Method for evaluating the bendability of molded bodies

Abstract

The present invention aims to provide a simpler method for evaluating the bendability of a molded article. [Solution] A method for evaluating bendability according to one embodiment of the present invention includes a measurement step of measuring the average Vickers hardness by applying a load of 10 gf to 50 gf to a cross section perpendicular to the surface of a molded body obtained by press-forming a steel plate, at a position 40 μm or less from the surface of the molded body; and an evaluation step of evaluating the bendability of the molded body based on the average Vickers hardness and a determination threshold.

Description

[Technical Field]

[0001] The present invention relates to a method for evaluating the bendability of a molded article. [Background technology]

[0002] Steel sheets are formed into various shapes depending on their application. As a technique for evaluating the bendability of a formed body, for example, Patent Document 1 discloses a method for evaluating the crack resistance of a metal sheet, characterized by having the following steps: (1) performing a bending test on a metal sheet in accordance with the bending test specified in VDA standard 238-100 and obtaining a load-bending angle curve showing the relationship between the load and bending angle of the metal sheet; and (2) evaluating the crack resistance of the metal sheet based on the relationship between the load and bending angle in the region exceeding the limit bending angle α in the load-bending angle curve.

[0003] Furthermore, Patent Document 2 discloses a method for evaluating crack resistance by correlating the maximum bending angle obtained by a VDA bending test on a pre-processed test piece with the crack score obtained by a crushing test of the member. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2016-080464 [Patent Document 2] Japanese Patent Publication No. 2021-179410 [Overview of the project] [Problems that the invention aims to solve]

[0005] Steel sheets are formed by hot working, such as hot stamping, or by cold working. However, the mechanical properties of the steel sheet may change before and after forming due to the thermal history during hot stamping or the introduction of strain during cold forming. Therefore, methods for predicting the bendability of a formed product based on the evaluation results of the mechanical properties of the steel sheet before pressing, such as the one described in Patent Document 1, may not be able to properly evaluate the quality of the mechanical properties of the formed product.

[0006] Furthermore, even when attempting to perform a bending test (VDA bending test) as defined in VDA standard 238-100 using a molded body, it is often impossible to prepare the 60mm square flat plate required as a test specimen for the VDA bending test. In particular, many automotive components have complex shapes, making it difficult to prepare the above-mentioned test specimen from automotive components. For this reason, in most cases, the VDA bending test cannot be directly applied to molded bodies containing automotive components. In addition, the technology described in Patent Document 2 requires pre-processing, which makes evaluation time-consuming.

[0007] This invention has been made in view of the above-described circumstances, and aims to provide a simpler method for evaluating the bendability of a molded article. [Means for solving the problem]

[0008] The gist of this invention is as follows: [1] A method for evaluating the bendability of a molded article according to one aspect of the present invention includes a measurement step of measuring the average Vickers hardness by applying a load of 10 gf to 50 gf to a cross section perpendicular to the surface of a molded article obtained by press-forming a steel sheet, at a position 40 μm or less from the surface of the molded article; and an evaluation step of evaluating the bendability of the molded article based on the average Vickers hardness and a determination threshold. [2] In the method for evaluating the bendability of a molded body described in [1] above, the threshold for determination may be determined based on the relationship between the maximum bending angle specified in VDA standard 238-100 for each of a plurality of test steel plates having a strength corresponding to the strength of the steel plate to be evaluated that constitutes the molded body, and the average Vickers hardness measured by applying a load of 10 gf to 50 gf to the cross section at a position 40 μm or less from the surface of the test steel plate, perpendicular to the surface of the test steel plate. [3] In the method for evaluating the bendability of a molded article described in [2] above, the test steel plate is a test steel plate before baking hardening, and the bendability of the molded article after baking hardening may be evaluated based on the above relationship. [4] In the method for evaluating the bendability of a molded article described in [2] or [3] above, at least two of the plurality of test steel plates may have different decarburization index values. [5] In the method for evaluating the bendability of a molded article described in any of [2] to [4] above, the metallographic structure at the center of the thickness direction of the steel sheet constituting the molded article, and the metallographic structure at the center of the thickness direction of at least two of the plurality of test steel sheets, may have a martensite fraction of 85% or more. [6] In the method for evaluating the bendability of a molded article described in [1] above, the threshold value for determination may be determined based on the relationship between the evaluation index for the bendability of the molded article and the average Vickers hardness measured by applying a load of 10 gf to 50 gf to a cross section perpendicular to the surface of a test steel sheet having a strength corresponding to the strength of the steel sheet to be evaluated that constitutes the molded article, at a position 40 μm or less from the surface of the test steel sheet. [7] In the method for evaluating the bendability of a molded article described in any of [2] to [6] above, the measurement position in the thickness direction of the Vickers hardness measured in the measurement step above and the measurement position in the thickness direction of the Vickers hardness used to determine the threshold value for judgment above may be the same. [8] In the method for evaluating the bendability of a molded article described in any of [1] to [7] above, the average Vickers hardness measured in the measurement step may be measured at a position of 10 μm or more and 40 μm or less from the surface of the molded article, and the average Vickers hardness used to determine the threshold for judgment may be measured at a position of 10 μm or more and 40 μm or less from the surface of the test steel plate. [9] In the method for evaluating the bendability of a molded article described in any of [1] to [8] above, the molded article may be an automotive molded member.

[10] In the method for evaluating the bendability of a molded article described in any of [1] to [9] above, the molded article may be made of hot-stamped material.

[11] In the method for evaluating the bendability of a molded article described in any of [1] to [9] above, the molded article may be made from a cold-worked material.

