COLD-ROLLED STEEL SHEET AND METHOD FOR PRODUCING SAME

Abstract

A cold-rolled steel sheet has a predetermined chemical composition, at a surface layer area which is in a range of 10 to 20 μm from a surface in a sheet thickness direction, a microstructure includes, by volume percentage, 10% to 95% of ferrite and a remainder of one or more selected from martensite, bainite, pearlite, cementite, and retained austenite, at a ¼ thickness position which is in a range of ⅛ to ⅜ of a sheet thickness from the surface in the sheet thickness direction, the microstructure includes, by volume percentage, 0% to 60% of ferrite, 0% to 3% of retained austenite, and a remainder of one or more selected from martensite and bainite, a volume percentage of tempered martensite in the martensite is 50% or more, a ratio of a dislocation density at the surface layer area to a dislocation density at the ¼ thickness position is 0.20 or more and less than 0.90, and a tensile strength is 1,180 MPa or more.

Description

TECHNICAL FIELD

The present invention relates to a cold-rolled steel sheet and a method for manufacturing the same.

Priority is claimed on Japanese Patent Application No. 2023-007231, filed on Jan. 20, 2023, the content of which is incorporated herein by reference.

BACKGROUND ART

Recently, as industrial technology fields are highly divided, materials used in each technology field is required to have special and advanced performance. For example, steel sheets for a vehicle are required to have high strength in order to improve fuel efficiency by reducing the weight of a vehicle body in consideration of the global environment. In a case where a high-strength steel sheet is applied to the vehicle body of a vehicle, a desired strength can be imparted to the vehicle body while reducing the sheet thickness of the steel sheet and reducing the weight of the vehicle body.

In addition, the high-strength steel sheet to be provided for a component for a vehicle is formed into a component shape, so that not only strength but also properties (formability) required for component forming such as elongation are required. In addition, since the components are joined by spot welding or the like, spot weldability is also required.

In general, strength and formability are in a trade-off relationship. In addition, there is a tendency that the weldability decreases when the strength is increased by increasing the content of C or other alloying elements.

As a steel sheet that has excellent spot weldability, high strength and excellent elongation, a dual phase steel sheet (hereinafter, also referred to as a DP steel sheet) mainly including a composite structure of a soft ferrite phase and a martensite phase or bainite phase which are hard structure is known.

In addition, in the case of a vehicle, emphasis is also placed on improving collision safety in order to secure the safety of occupants. In a collision of a vehicle, a steel sheet constituting a vehicle member is subjected to bending stress, and thus the steel sheet is required to have bending properties.

That is, in order to simultaneously achieve the reduction in weight of the vehicle body and the improvement in collision safety, there is a demand for a steel sheet having excellent bendability considering a collision, in addition to high strength, elongation, and spot weldability.

For such a problem, for example, Patent Document 1 discloses a thin steel sheet in which a component composition includes, by mass %, C: 0.10% or more and 0.35% or less, Si: 0.01% or more and 2.0% or less, Mn: 0.8% or more and 2.35% or less, P: 0.05% or less, S: 0.005% or less, Al: 0.005% or more and 0.10% or less, N: 0.0060% or less, and a remainder consisting of Fe and unavoidable impurities, and a steel structure in which a ferrite area ratio is 30% or less (including 0%), a bainite area ratio is 5% or less (including 0%), a martensite and tempered martensite area ratio is 70% or more (including 100%), a retained austenite area ratio is 2.0% or less (including 0%), a ratio of a dislocation density in a range of 0 to 20 μm from a surface of the steel sheet to a dislocation density in a center portion of a sheet thickness is 90% or more and 110% or less, and an average of the top 10% of a cementite particle size within a depth of 100 μm from the surface of the steel sheet is 300 nm or less, and a maximum warpage amount of the steel sheet when sheared in a longitudinal direction of the steel sheet with a length of 1 m is 15 mm or less.

Patent Document 1 describes that bendability is improved by controlling the dislocation density and the cementite particle size.

In addition, Patent Document 2 discloses a high-strength steel sheet having excellent delayed fracture resistance property, in which a chemical composition contains C: 0.10% to 0.40%, Si: 0.6% to 3.0%, Mn: 1.0% to 3.5%, Al: 3% or less (not including 0%), P: 0.15% or less (not including 0%), S: 0.02% or less (not including 0%), and a remainder consisting of iron and unavoidable impurities, martensite occupies 95 area % or more in an entire structure, and a predetermined relationship is satisfied in a structure from a position at a depth of 10 μm in a sheet thickness direction from a surface of the steel sheet to a ¼ depth position of the sheet thickness in terms of a prior austenite grain size, a dislocation density, a solute C concentration (mass %) in martensite, and a ratio of a length of a carbide precipitated at a prior γ grain boundary to a length of the prior γ grain boundary, and a tensile strength is 1,180 MPa or more.

Patent Document 2 describes that portions where shear deformation occurs during bending are dispersed in the steel material by refining the prior γ grains so that no local stress concentration portion is generated.

In addition, Patent Document 3 discloses a high-strength galvanized steel sheet having excellent collision absorption energy and a maximum tensile strength of 900 MPa or more, in which, at the inside of the steel sheet, a density of dislocations included in the steel sheet is 8×10¹¹ (pieces/mm²) or less, and a static-to-dynamic ratio (=FS2/FS1) composed of a ratio between a quasi-static strength (FS1) at a strain rate of 0.0067 (s⁻¹) and a dynamic strength (FS2) at a strain rate of 1,000 (s⁻¹) is 1.05 or more.

CITATION LIST

Patent Documents

• • Patent Document 1: PCT International Publication No. WO2020/026838
  • Patent Document 2: Japanese Unexamined Patent Application, First Publication No. 2013-104081
  • Patent Document 3: Japanese Patent No. 5487916

SUMMARY OF INVENTION

Technical Problem

However, Patent Document 1 relates to a method of reducing a variation in a bending fracture limit, and does not relate to a method of increasing an average value of the bending fracture limit.

Patent Document 2 discloses that the delayed fracture resistance property is improved by equalizing a strain dispersion during bending, but the bendability considering a collision is not evaluated.

In Patent Document 3, in order to increase the collision absorption energy, deformation resistance at a high strain rate is taken into consideration, but bending deformation at the time of collision is not taken into consideration.

That is, in the related art, a steel sheet having high strength and excellent elongation, spot weldability, and bendability has not been disclosed. An object of the present invention is to provide a steel sheet having high strength and excellent elongation, spot weldability, and bendability, and a method for manufacturing the same.

In the present invention, excellent bendability means that bending resistance is high and the bending fracture limit is high.

Solution to Problem

The present inventors studied a method for improving the bendability of a cold-rolled DP steel sheet having high strength, excellent elongation, and excellent spot weldability.

As a result, it was found that bending fracture can be suppressed by generating ferrite in a surface layer area. On the other hand, it was found that the bending strength decreases when ferrite is generated in the surface layer area. Therefore, as a result of further studies by the present inventors, it was found that the bending strength can be secured while suppressing bending fracture by controlling a dislocation density of the surface layer area in a predetermined range.

In addition, the present inventors found that in order to generate ferrite in the surface layer area and obtain a predetermined dislocation density, it is effective to perform atmosphere control in annealing and introduce strain into the surface layer area by leveling.

The present invention has been made in view of the above findings. The gist of the present invention is as follows.

[1] A cold-rolled steel sheet according to an aspect of the present invention has a chemical composition containing, by mass %: C: 0.060% to 0.300%; Si: 0.01% to 3.00%; Mn: 1.00% to 5.00%; sol.Al: 0.001% to 1.000%; P: 0.100% or less; S: 0.0100% or less; O: 0.1000% or less; N: 0.0100% or less; Ti: 0% to 0.200%; B: 0% to 0.0100%; Cr: 0% to 1.00%; Mo: 0% to 1.00%; Ni: 0% to 1.00%; Cu: 0% to 1.00%; Sn: 0% to 0.50%; Nb: 0% to 0.200%; V: 0% to 0.50%; W: 0% to 0.50%; Ca: 0% to 0.0100%; Mg: 0% to 0.0100%; Bi: 0% to 0.0100%; Sb: 0% to 0.100%; Zr: 0% to 0.0100%; REM: 0% to 0.0100%; and a remainder: Fe and impurities, in which Q obtained by Expression (1) is 2.3 or more, at a surface layer area which is in a range of 10 to 20 μm from a surface in a sheet thickness direction, a microstructure includes, by volume percentage, 10% to 95% of ferrite and a remainder of one or more selected from martensite, bainite, pearlite, cementite, and retained austenite, at a ¼ thickness position which is in a range of ⅛ to ⅜ of a sheet thickness from the surface in the sheet thickness direction, the microstructure includes, by volume percentage, 0% to 60% of ferrite, 0% to 3% of retained austenite, and a remainder of one or more selected from martensite and bainite, a volume percentage of tempered martensite in the martensite is 50% or more, a ratio of a dislocation density at the surface layer area to a dislocation density at the ¼ thickness position is 0.20 or more and less than 0.90, and a tensile strength is 1,180 MPa or more.

$\begin{array}{cc}Q=2.7\times \left[C\right]+0\text{.4}\times \left[\mathrm{Si}\right]+\left[\mathrm{Mn}\right]+0.45\times \left[\mathrm{Ni}\right]+0\text{.8}\times \left[\mathrm{Cr}\right]+2\text{.0}\times \left[\mathrm{Mo}\right]& \left(1\right)\end{array}$

In Expression (1), [Element symbol] is a content of the element indicated by the element symbol in mass % and 0 is substituted in a case where the element is not contained.

[2] In the cold-rolled steel sheet according to [1], a hot-dip galvanized layer may be included on the surface.

[3] In the cold-rolled steel sheet according to [2], the hot-dip galvanized layer may be a hot-dip galvannealed layer.

