Automotive undercarriage parts and their manufacturing methods

Abstract

The present invention provides an automotive undercarriage component and a method for manufacturing the same that can exhibit improved fatigue characteristics even at high strength. [Solution] An automotive undercarriage part is provided, which includes a molded body having a bent portion, wherein the ratio C1 / C2 of the average carbon concentration C1 from the surface of the bent portion to a thickness of 2 to 10 μm and the carbon concentration C2 at a position of 200 μm from the surface of the bent portion or molded body in the thickness direction is 0.85 or more, and the molded body contains 80% or more martensite by area ratio. A black coating containing 1.0 to 20.0 mass% carbon black and 80.0 to 99.0 mass% organic binder is applied to the surface at a density of 0.5 to 25.0 g / m² per side. 2 A method for manufacturing an automobile undercarriage part is provided, which includes the steps of: heating an unplated steel sheet coated with a certain amount of coating in an inert gas atmosphere or a reducing atmosphere of a gas furnace to a temperature above the austenite transformation completion temperature; forming the unplated steel sheet using a mold; and quenching the unplated steel sheet.

Description

[Technical Field]

[0001] The present invention relates to an automobile undercarriage component and a method for manufacturing the same. [Background technology]

[0002] In automotive suspension components, there is a demand for increased strength to reduce vehicle weight. However, because automotive suspension components are relatively thick, it is generally difficult to manufacture them by cold forming high-strength steel sheets.

[0003] Hot stamping (hot pressing) is a well-known technique for press forming materials that are difficult to cold form, such as high-strength steel sheets. Hot stamping is a hot forming technique in which the material to be formed is heated before forming. In this technique, because the material is heated before forming, the steel is soft and has good formability during the forming process. Therefore, even high-strength steel can be formed into complex shapes with high precision, and because quenching is performed simultaneously with forming using a press die, the formed steel is known to have sufficient strength.

[0004] In this regard, Patent Document 1 describes an Al-plated steel sheet for hot pressing, characterized by having an Al plating layer formed on one or both sides of the steel sheet, containing at least Al and having a surface roughness of Ra of 1.0 to 4.0 μm, and a surface film layer laminated on the Al plating layer, having an L* of 10 to 50, more specifically a black film containing carbon black. Furthermore, Patent Document 1 states that, according to the above configuration, it is possible to provide an Al-plated steel sheet for hot pressing that can be heated in about half the time compared to conventional methods using radiant heating, and is most suitable for rapid heating using radiation such as near-infrared rays, but it is also possible to halve the heating time even with atmospheric furnace heating, thereby increasing the productivity of the hot pressing process. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2011-149084 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] As described above, automotive suspension components are generally difficult to manufacture by cold forming high-strength steel sheets due to their relatively thick thickness. In addition, even when cold forming is possible by thinning the sheet, the fatigue properties may decrease somewhat in areas where stress is concentrated, such as bent sections, and in connection with this, further increases in strength may be required. On the other hand, the application of hot pressing (hot stamping), as described in Patent Document 1, to the manufacture of automotive suspension components with relatively thick sheet metal has not been sufficiently considered in conventional technology. In any case, in this field, there is a continuing need for automotive suspension components with improved fatigue properties in processed areas such as bent sections, regardless of whether they are cold-formed or hot-formed.

[0007] Therefore, the present invention aims to provide an automotive undercarriage component and a method for manufacturing the same, which, through a novel configuration, can exhibit improved fatigue characteristics even at high strength. [Means for solving the problem]

[0008] To achieve the above objective, the inventors focused on and investigated the metal structure of the surface layer in the bent portion of a molded body constituting an automobile suspension part. As a result, the inventors found that by controlling the carbon concentration in the surface layer of the bent portion to a predetermined range relative to the carbon concentration of the base material, the fatigue characteristics of the automobile suspension part are significantly improved, and thus completed the present invention.

[0009] The present invention, which has achieved the above objectives, is as follows. (1) An automotive undercarriage part comprising a molded body having at least one bent portion, wherein the ratio C1 / C2 of the average C concentration C1 from the surface of the at least one bent portion to a thickness of 2 to 10 μm and the C concentration C2 at a position 200 μm from the surface of the at least one bent portion in the thickness direction is 0.85 or more, and the molded body contains 80% or more martensite by area. (2) The automotive undercarriage part according to (1) above, characterized in that the ratio H1 / H2 of the Vickers hardness H1 at a position 20 μm in the thickness direction from the surface of the at least one bent portion to the Vickers hardness H2 at a position 200 μm in the thickness direction from the surface of the at least one bent portion is 0.900 to 1.200. (3) The automobile undercarriage part according to (1) or (2) above, characterized in that the molded body is made of a non-plated material. (4) An automobile undercarriage part according to any one of (1) to (3) above, characterized in that the ratio C1 / C3 of the average C concentration C1 from the surface of at least one bent portion to a thickness of 2 to 10 μm and the C concentration C3 at a position 30 μm from the surface of at least one bent portion to a thickness of 30 μm is 1.00 or more. (5) The automotive undercarriage part according to any one of the above items (1) to (4), characterized in that the molded body has a plate thickness of 1.6 to 5.0 mm. (6) The automotive undercarriage part according to any one of (1) to (5) above, characterized in that the molded body has a tensile strength of 1000 MPa or more. (7) The automotive undercarriage part according to any one of (1) to (6) above, characterized in that the molded body is a hot-stamped molded body. (8) A black coating containing 1.0 to 20.0% by mass of carbon black and 50.0 to 99.0% by mass of organic binder is applied to the surface, with a density of 0.5 to 25.0 g / m² per side. 2A non-plated steel sheet is coated so as to have an adhesion amount, and is heated in an inert gas atmosphere or a reducing atmosphere of a gas furnace to a temperature not lower than the austenite transformation completion temperature. The manufacturing method of an automotive underbody part includes a forming process of forming the non-plated steel sheet using a mold, and a quenching process of quenching the non-plated steel sheet. (9) The manufacturing method of the automotive underbody part according to (8) above, characterized in that the forming process and the quenching process are carried out simultaneously. (10) The manufacturing method of the automotive underbody part according to (8) above, characterized in that the heating process is carried out after the forming process, and then the quenching process is carried out.

Advantages of the Invention

[0010] According to the present invention, it is possible to provide an automotive underbody part that can exhibit improved fatigue characteristics and a manufacturing method thereof.

Brief Description of the Drawings

[0011] [Figure 1] It is a partial cross-sectional schematic view for explaining a bent portion of a molded body in an automotive underbody part.

