Molded body

Abstract

The present invention provides a molded body having excellent impact resistance in a bent part by virtue of a novel configuration.　A molded body (1) according to the present invention is characterized by having a bent part (2) formed from a steel plate, wherein the molded body (1) is characterized in that the M value of the average Taylor factor of the inner surface layer (21) of the bent part (2) is 3.300 or less.

Description

Molded body

[0001] The present invention relates to a formed article having a bent portion formed from a steel plate.

[0002] To achieve weight reduction in automobile suspension parts, the strength of the materials used is being increased. However, such increased strength generally leads to a decrease in formability. Therefore, there is a strong demand for materials that combine high strength with formability.

[0003] As such a material, for example, Patent Document 1 discloses a high-strength hot-rolled steel sheet with excellent bending workability, which is used for manufacturing automobile suspension parts, etc. Specifically, Patent Document 1 discloses a hot-rolled steel sheet containing C, Si, Mn, and sol. Al as chemical components, in which the sum of the average pole density of the orientation group consisting of {211}<111> to {111}<112> and the pole density of the crystal orientation {110}<001> is 0.5 or more and 6.0 or less in the surface region, and which has a tensile strength of 780 MPa or more and 1370 MPa or less.

[0004] Similarly, Patent Document 2 discloses a high-strength steel plate having excellent bending workability and fatigue properties as a steel plate used for manufacturing automobile suspension parts, etc. Specifically, Patent Document 2 discloses a high-strength steel plate having a base portion and a surface layer portion, in which the average Vickers hardness of the surface layer portion is 50 to 80% of the average Vickers hardness at a position halfway through the thickness of the high-strength hot-rolled steel plate, the arithmetic mean roughness Ra of the surface of the surface layer portion is 3.0 μm or less, the effective crystal grain size of the surface layer portion is 50.0 μm or less, and the Si content Si B and the Si content of the surface layer portion Si S The difference ΔSi = Si B -Si ｓ is 0.60% by mass or more, and the metal structure of the base portion is tempered martensite: 90% or more in area ratio.

[0005] Patent Document 3 discloses a high-strength steel sheet that has excellent bending properties for punched members with large clearances, as a steel sheet used in the manufacture of automobile suspension parts and the like. Specifically, Patent Document 3 discloses a high-strength steel sheet having a chemical composition containing 0.04 to 0.20% C, 0.6 to 1.5% Si, 1.0 to 3.0% Mn, 0.10% or less P, 0.030% or less S, 0.10% or less Al, 0.010% or less N, 0.01 to 1.0% each of one or more of Ti, Nb, and V, with the balance being iron and unavoidable impurities, a structure in which bainite accounts for more than 50% by area, an average grain size at a position 50 μm from the steel sheet surface in the sheet thickness depth direction is 2500 × [tensile strength TS (MPa)] - 0.85 μm or less, precipitates having a grain size of less than 20 nm precipitated in the steel have a C content of 0.005% by mass or more, and an arithmetic mean roughness Ra is 3.0 μm or less.

[0006] Furthermore, Patent Document 4 discloses a high-strength structural steel material having excellent cold bending properties as a steel sheet used for manufacturing automobile suspension parts, etc. Specifically, Patent Document 4 discloses a high-strength structural steel material comprising, by weight, C: 0.02 to 0.1%, Si: 0.01 to 0.6%, Mn: 1.7 to 2.5%, Al: 0.005 to 0.5% or less, P: 0.02% or less, S: 0.01% or less, N: 0.0015 to 0.015%, with the remainder being Fe and other unavoidable impurities, the steel material being microstructurally divided along the thickness direction into an outer surface layer portion and an inner center portion, the surface layer portion containing tempered bainite as a matrix structure, and the center portion containing bainitic ferrite as a matrix structure.

[0007] International Publication No. 2020 / 110843 International Publication No. 2020 / 203934 Japanese Patent Application Laid-Open No. 2017-150051 Special Publication No. 2022-514018

[0008] Generally, automotive suspension parts have bent portions formed by press-forming materials such as steel sheets into predetermined shapes suited to their respective purposes. Here, if the material has insufficient formability, the inside of the bent portion may be deformed in a woven manner during press-forming, resulting in the occurrence of fine cracks on the inner surface of the bent portion. Currently, the occurrence of such fine cracks does not have a significant effect on the impact resistance of the parts. However, in order to respond to future demands for further weight reduction and further complexity of automotive parts, it is desirable to develop parts with fewer such fine cracks and even better impact resistance.

[0009] Therefore, an object of the present invention is to provide a molded article having excellent impact resistance at a bent portion by using a novel structure.

[0010] The present invention includes the following aspects.

[0011] (Embodiment 1) A formed body having a bent portion formed from a steel plate, characterized in that the average Taylor factor M value of an inner surface layer of the bent portion is 3.300 or less.

[0012] (Aspect 2) The molded article according to aspect 1, wherein the Vickers hardness at a 1 / 4 position of the plate thickness of the steel plate is 250 HV or more.

[0013] (Aspect 3) The chemical composition of the steel sheet is, in mass%, C: 0.02 to 0.30%, Si: 0.03 to 2.00%, Mn: 0.50 to 3.00%, Al: 0.01 to 1.00%, Ti: 0.06 to 0.20%, P: 0.100% or less, S: 0.010% or less, N: 0.0100% or less, Nb: 0 to 0.10%, Ca: 0 to 0.0060%, Mo: 0 to 1.00%, Cr: 0 to 1.00%, V: 0 to 0.40%, Ni: 0 to 0.40%, B: 0 to 0.0020%, Cu: 0 to 1.00%, W: 0 to 1.00%, 3. The compact according to claim 1, wherein the content of Sn is 0 to 0.50%, Zr is 0 to 0.050%, and the balance is Fe and impurities.

[0014] According to the present invention, it is possible to provide a molded article having excellent impact resistance at a bent portion.

