Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
  • B22D11/12—Accessories for subsequent treating or working cast stock in situ
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals

Definitions

• the present invention relates to a panel.
• Patent Document 1 describes an exterior panel including a steel plate, the steel plate having a flat portion, in a surface region of the flat portion, a metallographic structure containing 80% or more ferrite by volume fraction, an average crystal grain size of the ferrite being 1.0 to 15.0 ⁇ m, and an intensity ratio X ODF ⁇ 001 ⁇ / ⁇ 111 ⁇ ,S between the ⁇ 001 ⁇ orientation and the ⁇ 111 ⁇ orientation of the ferrite being 0.30 or more and less than 3.50, and where uEl 1 is the uniform elongation measured using a tensile test piece cut out from the flat portion, and uEl 2 is the theoretical uniform elongation derived by a predetermined formula from the volume fractions, hardness, and average crystal grain size of ferrite and martensite in the metallographic structure of the inner region of the flat portion and the thickness of the flat portion, the ratio uEl 1 /uEl 2 is 0.44 to 0.80. Furthermore, Patent Document 1 teaches that the above configuration makes it possible to provide an exterior panel that
• Patent Document 2 describes a panel having a steel plate containing martensite, in which the surface roughness parameter (Sa) of a flat portion of the center portion of the panel is Sa ⁇ 0.500 ⁇ m, the martensite lath contains 15 or more precipitates with a major axis of 0.05 ⁇ m to 1.00 ⁇ m and an aspect ratio of 3 or more per ⁇ m, and the ratio YS 1 /YS 2 of the yield stress YS 1 measured using a tensile test piece cut from the flat portion to the yield stress YS 2 measured using a tensile test piece cut from the edge of the panel is 0.90 to 1.10.
• Patent Document 2 also teaches that the above configuration can provide an exterior panel that has excellent appearance after being formed from a material and excellent dent resistance.
• Patent Document 3 describes an automotive exterior panel part that includes a steel plate, in which the rolling direction of the steel plate extends along the left-right direction of the vehicle body in a plan view. Patent Document 3 also teaches that it is possible to provide an automotive exterior panel part with reduced ghost lines.
• the present invention aims to provide a panel with a new structure that is high in strength yet has an excellent appearance after molding.
• the inventors conducted research, focusing particularly on the metal structure of the steel plate that constitutes the panel and the surface texture of the panel.
• high strength can be achieved by including not only soft phases but also hard phases in the metal structure of the steel plate that constitutes the panel, and that an excellent appearance can be achieved even in a highly strengthened panel by appropriately selecting the metal structure of the steel plate, etc., so that two different parameters related to the panel's surface texture - more specifically, the surface texture aspect ratio Str and the surface roughness parameter Sa - are controlled within specific ranges at predetermined positions on the formed panel, thereby completing the present invention.
• the metal structure of the dual-phase steel plate in the flat portion is, in area%, soft phase: 75 to 97% and hard phase: 3 to 25%;
• the present invention makes it possible to provide panels that are high in strength yet have excellent appearance after molding.
• FIG. 1A and 1B are schematic diagrams showing a panel obtained by drawing in an example, in which FIG. 1A is a perspective view of the panel, and FIG. 1B is a perspective view of the panel of FIG. 1A as seen from the back side.
• a panel according to an embodiment of the present invention includes a dual-phase steel plate having a metal structure composed of a soft phase and a hard phase,
• the surface texture aspect ratio Str of the dual-phase steel plate in the flat portion of the center side portion of the panel is 0.50 to 1.00,
• the surface roughness parameter Sa of the dual-phase steel sheet at the flat portion in the center portion of the panel is 0.50 ⁇ m or less.
• the inventors conducted research, focusing particularly on the metallographic structure of the steel sheet that constitutes the panel and the surface texture of the panel.
• the desired high strength for example, a tensile strength of 400 MPa or more
• the high strength resulting from such a composite structure generally leads to a deterioration in the appearance of the panel after forming
• the inventors discovered that by appropriately selecting the metallographic structure, etc. of the composite phase steel sheet so that two different parameters related to the surface texture of the panel, more specifically the surface texture aspect ratio Str and the surface roughness parameter Sa, are controlled within specific ranges at predetermined positions on the panel after forming, it is possible to achieve an excellent appearance even in a high-strength panel.
• the surface texture aspect ratio Str is one of the spatial parameters of surface texture specified in JIS B0681-2:2018. It indicates the strength of surface anisotropy and is known to take values ranging from 0 to 1.00. Generally, as the Str value approaches 0, anisotropy becomes stronger, resulting in the appearance of streaks on the surface. Conversely, as the Str value approaches 1.00, the surface becomes isotropic and direction-independent.
• the inventors discovered that by appropriately selecting the metal structure, etc., of the dual-phase steel sheet used as the raw material, in addition to controlling Str, the surface roughness parameter Sa of the dual-phase steel sheet in the flat portion near the center of the panel can be controlled to within a range of 0.50 ⁇ m or less, thereby significantly suppressing or reducing the occurrence of poor appearance caused by minute irregularities on the panel surface, even when strain is imparted by press forming, particularly drawing forming, etc.
• the surface roughness parameter Sa refers to the average absolute value of z(x, y) in the reference area (A) defined in JIS B0681-2:2018, 4.1.7 "Arithmetic mean height of the scale limited surface.”
• panels according to embodiments of the present invention maintain high strength due to the hard phases contained together with soft phases in the metal structure of the dual-phase steel sheet.
• the surface texture aspect ratio Str and the surface roughness parameter Sa so that Str is within the range of 0.50 to 1.00 and Sa is 0.50 ⁇ m or less, it is possible to significantly suppress the occurrence of appearance defects such as ghost lines on the panel surface, even when strain is imparted by press forming, particularly drawing, etc.
• the present inventors have now discovered for the first time that even when the metal structure of the dual-phase steel sheet material contains hard phases, 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 ⁇ m or less and achieving a more isotropic surface texture with a surface texture aspect ratio Str of 0.50 or more. Therefore, panels according to embodiments of the present invention are particularly useful for automotive exterior panels, which require relatively high strength. Below, we will explain in more detail each component of the panel according to the embodiment of the present invention.
• a panel according to an embodiment of the present invention includes three portions, specifically (i) an edge portion, (ii) an end portion, and (iii) a center-side portion other than the edge portion and the end portion.
• the edge portion (i) is a portion that is bent by a hemming (HEM) process or fixed to another component by welding such as spot welding.
