Steel plates and outer panel components

Abstract

The steel sheet is characterized by having a metal structure having a predetermined chemical composition, containing, by area ratio, 70 to 95% bainite and 3 to 20% martensite, the total of bainite and martensite being 90% or more, the average misorientation within crystal grains surrounded by grain boundaries with a crystal misorientation of 15° or more being 0.60° or less, and the proportion of crystal grains containing two or more martensite grains within the grains being 90% or more.

Description

[Technical Field]

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

[0002] In order to reduce carbon dioxide emissions from automobiles, efforts are underway to reduce the weight of automobile bodies while ensuring safety by using high-strength steel sheets. While the strength of automotive steel sheets has been significantly improved for automobile frame components, steel sheets with a tensile strength of 300 MPa or less are still commonly used for exterior panel components such as doors and hoods, and progress in strengthening these components has been limited. These exterior panel components require high formability and appearance. Generally, increasing the strength of steel sheets reduces their formability and appearance after forming. Therefore, it is difficult to achieve both high strength and formability and appearance, especially appearance after forming, in high-strength steel sheets. Several approaches have been proposed to address these issues.

[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% or less 1.5% Mn+Cr, 4.1% - 2.3% Mn - 1.2% Cr ≤ 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 good 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 fraction of the main phase, ferrite, to 70% or more and the area fraction of the hard second phase containing martensite to 30% or less, it is possible to keep the strength, yield strength, yield ratio, and strength-ductility balance all within a good range.

[0004] In Patent Document 2, in terms of mass%, C: 0.0005 to 0.01%, Si: 0.2% or less, Mn: 0.1 to 1.5%, P: 0.03% or less, S: 0.005 to 0.03%, Ti: 0.02 to 0.1%, Al: 0.01 to 0.05%, N: 0.005% or less, Sb: 0.03% or less, Cu: more than 0.005% and 0.03% or less, and Ti* represented by Ti* = (Ti%) - 3.4×(N%) - 1.5×(S%) - 4×(C%) satisfies 0 < Ti* < 0.02, and further contains in the range satisfying (Sb%) ≥ (Cu%) / 5. The balance consists of Fe and inevitable impurities, and has a component composition. On both sides of the steel sheet, the content (mass%) of Ti element contained in precipitates with a size less than 20 nm in the plate thickness surface layer part from each surface to 10 μm is 9% or less of the total Ti content (mass%) in the steel sheet. A cold-rolled steel sheet is described. Further, in Patent Document 2, by setting the content (mass%) of Ti element contained in precipitates with a size less than 20 nm in the plate thickness surface layer part from each surface to 10 μm on both sides of the steel sheet to 9% or less of the total Ti content (mass%) in the steel sheet, the occurrence of appearance unevenness due to such fine Ti-based precipitates is avoided, and a cold-rolled steel sheet with excellent surface properties can be obtained. Furthermore, it is taught that the cold-rolled steel sheet can be suitably used for parts that require excellent surface quality after forming, mainly for the outer panels of automobiles.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[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 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. [Means for solving the problem]

[0008] To achieve the above object, the present inventors conducted research, focusing particularly on the metallographic structure of steel sheets. Specifically, the present inventors discovered that by forming the metallographic structure of a steel sheet having a predetermined chemical composition into 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 present inventors discovered that the steel sheet can achieve the desired high strength and formability, and that the generation of minute irregularities on the steel sheet surface can be significantly suppressed even when strain is imparted by press forming or the like, and thus completed the present invention.

[0009] The present invention, which has achieved the above object, is as follows. (1) In mass%, C: 0.03-0.08%, 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: 0~2.000%, Mo: 0 to 1.000%, Ti: 0 to 0.500% Nb: 0 to 0.500%, B: 0~0.0100%, Cu: 0-1.000%, Ni: 0 to 1.00% W: 0 to 0.100%, V: 0 to 1.000%, Ta: 0 to 0.100%, Co: 0 to 3.000%, Sn: 0 to 1.000%, Sb: 0 to 0.500% As: 0~0.050%, 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 has a chemical composition consisting of Fe and impurities, In terms of area ratio, Bainite: 70-95% Martensite: 3 to 20%, and At least one of ferrite, pearlite, and retained austenite: 0 to 10% in total; 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 by having 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: 0.0001~0.0100%, 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%, 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% The steel sheet according to (1) above, characterized in that it contains at least one of the following: (3) An outer panel member including the steel plate according to (1) or (2) above. [Effects of the Invention]

