Panel

Abstract

The present invention provides a panel which includes a composite structure steel sheet having a metal structure that is composed of a soft phase and a hard phase. This panel is characterized in that: the surface property aspect ratio Str of the composite structure steel sheet in a flat part of a center side portion of the panel is 0.50 to 1.00; and the surface roughness parameter Sa of the composite structure steel sheet in the flat part of the center side portion of the panel is 0.50 μm or less.

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 not only a soft phase but also a hard phase 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, etc. of the steel sheet 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 completed the present invention.

[0011] The present invention, which has achieved the above-mentioned object, is as follows: (1) A panel including a dual-phase steel sheet having a metallurgical structure composed of a soft phase and a hard phase, wherein the dual-phase steel sheet has a surface texture aspect ratio Str of 0.50 to 1.00 in a flat portion in a center portion of the panel, and a surface roughness parameter Sa of 0.50 μm or less in the flat portion in the center portion of the panel. (2) The panel described in (1) above, wherein the dual-phase steel sheet has a thinned portion in an area other than the flat portion that is thinner than the thickness of the flat portion. (3) The panel described in (1) or (2) above, wherein the panel is an outer panel of an automobile. (4) The panel described in any one of (1) to (3) above, wherein the dual-phase steel sheet is a painted steel sheet having a paint layer on at least one surface. (5) The panel described in any one of (1) to (4) above, wherein the thickness of the flat portion is 0.2 to 0.6 mm. (6) The panel according to any one of (1) to (5) above, characterized in that it has a tensile strength of 500 MPa or more. (7) The panel according to any one of (1) to (6) above, characterized in that the metallographic structure of the dual-phase steel plate in the flat portion is, in area %, 75 to 97% soft phase and 3 to 25% hard phase, and the standard deviation of the hard phase fraction in the direction perpendicular to the rolling direction is 0.75% or less. (8) The panel according to (7) above, characterized in that the dual-phase steel plate in the flat portion satisfies the following formula 1: (TS - 180,000 / TS) / Vm ≧ 35 ... formula 1, where TS is the tensile strength in MPa, and Vm is the hard phase fraction in area %.

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

[0013] 1A and 1B are schematic diagrams showing a panel obtained by drawing in an example, in which FIG. 1A is a perspective view of the panel, and FIG. 1B is a perspective view of the panel of FIG. 1A as seen from the back side.

[0014] <Panel> A panel according to an embodiment of the present invention includes a dual-phase steel plate having a metal structure composed of a soft phase and a hard phase, wherein the dual-phase steel plate has a surface texture aspect ratio Str of 0.50 to 1.00 in a flat portion in a central portion of the panel, and wherein the dual-phase steel plate has a surface roughness parameter Sa of 0.50 μm or less in the flat portion in the central portion of the panel.

[0015] In recent years, in response to demands for further improvements in fuel efficiency, there has been an increasing need to reduce the weight of not only structural components such as vehicle members, but also exterior panels such as roofs, hoods, doors, and tailgates. Unlike the structural components described above, these exterior panels are visible to the public, and therefore, not only are characteristics such as strength important, but also design and surface quality are important, and therefore, excellent appearance after forming is required. Meanwhile, in response to this demand for weight reduction, the steel sheets used for these exterior panels are also required to be stronger and thinner. However, increasing the strength and reducing the thickness increases the force that attempts to return to the original shape after forming (the springback phenomenon), resulting in a problem of lower surface quality compared to current products. Furthermore, in order to avoid surface defects known as surface distortions that occur during press forming and other processes, dual-phase steels with relatively low yield strength are often used for these exterior panels. However, in the case of multi-phase steels containing a soft phase consisting of ferrite and a hard phase consisting mainly of martensite, uneven deformation is likely to occur during processing such as press forming, in which the soft phase and its surroundings deform preferentially. This can result in minute irregularities on the panel surface after forming, resulting in appearance defects known as ghost lines. More specifically, during processing such as press forming, the soft phase consisting of ferrite undergoes large depressions, while the hard phase consisting mainly of martensite undergoes small depressions. Therefore, compared to the soft phase, the hard phase does not depression but rather rises to form a convex shape. As a result, variations in deformation occur, particularly in the direction perpendicular to the rolling direction of the panel, resulting in band-like ghost lines.

[0016] Therefore, the present inventors conducted studies 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 not only a soft phase but also a hard phase in the metallographic structure of the steel sheet constituting the panel. On the other hand, since the appearance of the panel after forming is generally deteriorated in association with the high strength resulting from such a dual phase, the present inventors further conducted studies focusing on the surface quality of the panel after forming in order to suppress such deterioration of the 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, etc. of the dual-phase steel sheet 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 panel after forming.

[0017] 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 occurrence of streaks and the like on the surface. Conversely, 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 press forming, particularly forming such as drawing, appropriately selecting the metal structure of the dual-phase steel sheet used as the raw material to control the surface texture aspect ratio Str of the dual-phase steel sheet in the flat portion near the center of the panel to be within the range of 0.50 to 1.00 is very 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 ghost lines, it is preferable that the minute irregularities on the panel surface are as isotropic as possible, and therefore the Str value is preferably closer to 1.00.

[0018] However, since there have been cases where simply appropriately controlling Str has not been enough to sufficiently improve the appearance after forming, the present inventors have conducted further studies related to the surface properties of panels. As a result, the present inventors have found that by appropriately selecting the metal structure, etc. of the dual-phase steel sheet used as the raw material, in addition to controlling Str, by controlling the surface roughness parameter Sa of the dual-phase steel sheet in the flat portion near the center of the panel to be within a range of 0.50 μm or less, it is possible to significantly suppress or reduce the occurrence of poor appearance caused by minute irregularities on the panel surface, even when strain is imparted by press forming, particularly drawing, etc. Here, the surface roughness parameter Sa refers to the average absolute value of z(x, y) in the reference area (A) defined in JIS B0681-2:2018, 4.1.7 "Arithmetic mean height of the scale limited surface."

[0019] Therefore, according to the panels according to the embodiments of the present invention, while maintaining sufficient high strength based on the hard phase contained together with the soft phase in the metallographic structure of the dual-phase 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 press forming, particularly drawing, or other forming processes. In particular, the present inventors have now discovered for the first time that even when the metallographic structure of the dual-phase steel sheet material contains a hard phase, 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 panels according to the embodiments of the present invention are particularly useful for application to automotive exterior panels, which require relatively high strength. Each component of the panel according to the embodiment of the present invention will be described in more detail below.

[0020] [Str of the dual-phase 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 dual-phase 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, the 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.

[0021] [Measurement of Str] The surface texture aspect ratio Str of a dual-phase 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 subjected to three-dimensional measurement 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.

[0022] [Sa of Dual-Phase Steel Sheet in Flat Portion of Center-Near Portion of Panel: 0.50 μm or Less] In an embodiment of the present invention, the surface roughness parameter Sa of the dual-phase steel sheet in the flat portion of the center-near portion of the panel after forming is controlled to 0.50 μm or less. As in the case of Str, when 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. The surface roughness parameter Sa is the average value of the height difference (absolute value) of each point with respect 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 in the flat portion of the center-near portion of the panel significantly improves the appearance of the formed high-strength panel due to the combined effect 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 in the flat portion of the center side 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 in the flat portion of the center side of the panel.

