Panel

Abstract

Provided is a sheet material comprising a steel sheet having a metal structure including martensite, the sheet material being characterized in that the surface property aspect ratio (Str) of the steel sheet in a flat portion of a center-side portion of the sheet material is 0.50-1.00, and the surface roughness parameter (Sa) of the steel sheet in the flat portion and an end portion of the center-side portion of the sheet material is 0.50 [mu] m or less.

Description

TECHNICAL FIELD

[0001] The present application relates to a panel. BACKGROUND

[0002] In recent years, in order to protect the global environment, improvement in fuel efficiency of automobiles is required. In order to make the vehicle body lightweight while ensuring safety, further high strength is required for a steel sheet for automobiles with respect to improvement in fuel efficiency of automobiles. Such a requirement for high strength is not limited to a cross member, a pillar, and the like as a structural member, but is also applied to an outer panel member (a hood, a fender, a door panel, a roof panel, and the like) of an automobile.

[0003] On the other hand, there is a tendency that the outer panel member of an automobile is more and more complicated in shape. If a steel sheet is made high in strength and thin in wall thickness in order to make the vehicle body lightweight, unevenness is easily generated on the surface of the steel sheet when formed into a complicated shape. If the surface is uneven, the appearance after forming is deteriorated. The outer panel member is important not only in strength and the like but also in design and surface quality, and thus it is required to be excellent in appearance after forming.

[0004] In association with this, in Patent Literature 1, an outer panel member is described, which is an outer panel member including a steel sheet, wherein the steel sheet has a flat portion, in a surface layer region of the flat portion, a metal structure contains ferrite at 80% or more in volume fraction, an average crystal grain diameter of the ferrite is 1.0 to 15.0 μm, a strength ratio X ODF{001} / {111},S of a {001} orientation to a {111} orientation of the ferrite is 0.30 or more and less than 3.50, and in a case where a uniform elongation measured with a tensile test piece cut out from the flat portion is set as uEl1, and a theoretical uniform elongation derived from the volume fraction of each of ferrite and martensite, hardness, and average crystal grain diameter in the metal structure of an inner region of the flat portion, and a sheet thickness of the flat portion by a prescribed formula is set as uEl2, uEl1 / uEl2 is 0.44 to 0.80. In addition, in Patent Literature 1, it is taught that the outer panel member after forming from a raw material is excellent in surface properties and excellent in dent resistance (also referred to as impact resistance) according to the above-described constitution.

[0005] In Patent Literature 2, an outer panel member is described, which is an outer panel member having a steel sheet containing martensite, wherein a flat portion of a center side portion of the outer panel member has a surface roughness parameter (Sa) of Sa≤0.500 μm, and in a lath of the martensite, 15 2The ratio YS1 / YS2 of the yield stress YS1 measured using the tensile test piece cut out from the flat portion to the yield stress YS2 measured using the tensile test piece cut out from the end portion of the panel member is 0.90 to 1.10. In addition, in Patent Document 2, it is taught that the panel member according to the above constitution can provide an appearance after forming from a raw material that is excellent and a dent resistance that is excellent.

[0006] In Patent Document 3, an outer panel member for a vehicle is described, which is an outer panel member for a vehicle provided with a steel sheet, wherein, in plan view, the rolling direction of the steel sheet extends along the left-right direction of the vehicle body. In addition, in Patent Document 3, it is taught that an outer panel member for a vehicle that reduces ghost lines can be provided.

[0007] Prior Art Documents Patent Documents Patent Document 1: International Publication No. 2022 / 004795 Patent Document 2: International Publication No. 2021 / 149810 Patent Document 3: International Publication No. 2023 / 026469 SUMMARY

[0008] Problems to be Solved by the Invention As described above, if the steel sheet is made high-strength and thin-walled for the purpose of weight reduction, unevenness is easily generated on the surface of the steel sheet when formed into a complex shape, and the appearance after forming is degraded. On the other hand, further weight reduction of the panel member is also required in the automobile industry and the like, and in order to achieve such weight reduction, it is necessary to make the panel member high-strength as before or higher. Therefore, there is a high demand for a panel member that can solve the problem of the appearance after forming even if high-strength equal to or higher than before is performed.

[0009] Therefore, the purpose of the present application is to provide a panel member whose appearance after forming is excellent even if high-strength, by a new constitution.

[0010] Means for Solving the Problems The present inventors have conducted research, focusing particularly on the metal structure of the steel sheet constituting the panel member and the surface properties of the panel member, in order to achieve the above purpose. As a result, the present inventors have found that high-strength is achieved by containing martensite in the metal structure of the steel sheet constituting the panel member, and by appropriately selecting the metal structure of the steel sheet and the conditions of press forming and the like in such a way that two different parameters related to the surface properties of the panel member, more specifically, the surface property aspect ratio Str and the surface roughness parameter Sa, are controlled within a specific range at a prescribed position of the panel member after forming, an excellent appearance can be achieved even in a high-strength panel member, and the present application has been completed.

[0011] The present application achieving the above object is as follows.

[0012] (1) A sheet member characterized by being a sheet member including a steel sheet having a metal structure containing martensite, wherein a surface aspect ratio Str of the steel sheet of a flat portion of a center side portion of the sheet member is 0.50 to 1.00, a surface roughness parameter Sa of the steel sheet of the flat portion and an end portion of the center side portion of the sheet member is 0.50 μm or less.

[0013] (2) The sheet member according to the above (1), characterized in that a ratio of an average KAM value of ferrite contained in the steel sheet to a volume fraction Vm of the martensite, that is, average KAM value / Vm is 1.8 or more.

[0014] (3) The sheet member according to the above (1) or (2), characterized in that the steel sheet is a coated steel sheet having a coating layer on at least one surface.

[0015] (4) The sheet member according to any one of the above (1) to (3), characterized in that a metal structure of the steel sheet of the flat portion consists of, in area %: ferrite: 75 to 95%, martensite: 5 to 25%, and at least one of bainite, pearlite, and residual austenite: 0 to 10% in total, an average inter-particle spacing of the martensite is 2.5 μm or less, a standard deviation of an area fraction of the martensite in a direction perpendicular to a rolling direction and a sheet thickness direction is 1.5% or less.

[0016] (5) The sheet member according to any one of the above (1) to (4), characterized in that a sheet thickness of the flat portion is 0.2 to 0.8 mm.

[0017] (6) The sheet member according to any one of the above (1) to (5), characterized by having a tensile strength of 400 to 900 MPa.

[0018] Effects of the Invention According to the present application, it is possible to provide a sheet member excellent in appearance even after high-strength forming. DETAILED DESCRIPTION

[0019] <Sheet Member> The sheet member of the embodiment of the present application is characterized by including a steel sheet having a metal structure containing martensite, The surface property aspect length-width ratio Str of the steel sheet of the flat portion of the center side portion of the above-described panel is 0.50 to 1.00, The surface roughness parameter Sa of the steel sheet of the flat portion and the end portion of the center side portion of the above-described panel is 0.50 μm or less.

[0020] As described above, in recent years, the demand for lightweight of the outer panel of an automobile (an engine hood, a fender, a door panel, a roof panel, etc.) is also increasing, and thus, as in the case of the structural member, high strength and thin wall are also required for these outer panels. On the other hand, in these outer panels, a DP (Dual Phase) steel having a relatively low yield strength is mostly used from the viewpoint of avoiding surface defects called surface strain generated at the time of press forming, etc. However, in the case of the DP steel in which a soft structure composed of ferrite and a hard structure composed of martensite are mixed, uneven deformation in which the soft structure and the periphery thereof preferentially deform is easily caused at the time of press forming, etc., and minute unevenness is generated on the surface of the panel after forming, thereby generating appearance defects called ghost lines. If explained in more detail, at the time of press forming, etc., the soft structure composed of ferrite is recessed by a large amount of deformation, and on the other hand, the hard structure composed of martensite is small in deformation amount. Therefore, the hard structure is not recessed but is raised in a convex manner as compared with the soft structure. As a result, the deviation in deformation amount is generated particularly in the rolling right angle direction of the panel, and the ghost lines are generated in a band shape (strip shape).

[0021] Therefore, the present inventors have paid attention to the metal structure of the steel sheet constituting the panel and the surface property of the panel and have conducted research in order to balance high strength and appearance after forming into a panel. First, the present inventors have found that by including martensite in the metal structure of the steel sheet constituting the panel, desired high strength, for example, a tensile strength of 400 MPa or more can be achieved. On the other hand, in association with such high strength, the appearance of the panel after forming is generally reduced, and thus the present inventors have paid attention to the surface property of the panel after forming and have further conducted research in order to suppress such reduction in appearance after forming. As a result, the present inventors have found that by appropriately selecting the metal structure of the steel sheet and the conditions of press forming, etc. in such a manner that two different parameters related to the surface property of the panel, more specifically, the surface property aspect length-width ratio Str and the surface roughness parameter Sa are controlled within a specific range at a prescribed position of the panel after forming, an excellent appearance can be achieved even in the panel after high strength.

[0022] More specifically, the surface property aspect ratio Str, which is one of the spatial parameters of the surface property prescribed in JIS B0681-2:2018, represents the strength of anisotropy of the surface and takes a value in the range of 0 to 1.00. Generally, when the value of Str approaches 0, the anisotropy becomes stronger, and striations and the like are generated on the surface, whereas when the value of Str approaches 1.00, the surface becomes isotropic regardless of the direction. As a result of the research by the present inventors, it was found that, in the case where strain is imparted by forming such as press forming, by appropriately selecting the metal structure of the steel sheet serving as a raw material and the conditions of press forming and the like, the surface property aspect ratio Str of the steel sheet of the flat portion of the center side portion of the panel is controlled in the range of 0.50 to 1.00, which is very effective in suppressing the generation of striae on the surface of the panel. The striae are related to the rib pattern of the surface of the panel, and therefore from the viewpoint of suppressing the generation of the striae, it is preferable that the slight unevenness of the surface of the panel be further isotropic, and therefore it is preferable that the value of Str be closer to 1.00.

[0023] However, by appropriately controlling only Str, the appearance after forming is sometimes not sufficiently improved, and therefore the present inventors further researched in association with the surface property of the panel. As a result, the present inventors found that, by appropriately selecting the metal structure of the steel sheet serving as a raw material and the conditions of press forming and the like, in addition to the control of Str, the surface roughness parameter Sa of the steel sheet of the flat portion of the center side portion of the panel and the end portion is controlled in the range of 0.50 μm or less, whereby even in the case where strain is imparted by forming such as press forming, the generation of appearance defects caused by slight unevenness of the surface of the panel can be significantly suppressed or reduced. Here, the surface roughness parameter Sa refers to the average of the absolute values of z(x, y) in the reference area (A) prescribed in 4.1.7 "arithmetical mean height of the scale limited surface" of JIS B0681-2:2018.

[0024] Therefore, the sheet member according to the embodiment of the present application sufficiently maintains high strength based on the martensite contained in the metal structure of the steel sheet, and controls the surface texture of the formed sheet member in the range of Str of 0.50 to 1.00 and Sa of 0.50 μm or less using 2 different parameters, the surface texture aspect ratio Str and the surface roughness parameter Sa, thereby being able to significantly suppress the occurrence of appearance defects such as ghost lines on the surface of the sheet member even in the case where strain is imparted by forming such as press forming. In particular, even in the case where the metal structure of the steel sheet as a raw material contains martensite, by controlling the surface roughness parameter Sa in the range of 0.50 μm or less and forming a further isotropic surface texture with the surface texture aspect ratio Str of 0.50 or more, it is possible to significantly improve the appearance of the high-strength sheet member after forming, and this fact is first clarified by the present inventors. Therefore, the sheet member according to the embodiment of the present application is particularly useful in the application to automobile outer panel members where the requirement for high strength is relatively high. Hereinafter, each component of the sheet member according to the embodiment of the present application is described in more detail.

