.5% or more in terms of area ratio to the entire structure, and the local Mn concentration is An ultra-high-strength steel sheet having an area ratio of 1.1 times or more of the total Mn content of 2% or more, a tensile strength of 1470 MPa or more, and excellent delayed fracture resistance at the cut edge. disclosed.

However, in recent years, when the level of demand has risen, there is a demand for further improvement in hydrogen embrittlement resistance while maintaining high tensile strength and total elongation. In particular, at present, there is a need to improve the hydrogen embrittlement resistance of the sheared portion.

As a result of diligent efforts to improve the hydrogen embrittlement resistance of sheared parts, the inventor obtained the following guidelines. It was found that the better the shape of the steel sheet, that is, the flatter the steel sheet, the better the hydrogen embrittlement resistance of the sheared portion. This is probably because shearing a non-flat steel sheet creates an angle between the punch and the steel sheet during shearing, increasing damage to the sheared portion.

In relation to this, the following documents, for example, are available as techniques for improving the shape of high-strength steel sheets.

Patent Document 4 describes an ultra-high-strength cold-rolled steel sheet having a predetermined chemical composition, a martensite single phase metal structure, a tensile strength of 980 MPa or more, and a flatness of 10 mm or less, and a method for producing the same. disclosed.

Patent Document 5 discloses a method for producing a high-strength cold-rolled steel sheet having a metal structure with a predetermined chemical composition and a tempered martensite content of 65 area% or more. An annealing step of heating and annealing in the phase region for 15 to 600 seconds, and after annealing, at an average cooling rate of 10 ° C./sec or less (excluding 0 ° C./sec) to the primary cooling stop temperature in the temperature range of 650 to 800 ° C. A primary cooling step of slow cooling, and an average cooling rate of 20 to 100 ° C./ A secondary cooling step of cooling in seconds, a tertiary cooling step of rapidly cooling from the secondary cooling stop temperature to room temperature at an average cooling rate of more than 100 ° C./s, and heating to a temperature range of 150 to 300 ° C. for 30 to 1500 seconds. A method for producing a high-strength cold-rolled steel sheet having an excellent steel sheet shape is disclosed, which includes a holding overaging treatment step in this order.

JP 2016-50343 A WO2016/152163 JP 2016-153524 A JP 2011-202195 A JP 2013-227657 A

However, the techniques disclosed in Patent Documents 4 and 5 do not improve the shape of the steel sheet with the intention of improving the hydrogen embrittlement resistance of the sheared portion. not enough to improve. In Patent Documents 4 and 5, the "maximum warpage height" is used as an index for evaluating the quality of the steel plate shape. It has been found that the hydrogen embrittlement resistance of is not necessarily excellent.

Therefore, an object of the present invention is to provide a cold-rolled steel sheet with improved resistance to hydrogen embrittlement while having high tensile strength and total elongation.

The present inventor believes that in order to improve the hydrogen embrittlement resistance of sheared parts, it is necessary to improve not the "maximum warpage height" of the steel sheet, but the "curvature", which is the amount that indicates the degree of curvature of the curved surface. I found something. Then, as a result of examining a method of manufacturing a steel sheet necessary for improving the curvature of the steel sheet, the following findings were obtained.
(1) In the hot rolling process, the edge portion is reheated after rough rolling. This suppresses fluctuations in the strength of the hot-rolled steel sheet in the width direction of the steel sheet. Furthermore, the steel sheet after finish rolling is wound up in an appropriate temperature range. As a result, the shape of the steel sheet after cold rolling is improved.
(2) In the cold rolling process, the forward tension and the backward tension in each rolling stand when passing through the rolling rolls are set to an appropriate range according to the yield strength of the hot-rolled steel sheet before cold rolling and the reduction ratio in each rolling stand. to control. Furthermore, the cumulative cold rolling reduction is controlled within an appropriate range. This improves the shape of the steel sheet after cold rolling.
(3) In the cooling process after heating and holding in the heat treatment process after the cold rolling process, the average cooling rate at 300 ° C. or less is limited to a predetermined range, gas is used as a coolant, and heat diffusion is prevented in the cooling process. Allow to cool to encourage. Furthermore, the average cooling rate between 300 and 700° C. and the cooling stop temperature must also be controlled within an appropriate range. In addition, the steel plate tension during cooling is controlled within an appropriate range. This improves the shape of the steel sheet after heat treatment.
When all of the above requirements (1) to (3) are satisfied, a steel sheet with an excellent level of shape that could not be achieved by existing techniques can be obtained.

The present invention has been realized based on the above findings, and is specifically as follows.
(1) in mass %,
C: 0.16 to 0.40%,
Si: 0.05 to 2.00%,
Mn: 0.50 to 4.00%,
P: 0.050% or less,
S: 0.0100% or less,
Al: 0.001 to 1.00%,
N: 0.0100% or less,
O: 0.0050% or less,
Cr: 0 to 2.00%,
Mo: 0 to 1.00%,
Cu: 0 to 1.00%,
Ni: 0 to 1.00%,
B: 0 to 0.0100%,
Co: 0 to 1.00%,
W: 0 to 1.00%,
Sn: 0 to 1.00%,
Sb: 0 to 1.00%,
Nb: 0 to 0.100%,
Ti: 0 to 0.200%,
V: 0 to 0.50%,
Ca: 0 to 0.0100%,
Mg: 0-0.0100%,
Ce: 0 to 0.0100%,
Zr: 0 to 0.0100%,
La: 0 to 0.0100%,
Hf: 0 to 0.0100%,
Bi: 0 to 0.0100%,
REM other than Ce and La: 0 to 0.0100%, and the balance: having a chemical composition consisting of Fe and impurities,
The steel structure in the range of 1/8 thickness to 3/8 thickness centering on 1/4 thickness from the surface is area%,
Martensite: 90.0-99.5%,
Ferrite: 0-5%,
Retained austenite: 0.5 to 7.0%, and the balance: bainite, and the proportion of tempered martensite in the total martensite is 80 to 100%,
The maximum value of the curvature 1/R obtained by shape measurement of an area of full width × length 300 mm and represented by the following formula (1) is 0.010 or less,
A cold-rolled steel sheet characterized by having a tensile strength of 1470 MPa or more.