[0023] [Measurement of Sa] The surface roughness parameter Sa of a dual-phase steel sheet at the flat portion of the center portion of a 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 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 the arithmetic mean height is determined. The arithmetic mean height thus obtained is determined as the surface roughness parameter Sa of the dual-phase steel sheet at the flat portion of the center portion of the panel.

[0024] [Dual-phase steel plate] The panel according to the embodiment of the present invention includes a dual-phase steel plate having a metallurgical structure composed of a soft phase and a hard phase. The panel according to the embodiment of the present invention includes at least a dual-phase steel plate having a metallurgical structure composed of a soft phase and a hard phase, and the flat portion of the center portion of the panel composed of the dual-phase steel plate has the above-mentioned characteristics. Therefore, the panel according to the embodiment of the present invention may partially include a material other than the dual-phase steel plate having a metallurgical structure composed of a soft phase and a hard phase. Preferably, the panel according to the embodiment of the present invention essentially consists of, consists of, or consists of a dual-phase steel plate having a metallurgical structure composed of a soft phase and a hard phase.

[0025] In the present invention, the term "soft phase" refers to ferrite. On the other hand, in the present invention, the term "hard phase" refers to a structure harder than the soft phase, ferrite, and includes or consists of at least one of martensite, bainite, tempered martensite, and pearlite, for example, at least one of martensite, bainite, tempered martensite, and pearlite. From the viewpoint of improving the strength of the dual-phase steel sheet, the hard phase preferably consists of at least one of martensite, bainite, and tempered martensite, or is at least one of them, and more preferably consists of martensite or is martensite. In an embodiment of the present invention, the metal structure of the dual-phase steel sheet preferably contains little retained austenite. Specifically, the retained austenite is preferably less than 1% or less than 0.5%, and more preferably 0%, in terms of area percentage. By including the above-described hard phase in the metal structure of the dual-phase steel sheet constituting the panel, it is possible to achieve a desired high strength. The area fraction of the hard phase in the metal structure may be appropriately selected depending on the desired strength of the panel, and is not particularly limited. For example, the area fraction of the hard phase in the metal structure may be 3% or more, 5% or more, 7% or more, 10% or more, or 12% or more. Similarly, the area fraction of the hard phase in the metal structure may be 25% or less, 22% or less, 20% or less, 18% or less, or 15% or less. In this regard, the area fraction of the soft phase in the metal structure may be 75% or more, 78% or more, 80% or more, 82% or more, or 85% or more. Similarly, the area fraction of the soft phase in the metal structure may be 97% or less, 95% or less, 93% or less, 90% or less, or 88% or less. The identification of the hard phase and the soft phase and the calculation of their area fractions are performed based on the description in the section "Identification of Metal Structure and Calculation of Area Fraction" below.

[0026] [Thinned Portion] In one preferred embodiment of the present invention, the dual-phase steel sheet has a thinned portion in a region other than the flat portion, the thickness of which is thinner than the flat portion. Panels according to embodiments of the present invention can be manufactured, for example, by a manufacturing method including drawing. When manufacturing a panel by drawing, for example, a flat portion and a ridge portion connected to the flat portion are formed, and a thinned portion thinner than the thickness of the flat portion is formed at the ridge portion due to drawing. According to panels according to embodiments of the present invention, even in drawing that forms such thinned portions, as described above, the surface texture of the formed panel can be significantly reduced by controlling the surface texture 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, thereby significantly reducing the occurrence of appearance defects such as ghost lines on the panel surface. The degree of thinning can be determined appropriately depending on the conditions of press forming, such as drawing, and the type and size of the part being manufactured, and is not particularly limited. For example, the thinning rate of the thinned portion may be 2 to 20%. The thickness reduction rate of the thinned portion may be, for example, 3% or more, 4% or more, 5% or more, or 7% or more. Similarly, the thickness reduction rate of the thinned portion may be, for example, 18% or less, 15% or less, 12% or less, 10% or less, or 8% or less. Here, the thickness reduction rate refers to the rate of reduction in plate thickness before and after forming. When it is difficult to measure the plate thickness before forming, the thickness reduction rate is calculated from the plate thickness of the flat portion and the plate thickness of the thinned portion using the following formula: Thickness reduction rate (%) = (plate thickness of flat portion - plate thickness of thinned portion) / plate thickness of flat portion × 100

[0027] [Paint Layer] In the panels according to the embodiments of the present invention, the dual-phase 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 is 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 therefore advantageous from the perspective of improving the dent resistance of the panel. When a dual-phase steel sheet having a metallurgical structure in which hard phases are uniformly dispersed, as described below in relation to preferred embodiments of the dual-phase steel sheet, is applied, the amount of bake hardening can be significantly increased due to the uniform dispersion of the hard phase 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.

[0028] When the dual-phase 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 automotive panels 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.

[0029] [Preferred Embodiments of Dual-Phase Steel Sheet] Hereinafter, a preferred embodiment of a dual-phase steel sheet having a metal structure composed of a soft phase and a hard phase, which is useful for realizing the surface texture of a panel in which, when formed by press forming, particularly drawing, the dual-phase steel sheet has a surface texture aspect ratio Str of 0.50 to 1.00 in the flat portion of the central portion and a surface roughness parameter Sa of 0.50 μm or less in the flat portion of the central portion, will be described in detail. However, these descriptions are intended to merely exemplify preferred dual-phase steel sheets for forming panels according to embodiments of the present invention, and are not intended to limit the present invention to embodiments using such specific dual-phase steel sheets.

[0030] In a preferred embodiment of the present invention, the dual-phase steel sheet is characterized in that the metal structure in the flat portion of the center side of the panel is, in area %, 75 to 97% soft phase and 3 to 25% hard phase, and the standard deviation of the hard phase fraction in the direction perpendicular to the rolling direction is 0.75% or less.

[0031] As mentioned above, in the case of dual-phase steels containing a soft phase consisting of ferrite and a hard phase mainly consisting of martensite, non-uniform deformation is likely to occur during processing such as press forming, in which the soft phase and its surroundings deform preferentially. This can result in minute irregularities on the panel surface after forming, resulting in appearance defects known as ghost lines. Meanwhile, as steel sheets become stronger, relatively large amounts of elements such as Mn are sometimes added to improve the hardenability of steel sheets. 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 elongated 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 phases are formed in the metallographic structure of the steel sheet after quenching. In this case, the occurrence of ghost lines becomes particularly noticeable.

