阅读说明：本技术 钢板及其制造方法 (Steel sheet and method for producing same ) 是由 久保雅宽 川田裕之 大塚研一郎 东昌史 于 2020-01-07 设计创作，主要内容包括：本发明的钢板的化学组成以质量％计含有C：0.0015％～0.0400％、Mn：0.20％～1.50％、P：0.010％～0.100％、Cr：0.001％～0.500％、Si：0.200％以下、S：0.020％以下、sol.Al：0.200％以下、N：0.0150％以下、Mo：0％～0.500％、B：0％～0.0100％、Nb：0％～0.200％、Ti：0％～0.200％、Ni：0％～0.200％及Cu：0％～0.100％,剩余部分包含铁及杂质,表层区域的金属组织以体积分率计包含90％以上的铁素体,在上述表层区域中,上述铁素体的平均晶体粒径为1.0～15.0μm,包含{001}取向与{111}取向的强度比X-(ODF{001}/{111},S)为0.30以上且低于3.50的织构。(The chemical composition of the steel sheet of the present invention contains, in mass%, C: 0.0015-0.0400%, Mn: 0.20% -1.50%, P: 0.010-0.100%, Cr: 0.001% -0.500%, Si: 0.200% or less, S: 0.020% or less, sol.Al: 0.200% or less, N: 0.0150% or less, Mo: 0% -0.500%, B: 0% -0.0100%, Nb: 0% -0.200%, Ti: 0% -0.200%, Ni: 0% to 0.200% and Cu: 0% to 0.100%, the balance being iron and impurities, the microstructure in the surface layer region containing not less than 90% ferrite in volume fraction, and the average crystal grain of the ferrite in the surface layer regionA bulk particle diameter of 1.0 to 15.0 μm, and a strength ratio X of {001} orientation to {111} orientation ODF{001}/{111}，S A texture of 0.30 or more and less than 3.50.)

1. A steel sheet characterized by having a chemical composition containing, in mass%:

C：0.0015％～0.0400％、

Mn：0.20％～1.50％、

P：0.010％～0.100％、

Cr：0.001％～0.500％、

si: less than 0.200 percent,

S: less than 0.020%,

Al: less than 0.200 percent,

N: less than 0.0150 percent,

Mo：0％～0.500％、

B：0％～0.0100％、

Nb：0％～0.200％、

Ti：0％～0.200％、

Ni: 0% to 0.200%, and

Cu：0％～0.100％，

the remaining part contains iron and impurities,

the microstructure in the surface layer region contains 90% or more of ferrite in terms of volume fraction,

in the region of the surface layer, the surface layer is,

the ferrite has an average crystal grain diameter of 1.0 to 15.0 μm,

intensity ratio X of {001} orientation to {111} orientation including the ferrite_{ODF{001}/{111}，S}A texture of 0.30 or more and less than 3.50.

2. The steel sheet according to claim 1, wherein the chemical composition comprises, in mass%:

Mo：0.001％～0.500％、

B：0.0001％～0.0100％、

Nb：0.001％～0.200％、

Ti：0.001％～0.200％、

ni: 0.001% -0.200%, and

Cu：0.001％～0.100％

any 1 or more of them.

3. The steel sheet according to claim 1 or 2, wherein in the inner region, a strength ratio X of {001} orientation to {111} orientation including ferrite_{ODF{001}/{111}，I}A texture of 0.001 or more and less than 1.00.

4. Steel sheet according to any one of claims 1 to 3, characterized in that the strength ratio X_{ODF{001}/{111}，S}Intensity ratio X to {001} orientation and {111} orientation of ferrite in inner region_{ODF{001}/{111}，I}Satisfies the following formula (1),

the average crystal grain size of the ferrite of the surface region is smaller than the average crystal grain size of the ferrite of the inner region,

－0.20＜X_{ODF{001}/{111}，S}－X_{ODF{001}/{111}，I}＜0.40 (1)。

5. a steel sheet according to any one of claims 1 to 4, having a plating layer on the surface.

6. A method for manufacturing a steel sheet, comprising the steps of:

a heating step of heating a steel slab having the chemical composition according to claim 1 to 1000 ℃ or higher;

a hot rolling step of hot rolling the slab so that the rolling completion temperature becomes 950 ℃ or lower to obtain a hot-rolled steel sheet;

subjecting the hot-rolled steel sheet after the hot rolling step to a residual stress in the surface thereof, i.e., σ_sA stress applying step of applying a stress so that the absolute value of the stress is 100 to 250 MPa;

performing a cumulative reduction ratio R on the hot-rolled steel sheet after the stress application step_CRA cold rolling step of obtaining a cold-rolled steel sheet by cold rolling 70 to 90%;

an annealing step of annealing the cold-rolled steel sheet so that the average heating rate is 1.5 to 10.0 ℃/second from 300 ℃ to a soaking temperature T1 ℃ satisfying the following formula (2), and then the steel sheet is held at the soaking temperature T1 ℃ for 30 to 150 seconds; and

a cooling step of cooling the cold-rolled steel sheet after the annealing step to a temperature range of 550 to 650 ℃ so that the average cooling rate at the soaking temperature T1 to 650 ℃ is 1.0 to 10.0 ℃/sec, and then cooling the cold-rolled steel sheet to a temperature range of 200 to 490 ℃ so that the average cooling rate is5 to 500 ℃/sec,

Ac₁+550－25×ln(σ_s)－4.5×R_CR≤T1≤Ac₁+550－25×ln(σ_s)－4×R_CR (2)

wherein said Ac in said formula (2)₁Represented by the following formula (3); the symbol of an element in the following formula (3) is the content of the element in mass%, and 0 is substituted when the element is not contained,

Ac₁＝723－10.7×Mn－16.9×Ni+29.1×Si+16.9×Cr (3)。

7. the method for producing a steel sheet according to claim 6, wherein the stress application step is performed at 40 to 500 ℃.

8. The method for producing a steel sheet according to claim 6 or 7, wherein a finish rolling start temperature in the hot rolling step is 900 ℃ or lower.

9. The method for manufacturing a steel sheet according to any one of claims 6 to 8, further comprising a holding step of holding the cold-rolled steel sheet after the cooling step at a temperature of 200 to 490 ℃ for 30 to 600 seconds.

Technical Field

The present invention relates to a steel sheet and a method for producing the same.

This application claims priority based on Japanese application No. 2019-025635, filed on 2019, month 02 and 15, the contents of which are incorporated herein by reference.

Background

In recent years, improvement in fuel efficiency of automobiles has been demanded for global environmental conservation. With respect to improvement of fuel efficiency of automobiles, steel sheets for automobiles are required to have further higher strength in order to reduce the weight of automobile bodies while ensuring safety. Such a demand for higher strength is increasing not only for beams, pillars, and the like, which are structural members, but also for outer panel parts (roofs, hoods, fenders, doors, and the like) of automobiles. In response to such a demand, material development has been carried out for the purpose of achieving both strength and elongation (moldability).

On the other hand, the molding of the outer panel member of the automobile tends to be complicated. If the steel sheet is strengthened to reduce the weight, it becomes difficult to process the steel sheet into a complicated shape. Further, when the steel sheet is thinned for weight reduction, irregularities are likely to be generated on the surface of the steel sheet when the steel sheet is formed into a complicated shape. When the surface is uneven, the molded appearance is degraded. The outer panel member is required to have not only properties such as strength but also excellent appearance after molding because design of a pattern and surface quality are important. The irregularities generated after forming described herein are irregularities generated on the surface of a formed member by forming even though there are no irregularities on the surface of a steel sheet after production, and are not necessarily suppressed even if the formability of the steel sheet is improved, and therefore, they are a major problem when applied to an outer panel of a high-strength steel sheet.