1/R: Curvature ρ ₁ and ρ ₂ : Principal curvatures on the curved surface (2) The chemical composition, in mass %,
Cr: 0.001 to 2.00%,
Mo: 0.001 to 1.00%,
Cu: 0.001 to 1.00%,
Ni: 0.001 to 1.00%,
B: 0.0001 to 0.0100%,
Co: 0.001 to 1.00%,
W: 0.001 to 1.00%,
Sn: 0.001 to 1.00%,
Sb: 0.001 to 1.00%,
Nb: 0.001 to 0.100%,
Ti: 0.001 to 0.200%,
V: 0.001 to 0.50%,
Ca: 0.0001 to 0.0100%,
Mg: 0.0001-0.0100%,
Ce: 0.0001 to 0.0100%,
Zr: 0.0001 to 0.0100%,
La: 0.0001 to 0.0100%,
Hf: 0.0001 to 0.0100%,
Bi: 0.0001 to 0.0100%, and REM other than Ce and La: 0.0001 to 0.0100%
The cold-rolled steel sheet according to (1) above, comprising one or more selected from the group consisting of:
(3) In a hydrogen embrittlement test in which the cold-rolled steel sheet is sheared, heat-treated at 170 ° C. for 10 minutes, and then immersed in an aqueous ammonium thiocyanate solution with a concentration of 0.3 g / L for 48 hours, the sheared surface The cold-rolled steel sheet according to (1) or (2) above, characterized in that cracks do not occur.
(4) The cold-rolled steel sheet according to any one of (1) to (3) above, having any one of an electrogalvanized layer, a hot-dip galvanized layer, and an alloyed hot-dip galvanized layer on the surface.
(5) (A) Hot rolling that includes rough rolling and finish rolling of a slab having the chemical composition described in (1) or (2) above, and that satisfies the following conditions (A1) to (A3): process,
(A1) the slab heating temperature is 1150° C. or higher;
(A2) Heating the width edge portion of the steel plate after rough rolling so that the temperature of the width edge portion is 10 to 150° C. higher than the temperature of the width center portion;
(A3) The coiling temperature is 450 to 650 ° C. (B) Cold rolling including cold rolling the obtained hot-rolled steel sheet using a tandem mill consisting of N-based (N ≥ 3) rolling stands A cold rolling process in which the cumulative cold rolling reduction is 30% or more and satisfies the following formulas (2) and (3):

R _k : reduction ratio at the k-th rolling stand Pb _k : backward tension at the k-th rolling stand Pf _k : forward tension at the k-th rolling stand σ _k-1 : after passing the k-1th rolling stand Flow stress of steel plate σ _k : Flow stress of steel plate after passing k-th rolling stand σ ₀ : Yield strength of hot-rolled steel plate ε _k : Cumulative strain after passing k-th rolling stand (C) Obtained A heat treatment step that satisfies the following conditions (C1) to (C3), including heat treating the cold-rolled steel sheet (C1) holding the cold-rolled steel sheet at Ac3 to 950 ° C for 10 seconds to 500 seconds (heating ),
(C2) performing a cooling treatment that satisfies (i) to (v) below;
(i) the cooling stop temperature T1 is 110 to 250° C.;
(ii) an average cooling rate between 300-700°C of 20-150°C/s;
(iii) an average cooling rate between T1 and 300° C. of 1.0 to 20° C./s and using a gas as a refrigerant;
(iv) between Ms and 700° C. and between T1 and less than Ms, cooling is performed at least once for 0.5 s or more each;
(v) The tension applied to the cold-rolled steel sheet is 5 to 20 MPa (C3) Holding at 200 to 300 ° C. for 100 to 1000 seconds (low temperature holding)
The method for producing a cold-rolled steel sheet according to any one of (1) to (3) above, comprising:

According to the present invention, it is possible to provide a cold-rolled steel sheet having a tensile strength of 1470 MPa or more and a high total elongation, and improved hydrogen embrittlement resistance.

FIG. 2 is a schematic diagram of shearing related to hydrogen embrittlement testing.

"Chemical composition"
First, the reasons for limiting the chemical composition of the steel sheet according to the embodiment of the present invention as described above will be described. In this specification, all "%" used to define the chemical composition are "% by mass" unless otherwise specified. In addition, in this specification, the term "to" indicating a numerical range is used to include the numerical values before and after it as lower and upper limits, unless otherwise specified.

[C: 0.16 to 0.40%]
C (carbon) is an essential element for ensuring the strength of the steel sheet. In order to sufficiently obtain such effects, the C content is made 0.16% or more. The C content may be 0.18% or more, 0.20% or more, or 0.22% or more. On the other hand, when C is contained excessively, workability such as press formability, weldability, and hydrogen embrittlement resistance may deteriorate. Therefore, the C content should be 0.40% or less. The C content may be 0.37% or less, 0.33% or less, or 0.30% or less.

[Si: 0.05 to 2.00%]
Si (silicon) is an element that suppresses the formation of iron carbide and contributes to the improvement of strength and formability. In order to sufficiently obtain these effects, the Si content should be 0.05% or more. The Si content may be 0.10% or more, 0.20% or more, or 0.40% or more. On the other hand, excessive addition may lower toughness, weldability, and hydrogen embrittlement resistance. Therefore, the Si content should be 2.00% or less. The Si content may be 1.60% or less, 1.30% or less, or 1.00% or less.

[Mn: 0.50 to 4.00%]
Mn (manganese) is a strong austenite stabilizing element, and is an effective element for increasing the strength of steel sheets. In order to sufficiently obtain such effects, the Mn content is made 0.50% or more. The Mn content may be 0.80% or more, 1.00% or more, or 1.30% or more. On the other hand, excessive addition may deteriorate workability such as press formability, weldability, and hydrogen embrittlement resistance. Therefore, the Mn content should be 4.0% or less. The Mn content may be 3.0% or less, 2.5% or less, or 2.0% or less.

[P: 0.050% or less]
Phosphorus (P) is a solid-solution-strengthening element that is effective in increasing the strength of steel sheets, but excessive addition degrades weldability and toughness. Therefore, the P content is limited to 0.050% or less. The P content is preferably 0.045% or less, 0.035% or less or 0.020% or less. Although the P content may be 0%, the lower limit is preferably set to 0.001% from the viewpoint of economy, because the cost of removing P increases in order to extremely reduce the P content.

[S: 0.0100% or less]
S (sulfur) is an element contained as an impurity, and forms MnS in steel to deteriorate toughness and hole expansibility. Therefore, the S content is limited to 0.0100% or less as a range in which deterioration of toughness and hole expansibility is not remarkable. The S content is preferably 0.0050% or less, 0.0040% or less, or 0.0030% or less. Although the S content may be 0%, the lower limit is preferably set to 0.0001% from the viewpoint of economy, because desulfurization cost increases to extremely reduce the S content.

[Al: 0.001 to 1.00%]
At least 0.001% of Al (aluminum) is added for deoxidizing steel. The Al content may be 0.005% or more, 0.01% or more, or 0.02% or more. On the other hand, even if Al is added excessively, the effect is saturated and not only does it unnecessarily lead to an increase in cost, but also the transformation temperature of the steel is increased, the load during hot rolling is increased, and as a result, the mechanical properties of the steel plate are reduced. There is Therefore, the upper limit of the Al content is 1.00%. The Al content may be 0.80% or less, 0.60% or less, or 0.30% or less.

[N: 0.0100% or less]
N (nitrogen) is an element contained as an impurity, and when the content is large, coarse nitrides are formed in the steel, which may deteriorate bendability and hole expandability. Therefore, the N content is limited to 0.0100% or less. The N content is preferably 0.0080% or less, 0.0060% or less or 0.0050% or less. Although the N content may be 0%, it is preferable to set the lower limit to 0.0001% from the viewpoint of economic efficiency, because the cost of removing N increases in order to extremely reduce the N content.