[0015] FIG. 1 is an end view of a cross section parallel to the thickness direction TP including a bent portion 2 of a formed body 1 according to one embodiment of the present invention. FIG. 2(a) is a cross-sectional SEM photograph of the inner surface layer in the bent portion of a formed body sample serving as a comparison standard, and FIG. 2(b) is a cross-sectional SEM photograph of the inner surface layer in the bent portion of a formed body sample manufactured using the same steel plate as in FIG. 2(a) but in a different bending direction. FIG. 3(a) is a cross-sectional SEM photograph of the inner surface layer in the bent portion of a formed body sample serving as a comparison standard, and FIG. 3(c) is a cross-sectional SEM photograph of the inner surface layer in the bent portion of a formed body sample manufactured using a different steel plate as in FIG. 2(a) but in the same bending direction as in FIG. 2(a). The formed body sample in FIG. 3(a) is the same as the formed body sample in FIG. 2(a). FIG. 4 is a schematic diagram illustrating a method for measuring the average Taylor factor of the inner surface layer in the bent portion. FIG. 5 is a schematic diagram for explaining the relationship between the bending axis AB and the rolling direction DR when a blank p is punched out from a hot-rolled steel sheet SP and subjected to bending.

[0016] To achieve the above-mentioned object, the inventors conducted detailed studies on the causes of cracks occurring in the inner surface layer of the bent portion of a formed body and on means for suppressing the occurrence of such cracks. As a result, the inventors discovered that the likelihood of cracks occurring in the inner surface layer of the bent portion is affected by the orientation of crystals developed in the surface layer of the steel sheet in the bent portion. Specifically, they discovered that the higher the average Taylor Factor (M value), which is determined by the relationship between the orientation of crystals in the surface layer of the steel sheet and the direction of strain, the more likely cracks will develop in the inner surface layer of the bent portion and the more likely brittle fracture will occur. Although the mechanism behind this is not entirely clear, it is thought that the larger the M value, i.e., the more likely dislocation slip will occur, the more likely shear bands in the surface layer, which are suggested to be related to cracks, will develop.

[0017] The present invention was completed based on these findings and includes the following aspects of the embodiments.

[0018] Hereinafter, a preferred embodiment of the molded article of the present invention will be described in detail with reference to the drawings. In this specification, various numerical ranges mean ranges including the upper and lower limits unless otherwise specified.

[0019] 1 is an end view of a cross section parallel to the plate thickness direction TP including a bent portion 2 in a formed body 1 according to one embodiment of the present invention. The formed body 1 of this embodiment is not particularly limited as long as it has a bent portion 2, and examples thereof include automobile suspension parts such as upper arms, lower arms, torsion beams, and stabilizers.

[0020] The formed body 1 is formed by forming a steel plate into a predetermined shape, and includes a predetermined bent portion 2 as shown in FIG. 1 and a plate-like portion 3 connected to the bent portion 2 but not bent. In FIG. 1 , the bent portion 2 has an inner surface layer 21 and an outer surface layer 22 that face each other in the plate thickness direction TP. The bent portion 2 functions as a rib to ensure the strength of the formed body 1 and is formed in a stress-generating portion, such as an edge portion, where stress transmitted from the outside is likely to concentrate. The bent portion 2 may be formed not only in such a stress-generating portion but also in other portions. For example, the bent portion 2 may be formed over substantially the entire outer peripheral edge of the formed body 1. Meanwhile, the plate-like portion 3 may be formed in a portion other than the bent portion 2. The plate-like portion 3 may have a flat structure that is the steel plate without being subjected to processing such as bending. Alternatively, the plate-like portion 3 may be subjected to processing other than bending to have a certain curved structure, an uneven structure, a structure with holes, or the like. In FIG. 1 , the symbol A Ｂ indicates the bending axis of the bent portion 2. Bending axis A Ｂ is a virtual axis when bending a steel plate. Ｂ is shown as a virtual axis extending in a direction perpendicular to a plane including the end face of the bent portion 2, and no actual member or member serving as the axis is arranged inside the steel plate.

[0021] The position where such a bent portion 2 is formed can be determined depending on the structure, shape, application, etc. of the part to which the molded body 1 is applied. For example, if the molded body 1 is a lower arm, which is a type of suspension part for an automobile, the maximum stress generation part in the lower arm, where stress is most likely to concentrate, is the edge part located on the tire side in the longitudinal direction of the lower arm, and it is preferable that the bent portion 2 is formed at a position that includes at least such a maximum stress generation part. Note that such a maximum stress generation part is also determined depending on the overall structure, shape, application, etc. of the molded body, such as an automobile suspension part.

[0022] The means for forming the bent portion 2 is not particularly limited, and the bent portion 2 can be formed using general processing means and processing conditions such as press forming (particularly cold press forming).

[0023] [Average Taylor factor (M value) of inner surface layer of bent portion is 3.300 or less] The formed body 1 of this embodiment has a characteristic configuration in which the average Taylor factor (M value) of the inner surface layer 21 of the bent portion 2 is 3.300 or less. As a result, cracks are less likely to occur in the inner surface layer 21 of the bent portion 2 of the formed body 1 when the steel sheet is press-formed, and as a result, the formed body has fewer cracks in the inner surface layer 21 of the bent portion 2.

[0024] In this specification, the inner surface layer of the bent portion refers to the region within the range from the surface of the steel sheet inside the bent portion to a depth of 1 / 8 of the sheet thickness in the sheet thickness direction. The same applies to the outer surface layer of the bent portion. Note that, if the steel sheet surface is painted or plated, the thickness of the painted or plated layer is excluded.

[0025] Here, a means for controlling the average Taylor factor M value of the inner surface layer 21 of the bent portion 2 to a range of 3.300 or less will be described. The inventors have investigated means for controlling this average Taylor factor M value and have found the following. Specifically, the inventors have found that when materials with high and low pre-bending M values ​​are deformed by the same bending process, the M values ​​of the materials decrease from their pre-bending values, but the relationship between the M values ​​of the materials remains the same as before the bending process. Furthermore, the inventors have found that when the relationship between the rolling direction of the steel sheet, which is the constituent material of the formed body, and the position where the bending process is to be performed changes, the M value before the bending process also changes, and as a result, the bending workability and the M value after the bending process also change. Based on these findings, the inventors have found that the M value after the bending process can be controlled within a predetermined range by adjusting the punching direction of the blank from the steel sheet (i.e., the blank cutting direction) so that the bent portion has a low M value below a certain level.

[0026] 2(a) is a cross-sectional SEM photograph of the inner surface layer at the bent portion of a molded body sample used as a comparison standard, and (b) is a cross-sectional SEM photograph of the inner surface layer at the bent portion of a molded body sample manufactured using the same steel plate as in (a) but with a different bending direction. In FIG. 2, the arrows indicate the cracks that have occurred.

