Panel

Abstract

There is provided a panel including a steel plate that has a metal structure containing martensite, wherein the surface property aspect ratio Str of the steel plate in a flat part in a center-side portion of the panel is 0.50 to 1.00, and the surface roughness parameter Sa of the steel plate in an end part and the flat part in the center-side portion of the panel is 0.50 μm or below.

Description

panel

[0001] The present invention relates to a panel.

[0002] In recent years, there has been a demand for improved fuel efficiency in automobiles to protect the global environment. To achieve this, automotive steel sheets are being required to have even higher strength in order to reduce the weight of vehicle bodies while ensuring safety. Such demands for higher strength are increasing not only for structural members and pillars, but also for automotive exterior panels (hoods, fender panels, door panels, roof panels, etc.).

[0003] On the other hand, the shapes of automotive exterior panel parts are becoming increasingly complex. When steel plates are made stronger and thinner to reduce weight, they are more likely to become uneven on the surface when formed into complex shapes. When unevenness occurs on the surface, the appearance after forming deteriorates. Since exterior panels require not only strength and other characteristics, but also design and surface quality, they are required to have excellent appearance after forming.

[0004] In this regard, Patent Document 1 discloses an exterior panel including a steel plate, the steel plate having a flat portion, and a metal structure in a surface layer region of the flat portion containing ferrite at a volume fraction of 80% or more, the average crystal grain size of the ferrite being 1.0 to 15.0 μm, and the intensity ratio X between the {001} orientation and the {111} orientation of the ferrite being ODF{001} / {111},S The patent document 1 also describes an exterior panel in which uEl1 / uEl2 is 0.44 to 0.80, where uEl1 is the uniform elongation measured on a tensile test piece cut out from the flat portion, and uEl2 is the theoretical uniform elongation calculated using a predetermined formula from the volume fraction, hardness, and average grain size of ferrite and martensite in the metal structure of the inner region of the flat portion and the thickness of the flat portion. Furthermore, the patent document 1 teaches that the above configuration can provide an exterior panel that has excellent surface quality and excellent dent resistance after being formed from a material.

[0005] Patent Document 2 discloses a panel having a steel plate containing martensite, in which the surface roughness parameter (Sa) in a flat portion of a central portion of the panel is Sa≦0.500 μm, and in the lath of the martensite, precipitates having a major axis of 0.05 μm to 1.00 μm and an aspect ratio of 3 or more are formed at a rate of 15 / μm. 2 The panel has the above-mentioned properties, and the ratio YS1 / YS2 of the yield stress YS1 measured using a tensile test piece cut out from the flat portion to the yield stress YS2 measured using a tensile test piece cut out from the end portion of the panel is 0.90 to 1.10. Furthermore, Patent Document 2 teaches that the above-mentioned configuration can provide an exterior panel that has excellent appearance after being molded from a material and excellent dent resistance.

[0006] Patent Document 3 describes an automotive exterior panel part including a steel plate, in which the rolling direction of the steel plate extends along the left-right direction of the vehicle body in a plan view. Patent Document 3 also teaches that an automotive exterior panel part with reduced ghost lines can be provided.

[0007] International Publication No. 2022 / 004795 International Publication No. 2021 / 149810 International Publication No. 2023 / 026469

[0008] As described above, when steel sheets are strengthened and thinned to reduce weight, unevenness is likely to occur on the surface of the steel sheet when it is formed into a complex shape, resulting in a poor appearance after forming. On the other hand, the automotive industry and other industries are also demanding further weight reduction of panels, and to achieve such weight reduction, panels must be made stronger than ever before. Therefore, there remains a high demand for panels that can solve the problem of appearance after forming even when strength is increased to the same level or higher than conventional panels.

[0009] Therefore, an object of the present invention is to provide a panel with a novel structure that is high in strength and has an excellent appearance after molding.

[0010] In order to achieve the above object, the present inventors have conducted research, focusing particularly on the metallographic structure of the steel sheet constituting the panel and the surface texture of the panel. As a result, the present inventors have discovered that high strength can be achieved by including martensite in the metallographic structure of the steel sheet constituting the panel, and that an excellent appearance can be achieved even in a highly strengthened panel by appropriately selecting the metallographic structure of the steel sheet and the conditions of press forming, etc., so that two different parameters related to the surface texture of the panel, more specifically, the surface texture aspect ratio Str and the surface roughness parameter Sa, are controlled within specific ranges at predetermined positions on the formed panel, and have thus completed the present invention.

[0011] The present invention, which has achieved the above-mentioned object, is as follows: (1) A panel including a steel sheet having a metallographic structure including martensite, wherein the surface texture aspect ratio Str of the steel sheet in a flat portion in a center portion of the panel is 0.50 to 1.00, and the surface roughness parameter Sa of the steel sheet in the flat portion in the center portion and at the edge of the panel is 0.50 μm or less. (2) The panel described in (1) above, wherein the ratio of the average KAM value of ferrite contained in the steel sheet to the volume fraction Vm of the martensite: average KAM value / Vm is 1.8 or more. (3) The panel described in (1) or (2) above, wherein the steel sheet is a painted steel sheet having a paint layer on at least one surface. (4) The panel according to any one of (1) to (3) above, characterized in that the metal structure of the steel sheet in the flat portion consists, by area percentage, of 75 to 95% ferrite, 5 to 25% martensite, and 0 to 10% in total of at least one of bainite, pearlite, and retained austenite, the average grain spacing of martensite is 2.5 μm or less, and the standard deviation of the area ratio of martensite in a direction perpendicular to the rolling direction and the sheet thickness direction is 1.5% or less. (5) The panel according to any one of (1) to (4) above, characterized in that the sheet thickness of the flat portion is 0.2 to 0.8 mm. (6) The panel according to any one of (1) to (5) above, characterized in that it has a tensile strength of 400 to 900 MPa.

[0012] According to the present invention, it is possible to provide a panel that has high strength and excellent appearance after molding.

[0013] <Panel> A panel according to an embodiment of the present invention includes a steel plate having a metal structure including martensite, wherein a surface texture aspect ratio Str of the steel plate in a flat portion in a central portion of the panel is 0.50 to 1.00, and a surface roughness parameter Sa of the steel plate in the flat portion and at an end portion in the central portion of the panel is 0.50 μm or less.

[0014] As mentioned above, there has been a growing need to reduce the weight of automotive exterior panels (hoods, fender panels, door panels, roof panels, etc.). Therefore, as with structural components, there is a demand for higher strength and thinner thicknesses in these exterior panels. On the other hand, in these exterior panels, DP (Dual Phase) steel, which has a relatively low yield strength, is often used to avoid surface defects called surface distortions that occur during press forming and the like. However, in the case of DP steel, which is a mixture of a soft structure made of ferrite and a hard structure made of martensite, uneven deformation occurs during processing such as press forming, in which the soft structure and its surroundings deform preferentially. This can lead to minute irregularities on the panel surface after forming, resulting in appearance defects known as ghost lines. To explain in more detail, during processing such as press forming, the soft structure made of ferrite undergoes a large amount of deformation, while the hard structure made of martensite undergoes a small amount of deformation. Therefore, the hard structure does not concave compared to the soft structure, but rather rises to form a convex shape. As a result, variations in the amount of deformation occur, particularly in the direction perpendicular to the rolling direction of the panel, and ghost lines appear in bands (stripes).

[0015] Therefore, the present inventors conducted research to achieve both high strength and good appearance after forming into a panel, focusing particularly on the metallographic structure of the steel sheet constituting the panel and the surface quality of the panel. First, the present inventors discovered that a desired high strength, for example, a tensile strength of 400 MPa or more, can be achieved by including martensite in the metallographic structure of the steel sheet constituting the panel. However, since such high strength generally leads to a deterioration in the appearance of the panel after forming, the present inventors conducted further research focusing on the surface quality of the formed panel in order to suppress such a deterioration in appearance after forming. As a result, the present inventors discovered that an excellent appearance can be achieved even in a high-strength panel by appropriately selecting the metallographic structure of the steel sheet and the press forming conditions, etc., so that two different parameters related to the surface quality of the panel, more specifically, the surface quality aspect ratio Str and the surface roughness parameter Sa, are controlled within specific ranges at predetermined positions on the formed panel.

[0016] More specifically, the surface texture aspect ratio Str is one of the spatial parameters of surface texture defined in JIS B0681-2:2018, and is known to indicate the strength of surface anisotropy, taking a value in the range of 0 to 1.00. Generally, as the Str value approaches 0, the anisotropy becomes stronger, resulting in the appearance of streaks on the surface, while as the Str value approaches 1.00, the surface becomes isotropic and independent of direction. As a result of research, the inventors have found that, when strain is imparted by forming, such as press forming, appropriately selecting the metal structure of the steel sheet as the raw material and the conditions of the press forming process to control the surface texture aspect ratio Str of the steel sheet in the flat portion near the center of the panel to within a range of 0.50 to 1.00 is highly effective in suppressing the occurrence of ghost lines on the panel surface. Since ghost lines are associated with streaks on the panel surface, from the viewpoint of suppressing the occurrence of the ghost lines, it is preferable that the minute irregularities on the panel surface are as isotropic as possible, and therefore it is preferable that the Str value is closer to 1.00.

[0017] However, since simply controlling Str appropriately may not be enough to sufficiently improve the appearance after forming, the inventors conducted further research into the surface properties of panels. As a result, the inventors discovered that by appropriately selecting the metal structure of the steel sheet material and the conditions of press forming, etc., in addition to controlling Str, by controlling the surface roughness parameter Sa of the steel sheet at the flat portion and edge of the center portion of the panel to within a range of 0.50 μm or less, it is possible to significantly suppress or reduce the occurrence of poor appearance due to minute irregularities on the panel surface, even when strain is imparted by press forming, etc. Here, the surface roughness parameter Sa refers to the average absolute value of z(x, y) in the reference region (A) defined in JIS B0681-2:2018, 4.1.7, "Arithmetic mean height of the scale-limited surface."

[0018] Therefore, according to the panel according to the embodiment of the present invention, while maintaining sufficient high strength based on the martensite contained in the metal structure of the steel sheet, the surface texture of the formed panel can be controlled using two different parameters, the surface texture aspect ratio Str and the surface roughness parameter Sa, so that Str is within the range of 0.50 to 1.00 and Sa is 0.50 μm or less. This significantly reduces the occurrence of appearance defects such as ghost lines on the panel surface, even when strain is imparted by forming, such as press forming. In particular, the present inventors have now discovered for the first time that even when the metal structure of the raw steel sheet contains martensite, the appearance of the formed high-strength panel can be significantly improved by controlling the surface roughness parameter Sa within the range of 0.50 μm or less and achieving a more isotropic surface texture with a surface texture aspect ratio Str of 0.50 or more. Therefore, the panel according to the embodiment of the present invention is particularly useful for application to automotive exterior panel parts, which require relatively high strength. Below, each component of the panel according to the embodiment of the present invention will be described in more detail.

