Exterior panel and automobile provided with the same

Abstract

An exterior panel includes a steel plate, wherein the steel plate has a flat portion, and in the surface region of the flat portion, the metallographic structure contains more than 80% ferrite by volume ratio, the average grain size of the ferrite being 1.0 μm to 15.0 μm, and the strength ratio of the ferrite being X ODF{001} / {111}，S If the uniform elongation measured in the tensile specimen cut from the flat portion is set as uEl1, and the theoretical uniform elongation derived from the volume ratio of ferrite and martensite in the metallographic structure of the internal region of the flat portion, hardness and average grain size, and the plate thickness of the flat portion is set as uEl2, then uEl1 / uEl2 is 0.44 to 0.80.

Description

Technical Field

[0001] This invention relates to an exterior panel and a vehicle equipped with the exterior panel. Background Technology

[0002] In recent years, to protect the Earth's environment, there has been a demand for improved fuel efficiency in automobiles. To achieve this, and in order to ensure safety while reducing vehicle weight, there is a need for further increases in the strength of automotive steel sheets. This demand for higher strength is not limited to structural components such as beams and pillars, but also extends to exterior panels (engine hood, fender panels, door panels, roof panels, etc.). To address these requirements, materials have been developed that balance strength and elongation (formability).

[0003] On the other hand, there is a growing trend towards increasingly complex shapes for automotive exterior panels. If high-strength steel is used to achieve weight reduction, it becomes difficult to process into complex shapes. Conversely, if thinner steel walls are used to achieve weight reduction, unevenness can easily occur on the surface of the steel when forming complex shapes. Such unevenness detracts from the final appearance. Exterior panels require not only strength and other properties but also aesthetic design, thus demanding excellent surface properties after forming.

[0004] Regarding the correlation between the surface properties and material characteristics of steel sheets used in exterior panels after forming, for example, Patent Document 1 discloses a ferritic thin steel sheet in which, in order to improve the surface properties after bulging, the area ratio of crystals having a crystal orientation within ±15° relative to the {001} plane parallel to the surface of the steel sheet is set to 0.25 or less, and the average grain size of the crystals is set to 25 μm or less.

[0005] Existing technical documents

[0006] Patent documents

[0007] Patent Document 1: Japanese Patent Application Publication No. 2016-156079 Summary of the Invention

[0008] The problem the invention aims to solve

[0009] In addition to requiring good surface properties after the raw materials are formed, automotive exterior panels also require good dent resistance. Dent resistance refers to the ease with which a dent (pit) remains after a localized load has been applied to the panel for some reason and the load has been removed. In actual car bodies, dents can occur when a door or other exterior panel is pressed forcefully with a finger or palm, or when it is hit by a flying stone during driving. Dents are caused by plastic deformation of the loaded areas of the panel.

[0010] Therefore, if the strain of the panel reaches a certain magnitude when a load is applied, the strain will remain after the load is removed, resulting in a pit. The minimum value of the load that causes a certain residual strain in the panel is called the pit load. Panels with a larger pit load have excellent pit resistance. Patent Document 1 does not disclose any method to improve pit resistance.

[0011] In view of the above background, one of the objects of the present invention is to provide an exterior panel with excellent surface properties and excellent dent resistance after being formed from raw materials, and an automobile having the exterior panel.

[0012] Solution for solving the problem

[0013] The present invention is based on the following exterior panel and a car equipped with the exterior panel.

[0014] (1) An exterior panel comprising a steel plate, wherein...

[0015] The steel plate has a flat portion.

[0016] In the surface region of the flat portion.

[0017] The metallographic structure contains more than 80% ferrite by volume.

[0018] The average grain size of ferrite is 1.0 μm to 15.0 μm.

[0019] The intensity ratio X of the {001} orientation to the {111} orientation of ferrite ODF{001} / {111}，S It is greater than 0.30 and less than 3.50.

[0020] In the case where the uniform elongation measured in the tensile specimen cut from the flat portion is set as uEl1, and the theoretical uniform elongation derived using the following formula (I) based on the volume ratio, hardness, and average grain size of ferrite and martensite in the metallographic structure of the internal region of the flat portion, as well as the plate thickness of the flat portion, is set as uEl2, then...

[0021] uEl1 / uEl2 is 0.44 to 0.80.

[0022] uEl2=-0.057Hf×Vf / 100-0.055Hm×Vm / 100-0.35d -1 / 2 -0.571h+27.2···(I)

[0023] The meanings of the symbols in equation (I) above are as follows:

[0024] Hf: Vickers hardness (HV0.01) of the ferrite in the internal region.

[0025] Hm: Vickers hardness (HV 0.01) of the martensite in the internal region.

[0026] Vf: The volume percentage of ferrite in the internal region (%)

[0027] Vm: The volume percentage of martensite in the internal region (%)

[0028] d: The average grain size (μm) in the internal region calculated from equation (II) below.

[0029] d=(df×Vf+dm×Vm) / (Vf+Vm)···(II)

[0030] df: Average grain size (μm) of ferrite in the internal region.

[0031] dm: Average grain size (μm) of martensite in the internal region.

[0032] h: The thickness of the flat section (mm).

[0033] (2) The outer panel according to (1) above, wherein,

[0034] The thickness of the flat section is 0.20 mm to 0.60 mm.

[0035] (3) The outer panel according to (1) or (2) above, wherein,

[0036] The tensile strength of the steel plate is 300MPa to 650MPa.

[0037] (4) An automobile having any of the exterior panels described in any one of (1) to (3) above.

[0038] The effects of the invention

[0039] According to the present invention, it is possible to provide an exterior panel with excellent surface properties and excellent dent resistance after being formed from raw materials. Attached Figure Description

[0040] Figure 1This is a graph illustrating an example of the relationship between the equivalent plastic strain (%) imparted by cold plastic processing and the pit load (anti-pit load) when a pit of 0.1 mm depth is formed.

[0041] Figure 2 This is a diagram illustrating the shape of the outer panel used in the embodiments. Detailed Implementation

[0042] The following text will first describe how the invention came to be, and then describe the embodiments in detail.

[0043] To achieve lightweighting of automobile bodies, the thinning of the walls of the body components that make up the automobile body is a continuous development. These body components include exterior panels. Exterior panels are integrally molded parts. Exterior panels are the external components of an automobile. Examples of exterior panels include the hood panel, fender panels, side panels, door panels, and roof panels.

