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

Technical Field

[0001] The present invention relates to a panel.

Background Art

[0002] In recent years, for environmental protection of the earth, improvement of fuel efficiency of automobiles has been demanded. Regarding the improvement of fuel efficiency of automobiles, for steel sheets for automobiles, in order to reduce the weight of the vehicle body while ensuring safety, further higher strength is required. Such a demand for higher strength is not limited to members and pillars which are structural members, but is also increasing for outer panel (hood, fender panel, door panel, roof panel, etc.) of automobiles.

[0003] On the other hand, the shaping of outer panel parts of automobiles tends to become increasingly complicated. When the steel sheet is made higher in strength and thinner for weight reduction, unevenness is likely to occur on the surface of the steel sheet when it is formed into a complicated shape. When unevenness occurs on the surface, the appearance after forming deteriorates. Since the outer panel is important not only for characteristics such as strength but also for design and surface quality, it is required to have an excellent appearance after forming.

[0004] In relation to this, in Patent Document 1, there is an exterior panel including a steel sheet, the steel sheet has a flat portion, in the surface layer region of the flat portion, the metal structure contains ferrite with a volume fraction of 80% or more, the average crystal grain size of ferrite is 1.0 to 15.0 μm, and the strength ratio X ODF{001} / {111},S between the {001} orientation and the {111} orientation of ferrite is 0.30 or more and less than 3.50. When the uniform elongation measured by a tensile test piece cut out from the flat portion is uEl1, and the theoretical uniform elongation derived from a predetermined formula from the volume fraction, hardness and average crystal grain size of ferrite and martensite in the metal structure of the internal region of the flat portion and the plate thickness of the flat portion is uEl2, an exterior panel in which uEl1 / uEl2 is 0.44 to 0.80 is described. Further, in Patent Document 1, it is taught that according to the above configuration, an exterior panel excellent in surface properties after being formed from a material and excellent in dent resistance can be provided.

[0005] Patent Document 2 describes a panel having a steel plate containing martensite, wherein the surface roughness parameter (Sa) of the flat portion on the central side of the panel is Sa ≤ 0.500 μm, and within the martensite lath, precipitates with a major axis of 0.05 μm to 1.00 μm and an aspect ratio of 3 or more are present at 15 particles / μm 2 A panel is described having the above characteristics, wherein the ratio YS1 / YS2 of the yield stress YS1 measured on a tensile test piece cut from the flat portion to the yield stress YS2 measured on a tensile test piece cut from the end of the panel is 0.90 to 1.10. Furthermore, Patent Document 2 teaches that according to the above configuration, an exterior panel can be provided that has excellent appearance after molding from the material and excellent dent resistance.

[0006] Patent Document 3 describes an automotive exterior panel part comprising a steel plate, wherein, in a plan view, the rolling direction of the steel plate extends along the left-right direction of the vehicle body. Furthermore, Patent Document 3 teaches that it is possible to provide an automotive exterior panel part with reduced ghost lines. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] International Publication No. 2022 / 004795 [Patent Document 2] International Publication No. 2021 / 149810 [Patent Document 3] International Publication No. 2023 / 026469 [Overview of the project] [Problems that the invention aims to solve]

[0008] As described above, when steel sheets are made thinner and stronger to reduce weight, irregularities tend to occur on the surface of the steel sheet when it is formed into complex shapes, resulting in a deterioration of the appearance after forming. On the other hand, the automotive industry and other sectors are also demanding further weight reduction of panels, and in order to achieve such weight reduction, it becomes necessary to make the panels stronger than ever before. Therefore, there is still a high demand for panels that can solve the problem of appearance after forming even when the strength is the same as or greater than that of conventional panels.

[0009] Therefore, the present invention aims to provide a panel with excellent appearance after molding, even while maintaining high strength, through a novel configuration. [Means for solving the problem]

[0010] To achieve the above objective, the inventors focused particularly on the metal structure of the steel sheet constituting the panel and the surface properties of the panel. As a result, the inventors discovered that by including not only a soft phase but also a hard phase in the metal structure of the steel sheet constituting the panel, high strength can be achieved. Furthermore, by appropriately selecting the metal structure of the steel sheet so that two different parameters related to the surface properties of the panel, more specifically the surface property aspect ratio Str and the surface roughness parameter Sa, are controlled within a specific range at a predetermined position on the panel after molding, an excellent appearance can be achieved even in a high-strength panel. Thus, the inventors completed the present invention.

[0011] The present invention, which has achieved the above objectives, is as follows. (1) A panel including a composite steel sheet having a metallic structure composed of a soft phase and a hard phase, The surface texture aspect ratio Str of the composite steel sheet in the flat portion of the central part of the panel is 0.50 to 1.00. A panel characterized in that the surface roughness parameter Sa of the composite steel sheet in the flat portion of the central part of the panel is 0.50 μm or less. (2) The panel according to (1) above, characterized in that the composite steel sheet has a thinned portion in a region other than the flat portion that is thinner than the thickness of the flat portion. (3) The panel described in (1) or (2) above, characterized in that it is an exterior panel of an automobile. (4) The panel according to any one of (1) to (3) above, characterized in that the composite steel sheet is a painted steel sheet having a paint layer on at least one surface. (5) The panel according to any one of the above items (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 the above items (1) to (5), characterized by having a tensile strength of 500 MPa or more. (7) The metallic structure of the composite steel sheet in the flat portion is such that, by area %, the soft phase is 75-97% and the hard phase is 3-25%. A panel according to any one of the above items (1) to (6), characterized in that 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 composite structure 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 units, and Vm is the hard phase fraction in area percentage units. [Effects of the Invention]

[0012] According to the present invention, it is possible to provide a panel that has excellent appearance after molding, even if it has high strength. [Brief explanation of the drawing]

[0013] [Figure 1] This is a schematic diagram showing a panel obtained by deep drawing in the embodiment, where (a) is a perspective view of the panel and (b) is a perspective view of the panel in (a) viewed from the back. [Modes for carrying out the invention]

[0014] <Panel> The panel according to an embodiment of the present invention includes a composite structure steel sheet having a metal structure composed of a soft phase and a hard phase, where the surface texture aspect ratio Str of the composite structure steel sheet in the flat part of the central 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 central side portion of the panel is 0.50 μm or less.

[0015] In recent years, in relation to the demand for further improvement in fuel efficiency, not only structural members such as members but also outer panel such as roofs, hoods, doors, back doors, etc. have an increasing need for weight reduction. These outer panels, unlike the above-described structural members, are in contact with the human eye, so not only characteristics such as strength but also design and surface quality are important, and therefore it is required to have excellent appearance after forming. On the other hand, in relation to such weight reduction requirements, further high strength and thin thickness are also required for the steel sheets used for these outer panels. However, when high strength and thin thickness are achieved, the force (springback phenomenon) that tries to return to the original state after forming becomes large, so there is a problem that the surface quality deteriorates compared to the current products. Also, in these outer panels, in view of avoiding surface defects called surface strains that occur during press forming or the like, dual-phase steel with a relatively low yield strength is often used. However, in the case of a composite structure steel in which a soft phase made of ferrite and a hard phase mainly composed of martensite or the like are mixed, non-uniform deformation is likely to occur in which the soft phase and its periphery are preferentially deformed during processing such as press forming, and minute unevenness occurs on the surface of the formed panel, so that an appearance defect called a ghost line may occur. More specifically, during processing such as press forming, the soft phase made of ferrite is greatly recessed in terms of the amount of deformation, while the hard phase mainly composed of martensite or the like has a small amount of deformation. Therefore, the hard phase does not dent compared to the soft phase and bulges so as to become convex. As a result, particularly in the rolling right angle direction of the panel, a variation in the amount of deformation occurs and ghost lines occur in a band (strip) shape.