[0019] [Str of the steel sheet in the flat portion of the center-side portion of the panel: 0.50 to 1.00] In an embodiment of the present invention, the surface texture aspect ratio Str of the steel sheet in the flat portion of the center-side portion of the panel after forming is controlled to 0.50 to 1.00. First, a panel according to an embodiment of the present invention includes three portions, specifically (i) an edge portion, (ii) an end portion, and (iii) a center-side portion other than the edge portion and the end portion. The edge portion (i) is a portion that is bent by a hemming (HEM) process or fixed to another component by welding such as spot welding. The end portion (ii) is a portion located toward the center of the panel from the edge portion and is a portion that is separated from the portion fixed to another component by hemming, welding, or the like. This end portion is located, for example, several millimeters toward the center of the panel from the edge portion and is a portion that is substantially unaffected by processing for fixing the panel to another component. In this case, "substantially unaffected" means that the change in properties due to processing for fixing the panel to other components is within a few percent. The central portion of (iii) above is the portion visible from the outside of the exterior, such as the exterior of an automobile. In this specification, the flat portion refers to a portion of the central portion of the panel with a curvature radius of 500 mm or greater. Furthermore, if a plating layer and / or a paint layer is present on the surface of the panel, the flat portion refers to the flat portion of the entire panel, including the plating layer and / or the paint layer. By controlling the surface texture aspect ratio Str of the flat portion within the range of 0.50 to 1.00, a more isotropic surface texture can be achieved, thereby enabling the provision of a panel with excellent appearance. As mentioned above, ghost lines are related to streaks on the panel surface. Therefore, from the perspective of suppressing the occurrence of ghost lines, it is preferable that the minute irregularities on the panel surface be isotropic. Therefore, from the viewpoint of further improving the appearance after molding, a higher Str is preferable, and may be, 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. There is no particular upper limit, but Str may be, for example, 0.95 or less, 0.90 or less, or 0.85 or less.

[0020] [Measurement of Str] The surface texture aspect ratio Str of the steel sheet in the flat portion of the center portion of the panel is determined as follows. First, a test piece is cut out from the flat portion of the center portion of the panel. Next, a 8 mm x 8 mm area on the surface of the cut-out sample (or the surface of the plating layer and / or paint layer if a plating layer and / or paint layer is present on the surface of the sample) is measured in three dimensions using a Keyence VK-X3000 white light interferometer. The measurement conditions are a measurement magnification of 10x, a resolution of 3 μm in the XY plane, and a resolution of 1 nm in the Z space plane, and the measurements are performed in a linked manner. Then, the measurement area is subjected to tilt correction using quadratic surface correction to remove the radius of curvature of the entire panel. Furthermore, a filtering process is performed to remove irregularities with a period of 0.8 mm or less, and Str is determined in accordance with the provisions of JIS B0681-2:2018.

[0021] [Sa of the steel sheet at the flat portion and edge of the center portion of the panel: 0.50 μm or less] In an embodiment of the present invention, the surface roughness parameter Sa of the steel sheet at the flat portion and edge of the center portion of the panel after forming is controlled to 0.50 μm or less. As in the case of Str, and when a plating layer and / or a paint layer is present on the surface of the panel, the flat portion and edge refer to the flat portion and edge of the entire panel, including the plating layer and / or the paint layer. The surface roughness parameter Sa is the average value of the height difference (absolute value) of each point relative to the average plane of the panel surface after strain is imparted during forming. In addition to controlling the surface texture aspect ratio Str described above, controlling the surface roughness parameter Sa to 0.50 μm or less at both the flat portion of the center portion of the panel and the edge of the panel significantly improves the appearance of the formed high-strength panel due to the combination of isotropic surface texture and lower surface roughness. From the viewpoint of further improving the appearance after molding, the lower Sa is the more preferable, and it may be, for example, 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 at each of the flat portion in the center side portion of the panel and the edge portion of the panel. Although there is no particular lower limit, Sa may be, for example, 0.05 μm or more, 0.10 μm or more, 0.15 μm or more, or 0.20 μm or more at each of the flat portion in the center side portion of the panel and the edge portion of the panel.

[0022] [Measurement of Sa] The surface roughness parameter Sa of the steel sheet at the flat portion and edge of the center portion of the panel is determined as follows. First, a test piece is cut out from the flat portion of the center portion of the panel, and a 8 mm x 8 mm area on the surface of the cut-out sample (or the surface of the plating layer and / or paint layer, if a plating layer and / or paint layer is present on the surface of the sample) is measured in three dimensions using a Keyence VK-X3000 white light interferometer. The measurement conditions are a measurement magnification of 10x, a resolution of 3 μm in the XY plane, and a resolution of 1 nm in the Z space plane, and the measurement is performed in a linked manner. Then, the measurement area is subjected to tilt correction using quadratic surface correction to remove the radius of curvature of the entire panel. Furthermore, a filtering process is performed to remove irregularities with a period of 0.8 mm or less, and the arithmetic mean height is determined. The arithmetic mean height thus obtained is determined as the surface roughness parameter Sa of the steel sheet at the flat portion of the center portion of the panel. The surface roughness parameter Sa of the steel sheet at the edge of the panel is determined in the same manner as above, except that the test piece is cut out from the edge of the panel instead of the flat portion of the center part of the panel.

[0023] [Steel Plate] A panel according to an embodiment of the present invention includes a steel plate having a metallurgical structure including martensite. The panel according to an embodiment of the present invention includes at least a steel plate having a metallurgical structure including martensite, and the flat portion and edge of the center portion of the panel formed from the steel plate may have the above-described characteristics. Therefore, the panel according to an embodiment of the present invention may partially include a material other than a steel plate having a metallurgical structure including martensite. Preferably, the panel according to an embodiment of the present invention essentially consists of, consists of, or is formed from a steel plate having a metallurgical structure including martensite. Martensite has a high dislocation density and is a hard structure. Therefore, by including martensite in the metallurgical structure of the steel plate forming the panel, it is possible to achieve a desired high strength. The area fraction of martensite in the metallurgical structure may be appropriately selected depending on the desired strength of the panel, and is not particularly limited, but may be, for example, 5% or more, 7% or more, 10% or more, or 13% or more. Similarly, the area fraction of martensite in the metallurgical structure may be, for example, 25% or less, 22% or less, 20% or less, 18% or less, or 15% or less. In the present invention, "martensite" includes not only as-quenched martensite (so-called fresh martensite) but also tempered martensite.

[0024] [Average KAM Value / Vm: 1.8 or More] In a preferred embodiment of the present invention, the ratio of the average KAM value of ferrite contained in the steel sheet constituting the panel to the volume fraction Vm of martensite (average KAM value / Vm) is controlled to 1.8 or more. The KAM (Kernel Average Misorientation) value tends to increase as strain accumulates, and is therefore generally known to be an effective indicator for evaluating strain distribution within crystal grains. On the other hand, the measured KAM value tends to increase as the proportion of martensite increases. Therefore, by dividing the average KAM value of ferrite contained in the steel sheet by the volume fraction Vm of martensite, it is possible to evaluate the degree of strain accumulation due to forming, such as press forming, without the influence of the martensite fraction. For panels used as exterior parts of automobiles, dent resistance, which indicates the resistance to dents caused by external stress, is known to be one of several important characteristics. To improve dent resistance, it is effective to make plastic deformation less likely to occur, i.e., to increase the yield stress. In this regard, in a preferred embodiment of the present invention, a relatively high strain can be introduced into the panel by performing press molding or other forming so that the average KAM value / Vm is 1.8 or greater. Introducing a relatively high strain into the panel allows a relatively large number of dislocations to be introduced into the panel, thereby increasing the amount of bake hardening during paint baking and improving the panel's yield stress. Here, bake hardening is a phenomenon in which interstitial elements (mainly carbon) migrate and adhere to dislocations introduced by press molding through a paint baking process at 100 to 220°C, thereby inhibiting their movement and increasing the panel's strength. This phenomenon is also known as strain aging. For these reasons, according to a preferred embodiment of the present invention, controlling the average KAM value / Vm to 1.8 or greater makes it possible to improve the dent resistance of the panel. From the viewpoint of further enhancing these effects, the higher the average KAM value / Vm, the more preferable, and may be, for example, 2.0 or more, 2.5 or more, 3.0 or more, 4.0 or more, or 5.0 or more. There is no particular upper limit, but the average KAM value / Vm may be, for example, 12.0 or less, 10.0 or less, or 8.0 or less.

[0025] [Measurement of Average KAM Value / Vm] The average KAM value is calculated by KAM (Kernel Average Misorientation) analysis in EBSD (Electron Backscatter Diffraction Patterns) measurement, a crystallography technique using an SEM. EBSD measurement involves measuring a 50 μm x 50 μm area from 1 / 8 to 3 / 8 of the thickness, centered at a position 1 / 4 of the thickness from the surface of the steel sheet, at intervals (pitch) of 0.05 μm. In this case, KAM analysis involves averaging the misorientation between one pixel at a measurement point and six adjacent pixels to obtain the value of the central pixel, allowing the creation of a map based on local crystal misorientation. Vm was defined as the volume fraction with a GAIQ (Grain Average Image Quality) of 4000 or more in EBSD measurements obtained under the same measurement conditions as the average KAM value. In this analysis, the IQ value representing the clarity of the Kikuchi pattern at a pixel at a measurement point was averaged within a grain boundary, where the region where the crystal orientation differed by 15° or more was considered to be a grain boundary, and the average value was determined as the value of the central pixel. Using this difference in clarity of the Kikuchi pattern, a phase map of ferrite and martensite with the grain boundary as its boundary can be created. The KAM map and the phase map obtained in this way were compared to calculate the average KAM value in the region corresponding to ferrite. Furthermore, the average KAM value / Vm in one field of view could be calculated by dividing the average KAM value by Vm. This process was performed for five fields of view, and the average value was defined as the average KAM value / Vm.

[0026] [Paint Layer] In the panels according to the embodiments of the present invention, the steel sheet may not have a paint layer, or may be a painted steel sheet having a paint layer on at least one surface. For example, if the surface quality of the panel is poor, a thick paint layer would be required to achieve a beautiful appearance. However, the panels according to the embodiments of the present invention have excellent surface quality, allowing the paint layer to be thin, which is therefore very advantageous from a cost perspective. In addition, the yield stress of the panel can be increased in relation to bake hardening during paint baking, which is also advantageous from the perspective of improving the dent resistance of the panel. When a steel sheet having a metallurgical structure in which martensite is finely and uniformly dispersed throughout, as described below in relation to preferred embodiments of the steel sheet, is used, the amount of bake hardening can be significantly increased due to the uniform dispersion of martensite containing a relatively large number of dislocations. Therefore, the yield stress can be similarly significantly increased, thereby further improving the dent resistance of the panel.

[0027] When the steel sheet has a plating layer, the paint layer can be formed on the plating layer. The paint layer is not particularly limited and may be any appropriate paint layer known to those skilled in the art. The thickness of the paint layer is also not particularly limited and may be, for example, 60 to 200 μm. The paint layers in an automotive panel generally include, in order from the steel sheet side, an electrodeposition paint layer, an intermediate paint layer, a base coat layer, and a clear coat layer. The thickness of the electrodeposition paint layer may be, for example, 10 to 40 μm, and the thickness of the intermediate paint layer may be, for example, 20 to 60 μm. Similarly, the thickness of the base coat layer may be, for example, 10 to 30 μm, and the thickness of the clear coat layer may be, for example, 20 to 80 μm.

[0028] [Preferred Embodiments of Steel Sheet] Hereinafter, a preferred embodiment of a steel sheet having a metal structure including martensite, which is useful for realizing the surface texture of a panel in which, when formed by press forming or the like, the surface texture aspect ratio Str of the steel sheet in the flat portion of the central portion is 0.50 to 1.00, and the surface roughness parameter Sa of the steel sheet in the flat portion of the central portion and at the edge is 0.50 μm or less, will be described in detail. However, these descriptions are intended to merely exemplify preferred steel sheets for constituting panels according to embodiments of the present invention, and are not intended to limit the present invention to embodiments using such specific steel sheets.