[12] In the method for evaluating the bendability of a molded article described in any of [1] to

[11] above, the molded article has a specific softening layer provided in the thickness direction from the surface, the hardness of the center in the thickness direction of the portion where the specific softening layer is provided is 400 Hv or more, the specific softening layer is a region having a hardness at least 10 Hv lower than the hardness of the center in the thickness direction of the portion where the specific softening layer is provided, the thickness of the specific softening layer is 2% or more and less than 20% of the thickness of the portion where the specific softening layer is provided, and the hardness of the specific softening layer on the surface is The specified softening layer is provided in the portion of the steel sheet that is 0.5 times or more and less than 0.9 times the hardness of the center in the thickness direction of the sheet. The specified softening layer has a first hardness change region in the thickness direction, which is the region from the surface to 40% of the thickness of the specified softening layer, and a second hardness change region, which is the region of the specified softening layer that is not the first hardness change region. The molded body may be made of a steel sheet in which the absolute value ΔHv1 of the hardness change in the thickness direction in the first hardness change region is greater than the absolute value ΔHv2 of the hardness change in the thickness direction in the second hardness change region.

[13] In the method for evaluating the bendability of a molded article described in

[12] above, the decarburization index value of the steel sheet constituting the molded article may be 0.085 or higher. [Effects of the Invention]

[0009] According to the above aspect of the present invention, the bendability of the molded body can be evaluated more simply, and the bendability can be evaluated with excellent reliability.

Brief Description of Drawings

[0010] [Figure 1] It is a diagram showing an example of the hardness change of a specific softened layer. [Figure 2] It is an example of a graph showing the relationship between the maximum bending angle measured in accordance with the bending test specified in VDA standard 238-100:2017 and the average Vickers hardness. [Figure 3] It is a schematic diagram of the molded body in the example. [Figure 4] It is a graph in which the average Vickers hardness and the maximum bending angle of a test piece cut out from the molded body are plotted on the graph shown in FIG. 2.

Modes for Carrying Out the Invention

[0011] Hereinafter, a method for evaluating the bendability of a molded body according to an embodiment of the present invention will be described with reference to the drawings. However, the present invention is not limited to the examples described below. In addition, the dimensions and ratios of each component in the drawings do not represent the actual dimensions and ratios of each component.

[0012] <Method for Evaluating Bendability of Molded Body> A method for evaluating the bendability of a molded body according to an embodiment of the present invention includes a measurement step of measuring the average Vickers hardness (cross-sectional hardness) by applying a load of 10 gf or more and 50 gf or less to a cross-section at a position of 40 μm or less from the surface of the molded body in a cross-section perpendicular to the surface of the molded body obtained by press-working a steel sheet, and an evaluation step of evaluating the bendability of the molded body based on the average Vickers hardness and a determination threshold value. Details will be described below.

[0013] (Measurement Step) In the measurement step, in a cross-section perpendicular to the surface of the molded body obtained by pressing a steel sheet, a load of 10 gf or more and 50 gf or less is applied to a position 40 μm or less from the surface of the molded body in the cross-section to measure the average Vickers hardness.

[0014] The cross-section for measuring the cross-sectional hardness is a cross-section in the direction perpendicular to the plate surface of the molded body (the plate thickness direction), and it may be cut out from any position more than 50 mm away from the end face of the molded body. However, when a sample cannot be collected from any position more than 50 mm away from the end face of the molded body, a position avoiding the end part may be used as the acquisition position of the cross-section. Note that the end face of the molded body refers to the outermost periphery in the longitudinal direction or the short-side direction of the molded body, and a surface having a cut part in that part.

[0015] The hardness test for measuring the average Vickers hardness is carried out using a micro-Vickers hardness tester in accordance with the method described in JIS Z 2244:2020. At a plurality of depth positions within the range from the surface of the molded body to 40 μm or less, Vickers hardness measurements are performed multiple times at each depth, and the average value thereof is taken as the cross-sectional hardness. For example, at three arbitrary depth positions within the above range, Vickers hardness at three points is measured at each depth, and the average value of the total nine points of Vickers hardness is taken as the average Vickers hardness. The interval between each measurement point shall be three times or more the size of the indentation. If the measurement position of the Vickers hardness is too close to the surface of the molded body, the indentation shape for calculating the hardness may not be obtained depending on the load, and accurate measurement may not be possible. Therefore, the position for measuring the Vickers hardness is preferably in the range of 10 μm or more and 40 μm or less from the surface of the molded body, and more preferably in the range of 20 μm or more and 40 μm or less from the surface of the molded body.

[0016] In measuring average Vickers hardness, a constant load is applied regardless of the measurement location. The applied load should be such that an appropriate indentation shape for calculating hardness can be obtained; for example, it can be selected from between 10 gf and 50 gf. In this case, it is preferable to select the largest possible load while still obtaining an appropriate indentation shape. A larger load results in a greater indentation depth at each measurement point, reducing the influence of error factors in the measured value. Therefore, measuring with a large load suppresses the variation in measurement errors among multiple measurements, improving the accuracy of the average value of multiple measurements.

[0017] The molded body is obtained by press-forming a steel sheet. The steel sheet constituting the molded body (the steel sheet that has been press-formed into a molded body) is not particularly limited and may be a steel sheet having a known chemical composition and metal structure. When the molded body is used as an automotive component, the steel sheet may have a tensile strength of 1470 MPa or higher (e.g., 1.5 GPa, 1.8 GPa, 2.0 GPa, 2.5 GPa or higher). The molded body may be a molded body after bake-hardening or a molded body before bake-hardening. That is, the steel sheet constituting the molded body may be a molded body after bake-hardening or a molded body before bake-hardening. In a bake-hardened molded body, the martensite is tempered, and the bendability is improved compared to a molded body before bake-hardening. Therefore, in the evaluation step described later, the bendability of the bake-hardened molded body can be evaluated by using a test steel sheet before bake-hardening, which has inferior bendability compared to the bake-hardened molded body.

[0018] The microstructure of the steel sheet constituting the molded body preferably has a total volume fraction of 85% or more of martensite, bainite, and tempered martensite at the center of the thickness direction. The center of the thickness direction refers to the range from 3 / 8 to 5 / 8 of the way from the surface in the thickness direction. If the total volume fraction of martensite, bainite, and tempered martensite is 85% or more, the steel sheet can be evaluated as applicable to molded bodies for applications requiring high strength. Furthermore, martensite, bainite, and tempered martensite can crack at their interfaces with other microstructures. If the total volume fraction of martensite, bainite, and tempered martensite is 85% or more, the proportion of other microstructures is small, making it less likely for cracks to occur at the interfaces between martensite, bainite, and tempered martensite and other microstructures. Therefore, when evaluating the bendability of the molded body based on the average Vickers hardness and a threshold value for evaluation, a more accurate evaluation can be made. Here, other microstructures refer to one or more of ferrite, pearlite, cementite, and retained austenite.

