Steel sheet

Abstract

Provided is a steel sheet that is characterized by having a prescribed chemical composition and having a metal structure which contains, in area%, 75% to 95% ferrite and 5% to 25% martensite and in which the total of ferrite and martensite is at least 90%, the average grain spacing of the martensite is not more than 2.5 μm, the standard deviation of the martensite area ratio in a direction perpendicular to the rolling direction and the sheet thickness direction is not more than 1.5%, and, in EBSD measurement, the ratio A / B between the proportion A of regions where the KAM value in the ferrite is less than 0.5° and the proportion B of regions where the KAM value in the ferrite is at least 1.0° is at least 0.60.

Description

steel plate

[0001] The present invention relates to a steel sheet, and more particularly to a steel sheet having excellent appearance, the main use of which is, for example, outer panel members of automobiles.

[0002] In order to reduce carbon dioxide emissions from automobiles, attempts are being made to reduce the weight of automobile bodies while ensuring safety by using high-strength steel sheets. While the increase in the strength of automotive steel sheets has been remarkable for automobile frame components, steel sheets with a tensile strength of 300 MPa or less are mainly used for exterior panel components such as doors and hoods, and the increase in strength has not progressed. High formability and appearance are required for such exterior panel components. Generally, increasing the strength of a steel sheet reduces its formability and appearance after forming. Therefore, it is difficult to achieve both strength and formability and appearance, especially appearance after forming, in high-strength steel sheets. Several means have been proposed to solve these problems.

[0003] For example, Patent Document 1 describes a steel sheet for hot-dip galvanizing, which contains, by mass%, 0.02 to 0.3% C, 0.1 to 2.0% Si, less than 1.0% Mn, more than 1.0 to 3.0% Cr, 0.02% or less P, 0.02% or less S, 0.014% or less Al, and 0.001 to 0.008% N, satisfying the following conditions: 2.5≦1.5Mn%+Cr%, 4.1−2.3Mn%−1.2Cr%≦Si%, with the balance being Fe and unavoidable impurities. Patent Document 1 also teaches that by optimizing the amounts of Mn, Cr, and Si added, it is possible to achieve both the workability of a steel sheet for hot-dip galvanizing having a tensile strength of 390 MPa or more and an appearance after processing that is usable as an automotive exterior panel. Furthermore, Patent Document 1 teaches that by setting the area ratio of the ferrite, which is the main phase, to 70% or more and the area ratio of the hard second phase containing martensite to 30% or less, it is possible to keep all of the strength, yield strength, yield ratio, and strength-ductility balance within a good range.

[0004] In Patent Document 2, the mass % of C is 0.0005 to 0.01%, Si is 0.2% or less, Mn is 0.1 to 1.5%, P is 0.03% or less, S is 0.005 to 0.03%, Ti is 0.02 to 0.1%, Al is 0.01 to 0.05%, N is 0.005% or less, Sb is 0.03% or less, and Cu is more than 0.005% and 0.03% or less, and Ti* is expressed by (Ti%) - 3.4 x (N%) - 1.5 x (S%) - 4 x (C%).

[0005] Patent Document 2 discloses a cold-rolled steel sheet having a composition containing Ti* in a range satisfying 0<Ti*<0.02 and further satisfying (Sb%)≧(Cu%) / 5, with the balance consisting of Fe and unavoidable impurities, wherein the content (mass%) of Ti element contained in precipitates less than 20 nm in size in the surface layer portion up to 10 μm from each surface on both sides of the steel sheet is 9% or less of the total Ti content (mass%) in the steel sheet. Patent Document 2 also teaches that by setting the content (mass%) of Ti element contained in precipitates less than 20 nm in size in the surface layer portion up to 10 μm from each surface on both sides of the steel sheet to 9% or less of the total Ti content (mass%) in the steel sheet, it is possible to avoid the occurrence of uneven appearance due to such fine Ti-based precipitates and obtain a cold-rolled steel sheet with excellent surface properties, and further teaches that the cold-rolled steel sheet can be suitably used for parts that require excellent surface quality after forming, such as automotive exterior panels.

[0005] Patent Document 1: JP 2009-249737 A, International Publication No. 2011 / 142473

[0006] For example, in the case of a dual-phase steel having a metallurgical structure including soft ferrite and hard martensite, as described in Patent Document 1, non-uniform deformation is likely to occur during processing such as press forming, in which the soft ferrite and its surroundings deform preferentially. Therefore, when such a dual-phase steel composed of a soft structure and a hard structure is used, minute irregularities may appear on the surface of the steel sheet after forming, resulting in appearance defects known as ghost lines. In this regard, for example, Patent Document 1 discusses improving formability and appearance after forming, mainly from the perspective of chemical composition, but does not necessarily provide sufficient consideration from the perspective of optimizing the metallurgical structure. Therefore, conventional steel sheets still have room for improvement in terms of formability and appearance after forming.

[0007] Therefore, an object of the present invention is to provide a steel sheet that has a novel structure and is capable of achieving both strength, formability, and good appearance after forming.

[0008] In order to achieve the above object, the present inventors have conducted studies focusing on the metallographic structure of steel sheets. As a result, the present inventors have found that by uniformly dispersing martensite contained in a predetermined proportion in the metallographic structure in both microregions and macroregions in the metallographic structure, a desired high strength can be achieved based on such a hard structure, and that even when strain is imparted by press forming or the like, the generation of minute irregularities on the steel sheet surface can be significantly suppressed. Furthermore, the present inventors have found that by controlling the KAM value, as determined by EBSD measurement, of ferrite, which is a soft structure in the metallographic structure, so as to satisfy predetermined requirements, formability can be significantly improved, and have completed the present invention.

[0009] The present invention, which has achieved the above object, is as follows. (1) In mass%, C: 0.030 to 0.100%, Mn: 1.00 to 2.80%, Si: 0.005 to 1.500%, Al: 1.000% or less, P: 0.100% or less, S: 0.0200% or less, N: 0.0150% or less, O: 0.0100% or less, Cr: 0 to 1.00%, Mo: 0 to 0.80%, B: 0 to 0.0100%, Ti: 0 to 0.200%, Nb: 0 to 0.200%, V: 0 to 0.500%, Ni: 0 to 1.00%, Cu: 0 to 1.00%, W: 0 to 1.00%, Ta: 0 to 0.10%, a chemical composition consisting of Co: 0 to 3.00%, Sn: 0 to 1.00%, Sb: 0 to 0.200%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, REM: 0 to 0.0100%, Bi: 0 to 0.0500%, As: 0 to 0.10%, and the balance: Fe and impurities, the chemical composition comprising, by area percentage, 75 to 95% ferrite and 5 to 25% martensite, the total of ferrite and martensite being 90% or more, the average grain spacing of martensite being 2.5 μm or less, and the standard deviation of the area ratio of martensite in the direction perpendicular to the rolling direction and the plate thickness direction being 1.5% or less, A steel sheet characterized by having a metal structure in which, in EBSD measurement, the ratio A / B, where A is the ratio of a region in ferrite where the KAM value is less than 0.5° and B is the ratio of a region in ferrite where the KAM value is 1.0° or more, is 0.60 or more.(2) The chemical composition is, in mass%, Cr: 0.001 to 1.00%, Mo: 0.001 to 0.80%, B: 0.0001 to 0.0100%, Ti: 0.001 to 0.200%, Nb: 0.001 to 0.200%, V: 0.001 to 0.500%, Ni: 0.001 to 1.00%, Cu: 0.001 to 1.00%, W: 0.001 to 1.00%, Ta: 0.001 to 0.10%, Co: 0.001 to 3.00%, Sn: 0.001 to 1.00%, Sb: 0.001 to 0.200%, The steel sheet according to (1) above, characterized in that it contains at least one of Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0100%, Zr: 0.0001 to 0.0100%, REM: 0.0001 to 0.0100%, Bi: 0.0001 to 0.0500%, and As: 0.001 to 0.10%. (3) The steel sheet according to (1) or (2) above, characterized in that the ferrite has an average grain size of 3.0 to 25.0 μm, the martensite has an average grain size of 1.0 to 5.0 μm, and the martensite has an average aspect ratio of 2.5 or more. (4) An outer panel member comprising the steel sheet according to any one of (1) to (3) above.

[0010] According to the present invention, it is possible to provide a steel sheet that can achieve both strength, formability, and good appearance after forming.

[0011] FIG. 2 is a schematic diagram illustrating the "direction perpendicular to the rolling direction and the plate thickness direction" in a steel plate.

[0012] <Steel Sheet> A steel sheet according to an embodiment of the present invention has, in mass %, C: 0.030 to 0.100%, Mn: 1.00 to 2.80%, Si: 0.005 to 1.500%, Al: 1.000% or less, P: 0.100% or less, S: 0.0200% or less, N: 0.0150% or less, O: 0.0100% or less, Cr: 0 to 1.00%, Mo: 0 to 0.80%, B: 0 to 0.0100%, Ti: 0 to 0.200%, Nb: 0 to 0.200%, V: 0 to 0.500%, Ni: 0 to 1.00%, Cu: 0 to 1.00%, W: 0 to 1.00%, a chemical composition consisting of Ta: 0-0.10%, Co: 0-3.00%, Sn: 0-1.00%, Sb: 0-0.200%, Ca: 0-0.0100%, Mg: 0-0.0100%, Zr: 0-0.0100%, REM: 0-0.0100%, Bi: 0-0.0500%, As: 0-0.10%, and the balance: Fe and impurities, the chemical composition comprising, by area percentage, 75-95% ferrite and 5-25% martensite, the total of ferrite and martensite being 90% or more, the average grain spacing of martensite being 2.5 μm or less, and the standard deviation of the area ratio of martensite in a direction perpendicular to the rolling direction and the plate thickness direction being 1.5% or less, The steel is characterized by having a metal structure in which, in EBSD measurement, the ratio A / B, where A is the ratio of the area in the ferrite where the KAM value is less than 0.5° and B is the ratio of the area in the ferrite where the KAM value is 1.0° or more, is 0.60 or more.

[0013] In recent years, there has been an increasing need to reduce the weight of automotive exterior panel components (roofs, hoods, fenders, doors, etc.). Therefore, just as with frame components, there is a demand for higher strength in these exterior panel components. On the other hand, dual-phase steel (DP steel), which has a relatively low yield strength, is often used for exterior panel components to avoid surface defects known as surface distortions that occur during press forming and other processes. However, in the case of DP steel, which contains a mixture of a soft structure made of ferrite and a hard structure made of martensite, non-uniform deformation occurs during processes such as press forming, in which the soft structure and its surrounding area deform preferentially. This can lead to minute irregularities on the surface of the steel sheet after forming, resulting in appearance defects known as ghost lines. More specifically, during processes such as press forming, the soft structure made of ferrite undergoes large depressions, while the hard structure made of martensite undergoes small deformations. Therefore, the hard structure does not depression compared to the soft structure, but rather protrudes. As a result, variations in deformation occur, particularly in the width direction of the steel sheet, resulting in the appearance of ghost lines in the form of bands (stripes) along the rolling direction of the steel sheet. On the other hand, with the increasing strength of steel sheets, elements such as Mn are sometimes added in relatively large amounts to improve the hardenability of the steel sheet. Mn is an element that is prone to streak-like segregation in steel sheets. More specifically, Mn-enriched regions such as center segregation and microsegregation are formed during casting, and these enriched regions are elongated in the rolling direction by hot rolling or cold rolling, resulting in Mn segregation in streak-like formation. Therefore, due to this Mn segregation, regions with high and low hardenability are present in the steel sheet. As a result, a relatively large number of striped hard structures are formed in the metallographic structure of the steel sheet after quenching. In this case, the occurrence of ghost lines is particularly noticeable. In contrast, if Mn segregation in steel sheets can be sufficiently suppressed, the formation of such striped hard structures can be reduced and the hard structures can be more uniformly dispersed in the metallographic structure. In this case, even when strain is imparted by press forming or the like, it is believed that the formation of minute irregularities on the steel sheet surface can be sufficiently reduced, thereby suppressing the occurrence of ghost lines.However, in reality, it is very difficult to reliably and sufficiently suppress Mn segregation, particularly when the amount of Mn added to the steel sheet increases with the demand for higher strength. In addition, since the formability itself decreases with such higher strength, it is generally very difficult to achieve both strength, formability, and appearance after forming.