[0044] Such exterior panels are formed by cutting, stamping, and painting cold-rolled steel sheets, followed by baking (bake hardening). In the steel sheet used as the raw material for such impact-absorbing components, it is preferable to have the lowest possible strain to ensure good surface properties after forming. This is because low strain allows for more uniform deformation of the entire sheet during stamping, resulting in smaller deviations in surface properties. However, low strain leads to lower yield stress after forming, thus reducing dent resistance.

[0045] The inventors of this application conducted in-depth research and obtained the following insights: by controlling the microstructure ratio, texture, and grain size of the surface region of the steel sheet, it is possible to achieve good surface properties after forming. They also obtained the following insights: by making the strain during steel sheet forming greater than the usual value, dent resistance is improved. Based on these research results, the present invention was conceived, thereby providing an exterior panel with excellent surface properties after forming and thus excellent dent resistance, and an automobile equipped with such an exterior panel.

[0046] The embodiments of the present invention will be described below.

[0047] <Exterior Panel>

[0048] The exterior panel used in this embodiment can be exemplified by the aforementioned exterior panel. The exterior panel is manufactured by the aforementioned manufacturing method. The exterior panel has a steel plate and a coating layer formed on the steel plate. The steel plate may have a coating on its surface or may not have a coating formed on its surface. Furthermore, when the steel plate has a coating, the "surface of the steel plate" refers to the surface of the steel plate base material excluding the coating. The exterior panel may also be made solely of steel plate without a coating layer.

[0049] The outer panel comprises three parts. Specifically, the outer panel comprises (i) an end edge, (ii) an end portion, and (iii) a center side portion that is the portion other than the end edge and the end portion.

[0050] The end portion described in (i) is either formed by bending through a hemming process (HEM) or fixed to other components by welding, such as spot welding. The end portion described in (ii) is located at the center of the outer panel from the aforementioned end portion, and is separate from the portion fixed to other components by hemming or welding. This end portion is located, for example, a few millimeters from the center of the outer panel from the aforementioned end portion, and is substantially unaffected by the processing used to fix the outer panel to other components. "Substantially unaffected" in this case means that the change in characteristics caused by the processing used to fix the outer panel to other components is within a few percent.

[0051] The central side portion described above (iii) is visually recognized as the exterior of the vehicle from the outside. In this specification, the portion with a radius of curvature of 500 mm or more in the central side portion of this exterior panel is referred to as a flat portion. That is, the steel sheet of this embodiment has a flat portion.

[0052] In this invention, controlling the metallographic structure and properties of the surface and internal regions of the flat portion is important. Furthermore, the definitions of the surface and internal regions vary depending on the plate thickness. Specifically, when the plate thickness of the flat portion is set to t, if t is 0.25 mm or less, the region in the flat portion extending from the surface of the steel plate to a depth of t / 4 in the thickness direction (depth direction) is defined as the surface region, and the region extending from a depth of t / 4 to t / 2 is defined as the internal region.

[0053] Furthermore, when t exceeds 0.25 mm but is less than 0.40 mm, the region from the surface to a depth of 50 μm is designated as the surface region, and the region from a depth of 50 μm to a depth of t / 4 is designated as the inner region. Conversely, when t exceeds 0.40 mm, the region from the surface to a depth of 50 μm is designated as the surface region, and the region from a depth of 50 μm to a depth of 100 μm is designated as the inner region.

[0054] <About the surface area>

[0055] The inventors of this application have learned through research that surface unevenness during forming is caused by uneven deformation due to inhomogeneous strength within micro-regions. In particular, it has been found that the metallographic structure of the surface region has a significant impact on the formation of surface unevenness. Therefore, in the steel sheet of this embodiment, the metallographic structure of the surface region is controlled as described below.

[0056] [Ferrite volume ratio: 80% or higher]

[0057] If the volume ratio of ferrite in the surface region is less than 80%, the surface quality grade of the formed steel sheet is prone to deterioration. Therefore, the volume ratio of ferrite is set to 80% or higher. Preferably, it is 90%, 95%, or 98% or higher. The metallographic structure of the surface region can also be entirely ferrite, so the upper limit can be set to 100%.

[0058] The remaining microstructure in the surface region is, for example, any one or more of pearlite, bainite, martensite, and tempered martensite. When the volume percentage of ferrite in the surface region is 100%, the volume percentage of these remaining microstructures is 0%. Furthermore, in this invention, the martensite includes tempered martensite.

[0059] The volume ratio of ferrite in the surface region is determined using the following method.

[0060] A metallographic (microstructure) observation sample (approximately the size of the steel plate thickness × 20 mm in the width direction × 20 mm in the direction orthogonal to the thickness and width directions) is collected from a position at W / 4 or 3W / 4 of the steel plate width (i.e., a position at a distance of W / 4 from either end of the steel plate in the width direction). Next, a section parallel to the width and thickness directions is ground as the observation surface. After etching with LePera reagent, the metallographic (microstructure) of this observation surface is observed using an optical microscope. The area ratio of ferrite in the surface region is then measured. Furthermore, in this invention, the width direction of the steel plate refers to the direction corresponding to the front-rear direction of the vehicle when the exterior panel is installed.

[0061] If LePera is etched and then observed under an optical microscope, the different structures can be distinguished by color. For example, bainite is observed as black, martensite (including tempered martensite) is observed as white, and ferrite is observed as gray. Therefore, it is easy to distinguish ferrite from other hard structures.

[0062] More specifically, for the aforementioned observation surface, a 10-field observation was performed at 500x magnification. The surface area of ​​the obtained optical microscope image was specified, and image analysis was performed using Adobe Photoshop CS5 image analysis software to determine the area ratio of ferrite.

[0063] As an image analysis method, for example, obtaining the maximum brightness value L of an image. max and minimum brightness value L min , will have a brightness of L max-0.3(L max -L min ) to L max A portion of a pixel is defined as a white area, which will have a brightness of L. min To L min +0.3(L max -L min The pixels in the image are defined as black areas, and the rest are defined as gray areas. The area ratio of ferrite in the gray areas is calculated. For a total of 10 viewing locations, image analysis is performed in the same way to measure the area ratio of ferrite, and these area ratios are averaged to calculate the average value. This average value is then set as the volume ratio of ferrite in the surface region.

