Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B62—LAND VEHICLES FOR TRAVELLING OTHERWISE THAN ON RAILS
  • B62D—MOTOR VEHICLES; TRAILERS
  • B62D29/00—Superstructures, understructures, or sub-units thereof, characterised by the material thereof
• • 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
• • 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
• • 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

Definitions

• Patent Document 1 describes an exterior panel including a steel plate, the steel plate having a flat portion, in which a metal structure in a surface region of the flat portion contains 80% or more ferrite by volume fraction, the average crystal grain size of the ferrite is 1.0 to 15.0 ⁇ m, and the intensity ratio X ODF ⁇ 001 ⁇ / ⁇ 111 ⁇ ,S between the ⁇ 001 ⁇ orientation and the ⁇ 111 ⁇ orientation of the ferrite is 0.30 or more and less than 3.50, and where uEl 1 is a uniform elongation measured using a tensile test piece cut out from the flat portion, and uEl 2 is a theoretical uniform elongation derived from a predetermined formula using the volume fractions, hardness and average crystal grain size of ferrite and martensite in the metal structure of the internal region of the flat portion and the sheet thickness of the flat portion, uEl 1 /uEl 2 is 0.44 to 0.80. Furthermore, Patent Document 1 teaches that the above-mentioned configuration makes it possible to provide an exterior panel
• Patent Document 2 describes a panel having a steel plate containing martensite, in which the surface roughness parameter (Sa) in the flat portion of the center portion of the panel is Sa ⁇ 0.500 ⁇ m, the martensite lath has 15 precipitates/ ⁇ m2 or more with a major axis of 0.05 ⁇ m to 1.00 ⁇ m and an aspect ratio of 3 or more, and the ratio YS 1 /YS 2 of the yield stress YS 1 measured using a tensile test piece cut out from the flat portion to the yield stress YS 2 measured using a tensile test piece cut out from the end portion 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 strong yet has excellent appearance after molding.
• the inventors conducted research focusing particularly on the metal structure of the steel plate constituting the panel and the surface properties of the panel.
• high strength can be achieved by including martensite in the metal structure of the steel plate constituting 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 and the conditions of press forming, etc. 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 specific ranges at a specified position on the panel after forming, and thus completed the present invention.
• a panel including a steel plate having a metal structure including martensite, The surface property aspect ratio Str of the steel plate in the flat portion of the center side portion of the panel is 0.50 to 1.00, A panel, characterized in that the surface roughness parameter Sa of the steel plate at the flat portion and end portions of the center side portion of the panel is 0.50 ⁇ m or less.
• the panel described in (1) above characterized in that the ratio of the average KAM value of ferrite contained in the steel plate to the volume fraction Vm of the martensite: average KAM value/Vm is 1.8 or more.
• the metal structure of the steel plate in the flat portion is, in terms of area%, Ferrite: 75-95%, martensite: 5 to 25%; and at least one of bainite, pearlite, and retained austenite: 0 to 10% in total;
• the average grain spacing of martensite is 2.5 ⁇ m or less
• the panel according to any one of (1) to (3) above characterized in that the standard deviation in the area ratio of martensite in the direction perpendicular to the rolling direction and the plate thickness direction is 1.5% or less.
• the present invention makes it possible to provide panels that are high in strength but have excellent appearance after molding.
• the panel according to the embodiment of the present invention includes a steel plate having a metal structure including martensite,
• the surface property aspect ratio Str of the steel plate in the flat portion of the center side portion of the panel is 0.50 to 1.00,
• the panel is characterized in that the surface roughness parameter Sa of the steel plate at the flat portion and end portions of the center side portion of the panel is 0.50 ⁇ m or less.
• DP steel which is a mixture of soft tissue made of ferrite and hard tissue made of martensite
• uneven deformation is likely to occur during processing such as press forming, in which the soft tissue and its surroundings are preferentially deformed, and fine irregularities are generated on the panel surface after forming, which can cause appearance defects called ghost lines.
• the soft tissue made of ferrite is greatly deformed and recessed, while the hard tissue made of martensite is less deformed. Therefore, the hard tissue does not recess as much as the soft tissue, but rises to be convex. As a result, the amount of deformation varies, especially in the direction perpendicular to the rolling of the panel, causing band-like ghost lines.
• the inventors therefore conducted a study focusing on the metal structure of the steel sheet constituting the panel and the surface properties of the panel in order to achieve both high strength and good appearance after forming into a panel.
• the desired high strength for example a tensile strength of 400 MPa or more
• the desired high strength can be achieved by including martensite in the metal structure of the steel sheet constituting the panel.
• the appearance of the panel after forming generally deteriorates in association with such high strength
• the inventors conducted further study focusing on the surface properties of the panel after forming in order to suppress such deterioration of the appearance after forming.
• the inventors discovered that an excellent appearance can be achieved even in a high-strength panel by appropriately selecting the metal structure of the steel sheet and the conditions of press forming, etc. 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.
• the surface texture aspect ratio Str is one of the spatial parameters of the surface texture defined in JIS B0681-2:2018, and is known to indicate the strength of the anisotropy of the surface and to take a value in the range of 0 to 1.00.
• the anisotropy becomes stronger and streaks and the like appear on the surface, while when the value of Str approaches 1.00, the surface becomes isotropic and independent of direction.
• the inventors have found that when strain is imparted by forming such as press forming, it is very effective in suppressing the occurrence of ghost lines on the panel surface to control the surface texture aspect ratio Str of the steel plate in the flat part of the central part of the panel to within the range of 0.50 to 1.00 by appropriately selecting the metal structure of the steel plate as the material and the conditions of the press forming, etc. Since ghost lines are related to streaks on the panel surface, from the perspective of suppressing the occurrence of said ghost lines, it is preferable that the minute irregularities on the panel surface are as isotropic as possible, and therefore it is preferable that the Str value is closer to 1.00.
• the inventors conducted further research into the surface properties of the panel. As a result, the inventors discovered that by appropriately selecting the metal structure of the steel plate, which is the raw material, and the conditions of press forming, etc., in addition to controlling Str, by controlling the surface roughness parameter Sa of the steel plate at the flat portion and end portion of the central portion of the panel to within a range of 0.50 ⁇ m or less, it is possible to significantly suppress or reduce the occurrence of poor appearance caused by minute irregularities on the panel surface, even when distortion is imparted by press 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.”
• the surface quality of the formed panel can be controlled using two different parameters, the surface quality 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 0.50 ⁇ m or less, thereby significantly suppressing the occurrence of appearance defects such as ghost lines on the panel surface even when distortion is imparted by forming such as press forming.
• the fact that the appearance of a high-strength panel after forming can be significantly improved by controlling the surface roughness parameter Sa to within the range of 0.50 ⁇ m or less and making the surface quality aspect ratio Str to be 0.50 or more, even when the metal structure of the steel sheet, which is the raw material, contains martensite, is now revealed for the first time by the present inventors. Therefore, the panel according to the embodiment of the present invention is particularly useful in application to automotive exterior panel parts, which have a relatively high demand for high strength. Below, each component of the panel according to the embodiment of the present invention will be described in more detail.
• the surface property aspect ratio Str of the steel plate in the flat portion of the center side portion of the panel after forming is controlled to 0.50 to 1.00.
• 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 center side portion other than the edge portion and the end portion.
• the edge portion of (i) is a portion that is bent by hemming (HEM) processing or fixed to another part by welding such as spot welding.
• the end portion of (ii) is a portion located on the center side of the panel from the edge portion, and is a portion that is out of the portion that is fixed to another part by hemming processing, welding, etc.
• This end portion is a portion that is, for example, a few mm toward the center side of the panel from the edge portion, and is a portion that is not substantially affected by processing for fixing the panel to another part. In this case, “substantially not affected” means that the amount of change in characteristics due to processing for fixing the panel to another part is within a few percent.
• the central portion of the above (iii) is a portion that is visible from the outside as an exterior, for example, the exterior of an automobile.
• the portion of the central portion of the panel having a radius of curvature of 500 mm or more is referred to as the flat portion.