[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. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 2 is a schematic diagram showing the morphology of bainite for each temperature range in which bainite is generated in an annealing process. [Figure 2]1 is a schematic diagram showing the metal structure of a steel plate according to an embodiment of the present invention. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0012] <Steel plate> The steel plate according to the embodiment of the present invention has, in mass%, C: 0.03-0.08%, 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: 0~2.000%, Mo: 0 to 1.000%, Ti: 0 to 0.500% Nb: 0 to 0.500%, B: 0~0.0100%, Cu: 0-1.000%, Ni: 0 to 1.00% W: 0 to 0.100%, V: 0 to 1.000%, Ta: 0 to 0.100%, Co: 0 to 3.000%, Sn: 0 to 1.000%, Sb: 0 to 0.500% As: 0~0.050%, 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 has a chemical composition consisting of Fe and impurities, In terms of area ratio, Bainite: 70-95% Martensite: 3 to 20%, and At least one of ferrite, pearlite, and retained austenite: 0 to 10% in total; The average misorientation within crystal grains surrounded by grain boundaries with a misorientation of 15° or more is 0.60° or less, and The metal structure is characterized in that 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 soft ferrite and hard martensite structures, is prone to nonuniform deformation, in which the soft structure and its surrounding area deform preferentially during press forming and other processes. This can lead to minute irregularities on the steel sheet surface, resulting in appearance defects known as ghost lines. More specifically, during press forming and other processes, the soft ferrite structure undergoes large depressions, while the hard martensite structure undergoes small deformations. Therefore, the hard structure does not depression compared to the soft structure, but rather protrudes. As a result, variations in deformation occur, particularly across the width of the steel sheet, resulting in band-like ghost lines. Meanwhile, as steel sheets become stronger, elements such as manganese (Mn) are often added in relatively large amounts to improve their hardenability. Mn is an element that tends to segregate in a streaky manner in steel sheets. More specifically, Mn-enriched regions, such as center segregation and microsegregation, are formed during casting. These enriched regions are then elongated in the rolling direction by hot rolling or cold rolling, resulting in the segregation of Mn in a streaky manner. This Mn segregation results in the presence of regions with high and low hardenability in the steel sheet. As a result, a relatively large number of streaky hard structures are formed in the metallographic structure of the steel sheet after quenching. In this case, the occurrence of ghost lines is particularly pronounced. However, if Mn segregation in steel sheets can be sufficiently suppressed, the formation of such streaky 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 minute irregularities on the steel sheet surface can be sufficiently reduced, and the occurrence of ghost lines can be suppressed. However, with the demand for higher strength, particularly when the Mn content in 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 bainite-based structure, rather than a ferrite-based structure as in conventional DP steels. As a result, as will be described in detail later with respect to a method for producing the 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 within predetermined crystal grains. More specifically, the proportion of crystal grains containing two or more martensite grains among predetermined crystal grains surrounded by grain boundaries with a crystal orientation misorientation of 15° or more (high-angle grain boundaries) 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, 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 the dislocation density within the crystal grains is low. Therefore, unlike conventional DP steels, 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 manufacture of steel sheets, for example, a desired metallographic structure is formed by heating a cold-rolled steel sheet to the ferrite-austenite two-phase region or the austenite single-phase region in the subsequent annealing process, followed by appropriate cooling. Therefore, when forming a specific metallographic structure primarily composed of bainite, it is extremely important to optimize the chemical composition of the steel sheet and 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 austenite single-phase region, during the annealing process. In this regard, FIG. 1(a) shows the initial state of bainite formation in the temperature range of 500 to 600°C, FIG. 1(b) shows the initial state of bainite formation in the temperature range of 450 to less than 500°C, and FIG. 1(c) shows the initial state of bainite formation in the 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 bainitic ferrite 2 formed from austenite grain boundaries 1 and bainitic ferrite 2. It can be seen that no carbides such as cementite precipitate between bainitic ferrite 2 and bainitic ferrite 2 or within bainitic ferrite 2 itself. This is thought to be due to the relatively high temperature range of 500 to 600°C, as well as 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.On the other hand, as shown in Figure 1(b), in the temperature range of 450 to less than 500°C, it can be seen that a large amount of carbides 4 such as cementite precipitates between bainitic ferrite 2 formed from austenite grain boundaries 1 and between 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 can be seen that not only do carbides 4 such as cementite precipitate between bainitic ferrite 2 formed from austenite grain boundaries 1, but also many carbides 4 precipitate within the bainitic ferrite 2 itself.

