Automotive undercarriage component

Abstract

In a cross-section (20CS) of a bent part (20) of an undercarriage component according to the present disclosure: an area ratio I{110}T of a {110} texture, an area ratio I{111}T of a {111} texture, and an area ratio I{100}T of a {100} texture satisfy expression (1); on the inner side of the bent part (20), an area ratio I{100}SI of the {100} texture in a surface layer region (20SI), and an area ratio I{100}C of the {100} texture in a central region (20C) satisfy expression (2); and an area ratio I{110}SI of the {110} texture in the surface layer region (20SI) and an area ratio I{110}SO of the {110} texture in the surface layer region (20SO) on the outer side of the bent part (20) satisfy expression (3).　(1): 0.70 ≤ I{110}T / (I{111}T + I{100}T) ≤ 1.50 (2): I{100}C - I{100}SI > 0.0 (3): I{110}SI - I{110}SO > 0.0

Description

Automotive undercarriage parts

[0001] This disclosure relates to undercarriage components, and more specifically, to undercarriage components suitable for automotive applications.

[0002] Automotive suspension components, such as upper arms, lower arms, torsion beams, and stabilizers, are formed by bending (cold pressing) steel sheets. These automotive suspension components may be subjected to external impacts, such as collisions, driving on uneven terrain or deteriorated pavement, and driving over curbs. Therefore, suspension components require excellent impact resistance, which is the property that suppresses the occurrence of cracks due to impacts.

[0003] Technology related to steel plates used as materials for undercarriage components is proposed in International Publication No. 2020 / 203934 (Patent Document 1) and Japanese Patent Application Publication No. 2017-150051 (Patent Document 2).

[0004] The steel plate used as the material for the undercarriage parts disclosed in Patent Document 1 comprises a base portion and a surface portion. The average Vickers hardness of the surface portion is 50-80% of the average Vickers hardness at half the thickness of the steel plate. Furthermore, the arithmetic mean roughness Ra of the surface of the surface portion is 3.0 μm or less, and the effective grain size of the surface portion is 50.0 μm or less. Si content of the base portion Ｂ and Si content of the surface layer Ｓ The difference is 0.60% or more in mass, and the area ratio of tempered martensite in the base metal structure is 90% or more. Patent Document 1 improves bendability by using a steel sheet having the above-described technical characteristics.

[0005] In the steel sheet disclosed in Patent Document 2, the bainite area ratio in the microstructure is greater than 50%, and the average particle size at a position of 50 μm from the surface of the steel sheet in the thickness direction is 2500 × [tensile strength TS (MPa)] - 0.85 μm or less. Furthermore, the carbon content of precipitates with a particle size of less than 20 nm in the steel sheet is 0.005 mass% or more. Furthermore, the arithmetic mean roughness Ra of the steel sheet surface is 3.0 μm or less. In Patent Document 2, the bendability is improved by using a steel sheet with the above configuration.

[0006] International Publication No. 2020 / 203934, Japanese Unexamined Patent Application Publication No. 2017-150051

[0007] In Patent Documents 1 and 2, although the mechanical properties of the steel sheet used as the material for the underbody parts have been studied, no study has been made on the underbody parts which are the final products. Therefore, in these patent documents, nothing has been studied regarding the impact resistance of the underbody parts.

[0008] An object of the present disclosure is to provide an underbody part for an automobile having excellent impact resistance.