[0025] [Str of the steel sheet of the flat portion of the center side portion of the sheet member: 0.50 to 1.00] In the embodiment of the present application, the surface property aspect ratio Str of the flat portion of the center side portion of the formed sheet member is controlled to be 0.50 to 1.00. First, the sheet member of the embodiment of the present application includes three portions, specifically, (i) an end edge portion, (ii) an end portion, and (iii) a center side portion other than the end edge portion and the end portion. The end edge portion of (i) is a portion bent by hemming (HEM) processing or fixed to other members by welding such as spot welding. The end portion of (ii) is a portion on the center side of the sheet member with respect to the end edge portion, and is a portion apart from the portion fixed to other members by hemming processing, welding, or the like. The end portion is a portion several mm advanced toward the center side of the sheet member from the end edge portion, and is a portion substantially not affected by the processing for fixing the sheet member to other members. The "substantially not affected" in this case means that the amount of change in properties caused by the processing for fixing the sheet member to other members is within several %. The center side portion of (iii) is a portion visually recognized from the outside as an exterior, for example, an exterior of an automobile. In this specification, a portion having a radius of curvature of 500 mm or more in the center side portion of the sheet member is referred to as a flat portion. In addition, in the case where a plating layer and / or a paint layer is present on the surface of the sheet member, the flat portion means a flat portion as a whole of the sheet member including the plating layer and / or the paint layer. By controlling the surface property aspect ratio Str of the flat portion to be within the range of 0.50 to 1.00, a further isotropic surface property can be formed, and in association therewith, a sheet member having an excellent appearance can be provided. As described above, a ghost line is related to the rib pattern of the surface of the sheet member, and therefore, from the viewpoint of suppressing the generation of the ghost line, it is preferable that the minute unevenness of the surface of the sheet member be isotropic. Therefore, from the viewpoint of further improving the appearance after forming, it is more preferable that the Str be higher, for example, 0.55 or more, 0.60 or more, 0.65 or more, 0.70 or more, 0.75 or more, or 0.80 or more. The upper limit is not particularly limited, and for example, the Str can be 0.95 or less, 0.90 or less, or 0.85 or less.

[0026] [Measurement of Str] The surface property aspect ratio Str of the steel sheet of the flat portion of the center side portion of the plate member is determined as follows. First, a test piece is cut out from the flat portion of the center side portion of the plate member, and then a 8 mm x 8 mm region in the surface of the cut test piece (in the case where a plated layer and / or a paint layer are present on the surface of the test piece, the surface of the plated layer and / or the paint layer) is subjected to three-dimensional measurement using a VK-X3000 white interferometer manufactured by KEYENCE Co. As to the measurement conditions at this time, the measurement magnification is set to 10 times, the resolution of the XY plane is set to 3 μm, and the resolution of the Z space is set to 1 nm, and measurement is continuously performed. Then, tilt correction based on quadric surface correction is performed on the measurement region, and the radius of curvature of the entire plate member is removed. Then, filter processing that removes irregularities with a cycle of 0.8 mm or less is performed, and Str is determined in accordance with the provisions of JIS B0681-2:2018.

[0027] [Surface roughness parameter Sa of the flat portion of the center side portion and the end portion of the steel sheet of the plate member: 0.50 μm or less] In the embodiment of the present application, the surface roughness parameter Sa of the steel sheet of the flat portion of the center side portion and the end portion of the plate member after forming is controlled to be 0.50 μm or less. As in the case of Str, and in the case where a plated layer and / or a paint layer are present on the surface of the plate member, the flat portion and the end portion refer to the flat portion and the end portion of the entire plate member including the plated layer and / or the paint layer. The surface roughness parameter Sa is the average value of the height difference (absolute value) of each point with respect to the average surface of the plate member surface to which strain is imparted at the time of forming. On the basis of the control of the surface property aspect ratio Str described above, by controlling the surface roughness parameter Sa to be 0.50 μm or less in both the flat portion of the center side portion of the plate member and the end portion of the plate member, the effect of the combination of the isotropic surface property and the lower surface roughness is utilized, and the appearance of the high-strength plate member after forming can be significantly improved. From the viewpoint of further improving the appearance after forming, the lower the Sa, the more preferable, and in each of the flat portion of the center side portion of the plate member and the end portion of the plate member, for example, Sa can be 0.48 μm or less, 0.45 μm or less, 0.42 μm or less, 0.40 μm or less, 0.38 μm or less, or 0.35 μm or less. The lower limit is not particularly limited, and in each of the flat portion of the center side portion of the plate member and the end portion of the plate member, Sa can be, for example, 0.05 μm or more, 0.10 μm or more, 0.15 μm or more, or 0.20 μm or more.

[0028] [Measurement of Sa] The surface roughness parameter Sa of the steel sheet of the flat portion of the center side portion and the end portion of the plate member is determined as follows. First, a test piece is cut out from the flat portion of the center side portion of the plate member, and a 8 mm x 8 mm region of the surface of the cut test piece (in the case where a plated layer and / or a paint layer are present on the surface of the test piece, the surface of the plated layer and / or the paint layer) is subjected to three-dimensional measurement using a VK-X3000 white interferometer manufactured by KEYENCE Co., Ltd. With respect to the measurement conditions at this time, the measurement magnification is set to 10 times, the resolution of the XY plane is set to 3 μm, and the resolution of the Z space is set to 1 nm, and measurement is continuously performed. Then, tilt correction based on quadric surface correction is performed on the measurement region, and the curvature radius of the entire plate member is removed. Further, then, filtering processing that removes irregularities having a period of 0.8 mm or less is performed, and the arithmetic average height is calculated. The arithmetic average height thus obtained is determined to be the surface roughness parameter Sa of the steel sheet of the flat portion of the center side portion of the plate member. Instead of cutting out a test piece from the flat portion of the center side portion of the plate member, a test piece is cut out from the end portion of the plate member, and the surface roughness parameter Sa of the steel sheet of the end portion of the plate member is determined in the same manner as described above.

[0029] [Steel sheet] The plate member of the embodiment of the present application includes a steel sheet having a metal structure including martensite. The plate member of the embodiment of the present application includes at least a steel sheet having a metal structure including martensite, and the flat portion of the center side portion and the end portion of the plate member formed of the steel sheet have the above-described characteristics. Therefore, the plate member of the embodiment of the present application can include a material other than the steel sheet having a metal structure including martensite in a portion thereof. It is preferable that the plate member of the embodiment of the present application be essentially formed of a steel sheet having a metal structure including martensite, or be formed of or constituted by the steel sheet. Martensite is a structure having a high dislocation density and being hard. Therefore, by containing martensite in the metal structure of the steel sheet constituting the plate member, desired high strength can be achieved. The area fraction of the martensite in the metal structure is appropriately selected according to the strength required of the plate member, and is not particularly limited, and can be, for example, 5% or more, 7% or more, 10% or more, or 13% or more. Similarly, the area fraction of the martensite in the metal structure can be, for example, 25% or less, 22% or less, 20% or less, 18% or less, or 15% or less. In the present application, "martensite" includes not only quenched martensite (so-called primary martensite), but also tempered martensite.

[0030] [Mean KAM value / Vm: 1.8 or more] In the preferred embodiment of the present application, the ratio of the average KAM value of ferrite contained in the steel sheet constituting the panel to the volume fraction of martensite, i.e., the average KAM value / Vm, is controlled to be 1.8 or greater. The KAM (Kernel Average Misorientation) value has a tendency to increase as strain accumulates, and thus is generally known to be an effective index for evaluating the distribution of strain within grains. On the other hand, when the proportion of martensite is large, there is a tendency for the measured KAM value to increase. Therefore, by dividing the average KAM value of ferrite contained in the steel sheet by the volume fraction of martensite Vm, the degree of accumulation of strain caused by forming such as press forming can be evaluated in a form in which the influence of the fraction of martensite is excluded. In a panel applied as an outer panel member of an automobile or the like, dent resistance, which indicates the difficulty of denting due to stress from the outside, is also known as one of several important characteristics. In order to improve the dent resistance, it is effective to make it difficult to produce plastic deformation, i.e., it is effective to increase the yield stress. In connection therewith, in the preferred embodiment of the present application, by performing forming such as press forming in a manner such that the average KAM value / Vm is 1.8 or greater, a relatively high amount of strain can be introduced into the panel. By introducing a relatively high amount of strain into the panel, a relatively large number of dislocations can be introduced into the panel, and as a result, the amount of bake hardening at the time of paint baking can be increased to increase the yield stress of the panel. Here, the so-called bake hardening is a phenomenon in which interstitial elements (mainly carbon) move and become fixed in dislocations introduced by press forming through paint baking treatment at 100 to 220°C, thereby hindering the movement thereof and increasing the strength, and is also called strain aging. For this reason, according to the preferred embodiment of the present application, by controlling the average KAM value / Vm to be 1.8 or greater, the dent resistance of the panel can be improved. From the viewpoint of further improving these effects, the average KAM value / Vm is preferably higher, and for example, can be 2.0 or greater, 2.5 or greater, 3.0 or greater, 4.0 or greater, or 5.0 or greater. The upper limit is not particularly limited, and for example, the average KAM value / Vm can be 12.0 or less, 10.0 or less, or 8.0 or less.

[0031] [Measurement of average KAM value / Vm] The average KAM value is calculated by KAM (Kernel Average Misorientation) analysis in the crystal analysis method using SEM, i.e., EBSD (Electron Back Scattering Diffraction Patterns) measurement. The EBSD measurement is performed on a range of 50 μm x 50 μm of 1 / 8 to 3 / 8 thickness at an interval (pitch) of 0.05 μm, with the center being a position of 1 / 4 of the thickness of the steel sheet from the surface of the steel sheet. At this time, in the KAM analysis, for each pixel of the measurement point, the orientation difference with the adjacent six pixels is averaged, and the value is taken as the value of the central pixel, and a distribution map of the local crystal orientation difference can be created. The Vm is set to a volume fraction of 4000 or more for the GAIQ (Grain Average Image Quality) in the EBSD measurement under the same measurement conditions as the average KAM value. At this time, in the GAIQ analysis, when a region having an orientation difference of 15° or more is set as a grain boundary (Grain), the IQ value indicating the sharpness of the Kikuchi pattern of each pixel of the measurement point is averaged within one grain boundary, and the value is taken as the value of the central pixel, and using the difference in the sharpness of the Kikuchi pattern, a phase distribution map of ferrite and martensite with the grain boundary as the boundary can be created. The average KAM value in the region corresponding to the ferrite is calculated by referring to the KAM distribution map and the phase distribution map thus obtained. Further, by dividing the average KAM value by the Vm, the average KAM value / Vm in one field of view can be calculated. This operation is performed in five fields of view, and the average value thereof is taken as the average KAM value / Vm.

[0032] [Coating layer] In the sheet member of the embodiment of the present application, the steel sheet can not have a coating layer, or can be a coated steel sheet having a coating layer on at least one surface. For example, in the case where the surface properties of the sheet member are poor, a relatively thick coating layer is required in order to obtain a beautiful appearance. However, the surface properties of the sheet member of the embodiment of the present application are excellent, and thus the coating layer can be made thin, and thus is also very advantageous from the viewpoint of cost. Further, the yield stress of the sheet member can be improved in association with the bake hardening at the time of coating baking, and thus is also advantageous from the viewpoint of improving the dent resistance of the sheet member. In association with the preferred embodiment of the steel sheet, in the case where a steel sheet having a metal structure in which the martensite is finely and uniformly dispersed throughout the entire body is applied, the amount of bake hardening can be significantly improved due to the uniform dispersion of the martensite containing a large amount of dislocations. Thus, the yield stress can also be significantly improved, and thus the dent resistance of the sheet member can be further improved.

[0033] In the case where the steel sheet has a plated layer, a coating layer can be formed on the plated layer. As the coating layer, there is no particular limitation, and it can be any appropriate coating layer known to those skilled in the art. The film thickness of the coating layer is also not particularly limited, and can be, for example, 60 to 200 μm. The coating layer in a panel for an automobile generally contains, in order from the steel sheet side, an electrodeposited coating layer, an intermediate coating layer, a base coating layer, and a clear coating layer. The thickness of the electrodeposited coating layer can be, for example, 10 to 40 μm, and the thickness of the intermediate coating layer can be, for example, 20 to 60 μm. Similarly, the thickness of the base coating layer can be, for example, 10 to 30 μm, and the thickness of the clear coating layer can be, for example, 20 to 80 μm.

[0034] [Preferred embodiment of steel sheet] Hereinafter, a preferred embodiment of a steel sheet having a metal structure containing martensite, which is useful for achieving a surface property of a panel in which the surface aspect ratio Str of the flat portion of the center side portion of the steel sheet is 0.50 to 1.00, and the surface roughness parameter Sa of the flat portion of the center side portion and the end portion of the steel sheet is 0.50 μm or less when formed by press forming or the like, will be described in detail. However, the intention of these descriptions is to simply exemplify a preferred steel sheet for a panel constituting an embodiment of the present application, and the intention is not to limit the present application to an embodiment using such a specific steel sheet.