[4] A method for manufacturing a cold-rolled steel sheet according to another aspect of the present invention includes: a hot rolling step of heating a slab having a chemical composition which contains, by mass %, C: 0.060% to 0.300%, Si: 0.01% to 3.00%, Mn: 1.00% to 5.00%, sol.Al: 0.001% to 1.000%, P: 0.100% or less, S: 0.0100% or less, O: 0.1000% or less, N: 0.0100% or less, Ti: 0% to 0.200%, B: 0% to 0.0100%, Cr: 0% to 1.00%, Mo: 0% to 1.00%, Ni: 0% to 1.00%, Cu: 0% to 1.00%, Sn: 0% to 0.50%, Nb: 0% to 0.200%, V: 0% to 0.50%, W: 0% to 0.50%, Ca: 0% to 0.0100%, Mg: 0% to 0.0100%, Bi: 0% to 0.0100%, Sb: 0% to 0.100%, Zr: 0% to 0.0100%, REM: 0% to 0.0100%, and a remainder: Fe and impurities, in which Q obtained by Expression (1) is 2.3 or more, and performing hot rolling so that a finish rolling completion temperature is 800° C. or higher and an Ar3 point or higher to obtain a hot-rolled steel sheet; a cooling step on run-out table of starting cooling of the hot-rolled steel sheet after the elapse of 1.0 second or longer from completion of the hot rolling, and cooling the hot-rolled steel sheet to a coiling temperature of 400° C. to 750° C. at an average cooling rate of 5.0° C./sec or higher; a coiling step of coiling the hot-rolled steel sheet after the cooling step on run-out table at the coiling temperature; a cold rolling step of performing cold rolling on the hot-rolled steel sheet after the coiling step so that a cumulative rolling reduction is 20% to 80% to obtain a cold-rolled steel sheet; an annealing step of heating the cold-rolled steel sheet after the cold rolling step to a soaking temperature of 750° C. to 1,000° C. and holding the cold-rolled steel sheet at the soaking temperature for 1 second or longer; a first cooling step of cooling the cold-rolled steel sheet after the annealing step from the soaking temperature to a first cooling stop temperature of 600° C. or lower at an average cooling rate of 10.0° C./sec or lower; a second cooling step of cooling the cold-rolled steel sheet after the first cooling step from the first cooling stop temperature to a second cooling stop temperature of an Ms point or lower at an average cooling rate of 1.0° C./sec or higher; a heat treatment step of slowly cooling the cold-rolled steel sheet after the second cooling step so that the time required for cooling the cold-rolled steel sheet from the second cooling stop temperature to 150° C. is 25 seconds or longer, or heating the cold-rolled steel sheet to a temperature range of 200° C. to 400° C. and holding the cold-rolled steel sheet in the temperature range for 10 seconds or longer; and a leveling step of applying a cumulative strain of 1.0% or more to each of front and rear surfaces of the cold-rolled steel sheet after the heat treatment step and setting an elongation ratio to 0.50% or less, in which in the annealing step, when the heating is performed in a temperature range of at least 650° C. to the soaking temperature, in an atmosphere in a furnace, P_(H2O)/P_(H2) that is a ratio of water vapor partial pressure P_(H2O) to hydrogen partial pressure P_(H2) is set to be in a range of 0.00010 to 2.00.

$\begin{array}{cc}Q=2.7\times \left[C\right]+0\text{.4}\times \left[\mathrm{Si}\right]+\left[\mathrm{Mn}\right]+0.45\times \left[\mathrm{Ni}\right]+0\text{.8}\times \left[\mathrm{Cr}\right]+2\text{.0}\times \left[\mathrm{Mo}\right]& \left(1\right)\end{array}$

In Expression (1), [Element symbol] is a content of the element indicated by the element symbol in mass % and 0 is substituted in a case where the element is not contained.

[5] The method for manufacturing a cold-rolled steel sheet according to [4] may further include a hot-dip galvanizing step of immersing the cold-rolled steel sheet in a hot-dip galvanizing bath having a bath temperature of 420° C. to 520° C. in a state that a temperature of the cold-rolled steel sheet is 420° C. to 520° C. to form a hot-dip galvanized layer during the second cooling step.

[6] The method for manufacturing a cold-rolled steel sheet according to [5] may further include an alloying treatment step of holding the cold-rolled steel sheet during the second cooling step and after the hot-dip galvanizing step at a temperature of 460° C. to 580° C. and alloying the hot-dip galvanized layer.

Advantageous Effects of Invention

According to the above aspect of the present invention, it is possible to provide a cold-rolled steel sheet having high strength and excellent elongation, spot weldability, and bendability, and a method for manufacturing the same.

DESCRIPTION OF EMBODIMENTS

A cold-rolled steel sheet according to an embodiment of the present invention (a cold-rolled steel sheet according to the present embodiment) and a method for manufacturing the same will be described.

A cold-rolled steel sheet according to the present embodiment has a predetermined chemical composition, in a surface layer area that is in a range of 10 to 20 μm from a surface in a sheet thickness direction, a microstructure contains, by volume percentage, 10% to 95% of ferrite and a remainder of one or more selected from martensite, bainite, pearlite, cementite, and retained austenite, at a ¼ thickness position which is in a range of ⅛ to ⅜ of a sheet thickness from the surface in the sheet thickness direction, a microstructure contains, by volume percentage, 0% to 60% of ferrite, 0% to 3% of retained austenite, and a remainder of one or more selected from martensite and bainite, a volume percentage of tempered martensite in the martensite is 50% or more, and a ratio of a dislocation density of the surface layer area to a dislocation density at the ¼ thickness position is 0.20 or more and less than 0.90.

In addition, the cold-rolled steel sheet according to the present embodiment has a tensile strength of 1,180 MPa or more.

The cold-rolled steel sheet according to the present embodiment includes a plated steel sheet including a plating layer on a surface.

Hereinafter, each will be described.

In the description, a range indicated by “to” includes values at both ends thereof as a lower limit and an upper limit of the range. However, numerical values indicated as “more than” or “less than” are not included in the range.

<Chemical Composition>

First, the chemical composition will be described.

In the present embodiment, % of the content of each element means mass %.

C: 0.060% to 0.300%

Carbon (C) is an essential element for high-strengthening of the steel sheet. From the viewpoint of increasing the strength, the C content is set to 0.060% or more. The C content is preferably set to 0.080% or more or 0.100% or more.

On the other hand, when the C content is excessive, bendability, press formability, and weldability deteriorate. Accordingly, the C content is set to 0.300% or less. The C content is preferably set to 0.250% or less, 0.200% or less, or 0.160% or less.

Si: 0.01% to 3.00%

Silicon (Si) is a solid solution strengthening element and is an effective element for increasing the strength of the steel sheet. In order to obtain this effect, a Si content is set to 0.01% or more. The Si content is set to preferably 0.10% or more, and more preferably 0.20% or more, 0.40% or more, or 0.60% or more.

On the other hand, in a case where the Si content is excessive, not only the chemical convertibility and wettability with hot-dip galvanizing of the steel sheet significantly deteriorate, but also the bendability deteriorates. Therefore, the Si content is set to 3.00% or less. Therefore, the Si content is set to preferably 2.00% or less, and more preferably 1.70% or less. In addition, from the viewpoint of suppressing the generation of retained austenite, the Si content is preferably set to 1.30% or less or 1.00% or less.

Mn: 1.00% to 5.00%

Manganese (Mn) is a strong austenite stabilizing element and is an effective element for improving the hardenability of the steel sheet. In order to obtain this effect, the Mn content is set to 1.00% or more. The Mn content is preferably set to 1.50% or more, 1.80% or more, or 2.20% or more.

On the other hand, when the Mn content is excessively high, bendability, weldability, and low temperature toughness deteriorate. Therefore, the Mn content is set to 5.00% or less. The Mn content is preferably set to 3.50% or less, 3.00% or less, or 2.70% or less.

Sol.Al: 0.001% to 1.000%

Aluminum (Al) is an effective element for deoxidizing steel. In order to obtain this effect, a sol.Al (acid-soluble Al) content is set to 0.001% or more. The sol.Al content is preferably set to 0.005% or more.

On the other hand, when the sol.Al content is excessive, the effect is saturated, and the cost increases. In addition, a transformation temperature of the steel increases, and a load during hot rolling increases. Therefore, the sol.Al content is set to 1.000% or less. The sol.Al content is preferably set to 0.800% or less, 0.500% or less, or 0.200% or less.

P: 0.100% or Less

Phosphorus (P) is an element that deteriorates the weldability and the toughness. When the P content is more than 0.100%, an adverse effect becomes significant. Therefore, the P content is set to 0.100% or less. The P content is more preferably set to 0.050% or less.

On the other hand, the P content is preferably as small as possible and may be 0%. However, P may be contained as an impurity, and the dephosphorization cost increases to extremely reduce the P content. In addition, P is a solid solution strengthening element and is an effective element for high-strengthening of the steel sheet. Therefore, the P content may be set to 0.001% or more.

S: 0.0100% or Less

Sulfur(S) is an element that forms MnS in steel and deteriorates the toughness and hole expansibility. When the S content is more than 0.0100%, the deterioration of the toughness and the hole expansibility becomes significant. For this reason, the S content is set to 0.0100% or less. The S content is set to preferably 0.0050% or less, and more preferably 0.0020% or less.

On the other hand, the S content is preferably small and may be 0%. However, S may be contained as an impurity, and the desulfurization cost increases in order to extremely reduce the S content. Therefore, the S content may be set to 0.0005% or more from the viewpoint of economic efficiency.

O: 0.1000% or Less

Oxygen (O) is an element contained as an impurity. When the O content is more than 0.1000%, coarse oxides are formed in steel, and the bendability and the hole expansibility decrease. Accordingly, the O content is set to 0.1000% or less. The O content is set to preferably 0.0100% or less, and more preferably 0.0050% or less. The O content may be 0%, but the O content may be set to 0.0001% or more from the viewpoint of the manufacturing cost.

N: 0.0100% or Less

Nitrogen (N) is an element contained as an impurity. When the N content is more than 0.0100%, coarse nitrides are formed in steel, and the bendability and hole expansibility deteriorate. Therefore, the N content is set to 0.0100% or less. The N content is preferably set to 0.0050% or less. The N content may be 0%, but the denitriding cost increases to extremely reduce the N content, and thus the N content may be set to 0.0005% or more or 0.0010% or more.

The cold-rolled steel sheet according to the present embodiment may contain the above elements and a remainder of Fe and impurities. However, in order to further improve various properties, one or more elements (optional elements) selected from Ti, B, Cr, Mo, Ni, Cu, Sn, Nb, V, W, Ca, Mg, Bi, Sb, Zr, and REM shown below may be contained. Since the optional elements may not be contained, the lower limit is 0%.

Ti: 0% to 0.200%

Titanium (Ti) is an element that suppresses formation of BN, which is a factor for reducing the hardenability, by fixing N as TiN in steel. In addition, Ti is an element that refines an austenite grain size during heating and improves the toughness. In a case of obtaining this effect, the Ti content is preferably set to 0.005% or more. The Ti content is preferably set to 0.010% or more.

On the other hand, when the Ti content is excessive, ductility of the steel sheet decreases. Accordingly, in a case where T is contained, the T content is set to 0.200% or less. The Ti content is preferably set to 0.050% or less.

B: 0% to 0.0100%

Boron (B) is an element that is segregated to austenite grain boundaries or ferrite/austenite grain boundaries during heating of the steel sheet and increases the hardenability of the steel by stabilizing the grain boundaries. In the case of obtaining this effect, the B content is preferably set to 0.0005% or more. The B content is preferably set to 0.0010% or more.

On the other hand, when the B content is excessive, the hardenability of the steel is impaired due to formation of a boride. Therefore, in a case where B is contained, the B content is set to 0.0100% or less. The B content is preferably 0.0050% or less or 0.0030% or less.

• • Cr: 0% to 1.00%
  • Mo: 0% to 1.00%
  • Ni: 0% to 1.00%
  • Cu: 0% to 1.00%
  • Sn: 0% to 0.50%

Chromium (Cr), molybdenum (Mo), nickel (Ni), copper (Cu), and tin (Sn) are all effective elements for high-strengthening of the steel sheet. Therefore, the elements may be contained as necessary.

In order to obtain the above effect, it is preferable to contain 0.001% or more, more preferably 0.01% or more, and even more preferably 0.05% or more of one or more selected from Cr, Mo, Ni, Cu, and Sn.

On the other hand, when these elements are excessively contained, the effect is saturated and the cost increases. Therefore, in a case where Cr, Mo, Ni, and Cu are contained, the Cr content, the Mo content, the Ni content, and the Cu content are all set to 1.00% or less, and the Sn content is set to 0.50% or less. The Cr content, the Mo content, the Ni content, and the Cu content are each preferably set to 0.60% or less, and the Sn content is preferably set to 0.30% or less.