[O: 0.0050% or less]
O (oxygen) is an element contained as an impurity, and if the content is large, it may form coarse oxides in the steel, deteriorating bendability and hole expandability. Therefore, the O content is limited to 0.0100% or less. The O content is preferably 0.0080% or less, 0.0060% or less or 0.0050% or less. Although the O content may be 0%, the lower limit is preferably 0.0001% from the viewpoint of manufacturing costs.

The basic chemical composition of the cold-rolled steel sheet according to the embodiment of the present invention and the slab used for its production are as described above. Furthermore, the cold-rolled steel sheet and slab may contain the following optional elements as necessary. In addition, the lower limit of the content when the arbitrary element is not included is 0%.

[Cr: 0-2.00%, Mo: 0-1.00%, Cu: 0-1.00%, Ni: 0-1.00%, B: 0-0.0100%, Co: 0- 1.00%, W: 0-1.00%, Sn: 0-1.00%, Sb: 0-1.00%, Nb: 0-0.100%, Ti: 0-0.200% and V: 0 to 0.50%]
Cr (chromium), Mo (molybdenum), Cu (copper), Ni (nickel), B (boron), Co (cobalt), W (tungsten), Sn (tin) and Sb (antimony) are all used for quenching steel. It is an element that is effective in increasing the strength of steel sheets by increasing the toughness. Nb (niobium), Ti (titanium), and V (vanadium) are alloy carbide forming elements, and contribute to increasing the strength of the steel sheet by precipitating as fine carbides in the steel sheet. Therefore, one or more of these elements may be added as required. However, excessive addition of these elements saturates the effect, unnecessarily leading to an increase in cost. Therefore, the contents are Cr: 0 to 2.00%, Mo: 0 to 1.00%, Cu: 0 to 1.00%, Ni: 0 to 1.00%, B: 0 to 0.0100% , Co: 0-1.00%, W: 0-1.00%, Sn: 0-1.00%, Sb: 0-1.00%, Nb: 0-0.100%, Ti: 0- 0.200% and V: 0-0.50%. Each element may be 0.001% or more, 0.005% or more, or 0.010% or more. In particular, the B content may be 0.0001% or more or 0.0005% or more.

[Ca: 0-0.0100%, Mg: 0-0.0100%, Ce: 0-0.0100%, Zr: 0-0.0100%, La: 0-0.0100%, Hf: 0- 0.0100%, Bi: 0 to 0.0100% and REM other than Ce and La: 0 to 0.0100%]
Ca (calcium), Mg (magnesium), Ce (cerium), Zr (zirconium), La (lanthanum), Hf (hafnium), and REMs (rare earth elements) other than Ce and La are used to finely disperse inclusions in steel. Bismuth (Bi) is an element that contributes to reducing the microsegregation of substitutional alloying elements such as Mn and Si in steel. One or more of these elements may be added, if necessary, because they each contribute to the improvement of the workability of the steel sheet. However, excessive addition causes deterioration of ductility. Therefore, the upper limit of its content is 0.0100%. Moreover, each element may be 0.0001% or more, 0.0005% or more, or 0.0010% or more.

In the cold-rolled steel sheet according to the embodiment of the present invention, the balance other than the above elements consists of Fe and impurities. Impurities are components and the like that are mixed due to various factors in the manufacturing process, including raw materials such as ores and scraps, when cold-rolled steel sheets are industrially manufactured.

"Steel structure of cold-rolled steel sheet"
Next, the steel structure of the cold-rolled steel sheet according to the embodiment of the present invention will be described.

[Martensite: 90.0 to 99.5%, ferrite: 0 to 5%, retained austenite: 0.5 to 7.0%, the balance: bainite, and the proportion of tempered martensite in all martensite: 80 ~100%]
The steel structure in the range of 1/8 thickness to 3/8 thickness centering on 1/4 thickness from the surface of the cold-rolled steel sheet is martensite: 90.0 to 99.5%, ferrite: 0 to 5%, retained austenite: 0.5-7.0%, and the balance: bainite, and the proportion of tempered martensite in the total martensite is 80-100%.

The desired tensile strength can be obtained by using mainly martensite (as-quenched martensite + tempered martensite). On the other hand, if the as-quenched martensite is large and the tempered martensite is small, the hydrogen embrittlement resistance deteriorates. Therefore, the area ratio of martensite is set to 90.0 to 99.5%, and the ratio of tempered martensite to the total martensite is set to 80 to 100%. The lower limit of the area ratio of martensite is preferably 93.0% or more, more preferably 95.0% or more. The upper limit of the area ratio of martensite may be 99.0% or less or 98.0% or less. The lower limit of the ratio of tempered martensite to all martensite is preferably 85% or more, more preferably 90% or more. The upper limit of the proportion of tempered martensite in the total martensite may be 98% or less or 95% or less.

If the ferrite content exceeds 5%, it becomes difficult to obtain the desired tensile strength. In addition, if ferrite, which is a soft structure, is present in the martensite-based structure, the non-uniformity of the structure increases, thereby promoting hydrogen embrittlement cracking. Therefore, the area ratio of ferrite is set to 0 to 5%. The upper limit of the area ratio of ferrite is preferably 4% or less, preferably 2% or less, and ideally 0%.

When retained austenite is included in the steel structure, the work hardening rate increases due to the TRIP (transformation-induced plasticity) effect, which improves ductility (that is, increases total elongation). On the other hand, when the retained austenite is contained excessively, the hydrogen embrittlement resistance deteriorates. Therefore, the area ratio of retained austenite is set to 0.5 to 7.0%. The lower limit of the area ratio of retained austenite is preferably 1.0% or more, and may be 2.0% or more. The upper limit of the area ratio of retained austenite is preferably 6.0% or less, and may be 5.0% or less or 4.0% or less.

The steel structure may contain residual structures in addition to martensite, ferrite and retained austenite. Bainite, for example, can be exemplified as the residual structure. The area ratio of the remaining tissue is exemplified as 0 to 9.5%.

[Method for measuring the area ratio of each tissue]
The area ratio of each structure other than retained austenite is evaluated by SEM-EBSD method (electron beam backscatter diffraction method) and SEM secondary electron image observation. First, a sample is collected by using a plate thickness section parallel to the rolling direction of the steel sheet as an observation surface, and the observation surface is mechanically polished to a mirror finish, and then electrolytically polished. Next, SEM-EBSD method for a total area of 3000 μm ² or more in one or more observation fields in the range of 1/8 thickness to 3/8 thickness centering on 1/4 thickness from the surface of the steel plate on the observation surface Crystal structure and orientation analysis are performed by "OIM Analysys 7.0" manufactured by TSL is used for analysis of data obtained by the EBSD method. Also, the distance between scores (step) is set to 0.03 to 0.20 μm. A grain boundary map is obtained with the boundary having a crystal orientation difference of 15 degrees or more as the grain boundary. Next, the same sample is subjected to nital etching. After that, a secondary electron image is taken using an FE-SEM for the same field of view as the field of view for crystal orientation analysis by EBSD. At this time, it is preferable to make a mark with a Vickers indentation or the like in advance. Finally, the grain boundary map and the secondary electron image are superimposed. Individual crystal grains surrounded by grain boundaries with an orientation difference of 15 degrees or more are classified according to the following criteria.