[0016] Therefore, in order to achieve both high strength and good appearance after forming into a panel, the inventors of the present invention focused on the metallographic structure of the steel sheet constituting the panel and the surface properties of the panel, and conducted investigations. First, the inventors of the present invention found that by including not only a soft phase but also a hard phase in the metallographic structure of the steel sheet constituting the panel, it is possible to achieve a desired increase in strength, for example, a tensile strength of 400 MPa or more. On the other hand, since the appearance of the panel generally deteriorates in relation to the increase in strength caused by such a composite structure, the inventors of the present invention further investigated the surface properties of the panel after forming in order to suppress such a deterioration in appearance after forming. As a result, the inventors of the present invention found that by appropriately selecting the metallographic structure of the composite structure steel sheet so that two different parameters related to the surface properties of the panel, more specifically, the surface property aspect ratio Str and the surface roughness parameter Sa, are controlled within a specific range at a predetermined position of the panel after forming, it is possible to achieve an excellent appearance even in a panel with increased strength.

[0017] More specifically, the surface property aspect ratio Str is one of the spatial parameters of the surface properties defined in JIS B0681-2:2018, indicates the strength of surface anisotropy, and is known to take values in the range of 0 to 1.00. Generally, when the value of Str approaches 0, the anisotropy becomes stronger and streaks or the like occur on the surface. On the other hand, when the value of Str approaches 1.00, the surface becomes isotropic and does not depend on the direction. As a result of investigations, the inventors of the present invention found that when strain is applied by forming such as press forming, particularly deep drawing forming, by appropriately selecting the metallographic structure of the composite structure steel sheet as the material, it is very effective in suppressing the occurrence of ghost lines on the panel surface to control the surface property aspect ratio Str of the composite structure steel sheet in the flat part on the center side of the panel within the range of 0.50 to 1.00. Since the ghost lines are related to the streak pattern on the panel surface, from the viewpoint of suppressing the occurrence of the ghost lines, it is preferable that the minute irregularities on the panel surface are more isotropic. Therefore, it is more preferable that the value of Str is closer to 1.00.

[0018] However, since simply controlling Str appropriately was sometimes insufficient to sufficiently improve the appearance after molding, the inventors conducted further studies related to the surface properties of the panel. As a result, the inventors found that by appropriately selecting the metal structure of the composite steel sheet material, in addition to controlling Str, the surface roughness parameter Sa of the composite steel sheet in the flat portion of the central part of the panel can be controlled to a range of 0.50 μm or less, thereby significantly suppressing or reducing the occurrence of appearance defects caused by minute irregularities on the panel surface, even when strain is applied by press forming, especially deep drawing. Here, the surface roughness parameter Sa refers to the absolute mean of z(x,y) in the reference region (A) as defined in JIS B0681-2:2018 4.1.7 "arithmetical mean height of the scale limited surface".

[0019] Therefore, according to the panel according to the embodiment of the present invention, while sufficiently maintaining high strength based on the hard phase contained together with the soft phase in the metal structure of the composite steel sheet, the surface properties of the formed panel can be controlled using two different parameters, the surface properties 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. This makes it possible to significantly suppress the occurrence of appearance defects such as ghost lines on the panel surface even when strain is applied by forming, particularly by forming such as press forming or deep drawing. In particular, the fact that even when the metal structure of the composite 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 to within the range of 0.50 μm or less and making the surface properties aspect ratio Str 0.50 or more more isotropic is something that the inventors have revealed for the first time. Therefore, the panel according to the embodiment of the present invention is particularly useful for application in automobile exterior panels where there is a relatively high demand for high strength. The following describes in more detail each component of the panel according to the embodiment of the present invention.

[0020] [Str: 0.50~1.00 of composite steel sheet in the flat area of ​​the central part of the panel] In embodiments of the present invention, the surface properties aspect ratio Str of the composite steel sheet in the flat portion of the central part of the panel after forming is controlled to 0.50 to 1.00. First, the panel according to the embodiment of the present invention includes three parts, specifically (i) an edge portion, (ii) an end portion, and (iii) a central portion other than the edge portion and the end portion. The edge portion (i) is a portion that is bent by hemming (HEM) or fixed to other parts by welding such as spot welding. The end portion (ii) is a portion located towards the center of the panel from the edge portion, and is a portion that is separate from the portion that is fixed to other parts by hemming or welding. This end portion is, for example, a few mm toward the center of the panel from the edge portion, and is a portion that is substantially unaffected by processing to fix the panel to other parts. In this case, "substantially unaffected" means that the amount of change in properties due to processing to fix the panel to other parts is within a few percent. The central portion (iii) is a part that is visible from the outside as an exterior, for example as the exterior of an automobile. In this specification, the portion of the panel's central side with a radius of curvature of 500 mm or more is referred to as the flat portion. Furthermore, if a plating layer and / or paint layer exists on the panel's surface, the flat portion refers to the flat portion of the entire panel including the plating layer and / or paint layer. By controlling the surface texture aspect ratio Str in the flat portion to within the range of 0.50 to 1.00, a more isotropic surface texture can be achieved, and consequently, a panel with superior appearance can be provided. As mentioned earlier, since ghost lines are related to the streaky pattern on the panel surface, from the viewpoint of suppressing the occurrence of such ghost lines, it is preferable that the minute irregularities on the panel surface are 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 higher, 0.60 or higher, 0.65 or higher, 0.70 or higher, 0.75 or higher, or 0.80 or higher. There is no particular upper limit, but for example, Str may be 0.95 or lower, 0.90 or lower, or 0.85 or lower.

[0021] [Measurement of Str] The surface texture aspect ratio Str of a composite steel sheet in the flat central portion of a panel is determined as follows. First, a test piece is cut from the flat central portion of the panel, and then a three-dimensional measurement is performed on an 8 mm × 8 mm area of ​​the surface of the cut-out sample (or the surface of the plating layer and / or paint layer if one exists on the sample surface) using a Keyence VK-X3000 white light interferometer. The measurement conditions are a 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 by linking the data. After that, the measurement area is corrected for tilt by 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 composite steel sheet in the flat area of ​​the central part of the panel: 0.50 μm or less] In the embodiment of the present invention, the surface roughness parameter Sa of the composite steel sheet in the flat portion of the central part of the panel after molding is controlled to 0.50 μm or less. Similar to the case of Str, if a plating layer and / or 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 paint layer. The surface roughness parameter Sa is the average value of the height difference (absolute value) of each point relative to the average plane of the panel surface after strain is applied during molding. In addition to controlling the surface property aspect ratio Str described above, controlling the surface roughness parameter Sa to 0.50 μm or less in the flat portion of the central part of the panel makes it possible to significantly improve the appearance of the high-strength panel after molding due to the effect based on the combination of isotropic surface properties and lower surface roughness. From the viewpoint of further improving the appearance after molding, a lower Sa is preferable, and in the flat portion of the central part of the panel, 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. The lower limit is not particularly limited, but 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 central part of the panel.

[0023] [Measurement of Sa] The surface roughness parameter Sa of the composite steel sheet in the flat central portion of the panel is determined as follows. First, a test piece is cut from the flat central portion of the panel, and a three-dimensional measurement is performed on an 8 mm × 8 mm area of ​​the surface of the cut sample (or the surface of the plating layer and / or paint layer if one exists on the surface of the sample) using a Keyence VK-X3000 white light interferometer. The measurement conditions at this time are a measurement magnification of 10x, a resolution of 3 μm in the XY plane, a resolution of 1 nm in the Z space plane, and the measurements are performed by linking the data. After that, the measurement area is corrected for tilt by 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 calculated. The arithmetic mean height obtained in this way is determined as the surface roughness parameter Sa of the composite steel sheet in the flat central portion of the panel.