Regarding the correlation between the post-forming appearance and the material properties of a steel sheet applied to an outer panel member, for example, patent document 1 discloses a ferritic thin steel sheet in which the surface integral rate of crystals having a crystal orientation within ± 15 ° from the {001} plane parallel to the surface of the steel sheet is set to 0.25 or less and the average grain size of the crystals is set to 25 μm or less in order to improve the surface properties after bulging.

However, patent document 1 relates to a ferritic steel sheet having a C content of 0.0060% or less. The present inventors have conducted studies and, as a result, have found that: in the case of a steel sheet having a higher C content than the steel sheet described in patent document 1, it is difficult to reduce the area fraction of crystals having a crystal orientation within ± 15 ° from the {001} plane parallel to the surface of the steel sheet. That is, the method of patent document 1 cannot satisfy both the improvement of the strength and the improvement of the surface properties after the processing.

For example, patent document 2 discloses a steel sheet having ferrite as a main phase, in which the X-ray random strength ratio in 1/4 layers of sheet thickness is controlled, and which has an excellent young's modulus in the direction perpendicular to rolling. However, patent document 2 does not disclose the relationship between the appearance and the structure after molding from the viewpoint of surface roughness and a pattern countermeasure.

That is, conventionally, there has been no proposal for a high-strength steel sheet having improved surface roughness after forming and excellent formability of pattern defects.

Documents of the prior art

Patent document

Patent document 1: japanese patent laid-open publication No. 2016-156079

Patent document 2: japanese patent laid-open publication No. 2012-233229

Disclosure of Invention

Problems to be solved by the invention

The present invention has been made in view of the above problems. The present invention addresses the problem of providing a high-strength steel sheet having excellent formability and capable of suppressing the occurrence of surface irregularities during forming, and a method for manufacturing the same.

Means for solving the problems

The present inventors have studied a method for solving the above problems.

The results are known as follows: the surface irregularities during molding are caused by uneven deformation during molding due to unevenness in strength in the microscopic region.

The present inventors have further conducted studies and, as a result, found that: in order to improve formability, the metal structure is controlled so that ferrite becomes a main phase, and the average grain size of ferrite and the texture (texture) of ferrite in the metal structure in the surface region are controlled to be different from those in the steel sheet, whereby the occurrence of surface irregularities during forming can be suppressed, and a steel sheet having excellent appearance (surface quality) after forming can be obtained.

In addition, the present inventors have conducted studies and found that: in order to control the microstructure of the surface region, it is effective to apply strain not after cold rolling but after hot rolling, and to set the subsequent cold rolling rate and heat treatment conditions according to the amount of work.

The present invention has been made based on the above-described knowledge, and the gist thereof is as follows.

[1]The steel sheet according to one aspect of the present invention has a chemical composition containing, in mass%, C: 0.0015-0.0400%, Mn: 0.20% -1.50%, P: 0.010-0.100%, Cr: 0.001% -0.500%, Si: 0.200% or less, S: 0.020% or less, sol.Al: 0.200% or less, N: 0.0150% or less, Mo: 0% -0.500%, B: 0% -0.0100%, Nb: 0% -0.200%, Ti: 0% -0.200%, Ni: 0% to 0.200%, and Cu: 0% to 0.100%, the balance being iron and impurities, the microstructure in the surface layer region containing not less than 90% of ferrite in volume fraction, the average crystal grain size of the ferrite in the surface layer region being 1.0 to 15.0 [ mu ] m, and the strength ratio X of the {001} orientation to the {111} orientation of the ferrite_{ODF{001}/{111}，S}A texture of 0.30 or more and less than 3.50.

[2] The steel sheet according to [1], wherein the chemical composition may include Mo: 0.001% -0.500%, B: 0.0001 to 0.0100%, Nb: 0.001-0.200%, Ti: 0.001% -0.200%, Ni: 0.001 to 0.200%, and Cu: any 1 or more of 0.001-0.100%.

[3]According to the above [1]Or [2]]The steel sheet may further include a strength ratio X of a {001} orientation to a {111} orientation of ferrite in the inner region_{ODF{001}/{111}，I}The texture is 0.001 or more and less than 1.0.

[4]According to the above [1]～[3]The steel sheet according to any one of claims, wherein the strength ratio X in the surface layer region_{ODF{001}/{111}，S}And the intensity ratio X of the {001} orientation to the {111} orientation of ferrite in the inner region_{ODF{001}/{111}，I}Satisfies the following formula (1),

the average crystal grain size of the ferrite in the surface region may be smaller than the average crystal grain size of the ferrite in the inner region.

－0.20＜X_{ODF{001}/{111}，S}－X_{ODF{001}/{111}，I}＜0.40 (1)

[5] The steel sheet according to any one of the above [1] to [4], wherein the surface thereof may have a plating layer.

[6]A method for manufacturing a steel sheet according to another aspect of the present invention includes the steps of: will have the above-mentioned [1]]The heating step of heating the steel slab having the chemical composition described in (1) to 1000 ℃ or higher; a hot rolling step of hot rolling the slab so that the rolling completion temperature becomes 950 ℃ or lower to obtain a hot-rolled steel sheet; subjecting the hot-rolled steel sheet after the hot rolling step to a residual stress in the surface thereof, i.e., σ_sA stress applying step of applying a stress so that the absolute value of the stress is 100 to 250 MPa; subjecting the hot-rolled steel sheet after the stress application step to a cumulative reduction ratio R_CRA cold rolling step of obtaining a cold-rolled steel sheet by cold rolling 70 to 90%; an annealing step of annealing the cold-rolled steel sheet so that the average heating rate is 1.5 to 10.0 ℃/sec from 300 ℃ to a soaking temperature T1 ℃ satisfying the following formula (2), and then the steel sheet is held at the soaking temperature T1 ℃ for 30 to 150 seconds; and cooling the cold-rolled steel sheet after the annealing step to a temperature range of 550 to 650 ℃ at an average cooling rate of 1.0 to 10.0 ℃/sec until the soaking temperature T1 to 650 ℃, and then adjusting the average cooling rate toA cooling step of cooling the substrate to a temperature range of 200 to 490 ℃ at a rate of 5 to 500 ℃/sec.

Ac₁+550－25×ln(σ_s)－4.5×R_CR≤T1≤Ac₁+550－25×ln(σ_s)－4×R_CR (2)

Wherein Ac is the one represented by the above formula (2)₁Is represented by the following formula (3). The symbol of an element in the following formula (3) is the content of the element in mass%, and 0 is substituted when the element is not contained.

Ac₁＝723－10.7×Mn－16.9×Ni+29.1×Si+16.9×Cr (3)

[7] The method of manufacturing a steel sheet according to the above [6], wherein the stress applying step may be performed at 40 to 500 ℃.

[8] The method for producing a steel sheet according to the above [6] or [7], wherein, in the hot rolling step, the finish rolling start temperature may be 900 ℃ or lower.

[9] The method for producing a steel sheet according to any one of the above [6] to [8], further comprising a holding step of holding the cold-rolled steel sheet after the cooling step at a temperature of 200 to 490 ℃ for 30 to 600 seconds.