[0035] The preferred embodiment of the present application is characterized in that the steel sheet has, at the flat portion of the center side portion of the panel, a metal structure in which: is composed of, in area %: ferrite: 75 to 95%, martensite: 5 to 25%, and at least one of bainite, pearlite, and residual austenite: 0 to 10% in total, the average particle spacing of the martensite is 2.5 μm or less, the standard deviation of the area ratio of the martensite in the direction perpendicular to the rolling direction and the sheet thickness direction is 1.5% or less.

[0036] As described above, in the case of the DP steel in which the soft structure composed of ferrite and the hard structure composed of martensite exist in mixture, sometimes, uneven deformation in which the soft structure and the periphery thereof preferentially deform is easily caused at the time of press forming or the like, and a slight unevenness is generated on the surface of the formed sheet, thereby appearance defects called ghost lines are generated. On the other hand, along with the high-strengthening of the steel sheet, in order to improve the hardenability of the steel sheet, sometimes, elements such as Mn are added more. Mn is an element which easily segregates in a banded shape in the steel sheet, and more specifically, a Mn concentration region such as a center segregation, a microsegregation is formed at the time of casting, and the concentration region extends in the rolling direction by hot rolling or cold rolling, thereby Mn segregates in a banded shape. Therefore, due to the segregation of such Mn, there are a region in which the hardenability is high and a region in which the hardenability is low in the steel sheet. As a result, in the metal structure of the steel sheet after quenching, a hard structure in a striped shape is more generated. In this case, the generation of the ghost lines becomes particularly remarkable. On the contrary, as long as the segregation of Mn in the steel sheet can be sufficiently suppressed, the generation of such a hard structure in a striped shape can be reduced and the hard structure can be more uniformly dispersed in the metal structure. In this case, it is considered that even if strain is imparted by press forming or the like, the following surface properties of the sheet can be achieved: the surface properties of the steel sheet in the flat portion of the center side portion have a length-width ratio Str of 0.50 to 1.00, and the surface roughness parameter Sa of the steel sheet in the flat portion of the center side portion and the end portion is 0.50 μm or less. Moreover, in association therewith, it is considered that the generation of the slight unevenness on the surface of the sheet can be sufficiently reduced, and the generation of the ghost lines can be suppressed. However, along with the requirement of high-strengthening, particularly in the case where the amount of Mn added in the steel sheet is more, it is very difficult to actually and sufficiently suppress the segregation of Mn.

[0037] Therefore, the present inventors have further studied means for improving the appearance after forming while achieving the desired high strength by optimizing the ratio of ferrite as soft structure and martensite as hard structure in the metal structure. Specifically, the present inventors have focused on the distribution state of martensite as hard structure in the metal structure, and more specifically, have studied controlling the distribution of martensite from a different viewpoint from the reduction of Mn segregation. As a result, as will be described in detail below with respect to the method of manufacturing the steel sheet, the present inventors have found that by constituting the metal structure in the steel sheet before final annealing with a structure mainly composed of bainite and / or martensite, and then performing final annealing of the steel sheet having such a metal structure under prescribed conditions, it is possible to uniformly disperse the martensite in both the microscopic region and the macroscopic region in the resulting metal structure, regardless of the presence or absence and degree of Mn segregation. More specifically, the present inventors have found that by performing final annealing of the steel sheet having a metal structure composed of bainite and / or martensite under prescribed conditions, it is possible to control the average particle spacing of the martensite to be 2.5 μm or less in the microscopic region, and to control the standard deviation of the area fraction of the martensite in the direction perpendicular to the rolling direction and the sheet thickness direction to be 1.5% or less in the macroscopic region. By controlling the average particle spacing of the martensite to be 2.5 μm or less, it is possible to make the hard structure dense and uniformly disperse in the microscopic region. Furthermore, by controlling the standard deviation of the area fraction of the martensite in the direction perpendicular to the rolling direction and the sheet thickness direction to be 1.5% or less, it is possible to significantly reduce the variation of the hard structure in the macroscopic region. By satisfying both of these requirements, it is possible to form a metal structure in which the martensite as hard structure is fine and uniformly dispersed throughout the steel sheet. As a result, according to the preferred embodiment of the present application, it is possible to make the amount of deformation of the steel sheet more uniform, particularly in the width direction, even at the time of forming such as press forming, and in association therewith, by achieving a surface property of the sheet part having the desired Str and Sa, it is possible to achieve an excellent appearance after forming in which appearance defects such as ghost lines are significantly suppressed. For example, even if the uniformity of the martensite in the microscopic region is ensured, if the uniformity of the martensite in the macroscopic region is not ensured, it is not possible to form a metal structure in which the martensite is fine and uniformly dispersed throughout the steel sheet. Likewise, even if the uniformity of the martensite in the macroscopic region is ensured, if the uniformity of the martensite in the microscopic region is not ensured, since the martensite can become locally unevenly present, it is not possible to form a metal structure in which the martensite is fine and uniformly dispersed throughout the steel sheet. Therefore, in the preferred embodiment of the present application, in order to achieve an excellent appearance after forming in which appearance defects such as ghost lines are significantly suppressed, it becomes necessary to satisfy both of the following requirements: control the average particle spacing of the martensite to be 2.5 μm or less, and control the standard deviation of the area fraction of the martensite in the direction perpendicular to the rolling direction and the sheet thickness direction to be 1.5% or less.

[0038] Although not intending to be bound by any particular theory, it is believed that in order to make the martensite fine and uniformly dispersed throughout the steel sheet in the resulting metal structure, it is extremely important to form a large number of austenite nucleation sites highly dispersed in advance during heating in the final annealing. In connection therewith, the martensite structure further has a lower structure of lath bundles, lath blocks, laths, etc. in the prior austenite grains, and thus is a structure having a large number of various interfaces inside, as compared with structures such as ferrite. The bainite is also a structure having a large number of various interfaces inside, as in the case of the martensite. Therefore, by constituting the metal structure in the steel sheet before the final annealing by the bainite and / or the martensite, it becomes possible to disperse and generate carbides that can become nucleation sites of austenite very much on these interfaces at the stage of heating such a metal structure in the final annealing. Therefore, it is believed that by generating a large number of carbides on the interfaces and further heating the temperature to the 2-phase region of ferrite and austenite, it becomes possible to generate austenite fine and uniformly throughout the steel sheet. Finally, by quenching the steel sheet having such a metal structure, the martensite is generated from these austenites, and thus in the resulting metal structure, the average particle spacing of the martensite is controlled to be 2.5 μm or less, and the standard deviation of the area fraction of the martensite in the direction perpendicular to the rolling direction and the sheet thickness direction is controlled to be 1.5% or less. That is, it is believed that a metal structure in which the martensite is uniformly dispersed in both the microscopic region and the macroscopic region can be obtained. It is believed that by carrying out such a heat treatment, it is possible to make the martensite fine and uniformly dispersed throughout the steel sheet to the extent that the effect of the Mn segregation is eliminated. It has been generally considered to study the control of the distribution of hard structures from the viewpoint of reducing the Mn segregation itself in the past, and thus it is extremely unexpected and surprising that it is not necessarily dependent on the presence or absence, degree of the Mn segregation, the fact that it is possible to make the martensite uniformly dispersed in both the microscopic region and the macroscopic region in the resulting metal structure.

[0039] According to the preferred embodiment of the present application, in addition to the above-described insight relating to suppression of the occurrence of ghost lines, by controlling the area ratio of ferrite, which is a soft structure, to be 75 to 95%, good formability can be ensured, and by controlling the area ratio of martensite, which is a hard structure, to be 5 to 25%, and further controlling the chemical composition of the steel sheet within a prescribed range, high strength, for example, a tensile strength of 400 MPa or more, can be reliably achieved. As a result thereof, the trade-off between high strength and appearance after forming into a panel can be achieved at a high level. Furthermore, the steel sheet of the preferred embodiment of the present application has a metal structure in which martensite is fine and uniformly dispersed throughout, as described above, and therefore, in the paint bake treatment after forming into a panel, the bake hardening amount can be significantly improved due to the uniform dispersion of the martensite containing a large amount of dislocations. Therefore, by using this steel sheet, the yield stress of the resulting panel can be significantly improved, and therefore, it is also very advantageous from the viewpoint of improving the dent resistance of the panel.

[0040] Hereinafter, first, the metal structure of the steel sheet of the preferred embodiment of the present application will be described in more detail. The structure fraction is expressed in terms of area ratio, and therefore, the unit "%" of the structure fraction means area %. In addition, the metal structure of the steel sheet refers to the metal structure of the steel sheet in the flat portion of the center side portion of the panel. In this flat portion, the degree of forming is also low, and therefore, the characteristics of the metal structure shown below do not change particularly before and after forming such as press forming.

[0041] [Ferrite: 75 to 95%] Ferrite is a soft structure, and therefore, is easily deformed, and contributes to the improvement of ductility. If the area ratio of ferrite is 75% or more, sufficient formability can be obtained. From the viewpoint of improving formability, the area ratio of ferrite is more preferably higher, and for example, can be 78% or more, 80% or more, 82% or more, or 85% or more. On the other hand, if ferrite is excessively contained, the desired strength cannot be achieved in the steel sheet in some cases. Therefore, the area ratio of ferrite is set to be 95% or less. The area ratio of ferrite can be 93% or less, 90% or less, or 87% or less.

[0042] [Martensite: 5 to 25%] Martensite is a hard structure with a high dislocation density, and therefore, is a structure that contributes to the improvement of tensile strength. By making the area ratio of martensite 5% or more, a tensile strength of, for example, 400 MPa or more can be achieved. From the viewpoint of improving strength, the area ratio of martensite is more preferably higher, and for example, can be 7% or more, 10% or more, or 13% or more. On the other hand, if the area ratio of martensite is 25% or less, formability and appearance can be ensured. The area ratio of martensite can be 22% or less, 20% or less, 18% or less, or 15% or less.

[0043] [At least one of bainite, pearlite, and residual austenite: total 0 to 10%] The remaining structure other than ferrite and martensite can also be 0% by area ratio, but in the case where the remaining structure is present, the remaining structure is at least one of bainite, pearlite, and residual austenite. From the viewpoint of ensuring the above-described effects based on ferrite and martensite, the area ratio of the remaining structure, that is, at least one of bainite, pearlite, and residual austenite, is 10% or less, for example, 9% or less, 8% or less, 7% or less, 6% or less, 5% or less, 4% or less, or 2% or less. On the other hand, in order to set the area ratio of the remaining structure to 0%, a high degree of control is required during the manufacturing process of the steel sheet, and thus sometimes leads to a decrease in yield. Therefore, the area ratio of the remaining structure can also be 0.5% or more or 1% or more.

[0044] [Identification of metal structure and calculation of area ratio] The identification of the metal structure and the calculation of the area ratio are performed by FE-SEM (field emission scanning electron microscope) and optical microscopy after etching using a nitric acid ethanol reagent or Le Pera solution, and X-ray diffraction. For a region of 100 μm x 100 μm in the steel sheet section in a direction perpendicular to the sheet surface, observation of the structure using FE-SEM and optical microscopy is performed at a magnification of 1000 to 50000 times. For any metal structure, the measurement site is set to three places, and the area ratio is determined by calculating the average of these measurement values. For example, in the case where it is not possible to secure a measurement region of 100 μm in the sheet thickness direction due to a thin sheet thickness, the length in the sheet thickness direction is reduced, and a measurement region of 10000 μm 2 The same applies to "a region of 100 μm x 100 μm" in the following description.

[0045] The area ratio of ferrite is found by observing a region of 100 μm x 100 μm in the range of 1 / 8 to 3 / 8 of the sheet thickness centered on the 1 / 4 position of the sheet thickness using an electron channel contrast image by FE-SEM (field emission scanning electron microscope). More specifically, it is possible to use image analysis software Image J, binarize ferrite and martensite according to the difference in brightness, and calculate the area fraction of ferrite. In the case of using Le Pera solution, the black portion of the image data is ferrite, and the white portion is martensite and residual austenite.