• • Nb: 0% to 0.200%
  • V: 0% to 0.50%
  • W: 0% to 0.50%

Niobium (Nb), vanadium (V), and tungsten (W) are carbide forming elements and are effective elements for increasing the strength of the steel sheet. Therefore, the elements may be contained as necessary.

In order to obtain the above effect, the Nb content is set to preferably 0.001% or more, more preferably 0.005% or more, and even more preferably 0.010% or more. In addition, the V content and/or the W content is preferably set to 0.01% or more.

On the other hand, the effect is saturated and the cost increases even when these elements are excessively contained. Therefore, in a case where Nb is contained, the Nb content is set to 0.200% or less, and the V content and the W content are both set to 0.50% or less. The Nb content is preferably set to 0.100% or less, and the V content and the W content are both preferably set to 0.30% or less.

• • Ca: 0% to 0.0100%
  • Mg: 0% to 0.0100%
  • Bi: 0% to 0.0100%
  • Sb: 0% to 0.100%
  • Zr: 0% to 0.0100%
  • REM: 0% to 0.0100%

Calcium (Ca), magnesium (Mg), antimony (Sb), zirconium (Zr), and a rare earth element (REM) are elements that contribute to fine dispersion of inclusions in steel, and bismuth (Bi) is an element that reduces microsegregation of substitutional alloying elements such as Mn and Si in steel. Each of the elements contributes to the improvement in bendability of the steel sheet. Therefore, the elements may be contained as necessary.

In order to obtain the above effect, it is preferable to contain 0.0001% or more, and more preferably 0.0010% or more of one or more selected from Ca, Mg, Bi, Sb, Zr, and REM.

On the other hand, when these elements are excessively contained, ductility deteriorates. Therefore, the Ca content, the Mg content, the Bi content, the Zr content, and the REM content are all set to 0.0100% or less, and the Sb content is set to 0.100% or less. The Ca content, the Mg content, the Bi content, the Zr content, and the REM content are all set to preferably 0.0060% or less, and the Sb content is set to preferably 0.080% or less.

Here, REM refers to a total of 17 elements of Sc, Y, and lanthanoids, and the REM content means the total content of these elements. Lanthanoids are industrially added in the form of mischmetal.

As described above, the chemical composition of the cold-rolled steel sheet according to the present embodiment contains C, Si, Mn, sol.Al, P, S, O, and N, and a remainder of Fe and impurities, or contains C, Si, Mn, sol.Al, P, S, O, and N, further contains one or more elements selected from Ti, B, Cr, Mo, Ni, Cu, Sn, Nb, V, W, Ca, Mg, Bi, Sb, Zr, and REM, and a remainder of Fe and impurities.

The impurities mean components that are mixed from raw materials such as ore and scrap or by other factors when a steel material is industrially manufactured and are allowed within a range that does not adversely affect the properties. The impurities are not limited to P, S, O, and N, and may include, for example, Co, As, Zn, In, Sr, Li, Re, Os, Ir, Tc, Pb, Se, Ta, H, Hf, and the like.

In the cold-rolled steel sheet according to the present embodiment, the chemical composition is controlled such that the contents of the elements are within the above-described ranges, and further, Q calculated from the C content, the Si content, the Mn content, the Ni content, the Cr content, and the Mo content by Expression (1) is 2.3 or more.

$\begin{array}{cc}Q=2.7\times \left[C\right]+0\text{.4}\times \left[\mathrm{Si}\right]+\left[\mathrm{Mn}\right]+0.45\times \left[\mathrm{Ni}\right]+0\text{.8}\times \left[\mathrm{Cr}\right]+2\text{.0}\times \left[\mathrm{Mo}\right]& \left(1\right)\end{array}$

In Expression (1), [Element Symbol] is a content of the element indicated by the element symbol in mass %.

Q calculated by Expression (1) is an index related to the hardenability of steel. When the value of Q is less than 2.3, the hardenability is insufficient, and a sufficient strength of the steel sheet cannot be obtained. Therefore, Q is 2.3 or more.

$\left(Q=2.7\times \left[C\right]+0\text{.4}\times \left[\mathrm{Si}\right]+\left[\mathrm{Mn}\right]+0.45\times \left[\mathrm{Ni}\right]+0\text{.8}\times \left[\mathrm{Cr}\right]+2\text{.0}\times \left[\mathrm{Mo}\right]\ge 2.3\right)$

The chemical composition of the cold-rolled steel sheet according to the present embodiment can be obtained by the following method.

The chemical composition of the steel sheet described above may be measured by a general chemical composition.

For example, the chemical composition may be measured using inductively coupled plasma-atomic emission spectrometry (ICP-AES). sol.Al may be measured by ICP-AES using a filtrate obtained by heating and decomposing a sample with an acid. In addition, C and S may be measured by using an infrared absorption method after combustion, N may be measured by using an inert gas-fusion thermal conductivity method, and O may be measured by using an inert gas fusion-nondispersive infrared absorption method. In a case where the steel sheet is provided with a plating layer on a surface, the chemical composition may be analyzed after removing the plating layer by mechanical grinding.

<Microstructure (Metallographic Structure)>

In the cold-rolled steel sheet according to the present embodiment, a surface layer area which is in a range of 10 to 20 μm from the surface in the sheet thickness direction (a position of 10 μm away from the surface in the sheet thickness direction to a position of 20 μm away from the surface in the sheet thickness direction) and a microstructure at ¼ thickness positions, which are ranges of ⅛ of the sheet thickness from the surface to ⅜ of the sheet thickness from the surface with a position of a ¼ sheet thickness from the surface in the sheet thickness direction set as a center, are controlled as follows, respectively.

In a case where the cold-rolled steel sheet is a plated steel sheet, the surface referred to herein is a surface of a steel sheet excluding the plating layer (base steel sheet).

[Volume Percentage of Each Phase in Microstructure of Surface Layer Area]

In the cold-rolled steel sheet according to the present embodiment, in the surface layer area which is in a range of 10 to 20 μm from the surface in the sheet thickness direction, the microstructure includes, by volume percentage, 10% to 95% of ferrite and a remainder of one or more selected from martensite, bainite, pearlite, cementite, and retained austenite.

When the volume percentage of ferrite is less than 10%, bending fracture cannot be sufficiently suppressed. The volume percentage of ferrite is preferably 12% or more. On the other hand, when the volume percentage of ferrite is more than 95%, the tensile strength and the bending strength of the steel sheet decrease. Therefore, the volume percentage of ferrite is 95% or less. The volume percentage of ferrite is preferably 80% or less.

The remainder may be one or more selected from martensite, bainite, pearlite, cementite, and retained austenite, but preferably does not include (is 0%) pearlite and cementite.

[Volume Percentage of Each Phase in Microstructure at ¼ Thickness Position]

In the cold-rolled steel sheet according to the present embodiment, at the ¼ thickness position which is in a range of ⅛ to ⅜ of the sheet thickness from the surface with a position of a ¼ sheet thickness from the surface in the sheet thickness direction set as a center, the microstructure includes, by volume percentage, 0% to 60% of ferrite, 0% to 3% of retained austenite, and a remainder of one or more selected from martensite and bainite.

Ferrite is not necessarily contained, but is a structure that is easily deformed and contributes to the improvement in elongation since it is a soft structure. Therefore, ferrite may be contained. However, when the volume percentage of ferrite is more than 60%, the volume percentage of the martensite and/or bainite included in the remainder is small, and sufficient tensile strength cannot be obtained. Therefore, the volume percentage of ferrite is set to 60% or less. The volume percentage of ferrite is preferably 40% or less, 30% or less, or 20% or less.

Retained austenite is a structure that contributes to the improvement in elongation (particularly, uniform elongation) by transformation-induced plasticity (TRIP) effect, but there is a concern that the spot weldability may be reduced.

Therefore, the volume percentage of retained austenite is set to 3% or less. The volume percentage of retained austenite is preferably 2% or less.

In the present embodiment, martensite includes quenched martensite (so-called fresh martensite) and tempered martensite obtained by tempering. Compared with the tempered martensite, the quenched martensite is brittle, and thus is likely to serve as an origin of fracture when plastic deformation such as bending is applied. Therefore, in order to secure desired bendability, the proportion of tempered martensite in the entire martensite by volume percentage in the steel sheet (represented by the ¼ thickness position) is set to 50% or more. The proportion of tempered martensite in the entire martensite is preferably set to 60% or more, 70% or more, or 80% or more. In addition, in the surface layer area, the proportion of tempered martensite in the entire martensite by volume percentage is preferably set to 30% or more. The proportion of tempered martensite is more preferably 40% or more or 50% or more.

The volume percentages of ferrite, bainite, martensite (tempered martensite and fresh martensite), pearlite, cementite, and retained austenite contained in the microstructure at the surface layer area and the ¼ thickness position can be measured by using the following methods.

A sample is collected with a cross section parallel to a rolling direction and the sheet thickness direction of the steel sheet set as an observed section, and the observed section is polished and subjected to nital etching.

Next, in a case of observing the structure at the ¼ thickness position, in a range of ⅛ of the sheet thickness from the surface to ⅜ of the sheet thickness from the surface with a position of a ¼ sheet thickness from the surface set as a center, a total of 5 visual fields are observed at a magnification of 5000-fold by a field emission scanning electron microscope (FE-SEM) by setting one visual field to 250 μm² or more. In addition, area ratios of ferrite, bainite, tempered martensite, fresh martensite, pearlite, cementite, and retained austenite are measured, and are regarded as the volume percentages.

In addition, in a case where the structure of the surface layer area is observed, the structure is observed in the same manner as at the ¼ thickness position in a range of 10 to 20 μm from the surface in the sheet thickness direction, the area ratios of ferrite, bainite, tempered martensite, fresh martensite, pearlite, cementite, and retained austenite are measured, and the area ratios are regarded as volume percentages.

Here, with regard to identification of each phase, a region which has a substructure in grains and in which carbides are precipitated with a plurality of variants is determined as tempered martensite. A region where cementite is precipitated in a lamellar form is determined as pearlite or cementite. A region in which luminance is low and no substructure is observed is determined as ferrite. A region where the luminance is high and the substructure is not revealed by etching is determined as fresh martensite or retained austenite. The remainder is determined as bainite. Each volume percentage is calculated by a point counting method and is set as the volume percentage of each structure.

A volume percentage of fresh martensite can be obtained by subtracting a volume percentage of retained austenite obtained by an EBSD method to be described later from a volume percentage of fresh martensite or retained austenite.

In the cold-rolled steel sheet according to the present embodiment, the volume percentage of the retained austenite in the surface layer area and the ¼ thickness position is evaluated by performing high-resolution crystal structure analysis by an electron backscatter diffraction method (EBSD method). Specifically, a sample is collected with a cross section parallel to a rolling direction and the sheet thickness direction of the steel sheet set as an observed section, and the observed section is polished and finished to a mirror surface. Furthermore, in order to remove a processed layer of a surface layer, electrolytic polishing or mechanical polishing using colloidal silica is performed.

Next, in each of the surface layer area and the ¼ thickness position of the steel sheet, crystal structure analysis is performed by the EBSD method on five visual fields at a magnification of 5,000-fold and a visual field size of 150 μm² or more. A distance between evaluation points (step) is set to 0.01 to 0.20 μm.

Data obtained by the EBSD method is analyzed by using “OIM Analysis 6.0” manufactured by TSL Solutions Inc. From the observation results at each position, a region determined to be FCC-iron is determined as retained austenite, and the volume percentage of retained austenite at each of the surface layer area and the ¼ thickness position is calculated.