Effects of the invention

The steel sheet according to the above aspect of the present invention can suppress the occurrence of surface irregularities even after various deformations due to press deformation, as compared with conventional materials. Therefore, the steel sheet according to the aspect of the present invention has excellent surface beauty, and contributes to improvement of clearness and design of the coating. The steel sheet of the present invention has high strength, and therefore can contribute to further weight reduction of an automobile, and also has excellent formability, and therefore can be applied to an outer panel member having a complicated shape. In the present invention, high strength means that the steel sheet has a tensile strength of 340MPa or more.

Further, according to the method for manufacturing a steel sheet of the above aspect of the present invention, it is possible to manufacture a high-strength steel sheet which is excellent in formability and in which occurrence of surface irregularities can be suppressed even after various deformations caused by press deformation.

Drawings

Fig. 1 is a graph showing the relationship between the surface texture after molding and the texture parameter.

Detailed Description

The chemical composition of the steel sheet according to an embodiment of the present invention (the steel sheet according to the present embodiment) contains, in mass%, C: 0.0015-0.0400%, Mn: 0.20% -1.50%, P: 0.010-0.100%, Cr: 0.001% -0.500%, Si: 0.200% or less, S: 0.020% or less, sol.Al: 0.200% or less, N: 0.0150% or less, Mo: 0% -0.500%, B: 0% -0.0100%, Nb: 0% -0.200%, Ti: 0% -0.200%, Ni: 0% to 0.200% and Cu: 0 to 0.100 percent, and the balance of iron and impurities.

In addition, the microstructure of the surface layer region of the steel sheet of the present embodiment contains 90% or more of ferrite in volume fraction, the average crystal grain size of the ferrite in the surface layer region is 1.0 to 15.0 μm, and the strength ratio X including the {001} orientation and the {111} orientation of the ferrite is_{ODF{001}/{111}，S}A texture of 0.30 or more and less than 3.50.

In the steel sheet of the present embodiment, it is preferable that the strength ratio X of the {001} orientation and the {111} orientation including ferrite in the internal region is larger_{ODF{001}/{111}，I}A texture of 0.001 or more and less than 1.00.

In the steel sheet of the present embodiment, it is preferable that the strength ratio X in the surface layer region is set to be higher than the strength ratio X in the surface layer region_{ODF{001}/{111}，S}And a strength ratio X of a {001} orientation to a {111} orientation of ferrite in the inner region_{ODF{001}/{111}，I}The surface layer region has ferrite having an average crystal grain size smaller than that of the inner region, and satisfies the following expression (1).

－0.20＜X_{ODF{001}/{111}，S}－X_{ODF{001}/{111}，I}＜0.40 (1)

The steel sheet of the present embodiment will be described in detail below. However, the present invention is not limited to the configuration disclosed in the present embodiment, and various modifications are possible without departing from the scope of the present invention. The numerical limitation ranges described below include lower and upper limits. For values expressed as "above" and "below," the values are not included in the range of values. The% with respect to the chemical composition all represents mass%. First, the reason for limiting the chemical composition of the steel sheet of the present embodiment will be described.

< about chemical composition >

[C：0.0015％～0.0400％]

C (carbon) is an element that improves the strength of the steel sheet. In addition, the {111} texture becomes easily developed with a decrease in the C content. In order to obtain desired strength and texture, the C content is set to 0.0015% or more. Preferably 0.0030% or more, more preferably 0.0060% or more.

On the other hand, if the C content exceeds 0.0400%, the formability of the steel sheet deteriorates. Therefore, the C content is set to 0.0400% or less. The C content is preferably 0.0300% or less, more preferably 0.0200% or less.

[Mn：0.20％～1.50％]

Mn (manganese) is an element that improves the strength of the steel sheet. Further, Mn is also an element that prevents cracking during hot rolling by fixing S (sulfur) in steel as MnS or the like. In order to obtain these effects, the Mn content is set to 0.20% or more. Preferably 0.30% or more.

On the other hand, if the Mn content exceeds 1.50%, the cold rolling load increases when cold rolling is performed at a high reduction ratio, and the productivity decreases. Further, since Mn segregation is likely to occur, the hard phase is likely to agglomerate after annealing, and pattern defects occur on the surface after molding. Therefore, the Mn content is set to 1.50% or less. Preferably 1.30% or less, more preferably 1.10% or less.

[P：0.010％～0.100％]

P (phosphorus) is an element that improves the strength of the steel sheet. In order to obtain a desired strength, the P content is set to 0.010% or more. Preferably 0.015% or more, and more preferably 0.020% or more.

On the other hand, if P is excessively contained in steel, cracking during hot rolling or cold rolling is promoted, and ductility and weldability of the steel sheet are reduced. Therefore, the P content is set to 0.100% or less. The P content is preferably set to 0.080% or less.

[Cr：0.001％～0.500％]

Cr (chromium) is an element that improves the strength of the steel sheet. The Cr content is set to 0.001% or more in order to obtain a desired strength. Preferably 0.050% or more.

On the other hand, if the Cr content exceeds 0.500%, the strength of the steel sheet to be subjected to cold rolling increases, and the cold rolling load increases when cold rolling is performed at a high reduction ratio. Further, the alloy cost increases. Therefore, the Cr content is set to 0.500% or less. Preferably 0.350% or less.

[ Si: 0.200% or less ]

Si (silicon) is a deoxidizing element of steel, and is an element effective for improving the strength of a steel sheet. However, if the Si content exceeds 0.200%, the scale-peeling property during production is lowered, and surface defects tend to occur in the product. Further, the cold rolling load increases when cold rolling is performed at a high reduction ratio, and productivity decreases. Further, weldability and deformability of the steel sheet are reduced. Therefore, the Si content is limited to 0.200% or less. Preferably 0.150% or less.

In order to reliably obtain the deoxidizing effect and the strength-improving effect of the steel, the Si content may be set to 0.005% or more.

[ S: 0.020% or less ]

S (sulfur) is an impurity. If S is excessively contained in the steel, elongated MnS is produced by hot rolling, and the deformability of the steel sheet is reduced. Therefore, the S content is limited to 0.020% or less. The S content is preferably small and therefore may be 0%, but if the current general refining (including secondary refining) is considered, the S content may be set to 0.002% or more.

Al: 0.200% or less ]

Al (aluminum) is a deoxidizing element of steel. However, if the sol.al content exceeds 0.200%, the scale-peeling property during production is lowered, and surface defects are likely to occur in the product. Further, weldability of the steel sheet is lowered. Therefore, the sol.al content is set to 0.200% or less. Preferably 0.150% or less.

In order to reliably obtain the deoxidation effect of the steel, the sol.al content may be set to 0.020% or more.

[ N: 0.0150% or less ]

N (nitrogen) is an impurity, and is an element that reduces the deformability of the steel sheet. Therefore, the N content is limited to 0.0150% or less. The N content is preferably small, and therefore may be 0%. However, when the current general refining (including secondary refining) is considered, the N content may be set to 0.0005% or more.

The steel sheet of the present embodiment may contain the above-described elements, and the remainder may contain Fe and impurities. However, in order to improve various properties, elements (arbitrary elements) shown below may be contained instead of a part of Fe. In order to reduce the alloy cost, it is not necessary to intentionally add these arbitrary elements to the steel, and therefore the lower limit of the content of these arbitrary elements is 0% each. The impurities are components that are unintentionally contained from the raw materials or from other manufacturing processes in the manufacturing process of the steel sheet.