Embodiments for Carrying Out the Invention

[0012] The automotive underbody part according to an embodiment of the present invention includes a molded body having at least one bent portion. The ratio C1 / C2 of the average C concentration C1 from the surface of the at least one bent portion in the thickness direction to 2 to 10 μm and the C concentration C2 at a position 200 μm in the thickness direction from the surface of the at least one bent portion is 0.85 or more, and the molded body contains 80% or more of martensite in terms of area ratio.

[0013] As mentioned earlier, automotive suspension components are generally thick, making it difficult to manufacture them by cold forming high-strength steel sheets. Even when cold forming is possible by thinning the sheet, the fatigue properties may decrease somewhat in areas where stress is concentrated, such as bends. Therefore, in addition to cold forming of high-strength steel sheets, other methods can be considered, such as forming the steel sheet in a relatively soft metallic state and then quenching it, or using hot stamping, which performs forming and quenching simultaneously. However, the application of hot stamping to the manufacture of automotive suspension components with relatively thick sheets has not been sufficiently studied in conventional technology.

[0014] In this study, the inventors focused on the microstructure of the surface layer of a molded body that constitutes an automotive undercarriage part, and investigated cases such as quenching after cold forming and hot stamping. As a result, they found that in both cases of quenching after cold forming and hot stamping, it is necessary to heat the material to a temperature above the austenite transformation completion temperature (the so-called Ac3 point) before quenching, and that during heating at such high temperatures, the carbon concentration in the surface layer decreases due to decarburization, and in connection with this, the fatigue properties of processed parts, especially bent sections, may deteriorate.

[0015] Therefore, the inventors conducted further studies, including on manufacturing methods, to suppress decarburization during high-temperature heating before quenching and thereby improve the fatigue characteristics of automotive undercarriage parts. As a result, as will be explained in detail later in relation to the manufacturing method, the inventors found that by applying a black coating containing carbon black or the like to the surface of a steel plate or a formed steel plate before high-temperature heating and then heating it at high temperature in a non-oxidizing atmosphere, decarburization from the surface layer of the steel plate or formed steel plate during high-temperature heating can be sufficiently suppressed or reduced. As a result, in the final automotive undercarriage part, the ratio C1 / C2 of the average C concentration C1 from the surface in the thickness direction from 2 to 10 μm from the surface to C concentration C2 at a position 200 μm from the surface in the thickness direction can be controlled to 0.85 or higher, and it was found that the fatigue characteristics of the automotive undercarriage part can be significantly improved by doing so.

[0016] While not intended to be bound by any particular theory, it is believed that applying a black coating to the surface of a steel sheet or formed steel sheet and heating it at high temperatures in a non-oxidizing atmosphere not only suppresses decarburization from the surface layer but also causes carburization from the black coating to the surface layer. Therefore, compared to simply suppressing decarburization, it is thought that the decrease in carbon concentration in the surface layer can be suppressed or reduced more significantly, and as a result, the hardness of the surface layer is improved and the fatigue properties are significantly improved. Conventionally, as described in Patent Document 1, it is known that a black coating containing carbon black or the like is applied to the surface of a steel sheet before high-temperature heating in hot pressing, from the viewpoint of shortening the heating time during heating. However, it was previously unknown that applying a black coating to the surface of a steel plate or formed steel plate constituting an automotive undercarriage part, and then heating it at a high temperature in a non-oxidizing atmosphere, suppresses decarburization and promotes carburization from the black coating to the surface layer, thereby allowing for appropriate control of the carbon concentration in the surface layer and significantly improving the fatigue characteristics of the resulting automotive undercarriage part. This fact has now been revealed for the first time by the present inventors. Based on this fact, according to the embodiments of the present invention, even with high strength, it is possible to significantly improve the fatigue characteristics of processed parts such as bent sections, where a decrease in fatigue strength is a concern, compared to conventional automotive undercarriage parts. The components of the automotive undercarriage part according to the embodiments of the present invention will be described in more detail below.

[0017] [Molded body] An automotive undercarriage part according to an embodiment of the present invention includes a molded body having at least one bent portion and containing 80% or more martensite by area ratio. An automotive undercarriage part according to an embodiment of the present invention includes at least such a molded body, and it is sufficient that at least one bent portion in the molded body has the above-described characteristics. Therefore, an automotive undercarriage part according to an embodiment of the present invention may include as part a molded body or material other than a molded body having at least one bent portion and containing 80% or more martensite by area ratio. Furthermore, the molded body according to an embodiment of the present invention is not particularly limited as long as it has at least one bent portion and contains 80% or more martensite by area ratio, and therefore may be a cold-formed body, a warm-formed body formed at a relatively low temperature of, for example, 300 to 400°C, or a hot-formed body, i.e., a hot-stamped molded body. In any case of a molded body, in order to generate 80% or more martensite by area ratio, it is necessary to heat it at a high temperature above the austenite transformation completion temperature and then quench it appropriately, and the carbon concentration of the surface layer may decrease due to decarburization during heating at such high temperatures. In contrast, in the case of molded articles according to the embodiments of the present invention, whether they are cold-formed articles, warm-formed articles, or hot-stamped articles, the carbon concentration in the surface layer of at least one bent portion can be appropriately controlled as described above, making it possible to significantly suppress the deterioration of fatigue properties caused by decarburization and the like.

[0018] [Martensite area ratio: 80% or more] The martensite area ratio of a molded article having at least one bend is 80% or more, as described above. Martensite is a very hard structure, and therefore, by including 80% or more martensite in the molded article by area ratio, it is possible to achieve high strength, specifically a tensile strength of 1000 MPa or more. In this regard, from the viewpoint of strength improvement, a higher martensite area ratio is preferable. For example, the martensite area ratio may be 85% or more, 90% or more, 95% or more, or 97% or more. The upper limit of the martensite area ratio is not particularly limited and may be 100%. For example, the martensite area ratio may be 99% or less or 98% or less. In the present invention, "martensite" includes not only as-quenched martensite (so-called fresh martensite) but also tempered martensite. The remaining structure is not particularly limited, but may consist of, for example, 20% or less of retained austenite, ferrite, bainite, and pearlite, or at least one of these.

[0019] [Identification of martensite and calculation of area ratio] The identification and calculation of martensite is performed as follows. First, a sample with a plate thickness cross-section perpendicular to the plate surface is taken, and this cross-section is used as the observation surface. This cross-section is polished, taking care to avoid leaving any scratches from polishing. Then, the strain introduced by surface polishing is removed by chemical polishing to obtain a cross-sectional observation sample for EBSD (Electron-Back-Scatter-Diffraction) analysis in which no change in crystal orientation has occurred due to polishing. Here, scratches in crystal orientation due to polishing refer to regions observed in a straight line penetrating the metal structure in the IPF map (Inverse-Pole-Figure) in EBSD analysis. Since such scratches are not inherent to the structure, it is naturally necessary to exclude them from the observation field of view. It is preferable to find a field of view that does not contain scratches, or if scratches inevitably appear in the field of view, to perform polishing again.