[0009] The underbody part of the present disclosure is an underbody part for an automobile, and includes a part body including a bent portion. The part body is made of steel and contains a bcc structure texture with an area ratio of 90% or more. The Vickers hardness of the part body is 250 HV or more. In a cross section perpendicular to the bending line of the bent portion, the area ratio I of the crystal plane within 18° of the azimuth difference from the {110} plane of the bcc structure texture with respect to the normal line of the cross section ｛１１０｝Ｔ and the area ratio I of the crystal plane within 18° of the azimuth difference from the {111} plane of the bcc structure texture with respect to the normal line of the cross section ｛１１１｝Ｔ and the area ratio I of the crystal plane within 18° of the azimuth difference from the {100} plane of the bcc structure texture with respect to the normal line of the cross section ｛１００｝Ｔ satisfy the formula (1). 0.70 ≤ I ｛１１０｝Ｔ / (I ｛１１１｝Ｔ + I ｛１００｝Ｔ ) ≤ 1.50 (1) Further, in the surface layer region on the inner side of the bend, which is a region from the surface on the inner side of the bend of the bent portion to a depth position of 1 / 3 of the plate thickness of the bent portion in the plate thickness direction in the cross section, the area ratio I of the crystal plane within 18° of the azimuth difference from the {100} plane of the bcc structure texture with respect to the normal line of the cross section ｛１００｝ＳＩ and, in the central region which is a region from the depth position of 1 / 3 of the plate thickness to the depth position of 2 / 3 of the plate thickness in the cross section, the area ratio I of the crystal plane within 18° of the azimuth difference from the {100} plane of the bcc structure texture with respect to the normal line of the cross section ｛１００｝Ｃ satisfy the formula (2). I ｛１００｝Ｃ - I ｛１００｝ＳＩ > 0.0 (2) Further, in the surface layer region on the inner side of the bend in the cross section, the area ratio I of the crystal plane within 18° of the azimuth difference from the {110} plane of the bcc structure texture with respect to the normal line of the cross section ｛１１０｝ＳＩIn the outer surface region of the bent section, which is the area from the outer surface of the bent section to a depth of 1 / 3 of the thickness of the bent section in the thickness direction, the area ratio I of crystal planes within an orientation difference of 18° from the {110} plane of the bcc structure with respect to the normal to the cross-section. ｛１１０｝ＳＯ And satisfies equation (3). ｛１１０｝ＳＩ -I ｛１１０｝ＳＯ >0.0 (3)

[0010] The suspension components disclosed herein offer excellent impact resistance.

[0011] Figure 1 is a schematic perspective view of the undercarriage part for an automobile according to this embodiment. Figure 2 is a perspective view of the undercarriage part of Figure 1 from the opposite side. Figure 3 is a cross-sectional view of the undercarriage part in Figure 2 taken along the line III-III. Figure 4 is an enlarged view of a cross-section perpendicular to the bending line 21 of the bent portion 20 shown in Figure 3. Figure 5 is a schematic diagram illustrating the angle between the bending line and the rolling direction of the steel sheet when punching out a blank material from a steel sheet during the cold press working process in the manufacturing process of the undercarriage part. Figure 6 is a schematic diagram of a test apparatus for illustrating the bending process using the V-block method. Figure 7 is a schematic diagram illustrating the manufacturing process following Figure 6. Figure 8 is a schematic diagram illustrating the impact resistance evaluation test in this embodiment.

[0012] To improve the impact resistance of automotive suspension components, the inventors first investigated the causes of reduced impact resistance. As a result, the following was found: As described above, suspension components are formed by bending. The formed suspension component comprises a component body including a bent portion. The component body is made of steel and contains a BCC structure in area ratio of 90% or more. During bending, stress can be applied to the bent portion from various directions. Therefore, if the anisotropy of the Lankford value (r value) of the bent portion is high, the plate thickness may locally decrease in the bent portion after bending. When an impact is applied to the suspension component from the outside, stress concentrates in the portion where the plate thickness has decreased. As a result, cracks occur in the portion where the plate thickness has decreased, reducing the impact resistance of the suspension component.

[0013] Based on the above findings, the inventors investigated means to suppress localized plate thickness reduction in the bent portion after bending. As a result, the inventors found that in a cross section perpendicular to the bend line of the bent portion, the {110} plane, {111} plane, and {100} plane of the bcc structure have different anisotropy trends in the r value. Based on this finding, the inventors considered that localized plate thickness reduction in the bent portion after bending could be suppressed by adjusting the area ratio of the {110} plane, the {111} plane, and the {100} plane in the cross section of the bent portion.