[Mo：0％～0.500％]

Mo (molybdenum) is an element that improves the strength of the steel sheet. If necessary, the resin composition may be contained in order to obtain a desired strength. In the case of obtaining the above-described effects, the Mo content is preferably set to 0.001% or more. More preferably, it is set to 0.010% or more.

On the other hand, if the Mo content exceeds 0.500%, the deformability of the steel sheet may be reduced. Further, the alloy cost increases. Therefore, the Mo content is set to 0.500% or less. Preferably 0.350% or less.

[B：0％～0.0100％]

B (boron) is an element that fixes carbon and nitrogen in steel to form fine carbonitrides. The fine carbonitride contributes to precipitation strengthening, structure control, fine grain strengthening, and the like of the steel. Therefore, B may be contained as necessary. In the case where the above-described effects are obtained, the B content is preferably set to 0.0001% or more.

On the other hand, if the B content exceeds 0.0100%, the above effects are saturated and the workability (deformability) of the steel sheet may be deteriorated. Further, since the strength of the steel sheet subjected to cold rolling by containing B increases, the cold rolling load when cold rolling is performed at a high reduction ratio increases. Therefore, when B is contained, the B content is set to 0.0100% or less.

[Nb：0％～0.200％]

Nb (niobium) is an element that fixes carbon and nitrogen in steel to form fine carbonitrides. Fine carbonitride of Nb contributes to precipitation strengthening, structure control, fine grain strengthening, and the like of steel. Therefore, Nb may be contained as necessary. In order to obtain the above-described effects, the Nb content is preferably set to 0.001% or more.

On the other hand, if the Nb content exceeds 0.200%, not only the above-described effects are saturated, but also the strength of the steel sheet to be cold-rolled increases, and the cold rolling load when cold-rolling at a high reduction ratio increases. Therefore, when Nb is contained, the Nb content is set to 0.200% or less.

[Ti：0％～0.200％]

Ti (titanium) is an element that fixes carbon and nitrogen in steel to form fine carbonitrides. The fine carbonitride contributes to precipitation strengthening, structure control, fine grain strengthening, and the like of the steel. Therefore, Ti may be contained as necessary. In the case where the above-described effects are obtained, the Ti content is preferably set to 0.001% or more.

On the other hand, if the Ti content exceeds 0.200%, not only the above-described effects are saturated, but also the strength of the steel sheet to be subjected to cold rolling increases, and the cold rolling load increases when cold rolling is performed at a high reduction ratio. Therefore, when Ti is contained, the Ti content is set to 0.200% or less.

[Ni：0％～0.200％]

Ni (nickel) is an element contributing to improvement of the strength of the steel sheet. Therefore, Ni may be contained as necessary. In the case where the above-described effects are obtained, the Ni content is preferably set to 0.001% or more.

On the other hand, if the Ni content becomes excessive, the strength of the steel sheet to be subjected to cold rolling increases, and the cold rolling load increases when cold rolling is performed at a high reduction ratio. Further, if Ni is excessively contained, the alloy cost increases. Therefore, even when Ni is contained, the Ni content is set to 0.200% or less.

[Cu：0％～0.100％]

Cu (copper) is an element stabilizing austenite, and therefore, by delaying the transformation from austenite to ferrite, the crystal grains are refined, contributing to the improvement of strength. Therefore, Cu may be contained as necessary. In the case of obtaining the above-described effects, the Cu content is preferably set to 0.001% or more.

On the other hand, if the Cu content exceeds 0.100%, not only the above-described effects are saturated, but also the strength of the steel sheet to be subjected to cold rolling increases, and the cold rolling load increases when cold rolling is performed at a high reduction ratio. Therefore, when Cu is contained, the Cu content is also set to 0.100% or less.

The chemical composition of the steel sheet may be measured by a general analytical method. For example, measurement may be performed by ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). C and S may be measured by a combustion-infrared absorption method, and N may be measured by an inert gas melting-thermal conductivity method. When the steel sheet has a plating layer on the surface, the chemical composition may be analyzed after the plating layer on the surface is removed by mechanical grinding.

< metallic texture with respect to the surface layer region >

In the steel sheet of the present embodiment, when the sheet thickness is set to t, the depth range from the surface to t/4 in the sheet thickness direction is divided into 2 regions, the depth range having the surface as a starting point and the depth position of 50 μm in the depth direction as an end point is set as the surface region, and the region closer to the center side of the steel sheet than the surface region is set as the inner region. When the thickness of the steel sheet is 0.20mm or less, a region of a depth from the surface to t/4 in the thickness direction is defined as a surface region, and a region of a depth from t/4 to t/2 is defined as an internal region. When the thickness of the steel sheet exceeds 0.40mm, the internal region is preferably set to a range from a position exceeding 50 μm in the thickness direction from the surface to a position 100 μm in the thickness direction from the surface.

The present inventors have conducted studies and, as a result, have found that: the surface irregularities during molding are caused by uneven deformation during molding due to unevenness in strength in the microscopic region. Knowing: in particular, the generation of surface irregularities has a large influence on the metal structure in the surface layer region. Therefore, in the steel sheet of the present embodiment, the metal structure in the surface region is controlled as follows.

[ ferrite containing 90% or more in volume fraction ]

If the volume fraction of ferrite in the surface layer region is less than 90%, the surface quality of the steel sheet after forming is likely to deteriorate. Therefore, the volume fraction of ferrite is set to 90% or more. Preferably 95% or more, or 98% or more. Since the entire microstructure in the surface layer region may be ferrite, the upper limit may be set to 100%.

The remaining structure in the surface layer region is, for example, 1 or more of pearlite, bainite, martensite, and tempered martensite. When the volume fraction of ferrite in the surface region is 100%, the volume fraction of these remaining portion structures is 0%.

The volume fraction of ferrite in the surface layer region is determined by the following method.

Samples for microstructure (microstructure) observation (approximately 20mm in the rolling direction x 20mm in the width direction x the thickness of the steel sheet) were sampled from a W/4 position or a 3W/4 position (i.e., a position W/4 in the width direction from either width-direction end of the steel sheet) of the steel sheet, and the microstructure (microstructure) was observed from a position where the thickness was 1/4 mm from the surface using an optical microscope, and the area fraction of ferrite from the surface of the steel sheet (the surface from which the plating layer was removed in the presence of the plating) to 50 μm was calculated. For the adjustment of the sample, the cross section of the sheet thickness in the direction perpendicular to the rolling direction (direction perpendicular to the rolling direction) was polished as an observation plane, and the sample was etched with a LePera reagent.

"microstructures" were classified according to an optical microscope photograph at 500 magnifications. When the optical microscope observation is performed after the LePera corrosion, for example, the microstructure of bainite in black, the microstructure of martensite (including tempered martensite) in white, and the microstructure of ferrite in gray are observed by color discrimination, and therefore, it is possible to easily discriminate ferrite from other hard microstructures.

Observation was performed at a magnification of 500 times over 10 fields from the surface of the steel sheet eroded by the LePera reagent to a position 1/4 of the sheet thickness in the sheet thickness direction from the surface, and the area from the surface to 50 μm of the obtained optical micrograph was specified, and image analysis was performed using image analysis software "Photoshop CS 5" manufactured by Adobe corporation to determine the area integral ratio of ferrite. As an image analysis method, for example, the maximum luminance value L of an image is obtained from an image_maxAnd a minimum luminance value L_minWill have a brightness from L_max－0.3×(L_max－L_min) To L_maxThe part of the pixels up to this point is defined as the white area, which will have a range from L_minTo L_min+0.3×(L_max－L_min) The area of the pixel (b) is defined as a black area, and the other area is defined as a gray area, and the area fraction of ferrite in the gray area is calculated. Since a white region is not observed when the ferrite area ratio is 100%, the ferrite fraction is set to 100% when the entire gray region is formed. The image analysis was performed in the same manner as described above for the observation fields of 10 sites in total to measure the area fraction of ferrite, and the area fractions were averaged to calculate an average value. The average value thereof was set as the volume fraction of ferrite in the surface layer region.