[0032] Therefore, the present inventors have investigated means for optimizing the ratio of soft and hard phases in the metal structure, more specifically, controlling the soft phase to 75 to 97% by area and the hard phase to 3 to 25% by area, thereby achieving the desired high strength and further improving the appearance after forming. Specifically, the present inventors conducted their investigations focusing on the distribution of the hard phase in the metal structure. As a result, the present inventors have found that, as will be described in detail later in connection with the manufacturing method of a dual-phase steel sheet, it is important to reduce the amount of equiaxed crystals in the solidification structure during the casting process and control the solidification structure to a columnar crystal structure. More specifically, the present inventors have found that when coarse equiaxed crystals are formed in the solidification structure during the casting process, even if the centerline segregation of Mn itself is small, negative segregation of Mn occurs, resulting in large variations in the hard phase fraction and possibly worsening the appearance after forming. In this regard, the present inventors have found that controlling the solidification structure to a columnar crystal structure can suppress negative segregation of Mn and reduce the central segregation of Mn during solidification, which is a cause of the formation of banded hard phases, thereby reducing the variation in the hard phase fraction in the metallographic structure. More specifically, the present inventors have found that controlling the solidification structure to a columnar crystal structure can reduce the standard deviation of the hard phase fraction in the transverse direction to 0.75% or less, thereby enabling the production of a dual-phase steel sheet having a metallographic structure in which the hard phase is uniformly dispersed. Here, the "transverse direction" refers to the direction perpendicular to the rolling direction and the plate thickness direction. A conventional countermeasure against center segregation has been to increase the equiaxed crystal fraction, which has been common technical knowledge among those skilled in the art (see, for example, Takaho Kawawa et al., "Tetsu to Hagane," Vol. 60 (1974) No. 5, pp. 486-500, and Hiroshi Kumai et al., "Tetsu to Hagane," Vol. 60 (1974) No. 7, pp. 894-914). Therefore, it is extremely unexpected and surprising that negative segregation and center segregation of Mn can be reduced by reducing the equiaxed crystals in the solidification structure and controlling the solidification structure to a columnar crystal structure.

[0033] In addition, the inventors have found that even when a certain degree of centerline segregation of Mn remains, by appropriately reducing the hard phase fraction within a range of 3 to 25%, the standard deviation of the hard phase fraction can be reduced to 0.75% or less, and as a result, it is possible to produce a dual-phase steel sheet having a metallographic structure in which the hard phase is uniformly dispersed. Therefore, according to a preferred embodiment of the present invention, by using a dual-phase steel sheet having a metallographic structure in which the hard phase is uniformly dispersed, the amount of deformation of the dual-phase 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 a panel having the desired Str and Sa, it is possible to achieve an excellent post-forming appearance in which appearance defects such as ghost lines are significantly suppressed.

[0034] 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 the soft phase to 75 to 97%, while controlling the area fraction of the hard phase to 3 to 25%. Furthermore, by controlling the chemical composition of the dual-phase steel sheet within a predetermined range, high strength, for example, a tensile strength of 400 MPa or more, preferably 500 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 dual-phase steel sheet according to a preferred embodiment of the present invention has a metal structure in which the hard phase is uniformly dispersed. 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 the hard phase containing a relatively large number of dislocations. Therefore, the use of this dual-phase steel sheet can significantly increase the yield stress of the resulting panel, which is therefore extremely advantageous from the perspective of improving the dent resistance of the panel.

[0035] First, the metallographic structure of a dual-phase steel sheet according to a preferred embodiment of the present invention will be described in more detail below. The unit "%" for the structure fraction means area %. The metallographic structure of the dual-phase steel sheet refers to the metallographic structure of the dual-phase steel sheet in the flat portion near the center of the panel. The degree of forming in this flat portion is low, and therefore the characteristics of the metallographic structure described below do not change significantly before and after forming, such as press forming.

[0036] [Soft Phase: 75 to 97% and Hard Phase: 3 to 25%] In a preferred embodiment of the present invention, the metallographic structure of the dual-phase steel sheet is, in area %, 75 to 97% soft phase and 3 to 25% hard phase. By making the metallographic structure such a dual-phase structure, high strength, more specifically a tensile strength of 400 MPa or more, preferably 500 MPa or more, can be achieved while suppressing poor appearance after forming. From the viewpoint of further increasing the strength of the dual-phase steel sheet, the area fraction of the hard phase may be, for example, 5% or more, 7% or more, 10% or more, or 12% or more. Similarly, the area fraction of the soft phase may be 95% or less, 93% or less, 90% or less, or 88%. On the other hand, from the viewpoint of further improving the appearance after forming, the area fraction of the hard phase may be, for example, 22% or less, 20% or less, 18% or less, or 15% or less. Similarly, the area fraction of the soft phase may be 78% or more, 80% or more, 82% or more, or 85% or more.

[0037] As described above, in the present invention, the "soft phase" refers to ferrite. On the other hand, in the present invention, the "hard phase" refers to a structure harder than the soft phase, which is ferrite, and includes or consists of at least one of martensite, bainite, tempered martensite, and pearlite, for example, and is particularly at least one of martensite, bainite, tempered martensite, and pearlite. From the viewpoint of improving the strength of the dual-phase steel sheet, the hard phase preferably consists of at least one of martensite, bainite, and tempered martensite, or is at least one of them, and more preferably consists of martensite or is martensite. In an embodiment of the present invention, it is preferable that the metal structure of the dual-phase steel sheet contains little retained austenite. Specifically, the retained austenite is preferably less than 1% or less than 0.5%, and more preferably 0%, by area.

[0038] [Identification of Metallographic Structure and Calculation of Area Fraction] Identification of the metallographic structure and calculation of the area fraction are performed as follows. First, a sample (e.g., 20 mm × 20 mm × sheet thickness) for observing the metallographic structure (microstructure) is taken from the dual-phase steel sheet. Next, the metallographic structure is observed from the surface to half the sheet thickness using a scanning electron microscope (SEM), and the area fraction of the hard phase is calculated from a position 50 μm from the steel sheet surface (the surface excluding the plating layer, if present) to a position half the sheet thickness. To prepare the sample, a cross section of the sheet thickness perpendicular to the sheet surface is polished as an observation surface and etched by nital etching. Next, the "microstructure" is classified from an SEM photograph at a magnification of 500 or 1000 times. Hard phases can be distinguished from each other based on differences in brightness.

[0039] The area from the surface of the steel plate etched by nital corrosion to the 1 / 2 position in the plate thickness direction is observed at a magnification of 500x or 1000x in 10 fields of view (3 or more fields if it is difficult to observe 10 fields of view due to the thin plate thickness), and the area fraction of the hard phase is determined using image analysis software Image J (Ver. 1.54f). The soft phase and hard phase are binarized based on the difference in brightness, and the area fraction of the hard phase is calculated. Image analysis is performed in the same manner as above for a total of 10 observed fields of view to measure the area fraction of the hard phase, and these area fractions are averaged to calculate the average value. This average value is the area fraction of the hard phase, and the remainder is the area fraction of ferrite. The observation area of ​​each field of view is 37,500 μm 2 For example, when the plate thickness is thin, the length in the plate thickness direction is reduced and the observation area is set to 37,500 μm 2 Furthermore, if it is necessary to measure the area fraction of retained austenite, the area fraction of retained austenite can be measured by X-ray diffraction on the above observation surface. Specifically, using Co-Kα radiation, the integrated intensity of a total of five peaks, α(200), α(211), γ(311), γ(200), and γ(220), at a 1 / 4 position in the sheet thickness direction is determined, and the volume fraction of retained austenite is calculated using the intensity averaging method. The obtained volume fraction of retained austenite is taken as the area fraction of retained austenite.