　In the secondary electron image, crystal grains in which neither the substructure nor the iron-based carbide are recognized and the crystal structure is BCC are judged to be ferrite. In the secondary electron image, crystal grains in which a substructure is observed and iron-based carbides are precipitated in a single variant, or crystal grains in which iron-based carbides are not observed are judged to be bainite. In the secondary electron image, crystal grains in which cementite is precipitated in lamellar form are judged to be pearlite. However, in principle, perlite is not included in the present invention. The remainder is judged to be martensite and retained austenite. By subtracting the area ratio of retained austenite, which will be described later, from the area ratio of the remainder, the area ratio of martensite is obtained. Of the remainder, crystal grains in which substructures are recognized and two or more iron-based carbides precipitated in multiple variants are recognized in the secondary electron image are judged to be tempered martensite.

The area ratio of retained austenite is calculated by measurement using X-rays. That is, mechanical polishing and chemical polishing are performed to remove the steel plate from the plate surface to the depth of 1/4 position in the plate thickness direction. Diffraction peaks of (200), (211) of the bcc phase and (200), (220), (311) of the fcc phase obtained using MoKα1 rays as characteristic X-rays for the polished sample From the integrated intensity ratio of , the structure fraction of retained austenite is calculated, and this is defined as the area ratio of retained austenite.

[Maximum value of curvature 1/R: 0.010 or less]
The cold-rolled steel sheet according to the embodiment of the present invention has a high strength, for example, a high strength of 1470 MPa or more, but has a very high flatness. Also in , the end face properties of the sheared portion are very good, and as a result, excellent hydrogen embrittlement resistance can be achieved. A steel sheet shape having such a high degree of flatness in the present invention is defined using the maximum value of the curvature 1/R corresponding to the reciprocal of the curvature radius R (mm). More specifically, the maximum value of the curvature 1/R in the present invention is defined by the following formula (1) using two principal curvatures ρ ₁ and ρ ₂ on the curved surface. The maximum value of the curvature 1/R is controlled to 0.010 or less.

Here, the curvature in the present invention is the larger absolute value of the principal curvatures ρ ₁ and ρ ₂ on the curved surface. The principal curvatures ρ ₁ , ρ ₂ are measured using a common shape measuring machine and estimated from three-dimensional geometric data with reduced measurement noise. For example, as a representative shape measuring machine, ATOS 3D scanner manufactured by GOM can be used for measurement. The curvature distribution in the cold-rolled steel sheet is obtained by measuring each point in an area of the entire width of the cold-rolled steel sheet and the length of 300 mm. In the present invention, the term "full width" refers to the length of the steel sheet in the direction perpendicular to the longitudinal direction of the cold-rolled steel sheet (cold-rolled coil). In the cold-rolled steel sheet obtained by the present invention, the maximum value of curvature distribution measured in this manner is 0.010 or less. For example, if the cold-rolled steel sheet is warped or wavy and the maximum value of the curvature distribution exceeds 0.010, an angle will be formed between the punch and the cold-rolled steel sheet during shearing, and the sheared part will be damaged. As a result, the hydrogen embrittlement resistance of the sheared portion deteriorates. The maximum value of curvature 1/R may be, for example, 0.008 or less, 0.006 or less, 0.004 or less, or 0.002 or less. The lower limit is not particularly limited, but the maximum value of the curvature 1/R is, for example, 0.0005 or more, 0.0006 or more, 0.0007 or more, 0.0008 or more, 0.0009 or more, or 0.001 or more. good too. According to the embodiment of the present invention, as described above, a very high flatness can be achieved in spite of the high strength of 1470 MPa or more, and the extremely high flatness exceeding 1800 MPa as specifically shown in the examples. Even with very high tensile strengths, it is possible to achieve flatness with a maximum value of curvature 1/R of 0.001. Therefore, for lower tensile strengths, e.g. closer to 1470 MPa, one skilled in the art will further reduce the maximum value of curvature 1/R, e.g. It will be readily appreciated that flatness can be achieved.

The measurement of the curvature distribution described above is not limited to any specific conditions regarding the timing of measurement and the like. Alternatively, it may be performed on as-manufactured cold-rolled steel sheets that have not undergone any specific mechanical flattening treatment. For example, in the case of a conventional cold-rolled steel sheet having a very high tensile strength of 1470 MPa or more, the maximum value of the curvature 1/R described above is controlled to 0.010 or less even if the flattening treatment is simply performed with a leveler or the like. is extremely difficult. In the embodiment of the present invention, a slab having a predetermined chemical composition is used to produce a cold-rolled steel sheet by appropriately controlling the conditions of the hot rolling process, the cold rolling process, and the heat treatment process, as will be described later in detail. By manufacturing, it is possible to achieve such a high flatness. In addition, when the cold-rolled steel sheet has a coating layer, the coating layer does not particularly affect the measurement of the curvature distribution. performed for

Next, the mechanical properties and the like of the cold-rolled steel sheet according to the embodiment of the present invention will be described.

[Tensile strength (TS)]
According to the cold-rolled steel sheet according to the embodiment of the present invention, excellent mechanical properties such as tensile strength (TS) of 1470 MPa or more can be achieved. Tensile strength is preferably 1490 MPa or more, more preferably 1500 MPa or more. Although the upper limit is not particularly limited, for example, the tensile strength may be 2000 MPa or less, 1900 MPa or less, or 1800 MPa or less.

[Total elongation (El)]
According to the cold-rolled steel sheet according to the embodiment of the present invention, high total elongation (El) can be achieved, and more specifically, total elongation of 6.0% or more can be achieved. The total elongation is preferably 7.0% or more, more preferably 8.0% or more. Although the upper limit is not particularly limited, for example, the total elongation may be 20.0% or less or 15.0% or less. Here, the tensile strength and total elongation of the cold-rolled steel sheet were obtained by collecting a JIS No. 5 tensile test piece from a direction perpendicular to the rolling direction of the steel sheet in the atmosphere at room temperature (25 ° C.), and specified in JIS Z 2241: 2011. It is measured by a tensile test.

[Hole expansion ratio (λ)]
According to the cold-rolled steel sheet according to the embodiment of the present invention, high hole expansibility can be achieved, and more specifically, a hole expansibility (λ) of 20% or more can be achieved. The hole expansion rate is preferably 25% or more, more preferably 30% or more. Although the upper limit is not particularly limited, for example, the hole expansion ratio may be 80.0% or less or 70.0% or less. The hole expansion rate (λ) is measured according to the Japan Iron and Steel Federation standard "JFS T 1001:1996 hole expansion test method".