[0064] [Average grain size of ferrite: 1.0 μm~15.0 μm]

[0065] If the average grain size of ferrite exceeds 15.0 μm, the surface properties after forming deteriorate. Therefore, the average grain size of ferrite in the surface region is set to 15.0 μm or less. Preferably, the average grain size of ferrite is set to 12.0 μm or less.

[0066] On the other hand, when the average grain size of ferrite is less than 1.0 μm, it easily agglomerates to form ferrite particles with a {001} orientation. Even if each ferrite particle with a {001} orientation is small, if these particles agglomerate, deformation is concentrated in the agglomerated area, thus deteriorating the surface properties after forming. Therefore, the average grain size of ferrite in the surface region is set to 1.0 μm or more. The average grain size of ferrite is preferably 3.0 μm or more, and more preferably 6.0 μm or more.

[0067] The average grain size of ferrite in the surface region can be determined using the following method. Specify the surface region of the aforementioned optical microscope photograph, and perform image analysis using Adobe Photoshop CS5 software, as described above, measuring the area ratio and number of ferrite particles. Then, calculate the total area of ​​ferrite based on the area of ​​the analyzed region and the area ratio of ferrite. Further, divide the total area of ​​ferrite by the number of ferrite particles to calculate the average area of ​​each ferrite particle. Calculate the equivalent circle diameter based on this average area, and set the obtained equivalent circle diameter as the average grain size of the ferrite.

[0068] [Strength Ratio X] ODF{001} / {111}，S [0.30 or higher and less than 3.50]

[0069] In the surface region, the intensity ratio X of the {001} orientation of ferrite to the {111} orientation is... ODF{001} / {111}，SThe value is above 0.30 and less than 3.50, which improves the surface properties of the formed steel plate. Although the reason is unclear, it is believed to be due to the suppression of uneven deformation of the surface by the interaction between the morphology of ferrite and the crystal orientation distribution.

[0070] If X ODF{001} / {111}，S When the value is less than 0.30, uneven deformation caused by the orientation distribution and strength difference of the various crystals in the material is easily generated, and the concentration of deformation towards the {001} orientation of the ferrite becomes significant. On the other hand, when X... ODF{001} / {111}，S When the strength is above 3.50, uneven deformation caused by the orientation distribution and strength difference of the crystals in the material is more likely to occur, and the unevenness of the steel plate surface is more likely to develop.

[0071] The intensity ratio X of the {001} orientation to the {111} orientation of ferrite in the surface region ODF{001} / {111}，S The following method can be used to determine the result using the EBSD (Electron Back Scattering Diffraction) method.

[0072] After collecting a sample for texture measurement from a position at W / 4 or 3W / 4 of the width W of the steel plate (i.e., a position at a distance of W / 4 from any end of the steel plate in the width direction), the steel plate is mechanically ground to form a measurement surface with a cross section parallel to both the width and thickness directions, including the surface area. Then, the strain on the measurement surface is removed by chemical grinding or electrolytic grinding for texture measurement.

[0073] Crystal orientation distribution was measured on the surface region of the sample using the EBSD method at intervals less than 0.5 μm. Ferrite was extracted using an IQ (Image Quality) map analyzed by EBSP-OIM (registered trademark, Electron Back Scatter Diffraction Pattern-Orientation Image Microscopy). Ferrite is characterized by a large IQ value, thus this method can easily distinguish ferrite from other metallographic structures. The IQ threshold was set such that the area ratio of ferrite calculated based on the aforementioned microstructure observation of LePera corrosion was consistent with the area ratio of ferrite calculated based on the IQ value.

[0074] Obtain X ODF{001} / {111}，S X ODF{001} / {111}，S It is represented by a three-dimensional texture (ODF, Orientation Distribution Functions) calculated using the crystal orientation of the extracted ferrite. The random strength ratio is the ratio of the maximum value of the random strength ratio of the {001} orientation group to the maximum value of the random strength ratio of the {111} orientation group (γ-fiber) in the cross-section. The random strength ratio is the ratio of the strength of the measured material to the strength of a randomly oriented material that does not exhibit aggregation towards a specific orientation; that is, it is the value obtained by dividing the strength of the measured material by the strength of the randomly oriented material. For example, when steel plates are rolled and annealed at a high pressure reduction rate of 70% or more, a well-developed texture results in an increased random strength ratio of the {111} orientation group (γ-fiber).

[0075] Here, {hkl} represents the direction of the normal to the plate surface when the sample was collected using the method described above. <hkl>In the parallel case, the orientation of a crystal is usually represented by (hkl) or {hkl}, indicating the orientation perpendicular to the plate surface. {hkl} is a general term for equivalent faces, while (hkl) refers to individual crystal faces. That is, in this embodiment, since a body-centered cubic (bcc) structure is being considered, faces such as (111), (-111), (1-11), (11-1), (-1-11), (-11-1), (1-1-1), and (-1-1-1) are equivalent and cannot be distinguished. In such cases, these orientations are collectively referred to as the {111} orientation group. ODF displays are also used to represent the orientation of other crystal structures with lower symmetry. Therefore, in ODF displays, each orientation is generally represented by (hkl)[uvw]. However, in this embodiment, the insight that the orientation of the normal direction of the plate surface has a significant impact on the development of the unevenness after forming is obtained, and the focus is on the orientation of the normal direction {hkl}.

[0076] <About the internal area>

[0077] [uEl1 / uEl2: 0.44~0.80]

[0078] uEl1 / uEl2 are factors affecting dent resistance and the surface properties of the exterior panel. The inventors of this application derived their concept from steel sheets with fewer surface irregularities, resulting in excellent surface properties after forming. From the viewpoint of improving dent resistance, they conceived of focusing on the relationship between the uniform elongation uEl1 and the theoretical uniform elongation uEl2 of the steel sheet. This focus on the theoretical uniform elongation uEl2 from the perspective of dent resistance is unprecedented.