• the end portion (ii) is a portion located toward the center of the panel from the edge portion and is a portion that is not fixed to another component by hemming, welding, or the like. This end portion is, for example, a location several millimeters toward the center of the panel from the edge portion and is a location that is substantially unaffected by processing for fixing the panel to another component. In this case, "substantially unaffected” means that the change in properties due to processing for fixing the panel to another component is within a few percent.
• the central portion of the panel (iii) is the portion visible from the outside as an exterior, for example, the exterior of an automobile.
• the flat portion refers to a portion of the central portion of the panel with a radius of curvature of 500 mm or more. Furthermore, when a plating layer and/or a paint layer is present on the surface of the panel, the flat portion refers to the flat portion of the entire panel, including the plating layer and/or the paint layer.
• the surface texture aspect ratio Str of the flat portion within the range of 0.50 to 1.00, a more isotropic surface texture can be achieved, thereby enabling the provision of a panel with excellent appearance.
• ghost lines are associated with streaks on the panel surface. Therefore, from the viewpoint of suppressing the occurrence of ghost lines, it is preferable that the minute irregularities on the panel surface be isotropic.
• Str is preferable, and may be, for example, 0.55 or more, 0.60 or more, 0.65 or more, 0.70 or more, 0.75 or more, or 0.80 or more. There is no particular upper limit, but Str may be, for example, 0.95 or less, 0.90 or less, or 0.85 or less.
• the surface texture aspect ratio Str of a dual-phase steel sheet in the flat portion of the center portion of the panel is determined as follows. First, a test piece is cut out from the flat portion of the center portion of the panel. Next, a 8 mm x 8 mm area on the surface of the cut-out sample (or the surface of the plating layer and/or paint layer, if a plating layer and/or paint layer is present on the surface of the sample) is measured in three dimensions using a Keyence VK-X3000 white light interferometer.
• the measurement conditions are a measurement magnification of 10x, a resolution of 3 ⁇ m in the XY plane, and a resolution of 1 nm in the Z space plane, and the measurements are performed in a linked manner. Then, the measurement area is subjected to tilt correction using quadratic surface correction to remove the radius of curvature of the entire panel. Furthermore, a filtering process is performed to remove irregularities with a period of 0.8 mm or less, and Str is determined in accordance with the provisions of JIS B0681-2:2018.
• the surface roughness parameter Sa of the dual-phase steel sheet in the flat portion of the center portion of the panel after forming is controlled to 0.50 ⁇ m or less.
• the flat portion refers to the flat portion of the entire panel including the plating layer and/or the paint layer.
• the surface roughness parameter Sa is the average value of the height difference (absolute value) of each point with respect to the average plane of the panel surface after strain is imparted during forming.
• the lower Sa is the more preferable, and it may be, for example, 0.48 ⁇ m or less, 0.45 ⁇ m or less, 0.42 ⁇ m or less, 0.40 ⁇ m or less, 0.38 ⁇ m or less, or 0.35 ⁇ m or less in the flat portion of the center side of the panel.
• Sa may be, for example, 0.05 ⁇ m or more, 0.10 ⁇ m or more, 0.15 ⁇ m or more, or 0.20 ⁇ m or more in the flat portion of the center side of the panel.
• the surface roughness parameter Sa of a dual-phase steel sheet in the flat portion of the center portion of a panel is determined as follows. First, a test piece is cut out from the flat portion of the center portion of the panel, and a 8 mm x 8 mm area on the surface of the cut-out sample (or the surface of the plating layer and/or paint layer, if a plating layer and/or paint layer is present on the surface of the sample) is measured in three dimensions using a Keyence VK-X3000 white light interferometer.
• the measurement conditions are a measurement magnification of 10x, a resolution of 3 ⁇ m in the XY plane, and a resolution of 1 nm in the Z space plane, and the measurements are performed in a linked manner. Then, the measurement area is subjected to tilt correction using quadratic surface correction to remove the radius of curvature of the entire panel. Furthermore, a filtering process is performed to remove irregularities with a period of 0.8 mm or less, and the arithmetic mean height is determined. The arithmetic mean height thus obtained is determined as the surface roughness parameter Sa of the dual-phase steel sheet in the flat portion of the center portion of the panel.
• the panel according to the embodiment of the present invention includes a dual-phase steel plate having a metallurgical structure composed of a soft phase and a hard phase.
• the panel according to the embodiment of the present invention includes at least a dual-phase steel plate having a metallurgical structure composed of a soft phase and a hard phase, and the flat portion of the center portion of the panel composed of the dual-phase steel plate has the above-mentioned characteristics. Therefore, the panel according to the embodiment of the present invention may partially include a material other than the dual-phase steel plate having a metallurgical structure composed of a soft phase and a hard phase.
• the panel according to the embodiment of the present invention essentially consists of, consists of, or consists of a dual-phase steel plate having a metallurgical structure composed of a soft phase and a hard phase.
• soft phase refers to ferrite.
• hard phase refers to a structure harder than the soft phase, ferrite, and includes or consists of at least one of martensite, bainite, tempered martensite, and pearlite, for example, and is particularly at least one of martensite, bainite, tempered martensite, and pearlite.
• the hard phase preferably consists of at least one of martensite, bainite, and tempered martensite, or is at least one of these, and more preferably consists of martensite or is martensite.
• the metal structure of the dual-phase steel plate preferably contains little retained austenite.
• the retained austenite is preferably less than 1% or less than 0.5%, and more preferably 0%, by area.
• the area fraction of the hard phase in the metal structure is not particularly limited and may be appropriately selected depending on the desired panel strength. For example, it may be 3% or more, 5% or more, 7% or more, 10% or more, or 12% or more.
• the area fraction of the hard phase in the metal structure may be 25% or less, 22% or less, 20% or less, 18% or less, or 15% or less.
• the area fraction of the soft phase in the metal structure may be 75% or more, 78% or more, 80% or more, 82% or more, or 85% or more.
• 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 dual-phase steel sheet has a thinned portion in a region other than the flat portion, the thickness of which is thinner than the thickness of the flat portion.
• Panels according to embodiments of the present invention can be manufactured, for example, by a manufacturing method including drawing.
• a manufacturing method including drawing for example, a flat portion and a ridge portion connected to the flat portion are formed, and a thinned portion thinner than the thickness of the flat portion is formed at the ridge portion due to drawing.
• the surface texture of the formed panel can be significantly reduced by controlling the surface texture using two different parameters, the surface texture aspect ratio Str and the surface roughness parameter Sa, so that Str is within the range of 0.50 to 1.00 and Sa is within the range of 0.50 ⁇ m or less, thereby significantly reducing the occurrence of appearance defects such as ghost lines on the panel surface.
• the degree of thinning can be determined appropriately depending on the conditions of press forming, such as drawing, and the type and size of the part to be manufactured, and is not particularly limited.