[0024] [Composite structure steel plate] A panel according to an embodiment of the present invention includes a composite steel sheet having a metallic structure composed of a soft phase and a hard phase. A panel according to an embodiment of the present invention includes at least a composite steel sheet having a metallic structure composed of a soft phase and a hard phase, and it is sufficient that the flat portion of the central part of the panel made of the composite steel sheet has the above-described characteristics. Therefore, a panel according to an embodiment of the present invention may include as part a material other than a composite steel sheet having a metallic structure composed of a soft phase and a hard phase. Preferably, a panel according to an embodiment of the present invention is essentially made of a composite steel sheet having a metallic structure composed of a soft phase and a hard phase, is made of such a composite steel sheet, or is composed of such a composite steel sheet.

[0025] In the present invention, "soft phase" means ferrite. On the other hand, in the present invention, "hard phase" means a structure that is harder than the soft phase which is ferrite, and for example, includes or consists of at least one of martensite, bainite, tempered martensite, and pearlite, and is particularly at least one of martensite, bainite, tempered martensite, and pearlite. From the viewpoint of improving the strength of the composite steel sheet, the hard phase preferably consists of at least one of martensite, bainite, and tempered martensite, or at least one of them, and more preferably consists of or is martensite. In embodiments of the present invention, it is preferable that the metallic structure of the composite steel sheet contains little retained austenite, specifically, the retained austenite is preferably less than 1% or less than 0.5% by area, and more preferably 0%. By including the above-mentioned hard phase in the metallic structure of the composite steel sheet that constitutes the panel, it is possible to achieve the desired high strength. The area fraction of the hard phase in the metal structure is not particularly limited and should be appropriately selected according to the required panel strength, but may be, for example, 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, for example, 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, for example, 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 soft phase and the calculation of their area fractions shall be carried out based on the description in [Identification of Metal Structure and Calculation of Area Fractions] below.

[0026] [Thickening area] In one preferred embodiment of the present invention, the composite steel sheet has a thinned portion in a region other than the flat portion that is thinner than the thickness of the flat portion. A panel according to an embodiment of the present invention can be manufactured, for example, by a manufacturing method including deep drawing. When a panel is manufactured by deep drawing, for example, a flat portion and a ridge portion continuous with the flat portion are formed, and in the ridge portion, a thinned portion thinner than the thickness of the flat portion is formed due to deep drawing. According to the panel according to an embodiment of the present invention, even in deep drawing in which such a thinned portion is formed, as described above, by controlling the surface properties of the panel after forming using two different parameters, the surface properties 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, it is possible to significantly suppress the occurrence of appearance defects such as ghost lines on the panel surface. The degree of thinning is not particularly limited and can be appropriately determined according to the press forming conditions such as deep drawing and the type and size of the part to be manufactured. For example, the thinning rate of the thinned portion may be 2 to 20%. The rate of wall thinning in the thinned section may be, for example, 3% or more, 4% or more, 5% or more, or 7% or more. Similarly, the rate of wall thinning in the thinned section may be, for example, 18% or less, 15% or less, 12% or less, 10% or less, or 8% or less. Here, the rate of wall thinning refers to the rate of decrease in plate thickness before and after forming. If it is difficult to measure the plate thickness before forming, the rate of wall thinning can be calculated from the plate thickness of the flat section and the plate thickness of the thinned section using the following formula. Thinning rate (%) = (Thickness of flat section - Thickness of thinned section) / Thickness of flat section × 100

[0027] [Paint layer] In the panel according to the embodiment of the present invention, the composite steel sheet may or may not have a paint layer, or it may be a painted steel sheet having a paint layer on at least one surface. For example, if the surface properties of the panel are not good, it is necessary to apply a thick paint layer to obtain a beautiful appearance. However, since the panel according to the embodiment of the present invention has excellent surface properties, the paint layer can be made thin, which is therefore very advantageous from the viewpoint of cost. In addition, the yield strength of the panel can be increased in relation to the bake hardening during paint baking, which is therefore also advantageous from the viewpoint of improving the dent resistance of the panel. When a composite steel sheet having a metallic structure in which the hard phase is uniformly dispersed, as described later in relation to a preferred embodiment of the composite 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 strength can be similarly significantly increased, making it possible to further improve the dent resistance of the panel.

[0028] If the composite 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 suitable 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. In automotive panels, the paint layer generally includes, in order from the steel sheet side, an electrodeposited paint layer, an intermediate paint layer, a base coat layer, and a clear coat layer. The thickness of the electrodeposited 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 Embodiment of Composite Structure Steel Sheet] The following describes in detail preferred embodiments of composite steel sheets having a metallic structure composed of a soft phase and a hard phase, which are useful for achieving a panel surface condition in which, when formed by press forming, particularly by deep drawing, the surface property aspect ratio Str of the composite steel sheet in the flat portion on the central side is 0.50 to 1.00, and the surface roughness parameter Sa of the composite steel sheet in the flat portion on the central side is 0.50 μm or less. However, these descriptions are intended merely as examples of preferred composite steel sheets for constructing panels according to embodiments of the present invention, and are not intended to limit the present invention to embodiments using such specific composite steel sheets.

[0030] In a preferred embodiment of the present invention, the composite steel sheet has a metallic structure in the flat portion of the central part of the panel, where the metallic structure is composed of soft phase: 75-97% and hard phase: 3-25% by area percentage. A key characteristic is that the standard deviation of the hard phase fraction in the direction perpendicular to the rolling process is 0.75% or less.

[0031] As mentioned earlier, in the case of composite steel with a soft phase consisting of ferrite and a hard phase mainly consisting of martensite, uneven deformation is likely to occur during processing such as press forming, where the soft phase and its surroundings deform preferentially. This can result in minute irregularities on the panel surface after forming, causing a cosmetic defect called a ghost line. On the other hand, with the increasing strength of steel sheets, relatively large amounts of elements such as Mn are sometimes added to improve the hardenability of the steel sheet. Mn is an element that tends to segregate in streaks in steel sheets. More specifically, during casting, Mn-enriched regions such as central segregation and microsegregation are formed, and as these enriched regions are stretched in the rolling direction by hot rolling or cold rolling, Mn segregates in streaks. As a result of this Mn segregation, there are regions with high hardenability and regions with low hardenability in the steel sheet. Consequently, a relatively large amount of striped hard phase is generated in the metal structure of the steel sheet after quenching. In this case, the occurrence of ghost lines becomes particularly noticeable.

[0032] Therefore, the inventors investigated means to improve the appearance after forming while achieving the desired high strength by optimizing the ratio of soft phase to hard phase in the metal structure, more specifically by controlling the soft phase to 75-97% and the hard phase to 3-25% in area percent. Specifically, the inventors focused on the distribution state of the hard phase in the metal structure. As a result, the inventors found that it is important to reduce equiaxed crystals in the solidification structure during the casting process and control the solidification structure to a columnar crystal structure, as will be explained in detail later in relation to the manufacturing method of composite steel sheets. More specifically, the inventors found that when coarse equiaxed crystals are generated in the solidification structure during the casting process, even if the central segregation of Mn itself is small, negative segregation of Mn occurs, which increases the variation in the hard phase fraction and can worsen the appearance defects after forming. In this regard, the inventors have found that by controlling the solidification structure to a columnar crystal structure, negative segregation of Mn can be suppressed, and central segregation of Mn during solidification, which is a factor in the formation of striped hard phases, can also be reduced, thereby reducing the variation in the hard phase fraction in the metal structure. More specifically, the inventors have found that by controlling the solidification structure to a columnar crystal structure, the standard deviation of the hard phase fraction in the direction perpendicular to rolling can be reduced to 0.75% or less, thereby enabling the production of composite steel sheets having a metal structure in which the hard phase is uniformly dispersed. Here, "direction perpendicular to rolling" refers to the direction perpendicular to the rolling direction and the thickness direction. Conventional countermeasures against central segregation involve increasing the equiaxed crystal fraction, which was common technical knowledge among those skilled in the art (see, for example, Takahiro Kawawa et al.: "Iron and Steel", Vol. 60 (1974) No. 5, pp. 486-500, and Hiroshi Kumai et al.: "Iron and Steel", Vol. 60 (1974) No. 7, pp. 894-914). Therefore, the fact that negative segregation and central segregation of Mn can be reduced by reducing equiaxed crystals in the solidification structure and controlling the solidification structure to a columnar crystal structure is extremely unexpected and astonishing.