[0014] Therefore, the present inventors first investigated a means for optimizing the chemical composition of a steel sheet and optimizing the ratio of ferrite, a soft structure, to martensite, a hard structure, in the metallographic structure, thereby ensuring a certain level of formability and achieving a desired high strength, while also improving the appearance after forming. Specifically, the present inventors focused on the distribution of martensite, a hard structure, in the metallographic structure, and more specifically, investigated controlling the martensite distribution from a perspective other than reducing Mn segregation. As a result, as will be described in detail later with respect to a method for producing a steel sheet, the present inventors discovered that by forming the metallographic structure of a steel sheet before final annealing into a structure mainly composed of bainite and / or martensite, and then final annealing the steel sheet having such a metallographic structure under predetermined conditions, it is possible to uniformly distribute martensite in both micro- and macro-regions in the final metallographic structure. Furthermore, even when strain is imparted by press forming or the like, the generation of minute irregularities on the steel sheet surface can be sufficiently reduced, and the occurrence of ghost lines can be successfully suppressed. More specifically, the inventors discovered that by final annealing a steel sheet having a metallographic structure consisting of bainite and / or martensite under predetermined conditions, it is possible to control the average particle spacing of martensite in the microregion to 2.5 μm or less, and to control the standard deviation of the area fraction of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction to 1.5% or less in the macroregion. By controlling the average particle spacing of martensite to 2.5 μm or less, it is possible to densely and uniformly disperse the hard structure in the microregion. By controlling the standard deviation of the area fraction of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction to 1.5% or less, it is possible to significantly reduce the variation in the hard structure in the macroregion. By satisfying both of these requirements, it is possible to form a metallographic structure in which martensite, a hard structure, is finely and uniformly dispersed throughout the steel sheet.As a result, the steel sheet according to the embodiment of the present invention can achieve a more uniform deformation amount, particularly in the width direction, during forming such as press forming, and can achieve an excellent post-forming appearance in which appearance defects such as ghost lines are significantly suppressed. For example, even if the uniformity of martensite is ensured in the microscopic region, if the uniformity of martensite is not ensured in the macroscopic region, it is impossible to form a metallographic structure in which martensite is finely and uniformly dispersed throughout the steel sheet. Similarly, even if the uniformity of martensite is ensured in the macroscopic region, if the uniformity of martensite is not ensured in the microscopic region, martensite may be locally present non-uniformly, and therefore it is impossible to form a metallographic structure in which martensite is finely and uniformly dispersed throughout the steel sheet. Therefore, in order to achieve an excellent post-forming appearance in which appearance defects such as ghost lines are significantly suppressed, it is necessary to satisfy both the requirements of controlling the average particle spacing of martensite to 2.5 μm or less and controlling the standard deviation of the area fraction of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction to 1.5% or less.

[0015] Here, in this specification, "a direction perpendicular to the rolling direction and the plate thickness direction" means a direction perpendicular to each of the "rolling direction" and the "plate thickness direction" as shown in FIG. 1, i.e., the "width direction of the steel plate."

[0016] Without intending to be bound by any particular theory, it is believed that in order to achieve a fine and uniform dispersion of martensite throughout the entire steel sheet in the metallographic structure of the final steel sheet, it is extremely important to disperse and form numerous austenite nucleation sites during heating in the final annealing. In this regard, a martensite structure has substructures such as packets, blocks, and laths within prior austenite grains, and therefore has many different interfaces within it compared to structures such as ferrite. Bainite, like martensite, also has many different interfaces within it. Therefore, by forming the metallographic structure of the steel sheet before final annealing with bainite and / or martensite, it is possible to generate a large number of dispersed carbides that can serve as austenite nucleation sites at these interfaces during heating of such a metallographic structure in the final annealing. Therefore, after generating many carbides at the interfaces, further heating to the two-phase region of ferrite and austenite is believed to enable the fine and uniform formation of austenite throughout the steel sheet. Finally, by rapidly cooling a steel sheet having such a metallographic structure, martensite is generated from the austenite, and in the final metallographic structure, the average particle spacing of martensite is controlled to 2.5 μm or less, and the standard deviation of the area fraction of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction is controlled to 1.5% or less. In other words, it is believed that a metallographic structure in which martensite is uniformly dispersed in both micro- and macro-regions can be obtained. By performing such heat treatment, it is believed that martensite can be finely and uniformly dispersed throughout the steel sheet to such an extent that the effects of Mn segregation are counteracted. Conventionally, it has been considered common to consider control of the distribution of hard structures from the perspective of reducing Mn segregation itself. Therefore, it is extremely unexpected and surprising that martensite can be uniformly dispersed in both micro- and macro-regions in the final metallographic structure, regardless of the presence or degree of Mn segregation.

[0017] Next, the inventors conducted further studies focusing on ferrite, which is a soft structure in the metal structure, in order to achieve both strength and formability. As a result, the inventors found that, in EBSD (electron backscatter diffraction) measurements, by controlling the ratio A / B, where A is the proportion of regions in ferrite where the KAM value is less than 0.5° and B is the proportion of regions in ferrite where the KAM value is 1.0° or more, to be 0.60 or more, the elongation of the steel sheet can be improved despite its high strength, and therefore the balance between strength and formability can be significantly improved.

[0018] While not intending to be bound by any particular theory, it is believed that controlling the KAM value to satisfy the above requirements can reduce the dislocation density in ferrite, thereby significantly improving the elongation of the steel sheet while maintaining high strength due to the presence of martensite, and as a result, significantly improving the balance between strength and formability. More specifically, the KAM (Kernel Average Misorientation) value tends to increase as strain accumulates, and is therefore generally known to be an effective index for evaluating strain distribution within crystal grains. Therefore, it is recognized that the higher the KAM value, the higher the dislocation density tends to be. While increasing the dislocation density is effective in improving strength, it generally reduces formability, such as elongation. Therefore, from the perspective of improving formability, it is generally preferable to reduce the dislocation density. In this regard, with regard to ferrite contained in a steel sheet, it is believed that the higher the ratio of lower KAM values, the smaller the accumulated strain and therefore the lower the dislocation density. Similarly, the lower the ratio of higher KAM values, the smaller the accumulated strain and therefore the lower the dislocation density. Therefore, in order to improve the formability of a steel sheet by reducing the dislocation density in ferrite, the inventors studied the KAM values ​​of ferrite calculated by EBSD measurement and their appropriate control. As a result, the inventors found that the elongation of a steel sheet can be significantly improved by increasing the ratio A of ferrite regions where the KAM value is less than 0.5° (to reduce the dislocation density) and decreasing the ratio B of ferrite regions where the KAM value is 1.0° or more (also to reduce the dislocation density).

[0019] According to the steel sheet of the present invention, a certain level of formability can be ensured by controlling the area fraction of ferrite, a soft structure, to 75 to 95%, while controlling the area fraction of martensite, a hard structure, to 5 to 25% and further controlling the chemical composition of the steel sheet within a predetermined range to ensure high strength, for example, a tensile strength of 540 MPa or more. In addition, as described above, by uniformly dispersing martensite in both the micro and macro regions in the metal structure, excellent post-forming appearance can be achieved with significantly reduced appearance defects such as ghost lines. Furthermore, by controlling the KAM value of ferrite to satisfy predetermined requirements, formability can be significantly improved. As a result, the steel sheet of the present invention can achieve a high level of strength, formability, and post-forming appearance.

[0020] Hereinafter, the steel sheet according to the embodiment of the present invention will be described in more detail. In the following description, the unit of content of each element, "%", means "mass %" unless otherwise specified. Furthermore, in this specification, "to" indicating a numerical range is used to mean that the numerical values ​​before and after it are included as the lower and upper limits, unless otherwise specified.

[0021] [C: 0.030 to 0.100%] C is an element that ensures a predetermined amount of martensite and improves the strength of the steel sheet. C is also an austenite-stabilizing element and is effective in lowering the Ac3 point. To fully obtain these effects, the C content is set to 0.030% or more. The C content may be 0.040% or more or 0.050% or more. On the other hand, excessive C content may make it impossible to control the average grain spacing of martensite within a desired range and / or impossible to control the standard deviation of the area fraction of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction within a desired range. For this reason, the C content is set to 0.100% or less. The C content may be 0.090% or less, 0.080% or less, 0.070% or less, or 0.060% or less.

[0022] [Mn: 1.00 to 2.80%] Mn is an element that improves hardenability and contributes to improving the strength of the steel sheet. Furthermore, Mn is also an austenite-stabilizing element and is effective in lowering the Ac3 point. To fully obtain these effects, the Mn content is set to 1.00% or more. The Mn content may be 1.10% or more, 1.20% or more, 1.30% or more, or 1.50% or more. In a preferred method for producing a steel sheet, which will be described later, in order to uniformly disperse martensite in both the microregion and the macroregion in the final metal structure, it is necessary for the metal structure of the steel sheet before final annealing to be composed of a structure mainly composed of bainite and / or martensite. For this reason, improving hardenability by adding Mn is also important for improving the appearance after forming. On the other hand, if Mn is contained excessively, the effects of Mn segregation cannot be sufficiently counteracted, and it may be impossible to control the average grain spacing of martensite within a desired range and / or the standard deviation of the area fraction of martensite in the direction perpendicular to the rolling direction and the thickness direction within a desired range. Therefore, the Mn content is set to 2.80% or less. The Mn content may be 2.60% or less, less than 2.50%, 2.49% or less, 2.48% or less, 2.47% or less, 2.46% or less, 2.45% or less, 2.44% or less, 2.42% or less, 2.40% or less, 2.20% or less, or 2.00% or less.

[0023] [Si: 0.005 to 1.500%] Si is an element that improves the strength of steel sheet through solid solution strengthening. To fully obtain this effect, the Si content is set to 0.005% or more. The Si content may be 0.010% or more, 0.050% or more, 0.100% or more, 0.200% or more, 0.300% or more, or 0.400% or more. On the other hand, excessive Si content may make it difficult to remove scale formed during hot rolling, which may lead to deterioration of appearance. Therefore, the Si content is set to 1.500% or less. Furthermore, because Si is a ferrite stabilizing element, reducing the Si content can lower the Ac3 point. Therefore, the Si content may be 1.200% or less, 1.000% or less, 0.800% or less, 0.700% or less, or 0.600% or less.

[0024] [Al: 1.000% or less] Al functions as a deoxidizer and is effective in increasing the strength of steel. The Al content may be 0%, but to fully obtain these effects, the Al content is preferably 0.001% or more. The Al content may be 0.005% or more, 0.010% or more, 0.025% or more, 0.050% or more, or 0.080% or more. On the other hand, excessive Al content may form coarse oxides and reduce toughness. Therefore, the Al content is set to 1.000% or less. Furthermore, since Al is a ferrite stabilizing element, reducing the Al content can lower the Ac3 point. Therefore, the Al content may be 0.800% or less, 0.700% or less, 0.600% or less, or 0.300% or less.

[0025] [P: 0.100% or less] P is an impurity element that embrittles welds and deteriorates galvanic properties. Therefore, the P content is set to 0.100% or less. The P content may be 0.060% or less, 0.040% or less, 0.020% or less, or 0.010% or less. The lower the P content, the more preferable it is, and the lower limit is not particularly limited and may be 0%. On the other hand, reducing the P content to less than 0.0001% in practical steel sheets significantly increases production costs, which is economically disadvantageous. Therefore, the P content may be 0.0001% or more, 0.0002% or more, 0.0005% or more, or 0.001% or more.