• the thinning rate of the thinned portion may be 2 to 20%.
• the thickness reduction rate of the thinned portion may be, for example, 3% or more, 4% or more, 5% or more, or 7% or more. Similarly, the thickness reduction rate of the thinned portion may be, for example, 18% or less, 15% or less, 12% or less, 10% or less, or 8% or less.
• the dual-phase steel sheet may not have a paint layer, or may be a painted steel sheet having a paint layer on at least one surface.
• a paint layer For example, if the surface quality of the panel is poor, a thick paint layer would be required to achieve a beautiful appearance.
• the panels according to the embodiments of the present invention have excellent surface quality, allowing the paint layer to be thin, which is therefore very advantageous from a cost perspective.
• the yield stress of the panel can be increased in relation to bake hardening during paint baking, which is therefore advantageous from the perspective of improving the dent resistance of the panel.
• the amount of bake hardening can be significantly increased due to the uniform dispersion of the hard phase containing a relatively large number of dislocations. Therefore, the yield stress can be similarly significantly increased, thereby further improving the dent resistance of the panel.
• a paint layer can be formed on the plating layer.
• the paint layer is not particularly limited and may be any appropriate paint layer known to those skilled in the art.
• the thickness of the paint layer is also not particularly limited and may be, for example, 60 to 200 ⁇ m.
• Paint layers in automotive panels generally include, from the steel sheet side, an electrodeposition paint layer, an intermediate paint layer, a base coat layer, and a clear coat layer.
• the thickness of the electrodeposition paint layer may be, for example, 10 to 40 ⁇ m, and the thickness of the intermediate paint layer may be, for example, 20 to 60 ⁇ m.
• the thickness of the base coat layer may be, for example, 10 to 30 ⁇ m
• the thickness of the clear coat layer may be, for example, 20 to 80 ⁇ m.
• a preferred embodiment of a dual-phase steel sheet having a metal structure composed of a soft phase and a hard phase which is useful for realizing the surface texture of a panel in which, when formed by press forming, particularly drawing, the dual-phase steel sheet has a surface texture aspect ratio Str of 0.50 to 1.00 in the flat portion of the central portion and a surface roughness parameter Sa of 0.50 ⁇ m or less in the flat portion of the central portion, will be described in detail.
• these descriptions are intended to merely exemplify preferred dual-phase 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 dual-phase steel sheets.
• the dual-phase steel plate has a metal structure in a flat portion of a center side portion of a panel, in terms of area%, of a soft phase: 75 to 97% and a hard phase: 3 to 25%,
• the standard deviation of the hard phase fraction in the direction perpendicular to the rolling direction is 0.75% or less.
• Mn-enriched regions such as center segregation and microsegregation are formed during casting, and these enriched regions are elongated in the rolling direction by hot rolling or cold rolling, resulting in Mn segregation in a streaky manner.
• This Mn segregation results in the presence of regions with high and low hardenability within the steel sheet.
• a relatively large number of streaky hard phases are formed in the metal structure of the steel sheet after quenching. In this case, the occurrence of ghost lines becomes particularly noticeable.
• the inventors investigated ways to optimize the ratio of soft and hard phases in the metal structure, more specifically, to control the soft phase to 75-97% by area and the hard phase to 3-25% by area, thereby achieving the desired high strength while also improving the appearance after forming.
• the inventors conducted their investigations focusing on the distribution of the hard phase in the metal structure.
• the inventors discovered that, as will be explained in detail later in connection with the manufacturing method of dual-phase steel sheet, it is important to reduce the amount of equiaxed crystals in the solidification structure during the casting process and control the solidification structure to a columnar crystal structure.
• the inventors discovered that when coarse equiaxed crystals are formed in the solidification structure during the casting process, even if the centerline segregation of Mn itself is small, negative segregation of Mn occurs, resulting in large variations in the hard phase fraction and potentially worsening the appearance after forming.
• the present inventors have found that controlling the solidification structure to a columnar crystal structure can suppress negative segregation of Mn and reduce the central segregation of Mn during solidification, which is a cause of the formation of banded hard phases, thereby reducing the variation in the hard phase fraction in the metallographic structure.
• controlling the solidification structure to a columnar crystal structure can reduce the standard deviation of the hard phase fraction in the transverse direction to 0.75% or less, thereby enabling the production of a dual-phase steel sheet having a metallographic structure in which the hard phase is uniformly dispersed.
• the "transverse direction” refers to the direction perpendicular to the rolling direction and the plate thickness direction.
• the conventional approach to preventing center segregation is to increase the equiaxed crystal fraction, which is common knowledge among those skilled in the art (see, for example, Takaho Kawawa et al., "Tetsu to Hagane," Vol. 60 (1974) No. 5, pp.
• the inventors have discovered that even when a certain degree of centerline Mn segregation remains, by appropriately reducing the hard phase fraction within the range of 3 to 25%, the standard deviation of the hard phase fraction can be reduced to 0.75% or less, and as a result, it is possible to produce a dual-phase steel sheet having a metallurgical structure in which the hard phase is uniformly dispersed. Therefore, according to a preferred embodiment of the present invention, by using a dual-phase steel sheet having a metallurgical structure in which the hard phase is uniformly dispersed, the amount of deformation of the dual-phase steel sheet can be made more uniform, particularly in the width direction, even during forming such as press forming. In connection with this, by realizing the surface properties of a panel having the desired Str and Sa, it is possible to achieve an excellent post-forming appearance in which appearance defects such as ghost lines are significantly suppressed.
• the dual-phase steel sheet according to a preferred embodiment of the present invention has a metal structure in which the hard phase is uniformly dispersed. Therefore, during the paint baking process after forming into a panel, the amount of bake hardening can be significantly increased due to the uniform dispersion of the hard phase, which contains a relatively large number of dislocations. Therefore, the use of this dual-phase steel sheet significantly increases the yield stress of the resulting panel, which is also highly advantageous from the perspective of improving the dent resistance of the panel.
• the metal structure of the dual-phase steel sheet refers to the metal structure of the dual-phase steel sheet in the flat portion near the center of the panel. The degree of forming in this flat portion is low, and therefore the characteristics of the metal structure described below do not change significantly before or after forming, such as press forming.
• the metallographic structure of the dual-phase steel sheet is, in area %, 75 to 97% soft phase and 3 to 25% hard phase.
• high strength more specifically a tensile strength of 400 MPa or more, preferably 500 MPa or more, can be achieved while suppressing poor appearance after forming.
• the area fraction of the hard phase may be, for example, 5% or more, 7% or more, 10% or more, or 12% or more.