[0033] In addition, the inventors have found that even when some degree of central segregation of Mn remains, the standard deviation of the hard phase fraction can be reduced to 0.75% or less by appropriately reducing the hard phase fraction within the range of 3 to 25%, and as a result, a composite steel sheet having a metallic structure in which the hard phase is uniformly dispersed can be manufactured. Therefore, according to a preferred embodiment of the present invention, by using a composite steel sheet having such a metallic structure in which the hard phase is uniformly dispersed, the amount of deformation of the composite steel sheet during forming, such as press forming, can be made more uniform, especially in the width direction. In connection with this, by realizing the surface properties of a panel having the desired Str and Sa, it becomes possible to achieve an excellent post-forming appearance in which appearance defects such as ghost lines are significantly suppressed.

[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 is ensured by controlling the area fraction of the soft phase to 75-97%, and the area fraction of the hard phase to 3-25%. Furthermore, by controlling the chemical composition of the composite steel sheet within a predetermined range, it becomes possible to reliably achieve high strength, such as a tensile strength of 400 MPa or more, preferably 500 MPa or more. As a result, it becomes possible to achieve a high level of both high strength and good appearance after forming into a panel. In addition, as described above, the composite steel sheet according to a preferred embodiment of the present invention has a metallic structure in which the hard phase is uniformly dispersed. Therefore, in the paint-baking treatment after forming into a panel, the amount of bake-hardened material can be significantly increased due to the uniform dispersion of the hard phase containing a relatively large number of dislocations. Consequently, by using this composite steel sheet, the yield strength of the resulting panel can be significantly increased, which is therefore very advantageous from the viewpoint of improving the dent resistance of the panel.

[0035] Below, we will first describe in more detail the microstructure of the composite steel sheet according to a preferred embodiment of the present invention. The unit "%" for microstructure fraction means area %. Furthermore, the microstructure of the composite steel sheet refers to the microstructure of the composite steel sheet in the flat portion of the central part of the panel. In this flat portion, the degree of forming is low, and therefore the characteristics of the microstructure shown below do not change particularly before and after forming such as press forming.

[0036] [Soft phase: 75-97% and hard phase: 3-25%] In a preferred embodiment of the present invention, the metallic structure of the composite steel sheet is, by area percentage, 75-97% for the soft phase and 3-25% for the hard phase. By having such a composite metallic structure, it is possible to achieve high strength, more specifically tensile strength of 400 MPa or more, preferably 500 MPa or more, while suppressing defects in appearance after forming. From the viewpoint of further increasing the strength of the composite 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 previously stated, in the present invention, "soft phase" means ferrite. On the other hand, in the present invention, "hard phase" means a structure that is harder than the soft phase which is ferrite, and for example, includes or consists of at least one of martensite, bainite, tempered martensite, and pearlite, and is particularly at least one of martensite, bainite, tempered martensite, and pearlite. From the viewpoint of improving the strength of the composite structure steel sheet, it is preferable that the hard phase consists of at least one of martensite, bainite, and tempered martensite, or at least one of them, and it is more preferable that it consists of martensite or is martensite. In embodiments of the present invention, it is preferable that the metallic structure of the composite structure steel sheet has little retained austenite, and specifically, it is preferable that the retained austenite is less than 1% or less than 0.5% in area percent, and more preferably 0%.

[0038] [Identification of metallographic structure and calculation of area fraction] The identification of the metal structure and calculation of its area fraction are performed as follows: First, a sample (size, for example, 20 mm × 20 mm × thickness) for observing the metal structure (microstructure) is taken from a composite steel sheet. Next, the metal structure is observed from the surface to half the thickness of the sheet using a scanning electron microscope (SEM), and the area fraction of the hard phase is calculated from 50 μm from the surface of the steel sheet (excluding the plating layer if plating is present) to half the thickness of the sheet. For sample preparation, the thickness cross section perpendicular to the sheet surface is polished as the observation surface and etched using Nital corrosion. Next, the "microstructure" is classified from SEM images at 500x or 1000x magnification. Hard phases can be distinguished by the difference in brightness.

[0039] For steel plates etched with Nital corrosion, the area from the surface to the halfway point in the thickness direction is observed at 500x or 1000x magnification for 10 fields of view (or 3 or more fields of view if 10 fields of view are difficult to observe due to the thinness of the plate). The area fraction of the hard phase is determined using the image analysis software Image J (Ver. 1.54f). The soft and hard phases are binarized based on the difference in brightness, and the area fraction of the hard phase is calculated. For a total of 10 observation fields, the area fraction of the hard phase is measured by image analysis in the same manner as above, and these area fractions are averaged to calculate the average value. This average value is taken as the area fraction of the hard phase, and the remainder is taken as the area fraction of ferrite. The observation area of ​​each field of view is 37500 μm². 2 For example, if the plate thickness is thin, the length in the thickness direction is reduced while maintaining an observation area of ​​37,500 μm². 2 We will ensure that this is secured. Furthermore, if it is necessary to measure the area fraction of retained austenite, it can be measured by X-ray diffraction on the observation surface described above. Specifically, using Co-Kα radiation, the integrated intensities of a total of five peaks, α(200), α(211), γ(311), γ(200), and γ(220), at a position 1 / 4 of the plate thickness direction are determined, and the volume fraction of retained austenite is calculated using the intensity averaging method. The obtained volume fraction of retained austenite is then taken as the area fraction of retained austenite.

[0040] [Standard deviation of hard phase fraction in the direction perpendicular to rolling: 0.75% or less] In a preferred embodiment of the present invention, the standard deviation of the hard phase fraction in the direction perpendicular to rolling is controlled to 0.75% or less in the metallic structure of the composite steel sheet in the flat portion. 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, the solidification structure, as well as Mn segregation, greatly influences the appearance defects after forming, such as ghost lines. For example, even if the central segregation of Mn is small, if coarse equiaxed crystals are formed in the solidification structure, negative segregation of Mn occurs, which can increase the variation in the hard phase fraction in the direction perpendicular to rolling and worsen the appearance defects after forming. However, in a preferred embodiment of the present invention, the standard deviation of the hard phase fraction in the direction perpendicular to rolling is 0.75% or less, that is, the variation in the hard phase fraction in the direction perpendicular to rolling is sufficiently reduced, so that the appearance defects after forming can be significantly suppressed. Here, "direction perpendicular to rolling" refers to the direction perpendicular to the rolling direction and the sheet thickness direction, as described above.

[0041] From the viewpoint of further improving the appearance after molding, a lower standard deviation of the hard phase fraction in the direction perpendicular to the rolling direction is preferable, for example, it may be 0.65% or less, 0.55% or less, or 0.45% or less. The lower limit is not particularly limited, but 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 the standard deviation of the hard phase fraction in the direction perpendicular to the rolling process] The standard deviation of the hard phase fraction in the microstructure perpendicular to the rolling direction is determined as follows: First, a scanning electron microscope (SEM) is used to observe a cross-section of the composite steel sheet in a flat section, parallel to the rolling direction and perpendicular to the sheet surface, between a position 50 μm from one sheet surface and a position 50 μm from the other sheet surface, at a magnification of 500x or 1000x, to obtain an SEM image. Similar to the case of the hard phase area fraction described above, image analysis software is used to analyze this SEM image, measuring the hard phase area fraction at 100 μm intervals within an 8 mm range perpendicular to the rolling direction of the composite steel sheet, and calculating its standard deviation. The observation range perpendicular to the rolling direction may be less than 8 mm or greater than 8 mm. However, the lower limit of the observation range for the standard deviation of the hard phase fraction perpendicular to the rolling direction is 4 mm, and the upper limit is 12 mm.