[0026] [S: 0.0200% or less] S is an impurity element that impairs weldability and also impairs manufacturability during casting and hot rolling. Therefore, the S content is set to 0.0200% or less. The S content may be 0.0150% or less, 0.0120% or less, 0.0100% or less, 0.0060% or less, or 0.0030% or less. The lower the S content, the more preferable it is, and the lower limit is not particularly limited and may be 0%. On the other hand, reducing the S content to less than 0.0001% in practical steel sheets significantly increases production costs, which is economically disadvantageous. Therefore, the S content may be 0.0001% or more, 0.0002% or more, or 0.0005% or more.

[0027] [N: 0.0150% or less] N is an element that causes blowholes during welding. Therefore, the N content is set to 0.0150% or less. The N content may be 0.0120% or less, 0.0100% or less, 0.0080% or less, or 0.0060% or less. The lower the N content, the more preferable it is, and the lower limit is not particularly limited and may be 0%. On the other hand, if the N content of a practical steel sheet is reduced to less than 0.0001%, the manufacturing cost will increase significantly, which is economically disadvantageous. Therefore, the N content may be 0.0001% or more, 0.0002% or more, or 0.0005% or more.

[0028] [O: 0.0100% or less] O is an element that causes blowholes during welding. Therefore, the O content is set to 0.0100% or less. The O content may be 0.0080% or less, 0.0050% or less, 0.0030% or less, or 0.0020% or less. The lower the O content, the more preferable it is, and the lower limit is not particularly limited and may be 0%. On the other hand, if the O content in a practical steel sheet is reduced to less than 0.0001%, the manufacturing cost will increase significantly, which is economically disadvantageous. Therefore, the O content may be 0.0001% or more, 0.0002% or more, or 0.0005% or more.

[0029] The basic chemical composition of the steel sheet according to the embodiment of the present invention is as described above. Furthermore, the steel sheet may contain at least one of the following optional elements in place of a portion of the remaining Fe, if necessary, for the purpose of improving properties. For example, the steel sheet may contain at least one of Cr: 0-1.00%, Mo: 0-0.80%, B: 0-0.0100%, Ti: 0-0.200%, Nb: 0-0.200%, V: 0-0.500%, Ni: 0-1.00%, Cu: 0-1.00%, W: 0-1.00%, Ta: 0-0.10%, Co: 0-3.00%, Sn: 0-1.00%, Sb: 0-0.200%, Ca: 0-0.0100%, Mg: 0-0.0100%, Zr: 0-0.0100%, REM: 0-0.0100%, Bi: 0-0.0500%, and As: 0-0.10%. These optional elements will be described in detail below.

[0030] [Cr: 0 to 1.00%] Cr, like Mn, is an element that improves hardenability and contributes to improving steel sheet strength. While the Cr content may be 0%, in order to obtain the above-mentioned effects, the Cr content is preferably 0.001% or more. The Cr content may be 0.01% or more, 0.10% or more, or 0.20% or more. On the other hand, excessive Cr content may saturate the effect and may result in an increase in manufacturing costs. Therefore, the Cr content is preferably 1.00% or less, and may be 0.80% or less, 0.60% or less, or 0.40% or less.

[0031] [Mo: 0 to 0.80%] Mo, like Cr, is an element that contributes to increasing the strength of steel sheets. This effect can be achieved even with a small amount. The Mo content may be 0%, but to achieve the above effect, the Mo content is preferably 0.001% or more. The Mo content may be 0.01% or more, 0.02% or more, 0.05% or more, or 0.10% or more. On the other hand, excessive Mo content may deteriorate hot workability and reduce productivity. Therefore, the Mo content is preferably 0.80% or less. The Mo content may be 0.60% or less, 0.50% or less, 0.40% or less, or 0.20% or less.

[0032] [B: 0 to 0.0100%] B is an element that suppresses the formation of ferrite and pearlite during the cooling process from austenite and promotes the formation of martensite. Furthermore, B is a beneficial element for increasing the strength of steel. These effects can be achieved even with trace amounts. The B content may be 0%, but to achieve the above effects, the B content is preferably 0.0001% or more. The B content may be 0.0005% or more or 0.0010% or more. On the other hand, excessive B content may decrease toughness and / or weldability. Therefore, the B content is preferably 0.0100% or less. The B content may be 0.0080% or less, 0.0050% or less, 0.0030% or less, or 0.0020% or less.

[0033] [Ti: 0 to 0.200%] Ti is an element effective in controlling the morphology of carbides. Ti can promote an increase in the strength of ferrite. The Ti content may be 0%, but to obtain these effects, the Ti content is preferably 0.001% or more. The Ti content may be 0.002% or more, 0.010% or more, 0.020% or more, or 0.040% or more. On the other hand, if Ti is contained excessively, the effect saturates and there is a risk of increasing manufacturing costs. Therefore, the Ti content is preferably 0.200% or less. The Ti content may be 0.150% or less, 0.100% or less, 0.080% or less, or 0.050% or less.

[0034] [Nb: 0 to 0.200%] Nb, like Ti, is an element effective in controlling the morphology of carbides and is also effective in refining the structure and improving the toughness of steel sheet. These effects can be obtained even with trace amounts. The Nb content may be 0%, but to obtain the above effects, the Nb content is preferably 0.001% or more. The Nb content may be 0.005% or more, 0.010% or more, 0.015% or more, 0.020% or more, or 0.040% or more. On the other hand, excessive Nb content may cause the formation of coarse carbides in the steel, reducing the toughness of the steel sheet. For this reason, the Nb content is preferably 0.200% or less. The Nb content may be 0.150% or less, 0.100% or less, 0.080% or less, or 0.050% or less.

[0035] [V: 0 to 0.500%] Like Ti, V is an element effective in controlling the morphology of carbides and is also effective in refining the structure to improve the toughness of the steel plate. The V content may be 0%, but to obtain the above effect, the V content is preferably 0.001% or more. The V content may be 0.005% or more, 0.010% or more, or 0.050% or more. On the other hand, excessive V content may cause the formation of a large amount of precipitates, which may reduce toughness. For this reason, the V content is preferably 0.500% or less. The V content may be 0.400% or less, 0.200% or less, or 0.100% or less.

[0036] [Ni: 0 to 1.00%] Ni is an element effective in improving the strength of steel sheet. The Ni content may be 0%, but to obtain the above effect, the Ni content is preferably 0.001% or more. The Ni content may be 0.01% or more or 0.05% or more. On the other hand, excessive Ni content may reduce the weldability of the steel sheet. For this reason, the Ni content is preferably 1.00% or less. The Ni content may be 0.80% or less, 0.40% or less, or 0.20% or less.

[0037] [Cu: 0 to 1.00%] Cu is an element that contributes to improving the strength of the steel sheet. This effect can be achieved even with a small amount. The Cu content may be 0%, but to achieve the above effect, the Cu content is preferably 0.001% or more. The Cu content may be 0.01% or more or 0.05% or more. On the other hand, excessive Cu content may cause red shortness and reduce productivity in hot rolling. Therefore, the Cu content is preferably 1.00% or less. The Cu content may be 0.80% or less, 0.60% or less, 0.30% or less, or 0.20% or less.

[0038] [W: 0 to 1.00%] W is an element effective in controlling the morphology of carbides and improving the strength of steel sheet. The W content may be 0%, but to obtain these effects, the W content is preferably 0.001% or more. The W content may be 0.01% or more or 0.05% or more. On the other hand, excessive W content may reduce weldability. For this reason, the W content is preferably 1.00% or less. The W content may be 0.80% or less, 0.40% or less, 0.20% or less, or 0.10% or less.

[0039] [Ta: 0 to 0.10%] Ta, like W, is an element effective in controlling the morphology of carbides and improving the strength of the steel sheet. The Ta content may be 0%, but to obtain these effects, the Ta content is preferably 0.001% or more. The Ta content may be 0.01% or more or 0.03% or more. On the other hand, even if Ta is contained in an excessive amount, the effect saturates, and containing more than necessary in the steel sheet increases the manufacturing cost. For this reason, the Ta content is preferably 0.10% or less. The Ta content may be 0.08% or less, 0.06% or less, or 0.04% or less.

[0040] [Co: 0 to 3.00%] Co, like Ni, is an element effective in improving the strength of steel sheet. The Co content may be 0%, but to obtain the above effect, the Co content is preferably 0.001% or more. The Co content may be 0.01% or more, 0.05% or more, or 0.10% or more. On the other hand, excessive Co content may deteriorate hot workability and increase raw material costs. For this reason, the Co content is preferably 3.00% or less. The Co content may be 2.00% or less, 1.00% or less, 0.50% or less, or 0.20% or less.

[0041] [Sn: 0 to 1.00%] Sn is an element that can be contained in steel sheet when scrap is used as a raw material for the steel sheet. Furthermore, Sn may cause ferrite embrittlement. Therefore, the lower the Sn content, the better, and it is preferably 1.00% or less. The Sn content may be 0.10% or less, 0.04% or less, or 0.02% or less. The Sn content may be 0%, but reducing the Sn content to less than 0.001% results in an excessive increase in refining costs. Therefore, the Sn content may be 0.001% or more, 0.005% or more, or 0.01% or more.

[0042] [Sb: 0 to 0.200%] Like Sn, Sb is an element that can be contained in steel sheet when scrap is used as a raw material for the steel sheet. Furthermore, Sb may strongly segregate at grain boundaries, potentially causing embrittlement of the grain boundaries. Therefore, the lower the Sb content, the better, and it is preferably 0.200% or less. The Sb content may be 0.100% or less, 0.040% or less, or 0.020% or less. The Sb content may be 0%, but reducing the Sb content to less than 0.001% would result in an excessive increase in refining costs. Therefore, the Sb content may be 0.001% or more, 0.005% or more, or 0.010% or more.

[0043] [Ca: 0 to 0.0100%] [Mg: 0 to 0.0100%] [Zr: 0 to 0.0100%] [REM: 0 to 0.0100%] Ca, Mg, Zr, and REM are elements that contribute to improving the formability of steel sheet. The Ca, Mg, Zr, and REM contents may be 0%, but to achieve these effects, the Ca, Mg, Zr, and REM contents are each preferably 0.0001% or more, and may be 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, excessive inclusion of these elements may reduce the ductility of the steel sheet. Therefore, the Ca, Mg, Zr, and REM contents are each preferably 0.0100% or less, and may be 0.0080% or less, 0.0060% or less, 0.0040% or less, or 0.0020% or less. In this specification, REM is a collective term for 17 elements: scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanides lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71, and the REM content is the total content of these elements.

[0044] [Bi: 0 to 0.0500%] Bi is an element that has the effect of improving formability by refining the solidification structure. The Bi content may be 0%, but to obtain this effect, the Bi content is preferably 0.0001% or more, and may be 0.0005% or more, 0.0010% or more, or 0.0030% or more. On the other hand, even if Bi is contained in an excessive amount, the effect saturates, and adding more Bi than necessary to the steel sheet increases the manufacturing cost. Therefore, the Bi content is preferably 0.0500% or less, and may be 0.0400% or less, 0.0200% or less, 0.0100% or less, or 0.0050% or less.

[0045] [As: 0 to 0.10%] Like Sn and Sb, As is an element that can be contained in steel sheet when scrap is used as the raw material for the steel sheet. Furthermore, As is an element that strongly segregates at grain boundaries, and the lower the As content, the better. The As content is preferably 0.10% or less, and may be 0.05% or less, 0.04% or less, or 0.02% or less. The As content may be 0%, but reducing the As content to less than 0.001% results in an excessive increase in refining costs. Therefore, the As content may be 0.001% or more, 0.005% or more, or 0.01% or more.