[Evaluation by hydrogen embrittlement test]
A cold-rolled steel sheet according to an embodiment of the present invention is characterized in that cracks do not occur in a hydrogen embrittlement test by the following method. Shearing is performed by the method shown in FIG. A sample of T (thickness) x 50W (width) x 50L (length) (unit: mm) is taken from the steel plate so as to include the portion where the maximum value of curvature 1/R is obtained. The shear angle θ is 1 degree, and the clearance CL is 0.15×T. A plate pressing pressure of at least 1 ton or more is applied. After cutting the above sample by shearing, the steel plate on the product side (plate holding side) is heat-treated at 170° C. for 10 minutes. Thereafter, the steel plate is immersed in an aqueous solution of ammonium thiocyanate at room temperature with a concentration of 0.3 g/L for 48 hours to introduce the generated hydrogen into the steel plate. After that, the sheared surface is observed with a microscope or the like to evaluate the presence or absence of cracks. Heat treatment at 170° C. for 10 minutes simulates heat treatment such as paint baking treatment.

[Thickness]
A cold-rolled steel sheet according to an embodiment of the present invention has a thickness of, for example, 0.5 to 3.0 mm. Although not particularly limited, the plate thickness may be 0.6 mm or more, 0.8 mm or more, or 1.0 mm or more. Similarly, the plate thickness may be 2.8 mm or less, 2.6 mm or less, or 2.3 mm or less.

[Width]
A cold-rolled steel sheet according to an embodiment of the present invention has a width of, for example, 500 mm or more. Although not particularly limited, the plate width may be 700 mm or more, 800 mm or more, or 900 mm or more. Although the upper limit of the plate width is not particularly limited, the plate width may be 2000 mm or less, 1800 mm or less, 1600 mm or less, 1400 mm or less, 1300 mm or less, 1200 mm or less, or 1100 mm or less.

[Plating layer]
The cold-rolled steel sheet according to the embodiment of the present invention may have a coating layer on both sides or one side, preferably both sides. The plating layer is typically exemplified by an electrogalvanizing layer, a hot-dip galvanizing layer, or an alloyed hot-dip galvanizing layer. These galvanized layers may have any composition known to those skilled in the art, and may contain additive elements such as Al and Mg in addition to Zn. Also, the amount of the plating layer to be deposited is not particularly limited, and may be a general amount of deposition.

<Manufacturing method>
Next, a method for manufacturing a cold-rolled steel sheet according to an embodiment of the present invention will be described. The following description is intended to illustrate a characteristic method for manufacturing the cold-rolled steel sheet according to the embodiment of the present invention, and the cold-rolled steel sheet is manufactured by the manufacturing method described below. It is not intended to be limited to

"(A) Hot rolling process"
First, the hot rolling process will be described.

[Slab heating temperature: 1150°C or higher]
In the hot rolling process, a slab having the same chemical composition as described above for cold rolled steel is heated before hot rolling and then subjected to rough and finish rolling. The heating temperature of the slab must be 1150° C. or higher, preferably 1200° C. or higher, in order to sufficiently dissolve borides, carbides, and the like. The steel slab to be used is preferably cast by a continuous casting method from the viewpoint of manufacturability, but may be produced by an ingot casting method or a thin slab casting method.

[rough rolling]
Rough rolling is performed on the heated slab before finish rolling. Rough rolling conditions are not particularly limited, but it is preferable to carry out rough rolling so that the total rolling reduction is 60% or more at 1050°C. If the total rolling reduction is less than 60%, recrystallization during hot rolling becomes insufficient, which may lead to heterogeneity in the structure of the hot-rolled steel sheet. The above total rolling reduction may be, for example, 90% or less.

[Heating the width edge part so that the temperature of the width edge part is 10 to 150° C. higher than the temperature of the width center part]
The width edge portion of the steel sheet that has completed rough rolling is reheated so that the temperature (Te) of the width edge portion is higher than the temperature (Tc) of the width center portion by 10 to 150°C. By performing such reheating, it is possible to suppress fluctuations in the strength of the hot-rolled steel sheet in the width direction, thereby manufacturing a hot-rolled steel sheet having a more uniform strength in the width direction. Therefore, in the subsequent cold rolling process, rolling can be uniformly performed over the entire width direction, and the shape of the steel sheet after cold rolling can be further improved. When such reheating is not performed, the width edge portion is hardened more than the width center portion because the subsequent cooling rate is higher in the width edge portion than in the width center portion. As a result, in the subsequent cold rolling process, a shape defect called "middle elongation" occurs in which the width center portion is elongated compared to the width edge portion. As a result, the curvature in the final product is degraded. On the other hand, if the width edge portion is excessively heated, the width edge portion becomes excessively soft, resulting in a shape defect called "ear wave" in which the edge portion extends from the center portion in the subsequent cold rolling process. In order to avoid these shape defects, the edge is heated so that the temperature of the width edge portion is 10 to 150° C. higher than the temperature of the width center portion. It is preferably 20 to 100°C, more preferably 40 to 90°C. Heating (reheating) of the width edge portion can be performed by any suitable means known to those skilled in the art and is not particularly limited, but can be performed using an edge heater, for example.

[Finish rolling]
After reheating the edge portion, finish rolling is performed. The conditions are not particularly limited. desirable. When the finish rolling entry temperature is lower than 950°C, the finish rolling exit temperature is lower than 850°C, or the total rolling reduction is higher than 95%, the texture of the hot rolled steel sheet develops, so the final product sheet Anisotropy in may become apparent. On the other hand, when the finish rolling entry temperature exceeds 1050 ° C., the finish rolling exit temperature exceeds 1000 ° C., or the total rolling reduction is less than 70%, the crystal grain size of the hot rolled steel sheet becomes coarse, and the final This may cause coarsening of the product plate structure.

[Winding temperature: 450 to 650°C]
In this method, the shape of the steel sheet after cold rolling can be improved by coiling the steel sheet after finish rolling at a coiling temperature of 450 to 650°C. If the coiling temperature is lower than 450°C, the strength of the hot-rolled steel sheet increases, and the shape of the steel sheet after cold rolling deteriorates. On the other hand, if the coiling temperature exceeds 650° C., cementite coarsens and undissolved cementite remains, which may impair workability.

[Pickling]
After hot rolling, if necessary, pickling is performed to remove scales. The pickling method should just follow a conventional method. In order to correct the shape of the hot-rolled coil or improve pickling properties, pretreatment such as skin pass rolling or shot blasting may be performed before pickling.

"(B) Cold rolling process"
Next, the cold rolling process will be explained.

[Cold rolling using a tandem mill consisting of N-group (N≧3) rolling stands]
In this method, the cold rolling process includes cold rolling the obtained hot rolled steel sheet using a tandem mill consisting of N (N≧3) rolling stands, and the cumulative cold rolling reduction is A cold rolling process is performed that is 30% or more and satisfies the following equations (2) and (3).

