Steel sheet and outer sheet member

Abstract

Provided is a steel sheet characterized by having a predetermined chemical composition and a metallographic structure, wherein, in terms of area fraction, the structure comprises 70% to 95% of bainite and 3% to 20% of martensite, the sum of bainite and martensite is 90% or above, the average intragranular misorientation of crystal grains surrounded by grain boundaries with a crystal orientation difference of 15° or above is 0.60° or below, and the proportion of crystal grains containing two or more martensite grains therein is 90% or above.

Description

Steel plates and outer panel components

[0001] The present invention relates to a steel plate and an outer panel member including the same.

[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 workability and a good appearance after work in a steel sheet for hot-dip galvanizing having a tensile strength of 390 MPa or more. 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] To achieve the above object, the inventors conducted research, focusing particularly on the metallographic structure of steel sheets. Specifically, the inventors discovered that by forming the metallographic structure of a steel sheet having a predetermined chemical composition from a structure mainly composed of bainite, rather than a structure mainly composed of ferrite as in conventional DP steels, martensite can be dispersed within predetermined crystal grains during the process of forming the structure, and the mean misorientation within the crystal grains can be reduced. As a result, the inventors discovered that the steel sheet can achieve the desired high strength and formability, and that even when strain is imparted by press forming or the like, the generation of minute irregularities on the steel sheet surface is significantly suppressed, thereby completing the present invention.

[0009] The present invention, which has achieved the above object, is as follows. (1) In mass%, C: 0.03 to 0.08%, Si: 0.10 to 1.50%, Mn: 0.50 to 3.00%, P: 0.1000% or less, S: 0.0200% or less, Al: 1.000% or less, N: 0.0200% or less, O: 0 to 0.020%, Cr: 0 to 2.000%, Mo: 0 to 1.000%, Ti: 0 to 0.500%, Nb: 0 to 0.500%, B: 0 to 0.0100%, Cu: 0 to 1.000%, Ni: 0 to 1.00%, W: 0 to 0.100%, V: 0 to 1.000%, Ta: 0 to 0.100%, A steel sheet having a chemical composition consisting of Co: 0 to 3.000%, Sn: 0 to 1.000%, Sb: 0 to 0.500%, As: 0 to 0.050%, Mg: 0 to 0.050%, Zr: 0 to 0.050%, Ca: 0 to 0.0500%, Y: 0 to 0.0500%, La: 0 to 0.0500%, Ce: 0 to 0.0500%, Bi: 0 to 0.0500%, and the balance: Fe and impurities, and consisting of, in area percentages, bainite: 70 to 95%, martensite: 3 to 20%, and at least one of ferrite, pearlite, and retained austenite: 0 to 10% in total, A steel sheet characterized by having a metal structure in which the average misorientation within crystal grains surrounded by grain boundaries with a crystal orientation misorientation of 15° or more is 0.60° or less, and the proportion of crystal grains containing two or more martensite grains within the crystal grains is 90% or more.(2) The chemical composition is, in mass%, Cr: 0.001 to 2.000%, Mo: 0.001 to 1.000%, Ti: 0.001 to 0.500%, Nb: 0.001 to 0.500%, B: 0.0001 to 0.0100%, Cu: 0.001 to 1.000%, Ni: 0.001 to 1.00%, W: 0.001 to 0.100%, V: 0.001 to 1.000%, Ta: 0.001 to 0.100%, Co: 0.001 to 3.000%, Sn: 0.001 to 1.000%, Sb: 0.001 to 0.500%, The steel sheet according to (1) above, characterized in that it contains at least one of As: 0.001 to 0.050%, Mg: 0.0001 to 0.050%, Zr: 0.0001 to 0.050%, Ca: 0.0001 to 0.0500%, Y: 0.0001 to 0.0500%, La: 0.0001 to 0.0500%, Ce: 0.0001 to 0.0500%, and Bi: 0.0001 to 0.0500%. (3) An outer panel member comprising the steel sheet according to (1) or (2) 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] 1 is a schematic diagram showing the morphology of bainite for each temperature range in which bainite is generated in an annealing process. FIG. 2 is a schematic diagram showing the metal structure of a steel plate according to an embodiment of the present invention.

[0012] <Steel Sheet> A steel sheet according to an embodiment of the present invention has, in mass %, C: 0.03 to 0.08%, Si: 0.10 to 1.50%, Mn: 0.50 to 3.00%, P: 0.1000% or less, S: 0.0200% or less, Al: 1.000% or less, N: 0.0200% or less, O: 0 to 0.020%, Cr: 0 to 2.000%, Mo: 0 to 1.000%, Ti: 0 to 0.500%, Nb: 0 to 0.500%, B: 0 to 0.0100%, Cu: 0 to 1.000%, Ni: 0 to 1.00%, W: 0 to 0.100%, V: 0 to 1.000%, Ta: 0 to 0.100%, A steel sheet having a chemical composition consisting of Co: 0 to 3.000%, Sn: 0 to 1.000%, Sb: 0 to 0.500%, As: 0 to 0.050%, Mg: 0 to 0.050%, Zr: 0 to 0.050%, Ca: 0 to 0.0500%, Y: 0 to 0.0500%, La: 0 to 0.0500%, Ce: 0 to 0.0500%, Bi: 0 to 0.0500%, and the balance: Fe and impurities, and consisting of, in area percentages, bainite: 70 to 95%, martensite: 3 to 20%, and at least one of ferrite, pearlite, and retained austenite: 0 to 10% in total, The metal structure is characterized in that the average misorientation within crystal grains surrounded by grain boundaries with a crystal orientation misorientation of 15° or more is 0.60° or less, and the proportion of crystal grains containing two or more martensite grains within the crystal grains is 90% or more.

[0013] Dual-phase steel (DP steel), which has a relatively low yield strength, is often used for exterior panel components such as doors and hoods to avoid surface defects known as surface distortions that occur during press forming. However, DP steel, which contains a mixture of a soft ferrite structure and a hard martensite structure, is prone to nonuniform deformation, in which the soft structure and its surrounding area deform preferentially during processing such as press forming. This can lead to minute irregularities on the steel sheet surface after forming, resulting in appearance defects known as ghost lines. More specifically, during processing such as press forming, the soft ferrite structure undergoes large depressions, while the hard martensite structure undergoes small depressions. 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 band-like ghost lines. Meanwhile, as steel sheets become stronger, elements such as Mn are sometimes added in relatively large amounts to improve their hardenability. Mn is an element that easily segregates in the form of stripes 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 the form of stripes. Therefore, due to this Mn segregation, regions with high and low hardenability are present in the steel sheet. As a result, relatively large amounts 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. On the other hand, 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, the formation of microscopic irregularities on the steel sheet surface can be sufficiently reduced, and it is believed that the occurrence of ghost lines can be suppressed. However, with the demand for higher strength, particularly when the amount of Mn added to steel sheets increases, it is actually very difficult to reliably and sufficiently suppress Mn segregation. In addition, as strength increases, formability itself also decreases, and it is generally very difficult to achieve both strength and formability, as well as good appearance after forming.

[0014] Therefore, the present inventors first optimized the chemical composition of the steel sheet and conducted studies, focusing particularly on the metallographic structure of the steel sheet. Specifically, the present inventors investigated the possibility of forming a metallographic structure of a steel sheet having a predetermined chemical composition with a structure mainly composed of bainite, rather than a structure mainly composed of ferrite as in conventional DP steel. 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 annealing a steel sheet containing a relatively large amount of Si at a relatively high temperature for a predetermined time after cold rolling and then appropriately cooling it, a structure mainly composed of bainite can be formed and martensite can be dispersed and present within predetermined crystal grains. More specifically, among predetermined crystal grains surrounded by grain boundaries (high-angle grain boundaries) with a crystal orientation difference of 15° or more, the proportion of crystal grains containing two or more martensite grains within the grains can be increased to 90% or more. The inventors have discovered that by forming a metal structure primarily composed of bainite, which is harder than ferrite, and further comprising martensite dispersed within the crystal grains, high strength, for example, a tensile strength of 400 MPa or more, can be achieved, and the deformation amount of the steel sheet can be made more uniform, particularly in the width direction, during forming such as press forming, thereby achieving an excellent post-forming appearance in which appearance defects such as ghost lines are significantly suppressed. In addition, the inventors have discovered that the crystal grains obtained as described above can have a relatively small average misorientation within the grains, and that by appropriately selecting the conditions for the annealing process, in particular, the average misorientation can be controlled to 0.60° or less. A small average misorientation within the grains generally means that the strain applied to the crystal grains is small, and therefore means that the dislocation density within the crystal grains is low. Therefore, unlike conventional DP steel, the steel sheet according to the embodiment of the present invention has a metal structure that is mainly composed of bainite, which is harder than ferrite. However, by controlling the average misorientation within crystal grains surrounded by grain boundaries with a crystal misorientation of 15° or more to 0.60° or less, the dislocation density within the grains is reduced, increasing the elongation of the steel sheet, and thereby making it possible to significantly improve the formability of the steel sheet.