[0017] In FIG. 1(a), the steel sheet is held at a relatively high temperature range of 500 to 600°C for a predetermined time, which promotes the formation of bainitic ferrite 2 and reduces the misorientation of each bainitic ferrite 2. In addition, unlike the cases of FIGS. 1(b) and 1(c), carbide 4 is not formed or its formation is sufficiently suppressed in FIG. 1(a). Therefore, carbon gradually concentrates in the untransformed austenite 3 present between the bainitic ferrite 2 due to the holding at the relatively high temperature range. It is also believed that the carbon concentration improves hardenability. Therefore, by subsequently performing appropriate cooling, not only is martensite formed on the austenite grain boundaries 1, but the untransformed austenite 3 present between the bainitic ferrite 2 and where carbon is concentrated can also be transformed into martensite. This makes it possible to obtain a metal structure consisting primarily of bainite with martensite dispersed within the bainite. In addition, as described above, it is believed that the dwell 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, surrounded by grain boundaries with a misorientation of 15° or more corresponding to the initial austenite grain boundaries 1, and 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 Figure 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 shown by thick lines with 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 with a sufficiently reduced average orientation difference within the grains.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, even though 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 orientation 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.

[0018] On the other hand, in the cases shown in FIGS. 1(b) and 1(c), a large amount of carbides 4 precipitates between the bainitic ferrite 2 and within the bainitic ferrite 2 itself, resulting in the precipitation of much of the untransformed austenite 3 that could be transformed to martensite being consumed by the precipitation of the carbides 4. In addition, carbon enrichment is insufficient, preventing the formation of sufficient untransformed austenite 3 with concentrated carbon between the bainitic ferrite 2 and the bainitic ferrite 2. Therefore, even if the steel sheet is subsequently subjected to a dwelling operation for a predetermined time and then appropriately cooled, 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 shown in FIGS. 1(a) and 2. In contrast, when the heat treatment described with reference to FIG. 1(a) is performed, a metallographic structure in which martensite is dispersed within predetermined crystal grains is obtained throughout the steel sheet. This suggests that martensite can be uniformly dispersed throughout the steel sheet, regardless of the presence 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 final metal structure, regardless of the presence 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 limit and upper limit, unless otherwise specified.

[0020] [C: 0.03-0.08%] 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 in an excessive amount, 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: 0.10~1.50%] 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. For this reason, 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: 0.50~3.00%] Mn is an element that improves hardenability and contributes to improving the strength of steel sheets. To fully achieve 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 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 galvanizability. 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, if the P content of practical steel sheets is reduced to less than 0.0001%, the manufacturing cost will increase significantly, 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, if the N content of a practical steel sheet is reduced to less than 0.0001%, the manufacturing cost will increase significantly, which is economically disadvantageous. Therefore, the N content may be 0.0001% or more, 0.0002% or more, or 0.0005% or more.

[0027] [O:0~0.020%] 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 of 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 ... Sn: 0-2.000%, Sn: 0-2.000%, Sn: 0-2.000%, Sn: 0-2.000%, Sn: 0-2.000%, Sn: 0- At least one of the following may be included: Cr: 0-1.000%, Sb: 0-0.500%, As: 0-0.050%, Mg: 0-0.050%, Zr: 0-0.050%, Ca: 0-0.0500%, Y: 0-0.0500%, La: 0-0.0500%, Ce: 0-0.0500%, and Bi: 0-0.0500%. These optional elements will be described in detail below.

[0029] [Cr:0~2.000%] Like Mn, Cr is an element that improves hardenability and contributes to improving the strength of the steel sheet. 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 effects 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: 0-1.000%] Like Cr, Mo 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: 0~0.500%] 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 increasing 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: 0~0.500%] Like Ti, Nb is an element effective in controlling the morphology of carbides and is also effective in refining the structure to improve the toughness of steel sheets. 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 sheet. 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: 0~0.0100%] 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 an element beneficial 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 result in a decrease in 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: 0-1.000%] Cu is an element that contributes to improving the strength of steel sheets. This effect can be achieved even with trace amounts. 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: 0-1.00%] 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:0~0.100%] W is an element effective in controlling the morphology of carbides and improving the strength of steel sheets. 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 deteriorate 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:0~1.000%] 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 steel sheet. The V content may be 0%, but to obtain the above effects, 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 steel sheets. The Ta content may be 0%, but to obtain these effects, the Ta content is preferably 0.001% or more. The Ta content may be 0.005% 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 Ta 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: 0-3.000%] 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: 0~1.000%] Sn is an element that can be contained in steel sheet when scrap is used as the 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: 0~0.500%] 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: 0~0.050%] Like Sn and Sb, As is an element that can be contained in a steel sheet when scrap is used as a raw material for the steel sheet. Furthermore, As is an element that strongly segregates at grain boundaries, and a lower As content is preferable. 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: 0~0.050%] Mg controls the morphology of sulfides and oxides and contributes to improving the bending formability of steel sheets. This effect can be achieved even with a small amount. The Mg content may be 0%, but to achieve 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 added, the effect saturates, and adding more Mg than necessary to steel sheets increases manufacturing costs. 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: 0~0.050%] 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 effects, 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 adding more Zr than necessary to 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: 0~0.0500%] [Y:0~0.0500%] [La:0~0.0500%] [Ce: 0~0.0500%] Ca, Y, La, and Ce are elements that can control the morphology of sulfides even in trace amounts. The Ca, Y, La, and Ce contents may be 0%, but 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 adding more than necessary to the steel sheet increases manufacturing costs. Therefore, the Ca, Y, La, and Ce contents are preferably 0.0500% or less, and may be 0.0200% or less, 0.0100% or less, or 0.0060% or less.