[Dislocation Density]

In the cold-rolled steel sheet according to the present embodiment, a ratio of the dislocation density of the surface layer area to the dislocation density at the ¼ thickness position is 0.20 or more and less than 0.90 (20% or more and less than 90%).

In the cold-rolled steel sheet according to the present embodiment, in order to suppress bending fracture, ferrite having excellent ductility is disposed in the surface layer area, and the bending strength is increased by dislocation strengthening of the surface layer area.

When the ratio of the dislocation density in a range of 0 to 20 μm from the surface of the steel sheet to the dislocation density at the ¼ thickness position is less than 0.20, the bending strength significantly decreases. Therefore, the ratio of the dislocation densities is set to 0.20 or more. The ratio of the dislocation densities is preferably 0.22 or more, 0.25 or more, or 0.30 or more.

On the other hand, when the ratio of the dislocation densities is 0.90 or more, bending fracture cannot be sufficiently suppressed. Therefore, the ratio of the dislocation densities is set to less than 0.90. The ratio of the dislocation densities is preferably less than 0.80 and more preferably 0.60 or less.

The dislocation densities at the surface layer area and the ¼ thickness position are obtained by the following method.

Two samples (a sample for measuring the dislocation density of the surface layer area and a sample for measuring the dislocation density at the ¼ thickness position) are collected so that a surface parallel to the rolled surface of the steel sheet is an observed section.

For the sample for measuring the dislocation density of the surface layer area, in a case where a plating layer is present on the surface, the plating layer is removed, and then the sample is adjusted so that a position of 10 to 20 μm from the surface can be measured by performing chemical polishing on the observed section.

For the sample for measuring the dislocation density at the ¼ thickness position, the sample is ground from the surface to the ¼ thickness position in the sheet thickness direction, and then chemical polishing is performed to remove a processed layer due to grinding, and the ¼ thickness position is adjusted to be exposed on the surface.

A strain of the steel sheet is measured by an X-ray diffractometer for each of the samples adjusted as described above. The measurement is performed by using an X-ray diffractometer, the diffraction intensities of a (110) plane, a (200) plane, a (211) plane, a (220) plane, and a (310) plane of alpha iron are measured by using CoKα-rays as an X-ray source, a half value width of a peak value of a reflection intensity of each crystal plane is obtained from the measurement chart, and a local strain s′ applied to the steel sheet is determined by Expressions (2) and (3).

$\begin{array}{cc}\beta \cos \theta /\lambda =0.9/D+2{\epsilon }^{\prime }\sin \theta /\lambda & \left(2\right)\end{array}$

Here, the meanings of the symbols are as follows.

• • β: Half value width of peak value (where a value corrected by Expression (3) is used)
  • θ: Diffraction angle
  • λ: Wavelength (0.1790 nm) of CoKα-rays
  • D: Crystallite size (dislocation cell, crystal grain size)
  • ε′: Local strain

$\begin{array}{cc}{\beta }^2=\beta m^2-\beta O^2& \left(3\right)\end{array}$

Here, the meanings of the symbols are as follows.

• • βm: Half value width of a peak of a sample for measuring the dislocation density
  • β0: Half value width of a peak of a sample without strain
  • β cos θ/λ is plotted against sin θ/λ, and ε′ and D are obtained from a slope and intercepts. The dislocation density ρ is determined from the obtained local strain ε′ by Expression (4):

$\begin{array}{cc}\rho =14.4{\epsilon }^{\mathrm{\prime 2}}/b^2& \left(4\right)\end{array}$

Here, the meanings of the symbols are as follows.

• • b: Burgers vector (0.248 nm).

[Plating Layer]

The cold-rolled steel sheet according to the present embodiment described above may include a hot-dip galvanized layer on the surface. Corrosion resistance is improved by providing the hot-dip galvanized layer on the surface.

For example, in a case where the cold-rolled steel sheet is used under a corrosive environment, there may be cases where the sheet thickness cannot be reduced to a certain sheet thickness or less even though high-strengthening is achieved because of concerns about perforation and the like. One of the purposes of the high-strengthening of the steel sheet is to reduce the weight by thinning. Therefore, even if a high-strength steel sheet is developed, an application range of a steel sheet with low corrosion resistance is limited. Therefore, the steel sheet is considered to be plated with a hot-dip galvanizing or the like having high corrosion resistance. The plating layer is, for example, a galvanized layer such as a hot-dip galvanized layer or an electrogalvanized layer. In addition, the galvanized layer may be a plating containing Si, Al, and/or Mg in addition to Zn.

In addition, the galvanized layer may be alloyed. In the alloyed hot-dip galvanized layer (hot-dip galvannealed layer), Fe is incorporated into the hot-dip galvanized layer by the alloying treatment, so that excellent weldability and coatability can be obtained.

In addition, upper layer plating may be performed on the galvanized layer in order to improve the coatability and the weldability. In addition, in the cold-rolled steel sheet according to the present embodiment, various treatments such as a chromate treatment, a phosphate treatment, a lubricity improvement treatment, and a weldability improvement treatment may be performed on the hot-dip galvanized layer.

<Mechanical Properties>

The cold-rolled steel sheet according to the present embodiment has high strength and excellent elongation, spot weldability, and bendability.

When the cold-rolled steel sheet is used for a vehicle component, a tensile strength is 1,180 MPa or more in consideration of a contribution to a weight reduction of the automobile. An upper limit of the tensile strength is not limited, but the tensile strength may be 1,500 MPa or less from the viewpoint of securing the spot weldability.

The cold-rolled steel sheet according to the present embodiment preferably has a sheet thickness (a sheet thickness of a base steel sheet excluding a plating layer) of 0.8 to 3.0 mm in consideration of application to vehicle components.

<Manufacturing Method>

The cold-rolled steel sheet according to the present embodiment can be manufactured by a manufacturing method including the following steps.

(I) A hot rolling step of heating a slab having a predetermined chemical composition and performing hot rolling so that a finish rolling completion temperature is 800° C. or higher and an Ar3 point or higher to obtain a hot-rolled steel sheet,

(II) A cooling step on run-out table of starting cooling of the hot-rolled steel sheet from completion of the hot rolling after the elapse of 1.0 second or longer and cooling the hot-rolled steel sheet to a coiling temperature of 400° C. to 750° C. at an average cooling rate of 5.0° C./sec or faster,

(III) A coiling step of coiling the hot-rolled steel sheet after the cooling step on run-out table at the coiling temperature,

(IV) A cold rolling step of performing cold rolling on the hot-rolled steel sheet after the coiling step so that a cumulative rolling reduction becomes 20% to 80% to obtain a cold-rolled steel sheet,

(V) An annealing step of heating the cold-rolled steel sheet after the cold rolling step to a soaking temperature of 750° C. to 1,000° C. and holding the cold-rolled steel sheet at the soaking temperature for 1 second or longer,

(VI) A first cooling step of cooling the cold-rolled steel sheet after the annealing step to a first cooling stop temperature of 600° C. or lower from the soaking temperature at an average cooling rate of 10.0° C./sec or slower,

(VII) A second cooling step of cooling the cold-rolled steel sheet after the first cooling step from the first cooling stop temperature to a second cooling stop temperature of an Ms point or lower at an average cooling rate of 1.0° C./sec or higher,

(VIII) A heat treatment step of slowly cooling the cold-rolled steel sheet after the second cooling step so that the time required from the second cooling stop temperature to 150° C. is 25 seconds or longer, or heating the cold-rolled steel sheet to a temperature range of 200° C. to 400° C. and holding at the temperature range for 10 seconds or longer, and

(IX) A leveling step of applying a cumulative strain of 1.0% or more to each of a front surface and a rear surface of the cold-rolled steel sheet after the heat treatment step and setting an elongation ratio to 0.50% or less.

Hereinafter, each step will be described.

[Hot Rolling Step]

In the hot rolling step, a slab having the same chemical composition as the cold-rolled steel sheet according to the present embodiment described above is heated and hot-rolled to obtain a hot-rolled steel sheet.

As the slab to be subjected to hot rolling, a continuous cast slab or a slab manufactured by a thin slab caster or the like can be used.

In the hot rolling step, the slab heating temperature is preferably set to 1,050° C. or higher. When the slab heating temperature is excessively low, a finish rolling temperature also decreases. The decrease in the finish rolling temperature may cause an increase in a rolling force to an excessive level, and there is a concern that rolling may be difficult or the resulting steel sheet may have a poor shape.

An upper limit of the slab heating temperature is not particularly specified, and the effects of the present invention are exhibited. However, it is not preferable to excessively increase the heating temperature for economic reasons. Therefore, the slab heating temperature is preferably set to 1,350° C. or lower.

In addition, in a case where a process such as continuous casting-direct rolling (CC-DR) in which hot rolling is immediately performed after casting is applied, it is not necessary to heat the slab in a case where the hot rolling can be started at a slab temperature of 1,050° C. or higher.

The hot rolling is performed so that a finish rolling completion temperature is 800° C. or higher and an Ar3 point or higher. When the finish rolling completion temperature is lower than 800° C. or lower than the Ar3 point, a rolling force during the finish rolling increases, and hot rolling becomes difficult.

Meanwhile, an upper limit of the finish rolling temperature is not limited. However, in a case where the finish rolling temperature is excessively high, a slab heating temperature needs to be excessively high in order to secure the temperature. Therefore, the finish rolling temperature is preferably set to 1000° C. or lower.

The Ar3 point (° C.) is calculated by the following expression.

$\mathrm{Ar}3=901-325\times \left[C\right]+33\times \left[\mathrm{Si}\right]-92\times \left(\left[\mathrm{Mn}\right]+\left[\mathrm{Ni}\right]/2+\left[\mathrm{Cr}\right]/2+\left[\mathrm{Cu}\right]/2+\left[\mathrm{Mo}\right]/2\right)+52\times \left[\mathrm{sol}.\mathrm{Al}\right]$

In the above expression, [element symbol] is the content [mass %] of each element. In addition, in a case where the element is not contained, 0 is substituted.

[Cooling Step on Run-Out Table]

[Coiling Step]

It is preferable that the hot-rolled steel sheet after the hot rolling step start to be cooled after the elapse of 1.0 second or longer from completion of the hot rolling, and is cooled to a coiling temperature of 400° C. to 750° C. at an average cooling rate of 5.0° C./sec or faster (cooling step on run-out table), and the hot-rolled steel sheet is coiled at the coiling temperature (coiling step).

When the time from completion of the hot rolling to the start of the cooling is shorter than 1.0 second, recrystallization of austenite becomes insufficient and steel sheet anisotropy is obvious, which is not preferable. In addition, when the average cooling rate from completion of the finish rolling to the coiling temperature is slower than 5.0° C./sec, ferritic transformation in a high-temperature range is promoted, and a hot-rolled sheet structure becomes coarse, which is not preferable.

In addition, when the coiling temperature is higher than 750° C., the thickness of oxides formed on a surface of the steel sheet excessively increases, and pickling properties deteriorate, which is not preferable. In order to increase the pickling properties, the coiling temperature is more preferably 720° C. or lower, and still more preferably 700° C. or lower. On the other hand, when the coiling temperature is lower than 400° C., the strength of the hot-rolled steel sheet excessively increases, and cold rolling becomes difficult, which is not preferable.