• the flat portion refers to the flat portion of the entire panel including the plating layer and/or the paint layer.
• the higher Str is, the more preferable, and it 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.
• the upper limit is not particularly limited, but for example, Str may be 0.95 or less, 0.90 or less, or 0.85 or less.
• the surface quality aspect ratio Str of the steel sheet in the flat part of the central part of the panel is determined as follows. First, a test piece is cut out from the flat part of the central part of the panel, and then a three-dimensional measurement is performed on an 8 mm x 8 mm area on the surface of the cut out sample (when a plating layer and/or a paint layer is present on the surface of the sample, the surface of the plating layer and/or the paint layer). The measurement conditions at this time are a measurement magnification of 10 times, 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.
• the measurement area is subjected to tilt correction by quadratic curve correction to remove the radius of curvature of the entire panel. Further, 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 steel sheet at the flat portion and end of the central portion of the panel after forming is controlled to 0.50 ⁇ m or less.
• the flat portion and end refer to the flat portion and end of the panel as a whole 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 surface of the panel surface after strain during forming is imparted.
• the effect based on the combination of isotropic surface quality and lower surface roughness makes it possible to significantly improve the appearance of the high-strength panel after 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 at each of the flat portion of the center side portion of the panel and the end portion 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 at each of the flat portion of the center side portion of the panel and the end portion of the panel.
• the surface roughness parameter Sa of the steel sheet at the flat portion and end portion of the central portion of the panel is determined as follows. First, a test piece is cut out from the flat portion of the central portion of the panel, and a three-dimensional measurement is performed on an 8 mm x 8 mm area on the surface of the cut-out sample (when a plating layer and/or a paint layer is present on the surface of the sample, the surface of the plating layer and/or the paint layer) using a VK-X3000 white light interferometer manufactured by Keyence Corporation.
• the measurement conditions at this time are a measurement magnification of 10 times, 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 by quadratic curve correction to remove the radius of curvature of the entire panel. Further, a filtering process is performed to remove irregularities with a period of 0.8 mm or less, and the arithmetic mean height is obtained. The arithmetic mean height obtained in this manner is determined as the surface roughness parameter Sa of the steel sheet at the flat portion of the central portion of the panel. The surface roughness parameter Sa of the steel sheet at the edge of the panel is determined in the same manner as above, except that the test piece is cut from the edge of the panel, rather than from the flat portion of the center part of the panel.
• the panel according to the embodiment of the present invention includes a steel plate having a metal structure including martensite.
• the panel according to the embodiment of the present invention includes at least a steel plate having a metal structure including martensite, and the flat portion and the end portion of the center side portion of the panel constituted by the steel plate may have the above-mentioned characteristics. Therefore, the panel according to the embodiment of the present invention may include a material other than the steel plate having a metal structure including martensite in a part thereof.
• the panel according to the embodiment of the present invention is essentially composed of a steel plate having a metal structure including martensite, or is composed of the steel plate or is composed of the steel plate. Martensite is a hard structure with a high dislocation density.
• the area ratio of martensite in the metal structure may be appropriately selected according to the required strength of the panel, and is not particularly limited, but may be, for example, 5% or more, 7% or more, 10% or more, or 13% or more. Similarly, the area ratio of martensite in the metal structure may be, for example, 25% or less, 22% or less, 20% or less, 18% or less, or 15% or less.
• the term "martensite” includes not only as-quenched martensite (so-called fresh martensite) but also tempered martensite.
• the ratio of the average KAM value of ferrite contained in the steel plate constituting the panel to the volume fraction Vm of martensite: average KAM value/Vm is controlled to 1.8 or more. It is generally known that the KAM (Kernel Average Misorientation) value tends to increase as strain accumulates, and is therefore an effective index for evaluating strain distribution within crystal grains. On the other hand, the measured KAM value tends to increase as the proportion of martensite increases.
• the dent resistance of the panel can be improved by controlling the average KAM value/Vm to 1.8 or more.
• the higher the average KAM value/Vm the more preferable it is, and it may be, for example, 2.0 or more, 2.5 or more, 3.0 or more, 4.0 or more, or 5.0 or more.
• the upper limit is not particularly limited, but for example, the average KAM value/Vm may be 12.0 or less, 10.0 or less, or 8.0 or less.
• KAM value/Vm measurement The average KAM value is calculated by KAM (Kernel Average Misorientation) analysis in EBSD (Electron Backscatter Diffraction Patterns) measurement, which is a crystal analysis method using a SEM.
• KAM Kernel Average Misorientation
• EBSD Electro Backscatter Diffraction Patterns
• the KAM analysis averages the orientation differences between one pixel at a measurement point and six adjacent pixels, and sets the value of the central pixel, and a map based on local crystal orientation differences can be created.
• Vm was defined as the volume fraction with a GAIQ (Grain Average Image Quality) of 4000 or more in the EBSD measurement obtained under the same measurement conditions as the average KAM value.
• the GAIQ analysis averages the IQ value representing the clarity of the Kikuchi pattern of one pixel at a measurement point within one grain boundary when the region where the crystal orientation differs by 15° or more is defined as a grain boundary (grain), and defines the value of the central pixel.
• a phase map of ferrite and martensite with the grain boundary as the boundary can be created.
• the KAM map and phase map obtained in this way are compared to calculate the average KAM value in the region corresponding to ferrite.
• the average KAM value is divided by Vm to calculate the average KAM value/Vm in one visual field. This operation is performed in five visual fields, and the average value is defined as the average KAM value/Vm.
• the steel sheet may not have a paint layer, or may be a painted steel sheet having a paint layer on at least one surface.
• the surface quality of the panel is not good, it is necessary to apply a thick paint layer to obtain a beautiful appearance.
• the paint layer can be thinned, which is very advantageous from the viewpoint of cost.
• the yield stress of the panel can be increased in relation to the bake hardening during paint baking, and therefore it is advantageous from the viewpoint of improving the dent resistance of the panel.
• the amount of bake hardening can be significantly increased due to the uniform dispersion of martensite containing a relatively large number of dislocations. Therefore, the yield stress can be similarly significantly increased, and the dent resistance of the panel can be further improved.
• 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 ⁇ m.
• the paint layers in an automotive panel generally include, in order from the steel sheet side, an electrodeposition paint layer, an intermediate paint layer, a base coat layer, and a clear coat layer.
• the thickness of the electrodeposition paint layer may be, for example, 10 to 40 ⁇ 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 steel sheet having a metal structure including martensite and useful for realizing a surface texture of a panel in which, when formed by press forming or the like, the surface texture aspect ratio Str of the steel sheet in the flat portion of the central portion is 0.50 to 1.00 and the surface roughness parameter Sa of the steel sheet in the flat portion of the central portion and at the end portion is 0.50 ⁇ m or less will be described in detail.
• these descriptions are intended to merely exemplify preferred steel sheets for constituting a panel according to an embodiment of the present invention, and are not intended to limit the present invention to an embodiment using such a specific steel sheet.
• the steel plate is provided at a flat portion of the center portion of the panel.
• Ferrite 75-95%
• martensite 5 to 25%
• at least one of bainite, pearlite, and retained austenite 0 to 10% in total
• the average grain spacing of martensite is 2.5 ⁇ m or less
• the steel sheet is characterized by having a metal structure in which the standard deviation in the area ratio of martensite in the direction perpendicular to the rolling direction and the plate thickness direction is 1.5% or less.
• Mn-enriched regions such as central segregation and microsegregation are formed during casting, and the enriched regions are stretched in the rolling direction by hot rolling or cold rolling, causing Mn to segregate in a streaky manner.
• Mn there are regions with high and low hardenability in the steel sheet.
• a relatively large amount of striped hard structures are generated in the metal structure of the steel sheet after quenching.
• the occurrence of ghost lines is particularly noticeable.
• the Mn segregation in the steel sheet can be sufficiently suppressed, the generation of such striped hard structures can be reduced and the hard structures can be more uniformly dispersed in the metal structure.