[0029] In a preferred embodiment of the present invention, the steel plate is characterized by having a metallographic structure in a flat portion of a center side portion of a panel, which is composed of, by area%, 75 to 95% ferrite, 5 to 25% martensite, and 0 to 10% in total of at least one of bainite, pearlite, and retained austenite, the average grain spacing of the martensite being 2.5 μm or less, and the standard deviation of the area ratio of martensite in a direction perpendicular to the rolling direction and the plate thickness direction being 1.5% or less.

[0030] As mentioned above, in the case of DP steel, which has a mixture of a soft structure made of ferrite and a hard structure made of martensite, uneven deformation is likely to occur during processing such as press forming, in which the soft structure and its surroundings deform preferentially. This can lead to minute irregularities on the panel surface after forming, resulting in appearance defects known as ghost lines. On the other hand, as steel sheets become stronger, relatively large amounts of elements such as Mn are sometimes added to improve the hardenability of the steel sheet. Mn is an element that tends to segregate in a streaky manner in steel sheets. More specifically, Mn-enriched regions such as center segregation and microsegregation are formed during casting, and these enriched regions are stretched in the rolling direction by hot rolling or cold rolling, resulting in Mn segregation in a streaky manner. Therefore, due to this Mn segregation, regions with high and low hardenability exist in the steel sheet. As a result, a relatively large number of striped hard structures are formed in the metal structure of the steel sheet after quenching. In this case, the occurrence of ghost lines becomes particularly noticeable. In contrast, if Mn segregation in steel sheets could be sufficiently suppressed, the formation of such striped hard structures would be reduced, enabling the hard structures to be more uniformly dispersed throughout the metal structure. In this case, even when strain is imparted by press forming or the like, it is believed possible to achieve a panel surface quality in which the surface quality aspect ratio Str of the steel sheet in the flat portion of the central portion is 0.50 to 1.00, and the surface roughness parameter Sa of the steel sheet in the flat portion of the central portion and at the edges is 0.50 μm or less. In relation to this, it is believed possible to sufficiently reduce the formation of minute irregularities on the panel surface and suppress the occurrence of ghost lines. However, in practice, it is extremely difficult to reliably and sufficiently suppress Mn segregation, especially when the Mn content in steel sheets increases with the demand for higher strength.

[0031] Therefore, the present inventors have investigated means for optimizing the ratio of ferrite, which is a soft structure, to martensite, which is a hard structure, in a metallographic structure to achieve a desired high strength while also improving the appearance after forming. Specifically, the present inventors have focused on the distribution state of martensite, which is a hard structure, in a metallographic structure, and more specifically, have investigated controlling the distribution of martensite from a viewpoint different from that of reducing Mn segregation. As a result, as will be described in detail later with respect to a method for producing a steel sheet, the present inventors have found that by forming a metallographic structure in a steel sheet before final annealing to a structure mainly composed of bainite and / or martensite, and then final annealing the steel sheet having such a metallographic structure under predetermined conditions, it is possible to uniformly distribute martensite in both microregions and macroregions in the final metallographic structure, regardless of the presence or degree of Mn segregation. More specifically, the inventors discovered that by final annealing a steel sheet having a metallographic structure consisting of bainite and / or martensite under predetermined conditions, the average particle spacing of martensite can be controlled to 2.5 μm or less in the microscopic region, and the standard deviation of the area fraction of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction can be controlled to 1.5% or less in the macroscopic region. By controlling the average particle spacing of martensite to 2.5 μm or less, the hard structure can be densely and uniformly dispersed in the microscopic region. Additionally, by controlling the standard deviation of the area fraction of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction to 1.5% or less, the variation in the hard structure in the macroscopic region can be significantly reduced. By satisfying both of these requirements, a metallographic structure in which martensite, a hard structure, is finely and uniformly dispersed throughout the steel sheet can be formed. As a result, according to a preferred embodiment of the present invention, the deformation amount of the steel sheet can be made more uniform, particularly in the width direction, even during forming such as press forming. In connection with this, by realizing the surface properties of the panel having the desired Str and Sa, it becomes possible to achieve an excellent post-forming appearance in which appearance defects such as ghost lines are significantly suppressed.For example, even if the uniformity of martensite is ensured in the microscopic region, if the uniformity of martensite is not ensured in the macroscopic region, it is impossible to form a metallographic structure in which martensite is finely and uniformly dispersed throughout the steel sheet. Similarly, even if the uniformity of martensite is ensured in the macroscopic region, if the uniformity of martensite is not ensured in the microscopic region, martensite may exist unevenly locally, making it impossible to form a metallographic structure in which martensite is finely and uniformly dispersed throughout the steel sheet. Therefore, in a preferred embodiment of the present invention, in order to achieve an excellent post-forming appearance in which appearance defects such as ghost lines are significantly suppressed, it is necessary to satisfy both requirements: controlling the average particle spacing of martensite to 2.5 μm or less, and controlling the standard deviation of the area fraction of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction to 1.5% or less.

[0032] Without intending to be bound by any particular theory, it is believed that in order to achieve a fine and uniform dispersion of martensite throughout the entire steel sheet in the metallographic structure of the final steel sheet, it is extremely important to form a large number of highly dispersed austenite nucleation sites during heating in the final annealing. In this regard, a martensite structure has substructures such as packets, blocks, and laths within prior austenite grains, and therefore has many different interfaces within it compared to structures such as ferrite. Bainite, like martensite, also has many different interfaces within it. Therefore, by forming the metallographic structure of the steel sheet before final annealing with bainite and / or martensite, it is possible to generate a large number of dispersed carbides that can serve as austenite nucleation sites at these interfaces during heating of such a metallographic structure in the final annealing. Therefore, it is believed that after generating many carbides at the interfaces, further heating to the two-phase region of ferrite and austenite enables the fine and uniform generation of austenite throughout the steel sheet. Finally, by rapidly cooling a steel sheet having such a metallographic structure, martensite is generated from the austenite, and in the final metallographic structure, the average particle spacing of martensite is controlled to 2.5 μm or less, and the standard deviation of the area fraction of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction is controlled to 1.5% or less. In other words, it is believed that a metallographic structure in which martensite is uniformly dispersed in both micro- and macro-regions can be obtained. By performing such heat treatment, it is believed that martensite can be finely and uniformly dispersed throughout the steel sheet to such an extent that the effects of Mn segregation are counteracted. Conventionally, it has been considered common to consider control of the distribution of hard structures from the perspective of reducing Mn segregation itself. Therefore, it is extremely unexpected and surprising that martensite can be uniformly dispersed in both micro- and macro-regions in the final metallographic structure, regardless of the presence or degree of Mn segregation.

[0033] According to a preferred embodiment of the present invention, in addition to the above-mentioned findings related to suppressing the occurrence of ghost lines, good formability can be ensured by controlling the area fraction of ferrite, a soft structure, to 75 to 95%, while controlling the area fraction of martensite, a hard structure, to 5 to 25%. Furthermore, by controlling the chemical composition of the steel sheet within a predetermined range, high strength, for example, a tensile strength of 400 MPa or more, can be reliably achieved. As a result, it is possible to achieve both high strength and excellent appearance after forming into a panel at a high level. In addition, as described above, the steel sheet according to a preferred embodiment of the present invention has a metal structure in which martensite is finely and uniformly dispersed throughout. Therefore, during the paint baking process after forming into a panel, the amount of bake hardening can be significantly increased due to the uniform dispersion of martensite containing a relatively large number of dislocations. Therefore, the use of this steel sheet significantly increases the yield stress of the resulting panel, which is therefore extremely advantageous from the perspective of improving the dent resistance of the panel.

[0034] First, the metallographic structure of the steel sheet according to a preferred embodiment of the present invention will be described in more detail below. The structure fraction is expressed as an area fraction, so the unit "%" for the structure fraction means area %. The metallographic structure of the steel sheet refers to the metallographic structure of the steel sheet in the flat portion near the center of the panel. The degree of forming in this flat portion is low, so the characteristics of the metallographic structure shown below do not change significantly before and after forming, such as press forming.

[0035] [Ferrite: 75 to 95%] Ferrite is a soft structure and therefore easily deformable, contributing to improved elongation. When the area ratio of ferrite is 75% or more, sufficient formability can be obtained. From the viewpoint of improving formability, a higher area ratio of ferrite is preferable, and may be, for example, 78% or more, 80% or more, 82% or more, or 85% or more. On the other hand, if ferrite is contained in an excessive amount, the steel sheet may not achieve the desired strength. Therefore, the area ratio of ferrite is set to 95% or less. The area ratio of ferrite may be 93% or less, 90% or less, or 87% or less.

[0036] [Martensite: 5 to 25%] Martensite is a hard structure with a high dislocation density, which contributes to improving tensile strength. By setting the area fraction of martensite to 5% or more, it is possible to achieve a tensile strength of, for example, 400 MPa or more. From the viewpoint of improving strength, the higher the area fraction of martensite, the more preferable it is, and it may be, for example, 7% or more, 10% or more, or 13% or more. On the other hand, if the area fraction of martensite is 25% or less, formability and appearance can be ensured. The area fraction of martensite may be 22% or less, 20% or less, 18% or less, or 15% or less.

[0037] [At least one of bainite, pearlite, and retained austenite: 0 to 10% in total] The area fraction of the remaining structure other than ferrite and martensite may be 0%, but if a remaining structure exists, the remaining structure is at least one of bainite, pearlite, and retained austenite. From the viewpoint of ensuring the above-described effects based on ferrite and martensite, the area fraction of the remaining structure, i.e., at least one of bainite, pearlite, and retained austenite, may be 10% or less in total, for example, 9% or less, 8% or less, 7% or less, 6% or less, 5% or less, 4% or less, or 2% or less. On the other hand, achieving a 0% area fraction of the remaining structure requires advanced control in the steel sheet manufacturing process, which may result in a decrease in yield. Therefore, the area fraction of the remaining structure may be 0.5% or more, or 1% or more.

[0038] [Identification of Metallic Structure and Calculation of Area Ratio] Identification of the metallic structure and calculation of the area ratio are performed using an FE-SEM (field emission scanning electron microscope), an optical microscope, and X-ray diffraction after corrosion using Nital reagent or Lepera solution. The observation of the structure using the FE-SEM and the optical microscope is performed at a magnification of 1,000 to 50,000 times on a 100 μm × 100 μm area in the cross section of the steel sheet perpendicular to the sheet surface. For each metallic structure, three measurement points are measured, and the area ratio is determined by calculating the average of the measured values. For example, if the sheet thickness is too thin to ensure a measurement area of ​​100 μm in the thickness direction, the length in the thickness direction is reduced and a measurement area of ​​10,000 μm is obtained. 2The same applies to the "100 μm×100 μm area" in the following explanation.

[0039] The ferrite area fraction is determined by observing a 100 μm × 100 μm region within a range of 1 / 8 to 3 / 8 of the plate thickness, centered at 1 / 4, in an electron channeling contrast image taken with an FE-SEM (field emission scanning electron microscope). More specifically, the image analysis software Image J is used to binarize ferrite and martensite based on differences in brightness, and the ferrite area fraction can be calculated. When using a Repeller liquid, the black parts of the image data represent ferrite, and the white parts represent martensite and retained austenite.