[0046] The area ratio of the martensite is calculated by the following procedure. First, the observation surface of the test piece is etched with Le Pera solution, and then the 100 μm x 100 μm region in the range of 1 / 8 to 3 / 8 of the plate thickness from the center of the 1 / 4 position of the plate thickness is observed with the FE-SEM. In the Le Pera etching, the martensite and the retained austenite are not etched, and thus the area ratio of the region not etched corresponds to the total area ratio of the martensite and the retained austenite. The area ratio of the martensite is calculated by subtracting the area ratio of the retained austenite measured by the X-ray diffraction method described later from the area ratio of the region not etched.

[0047] The area ratio of the retained austenite is calculated by the X-ray diffraction method. First, the test piece is removed from the plate surface to the 1 / 4 position of the plate thickness by mechanical polishing and chemical polishing. Then, the integrated intensity ratio of the diffraction peaks of the (200) and (211) of the bcc phase and the (200), (220), and (311) of the fcc phase obtained using Mo Kα rays is used to calculate the structure fraction of the retained austenite at the 1 / 4 position of the plate thickness. As the calculation method, the general 5-peak method is used. The calculated structure fraction of the retained austenite is determined as the area ratio of the retained austenite.

[0048] The identification and calculation of the area ratio of the bainite are performed by the following procedure. First, the observation surface of the test piece is etched with nitric acid ethanol reagent, and then the 100 μm x 100 μm region in the range of 1 / 8 to 3 / 8 of the plate thickness from the center of the 1 / 4 position of the plate thickness is observed with the FE-SEM. The bainite is identified in the observation region from the position and arrangement of the cementite contained in the structure. The bainite is classified into upper bainite and lower bainite, the upper bainite has cementite or retained austenite at the interface of the lath-shaped bainite ferrite. The lower bainite has cementite inside the lath-shaped bainite ferrite, the crystal orientation relationship between the bainite ferrite and the cementite is one kind, and the cementite has the same variant. Based on these characteristic points, the upper bainite and the lower bainite can be identified, respectively. In the present application, they are collectively referred to as bainite, and the area ratio of the identified bainite is calculated based on image analysis.

[0049] The identification and calculation of the area ratio of the pearlite are performed by the following procedure. First, the observation surface of the test piece is etched with nitric acid ethanol reagent, and then the 1 / 8 to 3 / 8 of the plate thickness from the center of the 1 / 4 position of the plate thickness is observed with the optical microscope at a magnification of 500 or 1000. In the observation image of the optical microscope, the region of dark contrast is identified as the pearlite, and the area ratio of the region is calculated based on image analysis.

[0050] [average particle spacing of the martensite: 2.5 μm or less] In the preferred embodiment of the present application, the average particle spacing of the martensite, which is the hard structure, is controlled to be 2.5 μm or less. The average particle spacing of the martensite is an index indicating the uniformity of the distribution of the hard structure in a microscopic region. The smaller the average particle spacing of the martensite, the more densely and uniformly the hard structure is dispersed, and thus it can be said that the uniformity is higher. The more uniform the deformation amount of the steel sheet at the time of press forming, particularly in the width direction of the steel sheet, the better the appearance of the steel sheet after press forming becomes. The deformation amount of the steel sheet is strongly affected by the distribution state of the hard structure, and thus in order to make the deformation amount of the steel sheet uniform in the width direction of the steel sheet, it is necessary to make the distribution of the hard structure in the metal structure uniform. By controlling the average particle spacing of the martensite to be 2.5 μm or less in addition to the control of the standard deviation of the area fraction of the martensite described later, it is possible to make the deformation amount of the steel sheet more uniform in the width direction at the time of forming such as press forming, and as a result, it is possible to achieve a good appearance after forming. The average particle spacing of the martensite is preferably 2.4 μm or less, more preferably 2.2 μm or less, and most preferably 2.0 μm or less or 1.8 μm or less. The lower limit is not particularly limited, but for example, the average particle spacing of the martensite can also be 0.5 μm or more, 0.8 μm or more, or 1.0 μm or more.

[0051] [Measurement of Average Particle Spacing of Martensite] The average particle spacing of the martensite is determined as follows. First, a test piece of a cross section of the steel sheet having a direction perpendicular to the plate surface is collected, and the cross section is set as an observation surface. In the observation surface, a range from the 1 / 8 position to the 3 / 8 position of the plate thickness centered on the 1 / 4 position of the plate thickness is set as an observation region of 100 μm x 100 μm, and the martensite is identified using an FE-SEM. Specifically, using an image analysis software Image J, the ferrite and the martensite are binarized according to the difference in brightness, and the martensite is identified. In the case of using Le Pera solution, the black portion of the image data is the ferrite, and the white portion not corroded by Le Pera is the total structure of the martensite and the residual austenite. However, in the steel sheet of the preferred embodiment of the present application, since the area fraction of the residual austenite is sufficiently low compared to the area fraction of the martensite, the white structure can be regarded as the martensite. Next, the distance between the centers (barycenters) of all adjacent martensite grains in the identified martensite is calculated as the particle spacing based on the image analysis, and the average of the calculated particle spacings is determined as the average particle spacing of the martensite (strictly speaking, the particle including the martensite and / or the residual austenite).

[0052] [Standard Deviation of Area Fraction of Martensite in Direction Perpendicular to Rolling Direction and Plate Thickness Direction is 1.5% or Less] In a preferred embodiment of the present application, the standard deviation of the area fraction of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction is controlled to be 1.5% or less. This standard deviation is an index indicating the uniformity of hard structures in the macro region. The appearance, which becomes a problem at the time of press forming, depends on the slight unevenness of the steel sheet surface caused by the difference in the amount of deformation in the width direction of the steel sheet. Therefore, if the deviation of the area fraction of hard structures contained in the sheet thickness in the direction perpendicular to the rolling direction and the sheet thickness direction is large, the amount of deformation in the width direction of the steel sheet differs, as a result of which the slight unevenness is generated on the steel sheet surface. Therefore, it is effective to reduce the standard deviation of the area fraction of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction, that is, the width direction of the steel sheet. More specifically, by controlling this standard deviation to be 1.5% or less in addition to the control of the average particle interval of martensite described earlier, the deviation of the amount of deformation in the width direction of the steel sheet can be further reduced even at the time of forming such as press forming, as a result of which a good appearance after forming can be achieved. The standard deviation of the area fraction of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction is preferably 1.4% or less, more preferably 1.2% or less, and most preferably 1.0% or less. The lower limit is not particularly limited, but for example, this standard deviation can be 0.1% or more, 0.3% or more, or 0.5% or more.

[0053] [Measurement of the standard deviation of the area fraction of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction] The standard deviation of the area fraction of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction is determined as follows. First, a metal structure image in the steel sheet cross section of a region of 50 mm in the direction perpendicular to the rolling direction and the sheet thickness direction is obtained. In the case of an image of 10 mm or less, a plurality of images can be obtained, and they can be connected to set to 50 mm. In the case where the rolling direction is not clear, the cross section is observed at orientations of 0°, 45°, 90°, and 135° with respect to an arbitrary direction, and the cross section in which the aspect ratio of the precipitates is highest is regarded as the cross section parallel to the rolling direction, and the direction perpendicular thereto is determined as the direction perpendicular to the rolling direction and the sheet thickness direction. Next, the obtained image is divided by every 100 μm (0.1 mm) in the direction perpendicular to the rolling direction, and the area fraction of martensite in the entire sheet thickness is calculated in each range divided. Based on the area fraction of martensite calculated by adding up 500 divided images, the standard deviation of the area fraction of martensite is calculated. This operation is performed for three regions different in the position of the rolling direction, and the average of the standard deviations respectively calculated is determined as the standard deviation of the area fraction of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction.

[0054] In a case where the rolling direction of the steel sheet is unclear, as a method of determining the rolling direction of the steel sheet, for example, the following method is employed. After the plate thickness section of the steel sheet is finished by mirror polishing, the S concentration is measured using an electron probe micro analyzer (EPMA). The measurement conditions are that the acceleration voltage is set to 15 kV and the measurement pitch is set to 1 μm, and a distribution image of a range of 100 μm (in the plate thickness direction) x 500 μm (in a direction perpendicular to the plate thickness direction) of the center portion of the plate thickness is measured. At this time, an extended region where the S concentration is high is determined to be an inclusion such as MnS. The observation can also be performed in multiple fields of view at the time of observation. Next, with the plate thickness section observed for the first time by the above method as a reference, sections parallel to the plane rotated every 5° in the range of 0° to 180° with the plate thickness direction as the axis are observed by the above method. The average value of the lengths of the long axes of the plurality of inclusions in each section obtained is calculated, and the section in which the average value of the lengths of the long axes of the inclusions is the largest is determined. The direction parallel to the direction of the long axes of the inclusions in this section is determined to be the rolling direction.

[0055] [average crystal grain diameter of ferrite: 3.0 to 25.0 μm] According to another preferred embodiment of the present application, the average crystal grain diameter of the ferrite in the metal structure is 3.0 to 25.0 μm. By controlling the average crystal grain diameter of the ferrite in such a fine range, the appearance of the formed plate member can be further improved. The average crystal grain diameter of the ferrite can also be 5.0 μm or more, 7.0 μm or more, 8.0 μm or more, 9.0 μm or more, or 10.0 μm or more. Similarly, the average crystal grain diameter of the ferrite can be 22.0 μm or less, 20.0 μm or less, 16.0 μm or less, 14.0 μm or less, or 12.0 μm or less.

[0056] The average crystal grain diameter of ferrite in the steel sheet is determined as follows. First, in a region from the surface to a position at 1 / 2 of the sheet thickness in the sheet thickness direction of the steel sheet after etching with Le Pera reagent, 10 fields of view are observed at a magnification of 500x or 1000x, image analysis is performed using an image analysis software of "Photoshop (registered trademark) CS5" manufactured by Adobe Inc., and the area fraction of ferrite and the number of ferrite particles in each field of view are calculated, respectively. Next, the area fraction of ferrite and the number of ferrite particles in the 10 fields of view are summed up, respectively, the summed area fraction of ferrite is divided by the summed number of ferrite particles, and thus the average area fraction per ferrite particle is calculated. The equivalent circle diameter is calculated from the average area fraction and the number of particles, and the obtained equivalent circle diameter is determined as the average crystal grain diameter of ferrite. The observation area is set to 150 μm in the sheet thickness direction and 250 μm in a direction perpendicular to the sheet thickness direction (the observation area at this time is 150 x 250 = 37500 μm 2 ). For example, in the case where it is not possible to secure a measurement region of 150 μm in the sheet thickness direction due to a thin sheet thickness, the length in the sheet thickness direction is set to be reduced and the observation area of 37500 μm 2 The average crystal grain diameter of martensite and the average aspect ratio of martensite are also determined in the same manner.

[0057] [average crystal grain diameter of martensite: 1.0 to 5.0 μm] According to another preferred embodiment of the present application, the average crystal grain diameter of martensite in the metal structure is 1.0 to 5.0 μm. By controlling the average crystal grain diameter of martensite in such a fine range, the appearance of the formed sheet member can be further improved. The average crystal grain diameter of martensite can be 1.2 μm or more, 1.5 μm or more, 1.7 μm or more, or 2.0 μm or more. Similarly, the average crystal grain diameter of martensite can be 4.7 μm or less, 4.5 μm or less, 4.2 μm or less, 4.0 μm or less, 3.8 μm or less, 3.6 μm or less, or 3.4 μm or less.

[0058] The average crystal grain size of the martensite is determined as follows. First, 10 fields of view are observed at a magnification of 500x or 1000x in a region of the steel sheet from the surface to a position at a depth of 1 / 2 of the sheet thickness after etching with Le Pera reagent, and image analysis is performed using the image analysis software "Photoshop (registered trademark) CS5" manufactured by Adobe Inc. to calculate the area fraction of the martensite and the number of particles of the martensite in each field of view, respectively. Next, the area fraction of the martensite and the number of particles of the martensite in the 10 fields of view are summed up, respectively, and the average area fraction per martensite particle is calculated by dividing the summed area fraction of the martensite by the summed number of particles of the martensite. The equivalent circle diameter is calculated from the average area fraction and the number of particles, and the obtained equivalent circle diameter is determined as the average crystal grain size of the martensite. The observation area is set to 150 μm in the sheet thickness direction and 250 μm in a direction perpendicular to the sheet thickness direction (the observation area at this time is 150 x 250 = 37500 μm 2 ).