[0015] Without intending to be bound by any particular theory, it is believed that in order to have the metallographic structure of the finally obtained steel sheet be mainly composed of bainite as described above, with martensite dispersed within predetermined crystal grains, and further to sufficiently reduce the average misorientation within the crystal grains, it is extremely important to optimize the chemical composition of the steel sheet and to appropriately control the heat treatment conditions in the annealing step. This will be explained in more detail below with reference to the drawings.

[0016] In the production of steel sheets, for example, a desired metal structure is formed by heating a cold-rolled steel sheet to a two-phase region of ferrite and austenite or a single-phase region of austenite in a subsequent annealing process, followed by appropriate cooling. Therefore, when forming a specific metal structure primarily composed of bainite, it is extremely important to optimize the chemical composition of the steel sheet and to appropriately control the heat treatment in the annealing process according to the chemical composition. FIG. 1 is a schematic diagram showing the morphology of bainite for each temperature range in which bainite forms during the annealing process. More specifically, it is a schematic diagram showing the morphology of bainite for each temperature range in which bainite forms after heating to a temperature, such as the single-phase region, during the annealing process. In this regard, FIG. 1( a) shows the initial state of bainite formation in a temperature range of 500 to 600°C, FIG. 1( b) shows the initial state of bainite formation in a temperature range of 450 to less than 500°C, and FIG. 1( c) shows the initial state of bainite formation in a temperature range less than 450°C. First, bainite is generally known to be a structure composed of ferrite, also known as bainitic ferrite, and fine carbides such as cementite. Referring to FIG. 1 , for example, when a steel sheet is heated to the austenite single-phase region above the Ac3 point and then cooled to a temperature range where bainite forms, bainitic ferrite 2 forms from austenite grain boundaries 1. As shown in FIG. 1( a), in the relatively high temperature range of 500 to 600°C, untransformed austenite 3 exists between the bainitic ferrite 2 formed from the austenite grain boundaries 1 and the bainitic ferrite 2. It can be seen that no carbides such as cementite precipitate between the bainitic ferrite 2 and the bainitic ferrite 2 or within the bainitic ferrite 2 itself. This is thought to be due to the addition of a relatively large amount of Si, specifically 0.10 mass% or more, which has the effect of delaying carbide formation, to the steel sheet in addition to the relatively high temperature range of 500 to 600°C.On the other hand, as shown in Figure 1(b), in the temperature range of 450 to less than 500°C, it is seen that a large amount of carbides 4 such as cementite precipitates between the bainitic ferrite 2 formed from the austenite grain boundaries 1 and between the bainitic ferrite 2, and therefore much of the untransformed austenite 3 that could be transformed into martensite is consumed by the precipitation of the carbides 4. Furthermore, as shown in Figure 1(c), in the relatively low temperature range of less than 450°C, it is seen that not only do carbides 4 such as cementite precipitate between the bainitic ferrite 2 formed from the austenite grain boundaries 1, but also many carbides 4 precipitate within the bainitic ferrite 2 itself.

[0017] In FIG. 1( a ), by retaining the steel sheet at a relatively high temperature range of 500 to 600°C for a predetermined time, it is possible to promote the formation of bainitic ferrite 2 and reduce the misorientation of each bainitic ferrite 2. In addition, in FIG. 1( a ), unlike the cases of FIGS. 1( b ) and 1 ( c ), carbides 4 are not formed or their formation is sufficiently suppressed. Therefore, carbon gradually concentrates in the untransformed austenite 3 present between the bainitic ferrite 2 due to retention at the relatively high temperature range. It is also believed that the concentration of carbon improves hardenability. Therefore, by subsequently performing appropriate cooling, not only is martensite formed on the austenite grain boundaries 1, but also the untransformed austenite 3 present between the bainitic ferrite 2 and where carbon is concentrated can be transformed into martensite. This makes it possible to obtain a metal structure that is primarily bainite with martensite dispersed within the bainite. In addition, as described above, it is believed that the dwell operation for a predetermined time at a relatively high temperature range of 500 to 600°C reduces the misorientation between the bainitic ferrite grains 2, while maintaining the misorientation at the austenite grain boundaries 1. As a result, in the finally obtained metallographic structure, it is possible to generate crystal grains containing multiple martensite grains, i.e., two or more martensite grains, at a rate of 90% or more relative to the total crystal grains, which are surrounded by grain boundaries with a crystallographic misorientation of 15° or more corresponding to the initial austenite grain boundaries 1, and it is also possible to make the average misorientation within the crystal grains relatively small, specifically, to control the average misorientation within the grains to 0.60° or less. Figure 2 is a schematic diagram showing the metallographic structure of a steel sheet according to an embodiment of the present invention. Referring to FIG. 2 , it can be seen that the metal structure of the steel plate according to the embodiment of the present invention is composed mainly of bainite 12, and not only do martensite grains 13 form on grain boundaries 11 indicated by thick lines and having a crystal orientation difference of 15° or more, but also two or more martensite grains 13 are formed dispersedly within the crystal grains surrounded by the grain boundaries 11, and the steel plate includes crystal grains 10 in which the average orientation difference within the grains is sufficiently reduced.Steel sheets having these characteristics achieve high strength and an excellent appearance after forming by forming a metal structure that is mainly composed of bainite with martensite dispersed inside certain crystal grains. In addition, unlike conventional DP steels, although the metal structure is mainly composed of bainite, which is harder than ferrite, by controlling the average misorientation within crystal grains surrounded by grain boundaries with a crystal misorientation of 15° or more to 0.60° or less, the dislocation density within the grains is reduced and the elongation of the steel sheet is increased, thereby making it possible to significantly improve the formability of the steel sheet.

[0018] 1(b) and 1(c), on the other hand, since a large amount of carbides 4 precipitates between the bainitic ferrite 2 and within the bainitic ferrite 2 itself, much of the untransformed austenite 3 that can be transformed to martensite is consumed by the precipitation of the carbides 4. In addition, carbon concentration is insufficient, and therefore, it is not possible to sufficiently form untransformed austenite 3 in which carbon is concentrated between the bainitic ferrite 2 and the bainitic ferrite 2. Therefore, even if a holding operation for a predetermined time is subsequently performed and then appropriate cooling is performed, it is not possible to obtain a metallographic structure containing a desired proportion of crystal grains in which martensite is highly dispersed and in which the mean intragranular orientation misorientation is sufficiently reduced, as in the cases of FIGS. 1(a) and 2. In contrast, when the heat treatment described in relation to FIG. 1(a) is performed, a metallographic structure in which martensite is dispersed within predetermined crystal grains is obtained throughout the steel sheet. Therefore, it is considered possible to uniformly distribute martensite throughout the steel sheet, regardless of the presence or absence or degree of Mn segregation. Conventionally, it has been considered common to consider controlling the distribution of hard structures from the viewpoint of reducing Mn segregation itself. Therefore, it is extremely unexpected and surprising that martensite can be uniformly dispersed in the finally obtained metal structure, regardless of the presence or absence or degree of Mn segregation.

[0019] 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.

[0020] [C: 0.03 to 0.08%] C is an element that ensures a predetermined amount of martensite and improves the strength of the steel sheet. To fully obtain this effect, the C content is set to 0.03% or more. The C content may be 0.04% or more or 0.05% or more. On the other hand, if C is contained excessively, the strength may become too high and the elongation may decrease. For this reason, the C content is set to 0.08% or less. The C content may be 0.07% or less or 0.06% or less.

[0021] [Si: 0.10 to 1.50%] Si is an element that improves the strength of steel sheet through solid solution strengthening. Si also has the effect of delaying the formation of carbides. To fully obtain these effects, the Si content is set to 0.10% or more. The Si content may be 0.15% or more, 0.20% or more, 0.30% or more, 0.50% or more, 0.60% or more, or 0.80% 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 1.50% or less. The Si content may be 1.45% or less, 1.40% or less, 1.20% or less, 1.00% or less, or 0.90% or less.