When the thickness of the steel sheet is 0.20mm or less, the above-described observation of the structure is performed in a region of a depth of t/4 in the thickness direction from the surface.

[ average grain size of ferrite 1.0 to 15.0 μm ]

If the average crystal grain size of ferrite exceeds 15.0. mu.m, the appearance after molding is deteriorated. Therefore, the average crystal grain size of ferrite in the surface region is set to 15.0 μm or less. Preferably, it is set to 12.0 μm or less.

On the other hand, if the average crystal grain size of ferrite is less than 1.0 μm, particles having the {001} orientation of ferrite tend to aggregate and form. Even if the particles having the {001} orientation of ferrite are small, when they are aggregated and generated, the deformation is concentrated on the aggregated portion, and therefore the appearance after molding is also degraded. Therefore, the average grain size of ferrite in the surface layer region is set to 1.0 μm or more. Preferably 3.0 μm or more, and more preferably 6.0 μm or more.

The average crystal grain size of ferrite in the surface region can be determined by the following method.

Similarly to the above, 10 field observations were made at a magnification of 500 times over the region from the surface of the steel sheet eroded by the LePera reagent to the position 1/4 of the sheet thickness in the sheet thickness direction from the surface, and the region from the surface of the steel sheet to 50 μm × 200 μm in the optical micrograph was selected, and image analysis was performed similarly to the above using image analysis software "Photoshop CS 5" manufactured by Adobe corporation, and the area fraction occupied by ferrite and the number of ferrite particles were calculated. The average surface area fraction of each ferrite particle was calculated by dividing the surface area fraction occupied by ferrite by the number of ferrite particles. From the average surface area fraction and the number of particles, an equivalent circle diameter was calculated, and the obtained equivalent circle diameter was set as an average crystal grain diameter of ferrite. When the thickness of the steel sheet is 0.20mm or less, a region of 200 μm from the surface of the steel sheet to t/4 in the optical micrograph is selected and subjected to image analysis.

Intensity ratio X of {001} orientation to {111} orientation of [ containing ferrite_{ODF{001}/{111}，S}A texture of 0.30 or more and less than 3.50]

In the surface layer region, the intensity ratio (the ratio of the maximum value of the X-ray random intensity ratio) of the {001} orientation to the {111} orientation including ferrite, that is, X_{ODF{001}/{111}，S}The texture is 0.30 or more and less than 3.50, and the appearance of the steel sheet after forming is improved. The reason is not clear, but it is considered that the uneven deformation in the surface is suppressed by the interaction between the existence form of ferrite and the crystal orientation distribution.

If X_{ODF{001}/{111}，S}If the amount is less than 0.30, uneven deformation is likely to occur due to the orientation distribution and strength difference of each crystal of the material, and the concentration of deformation toward the {001} vicinity of ferrite becomes significant. On the other hand, X_{ODF{001}/{111}，S}If the amount exceeds 3.50, uneven deformation is likely to occur due to the orientation distribution of each crystal of the material and the difference in strength, and the surface roughness of the steel sheet tends to develop.

Intensity ratio X of {001} orientation to {111} orientation of ferrite in surface layer region_{ODF{001}/{111}，S}The measurement can be obtained by the following method using an EBSD (Electron Back Scattering Diffraction) method.

For the sample subjected to the EBSD method, the steel sheet was polished by mechanical grinding, and then the sample was adjusted so that a cross section in the thickness direction including a range from the surface to a position 1/4 where the thickness is the thickness in the thickness direction becomes a measurement plane while removing strain by chemical polishing, electrolytic polishing, or the like, and the texture was measured. The sample collection position in the plate width direction was a position near the plate width position of W/4 or 3W/4 (a position distant from the end face of the steel plate by a distance of 1/4 times the plate width of the steel plate).

The crystal orientation distribution was measured by the EBSD method at a pitch of 0.5 μm or less in the region from the surface to 50 μm in the thickness direction of the steel sheet of the sample. When the thickness of the steel sheet is 0.20mm or less, the depth from the surface to t/4 in the thickness direction is measured. The ferrite was extracted using IQ (Image Quality) value mapping which can be analyzed by EBSP-OIM (registered trademark, Electron Back Scatter Pattern-Orientation Image microscope). Since ferrite has a characteristic of a large IQ value, it can be easily distinguished from other metal structures by this method. The threshold value of the IQ value is set so that the area fraction of ferrite calculated by the microstructure observation by the LePera corrosion described above coincides with the area fraction of ferrite calculated with the IQ value as a reference.

Is obtained to useThe ratio of the maximum value of the X-ray random intensity ratio of the {001} Orientation group to the maximum value of the X-ray random intensity ratio of the {111} Orientation group (γ -fiber) ((maximum value of the X-ray random intensity ratio of the {001} Orientation group/{ 111} Orientation group (γ -fiber)) in a section of [ [ phi 2] - ] 45 ° (ODF: Orientation Distribution Functions) expressed by the crystal Orientation of the extracted ferrite, that is, the maximum value of the X-ray random intensity ratio of the {001} Orientation group/{ 111} Orientation group (γ -fiber)), that is, X is X_{ODF{001}/{111}，S}. The X-ray random intensity ratio is a value obtained by measuring the diffraction intensity of a standard sample and the diffraction intensity of a sample which do not have a concentration in a specific orientation under the same condition by an X-ray diffraction method or the like, and dividing the diffraction intensity of the obtained sample by the diffraction intensity of the standard sample. For example, when a steel sheet is rolled and annealed at a high reduction ratio of 70% or more, the texture develops and the X-ray random strength of the {111} orientation group (γ -fiber) increases.

Here, { hkl } indicates that the normal direction of the plate surface was parallel to < hkl > when the sample was collected by the above-mentioned method. The orientation of the crystal is expressed as (hkl) or { hkl } in terms of the orientation generally perpendicular to the plate surface. { hkl } is a generic term for equivalent facets, (hkl) refers to the individual crystal facets. That is, in the present embodiment, since the body-centered cubic structure (bcc structure) is targeted, for example, the respective faces of (111), (-111), (1-11), (11-1), (-1-11), (-11-1), (1-1-1), and (-1-1-1) are equivalent and cannot be distinguished from each other. In such a case, these orientations are collectively referred to as a {111} orientation group. The ODF expression is also used for the expression of other orientations having a crystal structure with low symmetry, and therefore each orientation is generally expressed as (hkl) [ uvw ] in the ODF expression, but in the present embodiment, attention is paid to the normal direction orientation { hkl } which is a recognition that the normal direction orientation of the plate surface greatly affects the development of the unevenness after molding. { hkl } has the same meaning as (hkl).

In the case of a steel sheet having a plated layer in a product, the surface of the steel sheet from which the plated layer is removed is defined as a starting point of the surface layer region.

< metallic texture with respect to inner region >

In the steel sheet of the present embodiment, it is preferable to control the metal structure in the surface region as described above, and also to control the metal structure in the inner region in a range from a position exceeding 50 μm in the thickness direction from the surface to a position 1/4 (t/4 when the thickness is t) in the thickness direction from the surface (a range from the t/4 position to the t/2 position when the thickness of the steel sheet is 0.20mm or less).