[0059] [Average aspect ratio of martensite: 2.5 or more] According to another preferred embodiment of the present application, the average aspect ratio of the martensite in the metal structure is 2.5 or more. By controlling the average aspect ratio of the martensite to be 2.5 or more, a state in which a greater strain is imparted can be achieved, and the strength of the steel sheet can be improved. The average aspect ratio of the martensite can be 2.6 or more, 2.8 or more, or 3.0 or more. The upper limit is not particularly limited, and for example, the average aspect ratio of the martensite can be 6.0 or less, 5.0 or less, 4.0 or less, 3.8 or less, or 3.6 or less.

[0060] The average aspect ratio of the martensite is determined as follows. First, 10 fields of view are observed at a magnification of 500x or 1000x in a region of the steel sheet from the surface to a position at a depth of 1 / 2 of the sheet thickness after etching with Le Pera reagent, and image analysis is performed using the image analysis software "Photoshop (registered trademark) CS5" manufactured by Adobe Inc. to calculate the ratio of the long diameter to the short diameter, i.e., the aspect ratio, for each martensite grain. Next, the aspect ratios of all the martensite grains in the 10 fields of view are arithmetically averaged, and thereby the average aspect ratio of the martensite is determined. The observation area is set to 150 μm in the sheet thickness direction and 250 μm in a direction perpendicular to the sheet thickness direction (the observation area at this time is 150 x 250 = 37500 μm 2 ).

[0061] [Preferred chemical composition of steel sheet] As described above, the object of the present application is to provide a panel member excellent in appearance even after high-strength forming, by containing martensite in the metal structure of a steel sheet constituting the panel member, and controlling the surface texture of the panel member after forming in the range of Str of 0.50 to 1.00 and Sa of 0.50 μm or less using surface texture aspect ratio Str and surface roughness parameter Sa as two different parameters, thereby achieving the object. Therefore, it is known that the chemical composition of the steel sheet itself is not a necessary technical feature in achieving the object of the present application. Hereinafter, the preferred chemical composition of the steel sheet of the embodiment of the present application is described in detail, but the intention of these descriptions is to simply exemplify the preferred chemical composition of the steel sheet having, for example, a tensile strength of 400 to 900 MPa, and the intention is not to limit the present application to the steel sheet having such a specific chemical composition. In addition, in the following descriptions, "%" as the unit of the content of each element means "mass %" unless otherwise specified. Further, in the present specification, "~" indicating a numerical range is used with the meaning that the numerals recited before and after it are included as the lower limit value and the upper limit value, unless otherwise specified.

[0062] In the embodiment of the present application, for example, the steel sheet has a chemical composition consisting of, in mass %, C: 0.030 to 0.100 %, Si: 0.005 to 1.500 %, Mn: 0.70 to 3.00 %, P: 0.1000 % or less, S: 0.0200 % or less, Al: 1.000 % or less, N: 0.0200 % or less, O: 0.0100 % or less, Nb: 0 to 0.400 %, Cr: 0 to 1.00 %, Mo: 0 to 0.80 %, B: 0 to 0.0100 %, Ti: 0 to 0.200 %, V: 0 to 0.500 %, Ni: 0 to 1.00 %, Cu: 0 to 1.00 %, W: 0 to 1.00 %, Ta: 0 to 0.10 %, Co: 0 to 3.00 %, Sn: 0 to 1.00 %, Sb: 0 to 0.200 %, Ca: 0 to 0.0100 %, Mg: 0~0.0100% Zr: 0~0.0100% REM: 0~0.0100% Bi: 0~0.0500% As: 0~0.10%, and The remainder consists of Fe and impurities. The following provides a more detailed explanation of each element.

[0063] [C: 0.030~0.100%] Carbon (C) is an element that ensures a specified amount of martensite, increasing the strength of the steel sheet. To achieve this effect, the C content is set at 0.030% or more. The C content can be 0.040% or more, or 0.050% or more. On the other hand, if the C content is excessive, the strength may become too high, while the ductility may decrease. Therefore, the C content is set at 0.100% or less. The C content can also be 0.090% or less, 0.080% or less, 0.079% or less, 0.078% or less, 0.076% or less, 0.074% or less, 0.072% or less, 0.070% or less, or 0.060% or less.

[0064] [Si: 0.005~1.500%] Si is an element that increases the strength of steel sheets through solid solution strengthening. To achieve this effect, the Si content is set to 0.005% or more. The Si content can also be 0.010% or more, 0.100% or more, 0.200% or more, 0.300% or more, or 0.400% or more. On the other hand, if the Si content is excessive, it can sometimes be difficult to remove the oxide scale generated during hot rolling, leading to a deterioration in appearance. Therefore, the Si content is set to 1.500% or less. The Si content can also be 1.200% or less, 1.000% or less, 0.800% or less, 0.700% or less, or 0.600% or less.

[0065] [Mn: 0.70~3.00%] Mn is an element that improves hardenability and contributes to an increase in the strength of the steel sheet. In order to sufficiently obtain such an effect, the content of Mn is set to 0.70% or more. The content of Mn can also be 0.80% or more, 1.00% or more, 1.20% or more, or 1.50% or more. In the preferred manufacturing method of the steel sheet described later, in order to uniformly disperse the martensite in both the microscopic region and the macroscopic region in the metal structure of the steel sheet before final annealing, it is necessary to constitute the metal structure of the steel sheet before final annealing from a structure in which ferrite and / or martensite is the main body. Therefore, the improvement in hardenability due to the addition of Mn can also be said to be important in terms of improving the appearance after forming. On the other hand, if Mn is excessively contained, sometimes the ferrite transformation is excessively inhibited, and the desired amount of ferrite cannot be ensured, and the ductility decreases. Therefore, the content of Mn is set to 3.00% or less. The content of Mn can also be 2.80% or less, 2.50% or less, 2.20% or less, or 2.00% or less.

[0066] [P: 0.1000% or less] P is an impurity element that is an element that causes embrittlement of the welded portion and deterioration of plating properties. Therefore, the content of P is set to 0.1000% or less. The content of P can also be 0.0600% or less, 0.0400% or less, 0.0200% or less, or 0.0100% or less. The smaller the content of P, the more preferable, and the lower limit is not particularly limited and can be 0%. On the other hand, if the content of P is reduced to less than 0.0001% in a practical steel sheet, the manufacturing cost increases significantly, and it becomes economically disadvantageous. Therefore, the content of P can also be 0.0001% or more, 0.0002% or more, or 0.0005% or more.

[0067] [S: 0.0200% or less] S is an impurity element that is an element that hinders weldability and, in addition, hinders the manufacturability at the time of casting and at the time of hot rolling. Therefore, the content of S is set to 0.0200% or less. The content of S can also be 0.0150% or less, 0.0120% or less, 0.0100% or less, 0.0060% or less, or 0.0030% or less. The smaller the content of S, the more preferable, and the lower limit is not particularly limited and can be 0%. On the other hand, if the content of S is reduced to less than 0.0001% in a practical steel sheet, the manufacturing cost increases significantly, and it becomes economically disadvantageous. Therefore, the content of S can also be 0.0001% or more, 0.0002% or more, or 0.0005% or more.

[0068] [Al: 1.000% or less] Al is an element that functions as a deoxidizing agent and is an effective element for improving the strength of steel. The Al content can also be 0%, but in order to sufficiently obtain these effects, the Al content is preferably 0.001% or more. The Al content can also be 0.005% or more, 0.010% or more, 0.025% or more, or 0.050% or more. On the other hand, if Al is excessively contained, coarse oxides are sometimes formed, which lowers the toughness. Therefore, the Al content is set to 1.000% or less. The Al content can also be 0.800% or less, 0.600% or less, or 0.300% or less.

[0069] [N: 0.0200% or less] N is an element that becomes a cause of generation of blowholes at the time of welding. Therefore, the N content is set to 0.0200% or less. The N content can also be 0.0180% or less, 0.0150% or less, 0.0100% or less, 0.0080% or less, or 0.0060% or less. The N content is more preferably less, and the lower limit is not particularly limited and can also be 0%. On the other hand, if N is reduced to less than 0.0001% in a practical steel sheet, the manufacturing cost greatly increases, and it becomes economically disadvantageous. Therefore, the N content can also be 0.0001% or more, 0.0002% or more, or 0.0005% or more.

[0070] [O: 0.0100% or less] O is an element that becomes a cause of generation of blowholes at the time of welding. Therefore, the O content is set to 0.0100% or less. The O content can also be 0.0080% or less, 0.0050% or less, 0.0030% or less, or 0.0020% or less. The O content is more preferably less, and the lower limit is not particularly limited and can also be 0%. On the other hand, if O is reduced to less than 0.0001% in a practical steel sheet, the manufacturing cost greatly increases, and it becomes economically disadvantageous. Therefore, the O content can also be 0.0001% or more, 0.0002% or more, or 0.0005% or more.

[0071] The basic chemical composition of the steel sheet of the embodiments of the present application is as described above. Furthermore, the steel sheet can also contain at least one of the following optional elements in place of a part of the remaining portion of Fe as needed for the purpose of property improvement. For example, the steel sheet can contain at least one of Nb: 0 to 0.400%, Cr: 0 to 1.00%, Mo: 0 to 0.80%, B: 0 to 0.0100%, Ti: 0 to 0.200%, V: 0 to 0.500%, Ni: 0 to 1.00%, Cu: 0 to 1.00%, W: 0 to 1.00%, Ta: 0 to 0.10%, Co: 0 to 3.00%, Sn: 0 to 1.00%, Sb: 0 to 0.200%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, REM: 0 to 0.0100%, Bi: 0 to 0.0500%, and As: 0 to 0.10%. Hereinafter, these optional elements are described in detail.

[0072] [Nb: 0 to 0.400%] Nb is an element effective for the morphology control of carbides, and is also an element effective for the refinement of the structure to improve the toughness of the steel sheet. These effects can be obtained even in a small amount. The Nb content can also be 0%, but in order to obtain the above effects, the Nb content is preferably 0.001% or more. The Nb content can also be 0.005% or more or 0.010% or more. On the other hand, if Nb is excessively contained, coarse carbides or the like are sometimes generated in the steel to lower the toughness of the steel sheet. Therefore, the Nb content is preferably 0.400% or less. The Nb content can also be 0.200% or less, 0.100% or less, or 0.060% or less.

[0073] [Cr: 0 to 1.00%] Cr is an element that, like Mn, improves the hardenability and contributes to the improvement of the strength of the steel sheet. The Cr content can also be 0%, but in order to obtain the above effects, the Cr content is preferably 0.001% or more. The Cr content can also be 0.01% or more, 0.10% or more, or 0.20% or more. On the other hand, even if Cr is excessively contained, the effect is saturated, and this can lead to an increase in manufacturing cost. Therefore, the Cr content is preferably 1.00% or less, and can also be 0.80% or less, 0.60% or less, or 0.40% or less.

[0074] [Mo: 0 to 0.80%] Mo is an element that contributes to an increase in the high-temperature strength of the steel sheet as with Cr. The effect can be obtained even in a small amount. The Mo content can also be 0%, but in order to obtain the above effect, the Mo content is preferably 0.001% or more. The Mo content can also be 0.01% or more, 0.02% or more, 0.05% or more, or 0.10% or more. On the other hand, if Mo is excessively contained, the hot workability is sometimes reduced, and the productivity is reduced. Therefore, the Mo content is preferably 0.80% or less. The Mo content can also be 0.60% or less, 0.50% or less, 0.40% or less, or 0.20% or less.

[0075] [B: 0 to 0.0100%] B is an element that suppresses the generation of ferrite and pearlite and promotes the generation of martensite during cooling from austenite. In addition, B is an element that is beneficial to the high-strengthening of steel. These effects can be obtained even in a small amount. The B content can also be 0%, but in order to obtain the above effect, the B content is preferably 0.0001% or more. The B content can also be 0.0005% or more or 0.0010% or more. On the other hand, if B is excessively contained, the toughness and / or the weldability are sometimes reduced. Therefore, the B content is preferably 0.0100% or less. The B content can also be 0.0080% or less, 0.0050% or less, 0.0030% or less, or 0.0020% or less.