[0019] Furthermore, it is preferable that the total volume fraction of martensite, bainite, and tempered martensite in the surface layer of the steel sheet constituting the molded body is 75% or more. If the total volume fraction of martensite, bainite, and tempered martensite in the surface layer is 75% or more, the proportion of other metal structures is small, making it less likely for cracks to occur at the interface between martensite, bainite, and tempered martensite and other metal structures. Therefore, when a load is applied to the surface of the test piece, cracks at the above interface are suppressed, and the bendability can be evaluated more accurately. Here, other metal structures refer to one or more of ferrite, pearlite, cementite, and retained austenite. The surface layer here refers to the range from the surface to 50 μm in the thickness direction of the sheet.

[0020] The metalloid structure is determined by the following method. Specifically, for the metalloid structure of a molded body, a test specimen is taken from an arbitrary position at least 50 mm away from the end face of the molded body, in a cross section parallel to the rolling direction and the thickness direction, so that the metalloid structure in the surface layer and the center in the thickness direction can be observed. However, if it is not possible to take a sample from an arbitrary position at least 50 mm away from the end face of the molded body, the test specimen is taken from a position that avoids the end. The same applies to the metalloid structure of the test steel sheet. The end face of a molded body refers to the outermost periphery in the longitudinal or transverse direction of the molded body, and the surface that has a cut portion in that part. After polishing the cross-section of the above test specimen using silicon carbide paper from #600 to #1500, it is finished to a mirror surface using a liquid in which diamond powder with a particle size of 1 to 6 μm is dispersed in a diluent such as alcohol or pure water. Next, it is polished at room temperature with colloidal silica that does not contain alkaline solutions to remove the strain introduced into the surface of the sample. At an arbitrary position in the longitudinal direction (rolling direction) of the cross-section of the polished test specimen, with the center being at a position 1 / 2 of the plate thickness from the surface, the region extending 200 μm in the longitudinal direction and from a position 3 / 8 of the plate thickness from the surface to a position 5 / 8 of the plate thickness from the surface is measured by electron backscatter diffraction at measurement intervals of 0.1 μm to obtain the crystal orientation information of the region. The structure identified by the obtained crystal orientation information is defined as the metal structure at the center position in the thickness direction. Furthermore, the region extending 50 μm to a depth in the thickness direction from the surface and 200 μm in the longitudinal direction is measured by electron backscatter diffraction at measurement intervals of 0.1 μm to obtain crystal orientation information. The microstructure identified by the obtained crystal orientation information is defined as the metallic microstructure of the surface layer. The measurement is performed using a thermal field emission scanning electron microscope (JEOL JSM-7001F) and an EBSD detector (TSL DVC5 detector). The vacuum level inside the apparatus is 9.6 × 10⁻⁶. -5 The parameters will be set to Pa or less, the acceleration voltage to 15kV, the irradiation current level to 13, and the electron beam irradiation level to 62. During the measurement, "iron-α" and "iron-γ" will be set as the phases. Additionally, the same region as the EBSD measurement area will be observed at a magnification of 1000x or more using a thermal field emission scanning electron microscope (JEOL JSM-7001F).

[0021] To observe this region, Vickers indentations are imprinted at three of the four corners of the EBSD measurement area, within a range of 100 μm from each corner, so that the observation position can be identified. Then, surface contamination is polished off, leaving the microstructure of the observation surface, and nital etching is performed. Using the Vickers indentations as a guide, the same region as the EBSD observation surface can be observed. For contamination removal, methods such as buff polishing with alumina particles with a particle size of 0.1 μm or less, polishing with colloidal silica that does not contain alkaline solutions at room temperature, or Ar ion sputtering can be used.

[0022] Next, the crystal orientation information obtained from the EBSD measurement is used to calculate the area fraction of retained austenite using the "PhaseMap" function included in the "OIMAnalysis®" software that comes with the EBSD analyzer. In calculating the area fraction, crystals with an fcc crystal structure are identified as retained austenite.

[0023] Furthermore, for regions with a bcc crystal structure, the "GrainAverageMisorientation" function included in the "OIMAnalysis®" software attached to the EBSD analyzer is used to determine whether it is cementite, pearlite, ferrite, bainite, martensite, or tempered martensite, and the area fraction is measured. Specifically, under conditions where a boundary with a crystal orientation difference of 15° or more is defined as a grain boundary (15° grain boundary), regions with a GrainAverageMisorientation (GAM) value of 3.0° or less are determined to be ferrite. Furthermore, regions with a GAM value greater than 3.0° are determined to be martensite, bainite, or tempered martensite. In addition, the EBSD measurement results and the tissue images obtained by FE-SEM observation are superimposed using Vickers indentations as markers. For regions determined to be cementite and pearlite from the FE-SEM observation tissue images, the regions of retained austenite and other tissues determined by the "PhaseMap" function and the "GrainAverageMisorientation" function are determined to be cementite and pearlite, respectively. In this case, the locations of cementite and pearlite may also be identified by comparing the grain boundary map using 15° grain boundaries with the location of Vickers indentations.

[0024] In the molded article according to this embodiment, the area ratio and the volume ratio are assumed to be equal, and the area ratio obtained above is considered to be the volume ratio.

[0025] Preferably, the steel sheet constituting the molded body has a specific softening layer provided in the thickness direction from the surface on at least a portion of it. The specific softening layer is a region having a hardness at least 10 Hv lower than the hardness of the region on the thickness direction of the steel sheet that is closer to the center of the thickness direction than the specific softening layer (the region in the thickness direction of the steel sheet excluding the specific softening layer; hereinafter, this region may be referred to as the center of the thickness direction).