R _k : reduction ratio at the k-th rolling stand Pb _k : backward tension at the k-th rolling stand Pf _k : forward tension at the k-th rolling stand σ _k-1 : after passing the k-1th rolling stand Flow stress of steel plate σ _k : Flow stress of steel plate after passing k-th rolling stand σ ₀ : Yield strength of hot-rolled steel plate ε _k : Cumulative strain after passing k-th rolling stand

In the cold rolling process of the present invention, it is necessary to control the rolling reduction, forward tension/flow stress, and rear tension/flow stress in each rolling stand so as to satisfy the above formula (2). Forward tension and backward tension in each rolling stand in a tandem mill are parameters that are generally measured. Division Rolling Theory Subcommittee, 2010, p.264”, a detection roll can be placed on a steel plate during cold rolling to measure the tension from the vertical load. Also, the flow stress of the steel sheet during cold rolling is given by Equation (3). Here, σ ₀ is the flow stress of the steel sheet before passing through the first stand, that is, the yield strength of the hot-rolled steel sheet. σ ₀ is obtained by sampling a JIS No. 5 tensile test piece along the rolling direction from the width center portion of the hot-rolled steel sheet and performing a tensile test based on JIS Z 2241:2011. Equation (2) means that the value increases when a large reduction is applied in a state where the difference between the forward tension/flow stress and the rear tension/flow stress is large. In order to reduce equation (2), the difference between the forward tension/flow stress and the rear tension/flow stress should be reduced. By reducing the difference between the forward tension/flow stress and the rear tension/flow stress to satisfy the equation (2), it is possible to reliably suppress the phenomenon that the steel sheet slides on the rolling rolls, the so-called slip. , more stable cold rolling can be realized. As a result, the shape of the steel sheet after cold rolling can be improved.

When the left side of formula (2) is 3.0 or more, the shape of the steel sheet after cold rolling deteriorates significantly, and the curvature of the final product no longer satisfies formula (1). The smaller the left side of formula (2) is, the better. For example, less than 2.5 or less than 2.0 is desirable, and less than 1.0 is more preferable. Although the lower limit is not particularly limited, for example, the left side of Equation (2) may be 0.1 or more or 0.2 or more. Formula (2) is one preferable index for realizing stable cold rolling without rolling defects such as slip by well balancing the tension and the yield strength of the hot-rolled steel sheet before and after each rolling stand. Therefore, in order to realize such stable cold rolling, it is possible to use other control methods instead of the control method according to equation (2).

In the cold rolling process, in addition to satisfying formula (2), it is also important to make the cumulative cold rolling reduction ratio 30% or more in order to obtain a good steel plate shape with high flatness. If the cumulative cold rolling reduction is less than 30%, the shape of the steel sheet after cold rolling is not sufficiently improved, and as a result the curvature of the final product does not satisfy the formula (1). The cumulative cold rolling reduction may be 40% or more or 50% or more. Although the upper limit is not particularly limited, since excessive reduction causes an excessive rolling load and increases the burden on the cold rolling mill, the cumulative cold rolling reduction may be 75% or less or 70% or less.

"(C) Heat treatment process"
Next, the heat treatment process will be described.

[Heating and holding: holding at Ac 3 to 950 ° C. for 10 to 500 seconds]
The obtained cold-rolled steel sheet is subjected to a predetermined heat treatment in the heat treatment step. First, in order to sufficiently promote austenitization, heating is performed at Ac 3° C. or higher for 10 seconds or longer. If the heating temperature is less than Ac3°C or the holding time is less than 10 seconds, the austenitization is not sufficient, so the desired steel structure mainly composed of martensite cannot be obtained, and sufficient strength cannot be obtained. . On the other hand, if the heating temperature exceeds 950° C. or the holding time exceeds 500 seconds, the crystal grain size will become coarse, and in addition, fuel costs will increase and equipment will be damaged. Ac3 (°C) is calculated by the following formula. The mass % of the element concerned is substituted for the symbol of the element in the following formula. 0% by mass is substituted for elements that are not contained.
Ac3 (°C) = 912 - 230.5 x C + 31.6 x Si - 20.4 x Mn - 39.8 x Cu - 18.1 x Ni - 14.8 x Cr + 16.8 x Mo + 100.0 x Al

[Cooling stop temperature T1: 110 to 250° C.]
After heating, it is cooled to a range of 110 to 250°C. When T1 is less than 110°C, the area ratio of retained austenite is less than 0.5%, and the total elongation decreases. On the other hand, when the temperature exceeds 250°C, the proportion of tempered martensite in martensite becomes less than 80%, resulting in deterioration of hydrogen embrittlement resistance. The cooling stop temperature may be 120°C or higher and/or may be 220°C or lower.

[Average cooling rate between 300-700°C: 20-150°C/s]
By controlling the average cooling rate between 300 to 700 ° C. (average cooling rate 1) in the range of 20 to 150 ° C./s, it is possible to suppress the increase in temperature deviation in the steel plate, so the curvature of the steel plate can be reduced. improvement is possible. If the average cooling rate in the above section is less than 20°C/s, the martensite fraction becomes low and the desired tensile strength cannot be obtained. On the other hand, if it exceeds 150° C./s, the curvature of the steel sheet is deteriorated due to an increase in the temperature deviation within the steel sheet. It should be noted that the average cooling rate in the present invention is a rate including the cooling time described later.

[Average cooling rate between T1 and 300°C: 1.0 to 20°C/s and refrigerant: gas]
By setting the average cooling rate between T1 and 300°C (average cooling rate 2) to 1.0 to 20°C/s, and using a gas (for example, nitrogen gas) as a refrigerant for relatively gentle cooling, Since an increase in temperature deviation in the steel sheet can be suppressed, the curvature of the steel sheet can be improved. If the average cooling rate in the section is less than 1.0° C./s, the martensite fraction becomes low, making it impossible to obtain the desired tensile strength. On the other hand, if it exceeds 20° C./s, the curvature of the steel sheet deteriorates due to an increase in the temperature deviation within the steel sheet. Moreover, it is necessary to use a gas as the coolant from the viewpoint of reliably suppressing an increase in the temperature deviation within the steel sheet.

[Perform cooling for 0.5 s or more at least once between Ms and 700°C and between T1 and less than Ms]
Cooling is suspended in each interval between Ms and 700° C. and between T1 and less than Ms, and cooling is performed for 0.5 s or more. This treatment promotes heat transfer within the steel sheet and improves temperature unevenness within the steel sheet, thereby improving the curvature of the steel sheet. Ms (°C) is calculated by the following formula. The mass % of the element concerned is substituted for the symbol of the element in the following formula. 0% by mass is substituted for elements that are not contained.
Ms (°C) = 561-474 x C-33 x Mn-17 x Cr-21 x Mo-7.5 x Si + 10 x Co

[Tension applied to cold rolled steel sheet: 5 to 20 MPa]
During the cooling process, the applied tension on the cold-rolled steel sheet should be limited to 6-20 MPa. By controlling the tension within such a range, the flatness of the cold-rolled steel sheet can be improved, and the curvature of the finally obtained cold-rolled steel sheet can be improved. On the other hand, if the tension is outside the above range, the curvature of the cold-rolled steel sheet deteriorates. This tension may be 8 MPa or more. Likewise, this tension may be 16 MPa or less.