• the inventors therefore investigated means for optimizing the ratio of ferrite, which is a soft structure, and martensite, which is a hard structure, in the metal structure to achieve the desired high strength, while also improving the appearance after forming.
• the inventors focused on the distribution state of martensite, which is a hard structure in the metal structure, and more specifically, investigated controlling the distribution of martensite from a viewpoint different from that of reducing Mn segregation.
• the inventors discovered that by forming the metal structure in the steel sheet before final annealing with a structure mainly composed of bainite and/or martensite, and then final annealing the steel sheet having such a metal structure under specified conditions, it is possible to uniformly disperse martensite in both the microregion and the macroregion in the finally obtained metal structure, without necessarily depending on the presence or absence or the degree of Mn segregation.
• the inventors have found that by subjecting a steel sheet having a metal structure consisting of bainite and/or martensite to final annealing under predetermined conditions, the average grain spacing of martensite can be controlled to 2.5 ⁇ m or less in the micro region, and the standard deviation of the area ratio of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction can be controlled to 1.5% or less in the macro region.
• the average grain spacing of martensite can be densely and uniformly dispersed in the micro region.
• the deformation amount of the steel sheet can be made more uniform, particularly in the width direction, even during forming such as press forming, and in this regard, by realizing the surface properties of the 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 martensite structure has substructures such as packets, blocks, and laths in the prior austenite grains, and therefore has many different interfaces inside compared to structures such as ferrite. Bainite is also a structure that has many different interfaces inside, similar to the case of martensite.
• good formability is ensured by controlling the area ratio of ferrite, which is a soft structure, to 75-95%, while the area ratio of martensite, which is a hard structure, to 5-25%, and further controlling the chemical composition of the steel sheet within a predetermined range, thereby making it possible to reliably achieve high strength, for example, high strength with a tensile strength of 400 MPa or more. As a result, it is possible to achieve both high strength and appearance after forming into a panel at a high level.
• the steel sheet according to the preferred embodiment of the present invention has a metal structure in which martensite is finely and uniformly dispersed throughout, as described above, in the paint baking process after forming into a panel, the amount of bake hardening can be significantly increased due to the uniform dispersion of martensite containing a relatively large number of dislocations. Therefore, by using this steel sheet, the yield stress of the obtained panel can be significantly increased, which is also very advantageous from the viewpoint of improving the dent resistance of the panel.
• the structure fraction is expressed as an area percentage, so the unit "%" for the structure fraction means area %.
• the metal structure of the steel sheet refers to the metal structure of the steel sheet in the flat portion in the center part of the panel. The degree of forming is low in this flat portion, and therefore the characteristics of the metal structure shown below do not change significantly before and after forming such as press forming.
• ferrite Since ferrite is a soft structure, it is easily deformed and contributes to improving elongation. When the area ratio of ferrite is 75% or more, sufficient formability can be obtained. From the viewpoint of improving formability, the higher the area ratio of ferrite, the more preferable it is, and it may be, for example, 78% or more, 80% or more, 82% or more, or 85% or more. On the other hand, if ferrite is contained excessively, the desired strength may not be achieved in the steel plate. Therefore, the area ratio of ferrite is 95% or less. The area ratio of ferrite may be 93% or less, 90% or less, or 87% or less.
• Martensite is a structure with high dislocation density and hardness, which contributes to improving tensile strength.
• the area ratio of martensite By setting the area ratio of martensite to 5% or more, it is possible to achieve a tensile strength of, for example, 400 MPa or more. From the viewpoint of improving strength, the higher the area ratio of martensite, the more preferable it is, and it may be, for example, 7% or more, 10% or more, or 13% or more.
• the area ratio of martensite is 25% or less, it is possible to ensure formability and appearance.
• the area ratio of martensite may be 22% or less, 20% or less, 18% or less, or 15% or less.
• the remaining structure other than ferrite and martensite may be 0% in area ratio, but when the remaining structure exists, the remaining structure is at least one of bainite, pearlite, and retained austenite.
• the area ratio of the remaining structure i.e., at least one of bainite, pearlite, and retained austenite, may be 10% or less in total, for example, 9% or less, 8% or less, 7% or less, 6% or less, 5% or less, 4% or less, or 2% or less.
• the area ratio of the remaining structure 0% in order to make the area ratio of the remaining structure 0%, a high level of control is required in the manufacturing process of the steel plate, which may lead to a decrease in yield. Therefore, the area ratio of the remaining structure may be 0.5% or more, or 1% or more.
• Identification of the metal structure and calculation of the area ratio are performed by FE-SEM (field emission scanning electron microscope) and optical microscope after corrosion using Nital reagent or LePeller solution, and X-ray diffraction method.
• the structure observation by FE-SEM and optical microscope is performed at a magnification of 1000 to 50000 times for a 100 ⁇ m x 100 ⁇ m area in the steel plate cross section in the direction perpendicular to the plate surface.
• three measurement points are set, and the area ratio is determined by calculating the average value of the measured values.
• the ferrite area fraction is determined by observing a 100 ⁇ m x 100 ⁇ m region in the range of 1/8 to 3/8 of the plate thickness, centered at 1/4 of the plate thickness, in an electron channeling contrast image taken with a FE-SEM (field emission scanning electron microscope). More specifically, the image analysis software Image J is used to binarize ferrite and martensite based on differences in brightness, and the ferrite area fraction can be calculated. When Repeler liquid is used, the black parts of the image data are ferrite, and the white parts are martensite and retained austenite.
• the area ratio of martensite is determined using the following procedure. First, the observation surface of the sample is etched with LePeller solution, and then a 100 ⁇ m x 100 ⁇ m area is observed with an FE-SEM within the range of 1/8 to 3/8 of the plate thickness, centered at 1/4 of the plate thickness. In LePeller corrosion, martensite and retained austenite are not corroded, so the area ratio of the uncorroded area corresponds to the total area ratio of martensite and retained austenite. The area ratio of martensite is calculated by subtracting the area ratio of retained austenite, measured by the X-ray diffraction method described later, from the area ratio of this uncorroded area.
• the area fraction of retained austenite is calculated by X-ray diffraction. First, the sample is mechanically and chemically polished from the plate surface to a depth of 1/4 in the plate thickness direction. Next, at the 1/4 plate thickness position, the structural fraction of retained austenite is calculated from the integrated intensity ratio of the diffraction peaks of (200) and (211) of the bcc phase and (200), (220) and (311) of the fcc phase obtained using MoK ⁇ radiation. The general five-peak method is used for this calculation. The calculated structural fraction of retained austenite is determined as the area fraction of retained austenite.
• bainite and calculation of the area ratio are carried out as follows. First, the observation surface of the sample is corroded with Nital reagent, and then a 100 ⁇ m x 100 ⁇ m region within the range of 1/8 to 3/8 of the plate thickness, centered at 1/4 of the plate thickness, is observed with an FE-SEM. From the position of cementite contained inside the structure in this observation region and the arrangement of cementite, bainite is identified as follows. Bainite is classified into upper bainite and lower bainite, and in upper bainite, cementite or retained austenite exists at the interface of lath-shaped bainitic ferrite.
• lower bainite cementite exists inside lath-shaped bainitic ferrite, there is one type of crystal orientation relationship between bainitic ferrite and cementite, and cementite has the same variant. Based on these characteristic points, upper bainite and lower bainite can be identified. In the present invention, these are collectively called bainite, and the area ratio of identified bainite is calculated based on image analysis.
• the identification of pearlite and calculation of the area ratio are carried out using the following procedure. First, the observation surface of the sample is corroded with Nital reagent, and then the area between 1/8 and 3/8 of the plate thickness, centered at 1/4 of the plate thickness, is observed with an optical microscope at a magnification of 500 or 1000 times. Areas of dark contrast in the image observed with the optical microscope are identified as pearlite, and the area ratio of this area is calculated based on image analysis.
• the average particle spacing of the martensite which is a hard structure
• the average particle spacing of the martensite is an index that indicates the uniformity of the hard structure distribution in the micro region. The smaller the average particle spacing of the martensite, the more densely and uniformly the hard structure is dispersed, and therefore the higher the uniformity. The more uniform the deformation amount of the steel sheet during press forming is, particularly in the width direction of the steel sheet, the better the appearance after press forming.