[0079] By maintaining uEl1 / uEl2 within the range of 0.44 to 0.80, a moderately high pre-strain can be imparted to the steel sheet, and the stress reduction effect from thinning the sheet thickness is relatively small. This also reduces surface unevenness of the steel sheet. If uEl1 / uEl2 exceeds 0.80, sufficient pre-strain cannot be imparted, resulting in a low yield stress and making it difficult to ensure adequate resistance to pitting. On the other hand, if uEl1 / uEl2 is less than 0.44, the pre-strain in the steel sheet exceeds 6%, resulting in saturation of the yield stress increase effect. The stress reduction effect from thinning the sheet thickness becomes significant, thus worsening the resistance to pitting. Furthermore, the surface unevenness of the steel sheet increases, leading to a deterioration in the surface properties after forming.

[0080] The uniform elongation uEl1 can be determined by tensile testing according to JIS Z 2241:2011 using a JIS 5 specimen cut in a manner where the width and length directions of the flat portion of the steel plate are aligned.

[0081] Furthermore, the formula for calculating the theoretical uniform elongation uEl2 is disclosed in "Microstructure and Tensile Properties of Ferritic-Martensitic Steels" by Minoru Umemoto, Tsuyoshi Ushida, Hiroshi Kimura, and Takahiko Ogura, Japan Iron and Steel Institute Special Steel Research Committee, Deformation Characteristics Prediction and Control Department Meeting, Report, (1994), p. 305. Using this formula and experimental values ​​as references, the theoretical uniform elongation uEl2 is calculated from the following formula (I).

[0082] uEl2=-0.057Hf×Vf / 100-0.055Hm×Vm / 100-0.35d -1 / 2 -0.571h+27.2···(I)

[0083] The meanings of the symbols in equation (I) above are as follows.

[0084] Hf: Vickers hardness of ferrite in the internal region (HV0.01)

[0085] Hm: Vickers hardness of martensite in the internal region (HV 0.01)

[0086] Vf: Volume ratio of ferrite in the internal region (%)

[0087] Vm: Volume percentage of martensite in the internal region (%)

[0088] d: Average grain size (μm) in the internal region calculated from equation (II) below.

[0089] d=(df×Vf+dm×Vm) / (Vf+Vm)···(II)

[0090] df: Average grain size (μm) of ferrite in the internal region.

[0091] dm: Average grain size (μm) of martensite in the internal region.

[0092] h: Thickness of the flat section (mm)

[0093] Furthermore, the Vickers hardness Hf of ferrite and the Vickers hardness Hm of martensite were measured using a micro Vickers hardness tester according to JIS Z 2244:2009. Additionally, "HV0.01" refers to the hardness symbol obtained when a micro Vickers hardness test is performed with a test force of 0.098 N (10 gf). Measurements were taken at 10 points, and the average value was used.

[0094] The volume ratio of ferrite in the internal region was determined using the following method. First, a metallographic (microstructure) observation specimen (approximately 20 mm in the steel plate thickness × width × 20 mm in a direction orthogonal to the thickness and width directions) was collected from a position of W / 4 or 3W / 4 of the steel plate width W (i.e., a position at a distance of W / 4 from the end of either of the width directions in the width direction). Next, a section parallel to the width and thickness directions was ground as the observation surface, etched with LePera reagent, and the metallographic structure of this observation surface was observed using an optical microscope. Then, the area ratio of ferrite in the internal region was measured. Furthermore, as mentioned above, the width direction of the steel plate refers to the direction corresponding to the front-rear direction of the vehicle when the exterior panel is installed.

[0095] More specifically, for the aforementioned observation surface, a 10-field observation was performed at 500x magnification. The internal region of the obtained optical microscope image was specified, and image analysis was performed using Adobe Photoshop CS5 image analysis software. The average value of the ferrite area ratio was calculated using the same image analysis method as for the surface region, and this value was set as the volume ratio of ferrite in the internal region.

[0096] The volume ratio of martensite in the internal regions can be determined using the same method as the volume ratio of ferrite described above. Furthermore, as mentioned above, in this invention, tempered martensite is included within the martensite.

[0097] The following method can be used to determine the grain size of ferrite in the internal region.

[0098] The internal region of the aforementioned optical microscope photograph was selected, and image analysis was performed using Adobe Photoshop CS5 image analysis software in the same manner as described above, measuring the area ratio of ferrite and the number of ferrite particles. Then, the total area of ​​ferrite was calculated based on the area of ​​the analyzed region and the area ratio of ferrite. This total area was then divided by the number of ferrite particles to calculate the average area per ferrite particle. The equivalent circle diameter was calculated from this average area, and this obtained equivalent circle diameter was set as the average grain size of the ferrite.

[0099] The grain size measurement of martensite in the internal region is performed in the same manner as the grain size measurement of ferrite described above. Here, the grain size of martensite refers to the grain size equivalent to the grain size of untransformed austenite before the martensitic transformation.

[0100] A sample for measuring the thickness of the steel plate is obtained from the flat portion of the plate, and the thickness of the flat portion of the steel plate is obtained by measuring it with a micrometer.

[0101] <Regarding plate thickness>

[0102] [Thickness of the flat portion of the steel plate: 0.20mm~0.60mm]

[0103] There are no particular restrictions on the thickness of the flat portion of the steel plate, but it is preferably 0.20 mm to 0.60 mm. As long as the thickness of the flat portion is 0.20 mm or more, the thickness of the outer panel is sufficient, thus easily ensuring dent resistance. On the other hand, as long as the thickness of the flat portion is 0.60 mm or less, the weight of the outer panel can be reduced, making it easy to obtain a lightweight outer panel. Furthermore, as long as the thickness of the flat portion is within the above-mentioned range, uEl1 / uEl2 can be easily controlled within the specified range by applying appropriate pre-strain. The thickness of the flat portion of the steel plate is obtained by taking a sample for thickness measurement from the flat portion of the steel plate and measuring it with a micrometer.

[0104] <Regarding other features>

[0105] [Arithmetic mean ripple: below 0.50μm]

[0106] As mentioned above, good surface properties are required for the exterior panels. Therefore, in this invention, the arithmetic mean waviness Wa is used as an indicator of surface properties. Specifically, the arithmetic mean waviness Wa in the flat portion of the steel sheet is preferably 0.50 μm or less. Furthermore, the arithmetic mean waviness Wa can be measured according to JIS B 0601:2013 (ISO4287:1997, Amd.1:2009).