• the area fraction of the soft phase may be 95% or less, 93% or less, 90% or less, or 88%.
• the area fraction of the hard phase may be, for example, 22% or less, 20% or less, 18% or less, or 15% or less.
• the area fraction of the soft phase may be 78% or more, 80% or more, 82% or more, or 85% or more.
• the hard phase refers to a structure harder than the soft phase, which is ferrite, and includes or consists of at least one of martensite, bainite, tempered martensite, and pearlite, for example, and is particularly at least one of martensite, bainite, tempered martensite, and pearlite.
• the hard phase preferably consists of at least one of martensite, bainite, and tempered martensite, or is at least one of these, and more preferably consists of martensite or is martensite.
• the metal structure of the dual-phase steel plate contains little retained austenite; specifically, the retained austenite is preferably less than 1% or less than 0.5%, and more preferably 0%, by area.
• the identification of the metallographic structure and calculation of the area fraction are performed as follows. First, a sample (e.g., 20 mm ⁇ 20 mm ⁇ sheet thickness) for observing the metallographic structure (microstructure) is taken from the dual-phase steel sheet. Next, the metallographic structure is observed from the surface to half the sheet thickness using a scanning electron microscope (SEM), and the area fraction of the hard phase is calculated from a position 50 ⁇ m from the steel sheet surface (the surface excluding the plating layer, if present) to a position half the sheet thickness. To prepare the sample, a cross section of the sheet thickness perpendicular to the sheet surface is polished as the observation surface and etched by nital etching. Next, the "microstructure" is classified from an SEM photograph at a magnification of 500 or 1000 times. Hard phases can be distinguished from each other based on differences in brightness.
• SEM scanning electron microscope
• the area from the surface of the steel plate etched by nital corrosion to the half-thickness position in the plate thickness direction is observed at 500x or 1000x magnification in 10 fields of view (3 or more fields if it is difficult to observe 10 fields of view due to the thin plate thickness), and the area fraction of the hard phase is determined using image analysis software Image J (Ver. 1.54f).
• the soft phase and hard phase are binarized based on differences in brightness, and the area fraction of the hard phase is calculated.
• Image analysis is performed in the same manner as above for a total of 10 observation fields to measure the area fraction of the hard phase, and these area fractions are averaged to calculate an average value. This average value is the area fraction of the hard phase, and the remainder is the area fraction of ferrite.
• the observation area of each field of view is 37,500 ⁇ m 2.
• the area fraction of retained austenite can be measured by X-ray diffraction of the above observation surface. Specifically, using Co-K ⁇ radiation, the integrated intensities of five peaks, ⁇ (200), ⁇ (211), ⁇ (311), ⁇ (200), and ⁇ (220), at a quarter position in the sheet 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 defined as the area fraction of retained austenite.
• the standard deviation of the hard phase fraction in the transverse direction in the metallographic structure of the flat portion of the dual-phase steel sheet is controlled to 0.75% or less.
• the standard deviation of the hard phase fraction refers to the standard deviation of the area fraction of the hard phase itself.
• the standard deviation of the hard phase fraction in the transverse direction is 0.75% or less, i.e., the variation in the hard phase fraction in the transverse direction is sufficiently reduced, thereby significantly suppressing post-forming appearance defects.
• the "transverse direction” refers to the direction perpendicular to the rolling direction and the sheet thickness direction.
• the lower the standard deviation of the hard phase fraction in the direction perpendicular to the rolling direction the more preferable, and it may be, for example, 0.65% or less, 0.55% or less, or 0.45% or less.
• 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.
• the standard deviation of the hard phase fraction in the transverse direction of the rolling of the metallographic structure is determined as follows. First, a region between a position 50 ⁇ m from one steel sheet surface and a position 50 ⁇ m from the other steel sheet surface in a cross section of a flat portion of a dual-phase steel sheet parallel to the transverse direction of the rolling and perpendicular to the steel sheet surface is observed with a scanning electron microscope (SEM) at a magnification of 500x or 1000x to obtain an SEM photograph.
• SEM scanning electron microscope
• This SEM photograph is analyzed using image analysis software, as in the case of the hard phase area fraction described above, to measure the hard phase area fraction every 100 ⁇ m within an 8 mm range in the transverse direction of the rolling of the dual-phase steel sheet, and its standard deviation is calculated.
• the observation range in the transverse direction may be less than 8 mm or more than 8 mm.
• the lower limit of the observation range for the standard deviation of the hard phase fraction in the transverse direction is 4 mm
• the upper limit is 12 mm.
• the following method can be used to identify the transverse rolling direction of the dual-phase steel plate.
• the S concentration is measured using an electron probe microanalyzer (EPMA).
• the measurement conditions are an acceleration voltage of 15 kV and a measurement pitch of 1 ⁇ m, and a distribution image is measured over an area of 100 ⁇ m (thickness direction) x 500 ⁇ m (perpendicular to the thickness direction) at the center of the plate thickness.
• elongated areas with high S concentration are determined to be inclusions such as MnS. Observation may be performed from multiple fields of view.
• a plane parallel to the plane rotated in 5° increments between 0° and 180° around the thickness direction is observed using the above method.
• the average length of the major axes of the multiple inclusions in each cross section obtained is calculated for each cross section, and the cross section with the largest average value of the major axis length of the inclusions is identified.
• the direction parallel to the major axis of the inclusions in that cross section is determined to be the rolling direction.
• the dual-phase steel sheet in the flat portion satisfies the following formula 1. (TS-180,000/TS)/Vm ⁇ 35...Formula 1 where TS is the tensile strength in MPa and Vm is the hard phase fraction in area %, as determined in accordance with the description in [Identification of Metallographic Structure and Calculation of Area Fraction] above.
• Vm is controlled to a relatively low value in relation to TS. Therefore, even if a certain amount of central segregation of Mn remains, it becomes easier to suppress the variation in the hard phase fraction in the metal structure in the direction perpendicular to the rolling direction. As a result, it becomes relatively easy to achieve an excellent post-formed appearance in which appearance defects such as ghost lines are significantly suppressed.
• the present invention aims to provide a panel that has high strength but excellent appearance after forming.
• This objective is achieved by including a soft phase and a hard phase in the metallographic structure of the steel sheet constituting the panel, and controlling the surface texture of the formed panel using two different parameters, the surface texture aspect ratio Str and the surface roughness parameter Sa, so that Str is within the range of 0.50 to 1.00 and Sa is 0.50 ⁇ m or less. Therefore, it is clear that the chemical composition of the dual-phase steel sheet itself is not an essential technical feature for achieving the objectives of the present invention. Below, preferred chemical compositions of dual-phase steel sheets according to embodiments of the present invention are described in detail.