[0076] [Ti: 0 to 0.200%] Ti is an element that is effective for the morphology control of carbides. The strength increase of ferrite can be promoted by Ti. The Ti content can also be 0%, but in order to obtain these effects, the Ti content is preferably 0.001% or more. The Ti content can also be 0.002% or more, 0.010% or more, 0.020% or more, or 0.040% or more. On the other hand, even if Ti is excessively contained, the effect is saturated, and an increase in manufacturing cost can be caused. Therefore, the Ti content is preferably 0.200% or less, and can be 0.100% or less, 0.080% or less, or 0.050% or less.

[0077] [V: 0 to 0.500%] V is an element that is effective for the morphology control of carbides as with Ti, and is an element that is effective for the refinement of the structure to improve the toughness of the steel sheet. The V content can also be 0%, but in order to obtain the above effect, the V content is preferably 0.001% or more. The V content can also be 0.005% or more, 0.010% or more, or 0.050% or more. On the other hand, if V is excessively contained, a large amount of precipitates are sometimes generated, and the toughness is reduced. Therefore, the V content is preferably 0.500% or less. The V content can also be 0.400% or less, 0.200% or less, or 0.100% or less.

[0078] [Ni: 0 to 1.00%] [Cu: 0 to 1.00%] [W: 0 to 1.00%] Ni, Cu and W are elements effective for improving the strength of the steel sheet. The Ni, Cu and W contents can also be 0%, but in order to obtain such effects, the Ni, Cu and W contents are preferably 0.001% or more, and can be 0.01% or more or 0.05% or more, respectively. On the other hand, if these elements are excessively contained, the weldability of the steel sheet is sometimes reduced. Therefore, the Ni, Cu and W contents are preferably 1.00% or less, and can be 0.80% or less, 0.40% or less or 0.20% or less.

[0079] [Ta: 0 to 0.10%] Ta is an element effective for the control of the morphology of carbides and the improvement of the strength of the steel sheet, like W. The Ta content can also be 0%, but in order to obtain these effects, the Ta content is preferably 0.001% or more. The Ta content can also be 0.01% or more or 0.03% or more. On the other hand, even if Ta is excessively contained, the effect is saturated, and the excessive inclusion of Ta in the steel sheet leads to an increase in manufacturing cost. Therefore, the Ta content is preferably 0.10% or less. The Ta content can also be 0.08% or less, 0.06% or less or 0.04% or less.

[0080] [Co: 0 to 3.00%] Co is an element effective for the improvement of the strength of the steel sheet. The Co content can also be 0%, but in order to obtain the above effects, the Co content is preferably 0.001% or more. The Co content can also be 0.01% or more, 0.05% or more or 0.10% or more. On the other hand, if Co is excessively contained, the hot workability is sometimes reduced, and the raw material cost is also increased. Therefore, the Co content is preferably 3.00% or less. The Co content can also be 2.00% or less, 1.00% or less, 0.50% or less or 0.20% or less.

[0081] [Sn: 0 to 1.00%] Sn is an element that can be contained in the steel sheet in the case where scrap iron is used as a raw material for the steel sheet. In addition, Sn can cause embrittlement of ferrite. Therefore, the Sn content is more preferably less, and is preferably 1.00% or less. The Sn content can also be 0.10% or less, 0.040% or less or 0.02% or less. The Sn content can also be 0%, but the reduction of the Sn content to less than 0.001% leads to an excessive increase in refining cost. Therefore, the Sn content can also be 0.001% or more, 0.005% or more or 0.01% or more.

[0082] [Sb: 0 to 0.200%] Sb is an element that can be contained in the steel sheet in the case where scrap iron is used as a raw material of the steel sheet, like Sn. Further, Sb can strongly segregate at the grain boundaries to cause embrittlement of the grain boundaries. Therefore, the less the content of Sb is, the more preferable it is, and it is preferably 0.200% or less. The content of Sb can also be 0.100% or less, 0.040% or less, or 0.020% or less. The content of Sb can also be 0%, but reducing the content of Sb to less than 0.001% causes excessive increase in refining cost. Therefore, the content of Sb can also be 0.001% or more, 0.005% or more, or 0.010% or more.

[0083] [Ca: 0 to 0.0100%] [Mg: 0 to 0.0100%] [Zr: 0 to 0.0100%] [REM: 0 to 0.0100%] Ca, Mg, Zr, and REM are elements that contribute to improvement in formability of the steel sheet. The contents of Ca, Mg, Zr, and REM can also be 0%, but in order to obtain such effects, the contents of Ca, Mg, Zr, and REM are preferably 0.0001% or more, respectively, and can also be 0.0005% or more, 0.0010% or more, or 0.0015% or more, respectively. On the other hand, if these elements are excessively contained, the ductility of the steel sheet is sometimes reduced. Therefore, the contents of Ca, Mg, Zr, and REM are preferably 0.0100% or less, respectively, and can also be 0.0080% or less, 0.0060% or less, 0.0040% or less, or 0.0020% or less, respectively. REM in the present specification is a collective term of scandium (Sc) of atomic number 21, yttrium (Y) of atomic number 39, and 17 kinds of elements of lanthanum (La) of atomic number 57 to lutetium (Lu) of atomic number 71 as lanthanoid elements, and the content of REM is the total content of these elements.

[0084] [Bi: 0 to 0.0500%] Bi is an element that has an effect of improving formability by refining the solidification structure. The content of Bi can also be 0%, but in order to obtain such effects, the content of Bi is preferably 0.0001% or more, and can also be 0.0005% or more, 0.0010% or more, or 0.0030% or more. On the other hand, even if Bi is excessively contained, the effects are saturated, and containing Bi in the steel sheet more than necessary causes an increase in manufacturing cost. Therefore, the content of Bi is preferably 0.0500% or less, and can also be 0.0400% or less, 0.0200% or less, 0.0100% or less, or 0.0050% or less.

[0085] [As: 0 to 0.10%] As is an element that can be contained in the steel sheet in the case where scrap iron is used as a raw material of the steel sheet as well as Sn and Sb. Further, As is an element that strongly segregates at grain boundaries, and the less the As content, the more preferable. The As content is preferably 0.10% or less, and can be 0.04% or less or 0.02% or less. The As content can also be 0%, but reducing the As content to less than 0.001% results in excessive increase in refining cost. Therefore, the As content can also be 0.001% or more, 0.005% or more, or 0.01% or more.

[0086] In the steel sheet of the embodiment of the present application, the remainder other than the above-described elements is composed of Fe and impurities. The impurities are elements that are mixed from a steel raw material and / or in a steelmaking process, and are permitted to exist within a range that does not hinder the characteristics of the steel sheet of the embodiment of the present application.

[0087] The chemical composition of the steel sheet of the embodiment of the present application can be measured by a general analysis method. For example, for the chemical composition of the steel sheet, based on a test piece collected from a flat portion of a center side portion of a sheet piece, measurement can be performed using Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES). C and S can be measured using a combustion-infrared absorption method, N can be measured using a non-active gas melting-thermal conductivity method, and O can be measured using a non-active gas melting-non-dispersive infrared absorption method.

[0088] [Plating] The steel sheet of the embodiment of the present application can be a cold-rolled steel sheet, but can also include a plated layer on the surface for the purpose of improvement in corrosion resistance and the like. The plated layer can be any one of a hot-dip plated layer and an electroplated layer. That is, the steel sheet of the embodiment of the present application can be a cold-rolled steel sheet having a hot-dip plated layer or an electroplated layer on the surface thereof. The hot-dip plated layer includes, for example, a hot-dip galvanized layer (GI), an alloyed hot-dip galvanized layer (GA), a hot-dip aluminum plated layer, a hot-dip Zn-Al alloy plated layer, a hot-dip Zn-Al-Mg alloy plated layer, a hot-dip Zn-Al-Mg-Si alloy plated layer, and the like. The electroplated layer includes, for example, an electroplated zinc layer (EG), an electroplated Zn-Ni alloy layer, and the like. The plated layer is preferably a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, or an electroplated zinc layer. The attached amount of the plated layer is not particularly limited, and can be a general attached amount.

[0089] [Sheet thickness of steel sheet or sheet piece] The flat portion of the center side portion of the steel sheet and the corresponding panel is not particularly limited, and for example, has a sheet thickness of 0.2 to 2.0 mm. The sheet thickness can also be 0.3 mm or more or 0.4 mm or more. Similarly, the sheet thickness can also be 1.8 mm or less, 1.5 mm or less, 1.2 mm or less, 1.0 mm or less, or 0.8 mm or less. For example, by setting the sheet thickness to 0.2 mm or more, it is easy to maintain the shape of the formed product flat, and an additional effect of improving the dimensional accuracy and shape accuracy can be obtained. On the other hand, by setting the sheet thickness to 0.8 mm or less, the effect of lightening the member is significant. The sheet thickness of the steel sheet or the panel is measured with a micrometer.

[0090] [mechanical properties] [tensile strength: TS] According to the panel having the above-described characteristics, a high tensile strength, specifically a tensile strength of 400 MPa or more, can be achieved. The tensile strength is preferably 440 MPa or more or 490 MPa or more, and more preferably 540 MPa or more or 590 MPa or more. The upper limit is not particularly limited, and for example, the tensile strength can be 900 MPa or less, 860 MPa or less, or 800 MPa or less. The tensile strength is measured by collecting No. 5 tensile test pieces of JIS Z2241:2022 from the flat portion of the center side portion of the steel sheet of the panel, and performing a tensile test in accordance with JIS Z2241:2022.

[0091] [yield stress: YS] According to the panel having the above-described characteristics, a high yield stress, more specifically a yield stress of 300 MPa or more in the case where paint baking treatment is performed, can be achieved. The yield stress after paint baking treatment is preferably 350 MPa or more or 400 MPa or more, and more preferably 490 MPa or more or 540 MPa or more. The upper limit is not particularly limited, and for example, the yield stress after paint baking treatment can be 850 MPa or less, 800 MPa or less, or 750 MPa or less. The yield stress is measured by collecting No. 5 tensile test pieces of JIS Z2241:2022 from the flat portion of the center side portion of the steel sheet of the panel, and performing a tensile test in accordance with JIS Z2241:2022, and specifically, the 0.2% yield strength measured by the tensile test is determined as the yield stress.

[0092] The panel of the embodiment of the present application, although it can achieve a high strength, specifically a tensile strength of 400 MPa or more, can also maintain an excellent appearance after forming such as press forming. Therefore, the panel of the embodiment of the present application is very useful when used, for example, as an outer panel member such as a roof, an engine hood, a fender, and a door of a vehicle, which requires high designability in a car.

[0093] [manufacturing method of steel sheet and panel] Next, a preferred manufacturing method of the steel sheet and the plate member of the embodiment of the present application will be described. The following description is intended to exemplify a characteristic method for manufacturing the steel sheet and the plate member of the embodiment of the present application, and is not intended to limit the steel sheet and the plate member to those manufactured by the manufacturing method described below.

[0094] [Manufacturing method of steel sheet] The manufacturing method of the steel sheet of the embodiment of the present application is characterized in that it includes the following processes: a hot rolling process including: heating a slab having the chemical composition associated with the steel sheet described above to a temperature of 1100 to 1400°C to perform finish rolling, the finish rolling having an end temperature of 800 to 1350°C, and then performing coiling at a temperature of 500 to 700°C; an acid pickling process of performing acid pickling on the obtained hot-rolled steel sheet; a cold rolling process of performing cold rolling on the hot-rolled steel sheet after the acid pickling at a reduction ratio of 20 to 90%; a 1st annealing process of performing 1st annealing on the obtained cold-rolled steel sheet, the 1st annealing including: heating the cold-rolled steel sheet and holding at a maximum heating temperature of Ac3 to 950°C for 10 to 500 seconds, and then cooling to a cooling stop temperature of 350°C or lower at an average cooling speed in a temperature range of 500 to 700°C of 40°C / sec or more; and a 2nd annealing process of performing 2nd annealing on the cold-rolled steel sheet after the 1st annealing, the 2nd annealing including: heating the cold-rolled steel sheet and holding at a maximum heating temperature of (Ac1+20) to 820°C for 10 to 500 seconds, and then controlling the average cooling speed in a temperature range of 500 to 700°C to be 10°C / sec or more, and further controlling the average cooling speed in a temperature range of 200 to 500°C to be 40°C / sec or more. Each process will be described in more detail below.