[0107] Tensile strength of steel plate: 300MPa~650MPa

[0108] The preferred tensile strength of the steel plate is 300MPa to 650MPa. As long as the tensile strength of the steel plate is above 300MPa, it is easy to ensure the strength of the outer panel while simultaneously achieving a thinner outer panel. On the other hand, as long as the tensile strength of the steel plate is below 650MPa, it is easy to ensure the workability of the steel plate.

[0109] <About Steel Grades>

[0110] There are no particular limitations on the type of steel used in this embodiment, as long as it contains at least ferrite. Specifically, examples include (a) a ferritic single-phase steel plate and (b) a dual-phase steel plate (DP steel plate). Furthermore, in this embodiment, a ferritic single-phase steel plate refers to a steel plate with a ferrite volume ratio of 95% or more. Additionally, in this embodiment, a DP steel plate refers to a steel plate with a ferrite volume ratio of 80% or more and the remainder being a hard structure containing martensite.

[0111] Dual-phase steel sheets contain ferrite, which is a softer structure, and martensite, which is a harder structure, resulting in high strength and excellent machinability during panel forming. In DP steel sheets, the martensite and ferrite are distributed in a mosaic pattern, with the transformation-strengthened harder portion and the untransformed softer portion coexisting. Furthermore, when using DP steel sheets, deformation caused by cold plastic forming (stamping) mainly occurs in the softer ferrite structure. Therefore, DP steel sheets are preferred.

[0112] The preferred chemical compositions of (a) ferritic single-phase steel plates and (b) DP steel plates are shown below.

[0113] (a) Ferritic single-phase steel plate

[0114] Chemical composition, expressed in mass percent, contains

[0115] C: 0.0015~0.040%

[0116] Si: 0-0.20%

[0117] Mn: 0.20~1.50%

[0118] P: 0.010~0.100%

[0119] S: Below 0.020%

[0120] sol.Al: 0.20% or less,

[0121] N: below 0.0150%,

[0122] B: 0~0.010%

[0123] Mo: 0–0.50%

[0124] Ti: 0-0.20%

[0125] Nb: 0–0.20%

[0126] Cr: 0.001~0.50%

[0127] Ni: 0–0.20%, and

[0128] Cu: 0-0.10%, the remainder is Fe and impurities.

[0129] Here, impurities refer to components that are mixed into the steel plate during industrial manufacturing, primarily from raw materials such as ore or waste, due to various reasons in the manufacturing process, and are not intentionally added to the steel plate of this embodiment.

[0130] (b) DP steel plate

[0131] Chemical composition, expressed in mass percent, contains

[0132] C: 0.020~0.145%

[0133] Si: 0.010~3.00%

[0134] Mn: 0.45–2.25%

[0135] P: below 0.030%

[0136] S: Below 0.020%

[0137] sol.Al: 0.30% or less,

[0138] N: below 0.0100%

[0139] B: 0~0.0050%

[0140] Mo: 0–0.80%

[0141] Ti: 0-0.20%

[0142] Nb: 0–0.10%

[0143] Cr: 0–0.70%, and

[0144] Ni: 0-0.25%, the remainder is Fe and impurities.

[0145] Here, impurities refer to components that are mixed into the steel plate during industrial manufacturing, primarily from raw materials such as ore or waste, due to various reasons in the manufacturing process, and are not intentionally added to the steel plate of this embodiment.

[0146] <About the coating>

[0147] In the steel sheet of this embodiment, a coating may also be present on the surface. Having a coating on the surface improves corrosion resistance, and is therefore preferred. There are no particular limitations on suitable coatings, but examples include molten zinc plating, alloyed molten zinc plating, electroplating zinc, Zn-Ni plating (electroplated zinc alloy), Sn plating, Al-Si plating, alloyed electroplating zinc, molten zinc-aluminum alloy plating, molten zinc-aluminum-magnesium alloy plating, molten zinc-aluminum-magnesium alloy plating-Si plated steel sheet, and vapor-deposited zinc (Al).

[0148] <About the coating layer>

[0149] In this embodiment, a coating layer is formed on the surface of the steel plate. The coating layer is a visually perceptible part of the exterior panel. When a plating layer is formed, the coating layer is formed on top of the plating layer. In automotive exterior panels, the coating thickness is approximately 100 μm. The coating layer in the automotive exterior panel, from the steel plate side, sequentially includes an electrodeposited coating layer, a mid-coat coating layer, a primer coating layer, and a clear coat coating. The thickness of the electrodeposited coating layer is, for example, 15 μm to 20 μm. The thickness of the mid-coat coating layer is, for example, 25 μm to 35 μm. The thickness of the primer coating layer is 10 μm to 15 μm. The thickness of the clear coat coating is 30 μm to 40 μm.

[0150] <Regarding the manufacturing method>

[0151] Next, a preferred manufacturing method for the outer panel of this embodiment will be described. Regardless of the manufacturing method, the outer panel of this embodiment can achieve its effects as long as it has the above-described features. However, the following method can be manufactured stably and is therefore preferred.

[0152] [Manufacturing methods for steel plate raw materials]

[0153] First, the manufacturing methods of steel sheet raw materials that become raw materials for exterior panels will be explained for (a) ferritic single-phase steel sheet and (b) DP steel sheet respectively.

[0154] (a) Ferritic single-phase steel plate

[0155] When using ferritic single-phase steel sheets, the steel sheet raw material for the exterior panel of this embodiment can be manufactured using a manufacturing method that includes the following steps (ia-i) to (ia-vi).

[0156] (ia-i) Heating process: Heating the steel billet with the above chemical composition to above 1000°C;

[0157] (ia-ii) Hot rolling process: hot rolling of steel billets to obtain hot-rolled steel plates by hot rolling steel billets at a rolling end temperature of 950°C or below;

[0158] (ia-iii) Stress Imposition Process: Stress is imparted to the hot-rolled steel sheet after the hot rolling process to increase the residual stress σ at the surface. s In absolute terms, this ranges from 100 MPa to 250 MPa.

[0159] (ia-iv) Cold rolling process: The hot-rolled steel sheet after the stress-imposing process is subjected to R as the cumulative reduction rate. CR Cold-rolled steel sheets are obtained by cold rolling, which involves 70% to 90% of the material.

[0160] (ia-v) Annealing process: After the cold-rolled steel sheet is heated to a homogenization temperature T1°C of 300°C with an average heating rate of 1.5°C / second to 10.0°C / second, it is annealed at a homogenization temperature T1°C for 30 seconds to 150 seconds.