• the deformation amount of the steel sheet is strongly affected by the distribution state of the hard structure, in order to make the deformation amount of the steel sheet uniform in the width direction of the steel sheet, it is necessary to make the distribution of the hard structure in the metal structure uniform.
• the deformation amount of the steel sheet can be made more uniform in the width direction even during forming such as press forming, and as a result, a good post-forming appearance can be achieved.
• the average grain spacing of martensite is preferably 2.4 ⁇ m or less, more preferably 2.2 ⁇ m or less, and most preferably 2.0 ⁇ m or less or 1.8 ⁇ m or less. Although there is no particular lower limit, for example, the average grain spacing of martensite may be 0.5 ⁇ m or more, 0.8 ⁇ m or more, or 1.0 ⁇ m or more.
• the average grain spacing of martensite is determined as follows. First, a sample having a steel plate cross section perpendicular to the plate surface is taken, and the cross section is used as the observation surface. A region of 100 ⁇ m ⁇ 100 ⁇ m within the range of 1/8 to 3/8 of the plate thickness centered at 1/4 of the plate thickness is used as the observation region of this observation surface, and martensite is identified using FE-SEM. Specifically, ferrite and martensite are binarized based on the difference in brightness using image analysis software Image J, and martensite is identified.
• the black part of the image data is ferrite, and the white part that is not corroded by LePeller is the total structure of martensite and retained austenite.
• the area ratio of retained austenite is sufficiently low compared to the area ratio of martensite, so that the white structure can be regarded as martensite.
• the distance between the centers (centers of gravity) of all adjacent martensite grains among the identified martensite is calculated as the particle spacing based on image analysis, and the average value of the calculated particle spacings is determined as the average particle spacing of martensite (strictly speaking, particles containing martensite and/or retained austenite).
• Standard deviation in area ratio of martensite in the direction perpendicular to the rolling direction and the plate thickness direction is 1.5% or less
• the standard deviation in the area ratio of martensite in the direction perpendicular to the rolling direction and the plate thickness direction is controlled to 1.5% or less.
• the standard deviation is an index representing the uniformity of the hard structure in the macro region. The appearance, which is an issue during press forming, depends on the minute irregularities on the steel sheet surface caused by the difference in the amount of deformation in the width direction of the steel sheet.
• the standard deviation in the area ratio of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction is preferably 1.4% or less, more preferably 1.2% or less, and most preferably 1.0% or less.
• the lower limit is not particularly limited, but the standard deviation may be, for example, 0.1% or more, 0.3% or more, or 0.5% or more.
• the standard deviation in the area ratio of martensite in the direction perpendicular to the rolling direction and the plate thickness direction is determined as follows. First, a metal structure image of a steel plate cross section in a region of 50 mm in the direction perpendicular to the rolling direction and the plate thickness direction is obtained. In the case of an image of 10 mm or smaller, multiple images may be obtained and joined to make 50 mm.
• the cross section is observed at 0°, 45°, 90°, and 135° with respect to an arbitrary direction, and the cross section with the highest aspect ratio of the precipitates among them is determined as the cross section parallel to the rolling direction, and the direction perpendicular to it is determined as the direction perpendicular to the rolling direction and the plate thickness direction.
• the obtained image is divided into 100 ⁇ m (0.1 mm) in the direction perpendicular to the rolling direction, and the area ratio of martensite in the entire plate thickness is calculated for each divided range.
• the standard deviation in the area ratio of martensite is calculated. This operation is carried out for three regions at different positions in the rolling direction, and the average of the standard deviations found for each is determined as the standard deviation in the area ratio of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction.
• the following method is used, for example, to identify the rolling direction of the 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 in the range of 100 ⁇ m (thickness direction) x 500 ⁇ m (perpendicular to the thickness direction) at the center of the plate thickness.
• the extended area with a high S concentration is determined to be an inclusion such as MnS. Observation may be performed in multiple fields of view.
• a surface parallel to the surface rotated in 5° increments in the range of 0° to 180° around the thickness direction is observed by the above method.
• the average value of the length of the major axis of the multiple inclusions in each cross section obtained is calculated for each cross section, and the cross section in which the average value of the length of the major axis of the inclusions is the largest is identified.
• the direction parallel to the major axis of the inclusions in that cross section is determined to be the rolling direction.
• the average grain size of the ferrite in the metal structure is 3.0 to 25.0 ⁇ m.
• the average grain size of the ferrite may be 5.0 ⁇ m or more, 7.0 ⁇ m or more, 8.0 ⁇ m or more, 9.0 ⁇ m or more, or 10.0 ⁇ m or more.
• the average grain size of the ferrite may be 22.0 ⁇ m or less, 20.0 ⁇ m or less, 16.0 ⁇ m or less, 14.0 ⁇ m or less, or 12.0 ⁇ m or less.
• the average grain size of ferrite in the steel plate is determined as follows. First, 10 fields of view are observed at a magnification of 500x or 1000x in the region from the surface of the steel plate etched with LePeller's reagent to the 1/2 position in the plate thickness direction, and image analysis is performed using image analysis software "Photoshop (registered trademark) CS5" manufactured by Adobe, and the area fraction of ferrite and the number of ferrite particles in each field of view are calculated. Next, the area fraction of ferrite and the number of ferrite particles in the 10 fields of view are summed, and the total area fraction of ferrite is divided by the total number of ferrite particles to calculate the average area fraction per ferrite particle.
• the circle equivalent diameter is calculated, and the obtained circle equivalent diameter is determined as the average grain size of ferrite.
• the length in the plate thickness direction is reduced to ensure an observation area of 37,500 ⁇ m 2. The same applies to the method of determining the average grain size of martensite and the average aspect ratio of martensite.
• the average grain size of martensite in the metal structure is 1.0 to 5.0 ⁇ m.
• the average grain size of martensite may be 1.2 ⁇ m or more, 1.5 ⁇ m or more, 1.7 ⁇ m or more, or 2.0 ⁇ m or more.
• the average grain size of martensite may be 4.7 ⁇ m or less, 4.5 ⁇ m or less, 4.2 ⁇ m or less, 4.0 ⁇ m or less, 3.8 ⁇ m or less, 3.6 ⁇ m or less, or 3.4 ⁇ m or less.
• the average grain size of martensite is determined as follows. First, 10 visual fields are observed at a magnification of 500x or 1000x in the region from the surface of the steel plate etched with LePeller's reagent to the 1/2 position in the thickness direction, and image analysis is performed using image analysis software "Photoshop (registered trademark) CS5" manufactured by Adobe, and the area fraction of martensite and the number of martensite particles in each visual field are calculated. Next, the area fraction of martensite and the number of martensite particles in the 10 visual fields are summed, and the total area fraction of martensite is divided by the total number of martensite particles to calculate the average area fraction per martensite particle.
• the average aspect ratio of martensite in the metal structure is 2.5 or more.
• the average aspect ratio of martensite may be 2.6 or more, 2.8 or more, or 3.0 or more.
• the upper limit is not particularly limited, but for example, the average aspect ratio of martensite may be 6.0 or less, 5.0 or less, 4.0 or less, 3.8 or less, or 3.6 or less.
• the present invention aims to provide a panel that has excellent appearance after forming even if it has high strength, and achieves this objective by including martensite in the metal structure of the steel sheet constituting the panel, and controlling the surface quality of the panel after forming using two different parameters, the surface quality aspect ratio Str and the surface roughness parameter Sa, so that Str is in the range of 0.50 to 1.00 and Sa is in the range of 0.50 ⁇ m or less. Therefore, it is clear that the chemical composition of the steel sheet itself is not an essential technical feature for achieving the object of the present invention.