[0026] The thickness of the specific softening layer is preferably 2% or more and less than 20% of the plate thickness in the portion where the specific softening layer is provided. If the thickness of the specific softening layer is 20% or less of the plate thickness of the steel plate, the proportion of the specific softening layer in the steel plate is small, so the load-bearing capacity required for the manufactured molded body can be maintained. The thickness of the specific softening layer is preferably 17% or less of the plate thickness of the steel plate, and more preferably 14% or less. On the other hand, if the specific softening layer is provided on the surface over the entire area of ​​the steel plate, the deformation capacity of the specific softening layer can be fully exhibited if the thickness of the specific softening layer is 2% or more of the plate thickness of the steel plate. The thickness of the specific softening layer is preferably 5% or more of the plate thickness of the steel plate, and more preferably 8% or more.

[0027] The hardness of the specific softening layer is preferably 0.5 times or more and less than 0.9 times the hardness of the center in the thickness direction of the plate in the portion where the specific softening layer is provided. The surface hardness of the steel plate is measured by the Vickers hardness test described in JIS Z 2244-1:2020 on a cross-section obtained by cutting the steel plate along the thickness direction. At that time, the measurement point should be within 20 μm of the steel plate surface and the measurement should be taken so that the indentation is 10 μm or less. If the surface hardness is 0.5 times or more the hardness of the center in the thickness direction, the load-bearing capacity during impact, especially in the later stages of the stroke, can be improved. It is more preferable that the specific softening layer has a hardness of 0.6 times or more the hardness of the center in the thickness direction on the surface of the steel plate. On the other hand, if the surface hardness is less than 0.9 times the hardness of the center in the thickness direction, the deformability can be sufficiently improved. It is more preferable that the specific softening layer has a hardness of less than 0.8 times that of the center in the thickness direction of the steel plate on its surface.

[0028] The method for measuring the hardness at the center in the thickness direction of the plate is as follows: A cross-section perpendicular to the plate surface of the sample is taken, the measurement surface is prepared, and the sample is subjected to the hardness test. The preparation of the measurement surface is carried out in accordance with JIS Z 2244:2020. After polishing the measurement surface with silicon carbide paper from #600 to #1500, it is finished to a mirror surface using a liquid in which diamond powder with a particle size of 1 μm to 6 μm is dispersed in a diluent such as alcohol or pure water. The hardness test is carried out according to the method described in JIS Z 2244:2020. Using a micro-Vickers hardness tester, 10 measurements are taken at the 1 / 2 position of the plate thickness of the sample with a load of 1 kgf, at intervals of at least 3 times the indentation, and the average value is taken as the hardness at the center in the thickness direction of the plate.

[0029] As shown in Figure 1, it is preferable that the specific softening layer has a first hardness change region in the thickness direction, which is the region from the surface to 40% of the thickness of the specific softening layer, and a second hardness change region, which is the region of the specific softening layer that is not the first hardness change region. It is preferable that the absolute value ΔHv1 of the hardness change in the thickness direction in the first hardness change region is greater than the absolute value ΔHv2 of the hardness change in the thickness direction in the second hardness change region. If ΔHv1 is greater than ΔHv2, sufficient load characteristics can be obtained.

[0030] The absolute value ΔHv1 of the hardness change in the first hardness change region is preferably between 100Hv and less than 200Hv. If ΔHv1 is 100Hv or more, stress concentration during bending deformation can be further reduced, and the bending properties can be further improved. If ΔHv1 is less than 200Hv, the effect of reducing stress concentration during bending deformation is further enhanced, and better bending properties can be obtained. Therefore, when ΔHv1 is between 100Hv and less than 200Hv, good bending properties can be obtained, and the deformability of the molded body can be improved. Specifically, in the later stages of the stroke during impact, the drop in load from immediately after the load peak can be made more gradual. Thus, as described above, the absolute value ΔHv1 of the hardness change in the first hardness change region is preferably between 100Hv and less than 200Hv.

[0031] Next, the method for measuring the hardness in the first and second hardness change regions will be explained. A cross-section perpendicular to the plate surface of the sample is taken, and after the sample surface is prepared, it is subjected to the hardness test. The preparation of the measurement surface is carried out to minimize irregularities and prevent sagging near the surface in order to accurately measure the hardness near the surface of the sample. Here, the measurement surface is sputtered with an argon ion beam using a cross-section polisher manufactured by JEOL. At this time, in order to suppress the occurrence of streaky irregularities on the measurement surface, the argon ion beam is irradiated onto the measurement surface from 360 degrees using a sample rotation holder manufactured by JEOL.

[0032] For a sample with a prepared measurement surface, hardness is measured using a micro-Vickers hardness tester. Measurements are taken from the surface of the sample, in the region corresponding to the specific softened layer, in a direction perpendicular to the plate surface (thickness direction), with a load of 10 gf, at intervals of at least three times the indentation width. While the total number of measurement points will vary depending on the plate thickness, the number of measurement points for calculating ΔHv1 and ΔHv2 (described later) should be set as many as possible, ensuring sufficient spacing to avoid the influence of indentations, based on the description in JIS Z 2244:2020. The measurement position on the outermost surface of the sample should be within 20 μm of the surface (or, if a plating layer exists, directly beneath the plating layer or directly beneath the alloy layer between the plating layer and the base material). This is because the outermost surface of the base material has a large amount of soft tissue.

[0033] In the case of a sample in which specific softened layers are located on both sides of the center in the thickness direction, the same measurement is performed from the first surface side of the sample, and then again from the second surface side opposite the first surface.

[0034] ΔHv1 is calculated using the following procedure. Specifically, the hardness gradient Δa of the first hardness change region is calculated from all measurement points in the region from the surface of the sample to 40% of the total thickness of the specific softened layer (first hardness change region) using equation (1). Here, a i This is the percentage (%) of the distance from the surface at the i-th measurement point relative to the total thickness of the specific softened layer, c i is a i In this case, the Vickers hardness (Hv), n, is the sum of all measurement points included in the region from the surface to 40% of the total thickness of the specific softened layer (the first hardness change region).