[0046] [Bi: 0~0.0500%] Bi is an element that improves 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. 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: 70~95%] 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: 3~20%] 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, the higher the area fraction of martensite, the more preferable it is, and it 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] By ensuring that 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 can occur in steel sheets in which hard and soft structures 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 tissue: 0~10%] The area fraction of the remaining structure other than bainite and martensite may be 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-described effects based on bainite and martensite, the area fraction of the remaining structure, i.e., at least one of ferrite, pearlite, and retained austenite, is set to 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 a 0% area fraction of the remaining structure requires advanced control in the steel sheet manufacturing process, which may result in a decrease in yield. Therefore, the area fraction of the remaining structure may be 0.5% or more, or 1% or more.

[0053] [Identification of metal structure and calculation of area ratio] Identification of the metal structure and calculation of the area ratio are carried out using FE-SEM (field emission scanning electron microscope, for example, JEOL JSM-7200F, measured at an acceleration voltage of 15 kV) after corrosion using Nital reagent, an optical microscope, and X-ray diffraction. The observation of the structure using FE-SEM and an optical microscope is carried out at a magnification of 500 to 50,000 times on a 100 μm × 100 μm area in the cross section of the steel sheet perpendicular to the sheet surface. For each metal structure, three measurement points are measured, and the area ratio is determined by calculating the average of these measurement 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 while the measurement area is increased to 10,000 μm. 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 "plate thickness x / y position (where x and y are natural numbers satisfying x < y)" means the position that moves from the surface (plate surface) in the plate thickness direction of the steel plate toward the center of the steel plate by a distance (depth) of x / y of the plate thickness t in the plate thickness direction. For example, when the plate thickness t of the steel plate is 2.0 mm, the "plate thickness 1 / 8 position" means the position with a depth of 0.25 mm in the plate thickness direction from the surface of the steel plate. When the steel plate has a coating such as a plating layer on its surface, the "surface of the steel plate" means the interface between the steel plate and the coating, and the "plate thickness t" means the plate thickness of the steel plate (base material) excluding the coating.

[0055] The identification of bainite and the calculation of the area ratio are performed according to the following procedure. First, the observation surface of the sample is corroded with nital reagent, and then a 100 μm × 100 μm area within the range of plate thickness 1 / 8 to 3 / 8 centered on the plate thickness 1 / 4 is observed by FE-SEM. Based on the position and arrangement of cementite contained in the structure within this observation area, 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-like bainitic ferrite. In lower bainite, cementite exists inside the lath-like bainitic ferrite, the crystal orientation relationship between the bainitic ferrite and the cementite is of one type, and the cementite has the same variant. Based on these characteristic points, upper bainite and lower bainite can be identified respectively. In the present invention, these are collectively referred to as bainite, and the area ratio of the identified bainite is calculated based on image analysis.

[0056] The martensite area fraction is determined using the following procedure. First, the observation surface of the sample is etched with a nital reagent (3% nitric acid in ethanol). Next, a 100 μm × 100 μm area is observed using a field emission scanning electron microscope (FE-SEM) within the range from the 1 / 8 to 3 / 8 positions of the plate thickness, centered at the 1 / 4 position. Because martensite and retained austenite are not corroded by nital etching, the area fraction of the uncorroded area corresponds to the total area fraction of martensite and retained austenite. Specifically, the metal structure is binarized based on differences in brightness using the image analysis software Image J (Ver. 1.54f). The black areas in the image data represent ferrite, and the uncorroded white areas represent the combined structure of martensite and retained austenite. Therefore, the area fraction of ferrite is calculated from the area fraction of the black areas. The area fraction of martensite is calculated by subtracting the area fraction of retained austenite measured by X-ray diffraction (described later) from the area fraction of the uncorroded area. 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 using X-ray diffraction. First, the specimen is mechanically and chemically polished from the surface to a depth of 1 / 4 of the specimen thickness. Specifically, the specimen is thinned to the observation position by mechanical polishing, and then thinned to the target position by chemical polishing (with hydrofluoric acid). Next, the fraction of retained austenite is calculated from the integrated intensity ratio of the diffraction peaks of the (200) and (211) bcc phase and the (200), (220), and (311) fcc phase, obtained at the 1 / 4 specimen thickness position using MoKα radiation, using, for example, an X-ray diffractometer (RINT1500, X-ray output 40 kV-200 mA) manufactured by Rigaku Corporation. The common five-peak method is used for this calculation. The calculated fraction of retained austenite is determined as the area fraction of retained austenite.