The hot-rolled coil after the coiling step may be pickled by a known method as necessary. In addition, skin pass rolling may be carried out in order to straighten a shape of the hot-rolled coil and to improve the pickling properties.

[Cold Rolling Step]

In a cold rolling step, the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet. A cumulative rolling reduction in cold rolling is preferably 20% to 80%. When the cumulative rolling reduction is set to 20% or more, it is possible to refine an austenite grain size during heating in an annealing step to be described later. The cumulative rolling reduction in cold rolling is more preferably 30% or more. On the other hand, excessive reduction results in an excessive rolling force and an increase in the load on the cold rolling mill. The cumulative rolling reduction is more preferably 70% or less.

[Annealing Step]

In the annealing step, the cold-rolled steel sheet after the cold rolling step is heated to a soaking temperature (annealing temperature) of 750° C. to 1,000° C. and held at the soaking temperature for 1 second or longer. In addition, in the annealing step, when performing heating in a temperature range of at least 650° C. to the soaking temperature, the P_(H2O)/P_(H2) in the atmosphere in the furnace, which is a ratio of a water vapor partial pressure P_(H2O) to a hydrogen partial pressure P_(H2), is set to be in a range of 0.00010 to 2.00.

When the soaking temperature is lower than 750° C., the amount of austenite during soaking becomes insufficient, and a sufficient amount of a hard structure (martensite and bainite) cannot be secured during phase transformation in the subsequent cooling. Therefore, the soaking temperature is set to 750° C. or higher. The soaking temperature is preferably set to 770° C. or higher. On the other hand, when the soaking temperature is higher than 1,000° C., the grain size of austenite becomes coarse, and transformation during cooling is less likely to proceed, and it is difficult to sufficiently obtain a soft ferrite structure. Therefore, the soaking temperature is set to 1,000° C. or lower. The soaking temperature is preferably set to 900° C. or lower.

In addition, the holding time during the soaking is set to 1 second or longer. When the holding time is shorter than 1 second, there is a concern that austenitic transformation may not sufficiently occur. An upper limit of the holding time is not particularly limited. However, when the holding time is too long, the manufacturability of the steel sheet is impaired. Therefore, the holding time may be set to 1,000 seconds or shorter.

An average heating rate from 650° C. to the soaking temperature is not limited, but is preferably 5.0° C./sec or slower in order to secure an appropriate decarburization amount.

In addition, in the heating process of the annealing step, when heating to 650° C. to the soaking temperature is performed, the atmosphere in the furnace is controlled, so that a large amount of soft ferrite can be formed in the surface layer area in the subsequent first cooling step by a decarbonizing reaction.

When the P_(H2O)/P_(H2) in the furnace during the heating to the soaking temperature of 650° C. is less than 0.00010, it is not possible to generate a sufficient volume percentage of ferrite to improve the bendability. Therefore, P_(H2O)/P_(H2) is set to 0.00010 or more. P_(H2O)/P_(H2) is preferably 0.00020 or more, and more preferably 0.01 or more or 0.05 or more.

On the other hand, when P_(H2O)/P_(H2) is more than 2.00, decarburization excessively progresses, the thickness of a decarburized layer increases, and there is a concern that the bending strength may decrease. Therefore, P_(H2O)/P_(H2) is set to 2.00 or less. P_(H2O)/P_(H2) is preferably 1.50 or less and more preferably 1.20 or less.

[First Cooling Step]

[Second Cooling Step]

After the annealing step in which the cold-rolled steel sheet is held at the soaking temperature, the cold-rolled steel sheet is cooled in two stages.

That is, after the annealing step, a treatment including a first cooling step of cooling from the soaking temperature to the first cooling stop temperature and a second cooling step of cooling from the second cooling start temperature equal to the first cooling stop temperature to the second cooling stop temperature is performed.

In the first cooling step, the cold-rolled steel sheet is cooled from the soaking temperature to the first cooling stop temperature of 600° C. or lower at an average cooling rate of 10.0° C./sec or lower. This cooling promotes ferritic transformation in the surface layer area of the steel sheet.

In a case where the average cooling rate is faster than 10.0° C./sec or the cooling stop temperature is higher than 600° C., a sufficient volume percentage of ferrite cannot be generated in the surface layer area.

In the second cooling step performed after the first cooling step, the cold-rolled steel sheet is cooled from the second cooling start temperature equal to the first cooling stop temperature to a temperature equal to or lower than the Ms point (second cooling stop temperature) at an average cooling rate of 1.0° C./sec or faster.

When the average cooling rate is slower than 1.0° C./sec, the volume percentage of ferrite or the like increases, the volume percentage of martensite decreases, and there is a concern that sufficient tensile strength cannot be obtained. The average cooling rate in the second cooling step is preferably faster than 10.0° C./sec. An upper limit of the average cooling rate does not need to be particularly specified. However, in order to achieve a cooling rate of higher than 300° C./sec, special equipment is required. Therefore, the average cooling rate may be set to 300° C./sec or slower.

In addition, when the second cooling stop temperature is higher than the Ms point, there is a concern that martensite cannot be sufficiently obtained and the strength cannot be secured.

The Ms point (° C.) can be obtained by Expression (5).

$\begin{array}{cc}\mathrm{Ms}=550-361\times \left[C\right]-39\times \left[\mathrm{Mn}\right]-35\times \left[V\right]-20\times \left[\mathrm{Cr}\right]-17\left[\mathrm{Ni}\right]-10\times \left[\mathrm{Cu}\right]-5\times \left[\mathrm{Mo}\right]+30\times \left[\mathrm{sol}.\mathrm{Al}\right]& \left(5\right)\end{array}$

Here, [element symbol] in the expression indicates the content (mass %) of each element contained in the steel, and in a case where the content is 0, 0 is substituted into the expression and calculated.

[Heat Treatment Step]

After completion of the second cooling step, the cold-rolled steel sheet is subjected to a heat treatment in which the cold-rolled steel sheet is slowly cooled so that the time required from the second cooling stop temperature to 150° C. is 25 seconds or longer, or is heated to a temperature range of 200° C. to 400° C. and is held in the temperature range for 10 seconds or longer. By this heat treatment, martensite is tempered at least at the ¼ thickness position.

In the case of slow cooling, it is difficult to set the proportion of tempered martensite in the entire martensite to 50% or more when the time required to reach 150° C. is shorter than 25 seconds, and there is a concern that the bendability may deteriorate.

In the case of performing heating and holding (tempering), when the holding time is shorter than 10 seconds or the heat treatment temperature (holding temperature) is lower than 200° C., it is difficult to set the proportion of tempered martensite in the entire martensite to 50% or more, and there is a concern that the bendability may deteriorate. On the other hand, when the heat treatment temperature is higher than 400° C., martensite is excessively tempered, and it is difficult to secure a sufficient tensile strength.

[Galvanizing Step]

In a case where the cold-rolled steel sheet is a hot-dip galvanized steel sheet (in a case where a hot-dip galvanized layer is formed on the surface), a hot-dip galvanizing step may be further included in the middle of the second cooling step.

In a case where hot-dip galvanizing is performed during the second cooling step, as long as a condition in which the average cooling rate from the second cooling start temperature to the second cooling stop temperature of the Ms point or lower is 1.0° C./sec or faster is satisfied, the cold-rolled steel sheet may be immersed in a hot-dip galvanizing bath for a predetermined time in a state where the temperature of the cold-rolled steel sheet is in a predetermined range.

Specifically, during the second cooling, the cold-rolled steel sheet is immersed in a hot-dip galvanizing bath having a bath temperature of 420° C. to 520° C. in a state where the cold-rolled steel sheet has a temperature of 420° C. to 520° C. to form a hot-dip galvanized layer.

When the temperature of the cold-rolled steel sheet during immersion in the plating bath is lower than 420° C., heat extraction from the hot-dip galvanizing bath increases, which impairs productivity. In addition, when the steel sheet temperature during immersion in the plating bath or the plating bath temperature is higher than 520° C., pearlitic transformation occurs, and it is difficult to obtain a desired steel structure.

The time from heating of the steel sheet to a temperature of 420° C. to 520° C. to immersion of the steel sheet in the hot-dip galvanizing bath is not particularly specified, but is preferably 100 seconds or shorter from the viewpoint of productivity.

In addition, in a case where the galvanized layer is formed by performing electrogalvanizing, the electrogalvanizing may be performed after the heat treatment step. In addition, conditions for the electrogalvanizing may be conditions according to a known method.

[Alloying Treatment Step]

In a case where the hot-dip galvanized layer is subjected to an alloying treatment to form a hot-dip galvannealed layer, the cold-rolled steel sheet in which the hot-dip galvanized layer is formed during the second cooling step and after the hot-dip galvanizing step is held at a temperature of 460° C. to 580° C.

When the alloying treatment temperature (holding temperature) is lower than 460° C., the alloying reaction takes a long time, so that productivity is impaired. On the other hand, when the alloying treatment temperature is higher than 580° C., pearlitic transformation occurs, and it is difficult to obtain a desired steel structure.

[Leveling Step]

The cold-rolled steel sheet after each of the steps (after the heat treatment step, after the hot-dip galvanizing step, or after the alloying treatment step) is subjected to bending and unbending deformation to apply cumulative strain of 1.0% or more to front and rear surfaces.

Accordingly, dislocations can be introduced into the front and rear surfaces of the cold-rolled steel sheet, and the bending strength can be improved. In a case where the cumulative strain of the front and rear surfaces is less than 1.0%, it is not possible to obtain good bending strength. The cumulative strain is preferably 3.0% or more, and more preferably 10.0% or more.

An upper limit of the cumulative strain does not need to be limited, but is preferably 30.0% or less from the viewpoint of fatigue strength.

However, in the leveling step, it is necessary to set the overall elongation ratio to 0.50% or less. When the elongation ratio is more than 0.50%, the elongation of the cold-rolled steel sheet decreases. The elongation ratio in the leveling step is preferably 0.20% or less.

The above-described cumulative strain is obtained by accumulating strain histories obtained by attaching strain gauges to the front and rear surfaces. In a case where the cumulative strain is examined in advance with a steel sheet having the same strength or sheet thickness, the cumulative strain can be set to be the target cumulative strain by performing bending and unbending under the same conditions.

[Temper Rolling Step]

After the leveling step, temper rolling may be further performed for shape adjustment or the like. However, in a case where temper rolling is performed, the elongation ratio in the leveling step and the elongation ratio in the temper rolling are set to a total of 0.50% or less.

EXAMPLES

Slabs having the chemical compositions shown in Tables 1-1 and 1-2 were hot-rolled and coiled under conditions shown in Tables 2-1 and 2-2 to obtain hot-rolled steel sheets.

In addition, the hot-rolled steel sheets were cold-rolled under conditions shown in Tables 2-1 and 2-2 to obtain cold-rolled steel sheets.

The cold-rolled steel sheets were annealed under conditions shown in Tables 3-1 and 3-2, and then first cooling and second cooling were performed. Some of the cold-rolled steel sheets were immersed in a plating bath of 420° C. to 520° C. during the second cooling and hot-dip galvanizing was performed under conditions shown in Tables 3-1 and 3-2. In addition, some of the cold-rolled steel sheets subjected to hot-dip galvanizing were subjected to an alloying treatment at the alloying temperatures shown in Tables 3-1 and 3-2. Plating in which only hot-dip plating was performed was defined as GI, and plating in which an alloying treatment was also performed was defined as GA.