[0043] If the direction perpendicular to the rolling direction of a composite steel sheet is not clear, the following method can be used to determine the direction perpendicular to the rolling direction of the composite steel sheet. After finishing the thickness cross section of the composite steel sheet with mirror polishing, the S concentration is measured using an electron probe microanalyzer (EPMA). The measurement conditions are an acceleration voltage of 15kV and a measurement pitch of 1μm, and a distribution image is measured in a range of 100μm (thickness direction) × 500μm (direction perpendicular to the thickness direction) at the center of the thickness. At this time, stretched areas with high S concentration are identified as inclusions such as MnS. Multiple fields of view may be observed during the observation. Next, using the thickness cross section initially observed using the above method as a reference, the planes parallel to the planes rotated in 5° increments within the range of 0° to 180° around the thickness direction axis are observed in cross-section using the above method. The average length of the major axes of multiple inclusions in each obtained cross-section is calculated for each cross-section, and the cross-section with the maximum average length of the major axes of the inclusions is identified. The direction parallel to the long axis of the inclusion 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 even more preferable that the composite structure of the 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 units, and Vm is the hard phase fraction in area percentage units. Vm is determined based on the description in [Identification of Metallographic Structure and Calculation of Area Fraction] above.

[0045] When a composite steel sheet satisfies Equation 1 above, Vm is controlled to be relatively low in relation to TS. Therefore, even if some degree of central segregation of Mn remains, it becomes easier to suppress variations in the hard phase fraction in the direction perpendicular to rolling of the metal structure. As a result, it becomes relatively easy to achieve an excellent post-forming appearance with significantly suppressed appearance defects such as ghost lines. This effect becomes more pronounced as the value on the left side of Equation 1 increases. Therefore, the value on the left side of Equation 1 may be 38 or more, 40 or more, 42 or more, or 45. There is no particular upper limit, but for example, the value on the left side of Equation 1 may be 70 or less, 68 or less, or 65 or less.

[0046] [Preferred chemical composition of composite steel sheets] As described above, the present invention aims to provide a panel that is high in strength but has an excellent appearance after forming. This is achieved by including a soft phase and a hard phase in the metal structure of the steel sheet constituting the panel, and by controlling the surface properties of the panel after forming 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 composite steel sheet itself is not an essential technical feature for achieving the objective of the present invention. The following describes in detail preferred chemical compositions of composite steel sheets according to embodiments of the present invention, but these descriptions are intended to be merely examples of preferred chemical compositions of composite steel sheets having, for example, a tensile strength of 400 to 900 MPa, and are not intended to limit the present invention to composite steel sheets having such specific chemical compositions. In addition, in the following descriptions, "%", which is the unit of content of each element, means "mass%" unless otherwise specified. Furthermore, in this specification, unless otherwise specified, the "~" indicating a numerical range means that the numbers before and after it are included as the lower and upper limits.

[0047] In embodiments of the present invention, for example, the composite structure steel sheet is, by mass%, C: 0.030~0.100%, Mn: 1.00~2.50%, Si: 0.005~1.500%, P: 0.100% or less, S: 0.0200% or less, Al: 0.005~0.700%, N: 0.0150% or less, O: 0.0100% or less, 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%, REM: 0~0.0100%, and Remainder: Fe and impurities It is preferable to have a chemical composition consisting of the following. Each element will be described in more detail below.

[0048] [C:0.030~0.100%] Carbon (C) is an element that increases the strength of steel plates. To obtain this effect sufficiently, the C content should be 0.030% or more. The C content may also be 0.035% or more, 0.040% or more, or 0.050% or more. On the other hand, if the C content is excessive, the strength may become too high and the ductility may decrease. Therefore, the C content should be 0.100% or less. The C content may also be 0.095% or less, 0.090% or less, or 0.080% or less.

[0049] [Mn: 1.00~2.50%] Mn is an element that enhances the hardenability of steel and contributes to improving its strength. To fully obtain this effect, the Mn content should be 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 the Mn content is excessive, the ferrite transformation may be excessively suppressed, making it impossible to secure the desired amount of ferrite and potentially reducing elongation. Therefore, the Mn content should be 2.50% or less. The Mn content may be 2.25% or less, 2.00% or less, or 1.85% or less.

[0050] [Si: 0.005~1.500%] Si is a deoxidizing element for steel and also improves the strength of steel sheets through solid solution strengthening. To fully obtain these effects, the Si content should be 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, if the Si content is excessive, the scale detachability may decrease and surface defects may occur. Therefore, the Si content should be 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 introduced during the manufacturing process. Furthermore, 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 more time for refining, leading to decreased 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 sheet. Therefore, the P content should be 0.100% or less. The P content may also 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 introduced during the manufacturing process. The S content may be 0%. However, reducing the S content to less than 0.0001% requires more time for refining, leading to a decrease in 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 can form Mn sulfides, which can reduce the formability of the steel sheet, such as ductility, hole-expandability, stretch flangeability, and / or bendability. Therefore, the S content should be 0.0200% or less. The S content may also be 0.0100% or less, 0.0060% or less, or 0.0040% or less.

[0053] [Al:0.005~0.700%] Al (Al) is an element that functions as a deoxidizing agent and also improves the strength of steel sheets through solid solution strengthening. To obtain these effects to the fullest, the Al content should be 0.005% or more. The Al content may also be 0.010% or more, 0.020% or more, or 0.025% or more. On the other hand, if the Al content is excessive, the castability may deteriorate and productivity may decrease. Therefore, the Al content should be 0.700% or less. The Al content may also 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 introduced during the manufacturing process. The N content may be 0%. However, reducing the N content to less than 0.0001% requires more time for refining, leading to a decrease in productivity. Therefore, the N content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, if the N content is excessive, nitrides may form, which may reduce the formability of the steel sheet, such as ductility, hole-expandability, stretch flangeability, and / or bendability. Therefore, the N content should be 0.0150% or less. The N content may also 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 introduced during the manufacturing process. The O content may be 0%. However, reducing the O content to less than 0.0001% requires more time for refining, leading to a decrease in productivity. Therefore, the O content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, if the O content is excessive, coarse oxides may form, which may reduce the formability of the steel sheet, such as ductility, hole-expandability, stretch flangeability, and / or bendability. Therefore, the O content should be 0.0100% or less. The O content may also be 0.0070% or less, 0.0040% or less, or 0.0020% or less.

[0056] The basic chemical composition of the composite steel sheet according to the embodiment of the present invention is as described above. Furthermore, the composite steel sheet may contain at least one of the following optional elements in place of a portion of the remaining Fe, if necessary, for the purpose of improving its properties. For example, the composite steel sheet may contain at least one of the following: 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 will be described in detail below.

[0057] [Cr: 0~0.80%] Cr is an element that enhances the hardenability of steel and contributes to improving the strength of steel sheets. While the Cr content may be 0%, to obtain such effects, 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, if the Cr content is excessive, coarse Cr carbides that can become the starting point for fracture may form. Therefore, the Cr content is preferably 0.80% or less. The Cr content may also be 0.70% or less, 0.60% or less, or 0.50% or less.

[0058] [Mo: 0~0.50%] Mo is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of steel sheets. 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, if the Mo content is excessive, the hot workability may decrease, and productivity may decrease. 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: 0~0.0100%] B is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of steel sheets. The B content may be 0%, but to obtain this effect, it is preferable that the B content be 0.0001% or more. The B content may also be 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, if the B content is excessive, B precipitates may form, which may reduce the strength of the steel sheet. Therefore, it is preferable that the B content be 0.0100% or less. The B content may also be 0.0080% or less, 0.0060% or less, or 0.0030% or less.