[0022] [Mn: 0.50 to 3.00%] Mn is an element that improves hardenability and contributes to improving steel sheet strength. To fully obtain this effect, the Mn content is set to 0.50% or more. The Mn content may be 0.60% or more, 0.70% or more, 1.00% or more, 1.20% or more, or 1.50% or more. On the other hand, excessive Mn content may cause excessive martensitic transformation, resulting in reduced elongation, and / or may not be able to fully counteract the effects of Mn segregation, resulting in a poor appearance after forming. For this reason, the Mn content is set to 3.00% or less. The Mn content may be 2.90% or less, 2.80% or less, 2.50% or less, 2.20% or less, or 2.00% or less.

[0023] [P: 0.1000% or less] P is an impurity element that embrittles welds and deteriorates galvanic properties. Therefore, the P content is set to 0.1000% or less. The P content may be 0.0600% or less, 0.0200% or less, 0.0150% or less, or 0.0100% 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, or 0.0005% or more.

[0024] [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, or 0.0080% 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, if the S 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 S content may be 0.0001% or more, 0.0002% or more, or 0.0005% or more.

[0025] [Al: 1.000% or less] Al is an element that 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, or 0.050% or more. On the other hand, excessive Al content may form coarse oxides, which may reduce toughness. Therefore, the Al content is set to 1.000% or less. The Al content may be 0.800% or less, 0.600% or less, or 0.300% or less.

[0026] [N: 0.0200% or less] N is an element that causes blowholes during welding. Therefore, the N content is set to 0.0200% or less. The N content may be 0.0180% or less, 0.0150% 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, reducing the N content to less than 0.0001% in practical steel sheets significantly increases production costs and is economically disadvantageous. Therefore, the N content may be 0.0001% or more, 0.0002% or more, or 0.0005% or more.

[0027] [O: 0 to 0.020%] O is an element that causes blowholes during welding. Therefore, the O content is set to 0.020% or less. The O content may be 0.018% or less, 0.015% or less, 0.010% or less, or 0.008% 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.

[0028] 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 Cr: 0-2.000%, Mo: 0-1.000%, Ti: 0-0.500%, Nb: 0-0.500%, B: 0-0.0100%, Cu: 0-1.000%, Ni: 0-1.00%, W: 0-0.100%, V: 0-1.000%, Ta: 0-0.100%, Co: 0-3.000%, Sn At least one of the following may be included: 0 to 1.000%, Sb: 0 to 0.500%, As: 0 to 0.050%, Mg: 0 to 0.050%, Zr: 0 to 0.050%, Ca: 0 to 0.0500%, Y: 0 to 0.0500%, La: 0 to 0.0500%, Ce: 0 to 0.0500%, and Bi: 0 to 0.0500%. These optional elements will be described in detail below.

[0029] [Cr: 0 to 2.000%] Cr, like Mn, is an element that improves hardenability and contributes to improving steel sheet strength. While the Cr content may be 0%, to obtain the above-mentioned effects, the Cr content is preferably 0.001% or more. The Cr content may be 0.010% or more, 0.100% or more, or 0.200% or more. On the other hand, excessive Cr content may saturate the effect and increase manufacturing costs. Therefore, the Cr content is preferably 2.000% or less, and may be 1.800% or less, 1.500% or less, 1.000% or less, or 0.500% or less.

[0030] [Mo: 0 to 1.000%] 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 0.010% or more, 0.020% or more, 0.050% or more, or 0.100% or more. On the other hand, excessive Mo content may deteriorate hot workability and reduce productivity. For this reason, the Mo content is preferably 1.000% or less. The Mo content may be 0.800% or less, 0.400% or less, or 0.200% or less.

[0031] [Ti: 0 to 0.500%] 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 0.050% or more. On the other hand, if Ti is contained excessively, the effect saturates and there is a risk of an increase in manufacturing costs. Therefore, the Ti content is preferably 0.500% or less, and may be 0.400% or less, 0.200% or less, or 0.100% or less.

[0032] [Nb: 0 to 0.500%] Nb, like Ti, is an element effective in controlling the morphology of carbides and is also effective in refining the structure to improve the toughness of steel plate. 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 or 0.010% or more. On the other hand, excessive Nb content may generate coarse carbides in the steel, reducing the toughness of the steel plate. For this reason, the Nb content is preferably 0.500% or less. The Nb content may be 0.200% or less, 0.100% or less, or 0.060% or less.

[0033] [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.

[0034] [Cu: 0 to 1.000%] 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 0.005% or more, 0.010% or more, or 0.050% 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 1.000% or less. The Cu content may be 0.800% or less, 0.600% or less, 0.300% or less, or 0.100% or less.

[0035] [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 0.005% or more or 0.010% 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.

[0036] [W: 0 to 0.100%] 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 0.005% or more or 0.010% or more. On the other hand, excessive W content may reduce weldability. For this reason, the W content is preferably 0.100% or less. The W content may be 0.080% or less, 0.040% or less, or 0.020% or less.

[0037] [V: 0 to 1.000%] Like Ti and Nb, 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 sheet. 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 1.000% or less. The V content may be 0.400% or less, 0.200% or less, or 0.100% or less.

[0038] [Ta: 0 to 0.100%] 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 in order to obtain these effects, the Ta content is preferably 0.001% or more. The Ta content may be 0.005% or more or 0.010% or more. On the other hand, even if Ta is contained in an excessive amount, the effect saturates, and adding more than necessary to the steel sheet increases the manufacturing cost. For this reason, the Ta content is preferably 0.100% or less. The Ta content may be 0.080% or less, 0.040% or less, or 0.020% or less.

[0039] [Co: 0 to 3.000%] 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 0.005% or more, 0.010% or more, or 0.050% 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 3.000% or less. The Co content may be 2.000% or less, 1.000% or less, 0.500% or less, 0.200% or less, 0.100% or less, or 0.060% or less.

[0040] [Sn: 0 to 1.000%] 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 1.000% or less. The Sn content may be 0.100% or less, 0.040% or less, or 0.020% 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 0.010% or more.

[0041] [Sb: 0 to 0.500%] 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 0.500% or less. The Sb content may be 0.100% or less, 0.060% 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.

[0042] [As: 0 to 0.050%] 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 0.050% or less, and may be 0.040% or less, or 0.020% 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 0.010% or more.

[0043] [Mg: 0 to 0.050%] Mg controls the morphology of sulfides and oxides and contributes to improving the bending formability of steel sheet. This effect can be obtained even with a small amount. The Mg content may be 0%, but to obtain the above effect, the Mg content is preferably 0.0001% or more. The Mg content may be 0.0005% or more, 0.001% or more, or 0.005%. On the other hand, even if an excessive amount of Mg is contained, the effect saturates, and adding more Mg than necessary to the steel sheet increases the manufacturing cost. For this reason, the Mg content is preferably 0.050% or less. The Mg content may be 0.040% or less, 0.020% or less, or 0.010% or less.

[0044] [Zr: 0 to 0.050%] Zr is an element that can control the morphology of sulfides with a small amount. The Zr content may be 0%, but to obtain the above effect, the Zr content is preferably 0.0001% or more. The Zr content may be 0.0005% or more, 0.001% or more, or 0.005% or more. On the other hand, even if Zr is contained in an excessive amount, the effect saturates, and containing more Zr than necessary in the steel sheet increases the manufacturing cost. For this reason, the Zr content is preferably 0.050% or less. The Zr content may be 0.040% or less, 0.020% or less, or 0.010% or less.

[0045] [Ca: 0 to 0.0500%] [Y: 0 to 0.0500%] [La: 0 to 0.0500%] [Ce: 0 to 0.0500%] Ca, Y, La, and Ce are elements that can control the morphology of sulfides in trace amounts. The Ca, Y, La, and Ce contents may be 0%, but in order to obtain the above effects, the Ca, Y, La, and Ce contents are preferably 0.0001% or more, and may be 0.0005% or more, 0.0010% or more, 0.0020% or more, or 0.0030% or more. On the other hand, even if these elements are contained in excess, the effects saturate, and containing more than necessary in the steel sheet increases production costs. Therefore, the Ca, Y, La and Ce contents are each preferably 0.0500% or less, and may be 0.0200% or less, 0.0100% or less, or 0.0060% or less.

[0046] [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 0.0050% 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, or 0.0100% or less.

[0047] 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 whose presence is permitted to the extent that they do not impair the properties of the steel sheet according to the embodiment of the present invention.

