CN117751204A

CN117751204A - Cold-rolled steel sheet and method for producing same

Info

Publication number: CN117751204A
Application number: CN202280049812.8A
Authority: CN
Inventors: 池田亚梨纱
Original assignee: Nippon Steel and Sumitomo Metal Corp
Current assignee: Nippon Steel Corp
Priority date: 2021-07-21
Filing date: 2022-07-14
Publication date: 2024-03-22
Also published as: JPWO2023002910A1; KR20240023431A; WO2023002910A1

Abstract

The cold-rolled steel sheet has a predetermined chemical composition, and when a range of 1/8 to 3/8 of the sheet thickness from the surface in the sheet thickness direction is set as a t/4 portion and a range of 20 [ mu ] m from the surface in the sheet thickness direction is set as a surface layer portion, the microstructure in the t/4 portion includes, in terms of volume ratio: 0% -10.0% of residual austenite; and 1 or 2 of 90.0 to 100% of martensite and tempered martensite, wherein the ratio of the dislocation density of the surface layer portion to the dislocation density of the t/4 portion is 0.80 or more, the ratio of the hardness of the surface layer portion to the hardness of the t/4 portion is 0.90 or more, and the cold-rolled steel sheet has a tensile strength of 1310MPa or more.

Description

Cold-rolled steel sheet and method for producing same

Technical Field

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

The present application claims priority based on japanese patent application No. 2021-120895, 7-21 in 2021, the contents of which are incorporated herein by reference.

Background

Today, which is highly divided into industrial fields, special and high performance is required for materials used in each technical field. For example, in the case of steel sheets for automobiles, high strength is required for improvement of fuel efficiency due to weight reduction of automobile bodies from the viewpoint of global environment. When the high-strength steel sheet is applied to a vehicle body of an automobile, the sheet thickness of the steel sheet can be reduced to reduce the weight of the vehicle body, and a desired strength can be imparted to the vehicle body.

In recent years, there has been a demand for further increasing the height of steel sheets for automobiles, and there has been a demand for high strength steel sheets having a tensile strength of 1310MPa or more for cold-rolled steel sheets used for steel sheets for automobiles, particularly for body frame members.

For such a requirement, for example, patent document 1 discloses, as a high-strength steel sheet used for automobile parts and the like, a high-strength steel sheet excellent in delayed fracture resistance and having a tensile strength of 1470MPa or more, which has a predetermined composition, has a predetermined steel sheet structure mainly composed of martensite and bainite, and has an average number of inclusions having an average grain size of 5 μm or more in a cross section perpendicular to a rolling direction of 5.0 inclusions/mm ² The following is given.

Further, patent document 2 discloses a steel sheet having the following steel structure: the ratio of dislocation density to dislocation density in the central part of the plate thickness in the range of 0-20 [ mu ] m from the surface of the steel plate is 90-110%, and the average of the upper 10% of cementite grain size from the surface of the steel plate to a depth of 100 [ mu ] m is 300nm or less, wherein the maximum warpage amount of the steel plate when sheared by 1m in length along the length direction of the steel plate is 15mm or less. Patent document 2 discloses that the steel sheet has a tensile strength of 980MPa or more and also has a tensile strength of 2000MPa or more.

Patent document 3 discloses a high-strength steel sheet excellent in delayed fracture resistance, which has a chemical composition (C, si, mn, al, P, S) satisfying a predetermined range, contains iron and unavoidable impurities in the remainder, has a martensite content of 95 area% or more in the entire structure, satisfies a predetermined relational expression in the structure from a position 10 μm deep in the sheet thickness direction to a position 1/4 depth of the sheet thickness from the surface of the steel sheet, and has a tensile strength of 1180MPa or more.

Prior art literature

Patent literature

Patent document 1: japanese patent No. 6729835

Patent document 2: international publication No. 2020/026838

Patent document 3: japanese patent laid-open No. 2013-104081

Disclosure of Invention

Problems to be solved by the invention

As described above, conventionally, a high-strength steel sheet having a tensile strength of 1310MPa or more has been proposed. In general, the main structure of such a high-strength steel sheet is martensite and/or tempered martensite.

As a result of the studies by the inventors of the present invention, it was found that: in a high-strength steel sheet having a main structure of martensite or tempered martensite, when a load such as deformation is applied and the load is removed and then left for a certain period of time, and then the load is applied again, the flow stress when the load is applied again becomes lower than that when the load is initially applied (hereinafter, may be simply referred to as flow stress reduction). However, patent documents 1 to 3 have no study on the reduction of the flow stress when the load is applied again, and have room for improvement.

The present invention has been completed in view of the above. The present invention addresses the problem of providing a cold-rolled steel sheet which has a structure mainly composed of martensite and tempered martensite and which, when a load is applied and left for a certain period after removal of the load and then the load is applied again, can suppress the flow stress at the time of the load application again from becoming lower than the flow stress at the time of the load application initially (suppress the decrease in flow stress).

Means for solving the problems

The inventors of the present invention studied the cause of the occurrence of the above-described decrease in flow stress. The result is known: even if the structure is martensite and/or tempered martensite in the entire plate thickness direction, the flow stress is reduced when the dislocation density in the structure differs depending on the position in the plate thickness direction.

Further, the inventors of the present invention have further studied and as a result, found that: even when the difference in dislocation density at each position in the plate thickness direction is small, the flow stress may be reduced. The inventors of the present invention have further studied for this reason. The result is known: even if the difference in dislocation density in the thickness direction is small, the flow stress is reduced when the dislocation is mainly movable.

The present invention has been completed in view of the above-described knowledge. The gist of the present invention is as follows.