• the steel plate comprises, in mass%, C: 0.030-0.100%, Si: 0.005-1.500%, Mn: 0.70-3.00%, P: 0.1000% or less, S: 0.0200% or less, Al: 1.000% or less, N: 0.0200% or less, O: 0.0100% or less, Nb: 0 to 0.400%, Cr: 0-1.00%, Mo: 0 to 0.80%, B: 0 to 0.0100%, Ti: 0-0.200%, V: 0 to 0.500%, Ni: 0 to 1.00%, Cu: 0 to 1.00%, W: 0-1.00%, Ta: 0-0.10%, Co: 0-3.00%, Sn: 0-1.00%, Sb: 0 to 0.200%, Ca: 0-0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, REM: 0-0.0100%, Bi: 0-0.0500%, It is preferable that the chemical
• C is an element that secures a predetermined amount of martensite and improves the strength of the steel sheet.
• the C content is set to 0.030% or more.
• the C content may be 0.040% or more or 0.050% or more.
• the C content is set to 0.100% or less.
• the C content may be 0.090% or less, 0.080% or less, 0.079% or less, 0.078% or less, 0.076% or less, 0.074% or less, 0.072% or less, 0.070% or less, or 0.060% or less.
• Si is an element that improves the strength of steel sheet by solid solution strengthening.
• the Si content is set to 0.005% or more.
• the Si content may be 0.010% or more, 0.100% or more, 0.200% or more, 0.300% or more, or 0.400% or more.
• the Si content is set to 1.500% or less.
• the Si content may be 1.200% or less, 1.000% or less, 0.800% or less, 0.700% or less, or 0.600% or less.
• Mn is an element that improves hardenability and contributes to improving the strength of the steel sheet.
• the Mn content is set to 0.70% or more.
• the Mn content may be 0.80% or more, 1.00% or more, 1.20% or more, or 1.50% or more.
• the Mn content is set to 3.00% or less.
• the Mn content may be 2.80% or less, 2.50% or less, 2.20% or less, or 2.00% or less.
• P is an impurity element that embrittles welds and deteriorates galvanic properties. Therefore, the P content is set to 0.1000% or less.
• the P content may be 0.0600% or less, 0.0400% or less, 0.0200% or less, or 0.0100% or less.
• the P content may be 0.0001% or more, 0.0002% or more, or 0.0005% or more.
• S is an impurity element that impairs weldability and also impairs manufacturability during casting and hot rolling. Therefore, the S content is set to 0.0200% or less.
• the S content may be 0.0150% or less, 0.0120% or less, 0.0100% or less, 0.0060% or less, or 0.0030% or less.
• the S content may be 0.0001% or more, 0.0002% or more, or 0.0005% or more.
• Al is an element that functions as a deoxidizer and is effective in increasing the strength of steel.
• the Al content may be 0%, but in order to fully obtain these effects, the Al content is preferably 0.001% or more.
• the Al content may be 0.005% or more, 0.010% or more, 0.025% or more, or 0.050% or more.
• the Al content is set to 1.000% or less.
• the Al content may be 0.800% or less, 0.600% or less, or 0.300% or less.
• N is an element that causes blowholes during welding. Therefore, the N content is set to 0.0200% or less.
• the N content may be 0.0180% or less, 0.0150% or less, 0.0100% or less, 0.0080% or less, or 0.0060% or less.
• the N content may be 0.0001% or more, 0.0002% or more, or 0.0005% or more.
• O is an element that causes blowholes during welding. Therefore, the O content is set to 0.0100% or less.
• the O content may be 0.0080% or less, 0.0050% or less, 0.0030% or less, or 0.0020% or less.
• the O content may be 0.0001% or more, 0.0002% or more, or 0.0005% or more.
• the steel plate 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 the properties.
• the steel sheet may contain at least one of Nb: 0-0.400%, Cr: 0-1.00%, Mo: 0-0.80%, B: 0-0.0100%, Ti: 0-0.200%, V: 0-0.500%, Ni: 0-1.00%, Cu: 0-1.00%, W: 0-1.00%, Ta: 0-0.10%, Co: 0-3.00%, Sn: 0-1.00%, Sb: 0-0.200%, Ca: 0-0.0100%, Mg: 0-0.0100%, Zr: 0-0.0100%, REM: 0-0.0100%, Bi: 0-0.0500%, and As: 0-0.10%.
• These optional elements will be described in detail below.
• Nb is an element effective in controlling the morphology of carbides, and is also an element effective in refining the structure and improving the toughness of the steel plate. These effects can be obtained even with a small amount.
• the Nb content may be 0%, but in order to obtain the above effects, the Nb content is preferably 0.001% or more.
• the Nb content may be 0.005% or more or 0.010% or more.
• the Nb content is preferably 0.400% or less.
• the Nb content may be 0.200% or less, 0.100% or less, or 0.060% or less.
• Cr is an element that improves hardenability and contributes to improving the strength of the steel sheet, similar to Mn.
• the Cr content may be 0%, but in order to obtain the above effect, the Cr content is preferably 0.001% or more.
• the Cr content may be 0.01% or more, 0.10% or more, or 0.20% or more.
• the Cr content is preferably 1.00% or less, and may be 0.80% or less, 0.60% or less, or 0.40% or less.
• Mo is an element that contributes to improving the high-temperature strength of the steel sheet. This effect can be obtained even with a small amount.
• the Mo content may be 0%, but in order to obtain the above effect, the Mo content is preferably 0.001% or more.
• the Mo content may be 0.01% or more, 0.02% or more, 0.05% or more, or 0.10% or more.
• the Mo content is preferably 0.80% or less.
• the Mo content may be 0.60% or less, 0.50% or less, 0.40% or less, or 0.20% or less.
• B is an element that suppresses the formation of ferrite and pearlite during the cooling process from austenite and promotes the formation of martensite.
• B is an element that is beneficial for increasing the strength of steel. These effects can be obtained even with a small amount.
• the B content may be 0%, but in order to obtain the above effects, the B content is preferably 0.0001% or more.
• the B content may be 0.0005% or more or 0.0010% or more.
• the B content is preferably 0.0100% or less.
• the B content may be 0.0080% or less, 0.0050% or less, 0.0030% or less, or 0.0020% or less.
• Ti is an element effective in controlling the morphology of carbides. Ti can promote an increase in the strength of ferrite.
• the Ti content may be 0%, but in order to obtain these effects, the Ti content is preferably 0.001% or more.
• the Ti content may be 0.002% or more, 0.010% or more, 0.020% or more, or 0.040% or more.
• the Ti content is preferably 0.200% or less, and may be 0.100% or less, 0.080% or less, or 0.050% or less.
• V is an element effective in controlling the morphology of carbides, and is also effective in refining the structure to improve the toughness of the steel plate.
• the V content may be 0%, but in order to obtain the above effect, the V content is preferably 0.001% or more.
• the V content may be 0.005% or more, 0.010% or more, or 0.050% or more.
• the V content is preferably 0.500% or less.
• the V content may be 0.400% or less, 0.200% or less, or 0.100% or less.
• Ni, Cu and W are elements effective in improving the strength of the steel plate.
• the Ni, Cu and W contents may be 0%, but in order to obtain such effects, the Ni, Cu and W contents are preferably 0.001% or more, and may be 0.01% or more or 0.05% or more.
• the Ni, Cu and W contents are preferably 1.00% or less, and may be 0.80% or less, 0.40% or less, or 0.20% or less.
• Ta is an element effective in controlling the morphology of carbides and improving the strength of steel sheets.
• the Ta content may be 0%, but in order to obtain these effects, the Ta content is preferably 0.001% or more.
• the Ta content may be 0.01% or more or 0.03% or more.
• the Ta content is preferably 0.10% or less.
• the Ta content may be 0.08% or less, 0.06% or less, or 0.04% or less.
• Co is an element effective in improving the strength of steel sheet.
• the Co content may be 0%, but in order to obtain such an effect, the Co content is preferably 0.001% or more.
• the Co content may be 0.01% or more, 0.05% or more, or 0.10% or more.
• the Co content is preferably 3.00% or less.
• the Co content may be 2.00% or less, 1.00% or less, 0.50% or less, or 0.20% or less.
• Sn is an element that may be contained in the steel sheet when scrap is used as the raw material of the steel sheet. In addition, Sn may cause embrittlement of ferrite. Therefore, the lower the Sn content, the more preferable, and it is preferably 1.00% or less.