[0048] 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 inductively coupled plasma atomic emission spectrometry (ICP-AES). C and S 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.

[0049] [Bainite: 70 to 95%] Bainite is a relatively hard structure and can contribute to improving strength. Furthermore, in an embodiment of the present invention, the above-mentioned crystal grains primarily composed of bainite have a small average misorientation within the grains, and therefore, the inclusion of bainite can also contribute to improving the elongation of the steel sheet. When the area fraction of bainite is 70% or more, sufficient strength and formability can be obtained. From the viewpoint of further improving strength and formability, a higher area fraction of bainite is preferable, and may be, for example, 72% or more, 75% or more, 78% or more, 80% or more, or 82% or more. On the other hand, if bainite is contained in an excessive amount, the steel sheet may not achieve the desired strength and / or formability. Therefore, the area fraction of bainite is set to 95% or less. The area fraction of bainite may be 92% or less, 92% or less, 90% or less, or 87% or less.

[0050] [Martensite: 3 to 20%] Martensite is a hard structure with a high dislocation density, and therefore contributes to improving tensile strength. By setting the area fraction of martensite to 3% or more, a tensile strength of 400 MPa or more can be ensured. From the viewpoint of improving strength, a higher area fraction of martensite is preferable, and may be, for example, 5% or more, 7% or more, 10% or more, or 13% or more. On the other hand, when the area fraction of martensite is 20% or less, formability and appearance can be ensured. The area fraction of martensite may be 17% or less or 15% or less. In the present invention, "martensite" includes not only as-quenched martensite (so-called fresh martensite) but also tempered martensite.

[0051] [Total of Bainite and Martensite: 90% or More] When the total area ratio of bainite and martensite is 90% or more, the homogeneity of the metal structure as a whole is improved. That is, a metal structure mainly composed of bainite and martensite, which are hard structures, is obtained, and the soft structure is relatively reduced. This makes it possible to suppress the occurrence of ghost lines that may occur in a steel sheet in which a hard structure and a soft structure are mixed and unevenly distributed. To enhance this effect, the above-mentioned homogeneity is preferably high, and the total area ratio of bainite and martensite is preferably 92% or more, 94% or more, 96% or more, 98% or more, 99% or more, or even 100%.

[0052] [Remaining structure: 0 to 10%] The remaining structure other than bainite and martensite may have an area ratio of 0%, but if a remaining structure is present, the remaining structure is at least one of ferrite, pearlite, and retained austenite. From the viewpoint of ensuring the above-mentioned effects based on bainite and martensite, the area ratio of the remaining structure, i.e., at least one of ferrite, pearlite, and retained austenite, may be 10% or less in total, for example, 9% or less, 8% or less, 7% or less, 6% or less, 4% or less, or 2% or less. On the other hand, achieving an area ratio of the remaining structure 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.

[0053] [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.

[0054] 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 2.0 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. 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.

[0055] The identification of bainite and calculation of its area fraction are performed using the following procedure. First, the observation surface of the sample is corroded with Nital reagent. Next, a 100 μm × 100 μm region within the range of 1 / 8 to 3 / 8 of the plate thickness, centered at 1 / 4 of the plate thickness, is observed using an FE-SEM. Based on the position and arrangement of cementite contained within the structure in this observation region, bainite is identified as follows. Bainite is classified into upper bainite and lower bainite. In upper bainite, cementite or retained austenite exists at the interface of lath-shaped bainitic ferrite. In lower bainite, cementite exists within lath-shaped bainitic ferrite, the bainitic ferrite and cementite have one type of crystal orientation relationship, and the cementite has the same variant. Based on these characteristic features, upper bainite and lower bainite can be individually identified. In the present invention, these are collectively referred to as bainite, and the area fraction of the identified bainite is calculated based on image analysis.

[0056] The area fraction of martensite is 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 from the 1 / 8 position to the 3 / 8 position 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, which will be 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.

[0057] 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.

[0058] As described above in relation to the calculation of the area ratio of martensite, the area ratio of ferrite is calculated from the area ratio of the black region by binarizing the metal structure based on the difference in brightness using image analysis software Image J (Ver. 1.54f) in FE-SEM observation by nital corrosion.

[0059] The identification of pearlite and calculation of the area ratio are carried out as follows: First, the observation surface of the sample is corroded with Nital reagent, and then the range from 1 / 8 to 3 / 8 of the plate thickness, centered at 1 / 4 of the plate thickness, is observed with an optical microscope. Areas with dark contrast in the image observed with the optical microscope are identified as pearlite, and the area ratio of this area is calculated based on image analysis.

[0060] [Average misorientation within crystal grains surrounded by grain boundaries with a crystal orientation misorientation of 15° or more: 0.60° or less] In an embodiment of the present invention, the average misorientation within crystal grains surrounded by grain boundaries (high-angle grain boundaries) with a crystal orientation misorientation of 15° or more is controlled to 0.60° or less. The average misorientation within crystal grains surrounded by grain boundaries with a crystal orientation misorientation of 15° or more correlates with the dislocation density within the crystal grains, and a smaller average misorientation within the crystal grains indicates a lower dislocation density within the crystal grains. Therefore, according to the steel sheet according to an embodiment of the present invention, unlike conventional DP steel, even though the metallographic structure is mainly composed of bainite, which is harder than ferrite, by controlling the average misorientation within crystal grains surrounded by grain boundaries with a crystal orientation misorientation of 15° or more to 0.60° or less, the intragranular dislocation density is reduced, increasing the elongation of the steel sheet, and thereby significantly improving the formability of the steel sheet. From the viewpoint of improving the elongation and hence formability of the steel sheet, the smaller the average misorientation within the crystal grains, the more preferable, and may be, for example, 0.55° or less, 0.50° to 0.45°, 0.40° or less, 0.35° or less, or 0.30° or less. There is no particular lower limit, but, for example, the average misorientation within the crystal grains may be 0° or more, 0.10° or more, or 0.20° or more.

[0061] [Method for measuring the average misorientation within crystal grains surrounded by grain boundaries with a crystal orientation misorientation of 15° or more] The average misorientation within crystal grains surrounded by grain boundaries with a crystal orientation misorientation of 15° or more is measured by electron backscattered diffraction (EBSD). More specifically, first, a sample is taken from the steel sheet so that the thickness cross section perpendicular to the sheet surface serves as the observation surface. Next, at a depth of ¼ of the sheet thickness from the steel sheet surface, a region of 200 μm in the direction perpendicular to the sheet thickness direction and 100 μm in the sheet thickness direction is analyzed by EBSD at a measurement interval of 0.1 μm to obtain crystal orientation information. For example, if the sheet thickness is too thin to ensure a measurement region of 100 μm in the sheet thickness direction, the length in the sheet thickness direction is reduced and a measurement region of 20,000 μm is obtained. 2The EBSD analysis is performed at an analysis speed of 50 to 300 points per second using an apparatus consisting of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (HIKARI detector manufactured by TSL). Next, for the obtained crystal orientation information, regions surrounded by grain boundaries with a crystal orientation misorientation of 15° or more are defined as crystal grains, and the grain average misorientation (GAM) of the crystal grains is calculated. The crystal grains and the grain average misorientation defined above can be calculated using the software "OIM Analysis (registered trademark)" provided with the EBSD analysis apparatus. Here, the "grain average misorientation of crystal grains" refers to the misorientation between adjacent measurement points in a region surrounded by grain boundaries with a crystal orientation misorientation of 15° or more, averaged over all measurement points in the region. More specifically, the GAM value and area of ​​each crystal grain surrounded by grain boundaries with a crystal orientation misorientation of 15° or more in the measurement region are measured by EBSD analysis, and the GAM values ​​are weighted and averaged using this area to determine the "average misorientation within crystal grains surrounded by grain boundaries with a crystal orientation misorientation of 15° or more."