[0027] The formed body sample shown in Fig. 2(a) and the formed body sample shown in Fig. 2(b) use the same steel plate as the material, but the cutting direction of the steel plate, i.e., the punching direction of the blank from the steel plate, is different, and as a result, the M value before bending is different. Specifically, the formed body sample shown in Fig. 2(a) is different in the bending axis A when bending the blank when punching the blank from the steel plate. Ｂ The blank material is punched out so that the direction of the bending axis A crosses the rolling direction of the steel sheet, and the blank material is then formed. The formed sample shown in FIG. 2(b) is a sample of a formed body obtained by punching out a blank material from a steel sheet, and forming the blank material using the blank material. ＢThe blank material was punched out so that the bending axis A did not intersect with the rolling direction of the steel sheet, and the blank material was used to form the sample. Ｂ The relationship between the direction of rolling and the direction of rolling will be described in detail later in the description of the method for producing a compact.

[0028] The molded body sample shown in Figure 2(a) has an M value of 3.285 before bending and an M value of 3.268 after bending. On the other hand, the molded body sample shown in Figure 2(b) has an M value of 3.337 before bending and an M value of 3.312 after bending. Although the M values ​​of both the molded body samples in Figure 2(a) and (b) decrease after bending from the values ​​before bending, the magnitude relationship between them is maintained as before bending.

[0029] Furthermore, according to Figure 2, it can be seen that the molded body sample (b) having an M value of a certain level or more after bending has more cracks on the inner surface layer of the bent portion than the molded body sample (a) having an M value of a certain level or less after bending.

[0030] 3(a) is a cross-sectional SEM photograph of the inner surface layer at the bent portion of a molded body sample used as a comparison standard, and (c) is a cross-sectional SEM photograph of the inner surface layer at the bent portion of a molded body sample manufactured using a blank material punched in the same cutting direction as (a) from a different steel plate. Note that in FIG. 3 as well, multiple arrows indicate multiple cracks that have occurred.

[0031] The formed body sample shown in Fig. 3(a) and the formed body sample shown in Fig. 3(c) have the same cutting direction of the steel sheet, but the chemical composition of the steel sheet as a material is different, and as a result, the M value before bending is different. Note that since the formed body sample in Fig. 3(a) is the same as the formed body sample in Fig. 2(a), both the formed body samples shown in Fig. 3(a) and (c) have the same bending axis A when bending the blank when punching out the blank from the steel sheet. Ｂ A blank material was punched out so that the direction of the arrow intersects with the rolling direction of the steel sheet, and the blank material was used to form a compact sample.

[0032] As described above, the molded body sample shown in Figure 3(a) had an M value of 3.285 before bending and an M value of 3.268 after bending. On the other hand, the molded body sample shown in Figure 3(c) had an M value of 3.311 before bending and an M value of 3.309 after bending. In both the molded body samples in Figure 3(a) and (c), the M values ​​after bending decreased from the values ​​before bending, but the magnitude relationship between them was maintained as before bending.

[0033] 3, it can be seen that the molded body sample (c) has more cracks on the inner surface layer of the bent portion than the molded body sample (a), which has a smaller M value after bending than the molded body sample (c). However, the molded body sample shown in FIG. 3(c) has a smaller M value after bending than the molded body sample shown in FIG. 2(b), and as a result, it can be seen that the molded body sample shown in FIG. 3(c) has fewer cracks on the inner surface layer of the bent portion than the molded body sample shown in FIG. 2(b).

[0034] From a comparison of the formed body samples shown in (a) to (c) of FIG. 2 and FIG. 3, it can be seen that the average Taylor factor M value of the inner surface layer of the bent portion is the same as the bending axis A when punching out a blank from the steel sheet and bending it. Ｂ The influence of the relationship between the rolling direction of the steel sheet and the bending axis A is the largest, followed by the type of steel sheet, i.e., the chemical composition of the steel sheet. Ｂ By adjusting the rolling direction so that the angle between the rolling direction and the rolling direction is 45 degrees to 135 degrees, the average Taylor factor M value of the inner surface layer of the bent portion can be controlled within the above-mentioned predetermined range, and cracks in the inner surface layer of the bent portion can be reduced. Furthermore, by adjusting the texture of the steel sheet, i.e., by adjusting the chemical composition of the steel sheet, the M value of the steel sheet before bending can be more reliably reduced to a certain level or less, and therefore the average Taylor factor M value of the inner surface layer of the bent portion after bending can be more easily controlled within the above-mentioned predetermined range. As a result, cracks in the inner surface layer of the bent portion can be further reduced.

[0035] In addition, examples of means for making it easier to control the M value of the average Taylor factor of the inner surface layer of the bent portion within the above-mentioned specified range include means for softening at least the surface layer of a material such as a steel plate at least in the portion corresponding to the bent portion, thereby concentrating strain in the surface layer and lowering the M value after bending, and means for ensuring high formability of a material such as a steel plate, thereby applying large strain during bending and lowering the M value after bending.

[0036] In this embodiment, if the average Taylor factor M value of the inner surface layer of the bent portion exceeds 3.300, numerous cracks are likely to occur in the bent portion. Therefore, in order to obtain the effects of the present invention, it is necessary to control the M value to 3.300 or less. The M value may be 3.200 or less, 3.100 or less, or 3.000 or less. There is no particular restriction on the lower limit of the M value, but the effects of the present invention can be better obtained as the M value is lower. However, by definition, the M value cannot be less than 2.000. The M value may be 2.100 or more, 2.400 or more, or 2.700 or more.