[0058] As explained above in relation to the calculation of the area fraction of martensite, the area fraction of ferrite is calculated from the area fraction of the black areas in FE-SEM observations using nital etching, by binarizing the metal structure based on differences in brightness using the image analysis software Image J (Ver. 1.54f).

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

[0060] [Average misorientation within grains surrounded by grain boundaries with a 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 with a crystal orientation misorientation of 15° or more (high-angle grain boundaries) 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 steels, even though the metallographic structure is primarily 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 and the elongation of the steel sheet is increased, thereby making it possible to significantly improve 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 a grain surrounded by a grain boundary with a misorientation of 15° or more] The average misorientation within crystal grains surrounded by grain boundaries with a misorientation of 15° or more is measured by electron backscattered diffraction (EBSD). More specifically, a sample is first taken from the steel plate so that the observation surface is the cross section of the plate perpendicular to the plate surface. Next, at a depth of 1 / 4 of the plate thickness from the steel plate surface, an area of ​​200 μm in the direction perpendicular to the plate thickness and 100 μm in the plate thickness direction is analyzed by EBSD at a measurement interval of 0.1 μm to obtain crystal orientation information. For example, if the plate thickness is too thin to ensure a measurement area of ​​100 μm in the plate thickness direction, the length in the plate thickness direction is reduced and a measurement area of ​​20,000 μm is obtained. 2 The EBSD analysis was performed using a device consisting of a thermal field emission scanning electron microscope (JEOL JSM-7001F) and an EBSD detector (TSL HIKARI detector) at an analysis speed of 50 to 300 points per second. 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)" included with the EBSD analysis device. 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, EBSD analysis is used to measure the GAM value and area of ​​each crystal grain surrounded by grain boundaries with a crystal orientation misorientation of 15° or more in the above measurement area, 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] [The percentage of crystal grains that contain two or more martensite regions within the grains surrounded by grain boundaries with a crystal orientation misorientation of 15° or more: 90% or more] In an embodiment of the present invention, the proportion of crystal grains surrounded by grain boundaries with a crystal orientation misorientation of 15° or more, each containing two or more martensite grains, is controlled to 90% or more. As shown in FIG. 1 , bainitic ferrite forms from austenite grain boundaries. Therefore, in a structure primarily composed of bainite, although it is possible to transform austenite on grain boundaries into 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 crystal orientation misorientation of 15° or more corresponding to the initial austenite grain boundary 1, but also within the crystal grains surrounded by such grain boundaries. Furthermore, since crystal grains containing two or more martensite grains are present at 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 an embodiment 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 surrounded by grain boundaries with a crystal orientation misorientation of 15° or more that contain two or more martensite grains, the more preferable it is, and it may be, for example, 92% or more, 94% or more, or 96% or more. There is no particular upper limit, but for example, the proportion of crystal grains surrounded by grain boundaries with a crystal orientation misorientation of 15° or more that contain two or more martensite grains may be 100% or less, 99% or less, or 98% or less.

[0063] [Method for measuring the percentage of crystal grains that contain two or more martensite grains among crystal grains surrounded by grain boundaries with a crystal orientation misorientation of 15° or more] The proportion of crystal grains surrounded by grain boundaries with a misorientation of 15° or more that contain two or more martensite grains within them 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 misorientation of 15° or more," the region surrounded by grain boundaries with a 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 location. Finally, the proportion of crystal grains surrounded by grain boundaries with a misorientation of 15° or more that contain two or more martensite grains within them is determined by calculating the percentage of the number of crystal grains containing two or more martensite grains within the total number of crystal grains.

[0064] Plate Thickness The steel sheet according to the embodiment of the present invention is not particularly limited, but may have a thickness of, for example, 0.2 to 2.0 mm. Steel sheets 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 sheet 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 electroplated 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% to 15% by mass 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)] Steel sheets according to embodiments of the present invention can achieve high tensile strength (TS), specifically, a tensile strength of 400 MPa or more. 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, steel sheets according to embodiments of the present invention can achieve excellent formability, more specifically, a total elongation (El) of 15.0% or more. 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. The 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 obtained with the longitudinal direction of the test piece preferably parallel to the rolling direction of the steel sheet. If the rolling direction of the steel plate cannot be identified, JIS No. 5 test pieces may be taken from multiple directions within the steel plate plane. In this case, the multiple directions within the steel plate plane are obtained at regular angles within the steel plate plane, and the arithmetic mean of the multiple measured values ​​obtained is used as the tensile strength or total elongation of the steel plate. The multiple directions within the steel plate plane shall be four or more. For example, when measurements are taken from four directions within the steel plate plane, the angle between each direction shall be approximately 45°. If it is difficult to take JIS No. 5 test pieces, other test pieces described in JIS Z 2241:2022 may be used instead of JIS No. 5 test pieces.