[0046] In the steel sheet according to the embodiment of the present invention, the balance excluding the above elements consists of Fe and impurities. The impurities are elements that are mixed in from the steel raw materials and / or during the steelmaking process and are allowed to be present to an extent that does not impair the properties of the steel sheet according to the embodiment of the present invention.

[0047] The chemical composition of the steel sheet according to the embodiment of the present invention may be measured by a general analytical method. For example, the chemical composition of the steel sheet may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) on chips in accordance with JIS G 1201:2014. Specifically, for example, a 35 mm square test piece is obtained from the steel sheet at approximately 1 / 4 of the sheet thickness, and the composition can be determined by measuring it using a Shimadzu ICPS-8100 or similar measuring device under conditions based on a pre-created calibration curve. C and S, which cannot be measured by ICP-AES, may be measured using a combustion-infrared absorption method, N may be measured using an inert gas fusion-thermal conductivity method, and O may be measured using an inert gas fusion-non-dispersive infrared absorption method. If the steel sheet has a plating layer on its surface, the plating layer may be removed by mechanical grinding or the like before analyzing the chemical composition.

[0048] [Ferrite: 75 to 95%] Ferrite is a soft structure and therefore easily deformable, contributing to improved elongation. A certain level of formability can be ensured by setting the ferrite area ratio to 75% or more. From the viewpoint of improving formability, a higher ferrite area ratio is preferable, and may be, for example, 78% or more, 80% or more, 82% or more, or 85% or more. On the other hand, if ferrite is contained in an excessive amount, the steel sheet may not achieve the desired strength. Therefore, the ferrite area ratio is set to 95% or less. The ferrite area ratio may be 93% or less, 90% or less, or 87% or less.

[0049] [Martensite: 5 to 25%] Martensite is a hard structure with a high dislocation density, contributing to improved tensile strength. If the area fraction of martensite is excessively low, the desired strength may not be achieved, and / or uneven elongation may occur during press forming, resulting in the formation of a streak-like pattern known as stretcher strain. By setting the area fraction of martensite to 5% or more, a tensile strength of, for example, 540 MPa or more can be ensured without such problems. From the viewpoint of improving strength, a higher area fraction of martensite is preferable, and may be, for example, 7% or more, 8% or more, 10% or more, or 13% or more. On the other hand, if the area fraction of martensite is 25% or less, formability and appearance can be ensured. The area fraction of martensite may be 22% or less, 20% or less, 18% or less, or 15% or less. In the present invention, "martensite" encompasses not only as-quenched martensite (so-called fresh martensite) but also tempered martensite.

[0050] [Total of Ferrite and Martensite: 90% or More] If the total area ratio of ferrite and martensite is low, the area ratio of other remaining structures will be relatively high, which may result in uneven elongation during press forming and the formation of a streak-like pattern known as stretcher strain. Therefore, the total area ratio of ferrite and martensite is 90% or more. This reliably suppresses the occurrence of appearance defects such as stretcher strain and ensures the effects based on the ferrite and martensite described above. The total area ratio of ferrite and martensite may be 92% or more, 93% or more, 94% or more, 95% or more, 96% or more, 97% or more, 98% or more, 99% or more, or even 100%.

[0051] [Remaining Structure: 0 to 10% in Total] The remaining structure other than ferrite and martensite may have an area ratio of 0%. If a remaining structure is present, the remaining structure may be bainite, pearlite, retained austenite, or a combination of two or more thereof. In this embodiment, the remaining structure is presumed to be bainite, pearlite, retained austenite, or a combination of two or more thereof based on the chemical composition and the manufacturing method described below, but there is no need to identify or distinguish them. From the viewpoint of ensuring the above-described effects based on ferrite and martensite, the total area ratio of the remaining structure is preferably 10% or less, and may be, for example, 8% or less, 7% or less, 6% or less, 5% or less, 4% or less, 3% or less, 2% or less, or 1% or less. On the other hand, achieving a remaining structure area ratio of 0% requires advanced control in the steel sheet manufacturing process, which may result in a decrease in yield. Therefore, the area ratio of the remaining structure may be 0.5% or more or 1% or more.

[0052] [Identification of Metallic Structure and Calculation of Area Ratio] Identification of the metallic structure and calculation of the area ratio are performed using an FE-SEM (field emission scanning electron microscope, for example, JEOL JSM-7200F, measured at an acceleration voltage of 15 kV) after corrosion using a Nital reagent, an optical microscope, and X-ray diffraction. The observation of the structure using the FE-SEM and optical microscope is performed at a magnification of 500 to 50,000 times on a 100 μm × 100 μm area in the cross section of the steel sheet in a direction perpendicular to the sheet surface. For each metallic structure, three measurement points are measured, and the area ratio is determined by calculating the average of the measured values. For example, if the thickness of the steel sheet to be measured is too thin to ensure a measurement area of ​​100 μm in the thickness direction, the length in the thickness direction is reduced and a measurement area of ​​10,000 μm is obtained. 2 For example, a measurement area of ​​20 μm in the thickness direction and 500 μm in the direction perpendicular to the thickness direction may be observed. However, if the number of crystal grains in the thickness direction becomes too small, the measurement accuracy may decrease, so the measurement length in the thickness direction is set to 10 μm or more, preferably 50 μm or more. The same applies to the "100 μm × 100 μm area" in the following explanation.

[0053] In this specification, the term "plate thickness x / y position (where x and y are natural numbers satisfying x<y)" refers to a position moved in the plate thickness direction from the surface (plate surface) of the steel plate by a distance (depth) of x / y of the plate thickness t toward the center of the steel plate. For example, if the plate thickness t of the steel plate is 2 mm, the "plate thickness 1 / 8 position" refers to a position that is 0.25 mm deep in the plate thickness direction from the surface of the steel plate. Note that, when a steel plate has a coating such as a plating layer on its surface, the "surface of the steel plate" refers to the interface between the steel plate and the coating, and the "plate thickness t" refers to the thickness of the steel plate (base metal) excluding the coating.

[0054] The area fractions of ferrite and martensite are determined by the following procedure. First, the observation surface of the sample is etched with a nital reagent (3% nitric acid ethanol solution). Next, a 100 μm × 100 μm region within the range of 1 / 8 to 3 / 8 of the plate thickness, centered at the 1 / 4 position, is observed using an FE-SEM (field emission scanning electron microscope). Since martensite and retained austenite are not corroded by nital corrosion, the area fraction of the uncorroded region corresponds to the total area fraction of martensite and retained austenite. Specifically, using image analysis software Image J (Ver. 1.54f), the metal structure is binarized based on differences in brightness. The black portions of the image data represent ferrite, and the uncorroded white portions represent the combined structure of martensite and retained austenite. Therefore, the area fraction of ferrite is calculated from the area fraction of the black regions, and the area fraction of martensite is calculated by subtracting the area fraction of retained austenite measured by X-ray diffraction (described later) from the area fraction of the uncorroded region. The area fraction of martensite determined by this method also includes the area fraction of tempered martensite.

[0055] The area fraction of retained austenite is calculated by X-ray diffraction. First, the sample is removed from the plate surface to a depth of 1 / 4 in the plate thickness direction by mechanical polishing and chemical polishing. More specifically, the sample is thinned to the vicinity of the observation position by mechanical polishing, and then thinned to the target position by chemical polishing (with hydrofluoric acid). Next, using, for example, an X-ray diffractometer manufactured by Rigaku Corporation (RINT1500, X-ray output 40 kV-200 mA), the structure fraction of retained austenite is calculated from the integrated intensity ratio of the diffraction peaks of (200) and (211) of the bcc phase and (200), (220), and (311) of the fcc phase obtained at the 1 / 4 plate thickness position using MoKα radiation. The general five-peak method is used for this calculation. The calculated structure fraction of retained austenite is determined as the area fraction of retained austenite.

[0056] [Average Particle Spacing of Martensite: 2.5 μm or Less] In an embodiment of the present invention, the average particle spacing of martensite, which is a hard structure, is controlled to 2.5 μm or less. The average particle spacing of martensite is an index representing the uniformity of the hard structure distribution in the microregion. A smaller average particle spacing of martensite means that the hard structure is more densely and uniformly dispersed, and therefore, can be said to be higher uniformity. The appearance of the steel sheet after press forming becomes better when the deformation amount of the steel sheet during press forming is more uniform, particularly in the width direction of the steel sheet. Since the deformation amount of the steel sheet is strongly affected by the distribution state of the hard structure, in order to make the deformation amount of the steel sheet uniform in the width direction of the steel sheet, it is necessary to make the distribution of the hard structure in the metal structure uniform. In addition to controlling the standard deviation of the area fraction of martensite, which will be described later, by controlling the average particle spacing of martensite to 2.5 μm or less, the deformation amount of the steel sheet can be made more uniform in the width direction even during forming, such as press forming, and as a result, a good post-forming appearance can be achieved. The average grain spacing of martensite is preferably 2.4 μm or less, more preferably 2.2 μm or less, and most preferably 2.0 μm or less or 1.8 μm or less. There is no particular lower limit, but the average grain spacing of martensite may be, for example, 0.5 μm or more, 0.8 μm or more, or 1.0 μm or more.

[0057] [Measurement of Average Particle Spacing of Martensite] The average particle spacing of martensite is determined as follows. First, a sample having a steel sheet cross section perpendicular to the sheet surface is collected, and the cross section is used as the observation surface. A 100 μm × 100 μm region within the range of 1 / 8 to 3 / 8 sheet thickness positions, centered at the 1 / 4 sheet thickness position, is used as the observation region of this observation surface, and martensite is identified using an FE-SEM (e.g., a JSM-7200F manufactured by JEOL, measured at an acceleration voltage of 15 kV). Specifically, the metal structure is binarized based on differences in brightness using image analysis software Image J (Ver. 1.54f), and martensite is identified. Specifically, when a nital solution is used, the black areas in the image data represent ferrite, and the uncorroded white areas represent the combined structure of martensite and retained austenite. However, in the steel sheet according to the embodiment of the present invention, the area fraction of retained austenite is sufficiently low compared to the area fraction of martensite, so the white structure can be considered as martensite. Next, the distance between the centers (centers of gravity) of all adjacent martensite grains among the identified martensite grains is calculated as the particle spacing based on image analysis, and the average of the calculated particle spacings is obtained. This operation is repeated for the other two observation regions, and the average of the three obtained values ​​is determined as the average particle spacing of martensite (strictly speaking, particles containing martensite and / or retained austenite).

[0058] [Standard deviation of martensite area fraction in the direction perpendicular to the rolling direction and the thickness direction is 1.5% or less] In an embodiment of the present invention, the standard deviation of the martensite area fraction in the direction perpendicular to the rolling direction and the thickness direction is controlled to 1.5% or less. This standard deviation is an index representing the uniformity of the hard structure in the macro region. The appearance, which is an issue during press forming, depends on minute irregularities on the steel sheet surface caused by differences in the amount of deformation in the width direction of the steel sheet. Therefore, if there is a large variation in the area fraction of the hard structure contained within the steel sheet thickness in the direction perpendicular to the rolling direction and the thickness direction, a difference in the amount of deformation in the width direction of the steel sheet will occur, resulting in the generation of minute irregularities on the steel sheet surface. Therefore, it is effective to reduce the standard deviation of the martensite area fraction in the direction perpendicular to the rolling direction and the thickness direction, i.e., the width direction of the steel sheet. More specifically, by controlling the standard deviation to 1.5% or less in addition to the control of the average particle spacing of martensite as described above, it is possible to further reduce the variation in deformation amount in the width direction of the steel sheet even during forming such as press forming, and as a result, it is possible to achieve a good appearance after forming. The standard deviation of the area fraction of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction is preferably 1.4% or less, more preferably 1.2% or less, and most preferably 1.0% or less. There is no particular lower limit, but the standard deviation may be, for example, 0.1% or more, 0.3% or more, or 0.5% or more.