[0040] [Standard Deviation of Hard Phase Fraction in the Transverse Direction: 0.75% or Less] In a preferred embodiment of the present invention, in the metallographic structure of the flat portion of the dual-phase steel sheet, the standard deviation of the hard phase fraction in the transverse direction is controlled to 0.75% or less. The standard deviation of the hard phase fraction refers to the standard deviation of the area fraction of the hard phase itself. As described above, post-forming appearance defects such as ghost lines are significantly affected not only by Mn segregation but also by the solidification structure. For example, even when the centerline segregation of Mn is small, if coarse equiaxed crystals are formed in the solidification structure, negative segregation of Mn occurs, which increases the variation in the hard phase fraction in the transverse direction and may worsen the post-forming appearance defects. However, in a preferred embodiment of the present invention, the standard deviation of the hard phase fraction in the transverse direction is 0.75% or less, i.e., the variation in the hard phase fraction in the transverse direction is sufficiently reduced, thereby significantly suppressing post-forming appearance defects. Here, the "direction perpendicular to the rolling direction" refers to the direction perpendicular to the rolling direction and the plate thickness direction, as described above.

[0041] From the viewpoint of further improving the appearance after forming, the lower the standard deviation of the hard phase fraction in the direction perpendicular to the rolling direction, the more preferable, and it may be, for example, 0.65% or less, 0.55% or less, or 0.45% or less. Although there is no particular lower limit, for example, the standard deviation of the hard phase fraction in the direction perpendicular to the rolling direction may be 0.01% or more, 0.05% or more, 0.10% or more, 0.15% or more, or 0.20% or more.

[0042] [Measurement of Standard Deviation of Hard Phase Fraction in the Direction Transverse to the Rolling Direction] The standard deviation of the hard phase fraction in the direction transverse to the rolling direction of the metallographic structure is determined as follows. First, a region between a position 50 μm from one steel sheet surface and a position 50 μm from the other steel sheet surface in a cross section of a flat portion of a dual-phase steel sheet parallel to the direction transverse to the rolling direction and perpendicular to the steel sheet surface is observed with a scanning electron microscope (SEM) at a magnification of 500x or 1000x to obtain an SEM photograph. This SEM photograph is analyzed using image analysis software, as in the case of the hard phase area fraction described above, to measure the hard phase area fraction every 100 μm within an 8 mm range in the direction transverse to the rolling direction of the dual-phase steel sheet, and its standard deviation is calculated. The observation range in the direction transverse to the rolling direction may be less than 8 mm or more than 8 mm. However, the lower limit of the observation range for the standard deviation of the hard phase fraction in the direction transverse to the rolling direction is 4 mm, and the upper limit is 12 mm.

[0043] When the transverse direction of the rolling direction of a dual-phase steel plate is unclear, the following method, for example, is employed to identify the transverse direction of the rolling direction of the dual-phase steel plate. After mirror-polishing a thickness cross section of the dual-phase steel plate, the S concentration is measured using an electron probe microanalyzer (EPMA). The measurement conditions are 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 length of the major axes of the inclusions in each cross section is calculated, and the cross section with the largest average length of the major axes of the inclusions is identified. The direction parallel to the major axes of the inclusions in that cross section is determined to be the rolling direction.

[0044] [(TS - 180,000 / TS) / Vm ≥ 35] In a preferred embodiment of the present invention, it is more preferable that the dual phase steel sheet in the flat portion satisfies the following formula 1: (TS - 180,000 / TS) / Vm ≥ 35 Formula 1 Here, TS is the tensile strength in MPa, and Vm is the hard phase fraction in area %. Vm is determined based on the description in [Identification of Metallic Structure and Calculation of Area Fraction] above.

[0045] When a dual-phase steel sheet satisfies the above formula 1, Vm is controlled to a relatively low value in relation to TS. Therefore, even when a certain amount of center segregation of Mn remains, it becomes easier to suppress the variation in the hard phase fraction in the direction perpendicular to the rolling direction of the metallographic structure. As a result, it becomes relatively easy to achieve an excellent post-formation appearance in which appearance defects such as ghost lines are significantly suppressed. The larger the value of the left side of formula 1, the more remarkable this effect becomes. Therefore, the value of the left side of formula 1 may be 38 or more, 40 or more, 42 or more, or 45 or so. There is no particular upper limit, but the value of the left side of formula 1 may be 70 or less, 68 or less, or 65 or less, for example.

[0046] [Preferred Chemical Composition of Dual-Phase 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 a soft phase and a hard phase 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 dual-phase steel sheet itself is not an essential technical feature for achieving the objectives of the present invention. Below, preferred chemical compositions of dual-phase steel sheets according to embodiments of the present invention are described in detail. However, these descriptions are intended to merely exemplify preferred chemical compositions of dual-phase steel sheets having a tensile strength of, for example, 400 to 900 MPa, and are not intended to limit the present invention to dual-phase steel sheets having such specific chemical compositions. 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.

[0047] In an embodiment of the present invention, for example, the dual phase steel sheet has, in mass %, C: 0.030 to 0.100%, Mn: 1.00 to 2.50%, Si: 0.005 to 1.500%, P: 0.100% or less, S: 0.0200% or less, Al: 0.005 to 0.700%, N: 0.0150% or less, O: 0.0100% or less, Cr: 0 to 0.80%, Mo: 0 to 0.50%, B: 0 to 0.0100%, Ti: 0 to 0.100%, Nb: 0 to 0.100%, V: 0 to 0.50%, Ni: 0 to 1.00%, Cu: 0 to 1.00%, W: 0 to 1.00%, It is preferable that the alloy has a chemical composition consisting of Sn: 0 to 1.00%, Sb: 0 to 0.200%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, REM: 0 to 0.0100%, and the balance: Fe and impurities. Each element will be described in more detail below.

[0048] [C: 0.030 to 0.100%] C is an element that increases 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 0.035% or more, 0.040% or more, or 0.050% or more. On the other hand, if C is contained excessively, the strength may become too high and the elongation may decrease. Therefore, the C content is set to 0.100% or less. The C content may be 0.095% or less, 0.090% or less, or 0.080% or less.

[0049] [Mn: 1.00 to 2.50%] Mn is an element that improves the hardenability of steel and contributes to improving strength. To fully obtain this effect, the Mn content is set to 1.00% or more. The Mn content may be 1.20% or more, 1.30% or more, or 1.40% or more. On the other hand, if Mn is contained excessively, ferrite transformation may be excessively suppressed, making it impossible to secure the desired amount of ferrite, and elongation may decrease. Therefore, the Mn content is set to 2.50% or less. The Mn content may be 2.25% or less, 2.00% or less, or 1.85% or less.

[0050] [Si: 0.005 to 1.500%] Si is a deoxidizing element for steel and also an element that improves the strength of steel sheet through solid solution strengthening. To fully obtain these effects, the Si content is set to 0.005% or more. The Si content may be 0.010% or more, 0.050% or more, or 0.100% or more. On the other hand, excessive Si content may reduce the peelability of scale and cause surface defects. Therefore, the Si content is set to 1.500% or less. The Si content may be 1.000% or less, 0.500% or less, or 0.300% or less.