[0161] Ac1+550-25×ln(σ s -4.5×R CR ≤T1≤Ac1+550-25×ln(σ s )-4×R CR ···(IIIa)

[0162] In the above equation (IIIa), Ac1 is represented by the following equation (IV).

[0163] Ac1=723-10.7×Mn-16.9×Ni+29.1×Si+16.9×Cr···(IV)

[0164] (ia-vi) Cooling process: After the cold-rolled steel sheet after the annealing process is cooled to a temperature range of 550℃ to 650℃ at an average cooling rate of 1.0℃ / second to 10.0℃ / second at a homogenization temperature of T1℃ to 650℃, it is then cooled to a temperature range of 200℃ to 490℃ at an average cooling rate of 5℃ / s to 500℃ / s.

[0165] (b) DP steel plate

[0166] When using DP steel sheets, the steel sheet raw material for the exterior panel of this embodiment can be manufactured using a manufacturing method that includes the following steps (ib-i) to (ib-vi).

[0167] (ib-i) Heating process: The steel billet having the above chemical composition is heated to above 1000°C;

[0168] (ib-ii) Hot rolling process: hot rolling of steel billets at temperatures below 950°C to obtain hot-rolled steel plates;

[0169] (ib-iii) Stress imposition process: Stress is applied to the hot-rolled steel sheet after the hot-rolling process to increase the residual stress σ at the surface. s In absolute terms, this ranges from 150 MPa to 450 MPa.

[0170] (ib-iv) Cold rolling process: The hot-rolled steel sheet after the stress-imposing process is subjected to R as the cumulative reduction rate. CR Cold-rolled steel sheets are obtained by cold rolling, which involves 70% to 90% of the material.

[0171] (ib-v) Annealing process: After the cold-rolled steel sheet is heated to a homogenization temperature T1°C that satisfies the following formula (IIIb) at an average heating rate of 1.5°C / second to 10.0°C / second, it is annealed at a homogenization temperature T1°C for 30 seconds to 150 seconds.

[0172] 1275-27×ln(σ s -4.5×R CR ≤T1≤1275-25×ln(σ s )-4×R CR ...(IIIb); and

[0173] (ib-vi) Cooling process: After the cold-rolled steel sheet after the annealing process is cooled to a temperature range of 550°C to 650°C at an average cooling rate of 1.0°C / sec to 10.0°C / sec from T1°C to 650°C, it is cooled to a temperature range of 200°C to 490°C at an average cooling rate of 5.0°C / sec to 500.0°C / sec.

[0174] Furthermore, in either case (a) ferritic single-phase steel sheet or (b) DP steel sheet, stress is imparted during the stress-imposing process by grinding the hot-rolled steel sheet with a surface grinding brush after, for example, hot rolling or pickling. In this case, the surface residual stress can be controlled by varying the contact pressure of the grinding brush relative to the steel sheet surface, using a portable X-ray residual stress measuring device to measure the surface residual stress online, and ensuring that the surface residual stress remains within the aforementioned range. With residual stress imparted to the surface within the aforementioned range, a predetermined cold rolling, annealing, and cooling process is performed to obtain a steel sheet with ferrite and a predetermined hard phase distribution, the ferrite having a predetermined texture. Additionally, a plating process to form a coating on the surface can be performed after the cooling process.

[0175] [Method for manufacturing exterior panels from steel sheet raw materials]

[0176] This section describes an example of a method for manufacturing an exterior panel from steel sheet raw material that has undergone the aforementioned annealing process (final annealing). The steel sheet raw material can be either (a) a ferritic single-phase steel sheet or (b) a DP steel sheet.

[0177] The preferred method for manufacturing the outer panel from steel sheet raw materials in this embodiment includes the steps of performing cold plastic processing and heat treatment on the steel sheet raw materials after final annealing.

[0178] The method includes the following steps:

[0179] (ii-i) Punching process;

[0180] (ii-ii) The cold plastic forming process of the steel plate obtained by punching to become a steel component;

[0181] (ii-iii) Painting processes for coating steel components; and

[0182] (ii-iv) A heat treatment process in which the steel component is subjected to a heat treatment at a temperature T2 of 80°C to 200°C for 300 seconds to 1800 seconds.

[0183] Alternatively, the painting processes shown in (ii-iii) above can be omitted. Each process will be explained in detail.

[0184] (ii-i) Punching process

[0185] Steel plate raw materials are formed into blanks by punching processes that cut them into predetermined sizes.

[0186] (ii-ii) Cold Plastic Processing

[0187] Next, the blank is cold-plasticized to form the steel component before baking and painting. Specifically, the blank is deep-drawn as a cold-plasticization process to form the steel component before baking and painting. The shape of the steel component corresponds to the shape of the outer panel.

[0188] Steel components are formed by applying pre-strain to the entire blank through deep drawing. Typically, the pre-strain applied to the blank through deep drawing is less than 3%. However, in this invention, the strain applied through deep drawing needs to be set to 3% to 6%.

[0189] Figure 1 This is a graph illustrating an example of the relationship between the equivalent plastic strain (%) imparted by cold plastic forming (deep drawing) and the pit load (anti-pit load) when forming a pit with a depth of 0.1 mm. Figure 1 In the diagram, the horizontal axis represents the equivalent plastic strain. The vertical axis represents the value obtained by dividing the pit load (N) by the pit load when the equivalent plastic strain is 1%. Figure 1 The chart shows the results when using 590MPa duplex steel plates with a thickness of 0.4mm as an example.

[0190] As from Figure 1 The charts clearly show that the rate of increase in resistance to pitting load is low when the equivalent plastic strain is less than 3%. Furthermore, if the equivalent plastic strain exceeds 6%, the resistance to pitting load decreases with increasing plastic strain. Therefore, from the perspective of pitting resistance, the equivalent plastic strain is set at 3%–6%. By further imparting a pre-strain of more than 3%, the bake hardening amount can be sufficiently large. Baker hardening is a phenomenon also known as strain aging: intrusive elements (mainly carbon) move and become fixed to dislocations (line defects that are the basic process of plastic deformation) generated in the steel sheet due to cold plastic working (pre-strain), thereby hindering the movement of these dislocations and increasing strength.