After that, the heat treatment, the leveling, and the temper rolling were performed under conditions shown in Tables 3-1 and 3-2.

TABLE 1-1
Steel	Chemical composition (mass %, remainder: Fe and impurities)
type	C	Si	Mn	P	S	sol. Al	Ti	B	O	N	Cr	Mo	Ni	Cu
A	0.105	1.41	2.52	0.007	0.0018	0.031	0.025	0.0018	0.0027	0.0033	0.11	0.09
B	0.102	0.51	2.13	0.008	0.0026	0.021	0.022	0.0021	0.0021	0.0019
C	0.115	0.62	2.35	0.014	0.0023	0.019	0.021	0.0022	0.0019	0.0023	0.32
D	0.141	0.82	2.41	0.011	0.0021	0.012	0.031	0.0072	0.0036	0.0021
E	0.181	1.12	2.21	0.021	0.0032	0.016	0.131	0.0035	0.0038	0.0025			0.53
F	0.095	0.65	1.95	0.009	0.0017	0.443	0.028	0.0034	0.0023	0.0018				0.24
G	0.107	0.21	2.45	0.025	0.0014	0.025	0.015	0.0014	0.0018	0.0027
H	0.068	0.97	2.64	0.023	0.0035	0.041	0.019	0.0023	0.0015	0.0029
I	0.089	0.56	1.58	0.018	0.0022	0.036	0.027	0.0018	0.0022	0.0012	0.52	0.12
J	0.142	1.32	1.23	0.012	0.0025	0.024	0.023	0.0019	0.0013	0.0019		0.05	0.42
K	0.104	2.45	1.84	0.034	0.0017	0.032	0.019	0.0015	0.0014	0.0018
L	0.113	0.74	4.41	0.024	0.0029	0.017	0.031	0.0032	0.0024	0.0025
M	0.321	1.24	2.16	0.025	0.0014	0.019	0.024	0.0021	0.0019	0.0024
N	0.054	1.51	1.84	0.018	0.0026	0.029	0.014	0.0032	0.0012	0.0026
O	0.084	3.31	1.69	0.022	0.0017	0.025	0.028	0.0025	0.0024	0.0022
P	0.114	1.87	0.74	0.021	0.0031	0.028	0.024	0.0029	0.0021	0.0027	0.21	0.31
Q	0.097	0.77	5.41	0.019	0.0021	0.015	0.021	0.0031	0.0016	0.0034
R	0.102	0.74	1.84	0.021	0.0017	0.026			0.0023	0.0025	0.20	0.04
S	0.123	1.14	2.11	0.019	0.0011	0.021	0.029		0.0019	0.0014
T	0.115	0.54	2.31	0.022	0.0034	0.016	0.026	0.0152	0.0027	0.0016
U	0.094	0.67	1.64	0.021	0.0027	0.034	0.031	0.0024	0.0018	0.0019

TABLE 1-2
Steel	Chemical composition (mass %, remainder: Fe and impurities)		Ar3	Ms
type	Sn	Nb	V	W	Ca	Mg	Bi	Sb	Zr	REM	Q	(° C.)	(° C.)
A		0.012									3.6	674	412
B											2.6	690	431
C											3.2	654	411
D											3.1	661	405
E											3.4	652	390
F			0.13								2.5	724	446
G					0.0051		0.0019				2.8	649	417
H										0.0035	3.2	670	424
I								0.032			2.7	718	446
J						0.0056					2.4	765	444
K	0.22			0.34							3.1	780	442
L									0.0054	0.0021	5.0	484	338
M											3.5	640	350
N											2.6	766	460
O											3.2	829	455
P											2.6	835	475
Q											6.0	398	304
R											2.7	713	438
S											2.9	706	424
T											2.8	670	419
U											2.2	743	453

	TABLE 2-1
	Hot rolling step, cooling step on run-out table, coiling step	Cold rolling step
				Time from			Sheet		Sheet
			Rolling	completion of			thickness	Cumulative	thickness
		Heating	completion	rolling to	Cooling	Coiling	after hot	rolling	after cold
Experiment	Steel	temperature	temperature	starting of	rate	temperature	rolling	reduction	rolling
No.	type	[° C.]	[° C.]	cooling [S]	[° C./s]	[° C.]	[mm]	[%]	[mm]
1	A	1250	950	3.4	30.0	600	3.2	50	1.6
2	A	1250	950	3.4	30.0	600	3.2	50	1.6
3	A	1250	950	3.4	30.0	600	3.2	50	1.6
4	A	1250	950	3.4	30.0	600	3.2	50	1.6
5	A	1250	950	3.4	30.0	600	3.2	50	1.6
6	A	1250	950	3.4	30.0	600	3.2	50	1.6
7	A	1250	950	3.4	30.0	600	3.2	50	1.6
8	A	1250	950	3.4	30.0	600	3.2	50	1.6
9	A	1250	950	3.4	30.0	600	3.2	50	1.6
10	A	1250	950	3.4	30.0	600	3.2	50	1.6
11	A	1250	950	3.4	30.0	600	3.2	50	1.6
12	A	1250	950	3.4	30.0	600	3.2	50	1.6
13	A	1250	950	3.4	30.0	600	3.2	50	1.6
14	A	1250	950	3.4	30.0	600	3.2	50	1.6
15	A	1250	950	3.4	30.0	600	3.2	50	1.6
16	A	1250	950	3.4	30.0	600	4.5	64	1.6
17	A	1250	950	3.4	30.0	600	2.5	36	1.6
18	A	1250	950	3.4	30.0	600	3.2	50	1.6
19	A	1250	950	3.4	30.0	600	3.2	50	1.6
20	A	1250	950	3.4	30.0	600	3.2	50	1.6
21	A	1250	950	3.4	30.0	600	3.2	50	1.6
22	A	1250	950	3.4	30.0	600	3.2	50	1.6
23	A	1250	950	3.4	30.0	600	3.2	50	1.6
24	A	1250	950	3.4	30.0	600	3.2	50	1.6

	TABLE 2-2
	Hot rolling step, cooling step on run-out table, coiling step	Cold rolling step
				Time from			Sheet		Sheet
			Rolling	completion of			thickness	Cumulative	thickness
		Heating	completion	rolling to	Cooling	Coiling	after hot	rolling	after cold
Experiment	Steel	temperature	temperature	starting of	rate	temperature	rolling	reduction	rolling
No.	type	[° C.]	[° C.]	cooling [S]	[° C./s]	[° C.]	[mm]	[%]	[mm]
25	A	1250	950	3.4	30.0	600	3.2	50	1.6
26	B	1250	950	3.4	30.0	600	3.2	50	1.6
27	B	1250	950	3.4	30.0	600	3.2	50	1.6
28	B	1250	950	3.4	30.0	600	3.2	50	1.6
29	B	1250	950	3.4	30.0	600	3.2	50	1.6
30	C	1250	950	3.4	30.0	600	3.2	50	1.6
31	D	1250	950	3.4	30.0	600	3.2	50	1.6
32	E	1250	950	3.4	30.0	600	3.2	50	1.6
33	F	1250	950	3.4	30.0	600	3.2	50	1.6
34	G	1250	950	3.4	30.0	600	3.2	50	1.6
35	H	1250	950	3.4	30.0	600	3.2	50	1.6
36	I	1250	950	3.4	30.0	600	3.2	50	1.6
37	J	1250	950	3.4	30.0	600	3.2	50	1.6
38	K	1250	950	3.4	30.0	600	3.2	50	1.6
39	L	1250	950	3.4	30.0	600	3.2	50	1.6
40	M	1250	950	3.4	30.0	600	3.2	50	1.6
41	N	1250	950	3.4	30.0	600	3.2	50	1.6
42	O	1250	950	3.4	30.0	600	3.2	50	1.6
43	P	1250	950	3.4	30.0	600	3.2	50	1.6
44	Q	1250	950	3.4	30.0	600	3.2	50	1.6
45	R	1250	950	3.4	30.0	600	3.2	50	1.6
46	S	1250	950	3.4	30.0	600	3.2	50	1.6
47	T	1250	950	3.4	30.0	600	3.2	50	1.6
48	U	1250	950	3.4	30.0	600	3.2	50	1.6

	TABLE 3-1
	Annealing step
	Average
	heating rate	P(H2O)/P(H2)		First cooling step	Second cooling step
		from 650° C.	of atmosphere		Soaking		Cooling		Cooling
		to soaking	from 650° C.	Soaking	(holding)	Cooling	stop	Cooling	stop
Experiment	Steel	temperature	to soaking	temperature	time	rate	temperature	rate	temperature
No.	type	[° C./S]	temperature	[° C.]	[S]	[° C./S]	[° C.]	[° C./S]	[° C.]
1	A	1.1	0.70	820	60	1.1	580	1.3	200
2	A	0.9	1.34	824	194	1.0	545	85.2	251
3	A	1.2	0.73	910	22	7.9	570	9.9	202
4	A	1.2	0.34	768	52	6.8	539	178.2	342
5	A	1.1	1.86	848	78	0.8	573	27.5	207
6	A	1.2	0.56	832	139	5.8	576	74.9	206
7	A	0.8	0.77	810	78	3.7	537	157.5	188
8	A	0.9	0.96	813	179	4.7	582	191.1	223
9	A	0.8	0.00007	869	170	6.0	543	189.1	221
10	A	1.2	0.50	1035	26	1.0	503	41.8	152
11	A	1.0	2.24	835	106	6.2	512	99.0	217
12	A	1.2	0.56	859	37	13.2	481	164.4	82
13	A	1.2	0.25	787	50	1.8	632	111.5	105
14	A	1.1	0.35	852	104	7.0	579	0.8	190
15	A	1.1	0.85	818	58	0.9	573	161.1	430
16	A	0.8	0.24	830	129	5.1	585	65.8	260
17	A	0.9	0.09	860	14	6.3	500	63.9	72
18	A	1.0	0.94	793	190	8.4	507	105.2	60
19	A	1.3	0.06	868	17	8.4	545	15.4	117
20	A	1.0	0.40	869	106	1.3	517	62.8	211
21	A	1.2	0.54	864	49	3.0	554	6.8	274
22	A	1.1	0.40	864	107	0.4	531	103.2	166
23	A	0.9	0.40	809	63	1.8	538	119.5	184
24	A	1.3	0.05	796	35	7.4	483	59.9	356
	Heat treatment step	Leveling step,
	Time	temper rolling step
							required	Cumulative	Total
							from second	strain	elongation
	Plating step		cooling stop	of front	ratio in
			Temperature		Heat	Heat	temperature	and rear	leveling step
			before	Alloying	treatment	treatment	to 150° C. in	surfaces in	and temper
	Experiment	Plating	plating	temperature	temperature	time	slow cooling	leveling	rolling
	No.	type	[° C.]	[° C.]	[° C.]	[S]	[S]	step [%]	step [%]
	1	GA	460	530	300	30	—	15.0	0.18
	2	GA	460	576	262	186	—	9.6	0.10
	3	GA	460	527	280	117	—	18.1	0.29
	4	GA	460	475	—	—	60	16.4	0.11
	5	GA	460	520	—	—	37	12.0	0.17
	6	GA	460	538	260	74	—	2.0	0.22
	7	GA	460	486	335	158	—	8.0	0.29
	8	GA	460	527	282	84	—	13.4	0.31
	9	GA	460	514	323	89	—	10.8	0.29
	10	GA	460	547	377	215	—	13.4	0.29
	11	GA	460	499	329	53	—	19.4	0.22
	12	GA	460	462	373	91	—	12.1	0.29
	13	GA	460	460	244	134	—	18.4	0.10
	14	GA	460	561	221	67	—	16.2	0.29
	15	GA	460	553	—	—	65	12.6	0.20
	16	GA	460	565	291	111	—	10.2	0.26
	17	GA	460	505	280	263	—	19.5	0.28
	18	GA	460	515	180	198	—	17.0	0.28
	19	GA	460	553	430	135	—	14.3	0.12
	20	GA	460	470	—	—	23	11.0	0.15
	21	GA	460	557	345	124	—	0.8	0.22
	22	GA	460	484	227	69	—	14.7	0.72
	23	GI	460	—	217	245	—	12.6	0.29
	24	GI	460	—	—	—	82	10.4	0.24