[0051] [P: 0.100% or less] P is an element that is mixed in during the manufacturing process. P is also a solid-solution strengthening element. The P content may be 0%. However, reducing the P content to less than 0.0001% requires time for refining, resulting in reduced productivity. Therefore, the P content may be 0.0001% or more, 0.0005% or more, or 0.001% or more. On the other hand, excessive P content may reduce the toughness of the steel plate. Therefore, the P content is set to 0.100% or less. The P content may be 0.060% or less, 0.040% or less, or 0.020% or less.

[0052] [S: 0.0200% or less] S is an element that is mixed in during the manufacturing process. The S content may be 0%. However, reducing the S content to less than 0.0001% requires time for refining, resulting in reduced productivity. Therefore, the S content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, excessive S content may form Mn sulfides, which may reduce the formability of the steel sheet, such as ductility, hole expandability, stretch flangeability, and / or bendability. Therefore, the S content is set to 0.0200% or less. The S content may be 0.0100% or less, 0.0060% or less, or 0.0040% or less.

[0053] [Al: 0.005 to 0.700%] Al is an element that functions as a deoxidizer and also improves the strength of steel sheet through solid solution strengthening. To fully obtain these effects, the Al content is set to 0.005% or more. The Al content may be 0.010% or more, 0.020% or more, or 0.025% or more. On the other hand, excessive Al content may deteriorate castability and reduce productivity. Therefore, the Al content is set to 0.700% or less. The Al content may be 0.600% or less, 0.400% or less, 0.300% or less, 0.200% or less, or 0.100% or less.

[0054] [N: 0.0150% or less] N is an element that is mixed in during the manufacturing process. The N content may be 0%. However, reducing the N content to less than 0.0001% requires time for refining, resulting in reduced productivity. Therefore, the N content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, excessive N content may form nitrides, which may reduce the formability of the steel sheet, such as ductility, hole expandability, stretch flangeability, and / or bendability. Therefore, the N content is set to 0.0150% or less. The N content may be 0.0100% or less, 0.0080% or less, or 0.0050% or less.

[0055] [O: 0.0100% or less] O is an element that is mixed in during the manufacturing process. The O content may be 0%. However, reducing the O content to less than 0.0001% requires time for refining, resulting in reduced productivity. Therefore, the O content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, excessive O content may form coarse oxides, which may reduce the formability of the steel sheet, such as ductility, hole expandability, stretch flangeability, and / or bendability. Therefore, the O content is set to 0.0100% or less. The O content may be 0.0070% or less, 0.0040% or less, or 0.0020% or less.

[0056] The basic chemical composition of the dual-phase steel sheet according to the embodiment of the present invention is as described above. Furthermore, the dual-phase steel sheet may contain at least one of the following optional elements in place of a portion of the remaining Fe, as needed, for the purpose of improving properties. For example, the dual-phase steel sheet may contain at least one of Cr: 0-0.80%, Mo: 0-0.50%, B: 0-0.0100%, Ti: 0-0.100%, Nb: 0-0.100%, V: 0-0.50%, Ni: 0-1.00%, Cu: 0-1.00%, W: 0-1.00%, Sn: 0-1.00%, Sb: 0-0.200%, Ca: 0-0.0100%, Mg: 0-0.0100%, Zr: 0-0.0100%, and REM: 0-0.0100%. These optional elements are described in detail below.

[0057] [Cr: 0 to 0.80%] Cr is an element that improves the hardenability of steel and contributes to improving the strength of steel sheet. The Cr content may be 0%, but to achieve this effect, the Cr content is preferably 0.001% or more, and more preferably 0.01% or more. The Cr content may be 0.10% or more, 0.20% or more, or 0.30% or more. On the other hand, excessive Cr content may cause the formation of coarse Cr carbides that serve as fracture initiation points. Therefore, the Cr content is preferably 0.80% or less. The Cr content may be 0.70% or less, 0.60% or less, or 0.50% or less.

[0058] [Mo: 0 to 0.50%] Mo is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of the steel sheet. The Mo content may be 0%, but to obtain this effect, the Mo content is preferably 0.001% or more, and more preferably 0.01% or more. The Mo content may be 0.05% or more or 0.07% or more. On the other hand, excessive Mo content may deteriorate hot workability and reduce productivity. Therefore, the Mo content is preferably 0.50% or less. The Mo content may be 0.40% or less, 0.30% or less, or 0.20% or less.

[0059] [B: 0 to 0.0100%] B is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of the steel sheet. The B content may be 0%, but to obtain this effect, the B content is preferably 0.0001% or more. The B content may be 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, excessive B content may cause the formation of B precipitates, which may reduce the strength of the steel sheet. Therefore, the B content is preferably 0.0100% or less. The B content may be 0.0080% or less, 0.0060% or less, or 0.0030% or less.

[0060] [Ti: 0 to 0.100%] Ti is an element that has the effect of reducing the amounts of S, N, and O, which generate coarse inclusions that act as fracture initiation sites. Ti also precipitates finely in steel as carbides, etc., and improves the strength of the steel through precipitation strengthening. While the Ti content may be 0%, to obtain these effects, the Ti content is preferably 0.001% or more. The Ti content may be 0.005% or more, 0.007% or more, or 0.010% or more. On the other hand, excessive Ti content may form coarse Ti sulfides, Ti nitrides, and / or Ti oxides, which may reduce the formability of the steel sheet. Therefore, the Ti content is preferably 0.100% or less. The Ti content may be 0.080% or less, 0.060% or less, or 0.030% or less.

[0061] [Nb: 0 to 0.100%] Nb is an element that contributes to improving strength through precipitation strengthening. The Nb content may be 0%, but to obtain this effect, the Nb content is preferably 0.001% or more. The Nb content may be 0.005% or more, 0.007% or more, or 0.010% or more. On the other hand, excessive Nb content may increase non-recrystallized ferrite, thereby reducing the formability of the steel sheet. Therefore, the Nb content is preferably 0.100% or less. The Nb content may be 0.060% or less, 0.040% or less, or 0.030% or less.

[0062] [V: 0 to 0.50%] V is an element that contributes to improving the strength of steel sheets due to strengthening by precipitates, grain refinement strengthening by suppressing the growth of crystal grains in soft phases, and / or dislocation strengthening by suppressing recrystallization. The V content may be 0%, but to obtain these effects, the V content is preferably 0.001% or more, more preferably 0.005% or more. The V content may be 0.01% or more or 0.02% or more. On the other hand, excessive V content may precipitate a large amount of carbonitrides, reducing the formability of the steel sheet. Therefore, the V content is preferably 0.50% or less. The V content may be 0.40% or less, 0.20% or less, or 0.10% or less.

[0063] [Ni: 0 to 1.00%] Ni is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of the steel sheet. The Ni content may be 0%, but to obtain such an effect, the Ni content is preferably 0.001% or more, and more preferably 0.01% or more. The Ni content may be 0.03% or more or 0.05% or more. On the other hand, excessive Ni content may reduce the weldability of the steel sheet. Therefore, the Ni content is preferably 1.00% or less. The Ni content may be 0.60% or less, 0.40% or less, or 0.20% or less.

[0064] [Cu: 0 to 1.00%] Cu is an element that exists in steel in the form of fine particles and contributes to improving the strength of the steel sheet. The Cu content may be 0%, but to obtain such an effect, the Cu content is preferably 0.001% or more, and more preferably 0.01% or more. The Cu content may be 0.03% or more or 0.05% or more. On the other hand, excessive Cu content may reduce the weldability of the steel sheet. Therefore, the Cu content is preferably 1.00% or less. The Cu content may be 0.60% or less, 0.40% or less, or 0.20% or less.