• the dual phase steel plate comprises, in 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 to 0.50%, B: 0 to 0.0100%, Ti: 0 to 0.100%, Nb: 0 to 0.100%, V: 0 to 0.50%, Ni: 0 to 1.00%, Cu: 0 to 1.00%, W: 0-1.00%, Sn: 0-1.00%, Sb: 0 to 0.200%, Ca: 0-0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, It is preferable that the chemical composition of the alloy be REM: 0 to 0.0100%, and the balance: Fe and impurities.
• the chemical composition of the alloy be REM
• C is an element that increases the strength of steel sheet. To fully obtain this effect, the C content is set to 0.030% or more. The C content may be 0.035% or more, 0.040% or more, or 0.050% or more. On the other hand, if C is contained excessively, the strength may become too high and the elongation may decrease. Therefore, the C content is set to 0.100% or less. The C content may be 0.095% or less, 0.090% or less, or 0.080% or less.
• Mn is an element that improves the hardenability of steel and contributes to improving strength. To fully obtain this effect, the Mn content is set to 1.00% or more. The Mn content may be 1.20% or more, 1.30% or more, or 1.40% or more. On the other hand, if Mn is contained excessively, ferrite transformation may be excessively suppressed, making it impossible to secure the desired amount of ferrite, and elongation may decrease. Therefore, the Mn content is set to 2.50% or less. The Mn content may be 2.25% or less, 2.00% or less, or 1.85% or less.
• Si is a deoxidizing element for steel and also an element that improves the strength of steel sheet through solid solution strengthening. To fully obtain these effects, the Si content is set to 0.005% or more. The Si content may be 0.010% or more, 0.050% or more, or 0.100% or more. On the other hand, excessive Si content may reduce the peelability of scale and cause surface defects. Therefore, the Si content is set to 1.500% or less. The Si content may be 1.000% or less, 0.500% or less, or 0.300% or less.
• P is an element that is mixed in during the manufacturing process.
• P is also a solid-solution strengthening element.
• the P content may be 0%.
• reducing the P content to less than 0.0001% requires time for refining, resulting in reduced productivity. Therefore, the P content may be 0.0001% or more, 0.0005% or more, or 0.001% or more.
• excessive P content may reduce the toughness of the steel plate. Therefore, the P content is set to 0.100% or less.
• the P content may be 0.060% or less, 0.040% or less, or 0.020% or less.
• S is an element that is mixed in during the manufacturing process.
• the S content may be 0%.
• reducing the S content to less than 0.0001% requires time for refining, resulting in reduced productivity. Therefore, the S content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more.
• excessive S content may form Mn sulfides, which may reduce the formability of the steel sheet, such as ductility, hole expandability, stretch flangeability, and/or bendability. Therefore, the S content is set to 0.0200% or less.
• the S content may be 0.0100% or less, 0.0060% or less, or 0.0040% or less.
• Al is an element that functions as a deoxidizer and also improves the strength of steel sheet through solid solution strengthening. To fully obtain these effects, the Al content is set to 0.005% or more.
• the Al content may be 0.010% or more, 0.020% or more, or 0.025% or more.
• the Al content is set to 0.700% or less.
• the Al content may be 0.600% or less, 0.400% or less, 0.300% or less, 0.200% or less, or 0.100% or less.
• N is an element that is mixed in during the manufacturing process.
• the N content may be 0%.
• reducing the N content to less than 0.0001% requires time for refining, resulting in reduced productivity. Therefore, the N content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more.
• excessive N content may form nitrides, which may reduce the formability of the steel sheet, such as ductility, hole expandability, stretch flangeability, and/or bendability. Therefore, the N content is set to 0.0150% or less.
• the N content may be 0.0100% or less, 0.0080% or less, or 0.0050% or less.
• O is an element that is mixed in during the manufacturing process.
• the O content may be 0%.
• reducing the O content to less than 0.0001% requires time for refining, resulting in reduced productivity. Therefore, the O content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more.
• excessive O content may form coarse oxides, which may reduce the formability of the steel sheet, such as ductility, hole expandability, stretch flangeability, and/or bendability. Therefore, the O content is set to 0.0100% or less.
• the O content may be 0.0070% or less, 0.0040% or less, or 0.0020% or less.
• the dual-phase steel sheet may contain at least one of the following optional elements in place of a portion of the remaining Fe, as needed, for the purpose of improving properties.
• the dual-phase steel sheet may contain at least one of Cr: 0-0.80%, Mo: 0-0.50%, B: 0-0.0100%, Ti: 0-0.100%, Nb: 0-0.100%, V: 0-0.50%, Ni: 0-1.00%, Cu: 0-1.00%, W: 0-1.00%, Sn: 0-1.00%, Sb: 0-0.200%, Ca: 0-0.0100%, Mg: 0-0.0100%, Zr: 0-0.0100%, and REM: 0-0.0100%.
• These optional elements are described in more detail below.
• Cr is an element that improves the hardenability of steel and contributes to improving the strength of steel sheets.
• the Cr content may be 0%, but to achieve this effect, the Cr content is preferably 0.001% or more, and more preferably 0.01% or more.
• the Cr content may be 0.10% or more, 0.20% or more, or 0.30% or more.
• excessive Cr content may cause the formation of coarse Cr carbides that serve as fracture initiation points. Therefore, the Cr content is preferably 0.80% or less.
• the Cr content may be 0.70% or less, 0.60% or less, or 0.50% or less.
• Mo is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of the steel sheet.
• the Mo content may be 0%, but to obtain such effects, the Mo content is preferably 0.001% or more, more preferably 0.01% or more.
• the Mo content may be 0.05% or more or 0.07% or more.
• 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.
• B is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of the steel sheet.
• the B content may be 0%, but to obtain this effect, the B content is preferably 0.0001% or more.
• the B content may be 0.0005% or more, 0.0010% or more, or 0.0015% or more.
• excessive B content may cause B precipitates to form, reducing the strength of the steel sheet. Therefore, the B content is preferably 0.0100% or less.
• the B content may be 0.0080% or less, 0.0060% or less, or 0.0030% or less.
• Ti is an element that has the effect of reducing the amounts of S, N, and O, which generate coarse inclusions that act as fracture initiation sites. Ti also precipitates finely in steel as carbides, etc., and improves the strength of steel through precipitation strengthening.
• the Ti content may be 0%, but to obtain these effects, the Ti content is preferably 0.001% or more.
• the Ti content may be 0.005% or more, 0.007% or more, or 0.010% or more.