[0035]

number

[0036] Here, Δa: Gradient of hardness change in the thickness direction in the first hardness change region (Hv / %) a i: The ratio (%) of the distance from the surface at the i-th measurement point to the thickness of the entire specific softened layer c i :a i Vickers hardness (Hv) at n: The sum of all measurement points included in the first hardness change region on the first surface side is.

[0037] In the case of a sample in which specific softened layers are arranged on both sides of the center in the plate thickness direction, based on the hardness measurement results from the first surface side, Δa1 on the first surface side is calculated, and further, based on the hardness measurement results from the second surface side, Δa2 on the second surface side is calculated. The arithmetic mean of Δa1 and Δa2 can be taken as Δa.

[0038] ΔHv1 can be obtained by multiplying Δa obtained by Equation (1) by the ratio of the thickness in the plate thickness direction of the first hardness change region to the thickness of the entire specific softened layer.

[0039] ΔHv2 is calculated by the following procedure. That is, from all the measurement points included in the region from 40% to 100% of the thickness of the entire specific softened layer on the surface side of the sample (the second hardness change region), the hardness gradient ΔA of the second hardness change region is calculated by Equation (2). Here, A i is the ratio (%) of the distance from the surface at the i-th measurement point to the thickness of the entire specific softened layer, Ci is the Vickers hardness (Hv) at Ai, and N is the sum of all the measurement points included in the region from 40% to 100% of the thickness of the entire specific softened layer on the surface side (the second hardness change region).

[0040] [Number]

[0041] Here, ΔA: The gradient of the change in hardness in the plate thickness direction in the second hardness change region (Hv / %) A i : The ratio (%) of the distance from the surface at the i-th measurement point to the thickness of the entire specific softened layer C<000001​i Vickers hardness (Hv) N: Sum of all measurement points included in the second hardness change region on the first surface side. That is the case.

[0042] In the case of a sample in which specific softening layers are arranged on both sides of the center in the thickness direction, ΔA1 of the first surface is calculated based on the hardness measurement result from the first surface, and then ΔA2 of the second surface is calculated based on the hardness measurement result from the second surface. The arithmetic mean of ΔA1 and ΔA2 can be taken as ΔA.

[0043] ΔHv2 can be obtained by multiplying ΔA, obtained by equation (2), by the ratio of the thickness in the plate thickness direction of the second hardness change region to the total thickness of the specific softened layer.

[0044] The hardness at the center in the thickness direction of the portion where the specific softening layer is provided is preferably 400 Hv or higher. A molded body formed from a steel plate with a Vickers hardness of 400 Hv or higher at the center in the thickness direction exhibits a significant improvement in deformability due to the specific softening layer. The Vickers hardness at the center in the thickness direction of the steel plate is preferably 500 Hv or higher, and more preferably 600 Hv or higher. There is no particular upper limit to the hardness at the center in the thickness direction, but considering formability, etc., the hardness at the center in the thickness direction is preferably 900 Hv or lower, and more preferably 800 Hv or lower.

[0045] The thickness of the steel sheet constituting the molded body is, for example, 0.5 mm or more or 1.0 mm or more. Alternatively, the thickness of the steel sheet may be, for example, 3.5 mm or less or 2.9 mm or less.

[0046] The press working technique is not particularly limited, and various known processing techniques may be applied to the press working, and it may be either hot working (e.g., hot pressing (hot stamping)) or cold working (e.g., cold pressing).

[0047] Hot-stamped materials and cold-worked materials can be easily distinguished by measuring the hardness of the bent and bulging parts of the component. Specifically, cold-worked materials are formed into parts by cold working steel sheets. As a result, the bent and bulging areas undergo work hardening, and it is known that the hardness of these work-hardened areas is higher than, for example, the flat, unworked parts of the component. On the other hand, hot-stamped materials are produced by heating the material to the austenite single-phase region of about 900°C, where strength is greatly reduced, and then forming it in the hot-worked state while it is still in the low-strength austenite phase. During cooling in a mold, it undergoes martensitic transformation, resulting in a high-hardness component. Generally, the work hardening introduced during hot working is extremely small compared to the work hardening introduced by cold working, so the hardness of the bent and bulging parts of a hot-stamped component is equivalent to that of the unworked parts. For this reason, they can be easily distinguished by measuring the hardness of the bent and bulging parts of the component. Furthermore, since the hardness during cold working depends on the strain, even for parts without unprocessed (flat) sections, it is possible to confirm whether work hardening has occurred, i.e., whether the material is cold-worked, by measuring the hardness of areas with different levels of strain.

[0048] The molded articles subject to bendability evaluation are not particularly limited, and various molded articles can be used for bendability evaluation. Examples of molded articles include automotive molded members, which are structural members that can constitute an automotive frame as a cabin frame or impact-absorbing frame. Examples of cabin frames include roof center reinforcement, roof side rails, B-pillars, side sills, tunnels, A-pillar lowers, A-pillar uppers, kick reinforcement, floor cross members, under reinforcement, and front headers. Examples of impact-absorbing frames include rear side members, apron upper members, bumper reinforcement, crash boxes, and front side members.

[0049] Furthermore, it is preferable that the steel sheet constituting the molded body has a decarburization index value of 0.085 or higher. If the decarburization index value is 0.085 or higher, the deformability of the surface layer is improved, so that the load-bearing capacity can be improved while maintaining excellent bendability. Therefore, it is preferable that the decarburization index value of the steel sheet is 0.085 or higher. More preferably, the decarburization index value of the steel sheet is 0.140 or higher, or 0.180 or higher. The upper limit of the decarburization index value is 1.000 from the method of calculating the decarburization index value, but in order to improve the load-bearing capacity while maintaining excellent bendability, the decarburization index value is even more preferably 0.60 or lower, or 0.50 or lower, or 0.40 or lower.

[0050] The decarburization index can be determined by the following method. The elemental concentration distribution in the thickness direction of a molded body will be measured using a Glow Discharge Optical Emission Spectrometry (GD-OES) analyzer. The measurement range will be from the surface of the molded body to a position 200 μm from the surface (200 μm depth), and the measurement interval will be 0.02 μm or less. Measurements will be performed for all elements contained in the molded body.