• the Sn content may be 0.10% or less, 0.040% or less, or 0.02% or less.
• the Sn content may be 0%, but reducing the Sn content to less than 0.001% will lead to an excessive increase in refining costs. Therefore, the Sn content may be 0.001% or more, 0.005% or more, or 0.01% or more.
• Sb is an element that may be contained in a steel sheet when scrap is used as a raw material for the steel sheet.
• Sb may strongly segregate at grain boundaries, which may lead to embrittlement of the grain boundaries.
• the Sb content may be 0.100% or less, 0.040% or less, or 0.020% or less.
• the Sb content may be 0%, but reducing the Sb content to less than 0.001% will lead to an excessive increase in refining costs. For this reason, the Sb content may be 0.001% or more, 0.005% or more, or 0.010% or more.
• Ca, Mg, Zr and REM are elements that contribute to improving the formability of the steel sheet.
• the Ca, Mg, Zr and REM contents may be 0%, but in order to obtain such effects, the Ca, Mg, Zr and REM contents are preferably 0.0001% or more, and may be 0.0005% or more, 0.0010% or more, or 0.0015% or more.
• the ductility of the steel sheet may decrease.
• the Ca, Mg, Zr and REM contents are preferably 0.0100% or less, and may be 0.0080% or less, 0.0060% or less, 0.0040% or less, or 0.0020% or less.
• REM is a collective term for 17 elements: scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanides lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71, and the REM content is the total content of these elements.
• Bi is an element that has the effect of improving formability by refining the solidification structure.
• the Bi content may be 0%, but in order to obtain such an effect, the Bi content is preferably 0.0001% or more, and may be 0.0005% or more, 0.0010% or more, or 0.0030% or more.
• the Bi content is preferably 0.0500% or less, and may be 0.0400% or less, 0.0200% or less, 0.0100% or less, or 0.0050% or less.
• As is an element that may be contained in a steel sheet when scrap is used as a raw material for the steel sheet.
• As is an element that strongly segregates at grain boundaries, and the lower the As content, the better.
• the As content is preferably 0.10% or less, and may be 0.04% or less or 0.02% or less.
• the As content may be 0%, but reducing the As content to less than 0.001% leads to an excessive increase in refining costs. For this reason, the As content may be 0.001% or more, 0.005% or more, or 0.01% or more.
• the remainder excluding the above elements consists of Fe and impurities.
• Impurities are elements that are mixed in from the steel raw materials and/or during the steelmaking process, and whose presence is permitted to the extent that they do not impair the properties of the steel plate according to the embodiment of the present invention.
• the chemical composition of the steel plate according to the embodiment of the present invention may be measured by a general analytical method.
• the chemical composition of the steel plate may be measured using inductively coupled plasma atomic emission spectrometry (ICP-AES) based on a test piece taken from the flat part of the center part 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.
• the steel sheet according to the embodiment of the present invention may be a cold-rolled steel sheet, but may further include a plating layer on the surface for the purpose of improving corrosion resistance or the like.
• the plating layer may be either a hot-dip plating layer or an electroplating layer. That is, the steel sheet according to the embodiment of the present invention may be a cold-rolled steel sheet having a hot-dip plating layer or an electroplating layer on its surface.
• the hot-dip plating layer includes, for example, a hot-dip galvanized layer (GI), a hot-dip 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, a hot-dip Zn-Al-Mg-Si alloy plating layer, and the like.
• the electroplating layer includes, for example, an electrogalvanized layer (EG), an electrogalvanized Zn-Ni alloy plating layer, and the like.
• the plating layer is a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, or an electrogalvanized layer.
• the coating weight of the plating layer is not particularly limited and may be a general coating weight.
• the flat portion of the central side of the steel plate and the corresponding panel has a plate thickness of, for example, 0.2 to 2.0 mm, but is not particularly limited thereto.
• the plate thickness may be 0.3 mm or more or 0.4 mm or more.
• the plate thickness may be 1.8 mm or less, 1.5 mm or less, 1.2 mm or less, 1.0 mm or less, or 0.8 mm or less.
• the plate thickness 0.2 mm or more it becomes easier to maintain the shape of the molded product flat, and an additional effect of improving the dimensional accuracy and shape accuracy can be obtained.
• the plate thickness 0.8 mm or less the weight reduction effect of the member becomes remarkable.
• the plate thickness of the steel plate or panel is measured by a micrometer.
• a high tensile strength specifically a tensile strength of 400 MPa or more can be achieved.
• the tensile strength is preferably 440 MPa or more or 490 MPa or more, more preferably 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 tensile test piece No. 5 of JIS Z2241:2022 from the steel plate at the flat part of the center side part of the panel and performing a tensile test in accordance with JIS Z2241:2022.
• yield stress: YS According to the panel having the above characteristics, a high yield stress, more specifically, a yield stress of 300 MPa or more can be achieved when paint baking is performed.
• the yield stress after paint baking is preferably 350 MPa or more or 400 MPa or more, more preferably 490 MPa or more or 540 MPa or more.
• the upper limit is not particularly limited, but for example, the yield stress after paint baking may be 850 MPa or less, 800 MPa or less, or 750 MPa or less.
• the yield stress is measured by taking a No.
• the panels according to the embodiments of the present invention can achieve high strength, specifically a tensile strength of 400 MPa or more, yet maintain an excellent appearance after molding such as press molding. For this reason, the panels according to the embodiments of the present invention are very useful for use as exterior panel parts such as roofs, hoods, fenders, and doors, which require high design quality in automobiles.
• the method for producing a steel sheet according to an embodiment of the present invention includes: a hot rolling process comprising: heating a slab having the chemical composition described above in relation to the steel plate to a temperature of 1100-1400°C, finish rolling it and then coiling it at a temperature of 500-700°C, the end temperature of said finish rolling being 800-1350°C; a pickling step of pickling the obtained hot-rolled steel sheet; a cold rolling process in which the pickled hot-rolled steel sheet is cold-rolled at a rolling reduction of 20 to 90%; A step of subjecting the obtained cold-rolled steel sheet to primary annealing, the primary annealing includes heating the cold-rolled steel sheet and holding it at a maximum heating temperature of Ac3 to 950 ° C.
• a step of subjecting the cold-rolled steel sheet after the primary annealing to secondary annealing, the secondary annealing includes heating the cold-rolled steel sheet and holding it at a maximum heating temperature of (Ac1 + 20) to 820 ° C. for 10 to 500 seconds, then controlling the average cooling rate in the temperature range of 500 to 700 ° C. to 10 ° C./second or more, and further controlling the average cooling rate in the temperature range of 200 to 500 ° C. to 40 ° C./second or more.
• Each step will be described in more detail below.
• a slab having the chemical composition described above in relation to the steel plate is heated.
• the slab used is preferably cast by a continuous casting method from the viewpoint of productivity, but may be manufactured by an ingot casting method or a thin slab casting method.
• the slab used contains a relatively large amount of alloying elements in order to obtain a high-strength steel plate. For this reason, it is necessary to heat the slab before subjecting it to hot rolling to dissolve the alloying elements in the slab. If the heating temperature is less than 1100°C, the alloying elements may not be sufficiently dissolved in the slab, leaving coarse alloy carbides, which may cause embrittlement cracking during hot rolling. For this reason, the heating temperature is preferably 1100°C or higher.
• the upper limit of the heating temperature is not particularly limited, but is preferably 1400°C or lower from the viewpoint of the capacity and productivity of the heating equipment.
• the heated slab may be subjected to rough rolling before finish rolling in order to adjust the plate thickness, etc.
• the conditions of the rough rolling are not particularly limited as long as the desired sheet bar dimensions can be secured.
• the heated slab or the slab that has been rough-rolled as necessary is then subjected to finish rolling. Since the slab used as described above contains a relatively large amount of alloying elements, it is necessary to increase the rolling load during hot rolling. For this reason, it is preferable to perform hot rolling at a high temperature.