[0037] (Method for Measuring the Average Taylor Factor M of the Inner Surface Layer of the Bent Portion) The average Taylor factor M of the inner surface layer of the bent portion can be calculated by measuring a cross section perpendicular to the bending axis and including the entire thickness of the sample using EBSD. FIG. 4 is a schematic diagram for explaining a method for measuring the average Taylor factor of the inner surface layer of the bent portion 2. To measure the average Taylor factor of the inner surface layer of the bent portion 2, first, as shown in FIG. 4 , the crystal orientation is measured from a rectangular measurement region MR centered on the vertex of the inner surface of the bent portion 2 in a cross section perpendicular to the bending axis and including the entire thickness of the sample. The surface of the sample including the measurement region MR is finished by mirror polishing followed by electrolytic polishing. For the measurement, an EBSD analysis device composed of a thermal field emission scanning electron microscope (e.g., JEOL JSM-7200F) and an EBSD detector (e.g., EDAX Velocity (registered trademark) ultra-high-speed EBSD detector) is used. At this time, the degree of vacuum inside the device was 9.6×10 －５The pressure is set to 0.1 Pa or less, the acceleration voltage is 25 kV, the probe current level is 15-18, and the WD (Working Distance) is 15 mm. As shown in FIG. 4, the measurement region MR is set as a rectangular region with a thickness-wise length that fits within the surface region, i.e., a thickness-wise length that is less than 1 / 8 of the thickness, and a length of 750 μm perpendicular to the thickness-wise length. For example, if the thickness of the sample to be measured is 3 mm, the measurement region MR may be set as a rectangular region with a thickness-wise short side of 300 μm and a thickness-wise long side of 750 μm perpendicular to the thickness-wise length. In this case, the measurement step is set to 2 μm. From the multiple crystal orientation information obtained in this way, the average Taylor factor is calculated using EBSD analysis software. The analysis software used is "OIM Analysis" (registered trademark) version 7.3.1 manufactured by TSL. Note that because the bent section is subject to strain and measurement accuracy may be reduced, only points with a CI value (accuracy index) greater than 0.05 are used. Furthermore, to ensure the accuracy of the average Taylor factor, at least 2,000 measurement points with a CI value of 0.05 or greater must be obtained. To achieve this, the detector exposure time must be set sufficiently long or the magnification of the SEM observation must be sufficiently high. If the measurement region MR cannot fit into a single observation image due to the high magnification of the SEM observation, the measurement region MR may be divided into multiple regions for measurement. When calculating the Taylor factor, calculations are performed assuming 12 types of slip systems, including the {110}<111> system, which is the primary slip system in BCC. Furthermore, the deformation of the inner surface layer of the bent section is calculated using a tensor assuming compressive deformation under plane strain conditions. In other words, it is assumed that expansion to maintain a constant volume occurs only in the thickness direction in response to the compression caused by bending deformation. For example, when the x-axis is parallel to the bending axis, the z-axis is in the thickness direction, and the y-axis is perpendicular to both the bending axis and the thickness direction, the diagonal components of x, y, and z of the given tensor are 0, -1, and 1, respectively, and all off-diagonal components are 0.

[0038] (Minimum radius of curvature r of bent portion) In the formed body 1 of this embodiment, the minimum radius of curvature r on the inside of the bent portion 2 is not particularly limited. From the viewpoint of effectively utilizing the limited design space, the minimum radius of curvature r may be 8.0 mm or less, 5.0 mm or less, or 3.0 mm or less. Furthermore, from the viewpoint of suppressing wrinkles and cracks that may occur on the surface during bending, the minimum radius of curvature r may be 0.8 mm or more, 1.2 mm or more, 1.6 mm or more, or 2.0 mm or more.

[0039] The steel sheet used in the formed body of this embodiment is not particularly limited in terms of other requirements for the steel sheet, as long as the average Taylor factor M value of the inner surface layer of the bent portion of the formed body can be adjusted to fall within the above-mentioned predetermined range. For example, the strength, thickness, etc. of the steel sheet can be the predetermined strength, thickness, etc. required for the part to which the formed body is applied.

[0040] [Vickers hardness of steel plate at 1 / 4 thickness position is 250 HV or more] Examples of the strength of steel plate suitable for application of the present invention include a tensile strength of 780 MPa or more. The Vickers hardness of such a high-strength steel plate at 1 / 4 thickness position is greater than 250 HV. Normally, as the strength of such steel plate increases, cracks are more likely to occur in the inner surface layer of the bent portion after bending. However, in the formed body 1 of this embodiment, even when such a steel plate having a Vickers hardness of 250 HV or more at 1 / 4 thickness position is used, the average Taylor factor M value of the inner surface layer of the bent portion is 3.300 or less, thereby reducing cracks in the inner surface layer of the bent portion.

[0041] The 1 / 4 position in the thickness direction of the steel plate means a position in the thickness direction of the steel plate, starting from the surface of the steel plate, that is a distance that is 1 / 4 of the total thickness of the steel plate from the surface of the steel plate.

[0042] The Vickers hardness at the 1 / 4 position of the steel plate may be 252 HV or more, 255 HV or more, 258 HV or more, or 260 HV or more from the viewpoint of the impact resistance of the formed body. The upper limit of the Vickers hardness at the 1 / 4 position of the steel plate is not particularly limited. The Vickers hardness at the 1 / 4 position of the steel plate may be 450 HV or less, 400 HV or less, 350 HV or less, or 300 HV or less.

[0043] The present invention can also be applied to steel sheets having a Vickers hardness of less than 250 HV. Such steel sheets have low tensile strength and therefore excellent bending workability, so by applying the present invention, it is possible to more reliably suppress the occurrence of cracks.

[0044] (Method for measuring Vickers hardness at 1 / 4 position of steel plate thickness) The Vickers hardness at 1 / 4 position of steel plate thickness can be measured in accordance with JIS Z 2244:2009. The Vickers hardness at 1 / 4 position of steel plate thickness can be obtained by measuring 10 times at 1 / 4 position of the plate thickness of the steel plate at a location other than the bent portion with a load of 1 kgf (approximately 9.80 N) and taking the average value of the 10 measured values. In this case, the distance between the measurement positions must be at least three times the distance of the indentation.

[0045] The thickness of the steel plate may be, for example, 1.5 mm or more. The thickness of the steel plate may be 1.8 mm or more, 2.0 mm or more, or 2.2 mm or more. There is no particular upper limit to the thickness of the steel plate, but from the viewpoint of ensuring good bending workability, the thickness of the steel plate may be 6.0 mm or less, 5.5 mm or less, 5.0 mm or less, 4.5 mm or less, or 4.0 mm or less.

[0046] The thickness of the steel plate is measured with a micrometer from a relatively smooth position on the part (formed body). Note that the measurement does not include locally thinned portions of the plate due to thinning caused by press working, etc.

[0047] (Chemical Composition) In this embodiment, the steel plate used for the formed body 1 has the following composition: C: 0.03 to 0.30%, Si: 0.03 to 2.00%, Mn: 0.50 to 3.00%, Al: 0.01 to 1.00%, Ti: 0.06 to 0.20%, P: 0.100% or less, S: 0.010% or less, N: 0.0100% or less, Nb: 0 to 0.10%, Ca: 0 to 0.0060%, Mo: 0 to 1.00%, Cr: 0 to 1.00%, V: 0 to 0.40%, Ni: 0 to 0.40%, B: 0 to 0.0020%, Cu: 0 to 1.00%, W: 0 to 1.00%, It preferably has a specific chemical composition of Sn: 0 to 0.50%, Zr: 0 to 0.050%, and the balance: Fe and impurities.