[0059] [Measurement of Standard Deviation in Area Fraction of Martensite in Directions Perpendicular to the Rolling Direction and Plate Thickness Direction] The standard deviation in the area fraction of martensite in the direction perpendicular to the rolling direction and plate thickness direction is determined as follows. First, a metallographic image of the steel plate cross section in a region of 50 mm in the direction perpendicular to the rolling direction and plate thickness direction is acquired. In the case of an image of 10 mm or smaller, multiple images may be acquired and stitched together to form a 50 mm image. Next, the acquired image is divided into 100 μm (0.1 mm) sections in the direction perpendicular to the rolling direction and plate thickness direction, and the area fraction of martensite in each divided region is calculated in the range from the 3 / 8 position to the 5 / 8 position in the plate thickness direction. The standard deviation in the area fraction of martensite is calculated based on the martensite area fraction calculated from each of the 500 divided images. The area fraction of martensite in each divided region is calculated according to the procedure described in the section [Identification of Metallographic Structure and Calculation of Area Fraction]. For convenience, the area ratio of retained austenite may be measured in a cross section of the steel plate in a region of 50 mm in a direction perpendicular to the rolling direction and the plate thickness direction, instead of the measurement results in each divided region.

[0060] When the rolling direction of the steel sheet is unclear, the following method can be used to identify the rolling direction of the steel sheet, for example. After mirror-polishing the thickness cross section of the steel sheet, the S concentration is measured using an electron probe microanalyzer (EPMA, for example, a JXA-8230 manufactured by JEOL, measured at an acceleration voltage of 15 kV and a measurement pitch of 1 μm). The measurement conditions are an acceleration voltage of 15 kV and a measurement pitch of 1 μm, and a distribution image is measured over an area of ​​100 μm (thickness direction) × 500 μm (direction perpendicular to the thickness direction) at the center of the sheet thickness. At this time, an elongated region with a high S concentration is determined to be an inclusion such as MnS. Observation may be performed from multiple fields of view. Next, using the thickness cross section initially observed by the above method as a reference, a plane parallel to a plane rotated in 5° increments in the range of 0° to 180° around the thickness direction as an axis is observed by the above method. The average length of the major axes of the inclusions in each cross section is calculated, and the cross section with the largest average length of the major axes of the inclusions is identified. The direction parallel to the major axes of the inclusions in that cross section is determined to be the rolling direction.

[0061] [Ratio A / B of the ratio A of the region in the ferrite where the KAM value is less than 0.5° to the ratio B of the region in the ferrite where the KAM value is 1.0° or more: 0.60 or more] In an embodiment of the present invention, in an EBSD (electron backscatter diffraction) measurement, the ratio A / B of the ratio A of the region in the ferrite where the KAM value is less than 0.5° to the ratio B of the region in the ferrite where the KAM value is 1.0° or more is controlled to be 0.60 or more. As mentioned above, the KAM (Kernel Average Misorientation) value tends to increase as strain accumulates, and therefore it is recognized that the higher the KAM value, the higher the dislocation density tends to be. For this reason, in EBSD measurement, the ratio A of the region in the ferrite where the KAM value is less than 0.5° is increased (in the direction of decreasing dislocation density), while the ratio B of the region in the ferrite where the KAM value is 1.0° or more is decreased (in the direction of decreasing dislocation density as well), thereby controlling the ratio A / B to be larger, more specifically, by controlling the ratio A / B to be 0.60 or more, the dislocation density in the ferrite can be sufficiently reduced, thereby making it possible to significantly improve the elongation of the steel sheet. As a result, according to the steel sheet according to the embodiment of the present invention, by controlling the area ratio of martensite, which is a hard structure, to 5 to 25%, and further controlling the chemical composition of the steel sheet within a predetermined range, high strength, for example, high strength with a tensile strength of 540 MPa or more, is ensured, and by controlling the area ratio of ferrite, which is a soft structure, to 75 to 95%, a certain level of formability is ensured, and further by controlling the ratio A / B, where A is the proportion of the region in ferrite where the KAM value is less than 0.5° and B is the proportion of the region in ferrite where the KAM value is 1.0° or more, to 0.60 or more, it is possible to significantly improve the balance between strength and formability.

[0062] From the viewpoint of further improving the balance between strength and formability by further improving the elongation of the steel sheet, the ratio A / B of the ratio A of the region in ferrite where the KAM value is less than 0.5° to the ratio B of the region in ferrite where the KAM value is 1.0° or more is preferably as high as possible, and may be, for example, 0.62 or more, 0.64 or more, 0.66 or more, 0.68 or more, or 0.70 or more. The upper limit is not particularly limited, and the ratio A / B may be, for example, 0.85 or less, 0.80 or less, or 0.75 or less.

[0063] [Measurement of the A / B ratio] The KAM value of ferrite is calculated by KAM analysis and GAIQ analysis in EBSD measurement, a crystal analysis method using an SEM. A field emission scanning electron microscope (e.g., JEOL's "JSM-7001F") is used as the SEM observation device, and EBSD analysis can be performed using, for example, TSL's "OIM Analysis 7." In the EBSD analysis, a 50 μm x 50 μm range from the 1 / 8 to 3 / 8 thickness positions, centered at the 1 / 4 thickness position from the surface of the steel sheet, is analyzed at intervals (pitch) of 0.05 μm. KAM analysis is an analysis in which the average value of the misorientation (°) between a "certain pixel" that is the measurement point and all neighboring pixels is used as the KAM value of the "certain pixel," and a KAM map based on the local crystal misorientation can be created. The KAM value in the ferrite was analyzed by such KAM analysis. The determination of the region where ferrite exists in the EBSD measurement results is performed by the GAIQ analysis described below.

[0064] Based on the GAIQ (Grain Average Image Quality) values ​​obtained by GAIQ analysis of EBSD measurement results obtained under the same measurement conditions as the KAM values, the region is divided into a region with a relatively high GAIQ value and a region with a relatively low GAIQ value. The region with a relatively low GAIQ value is determined to correspond to a region where ferrite is present. Furthermore, the region with a relatively low GAIQ value can be determined to correspond to a region where a hard phase other than ferrite is present. In this case, the GAIQ analysis is an analysis in which the average value within a single crystal grain of the IQ value representing the clarity of the Kikuchi pattern at a "certain pixel" that is the measurement point is taken as the GAIQ value of the crystal grain. Here, a crystal grain is defined as a region surrounded by a grain boundary, which is the boundary between regions where the crystal orientations differ by 15° or more.

[0065] In the region corresponding to the ferrite determined by the GAIQ analysis, the ratio A / B is determined from the ratio A of the region where the KAM value is less than 0.5° and the ratio B of the region where the KAM value is 1.0° or more, and the ratio A / B in one visual field can be calculated. Similar EBSD measurement, KAM analysis, and GAIQ analysis are performed in five different visual fields, and the average value is determined as the ratio A / B of the ratio A of the region in the ferrite where the KAM value is less than 0.5° and the ratio B of the region in the ferrite where the KAM value is 1.0° or more.

[0066] [Average grain size of ferrite: 3.0 to 25.0 μm] According to a preferred embodiment of the present invention, the average grain size of ferrite in the metal structure is 3.0 to 25.0 μm. By controlling the average grain size of ferrite within such a fine range, it is possible to further improve the appearance of the steel sheet, particularly the appearance after forming. The average grain size of ferrite may be 5.0 μm or more, 7.0 μm or more, 8.0 μm or more, 9.0 μm or more, or 10.0 μm or more. Similarly, the average grain size of ferrite may be 22.0 μm or less, 20.0 μm or less, 16.0 μm or less, 14.0 μm or less, or 12.0 μm or less.

[0067] The average grain size of ferrite in a steel sheet is determined as follows. First, the circle-equivalent diameters of all grains (ferrite grains) located in the region corresponding to the ferrite determined by the GAIQ analysis in the EBSD measurement results are calculated. Here, the grains are defined as regions surrounded by grain boundaries, which are boundaries between regions whose crystal orientations differ by 15° or more. Next, the value obtained by arithmetically averaging these diameters is determined as the average grain size of ferrite.

[0068] [Average grain size of martensite: 1.0 to 5.0 μm] According to a preferred embodiment of the present invention, the average grain size of martensite in the metal structure is 1.0 to 5.0 μm. By controlling the average grain size of martensite within such a fine range, it is possible to further improve the appearance of the steel sheet, particularly the appearance after forming. The average grain size of martensite may be 1.2 μm or more, 1.5 μm or more, 1.7 μm or more, or 2.0 μm or more. Similarly, the average grain size of martensite may be 4.7 μm or less, 4.5 μm or less, 4.2 μm or less, 4.0 μm or less, 3.8 μm or less, 3.6 μm or less, or 3.4 μm or less.

[0069] The average grain size of martensite is determined as follows. First, a sample having a steel sheet cross section perpendicular to the sheet surface is collected, and the cross section is used as the observation surface. A 100 μm × 100 μm region within the range of 1 / 8 to 3 / 8 sheet thickness positions, centered at the 1 / 4 sheet thickness position, is used as the observation region of this observation surface, and martensite is identified using an FE-SEM (e.g., a JSM-7200F manufactured by JEOL, measured at an acceleration voltage of 15 kV). Specifically, the metal structure is binarized based on differences in brightness using image analysis software Image J (Ver. 1.54f), and martensite is identified. Specifically, when a nital solution is used, the black portions of the image data represent ferrite, and the uncorroded white portions represent the combined structure of martensite and retained austenite. However, in the steel sheet according to the embodiment of the present invention, the area ratio of retained austenite is sufficiently low compared to the area ratio of martensite, so the white structure can be considered as martensite. Next, the circle-equivalent diameters of all the identified martensite are calculated, and this operation is repeated for the other two observation regions to calculate the arithmetic mean of the circle-equivalent diameters of all the martensite obtained in the three observation regions, and the obtained value is determined as the average crystal grain size of the martensite (strictly speaking, particles including martensite and / or retained austenite).

[0070] [Average aspect ratio of martensite: 2.5 or more] According to a preferred embodiment of the present invention, the average aspect ratio of martensite in the metal structure is 2.5 or more. By controlling the average aspect ratio of martensite to 2.5 or more, a state in which greater strain is imparted can be achieved, thereby improving the strength of the steel sheet. The average aspect ratio of martensite may be 2.6 or more, 2.8 or more, or 3.0 or more. There is no particular upper limit, but the average aspect ratio of martensite may be 4.0 or less, 3.8 or less, or 3.6 or less, for example.

[0071] The average aspect ratio of martensite is determined as follows. First, the aspect ratios of all martensite grains are calculated using image analysis software Image J (Ver. 1.54f) for image data of one observation region obtained when measuring the average grain size of martensite. The aspect ratios of particles (crystal grains) on the image can be measured using a function built into the image analysis software Image J (Ver. 1.54f). Next, this operation is performed for the other two observation regions, and the aspect ratios of all martensite grains obtained in the three observation regions are arithmetically averaged. The obtained value is determined as the average aspect ratio of martensite (strictly speaking, particles containing martensite and / or retained austenite).

[0072] [Thickness] The steel plate according to the embodiment of the present invention has a thickness of, for example, 0.2 to 2.0 mm, but is not particularly limited thereto. Steel plates having such a thickness are suitable for use as materials for exterior panel components such as doors and hoods. The thickness may be 0.3 mm or more, 0.4 mm or more, or 0.6 mm or more. Similarly, the thickness may be 1.8 mm or less, 1.5 mm or less, 1.2 mm or less, or 1.0 mm or less. For example, by setting the thickness to 0.2 mm or more, it becomes easier to maintain the shape of the molded product flat, and additional effects such as improved dimensional accuracy and shape accuracy can be obtained. On the other hand, by setting the thickness to 1.0 mm or less, the weight reduction effect of the component is significant. The thickness of the steel plate is measured using a micrometer.