[0014] Based on the above considerations, the inventors conducted further investigations. As a result, in a cross section perpendicular to the bending line of the bent portion, the area ratio I of crystal planes within an orientation difference of 18° from the {110} plane of the bcc structure with respect to the normal of the cross section ｛１１０｝Ｔ And, the area fraction I of crystal planes within an orientation difference of 18° from the {111} plane of the bcc structure with respect to the normal of the cross-section. ｛１１１｝Ｔ And, the area fraction I of crystal planes within an orientation difference of 18° from the {100} plane of the bcc structure with respect to the normal of the cross-section. ｛１００｝Ｔ We found that if equation (1) is satisfied, localized plate thickness reduction in the bent portion after bending can be sufficiently suppressed. 0.70 ≤ I ｛１１０｝Ｔ / (I ｛１１１｝Ｔ +I ｛１００｝Ｔ ) ≤ 1.50 (1)

[0015] However, even when the bent portion satisfied equation (1), sufficient impact resistance could still not be obtained in the undercarriage components. In particular, when the undercarriage components were subjected to external impact, cracks, such as cleavage, sometimes occurred on the inner surface of the bent portion. Therefore, the inventors conducted further investigations. As a result, the following findings were obtained.

[0016] In the cross section perpendicular to the bending line of the aforementioned bend, the {100} plane is a crystal plane that is easily cleaved. Therefore, even if equation (1) is satisfied, if the area ratio of the {100} plane is high in the surface region on the inside of the bend, cracks due to cleavage are likely to occur in that surface region when an external impact is applied. On the other hand, the {110} plane is a crystal plane that is not easily cleaved. Therefore, if the area ratio of the {110} plane is high in the surface region on the inside of the bend, cracks due to cleavage are suppressed in that surface region when an external impact is applied.

[0017] Furthermore, in the cross-section perpendicular to the bending line of the aforementioned bent section, tensile residual stress is generated in the surface region on the inside of the bend due to bending and springback. Therefore, cracks such as cleavage are more likely to occur in the surface region on the inside of the bend.

[0018] Based on the above findings, the inventors conducted further investigations. As a result, the inventors found that if the following equations (2) and (3) are satisfied, even when an external impact is applied, the occurrence of cracks is suppressed in the surface layer on the inside of the bend of the bent portion, and impact resistance is obtained. (I) Area ratio I of crystal planes within an orientation difference of 18° from the {100} plane of the bcc structure with respect to the normal of the cross-section in the surface layer region, which is the region from the inside of the bend of the bent portion to a depth of 1 / 3 of the thickness of the bent portion in the thickness direction of the cross-section perpendicular to the bending line of the bent portion. ｛１００｝ＳＩ And, in the central region of the cross-section, which is the area from the 1 / 3 depth position to the 2 / 3 depth position of the plate thickness, the area ratio I of crystal planes within an orientation difference of 18° from the {100} plane of the bcc structure relative to the normal of the cross-section. ｛１００｝Ｃ And satisfies equation (2). ｛１００｝Ｃ -I ｛１００｝ＳＩ >0.0 (2) (II) Area fraction I of crystal planes within an orientation difference of 18° from the {110} plane of the bcc structure with respect to the normal of the cross section, in the surface layer region on the inside of the bend of the bend, in a cross section perpendicular to the bending line of the bend. ｛１１０｝ＳＩ In the outer surface region of the bent section, which is the area from the outer surface of the bent section to a depth of 1 / 3 of the thickness of the bent section in the thickness direction, the area ratio I of crystal planes within an orientation difference of 18° from the {110} plane of the bcc structure with respect to the normal to the cross-section.｛１１０｝ＳＯ And satisfies equation (3). ｛１１０｝ＳＩ -I ｛１１０｝ＳＯ >0.0 (3)

[0019] Based on the above technical concept, the automotive undercarriage component of this embodiment has the following technical features.