	TABLE 3-2
	Annealing step
	Average
	heating rate	P(H2O)/P(H2)		First cooling step	Second cooling step
		from 650° C.	of atmosphere		Soaking		Cooling		Cooling
		to soaking	from 650° C.	Soaking	(holding)	Cooling	stop	Cooling	stop
Experiment	Steel	temperature	to soaking	temperature	time	rate	temperature	rate	temperature
No.	type	[° C./S]	temperature	[° C.]	[S]	[° C./S]	[° C.]	[° C./S]	[° C.]
25	A	1.0	0.28	826	32	8.3	566	129.0	327
26	B	0.9	1.07	785	195	6.3	558	193.2	160
27	B	1.0	0.39	817	13	4.2	586	164.5	271
28	B	1.1	0.39	802	93	0.2	583	88.2	55
29	B	0.9	0.80	868	166	1.3	540	37.0	209
30	C	1.0	0.40	827	193	3.0	563	151.6	237
31	D	1.3	0.31	845	21	8.1	510	81.8	142
32	E	1.0	0.93	809	132	7.1	560	145.5	172
33	F	1.2	0.79	843	166	3.2	524	48.9	135
34	G	0.9	0.08	782	192	0.3	564	95.1	50
35	H	1.2	1.04	784	139	0.9	572	96.6	59
36	I	1.2	0.52	830	85	6.2	584	83.9	194
37	J	1.3	0.72	783	90	8.5	516	46.6	112
38	K	1.1	0.76	780	103	1.6	589	56.7	193
39	L	1.2	0.06	798	162	7.4	568	12.0	241
40	M	1.1	0.39	800	51	7.0	525	72.9	160
41	N	1.1	1.02	832	81	7.6	529	175.5	274
42	O	1.0	0.64	799	123	6.7	572	144.5	227
43	P	0.9	0.34	833	174	6.8	489	128.9	130
44	Q	0.9	0.59	850	130	1.1	551	167.1	234
45	R	1.0	0.38	837	67	7.7	584	33.3	216
46	S	0.9	0.91	862	183	6.8	580	121.8	89
47	T	1.2	0.58	849	153	0.3	561	146.3	84
48	U	1.3	0.79	843	69	2.1	514	137.3	159
	Heat treatment step	Leveling step,
	Time	temper rolling step
							required	Cumulative	Total
							from second	strain	elongation
	Plating step		cooling stop	of front	ratio in
			Temperature		Heat	Heat	temperature	and rear	leveling step
			before	Alloying	treatment	treatment	to 150° C. in	surfaces in	and temper
	Experiment	Plating	plating	temperature	temperature	time	slow cooling	leveling	rolling
	No.	type	[° C.]	[° C.]	[° C.]	[S]	[S]	step [%]	step [%]
	25	—	—	—	—	—	45	19.1	0.28
	26	GA	460	498	251	34	—	13.6	0.11
	27	GA	460	547	241	163	—	0.8	0.16
	28	GA	460	578	356	279	—	0.6	0.11
	29	GA	460	524	373	165	—	0.5	0.30
	30	GA	460	548	340	52	—	16.6	0.16
	31	GI	460	—	222	288	—	13.6	0.28
	32	GA	460	549	255	22	—	17.4	0.27
	33	GA	460	505	252	210	—	18.2	0.18
	34	—	—	—	330	286	—	13.8	0.19
	35	GA	460	496	301	257	—	14.6	0.18
	36	GA	460	535	299	238	—	18.2	0.19
	37	GA	460	568	228	130	—	12.1	0.24
	38	GA	460	521	241	68	—	17.4	0.11
	39	GA	460	553	373	259	—	16.5	0.13
	40	GA	460	560	380	211	—	18.7	0.27
	41	GA	460	475	301	232	—	16.0	0.19
	42	GA	460	473	240	295	—	10.3	0.17
	43	GA	460	489	300	26	—	15.2	0.24
	44	GA	460	465	271	196	—	12.9	0.10
	45	GA	460	466	267	120	—	15.9	0.19
	46	GA	460	523	266	273	—	9.3	0.22
	47	GA	460	521	347	294	—	16.7	0.25
	48	GA	460	548	313	24	—	10.6	0.29

A surface layer area and a structure at the ¼ thickness position of the obtained cold-rolled steel sheet were observed by the above-described method, and the volume percentage of each phase was measured. The results are shown in Tables 4-1 and 4-2. In the table, Vα is the volume percentage of ferrite, VP is a volume percentage of pearlite, Vθ is a volume percentage of cementite, VB is a volume percentage of bainite, VM is a volume percentage of martensite, Vγ is a volume percentage of retained austenite, and VTM/VM is a ratio of the volume of tempered martensite in martensite.

In addition, a ratio of a dislocation density of the surface layer area to a dislocation density at the ¼ thickness position was measured for the obtained cold-rolled steel sheet with the above-described method.

The results are shown in Tables 4-1 and 4-2.

In addition, from the obtained cold-rolled steel sheets, tensile properties, bendability, and weldability were evaluated as follows.

<Tensile Properties>

JIS No. 5 tensile test pieces were collected from a direction perpendicular to a rolling direction and a thickness direction (width direction) of the cold-rolled steel sheets, and a tensile test was performed in accordance with JIS Z 2241:2011 to measure a tensile strength (TS), a yield strength (YS), and a total elongation (EL).

In a case where the tensile strength was 1180 MPa or more, it was determined that the steel sheet had high strength.

In addition, in a case where the total elongation was 9.0% or more, the elongation was determined to be excellent.

<Bendability>

A bending test was performed in accordance with 238-100 established by the Verband der Automobilindustrie (VDA) (German Association of the Automotive Industry), and a bending angle was evaluated.

In addition, a reaction force of a stroke of 2 mm was defined as the bending strength.

In a case where the bending angle was 80° or more and the bending strength was 6.0 kN or more, the bendability was determined to be excellent (the bending resistance is high and the bending fracture limit is high).

<Weldability>

The weldability was evaluated by performing a continuous spot point test of spot welding.

In welding conditions in which a diameter of a molten portion was 5.3 to 5.7 times the square root of the sheet thickness, spot welding was continuously performed 1,000 times, the diameter of the molten portion was compared between d1 at the first point and d1000 at the 1,000^th point, and a case where d1000/d1 was 0.90 or more was determined to have excellent weldability (O in the tables). On the other hand, in a case where d1000/d1 was less than 0.90, the weldability was determined to be insufficient (X in the tables).

	TABLE 4-1
	Structure
Experiment	Steel	Surface layer area (volume %)	¼ thickness position (volume %)	Dislocation
No.	type	Vα	VB	VM	VTM/VM	Vγ	VP	Vθ	Vα	VB	VM	VTM/VM	Vγ	density ratio
1	A	72	2	26	76	0	0	0	16	10	74	83	0	0.56
2	A	63	6	30	56	1	1	0	1	6	92	66	1	0.20
3	A	33	5	61	81	0	0	0	18	17	64	95	0	0.32
4	A	49	24	23	63	1	0	1	0	0	98	89	2	0.23
5	A	75	1	22	40	2	0	0	9	10	78	59	3	0.24
6	A	45	9	46	55	0	0	0	12	1	87	90	0	0.61
7	A	71	13	16	44	0	0	0	18	0	82	77	0	0.48
8	A	17	23	58	42	2	0	0	1	11	86	79	2	0.50
9	A	6	22	71	40	1	0	0	5	20	73	73	2	0.92
10	A	0	35	64	39	1	0	0	0	13	86	75	1	0.47
11	A	98	1	1	47	0	0	0	51	12	34	70	3	0.41
12	A	3	31	65	39	1	0	0	2	9	87	74	2	0.59
13	A	44	21	33	55	2	0	0	63	1	34	62	2	0.78
14	A	42	26	31	41	1	0	0	65	8	26	79	1	0.58
15	A	14	61	24	67	1	0	0	64	7	28	89	1	0.41
16	A	30	21	47	59	2	0	0	20	9	69	79	2	0.72
17	A	41	3	54	41	2	0	0	1	3	94	65	3	0.27
18	A	32	13	55	0	0	0	0	24	14	61	21	0	0.75
19	A	30	16	52	1	2	0	0	22	6	71	92	2	0.70
20	A	27	8	62	0	1	1	0	16	13	70	34	2	0.75
21	A	31	31	37	68	1	0	0	2	6	90	93	2	0.15
22	A	55	19	25	39	2	0	0	1	3	93	62	3	0.94
23	A	20	29	50	60	0	0	0	10	22	68	86	0	0.69
24	A	24	0	76	75	0	0	0	11	4	84	80	0	0.41
	Material characteristics
					Bending	Bending
	Experiment	YS	TS	EL	strength	angle		Present Example/
	No.	(MPa)	(MPa)	(%)	(kN)	(°)	Weldability	Comparative Example
	1	968	1236	12.6	6.3	112	◯	Present Example
	2	1038	1253	12.3	8.4	101	◯	Present Example
	3	795	1240	10.0	6.8	111	◯	Present Example
	4	950	1217	10.8	7.9	89	◯	Present Example
	5	790	1184	10.0	8.1	95	◯	Present Example
	6	1122	1219	12.0	6.4	92	◯	Present Example
	7	991	1261	12.2	6.3	102	◯	Present Example
	8	982	1277	11.7	6.3	82	◯	Present example
	9	929	1249	13.3	7.8	62	◯	Comparative Example
	10	1047	1420	7.2	7.4	111	◯	Comparative Example
	11	781	1268	10.4	5.5	90	◯	Comparative Example
	12	973	1243	6.7	6.3	86	◯	Comparative Example
	13	913	1130	10.3	8.5	104	◯	Comparative Example
	14	932	1162	14.7	6.7	88	◯	Comparative Example
	15	835	1106	9.5	6.9	107	◯	Comparative Example
	16	838	1207	12.4	8.0	98	◯	Present Example
	17	908	1215	10.9	6.3	102	◯	Present Example
	18	670	1180	7.8	5.7	64	◯	Comparative Example
	19	801	1064	12.0	6.1	99	◯	Comparative Example
	20	624	1375	8.4	6.4	56	◯	Comparative Example
	21	900	1263	10.3	7.0	73	◯	Comparative Example
	22	1109	1194	7.6	7.6	76	◯	Comparative Example
	23	945	1205	12.5	8.4	89	◯	Present Example
	24	1022	1214	13.7	8.5	83	◯	Present Example