[0067] Despite having high strength, specifically a tensile strength of 400 MPa or more, the steel sheet according to the present invention maintains 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 that require both strength, formability, and good appearance after forming. 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 aforementioned chemical composition and metallographic characteristics. In portions of a steel sheet that have undergone relatively little processing, such as press forming, the metallographic characteristics do not change significantly before and after forming. A relatively little 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] <Steel sheet manufacturing method> 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 manufacturing a steel sheet according to an embodiment of the present invention includes: a hot rolling process comprising: 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 temperature at the end of the finish rolling is 800 to 1000°C; a pickling step of pickling the obtained hot-rolled steel sheet; A cold rolling process in which the pickled hot-rolled steel sheet is cold-rolled at a reduction ratio of 20 to 90%; An annealing step including annealing the obtained cold-rolled steel sheet, which 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 in the 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 plate is heated. From the viewpoint of productivity, the slab used is preferably cast by a continuous casting method, 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 plate. Therefore, the slab must be heated before being subjected to hot rolling to dissolve the alloying elements. 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] This method requires appropriate control of 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 layered dispersion of alloying element-enriched and -depleted regions in the thickness direction affects the metallographic structure after hot rolling and the metallographic structure after cold rolling and annealing. With this method, the metallographic structure cannot be properly engineered. Therefore, the final metallographic structure may not be able 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 the proportion of crystal grains containing two or more martensite grains within the crystal grains may not be able to be controlled 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, it may not be sufficiently removed even by such descaling. In this case, the scale that is not removed and remains on the surface is pushed 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] [Finishing rolling and winding] The heated slab, or the slab that has been subjected to rough rolling 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, if the finish rolling completion temperature exceeds 1000°C, a device for heating the steel sheet is required 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. By setting the coiling temperature to 500 to 700°C, the growth of oxide scale can be suppressed. If the coiling temperature is less than 500°C, the strength of the obtained hot-rolled steel sheet will be too high, 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 final metal structure may not be able to be composed mainly of bainite.

[0075] [Pickling process] 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 820-950°C: 60-500 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 metallographic 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 metallographic 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 formed. 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 the maximum heating temperature to the cooling stop temperature of 500-600°C: 10°C / sec or more] After holding 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 will be generated during cooling, making it impossible to achieve a bainite area ratio of 70% or more and / or to keep the residual structure at 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 Figure 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 grains to 90% or more.

[0079] [(c) Dwell time in the temperature range of 500-600°C: 100-600 seconds] The cold-rolled steel sheet cooled to a cooling stop temperature of 500 to 600°C is then dwelled in a temperature range of 500 to 600°C (dwell temperature) for 100 to 600 seconds. In this manufacturing method, dwelling in a temperature range of 500 to 600°C includes 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 a relatively high temperature range of 500 to 600°C for 100 to 600 seconds, as explained above in relation to FIG. 1(a), it is possible to promote the formation of bainitic ferrite and reduce the misorientation of each bainitic ferrite. In addition, unlike the cases shown in Figures 1(b) and (c), carbides are not formed between bainitic ferrite or within the bainitic ferrite itself, or their formation is sufficiently suppressed. This results in a gradual enrichment of carbon in the untransformed austenite between each bainitic ferrite. 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 each bainitic ferrite into martensite. This makes it possible to obtain a metallographic structure consisting primarily 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 misorientation of 15° or more corresponding to the initial austenite grain boundaries can be increased to 90% or more, and the average misorientation within the crystal grains can be controlled to 0.60° or less.

[0080] If the dwell temperature is below 500°C, a large amount of carbides precipitate between bainitic ferrite and / or within the bainitic ferrite itself, consuming much of the untransformed austenite that could be transformed to martensite. In addition, carbon enrichment is insufficient, preventing the formation of sufficient carbon-enriched untransformed austenite 3 between bainitic ferrite 2, as shown in Figure 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 within the crystal grains to 0.60° or less. On the other hand, if the dwell temperature exceeds 600°C, excessive 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 formation 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 final 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 formed in the carbon-enriched regions of austenite. In this case, the desired area fraction of the metallographic structure will not be achieved in the final 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 at an average cooling rate of 40°C / s or more in a temperature range of 200 to 500°C. Cooling under these conditions allows martensite to be properly formed from untransformed austenite, where carbon is concentrated and present between bainitic ferrite particles. As a result, it becomes possible to form crystal grains containing two or more martensite grains within the crystal grains surrounded by grain boundaries with a crystal orientation misorientation of 15° or more, at a rate of 90% or more of the total crystal grains. If the average cooling rate in the temperature range of 200 to 500°C is less than 40°C / s, the desired martensite area ratio cannot be achieved in the final metal structure, and / or the rate of the crystal grains containing two or more martensite grains within the crystal grains surrounded by grain boundaries with a crystal orientation misorientation of 15° or more cannot be formed at a rate of 90% or more of the total crystal grains. Therefore, the average cooling rate in the temperature range of 200 to 500°C must be 40°C / second or more, and the upper limit is, for example, 200°C / second or less, and preferably 80°C / second or less.