[0060] [Ti: 0~0.100%] Ti is an element that reduces the amount of S, N, and O, which generate coarse inclusions that act as fracture initiation points. Furthermore, Ti precipitates finely in steel as carbides, etc., and improves the strength of steel through precipitation strengthening. While the Ti content may be 0%, it is preferable that the Ti content be 0.001% or higher to obtain these effects. The Ti content may also be 0.005% or higher, 0.007% or higher, or 0.010% or higher. On the other hand, excessive Ti content can lead to the formation of coarse Ti sulfides, Ti nitrides, and / or Ti oxides, reducing the formability of the steel sheet. Therefore, the Ti content is preferably 0.100% or lower. The Ti content may also be 0.080% or lower, 0.060% or lower, or 0.030% or lower.

[0061] [Nb: 0~0.100%] Nb is an element that contributes to improving strength through precipitation strengthening. The Nb content may be 0%, but to obtain this effect, it is preferable that the Nb content be 0.001% or more. The Nb content may also be 0.005% or more, 0.007% or more, or 0.010% or more. On the other hand, if the Nb content is excessive, the amount of unrecrystallized ferrite may increase, which may reduce the formability of the steel sheet. Therefore, it is preferable that the Nb content be 0.100% or less. The Nb content may also be 0.060% or less, 0.040% or less, or 0.030% or less.

[0062] [V: 0 ~ 0.50%] V is an element that contributes to improving the strength of steel sheets through strengthening by precipitates, strengthening by refining due to the suppression of grain growth in the soft phase, and / or dislocation strengthening due to the suppression of recrystallization. The V content may be 0%, but in order to obtain these effects, the V content is preferably 0.001% or more, and more preferably 0.005% or more. The V content may be 0.01% or more or 0.02% or more. On the other hand, if the V content is excessive, a large amount of carbonitrides may precipitate, which may reduce 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: 0~1.00%] Ni is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of steel sheets. The Ni content may be 0%, but to obtain this 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, if the Ni content is excessive, the weldability of the steel sheet may decrease. 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: 0~1.00%] Cu is an element that exists in steel in the form of fine particles and contributes to improving the strength of steel sheets. 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, if the Cu content is excessive, the weldability of the steel sheet may decrease. 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: 0~1.00%] W is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of steel sheets. 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, if the W content is excessive, the hot workability may decrease and productivity may decrease. 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: 0~1.00%] Sn is an element that suppresses grain coarsening and contributes to improving the strength of steel sheets. The Sn content may be 0%, but to obtain this 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: 0~0.200%] Sb is an element that suppresses grain coarsening and contributes to improving the strength of steel sheets. The Sb content may be 0%, but to obtain this effect, it is preferable that the Sb content be 0.001% or more. The Sb content may also be 0.003% or more, 0.005% or more, or 0.010% or more. On the other hand, if the Sb content is excessive, it may cause embrittlement of the steel sheet. Therefore, it is preferable that the Sb content be 0.200% or less. The Sb content may also be 0.150% or less, 0.100% or less, 0.050% or less, or 0.020% or less.

[0068] [Ca: 0~0.0100%] [Mg: 0~0.0100%] [Zr:0~0.0100%] [REM:0~0.0100%] Ca, Mg, Zr, and REM are elements that contribute to improving the formability of steel sheets. The content of Ca, Mg, Zr, and REM may be 0%, but to obtain such an effect, it is preferable that the content of Ca, Mg, Zr, and REM be 0.0001% or more, 0.0010% or more, or 0.0015% or more, respectively. On the other hand, if these elements are included in excess, the ductility of the steel sheet may decrease. Therefore, it is preferable that the content of Ca, Mg, Zr, and REM be 0.0100% or less, respectively. The content of Ca, Mg, Zr, and REM may be 0.0080% or less, 0.0060% or less, or 0.0030% or less, respectively. In this specification, REM refers to the collective term for 17 elements, including scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanides from 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 composite steel sheet according to an embodiment of the present invention, the remainder of the elements other than those mentioned above consists of Fe and impurities. Here, impurities are components that are mixed in during the industrial production of composite steel sheets due to various factors in the manufacturing process, including raw materials such as ore and scrap. 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 amount of impurities may be 0.100% or less.

[0070] The chemical composition of the composite steel sheet according to the embodiment of the present invention can be measured by general analytical methods. For example, the chemical composition of the composite steel sheet can be measured using inductively coupled plasma-atomic emission spectroscopy (ICP-AES) based on a test piece taken from the flat portion of the central part of the panel. C and S can be measured using the combustion-infrared absorption method, N can be measured using the inert gas fusion-thermal conductivity method, and O can be measured using the inert gas fusion-nondispersive infrared absorption method.

[0071] [Index A: 0.45% or more] In a preferred embodiment of the present invention, the chemical composition of the composite steel sheet is such that the index A represented by the following formula 2 is 0.45% or more. A=[Si]+10[P]+0.6[Al]+8[Ti]+9[Nb]...Equation 2 Here, [Si], [P], [Al], [Ti], and [Nb] represent the content of each element in mass percent, and 0% indicates that an element is not present.

[0072] Index A is determined by the content of solid solution strengthening elements Si, P, and Al, and precipitation strengthening elements Ti and Nb. In order to increase the strength of composite steel sheets, it is generally preferable to increase the hard phase fraction. However, increasing the hard phase fraction tends to increase the variation in the hard phase fraction in the direction perpendicular to rolling. 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 direction perpendicular to rolling of the final metal structure to 0.75% or less, it is necessary to further sufficiently reduce the central segregation of Mn in the casting process. This requires strict control of manufacturing conditions, which increases the burden on manufacturing. In this regard, the inventors have found that by utilizing solid solution strengthening with Si, P, and Al, and precipitation strengthening with Ti and Nb, and more specifically by controlling index A, represented by the above formula 2, to 0.45% or more, it is possible to reduce the hard phase fraction while maintaining high strength. As a result, the inventors have found that even when some degree of central segregation of Mn remains, it is relatively easy to reduce the standard deviation of the hard phase fraction in the direction perpendicular to rolling to 0.75% or less by appropriately reducing the hard phase fraction within the range of 3 to 25%, thereby enabling the production of composite steel sheets having a metallic structure in which the hard phase is uniformly dispersed.

[0073] From the viewpoint of further increasing the strength of the composite steel sheet, a larger index A is preferable, and 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. There is no particular upper limit, but 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 composite 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 galvanized layer or an electroplated layer. That is, the composite steel sheet according to the embodiment of the present invention may be a cold-rolled steel sheet having a hot-dip galvanized layer or an electroplated layer on its surface. The hot-dip galvanized layer includes, for example, a hot-dip galvanized layer (GI), an alloyed hot-dip galvanized layer (GA), a hot-dip aluminum galvanized layer, a hot-dip Zn-Al alloy galvanized layer, a hot-dip Zn-Al-Mg alloy galvanized layer, a hot-dip Zn-Al-Mg-Si alloy galvanized layer, and the like. The electroplated layer includes, for example, an electroplated zinc galvanized layer (EG), an electroplated Zn-Ni alloy galvanized layer, and the like. Preferably, the plating layer is a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, or an electroplated zinc galvanized layer. The amount of the plating layer is not particularly limited and may be a general amount.

[0075] [Thickness of composite steel sheet or panel] The flat portion of the central side of the composite steel sheet and the corresponding panel is not particularly limited, but 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 flatness of the molded product shape, 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 effect of reducing the weight of the component becomes significant. The thickness of the composite steel sheet or panel is measured with a micrometer.

[0076] [Mechanical properties] [Tensile strength: TS] Panels having the above characteristics can achieve high tensile strength, specifically 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. There is no particular upper limit, but for example, the tensile strength may be 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 specimen of JIS Z2241:2022 from the composite structure steel sheet in the flat part of the central portion of the panel and performing 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, while maintaining an excellent appearance after molding, such as press molding, and especially deep drawing. For this reason, the panels according to the embodiments of the present invention are extremely useful for use as, for example, automobile exterior panels, and more specifically, automobile exterior panels such as roofs, hoods, fenders, and doors, where a high level of design is required in automobiles.