[0062] [Proportion of Crystal Grains Surrounded by Grain Boundaries with a Crystallographic Misorientation of 15° or More Containing Two or More Martensite Regions Within the Grain: 90% or More] In an embodiment of the present invention, the proportion of crystal grains surrounded by grain boundaries with a crystallographic misorientation of 15° or more containing two or more martensite grains within the grains is controlled to be 90% or more. As shown in FIG. 1 , bainitic ferrite forms from austenite grain boundaries. Therefore, in a structure mainly composed of bainite, although it is possible to transform austenite on grain boundaries to martensite, it is difficult to uniformly form martensite throughout the steel sheet. However, in an embodiment of the present invention, as specifically described particularly with reference to FIG. 1( a), martensite not only forms on grain boundaries with a crystallographic misorientation of 15° or more corresponding to the initial austenite grain boundary 1, but also within the crystal grains surrounded by the grain boundaries. In addition, since crystal grains containing two or more martensite grains are present in a proportion of 90% or more of the total crystal grains, and such a metallographic structure is obtained throughout the steel sheet, it can be understood that in embodiments of the present invention, martensite is uniformly dispersed throughout the steel sheet. Therefore, due to the uniform dispersion of martensite, 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, the occurrence of ghost lines can be suppressed, and an excellent post-forming appearance can be achieved. From the viewpoint of further improving the post-forming appearance, the higher the proportion of crystal grains containing two or more martensite grains among crystal grains surrounded by grain boundaries with a crystal orientation misorientation of 15° or more, the more preferable it is, and it may be, for example, 92% or more, 94% or more, or 96% or more. While the upper limit is not particularly limited, for example, the proportion of crystal grains containing two or more martensite grains among crystal grains surrounded by grain boundaries with a crystal orientation misorientation of 15° or more may be 100% or less, 99% or less, or 98% or less.

[0063] [Method for Measuring the Proportion of Crystal Grains Surrounded by Grain Boundaries with a Crystallographic Misorientation of 15° or More Containing Two or More Martensite Grains Within the Grain] The proportion of crystal grains surrounded by grain boundaries with a crystallographic misorientation of 15° or more containing two or more martensite grains within the grain is measured by combining EBSD analysis and SEM observation. More specifically, as described in the section [Method for Measuring the Average Misorientation Within Crystal Grains Surrounded by Grain Boundaries with a Crystallographic Misorientation of 15° or More], the region surrounded by grain boundaries with a crystallographic misorientation of 15° or more is first defined as a crystal grain. Next, the field of view observed by EBSD is observed with SEM to identify the martensite structure and confirm its generation location. Finally, the proportion (%) of the number of crystal grains containing two or more martensite grains within the grain to the total number of crystal grains is calculated, thereby determining the proportion of crystal grains containing two or more martensite grains within the grain surrounded by grain boundaries with a crystallographic misorientation of 15° or more.

[0064] [Thickness] The steel plate according to the embodiment of the present invention is not particularly limited, but has a thickness of, for example, 0.2 to 2.0 mm. Steel plates having such a thickness are suitable for use as materials for covering members such as automobile doors and hoods. The thickness may be 0.3 mm or more, or 0.4 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. The thickness of the steel plate is measured using a micrometer.

[0065] [Plating] The steel sheet according to the embodiment of the present invention may further have a plating layer on its surface for the purpose of improving corrosion resistance, etc. The plating layer may be any appropriate plating layer, for example, a hot-dip plating layer or an electroplating layer. The hot-dip plating layer may be, for example, a hot-dip galvanized layer, a hot-dip zinc alloy plating layer (a hot-dip plating layer composed of an alloy of zinc and additional elements such as Si and Al), or a galvannealed layer (alloyed plating layer) obtained by alloying these platings. The hot-dip galvanized layer and hot-dip zinc alloy plating layer preferably contain less than 7% by mass of Fe, and the alloyed plating layer preferably contains 7% by mass or more and 15% by mass or less of Fe. In the hot-dip galvanized layer, hot-dip zinc alloy plating layer, and alloyed plating layer, the components other than zinc and Fe are not particularly limited, and various compositions within the usual range can be adopted. The plating layer may also be, for example, an aluminum plating layer. The coating weight of the plating layer is not particularly limited and may be a common coating weight.

[0066] [Mechanical Properties] [Tensile Strength (TS) and Total Elongation (El)] According to the steel sheet of the embodiment of the present invention, high tensile strength (TS), specifically a tensile strength of 400 MPa or more, can be achieved. The tensile strength is preferably 440 MPa or more or 480 MPa or more, more preferably 540 MPa or more or 600 MPa or more. The upper limit is not particularly limited, but for example, the tensile strength may be 980 MPa or less or 900 MPa or less. Similarly, according to the steel sheet of the embodiment of the present invention, excellent formability can be achieved, more specifically, a total elongation (El) of 15.0% or more can be achieved. The total elongation is preferably 16.0% or more, more preferably 16.5% or more or 17.0% or more. The upper limit is not particularly limited, but for example, the total elongation may be 35.0% or less or 30.0% or less. Tensile strength and total elongation are measured by conducting a tensile test in accordance with JIS Z 2241:2022 using a JIS No. 5 test piece taken from a position where the longitudinal direction of the test piece is preferably parallel to the rolling direction of the steel sheet. If the rolling direction of the steel sheet cannot be identified, JIS No. 5 test pieces may be taken from multiple directions within the steel sheet plane. In this case, measurements are taken from multiple directions within the steel sheet plane at regular angles within the steel sheet plane, and the arithmetic mean of the multiple measurements is used as the tensile strength or total elongation of the steel sheet. The multiple directions within the steel sheet plane should be four or more. For example, when measurements are taken from four directions within the steel sheet plane, the angle between each direction should be approximately 45°. If it is difficult to take a JIS No. 5 test piece, other test pieces described in JIS Z 2241:2022 can be used instead of the JIS No. 5 test piece.

[0067] Despite having high strength, specifically a tensile strength of 400 MPa or more, the steel sheet according to the present invention can maintain excellent formability and appearance even after forming, such as press forming. Therefore, the steel sheet according to the present invention is particularly useful for use in components in technical fields where strength, formability, and appearance after forming are all 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 present invention. Examples of automotive exterior panel members include roofs, hoods, fenders, and doors, which require high design quality. These exterior panel members, particularly automotive exterior panel members, may comprise the steel sheet according to the present invention in at least a portion thereof, and therefore, at least a portion of these exterior panel members satisfies the above-described chemical composition and metallographic characteristics. In portions of a steel sheet that have been processed relatively lightly in forming, such as press forming, the characteristics of the metallographic structure do not change significantly before and after forming. A relatively lightly processed portion of a steel sheet is determined by characteristics such as a smooth shape that has not been subjected to deformation, such as bending, and a small rate of change in thickness.

[0068] <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.

[0069] A method for producing a steel sheet according to an embodiment of the present invention comprises: a hot rolling step including heating a slab having the chemical composition described above in relation to the steel sheet at a temperature of 1100 to 1350°C for 300 to 600 minutes, finish-rolling the slab, and then coiling the slab at a temperature of 500 to 700°C, wherein the finish-rolling completion temperature is 800 to 1000°C; a pickling step of pickling the obtained hot-rolled steel sheet; a cold rolling step of cold-rolling the pickled hot-rolled steel sheet at a rolling reduction of 20 to 90%; and an annealing step of annealing the obtained cold-rolled steel sheet, wherein the annealing satisfies the following conditions (a) to (d): (a) heating the cold-rolled steel sheet and holding it at a maximum heating temperature of 820 to 950°C for 60 to 500 seconds; (b) cooling the cold-rolled steel sheet from the maximum heating temperature to a cooling stop temperature of 500 to 600°C at an average cooling rate of 10°C / second or more; (c) holding the cold-rolled steel sheet in a temperature range of 500 to 600°C for 100 to 600 seconds, and (d) cooling the cold-rolled steel sheet in a temperature range of 200 to 500°C at an average cooling rate of 40°C / second or more. Each step will be described in detail below.

[0070] [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 1350°C or lower from the viewpoint of the capacity of the heating equipment and productivity.

[0071] In this method, it is necessary to appropriately control not only the slab heating temperature but also the slab heating time. More specifically, regardless of the casting method, such as continuous casting, segregation regions where alloying elements such as Mn are concentrated generally exist within the slab during solidification. These segregation regions are elongated in the rolling direction by subsequent hot rolling, resulting in layers of alloying element-enriched and -depleted regions dispersed in the thickness direction. Naturally, the state in which the alloying element-enriched and -depleted regions are dispersed in the thickness direction affects the formation of the metallographic structure after hot rolling and the metallographic structure after cold rolling and annealing. With this method, since the metallographic structure cannot be appropriately engineered, it may be impossible to control the average intragranular misorientation of crystal grains surrounded by grain boundaries with a crystallographic misorientation of 15° or more to 0.60° or less, and / or it may be impossible to control the proportion of crystal grains containing two or more martensite grains within the crystal grains to 90% or more.