[0083] [Plating process] The surface of the obtained cold-rolled steel sheet may be subjected to a plating treatment for the purpose of improving corrosion resistance, etc. The plating treatment may be a treatment such as hot-dip plating, alloying hot-dip plating, or electroplating. For example, the steel sheet may be subjected to hot-dip galvanizing treatment as the plating treatment, or may be subjected to alloying treatment after hot-dip galvanizing treatment. The specific conditions for the plating treatment and alloying treatment are not particularly limited and may be any appropriate conditions known to those skilled in the art. For example, in the hot-dip galvanizing treatment, 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 a temperature 40°C lower than the hot-dip galvanizing bath temperature (hot-dip galvanizing bath temperature -40°C) to a temperature 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 treatment, 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. [Example]

[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, and these slabs were heated under the heating temperature and heating time conditions shown in Table 2 and 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 rolling 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] [Table 1-1]

[0088] [Table 1-2]

[0089] [Table 2]

[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 a grain" in Figure 3 refers to the average misorientation within a grain of crystal grains surrounded by grain boundaries with a crystal misorientation of 15° or more, and is measured as explained in the section "Method for measuring the average misorientation within a grain of crystal grains surrounded by grain boundaries with a crystal misorientation of 15° or more."

[0092] [Ratio of crystal grains containing two or more martensite grains] The "proportion of crystal grains containing two or more martensite grains" in Figure 3 refers to the proportion of crystal grains that contain two or more martensite grains among crystal grains surrounded by grain boundaries with a crystal orientation misorientation of 15° or more, and is measured as explained in the section "Method for measuring the proportion of crystal grains that contain two or more martensite grains among crystal grains surrounded by grain boundaries with a crystal orientation misorientation of 15° or more."

[0093] [Tensile strength (TS) and total elongation (El)] Tensile strength (TS) and total elongation (El) were measured by taking No. 5 tensile test pieces according to JIS Z2241:2022 from the steel sheets, with the longitudinal direction perpendicular to the rolling direction and the thickness direction, and conducting tensile tests in accordance with JIS Z2241:2022.

[0094] [Appearance after molding] The appearance after forming was evaluated using pressed parts made by press-forming steel plates blanked to 600 mm square so that the radius of curvature R at the center was 1200 mm. The pressed parts were subjected to a 2.5% strain during press forming. The surface of the center 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 almost parallel to the rolling direction were judged to be ghost lines and evaluated. A random 100 mm x 100 mm area in the center of the pressed part was visually inspected; if no streaks were observed, it was judged as passing (OK), and if streaks were observed, it was judged as failing (NG). In this test, the appearance after forming was evaluated using a press part simulating a door outer, but the evaluation subject can also be a formed part that can be assumed 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 can also be evaluated, and equivalent evaluations can be made using these test methods. In the case of test pieces taken from 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 being able to achieve both strength, formability, and appearance after forming. The results are shown in Table 3.

[0096] [Table 3]