[0051] When a molded body has a coating on its surface, the surface referred to here is the interface between the coating and the base material. In cases where the surface has a coating, the coating is partially or completely removed by mechanical polishing or chemical polishing so that measurements can be taken up to a depth of 200 μm from the surface of the base material (the interface between the base material and the coating) before being subjected to GD-OES measurement. In GD-OES measurement, the region where the Fe concentration (Fe content) is 90% by mass or more is considered the base material, and the measurement point where the Fe concentration first reaches 90% by mass from the surface is considered the surface of the base material. Next, the average value of the measured C concentration (C content) (more than 1000 points) from a position 180 μm (180 μm depth) to a depth 200 μm from the surface of the molded body is calculated, and this average value is considered to be the C concentration in the part unaffected by decarburization. However, if measurements can be taken to a depth where it can be determined that the carbon content has reached the same level as the average carbon content of the base material (the portion unaffected by decarburization), the measurement range may be extended to the surface beyond the 200 μm depth (however, measurements must be taken from 50 μm or more from the surface). In that case, if the measured carbon concentration in the region from the deepest point to 20 μm from that deepest point towards the surface has an absolute difference of 0.05 mass% or less between the average carbon concentration in the region from the deepest point to 20 μm towards the surface and the maximum measured carbon concentration in the region from the deepest point to 20 μm towards the surface, and the absolute difference of 0.05 mass% or less between the average carbon concentration in the region from the deepest point to 20 μm towards the surface and the minimum measured carbon concentration in the region from the deepest point to 20 μm towards the surface, then the average carbon concentration in the region from the deepest point to 20 μm towards the surface may be considered as the carbon concentration in the portion unaffected by decarburization. If the deepest point is 120 μm, then "the measured C concentration in the region from the deepest point to 20 μm from the surface" means the C concentration contained within the area from 100 μm to 120 μm.

[0052] In the region from the surface of the molded body to the area where decarburization does not affect the C concentration, the decrease in C concentration per unit depth (the value obtained by subtracting the C concentration at each measurement point from the C concentration in the area where decarburization does not affect) is calculated, and the integral of the product of the unit depth and the decrease in C concentration is taken as the area of ​​the C-deficient region (Area A). Here, unit depth refers to the measurement interval of GD-OES. Next, the product of the C concentration in the area where decarburization does not affect and 200 (μm) is taken as the reference area (Area B), and the value obtained by dividing the C-deficient area (Area A) by the reference area (Area B) (Area A / Area B) is taken as the decarburization index value.

[0053] (Evaluation Steps) In the evaluation step, the bendability of the molded body is evaluated based on the average Vickers hardness (sectional hardness) and a threshold value for judgment.

[0054] The threshold value for judgment should be a value based on a correlation with the average Vickers hardness of the molded body. An example of a threshold value for judgment is described below. The threshold value for judgment is determined, for example, based on the relationship between the maximum bending angle measured in accordance with the bending test (VDA bending test) specified in VDA standard 238-100:2017 for each of several test steel plates having a strength corresponding to the strength of the steel plate to be evaluated, which is the steel plate constituting the molded body, and the average Vickers hardness measured by applying a load of 10 gf to 50 gf to the cross section at a position 40 μm or less from the surface of the test steel plate, perpendicular to the surface of the test steel plate. The maximum bending angle measured in accordance with the bending test specified in VDA standard 238-100:2017 refers to the bending angle when a load of the magnitude of the maximum load in the VDA bending test -60 N is applied. In the following, the maximum bending angle measured in accordance with the bending test specified in VDA standard 238-100:2017 may simply be referred to as the maximum bending angle.

[0055] The test steel plate is used to obtain the relationship between the maximum bending angle in a VDA bending test and the average Vickers hardness measured by applying a load of 10 gf to 50 gf to the cross-section at a position 40 μm or less from the surface. The test steel plate used has a strength corresponding to the strength of the steel plate to be evaluated that constitutes the molded body. The test steel plate may be a steel plate before baking hardening or a steel plate after baking hardening.

[0056] The average Vickers hardness is measured on a test specimen cut from the test steel plate using the test method described in the measurement step.

[0057] Figure 2 shows an example of a graph illustrating the relationship between the maximum bending angle measured in accordance with the bending test specified in VDA standard 238-100:2017 and the average Vickers hardness. Figure 2(A) is a graph plotting the average Vickers hardness (average Vickers hardness) of a total of nine points measured at three points each at 20 μm, 30 μm, and 40 μm from the surface, with a load of 20 gf, and the maximum bending angle of the molded body before baking hardening. Figure 2(B) is a graph plotting the average Vickers hardness of three points measured at 75 μm from the surface, with a load of 20 gf, and the maximum bending angle of the molded body before baking hardening. Figure 2(C) is a graph showing the average Vickers hardness (average Vickers hardness) of a total of nine points measured at three points each at 20 μm, 30 μm, and 40 μm from the surface, with a load of 20 gf, and plotted against the maximum bending angle of the molded body after baking hardening. Similar plots in Figures 2(A) to (C) are for steel sheets with the same strength but differing in at least one of the following: plate thickness, metal structure, and decarburization index value. For example, "1.5G" in the legend represents a 1.5 GPa grade steel sheet. The maximum bending angle was measured for a molded body with a ridge of 30 mm in length. White plots in the figure represent plots for surface softening material with a specific softening layer. As shown in Figure 2(A), a correlation is observed between the maximum bending angle and the average Vickers hardness. Therefore, the average Vickers hardness at which the desired maximum bending angle is achieved can be used as the threshold for determination. For example, based on this correlation, if the average Vickers hardness of the steel sheets constituting the molded body measured in the measurement step is smaller than the desired threshold for judgment, it can be determined that the bendability of the molded body is good. On the other hand, as shown in Figure 2(B), no correlation is observed between the average value of the Vickers hardness measured at three points 75 μm from the surface and the maximum bending angle, even among steel sheets with different strengths. Furthermore, as shown in Figure 2(C), a correlation is observed between the average Vickers hardness and the maximum bending angle even for molded bodies that have undergone bake-hardening. Therefore, the bendability of molded bodies after bake-hardening can be evaluated using test steel sheets without bake-hardening.