[0072] Therefore, it is important to heat the slab at 1100 to 1350°C for a long period of time to eliminate or sufficiently reduce the segregation of alloying elements such as Mn formed during slab solidification. This is because eliminating or sufficiently reducing the segregation of alloying elements before hot rolling makes it possible to reliably eliminate the layered dispersion of enriched and dilute regions of alloying elements caused by the segregation during subsequent hot rolling. In this method, to reliably obtain this effect, the slab heating time must be 300 minutes or longer. The slab heating time is preferably 330 minutes or longer, more preferably 350 minutes or longer. On the other hand, if the slab heating time is excessively long, the scale formed on the slab surface becomes too thick. This scale is removed by descaling using high-pressure water or the like during the hot rolling process, but if the scale becomes too thick, even such descaling may not be sufficient to remove it. In this case, the scale that is not removed and remains on the surface is pressed into the steel sheet surface during the subsequent hot rolling, causing scale defects and deteriorating the appearance of the steel sheet. Therefore, the heating time of the slab is set to 600 minutes or less, preferably 500 minutes or less, and more preferably 470 minutes or less.

[0073] [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.

[0074] [Finish Rolling and Coiling] The heated slab, or the slab that has been rough-rolled as needed, is then subjected to finish rolling. If the finish rolling completion temperature (finishing temperature) is less than 800°C, the steel sheet will be rolled in the two-phase region of ferrite and austenite, resulting in a deterioration in the shape of the steel sheet. If the shape of the steel sheet deteriorates, it may be difficult to thread the sheet after hot rolling. For this reason, the finish rolling completion temperature is set to 800°C or higher. On the other hand, setting the finish rolling completion temperature to more than 1000°C requires a device to heat the steel sheet in the process from the end of slab heating to the completion of finish rolling, which requires high costs. Therefore, the finish rolling completion temperature is set to 1000°C or lower. Next, the finish-rolled hot-rolled steel sheet is coiled at a coiling temperature of 500 to 700°C. Setting the coiling temperature to 500 to 700°C can suppress the growth of oxide scale. If the coiling temperature is less than 500°C, the strength of the obtained hot-rolled steel sheet will increase, and the cold rolling load will become excessive, which may make rolling difficult. On the other hand, if the coiling temperature is more than 700°C, the metal structure that is finally obtained may not be constituted by a structure mainly composed of bainite.

[0075] [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.

[0076] [Cold Rolling Process] The pickled hot-rolled steel sheet is cold-rolled at a reduction of 20 to 90% in the cold rolling process. By setting the cold rolling reduction 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. Furthermore, if the flatness of the cold-rolled steel sheet is not sufficiently ensured, it may be difficult to carry out the subsequent annealing process. On the other hand, by setting the cold rolling reduction 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 per pass are not particularly limited, and may be appropriately set so that the reduction of the entire cold rolling is within the above range.

[0077] [Annealing Process] [(a) Holding Time at Maximum Heating Temperature of 820 to 950°C: 60 to 500 Seconds] The obtained cold-rolled steel sheet is heated in the annealing process and held at a maximum heating temperature of 820 to 950°C for 60 to 500 seconds. Holding at a temperature of 820°C or higher for a sufficient time can promote and stabilize austenitization. Therefore, it is possible to reliably form a metal structure in the steel sheet mainly composed of bainite during subsequent cooling. If the maximum heating temperature is lower than 820°C or the holding time is shorter than 60 seconds, austenitization is insufficient, and the metal structure in the steel sheet cannot be formed mainly of bainite even after subsequent cooling. In other words, the area ratio of bainite cannot be increased to 70% or more. In addition, martensite may not be sufficiently generated. On the other hand, since holding at a higher temperature and for a longer period of time reduces productivity, the maximum heating temperature in the annealing process is set to 950°C or lower, and the holding time is set to 500 seconds or lower.

[0078] [(b) Average Cooling Rate from Maximum Heating Temperature to Cooling Stop Temperature of 500 to 600°C: 10°C / sec or More] After holding at the maximum heating temperature for a predetermined time, the cold-rolled steel sheet is cooled from the maximum heating temperature to a cooling stop temperature of 500 to 600°C at an average cooling rate of 10°C / sec or more. Cooling under these conditions can suppress ferrite transformation during cooling. If the average cooling rate is less than 10°C / sec or the cooling stop temperature exceeds 600°C, excessive ferrite is generated during cooling, making it impossible to achieve a bainite area ratio of 70% or more and / or to achieve a residual structure of 10% or less. Therefore, the average cooling rate must be 10°C / sec or more, with the upper limit preferably being 100°C / sec. On the other hand, if the cooling stop temperature is less than 500°C, it is not possible to create a structure such as that shown in FIG. 1(a), and therefore it becomes impossible to control the average misorientation within crystal grains surrounded by grain boundaries with a crystal misorientation of 15° or more to 0.60° or less, and / or it becomes impossible to control the proportion of crystal grains containing two or more martensite grains within the crystal grains to 90% or more.

[0079] [(c) Dwell Time in Temperature Range of 500 to 600°C: 100 to 600 Seconds] The cold-rolled steel sheet cooled to the cooling stop temperature of 500 to 600°C is then dwelled in the temperature range of 500 to 600°C (dwell temperature) for 100 to 600 seconds. In this production method, dwelling in the temperature range of 500 to 600°C encompasses not only cases where the temperature of the cold-rolled steel sheet is held at a constant temperature within the range of 500 to 600°C, but also cases where the temperature of the cold-rolled steel sheet is held while fluctuating within the temperature range of 500 to 600°C. By dwelling the cold-rolled steel sheet in the relatively high temperature range of 500 to 600°C for 100 to 600 seconds, as described above in connection with FIG. 1(a), it is possible to promote the generation of bainitic ferrite and reduce the misorientation of each bainitic ferrite. 1(b) and 1(c) , carbides are not formed between bainitic ferrite grains or within the bainitic ferrite grains themselves, or their formation is sufficiently suppressed. This results in a gradual enrichment of carbon in the untransformed austenite between the bainitic ferrite grains. Furthermore, because the enrichment of carbon improves hardenability, subsequent appropriate cooling not only produces martensite on the austenite grain boundaries, but also transforms the untransformed austenite between the bainitic ferrite grains into martensite. This makes it possible to obtain a metallographic structure that is primarily composed of bainite with martensite dispersed within the bainite. As a result, in the final metallographic structure, the proportion of crystal grains containing two or more martensite grains within crystal grains surrounded by grain boundaries with a crystallographic misorientation of 15° or more corresponding to the initial austenite grain boundaries can be increased to 90% or more, and the average intragranular misorientation of the crystal grains can be controlled to 0.60° or less.

[0080] If the dwell temperature is less than 500°C, a large amount of carbides precipitate between bainitic ferrite and / or within the bainitic ferrite itself, resulting in the consumption of much of the untransformed austenite that could be transformed to martensite. In addition, carbon concentration is insufficient, making it impossible to sufficiently form untransformed austenite 3, in which carbon is concentrated, between bainitic ferrite 2 and bainitic ferrite 2, as shown in FIG. 1(a). As a result, in the final metallographic structure, it becomes impossible to generate 90% or more of crystal grains surrounded by grain boundaries with a crystallographic misorientation of 15° or more, each containing two or more martensite grains, and / or it becomes impossible to control the average crystallographic misorientation of the crystal grains to 0.60° or less. On the other hand, if the dwell temperature exceeds 600°C, a large amount of ferrite is formed, making it impossible to achieve the desired area fraction of the metallographic structure in the final steel sheet.

[0081] Furthermore, if the dwell time is less than 100 seconds, the generation of bainitic ferrite will be insufficient and / or the misorientation between the bainitic ferrite grains will not be sufficiently reduced. In this case, the desired bainite area fraction will not be achieved in the finally obtained metallographic structure, or the proportion of crystal grains containing two or more martensite grains surrounded by grain boundaries with a crystallographic misorientation of 15° or more will not be 90% or more of the total crystal grains, and / or the average misorientation within the crystal grains will not be controlled to 0.60° or less. On the other hand, if the dwell time is more than 600 seconds, a relatively large amount of pearlite may be generated from the carbon-enriched regions of austenite. In this case, the desired area fraction of the metallographic structure will not be achieved in the finally obtained steel sheet, and the strength of the steel sheet will also be reduced.