[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, because the Si content was low, a relatively large amount of carbides such as cementite precipitated 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 misorientation of 15° or more (high-angle grain boundaries) decreased, resulting in a poor appearance after forming. In Comparative Example 24, the high Si content prevented adequate removal of scale formed during hot rolling, resulting in a poor appearance after forming. In Comparative Example 25, TS decreased due to the low Mn content. In addition, the total area fraction 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 increased the risk of embrittlement cracking due to the residual coarse alloy carbides. Therefore, hot rolling was not performed. In Comparative Example 28, the finish rolling temperature was low, which resulted in a deterioration in the shape of the steel sheet, making it impossible to thread the sheet after hot rolling. In Comparative Example 29, the coiling temperature was low, which increased the strength of the hot-rolled steel sheet, resulting in an excessive cold-rolling load and making it impossible to properly complete the cold-rolling process. In Comparative Example 30, the coiling temperature was high, which prevented the final metal structure from being composed primarily 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 each grain decreased, resulting in poor appearance after forming. In Comparative Example 31, the reduction rate in the cold-rolling process was low, which prevented the cold-rolled steel sheet from having sufficient flatness, making it impossible 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 could not be sufficiently formed. As a result, the 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 could not be 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 process to the cooling stop temperature of 500 to 600°C was slow, so that excessive ferrite was formed during cooling, and the metal structure in the steel sheet could not be composed of a structure mainly composed of bainite, and martensite could not be dispersed and present inside the crystal grains during the process of creating this structure. 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 metallographic 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, which consumed much of the untransformed austenite that could transform 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 annealing temperature resulted in the formation of a large amount of ferrite, making it impossible to form a metallographic structure primarily composed of bainite. Furthermore, the process of developing this structure failed to allow martensite to be dispersed within the crystal grains. As a result, the proportion of crystal grains surrounded by high-angle boundaries that contained two or more martensite grains within them decreased, resulting in poor appearance after forming. In Comparative Example 40, the short annealing time resulted in the failure to achieve the desired bainite area fraction in the final metallographic structure. The proportion of crystal grains surrounded by high-angle boundaries that contained two or more martensite grains within them also decreased, and the mean misorientation within these crystal grains could not be controlled to 0.60° or less. As a result, El decreased and the appearance after forming deteriorated. In Comparative Example 41, the long annealing time 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 formation was also prevented. As a result, the TS decreased, and the proportion of crystal grains surrounded by high-angle grain boundaries that contained two or more martensite grains within the grains 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 the proportion of crystal grains surrounded by high-angle grain boundaries containing two or more martensite grains within each grain decreased. As a result, the TS decreased and the appearance after forming deteriorated.

[0100] In contrast, all of the steel sheets according to the examples of the present invention have a predetermined chemical composition, form a microstructure primarily composed of bainite, and increase the proportion of crystal grains surrounded by high-angle grain boundaries containing two or more martensite grain regions to 90% or more. This achieves high tensile strength of 400 MPa or more, and significantly reduces the occurrence of ghost lines by suppressing the generation of minute irregularities on the steel sheet surface even when strain is applied during 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 is increased, thereby significantly improving the formability of the steel sheet. Furthermore, when a residual structure is present in the examples of the present invention, the residual structure is 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 varied. 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] [Table 4]

[0103] Referring to Table 4, in Comparative Examples 45 and 46, the short heating time of the slab 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 heating time of the slab caused 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 scale remaining on the surface was pressed 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 bainite-based structure was formed, and the proportion of crystal grains surrounded by high-angle boundaries that contained two or more martensite grain regions was increased to 90% or more. This resulted in a 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. In addition, by controlling the average misorientation of crystal grains surrounded by high-angle boundaries to 0.60° or less, the elongation of the steel sheet was increased, thereby significantly improving the formability of the steel sheet. [Explanation of symbols]

[0105] 1 Austenite grain boundary 2 Bainitic ferrite 3 Untransformed austenite 4. Carbide 10 grains 11 Grain boundaries with a crystal orientation difference of 15° or more 12 Bainite 13 Martensite grains

Claims

1. In mass%, C: 0.03-0.08%, Si: 0.10 to 1.50%, Mn: 0.50-3.00%, 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: 0-2.000%, Mo: 0-1.000%, 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: 0-0.100%, V: 0-1.000%, Ta: 0-0.100%, Co: 0-3.000%, Sn: 0-1.000%, Sb: 0 to 0.500%, As: 0 to 0.050%, Mg: 0 to 0.050%, Zr: 0 to 0.050%, Ca: 0-0.0500%, Y: 0 to 0.0500%, La: 0 to 0.0500%, Ce: 0 to 0.0500%, Bi: 0 to 0.0500%, and The balance has a chemical composition consisting of Fe and impurities, In terms of area ratio, Bainite: 70-95%, and Martensite: 3 to 20%, and The sum of bainite and martensite is 90% or more, 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: 0.001-2.000%, Mo: 0.001 to 1.000%, Ti: 0.001 to 0.500%, Nb: 0.001-0.500%, B: 0.0001 to 0.0100%, Cu: 0.001 to 1.000%, Ni: 0.001 to 1.00%, W: 0.001-0.100%, V: 0.001-1.000%, Ta: 0.001 to 0.100%, Co: 0.001 to 3.000%, Sn: 0.001 to 1.000%, Sb: 0.001 to 0.500%, As: 0.001 to 0.050%, Mg: 0.0001-0.050%, Zr: 0.0001 to 0.050%, Ca: 0.0001-0.0500%, Y: 0.0001-0.0500%, La: 0.0001 to 0.0500%, Ce: 0.0001 to 0.0500%, and Bi:0.0001~0.0500% The steel sheet according to claim 1, characterized in that it contains at least one of the following:

3. An outer panel member of an automobile comprising the steel sheet according to claim 1 or 2.

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