[0040] The area fraction of martensite is determined by the following procedure. First, the observation surface of the sample is etched with LePeller solution, and then a 100 μm × 100 μm region within the range of 1 / 8 to 3 / 8 of the plate thickness, centered at 1 / 4 of the plate thickness, is observed with an FE-SEM. Because LePeller corrosion does not corrode martensite or retained austenite, the area fraction of the uncorroded region corresponds to the total area fraction of martensite and retained austenite. The area fraction of martensite is calculated by subtracting the area fraction of retained austenite measured by X-ray diffraction, which will be described later, from the area fraction of the uncorroded region.

[0041] The area fraction of retained austenite is calculated by X-ray diffraction. First, the sample is removed from the plate surface to a depth of 1 / 4 in the plate thickness direction by mechanical polishing and chemical polishing. Next, at the 1 / 4 plate thickness position, the structural fraction of retained austenite is calculated from the integrated intensity ratio of the diffraction peaks of (200) and (211) of the bcc phase and (200), (220), and (311) of the fcc phase obtained using MoKα radiation. A common five-peak method is used for this calculation. The calculated structural fraction of retained austenite is determined as the area fraction of retained austenite.

[0042] The identification of bainite and calculation of its area fraction are performed using the following procedure. First, the observation surface of the sample is corroded with Nital reagent. Next, a 100 μm × 100 μm region within the range of 1 / 8 to 3 / 8 of the plate thickness, centered at 1 / 4 of the plate thickness, is observed using an FE-SEM. Based on the position and arrangement of cementite contained within the structure in this observation region, bainite is identified as follows. Bainite is classified into upper bainite and lower bainite. In upper bainite, cementite or retained austenite exists at the interface of lath-shaped bainitic ferrite. In lower bainite, cementite exists within lath-shaped bainitic ferrite, the bainitic ferrite and cementite have one type of crystal orientation relationship, and the cementite has the same variant. Based on these characteristic features, upper bainite and lower bainite can be individually identified. In the present invention, these are collectively referred to as bainite, and the area fraction of the identified bainite is calculated based on image analysis.

[0043] The identification of pearlite and calculation of the area ratio are carried out as follows: First, the observation surface of the sample is corroded with Nital reagent, and then the range of 1 / 8 to 3 / 8 of the plate thickness, centered at 1 / 4 of the plate thickness, is observed with an optical microscope at a magnification of 500 or 1000 times. Areas with dark contrast in the image observed with the optical microscope are identified as pearlite, and the area ratio of this area is calculated based on image analysis.

[0044] [Average Particle Spacing of Martensite: 2.5 μm or Less] In a preferred embodiment of the present invention, the average particle spacing of martensite, which is a hard structure, is controlled to 2.5 μm or less. The average particle spacing of martensite is an index that represents the uniformity of the hard structure distribution in the microregion. The smaller the average particle spacing of martensite, the more densely and uniformly the hard structure is dispersed, and therefore the higher the uniformity. The more uniform the deformation amount of the steel sheet during press forming, particularly in the width direction of the steel sheet, the better the appearance after press forming. Since the deformation amount of the steel sheet is strongly affected by the distribution state of the hard structure, in order to make the deformation amount of the steel sheet uniform in the width direction of the steel sheet, it is necessary to make the distribution of the hard structure in the metal structure uniform. In addition to controlling the standard deviation of the area fraction of martensite, which will be described later, by controlling the average particle spacing of martensite to 2.5 μm or less, the deformation amount of the steel sheet can be made more uniform in the width direction even during forming, such as press forming, and as a result, a good post-forming appearance can be achieved. The average grain spacing of martensite is preferably 2.4 μm or less, more preferably 2.2 μm or less, and most preferably 2.0 μm or less or 1.8 μm or less. There is no particular lower limit, but the average grain spacing of martensite may be, for example, 0.5 μm or more, 0.8 μm or more, or 1.0 μm or more.

[0045] [Measurement of Average Particle Spacing of Martensite] The average particle spacing of martensite is determined as follows. First, a sample having a steel sheet cross section perpendicular to the sheet surface is collected, and the cross section is used as the observation surface. A 100 μm × 100 μm region within the range of 1 / 8 to 3 / 8 of the sheet thickness, centered at 1 / 4 of the sheet thickness, is used as the observation region of this observation surface, and martensite is identified using an FE-SEM. Specifically, ferrite and martensite are binarized based on differences in brightness using image analysis software Image J, and martensite is identified. When a RePeller solution is used, the black areas in the image data represent ferrite, and the white areas not corroded by the RePeller represent the combined structure of martensite and retained austenite. In the steel sheet according to a preferred embodiment of the present invention, the area ratio of retained austenite is sufficiently low compared to the area ratio of martensite, so the white structure can be considered as martensite. Next, the distance between the centers (centers of gravity) of all adjacent martensite grains among the identified martensite grains is calculated as the particle spacing based on image analysis, and the average value of the calculated particle spacings is determined as the average particle spacing of martensite (strictly speaking, particles containing martensite and / or retained austenite).

[0046] [Standard deviation in area fraction of martensite in the direction perpendicular to the rolling direction and the thickness direction is 1.5% or less] In a preferred embodiment of the present invention, the standard deviation in the area fraction of martensite in the direction perpendicular to the rolling direction and the thickness direction is controlled to 1.5% or less. This standard deviation is an index representing the uniformity of the hard structure in the macro region. The appearance, which is an issue during press forming, depends on minute irregularities on the steel sheet surface caused by differences in the amount of deformation in the width direction of the steel sheet. Therefore, if there is a large variation in the area fraction of the hard structure contained within the thickness direction in the direction perpendicular to the rolling direction and the thickness direction, a difference in the amount of deformation in the width direction of the steel sheet will occur, resulting in the generation of minute irregularities on the steel sheet surface. Therefore, it is effective to reduce the standard deviation in the area fraction of martensite in the direction perpendicular to the rolling direction and the thickness direction, i.e., the width direction of the steel sheet. More specifically, by controlling the standard deviation to 1.5% or less in addition to the control of the average particle spacing of martensite as described above, it is possible to further reduce the variation in deformation amount in the width direction of the steel sheet even during forming such as press forming, and as a result, it is possible to achieve a good appearance after forming. The standard deviation of the area fraction of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction is preferably 1.4% or less, more preferably 1.2% or less, and most preferably 1.0% or less. There is no particular lower limit, but the standard deviation may be, for example, 0.1% or more, 0.3% or more, or 0.5% or more.

[0047] [Measurement of standard deviation in area fraction of martensite in the direction perpendicular to the rolling direction and the thickness direction] The standard deviation in the area fraction of martensite in the direction perpendicular to the rolling direction and the thickness direction is determined as follows. First, a metallographic image of the steel sheet cross section in a region of 50 mm in the direction perpendicular to the rolling direction and the thickness direction is obtained. In the case of an image of 10 mm or smaller, multiple images may be obtained and joined to form a 50 mm image. When the rolling direction is unknown, the cross section is observed at angles of 0°, 45°, 90°, and 135° relative to an arbitrary direction, and the cross section with the highest aspect ratio of precipitates is identified as the cross section parallel to the rolling direction, and the direction perpendicular to this is identified as the direction perpendicular to the rolling direction and the thickness direction. Next, the obtained image is divided into 100 μm (0.1 mm) sections in the direction perpendicular to the rolling direction, and the area fraction of martensite throughout the thickness of the sheet is calculated for each divided area. The standard deviation of the martensite area fraction is calculated based on the martensite area fraction calculated from each of the 500 divided images in total. This operation is performed for three regions at different positions in the rolling direction, and the average of the standard deviations calculated for each region is determined as the standard deviation of the martensite area fraction in the direction perpendicular to the rolling direction and the sheet thickness direction.

[0048] When the rolling direction of a steel sheet is unclear, the following method, for example, is employed to identify the rolling direction of the steel sheet. After mirror-polishing a thickness cross-section of the steel sheet, the S concentration is measured using an electron probe microanalyzer (EPMA). Measurement conditions include an acceleration voltage of 15 kV and a measurement pitch of 1 μm, and a distribution image is measured over a range of 100 μm (thickness direction) × 500 μm (direction perpendicular to the thickness direction) at the center of the thickness. At this time, an elongated region with a high S concentration is determined to be an inclusion such as MnS. Observation may be performed from multiple fields of view. Next, using the thickness cross-section initially observed by the above method as a reference, a plane parallel to a plane rotated in 5° increments in the range of 0° to 180° around the thickness direction is observed by the above method. The average value of the major axis lengths of the multiple inclusions in each obtained cross-section is calculated for each cross-section, and the cross-section with the largest average major axis length of the inclusions is identified. The direction parallel to the longitudinal axis of the inclusions in the cross section is determined to be the rolling direction.

[0049] [Average grain size of ferrite: 3.0 to 25.0 μm] According to another preferred embodiment of the present invention, the average grain size of ferrite in the metal structure is 3.0 to 25.0 μm. By controlling the average grain size of ferrite within such a fine range, it is possible to further improve the appearance of the formed panel. The average grain size of ferrite may be 5.0 μm or more, 7.0 μm or more, 8.0 μm or more, 9.0 μm or more, or 10.0 μm or more. Similarly, the average grain size of ferrite may be 22.0 μm or less, 20.0 μm or less, 16.0 μm or less, 14.0 μm or less, or 12.0 μm or less.

[0050] The average grain size of ferrite in a steel sheet is determined as follows. First, 10 visual fields are observed at a magnification of 500x or 1000x in the region from the surface of a steel sheet etched with LePeller's reagent to a position halfway through the sheet thickness in the sheet thickness direction. Image analysis is performed using image analysis software such as Adobe Photoshop (registered trademark) CS5, and the area fraction of ferrite and the number of ferrite particles in each visual field are calculated. Next, the area fraction of ferrite and the number of ferrite particles in the 10 visual fields are added together, and the total area fraction of ferrite is divided by the total number of ferrite particles to calculate the average area fraction per ferrite particle. From this average area fraction and the number of particles, the equivalent circle diameter is calculated, and the resulting equivalent circle diameter is determined as the average grain size of ferrite. The observation area is 150 μm in the sheet thickness direction and 250 μm in the direction perpendicular to the sheet thickness direction (the observation area in this case is 150 × 250 = 37,500 μm). ２ For example, if the plate thickness is too thin to ensure a measurement area of ​​150 μm in the plate thickness direction, the length in the plate thickness direction is reduced to obtain an observation area of ​​37,500 μm. 2 The same applies to the method of determining the average grain size of martensite and the average aspect ratio of martensite.

[0051] [Average grain size of martensite: 1.0 to 5.0 μm] According to another preferred embodiment of the present invention, the average grain size of martensite in the metal structure is 1.0 to 5.0 μm. Controlling the average grain size of martensite within such a fine range can further improve the appearance of the formed panel. The average grain size of martensite may be 1.2 μm or more, 1.5 μm or more, 1.7 μm or more, or 2.0 μm or more. Similarly, the average grain size of martensite may be 4.7 μm or less, 4.5 μm or less, 4.2 μm or less, 4.0 μm or less, 3.8 μm or less, 3.6 μm or less, or 3.4 μm or less.