[0073] [Plating] The steel sheet according to the embodiment of the present invention is a cold-rolled steel sheet, but may further include a plating layer on its surface for the purpose of improving corrosion resistance, etc. The plating layer may be either a hot-dip plating layer or an electroplated layer. That is, the steel sheet according to the embodiment of the present invention may be a cold-rolled steel sheet having a hot-dip plating layer or an electroplated layer on its surface. Examples of the hot-dip plating layer include a hot-dip galvanized layer (GI), a galvannealed layer (GA), a hot-dip aluminum plating layer, a hot-dip Zn—Al alloy plating layer, a hot-dip Zn—Al—Mg alloy plating layer, and a hot-dip Zn—Al—Mg—Si alloy plating layer. Examples of the electroplated layer include an electrogalvanized layer (EG), an electrolytic Zn—Ni alloy plating layer, etc. Preferably, the plating layer is a hot-dip galvanized layer, a hot-dip galvannealed layer, or an electrogalvanized layer. The coating weight of the coating layer is not particularly limited and may be a general coating weight.

[0074] [Mechanical Properties] [Tensile Strength (TS) and Total Elongation (El)] The steel sheet according to the embodiment of the present invention can achieve high tensile strength (TS), specifically, a tensile strength of 540 MPa or more. The tensile strength is preferably 570 MPa or more, more preferably 600 MPa or more. The upper limit is not particularly limited, and for example, the tensile strength may be 980 MPa or less, 900 MPa or less, 850 MPa or less, 830 MPa or less, 810 MPa or less, or 800 MPa or less. Setting the tensile strength to 850 MPa or less has the advantage of easily ensuring formability during press working of the steel sheet. Similarly, the steel sheet according to the embodiment of the present invention can achieve excellent elongation, more specifically, a total elongation (El) of 20.0% or more. The total elongation is preferably 22.0% or more, more preferably 25.0% or more. The upper limit is not particularly limited, and for example, the total elongation may be 40.0% or less, 35.0% or less, or 30.0% or less. The tensile strength and total elongation are measured by taking a No. 5 tensile test piece of JIS Z2241:2022 from the steel plate, with the longitudinal direction being perpendicular to the rolling direction and the plate thickness direction, and conducting a tensile test in accordance with JIS Z2241:2022.

[0075] [Product of Tensile Strength and Total Elongation (TS×El)] Furthermore, according to the steel sheet of the embodiment of the present invention, excellent formability can be achieved despite high strength, that is, an excellent balance between strength and formability can be achieved at a high level. More specifically, according to the steel sheet of the embodiment of the present invention, a product of tensile strength and total elongation (TS×El) of 14,000 MPa·% or more can be achieved. TS×El is preferably 15,000 MPa·% or more, more preferably 16,000 MPa·% or more. There is no particular upper limit, but TS×El may be, for example, 20,000 MPa·% or less, 19,000 MPa·% or less, or 18,500 MPa·% or less.

[0076] As described above, the steel sheet according to the embodiment of the present invention is capable of achieving excellent formability and appearance after forming despite its high strength. Therefore, it is possible to reliably achieve a high level of both high strength (e.g., a tensile strength of 540 MPa or more) and excellent formability in press working and appearance after forming. Therefore, the steel sheet according to the embodiment of the present invention is particularly useful for use in parts in technical fields where both of these properties are required. In a preferred embodiment, an exterior panel member, particularly an automotive exterior panel member, is provided that includes the steel sheet according to the embodiment of the present invention. Examples of automotive exterior panel members include roofs, hoods, fenders, doors, and the like, which require high design quality. These exterior panel members, particularly automotive exterior panel members, may comprise the steel sheet according to the embodiment of the present invention in at least a portion thereof, and therefore, at least a portion of these exterior panel members satisfies the chemical composition and metallographic characteristics described above. In portions of the steel sheet that are not in direct contact with a mold during press forming or other forming and that are subjected to a relatively low degree of processing, the characteristics of the metallographic structure do not change significantly before and after forming.

[0077] <Method for manufacturing steel sheet> Next, a preferred method for manufacturing a steel sheet according to an embodiment of the present invention will be described. The following description is intended to exemplify a characteristic method for manufacturing a steel sheet according to an embodiment of the present invention, but is not intended to limit the steel sheet to one manufactured by the manufacturing method described below.

[0078] A method for producing a steel sheet according to an embodiment of the present invention includes: a hot rolling step, which includes heating a slab having the chemical composition described above in relation to the steel sheet to a temperature of 1100 to 1300°C, finish-rolling the slab, and then coiling the slab at a temperature of 500 to 670°C, wherein the finish rolling temperature is 800 to 1250°C; a pickling step, which pickles the obtained hot-rolled steel sheet; a cold rolling step, which cold-rolls the pickled hot-rolled steel sheet at a rolling reduction of 20 to 90%; a primary annealing step, which includes heating the obtained cold-rolled steel sheet to a temperature of Ac3+10°C or higher, and then cooling the cold-rolled steel sheet at an average cooling rate CR of 30 to 200°C / sec to a cooling stop temperature of 350°C or lower, and holding the cold-rolled steel sheet at a temperature T1 of 600°C or lower, wherein CR, T1, and an index X expressed by the following formula 1 or 2 satisfy the following formula 3; The method is characterized by including a secondary annealing step, which includes heating the cold-rolled steel sheet to a maximum heating temperature T2 of Ac1 to (Ac3-10)°C at an average heating rate HR°C / sec, and then holding the maximum heating temperature T2 for 10 to 500 seconds, wherein HR, T2, and an index X expressed by the following formula 1 or 2 satisfy the following formula 4: When [B]≧0.0005%, Index X=0.23+0.5[Mo]+[Ti]+5[Nb]...Equation 1 When [B]<0.0005%, Index X=0.2+0.3[Mo]+[Ti]+3[Nb]...Equation 2 (5CR+T1) / Index X≧1400...Equation 3 (1000 / HR+T2) / Index X≧3200...Equation 4 Here, [B], [Mo], [Ti], and [Nb] are the contents [mass%] of each element, and are 0% when the element is not contained.

[0079] [Hot Rolling Process] [Slab Heating] First, a slab having the chemical composition described above in relation to the steel sheet is heated. The slab used is preferably cast by a continuous casting method from the viewpoint of productivity, but may also be produced by an ingot casting method or a thin slab casting method. The slab used contains a relatively large amount of alloying elements in order to obtain a high-strength steel sheet. Therefore, the slab needs to be heated before being subjected to hot rolling to dissolve the alloying elements in the slab. If the heating temperature is less than 1100°C, the alloying elements may not be sufficiently dissolved in the slab, leaving coarse alloy carbides, which may cause embrittlement cracking during hot rolling. Therefore, the heating temperature is preferably 1100°C or higher. The upper limit of the heating temperature is not particularly limited, but is preferably 1300°C or lower from the viewpoint of the capacity of the heating equipment and productivity.

[0080] [Rough rolling] In this method, for example, the heated slab may be subjected to rough rolling before finish rolling in order to adjust the plate thickness, etc. The conditions for rough rolling are not particularly limited as long as the desired sheet bar dimensions can be secured.

[0081] [Finish Rolling] The heated slab, or the slab that has been rough-rolled as needed, is then subjected to finish rolling. As described above, the slab used contains a relatively large amount of alloying elements, so it is necessary to increase the rolling load during hot rolling. For this reason, it is preferable to perform hot rolling at a high temperature. In particular, the end temperature of finish rolling is important in terms of controlling the metal structure of the steel sheet. If the end temperature of finish rolling is low, the metal structure may become non-uniform and formability may decrease. For this reason, the end temperature of finish rolling is set to 800°C or higher. On the other hand, in order to suppress coarsening of austenite, the end temperature of finish rolling is set to 1250°C or lower.

[0082] [Coiling] Next, the finish-rolled hot-rolled steel sheet is coiled at a coiling temperature of 500 to 670°C. By appropriately controlling the coiling temperature within this temperature range, scale growth can be suppressed and the metallographic structure can be made fine and uniform, which is important for obtaining a desired dispersion state of martensite in the final metallographic structure. If the coiling temperature exceeds 670°C, alloying elements will concentrate in the cementite in the metallographic structure, and undissolved carbides will remain during heating in the subsequent primary annealing step. As a result, the metallographic structure cannot be formed from a structure mainly composed of bainite and / or martensite in the primary annealing step, and the desired dispersion state of martensite cannot be obtained even in the subsequent secondary annealing step. More specifically, even after the subsequent secondary annealing step, it is not possible to control the average grain spacing of martensite to 2.5 μm or less, and / or it is not possible to control the standard deviation of the area ratio of martensite in the direction perpendicular to the rolling direction and the plate thickness direction to 1.5% or less, i.e., it is not possible to obtain a metal structure in which martensite is uniformly dispersed in both micro- and macro-regions. In this case, it is not possible to sufficiently suppress the occurrence of ghost lines, etc., and the appearance after forming is deteriorated.

[0083] [Pickling Step] Next, the obtained hot-rolled steel sheet is pickled to remove oxide scale formed on the surface of the hot-rolled steel sheet. Pickling may be carried out under conditions suitable for removing oxide scale, and may be carried out once or in multiple steps to ensure complete removal of oxide scale.

[0084] [Cold Rolling Process] The pickled hot-rolled steel sheet is cold-rolled at a reduction ratio of 20 to 90% in the cold rolling process. By setting the cold rolling reduction ratio to 20% or more, the shape of the cold-rolled steel sheet can be kept flat, and a decrease in ductility in the final product can be suppressed. On the other hand, by setting the cold rolling reduction ratio to 90% or less, it is possible to prevent the rolling load from becoming excessively large, making rolling difficult. The number of rolling passes and the reduction ratio per pass are not particularly limited, and may be appropriately set so that the reduction ratio of the entire cold rolling is within the above range.

[0085] [Primary annealing step] The obtained cold-rolled steel sheet is subjected to primary annealing in the subsequent primary annealing step. Specifically, the primary annealing step includes heating the cold-rolled steel sheet to a temperature of Ac3+10°C or higher, then cooling it at an average cooling rate CR of 30 to 200°C / sec to a cooling stop temperature of 350°C or lower, and holding it at a temperature T1 of 600°C or lower, wherein CR, T1, and an index X represented by the following formula 1 or 2 satisfy the following formula 3. When [B]≧0.0005%, Index X=0.23+0.5[Mo]+[Ti]+5[Nb]...Equation 1 When [B]<0.0005%, Index X=0.2+0.3[Mo]+[Ti]+3[Nb]...Equation 2 (5CR+T1) / Index X≧1400...Equation 3 Here, [B], [Mo], [Ti], and [Nb] are the contents [mass%] of each element, and are 0% when the element is not contained.