[0048] The chemical composition of the steel sheet will be described in more detail below. In the following description, the unit of "%" for the content of each element means "mass %" unless otherwise specified.

[0049] [C: 0.02 to 0.30%] C is an element that increases the strength of the steel sheet. To fully obtain this effect, the C content is preferably 0.02% or more. The C content may be 0.03% or more, 0.04% or more, or 0.05% or more. On the other hand, from the viewpoint of ensuring good bending workability during forming, particularly during cold press forming, the C content is preferably 0.30% or less. The C content may be 0.25% or less, 0.20% or less, or 0.15% or less.

[0050] [Si: 0.03 to 2.00%] Si is a deoxidizing element for steel and a solid-solution strengthening element effective in increasing the strength of steel sheet without impairing its ductility. To fully obtain this effect, the Si content is preferably 0.03% or more. The Si content may be 0.05% or more, 0.08% or more, or 0.10% or more. On the other hand, from the viewpoint of suppressing ferrite formation and ensuring a predetermined strength, or from the viewpoint of ensuring good bending workability, the Si content is preferably 2.00% or less. The Si content may be 1.80% or less, 1.60% or less, or 1.40% or less.

[0051] [Mn: 0.50 to 3.00%] Mn is an element that improves the hardenability of steel and contributes to improving strength. To fully obtain this effect, the Mn content is preferably 0.50% or more. The Mn content may be 0.60% or more, 0.80% or more, or 1.00% or more. On the other hand, from the viewpoint of suppressing the non-uniformity of the structure due to microsegregation of Mn and ensuring good bending workability, the Mn content is preferably 3.00% or less. The Mn content may be 2.80% or less, 2.60% or less, or 2.40% or less.

[0052] [Al: 0.01 to 1.00%] Al is an element that functions as a deoxidizer and is a solid-solution strengthening element that is effective in increasing the strength of steel. Al also suppresses the formation of carbides and facilitates the formation of retained austenite. In order to fully obtain these effects, the Al content is preferably 0.01% or more. The Al content may be 0.05% or more, 0.10% or more, or 0.15% or more. On the other hand, from the viewpoint of suppressing the formation of coarse precipitates at the grain boundaries of prior austenite grains and ensuring good bending workability, the Al content is preferably 1.00% or less. The Al content may be 0.80% or less, 0.60% or less, or 0.40% or less.

[0053] [Ti: 0.06 to 0.20%] Ti is an element that controls the morphology of carbides and increases the strength of ferrite. To fully obtain this effect, the Ti content is preferably 0.06% or more. The Ti content may be 0.08% or more or 0.10% or more. On the other hand, from the viewpoint of suppressing the formation of coarse Ti oxides or TiN and ensuring good bending workability, the Ti content is preferably 0.20% or less. The Ti content may be 0.18% or less, 0.16% or less, or 0.14% or less.

[0054] [P: 0.100% or less] P is an element that is mixed in during the manufacturing process and is an impurity. Furthermore, P segregates at prior austenite grain boundaries, which may reduce the formability of the steel sheet due to grain boundary embrittlement. Therefore, the lower the P content, the better. The P content may be 0%. However, from the viewpoint of shortening the refining time and ensuring good productivity, the P content may be 0.0001% or more, 0.0005% or more, or 0.001% or more. On the other hand, from the viewpoint of suppressing a decrease in the toughness of the steel sheet and ensuring good bending workability, the P content may be 0.100% or less. The P content may be 0.080% or less, 0.060% or less, or 0.040% or less.

[0055] [S: 0.010% or less] S is an element that is mixed in during the manufacturing process and is an impurity. Furthermore, S may generate non-metallic inclusions such as MnS in the steel, which may increase the hardness and reduce the ductility of the steel sheet. Therefore, the lower the S content, the better. The S content may be 0%. However, from the viewpoint of shortening the refining time and ensuring good productivity, the S content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, from the viewpoint of ensuring good bending workability, the S content may be 0.010% or less. The S content may be 0.008% or less, 0.006% or less, or 0.004% or less.

[0056] [N: 0.0100% or less] N is an element that is mixed in during the manufacturing process. Like C, N is an element that is effective in increasing the strength of steel, but it also affects the occurrence of dislocation cross-slip during forming, particularly cold press forming. If the N content is high, strain concentration cannot be suppressed during steel sheet forming, causing voids to occur, thereby reducing formability. From the perspective of ensuring good formability, the lower the N content, the better. The N content may be 0%. However, from the perspective of shortening refining time and ensuring good productivity, the N content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, from the perspective of ensuring good bending workability, the N content may be 0.0100% or less. The N content may be 0.0080% or less, 0.0060% or less, or 0.0050% or less.

[0057] In this embodiment, the preferred basic chemical composition of the steel sheet used for the formed body 1 is as described above. Furthermore, in this embodiment, the steel sheet may contain one or more of the following optional elements in place of a portion of the remaining Fe, as necessary. These optional elements will be described in detail below.

[0058] [Nb: 0 to 0.10%] Nb is an element effective in controlling the morphology of carbides, and is also effective in refining crystal grains and improving the toughness and bending workability of steel sheets. The Nb content may be 0%, but to fully obtain these effects, the Nb content may be 0.001% or more. The Nb content may be 0.005% or more, 0.007% or more, or 0.010% or more. On the other hand, from the viewpoint of suppressing excessive strengthening, the Nb content may be 0.10% or less. From the viewpoint of economy, the Nb content may be 0.08% or less, 0.06% or less, or 0.04% or less.

[0059] [Ca: 0 to 0.0060%] Ca is an element that contributes to finely dispersing inclusions and enhancing toughness. That is, Ca is an element that contributes to improving the formability of the steel sheet. The Ca content may be 0%, but to fully obtain this effect, the Ca content may be 0.0001% or more. The Ca content may be 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, from the viewpoint of ensuring good bending workability, the Ca content is preferably 0.0060% or less. The Ca content may be 0.0050% or less, 0.0040% or less, or 0.0030% or less.

[0060] [Mo: 0 to 1.00%] Mo is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of the steel sheet. The Mo content may be 0%, but to fully obtain this effect, the Mo content may be 0.001% or more. The Mo content may be 0.01% or more, 0.05% or more, or 0.10% or more. On the other hand, from the viewpoint of suppressing deterioration of bending workability due to the formation of coarse Mo carbides, the Mo content may be 1.00% or less. The Mo content may be 0.80% or less, 0.60% or less, or 0.40% or less.