[0052] The average grain size of martensite is determined as follows. First, 10 visual fields are observed at a magnification of 500x or 1000x in the region from the surface of a steel sheet etched with LePeller's reagent to a position halfway through the thickness in the thickness direction. Image analysis is performed using image analysis software such as Adobe Photoshop (registered trademark) CS5, and the area fraction of martensite and the number of martensite particles in each visual field are calculated. Next, the area fraction of martensite and the number of martensite particles in the 10 visual fields are summed, and the total area fraction of martensite is divided by the total number of martensite particles to calculate the average area fraction per martensite particle. From this average area fraction and the number of particles, the equivalent circle diameter is calculated, and the resulting equivalent circle diameter is determined as the average grain size of martensite. The observation area is 150 μm in the thickness direction and 250 μm in the direction perpendicular to the thickness direction (the observation area in this case is 150 × 250 = 37,500 μm). ２ )

[0053] [Average aspect ratio of martensite: 2.5 or more] According to another preferred embodiment of the present invention, the average aspect ratio of martensite in the metal structure is 2.5 or more. By controlling the average aspect ratio of martensite to 2.5 or more, a state in which greater strain is imparted can be achieved, thereby improving the strength of the steel sheet. The average aspect ratio of martensite may be 2.6 or more, 2.8 or more, or 3.0 or more. There is no particular upper limit, but the average aspect ratio of martensite may be, for example, 6.0 or less, 5.0 or less, 4.0 or less, 3.8 or less, or 3.6 or less.

[0054] The average aspect ratio of martensite is determined as follows. First, 10 visual fields are observed at a magnification of 500x or 1000x in the region from the surface of a steel sheet etched with LePeller's reagent to a position halfway through the thickness in the thickness direction. Image analysis is performed using image analysis software such as Adobe Photoshop (registered trademark) CS5, and the aspect ratio, which is the ratio of the major axis to the minor axis, of each martensite grain is calculated. Next, the average aspect ratio of martensite is determined by arithmetically averaging the aspect ratios of all martensite grains in the 10 visual fields. The observation area is 150 μm in the thickness direction and 250 μm in the direction perpendicular to the thickness direction (the observation area in this case is 150 × 250 = 37,500 μm). ２ )

[0055] [Preferable Chemical Composition of Steel Sheet] As described above, the present invention aims to provide a panel that has high strength but excellent appearance after forming. This objective is achieved by including martensite in the metallographic structure of the steel sheet constituting the panel and controlling the surface texture of the formed panel using two different parameters, the surface texture aspect ratio Str and the surface roughness parameter Sa, so that Str is within the range of 0.50 to 1.00 and Sa is within the range of 0.50 μm or less. Therefore, it is clear that the chemical composition of the steel sheet itself is not an essential technical feature for achieving the objectives of the present invention. Below, we will explain in detail the preferred chemical composition of the steel sheet according to the embodiment of the present invention. However, these explanations are intended to merely exemplify the preferred chemical composition of a steel sheet having a tensile strength of, for example, 400 to 900 MPa and are not intended to limit the present invention to a steel sheet having such a specific chemical composition. Furthermore, in the following description, the unit of content of each element, "%," means "mass %" unless otherwise specified. Furthermore, in this specification, unless otherwise specified, the term "to" indicating a range of values ​​is used to mean that the values ​​before and after it are included as the lower limit and upper limit.

[0056] In an embodiment of the present invention, for example, the steel plate contains, 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%, It preferably has a chemical composition consisting of Ta: 0-0.10%, Co: 0-3.00%, Sn: 0-1.00%, Sb: 0-0.200%, Ca: 0-0.0100%, Mg: 0-0.0100%, Zr: 0-0.0100%, REM: 0-0.0100%, Bi: 0-0.0500%, As: 0-0.10%, and the balance: Fe and impurities. Each element will be described in more detail below.

[0057] [C: 0.030 to 0.100%] C is an element that ensures a predetermined amount of martensite and improves the strength of the steel sheet. To fully obtain this effect, the C content is set to 0.030% or more. The C content may be 0.040% or more or 0.050% or more. On the other hand, excessive C content may result in excessively high strength and reduced elongation. For this reason, the C content is set to 0.100% or less. The C content may be 0.090% or less, 0.080% or less, 0.079% 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.

[0058] [Si: 0.005 to 1.500%] Si is an element that improves the strength of steel sheet through solid solution strengthening. To fully obtain this effect, the Si content is set to 0.005% or more. The Si content may be 0.010% or more, 0.100% or more, 0.200% or more, 0.300% or more, or 0.400% or more. On the other hand, excessive Si content may make it difficult to remove scale formed during hot rolling, which may lead to deterioration of appearance. Therefore, the Si content is set to 1.500% or less. The Si content may be 1.200% or less, 1.000% or less, 0.800% or less, 0.700% or less, or 0.600% or less.

[0059] [Mn: 0.70 to 3.00%] Mn is an element that improves hardenability and contributes to improving steel sheet strength. To fully achieve this effect, the Mn content is set to 0.70% or more. The Mn content may be 0.80% or more, 1.00% or more, 1.20% or more, or 1.50% or more. In a preferred steel sheet manufacturing method described later, in order to uniformly disperse martensite in both the microregion and the macroregion in the final metal structure, it is necessary for the metal structure of the steel sheet before final annealing to be composed of a structure mainly composed of bainite and / or martensite. For this reason, improving hardenability by adding Mn is also important for improving the appearance after forming. On the other hand, excessive Mn content excessively suppresses ferrite transformation, making it impossible to secure the desired amount of ferrite, which may result in reduced elongation. Therefore, the Mn content is set to 3.00% or less. The Mn content may be 2.80% or less, 2.50% or less, 2.20% or less, or 2.00% or less.

[0060] [P: 0.1000% or less] P is an impurity element that embrittles welds and deteriorates galvanic properties. Therefore, the P content is set to 0.1000% or less. The P content may be 0.0600% or less, 0.0400% or less, 0.0200% or less, or 0.0100% or less. The lower the P content, the more preferable it is, and the lower limit is not particularly limited and may be 0%. On the other hand, reducing the P content to less than 0.0001% in practical steel sheets significantly increases production costs, which is economically disadvantageous. Therefore, the P content may be 0.0001% or more, 0.0002% or more, or 0.0005% or more.

[0061] [S: 0.0200% or less] S is an impurity element that impairs weldability and also impairs manufacturability during casting and hot rolling. Therefore, the S content is set to 0.0200% or less. The S content may be 0.0150% or less, 0.0120% or less, 0.0100% or less, 0.0060% or less, or 0.0030% or less. The lower the S content, the more preferable it is, and the lower limit is not particularly limited and may be 0%. On the other hand, reducing the S content to less than 0.0001% in practical steel sheets significantly increases production costs, which is economically disadvantageous. Therefore, the S content may be 0.0001% or more, 0.0002% or more, or 0.0005% or more.

[0062] [Al: 1.000% or less] Al is an element that functions as a deoxidizer and is effective in increasing the strength of steel. The Al content may be 0%, but to fully obtain these effects, the Al content is preferably 0.001% or more. The Al content may be 0.005% or more, 0.010% or more, 0.025% or more, or 0.050% or more. On the other hand, excessive Al content may form coarse oxides, which may reduce toughness. Therefore, the Al content is set to 1.000% or less. The Al content may be 0.800% or less, 0.600% or less, or 0.300% or less.

[0063] [N: 0.0200% or less] N is an element that causes blowholes during welding. Therefore, the N content is set to 0.0200% or less. The N content may be 0.0180% or less, 0.0150% or less, 0.0100% or less, 0.0080% or less, or 0.0060% or less. The lower the N content, the more preferable it is, and the lower limit is not particularly limited and may be 0%. On the other hand, reducing the N content to less than 0.0001% in practical steel sheets significantly increases production costs and is economically disadvantageous. Therefore, the N content may be 0.0001% or more, 0.0002% or more, or 0.0005% or more.

[0064] [O: 0.0100% or less] O is an element that causes blowholes during welding. Therefore, the O content is set to 0.0100% or less. The O content may be 0.0080% or less, 0.0050% or less, 0.0030% or less, or 0.0020% or less. The lower the O content, the more preferable it is, and the lower limit is not particularly limited and may be 0%. On the other hand, if the O content in a practical steel sheet is reduced to less than 0.0001%, the manufacturing cost will increase significantly, which is economically disadvantageous. Therefore, the O content may be 0.0001% or more, 0.0002% or more, or 0.0005% or more.

[0065] The basic chemical composition of the steel sheet according to the embodiment of the present invention is as described above. Furthermore, the steel sheet may contain at least one of the following optional elements in place of a portion of the remaining Fe, if necessary, for the purpose of improving properties. For example, the steel sheet may contain at least one of Nb: 0-0.400%, Cr: 0-1.00%, Mo: 0-0.80%, B: 0-0.0100%, Ti: 0-0.200%, V: 0-0.500%, Ni: 0-1.00%, Cu: 0-1.00%, W: 0-1.00%, Ta: 0-0.10%, Co: 0-3.00%, Sn: 0-1.00%, Sb: 0-0.200%, Ca: 0-0.0100%, Mg: 0-0.0100%, Zr: 0-0.0100%, REM: 0-0.0100%, Bi: 0-0.0500%, and As: 0-0.10%. These optional elements will be described in detail below.

[0066] [Nb: 0 to 0.400%] Nb is an element effective in controlling the morphology of carbides and is also effective in refining the structure to improve the toughness of steel plate. These effects can be obtained even with trace amounts. The Nb content may be 0%, but to obtain the above effects, the Nb content is preferably 0.001% or more. The Nb content may be 0.005% or more or 0.010% or more. On the other hand, excessive Nb content may generate coarse carbides in the steel, reducing the toughness of the steel plate. For this reason, the Nb content is preferably 0.400% or less. The Nb content may be 0.200% or less, 0.100% or less, or 0.060% or less.

[0067] [Cr: 0 to 1.00%] Cr, like Mn, is an element that improves hardenability and contributes to improving steel sheet strength. While the Cr content may be 0%, in order to obtain the above-mentioned effects, the Cr content is preferably 0.001% or more. The Cr content may be 0.01% or more, 0.10% or more, or 0.20% or more. On the other hand, excessive Cr content may saturate the effect and may result in an increase in manufacturing costs. Therefore, the Cr content is preferably 1.00% or less, and may be 0.80% or less, 0.60% or less, or 0.40% or less.

[0068] [Mo: 0 to 0.80%] Mo, like Cr, is an element that contributes to improving the high-temperature strength of steel sheet. This effect can be achieved even with a small amount. The Mo content may be 0%, but to achieve the above effect, the Mo content is preferably 0.001% or more. The Mo content may be 0.01% or more, 0.02% or more, 0.05% or more, or 0.10% or more. On the other hand, excessive Mo content may deteriorate hot workability and reduce productivity. For this reason, the Mo content is preferably 0.80% or less. The Mo content may be 0.60% or less, 0.50% or less, 0.40% or less, or 0.20% or less.

[0069] [B: 0 to 0.0100%] B is an element that suppresses the formation of ferrite and pearlite during the cooling process from austenite and promotes the formation of martensite. Furthermore, B is a beneficial element for increasing the strength of steel. These effects can be achieved even with trace amounts. The B content may be 0%, but to achieve the above effects, the B content is preferably 0.0001% or more. The B content may be 0.0005% or more or 0.0010% or more. On the other hand, excessive B content may decrease toughness and / or weldability. Therefore, the B content is preferably 0.0100% or less. The B content may be 0.0080% or less, 0.0050% or less, 0.0030% or less, or 0.0020% or less.

[0070] [Ti: 0 to 0.200%] Ti is an element effective in controlling the morphology of carbides. Ti can promote an increase in the strength of ferrite. The Ti content may be 0%, but to obtain these effects, the Ti content is preferably 0.001% or more. The Ti content may be 0.002% or more, 0.010% or more, 0.020% or more, or 0.040% or more. On the other hand, if Ti is contained excessively, the effect saturates and there is a risk of an increase in manufacturing costs. Therefore, the Ti content is preferably 0.200% or less, and may be 0.100% or less, 0.080% or less, or 0.050% or less.