[0078] <Manufacturing method for composite steel sheets and panels> Next, preferred manufacturing methods for composite steel sheets and panels according to embodiments of the present invention will be described. The following description is intended to illustrate characteristic methods for manufacturing composite steel sheets and panels according to embodiments of the present invention, and is not intended to limit the composite steel sheets and panels to those manufactured by the manufacturing methods described below.

[0079] <Manufacturing method for composite steel sheets> [Casting Process] A method for manufacturing a composite steel sheet 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 a composite steel sheet. Such a casting step includes performing light reduction using a continuous casting machine equipped with a plurality of adjacent reduction rolls in the slab transport direction, wherein the roll pitch of adjacent reduction rolls is 290 mm or less. In this specification, light reduction refers to reduction having a reduction gradient of 0.6 mm or more per meter in the casting direction.

[0080] In the composite structure steel sheet according to the embodiment of the present invention, as described above, it is preferable to have a unique metal structure in which the variation in the hard phase fraction in the direction perpendicular to rolling is small, more specifically, a metal structure in which the standard deviation of the hard phase fraction in the direction perpendicular to rolling is reduced to 0.75% or less. In order to obtain such a metal structure, it is very important to control the solidification structure during casting so that it becomes a columnar crystal structure. Specifically, in the casting process, by setting the superheat ΔT (difference between the molten steel temperature and the solidification temperature of the molten steel) of the molten steel having the above-mentioned 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 ratio of 15% or less, and also to suppress central 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 central segregation involve minimizing the superheat ΔT (to at least less than 25°C) and increasing the equiaxed crystallinity (to at least more than 15%). However, such conventional measures do not provide sufficient improvement. In this manufacturing method, casting conditions that are completely different from conventional measures, namely a superheat ΔT of 25°C or higher and a segment pressing force of 450 tons or more, are employed to control the solidification structure to a columnar crystal structure, thereby suppressing negative segregation and central segregation of Mn.

[0082] The equiaxed crystallinity (%) can be calculated by taking an etch print of the slab's thickness cross-section in the width direction, visually observing the boundary between the columnar and equiaxed crystal structures, measuring the thickness of the equiaxed crystal structure (mm) at the center of the slab's thickness and the slab's thickness (mm), and then dividing the equiaxed crystal structure thickness by the slab's thickness and multiplying by 100.

[0083] Furthermore, in the casting process, performing light reduction using a continuous casting machine in which the roll pitch of adjacent reduction rolls is 290 mm or less can suppress the flow of molten steel during solidification and reduce the concentration of Mn in the center. This can suppress the central segregation of Mn. It is more preferable that the roll pitch of adjacent reduction rolls be 280 mm or less.

[0084] [Other processes] This manufacturing method may include other processes in addition to the casting process described above, such as hot rolling, cold rolling, annealing, and cooling. Furthermore, this manufacturing method may optionally include a plating process. These processes are not particularly limited, and any appropriate conditions can be appropriately selected and carried out in relation to the composite steel sheet to obtain a metal structure containing the soft phase and hard phase described above in predetermined area fractions. Preferred conditions for these processes will be briefly described below.

[0085] [Hot rolling process] Prior to hot rolling, it is preferable to heat the slab to 1100°C or higher. By heating 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 economic standpoint, it is preferable to heat to less than 1300°C. In the hot rolling process, rough rolling and finish rolling are performed on the heated slab. The hot-rolled steel sheet obtained in this way is wound at a winding temperature of, for example, 450 to 650°C.

[0086] The finish rolling completion temperature is preferably 950°C or lower. By setting the finish rolling completion temperature to 950°C or lower, the average 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 winding temperature to 450-650°C, the average 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 appropriate pickling treatment to remove scale, and then subjected to the cold-rolling process. In the cold-rolling process, it is preferable to cold-roll the hot-rolled steel sheet so that, for example, the cumulative reduction ratio is 50 to 90%. By controlling the cumulative reduction ratio within this range, it is possible to secure the desired sheet thickness, ensure sufficient uniformity of the material in the width direction of the sheet, and prevent the rolling load from becoming excessive and making rolling difficult.

[0088] [Annealing process] In the annealing process, it is preferable to heat and hold the cold-rolled steel sheet to a soaking temperature of 750-900°C. By setting the soaking temperature to 750°C or higher, the recrystallization of ferrite and the reverse transformation from ferrite to austenite are sufficiently advanced, making it possible to obtain the desired metallic structure in the final product. On the other hand, by setting the soaking temperature to 900°C or lower, the crystal grains are densified, and sufficient strength can be obtained.

[0089] [Cooling process] In the cooling process, the cold-rolled steel sheet is cooled after the annealing process. In the cooling process, it is preferable to cool the cold-rolled steel sheet so that the average cooling rate from the soaking temperature is 5 to 50°C / second. By setting the average cooling rate to 5°C / second or higher, excessive transformation to ferrite is suppressed, and the amount of hard phase such as martensite is increased, allowing the desired strength to be obtained. Furthermore, by setting the average cooling rate to 50°C / second or lower, the steel sheet can be cooled more uniformly in the width direction.

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

[0091] <Panel manufacturing method> The method for manufacturing a panel according to an embodiment of the present invention is: A blanking process is performed on the composite structure steel sheet obtained above. A forming process for forming blanked composite steel sheets into steel parts, and Optional painting process for painted molded steel parts. This includes the following. Each step will be explained in more detail below.

[0092] [Blanking process] In the blanking process, the composite steel sheet obtained above is subjected to a blanking process in which it is cut to a predetermined size. The blanking process can be carried out by any suitable means known to those skilled in the art, such as punching by press.

[0093] [Molding process] The blanked composite steel sheet (blank) is formed into a steel part by press forming or the like in the next forming process. Examples of press forming include bending and deep drawing. Here, the amount of strain applied by press forming must be appropriately controlled. If the amount of strain is small, although it does not necessarily negatively affect the appearance after forming, the introduction of dislocations may be insufficient. In this case, the amount of bake hardening during paint baking decreases, and the yield stress cannot be sufficiently increased. As a result, the dent resistance of the final product decreases. Therefore, from the viewpoint of improving dent resistance, it is preferable that the amount of strain applied in the forming process be 2.0% or more in the flat portion of the central part 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 decrease in the appearance after forming. Therefore, from the viewpoint of improving the appearance after forming, it is preferable that the amount of strain applied in the forming process be 5.0% or less in the flat portion of the central part of the panel.

[0094] [Painting Process] The formed steel parts are optionally painted in the following painting process, preferably with a paint-baking treatment. This painting process includes, for example, three types of painting: electrodeposition coating, intermediate coating, and topcoat coating (base and clear coating). Water-based or solvent-based paints are used for painting. In electrodeposition coating, the steel parts are submerged in an electrodeposition tank containing paint, and the entire surface of the steel parts is coated with electrodeposition paint. In intermediate coating, the entire surface of the steel parts is coated by spraying paint from a spray nozzle using a painting robot or by a worker. In topcoat coating, the entire surface of the steel parts is coated by spraying paint from a spray nozzle using a painting robot or by a worker. As a result, the surface of the steel parts is covered with a paint layer with a film thickness of 60 to 200 μm.

[0095] [Paint baking process] Paint baking treatment is a baking and drying treatment to bake the paint layer onto the steel part, and also a treatment to bake and harden the steel part. Paint baking treatment may be performed in the painting process after electrodeposition coating and before the intermediate coating, or between multiple intermediate coatings, or after the intermediate coating and before the topcoat, or between multiple topcoat coatings, or after the topcoat.