[0058] To obtain a correlation between the maximum bending angle and the average Vickers hardness, the maximum bending angle and average Vickers hardness of two or more test steel plates with equal thickness but different factors influencing the maximum bending are measured in the VDA bending test. Factors influencing the maximum bending and average Vickers hardness include, for example, strength, microstructure, and decarburization index value. It is preferable that at least two of the test steel plates have different decarburization index values.

[0059] To more accurately evaluate the bendability of the molded body, it is preferable that the test steel sheets have a metallographic structure corresponding to the metallographic structure of the steel sheets constituting the molded body. Therefore, if the total volume percentage of martensite, bainite, and tempered martensite in the metallographic structure at the center of the thickness direction of the steel sheets constituting the molded body is 85% or more, it is preferable that the metallographic structure of at least two of the multiple test steel sheets has a total volume percentage of martensite, bainite, and tempered martensite of 85% or more. Furthermore, if the total volume percentage of martensite, bainite, and tempered martensite in the metallographic structure of the surface layer of the steel sheets constituting the molded body is 75% or more, it is preferable that the metallographic structure of at least two of the multiple test steel sheets has a total volume percentage of martensite, bainite, and tempered martensite of 75% or more. The microstructure of the test steel sheet is determined by the same method as the method for determining the microstructure of the molded body described above.

[0060] To evaluate bendability more accurately, it is preferable that all of the test steel sheets have different decarburization index values, that the total volume ratio of martensite, bainite, and tempered martensite in the metal structure at the center in the thickness direction is 85% or more, and that they have the same thickness. Furthermore, it is preferable that the steel sheet constituting the formed body has the total volume ratio of martensite, bainite, and tempered martensite in the metal structure at the center in the thickness direction is 85% or more, and that it has the same thickness as the test steel sheets. The decarburization index value of the test steel plate is determined by the method described above.

[0061] According to this embodiment, the bendability of a molded body with a shape that cannot be directly subjected to the VDA bending test can be evaluated. Therefore, the bendability of a molded body can be evaluated more easily.

[0062] The embodiments of the present invention have been described above. The embodiments described above are merely illustrative, and any configuration that is substantially identical to the technical idea described in the claims of the present invention and achieves similar effects is included within the technical scope of the present invention.

[0063] For example, in the above embodiment, a linearly approximated approximation line is used to evaluate the bending properties, but in addition to a straight line, an approximation curve that is appropriately approximated according to the plot may also be used.

[0064] Furthermore, while the threshold for judgment is determined based on the relationship between the maximum bending angle and the average Vickers hardness, the threshold may also be determined based on the relationship between an evaluation index for the bendability of the molded article and the average Vickers hardness. An evaluation index for the bendability of the molded article is, for example, the stroke of a three-point bending test. The stroke of a three-point bending test correlates with the maximum bending angle specified in VDA standard 238-100:2017. Therefore, the threshold for judgment may also be determined based on the relationship between the stroke of a three-point bending test and the average Vickers hardness. [Examples]

[0065] The present invention will be described in more detail below with reference to examples. The examples described below are merely examples of the present invention and do not limit it.

[0066] (Example 1) As shown in Table 1, steel plates with strengths of 1.5 GPa, 1.8 GPa, 2.0 GPa, or 2.5 GPa, and a thickness of 1.6 mm were prepared, either with or without surface softening. In the table, "Surface Softening 1" indicates that the steel plate was heat-treated at 800°C for 10 minutes under a city gas atmosphere, followed by heating at 900°C for 4 minutes under a nitrogen atmosphere, and then heat-treated by mold cooling. "Surface Softening 2" indicates that the steel plate was heat-treated at 800°C for 20 minutes under a city gas atmosphere, followed by heating at 900°C for 4 minutes under a nitrogen atmosphere, and then heat-treated by mold cooling. "No Surface Softening" indicates that only heat treatment at 900°C for 4 minutes under a nitrogen atmosphere was performed. Each test steel plate was manufactured in this manner. In Table 1, the decarburization index, metal structure, hardness at the center in the thickness direction, hardness of the specific softened layer, thickness of the specific softened layer, ΔHv1, and ΔHv2 are values ​​measured by the method described above.

[0067] [Table 1]

[0068] Table 2 shows the cross-sectional hardness and maximum bending angle of each test steel plate shown in Table 1. Table 3 shows the cross-sectional hardness and maximum bending angle of each test steel plate after a bake-hardening treatment at 170°C for 20 minutes. For the Vickers hardness test to calculate the average Vickers hardness, the average value of nine Vickers hardness measurements (3 points each at 20 μm, 30 μm, and 40 μm from the surface) was measured. Additionally, the average value of three Vickers hardness measurements taken at 75 μm from the surface was also measured. The applied load was 20 gf.

[0069] [Table 2]

[0070] [Table 3]

[0071] Furthermore, using steel plates with strengths of 1.5 GPa, 1.8 GPa, 2.0 GPa, or 2.5 GPa and a thickness of 1.6 mm, molded bodies with the shape shown in Figure 3 were manufactured by hot stamping. The heating conditions during hot stamping were as follows: heat treatment was performed at 800°C for 20 minutes under a city gas atmosphere, followed by heating at 900°C for 4 or 10 minutes under a nitrogen atmosphere, as shown in Table 4, and then heat treatment by mold cooling. Test pieces were cut from the top plate portion of the obtained molded bodies, and each test piece was subjected to a VDA bending test and a Vickers hardness test to calculate the average Vickers hardness of the cross-section. The Vickers hardness test was performed in the same manner as described above, and the average Vickers hardness was calculated.