[0065] [W: 0 to 1.00%] W is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of the steel sheet. The W content may be 0%, but to obtain this effect, the W content is preferably 0.001% or more, and more preferably 0.01% or more. The W content may be 0.02% or more or 0.10% or more. On the other hand, excessive W content may deteriorate hot workability and reduce productivity. Therefore, the W content is preferably 1.00% or less. The W content may be 0.80% or less, 0.50% or less, or 0.20% or less.

[0066] [Sn: 0 to 1.00%] Sn is an element that suppresses coarsening of crystal grains and contributes to improving the strength of the steel sheet. The Sn content may be 0%, but to obtain such an effect, the Sn content is preferably 0.001% or more, and more preferably 0.01% or more. The Sn content may be 0.05% or more or 0.08% or more. On the other hand, excessive Sn content may cause embrittlement of the steel sheet. Therefore, the Sn content is preferably 1.00% or less. The Sn content may be 0.80% or less, 0.50% or less, or 0.20% or less.

[0067] [Sb: 0 to 0.200%] Sb is an element that suppresses coarsening of crystal grains and contributes to improving the strength of the steel sheet. The Sb content may be 0%, but to obtain this effect, the Sb content is preferably 0.001% or more. The Sb content may be 0.003% or more, 0.005% or more, or 0.010% or more. On the other hand, excessive Sb content may cause embrittlement of the steel sheet. Therefore, the Sb content is preferably 0.200% or less. The Sb content may be 0.150% or less, 0.100% or less, 0.050% or less, or 0.020% or less.

[0068] [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 preferably 0.0001% or more. The Ca, Mg, Zr, and REM contents may be 0.0005% or more, 0.0010% or more, or 0.0015% or more, respectively. 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 preferably 0.0100% or less. The Ca, Mg, Zr, and REM contents may be 0.0080% or less, 0.0060% or less, or 0.0030% or less, respectively. 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. The REM content is the total content of these elements.

[0069] In the dual-phase steel sheet according to the embodiment of the present invention, the balance other than the above elements consists of Fe and impurities. Here, the impurities refer to components that are mixed in due to various factors in the manufacturing process, including raw materials such as ore and scrap, when industrially manufacturing the dual-phase steel sheet. Examples of impurities include H, Na, Cl, Co, Zn, Ga, Ge, As, Se, Y, Tc, Ru, Rh, Pd, Ag, Cd, In, Te, Cs, Ta, Re, Os, Ir, Pt, Au, Pb, Bi, and Po. The total impurities may be 0.100% or less.

[0070] The chemical composition of the dual-phase 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 dual-phase steel sheet may be measured using inductively coupled plasma atomic emission spectrometry (ICP-AES) based on a test piece 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.

[0071] [Index A: 0.45% or More] In a preferred embodiment of the present invention, the chemical composition of the dual-phase steel sheet has an index A represented by the following formula 2 of 0.45% or more: A=[Si]+10[P]+0.6[Al]+8[Ti]+9[Nb] Formula 2 Here, [Si], [P], [Al], [Ti], and [Nb] are the contents of each element in mass%, and are 0% when the element is not contained.

[0072] The index A is determined by the contents of the solid-solution strengthening elements Si, P, and Al and the precipitation strengthening elements Ti and Nb. Increasing the hard phase fraction is generally preferable to increase the strength of dual-phase steel sheets. However, increasing the hard phase fraction tends to increase the variation in the hard phase fraction in the transverse direction. Therefore, even in the case of a high hard phase fraction, in order to control the standard deviation of the hard phase fraction in the transverse direction of the rolling of the final metallographic structure to 0.75% or less, it is necessary to further sufficiently reduce the center segregation of Mn in the casting process. This requires strict control of manufacturing conditions, which increases the manufacturing burden. In this regard, the inventors have discovered that by utilizing solid-solution strengthening by Si, P, and Al and precipitation strengthening by Ti and Nb, more specifically, by controlling the index A expressed by the above formula 2 to 0.45% or more, the hard phase fraction can be reduced while maintaining high strength. As a result, the inventors have found that even in cases where centerline segregation of Mn remains to some extent, by appropriately reducing the hard phase fraction within the range of 3 to 25%, it is possible to relatively easily reduce the standard deviation of the hard phase fraction in the direction perpendicular to the rolling direction to 0.75% or less, and thereby to manufacture a dual-phase steel sheet having a metal structure in which the hard phase is uniformly dispersed.

[0073] From the viewpoint of further increasing the strength of the dual-phase steel sheet, the larger the index A, the more preferable, and it may be, for example, 0.48% or more, 0.50% or more, 0.52% or more, 0.55% or more, 0.58% or more, 0.60% or more, 0.62% or more, or 0.65% or more. The upper limit is not particularly limited, and for example, index A may be 2.00% or less, 1.80% or less, 1.50% or less, 1.30% or less, or 1.00% or less.

[0074] [Plating] The dual-phase 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 dual-phase 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 galvannealed layer, or an electrogalvanized layer. The coating weight of the coating layer is not particularly limited and may be a general coating weight.

[0075] [Thickness of Dual-Phase Steel Plate or Panel] The flat portion of the center portion of the dual-phase steel plate and the corresponding panel is not particularly limited, and may have a thickness of, for example, 0.2 to 2.0 mm. 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, 0.8 mm or less, or 0.6 mm or less. For example, by setting the thickness to 0.2 mm or more, it becomes easier to maintain the shape of the molded product flat, and additional effects such as improved dimensional accuracy and shape accuracy can be obtained. On the other hand, by setting the thickness to 0.8 mm or less or 0.6 mm or less, the weight reduction effect of the component is significant. The thickness of the dual-phase steel plate or panel is measured using a micrometer.

[0076] [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 500 MPa, 540 MPa or more, or 590 MPa or more. The upper limit is not particularly limited, 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 dual-phase steel plate in the flat portion of the center portion of the panel and conducting a tensile test in accordance with JIS Z2241:2022.

[0077] 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 press forming, particularly drawing, etc. Therefore, the panels according to the embodiments of the present invention are very useful for use as, for example, outer panel panels of automobiles, more specifically, outer panel panels of automobiles such as roofs, hoods, fenders, and doors, which require high design quality.

[0078] <Method for manufacturing dual phase steel sheet and panel> Next, a preferred method for manufacturing a dual phase steel sheet and 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 dual phase steel sheet and panel according to an embodiment of the present invention, but is not intended to limit the dual phase steel sheet and panel to those manufactured by the manufacturing method described below.

[0079] <Method for manufacturing dual-phase steel plate> [Casting step] A method for manufacturing dual-phase steel plate according to an embodiment of the present invention includes a casting step of casting a slab having the specific chemical composition described above in relation to the dual-phase steel plate. The casting step includes performing soft reduction using a continuous casting machine equipped with multiple reduction rolls adjacent to each other in the slab transport direction, the roll pitch between adjacent reduction rolls being 290 mm or less. In this specification, soft reduction refers to a reduction gradient of 0.6 mm or more per meter in the casting direction.