[0020] The above-mentioned EBSD analysis sample is subjected to electron backscatter analysis. While the EBSD analysis conditions can be within the bounds of common sense for those skilled in the art, a detailed example is described below. After inserting the sample into an EBSD-capable FE-SEM, the polished surface of the sample is tilted 60-70° relative to the electron beam incidence direction. The tilt direction of the sample must be such that the cross-section of the object to be measured faces the EBSD detector that will be inserted later. After tilting, the EBSD detector is inserted into the FE-SEM chamber and brought close to the sample. The position of the EBSD detector should be such that the electron beam detection sensitivity is high, but it is desirable to bring it as close as possible without colliding with the FE-SEM chamber, the sample, or the jig or table holding the sample. Subsequently, the electron beam detection sensitivity of the EBSD detector is adjusted. Since the adjustment of the detection sensitivity depends on the performance of the electron gun and EBSD detector of the FE-SEM used, it is desirable to perform the adjustment within the bounds of common sense for those skilled in the art. The adjustment should ultimately result in conditions that allow for clear observation of the EBSD pattern. Subsequently, the electron beam is irradiated onto the observation field at intervals of 0.3 μm step size, and EBSD patterns are collected at each measurement point. Based on the EBSD patterns at each measurement point, indexing and crystal orientation calculation are performed. For indexing and crystal orientation calculation, it is desirable to use APEX software from AMETEK. The EBSD data obtained in this way is analyzed using OIMAnalysis software (Orientation Imaging Microscopy), version 7 or later, which is AMETEK's EBSD data analysis software. The obtained EBSD data is opened in OIMAnalysis, and only regions with a CI value (Confidence Index) of 0.1 or higher are extracted. The CI value is an indicator of the reliability of the indexing and crystal orientation analysis results. Areas with a CI value lower than 0.1 are highly likely to be regions where the orientations of contaminants on the sample surface or grain boundaries overlap during electron beam irradiation, and can be considered regions that are not the original crystal orientation of the metal microstructure. Subsequently, the area ratio of regions with a GAM value (Grain Average Misorientation) of 0.5° or higher is calculated. A grain boundary refers to a boundary line where the azimuthal difference between measurement points is 15° or more.Here, the GAM value is the average orientation difference between measurement points in the region surrounded by grain boundaries. Microstructures formed at low temperatures, such as martensite, are characterized by the occurrence of orientation differences within grains due to transformation strain, making identification possible using the GAM value. Such investigations are carried out at five or more locations within a 100 × 100 μm field of view centered at a point 1 / 4 of the plate thickness from the surface of the steel plate, and the average area ratio is derived.

[0021] Retained austenite is formed at sub-nanometer levels between the martensite laths. Therefore, with the resolution required for EBSD analysis, it is difficult to separate the two, and the average area ratio of the region with a GAM value of 0.5° or higher obtained by the method described above is the combined area ratio of martensite and retained austenite. For this reason, the area ratio of retained austenite measured by the procedure described later is subtracted from the area ratio obtained by the method described above, and the resulting value is determined as the area ratio of martensite at the 1 / 4 thickness position. As described above, the martensite in the molded article according to the embodiment of the present invention is a structure that may include tempered martensite, and no particular distinction is made regarding the tempered state of the martensite.

[0022] [Residual austenite] The area fraction of retained austenite is calculated by measurement using X-rays. First, the material from the surface of the sample up to 1 / 4 of the thickness in the thickness direction is removed by mechanical and chemical polishing. Next, the integral intensity ratio of the diffraction peaks (200), (211) of the bcc phase and (200), (220), (311) of the fcc phase, obtained using MoKα rays as characteristic X-rays on the polished sample, is used to calculate the structural fraction of retained austenite, and this is determined as the area fraction of retained austenite.

[0023] [Other Organizations] The area percentage of other structures is determined by subtracting the area percentage of martensite and retained austenite obtained above from 100%. Other structures may include, for example, at least one of ferrite, bainite, and pearlite, or at least one of them, but in this invention, the identification of these structures and the determination of the area percentage of each structure are not particularly necessary to achieve the objectives of this invention. If there is any need, it is not difficult to identify them using methods commonly applied by those skilled in the art.

[0024] [At least one bend] In an embodiment of the present invention, the above-described molded body has at least one bent portion. Figure 1 is a schematic partial cross-sectional view illustrating the bent portion of a molded body in an automotive suspension part. Referring to Figure 1, the molded body 1 includes, for example, a bent portion 2 and a flat portion 3, and has an inner surface IS and an outer surface OS. Here, perpendicular lines are drawn from points B and C on the outer surface OS to the inner surface IS, and the intersection points of these lines with the inner surface IS are denoted as D and E, respectively. These points D and E are the boundaries between the bent portion 2 and the flat portion 3 on the inner surface IS of the automotive suspension part 1. The intersection point obtained by further extending line segments BD and CE inward is denoted as A. Here, the radius of curvature inside the bent portion 2 approximated by an arc centered at point A and passing through points D and E is denoted as r. In the present invention, the portion where r is 25 mm or less is defined as the bent portion 2.

[0025] In automotive suspension parts, including high-strength molded articles having a metallic structure with a martensitic area ratio of 80% or more, the presence of such bends tends to result in somewhat reduced fatigue characteristics compared to flat areas. In addition, the carbon concentration in the surface layer may decrease due to decarburization during high-temperature heating before quenching. However, in automotive suspension parts according to the embodiment of the present invention, as described above, the fatigue characteristics can be significantly improved by controlling the ratio C1 / C2 between the average carbon concentration C1 from the surface to 2 to 10 μm in the thickness direction from the surface of the bend to 0.85 or higher. In automotive suspension parts according to the embodiment of the present invention, it is sufficient that the molded article has at least one of the above-mentioned bends. If the molded article has multiple bends, it is sufficient that the ratio C1 / C2 between the carbon concentration C2 at 200 μm in the thickness direction from the surface is controlled to 0.85 or higher in at least one of the multiple bends. This is because, for example, the bending sections that are subjected to relatively high stresses depending on the usage conditions can be identified, and if the ratio C1 / C2 is controlled to 0.85 or higher in at least those bending sections, the fatigue characteristics will be reliably improved. Furthermore, the areas where fatigue is a problem in bending sections are often the inner surface of the bend, and if one of the two surfaces must be selected, it is preferable that the inner surface of the bend satisfies the C concentration.