• excessive Ti content may form coarse Ti sulfides, Ti nitrides, and/or Ti oxides, which may reduce the formability of the steel sheet. Therefore, the Ti content is preferably 0.100% or less.
• the Ti content may be 0.080% or less, 0.060% or less, or 0.030% or less.
• Nb is an element that contributes to improving strength through precipitation strengthening.
• the Nb content may be 0%, but to obtain such an effect, the Nb content is preferably 0.001% or more.
• the Nb content may be 0.005% or more, 0.007% or more, or 0.010% or more.
• excessive Nb content may increase unrecrystallized ferrite, thereby reducing the formability of the steel sheet. Therefore, the Nb content is preferably 0.100% or less.
• the Nb content may be 0.060% or less, 0.040% or less, or 0.030% or less.
• V is an element that contributes to improving the strength of steel sheets due to strengthening by precipitates, grain refinement strengthening by suppressing the growth of crystal grains in soft phases, and/or dislocation strengthening by suppressing recrystallization.
• the V content may be 0%, but to obtain these effects, the V content is preferably 0.001% or more, more preferably 0.005% or more.
• the V content may be 0.01% or more or 0.02% or more.
• excessive V content may precipitate a large amount of carbonitrides, reducing the formability of the steel sheet. Therefore, the V content is preferably 0.50% or less.
• the V content may be 0.40% or less, 0.20% or less, or 0.10% or less.
• Ni is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of the steel sheet.
• the Ni content may be 0%, but to obtain such effects, the Ni content is preferably 0.001% or more, more preferably 0.01% or more.
• the Ni content may be 0.03% or more or 0.05% or more.
• 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.
• Cu is present in steel in the form of fine particles and is an element that contributes to improving the strength of the steel sheet.
• the Cu content may be 0%, but to obtain such an effect, the Cu content is preferably 0.001% or more, more preferably 0.01% or more.
• the Cu content may be 0.03% or more or 0.05% or more.
• excessive Cu content may reduce the weldability of the steel sheet. Therefore, the Cu content is preferably 1.00% or less.
• the Cu content may be 0.60% or less, 0.40% or less, or 0.20% or less.
• W is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of the steel sheet.
• the W content may be 0%, but to obtain such effects, the W content is preferably 0.001% or more, more preferably 0.01% or more.
• the W content may be 0.02% or more or 0.10% or more.
• 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.
• Sn is an element that suppresses coarsening of crystal grains and contributes to improving the strength of the steel sheet.
• the Sn content may be 0%, but to obtain such an effect, the Sn content is preferably 0.001% or more, more preferably 0.01% or more.
• the Sn content may be 0.05% or more or 0.08% or more.
• 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.
• Sb is an element that suppresses coarsening of crystal grains and contributes to improving the strength of the steel sheet.
• the Sb content may be 0%, but to obtain such an effect, the Sb content is preferably 0.001% or more.
• the Sb content may be 0.003% or more, 0.005% or more, or 0.010% or more.
• excessive Sb content may cause embrittlement of the steel sheet. Therefore, the Sb content is preferably 0.200% or less.
• the Sb content may be 0.150% or less, 0.100% or less, 0.050% or less, or 0.020% or less.
• Ca, Mg, Zr, and REM are elements that contribute to improving the formability of steel sheets.
• the Ca, Mg, Zr, and REM contents may be 0%, but to achieve these effects, the Ca, Mg, Zr, and REM contents are preferably 0.0001% or more.
• the Ca, Mg, Zr, and REM contents may be 0.0005% or more, 0.0010% or more, or 0.0015% or more.
• excessive inclusion of these elements may reduce the ductility of the steel sheet.
• the Ca, Mg, Zr, and REM contents are preferably 0.0100% or less.
• the Ca, Mg, Zr, and REM contents may be 0.0080% or less, 0.0060% or less, or 0.0030% or less, respectively.
• REM is a collective term for 17 elements: scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanides lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71.
• the REM content is the total content of these elements.
• the remainder other than the above elements consists of Fe and impurities.
• impurities refer to components that are mixed in during the industrial production of dual-phase steel sheet due to various factors in the manufacturing process, including raw materials such as ore and scrap.
• 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 impurity content may be 0.100% or less.
• the chemical composition of the dual-phase steel sheet according to an embodiment of the present invention may be measured using a common analytical method.
• the chemical composition of the dual-phase steel sheet may be measured using inductively coupled plasma atomic emission spectrometry (ICP-AES) on a test specimen taken from the flat portion of the center of the panel.
• ICP-AES inductively coupled plasma atomic emission spectrometry
• C and S may be measured using the combustion-infrared absorption method, N using the inert gas fusion-thermal conductivity method, and O using the inert gas fusion-non-dispersive infrared absorption method.
• index A 0.45% or more
• the chemical composition of the dual-phase steel sheet is such that index A, represented by the following formula 2, is 0.45% or more.
• A [Si]+10[P]+0.6[Al]+8[Ti]+9[Nb]...Formula 2
• [Si], [P], [Al], [Ti] and [Nb] are the contents of each element in mass %, and when no element is contained, it is 0%.
• Index A is determined by the content of the solid-solution strengthening elements Si, P, and Al, and the precipitation strengthening elements Ti and Nb.
• Increasing the hard phase fraction is generally preferable to increase the strength of dual-phase steel sheets.
• increasing the hard phase fraction tends to increase the variation in the hard phase fraction in the direction perpendicular to the rolling direction. Therefore, even in the case of a high hard phase fraction, in order to control the standard deviation of the hard phase fraction in the direction perpendicular to the rolling direction of the final metal structure to 0.75% or less, it is necessary to further sufficiently reduce the center segregation of Mn in the casting process. This requires strict control of manufacturing conditions, which increases the manufacturing burden.
• the inventors have discovered that by utilizing solid-solution strengthening by Si, P, and Al and precipitation strengthening by Ti and Nb, more specifically by controlling Index A expressed by the above formula 2 to 0.45% or more, it is possible to reduce the hard phase fraction while maintaining high strength.
• the inventors discovered that even when some degree of centerline segregation of Mn remains, by appropriately reducing the hard phase fraction within the range of 3-25%, it is possible to relatively easily reduce the standard deviation of the hard phase fraction in the direction perpendicular to the rolling direction to 0.75% or less, thereby making it possible to produce a dual-phase steel sheet having a metal structure in which the hard phase is uniformly dispersed.
• index A 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.
• 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.
• the dual-phase steel sheet according to the embodiment of the present invention may be a cold-rolled steel sheet, but may further include a plating layer on its surface for the purpose of improving corrosion resistance, etc.