[0096] The temperature and time of the paint baking process are preferably controlled so that the drying parameter P, represented by the following formula 3, is within the range of 7500 to 10000. The specific temperature and time of the paint baking process can be appropriately selected from, for example, the range of 100 to 220°C and 20 to 60 minutes, within the range that satisfies the following formula 3. P=(T+273)×(17.7+log(t))...Equation 3 In the formula, T is the temperature (°C) of the paint baking process, and t is the time (seconds) of the paint baking process. When multiple paint curing processes are performed, it is preferable to control the total time for each curing process to be within the range of 20 to 60 minutes. Similarly, it is preferable to control the drying parameter P so that the cumulative value of the drying parameters calculated from the temperature and time of each curing process is within the range of 7500 to 10000. If the drying parameter P is less than 7500, the amount of curing will decrease, and as a result, the yield strength after curing may not be sufficiently increased. In this case, the dent resistance of the final product will decrease. On the other hand, if the drying parameter P exceeds 10000, the yield strength after curing may decrease due to excessive curing, and similarly, the dent resistance of the final product may decrease.

[0097] Panels manufactured by the above manufacturing method achieve high strength by including both a soft and hard phase 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 0.50 μm or less. This significantly suppresses the occurrence of appearance defects such as ghost lines on the panel surface, even when strain is applied by forming such as press forming. Furthermore, when the panel is subjected to paint and bake treatment, the yield strength 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 sector, where high strength, excellent post-forming appearance, and 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 in any way to these examples. [Examples]

[0099] In the following embodiments, panels according to the present invention were manufactured under various conditions, and the properties of the obtained panels were investigated.

[0100] [Manufacturing of composite steel sheets] First, using a continuous casting machine equipped with multiple reduction rolls arranged with a roll pitch of 280 mm or less, a light reduction method was performed with a reduction gradient of 0.6 mm or more per meter in the casting direction, resulting in the casting of slabs with the chemical composition shown in Table 1 and a thickness of 200-300 mm. The remainder other than the components shown in Table 1 consists of Fe and impurities. In Table 2, casting condition (I) is "superheat ΔT ≥ 25℃", and casting condition (II) is "segment pressing force ≥ 450 tons". In each example, Table 2 shows the cases where these conditions are met (indicated as "OK") and where they are not met (indicated as "NG").

[0101] Next, the obtained slab was subjected to a hot rolling process (heating temperature 1200°C, finish rolling completion temperature 900°C, and winding temperature 550°C), a cold rolling process (cumulative reduction ratio 80%), an annealing process (soaking temperature 800°C), and a cooling process (average cooling rate 10°C / second) 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), an alloyed hot-dip galvanized layer (GA), or an electro-galvanized layer (EG). Furthermore, when the chemical composition of samples taken from the manufactured cold-rolled steel sheet was analyzed, there was no change from the chemical composition of the slab shown in Table 1.

[0102] [Panel manufacturing] Next, the obtained cold-rolled steel sheet or plated steel sheet was subjected to blanking to cut it into blanks of a predetermined size, and then deep drawing was performed on the blanks to obtain steel parts in the panel shape shown in Figure 1. Figure 1 is a schematic diagram showing the panel obtained by deep drawing in the embodiment, where Figure 1(a) is a perspective view of the panel and Figure 1(b) is a perspective view of the panel of Figure 1(a) viewed from the back. In Figure 1, the strain of the flat section with a radius of curvature of 1200 mm was 5.0% or less. In addition, the thinning rate of the ridge section (thinned section) shown by the thick line in Figure 1(a) was approximately 5% in all embodiments and comparative examples.

[0103] [Table 1]

[0104] [Table 2]

[0105] The characteristics of the obtained panels were measured and evaluated by the following method.

[0106] [Appearance Evaluation] The appearance of the panels after deep drawing was evaluated based on the degree of ghost lines that appeared on the panel surface after deep drawing. For each panel after deep drawing, the same flat area and edge were ground with a grinding wheel, and the linear streaks extending approximately parallel to the rolling direction that appeared on the surface were judged to be ghost lines and evaluated. Specifically, the panel surface was visually inspected, and the appearance after deep drawing was evaluated as follows. If the evaluation was AA or A, it was judged to be a passing grade as the appearance after forming was excellent. AA: No grooves on the flat parts and edges of the panel. A: No grooves on the flat parts of the panel, grooves on the edges of the panel. B: The flat surface of the panel has a streaky pattern.

[0107] The results are shown in Table 2. In the metallographic structures shown in Table 2, the hard phase contained at least one of martensite, bainite, tempered martensite, and pearlite, or at least one of these. Furthermore, X-ray diffraction measurements of retained austenite showed that the area percentage of retained austenite was less than 1% in all cases.

[0108] Referring to Tables 1-3, in Comparative Examples 3 and 16, the low superheat ΔT in the casting process resulted in a high equiaxed crystallinity in the solidified structure, which is thought to have prevented sufficient suppression of negative segregation and / or central segregation of Mn. As a result, the variation in the hard phase fraction in the direction perpendicular to rolling increased, causing the Sa of the composite steel sheet in the flat portion of the panel to exceed 0.50 μm and / or the Str of the composite steel sheet in the flat portion of the panel to fall below 0.50, resulting in a deterioration of the appearance after forming. In Comparative Examples 4 and 17, the low superheat ΔT in the casting process, coupled with the failure to satisfy the segment pressing force requirement of 450 tons or more, similarly resulted in a high equiaxed crystallinity in the solidified structure. As a result, the variation in the hard phase fraction in the direction perpendicular to rolling increased, causing the Sa of the composite steel sheet in the flat portion of the panel to exceed 0.50 μm and the Str of the composite steel sheet in the flat portion of the panel to fall below 0.50, resulting in a deterioration of the appearance after forming. In Comparative Example 12, the segment pressing force during the casting process was low, resulting in a similarly high equiaxed crystallinity in the solidification structure. As a result, the variation in the hard phase fraction perpendicular to the rolling direction increased, and the Sa of the composite steel sheet in the flat portion of the panel exceeded 0.50 μm, leading to a deterioration in the appearance after forming. In Comparative Examples 23 and 24, it is thought 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, failing to satisfy Equation 1. As a result, the variation in the hard phase fraction perpendicular to the rolling direction increased, the Sa of the composite steel sheet in the flat portion of the panel exceeded 0.50 μm, and the Str of the composite steel sheet in the flat portion of the panel fell below 0.50, leading to a deterioration in the appearance after forming.

[0109] In contrast, in all the panels according to the invention examples, high strength was achieved by including not only a soft phase but also a hard phase in the metal structure of the steel sheet constituting the panel, for example, achieving a tensile strength of 400 MPa or more. At the same time, the surface properties of the panel were controlled so that Str was within the range of 0.50 to 1.00 and Sa was 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 was applied by deep drawing.

Claims

1. A panel including a composite steel sheet having a metallic structure composed of a soft phase and a hard phase, The surface texture aspect ratio Str of the composite steel sheet in the flat portion of the central part of the panel is 0.50 to 1.

00. A panel characterized in that the surface roughness parameter Sa of the composite steel sheet in the flat portion of the central part of the panel is 0.50 μm or less.

2. The panel according to claim 1, characterized in that the composite steel sheet has a thinned portion in a region other than the flat portion that is thinner than the thickness of the flat portion.

3. The panel according to claim 1 or 2, characterized in that it is an exterior panel for an automobile.

4. The panel according to claim 1 or 2, characterized in that the composite steel sheet is a painted steel sheet having a paint layer on at least one surface.

5. The panel according to claim 1 or 2, characterized in that the thickness of the flat portion is 0.2 to 0.6 mm.

6. The panel according to claim 1 or 2, characterized by having a tensile strength of 500 MPa or more.

7. The metallic structure of the composite steel sheet in the flat portion is such that, by area percentage, the soft phase is 75-97% and the hard phase is 3-25%. The panel according to claim 1 or 2, characterized in that 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, characterized in that the composite 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 percentage.

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