[0080] As described above, the dual-phase steel sheet according to the embodiment of the present invention preferably has a unique metallographic structure with a small variation in the hard phase fraction in the direction transverse to the rolling direction, more specifically, a metallographic structure in which the standard deviation of the hard phase fraction in the direction transverse to the rolling direction is reduced to 0.75% or less. To obtain such a metallographic structure, it is very important to control the solidification structure during casting to be columnar. Specifically, in the casting process, by setting the superheat ΔT (the difference between the molten steel temperature and the solidification temperature of the molten steel) of the molten steel having the specific chemical composition to 25°C or more and further setting the segment pressing force to 450 tons or more, it is possible to control the solidification structure to a columnar crystal structure with an equiaxed crystal fraction of 15% or less and suppress center segregation. The superheat ΔT is preferably 30°C or more, and more preferably 40°C or less. The molten steel temperature is the temperature of the molten steel in the tundish and can be determined by actual measurement. The solidification temperature can be determined from the chemical composition of the molten steel using a known solidification temperature estimation formula.

[0081] Conventional measures to improve centerline segregation have been to minimize the superheat ΔT (at least below 25°C) and increase the equiaxed crystal fraction (to at least 15%), but these conventional measures do not provide sufficient improvement. In the present production method, casting conditions that are completely different from conventional measures, namely, unique casting conditions of setting the superheat ΔT to 25°C or higher and setting the segment pressing force to 450 tons or higher, are adopted to control the solidification structure to a columnar crystal structure, thereby suppressing negative segregation and centerline segregation of Mn.

[0082] The equiaxed crystal ratio (%) can be calculated by taking an etched print of a cross section of the thickness direction of the slab in the width direction, visually determining the boundary between the columnar crystal structure and the equiaxed crystal structure, measuring the thickness (mm) of the equiaxed crystal structure at the center of the thickness of the slab and the thickness (mm) of the slab, and dividing the thickness of the equiaxed crystal structure by the thickness of the slab and multiplying the result by 100.

[0083] Furthermore, in the casting process, soft reduction is performed using a continuous casting machine in which the roll pitch of adjacent reduction rolls is 290 mm or less, thereby suppressing the flow of molten steel during solidification and reducing the concentration of Mn in the center. This makes it possible to suppress center segregation of Mn. It is more preferable that the roll pitch of adjacent reduction rolls is 280 mm or less.

[0084] [Other Steps] In addition to the casting step, the present manufacturing method may include other steps such as a hot rolling step, a cold rolling step, an annealing step, and a cooling step. Furthermore, the present manufacturing method may optionally include a plating step. These steps are not particularly limited, and may be carried out under any appropriate conditions appropriately selected so as to obtain a metal structure containing the soft phase and hard phase at predetermined area fractions as described above in relation to the dual-phase steel sheet. Preferred conditions for these steps will be briefly described below.

[0085] [Hot Rolling Process] Prior to hot rolling, the slab is preferably heated to 1100°C or higher. By setting the heating temperature to 1100°C or higher, the rolling reaction force during hot rolling does not become excessively large, making it easier to obtain the desired product thickness. There is no particular upper limit to the heating temperature, but from an economical viewpoint, the heating temperature is preferably less than 1300°C. In the hot rolling process, the heated slab is subjected to rough rolling and finish rolling. The hot-rolled steel sheet obtained in this manner is coiled at a coiling temperature of, for example, 450 to 650°C.

[0086] The finish rolling end temperature is preferably 950°C or lower. By setting the finish rolling end temperature to 950°C or lower, the average crystal grain size of the hot-rolled steel sheet and the final product can be reduced, ensuring sufficient yield strength and high surface quality after forming. Furthermore, by setting the coiling temperature to 450 to 650°C, the average crystal grain size can be reduced and scale growth can be suppressed.

[0087] [Cold Rolling Process] The hot-rolled steel sheet obtained by the hot-rolling process is subjected to an appropriate pickling treatment to remove scale, and then subjected to the cold-rolling process. In the cold-rolling process, for example, it is preferable to cold-roll the hot-rolled steel sheet so that the cumulative reduction is 50 to 90%. By controlling the cumulative reduction within this range, it is possible to ensure the desired sheet thickness and further ensure sufficient uniformity of the material in the sheet width direction, while preventing the rolling load from becoming excessive and making the rolling difficult.

[0088] [Annealing Process] In the annealing process, it is preferable to perform an annealing treatment in which the cold-rolled steel sheet is heated to a soaking temperature of 750 to 900°C and maintained at that temperature. By setting the soaking temperature to 750°C or higher, it is possible to sufficiently promote the recrystallization of ferrite and the reverse transformation from ferrite to austenite, thereby obtaining the desired metal structure in the final product. On the other hand, by setting the soaking temperature to 900°C or lower, it is possible to densify the crystal grains and obtain sufficient strength.

[0089] [Cooling Step] In the cooling step, the cold-rolled steel sheet after the annealing step is cooled. In the cooling step, the cold-rolled steel sheet is preferably cooled so that the average cooling rate from the soaking temperature is 5 to 50°C / s. By setting the average cooling rate to 5°C / s or more, excessive transformation to ferrite is suppressed and the amount of hard phases such as martensite produced is increased, thereby achieving the desired strength. Furthermore, by setting the average cooling rate to 50°C / s or less, the steel sheet can be cooled more uniformly in the width direction.

[0090] [Plating Step] For the purpose of improving corrosion resistance, etc., the surface of the obtained cold-rolled steel sheet may be subjected to a plating treatment, if necessary. Examples of plating treatments include hot-dip plating, alloying hot-dip plating, and electroplating. For example, the steel sheet surface may be subjected to hot-dip galvanizing treatment as the plating treatment, or an alloying treatment may be performed after the hot-dip galvanizing treatment. The specific conditions for the plating treatment and the alloying treatment are not particularly limited, and any appropriate conditions known to those skilled in the art may be adopted. For example, the alloying temperature may be 450 to 600°C.

[0091] <Panel manufacturing method> A panel manufacturing method according to an embodiment of the present invention includes: a blanking step of blanking the dual-phase steel sheet obtained above; a forming step of forming the blanked dual-phase 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.

[0092] [Blanking Step] In the blanking step, the dual-phase steel sheet obtained above is subjected to blanking processing, in which the sheet 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.

[0093] [Forming Process] The blanked dual-phase steel sheet (blank) is then formed into a steel part by press forming or the like in the subsequent forming process. Examples of press forming include bending and drawing. Here, the amount of strain imparted by press forming must be appropriately controlled. A small amount of strain does not necessarily adversely affect the appearance after forming, but it may result in insufficient dislocation introduction. In this case, the amount of bake hardening during paint baking decreases, making it impossible to sufficiently increase the yield stress. As a result, the dent resistance of the final product decreases. Therefore, from the perspective of improving dent resistance, the amount of strain imparted in the forming process is preferably 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 of the final product, resulting in a deterioration of the appearance after forming. Therefore, from the perspective of improving the appearance after forming, the amount of strain imparted in the forming process is preferably 5.0% or less in the flat portion of the center portion of the panel.

[0094] [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.

[0095] [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.