Intensity ratio X of {001} orientation to {111} orientation of [ containing ferrite_{ODF{001}/{111}，I}A texture of 0.001 or more and less than 1.00]

In the inner region, the intensity ratio (the maximum value of the X-ray random intensity ratio) of the {001} orientation to the {111} orientation including ferrite, that is, X_{ODF{001}/{111}，I}A texture of 0.001 or more and less than 1.00 is preferable because the appearance of the steel sheet after forming can be further improved.

[ intensity ratio X_{ODF{001}/{111}，S}To intensity ratio X_{ODF{001}/{111}，I}Satisfy the formula (1) (-0.20 < X)_{ODF{001}/{111}，S}－X_{ODF{001}/{111}，I}< 0.40), the average crystal grain size of ferrite in the surface region is smaller than the average crystal grain size of ferrite in the inner region]

If the ferrite in the surface layer region has a strength ratio X_{ODF{001}/{111}，S}Strength ratio X to ferrite of inner region_{ODF{001}/{111}，I}Satisfying the following expression (1), and the average crystal grain size of ferrite in the surface region is smaller than the average crystal grain size of ferrite in the inner region, uneven deformation is more suppressed in the surface region, which is preferable.

－0.20＜X_{ODF{001}/{111}，S}－X_{ODF{001}/{111}，I}＜0.40(1)

The average crystal grain size in the internal region can be obtained by using a steel sheet etched with a LePera reagent, selecting a range from a position exceeding 50 μm in the thickness direction from the surface to a position 1/4 in the thickness direction from the surface, and analyzing by the same method as in the surface region.

The ferrite texture in the inner region can also be obtained by selecting a range from a position exceeding 50 μm in the thickness direction from the surface to a position 1/4 in the thickness direction from the surface of the sample by using the EBSD method described above and analyzing the selected range by the same method as that for the surface region.

When the thickness of the steel sheet is 0.20mm or less, a range from the t/4 position to the t/2 position is selected and analyzed.

< regarding the sheet thickness >

The thickness of the steel sheet of the present embodiment is not particularly limited. However, when applied to an outer panel member, if the plate thickness exceeds 0.55mm, the contribution to weight reduction of the member is small. Further, the thickness is less than 0.12mm, and the rigidity may be a problem. Therefore, the thickness is preferably 0.12 to 0.55 mm.

The thickness of the steel sheet was obtained by sampling the sheet from the end in the longitudinal direction of the steel sheet coil, further obtaining a sample for measuring the thickness from a position 300mm in the width direction from the end, and measuring the sample with a micrometer.

< with respect to plating layer >

The steel sheet of the present embodiment may have a plated layer on the surface. The surface plating is preferable because it improves corrosion resistance.

The plating to be applied is not particularly limited, and examples thereof include hot dip galvanizing, galvannealed zinc, electrogalvanizing, Zn — Ni plating (galvannealed zinc), Sn plating, Al — Si plating, galvannealed zinc, hot dip galvanizing-aluminum alloy, hot dip galvanizing-aluminum-magnesium alloy-Si steel sheet, zinc vapor deposited Al.

< production method >

Next, a preferred method for producing the steel sheet of the present embodiment will be described. The steel sheet of the present embodiment can obtain the effects as long as it has the above-described characteristics regardless of the production method. However, the following method is preferable because the production can be stably performed.

Specifically, the steel sheet of the present embodiment can be produced by a production method including the following steps (i) to (vi).

(i) A heating step of heating the billet having the above chemical composition to 1000 ℃ or higher;

(ii) a hot rolling step of hot rolling the slab so that the rolling completion temperature becomes 950 ℃ or lower to obtain a hot-rolled steel sheet;

(iii) the residual stress in the surface of the hot-rolled steel sheet after the hot-rolling step, i.e., [ sigma ]_sA stress applying step of applying a stress so that the absolute value of the stress is 100 to 250 MPa;

(iv) subjecting the hot-rolled steel sheet after the stress application step to a cumulative reduction ratio R_CRA cold rolling step of obtaining a cold-rolled steel sheet by cold rolling 70 to 90%;

(v) an annealing step of annealing the cold-rolled steel sheet so that the average heating rate is 1.5 to 10.0 ℃/sec from 300 ℃ to a soaking temperature T1 ℃ satisfying the following formula (2), and then the steel sheet is held at the soaking temperature T1 ℃ for 30 to 150 seconds;

Ac₁+550－25×ln(σ_s)－4.5×R_CR≤T1≤Ac₁+550－25×ln(σ_s)－4×R_CR (2)

(wherein Ac is the one represented by the above formula (2)₁By the formula (3) (Ac)₁723-10.7 xmn-16.9 xni +29.1 xsi +16.9 xcr). )

(vi) And a cooling step of cooling the cold-rolled steel sheet after the annealing step to a temperature range of 550 to 650 ℃ so that the average cooling rate from the soaking temperature T1 to 650 ℃ is 1.0 to 10.0 ℃/sec, and then cooling the cold-rolled steel sheet to a temperature range of 200 to 490 ℃ so that the average cooling rate is5 to 500 ℃/sec.

In addition, in order to obtain the tempering effect of the hard phase present in a small amount, a manufacturing method further including the following steps may be set.

(vii) And a holding step of holding the cold-rolled steel sheet after the cooling step in a temperature range of 200 to 490 ℃ for 30 to 600 seconds.

Hereinafter, each step will be explained.

[ heating Process ]

In the heating step, a billet having a predetermined chemical composition is heated to 1000 ℃ or higher before rolling. If the heating temperature is less than 1000 ℃, the rolling reaction force increases in the subsequent hot rolling, and sufficient hot rolling may not be performed, and a target product thickness may not be obtained. Alternatively, the shape of the sheet may deteriorate, and the sheet may not be wound.

The upper limit of the heating temperature is not necessarily limited, but it is economically unfavorable to set the heating temperature to an excessively high temperature. Thus, the billet heating temperature is preferably set to be lower than 1300 ℃. The billet to be subjected to the heating step is not limited. For example, a billet produced by melting molten steel having the above chemical composition in a converter, an electric furnace, or the like and then performing a continuous casting method can be used. Instead of the continuous casting method, an ingot casting method, a thin slab casting method, or the like may be employed.

[ Hot Rolling Process ]

In the hot rolling step, the slab heated to 1000 ℃ or higher in the heating step is hot-rolled and wound to obtain a hot-rolled steel sheet.

When the rolling completion temperature exceeds 950 ℃, the average crystal grain size of the hot-rolled steel sheet becomes too large. In this case, the average crystal grain size of the final product sheet also becomes large, which is not preferable because it causes a decrease in yield strength and a deterioration in surface quality after molding. Therefore, the rolling completion temperature is set to 950 ℃ or lower.

In order to refine the crystal grain size of the steel sheet and improve the surface quality, the finish rolling start temperature is preferably set to 900 ℃ or lower. More preferably 850 ℃ or lower. In addition, the rolling start temperature is preferably 700 ℃ or higher, and more preferably 750 ℃ or higher, from the viewpoint of reducing the rolling load during hot rolling.

When the temperature change in the hot rolling step (finish rolling end temperature-finish rolling start temperature) is +5 ℃ or more, recrystallization is promoted by heat generated by working in the hot rolling step, and the crystal grains are refined, which is preferable.