[0082] [(d) Average Cooling Rate in the Temperature Range of 200 to 500°C: 40°C / sec or More] Finally, the cold-rolled steel sheet is cooled in the temperature range of 200 to 500°C at an average cooling rate of 40°C / sec or more. By cooling under these conditions, martensite can be appropriately generated from untransformed austenite in which carbon is concentrated between each bainitic ferrite. As a result, it is possible to generate crystal grains containing two or more martensite grains within crystal grains surrounded by grain boundaries with a crystal orientation misorientation of 15° or more at a rate of 90% or more relative to the total crystal grains. If the average cooling rate in the temperature range of 200 to 500°C is less than 40°C / sec, the desired martensite area ratio cannot be achieved in the final metal structure, and / or the rate of crystal grains containing two or more martensite grains within crystal grains surrounded by grain boundaries with a crystal orientation misorientation of 15° or more cannot be generated at a rate of 90% or more relative to the total crystal grains. Therefore, the average cooling rate in the temperature range of 200 to 500° C. must be 40° C. / sec or more, with the upper limit being, for example, 200° C. / sec or less, preferably 80° C. / sec or less.

[0083] [Plating Step] The surface of the obtained cold-rolled steel sheet may be plated for purposes such as improving corrosion resistance. The plating process may be hot-dip galvanizing, alloying galvanizing, electroplating, or the like. For example, the steel sheet may be subjected to hot-dip galvanizing as the plating process, or may be subjected to alloying after the hot-dip galvanizing process. The specific conditions for the plating and alloying processes are not particularly limited and may be any appropriate conditions known to those skilled in the art. For example, in the hot-dip galvanizing process, the temperature of the sheet immersed in the plating bath (the temperature of the steel sheet when immersed in the hot-dip galvanizing bath) is preferably in the range from 40°C lower than the hot-dip galvanizing bath temperature (hot-dip galvanizing bath temperature -40°C) to 50°C higher than the hot-dip galvanizing bath temperature (hot-dip galvanizing bath temperature +50°C). When the hot-dip galvanized layer is subjected to alloying, the steel sheet on which the hot-dip galvanized layer has been formed is preferably heated to a temperature range of 400 to 600°C.

[0084] 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.

[0085] In the following examples, steel sheets according to the embodiments of the present invention were produced under various conditions, and the tensile strength, formability, and appearance after forming of the resulting steel sheets were examined.

[0086] [Example A] First, molten steel was cast by a continuous casting method to form slabs having various chemical compositions shown in Table 1. These slabs were heated under the heating temperature and heating time conditions shown in Table 2 and then hot-rolled. Hot rolling was performed by rough rolling and finish rolling, and the finish rolling completion temperature and coiling temperature were as shown in Table 2. Next, the obtained hot-rolled steel sheets were pickled and then cold-rolled at the reduction shown in Table 2 to obtain cold-rolled steel sheets having a thickness of 0.4 mm. Next, the obtained cold-rolled steel sheets were annealed under the conditions shown in Table 2. Finally, hot-dip galvanizing was appropriately performed as a plating treatment, and some of the slabs were further subjected to an alloying treatment at a predetermined alloying temperature of 400 to 600°C.

[0087]

[0088]

[0089]

[0090] The properties of the obtained steel sheets were measured and evaluated by the following methods.

[0091] [Average misorientation within grains] The "average misorientation within grains" in Figure 3 means the average misorientation within crystal grains surrounded by grain boundaries with a crystal misorientation of 15° or more, and is measured as described in the section [Method for measuring the average misorientation within crystal grains surrounded by grain boundaries with a crystal misorientation of 15° or more].

[0092] [Proportion of crystal grains containing two or more martensite grains within the grain] In Figure 3, "proportion of crystal grains containing two or more martensite grains within the grain" means the proportion of crystal grains that contain two or more martensite grains within the grain, among crystal grains surrounded by grain boundaries with a crystal orientation difference of 15° or more, and is measured as described in the section [Method for measuring the proportion of crystal grains that contain two or more martensite grains within the grain, among crystal grains surrounded by grain boundaries with a crystal orientation difference of 15° or more].

[0093] [Tensile strength (TS) and total elongation (El)] The tensile strength (TS) and total elongation (El) were measured by taking a No. 5 tensile test piece according to 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.

[0094] [Post-Forming Appearance] The post-forming appearance was evaluated using a pressed part obtained by press-forming a blanked steel plate into 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 pressed part during the press forming. The surface of the central part of the pressed 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 evaluated. An arbitrary 100 mm x 100 mm area in the center of the pressed part was visually inspected. Cases where no streaks were observed were judged as pass (OK), and cases where streaks were observed were judged as fail (NG). In this test, the appearance after forming was evaluated using a press member simulating a door outer, but the evaluation object may also be a formed member that can be estimated to have been given a 2.5% strain by press forming, or a test piece taken from a steel plate to which a 2.5% pre-strain has been similarly given may also be used, and equivalent evaluations can be made using these test methods. In the case of a test piece taken from a steel plate, a JIS No. 5 test piece with the longitudinal direction perpendicular to the rolling direction and the plate thickness direction and given a 2.5% pre-strain can be evaluated.

[0095] Steel sheets with a tensile strength of 400 MPa or more, a total elongation of 15.0% or more, and a passing evaluation of appearance after forming were evaluated as steel sheets that could achieve both strength, formability, and appearance after forming. The results are shown in Table 3.

[0096]

[0097] Referring to Tables 1 to 3, in Comparative Example 21, TS decreased due to the low C content. In Comparative Example 22, El decreased due to excessive strengthening caused by the high C content. In Comparative Example 23, TS decreased due to the low Si content. In addition, it is believed that the low Si content caused relatively large amounts of carbides such as cementite to precipitate during the manufacturing process, and much of the untransformed austenite that could be transformed into martensite was consumed by the precipitation of these carbides. As a result, the proportion of crystal grains containing two or more martensite grains among those surrounded by grain boundaries with a crystal orientation difference of 15° or more (high-angle grain boundaries) decreased, resulting in poor appearance after forming. In Comparative Example 24, the high Si content prevented adequate removal of scale formed during hot rolling, resulting in poor appearance after forming. In Comparative Example 25, TS decreased due to the low Mn content. In addition, the total area ratio of bainite and martensite decreased due to the poor hardenability caused by the low Mn content. As a result, a relatively large amount of residual structure was generated, which reduced the homogeneity of the metal structure as a whole, making it impossible to sufficiently suppress the occurrence of ghost lines, and resulting in a poor appearance after forming. In Comparative Example 26, it is believed that the high Mn content made it impossible to sufficiently counteract the effects of Mn segregation. As a result, the appearance after forming was poor.

[0098] In Comparative Example 27, the slab heating temperature was low, which resulted in a risk of embrittlement cracking due to the remaining coarse alloy carbides, and therefore hot rolling was not performed. In Comparative Example 28, the finish rolling completion temperature was low, which resulted in a deterioration in the shape of the steel sheet, and therefore it was not possible to pass the sheet after hot rolling. In Comparative Example 29, the coiling temperature was low, which resulted in an increase in strength of the hot-rolled steel sheet, which resulted in an excessive cold-rolling load, and therefore it was not possible to properly complete the cold-rolling process. In Comparative Example 30, the coiling temperature was high, which resulted in an inability to form the final metal structure mainly composed of bainite, and therefore it was not possible to form martensite dispersedly within the crystal grains during the process of developing this structure. As a result, the proportion of crystal grains containing two or more martensite grains within the crystal grains surrounded by high-angle grain boundaries decreased, resulting in a poor appearance after forming. In Comparative Example 31, the reduction rate in the cold-rolling process was low, which resulted in the cold-rolled steel sheet not being sufficiently flat, and therefore it was not possible to perform the subsequent annealing process. In Comparative Example 32, the rolling reduction in the cold rolling step was too high, resulting in an excessively large rolling load, and the cold rolling step could not be completed properly. In Comparative Example 33, the maximum heating temperature in the annealing step was low, which is thought to have resulted in insufficient austenitization. In relation to this, the subsequent cooling did not result in a metal structure in the steel sheet that was primarily composed of bainite, and martensite was not sufficiently formed. As a result, TS decreased, and the proportion of crystal grains surrounded by high-angle grain boundaries that contained two or more martensite grains within each grain decreased, resulting in poor appearance after forming. In Comparative Example 34, the holding time at the maximum heating temperature in the annealing step was too short, which is also thought to have resulted in insufficient austenitization. In relation to this, the subsequent cooling did not result in a metal structure in the steel sheet that was primarily composed of bainite, and martensite was not dispersed and present within the crystal grains during the process of forming the structure. As a result, the proportion of crystal grains containing two or more martensite grains among the crystal grains surrounded by high-angle grain boundaries decreased, resulting in a poor appearance after forming.In Comparative Example 35, the average cooling rate from the maximum heating temperature in the annealing step to the cooling stop temperature of 500 to 600°C was slow, which resulted in excessive ferrite formation during cooling, making it impossible to form a metal structure in the steel sheet mainly composed of bainite, and in the process of creating this structure, it was impossible to make martensite dispersed within the crystal grains. As a result, TS decreased, and the proportion of crystal grains surrounded by high-angle grain boundaries that contained two or more martensite grains within each grain decreased, resulting in a poor appearance after forming.