[0096] The temperature and time of the paint baking treatment are preferably controlled so that the drying parameter P, expressed by the following formula 3, 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 3: P = (T + 273) × (17.7 + log(t)) ... Formula 3, where 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.

[0097] Panels manufactured by the above manufacturing method achieve high strength by including not only soft phases but also hard phases in the metal structure of the steel sheet constituting 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 even excellent dent resistance are required.

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

[0099] 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.

[0100] [Manufacturing of Dual-Phase Steel Plate] First, a continuous casting machine equipped with a plurality of reduction rolls arranged at a roll pitch of 280 mm or less was used to cast a slab having the chemical composition shown in Table 1 and a thickness of 200 to 300 mm by a continuous casting method in which soft reduction was performed with a reduction gradient of 0.6 mm or more per meter in the casting direction. The balance other than the components shown in Table 1 is Fe and impurities. In Table 2, casting condition (I) is the condition of "superheat ΔT≧25°C," and casting condition (II) is the condition of "segment pressing force≧450 tons." For each example, Table 2 shows the cases where these conditions are met (denoted as "OK") and the cases where they are not met (denoted as "NG").

[0101] Next, the obtained slab was subjected to a hot rolling process (heating temperature 1200°C, finish rolling end temperature 900°C, and coiling temperature 550°C), a cold rolling process (cumulative reduction 80%), an annealing process (soaking temperature 800°C), and a cooling process (average cooling rate 10°C / sec) to produce a cold-rolled steel sheet with a thickness of 0.4 mm. The surface of the obtained cold-rolled steel sheet was appropriately plated to form a hot-dip galvanized layer (GI), a galvannealed layer (GA), or an electrogalvanized layer (EG). In addition, when the chemical composition of a sample taken from the produced cold-rolled steel sheet was analyzed, it was found to be unchanged from the chemical composition of the slab shown in Table 1.

[0102] [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 drawn to obtain steel parts having the panel shape shown in FIG. 1 . FIG. 1 is a schematic diagram showing a panel obtained by drawing in an example, where FIG. 1(a) is a perspective view of the panel, and FIG. 1(b) is a perspective view of the panel of FIG. 1(a) as seen from the back. In FIG. 1 , the amount of strain in the flat portion with a curvature radius of 1200 mm was 5.0% or less. Furthermore, the thinning rate of the ridge line portion (thinned portion) indicated by the thick line in FIG. 1(a) was approximately 5% in all examples and comparative examples.

[0103]

[0104]

[0105] The properties of the obtained panel were measured and evaluated by the following methods.

[0106] [Evaluation of Appearance] The appearance of the panel after drawing was evaluated based on the degree of ghost lines that appeared on the panel surface after drawing. The flat portion and edge of each panel after drawing were ground with a grindstone at the same location, and linear streaks that appeared on the surface and extended approximately parallel to the rolling direction were judged to be ghost lines and evaluated. Specifically, the panel surface was visually inspected, and the appearance after drawing was evaluated as follows, and cases where the evaluation was AA or A were judged to be excellent in appearance after forming and to be acceptable. AA: No streaks on the flat portion and edge of the panel A: No streaks on the flat portion of the panel, streaks present on the edge of the panel B: Streaks present on the flat portion of the panel

[0107] The results are shown in Table 2. In the metal structures shown in Table 2, the hard phase contained at least one of martensite, bainite, tempered martensite, and pearlite, or was at least one of these. Furthermore, as a result of measuring the retained austenite by X-ray diffraction, the area ratio of the retained austenite was less than 1% in all examples.

[0108] Referring to Tables 1 to 3, in Comparative Examples 3 and 16, the superheat ΔT in the casting process was low, which resulted in a high equiaxed crystal fraction in the solidification structure, and it is believed that negative segregation and / or center segregation of Mn could not be sufficiently suppressed. As a result, the hard phase fraction in the direction perpendicular to the rolling direction increased, and the Sa of the dual-phase steel plate in the flat portion of the panel exceeded 0.50 μm and / or the Str of the dual-phase steel plate in the flat portion of the panel was less than 0.50, resulting in poor appearance after forming. In Comparative Examples 4 and 17, the superheat ΔT in the casting process was low, and the segment pressing force did not satisfy the condition of 450 tons or more, which similarly resulted in a high equiaxed crystal fraction in the solidification structure. As a result, the hard phase fraction in the direction perpendicular to the rolling direction increased, and the Sa of the dual-phase steel plate in the flat portion of the panel exceeded 0.50 μm and the Str of the dual-phase steel plate in the flat portion of the panel was less than 0.50, resulting in poor appearance after forming. In Comparative Example 12, the segment pressing force during the casting process was low, which similarly resulted in a high equiaxed crystal fraction in the solidified structure. As a result, the hard phase fraction in the direction perpendicular to the rolling direction increased, and the Sa of the dual-phase steel sheet in the flat portion of the panel exceeded 0.50 μm, resulting in poor appearance after forming. In Comparative Examples 23 and 24, it is believed that some central segregation of Mn remained during the manufacturing process. In addition, in Comparative Examples 23 and 24, the hard phase fraction was somewhat high, which did not satisfy Equation 1. As a result, the hard phase fraction in the direction perpendicular to the rolling direction increased, and the Sa of the dual-phase steel sheet in the flat portion of the panel exceeded 0.50 μm and the Str of the dual-phase steel sheet in the flat portion of the panel was less than 0.50, resulting in poor appearance after forming.

[0109] In contrast to this, in the panels according to all of the invention examples, the metal structure of the steel plate that makes up the panel contains not only soft phases but also hard phases, thereby achieving high strength, for example a tensile strength of 400 MPa or more, while controlling the surface properties of the panel so that Str is within the range of 0.50 to 1.00 and Sa is 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 drawing.

Claims

1. A panel including a dual-phase steel plate having a metal structure composed of a soft phase and a hard phase, wherein the surface texture aspect ratio Str of the dual-phase steel plate in the flat portion of the center portion of the panel is 0.50 to 1.00, and the surface roughness parameter Sa of the dual-phase steel plate in the flat portion of the center portion of the panel is 0.50 μm or less.

2. A panel according to claim 1, characterized in that the dual phase steel plate has a thinned portion in an area other than the flat portion, the thickness of which is thinner than the thickness of the flat portion.

3. A panel according to claim 1 or 2, characterized in that it is an outer panel of an automobile.

4. A panel according to any one of claims 1 to 3, characterized in that the dual-phase steel plate is a painted steel plate having a paint layer on at least one surface.

5. A panel according to any one of claims 1 to 4, characterized in that the thickness of the flat portion is 0.2 to 0.6 mm.

6. A panel according to any one of claims 1 to 5, characterized in that it has a tensile strength of 500 MPa or more.

7. A panel according to any one of claims 1 to 6, characterized in that the metal structure of the dual-phase steel plate in the flat portion is, in area %, soft phase: 75 to 97% and hard phase: 3 to 25%, and the standard deviation of the hard phase fraction in the direction perpendicular to the rolling direction is 0.75% or less.

8. The panel according to claim 7, wherein the dual-phase steel sheet in the flat portion satisfies the following formula 1: (TS-180,000 / TS) / Vm≧35 ... formula 1, where TS is the tensile strength in MPa, and Vm is the hard phase fraction in area %.

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