[0072] [Table 4]

[0073] Figure 2 shows a graph illustrating the relationship between the maximum bending angle measured in accordance with the bending test specified in VDA standard 238-100:2017 and the average Vickers hardness. Figure 2(A) is a graph plotting the average Vickers hardness (average Vickers hardness) of a total of nine points measured at three points each at 20 μm, 30 μm, and 40 μm from the surface, with a load of 20 gf, and the maximum bending angle of the molded body before baking hardening. Figure 2(B) is a graph plotting the average Vickers hardness of three points measured at 75 μm from the surface, with a load of 50 gf, and the maximum bending angle of the molded body before baking hardening. Figure 2(C) is a graph showing the average Vickers hardness (average Vickers hardness) of a total of nine points measured at three points each at 20 μm, 30 μm, and 40 μm from the surface, with a load of 20 gf, and plotting the maximum bending angle of the molded body after baking hardening. Similar plots in Figures 2(A) to (C) are plots for steel plates with equal strength but differing in at least one of the following: plate thickness, metal structure, and decarburization index value. The maximum bending angle was measured for molded bodies with a ridge of 30 mm in length. White plots in the figure are plots for surface softening materials with a specific softening layer.

[0074] As shown in Figure 2(A), a correlation was found between the maximum bending angle and the average Vickers hardness. Therefore, it was found that the average Vickers hardness at which the desired maximum bending angle is achieved can be used as the threshold for evaluation. On the other hand, as shown in Figure 2(B), no correlation was found between the average value of the Vickers hardness measured at three points 75 μm from the surface and the maximum bending angle among steel plates of different strengths. Furthermore, as shown in Figure 2(C), a correlation was found between the average Vickers hardness and the maximum bending angle even for molded bodies that had undergone bake-hardening. Therefore, it was found that the bendability of molded bodies after bake-hardening can be evaluated using test steel plates without bake-hardening.

[0075] Furthermore, Figure 4 shows a graph plotting the surface hardness and maximum bending angle of the test specimens on the graph in Figure 2(A). As shown in Figure 4, the average Vickers hardness and maximum bending angle of the test specimens cut from the molded body could be approximated by the same approximation line. Therefore, it was found that the bendability of the molded body can be evaluated by comparing the average Vickers hardness with a threshold value.

Claims

1. A measurement step in which, in a cross section perpendicular to the surface of a molded body obtained by press-forming a steel plate, a load of 10 gf to 50 gf is applied to the cross section at a position 40 μm or less from the surface of the molded body and the average Vickers hardness is measured, A method for evaluating the bendability of a molded article, comprising an evaluation step of evaluating the bendability of the molded article based on the average Vickers hardness and a determination threshold.

2. The method for evaluating the bendability of a molded body according to claim 1, wherein the threshold for determination is determined based on the relationship between the maximum bending angle specified in VDA standard 238-100 for each of a plurality of test steel plates having a strength corresponding to the strength of the steel plate to be evaluated that constitutes the molded body, and the average Vickers hardness measured by applying a load of 10 gf to 50 gf to the cross section at a position 40 μm or less from the surface of the test steel plate, perpendicular to the surface of the test steel plate.

3. The aforementioned test steel plate is a test steel plate before baking and hardening. A method for evaluating the bendability of a molded article according to claim 2, wherein the bendability of the molded article after baking and hardening is evaluated based on the aforementioned relationship.

4. The method for evaluating the bendability of a molded article according to claim 2, wherein at least two of the plurality of test steel plates have different decarburization index values.

5. The method for evaluating the bendability of a molded article according to claim 2, wherein the metal structure at the center of the thickness direction of the steel sheet constituting the molded article, and the metal structure at the center of the thickness direction of at least two of the plurality of test steel sheets, have a martensite fraction of 85% or more.

6. The method for evaluating the bendability of a molded article according to claim 1, wherein the threshold for determination is determined based on the relationship between an evaluation index for the bendability of the molded article and the average Vickers hardness measured by applying a load of 10 gf to 50 gf to a cross section perpendicular to the surface of a test steel plate having a strength corresponding to the strength of the steel plate to be evaluated constituting the molded article, at a position 40 μm or less from the surface of the test steel plate.

7. The method for evaluating the bendability of a molded article according to claim 2, wherein the measurement position in the thickness direction of the Vickers hardness measured in the measurement step is the same as the measurement position in the thickness direction of the Vickers hardness used to determine the threshold value for determination.

8. The average Vickers hardness measured in the above measurement step is measured at a position between 10 μm and 40 μm from the surface of the molded body. The method for evaluating the bendability of a molded article according to any one of claims 2 to 7, wherein the average Vickers hardness used to determine the threshold for judgment is measured at a position of 10 μm or more and 40 μm or less from the surface of the test steel plate.

9. The method for evaluating the bendability of a molded article according to claim 1, wherein the molded article is a molded member for automobiles.

10. The method for evaluating the bendability of a molded article according to claim 1, wherein the molded article is a hot-stamped material.

11. The method for evaluating the bendability of a molded article according to claim 1, wherein the molded article is a cold-worked material.

12. The molded body has a specific softening layer provided in the thickness direction from the surface, the hardness of the center in the thickness direction of the portion where the specific softening layer is provided is 400 Hv or more, the specific softening layer is a region having a hardness at least 10 Hv lower than the hardness of the center in the thickness direction of the portion where the specific softening layer is provided, the thickness of the specific softening layer is 2% or more and less than 20% of the thickness of the portion where the specific softening layer is provided, and the hardness of the specific softening layer on the surface is at least 10 Hv lower than the hardness of the center in the thickness direction of the portion where the specific softening layer is provided. A method for evaluating the bendability of a molded article according to claim 1, wherein the molded article is made of a steel plate in which the hardness of the core is 0.5 times or more and less than 0.9 times, the specific softened layer has a first hardness change region in the thickness direction of the plate, which is the region from the surface to 40% of the thickness of the specific softened layer, and a second hardness change region of the specific softened layer that is not the first hardness change region, and the absolute value ΔHv1 of the hardness change in the thickness direction in the first hardness change region is greater than the absolute value ΔHv2 of the hardness change in the thickness direction in the second hardness change region.

13. The method for evaluating the bendability of a molded body according to claim 12, wherein the decarburization index value of the steel sheet constituting the molded body is 0.085 or higher.

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