[0071] [V: 0 to 0.500%] Like Ti, V is an element effective in controlling the morphology of carbides and is also effective in refining the structure to improve the toughness of the steel plate. The V content may be 0%, but to obtain the above effect, the V content is preferably 0.001% or more. The V content may be 0.005% or more, 0.010% or more, or 0.050% or more. On the other hand, excessive V content may cause the formation of a large amount of precipitates, which may reduce toughness. For this reason, the V content is preferably 0.500% or less. The V content may be 0.400% or less, 0.200% or less, or 0.100% or less.

[0072] [Ni: 0 to 1.00%] [Cu: 0 to 1.00%] [W: 0 to 1.00%] Ni, Cu, and W are elements effective in improving the strength of steel sheet. The Ni, Cu, and W contents may be 0%, but to obtain such effects, the Ni, Cu, and W contents are preferably 0.001% or more, and may be 0.01% or more or 0.05% or more. On the other hand, excessive inclusion of these elements may reduce the weldability of the steel sheet. For this reason, the Ni, Cu, and W contents are preferably 1.00% or less, and may be 0.80% or less, 0.40% or less, or 0.20% or less.

[0073] [Ta: 0 to 0.10%] Ta, like W, is an element effective in controlling the morphology of carbides and improving the strength of the steel sheet. The Ta content may be 0%, but to obtain these effects, the Ta content is preferably 0.001% or more. The Ta content may be 0.01% or more or 0.03% or more. On the other hand, even if Ta is contained in an excessive amount, the effect saturates, and containing more than necessary in the steel sheet increases the manufacturing cost. For this reason, the Ta content is preferably 0.10% or less. The Ta content may be 0.08% or less, 0.06% or less, or 0.04% or less.

[0074] [Co: 0 to 3.00%] Co is an element effective in improving the strength of steel sheet. The Co content may be 0%, but to obtain this effect, the Co content is preferably 0.001% or more. The Co content may be 0.01% or more, 0.05% or more, or 0.10% or more. On the other hand, excessive Co content may deteriorate hot workability and increase raw material costs. For this reason, the Co content is preferably 3.00% or less. The Co content may be 2.00% or less, 1.00% or less, 0.50% or less, or 0.20% or less.

[0075] [Sn: 0 to 1.00%] Sn is an element that can be contained in steel sheet when scrap is used as a raw material for the steel sheet. Furthermore, Sn may cause ferrite embrittlement. Therefore, the lower the Sn content, the better, and it is preferably 1.00% or less. The Sn content may be 0.10% or less, 0.040% or less, or 0.02% or less. The Sn content may be 0%, but reducing the Sn content to less than 0.001% results in an excessive increase in refining costs. Therefore, the Sn content may be 0.001% or more, 0.005% or more, or 0.01% or more.

[0076] [Sb: 0 to 0.200%] Like Sn, Sb is an element that can be contained in steel sheet when scrap is used as a raw material for the steel sheet. Furthermore, Sb may strongly segregate at grain boundaries, potentially causing embrittlement of the grain boundaries. Therefore, the lower the Sb content, the better, and it is preferably 0.200% or less. The Sb content may be 0.100% or less, 0.040% or less, or 0.020% or less. The Sb content may be 0%, but reducing the Sb content to less than 0.001% would result in an excessive increase in refining costs. Therefore, the Sb content may be 0.001% or more, 0.005% or more, or 0.010% or more.

[0077] [Ca: 0 to 0.0100%] [Mg: 0 to 0.0100%] [Zr: 0 to 0.0100%] [REM: 0 to 0.0100%] Ca, Mg, Zr, and REM are elements that contribute to improving the formability of steel sheet. The Ca, Mg, Zr, and REM contents may be 0%, but to achieve these effects, the Ca, Mg, Zr, and REM contents are each preferably 0.0001% or more, and may be 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, excessive inclusion of these elements may reduce the ductility of the steel sheet. Therefore, the Ca, Mg, Zr, and REM contents are each preferably 0.0100% or less, and may be 0.0080% or less, 0.0060% or less, 0.0040% or less, or 0.0020% or less. In this specification, REM is a collective term for 17 elements: scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanides lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71, and the REM content is the total content of these elements.

[0078] [Bi: 0 to 0.0500%] Bi is an element that has the effect of improving formability by refining the solidification structure. The Bi content may be 0%, but to obtain this effect, the Bi content is preferably 0.0001% or more, and may be 0.0005% or more, 0.0010% or more, or 0.0030% or more. On the other hand, even if Bi is contained in an excessive amount, the effect saturates, and adding more Bi than necessary to the steel sheet increases the manufacturing cost. Therefore, the Bi content is preferably 0.0500% or less, and may be 0.0400% or less, 0.0200% or less, 0.0100% or less, or 0.0050% or less.

[0079] [As: 0 to 0.10%] Like Sn and Sb, As is an element that can be contained in steel sheet when scrap is used as the raw material for the steel sheet. Furthermore, As is an element that strongly segregates at grain boundaries, and the lower the As content, the better. The As content is preferably 0.10% or less, and may be 0.04% or less, or 0.02% or less. The As content may be 0%, but reducing the As content to less than 0.001% results in an excessive increase in refining costs. Therefore, the As content may be 0.001% or more, 0.005% or more, or 0.01% or more.

[0080] In the steel sheet according to the embodiment of the present invention, the balance excluding the above elements consists of Fe and impurities. The impurities are elements that are mixed in from the steel raw materials and / or during the steelmaking process and are allowed to be present to an extent that does not impair the properties of the steel sheet according to the embodiment of the present invention.

[0081] The chemical composition of the steel sheet according to the embodiment of the present invention may be measured by a general analytical method. For example, the chemical composition of the steel sheet may be measured using inductively coupled plasma atomic emission spectrometry (ICP-AES) based on a test specimen collected from a flat portion of the center portion of the panel. C and S may be measured using a combustion-infrared absorption method, N may be measured using an inert gas fusion-thermal conductivity method, and O may be measured using an inert gas fusion-non-dispersive infrared absorption method.

[0082] [Plating] The steel sheet according to the embodiment of the present invention may be a cold-rolled steel sheet, but may further include a plating layer on its surface for the purpose of improving corrosion resistance, etc. The plating layer may be either a hot-dip plating layer or an electroplated layer. That is, the steel sheet according to the embodiment of the present invention may be a cold-rolled steel sheet having a hot-dip plating layer or an electroplated layer on its surface. Examples of the hot-dip plating layer include a hot-dip galvanized layer (GI), a galvannealed layer (GA), a hot-dip aluminum plating layer, a hot-dip Zn—Al alloy plating layer, a hot-dip Zn—Al—Mg alloy plating layer, and a hot-dip Zn—Al—Mg—Si alloy plating layer. Examples of the electroplated layer include an electrogalvanized layer (EG), an electrolytic Zn—Ni alloy plating layer, etc. Preferably, the plating layer is a hot-dip galvanized layer, a hot-dip galvannealed layer, or an electrogalvanized layer. The coating weight of the coating layer is not particularly limited and may be a general coating weight.

[0083] [Thickness of Steel Plate or Panel] The flat portion of the central portion of the steel plate and the corresponding panel has a thickness of, for example, 0.2 to 2.0 mm, but is not particularly limited thereto. The thickness may be 0.3 mm or more, or 0.4 mm or more. Similarly, the thickness may be 1.8 mm or less, 1.5 mm or less, 1.2 mm or less, 1.0 mm or less, or 0.8 mm or less. For example, by making the thickness 0.2 mm or more, it becomes easier to maintain the shape of the molded product flat, and additional effects such as improved dimensional accuracy and shape accuracy can be obtained. On the other hand, by making the thickness 0.8 mm or less, the weight reduction effect of the component is significant. The thickness of the steel plate or panel is measured using a micrometer.

[0084] [Mechanical Properties] [Tensile Strength: TS] A panel having the above characteristics can achieve high tensile strength, specifically a tensile strength of 400 MPa or more. The tensile strength is preferably 440 MPa or more or 490 MPa or more, more preferably 540 MPa or more or 590 MPa or more. There is no particular upper limit, but the tensile strength may be, for example, 900 MPa or less, 860 MPa or less, or 800 MPa or less. The tensile strength is measured by taking a No. 5 tensile test piece according to JIS Z2241:2022 from the steel plate in the flat portion of the center portion of the panel and conducting a tensile test in accordance with JIS Z2241:2022.

[0085] [Yield Stress: YS] A panel having the above characteristics can achieve a high yield stress, more specifically, a yield stress of 300 MPa or more when subjected to a paint baking treatment. The yield stress after paint baking is preferably 350 MPa or more or 400 MPa or more, more preferably 490 MPa or more or 540 MPa or more. There is no particular upper limit, but for example, the yield stress after paint baking may be 850 MPa or less, 800 MPa or less, or 750 MPa or less. The yield stress is measured by taking a No. 5 tensile test piece according to JIS Z2241:2022 from the steel plate in the flat portion of the center portion of the panel and conducting a tensile test in accordance with JIS Z2241:2022. Specifically, the 0.2% proof stress measured by the tensile test is determined as the yield stress.

[0086] The panels according to the embodiments of the present invention can achieve high strength, specifically a tensile strength of 400 MPa or more, yet maintain an excellent appearance after molding, such as press molding, etc. Therefore, the panels according to the embodiments of the present invention are very useful for use as exterior panel parts, such as roofs, hoods, fenders, and doors, which require high design quality in automobiles.

[0087] <Method for manufacturing steel plate and panel> Next, a preferred method for manufacturing a steel plate and a panel according to an embodiment of the present invention will be described. The following description is intended to exemplify a characteristic method for manufacturing a steel plate and a panel according to an embodiment of the present invention, but is not intended to limit the steel plate and the panel to those manufactured by the manufacturing method described below.

[0088] <Method for manufacturing steel sheet> A method for manufacturing a steel sheet according to an embodiment of the present invention comprises: a hot rolling step including heating a slab having the chemical composition described above in relation to the steel sheet to a temperature of 1100 to 1400°C, finish-rolling the slab, and then coiling the slab at a temperature of 500 to 700°C, wherein the finish rolling temperature is 800 to 1350°C; a pickling step of pickling the obtained hot-rolled steel sheet; a cold rolling step of cold-rolling the pickled hot-rolled steel sheet at a rolling reduction of 20 to 90%; a primary annealing step of heating the cold-rolled steel sheet and holding it at a maximum heating temperature of Ac3 to 950°C for 10 to 500 seconds, and then cooling the cold-rolled steel sheet to a cooling stop temperature of 350°C or less by controlling the average cooling rate in the temperature range of 500 to 700°C to be 40°C / second or more; The method is characterized by including a secondary annealing step in which the cold-rolled steel sheet after primary annealing is subjected to secondary annealing, the secondary annealing step comprising heating the cold-rolled steel sheet and holding it at a maximum heating temperature of (Ac1+20) to 820°C for 10 to 500 seconds, then controlling the average cooling rate in a temperature range of 500 to 700°C to 10°C / second or more, and further controlling the average cooling rate in a temperature range of 200 to 500°C to 40°C / second or more. Each step will be described in more detail below.