[0026] Furthermore, in the automotive undercarriage parts according to the embodiments of the present invention, the molded body may include not only bent portions but also trim portions and end faces formed by trimming or drilling. These portions also tend to have somewhat lower fatigue characteristics compared to flat portions. Therefore, especially when high-temperature heating and quenching are performed after these processes, applying a black coating to the trim portions and end faces before high-temperature heating can sufficiently suppress or reduce decarburization from these portions. In such cases, it is possible to improve the fatigue characteristics of the entire automotive undercarriage part or the entire molded body. For example, when trimming or drilling is performed after quenching, the trim portions and drilled end faces do not exist during high-temperature heating before quenching, so no decarburization occurs from these portions. On the other hand, since decarburization is suppressed on the front and back surfaces around the trim portions and end faces by applying a black coating before high-temperature heating, the fatigue characteristics of the trim portions and end faces can be improved compared to cases where such suppression of decarburization is not performed. Furthermore, in the automotive suspension parts according to the embodiments of the present invention, it is not necessary for the ratio C1 / C2 in the entire automotive suspension part or the surface layer of the molded body to be controlled to 0.85 or higher. Only one or more specific bends where a decrease in fatigue characteristics is a concern may be controlled to have a ratio C1 / C2 of 0.85 or higher.

[0027] [Ratio C1 / C2: 0.85 or more] In the automotive undercarriage parts according to the embodiment of the present invention, as described above, the ratio C1 / C2 of the average C concentration C1 from the surface of at least one bent portion to a thickness of 2 to 10 μm and the C concentration C2 at a position 200 μm from the surface of the at least one bent portion or molded body in the thickness direction is controlled to 0.85 or higher. By maintaining the C concentration of the surface layer in the bent portion at a sufficiently high level compared to the C concentration of the base material, the hardness of the surface layer in the bent portion can be improved, and as a result, the fatigue characteristics of the bent portion can be significantly improved. From the viewpoint of further improving the fatigue characteristics of the bent portion, a higher ratio C1 / C2 is preferable, and may be, for example, 0.86 or higher, 0.88 or higher, 0.90 or higher, 0.92 or higher, or 0.94 or higher. There is no particular upper limit, but for example, the ratio C1 / C2 may be 1.10 or lower, 1.05 or lower, 1.03 or lower, 1.00 or lower, or 0.98 or lower.

[0028] [Ratio C1 / C3:1.00 or more] In a preferred embodiment of the present invention, the ratio C1 / C3 of the average C concentration C1 from 2 to 10 μm in the thickness direction from the surface of at least one bent portion to the C concentration C3 at a position 30 μm in the thickness direction from the surface of the at least one bent portion is controlled to 1.00 or higher. As will be explained in detail later in relation to the manufacturing method, by applying a black coating to the surface of a steel sheet or a formed steel sheet and heating it at a high temperature in a non-oxidizing atmosphere, it is thought that not only is decarburization from the surface layer suppressed, but carburization from the black coating to the surface layer also occurs. The effect of such carburization is particularly pronounced in the surface layer closer to the surface. For this reason, when comparing the average C concentration C1 from 2 to 10 μm in the thickness direction from the surface of at least one bent portion to the C concentration C3 at a position 30 μm in the thickness direction from the surface of the at least one bent portion, the value of C1 may be equal to or higher than the value of C3. According to a preferred embodiment of the present invention, by controlling the ratio C1 / C3 to 1.00 or higher, the hardness of the surface layer in the bent portion can be further improved compared to the case where the ratio C1 / C3 is less than 1.00, and as a result, the fatigue characteristics of the bent portion can be improved more significantly. From the viewpoint of further improving the fatigue characteristics of the bent portion, a higher ratio C1 / C3 is preferable, and may be, for example, 1.01 or higher, 1.02 or higher, 1.03 or higher, or 1.04 or higher. There is no particular upper limit, but for example, the ratio C1 / C3 may be 1.10 or lower, 1.08 or lower, 1.07 or lower, 1.06 or lower, or 1.05 or lower.

[0029] [Measurement of ratios C1 / C2 and C1 / C3] The ratios C1 / C2 and C1 / C3 are measured using an EPMA (electron probe microanalyzer) as follows: First, the cross-section in the thickness direction of the bent portion of a molded body constituting an automotive suspension part is polished to a mirror finish. Then, using a SEM-EPMA, the acceleration voltage is 15kV and the irradiation current is 5×10⁻¹⁰. -7Under condition A, line analysis is performed at 0.5 μm intervals in the thickness direction from the surface of the bent portion. Next, the average value of the C concentration from 2 to 10 μm in the thickness direction from the surface of the bent portion is determined as C1, then the average value of the C concentration from 29 to 31 μm in the thickness direction from the surface of the bent portion is determined as the C concentration at the 30 μm position, and similarly, the average value of the C concentration from 199 to 201 μm in the thickness direction from the surface of the bent portion is determined as the C concentration at the 200 μm position, C2. C2 corresponds to the C concentration of the base material. Therefore, if it is difficult to measure the C concentration at the 200 μm position in the thickness direction from the surface of the bent portion, the average value of the C concentration from 199 to 201 μm in the thickness direction from the surface of another part of the molded body, for example, a flat portion, may be determined as the C concentration at the 200 μm position, C2. Finally, the ratios C1 / C2 and C1 / C3 are determined based on the obtained C1, C2, and C3.

[0030] [Ratio H1 / H2:0.900~1.200] In a preferred embodiment of the present invention, the ratio H1 / H2 of the Vickers hardness H1 at a position 20 μm in the thickness direction from the surface of at least one bent portion to the Vickers hardness H2 at a position 200 μm in the thickness direction from the surface of the at least one bent portion or molded body is controlled to be within the range of 0.900 to 1.200. By controlling the ratio H1 / H2 to be within the range of 0.900 to 1.200, the fatigue characteristics of the bent portion can be more reliably improved, and as a result, the fatigue characteristics of the bent portion can be more reliably improved. From the viewpoint of further improving the fatigue characteristics of the bent portion, a higher ratio H1 / H2 is preferable, and may be, for example, 0.920 or higher, 0.940 or higher, 0.960 or higher, 0.980 or higher, 1.000 or higher, or 1.020 or higher. Similarly, the ratio H1 / H3 may be 1.180 or less, 1.160 or less, 1.140 or less, 1.120 or less, 1.100 or less, or 1.080 or less.