[1] The cold-rolled steel sheet according to an embodiment of the present invention has the following chemical composition, and contains C in mass%: 0.150 to 0.500 percent of Si:0.01 to 2.00 percent of Mn:0.50 to 3.00 percent of P:0.0200% or less, S: less than 0.0200%, al:0.100% or less, N: less than 0.0200%, O: less than 0.020%, ni:0 to 1.000 percent of Mo:0 to 1.000 percent of Cr:0 to 2.000 percent, B:0 to 0.010 percent, as:0 to 0.050 percent, co:0 to 0.500 percent of Ti:0 to 0.500 percent of Nb:0 to 0.500 percent, V:0 to 0.500 percent of Cu:0 to 0.500 percent, W:0 to 0.100 percent, ta:0 to 0.100 percent of Ca:0 to 0.050 percent, mg:0 to 0.050 percent, la:0 to 0.050 percent, ce:0 to 0.050 percent, Y:0 to 0.050 percent, zr:0 to 0.050 percent, sb:0 to 0.050 percent of Sn:0 to 0.050% and the remainder: fe and impurities, wherein when a range of 1/8 to 3/8 of a plate thickness from a surface in a plate thickness direction is set as a t/4 portion and a range of 20 μm from the surface in the plate thickness direction is set as a surface layer portion, a microstructure in the t/4 portion includes in volume ratio: 0% -10.0% of residual austenite; and 1 or 2 of 90.0 to 100% of martensite and tempered martensite, wherein the ratio of the dislocation density of the surface layer portion to the dislocation density of the t/4 portion is 0.80 or more, the ratio of the hardness of the surface layer portion to the hardness of the t/4 portion is 0.90 or more, and the tensile strength of the cold-rolled steel sheet is 1310MPa or more.

[2] The cold-rolled steel sheet according to [1], wherein the surface of the cold-rolled steel sheet may have a coating layer formed of zinc, aluminum, magnesium, or an alloy of one or more of them.

[3] The method for producing a cold-rolled steel sheet according to another aspect of the present invention comprises the steps of: a hot rolling step of hot rolling a slab having the chemical composition described in [1] to obtain a hot-rolled steel sheet, and coiling the hot-rolled steel sheet in a state where the temperature of the widthwise central portion is more than 600 ℃ and 700 ℃ or less and the temperature of the edge portion at a position 20mm away from the widthwise end portion is 600 ℃ or less; a cold rolling step of pickling the hot-rolled steel sheet after the hot rolling step, and cold-rolling the hot-rolled steel sheet at a reduction of 30 to 90% to obtain a cold-rolled steel sheet; an annealing step of heating the cold-rolled steel sheet to an annealing temperature exceeding Ac3 ℃, holding the cold-rolled steel sheet at the annealing temperature, and cooling the cold-rolled steel sheet after the holding to the cooling stop temperature so that an average cooling rate up to 400 ℃ is 10 ℃/sec or more and an average cooling rate from 400 ℃ to 100 ℃ or less is 15 ℃/sec or more; a heat treatment step of heating the cold-rolled steel sheet after the annealing step to a temperature range of 200 to 350 ℃ and maintaining the temperature range; and a skin pass rolling step of skin pass rolling the cold-rolled steel sheet after the heat treatment step at a reduction ratio of 0.1% or more, wherein a difference between a thickness of a center portion and a thickness of an edge portion in a width direction of the cold-rolled steel sheet after the cold-rolling step is 10 [ mu ] m or more.

[4] The method of producing a cold-rolled steel sheet according to [3], wherein in the annealing step, a film layer formed of zinc, aluminum, magnesium, or an alloy of one or more of them may be formed on the front and rear surfaces of the cold-rolled steel sheet.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the above aspect of the present invention, it is possible to provide a cold-rolled steel sheet having a structure mainly composed of martensite and tempered martensite, and capable of suppressing the flow stress at the time of re-loading from being lower than the flow stress at the time of initial loading when the load is applied again after the load is applied and the load is removed and then the load is applied again, and a method for manufacturing the same.

Detailed Description

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

The cold-rolled steel sheet of the present embodiment has a predetermined chemical composition, and when a range of 1/8 to 3/8 of the sheet thickness from the surface in the sheet thickness direction is defined as t/4 and a range of 20 μm from the surface in the sheet thickness direction is defined as a surface layer, the microstructure (metal structure) in the t/4 includes, in terms of volume ratio: 0% -10.0% of residual austenite; and 1 or 2 of 90.0 to 100% of martensite and tempered martensite, wherein the ratio of the dislocation density of the surface layer portion to the dislocation density of the t/4 portion is 0.80 or more, and the ratio of the hardness of the surface layer portion to the hardness of the t/4 portion is 0.90 or more. The cold-rolled steel sheet has a tensile strength of 1310MPa or more.

These will be described separately below.

In the description, regarding the range indicated by the clamps "-" in principle, the values at both ends thereof are included in the range as a lower limit value and an upper limit value. However, values expressed as "above" and "below" 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 "% by mass".

C：0.150～0.500％

C is an element necessary for increasing the strength of the steel sheet in relation to the hardness of martensite and tempered martensite. In order to obtain a tensile strength of 1310MPa or more, it is necessary that the C content is at least 0.150% or more. Therefore, the C content is set to 0.150% or more. The C content is preferably 0.180% or more, more preferably 0.200% or more.

On the other hand, if the C content exceeds 0.500%, weldability deteriorates, and formability deteriorates. Therefore, the C content is set to 0.500% or less. The C content is preferably 0.350% or less, more preferably 0.300% or less.

Si：0.01～2.00％

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

On the other hand, if the Si content becomes excessive, formability decreases, and wettability with the plating layer decreases. Therefore, the Si content is set to 2.00% or less. The Si content is preferably set to 1.80% or less, more preferably 1.70% or less.

Mn：0.50～3.00％

Mn is an element that enhances hardenability, and is an element that promotes the formation of martensite. If the Mn content is less than 0.50%, it becomes difficult to obtain the objective microstructure. Therefore, the Mn content is set to 0.50% or more.

On the other hand, if the Mn content becomes excessive, the effect of improving hardenability due to Mn segregation is reduced, and the raw material cost increases. Therefore, the Mn content is 3.00% or less. The Mn content is preferably 2.80% or less.

P: less than 0.0200%

P is an element contained in steel as an impurity, and is an element segregated at grain boundaries to embrittle the steel. Therefore, the smaller the P content, the more preferable the P content is, but the P content may be 0.0200% or less in consideration of the P removal time and cost. The P content is preferably 0.0150% or less, more preferably 0.0100% or less.

The P content may be 0.0001% or more from the viewpoint of cost such as refining.

S: less than 0.0200%

S is an element contained in steel as an impurity, and is an element that forms sulfide-based inclusions and deteriorates formability of a steel sheet. Therefore, the smaller the S content, the more preferable the S content is, but the S content may be 0.0200% or less in consideration of the removal time and cost of S. The S content is preferably 0.0100% or less, more preferably 0.0050% or less, and still more preferably 0.0030% or less.