• the end temperature of the finish rolling is important in terms of controlling the metal structure of the steel sheet. If the end temperature of the finish rolling is low, the metal structure may become non-uniform and the formability may decrease. For this reason, the end temperature of the finish rolling is 800°C or higher. On the other hand, in order to suppress the coarsening of austenite, the end temperature of the finish rolling is 1350°C or lower.
• the end temperature of the finish rolling is preferably 950 to 1050°C.
• the finish-rolled hot-rolled steel sheet is coiled at a coiling temperature of 500 to 700°C.
• the coiling temperature is preferably 550 to 630°C.
• the obtained hot-rolled steel sheet is pickled to remove the oxide scale formed on the surface of the hot-rolled steel sheet.
• the pickling may be performed once or may be performed multiple times to ensure the removal of the oxide scale, as long as the pickling is performed under conditions suitable for removing the oxide scale.
• the pickled hot-rolled steel sheet is cold-rolled at a reduction of 20 to 90% in the cold rolling process.
• the reduction of the cold rolling is preferably 70 to 90%.
• the number of rolling passes and the reduction of each pass are not particularly limited, and may be appropriately set so that the reduction of the entire cold rolling is within the above range.
• the obtained cold-rolled steel sheet is heated in the next primary annealing step, held at a maximum heating temperature of Ac3 to 950 ° C for 10 to 500 seconds, and then cooled to a cooling stop temperature of 350 ° C or less by controlling the average cooling rate in the temperature range of 500 to 700 ° C to 40 ° C / sec or more.
• the Ac3 point ( ° C) is obtained from the thermal expansion of a small piece cut from the cold-rolled steel sheet during heating from room temperature to 1000 ° C at 10 ° C / sec.
• the metal structure in the steel sheet after cooling can be reliably composed of a structure mainly composed of bainite and / or martensite, for example, full bainite or full martensite.
• the structure mainly composed of bainite and/or martensite refers to a structure containing at least one of bainite and martensite in a total area ratio of 90% or more
• full bainite refers to a structure composed of 100% bainite in area ratio
• full martensite refers to a structure composed of 100% martensite in area ratio.
• Bainite and/or martensite structures have many different interfaces inside compared to structures such as ferrite. Therefore, by forming the metal structure of the steel sheet before the secondary annealing process, i.e., the final annealing process, from bainite and/or martensite, it becomes possible to disperse and generate a very large number of carbides that can become nucleation sites of austenite on these interfaces during the heating of such a metal structure in the secondary annealing process.
• austenite is generated finely and uniformly throughout the steel sheet from the nucleation sites dispersed in such a large number, and then martensite is generated from the austenite, so that in the metal structure obtained after the secondary annealing, the average grain spacing of martensite is controlled to 2.5 ⁇ m or less, and the standard deviation in the area ratio of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction is controlled to 1.5% or less.
• the maximum heating temperature in the first annealing process is below the Ac3 point or the holding time is less than 10 seconds, austenitization will be insufficient, and the metal structure in the steel sheet will not be composed of a structure mainly composed of bainite and/or martensite even after subsequent cooling. In other words, the total area ratio of bainite and martensite cannot be made 90% or more.
• the maximum heating temperature in the first annealing process is set to 950°C or less and the holding time is set to 500 seconds or less.
• the maximum heating temperature is preferably 870 to 950°C, and the holding time is preferably 50 to 100 seconds.
• the average cooling rate in the temperature range of 500 to 700°C in the first annealing process is less than 40°C/sec or the cooling stop temperature exceeds 350°C, ferrite will be generated during cooling, and the total area ratio of bainite and martensite in the metal structure of the steel sheet cannot be made 90% or more. Therefore, the average cooling rate must be 40°C/sec or more, and is preferably 70°C/sec.
• the upper limit is preferably 300°C/sec or less, and more preferably 150°C/sec or less.
• the lower limit of the cooling stop temperature is not particularly limited, and for example, the cooling stop temperature may be room temperature (25°C) or higher, and is preferably 200°C or higher. Similarly, the cooling stop temperature is preferably 300°C or lower.
• carbides can be dispersed and generated on many interfaces contained inside the bainite and/or martensite in the metal structure.
• austenite can be generated finely and uniformly from the carbides throughout the steel sheet while maintaining the state in which the carbides are dispersed on the interface.
• martensite can be appropriately generated from the finely dispersed austenite, and as a result, the average particle spacing of martensite is controlled to 2.5 ⁇ m or less, and the standard deviation in the area ratio of martensite in the direction perpendicular to the rolling direction and the plate thickness direction is controlled to 1.5% or less. In other words, it is possible to achieve a metal structure in which martensite is uniformly dispersed in both micro and macro regions.
• the maximum heating temperature in the secondary annealing process is less than Ac1+20°C or the holding time is less than 10 seconds, the desired metal structure as described above cannot be obtained.
• the maximum heating temperature exceeds 820°C, the area ratio of austenite becomes too high, and the area ratio of ferrite cannot be made 75% or more.
• the holding time exceeds 500 seconds, the austenite grains become coarse, and the martensite grains obtained by the subsequent cooling are also relatively coarse.
• the maximum heating temperature is preferably 760 to 800°C, and the holding time is preferably 20 to 100 seconds.
• the average cooling rate in the temperature range of 500 to 700 ° C. in the secondary annealing process is less than 10 ° C. / sec, the transformation from austenite to bainite, etc. is promoted, and even if the subsequent cooling is performed appropriately, the desired amount of martensite may not be obtained. In this case, the desired strength cannot be achieved and / or uniform dispersion of martensite, especially in microscopic regions, cannot be achieved. Therefore, the average cooling rate in the temperature range of 500 to 700 ° C. must be 10 ° C. / sec or more, preferably 40 ° C. / sec or more, with an upper limit of, for example, 200 ° C. / sec or less, preferably 60 ° C. / sec or less.
• the average cooling rate in the temperature range of 200 to 500 ° C. is less than 40 ° C. / sec, the transformation from austenite to martensite cannot be promoted, and similarly, the production of other structures such as bainite increases. Therefore, the average cooling rate in the temperature range of 200 to 500°C must be 40°C/sec or more, preferably 50°C/sec or more, with an upper limit of, for example, 200°C/sec or less, preferably 80°C/sec or less.
• the above method produces a steel sheet according to an embodiment of the present invention by two annealing treatments including a primary annealing and a secondary annealing, but the steel sheet according to an embodiment of the present invention is not necessarily limited to one produced by such a method, and for example, it is also possible to produce it by a single annealing treatment. More specifically, by forming the metal structure of the steel sheet after the hot rolling process with full bainite or full martensite, it is possible to omit the primary annealing described above. However, in this case, it is necessary to appropriately control the cooling conditions and coiling temperature after hot rolling, and it is also important to control the reduction ratio in the subsequent cold rolling. This is because if the reduction ratio in cold rolling is high, recrystallization occurs during heating in the subsequent annealing process, and the metal structure formed in the hot rolling process cannot be maintained.
• a plating treatment may be applied to the surface of the obtained cold-rolled steel sheet.
• the plating treatment may be a treatment such as hot-dip plating, alloying hot-dip plating, or electroplating.
• the steel sheet may be subjected to hot-dip galvanizing treatment as the plating treatment, or the alloying treatment may be performed after the hot-dip galvanizing treatment.
• the specific conditions of the plating treatment and the alloying treatment are not particularly limited, and may be any appropriate conditions known to those skilled in the art.
• the temperature of the plate immersed in the plating bath is preferably in the range from a temperature 40°C lower than the hot-dip galvanizing bath temperature (hot-dip galvanizing bath temperature -40°C) to a temperature 50°C higher than the hot-dip galvanizing bath temperature (hot-dip galvanizing bath temperature +50°C).
• the hot-dip galvanizing layer is subjected to an alloying treatment, the steel sheet on which the hot-dip galvanizing layer is formed is preferably heated to a temperature range of 460 to 600°C, more preferably to a temperature range of 480 to 550°C.
• the method for manufacturing a panel according to an embodiment of the present invention includes the steps of: a blanking step of blanking the steel plate obtained above; a forming step of forming the blanked 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 steel plate obtained above is subjected to blanking processing in which the steel plate is cut to a predetermined size.