[0031] [Measurement of H1 / H2 ratio] The ratio H1 / H2 is measured using a micro-Vickers hardness tester as follows: First, a sample is taken from the cross-section in the thickness direction of the bend in a molded body constituting an automobile suspension part, and the observation surface is mirror-polished. Using a micro-Vickers hardness tester, the Vickers hardness at a position 20 μm from the surface of the bend in the thickness direction is measured with an indentation load of 50 gf. Then, the Vickers hardness at a total of three points is measured at the same position on a line perpendicular to the thickness direction with an indentation load of 50 gf, and the average value of these measurements is determined as H1. Similarly, the Vickers hardness at a position 200 μm from the surface of the bend in the thickness direction is measured with an indentation load of 50 gf. Then, the Vickers hardness at a total of three points is measured at the same position on a line perpendicular to the thickness direction with an indentation load of 50 gf, and the average value of these measurements is determined as H2. Since H2 corresponds to the Vickers hardness of the base material, it is not necessarily limited to a position 200 μm from the surface of the bent portion in the thickness direction, but may also be the Vickers hardness at a position 200 μm from the surface of another part of the molded body, such as a flat portion. Finally, the ratio H1 / H2 is determined based on the obtained H1 and H2.

[0032] [Non-plated material] In a preferred embodiment of the present invention, the molded body is made of an unplated material. In the assembly of automotive undercarriage parts, welding such as arc welding is used, but when plated materials are arc welded, blowholes may occur in the case of zinc plating, for example, or slag may be generated in the case of aluminum plating, reducing corrosion resistance after painting. Therefore, by using an unplated material for the molded body, it is possible to reliably eliminate the occurrence of the above-mentioned problems when assembling automotive undercarriage parts. On the other hand, when using an unplated material, the effect of decarburization during high-temperature heating is generally greater compared to the case of plated materials. However, in the automotive undercarriage parts according to the embodiment of the present invention, by devising the manufacturing method, the ratio C1 / C2 of the average C concentration C1 from the surface in the thickness direction from 2 to 10 μm in the bent portion of the molded body to the C concentration C2 at a position 200 μm from the surface in the thickness direction can be controlled to 0.85 or more, thereby significantly improving the fatigue characteristics. Therefore, according to a preferred embodiment of the present invention, by using an unplated material as the molded body, it is possible to achieve improved fatigue characteristics without the disadvantages of using plated materials.

[0033] [Identification of unplated materials] If it is not possible to determine that a material is unplated by visual inspection, etc., the identification of the unplated material is performed using EPMA as follows: First, the cross-section in the thickness direction of the molded body constituting the automotive undercarriage part is finished with mirror polishing, and then, using SEM-EPMA, the acceleration voltage: 15kV and irradiation current: 5×10 -7 Under condition A, the concentrations of Zn and Al are measured at five locations 2 μm from the surface in the thickness direction. Then, if the average concentration of Zn and Al at all five locations is 5% by mass or less, the material is determined to be unplated, meaning no plating is present.

[0034] [plate thickness] The thickness of the molded body is not particularly limited, but may be, for example, 1.6 to 5.0 mm. The thickness may be 1.8 mm or more, 2.0 mm or more, 2.3 mm or more, or 2.5 mm or more. Similarly, the thickness may be 4.5 mm or less, 4.0 mm or less, 3.5 mm or less, or 3.0 mm or less.

[0035] [Tensile Strength (TS)] The tensile strength (TS) of the molded article is not particularly limited, but may be, for example, 1000 MPa or higher. To obtain such a very high tensile strength, it is necessary to heat the article at a temperature above the austenite transformation completion temperature and then perform appropriate quenching. During heating at such high temperatures, the carbon concentration in the surface layer may decrease due to decarburization. In contrast, the molded article according to the embodiment of the present invention can appropriately control the carbon concentration in the surface layer at at least one bent portion as described above, so even with a high strength of 1000 MPa or higher, it is possible to significantly suppress the decrease in fatigue properties caused by decarburization and the like. The tensile strength of the molded article is preferably 1080 MPa or higher, 1180 MPa or higher, 1250 MPa or higher, 1300 MPa or higher, or 1470 MPa or higher. The upper limit is not particularly limited, but for example, the tensile strength may be 2000 MPa or lower, 1800 MPa or lower, or 1600 MPa or lower. Tensile strength is determined by taking a JIS No. 5 test specimen from a position at least 1 mm away from the bend or hole of the molded body and performing a tensile test in accordance with JIS Z 2241:2022. If it is difficult to obtain a JIS No. 5 test specimen, a JIS No. 13B test specimen or a micro-test specimen with a similar shape to the JIS No. 13B test specimen may be used.

[0036] [Preferred chemical composition of the molded article] As described above, the present invention aims to provide automotive undercarriage parts that exhibit improved fatigue characteristics even with high strength. This objective is achieved by controlling the ratio C1 / C2 between the average C concentration C1 from the surface of at least one bend to 2 to 10 μm in the thickness direction and the C concentration C2 at a position 200 μm from the surface of the at least one bend to 0.85 or higher in a molded article containing 80% or more martensite. Therefore, it is clear that the chemical composition of the molded article itself is not an essential technical feature for achieving the objective of the present invention. The following describes preferred chemical compositions of molded articles according to embodiments of the present invention, but these descriptions are intended to be merely examples of preferred chemical compositions for obtaining, for example, a metal structure containing 80 area% or more of martensite or a tensile strength of 1000 MPa or higher, and the present invention is not intended to be limited to molded articles having such specific chemical compositions. In addition, in the following descriptions, "%", which is the unit of content of each element, means "mass%" unless otherwise specified. Furthermore, in this specification, unless otherwise specified, the "~" indicating a numerical range means that the numbers before and after it are included as the lower and upper limits.

[0037] In a particular embodiment of the present invention, for example, the chemical composition of the molded article is, in mass%, C: 0.100~0.400%, Si: 0.01~2.50%, Mn: 1.00~4.00%, P: 0.050% or less, S: 0.0100% or less, N: 0~0.0100%, O: 0~0.0100%, Al: 0-1,500%, Cr: 0~1.00%, Mo: 0~1.00%, Cu: 0~1.00%, Ni: 0~1.00%, Co: 0~1.00%, W: 0~1.00%, B: 0~0.0050%, Ta: 0~1.000%, Sn: 0~1.000%, Sb: 0~0.500%, Ti: 0~0.100%, Nb: 0~0.200%, V: 0~1.00%, As: 0~0.100%, Zn: 0~1,000%, Ca: 0~0.0100%, Mg: 0~0.0100%, Zr: 0~0.0100%, Hf: 0~0.0100%, Sr: 0~0.0100%, Bi: 0~0.0100%, REM: 0~0.0100%, and The remainder has a chemical composition consisting of Fe and impurities. Each element is described in more detail below.