• the plating layer may be either a hot-dip plating layer or an electroplated layer. That is, the dual-phase steel sheet according to the embodiment of the present invention may be a cold-rolled steel sheet having a hot-dip plating layer or an electroplated layer on its surface.
• the hot-dip plating layer examples include a hot-dip galvanized layer (GI), a galvannealed layer (GA), a hot-dip aluminum plating layer, a hot-dip Zn—Al alloy plating layer, a hot-dip Zn—Al—Mg alloy plating layer, and a hot-dip Zn—Al—Mg—Si alloy plating layer.
• the electroplated layer examples include an electrogalvanized layer (EG), an electrolytic Zn—Ni alloy plating layer, etc.
• the plating layer is a hot-dip galvanized layer, a galvannealed layer, or an electrogalvanized layer.
• the coating weight of the coating layer is not particularly limited and may be a general coating weight.
• the flat portion of the center portion of the dual-phase steel plate and the corresponding panel may have a thickness of, for example, 0.2 to 2.0 mm, but is not particularly limited thereto.
• the thickness may be 0.3 mm or more or 0.4 mm or more.
• 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.
• the thickness of the dual-phase steel plate or panel is measured using a micrometer.
• a panel having the above characteristics can achieve high tensile strength, specifically a tensile strength of 400 MPa or more.
• the tensile strength is preferably 440 MPa or more or 490 MPa or more, more preferably 500 MPa, 540 MPa or more, or 590 MPa or more.
• the upper limit is not particularly limited, but the tensile strength may be, for example, 900 MPa or less, 860 MPa or less, or 800 MPa or less.
• the tensile strength is measured by taking a No. 5 tensile test piece according to JIS Z2241:2022 from the dual-phase steel plate in the flat portion of the center portion of the panel and conducting a tensile test in accordance with JIS Z2241:2022.
• the panels according to the embodiments of the present invention are capable of achieving high strength, specifically a tensile strength of 400 MPa or more, yet maintain an excellent appearance after press forming, particularly drawing, and other forming processes. For this reason, the panels according to the embodiments of the present invention are highly useful for use as, for example, outer panel panels for automobiles, and more specifically, as outer panel panels for automobiles such as roofs, hoods, fenders, and doors, which require high levels of design.
• a method for producing a dual-phase 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 the dual-phase steel sheet.
• the casting step includes soft reduction using a continuous casting machine having a plurality of reduction rolls adjacent to each other in the slab transport direction, the roll pitch between the adjacent reduction rolls being 290 mm or less.
• soft reduction refers to a reduction gradient of 0.6 mm or more per meter in the casting direction.
• the dual-phase steel sheet according to an embodiment of the present invention preferably has a unique metal structure with small variation in the hard phase fraction in the direction perpendicular to the rolling direction, more specifically, a metal structure in which the standard deviation of the hard phase fraction in the direction perpendicular to the rolling direction is reduced to 0.75% or less.
• a metal structure in which the standard deviation of the hard phase fraction in the direction perpendicular to the rolling direction is reduced to 0.75% or less.
• the superheat ⁇ T (the difference between the molten steel temperature and the solidification temperature of the molten steel) of molten steel having the specific chemical composition described above to 25°C or higher and further setting the segment pressing force to 450 tons or higher during the casting process, it is possible to control the solidification structure to a columnar crystal structure with an equiaxed crystal fraction of 15% or less and suppress center segregation.
• Superheat ⁇ T is preferably 30°C or higher, and more preferably 40°C or lower.
• 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 also be determined from the chemical composition of the molten steel using a known solidification temperature estimation formula.
• the equiaxed crystal ratio (%) can be calculated by taking an etched print of the slab's cross-section in the thickness direction, visually determining the boundary between the columnar crystal structure and the equiaxed crystal structure, measuring the thickness (mm) of the equiaxed crystal structure at the center of the slab's thickness and the thickness (mm) of the slab, and dividing the thickness of the equiaxed crystal structure by the slab thickness and multiplying the result by 100.
• the roll pitch of adjacent reduction rolls is 290 mm or less, which suppresses the flow of molten steel during solidification and reduces the concentration of Mn in the center. This makes it possible to suppress central segregation of Mn. It is more preferable that the roll pitch of adjacent reduction rolls is 280 mm or less.
• the present manufacturing method may include other steps such as a hot rolling step, a cold rolling step, an annealing step, and a cooling step. Furthermore, the present manufacturing method may optionally include a plating step. These steps are not particularly limited, and may be carried out under any appropriate conditions appropriately selected so as to obtain a metal structure containing the soft phase and hard phase at predetermined area fractions as described above in relation to the dual-phase steel sheet. Preferred conditions for these steps will be briefly described below.
• the slab Prior to hot rolling, the slab is preferably heated to 1100°C or higher.
• the heating temperature is preferably less than 1300°C.
• the heated slab is subjected to rough rolling and finish rolling.
• the hot-rolled steel sheet obtained in this manner is coiled at a coiling temperature of, for example, 450 to 650°C.
• the finish rolling end temperature be 950°C or lower.
• the finish rolling end temperature be 950°C or lower.
• the hot-rolled steel sheet obtained by the hot-rolling process is subjected to an appropriate pickling treatment to remove scale, and then to a cold-rolling process.
• the hot-rolled steel sheet is preferably cold-rolled so that the cumulative reduction is, for example, 50 to 90%. By controlling the cumulative reduction within this range, it is possible to ensure the desired sheet thickness and sufficient uniformity of the material in the sheet width direction, while preventing the rolling load from becoming excessive and making the rolling difficult.
• the annealing step it is preferable to perform an annealing treatment in which the cold-rolled steel sheet is heated to a soaking temperature of 750 to 900°C and maintained at that temperature.
• a soaking temperature 750 to 900°C or higher, it is possible to sufficiently promote the recrystallization of ferrite and the reverse transformation from ferrite to austenite, thereby obtaining the desired metal structure in the final product.
• the soaking temperature to 900°C or lower, it is possible to densify the crystal grains and obtain sufficient strength.
• the cold-rolled steel sheet after the annealing step is cooled.
• the cold-rolled steel sheet is preferably cooled so that the average cooling rate from the soaking temperature is 5 to 50°C/s.
• the average cooling rate is set to 5°C/s or more, excessive transformation to ferrite is suppressed and the amount of hard phases such as martensite produced is increased, thereby achieving the desired strength.
• the average cooling rate is set to 50°C/s or less, the steel sheet can be cooled more uniformly in the width direction.
• the surface of the obtained cold-rolled steel sheet may be subjected to a plating treatment, if necessary.
• plating treatments include hot-dip plating, alloying hot-dip plating, and electroplating.