[0191] (ii-iii) Painting process

[0192] Next, the steel component is coated. This coating process includes, for example, three types: electrodeposition coating, intermediate coating, and topcoat coating (primer and clear coat). Water-based or solvent-based paints are used in the coating. In the electrodeposition coating process, the entire surface of the steel component is coated with electrodeposition paint while the steel component is immersed in an electrodeposition tank containing the paint. In the intermediate coating process, paint is sprayed from nozzles onto the steel component using a coating robot or manually by a worker, thus applying the intermediate coating to the entire surface of the steel component. In the topcoat coating process, paint is sprayed from nozzles onto the steel component using a coating robot or manually by a worker, thus applying the topcoat to the entire surface of the steel component. As a result, the surface of the steel component is composed of a coating film with a thickness of approximately 100 μm.

[0193] (ii-iv) Heat treatment process

[0194] The heat treatment in this process is a baking and drying process used to bake the coating film onto the steel component, and it is also a process to bake harden the steel component. The heat treatment process can be performed in the coating process either after electrodeposition coating and before intermediate coating, between multiple intermediate coatings, after intermediate coating and before topcoat coating, between multiple topcoat coatings, or after topcoat coating.

[0195] As described above, the temperature T2 of the steel component in the heat treatment process is set within the range of 80°C to 200°C. Since the temperature T2 in the heat treatment process is above the lower limit mentioned above, the coating can be reliably baked onto the steel component, and the hardening treatment of the steel component can be performed more reliably. Furthermore, if the temperature T2 exceeds the upper limit mentioned above, the cost of the outer panel manufacturing process will increase. Therefore, the upper limit of the temperature is kept below 200°C.

[0196] As described above, the holding time at temperature T2 during heat treatment is set within the range of 300 seconds to 1800 seconds. The holding time in the heat treatment process is above the lower limit mentioned above, thereby enabling reliable baking of the coating onto the steel component and more reliable hardening treatment of the steel component. Furthermore, if the holding time exceeds 1800 seconds, it will increase the cost of the exterior panel manufacturing process. Therefore, the holding time is set to 1800 seconds or less.

[0197] In the heat treatment process, the steel component is continuously held at a constant temperature T2 within the aforementioned temperature range for 300 to 1800 seconds. The holding time of temperature T2 in the heat treatment process is above the lower limit mentioned above, thereby achieving a baking effect on the coating. Furthermore, if the holding time of temperature T2 exceeds the upper limit mentioned above, the manufacturing cost of the outer panel will increase. Therefore, the holding time of temperature T2 is set to 1800 seconds or less.

[0198] The outer panel of this embodiment is completed through the above processes.

[0199] In this embodiment, the steel sheet undergoes appropriate strain during the cold rolling process, thereby increasing the amount of work hardening and thus increasing resistance to dent loads. This allows for the creation of an exterior panel with excellent surface properties and superior dent resistance from the point of origin after raw material forming.

[0200] Example

[0201] The embodiments of the present invention will now be described. The conditions in the embodiments are examples of conditions adopted to confirm the feasibility and effectiveness of the present invention, and the present invention is not limited to these examples of conditions. The present invention can employ various conditions as long as it achieves its objectives without departing from the spirit of the present invention.

[0202] Steel billets A to C with the chemical compositions shown in Table 1 were melted and continuously cast to produce slabs with a thickness of 240 mm to 300 mm. The obtained slabs were heated to the temperatures shown in Table 2. The heated slabs were then hot-rolled and coiled under the conditions shown in Table 2.

[0203] Next, the hot-rolled steel sheet is uncoiled to impart stress. At this time, while using a portable X-ray residual stress measuring device to measure the surface residual stress online at the processing temperature (steel sheet temperature) shown in Table 2, the contact pressure of the grinding brush relative to the steel sheet surface is varied to obtain the residual stress σ shown in Table 2. s Then, using the cumulative reduction rate R shown in Table 2... CR Steel plates A1, A2, B1, and C1 are obtained by cold rolling.

[0204] Next, annealing and cooling were performed under the conditions shown in Table 3. Additionally, a portion of the steel sheet was plated to form a coating on the surface. In Table 3, CR indicates no plating, and GA indicates alloyed molten zinc plating.

[0205] [Table 1]

[0206] Table 1

[0207]

[0208] [Table 2]

[0209] Table 2

[0210]

[0211] * indicates that it is outside the preferred scope of the present invention.

[0212] [Table 3]

[0213] Table 3

[0214]

[0215] * indicates that it is outside the preferred scope of the present invention.

[0216] Next, steel plates A1, A2, B1, and C1 are cold-formed into the shape of the outer panel. Specifically, 600mm square steel plates are formed into... Figure 2 The panel shown has a central curvature radius R of 1200 mm. Furthermore, Figure 2 The width direction shown corresponds to the front-to-rear direction of the vehicle when the exterior panel is installed. Furthermore, the cold plastic forming method used at this time is shown in Table 4. Next, the parts formed into the shape of the exterior panel are heat-treated (baked to harden) to produce the exterior panel parts A1a-A1d, A2a, B1a, and C1a-C1d. The temperature of the parts during heat treatment and the holding time at that temperature are shown in Table 4.

[0217] The metallographic structure and texture of the surface and internal regions of the obtained components A1a-A1d, A2a, B1a, and C1a-C1d were observed and measured using the methods described above. The results are shown in Table 5.

[0218] In addition, for the flat portion of the outer panel, as shown in Table 6, the pre-strain, uniform elongation uEl1, theoretical uniform elongation uEl2, ratio uEl1 / uEl2, ultimate yield stress (YS+WH+BH), tensile strength TS, and plate thickness of the flat portion were measured using the methods described above. Furthermore, the ultimate yield stress refers to the yield stress of the outer panel after heat treatment, which is the sum of the yield stress YS of the steel plate before cold plastic forming, the strain hardening amount WH increased by cold plastic forming, and the bake hardening amount BH increased by heat treatment. The ultimate yield stress was measured by performing the above-described tensile test on the flat portion of the outer panel. The steel grades in Table 6 are F steel (ferritic single-phase steel plate) and DP steel (dual-phase steel plate).

[0219] [Table 4]

[0220] Table 4

[0221]

[0222] * indicates that it is outside the preferred scope of the present invention.