[0038] [C:0.100~0.400%] Carbon (C) is an effective element for increasing the strength of steel plates. To fully obtain this effect, the C content is preferably 0.100% or more, and may be 0.140% or more, 0.160% or more, 0.180% or more, 0.200% or more, or 0.220% or more. On the other hand, the C content is preferably 0.400% or less, and may be 0.350% or less, 0.320% or less, 0.300% or less, or 0.280% or less.

[0039] [Si: 0.01~2.50%] Si is an element that contributes to improved strength and moldability. To fully obtain these effects, the Si content is preferably 0.01% or more, and may be 0.05% or more, 0.10% or more, or 0.20% or more. On the other hand, the Si content is preferably 2.50% or less, and may be 2.00% or less, 1.50% or less, 1.00% or less, or 0.50% or less.

[0040] [Mn: 1.00~4.00%] Mn is an element that enhances hardenability and contributes to improved strength. To fully obtain this effect, the Mn content is preferably 1.00% or more, and may be 1.10% or more or 1.20% or more. On the other hand, the Mn content is preferably 4.00% or less, and may be 3.00% or less, 2.00% or less or 1.50% or less.

[0041] [P: 0.050% or less, S: 0.0100% or less, N: 0.0100% or less, and O: 0.0100% or less] P, S, N, and O are elements contained as impurities, and their content is limited to 0.050% or less, 0.0100% or less, 0.0100% or less, and 0.0100% or less, respectively. Preferably, the P, S, N, and O content is 0.020% or less, 0.0020% or less, 0.0050% or less, and 0.0050% or less, respectively.

[0042] The basic chemical composition of the molded article according to the embodiment of the present invention is as described above. Furthermore, the molded article may optionally contain at least one of the following optional elements in place of a portion of the remaining Fe. For example, the molded article may have the following composition: Al: 0-1.500%, Cr: 0-1.00%, Mo: 0-1.00%, Cu: 0-1.00%, Ni: 0-1.00%, Co: 0-1.00%, W: 0-1.00%, B: 0-0.0050%, Ta: 0-1.000%, Sn: 0-1.000%, Sb: 0-0.500%, Ti: 0-0.100%, Nb: 0-0. It may contain at least one of the following elements: 0.200%, V: 0-1.00%, As: 0-0.100%, Zn: 0-1.000%, Ca: 0-0.0100%, Mg: 0-0.0100%, Zr: 0-0.0100%, Hf: 0-0.0100%, Sr: 0-0.0100%, Bi: 0-0.0100%, and REM: 0-0.0100%. The content of each of these optional elements may be 0.0001% or more, or 0.001% or more. REM is a collective term for 17 elements: Sc (atomic number 21), Y (atomic number 39), and lanthanides from La (atomic number 57) to Lu (atomic number 71), and the REM content is the total content of these elements.

[0043] In the molded article according to the embodiment of the present invention, the remainder other than the above-mentioned elements consists of Fe and impurities. Impurities include components that are mixed in due to various factors in the manufacturing process, such as raw materials like ore and scrap, when the molded article is manufactured industrially, as well as components that are included in a range that does not affect the effects of the present invention.

[0044] The chemical composition of the steel sheet according to the embodiment of the present invention can be measured by general analytical methods. For example, the chemical composition of the molded body can be measured using inductively coupled plasma-atomic emission spectrometry (ICP-AES). C and S can be measured using the combustion-infrared absorption method, N can be measured using the inert gas fusion-thermal conductivity method, and O can be measured using the inert gas fusion-nondispersive infrared absorption method.

[0045] The automotive suspension components according to the embodiments of the present invention are not particularly limited, but may include lower arms, trailing arms, and the like. These automotive suspension components only need to include a molded body according to the embodiments of the present invention in at least a portion of their components, and therefore at least a portion of these components will satisfy the characteristics of the molded body described above.

[0046] <Manufacturing method for automotive undercarriage parts> Next, preferred manufacturing methods for automotive undercarriage components according to embodiments of the present invention will be described. The following description is intended to illustrate characteristic methods for manufacturing automotive undercarriage components according to embodiments of the present invention, and is not intended to limit the automotive undercarriage components to those manufactured by the manufacturing methods described below.

[0047] A method for manufacturing automotive undercarriage parts according to an embodiment of the present invention involves applying a black coating to the surface containing 1.0 to 20.0% by mass of carbon black and 80.0 to 99.0% by mass of an organic binder, with a density of 0.5 to 25.0 g / m² per side. 2The process is characterized by including a heating step in which an unplated steel sheet coated to a certain amount is heated to a temperature above the austenite transformation completion temperature in an inert gas atmosphere or a reducing atmosphere of a gas furnace, a forming step in which the unplated steel sheet is formed using a mold, and a quenching step in which the unplated steel sheet is hardened. Each step will be described in detail below.

[0048] [Heating process] First, before the heating process, a black coating is applied to the surface, preferably both sides, of the unplated steel sheet. Specifically, the coating contains 1.0 to 10.0% by mass of carbon black, 1.0 to 30.0% by mass of an organic binder, an optional other component such as an oxide, and 60.0 to 98.0% by mass of water, and the amount of black coating applied after drying is 0.5 to 25.0 g / m² per side. 2 The coating is applied to the surface, preferably both sides, of an unplated steel sheet, and then dried at a temperature of 70 to 120°C, resulting in a black coating on the surface, preferably both sides, containing 1.0 to 20.0% by mass of carbon black, 50.0 to 99.0% by mass of an organic binder, and other optional components such as oxides, at a density of 0.5 to 25.0 g / m² per side. 2 An unplated steel sheet is obtained with the coating amount. The drying method is not particularly limited and may be any suitable method known to those skilled in the art. The unplated steel sheet is not particularly limited and may be any suitable unplated steel sheet known to those skilled in the art, for example, an unplated steel sheet having the preferred chemical composition described in relation to molded bodies, and in particular, an unplated hot-rolled steel sheet. Whether or not it is an unplated steel sheet is determined by the method described above in relation to the identification of unplated materials.