From the viewpoint of cost such as refining, the S content may be 0.0001% or more.

Al: less than 0.100%

Al is an element having a deoxidizing effect on molten steel. The cold-rolled steel sheet according to the present embodiment does not necessarily need to contain Al, and may contain Al in an amount of 0% for deoxidization. In this case, the Al content is preferably 0.001% or more. Since Al has an effect of improving the stability of austenite, al may be contained in order to obtain retained austenite, similarly to Si.

On the other hand, if the Al content is too high, not only surface defects due to alumina are likely to occur, but also the transformation point is greatly increased, and the volume ratio of ferrite increases. In this case, it is difficult to obtain a desired metal structure, and a sufficient tensile strength cannot be obtained. Therefore, the Al content is set to 0.100% or less. The Al content is preferably 0.050% or less, more preferably 0.040% or less, and still more preferably 0.030% or less.

N: less than 0.0200%

N is an element that can be contained in steel as an impurity, and is an element that generates coarse precipitates and deteriorates formability. Therefore, the N content is set to 0.0200% or less. The N content is preferably 0.0100% or less, more preferably 0.0060% or less. The smaller the N content, the more preferable the N content is, but the N content may be 0.0001% or more from the viewpoint of cost such as refining.

O: less than 0.020%

O is an element contained as an impurity. If the O content exceeds 0.020%, coarse oxides are formed in the steel and formability is lowered. Therefore, the O content is set to 0.020% or less. The O content is preferably set to 0.010% or less, more preferably to 0.005% or less. The O content may be 0% or more, but from the viewpoint of cost such as refining, the O content may be 0.0001% or more, or 0.001% or more.

In the chemical composition of the cold-rolled steel sheet according to the present embodiment, the balance excluding the above elements is Fe and impurities as essential components. The impurities are elements which are mixed from the steel raw material and/or during the steelmaking process and which are allowed to exist within a range that does not significantly deteriorate the properties of the cold-rolled steel sheet according to the present embodiment.

On the other hand, the chemical composition of the cold-rolled steel sheet according to the present embodiment may contain 1 or 2 or more kinds selected from Ni, mo, cr, B, as, co, ti, nb, V, cu, W, ta, ca, mg, la, ce, Y, zr, sb, sn in the range described later in order to improve various properties, instead of part of Fe. These elements may not be contained, and thus the lower limit is 0%. If the content is within the range described later, the effect of the cold-rolled steel sheet of the present embodiment is not impaired even if these elements are contained as impurities.

Ni：0～1.000％

Mo：0～1.000％

Cr：0～2.000％

B：0～0.010％

As：0～0.050％

Ni, mo, cr, B and As are elements that improve hardenability and contribute to the strength of the steel sheet. Therefore, these elements may be contained. In order to sufficiently obtain the above-described effects, it is preferable to set the Ni content, mo content, cr content to 0.010% or more, B content to 0.0001% or more, and/or As content to 0.001% or more. More preferably, the Ni content, mo content, cr content is 0.050% or more, the B content is 0.001% or more, and the As content is 0.005% or more. It is not necessary to obtain the above-described effects. Therefore, the lower limits of Ni content, mo content, cr content, B content, as content, which are 0% are not particularly limited.

On the other hand, even if these elements are excessively contained, the effect due to the above-described action is saturated and becomes uneconomical. Therefore, when these elements are contained, the Ni content and Mo content are set to 1.000% or less, the Cr content is set to 2.000% or less, the B content is set to 0.010% or less, and the As content is set to 0.050% or less. The Ni content and the Mo content are preferably 0.500% or less, the Cr content is preferably 1.000% or less, the B content is preferably 0.0060% or less, and the As content is 0.030% or less.

Co：0～0.500％

Co is an element effective for improving the strength of a steel sheet. The Co content may be 0%, but in order to obtain the above-described effects, the Co content is preferably 0.010% or more, more preferably 0.100% or more.

On the other hand, if the Co content is too large, the elongation of the steel sheet may decrease, and the formability may decrease. Therefore, the Co content is set to 0.500% or less.

Ti：0～0.500％

Nb：0～0.500％

V：0～0.500％

Cu：0～0.500％

W：0～0.100％

Ta：0～0.100％

Ti, nb, V, cu, W, ta is an element that has an effect of improving the strength of a steel sheet by precipitation hardening. Therefore, these elements may be contained. In order to sufficiently obtain the above-described effects, it is preferable that 1 or more of Ti, nb, V, cu, W, ta are contained, and the content of each is 0.001% or more.

On the other hand, if these elements are excessively contained, the recrystallization temperature increases, and the metal structure of the cold-rolled steel sheet becomes uneven, which deteriorates formability. Therefore, the Ti content, nb content, V content, cu content are each set to 0.500% or less. The W content and the Ta content are each set to 0.100% or less.

Ca：0～0.050％

Mg：0～0.050％

La：0～0.050％

Ce：0～0.050％

Y：0～0.050％

Zr：0～0.050％

Sb：0～0.050％

Ca. Mg, la, ce, Y, zr, sb is an element contributing to fine dispersion of inclusions in steel, and is an element contributing to improvement of formability of steel sheet by the fine dispersion. Therefore, these elements may be contained. In order to obtain the above effect, it is preferable to contain 1 or more of Ca, mg, la, ce, Y, zr, sb and to set the content of each to 0.001% or more.

On the other hand, if these elements are excessively contained, ductility deteriorates. Therefore, the content of Ca, mg, la, ce, Y, zr, sb is set to 0.050% or less.

Sn：0～0.050％

Sn is an element that suppresses coarsening of crystal grains and contributes to improvement of strength of the steel sheet. Therefore, sn may be contained.

On the other hand, sn is an element that may cause deterioration of cold formability of the steel sheet due to embrittlement of ferrite. If the Sn content exceeds 0.050%, the adverse effect becomes remarkable, and therefore the Sn content is made 0.050% or less. The Sn content is preferably 0.040% or less.

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

For example, according to JISG1201 (2014), the cut powder may be measured by ICP-AES (inductively coupled plasma-atomic emission Spectrometry; inductively Coupled Plasma-Atomic Emission Spectrometry). In this case, the chemical composition is an average content in the whole plate thickness. The measurement of C and S which cannot be performed by ICP-AES may be performed by a combustion-infrared absorption method, the measurement of N may be performed by an inert gas fusion-thermal conductivity method, and the measurement of O may be performed by an inert gas fusion-non-dispersive infrared absorption method.