[0089] [Hot Rolling Process] [Slab Heating] First, a slab having the chemical composition described above in relation to the steel sheet is heated. The slab used is preferably cast by a continuous casting method from the viewpoint of productivity, but may also be produced by an ingot casting method or a thin slab casting method. The slab used contains a relatively large amount of alloying elements in order to obtain a high-strength steel sheet. Therefore, the slab needs to be heated before being subjected to hot rolling to dissolve the alloying elements in the slab. If the heating temperature is less than 1100°C, the alloying elements may not be sufficiently dissolved in the slab, leaving coarse alloy carbides, which may cause embrittlement cracking during hot rolling. Therefore, the heating temperature is preferably 1100°C or higher. The upper limit of the heating temperature is not particularly limited, but is preferably 1400°C or lower from the viewpoint of the capacity of the heating equipment and productivity.

[0090] [Rough rolling] In this method, for example, the heated slab may be subjected to rough rolling before finish rolling in order to adjust the plate thickness, etc. The conditions for rough rolling are not particularly limited as long as the desired sheet bar dimensions can be secured.

[0091] [Finish Rolling and Coiling] The heated slab, or the slab that has been rough-rolled as needed, is then subjected to finish rolling. Because the slab used as described above contains a relatively large amount of alloying elements, a large rolling load is required during hot rolling. For this reason, hot rolling is preferably performed at a high temperature. The finish rolling temperature is particularly important in terms of controlling the metal structure of the steel sheet. If the finish rolling temperature is low, the metal structure may become non-uniform, resulting in reduced formability. For this reason, the finish rolling temperature is set to 800°C or higher. On the other hand, to suppress coarsening of austenite, the finish rolling temperature is set to 1350°C or lower. The finish rolling temperature is preferably 950 to 1050°C. Next, the finish-rolled hot-rolled steel sheet is coiled at a coiling temperature of 500 to 700°C. By setting the coiling temperature to 500 to 700°C, the growth of oxide scale can be suppressed. The coiling temperature is preferably 550 to 630°C.

[0092] [Pickling Step] Next, the obtained hot-rolled steel sheet is pickled to remove oxide scale formed on the surface of the hot-rolled steel sheet. Pickling may be carried out under conditions suitable for removing oxide scale, and may be carried out once or in multiple steps to ensure complete removal of oxide scale.

[0093] [Cold Rolling Process] The pickled hot-rolled steel sheet is cold-rolled at a reduction of 20 to 90% in the cold rolling process. By setting the cold rolling reduction to 20% or more, the shape of the cold-rolled steel sheet can be kept flat, and a decrease in ductility in the final product can be suppressed. On the other hand, by setting the cold rolling reduction to 90% or less, it is possible to prevent the rolling load from becoming excessively large, making rolling difficult. The cold rolling reduction is preferably 70 to 90%. The number of rolling passes and the reduction per pass are not particularly limited, and may be appropriately set so that the reduction of the entire cold rolling is within the above range.

[0094] [Primary annealing step] The obtained cold-rolled steel sheet is heated in the subsequent primary annealing step, held at a maximum heating temperature of Ac3 to 950°C for 10 to 500 seconds, and then cooled to a cooling stop temperature of 350°C or less by controlling the average cooling rate in the temperature range of 500 to 700°C to be 40°C / second or more. Here, the Ac3 point (°C) is determined by cutting a small piece from the cold-rolled steel sheet and measuring the thermal expansion of the small piece during heating from room temperature to 1000°C at 10°C / second. Holding the steel sheet at a temperature of the Ac3 point or higher for a sufficient time promotes austenitization, and then rapidly cooling the steel sheet to a temperature of 350°C or less enables the metal structure in the cooled steel sheet to be reliably composed of a structure mainly composed of bainite and / or martensite, for example, full bainite or full martensite. Here, a structure mainly composed of bainite and / or martensite refers to a structure containing at least one of bainite and martensite in a total area ratio of 90% or more, full bainite refers to a structure consisting of 100% bainite in area ratio, and full martensite refers to a structure consisting of 100% martensite in area ratio. A bainite and / or martensite structure has many different interfaces inside it compared to structures such as ferrite. Therefore, by forming the metal structure of a steel sheet before the secondary annealing process, i.e., the final annealing process, from bainite and / or martensite, it becomes possible to generate a very large number of dispersed carbides that can serve as austenite nucleation sites at these interfaces when such a metal structure is heated in the secondary annealing. As a result, by generating austenite finely and uniformly throughout the steel sheet from these dispersed nucleation sites and then generating martensite from this austenite, the average particle spacing of martensite in the metallographic structure obtained after secondary annealing is controlled to 2.5 μm or less, and the standard deviation of the area fraction of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction is controlled to 1.5% or less. In other words, it is possible to achieve a metallographic structure in which martensite is uniformly dispersed in both micro- and macro-regions.

[0095] If the maximum heating temperature in the primary annealing step is below the Ac3 point or the holding time is less than 10 seconds, austenitization will be insufficient, and the metal structure in the steel sheet will not be composed of a structure mainly composed of bainite and / or martensite even after subsequent cooling. That is, the total area ratio of bainite and martensite will not be 90% or more. On the other hand, since heating and holding at higher temperatures and for longer periods reduces productivity, the maximum heating temperature in the primary annealing step is set to 950°C or less, and the holding time is set to 500 seconds or less. The maximum heating temperature is preferably 870 to 950°C, and the holding time is preferably 50 to 100 seconds.

[0096] Furthermore, if the average cooling rate in the temperature range of 500 to 700°C in the primary annealing step is less than 40°C / sec or the cooling stop temperature exceeds 350°C, ferrite will form during cooling, and the metal structure in the steel sheet will not be able to have a total area ratio of bainite and martensite of 90% or more. Therefore, the average cooling rate must be 40°C / sec or more, preferably 70°C / sec. The upper limit is preferably 300°C / sec or less, more preferably 150°C / sec or less. On the other hand, the lower limit of the cooling stop temperature is not particularly limited. For example, the cooling stop temperature may be room temperature (25°C) or higher, preferably 200°C or higher. Similarly, the cooling stop temperature is preferably 300°C or lower.

[0097] [Secondary Annealing Process (Final Annealing Process)] The cold-rolled steel sheet after the primary annealing is heated again in the subsequent secondary annealing process, where it is 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 10°C / sec or more, and further controlling the average cooling rate in the temperature range of 200 to 500°C to 40°C / sec or more. Here, the Ac1 point (°C), as in the case of the Ac3 point, is determined by cutting a small piece from the cold-rolled steel sheet and measuring the thermal expansion of the small piece during heating from room temperature to 1000°C at 10°C / sec. First, in the stage where the steel sheet after the primary cooling is heated to a maximum heating temperature of (Ac1+20) to 820°C, carbides can be dispersed and generated on many interfaces contained within the bainite and / or martensite in the metal structure. Next, by heating for 10 to 500 seconds at the maximum heating temperature corresponding to the two-phase region of ferrite and austenite, austenite can be generated finely and uniformly throughout the steel sheet from the carbides while maintaining the state in which the carbides are dispersed at the interfaces. Finally, by controlling the average cooling rate in the temperature range of 500 to 700°C to 10°C / sec or more and further controlling the average cooling rate in the temperature range of 200 to 500°C to 40°C / sec or more, martensite can be appropriately generated from the finely dispersed austenite, resulting in an average particle spacing of martensite of 2.5 μm or less and a standard deviation in the area fraction of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction of 1.5% or less. In other words, it is possible to achieve a metal structure in which martensite is uniformly dispersed in both micro- and macro-regions.

[0098] If the maximum heating temperature in the secondary annealing step is less than Ac1+20°C or the holding time is less than 10 seconds, the desired metal structure described above cannot be obtained. On the other hand, if the maximum heating temperature exceeds 820°C, the austenite area ratio becomes too high, making it impossible to achieve a ferrite area ratio of 75% or more. Furthermore, due to the high temperature, it becomes impossible to maintain the state in which carbides are dispersed at the interface, and the final metal structure cannot achieve uniform dispersion of martensite in both the microregion and the macroregion. Furthermore, if the holding time exceeds 500 seconds, the austenite grains become coarse, and the martensite grains obtained by subsequent cooling also become relatively coarse. In such cases, it is impossible to obtain a fine martensite structure in which the average particle spacing of martensite is controlled to 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.

[0099] Furthermore, if the average cooling rate in the temperature range of 500 to 700°C in the secondary annealing step is less than 10°C / sec, the transformation from austenite to bainite or the like is accelerated, and even if subsequent cooling is performed appropriately, the desired amount of martensite may not be obtained. In this case, the desired strength may not be achieved and / or uniform dispersion of martensite, particularly in microscopic regions, may not be achieved. Therefore, the average cooling rate in the temperature range of 500 to 700°C must be 10°C / sec or more, preferably 40°C / sec or more, with an upper limit of, 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 transformation from austenite to martensite cannot be accelerated, and similarly, the formation of other structures such as bainite increases. Therefore, the average cooling rate in the temperature range of 200 to 500°C must be 40°C / sec or more, preferably 50°C / sec or more, with the upper limit being, for example, 200°C / sec or less, preferably 80°C / sec or less.

[0100] The above-described method produces a steel sheet according to an embodiment of the present invention by two annealing treatments, including a primary annealing and a secondary annealing. However, the steel sheet according to an embodiment of the present invention is not necessarily limited to one produced by such a method, and can also be produced, for example, by a single annealing treatment. More specifically, by configuring the metallographic structure of the steel sheet after the hot rolling process to full bainite or full martensite, it is possible to omit the above-described primary annealing. However, in this case, it is necessary to appropriately control the cooling conditions and coiling temperature after hot rolling, and it is also important to control the reduction ratio in the subsequent cold rolling. This is because, if the reduction ratio in cold rolling is too high, recrystallization occurs during heating in the subsequent annealing process, making it impossible to maintain the metallographic structure formed in the hot rolling process.

[0101] [Plating Step] The surface of the obtained cold-rolled steel sheet may be plated for purposes such as improving corrosion resistance. The plating process may be hot-dip galvanizing, alloying galvanizing, electroplating, or the like. For example, the steel sheet may be subjected to hot-dip galvanizing as the plating process, or may be subjected to alloying after the hot-dip galvanizing process. The specific conditions for the plating and alloying processes are not particularly limited and may be any appropriate conditions known to those skilled in the art. For example, in the hot-dip galvanizing process, the temperature of the sheet immersed in the plating bath (the temperature of the steel sheet when immersed in the hot-dip galvanizing bath) is preferably in the range from 40°C lower than the hot-dip galvanizing bath temperature (hot-dip galvanizing bath temperature -40°C) to 50°C higher than the hot-dip galvanizing bath temperature (hot-dip galvanizing bath temperature +50°C). When the hot-dip galvanized layer is subjected to alloying, the steel sheet on which the hot-dip galvanized layer has been formed is preferably heated to a temperature range of 460 to 600°C, more preferably to a temperature range of 480 to 550°C.

[0102] <Panel manufacturing method> A panel manufacturing method according to an embodiment of the present invention includes: a blanking step of blanking the steel sheet obtained above; a forming step of forming the blanked steel sheet into a steel part; and optionally a painting step of painting the formed steel part. Each step will be described in more detail below.

[0103] [Blanking Step] In the blanking step, the steel plate obtained above is subjected to blanking processing, in which the steel plate is cut to a predetermined size. The blanking processing can be performed by any appropriate means known to those skilled in the art, such as punching using a press.