[0049] The carbon black content in the black film (i.e., after drying) may be 3.0% by mass or more, 5.0% by mass or more, 8.0% by mass or more, or 10.0% by mass or more, and / or 18.0% by mass or less, 15.0% by mass or less, or 12.0% by mass or less. Similarly, the content of the organic binder in the black film may be 55.0% by mass or more, 60.0% by mass or more, 65.0% by mass or more, 70.0% by mass or more, 75.0% by mass or more, 80.0% by mass or more, 82.0% by mass or more, 85.0% by mass or more, or 88.0% by mass or more, and / or 97.0% by mass or less, 95.0% by mass or less, 92.0% by mass or less, or 90.0% by mass or less. The organic binder is not particularly limited, and includes, for example, polyurethane, polyester, acrylic, and any combination thereof. By using these organic binders, a black film with good adhesion can be obtained. From the viewpoint of further enhancing the decarburization suppression effect and even the carburization promotion effect by the black film, the larger the adhesion amount of the black film, the more preferable. For example, 0.6 g / m 2 or more, 0.7 g / m 2 or more, 0.8 g / m 2 or more, 1.0 g / m 2 or more, 2.0 g / m 2 or more, 3.0 g / m 2 or more, 5.0 g / m 2 or more, 8.0 g / m 2 or more, or 10.0 g / m 2 or more may be acceptable. On the other hand, if the black film is excessively adhered, the effect may saturate and the manufacturing cost may increase. Therefore, the adhesion amount of the black film is 25.0 g / m 2 or less, for example, 22.0 g / m 2 or less, 20.0 g / m 2 or less, 18.0 g / m 2 or less, 15.0 g / m 2 or less, or 12.0 g / m 2 or less may be acceptable.

[0050] In the heating process, the unplated steel sheet coated with a black film is heated to a temperature above the austenite transformation completion temperature, i.e., above the Ac3 point, in an inert gas atmosphere or a reducing atmosphere in a gas furnace. By heating the unplated steel sheet coated with a black film to a temperature above the austenite transformation completion temperature in a non-oxidizing atmosphere such as an inert gas atmosphere or a reducing atmosphere in a gas furnace, it is possible to sufficiently suppress or reduce decarburization from the surface layer of the unplated steel sheet due to the black film, even under high-temperature heating. As a result, in the final automotive undercarriage part, the ratio C1 / C2 of the average carbon concentration C1 from the surface to a thickness of 2 to 10 μm in the thickness direction from the surface can be controlled to 0.85 or higher, thereby significantly improving the fatigue characteristics of the automotive undercarriage part. For example, in an oxidizing atmosphere where the air-fuel ratio of the atmosphere or gas combustion atmosphere exceeds 1.00, even if a black coating is applied to the surface of an unplated steel sheet in an appropriate amount, it becomes impossible to sufficiently suppress or reduce decarburization from the surface layer of the unplated steel sheet. As a result, it becomes impossible to control the ratio C1 / C2 of the average carbon concentration C1 from the surface to a thickness of 2 to 10 μm in the thickness direction from the surface of at least one bend in the resulting automotive undercarriage part to 0.85 or higher. Therefore, in this manufacturing method, the combination of applying a black coating and heating in a non-oxidizing atmosphere such as an inert gas atmosphere or a reducing atmosphere of a gas furnace is extremely important.

[0051] The inert gas atmosphere may be, for example, an N2 atmosphere or an Ar atmosphere. On the other hand, when using a gas furnace, a reducing atmosphere can be created, for example, by adjusting the air-fuel ratio of the gas combustion atmosphere to 0.75 to 0.90, preferably 0.80 to 0.85. As for heating methods, in addition to gas furnaces, examples include electric furnaces, electrostatic heating, high-frequency heating, induction heating, etc. Furthermore, the austenite transformation completion temperature (Ac3 point) is calculated by the following formula based on the chemical composition of the unplated steel sheet (the same chemical composition as the final molded body). Substitute the mass % of the element for the element symbol in the following formula. Substitute 0 mass % for elements that are not present. Ac3(℃)=912-230.5C+31.6Si-20.4Mn For example, in hot stamping, the material is generally heated to a temperature range of 800-1000°C and then held for a predetermined time, for example, 60-600 seconds. During the holding period, the temperature of the unplated steel sheet may be varied or kept constant within the 800-1000°C range.

[0052] [Molding process] In the forming process, an unplated steel sheet is formed using a die into any shape suitable for a desired automotive undercarriage part. The forming is not particularly limited and may be cold forming, warm forming performed at a relatively low temperature of, for example, 300-400°C, or hot forming, i.e., hot stamping. If the forming is cold forming, the conditions are not particularly limited and any suitable conditions known to those skilled in the art may be selected. In the case of warm forming, it is possible to carry out the process under the same conditions as cold forming, except that the forming is performed at a relatively low temperature of 300-400°C. In addition, in the case of cold forming or warm forming, it is preferable to perform a heating process after the forming process, followed by a quenching process which will be described later. On the other hand, in the case of hot stamping, the forming process and the quenching process which will be described later are carried out simultaneously.

[0053] [Heat treatment process] Finally, in the quenching process, the unplated steel sheet is quenched. The quenching conditions are not particularly limited as long as the martensite area ratio in the final formed body is 80% or more. For example, in the case of cold forming or warm forming, the quenching process is preferably carried out by heating to above the Ac3 point and then rapidly cooling to below the austenite transformation onset temperature at an average cooling rate of 20°C / second or more. On the other hand, in the case of hot stamping, the quenching process can be carried out simultaneously with the forming process by any suitable method known to those skilled in the art. After heating and holding in the furnace, the unplated steel sheet is removed from the furnace, and then, after the unplated steel sheet reaches a predetermined temperature, for example, 850°C or lower, forming and quenching can be carried out under normal conditions, and are not particularly limited, but can be cooled to a temperature range of, for example, 250°C or lower by mold cooling at an average cooling rate of 20°C / second or more.

[0054] Optionally, shot blasting or the like may be applied to the unplated steel sheet (molded body) after the quenching process. In particular, in the case of hot stamping, the unplated steel sheet, with a black coating applied, is held at a high temperature of 800 to 1000°C for 60 to 600 seconds as described above. Therefore, scale may form on the surface of the molded body after hot stamping. Accordingly, by applying shot blasting under appropriate conditions, this scale can be reliably removed.

[0055] The present invention will be described in more detail below with reference to examples, but the present invention is not limited in any way to these examples. [Examples]

[0056] In the following examples, hot stamping was performed under various conditions, and the fatigue characteristics of the resulting molded articles were investigated.

[0057] First, an unplated hot-rolled steel sheet having dimensions of 150 mm × 200 mm × 4.5 mm and a chemical composition of C: 0.225%, Si: 0.25%, Mn: 1.25%, P: 0.007%, S: 0.0003%, with the remainder being Fe and impurities, was coated on both sides with a paint containing 3.6 mass% carbon black, 26.4 mass% organic binder, and 70.0 mass% water in various amounts. The sheets were then dried at 80°C to obtain unplated hot-rolled steel sheets with black coatings of varying adhesion amounts as shown in Table 1. However, no black coating was applied to Comparative Examples 1 and 2. The carbon black and organic binder content in each black coating after drying was 12.0% and 88.0%, respectively. The Ac3 point of the unplated hot-rolled steel sheet with the above chemical composition is 843°C.