In order to refine the crystal grains, the winding temperature in the winding step is preferably 750 ℃ or lower, and more preferably 650 ℃ or lower. In addition, the coiling temperature is preferably 450 ℃ or more, and more preferably 500 ℃ or more, from the viewpoint of reducing the strength of the steel sheet to be subjected to cold rolling.

[ stress applying step ]

In the stress applying step, the hot-rolled steel sheet after hot rolling is subjected to stress application according to the residual stress σ in the surface_sStress is applied so that the absolute value of the stress is 100 to 250 MPa. For example, stress can be applied by grinding a hot-rolled steel sheet using a surface grinding brush after hot rolling or pickling. In this case, the contact pressure between the grinding brush and the surface of the steel sheet may be changed, the surface layer residual stress may be measured online using a portable X-ray residual stress measuring device, and the control may be performed so that the surface layer residual stress falls within the above range. By performing cold rolling, annealing, and cooling in a state where the surface is given a residual stress within the above range, a steel sheet containing ferrite having a desired texture can be obtained.

If residual stress σ_sBelow 100MPa, or above 250MPa, the desired texture cannot be obtained after the subsequent cold rolling, annealing and cooling. Further, when residual stress is applied after cold rolling without hot rolling, the residual stress is widely distributed in the thickness direction, and therefore, a desired metal structure cannot be obtained only in the surface layer of the material.

The method of applying residual stress to the surface of the hot-rolled steel sheet is not limited to the above-described grinding brush, and for example, there is a method of performing surface grinding such as shot blasting or machining. However, in the case of shot blasting, there is a possibility that fine irregularities are generated on the surface by collision of the projection material, or flaws are generated in the subsequent cold rolling or the like by biting of the projection material. Therefore, the stress is preferably imparted by grinding with a brush.

Further, the rolling reduction by the rolls such as the surface smoothing rolling imparts stress to the entire thickness direction of the steel sheet, and a desired hard phase distribution and texture cannot be obtained only in the surface layer of the material.

The stress applying step is preferably performed at a steel sheet temperature of 40 to 500 ℃. By performing the heating in this temperature range, residual stress can be efficiently applied to the range to be the surface layer region, and cracking of the hot-rolled steel sheet due to the residual stress can be suppressed.

[ Cold Rolling Process ]

In the cold rolling step, the cumulative reduction rate R is performed_CRCold rolling the steel sheet to 70 to 90%. By cold rolling the hot-rolled steel sheet to which a predetermined residual stress is applied at the above-described cumulative reduction ratio, ferrite having a desired texture can be obtained after annealing and cooling.

Cumulative reduction ratio R_CRIf the content is less than 70%, the texture of the cold-rolled steel sheet is not sufficiently developed, and thus a desired texture cannot be obtained after annealing. Further, the cumulative reduction ratio R_CRIf the content exceeds 90%, the texture of the cold-rolled steel sheet is excessively developed, and the desired texture cannot be obtained after annealing. Further, the rolling load increases, and the uniformity of the material in the plate width direction decreases. Further, the production stability is also lowered. Therefore, the cumulative reduction R in the cold rolling_CRThe content is set to 70 to 90%.

[ annealing step ]

In the annealing step, with Ac₁Residual stress applied in the stress applying step, and cumulative reduction R in the cold rolling step_CRHeating the cold-rolled steel plate to a soaking temperature T1 ℃ at a corresponding average heating speed to be matched with Ac₁Residual stress applied in the stress applying step, and cumulative reduction R in the cold rolling step_CRThe corresponding soaking temperature is maintained.

Specifically, in the annealing step, the cold-rolled steel sheet is heated so that the average heating rate from 300 ℃ to a soaking temperature T1 ℃ satisfying the following formula (2) is 1.5 to 10.0 ℃/sec, and then is held at the soaking temperature T1 ℃ for 30 to 150 seconds.

Ac₁+550－25×ln(σ_s)－4.5×R_CR≤T1≤Ac₁+550－25×ln(σ_s)－4×R_CR (2)

Wherein Ac is the one represented by the above formula (2)₁Is represented by the following formula (3). The symbol of an element in the following formula (3) is the content of the element in mass%, and 0 is substituted when the element is not included.

Ac₁＝723－10.7×Mn－16.9×Ni+29.1×Si+16.9×Cr (3)

If the average heating rate is less than 1.5 ℃/sec, the heating takes time, and the productivity is lowered, which is not preferable. Further, when the average heating rate exceeds 10.0 ℃/sec, the uniformity of the temperature in the plate width direction is lowered, which is not preferable.

If the soaking temperature T1 is lower than the left side of the formula (2), recrystallization of ferrite and reverse transformation from ferrite to austenite do not proceed sufficiently, and a desired texture cannot be obtained. Further, the strength difference between non-recrystallized grains and recrystallized grains is not preferable because it promotes uneven deformation during molding. On the other hand, if the soaking temperature T1 is higher than the right side of the formula (2), recrystallization of ferrite and reverse transformation from ferrite to austenite sufficiently proceed, but the crystal grains are coarsened, and a desired texture cannot be obtained, which is not preferable.

The average heating rate is determined by (heating end temperature-heating start temperature)/(heating time).

[ Cooling Process ]

In the cooling step, the cold-rolled steel sheet after soaking in the annealing step is cooled. During cooling, the steel sheet is cooled to a temperature range of 550-650 ℃ so that the average cooling rate from the soaking temperature T1-650 ℃ is 1.0-10.0 ℃/s, and then cooled to a temperature range of 200-490 ℃ so that the average cooling rate is 5-500 ℃/s.

If the average cooling rate up to T1 to 650 ℃ is less than 1.0 ℃/sec, a desired metal structure cannot be obtained in the surface layer region. On the other hand, if the average cooling rate exceeds 10.0 ℃, ferrite transformation does not sufficiently proceed, and a desired volume fraction of ferrite cannot be obtained.

Further, if the average cooling rate from the temperature range to the temperature range of 200 to 490 ℃ after cooling to the temperature range of 550 to 650 ℃ is less than 5 ℃/sec, a desired texture cannot be obtained in the ferrite. On the other hand, setting to more than 500 ℃/sec is difficult in terms of equipment constraints, so the upper limit is set to 500 ℃/sec.

The average cooling rate was determined by (cooling start temperature-cooling end temperature)/(cooling time).

[ holding step ]

The cold-rolled steel sheet cooled to 200 to 490 ℃ may be kept at the temperature range for 30 to 600 seconds.

It is preferable to maintain the temperature range for a predetermined time because the tempering effect of the hard phase existing in a slight amount can be obtained.

The cold-rolled steel sheet cooled to 200 to 490 ℃ or the cold-rolled steel sheet after the holding step may be cooled to room temperature at 10 ℃/sec or more.

The cold-rolled steel sheet obtained by the above-described method may be further subjected to a plating step of forming a plating layer on the surface. Examples of the plating step include the following steps.

[ electroplating Process ]

[ alloying Process ]

The cold-rolled steel sheet after the cooling step or after the holding step may be plated to form a plated layer on the surface. The plating method is not particularly limited. The conditions may be determined according to the required characteristics (corrosion resistance, adhesion, etc.).

The plated metal may be alloyed by heating the cold-rolled steel sheet after the plating.

[ Hot Dip galvanizing step ]

[ alloying Process ]

The cold-rolled steel sheet after the cooling step or after the holding step may be subjected to hot-dip galvanizing to form a hot-dip galvanized layer on the surface. The hot dip galvanizing method is not particularly limited. The conditions may be determined according to the required characteristics (corrosion resistance, adhesion, etc.).