[0104] [Forming Process] The blanked steel sheet (blank) is formed into a steel part in the subsequent forming process. For example, if bending is performed in a direction perpendicular to the rolling direction of the blank to introduce strain in the direction perpendicular to the rolling, it may be impossible to appropriately control Str and / or Sa within a predetermined range in the formed panel. In such cases, the occurrence of appearance defects such as ghost lines becomes significant. Therefore, in the forming process, it is preferable to perform forming so that the rolling direction of the blank coincides with the bending direction in which the largest curvature is formed. By performing such bending, even in the case of a high-strength steel sheet containing martensite, the introduction of strain in the direction perpendicular to the rolling can be sufficiently reduced, making it possible to suppress or reduce the occurrence of appearance defects such as ghost lines.

[0105] The amount of strain imparted during the forming process must also be appropriately controlled. While a small amount of strain does not necessarily adversely affect the post-forming appearance, it may result in insufficient dislocation introduction. In this case, the amount of bake hardening during paint baking is reduced, making it impossible to sufficiently increase the yield stress. As a result, the dent resistance of the final product is reduced. Therefore, from the perspective of improving dent resistance, it is preferable that the amount of strain imparted during the forming process be 2.0% or more in the flat portion of the center portion of the panel. On the other hand, applying excessive strain increases the surface roughness parameter Sa of the flat portion and / or edge of the final product, resulting in a deterioration of the post-forming appearance. Therefore, from the perspective of improving the post-forming appearance, it is preferable that the amount of strain imparted during the forming process be 5.0% or less in the flat portion of the center portion of the panel.

[0106] [Painting Process] The formed steel part is optionally painted in the next painting process, preferably by a paint baking treatment. This painting process includes, for example, three types of painting: electrodeposition painting, intermediate painting, and top painting (base and clear coating). Water-based paints or solvent-based paints are used for painting. In the electrodeposition painting process, the steel part is submerged in an electrodeposition tank containing paint, and the entire surface of the steel part is electroplated. In the intermediate painting process, the entire surface of the steel part is coated with the intermediate paint by spraying the paint onto the steel part from a spray nozzle using a painting robot or manually by a worker. In the top painting process, the entire surface of the steel part is coated with the top coat by spraying the paint onto the steel part from a spray nozzle using a painting robot or manually by a worker. As a result, the surface of the steel part is covered with a paint layer with a thickness of 60 to 200 μm.

[0107] [Paint baking treatment] Paint baking treatment is a baking and drying treatment for baking a paint layer onto a steel part and for baking and hardening the steel part. The paint baking treatment may be performed after electrodeposition coating and before intermediate coating, between intermediate coatings that are performed multiple times, after intermediate coating and before topcoat coating, between topcoat coatings that are performed multiple times, or after topcoat coating.

[0108] The temperature and time of the paint baking treatment are preferably controlled so that the drying parameter P, expressed by the following formula 1, is within the range of 7,500 to 10,000. The specific temperature and time of the paint baking treatment may be appropriately selected, for example, from the ranges of 100 to 220°C and 20 to 60 minutes, so long as they satisfy the following formula 1: P = (T + 273) × (17.7 + log(t)) ... Formula 1. In this formula, T is the temperature (°C) of the paint baking treatment, and t is the time (seconds) of the paint baking treatment. When the paint baking treatment is performed multiple times, the total time of each paint baking treatment is preferably controlled so that it is within the range of 20 to 60 minutes. Similarly, the drying parameter P is preferably controlled so that the cumulative value of the drying parameter calculated from the temperature and time of each paint baking treatment is within the range of 7,500 to 10,000. If the drying parameter P is less than 7,500, the amount of bake hardening decreases, and as a result, the yield stress after bake hardening may not be sufficiently increased. In this case, the dent resistance of the final product will be reduced. On the other hand, if the drying parameter P exceeds 10,000, excessive baking treatment will result in a reduction in the yield stress after bake hardening, which may also result in a reduction in the dent resistance of the final product.

[0109] Panels manufactured by the above manufacturing method achieve high strength by including martensite in the metal structure of the steel sheet that constitutes the panel, while controlling the surface properties of the panel so that Str is within the range of 0.50 to 1.00 and Sa is within the range of 0.50 μm or less, thereby significantly suppressing the occurrence of appearance defects such as ghost lines on the panel surface even when strain is imparted by forming such as press forming. Furthermore, when the panel is subjected to a paint baking process, the yield stress can be significantly increased by bake hardening, thereby improving the dent resistance of the panel. Therefore, panels manufactured by the above manufacturing method are particularly useful in the automotive field, where high strength, excellent post-forming appearance, and excellent dent resistance are required.

[0110] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples in any way.

[0111] In the following examples, panels according to embodiments of the present invention were produced under various conditions, and the properties of the resulting panels were investigated.

[0112] [Manufacturing of Steel Sheets] 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 predetermined temperature of 1100 to 1400°C and hot-rolled. Hot rolling was performed by rough rolling and finish rolling, and the finish rolling end temperature and coiling temperature were as shown in Table 2. Next, the obtained hot-rolled steel sheets were pickled and then cold-rolled at the reduction shown in Table 2 to obtain cold-rolled steel sheets having a thickness of 0.4 mm. Next, the obtained cold-rolled steel sheets were subjected to primary annealing and secondary annealing under the conditions shown in Table 2. Finally, hot-dip galvanizing was appropriately performed as a plating treatment, and some of the slabs were further subjected to alloying treatment at the alloying temperatures shown in Table 2.

[0113] [Manufacturing of Panels] Next, the obtained cold-rolled steel sheet or plated steel sheet was blanked and cut into blanks of a predetermined size, and then the blanks were press-formed under the conditions shown in Table 2 to obtain steel parts in the shape of panels. Here, "OK" for the bending direction in Table 2 means that the rolling direction of the blank and the bending direction in which the largest curvature is formed are consistent. The obtained steel parts were washed, immersed for 3 minutes in a 40°C aqueous solution containing 3 mass% of an aqueous alkaline degreasing agent (FC-301 manufactured by Nippon Parkerizing Co., Ltd.) for degreasing, and then washed with water and dried. Next, 3 g / m 2After pretreatment with zinc phosphate (Palbond 3020 (trade name) manufactured by Nippon Parkerizing Co., Ltd.), electrodeposition coating was performed using a cationic electrodeposition paint manufactured by Nippon Paint Co., Ltd. at a slope current of 160 V. Subsequently, an intermediate coat (EP Primer 1405(A) manufactured by Kansai Paint Co., Ltd.) and a top coat (Magicron 1000 manufactured by Kansai Paint Co., Ltd.) were spray-coated. After electrodeposition coating, intermediate coating, and top coat, the temperature and time of the paint baking treatment were appropriately selected from the ranges of 100 to 220°C and 20 to 60 minutes to ensure that each paint was dried appropriately. A paint baking treatment was performed according to the drying parameter P shown in Table 2, resulting in a panel with a paint layer having the film thickness shown in Table 2. The drying parameter P shown in Table 2 is the cumulative sum of the drying parameters calculated from the temperature and time of each paint baking treatment after electrodeposition coating, intermediate coating, and top coat.

[0114]

[0115]

[0116] The properties of the obtained panel were measured and evaluated by the following methods.

[0117] [Evaluation of Appearance] The appearance after molding into a panel was evaluated based on the degree of ghost lines that appeared on the surface of the door outer after molding. The press-molded surface was grinded, and stripes spaced a few millimeters apart that appeared on the surface were judged to be ghost lines. The appearance was scored from 1 to 5 based on the following criteria, depending on the degree of streak occurrence. A random 100 mm x 100 mm area was visually inspected. A score of "1" was given if no streak was observed; a score of "2" was given if the maximum length of the streak was 20 mm or less; a score of "3" was given if the maximum length of the streak was greater than 20 mm but less than 50 mm; a score of "4" was given if the maximum length of the streak was greater than 50 mm but less than 70 mm; and a score of "5" was given if the maximum length of the streak was greater than 70 mm. A score of "3" or less was deemed to be excellent in appearance after molding and therefore passed. A score of "4" or more was deemed to be poor in appearance after molding and therefore failed. The results are shown in Table 3.

[0118]

[0119] Referring to Tables 1 to 3, in Comparative Example 3, the amount of strain imparted in the panel forming process was large, resulting in the Sa of the steel sheet in the flat portion of the panel exceeding 0.50 μm, and as a result, the appearance after forming was poor. In Comparative Example 4, it is believed that the long holding time in the secondary annealing process of steel sheet production caused the austenite grains to coarsen. In relation to this, the average particle spacing of martensite in the metallographic structure obtained after secondary annealing exceeded 2.5 μm, and it was not possible to obtain a steel sheet having a metallographic structure in which martensite is finely and uniformly dispersed throughout. As a result, the Sa of the steel sheet in the flat portion and edge of the formed panel exceeded 0.50 μm, resulting in poor appearance after forming. In Comparative Example 9, it is believed that the maximum heating temperature in the primary annealing process was low, resulting in insufficient austenitization, and that the metallographic structure in the steel sheet could not be composed of a structure mainly composed of bainite and / or martensite, even after subsequent cooling. In this regard, the standard deviation of the area fraction of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction in the metallographic structure obtained after the secondary annealing exceeded 1.5%, and it was not possible to obtain a steel sheet having a metallographic structure in which martensite is finely and uniformly dispersed throughout. As a result, the Str of the steel sheet at the flat portion of the formed panel was less than 0.50, and the appearance after forming was degraded.

[0120] In contrast, in all of the panels according to the invention, the metal structure of the steel sheet constituting the panel was enriched with martensite to achieve high strength, for example, a tensile strength of 400 MPa or more, while the surface properties of the panel were controlled so that Str was 0.50 to 1.00 and Sa was 0.50 μm or less, thereby significantly suppressing the occurrence of appearance defects such as ghost lines on the panel surface even when strain was imparted by forming, such as press forming. In particular, in Examples 1, 2, 5, 6, and 8, steel sheets having a metal structure in which martensite is finely and uniformly dispersed throughout, i.e., steel sheets having a metal structure in which the average particle spacing of martensite is controlled to 2.5 μm or less and the standard deviation of the area fraction of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction is controlled to 1.5% or less, were used, and by forming the steel sheets so that the average KAM value / Vm was 1.8 or more, a YS of 350 MPa or more was achieved, thereby significantly improving dent resistance. This is thought to be due to the fact that by applying appropriate strain to a steel sheet in which martensite is uniformly dispersed, many dislocations are uniformly introduced, thereby increasing the amount of bake hardening.

Claims

1. A panel including a steel plate having a metal structure including martensite, wherein the surface property aspect ratio Str of the steel plate in a flat portion of a center portion of the panel is 0.50 to 1.00, and the surface roughness parameter Sa of the steel plate in the flat portion of the center portion and at the ends of the panel is 0.50 μm or less.

2. The panel described in claim 1, characterized in that the ratio of the average KAM value of ferrite contained in the steel plate to the volume fraction Vm of the martensite: average KAM value / Vm is 1.8 or more.

3. A panel according to claim 1 or 2, characterized in that the steel plate is a painted steel plate having a paint layer on at least one surface.

4. The panel according to claim 1 or 2, characterized in that the metal structure of the steel plate in the flat portion is composed, by area percentage, of ferrite: 75-95%, martensite: 5-25%, and at least one of bainite, pearlite, and retained austenite: 0-10% in total, the average grain spacing of martensite is 2.5 μm or less, and the standard deviation of the area ratio of martensite in the direction perpendicular to the rolling direction and the plate thickness direction is 1.5% or less.

5. The panel according to claim 1 or 2, wherein the thickness of the flat portion is 0.2 to 0.8 mm.

6. A panel according to claim 1 or 2, characterized in that it has a tensile strength of 400 to 900 MPa.

Images