When the steel sheet has a coating layer on the surface, the coating layer may be removed by mechanical grinding or the like and then the chemical composition may be analyzed. In the case where the coating layer is a plating layer, the plating layer may be removed by dissolving the plating layer in an acid solution to which an inhibitor for inhibiting corrosion of the steel sheet is added.

[ microstructure (Metal Structure) ]

In this embodiment, a range from a position 1/8 to a position 3/8 of the plate thickness with respect to the surface centered at a position 1/4 of the plate thickness in the plate thickness direction is defined as a t/4 portion ((1/4) t portion), and a range from the surface to 20 μm in the plate thickness direction is defined as a surface layer portion.

Microstructure of section [ t/4 ]: the volume ratio comprises: 0% -10.0% of residual austenite; and 90.0% to 100% of 1 or 2 of martensite and tempered martensite ]

The retained austenite contributes to improvement of formability of the steel sheet by improving the uniform elongation of the steel sheet by utilizing the TRIP effect. Therefore, retained austenite (retained γ) may be contained. In order to obtain the above-described effect, the volume ratio of the retained austenite is preferably set to 1.0% or more. The volume ratio of the retained austenite is more preferably 2.0% or more, and still more preferably 3.0% or more.

On the other hand, if the volume fraction of the retained austenite becomes excessive, the grain size of the retained austenite becomes large. Such a residual austenite having a large grain size becomes coarse and hard martensite after deformation. In this case, the initiation point of the crack is easily generated, and the formability is lowered. Therefore, the volume ratio of the retained austenite is set to 10.0% or less. The volume ratio of the retained austenite is preferably 8.0% or less, more preferably 7.0% or less.

The structure other than the retained austenite includes 1 or 2 of martensite and tempered martensite.

Martensite (so-called primary martensite) and tempered martensite are collections of lath-shaped grains, which contribute significantly to the strength improvement. Accordingly, the cold-rolled steel sheet of the present embodiment contains 90.0 to 100% by volume of martensite and tempered martensite.

Tempered martensite is a hard structure including fine iron-based carbide inside by tempering, unlike martensite. The tempered martensite has a small contribution to the improvement of strength as compared with martensite, but is a structure which is not brittle and has ductility, so that when further improvement of formability is desired, it is preferable to increase the volume fraction of the tempered martensite. For example, the volume fraction of tempered martensite is 85.0% or more.

On the other hand, when high strength is desired, the volume fraction of martensite is preferably increased.

The microstructure may contain bainite in addition to retained austenite, martensite and tempered martensite. Preferably, ferrite and pearlite are not included.

The volume fraction of each structure in the microstructure of the t/4 portion of the cold-rolled steel sheet according to the present embodiment was measured as follows.

That is, regarding the volume fractions of ferrite, bainite, martensite, tempered martensite, and pearlite, test pieces were taken from arbitrary positions in the rolling direction and the width direction of the steel sheet, the longitudinal section parallel to the rolling direction (section parallel to the plate thickness direction) was polished, and the structure developed by the ethanol nitrate etching in the range of 1/8 to 3/8 (t/4 section) from the surface by SEM observation. In SEM observation, 5 fields of view of 30. Mu.m.times.50. Mu.m, were observed at a magnification of 3000 times, and the area ratio of each tissue was measured from the observed image to calculate an average value. Since there is no structural change in the direction perpendicular to the rolling direction (the steel sheet width direction), the area ratio of the longitudinal section parallel to the rolling direction is equal to the volume ratio, and the area ratio obtained in the structural observation is taken as the volume ratio.

In the measurement of the area ratio of each structure, the region where the lower structure is not displayed and the brightness is low was set as ferrite. In addition, the region where the lower structure is not exhibited and the brightness is high is set to be martensite or retained austenite. The region where the lower structure is exhibited is set to be tempered martensite or bainite.

Bainite and tempered martensite can be distinguished by further careful observation of carbides within the grains.

Specifically, tempered martensite is composed of laths of martensite and cementite generated inside the laths. In this case, there are 2 or more kinds of crystal orientation relations between the martensite lath and cementite, and therefore cementite constituting tempered martensite has a plurality of modifications. On the other hand, bainite is divided into upper and lower bainite. The upper bainite is composed of lath-shaped bainitic ferrite and cementite generated at the lath interface, and thus can be easily distinguished from tempered martensite. The lower bainite is composed of lath-shaped bainitic ferrite and cementite generated inside the lath. In this case, the crystal orientation relationship between bainitic ferrite and cementite is 1 type, unlike tempered martensite, and cementite constituting lower bainite has the same modification. Thus, lower bainite can be distinguished from tempered martensite based on variants of cementite.

On the other hand, martensite and retained austenite cannot be clearly distinguished by SEM observation. Therefore, the volume fraction of martensite is calculated by subtracting the volume fraction of retained austenite calculated by a method described later from the volume fraction of the structure determined to be martensite or retained austenite.

Regarding the volume fraction of retained austenite, test pieces were taken from arbitrary positions of the steel sheet, the rolled surface was chemically polished from the steel sheet surface to a position of 1/4 of the sheet thickness, and the area integral intensities of ferrite (200), (210) and austenite (200), (220) and (311) obtained by using mokα rays were quantified.

[ ratio of dislocation density in surface layer portion to dislocation density in t/4 portion: 0.80 or more ]

The microstructure of the cold-rolled steel sheet according to the present embodiment mainly includes martensite and/or tempered martensite in which martensite is tempered.

Martensite is obtained by holding and then quenching a steel sheet in an austenite single-phase region, but when the steel sheet is cooled by a general method, the martensite varies in structure characteristics (for example, dislocation density included) depending on the position in the sheet thickness direction of the steel sheet. The difference results from the difference in the time points of the phase transition. That is, during cooling, the temperature of the region near the surface of the steel sheet first decreases, and then the temperature of the inside of the steel sheet decreases. Therefore, transformation from austenite to martensite first occurs on the surface layer side of the steel sheet. Since the martensite phase transformation is an exothermic reaction, the martensite generated on the surface layer side is maintained at a high temperature for a long period of time as compared with the martensite in the inner portion, and tempering proceeds. If tempered, the dislocation density in martensite decreases.