Further, the cold-rolled steel sheet after hot dip galvanizing may be heat-treated to alloy the plating layer. When alloying is performed, the cold-rolled steel sheet is preferably heat-treated at a temperature of 400 to 600 ℃ for 3 to 60 seconds.

According to the above-described manufacturing method, the steel sheet of the present embodiment can be obtained.

Examples

Next, an embodiment of the present invention will be explained. The conditions in the examples are conditions employed for confirming the feasibility and effects of the present invention, and the present invention is not limited to the conditions. Various conditions can be adopted in the present invention as long as the object of the present invention is achieved without departing from the gist of the present invention.

Steels having chemical compositions shown in steel slabs nos. a to T of table 1 were melted and continuously cast to produce slabs 240 to 300mm in thickness. The resulting slab was heated to the temperature shown in the table. The heated slab was hot-rolled under the conditions shown in table 2, and was wound.

Thereafter, the coil is unwound, and stress is applied to the hot-rolled steel sheet. At this time, the machining temperature (steel plate temperature) shown in table 2 was used, and the surface layer residual stress was measured on-line, while the residual stress σ shown in table 2 was obtained as the residual stress_sThe contact pressure between the grinding brush and the surface of the steel plate is changed. Thereafter, the cumulative reduction R shown in Table 2 was used_CRThe steel sheets A1 to T1 were obtained by cold rolling.

The "hot rolling process temperature change" in table 2 indicates a temperature change (finish rolling end temperature-finish rolling start temperature) in the hot rolling process. In table 2, the residual stress σ is described in the example where the stress applying step is not performed (the example where ". about.1" is described in the column of "steel sheet temperature")_sBut the residual stress σ is considered to be_sIs a residual stress caused by the non-uniformity of the cooling rate when the steel sheet is cooled.

Then, annealing and cooling were performed under the conditions shown in tables 3A and 3B, and a part of the steel sheet was further held at 200 to 490 ℃ for 30 to 600 seconds. After cooling or holding, let cool to room temperature. Thereafter, various kinds of plating were performed on a part of the steel sheet, and a plated layer was formed on the surface. In tables 3A and 3B, CR indicates no plating, GI indicates hot-dip galvanizing, GA indicates alloying hot-dip galvanizing, EG indicates electroplating, EGA indicates alloying electrogalvanizing, and Sn, Zn — Al — Mg, Al — Si, and the like indicate plating containing these elements. In tables 3A and 3B, the phosphate treatment EG indicates that the phosphate treatment electrogalvanizing was performed, and the lubrication treatment GA indicates that the lubrication treatment alloying hot dip galvanizing was performed.

The obtained product sheets No. a1a to T1a were subjected to the observation of the metal structure of the surface region and the internal region, and X by the above-described method_{ODF{001}/{111}，S}、X_{ODF{001}/{111}，I}And measurement of the sheet thickness. The results are shown in tables 4A and 4B.

[ evaluation of tensile Strength ]

The tensile strength of the product plate thus obtained was determined by a tensile test in accordance with JIS Z2241 using a JIS5 test piece cut out in a direction perpendicular to the rolling direction. As a result, the tensile strength of the product plate of all the invention examples was 340MPa or more.

[ evaluation of surface Properties of Steel sheet ]

Further, the surface properties of the steel sheet were evaluated for the produced product sheet.

Specifically, the surface of the produced steel sheet was observed by visual observation, and the surface properties were evaluated. The evaluation criteria for the surface properties of the steel sheet are set as follows.

A: no pattern (more preferably, it can be used as an exterior material)

B: allowable generation of minute pattern (usable as exterior material)

C: unacceptable pattern formation (usable as a member, but not as an exterior material)

D: significant pattern defects (not available as part)

[ test for Forming Steel sheet ]

For the manufactured product plate, a molding test was performed.

For the forming, the steel sheet whose surface properties were measured was given a plastic strain of 10% in the rolling width direction by a cylinder drawing test by the Marciniak method using a deep drawing machine, a cylinder punch with a diameter of 50mm, and a cylinder die with a diameter of 54 mm.

A test piece of 100mm in the rolling width direction X50 mm in the rolling direction was prepared from the portion deformed by the forming, and the arithmetic average height Pa of the cross-sectional curve specified in JIS B0601(2001) was measured in a direction perpendicular to the rolling direction in accordance with JIS B0633(2001) standard. The evaluation was performed on a portion deformed by molding, and the evaluation length was set to 30 mm.

Further, a test piece of 100mm in the rolling width direction × 50mm in the rolling direction was prepared on the flat portion of the molded article, and the arithmetic average height Pa of the cross-sectional curve specified in JIS B0601(2001) was measured in the direction perpendicular to the rolling direction in accordance with JIS B0633(2001) standard. The evaluation length was set to 30 mm.

The roughness increase Δ Pa was calculated using Pa of the molded product and Pa of the steel sheet obtained in the above measurement test (Δ Pa — Pa of the molded product Pa — Pa of the steel sheet).

The surface properties of the steel sheet after forming were evaluated based on Δ Pa. The evaluation criteria were set as follows.

A: Δ Pa. ltoreq.0.25 μm (more preferably, usable as a covering material)

B: 0.25 μm < Δ Pa ≦ 0.35 μm (usable as a sheathing material)

C: 0.35 μm < Δ Pa ≦ 0.55 μm (usable as a member, but not as an exterior material)

D: 0.55 μm < Δ Pa (not usable as a member)

[ comprehensive evaluation ]

The overall evaluation criteria for surface properties was set such that the lower of the 2 evaluations (evaluation of surface properties of steel sheet, evaluation of surface properties after forming) was the overall evaluation. When the overall evaluation is C or D, the exterior material or member is set to be unusable and determined as defective.

A: more preferably, the outer member is used.

B: can be used as an exterior material.

C: it cannot be used as an exterior material.

D: cannot be used as a component.

The test results are shown in tables 4A and 4B.

TABLE 2

Underlining is indicated as being outside the scope of the invention.

The symbol 1 indicates that the stress application step was not performed.

As shown in tables 1 to 4B, with respect to the chemical composition, the metal structure of the surface region and X_{ODF{001}/{111}，S}In the examples (examples) within the scope of the present invention, the overall evaluation is a or B, and the formation of surface irregularities at the stage of the steel sheet and after the working is suppressed. On the other hand, with respect to the chemical composition, the metal structure of the surface region and X_{ODF{001}/{111}，S}In the case where any one of the above-mentioned members deviates from the scope of the present invention (comparative example), the pattern or the unevenness is generated at the stage of the steel sheet or after the steel sheet is formed, and the steel sheet cannot be used as an exterior material or a member.

Fig. 1 is a graph showing the relationship between the surface texture after molding and the texture parameter obtained in this example. ■ in FIG. 1 shows an example where the ferrite in the surface layer region has an average crystal grain size exceeding 15.0. mu.m.

When FIG. 1 is observed, it is found that the texture parameter is within the range of the present invention (intensity ratio X of {001} orientation to {111} orientation of ferrite)_{ODF{001}/{111}，S}0.30 or more and less than 3.50) is excellent in surface properties after molding.

Industrial applicability

The steel sheet according to the aspect of the present invention can produce a high-strength steel sheet having excellent formability and in which the occurrence of surface irregularities can be suppressed even after various deformations caused by press deformation. Therefore, the industrial applicability is high.