[0061] [Cr: 0 to 1.00%] Cr is an element that improves the hardenability of steel and contributes to improving the strength of the steel sheet. The Cr content may be 0%, but to fully obtain this effect, the Cr content may be 0.001% or more. The Cr content may be 0.01% or more, 0.10% or more, 0.20% or more, or 0.30% or more. On the other hand, from the viewpoint of suppressing a decrease in bending workability due to the formation of coarse Cr carbides, the Cr content may be 1.00% or less. The Cr content may be 0.80% or less, 0.60% or less, or 0.50% or less.

[0062] [V: 0 to 0.40%] V is an element effective in controlling the morphology of carbides and is also effective in refining crystal grains to improve the toughness and bending workability of steel sheets. The V content may be 0%, but to fully obtain such effects, the V content may be 0.001% or more. The V content may be 0.005% or more, 0.01% or more, or 0.02% or more. On the other hand, from the viewpoint of suppressing a decrease in formability of the steel sheet due to the precipitation of a large amount of carbonitrides, the V content may be 0.40% or less. The V content may be 0.30% or less, 0.20% or less, or 0.10% or less.

[0063] [Ni: 0 to 0.40%] Ni is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of the steel sheet. The Ni content may be 0%, but to fully obtain this effect, the Ni content may be 0.001% or more. The Ni content may be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, from the viewpoint of suppressing a decrease in ductility of the steel sheet and ensuring good bending workability, the Ni content may be 0.40% or less. The Ni content may be 0.30% or less, 0.25% or less, or 0.20% or less.

[0064] [B: 0 to 0.0020%] B is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of the steel sheet. The B content may be 0%, but to fully obtain this effect, the B content may be 0.0001% or more. The B content may be 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, from the viewpoint of suppressing a decrease in bending workability caused by the generation of coarse B oxides that become the origin of voids during steel sheet forming, the B content may be 0.0020% or less. The B content may be 0.0018% or less, 0.0016% or less, or 0.0015% or less.

[0065] [Cu: 0 to 1.00%] Cu is an element that exists in steel in the form of fine particles and contributes to improving the strength of the steel sheet. The Cu content may be 0%, but to fully obtain this effect, the Cu content may be 0.001% or more. The Cu content may be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, from the viewpoint of suppressing a decrease in the bending workability of the steel sheet due to the precipitation of a large amount of coarse precipitates or inclusions, the Cu content may be 1.00% or less. The Cu content may be 0.80% or less, 0.60% or less, or 0.40% or less.

[0066] [W: 0 to 1.00%] W is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of the steel sheet. The W content may be 0%, but to fully obtain this effect, the W content may be 0.001% or more or 0.005% or more. On the other hand, from the viewpoint of preventing a decrease in productivity due to a decrease in bending workability, the W content is set to 1.00% or less. The W content may be 0.08% or less.

[0067] [Sn: 0 to 0.50%] Sn is an element that suppresses coarsening of crystal grains and contributes to improving the strength of the steel sheet. The Sn content may be 0%, but to fully obtain this effect, the Sn content may be 0.001% or more. The Sn content may be 0.01% or more, 0.05% or more, or 0.08% or more. On the other hand, from the viewpoint of suppressing a decrease in the bending workability of the steel sheet due to an increase in coarse precipitates and inclusions, the Sn content may be 0.50% or less. The Sn content may be 0.40% or less, 0.30% or less, or 0.20% or less.

[0068] [Zr: 0 to 0.050%] Zr is an element that contributes to improving the formability of the steel sheet. The Zr content may be 0%, but to fully obtain this effect, the Zr content may be 0.0001% or more. The Zr content may be 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, from the viewpoint of suppressing a decrease in ductility of the steel sheet and ensuring good bending workability, the Zr content may be 0.050% or less. The Zr content may be 0.040% or less, 0.030% or less, or 0.020% or less.

[0069] [Balance: Fe and Impurities] In this embodiment, the balance of the steel sheet other than the above elements consists of Fe and impurities. Here, the impurities refer to components that are mixed in due to various factors in the manufacturing process, including raw materials such as ore and scrap, when industrially manufacturing steel sheet. Examples of impurities include H, O, Na, Cl, Co, Zn, Ga, Ge, As, Se, Y, Tc, Ru, Rh, Pd, Ag, Cd, In, Te, Cs, Ta, Re, Os, Ir, Pt, Au, Pb, Bi, Sb, and Po. The impurities may be contained in a total amount of 0.100% or less.

[0070] Here, the chemical composition of the steel sheet can be measured by a common analytical method. For example, the chemical composition of the steel sheet can be measured using inductively coupled plasma atomic emission spectrometry (ICP-AES). C and S can be measured using a combustion-infrared absorption method, N can be measured using an inert gas fusion-thermal conductivity method, and O can be measured using an inert gas fusion-non-dispersive infrared absorption method.

[0071] As described above, the molded article 1 of this embodiment has few cracks on the inner surface layer of the bent portion, and therefore, the molded article of this embodiment can be applied to various automobile suspension parts that require high impact resistance, such as upper arms, lower arms, torsion beams, and stabilizers.

[0072] <Method for manufacturing molded body> Next, a description will be given of a preferred method for manufacturing the molded body 1, which is one embodiment of the present invention. Note that the following description is intended to exemplify a characteristic method for manufacturing the molded body 1, which is one embodiment of the present invention, and is not intended to limit the molded body 1 to those manufactured by the manufacturing method described below.

[0073] In manufacturing the formed body 1 of this embodiment, first, a steel plate is manufactured.

[0074] (Method for manufacturing steel sheet) In the present embodiment, the method for manufacturing the steel sheet is not particularly limited, but a method that can produce a steel sheet having the above-mentioned specific chemical composition is preferred. An example of a method for manufacturing such a steel sheet is a method that includes a casting step of casting a slab having the above-mentioned specific chemical composition, and a hot rolling step of hot rolling the cast slab. Preferred conditions for these steps will be described below.

[0075] In the method for producing a steel sheet according to the present embodiment, the casting step is a step of casting a slab having the specific chemical composition described above. The casting step uses a continuous casting machine having a plurality of reduction rolls adjacent to each other in the conveying direction of the slab, and the roll pitch between the adjacent reduction rolls is 290 mm or less.