If there is such a difference in dislocation density, when a load such as deformation is applied and left for a certain period after the load is removed, and then the load is applied again, the flow stress when the load is applied again becomes lower than the flow stress when the load is initially applied. Therefore, in the cold-rolled steel sheet according to the present embodiment, the dislocation density (ρ _s ) Dislocation density (ρ) with t/4 portion _t/4 ) Ratio (ρ) _s /ρ _t/4 ) Is 0.80 or more. (ρ) _s /ρ _t/4 ) Preferably 0.85 or more, more preferably 0.90 or more.

the dislocation density at t/4 portion is preferably 5.2X10 ¹⁵ m ^-2 The above. Therefore, consider the above ρ _s /ρ _t/4 The dislocation density of the surface layer portion is preferably 4.2X10 ¹⁵ m ^-2 The above.

The dislocation density at each position was obtained by the following method.

A sample with a thickness of 20 μm from the surface and a sample with a thickness of 1/4 from the surface were prepared by taking a position of 20 μm from the surface of the steel sheet as a representative structure of the surface layer portion and a position of 1/4 from the surface as a representative structure of the t/4 portion, and the respective ground surfaces were subjected to X-ray diffraction after strain was removed by chemical polishing. From an X-ray diffraction curve obtained by X-ray diffraction, a dislocation density at a position 20 μm from the surface and a dislocation density at a position 1/4 of the plate thickness from the surface were obtained using a modified Williamson-Hall method and a modified Warren-Averbach method. Specifically, the dislocation density was obtained according to the method described in ISIJ int.vol.50 (2010) pages 875 to 882. The dislocation density at a position 20 μm from the surface was set as the dislocation density in the surface layer portion, and the dislocation density at a position 1/4 of the thickness from the surface was set as the dislocation density in the t/4 portion.

[ ratio of hardness of surface layer portion to hardness of t/4 portion: 0.90 or more ]

As described above, even if the difference in dislocation density in the thickness direction is reduced, when the dislocation is mainly a movable dislocation, the flow stress when the load is applied again after the load is removed by applying the load that causes deformation for a certain period of time, and then the load is applied again, becomes lower than the flow stress when the load is initially applied.

Therefore, in the cold-rolled steel sheet according to the present embodiment, dislocation in the surface layer portion is particularly immobilized. In the present embodiment, as an index of whether or not dislocation is immobilized, a ratio of the hardness of the surface layer portion to the hardness of the t/4 portion is used.

At (ρ) _s /ρ _t/4 ) When the ratio of the hardness of the surface layer portion to the hardness of the t/4 portion is 0.80 or more, dislocation is immobilized, and a decrease in flow stress can be prevented. The ratio of the hardness is preferably 0.92 or more, more preferably 0.94 or more, and still more preferably 0.95 or more.

As described later, the immobilisation of dislocations can be achieved by skin pass rolling of a steel sheet in which a sheet thickness difference is intentionally set.

Since the tensile strength and the hardness are related, the hardness of the t/4 portion is preferably 360Hv or more. Therefore, considering the preferable range of the ratio of the hardness of the surface layer portion to the hardness of the t/4 portion, the hardness of the surface layer portion is preferably 324Hv or more.

The hardness was determined by the following method.

A cut surface perpendicular to the rolling direction of the steel sheet and parallel to the sheet thickness direction is formed, and mirror polishing is performed. Next, on the cut surface, 4 points of vickers hardness were measured on the basis of jis z2244-1 (2020) at a position of 20 μm from the surface and a position of 1/4 of the plate thickness from the surface of the steel plate. The load in the vickers hardness measurement was set to 2kgf. The average value of the hardness measurement values at a position 20 μm from the surface of the steel sheet was set as the hardness of the surface layer portion, and the average value of the hardness measurement values at a position 1/4 of the sheet thickness from the surface was set as the hardness of the t/4 portion.

The cold-rolled steel sheet according to the present embodiment may have a coating layer containing zinc, aluminum, or magnesium or an alloy of one or more of these, or a coating layer formed of zinc, aluminum, or magnesium or an alloy of one or more of these (which is allowed to contain impurities, etc.) on the surface (one or both sides).

By providing the surface with a coating layer, corrosion resistance is improved. In the case of steel sheets for automobiles, if there is a concern about perforation due to corrosion, there is a possibility that the steel sheets cannot be thinned to a certain plate thickness or less even if the steel sheets are made high-strength. One of the purposes of increasing the strength of steel sheets is to reduce the weight due to the reduction in thickness, and therefore, even if high-strength steel sheets are developed, the application sites are limited if the corrosion resistance is low. As a method for solving these problems, forming a coating layer on the surface is considered to improve corrosion resistance.

Examples of the coating layer include a hot dip galvanized layer, an alloyed hot dip galvanized layer, an electro-galvanized layer, an aluminum plating layer, a Zn-Al alloy plating layer, an Al-Mg alloy plating layer, and a Zn-Al-Mg alloy plating layer.

When the surface has a coating layer, the surface to be the reference of the t/4 section and the like is the surface of the base metal from which the coating layer is removed.

[ tensile Strength ]

In the cold-rolled steel sheet of the present embodiment, the Tensile Strength (TS) is at least 1310MPa as a strength contributing to the weight saving of the automobile body. From the viewpoint of impact absorbability, the tensile strength is preferably 1400MPa or more, more preferably 1470MPa or more.

The upper limit is not necessarily limited, but if the tensile strength is increased, the formability may be lowered, so that the tensile strength may be 2000MPa or less.

< manufacturing method >)

The cold-rolled steel sheet according to the present embodiment can obtain the effects of the present invention as long as the cold-rolled steel sheet has the above-described characteristics, regardless of the manufacturing method, but can be stably manufactured by the following manufacturing method.

Specifically, the cold-rolled steel sheet according to the present embodiment can be produced by a production method including the following steps (I) to (VI).