• the blanking processing can be performed by any suitable means known to those skilled in the art, such as punching with a press.
• the blanked steel plate (blank) is formed into a steel part in the next forming process.
• the forming process it is preferable to perform forming so that the rolling direction of the blank coincides with the bending direction in which the largest curvature is formed.
• the amount of strain imparted in the forming process must also be appropriately controlled. If the amount of strain is small, it does not necessarily have a detrimental effect on the appearance after forming, but the introduction of dislocations may be insufficient. In this case, the amount of bake hardening during painting is reduced and the yield stress cannot be sufficiently increased. As a result, the dent resistance of the final product is reduced. Therefore, from the viewpoint of improving dent resistance, it is preferable that the amount of strain imparted in the forming process is 2.0% or more in the flat part of the center part of the panel. On the other hand, the application of excessive strain increases the surface roughness parameter Sa of the flat part and/or edge of the final product, which results in a deterioration of the appearance after forming. Therefore, from the viewpoint of improving the appearance after forming, it is preferable that the amount of strain imparted in the forming process is 5.0% or less in the flat part of the center part of the panel.
• the formed steel parts are optionally painted in the next painting step, preferably by painting and baking.
• This painting includes, for example, three types of painting: electrocoating, intermediate coating, and top coating (base and clear coating).
• Water-based paint or solvent-based paint is used for the painting.
• electrocoating the steel parts are submerged in an electrocoating tank containing paint, and electrocoating is applied to the entire surface of the steel parts.
• intermediate coating the paint is sprayed from a spray nozzle onto the steel parts by a painting robot or manually by a worker, so that the intermediate coating is applied to the entire surface of the steel parts.
• top coating the paint is sprayed from a spray nozzle onto the steel parts by a painting robot or manually by a worker, so that the top coating is applied to the entire surface of the steel parts.
• a paint layer having a thickness of 60 to 200 ⁇ m.
• the paint baking treatment is a baking and drying treatment for baking a paint layer onto the steel part and for baking and hardening the steel part.
• the paint baking treatment may be performed after electrodeposition painting and before undercoat painting, between undercoat painting and undercoat painting which are performed multiple times, after undercoat painting and before topcoat painting, between topcoat painting and topcoat painting which are performed multiple times, or after topcoat painting.
• the temperature and time of the paint baking treatment are preferably controlled so that the drying parameter P, expressed by the following formula 1, is within the range of 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 1, for example, from the ranges of 100 to 220° C. and 20 to 60 minutes.
• P (T+273) ⁇ (17.7+log(t))...Formula 1
• T is the temperature of the paint baking process (° C.)
• t is the time of the paint baking process (seconds).
• it is preferable to control the total time of each paint baking process to be in the range of 20 to 60 minutes.
• the drying parameter P it is preferable to control the drying parameter P 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, the yield stress after bake hardening decreases due to excessive baking, and the dent resistance of the final product may also decrease.
• a panel manufactured by the above manufacturing method high strength is achieved by including martensite in the metal structure of the steel plate that constitutes the panel, while the surface properties of the panel are controlled 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, making it possible to significantly suppress the occurrence of appearance defects such as ghost lines on the panel surface even when distortion is imparted by molding such as press molding. Furthermore, when the panel is paint-baked, the yield stress can be significantly increased by bake hardening, improving the dent resistance of the panel. Therefore, panels manufactured by the above manufacturing method are particularly useful for use in the automotive field, where high strength, excellent post-molding appearance, and even excellent dent resistance are required.
• molten steel was cast by a continuous casting method to form slabs having various chemical compositions shown in Table 1, and these slabs were heated to a predetermined temperature of 1100 to 1400 ° C. and hot-rolled.
• Hot rolling was performed by rough rolling and finish rolling, and the finishing temperature and coiling temperature of the finish rolling were as shown in Table 2.
• the obtained hot-rolled steel sheet was pickled, and then cold-rolled at the rolling reduction shown in Table 2 to obtain a cold-rolled steel sheet having a sheet thickness of 0.4 mm.
• the obtained cold-rolled steel sheet was subjected to primary annealing and secondary annealing under the conditions shown in Table 2.
• hot-dip galvanizing was appropriately performed as a plating treatment, and some of them were further subjected to alloying treatment at the alloying temperature shown in Table 2.
• the obtained cold-rolled steel sheet or plated steel sheet was blanked and cut into blanks of a predetermined size, and then the blanks were press-formed under the conditions shown in Table 2 to obtain steel parts in the shape of a panel.
• "OK" in the bending direction in Table 2 means that the rolling direction of the blank coincides with the bending direction in which the largest curvature is formed.
• the obtained steel parts were washed, immersed in a 40°C aqueous solution containing 3% by mass of an aqueous alkaline degreaser (FC-301 manufactured by Nippon Parkerizing Co., Ltd.) for 3 minutes to degrease, and then washed with water and dried.
• the steel parts were pretreated with 3 g/ m2 zinc phosphate treatment (Palbond 3020 (trade name) manufactured by Nippon Parkerizing Co., Ltd.), and then electrocoated with a cationic electrocoating paint manufactured by Nippon Paint Co., Ltd. at a slope current of 160 V. Thereafter, an undercoat (EP Primer 1405(A) manufactured by Kansai Paint Co., Ltd.) and a topcoat (Magicron 1000 manufactured by Kansai Paint Co., Ltd.) were spray painted.
• 3 g/ m2 zinc phosphate treatment Palbond 3020 (trade name) manufactured by Nippon Parkerizing Co., Ltd.
• a cationic electrocoating paint manufactured by Nippon Paint Co., Ltd. at a slope current of 160 V.
• an undercoat EP Primer 1405(A) manufactured by Kansai Paint Co., Ltd.
• a topcoat Magnicron 1000 manufactured by Kansai Paint Co., Ltd.
• the temperature and time of the paint baking treatment were appropriately selected from the ranges of 100 to 220°C and 20 to 60 minutes so that each paint was dried appropriately, and the paint baking treatment was carried out with the drying parameter P shown in Table 2, to obtain a panel having a paint layer with the thickness shown in Table 2.
• the drying parameter P shown in Table 2 is the cumulative total of the drying parameters calculated from the temperature and time of each paint baking treatment after the electrodeposition coating, the undercoat and the topcoat.
• the characteristics of the obtained panels were measured and evaluated using the following methods.
• Comparative Example 3 the amount of strain applied in the panel forming process was large, so that the Sa of the steel sheet in the flat part of the panel exceeded 0.50 ⁇ m, resulting in a poor appearance after forming.
• Comparative Example 4 it is believed that the austenite grains became coarse because the holding time in the secondary annealing process of steel sheet manufacturing was long.
• the average particle spacing of martensite in the metal structure obtained after secondary annealing exceeded 2.5 ⁇ m, and it was not possible to obtain a steel sheet having a metal structure in which martensite is finely and uniformly dispersed throughout.
• the metal structure of the steel sheet constituting the panel was made to contain martensite, thereby achieving high strength, for example a tensile strength of 400 MPa or more, while controlling the surface properties of the panel so that Str was within the range of 0.50 to 1.00 and Sa was within the range of 0.50 ⁇ m or less, thereby significantly suppressing the occurrence of appearance defects such as ghost lines on the panel surface even when strain was imparted by forming such as press forming.
• a steel sheet having a metal structure in which martensite is finely and uniformly dispersed throughout i.e., a steel sheet having a metal structure in which the average grain spacing of martensite is controlled to 2.5 ⁇ m or less and the standard deviation in the area ratio of martensite in the direction perpendicular to the rolling direction and the plate thickness direction is controlled to 1.5% or less, was used, and formed so that the average KAM value/Vm was 1.8 or more, thereby achieving a YS of 350 MPa or more, and therefore significantly improving dent resistance.
• This is thought to be due to the fact that by applying appropriate strain to a steel sheet in which martensite is uniformly dispersed, many dislocations are uniformly introduced, increasing the amount of bake hardening.

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