[0095] [Hot rolling process] [Heating of slab] First, a slab having the chemical composition explained above in association with the steel sheet is heated. The slab used is preferably cast in a continuous casting method from the viewpoint of productivity, but can also be produced by an ingot casting method or a thin slab casting method. In order to obtain a high-strength steel sheet, the slab used contains alloy elements in a relatively large amount. Therefore, it is necessary to heat the slab before supplying it to hot rolling so as to cause the alloy elements to be solid-solved in the slab. If the heating temperature is lower than 1100°C, sometimes the alloy elements are not sufficiently solid-solved in the slab and coarse alloy carbides remain, causing embrittlement cracking in 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 1400°C or lower from the viewpoint of the capacity of the heating equipment and productivity.

[0096] [Coarse rolling] In the present method, for example, for the heated slab, coarse rolling can also be performed before finish rolling in order to adjust the sheet thickness or the like. The coarse rolling is not particularly limited as long as the desired thin slab size can be ensured.

[0097] [Finish rolling / coiling] The heated slab or the slab subjected to coarse rolling as necessary in addition thereto is then subjected to finish rolling. The slab used as described above contains alloy elements in a relatively large amount, and therefore it is necessary to increase the rolling load at the time of hot rolling. Therefore, the hot rolling is preferably performed at a high temperature. In particular, the finish rolling end temperature is important in terms of controlling the microstructure of the steel sheet. If the finish rolling end temperature is low, sometimes the microstructure becomes non-uniform and the formability decreases. Therefore, the finish rolling end temperature is set to 800°C or higher. On the other hand, in order to suppress the coarsening of austenite, the finish rolling end temperature is set to 1350°C or lower. The finish rolling end temperature is preferably 950 to 1050°C. Next, the hot-rolled steel sheet after finish rolling is coiled at a coiling temperature of 500 to 700°C. By setting the coiling temperature to 500 to 700°C, the growth of the scale can be suppressed. The coiling temperature is preferably 550 to 630°C.

[0098] [Pickling step] Next, the obtained hot-rolled steel sheet is subjected to pickling in order to remove the scale formed on the surface of the hot-rolled steel sheet. The pickling can be performed once under appropriate conditions for removing the scale, or can be performed in multiple stages in order to reliably remove the scale.

[0099] [Cold rolling step] The pickled hot-rolled steel sheet is cold-rolled in the cold-rolling step at a reduction ratio of 20 to 90%. By setting the cold-rolling reduction ratio to 20% or more, the shape of the cold-rolled steel sheet can be kept flat, and the decrease in ductility in the final product can be suppressed. On the other hand, by setting the cold-rolling reduction ratio to 90% or less, the rolling load can be prevented from becoming excessively large and the rolling can be prevented from becoming difficult. The cold-rolling reduction ratio is preferably 70 to 90%. The number of rolling passes and the reduction ratio per pass are not particularly limited, and can be appropriately set in a manner that the overall cold-rolling reduction ratio falls within the above range.

[0100] [1st annealing step] The obtained cold-rolled steel sheet is heated in the following 1st annealing step, held at a maximum heating temperature of Ac3 to 950°C for 10 to 500 seconds, and then cooled at an average cooling rate of 40°C / sec or more in the temperature range of 500 to 700°C to a cooling stop temperature of 350°C or less. Here, the Ac3 point (°C) is found from a small piece cut out from the cold-rolled steel sheet, based on thermal expansion in heating from room temperature to 1000°C at 10°C / sec. By holding at a temperature of Ac3 or more for a sufficient time, austenitization is promoted, and by subsequent rapid cooling to a temperature of 350°C or less, the metal structure of the steel sheet after cooling can be reliably composed of a structure in which bainite and / or martensite is the main component, such as full bainite or full martensite. Here, the structure in which bainite and / or martensite is the main component means a structure containing 90% or more of at least one of bainite and martensite in terms of the total area ratio, the full bainite means a structure composed of 100% of bainite in terms of the area ratio, and the full martensite means a structure composed of 100% of martensite in terms of the area ratio. The bainite and / or martensite structure is a structure having a large number of various interfaces in the interior compared to structures such as ferrite. Therefore, by composing the metal structure of the steel sheet before the 2nd annealing step, i.e., the final annealing step, of bainite and / or martensite, it becomes possible to generate carbides that can be nucleation sites for austenite very much dispersedly on these interfaces at the stage of heating such a metal structure in the 2nd annealing. As a result, by generating austenite finely and uniformly in the entire steel sheet from such a large number of dispersed nucleation sites, and then generating martensite from these austenites, in the metal structure obtained after the 2nd annealing, the average particle spacing of the martensite is controlled to 2.5 μm or less, and the standard deviation of the area ratio of the martensite in the direction perpendicular to the rolling direction and the sheet thickness direction is controlled to 1.5% or less. That is, it becomes possible to achieve a metal structure in which the martensite is uniformly dispersed in both the microscopic region and the macroscopic region.

[0101] If the maximum heating temperature in the 1st annealing step is lower than the Ac3 point or the holding time is less than 10 seconds, the austenitization is insufficient, and even if the subsequent cooling is performed, the metal structure in the steel sheet cannot be composed of a structure mainly of bainite and / or martensite. That is, it is not possible to set the total area ratio of bainite and martensite to be 90% or more. On the other hand, heating and holding at a higher temperature and for a longer time would decrease the productivity, and therefore the maximum heating temperature in the 1st annealing step is set to be 950°C or less and the holding time is set to be 500 seconds or less. The maximum heating temperature is preferably 870 to 950°C and the holding time is preferably 50 to 100 seconds.

[0102] Further, if the average cooling rate in the temperature range of 500 to 700°C in the 1st annealing step is less than 40°C / sec or the cooling stop temperature exceeds 350°C, ferrite is generated in the cooling, and it is not possible to set the metal structure in the steel sheet to be 90% or more in the total area ratio of bainite and martensite. Therefore, the average cooling rate needs to be set to be 40°C / sec or more, and is preferably 70°C / sec. The upper limit is preferably 300°C / sec or less, and more preferably 150°C / sec or less. On the other hand, the lower limit of the cooling stop temperature is not particularly limited, and for example, the cooling stop temperature can be 25°C or more, and is preferably 200°C or more. Similarly, the cooling stop temperature is preferably 300°C or less.

[0103] [2nd annealing step (final annealing step)] The cold-rolled steel sheet after the first annealing is heated again in the following second annealing process, held at a maximum heating temperature of (Ac1+20) to 820°C for 10 to 500 seconds, and then cooled by controlling the average cooling rate in the temperature range of 500 to 700°C to be 10°C / sec or more and further controlling the average cooling rate in the temperature range of 200 to 500°C to be 40°C / sec or more. Here, for the Ac1 point (°C), as in the case of the Ac3 point, a small piece is cut out from the cold-rolled steel sheet, and the thermal expansion in heating from room temperature to 1000°C of the small piece is measured. First, at the stage of heating the steel sheet after the first cooling to the maximum heating temperature of (Ac1+20) to 820°C, carbides can be dispersedly generated on the interfaces contained in the large amount in the bainite and / or the martensite in the metal structure. Then, by holding at the maximum heating temperature corresponding to the two-phase region of ferrite and austenite for 10 to 500 seconds, the state of dispersing the carbides on the interfaces can be maintained, and the austenite can be finely and uniformly generated in the steel sheet as a whole from the carbides. Finally, by controlling the average cooling rate in the temperature range of 500 to 700°C to be 10°C / sec or more and further controlling the average cooling rate in the temperature range of 200 to 500°C to be 40°C / sec or more, the martensite can be suitably generated from the finely dispersed austenite, as a result of which the average particle spacing of the martensite is controlled to be 2.5 μm or less, and the standard deviation of the area ratio of the martensite in the direction perpendicular to the rolling direction and the sheet thickness direction is controlled to be 1.5% or less. That is, the metal structure in which the martensite is uniformly dispersed in both the microscopic region and the macroscopic region can be achieved.

[0104] If the maximum heating temperature in the second annealing process is lower than Ac1+20°C or the holding time is lower than 10 seconds, the desired metal structure as described above cannot be obtained. On the other hand, in the case where the maximum heating temperature exceeds 820°C, the area ratio of the austenite becomes too high, and it is not possible to set the area ratio of the ferrite to be 75% or more. Further, due to the high temperature, it becomes impossible to maintain the state of dispersing the carbides on the interfaces, and in the finally obtained metal structure, it becomes impossible to achieve the uniform dispersion of the martensite in both the microscopic region and the macroscopic region. In addition, if the holding time exceeds 500 seconds, the austenite grains become coarse, and the martensite grains obtained by the subsequent cooling also become relatively coarse. In such a case, it is not possible to obtain a fine martensite structure in which the average particle spacing of the martensite is controlled to be 2.5 μm or less. The maximum heating temperature is preferably 760 to 800°C, and the holding time is preferably 20 to 100 seconds.

[0105] Further, if the average cooling rate in the temperature range of 500 to 700°C in the 2nd annealing process is less than 10°C / sec, the phase transformation from austenite to bainite or the like is promoted, and even if the subsequent cooling is properly performed, sometimes the desired amount of martensite is not obtained. In this case, the desired strength cannot be achieved, and / or particularly the uniform dispersion of martensite in the micro region cannot be achieved. Therefore, the average cooling rate in the temperature range of 500 to 700°C needs to be set to 10°C / sec or more, preferably 40°C / sec or more, and the upper limit is, for example, 200°C / sec or less, preferably 60°C / sec or less. On the other hand, if the average cooling rate in the temperature range of 200 to 500°C is less than 40°C / sec, the phase transformation from austenite to martensite cannot be promoted, and similarly, the generation of other structures such as bainite or the like increases. Therefore, the average cooling rate in the temperature range of 200 to 500°C needs to be set to 40°C / sec or more, preferably 50°C / sec or more, and the upper limit is, for example, 200°C / sec or less, preferably 80°C / sec or less.

[0106] The above-described method is a method of manufacturing the steel sheet of the embodiment of the present application by two annealing processes including the 1st annealing and the 2nd annealing, but the steel sheet of the embodiment of the present application is not necessarily limited to being manufactured by such a method, and for example, can be manufactured by a 1st annealing process. More specifically, by the metal structure of the steel sheet after the hot rolling process consisting of full bainite or full martensite, the above-described 1st annealing can be omitted. However, in this case, the cooling conditions after the hot rolling, the coiling temperature, and the like need to be properly controlled, and in addition, the control of the reduction rate in the subsequent cold rolling becomes important. This is because, if the reduction rate in the cold rolling becomes high, recrystallization occurs at the time of heating in the subsequent annealing process, and the metal structure formed in the hot rolling process cannot be maintained.

[0107] [Plating Process] In order to improve corrosion resistance or the like, plating treatment can be performed on the surface of the obtained cold-rolled steel sheet. The plating treatment can be hot-dip plating, alloying hot-dip plating, electroplating, or the like. For example, as the plating treatment, the steel sheet can be subjected to hot-dip galvanizing treatment, or alloying treatment can be performed after the hot-dip galvanizing treatment. The specific conditions of the plating treatment and the alloying treatment are not particularly limited, and can be any appropriate conditions known to those skilled in the art. For example, in the hot-dip galvanizing treatment, the plating bath immersion plate temperature (the temperature of the steel sheet when immersed in the hot-dip galvanizing bath) is preferably in the temperature range from 40°C lower than the temperature of the hot-dip galvanizing bath (hot-dip galvanizing bath temperature - 40°C) to 50°C higher than the temperature of the hot-dip galvanizing bath (hot-dip galvanizing bath temperature + 50°C). In the case where alloying treatment is performed on the hot-dip galvanized layer, the steel sheet on which the hot-dip galvanized layer is formed is preferably heated to a temperature range of 460 to 600°C, and more preferably to a temperature range of 480 to 550°C.

[0108] <Method for manufacturing a plate member> The method for manufacturing a plate member of an embodiment of the present application includes the following steps: a punching step of punching the above-obtained steel sheet; a forming step of forming the punched steel sheet into a steel member; and an optional painting step of painting the formed steel member. Hereinafter, each step is described in more detail.

[0109] [The punching step] In the punching step, punching processing of cutting the above-obtained steel sheet into a prescribed size is performed. The punching processing can be performed by any appropriate means known to those skilled in the art, such as blanking based on a press machine, for example.