(I) A hot rolling step of hot rolling a slab having a predetermined chemical composition to form a hot-rolled steel sheet, and coiling the hot-rolled steel sheet in a state where the temperature of the widthwise central portion is more than 600 ℃ and 700 ℃ or less and the temperature of the edge portion at a position 20mm away from the widthwise end portion is 600 ℃ or less;

(III) a cold rolling step of pickling the hot-rolled steel sheet after the hot rolling step, and cold-rolling the hot-rolled steel sheet at a reduction of 30 to 90% to obtain a cold-rolled steel sheet;

(IV) an annealing step of heating the cold-rolled steel sheet to an annealing temperature exceeding Ac3 ℃, holding the cold-rolled steel sheet at the annealing temperature, and cooling the cold-rolled steel sheet after the holding to the cooling stop temperature so that an average cooling rate up to 400 ℃ is 10 ℃/sec or more and an average cooling rate from 400 ℃ to 100 ℃ or less is 15 ℃/sec or more;

(V) a heat treatment step of heating the cold-rolled steel sheet after the annealing step to a temperature range of 200 to 350 ℃ and maintaining the temperature range;

and (VI) a skin finishing step of skin finishing the cold-rolled steel sheet after the heat treatment step at a reduction ratio of 0.1% or more.

In the method for producing a cold-rolled steel sheet according to the present embodiment, the difference between the thickness of the center portion and the thickness of the edge portion in the width direction of the cold-rolled steel sheet after the cold-rolling step is 10 μm or more.

The following will explain each step.

[ Hot Rolling Process ]

In the hot rolling step, a slab having the same chemical composition as the cold-rolled steel sheet of the present embodiment is hot-rolled to obtain a hot-rolled steel sheet. The hot rolling is preferably performed under such conditions that the finishing temperature of the finish rolling becomes Ac3 ℃ or higher in order to satisfy the temperature at the time of coiling to be described later. The upper limit of the finish temperature of the finish rolling is not particularly limited, but is generally 950 ℃ or lower.

The hot-rolled steel sheet is coiled in a state where the temperature of the widthwise central portion is more than 600 ℃ and 700 ℃ or less and the temperature of the edge portion at a position 20mm away from the widthwise end portion is 600 ℃ or less.

In order to reduce the winding temperature of the edge portion to be lower than that of the widthwise central portion, the edge portion is cooled so that the cooling rate of the edge portion is increased as compared with that of the central portion. For example, the edge portion of the steel sheet after hot rolling may be water-cooled, or in the case of water-cooling the entire steel sheet, the amount of cooling water at the edge portion may be set to be larger than that at the widthwise central portion.

When the edge portion is cooled by water and then wound, the edge portion is tempered by heat transfer from the widthwise central portion having a higher temperature, and therefore is softer than the widthwise central portion. As a result, the strength of the edge portion becomes lower than the strength of the widthwise central portion in a state of being cooled to around room temperature.

By cold rolling the steel sheet having such a strength difference in the width direction as described later, a difference in sheet thickness occurs between the widthwise central portion and the edge portion of the steel sheet.

If the winding temperature of the widthwise central portion exceeds 700 ℃, the widthwise central portion is softened. If the winding temperature in the widthwise central portion is 600 ℃ or lower, the temperature difference from the edge portion becomes small, or the edge portion cannot be sufficiently tempered. The winding temperature in the central portion is preferably 620 ℃ or higher.

In addition, if the winding temperature of the edge portion exceeds 600 ℃, the softening effect by tempering cannot be sufficiently obtained. If the coiling temperature of the edge portion is 400 ℃ or lower, the edge portion is tempered by heat transfer from the widthwise central portion, but the strength increases to increase the cold rolling load, and cracks may occur. The winding temperature at the edge portion is therefore preferably over 400 ℃, more preferably 450 ℃ or higher.

When the difference in thickness between the widthwise central portion and the edge portion is to be made larger, the difference in winding temperature between the widthwise central portion and the edge portion is preferably 50 ℃ or higher, more preferably 75 ℃ or higher, and still more preferably 100 ℃ or higher.

The method for producing the slab to be hot-rolled is not particularly limited. In the preferred method for producing a slab as exemplified, steel having the above-mentioned chemical composition is melted by a known means and then cast into a steel ingot by a continuous casting method, or a steel ingot is cast by an arbitrary casting method and then cast into a steel slab by a cogging rolling method or the like. In the continuous casting step, it is preferable to generate external additional flow such as electromagnetic stirring in the molten steel in the mold in order to suppress occurrence of surface defects due to inclusions. The temporarily cooled product may be reheated for hot rolling, or the continuously cast high-temperature steel ingot or the bloom rolled high-temperature steel ingot may be directly hot rolled, heat-insulated for hot rolling, or auxiliary heated for hot rolling. In the present embodiment, such a steel ingot and a steel slab are collectively referred to as a "slab" as a hot rolled material.

[ Cold Rolling Process ]

In the cold rolling step, the hot-rolled steel sheet after the hot rolling step is pickled, and cold-rolled at a reduction of 30 to 90% to obtain a cold-rolled steel sheet.

The acid washing conditions are not particularly limited, and known conditions may be used.

In the cold rolling step, a steel sheet having a difference in sheet thickness in the width direction (cold-rolled steel sheet) is obtained by cold rolling a steel sheet having a difference in strength in the width direction.

If the rolling reduction (cumulative rolling reduction) of the cold rolling is less than 30%, a sufficient plate thickness difference cannot be provided. If the reduction ratio of the cold rolling exceeds 90%, the cold rolling load becomes excessively large, and cold rolling becomes difficult.

In the method for producing a cold-rolled steel sheet according to the present embodiment, a cold-rolled steel sheet having a difference between the thickness of the center portion and the thickness of the edge portion in the width direction of the cold-rolled steel sheet after the cold-rolling step of 10 μm or more is obtained through the hot-rolling step and the cold-rolling step. The thickness difference is preferably 15 μm or more.

The upper limit of the plate thickness difference is not limited, but if the plate thickness difference is large, cracks may be generated from a portion where the plate thickness is thin, and hole expansibility may be reduced. Therefore, from the viewpoint of formability, the plate thickness difference may be 55 μm or less.

The thickness of the center portion and the thickness of the edge portion in the width direction can be measured by providing a scanning type thickness meter on the outlet side of the cold rolling mill.

[ Width trimming procedure ]

After cutting, if the difference between the plate thickness of the widthwise central portion and the plate thickness of the edge portion is 10 μm or more, a width trimming may be performed in which an arbitrary width is cut from the widthwise end portion of the steel sheet.

By trimming the width, even when cracks or flaws occur at the end of the cold-rolled steel sheet, the steel sheet can be supplied to the next step by cutting off the end, and this is preferable in terms of cost and yield.