[0223] [Table 5]

[0224]

[0225] [Table 6]

[0226] Table 6

[0227]

[0228] The underlined part indicates that it is outside the scope of this invention.

[0229] * indicates that it is outside the preferred scope of the present invention.

[0230] [Dent Resistance Evaluation]

[0231] The dent resistance of the manufactured exterior panels was evaluated. Specifically, dent resistance is affected by the yield stress and thickness of the steel sheet. Therefore, the ultimate yield stress (YS+WH+BH) of the flat central portion of the manufactured exterior panel multiplied by the panel thickness was calculated. 1.7 The dent resistance index is calculated. This dent resistance index is also calculated for the flat central portion of the reference product. Additionally, using component A1b as a reference product, the ratio (dent resistance index of the outer panel / dent resistance index of the reference product) is calculated as a proportional index. It is assumed that dent resistance is good when this proportional index is 1.00 or higher.

[0232] [Surface property evaluation]

[0233] The surface properties of the manufactured exterior panel were evaluated. Specifically, the arithmetic mean waviness Wa was measured in a 3 mm square area on the flat portion of the exterior panel. The arithmetic mean waviness was measured according to the standards described above. Furthermore, it was assumed that the surface properties after molding were good if the arithmetic mean waviness Wa was 0.50 μm or less.

[0234] [Overall Evaluation]

[0235] Products with good dent resistance and good surface properties were designated as rating A, while products with poor dent resistance and poor surface properties were designated as rating B. These results are shown in Table 7.

[0236] [Table 7]

[0237] Table 7

[0238]

[0239] As shown in Tables 5 to 7, the metallographic structure and X-ray diffraction patterns in the surface region are... ODF{001} / {111}，S In the present invention examples where uEl1 / uEl2 are within the scope of the present invention, the overall evaluation is A, suppressing the formation of surface irregularities after processing and exhibiting excellent anti-pitting properties. On the other hand, regarding the metallographic structure of the surface region, X... ODF{001} / {111}，S And any one or more of uEl1 / uEl2 that are outside the scope of the present invention, after molding, produce texture or unevenness, thus being unsuitable as an outer panel, or having poor resistance to dents.

[0240] Specifically, for component A1c, the strain in the flat portion is as small as 2%, and uEl1 / uEl2 is outside the scope of this invention. Component A1c exhibits poor resistance to pitting. For component A1d, the strain in the flat portion is as large as 7%, and uEl1 / uEl2 is outside the scope of this invention. Component A1d exhibits poor resistance to pitting and poor surface properties. For component A2a, X ODF{001} / {111}，S The components uEl1 / uEl2 are outside the scope of this invention, with the strain in the flat portion being as small as 2%. Such a component A2a has poor resistance to pitting and poor surface properties.

[0241] For component C1c, the strain in the flat portion is as small as 2%, and uEl1 / uEl2 is outside the scope of this invention. Component C1c has poor resistance to pitting. For component C1d, the strain in the flat portion is as large as 7%, and uEl1 / uEl2 is outside the scope of this invention. Component C1d has poor resistance to pitting and poor surface properties.

[0242] Industrial availability

[0243] According to the present invention, an exterior panel with excellent surface properties and excellent dent resistance after being formed from raw materials can be provided. Therefore, the present invention can be widely used as an exterior panel and in automobiles equipped with such an exterior panel.< / hkl>

Claims

1. An exterior panel comprising a steel plate, wherein, The steel plate has a flat portion. The chemical composition of the steel plate is either (a) or (b): （a） The chemical composition of the steel plate, expressed in % by mass, contains C：0.0015~0.040%、 Si: 0~0.20% Mn: 0.20~1.50%, P：0.010~0.100%、 S: Below 0.020% sol.Al: 0.20% or less, N: below 0.0150%, B：0~0.010%、 Mo: 0~0.50%, Ti: 0~0.20%, Nb: 0~0.20%, Cr：0.001~0.50%、 Ni: 0~0.20%, and Cu: 0~0.10%, the remainder is Fe and impurities. （b） The chemical composition of the steel plate, expressed in % by mass, contains C：0.020~0.145%、 Si: 0.010~3.00% Mn: 0.45~2.25%, P: below 0.030% S: Below 0.020% sol.Al: 0.30% or less, N: below 0.0100% B：0~0.0050%、 Mo: 0~0.80%, Ti: 0~0.20%, Nb: 0~0.10%, Cr: 0~0.70%, and Ni: 0~0.25%, the remainder is Fe and impurities. In the surface region of the flat portion. The metallographic structure contains more than 80% ferrite by volume. The average grain size of ferrite is 1.0 μm to 15.0 μm. The intensity ratio X of the {001} orientation to the {111} orientation of the ferrite is 0.30 or greater but less than 3.

50. ODF｛001｝ / ｛111｝，S is 0.30 or greater but less than 3.50, In the case where the uniform elongation measured in the tensile specimen cut from the flat portion is set as uEl1, and the theoretical uniform elongation derived using the following formula (I) based on the volume ratio, hardness, and average grain size of ferrite and martensite in the metallographic structure of the internal region of the flat portion, as well as the plate thickness of the flat portion, is set as uEl2, then... uEl1 / uEl2 is 0.44~0.80, uEl2 = -0.057 Hf x Vf / 100 - 0.055 Hm x Vm / 100 - 0.35 d -1 / 2 -0.571 h + 27.2... (I) The meanings of the symbols in equation (I) above are as follows: Hf: Vickers hardness (HV0.01) of the ferrite in the internal region. Hm: Vickers hardness (HV 0.01) of the martensite in the internal region. Vf: The volume percentage of ferrite in the internal region (%) Vm: The volume percentage of martensite in the internal region (%) d: The average grain size in the internal region calculated from equation (II) below. d=(df×Vf+dm×Vm) / (Vf+Vm)…(II) df: Average grain size of ferrite in the internal region dm: Average grain size of martensite in the internal region h: The thickness of the flat section. The unit for particle size is μm, and the unit for plate thickness is mm.

2. The outer panel according to claim 1, wherein, The thickness of the flat section is 0.20mm to 0.60mm.

3. The outer panel according to claim 1 or 2, wherein, The tensile strength of the steel plate is 300MPa~650MPa.

4. An automobile having an exterior panel as described in any one of claims 1 to 3.

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