[0058] Next, the unplated hot-rolled steel sheet coated with a black film was heated to 900°C in an electric or gas furnace in the atmosphere shown in Table 1 and held for 120 seconds. Then, the unplated hot-rolled steel sheet was removed from the electric or gas furnace and rapidly cooled by sandwiching it between flat die plates at room temperature. Finally, shot blasting was performed to remove scale from the surface and obtain a hot-stamped molded body. In this example, no bending was performed, and therefore C1, C2, C3, H1, and H2 shown in Table 1 all correspond to measurements taken from the surface of the flat portion. However, by referring to the disclosure herein, it will be fully understood that regardless of whether bending is performed or not, if decarburization is not sufficiently suppressed, and therefore the carbon concentration in the surface layer is relatively low, the fatigue properties will decrease, and that the results of this example can also be applied to molded bodies having bent portions, especially hot-stamped molded bodies.

[0059] [Table 1]

[0060] The properties of the obtained hot-stamped molded articles were measured and evaluated by the following method.

[0061] [Tensile Strength (TS)] Tensile strength (TS) was measured by taking a JIS 13B test specimen from a hot-stamped molded body that had undergone the same tests as above, and performing a tensile test in accordance with JIS Z 2241:2022.

[0062] [Evaluation of fatigue characteristics] The fatigue characteristics were determined by performing a planar bending fatigue test 200,000 times on the hot-stamped molded product obtained above, according to JIS Z 2275:1978, and the resulting fatigue strength σ 20 The evaluation was performed using the following method. The stress ratio was set to -1. More specifically, although the same steel grade was used in each inventive example and comparative example, the TS was not perfectly identical in all inventive examples and comparative examples, so σ 20 The value obtained by dividing by the TS of each example (σ 20 Fatigue characteristics were evaluated using / TS.

[0063] σ 20 Automotive suspension components with a / TS value of 0.400 or higher were evaluated as exhibiting improved fatigue characteristics. The results are shown in Table 1.

[0064] Referring to Table 1, in Comparative Examples 1 and 2, where no black coating was applied, it is thought that decarburization from the surface layer could not be sufficiently suppressed despite heating in a non-oxidizing atmosphere. As a result, the ratio C1 / C2 was less than 0.85, and the fatigue properties deteriorated. In Comparative Examples 3 and 6, although the black coating was applied in sufficient quantity, it is thought that decarburization from the surface layer could not be sufficiently suppressed because it was heated in an oxidizing atmosphere. As a result, the ratio C1 / C2 was less than 0.85, and the fatigue properties deteriorated. In Comparative Example 8, it is thought that decarburization from the surface layer could not be sufficiently suppressed because the amount of black coating applied was insufficient. As a result, the ratio C1 / C2 was less than 0.85, and the fatigue properties deteriorated.

[0065] In contrast, in all of the inventions, by applying a black coating to the surface of an unplated hot-rolled steel sheet in an appropriate amount and heating it at a high temperature in a non-oxidizing atmosphere, decarburization from the surface layer of the unplated hot-rolled steel sheet during high-temperature heating can be sufficiently suppressed or reduced, and as a result the C1 / C2 ratio can be controlled to 0.85 or higher, and relatedly the fatigue properties can be significantly improved. In particular, the amount of black coating applied was 0.7 g / m² per side. 2 In the above-mentioned examples 4, 5, 7, and 10-14, the ratio C1 / C3 is 1.00 or greater and σ 20 Compared to Invention Example 9, in which the / TS ratio was 0.410 or higher and the ratio C1 / C3 was less than 1.00, the fatigue characteristics were significantly improved. This result suggests that increasing the amount of black coating not only suppressed decarburization but also promoted carburization. Furthermore, in all Invention Examples and Comparative Examples, the martensite area ratio in the metal structure of the hot-stamped molded articles was 97% or higher. [Explanation of Symbols]

[0066] 1 Molded body 2. Bent section 3 Flat area IS inner surface OS outer surface r inner radius of curvature

Claims

1. An automotive undercarriage part comprising a molded body having at least one bent portion, wherein the ratio C1 / C2 of the average C concentration C1 from 2 to 10 μm in the thickness direction from the surface of the at least one bent portion to the C concentration C2 at a position 200 μm in the thickness direction from the surface of the at least one bent portion is 0.85 or more, and the molded body contains 80% or more martensite by area.

2. The automotive undercarriage part according to claim 1, characterized in that the ratio H1 / H2 of the Vickers hardness H1 at a position 20 μm in the thickness direction from the surface of the at least one bent portion to the Vickers hardness H2 at a position 200 μm in the thickness direction from the surface of the at least one bent portion is 0.900 to 1.

200.

3. The automotive undercarriage part according to claim 1 or 2, characterized in that the molded body is made of a non-plated material.

4. The automotive undercarriage part according to claim 1 or 2, characterized in that the ratio C1 / C3 of the average C concentration C1 from the surface of at least one bent portion to a thickness of 2 to 10 μm and the C concentration C3 at a position 30 μm from the surface of at least one bent portion to a thickness of 30 μm is 1.00 or more.

5. The automotive undercarriage part according to claim 1 or 2, characterized in that the molded body has a plate thickness of 1.6 to 5.0 mm.

6. The automotive undercarriage part according to claim 1 or 2, characterized in that the molded body has a tensile strength of 1000 MPa or more.

7. The automotive undercarriage part according to claim 1 or 2, characterized in that the molded body is a hot-stamped molded body.

8. The surface is coated with a black film containing 1.0 to 20.0% by mass of carbon black and 50.0 to 99.0% by mass of an organic binder, at a density of 0.5 to 25.0 g / m² per side. 2 A method for manufacturing an automobile undercarriage part, comprising: a heating step of heating an unplated steel sheet coated with a certain amount of coating in an inert gas atmosphere or a reducing atmosphere of a gas furnace to a temperature above the austenite transformation completion temperature; a forming step of shaping the unplated steel sheet using a mold; and a quenching step of quenching the unplated steel sheet.

9. A method for manufacturing an automobile undercarriage part according to claim 8, characterized in that the molding step and the quenching step are carried out simultaneously.

10. A method for manufacturing an automobile undercarriage part according to claim 8, characterized in that the heating step and the quenching step are carried out after the molding step.

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