[0086] The Ac3 point (°C) is determined by cutting a small piece from a cold-rolled steel sheet and measuring the thermal expansion of the piece during heating from room temperature to 1000°C at 10°C / s. Heating the cold-rolled steel sheet to a temperature of Ac3+10°C or higher promotes austenitization, and then appropriately cooling it, i.e., cooling it at an average cooling rate CR of 30 to 200°C / s to a cooling stop temperature of 350°C or lower, ensures that the metal structure in the cooled steel sheet is composed of a structure primarily composed of bainite and / or martensite, for example, full bainite or full martensite. Here, a structure primarily composed of bainite and / or martensite refers to a structure containing at least one of bainite and martensite in a total area fraction of 90% or more. Full bainite refers to a structure composed of 100% bainite by area, and full martensite refers to a structure composed of 100% martensite by area. A bainite and / or martensite structure has many different interfaces within it compared to structures such as ferrite. Therefore, by forming the metallographic structure of a steel sheet before the secondary annealing process, i.e., the final annealing process, with a structure mainly composed of bainite and / or martensite, it is possible to generate a large number of dispersed carbides that can serve as austenite nucleation sites at these interfaces during heating of such a metallographic structure in the secondary annealing. As a result, austenite is finely and uniformly generated throughout the steel sheet from such dispersed nucleation sites, and then martensite is generated from this austenite. In the metallographic structure obtained after the secondary annealing, the average particle spacing of martensite is controlled to 2.5 μm or less, and the standard deviation of the area fraction of martensite in the directions perpendicular to the rolling direction and the sheet thickness direction is controlled to 1.5% or less. In other words, it is possible to achieve a metallographic structure in which martensite is uniformly dispersed in both microregions and macroregions.

[0087] If the heating temperature in the primary annealing step is lower than Ac3+10°C, austenitization will be insufficient, and the metallographic structure in the steel sheet will not be composed mainly of bainite and / or martensite even after subsequent cooling. That is, the total area fraction of bainite and martensite will not be 90% or more. As a result, in the final metallographic structure, it will be impossible to control the average grain spacing of martensite to 2.5 μm or less, and / or the standard deviation of the area fraction of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction to 1.5% or less. In this case, the occurrence of ghost lines and the like will not be sufficiently suppressed, resulting in poor appearance after forming. On the other hand, because heating at higher temperatures reduces productivity, it is preferable that the heating temperature in the primary annealing step be 1050°C or less. The holding time at the above heating temperature is preferably 10 to 500 seconds.

[0088] In the primary annealing step, in addition to the heating of the cold-rolled steel sheet, the subsequent cooling and holding are also important. That is, the cold-rolled steel sheet is cooled to a cooling stop temperature of 350°C or less at an average cooling rate CR of 30 to 200°C / s, reheated as necessary, and then held at a temperature T1 of 600°C or less. It is also important to control CR, T1, and the index X expressed by the above formula 1 or 2 so that they satisfy the above formula 3. The first stage of cooling reliably transforms the metal structure in the steel sheet into a structure mainly composed of bainite and / or martensite, and the second stage of holding at least partially tempers these structures, thereby reducing the dislocation density in the structure. As a result, in the finally obtained metal structure, the ratio A / B (the ratio A of the region in ferrite where the KAM value is less than 0.5° and the ratio B of the region in ferrite where the KAM value is 1.0° or more) can be controlled to 0.60 or more, thereby sufficiently reducing the dislocation density in ferrite and significantly improving the balance between strength and formability.

[0089] More specifically, Mo, Ti, and Nb have the effect of suppressing dislocation recovery and recrystallization, and these effects are enhanced by combination with B. Therefore, in order to reduce the dislocation density in ferrite in the final structure, it is necessary to appropriately determine the average cooling rate CR and the holding temperature T1 after heating in the first annealing step while taking into account the effects of Mo, Ti, Nb, and B. The inventors therefore investigated the effects of these elements, the average cooling rate CR, and the holding temperature T1 on the dislocation density in ferrite in the final structure. As a result, the inventors found that the dislocation density in the metal structure after the first annealing step can be sufficiently reduced by using an index X calculated by the following formula 1 or 2 depending on the B content and selecting the average cooling rate CR and the holding temperature T1 in the first annealing step so that the value calculated by the left side of the following formula 3 is 1400 or more. In this regard, the inventors have found that the dislocation density in ferrite in the final structure can be reduced to a level where the ratio A / B of the KAM value is 0.60 or more. When [B] ≧ 0.0005%, Index X = 0.23 + 0.5[Mo] + [Ti] + 5[Nb] ... Equation 1 When [B] < 0.0005%, Index X = 0.2 + 0.3[Mo] + [Ti] + 3[Nb] ... Equation 2 (5CR + T1) / Index X ≧ 1400 ... Equation 3 Here, [B], [Mo], [Ti], and [Nb] are the contents [mass %] of each element, and are 0% when the element is not contained.

[0090] From the above formulas 1 to 3, it can be seen that Nb has the strongest effect of suppressing dislocation recovery, followed by Ti, and Mo has the weakest effect. In any case, from the above formulas 1 to 3, as the contents of Mo, Ti, and Nb increase, the value of index B increases, and dislocation recovery is delayed. In this case, it can be seen that it is necessary to increase CR and T1 in order to reduce dislocation density. In relation to this, for example, if the value of the left side of formula 3, i.e., (5CR + T1) / index X, is less than 1400, the dislocation density in the metal structure after the primary annealing process cannot be sufficiently reduced. As a result, in the finally obtained metal structure, it becomes impossible to control the ratio A / B, where A is the proportion of the region in ferrite where the KAM value is less than 0.5° and B is the proportion of the region in ferrite where the KAM value is 1.0° or more, to 0.60 or more. From the viewpoint of further improving the balance between strength and formability, the higher the value of (5CR + T1) / index X, the more preferable, for example, 1500 or more is preferable. The upper limit is not particularly limited, but for example, the value of (5CR + T1) / index X may be 3000 or less or 2800 or less. The average cooling rate CR is set to 30°C / s or more to ensure that the metal structure in the steel sheet after cooling is composed mainly of bainite and / or martensite. From this perspective, a higher average cooling rate CR is preferable from the viewpoint of increasing the value of (5CR + T1) / index X. However, if CR becomes too high, the shape of the steel sheet may deteriorate. Therefore, CR is set to 200°C / s or less. Similarly, from the viewpoint of increasing the value of (5CR + T1) / index X, a higher holding temperature T1 is preferable. However, if T1 becomes too high, it may not be possible to achieve a metal structure in which martensite is uniformly dispersed in both micro- and macro-regions after secondary annealing. Therefore, T1 is set to 600°C or less. The holding time at T1 is not particularly limited, but is preferably, for example, 50 to 600 seconds.

[0091] [Secondary annealing step (final annealing step)] The cold-rolled steel sheet after the primary annealing is heated again in the subsequent secondary annealing step. Specifically, the secondary annealing step includes heating the cold-rolled steel sheet to a maximum heating temperature T2 of Ac1 to (Ac3-10)°C at an average heating rate HR°C / sec, and then holding the temperature at the maximum heating temperature T2 for 10 to 500 seconds, wherein HR, T2, and an index X represented by the following formula 1 or 2 satisfy the following formula 4: When [B]≧0.0005%, Index X=0.23+0.5[Mo]+[Ti]+5[Nb] Formula 1 When [B]<0.0005%, Index X=0.2+0.3[Mo]+[Ti]+3[Nb] Formula 2 (1000 / HR+T2) / Index X≧3200 Formula 4 Here, [B], [Mo], [Ti], and [Nb] are the contents [mass%] of each element, and are 0% when the element is not contained.

[0092] As with the Ac3 point, the Ac1 point (°C) is determined by cutting a small piece from a cold-rolled steel sheet and measuring the thermal expansion of the small piece during heating from room temperature to 1,000°C at a rate of 10°C / second. First, the steel sheet after primary cooling is heated to a maximum heating temperature T2 of Ac1 to (Ac3-10)°C, whereby carbides can be generated and dispersed on many interfaces contained within bainite and / or martensite in the metal structure. Next, by holding the steel sheet at a maximum heating temperature T2 of Ac1 to (Ac3-10)°C, which corresponds to the two-phase region of ferrite and austenite, for 10 to 500 seconds, austenite can be generated finely and uniformly from the carbides throughout the steel sheet while maintaining the state in which the carbides are dispersed on the interfaces. Finally, by appropriately cooling the steel sheet, for example, by cooling at an average cooling rate of 10°C / sec or more in a temperature range up to 500°C, martensite can be appropriately generated from the finely dispersed austenite, and as a result, the average particle spacing of martensite is controlled to 2.5 µm or less, and the standard deviation of the area fraction of martensite in the direction perpendicular to the rolling direction and the plate thickness direction is controlled to 1.5% or less. In other words, it is possible to achieve a metal structure in which martensite is uniformly dispersed in both micro- and macro-regions.

[0093] If the heating temperature in the secondary annealing step is less than Ac1°C or the holding time is less than 10 seconds, the desired metal structure as described above cannot be obtained. On the other hand, if the heating temperature exceeds (Ac3-10)°C, the austenite grains will coarsen, and furthermore, due to the high temperature, it will be impossible to maintain the state in which carbides are dispersed at the interfaces. As a result, the average aspect ratio of martensite in the final metal structure will be small, and / or it will be impossible to achieve uniform dispersion of martensite in both the microregion and the macroregion. Furthermore, if the holding time exceeds 500 seconds, the austenite grains will also coarsen, and the martensite grains obtained by subsequent cooling will also be relatively coarse. In such cases, similarly, the average aspect ratio of martensite will be small, and / or it will be impossible to achieve uniform dispersion of martensite in both the microregion and the macroregion.

[0094] In the secondary annealing step, in addition to the above-described controls, it is also important to control the average heating rate HR, the maximum heating temperature T2, and the index X represented by the above formula 1 or 2 so as to satisfy the above formula 4. Such control can promote recrystallization of the metal structure, thereby making it possible to reduce the dislocation density in the metal structure. As a result, in the finally obtained metal structure, the ratio A / B of the proportion A of the region in ferrite where the KAM value is less than 0.5° to the proportion B of the region in ferrite where the KAM value is 1.0° or more can be controlled to 0.60 or more, thereby sufficiently reducing the dislocation density in the metal structure, particularly the dislocation density in ferrite, and making it possible to significantly improve the balance between strength and formability.

[0095] More specifically, as described above, Mo, Ti, and Nb have the effect of suppressing dislocation recovery and recrystallization, and these effects are enhanced by combination with B. Therefore, in order to reduce the dislocation density in ferrite in the final structure, it is necessary to appropriately determine the average heating rate HR and maximum heating temperature T2 during heating in the secondary annealing step while taking into account the effects of Mo, Ti, Nb, and B. The inventors therefore investigated the effects of these elements, the average heating rate HR, and the maximum heating temperature T2 on the dislocation density in ferrite in the final structure. As a result, the inventors found that recrystallization of the metallic structure can be promoted by using an index X calculated by the following formula 1 or 2 depending on the B content and selecting the average heating rate HR and maximum heating temperature T2 in the secondary annealing step so that the value calculated by the left side of the following formula 4 is 3200 or more. In this regard, the inventors found that the dislocation density in ferrite in the final structure can be reduced to a level at which the ratio A / B of the KAM value is 0.60 or more. When [B]≧0.0005%, Index X=0.23+0.5[Mo]+[Ti]+5[Nb] Formula 1 When [B]<0.0005%, Index X=0.2+0.3[Mo]+[Ti]+3[Nb] Formula 2 (1000 / HR+T2) / Index X≧3200 Formula 4 Here, [B], [Mo], [Ti], and [Nb] are the contents [mass%] of each element, and are 0% when the element is not contained.