[0076] (Hot rolling process) The hot rolling process is a process of hot rolling a cast slab. In the hot rolling process, it is preferable to heat the slab to 1200°C or higher prior to hot rolling. By setting the heating temperature to 1200°C or higher, the rolling reaction force does not become excessively large during hot rolling, making it easier to obtain the desired thickness. There is no particular upper limit to the heating temperature, but from an economical viewpoint, it is preferable that the heating temperature be 1300°C or lower.

[0077] In the hot rolling process, the heated slab is subjected to rough rolling and finish rolling. By appropriately selecting various conditions for the rough rolling and finish rolling, the steel sheet can have the above-mentioned preferred specific Vickers hardness and the average Taylor factor M value of the inner surface layer of the bent portion 2 formed after bending can be easily controlled within the above-mentioned specific range.

[0078] In the hot rolling process, the starting temperature of rough rolling is, for example, 1150°C or lower. When the starting temperature of rough rolling is 1150°C or lower, the influence of heat removal by the rolling rolls is reduced, and the steel sheet can be rolled uniformly on both sides. On the other hand, the starting temperature of rough rolling is, for example, 1050°C or higher. When the starting temperature of rough rolling is 1050°C or higher, it is possible to control the rolling reaction force so that it does not become excessively large.

[0079] In the hot rolling process, the end temperature of finish rolling is, for example, 800°C or higher. When the end temperature of finish rolling is 800°C or higher, the average crystal grain size of the hot-rolled steel sheet and the final product can be reduced, thereby ensuring sufficient yield strength. On the other hand, although there is no particular upper limit for the end temperature of finish rolling, from an economical viewpoint, the end temperature of finish rolling is, for example, 980°C or lower.

[0080] The diameter of the rolling rolls used in the hot rolling step is, for example, 100 mm or more. When the diameter of the rolling rolls is 100 mm or more, strain is less likely to concentrate on the surface in contact with the rolling rolls, and the steel sheet can be rolled uniformly on both sides. On the other hand, the upper limit of the diameter of the rolling rolls is not particularly limited, but from an economical viewpoint, it is, for example, 700 mm or less. The rolling rolls used in the hot rolling step may be heated in advance. When the rolling rolls are heated in advance, heat removal from the steel sheet by the rolling rolls is suppressed, and unevenness in heat removal from the steel sheet by the rolling rolls can be reduced.

[0081] The hot-rolled steel sheet obtained in the hot rolling process is coiled at a coiling temperature of, for example, 450 to 600°C. By setting the coiling temperature to 450°C or higher, the strength of the hot-rolled steel sheet does not become excessively high, and deterioration of bending workability can be suppressed. On the other hand, by setting the coiling temperature to 600°C or lower, coarse ferrite and pearlite are less likely to be formed in the structure of the hot-rolled steel sheet, and bainite is more likely to be obtained in the structure of the hot-rolled steel sheet, thereby improving the strength of the steel sheet.

[0082] The coiling temperature of the hot-rolled steel sheet may be, for example, 450°C or lower. By setting the coiling temperature to 450°C or lower, a large amount of structures such as bainite and martensite are formed, making it easier to ensure the desired strength even with the addition of a small amount of alloying elements. The coiling temperature may be, for example, 200°C or lower. By setting the coiling temperature to 200°C or lower, a large amount of martensite is formed, making it possible to improve the strength of the steel sheet even with a small amount of alloying elements added.

[0083] (Skin-pass rolling process) The hot-rolled steel sheet obtained by the hot-rolling process may be subjected to skin-pass rolling for the purpose of correcting the shape. Note that, since skin-pass rolling is not included in cold rolling, the steel sheet after skin-pass rolling is also a hot-rolled steel sheet.

[0084] A steel sheet obtained by hot rolling or a steel sheet obtained by skin-pass rolling (hereinafter, these are collectively referred to as "hot-rolled steel sheet") may be subjected to an optional treatment step such as a plating treatment, if necessary.

[0085] The hot-rolled steel sheet obtained by the above-described manufacturing method is then subjected to the next forming step. That is, the steel sheet used for the formed body of this embodiment is preferably a hot-rolled steel sheet, since the effects of the present invention can be more reliably obtained.

[0086] (Forming Process) The forming process includes a punching process of punching a steel plate into a predetermined shape, and a bending process of bending the blank material punched into the predetermined shape.

[0087] The punching process is a process of punching out a blank material having a shape in plan view corresponding to the shape of the formed body 1 before bending from a rolled steel sheet, i.e., a hot-rolled steel sheet.

[0088] Here, FIG. 5 shows the bending axis A when a blank p is punched out from a hot-rolled steel plate SP and subjected to bending. Ｂ 5 is a schematic diagram for explaining the relationship between the rolling direction DR and the bending axis A when punching a blank p from the hot-rolled steel sheet SP after the rolling process. Ｂ The punching direction of the blank p (i.e., the blank cutting direction) is adjusted so that the bending axis A intersects with the rolling direction DR of the hot-rolled steel sheet SP. Ｂ The sheet cutting direction is adjusted so that the angle θ formed by the bending axis A of the portion to be bent 2 in the hot-rolled steel sheet SP is 45 degrees to 135 degrees. Ｂ The hot-rolled steel sheet SP is subjected to a punching process and a bending process so that the direction of the cutting edge intersects with the rolling direction DR of the hot-rolled steel sheet SP at an angle of 45 degrees to 135 degrees.

[0089] In FIG. 5, the blank p is shown in a rectangular shape in plan view for convenience, but in reality, it has a shape in plan view that corresponds to the formed body 1 before bending.

[0090] As described above, the bending axis A of the portion to be bent 2 in the hot-rolled steel sheet SP Ｂ By performing the punching process and bending process of the hot-rolled steel sheet SP so that the average Taylor factor M value of the inner surface layer of the bent portion intersects with the rolling direction DR of the hot-rolled steel sheet SP, the average Taylor factor M value of the inner surface layer of the bent portion can be controlled within the above-mentioned specified range.

[0091] In this embodiment, the blank p is cut in the direction of the bending axis A. Ｂ and the rolling direction DR is not particularly limited as long as it is an angle of 45 degrees to 135 degrees, but from the viewpoint of impact resistance, it may be in the range of 60 degrees or more and 120 degrees or less, 85 degrees or more and 95 degrees or less, or 90 degrees.