[Low temperature retention: 100 to 1000 seconds between 200 and 300° C.]
After cooling to the cooling stop temperature T1, the temperature is kept between 200 and 300° C. for 100 to 1000 seconds. Thereby, residual austenite can be obtained by distributing carbon in untransformed austenite. If the temperature is less than 200°C or the holding time is less than 100 seconds, the desired amount of retained austenite cannot be obtained. On the other hand, if the temperature exceeds 300° C. or the holding time exceeds 1000 seconds, the desired steel structure cannot be obtained, resulting in the desired tensile strength and total elongation.

The cold-rolled steel sheet obtained by the cold-rolled steel sheet manufacturing method according to the embodiment of the present invention may be subjected to a post-process such as a plating process for forming a coating layer on one or both sides of the cold-rolled steel sheet. . A post-process such as a plating process can be performed by a conventional method.

Examples of cold-rolled steel sheets according to embodiments of the present invention will be described below. The conditions in the examples are examples of conditions adopted for confirming the feasibility and effects of the present invention. The present invention is not limited to this one conditional example. Various conditions can be adopted in the present invention as long as the object of the present invention is achieved without departing from the gist of the present invention.

First, steel having the chemical composition shown in Table 1 was cast to produce a slab. The balance other than the components shown in Table 1 is Fe and impurities. These slabs were subjected to hot rolling including rough rolling and finish rolling under the conditions shown in Table 2 to produce hot rolled steel sheets. Heating (reheating) of the width edge portion after rough rolling was performed using an edge heater. Next, the hot-rolled steel sheet was pickled to remove surface scales, and cold-rolled under the conditions shown in Table 2 using a tandem mill consisting of five rolling stands. The sheet thickness after cold rolling was 1.6 mm, and the sheet width was 1000 mm. Finally, the obtained cold-rolled steel sheets were heat-treated under the conditions shown in Table 2. Cooling between the cooling stop temperature T1 and 300° C. was carried out at a predetermined average cooling rate (average cooling rate 2 in Table 2) using nitrogen gas (water in Comparative Example 24) as a coolant.

From the steel sheet thus obtained, a JIS No. 5 tensile test piece was taken from the direction perpendicular to the rolling direction of the steel sheet in the air at room temperature (25°C), and a tensile test was performed in accordance with JIS Z 2241:2011. Tensile strength (TS) and total elongation (El) were measured. In addition, the "JFS T 1001: 1996 hole expansion test method" of the Japan Iron and Steel Federation standard was performed to measure the hole expansion rate (λ).

The maximum value of curvature 1/R was determined as follows. First, cold-rolled steel sheets, which have not been subjected to a specific mechanical flattening process, are measured at each point in an area of full width x 300 mm length using an ATOS 3D scanner manufactured by GOM. Curvature distribution in the steel plate was obtained. Next, in the curvature distribution thus measured, the larger absolute value of the principal curvatures ρ ₁ and ρ ₂ was determined as the maximum value of the curvature 1/R.

The hydrogen embrittlement resistance was evaluated by the hydrogen embrittlement test using the shearing shown in Fig. 1. Specifically, first, a sample of T (thickness) × 50W (width) × 50L (length) (unit: mm) was taken from the steel plate so as to include a portion where the maximum value of curvature 1/R was obtained. . The shear angle θ was 1 degree, the clearance CL was 0.15×T, and the plate pressing pressure was 1 ton or more. After cutting the above sample by shearing, the steel plate on the product side (plate holding side) was heat-treated at 170° C. for 10 minutes. Thereafter, the steel sheet was immersed in an aqueous ammonium thiocyanate solution having a concentration of 0.3 g/L and a concentration of 3 g/L at room temperature for 48 hours to introduce hydrogen into the steel sheet. After that, the sheared surface was observed with a microscope to evaluate the presence or absence of cracks. Those in which cracks were observed at 0.3 g / L were x (failed), cracks were not observed at 0.3 g / L, but cracks were observed at 3 g / L, ○ (accepted), 0 3 g/L and 3 g/L were evaluated as ⊚ (accepted) when cracks were not observed.

In the case where TS is 1470 MPa or more, El is 6.0% or more, and hydrogen embrittlement resistance is acceptable, cold steel with improved hydrogen embrittlement resistance while having high tensile strength and total elongation It was evaluated as a rolled steel sheet. Table 3 shows the results.

Referring to Table 3, in Comparative Example 2, the maximum value of the curvature 1/R increased and the hydrogen embrittlement resistance deteriorated because the expression (2) was not satisfied in the cold rolling process. In Comparative Examples 3 and 12, the difference between the temperature of the width edge portion and the temperature of the width center portion of the steel plate after rough rolling was not appropriate in the hot rolling process, so the maximum value of the curvature 1/R increased, and the hydrogen resistance increased. Embrittlement properties decreased. In Comparative Example 4, since the cumulative cold rolling reduction was low in the cold rolling process, the maximum value of the curvature 1/R was increased and the hydrogen embrittlement resistance was lowered. In Comparative Example 5, since the cooling stop temperature T1 was low in the heat treatment step, retained austenite was not sufficiently formed, and El decreased. In Comparative Example 6, since the average cooling rate between 300° C. and 700° C. (average cooling rate 1) was slow in the heat treatment step, sufficient martensite was not generated, resulting in a decrease in TS. In Comparative Example 7, since the average cooling rate (average cooling rate 2) between T1 and 300° C. was high in the heat treatment step, the maximum value of the curvature 1/R increased and the hydrogen embrittlement resistance decreased. In Comparative Examples 8 and 19, since the tension applied to the cold-rolled steel sheet in the heat treatment step was not appropriate, the maximum value of the curvature 1/R increased and the hydrogen embrittlement resistance decreased. In Comparative Examples 9, 10 and 23, the desired steel structure was not obtained and TS and/or El decreased because the temperature or time of low temperature holding in the heat treatment process was not appropriate. In Comparative Example 11, since the temperature of heating and holding in the heat treatment step was low, sufficient martensite was not generated, resulting in a decrease in TS.