[0096] From the above formulas 1, 2, and 4, it can be seen that Nb has the strongest effect of suppressing recrystallization, followed by Ti, and Mo has the weakest effect. In any case, from the above formulas 1, 2, and 4, it can be seen that as the contents of Mo, Ti, and Nb increase, the value of index B increases, and recrystallization is suppressed. In this case, it can be seen that in order to promote recrystallization and reduce dislocation density, it is necessary to slow down HR and / or increase T2. In this regard, for example, if the value of the left side of formula 4, i.e., (1000 / HR + T2) / index X, is less than 3200, it becomes impossible to control the ratio A / B, where A is the proportion of the ferrite region where the KAM value is less than 0.5° and B is the proportion of the ferrite region where the KAM value is 1.0° or more, to 0.60 or more in the final metallographic structure. In this manufacturing method, it is important to appropriately reduce the dislocation density in the metallographic structure in both the first annealing step and the second annealing step. For example, if the dislocation density is not sufficiently reduced in the first annealing step, even if recrystallization is promoted in the second annealing step, the dislocation density in ferrite in the final structure may not be reduced to the desired level. In such cases, the ratio A / B, which is the ratio A of the region in ferrite where the KAM value is less than 0.5° to the ratio B of the region in ferrite where the KAM value is 1.0° or more, cannot be controlled to 0.60 or more, and as a result, the balance between strength and formability cannot be sufficiently improved. From the viewpoint of further improving the balance between strength and formability, the higher the value of (1000 / HR+T2) / index X, the more preferable it is, for example, 3300 or more. The upper limit is not particularly limited, but the value of (1000 / HR+T2) / index X may be 6000 or less or 5500 or less. Furthermore, in order to increase the value of (1000 / HR+T2) / index X, the slower the average heating rate HR, the more preferable it is. However, if the HR is too slow, productivity decreases, so it is preferable to appropriately select the HR within the range of, for example, 1.0 to 30.0° C. / sec.

[0097] [Plating Step] For the purpose of improving corrosion resistance, etc., the surface of the obtained cold-rolled steel sheet may be subjected to a plating treatment, if necessary. The plating treatment may be a treatment such as hot-dip plating, alloying hot-dip plating, or electroplating. For example, the steel sheet may be subjected to hot-dip galvanizing treatment as the plating treatment, or the hot-dip galvanizing treatment may be followed by an alloying treatment. The specific conditions for the plating treatment and the alloying treatment are not particularly limited and may be any appropriate conditions known to those skilled in the art. For example, the alloying temperature may be 450 to 600°C.

[0098] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples in any way.

[0099] First, a slab having the chemical composition shown in Table 1 and a thickness of 200 to 300 mm was cast by a continuous casting method. The balance other than the components shown in Table 1 was Fe and impurities. Next, the obtained slab was heated to 1230°C and then hot rolled. Hot rolling was performed by rough rolling and finish rolling, with the finish rolling temperature being 910°C and the coiling temperature being 550°C. Next, the obtained hot-rolled steel sheet was pickled and then cold-rolled at a reduction ratio of 84% to obtain a cold-rolled steel sheet having a thickness of 0.4 mm.

[0100] Next, the obtained cold-rolled steel sheet was subjected to primary annealing. Specifically, the cold-rolled steel sheet was heated to a temperature of (Ac3+10)°C and held there for 100 seconds, then cooled to a cooling stop temperature of 250°C at an average cooling rate CR shown in Table 2, reheated as necessary, and then held at a temperature T1 shown in Table 2 for 200 seconds. The cold-rolled steel sheet after the primary annealing was subjected to secondary annealing. Specifically, the cold-rolled steel sheet was heated to a maximum heating temperature T2 also shown in Table 2 at an average heating rate HR shown in Table 2, then held at that heating temperature for 100 seconds, and further cooled to 500°C at an average cooling rate of 10°C. Finally, the surface of the obtained cold-rolled steel sheet was subjected to a plating treatment as necessary to appropriately form a hot-dip galvanized layer (GI), a galvannealed layer (GA), or an electrogalvanized layer (EG).

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[0104] The properties of the obtained steel sheets were measured and evaluated by the following methods.

[0105] [Tensile strength (TS), total elongation (El), and product of tensile strength and total elongation (TS × El)] Tensile strength (TS) and total elongation (El) were measured by taking a No. 5 tensile test piece of JIS Z2241:2022 from the steel plate, with the longitudinal direction being perpendicular to the rolling direction and the plate thickness direction, and conducting a tensile test in accordance with JIS Z2241:2022. TS × El was calculated based on the obtained TS and El.

[0106] [Post-Forming Appearance] The post-forming appearance was evaluated using a press-formed part obtained by press-forming a steel plate blanked to a 600 mm square so that the radius of curvature R at the center was 1200 mm. A strain of 2.5% was imparted to the press-formed part. The surface of the center of the press-formed part was ground with a grindstone in a direction perpendicular to the rolling direction of the steel plate, and the surface was observed. Linear streaks that appeared on the surface and extended approximately parallel to the rolling direction were judged to be ghost lines, and the degree of occurrence of the streaks was scored from 1 to 5. A random 100 mm x 100 mm area in the center of the pressed part was visually inspected, and the evaluation scores were assigned as follows: "1" if no streaks were observed; "2" if the maximum length of the streaks was 20 mm or less; "3" if the maximum length of the streaks was more than 20 mm but not more than 50 mm; "4" if the maximum length of the streaks was more than 50 mm but not more than 70 mm; and "5" if the maximum length of the streaks was more than 70 mm. A rating of "3" or less was deemed to be excellent in appearance after forming and thus passed. On the other hand, a rating of "4" or more was deemed to be poor in appearance after forming and therefore failed. In this test, the appearance after forming was evaluated using a pressed part simulating a door outer. However, a formed part that can be estimated to have been subjected to 2.5% strain by press forming may also be evaluated, or a test specimen taken from a steel plate to which a 2.5% pre-strain was similarly applied may also be evaluated. Similar evaluations can be performed using such test methods. In the case of test pieces taken from steel plates, JIS No. 5 test pieces with the longitudinal direction perpendicular to the rolling direction and the plate thickness direction and pre-strained by 2.5% can be evaluated.

[0107] Steel sheets with TS of 540 MPa or more, TS×El of 14,000 MPa·% or more, and an appearance after forming rating of 3 or less were evaluated as steel sheets that could achieve both strength, formability, and appearance after forming. The results are shown in Table 2. In Table 2, "KAM value ratio A / B" means "the ratio A / B of the ratio A of the region in ferrite where the KAM value is less than 0.5° in EBSD measurement to the ratio B of the region in ferrite where the KAM value is 1.0° or more."

[0108] With reference to Tables 1 and 2, it is believed that in Comparative Example 4, the dislocation density in the metal structure could not be sufficiently reduced because Equation 3 was not satisfied in the first annealing step. As a result, the ratio A / B related to the KAM value of ferrite in the final structure was less than 0.60, and TS×El decreased. In Comparative Examples 5, 15, 36, and 37, it is believed that the dislocation density in the metal structure could not be sufficiently reduced because Equation 4 was not satisfied in the second annealing step. As a result, the ratio A / B related to the KAM value of ferrite in the final structure was similarly less than 0.60, and TS×El decreased. In Comparative Examples 16 and 22, it is believed that the dislocation density in the metal structure could not be sufficiently reduced because Equation 3 and 4 were not satisfied in the first annealing step and the second annealing step. As a result, the ratio A / B related to the KAM value of ferrite in the final structure was similarly less than 0.60, and TS×El decreased. In Comparative Examples 6, 17, and 23, the maximum heating temperature T2 in the secondary annealing step was higher than (Ac3-10)°C, which is thought to have caused the austenite grains to become coarse and, furthermore, made it impossible to maintain the state in which carbides were dispersed on the interfaces due to the high temperature. As a result, it was not possible to achieve uniform dispersion of martensite in both the microregion and the macroregion in the final structure, and the appearance after forming was deteriorated.

[0109] In Comparative Examples 30 and 31, the high C or Mn content resulted in a standard deviation of the martensite area fraction in the direction perpendicular to the rolling direction and the thickness direction exceeding 1.5%, resulting in poor post-forming appearance. In Comparative Example 32, the high Si content prevented proper removal of scale formed by hot rolling. As a result, the pre-forming appearance was poor, and post-forming appearance was not evaluated. In Comparative Examples 33 and 34, the low C or Mn content prevented sufficient strength. In particular, in Comparative Example 34, the martensite area fraction was 0%, resulting in uneven elongation during press forming, resulting in the formation of a streaky pattern known as stretcher strain (SS), and therefore poor post-forming appearance. In Comparative Example 35, the low total area fraction of ferrite and martensite resulted in the formation of a relatively large residual structure. As a result, similarly uneven elongation during press forming resulted in stretcher strain (SS), and therefore poor post-forming appearance.

[0110] In contrast, all of the steel sheets according to the examples of the present invention had a predetermined chemical composition and furthermore, by appropriately controlling the proportions of ferrite and martensite in the metallographic structure, a TS of 540 MPa or more was achieved, and the average grain spacing of martensite was controlled to 2.5 μm or less in the microscopic region, while the standard deviation of the area fraction of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction was controlled to 1.5% or less in the macroscopic region. This suppressed the generation of minute irregularities on the steel sheet surface and significantly suppressed the occurrence of ghost lines, even when strain was imparted by press forming. In addition, in all of the steel sheets according to the examples of the present invention, by controlling the ratio A / B of the proportion A of the ferrite regions having a KAM value of less than 0.5° to the proportion B of the ferrite regions having a KAM value of 1.0° or more in EBSD measurement to be 0.60 or more, the elongation of the steel sheet could be improved despite the high strength of the TS of 540 MPa or more, and therefore the balance between strength and formability could be significantly improved. When the cross-sections of the metal structures of the cold-rolled steel sheets before secondary annealing according to all of the invention examples were observed, they were all composed of martensite with an area ratio of 90% or more.

Claims

1. By mass percentage, C: 0.030 to 0.100%, Mn: 1.00 to 2.80%, Si: 0.005 to 1.500%, Al: 1.000% or less, P: 0.100% or less, S: 0.0200% or less, N: 0.0150% or less, O: 0.0100% or less, Cr: 0 to 1.00%, Mo: 0 to 0.80%, B: 0 to 0.0100%, Ti: 0 to 0.200%, Nb: 0 to 0.200%, V: 0 to 0.500%, Ni: 0 to 1.00%, Cu: 0 to 1.00%, W: 0 to 1.00%, Ta: 0 to 0.10%, Co: 0 to 3.00%, Sn: 0 to 1.00%, Sb: 0 to 0.200%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, REM: 0 to 0.0100%, Bi: 0 to 0.0500%, As: 0 to 0.10%, and the balance consists of Fe and impurities, having a chemical composition; by area percentage, ferrite: 75 to 95%, and martensite: 5 to 25%, and the total of ferrite and martensite is 90% or more, the average interparticle spacing of martensite is 2.5 μm or less, the standard deviation of the area ratio of martensite in the direction perpendicular to the rolling direction and the plate thickness direction is 1.5% or less, and in the EBSD measurement, the ratio A / B of the ratio A of the region where the KAM value is less than 0.5° in ferrite to the ratio B of the region where the KAM value is 1.0° or more in ferrite is 0.60 or more, and having a metallographic structure, a steel sheet characterized by the above.

2. The chemical composition, in mass %, is as follows: Cr: 0.001 to 1.00%, Mo: 0.001 to 0.80%, B: 0.0001 to 0.0100%, Ti: 0.001 to 0.200%, Nb: 0.001 to 0.200%, V: 0.001 to 0.500%, Ni: 0.001 to 1.00%, Cu: 0.001 to 1.00%, W: 0.001 to 1.00%, Ta: 0.001 to 0.10%, Co: 0.001 to 3.00%, Sn: 0.001 to 1.00%, Sb: 0.001 to 0.200%, Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0100%, Zr: 0.0001 to 0.0100%, REM: 0.0001 to 0.0100%, Bi: 0.0001 to 0.0500%, and As: 0.001 to 0.10%, and the steel sheet according to claim 1 is characterized by containing at least one of them.

3. The average crystal grain size of the ferrite is 3.0 to 25.0 μm, the average crystal grain size of the martensite is 1.0 to 5.0 μm, and the average aspect ratio of the martensite is 2.5 or more. The steel sheet according to claim 1 or 2 is characterized by this.

4. An outer plate member including the steel sheet according to any one of claims 1 to 3.

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