	TABLE 4-2
	Structure
Experiment	Steel	Surface layer area (volume %)	¼ thickness position (volume %)	Dislocation
No.	type	Vα	VB	VM	VTM/VM	Vγ	VP	Vθ	Vα	VB	VM	VTM/VM	Vγ	density ratio
25	A	74	6	18	50	1	0	0	14	11	73	88	2	0.41
26	B	39	29	31	46	1	0	0	5	2	92	76	1	0.52
27	B	53	12	34	62	1	0	0	10	4	85	91	1	0.15
28	B	52	8	38	82	2	0	0	22	2	74	84	2	0.16
29	B	58	11	29	43	2	0	0	4	9	84	79	3	0.12
30	C	15	39	45	92	1	0	0	5	21	72	96	2	0.31
31	D	70	7	21	53	1	0	0	8	5	84	68	2	0.57
32	E	36	19	44	51	1	0	0	4	18	77	84	1	0.54
33	F	54	2	43	40	1	0	0	8	3	88	69	1	0.66
34	G	33	7	60	85	0	0	0	8	22	70	89	1	0.65
35	H	64	11	22	73	2	0	0	20	11	67	78	2	0.27
36	I	79	5	15	50	0	0	0	4	15	81	77	0	0.26
37	J	65	1	34	61	1	0	0	7	2	90	70	1	0.38
38	K	42	14	44	51	1	0	0	6	5	88	82	1	0.34
39	L	67	14	18	61	1	0	0	22	3	74	90	1	0.70
40	M	36	10	41	76	13	0	0	21	2	60	98	17	0.41
41	N	54	4	42	51	1	0	0	8	4	87	87	1	0.54
42	O	19	37	43	51	1	0	0	4	6	88	60	2	0.67
43	P	78	7	15	49	0	0	0	13	11	76	62	0	0.56
44	Q	47	7	45	55	1	0	0	7	1	90	71	2	0.70
45	R	40	27	31	37	2	0	0	28	9	60	59	3	0.41
46	S	22	28	48	65	2	0	0	22	5	72	96	2	0.42
47	T	30	54	15	48	1	0	0	59	16	24	69	2	0.61
48	U	40	30	29	55	0	0	0	56	6	37	80	1	0.32
	Material characteristics
					Bending	Bending
	Experiment	YS	TS	EL	strength	angle		Present Example/
	No.	(MPa)	(MPa)	(%)	(kN)	(°)	Weldability	Comparative Example
	25	798	1270	13.4	7.5	113	◯	Present Example
	26	796	1271	10.5	8.3	101	◯	Present Example
	27	858	1237	9.9	5.6	108	◯	Comparative Example
	28	795	1253	13.4	5.1	91	◯	Comparative Example
	29	883	1210	13.0	4.8	94	◯	Comparative Example
	30	877	1246	11.3	6.7	106	◯	Present Example
	31	1009	1258	12.5	6.5	85	◯	Present Example
	32	933	1256	9.4	7.2	109	◯	Present Example
	33	1001	1198	9.7	6.3	84	◯	Present Example
	34	975	1193	11.0	7.5	112	◯	Present Example
	35	1013	1271	11.5	6.2	98	◯	Present Example
	36	884	1218	9.6	7.9	112	◯	Present Example
	37	982	1235	11.3	8.1	108	◯	Present Example
	38	1055	1280	12.2	8.1	100	◯	Present Example
	39	810	1225	11.2	7.0	112	◯	Present Example
	40	820	1263	21.4	6.7	81	X	Comparative Example
	41	1105	1124	13.5	6.5	98	◯	Comparative Example
	42	971	1230	10.2	6.9	68	◯	Comparative Example
	43	873	1160	12.9	6.1	96	◯	Comparative Example
	44	796	1266	11.6	6.6	57	X	Comparative Example
	45	795	1204	14.2	7.8	82	◯	Present Example
	46	872	1223	11.7	6.9	111	◯	Present Example
	47	790	1146	10.7	6.7	81	◯	Comparative Example
	48	844	1097	11.1	6.3	100	◯	Comparative Example

As can be seen from Tables 1-1 to 4-2, in the present examples, the chemical composition, the microstructure at the surface layer area and at the ¼ thickness position were within ranges of the present invention, and the ratio of the dislocation density at the surface layer area to the dislocation density at the ¼ thickness position and the tensile strength were also within the ranges of the present invention. As a result, the cold-rolled steel sheet had high strength and excellent elongation, spot weldability, and bendability.

In contrast, in comparative examples, one or more of the chemical composition, the microstructure at the surface layer area and the ¼ thickness position, the ratio of the dislocation density at the surface layer area to the dislocation density at the ¼ thickness position, and the tensile strength were outside the ranges of the present invention, and one or more of the strength, the elongation, the spot weldability, and the bendability were inferior.

INDUSTRIAL APPLICABILITY

According to the present invention, it is possible to provide a cold-rolled steel sheet having high strength and excellent elongation, spot weldability, and bendability, and a method for manufacturing the same. Therefore, the present invention has high industrial applicability.

Claims

1. A cold-rolled steel sheet having a chemical composition containing, by mass %: Q = 2. 7 × [ C ] + 0.4 × [ Si ] + [ Mn ] + 0. 4 ⁢ 5 × [ Ni ] + 0.8 × [ Cr ] + 2.0 × [ Mo ] ( 1 )

C: 0.060% to 0.300%;

Si: 0.01% to 3.00%;

Mn: 1.00% to 5.00%;

sol.Al: 0.001% to 1.000%;

P: 0.100% or less;

S: 0.0100% or less;

O: 0.1000% or less;

N: 0.0100% or less;

Ti: 0% to 0.200%;

B: 0% to 0.0100%;

Cr: 0% to 1.00%;

Mo: 0% to 1.00%;

Ni: 0% to 1.00%;

Cu: 0% to 1.00%;

Sn: 0% to 0.50%;

Nb: 0% to 0.200%;

V: 0% to 0.50%;

W: 0% to 0.50%;

Ca: 0% to 0.0100%;

Mg: 0% to 0.0100%;

Bi: 0% to 0.0100%;

Sb: 0% to 0.100%;

Zr: 0% to 0.0100%;

REM: 0% to 0.0100%; and

a remainder: Fe and impurities,

wherein Q obtained by Expression (1) is 2.3 or more,

at a surface layer area which is in a range of 10 to 20 μm from a surface in a sheet thickness direction, a microstructure includes, by volume percentage, 10% to 95% of ferrite and a remainder of one or more selected from martensite, bainite, pearlite, cementite, and retained austenite,

at a ¼ thickness position which is in a range of ⅛ to ⅜ of a sheet thickness from the surface in the sheet thickness direction, the microstructure includes, by volume percentage, 0% to 60% of ferrite, 0% to 3% of retained austenite, and a remainder of one or more selected from martensite and bainite,

a volume percentage of tempered martensite in the martensite is 50% or more,

a ratio of a dislocation density at the surface layer area to a dislocation density at the ¼ thickness position is 0.20 or more and less than 0.90, and

a tensile strength is 1,180 MPa or more,

in Expression (1), [Element symbol] is a content of the element indicated by the element symbol in mass % and 0 is substituted in a case where the element is not contained.

2. The cold-rolled steel sheet according to claim 1, wherein a hot-dip galvanized layer is included on the surface.

3. The cold-rolled steel sheet according to claim 2, wherein the hot-dip galvanized layer is a hot-dip galvannealed layer.

4. A method for manufacturing a cold-rolled steel sheet, the method comprising: Q = 2. 7 × [ C ] + 0.4 × [ Si ] + [ Mn ] + 0. 4 ⁢ 5 × [ Ni ] + 0.8 × [ Cr ] + 2.0 × [ Mo ] ( 1 )

a hot rolling step of heating a slab having a chemical composition which contains, by mass %, C: 0.060% to 0.300%, Si: 0.01% to 3.00%, Mn: 1.00% to 5.00%, sol.Al: 0.001% to 1.000%, P: 0.100% or less, S: 0.0100% or less, O: 0.1000% or less, N: 0.0100% or less, Ti: 0% to 0.200%, B: 0% to 0.0100%, Cr: 0% to 1.00%, Mo: 0% to 1.00%, Ni: 0% to 1.00%, Cu: 0% to 1.00%, Sn: 0% to 0.50%, Nb: 0% to 0.200%, V: 0% to 0.50%, W: 0% to 0.50%, Ca: 0% to 0.0100%, Mg: 0% to 0.0100%, Bi: 0% to 0.0100%, Sb: 0% to 0.100%, Zr: 0% to 0.0100%, REM: 0% to 0.0100%, and a remainder: Fe and impurities, in which Q obtained by Expression (1) is 2.3 or more, and performing hot rolling so that a finish rolling completion temperature is 800° C. or higher and an Ar3 point or higher to obtain a hot-rolled steel sheet;

a cooling step on run-out table of starting cooling of the hot-rolled steel sheet after the elapse of 1.0 second or longer from completion of the hot rolling, and cooling the hot-rolled steel sheet to a coiling temperature of 400° C. to 750° C. at an average cooling rate of 5.0° C./sec or higher;

a coiling step of coiling the hot-rolled steel sheet after the cooling step on run-out table at the coiling temperature;

a cold rolling step of performing cold rolling on the hot-rolled steel sheet after the coiling step so that a cumulative rolling reduction is 20% to 80% to obtain a cold-rolled steel sheet;

an annealing step of heating the cold-rolled steel sheet after the cold rolling step to a soaking temperature of 750° C. to 1,000° C. and holding the cold-rolled steel sheet at the soaking temperature for 1 second or longer;

a first cooling step of cooling the cold-rolled steel sheet after the annealing step from the soaking temperature to a first cooling stop temperature of 600° C. or lower at an average cooling rate of 10.0° C./sec or lower;

a second cooling step of cooling the cold-rolled steel sheet after the first cooling step from the first cooling stop temperature to a second cooling stop temperature of an Ms point or lower at an average cooling rate of 1.0° C./sec or higher;

a heat treatment step of slowly cooling the cold-rolled steel sheet after the second cooling step so that the time required for cooling the cold-rolled steel sheet from the second cooling stop temperature to 150° C. is 25 seconds or longer, or heating the cold-rolled steel sheet to a temperature range of 200° C. to 400° C. and holding the cold-rolled steel sheet in the temperature range for 10 seconds or longer; and

a leveling step of applying a cumulative strain of 1.0% or more to each of front and rear surfaces of the cold-rolled steel sheet after the heat treatment step and setting an elongation ratio to 0.50% or less,

wherein in the annealing step, when the heating is performed in a temperature range of at least 650° C. to the soaking temperature, in an atmosphere in a furnace, P(H2O)/P(H2) that is a ratio of water vapor partial pressure P(H2O) to hydrogen partial pressure P(H2) is set to be in a range of 0.00010 to 2.00,

in Expression (1), [Element symbol] is a content of the element indicated by the element symbol in mass % and 0 is substituted in a case where the element is not contained.

5. The method for manufacturing a cold-rolled steel sheet according to claim 4, further comprising:

a hot-dip galvanizing step of immersing the cold-rolled steel sheet in a hot-dip galvanizing bath having a bath temperature of 420° C. to 520° C. in a state that a temperature of the cold-rolled steel sheet is 420° C. to 520° C. to form a hot-dip galvanized layer during the second cooling step.

6. The method for manufacturing a cold-rolled steel sheet according to claim 5, further comprising:

an alloying treatment step of holding the cold-rolled steel sheet during the second cooling step and after the hot-dip galvanizing step at a temperature of 460° C. to 580° C. and alloying the hot-dip galvanized layer.