[ annealing Process ]

In the annealing step, the cold-rolled steel sheet after the cold rolling step or after the width trimming step as needed is heated to an annealing temperature exceeding Ac3 ℃ and is held at the annealing temperature.

If the annealing temperature is Ac3 ℃ or lower, the microstructure does not undergo austenite transformation sufficiently, and a desired microstructure mainly composed of martensite cannot be obtained after the annealing step.

On the other hand, excessive high-temperature heating to an annealing temperature exceeding 900 ℃ causes an increase in manufacturing cost. Therefore, the annealing temperature is preferably set to 900 ℃ or lower.

The temperature (. Degree. C.) at Ac3 point can be determined by the following method.

Ac3＝910-(203×C ^1/2 )+44.7×Si-30×Mn+700×P-20×Cu-15.2×Ni-

11×Cr+31.5×Mo+400×Ti+104×V+120×Al

The symbol of the element contained in the formula refers to the content of the element contained in the steel sheet in mass%.

The holding time at the annealing temperature is preferably 40 to 135 seconds.

If the holding time is less than 40 seconds, austenitization may not proceed sufficiently. In addition, if the holding time exceeds 135 seconds, productivity is lowered.

The cold-rolled steel sheet is cooled to a cooling stop temperature such that the average cooling rate up to 400 ℃ is 10 ℃/sec or more and the average cooling rate from 400 ℃ to 100 ℃ or less is 15 ℃/sec or more.

If the average cooling rate up to 400 ℃ is less than 10 ℃/sec, ferrite may be generated in the microstructure. In addition, when the average cooling rate from 400 ℃ to the cooling stop temperature (100 ℃ or lower) is lower than 15 ℃/sec or the cooling stop temperature exceeds 100 ℃, bainite may be formed in the microstructure. In these cases, the desired microstructure cannot be obtained.

In the annealing step, a film layer made of zinc, aluminum, or magnesium, or an alloy of one or more of these is formed on the surface (one side surface or both side surfaces) of the cold-rolled steel sheet.

In the case of forming a coating layer, for example, if hot dip plating is used, the steel sheet may be immersed in a plating bath during cooling to form a hot dip coating layer on the surface thereof, and the steel sheet may be kept at a temperature of about 450 to 470 ℃ for 10 to 40 seconds, with an average cooling rate of 10 ℃/sec or more up to 400 ℃ and an average cooling rate of 15 ℃/sec or more up to a cooling stop temperature of 400 ℃ to 100 ℃.

In the case of performing the alloying treatment on the hot dip coating layer, it is preferable that the steel sheet, in which the hot dip galvanized layer is formed by immersing the steel sheet in a plating bath, is heated to a temperature range (alloying temperature) of 470 to 550 ℃, and kept in that temperature range for 10 to 40 seconds. If the alloying temperature is less than 470 ℃, there is a possibility that the alloying does not proceed sufficiently. On the other hand, if the alloying temperature exceeds 550 ℃, the alloying proceeds excessively, and there is a possibility that the Fe concentration in the plating layer exceeds 15% due to the generation of Γ phase, resulting in deterioration of corrosion resistance. More preferably, the alloying temperature is 480 ℃ or higher. Further, the alloying temperature is more preferably 540 ℃ or lower.

[ Heat treatment Process ]

By cooling in the annealing step (cooling after holding), austenite which has not been transformed into martensite is transformed into a microstructure of the cold-rolled steel sheet. However, some of the austenite may not be transformed into retained austenite.

Such a cold-rolled steel sheet is subjected to a heat treatment of heating to a temperature range of 200 to 350 ℃ and holding the sheet in the temperature range.

By this heat treatment, part or all of the martensite becomes tempered martensite. In the case of forming a microstructure mainly composed of tempered martensite, the holding time is preferably 1 second or longer.

If the heating temperature is less than 200 ℃, the martensite may not be sufficiently tempered, and satisfactory changes in microstructure and mechanical properties may not be brought about. If the heating temperature exceeds 350 ℃, the dislocation density in tempered martensite may decrease, resulting in a decrease in tensile strength.

[ skin finishing Process ]

In the skin pass rolling step, the cold rolled steel sheet after the heat treatment step is subjected to skin pass rolling at a reduction of 0.1% or more.

As described above, the cold-rolled steel sheet after the heat treatment step has a thickness difference of 10 μm or more between the widthwise central portion and the edge portion.

When performing skin pass rolling on such a cold-rolled steel sheet, the sheet thickness difference causes the steel sheet to be bitten into the roll at a predetermined angle rather than being perpendicular to the longitudinal direction of the roll. The rolling reduction can be arbitrarily selected in the setting of the skin pass mill, but in the case of having a plate thickness difference, the amount of strain introduced into the surface layer portion can be further increased than the amount of strain assumed to be introduced by the rolling reduction set in the case of having a uniform plate thickness.

In the present embodiment, by performing skin pass rolling on a cold-rolled steel sheet having a predetermined sheet thickness difference, strain is introduced into the surface layer portion, and dislocation density in the surface layer portion can be increased and dislocation can be immobilized.

However, if the rolling reduction is less than 0.1%, a sufficient effect cannot be obtained, and therefore the rolling reduction is made to be 0.1% or more. The upper limit of the reduction ratio is not limited, but if it exceeds 1.5%, the productivity is significantly lowered, and thus is preferably set to less than 1.5%.

Generally, no skin pass rolling is performed on a steel sheet having a tensile strength of 1310MPa or more and a rolling reduction of 0.1% or more, but in the present embodiment, skin pass rolling is performed with a rolling reduction of 0.1% or more based on the novel findings found by the inventors of the present invention.

Examples

Slabs (steel grades a to W) having the chemical compositions shown in tables 1-1 to 1-2 (unit is mass%, and the remainder is Fe and impurities) were produced by continuous casting.

Using these slabs, hot rolling was performed so that the finish temperature of finish rolling became Ac3 ℃ or higher, and cooling conditions of the central portion and the edge portion were changed, whereby coiling was performed under the conditions shown in table 2-1, to obtain hot-rolled steel sheets.

These hot-rolled steel sheets were cold-rolled under the conditions shown in Table 2-1 to obtain cold-rolled steel sheets having sheet thickness differences shown in Table 2-1.

These cold-rolled steel sheets were annealed, heat-treated, and skin-pass rolled under the conditions shown in Table 2-2.