[0110] [The forming step] The punched steel sheet (blank) is formed into a steel member in the following forming step. For example, if bending processing is performed in a direction at right angles to the rolling direction of the blank and strain is introduced in the right-angled direction of rolling, it is sometimes not possible to appropriately control Str and / or Sa in the formed plate member within a prescribed range. In such a case, the occurrence of appearance defects such as a ghost line becomes significant. Therefore, in the forming step, it is preferable to perform forming in such a manner that the rolling direction of the blank coincides with the direction of bending processing in which the maximum curvature is formed. By performing such bending processing, even in the case of a high-strength steel sheet containing martensite, introduction of strain in the right-angled direction of rolling can be sufficiently reduced, and thus the occurrence of appearance defects such as a ghost line can be suppressed or reduced.

[0111] In addition, the amount of strain imparted in the forming step also needs to be appropriately controlled. When the amount of strain is small, although the appearance after forming is not necessarily adversely affected, sometimes the introduction of dislocations becomes insufficient. In this case, the amount of bake hardening at the time of paint baking decreases, and the yield stress cannot be sufficiently increased. As a result, the dent resistance of the final product decreases. Therefore, from the viewpoint of improving the dent resistance, the amount of strain imparted in the forming step is preferably set to 2.0% or more at the flat portion of the center side portion of the plate member. On the other hand, imparting of excessive strain increases the surface roughness parameter Sa of the flat portion and / or the end portion of the final product, which results in a decrease in the appearance after forming. Therefore, from the viewpoint of improving the appearance after forming, the amount of strain imparted in the forming step is preferably set to 5.0% or less at the flat portion of the center side portion of the plate member.

[0112] [The painting step] The shaped steel member is coated in an optional subsequent coating step, preferably a coating bake treatment is performed. The coating includes, for example, three types of coating, namely, electrodeposition coating, intermediate coating, and top coating (base coating and clear coating). The coating uses water-based paint or solvent-based paint. In the electrodeposition coating, the entire surface of the steel member is subjected to electrodeposition coating in a state where the steel member is immersed in an electrodeposition tank in which paint is stored. In the intermediate coating, the entire surface of the steel member is subjected to intermediate coating by spraying paint from a spray nozzle by a coating robot or manual work of a worker. In the top coating, the entire surface of the steel member is subjected to top coating by spraying paint from a spray nozzle by a coating robot or manual work of a worker. Thus, the surface of the steel member is covered with a paint layer having a film thickness of 60 to 200 μm.

[0113] [Coating bake treatment] The coating bake treatment is a baking and drying treatment for baking a paint layer on the steel member, and is a treatment for bake hardening the steel member. The coating bake treatment can be performed after the electrodeposition coating and before the intermediate coating in the coating step, between the intermediate coating and the intermediate coating performed a plurality of times, after the intermediate coating and before the top coating, between the top coating and the top coating performed a plurality of times, or after the top coating.

[0114] The temperature and time of the coating bake treatment are preferably controlled in a manner such that the drying parameter P shown in the following Formula 1 is in the range of 7500 to 10000. The specific temperature and time of the coating bake treatment are appropriately selected from the range satisfying the following Formula 1, for example, the range of 100 to 220°C and 20 to 60 minutes.

[0115] P = (T + 273) x (17.7 + log(t)) Formula 1 In the formula, T is the temperature (°C) of the coating bake treatment, and t is the time (seconds) of the coating bake treatment.

[0116] In the case where the coating bake treatment is performed a plurality of times, it is preferable to control in a manner such that the total of the times of the respective coating bake treatments is in the range of 20 to 60 minutes. Similarly, the drying parameter P is preferably controlled in a manner such that the value obtained by accumulating the drying parameters calculated from the temperature and time of the respective coating bake treatments is in the range of 7500 to 10000. If the drying parameter P is less than 7500, the amount of bake hardening decreases, and as a result, the yield stress after bake hardening sometimes cannot be sufficiently increased. In this case, the dent resistance of the final product decreases. On the other hand, if the drying parameter P exceeds 10000, the yield stress after bake hardening decreases due to excessive bake treatment, and there is also a case where the dent resistance of the final product decreases.

[0117] According to the sheet member manufactured by the above manufacturing method, high strength is achieved by containing martensite in the metal structure of the steel sheet constituting the sheet member, and by controlling the surface properties of the sheet member to be in the range of Str of 0.50 to 1.00 and Sa of 0.50 μm or less, even in the case where strain is imparted by forming such as press forming, occurrence of appearance defects such as ghost lines on the surface of the sheet member can be significantly suppressed. Furthermore, in the case where the sheet member is subjected to paint baking treatment, the yield stress can be significantly increased by bake hardening, and thus the dent resistance of the sheet member can be improved. Therefore, the sheet member manufactured by the above manufacturing method is particularly useful in the automobile field where high strength is required and excellent post-forming appearance and further excellent dent resistance are required.

[0118] Hereinafter, the present application will be described in more detail by way of examples, but the present application is not limited by these examples at all.

[0119] Example In the following examples, sheet members of the embodiments of the present application were manufactured under various conditions, and the properties of the obtained sheet members were investigated.

[0120] [Manufacture of Steel Sheet] 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 to a prescribed temperature of 1100 to 1400°C to perform hot rolling. The hot rolling was performed by conducting rough rolling and finish rolling, and the finish rolling end temperature and the coiling temperature were as shown in Table 2. Next, the obtained hot-rolled steel sheets were subjected to pickling, and then cold rolling was performed at the reduction rates shown in Table 2 to obtain cold-rolled steel sheets having a sheet thickness of 0.4 mm. Next, the obtained cold-rolled steel sheets were subjected to 1-time annealing and 2-time annealing under the conditions shown in Table 2. Finally, as a plating treatment, hot dip galvanizing was appropriately performed, and further, alloying treatment was performed at the alloying temperatures shown in Table 2 for several of the steel sheets.

[0121] [Manufacture of Sheet Member] Next, the obtained cold-rolled steel sheets or plated steel sheets were subjected to punching processing to cut into blanks of a prescribed size, and then press forming was performed on the blanks under the conditions shown in Table 2 to obtain steel members in the shape of a sheet member. Here, "OK" in the bending processing direction in Table 2 means that the rolling direction of the blank coincides with the direction of the bending processing where the maximum curvature is formed. The obtained steel members were cleaned, and after degreasing by immersion in a 40°C aqueous solution containing 3 mass% of an aqueous alkaline degreasing agent (FC-301 manufactured by Nihon Parkerizing Co., Ltd.) for 3 minutes, water washing and drying were performed. Next, the steel members were subjected to zincate treatment at 40°C for 3 g / m 2After the pretreatment of zinc phosphate treatment (Nihon Parkerizing (K.K.) PALBOND 3020 (trade name)), the cationic electrodeposition paint manufactured by Nippon Paint Co., Ltd. was subjected to electrodeposition coating with a ramp voltage of 160 V. Then, the spray coating of the intermediate coating (EP Primer 1405 (A) manufactured by Kansai Paint Co., Ltd.) and the top coating (MAGICRON 1000 manufactured by Kansai Paint Co., Ltd.) was performed. In order to appropriately dry each paint after the electrodeposition coating, the intermediate coating, and the top coating, the temperature and the time of the coating bake treatment were appropriately selected from the range of 100 to 220°C and 20 to 60 minutes, and the coating bake treatment was performed with the drying parameters P shown in Table 2 to obtain a plate member having a paint layer with the film thickness shown in Table 2. The drying parameters P shown in Table 2 were obtained by accumulating the drying parameters calculated from the temperature and the time of each coating bake treatment after the electrodeposition coating, the intermediate coating, and the top coating.

[0122] The properties of the obtained plate member were measured and evaluated by the following methods.

[0123] [evaluation of appearance] The appearance after the forming into a plate member was evaluated by the degree of the ghost line generated on the surface of the formed outer panel of a door. The surface after the press forming was polished with a grinding wheel, and the stripe pattern of several mm intervals generated on the surface was judged as the ghost line, and the score was given in 1 to 5 based on the following criteria corresponding to the degree of generation of the rib pattern. The arbitrary region of 100 mm x 100 mm was visually confirmed, and the case where the rib pattern was not confirmed at all was set to "1", the case where the maximum length of the rib pattern was 20 mm or less was set to "2", the case where the maximum length of the rib pattern exceeded 20 mm and was 50 mm or less was set to "3", the case where the maximum length of the rib pattern exceeded 50 mm and was 70 mm or less was set to "4", and the case where the maximum length of the rib pattern exceeded 70 mm was set to "5". The case evaluated as "3" or less was regarded as excellent in the appearance after the forming, and was determined to be acceptable. On the other hand, the case evaluated as "4" or more was regarded as poor in the appearance after the forming, and was determined to be unacceptable. The results thereof are shown in Table 3.

[0124] If referring to Tables 1 to 3, in Comparative Example 3, the amount of strain imparted in the forming process of the plate member is large, and therefore the Sa of the steel sheet of the flat portion of the plate member exceeds 0.50 μm, and as a result, the appearance after forming is reduced. In Comparative Example 4, it is considered that the austenite grains are coarsened due to the long holding time in the second annealing process of the steel sheet production. In association therewith, in the metal structure obtained after the second annealing, the average particle spacing of the martensite exceeds 2.5 μm, and a steel sheet having a metal structure in which the martensite is fine and uniformly dispersed throughout the whole cannot be obtained. As a result, the Sa of the steel sheet of the flat portion and the end portion of the plate member after forming exceeds 0.50 μm, and the appearance after forming is reduced. In Comparative Example 9, it is considered that the austenitization is insufficient due to the low maximum heating temperature of the first annealing process, and even if cooled thereafter, the metal structure in the steel sheet is not constituted of a structure in which the bainite and / or the martensite is the main component. In association therewith, in the metal structure obtained after the second annealing, the standard deviation of the area fraction of the martensite in the direction perpendicular to the rolling direction and the sheet thickness direction exceeds 1.5%, and a steel sheet having a metal structure in which the martensite is fine and uniformly dispersed throughout the whole cannot be obtained. As a result, the Str of the steel sheet of the flat portion of the plate member after forming is less than 0.50, and the appearance after forming is reduced.

[0125] In contrast to this, in the plate members of all of the inventive examples, high strength, for example, a tensile strength of 400 MPa or more, is achieved by including the martensite in the metal structure of the steel sheet constituting the plate member, and by controlling the surface properties of the plate member to be in the range where the Str is 0.50 to 1.00 and the Sa is 0.50 μm or less, even in the case where strain is imparted by forming such as press forming, the occurrence of appearance defects such as lines on the surface of the plate member can be significantly suppressed. In particular, in Inventive Examples 1, 2, 5, 6, and 8, a steel sheet having a metal structure in which the martensite is fine and uniformly dispersed throughout the whole, that is, a steel sheet having a metal structure in which the average particle spacing of the martensite is controlled to be 2.5 μm or less and the standard deviation of the area fraction of the martensite in the direction perpendicular to the rolling direction and the sheet thickness direction is controlled to be 1.5% or less, is used, and the forming is performed in such a manner that the average KAM value / Vm becomes 1.8 or more, and therefore a YS of 350 MPa or more can be achieved, and therefore the dent resistance is significantly improved. It is considered that this is because, by imparting appropriate strain to the steel sheet in which the martensite is uniformly dispersed, a large amount of dislocations can be uniformly introduced, and the amount of bake hardening is increased.

Claims

1. A sheet metal component, characterized in that, It is a plate comprising a steel sheet having a metallic microstructure containing martensite, wherein, The aspect ratio Str of the flat portion of the central side of the plate is 0.50~1.

00. The surface roughness parameter Sa of the flat portion of the central side part and the end portion of the steel plate is less than 0.50 μm.

2. The plate component according to claim 1, characterized in that, The ratio of the average KAM value of ferrite in the steel plate to the volume fraction Vm of martensite, i.e., average KAM value / Vm, is 1.8 or higher.

3. The plate component according to claim 1 or 2, characterized in that, The steel plate is a coated steel plate having a coating layer on at least one surface.

4. The plate component according to claim 1 or 2, characterized in that, The metal structure of the steel plate in the flat section is composed of the following, in percentage of area: Ferrite: 75~95%, Martensite: 5~25%, and At least one of bainite, pearlite, and retained austenite: totaling 0-10%. The average particle spacing of martensite is less than 2.5 μm. The standard deviation of the martensite area ratio in the direction perpendicular to the rolling direction and the plate thickness direction is less than 1.5%.

5. The plate component according to claim 1 or 2, characterized in that, The thickness of the flat section is 0.2~0.8mm.

6. The plate component according to claim 1 or 2, characterized in that, It has a tensile strength of 400~900MPa.

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