[0092] Depending on the type of part to which the formed body is applied, the direction of extension of the bending axis in the formed body may not be constant. Specifically, the bending axis may be curved, or there may be multiple bending axes. In such cases, the punching direction of the blank (i.e., the blank cutting direction) may be set to match the part where crack reduction is most desired, taking into account the expected direction of stress in the usage environment of the part to which the formed body is applied and the dimensions of the bent part. In addition, when there are multiple bending axes, the bending axes may be parallel to each other or may intersect at an angle other than perpendicular.

[0093] The bending process is a process of bending a blank material that has been punched into a predetermined shape. In the bending process, the means for bending the blank material is not particularly limited, and examples thereof include press forming. Furthermore, in the bending process, such a bending means as press forming may be performed only once, or may be performed in stages multiple times.

[0094] In the above-described embodiment, the bending process is performed on the blank material after it has been punched into a predetermined shape in the punching process, but this is not limited to this. The bending process may also be performed by bending predetermined locations of the blank material while punching out the blank material in the punching process. That is, the punching process and the bending process may be performed in parallel, or the bending process may be performed after the punching process as described above.

[0095] The molded body obtained by the above molding process may be subjected to a painting process or other surface treatment process for the purpose of improving the appearance and design.

[0096] The present invention is not limited to the above-described embodiments or the following examples, and appropriate combinations, substitutions, modifications, etc. are possible within the scope that does not deviate from the object and intent of the present invention.

[0097] The present invention will be explained in more detail below by way of examples, but the present invention is not limited to these examples.

[0098] In the following examples, a molded article according to one embodiment of the present invention was produced under various conditions, and the impact resistance of the bent portion of the obtained molded article was examined.

[0099] (Production of Steel Plate) Slabs having the chemical compositions shown in Table 1 below were cast by a continuous casting method using a continuous casting machine equipped with a plurality of reduction rolls arranged at a roll pitch of 290 mm or less.

[0100] Next, the obtained slab was subjected to a hot rolling process. Specifically, the slab was heated to a temperature of 1250°C, and rough rolling and finish rolling were performed. The starting temperature of rough rolling was 1050°C, and the ending temperature of finish rolling was 940°C to 980°C. The coiling temperature of the obtained hot-rolled steel sheet was 450°C to 480°C. In this way, a hot-rolled steel sheet having a thickness of 3 mm was obtained.

[0101] Analysis of the chemical composition of samples taken from the obtained hot-rolled steel sheets confirmed that there was no change from the chemical composition of the slab. The chemical compositions of the obtained hot-rolled steel sheets are shown in Table 1 below, with steel types A to H. The underlines next to various values ​​in Table 1 indicate that the conditions were not preferable for producing the formed body of the present invention.

[0102]

[0103] (Production of formed body) From the obtained hot-rolled steel sheet, the sheet is cut in the sheet cutting direction, i.e., the bending axis A of the portion to be bent is ＢA rectangular blank material having a width of 30 mm and a length of 100 mm was punched out so that the angle θ between the bending direction DR and the rolling direction DR was 0 degrees, 15 degrees, 30 degrees, 45 degrees, or 90 degrees. Two adjacent blank materials were punched out, and one of them was used to measure the average Taylor factor M value of the inner surface layer of the bent portion. The other was used to prepare a compact and evaluate its impact resistance.

[0104] The obtained rectangular blanks were then press-molded in accordance with JIS Z2248:2006 "6.3V block method" to produce compacts. The punch used for this press molding had a bending angle of 90 degrees. In this way, compacts No. 1 to 8 shown in Table 2 were obtained.

[0105] For the formed bodies Nos. 1 to 8 obtained as described above, the average Taylor factor M value of the inner surface layer of the bent portion was measured according to the above-mentioned "Method for measuring the average Taylor factor M value of the inner surface layer of the bent portion," and further, the Vickers hardness was measured according to the above-mentioned "Method for measuring Vickers hardness at a 1 / 4 position of the plate thickness of a steel plate." Then, the formed bodies Nos. 1 to 8 were subjected to the following impact resistance test, and the impact resistance of the bent portion of these formed bodies was evaluated. The measurement results and evaluation results are shown in Table 2 below.

[0106] (Impact Resistance Test) First, the molded body to be tested is cooled to -40°C. Next, the cooled molded body is placed on a horizontal test bench with the outer surface of the bent portion facing up. Then, a cone weighing approximately 120 kg is placed at a height of 0.18 m from the surface of the test bench and allowed to fall freely, colliding with the outer surface of the bent portion of the molded body. Note that before being allowed to fall freely, the cone is placed with its bottom surface facing downward. In addition, the cone used has an area large enough to cover the entire bent portion of the part. Then, the condition of the molded body after the cone has collided with the bent portion of the molded body is observed, and those that do not break in the bent portion are judged to have "excellent impact resistance," and those that do break in the bent portion are judged to have "poor impact resistance."

[0107] In Table 2, the judgment of "excellent impact resistance" is indicated by "◯" and the judgment of "poor impact resistance" is indicated by "X." The underlines next to various values ​​in Table 2 indicate that the values ​​are outside the range of the present invention or that the moldings of the present invention cannot be obtained under the manufacturing conditions.

[0108]

[0109] As shown in Table 2, it was found that the molded articles of the examples of the present invention had excellent impact resistance at the bent portions, while the molded articles of the comparative examples had poor impact resistance at the bent portions.

[0110] 1 Molded object 2 Bending part 21 Inner surface layer 22 Outer surface layer

Claims

1. A formed body having a bent portion formed from a steel plate, characterized in that the average Taylor factor M value of the inner surface layer of the bent portion is 3.300 or less.

2. The molded article according to claim 1, characterized in that the Vickers hardness at the 1 / 4 position of the plate thickness of the steel plate is 250 HV or more.

3. The chemical composition of the steel plate is, in mass%, C: 0.02-0.30%, Si: 0.03-2.00%, Mn: 0.50-3.00%, Al: 0.01-1.00%, Ti: 0.06-0.20%, P: 0.100% or less, S: 0.010% or less, N: 0.0100% or less, Nb: 0-0.10%, Ca: 0-0.0060%, Mo: 0-1.00%, Cr: 0-1.00%, V: 0-0.40%, Ni: 0-0.40%, B: 0-0.0020%, Cu: 0-1.00%, W: 0-1.00%, 3. The molded body according to claim 1, characterized in that Sn: 0 to 0.50%, Zr: 0 to 0.050%, and the balance: Fe and impurities.

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