In addition, a part of the cold-rolled steel sheet is hot dip galvanized during annealing. By varying the holding temperature after immersion in the plating bath, a part of the plating layer is alloyed. The holding time was set to 10 to 40 seconds. In Table 3-1, GI indicates that a hot dip galvanized layer was formed, and GA indicates that an alloyed hot dip galvanized layer was formed.

From the obtained cold-rolled steel sheet, the microstructure of the t/4 portion was observed by the above method, and the volume fractions of martensite, tempered martensite, bainite, retained austenite, ferrite, and pearlite were measured.

The results of measuring the volume fractions of martensite, tempered martensite, bainite, and retained austenite are shown in Table 3-1. Although not shown in the table, nos. 38 and 39 generate ferrite in addition to martensite, tempered martensite, bainite, and retained austenite.

The dislocation density and hardness of the surface layer portion and t/4 portion were measured from the obtained cold-rolled steel sheet by the method described above.

The results are shown in Table 3-2. However, for the expression of dislocation density, "5.0E+15" in the table means 5.0X10 ¹⁵ In the case of YE+X, the other is Y×10 in the same way ^x 。

The tensile strength of the obtained cold-rolled steel sheet was also obtained. The tensile strength was obtained by the following method.

Tensile Strength (TS) is determined by: a JIS No. 5 test piece was used from the direction in which the longitudinal direction of the test piece was parallel to the rolling direction of the steel sheet, and a tensile test was performed in accordance with JIS Z2241 (2011).

The results are shown in Table 3-2.

In order to evaluate the reduction in flow stress, a post-prestrain tensile test was performed by the following collar.

The change in flow stress was evaluated by: a test piece of JIS No. 5 was taken from a direction in which the longitudinal direction of the test piece was parallel to the rolling direction of the steel sheet, and after 0.1% of the pre-strain was applied to the test piece in accordance with JIS Z2241 (2011), the test piece was left for 1 day after the load was removed, and the stress at the time of re-stretching the test piece was compared with the stress at the time of 0.1% of the pre-strain load. The case where the flow stress increases when the strain is applied again, the same case, or the case where the reduction amount is smaller than 40MPa is set as a (excellent), the case where the reduction amount is 40MPa or more and less than 80MPa is set as B (good), and the case where the reduction amount is 80MPa or more is set as C (not meeting the target).

The results are shown in Table 3-2.

[ Table 1-1]

[ tables 1-2]

[ Table 2-1]

[ Table 2-2]

[ Table 3-1]

[ Table 3-2]

As is clear from tables 1-1 to 3-2, the invention examples Nos. 1 to 30 have a tensile strength of 1310MPa or more, and the decrease in flow stress is suppressed to be lower than 80MPa.

On the other hand, in comparative examples Nos. 31 to 45, a tensile strength of 1310MPa or more was not obtained, or a decrease in flow stress of 80MPa or more was observed.

Claims

1. A cold-rolled steel sheet characterized by having the following chemical composition, in mass%:

C：0.150～0.500％、

Si：0.01～2.00％、

Mn：0.50～3.00％、

p: less than 0.0200 percent,

S: less than 0.0200 percent,

Al:0.100% or less,

N: less than 0.0200 percent,

O: less than 0.020%,

Ni：0～1.000％、

Mo：0～1.000％、

Cr：0～2.000％、

B：0～0.010％、

As：0～0.050％、

Co：0～0.500％、

Ti：0～0.500％、

Nb：0～0.500％、

V：0～0.500％、

Cu：0～0.500％、

W：0～0.100％、

Ta：0～0.100％、

Ca：0～0.050％、

Mg：0～0.050％、

La：0～0.050％、

Ce：0～0.050％、

Y：0～0.050％、

Zr：0～0.050％、

Sb：0～0.050％、

Sn:0 to 0.050%, and

the remainder: fe and impurities are mixed in the alloy,

when a range of 1/8 to 3/8 of the plate thickness from the surface in the plate thickness direction is set as a t/4 portion and a range of 20 μm from the surface in the plate thickness direction is set as a surface layer portion, the microstructure in the t/4 portion includes, in terms of volume ratio: 0% -10.0% of residual austenite; and 90.0% -100% of 1 or 2 of martensite and tempered martensite,

the ratio of the dislocation density of the surface layer portion to the dislocation density of the t/4 portion is 0.80 or more,

The ratio of the hardness of the surface layer portion to the hardness of the t/4 portion is 0.90 or more,

the tensile strength of the cold-rolled steel sheet is 1310MPa or more.

2. The cold-rolled steel sheet according to claim 1, wherein the surface has a coating layer formed of zinc, aluminum or magnesium or an alloy of one or more of them.

3. A method for producing a cold-rolled steel sheet, characterized by comprising the steps of:

a hot rolling step of hot rolling a slab having the chemical composition according to claim 1 to obtain a hot-rolled steel sheet, and coiling the hot-rolled steel sheet in a state where the temperature of the widthwise central portion is more than 600 ℃ and 700 ℃ or less and the temperature of the edge portion at a position 20mm away from the widthwise end portion is 600 ℃ or less;

a cold rolling step of pickling the hot-rolled steel sheet after the hot rolling step, and cold-rolling the hot-rolled steel sheet at a reduction of 30 to 90% to obtain a cold-rolled steel sheet;

an annealing step of heating the cold-rolled steel sheet to an annealing temperature exceeding Ac3 ℃, holding the cold-rolled steel sheet at the annealing temperature, and cooling the cold-rolled steel sheet to the cooling stop temperature so that an average cooling rate up to 400 ℃ is 10 ℃/sec or more and an average cooling rate from 400 ℃ to 100 ℃ or less is 15 ℃/sec or more;

A heat treatment step of heating the cold-rolled steel sheet after the annealing step to a temperature range of 200 to 350 ℃ and maintaining the steel sheet in the temperature range; and

a skin finishing step of skin finishing the cold-rolled steel sheet after the heat treatment step at a reduction of 0.1% or more,

the difference between the thickness of the center portion and the thickness of the edge portion in the width direction of the cold-rolled steel sheet after the cold-rolling step is 10 [ mu ] m or more.

4. The method of producing a cold-rolled steel sheet according to claim 3, wherein a film layer made of zinc, aluminum, magnesium, or an alloy of one or more of these is formed on the front and rear surfaces of the cold-rolled steel sheet in the annealing step.