• the steel sheet surface may be subjected to hot-dip galvanizing treatment as the plating treatment, or hot-dip galvanizing treatment may be followed by alloying treatment.
• Specific conditions for the plating treatment and alloying treatment are not particularly limited, and any appropriate conditions known to those skilled in the art may be employed.
• the alloying temperature may be 450 to 600°C.
• a method for manufacturing a panel according to an embodiment of the present invention includes the steps of: a blanking step of blanking the dual-phase steel plate obtained above; a forming step of forming the blanked dual-phase steel plate into a steel part, and optionally a painting step of painting the formed steel part. Each step will be described in more detail below.
• the dual-phase steel plate obtained above is subjected to blanking processing, in which the plate is cut to a predetermined size.
• the blanking processing can be performed by any appropriate means known to those skilled in the art, such as punching using a press.
• the blanked dual-phase steel sheet (blank) is then formed into a steel part by press forming or the like in the next forming process.
• press forming include bending and drawing.
• the amount of strain imparted by press forming must be appropriately controlled. A small amount of strain does not necessarily adversely affect the appearance after forming, but it may result in insufficient dislocation introduction. In this case, the amount of bake hardening during paint baking decreases, making it impossible to sufficiently increase the yield stress. As a result, the dent resistance of the final product decreases. Therefore, from the perspective of improving dent resistance, the amount of strain imparted in the forming process is preferably 2.0% or more in the flat portion of the center portion of the panel.
• the amount of strain imparted in the forming process is preferably 5.0% or less in the flat portion of the center portion of the panel.
• the formed steel part is optionally painted in the next painting step, preferably by a paint baking process.
• This painting includes, for example, three types of painting: electrodeposition painting, intermediate painting, and top painting (base and clear coating). Water-based paints or solvent-based paints are used for painting.
• electrodeposition painting the steel part is submerged in an electrodeposition tank containing paint, and electrodeposition painting is applied to the entire surface of the steel part.
• intermediate painting the intermediate paint is applied to the entire surface of the steel part by spraying paint onto the steel part from a spray nozzle using a painting robot or manually by a worker.
• top paint is applied to the entire surface of the steel part by spraying paint onto the steel part from a spray nozzle using a painting robot or manually by a worker.
• the surface of the steel part is covered with a paint layer with a thickness of 60 to 200 ⁇ m.
• the paint baking treatment is a baking and drying treatment for baking a paint layer onto a steel part and for baking and hardening the steel part.
• the paint baking treatment may be performed after electrodeposition coating and before intermediate coating, between intermediate coatings that are performed multiple times, after intermediate coating and before topcoat coating, between topcoat coatings that are performed multiple times, or after topcoat coating.
• the temperature and time of the paint baking treatment are preferably controlled so that the drying parameter P, expressed by the following formula 3, is within the range of 7500 to 10000.
• the specific temperature and time of the paint baking treatment may be appropriately selected within the ranges that satisfy the following formula 3, for example, from the ranges of 100 to 220°C and 20 to 60 minutes.
• P (T+273) ⁇ (17.7+log(t))...Formula 3
• T is the temperature (°C) of the paint baking treatment
• t is the time (seconds) of the paint baking treatment.
• the total time for each paint baking process is preferably controlled to be in the range of 20 to 60 minutes.
• the drying parameter P is preferably controlled so that the cumulative value of the drying parameter calculated from the temperature and time of each paint baking process is in the range of 7500 to 10000. If the drying parameter P is less than 7500, the amount of bake hardening decreases, and as a result, the yield stress after bake hardening may not be sufficiently increased. In this case, the dent resistance of the final product decreases. On the other hand, if the drying parameter P exceeds 10000, excessive baking may cause a decrease in the yield stress after bake hardening, which may also result in a decrease in the dent resistance of the final product.
• Panels manufactured using the above manufacturing method achieve high strength by including not only soft phases but also hard phases in the metal structure of the steel plate that makes up the panel, while controlling the panel's surface properties so that Str is within the range of 0.50 to 1.00 and Sa is 0.50 ⁇ m or less makes it possible to significantly suppress the occurrence of appearance defects such as ghost lines on the panel surface, even when strain is imparted by forming such as press forming. Furthermore, when the panel is painted and baked, bake hardening can significantly increase the yield stress, thereby improving the panel's dent resistance. Therefore, panels manufactured using the above manufacturing method are particularly useful in the automotive field, where high strength, excellent post-forming appearance, and even excellent dent resistance are required.
• casting condition (I) is the condition of "superheat ⁇ T ⁇ 25°C”
• casting condition (II) is the condition of "segment pressing force ⁇ 450 tons.”
• Table 2 shows the cases where these conditions were met (denoted “OK”) and the cases where they were not met (denoted "NG”), respectively.
• the resulting slab was then subjected to a hot rolling process (heating temperature 1200°C, finish rolling end temperature 900°C, and coiling temperature 550°C), a cold rolling process (cumulative reduction 80%), an annealing process (soaking temperature 800°C), and a cooling process (average cooling rate 10°C/sec) to produce a cold-rolled steel sheet with a thickness of 0.4 mm.
• the surface of the resulting cold-rolled steel sheet was then appropriately plated to form a hot-dip galvanized layer (GI), a galvannealed layer (GA), or an electrogalvanized layer (EG).
• GI hot-dip galvanized layer
• GA galvannealed layer
• EG electrogalvanized layer
• FIG. 1 is a schematic diagram showing a panel obtained by drawing in an example, where FIG. 1( a) is a perspective view of the panel, and FIG. 1( b) is a perspective view of the panel of FIG. 1( a) as seen from the back.
• the amount of strain in the flat portion having a curvature radius of 1200 mm was 5.0% or less.
• the thinning rate of the ridge line portion (thinned portion) indicated by the thick line in FIG. 1( a) was approximately 5% in all examples and comparative examples.
• the hard phase contained at least one of martensite, bainite, tempered martensite, and pearlite, or was at least one of these. Furthermore, measurement of retained austenite using X-ray diffraction showed that the area ratio of retained austenite was less than 1% in all examples.
• the metal structure of the steel sheet that makes up the panel contains not only soft phases but also hard phases, thereby achieving high strength, for example a tensile strength of 400 MPa or more.
• Str is within the range of 0.50 to 1.00
• Sa is within the range of 0.50 ⁇ m or less, it is possible to significantly reduce the occurrence of appearance defects such as ghost lines on the panel surface, even when strain is imparted by drawing.

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• Metal Rolling (AREA)

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• 2025
  • 2025-01-24 WO PCT/JP2025/002145 patent/WO2025182368A1/ja active Pending
  • 2025-01-24 JP JP2025566025A patent/JP7839443B2/ja active Active

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