[0099] In Comparative Example 36, the cooling stop temperature from the maximum heating temperature was low, which reduced the proportion of crystal grains containing two or more martensite grains within the crystal grains surrounded by high-angle grain boundaries, resulting in poor appearance after forming. In Comparative Example 37, the cooling stop temperature from the maximum heating temperature was high, which resulted in excessive ferrite formation during cooling. This prevented the steel sheet from forming a metal structure primarily composed of bainite, and martensite could not be dispersed within the crystal grains during the process of developing this structure. As a result, the proportion of crystal grains containing two or more martensite grains within the crystal grains surrounded by high-angle grain boundaries reduced, resulting in poor appearance after forming. In Comparative Example 38, the annealing temperature was low, which presumably resulted in the precipitation of relatively large amounts of carbides such as cementite, and the precipitation of these carbides consumed much of the untransformed austenite that could be transformed to martensite. As a result, the average misorientation within the crystal grains surrounded by high-angle grain boundaries could not be controlled to 0.60° or less, resulting in a decrease in El. In Comparative Example 39, the high dwell temperature in the annealing process resulted in the formation of a large amount of ferrite, making it impossible to form a metallographic structure in the steel sheet primarily composed of bainite. This prevented martensite from being dispersed within the crystal grains during the process of developing this structure. As a result, the proportion of crystal grains surrounded by high-angle grain boundaries that contained two or more martensite grains within them decreased, resulting in poor appearance after forming. In Comparative Example 40, the short dwell time in the annealing process prevented the desired bainite area fraction from being achieved in the final metallographic structure. This reduced the proportion of crystal grains surrounded by high-angle grain boundaries that contained two or more martensite grains within them. Furthermore, it was not possible to control the average misorientation within these crystal grains to 0.60° or less. As a result, El decreased and the appearance after forming deteriorated. In Comparative Example 41, the long dwell time in the annealing process resulted in the formation of a relatively large amount of pearlite, making it impossible to achieve the desired area fraction of the metallographic structure, and sufficient martensite was not produced. As a result, TS decreased, and the proportion of crystal grains containing two or more martensite grains among crystal grains surrounded by high-angle grain boundaries decreased, resulting in a poor appearance after forming.In Comparative Example 42, the average cooling rate in the annealing step in the temperature range of 200 to 500°C was slow, so the desired martensite area ratio could not be achieved in the final metal structure, and as a result, the proportion of crystal grains surrounded by high-angle grain boundaries containing two or more martensite grains within each grain decreased, resulting in a decrease in TS and a deterioration in the appearance after forming.

[0100] In contrast, all of the steel sheets according to the examples of the present invention had a predetermined chemical composition, formed a structure primarily composed of bainite, and increased the proportion of crystal grains surrounded by high-angle grain boundaries containing two or more martensite grain regions to 90% or more. This resulted in a high tensile strength of 400 MPa or more, and significantly reduced the generation of minute irregularities on the steel sheet surface and the occurrence of ghost lines, even when strain was imparted by press forming. Furthermore, by controlling the mean misorientation of crystal grains surrounded by high-angle grain boundaries to 0.60° or less, the elongation of the steel sheet was increased, thereby significantly improving the formability of the steel sheet. Furthermore, when a residual structure was present in the examples of the present invention, the residual structure was at least one of ferrite, pearlite, and retained austenite.

[0101] [Example B] In this example, the effect of the slab heating time on the metallographic structure of the steel sheet was investigated. Specifically, steel sheets were produced under the same conditions as Inventive Example 1 in Example A, except that the slab heating time was changed in various ways. The results are shown in Table 4 below. For ease of comparison, Inventive Example 1 in Table 3 is also shown in Table 4.

[0102]

[0103] Referring to Table 4, in Comparative Examples 45 and 46, the short slab heating time prevented sufficient reduction of the segregation of alloying elements such as Mn formed during slab solidification. This presumably resulted in layered dispersion of enriched and depleted alloying elements during subsequent hot rolling due to the segregation. As a result, the subsequent cold rolling and annealing processes did not adequately refine the metal structure, reducing the proportion of crystal grains surrounded by high-angle grain boundaries containing two or more martensite grains, resulting in poor appearance after forming. On the other hand, in Comparative Example 47, the long slab heating time resulted in excessive scale growth on the slab surface, which was presumably insufficient to be removed by subsequent descaling using high-pressure water. As a result, the unremoved scale remaining on the surface was pushed into the steel sheet surface during subsequent hot rolling, resulting in significant scale defects. For this reason, no processes subsequent to the hot rolling process were performed on Comparative Example 47.

[0104] In contrast, in Examples 1, 43, and 44, in which the slab heating time was controlled to 300 to 600 minutes, a structure mainly composed of bainite was formed, and the proportion of crystal grains surrounded by high-angle grain boundaries that contained two or more martensite grain regions was increased to 90% or more. This resulted in a high tensile strength of 400 MPa or more, and the occurrence of ghost lines was significantly suppressed by suppressing the generation of minute irregularities on the steel sheet surface even when strain was imparted by press forming. In addition, by controlling the average misorientation within crystal grains surrounded by high-angle grain boundaries to 0.60° or less, the elongation of the steel sheet was increased, thereby significantly improving the formability of the steel sheet.

[0105] 1 austenite grain boundary 2 bainitic ferrite 3 untransformed austenite 4 carbide 10 crystal grain 11 grain boundary with crystal orientation difference of 15° or more 12 bainite 13 martensite grain

Claims

1. In mass%, C: 0.03 to 0.08%, Si: 0.10 to 1.50%, Mn: 0.50 to 3.00%, P: 0.1000% or less, S: 0.0200% or less, Al: 1.000% or less, N: 0.0200% or less, O: 0 to 0.020%, Cr: 0 to 2.000%, Mo: 0 to 1.000%, Ti: 0 to 0.500%, Nb: 0 to 0.500%, B: 0 to 0.0100%, Cu: 0 to 1.000%, Ni: 0 to 1.00%, W: 0 to 0.100%, V: 0 to 1.000%, Ta: 0 to 0.100%, It has a chemical composition consisting of Co: 0 to 3.000%, Sn: 0 to 1.000%, Sb: 0 to 0.500%, As: 0 to 0.050%, Mg: 0 to 0.050%, Zr: 0 to 0.050%, Ca: 0 to 0.0500%, Y: 0 to 0.0500%, La: 0 to 0.0500%, Ce: 0 to 0.0500%, Bi: 0 to 0.0500%, and the balance: Fe and impurities, and contains, in area ratios, 70 to 95% bainite and 3 to 20% martensite, and the total of bainite and martensite is 90% or more, and the average misorientation within crystal grains surrounded by grain boundaries with a misorientation of 15° or more is 0.60° or less, and A steel plate characterized in that it has a metal structure in which the proportion of crystal grains containing two or more martensite grains within the crystal grains is 90% or more.

2. The chemical composition is, in mass%, Cr: 0.001 to 2.000%, Mo: 0.001 to 1.000%, Ti: 0.001 to 0.500%, Nb: 0.001 to 0.500%, B: 0.0001 to 0.0100%, Cu: 0.001 to 1.000%, Ni: 0.001 to 1.00%, W: 0.001 to 0.100%, V: 0.001 to 1.000%, Ta: 0.001 to 0.100%, Co: 0.001 to 3.000%, Sn: 0.001 to 1.000%, Sb: 0.001 to 0.500%, 2. The steel sheet according to claim 1, comprising at least one of As: 0.001 to 0.050%, Mg: 0.0001 to 0.050%, Zr: 0.0001 to 0.050%, Ca: 0.0001 to 0.0500%, Y: 0.0001 to 0.0500%, La: 0.0001 to 0.0500%, Ce: 0.0001 to 0.0500%, and Bi: 0.0001 to 0.0500%.

3. An outer panel member comprising the steel plate according to claim 1 or 2.

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