In Comparative Example 13, since T1 was high in the heat treatment process, the proportion of tempered martensite in martensite was small, and the hydrogen embrittlement resistance was lowered. In Comparative Example 14, the average cooling rate 1 was high in the heat treatment process, so the temperature deviation in the steel sheet increased, and as a result, the maximum value of the curvature 1/R increased and the hydrogen embrittlement resistance decreased. In Comparative Example 15, since the average cooling rate 2 was slow in the heat treatment step, sufficient martensite was not generated, resulting in a decrease in TS. In Comparative Example 16, it is considered that the strength of the hot-rolled steel sheet was increased because the coiling temperature was low in the hot-rolling process. As a result, the shape of the steel sheet after cold rolling deteriorated, and the hydrogen embrittlement resistance decreased. In Comparative Examples 17 and 18, it is considered that temperature unevenness occurred in the steel sheet because the steel sheet was not cooled appropriately between Ms and 700° C. or between T1 and less than Ms in the heat treatment process. As a result, the maximum value of curvature 1/R increased and the hydrogen embrittlement resistance decreased. In Comparative Example 24, cooling between T1 and 300° C. was performed using water as a coolant in the heat treatment step, so the average cooling rate 2 was increased, and appropriate air cooling was also performed between T1 and less than Ms. I didn't. As a result, the temperature deviation in the steel sheet increased, the maximum value of the curvature 1/R increased, and the hydrogen embrittlement resistance decreased. In Comparative Example 45, since the Si content was low, retained austenite was not generated sufficiently, and El decreased. In Comparative Example 46, since the Mn content was low, sufficient martensite was not generated, resulting in a decrease in TS. In Comparative Example 47, the TS decreased due to the low C content. In Comparative Examples 48-50, the hydrogen embrittlement resistance decreased due to the high C, Mn or Si content.

In contrast, in Examples 1, 20 to 22, and 25 to 44 of the present invention, by having a predetermined chemical composition and steel structure and further controlling the maximum value of the curvature 1/R to 0.010 or less, , a cold-rolled steel sheet having high tensile strength and total elongation and improved hydrogen embrittlement resistance could be obtained.

Claims (5)

1. in % by mass,
   C: 0.16 to 0.40%,
   Si: 0.05 to 2.00%,
   Mn: 0.50 to 4.00%,
   P: 0.050% or less,
   S: 0.0100% or less,
   Al: 0.001 to 1.00%,
   N: 0.0100% or less,
   O: 0.0050% or less,
   Cr: 0 to 2.00%,
   Mo: 0 to 1.00%,
   Cu: 0 to 1.00%,
   Ni: 0 to 1.00%,
   B: 0 to 0.0100%,
   Co: 0 to 1.00%,
   W: 0 to 1.00%,
   Sn: 0 to 1.00%,
   Sb: 0 to 1.00%,
   Nb: 0 to 0.100%,
   Ti: 0 to 0.200%,
   V: 0 to 0.50%,
   Ca: 0 to 0.0100%,
   Mg: 0-0.0100%,
   Ce: 0 to 0.0100%,
   Zr: 0 to 0.0100%,
   La: 0 to 0.0100%,
   Hf: 0 to 0.0100%,
   Bi: 0 to 0.0100%,
   REM other than Ce and La: 0 to 0.0100%, and the balance: having a chemical composition consisting of Fe and impurities,
   The steel structure in the range of 1/8 thickness to 3/8 thickness centering on 1/4 thickness from the surface is area%,
   Martensite: 90.0-99.5%,
   Ferrite: 0-5%,
   Retained austenite: 0.5 to 7.0%, and the balance: bainite, and the proportion of tempered martensite in the total martensite is 80 to 100%,
   The maximum value of the curvature 1/R obtained by shape measurement of an area of full width × length 300 mm and represented by the following formula (1) is 0.010 or less,
   A cold-rolled steel sheet characterized by having a tensile strength of 1470 MPa or more.
   

   

   1/R: curvature ρ ₁ and ρ ₂ : principal curvatures on the curved surface
2. The chemical composition, in mass %,
   Cr: 0.001 to 2.00%,
   Mo: 0.001 to 1.00%,
   Cu: 0.001 to 1.00%,
   Ni: 0.001 to 1.00%,
   B: 0.0001 to 0.0100%,
   Co: 0.001 to 1.00%,
   W: 0.001 to 1.00%,
   Sn: 0.001 to 1.00%,
   Sb: 0.001 to 1.00%,
   Nb: 0.001 to 0.100%,
   Ti: 0.001 to 0.200%,
   V: 0.001 to 0.50%,
   Ca: 0.0001 to 0.0100%,
   Mg: 0.0001-0.0100%,
   Ce: 0.0001 to 0.0100%,
   Zr: 0.0001 to 0.0100%,
   La: 0.0001 to 0.0100%,
   Hf: 0.0001 to 0.0100%,
   Bi: 0.0001 to 0.0100%, and REM other than Ce and La: 0.0001 to 0.0100%
   The cold-rolled steel sheet according to claim 1, comprising one or more selected from the group consisting of
3. In a hydrogen embrittlement test in which the cold-rolled steel sheet is sheared, heat-treated at 170° C. for 10 minutes, and then immersed in an ammonium thiocyanate aqueous solution with a concentration of 0.3 g / L for 48 hours, cracks occur on the sheared surface. The cold-rolled steel sheet according to claim 1 or 2, characterized in that it does not.
4. The cold-rolled steel sheet according to any one of claims 1 to 3, having any one of an electrogalvanized layer, a hot-dip galvanized layer, and an alloyed hot-dip galvanized layer on the surface.
5. (A) A hot rolling step that includes rough rolling and finish rolling of a slab having the chemical composition according to claim 1 or 2 and satisfies the following conditions (A1) to (A3):
   (A1) the slab heating temperature is 1150° C. or higher;
   (A2) Heating the width edge portion of the steel plate after rough rolling so that the temperature of the width edge portion is 10 to 150° C. higher than the temperature of the width center portion;
   (A3) The coiling temperature is 450 to 650 ° C. (B) Cold rolling including cold rolling the obtained hot-rolled steel sheet using a tandem mill consisting of N-based (N ≥ 3) rolling stands A cold rolling process in which the cumulative cold rolling reduction is 30% or more and satisfies the following formulas (2) and (3):
   

   

   

   R _k : reduction ratio at the k-th rolling stand Pb _k : backward tension at the k-th rolling stand Pf _k : forward tension at the k-th rolling stand σ _k-1 : after passing the k-1th rolling stand Flow stress of steel plate σ _k : Flow stress of steel plate after passing k-th rolling stand σ ₀ : Yield strength of hot-rolled steel plate ε _k : Cumulative strain after passing k-th rolling stand (C) Obtained A heat treatment step that satisfies the following conditions (C1) to (C3), including heat treating the cold-rolled steel sheet (C1) holding the cold-rolled steel sheet at Ac3 to 950 ° C for 10 seconds to 500 seconds (heating ),
   (C2) performing a cooling treatment that satisfies (i) to (v) below;
   (i) the cooling stop temperature T1 is 110 to 250° C.;
   (ii) an average cooling rate between 300-700°C of 20-150°C/s;
   (iii) an average cooling rate between T1 and 300° C. of 1.0 to 20° C./s and using a gas as a refrigerant;
   (iv) between Ms and 700° C. and between T1 and less than Ms, cooling is performed at least once for 0.5 s or more each;
   (v) The tension applied to the cold-rolled steel sheet is 5 to 20 MPa (C3) Holding at 200 to 300 ° C. for 100 to 1000 seconds (low temperature holding)
   The method for producing a cold-rolled steel sheet according to any one of claims 1 to 3, comprising:

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