[0020] The first embodiment of the undercarriage component is an undercarriage component for an automobile, comprising a component body including a bent portion. The component body is made of steel and contains 90% or more of a bcc structure by area ratio. The Vickers hardness of the component body is 250 HV or more. In a cross section perpendicular to the bending line of the bent portion, the area ratio of crystal planes within an orientation difference of 18° from the {110} plane of the bcc structure with respect to the normal to the cross section is I ｛１１０｝Ｔ And, the area fraction I of crystal planes within an orientation difference of 18° from the {111} plane of the bcc structure with respect to the normal of the cross-section. ｛１１１｝Ｔ And, the area fraction I of crystal planes within an orientation difference of 18° from the {100} plane of the bcc structure with respect to the normal of the cross-section. ｛１００｝Ｔ The equation (1) is satisfied. 0.70 ≤ I ｛１１０｝Ｔ / (I ｛１１１｝Ｔ +I ｛１００｝Ｔ ) ≤ 1.50 (1) Furthermore, in the inner surface region of the cross section, which is the region from the inner surface of the bent portion to a depth of 1 / 3 of the thickness of the bent portion in the thickness direction, the area ratio I of crystal planes within an orientation difference of 18° from the {100} plane of the bcc structure with respect to the normal of the cross section ｛１００｝ＳＩ And, in the central region of the cross-section, which is the area from the 1 / 3 depth position to the 2 / 3 depth position of the plate thickness, the area ratio I of crystal planes within an orientation difference of 18° from the {100} plane of the bcc structure relative to the normal of the cross-section. ｛１００｝Ｃ And satisfies equation (2). ｛１００｝Ｃ -I ｛１００｝ＳＩ >0.0 (2) Furthermore, in the surface region on the inside of the cross section, the area ratio I of crystal planes within an orientation difference of 18° from the {110} plane of the bcc structure with respect to the normal of the cross section. ｛１１０｝ＳＩ In the outer surface region of the bent section, which is the area from the outer surface of the bent section to a depth of 1 / 3 of the thickness of the bent section in the thickness direction, the area ratio I of crystal planes within an orientation difference of 18° from the {110} plane of the bcc structure with respect to the normal to the cross-section.｛１１０｝ＳＯ And satisfies equation (3). ｛１１０｝ＳＩ -I ｛１１０｝ＳＯ >0.0 (3)

[0021] In this embodiment, the orientation difference refers to the orientation difference (°) of the normal of the {xyz} plane (x, y, z are integers) of the bcc structural material with respect to the normal of the cross-section.

[0022] The second form of the undercarriage component is the same as the first form of the undercarriage component, wherein the component body comprises a top plate and vertical walls that rise in the direction of the thickness of the top plate. The bent portion is positioned between the top plate and the vertical walls and is connected to the top plate and the vertical walls.

[0023] Embodiments of this disclosure will be described below with reference to the drawings. In each drawing, the same or equivalent components are denoted by the same reference numerals, and the same description will not be repeated.

[0024] [Configuration of Automotive Undercarriage Components] Figures 1 and 2 are schematic perspective views of the automotive undercarriage component 100 of this embodiment. Figure 2 is a perspective view of the undercarriage component 100 of Figure 1 from the opposite side. The undercarriage component 100 includes, for example, suspension arms such as upper arms and lower arms, a torsion beam, and a stabilizer. In this embodiment, an example in which the undercarriage component 100 is a lower arm will be described.

[0025] Referring to Figures 1 and 2, the undercarriage component 100 comprises a component body 10. In the undercarriage component 100 shown in Figures 1 and 2, the component body 10 is a curved member in plan view. Plan view refers to a method of viewing the undercarriage component 100 in a direction substantially perpendicular to the horizontal plane (vertical direction) when the undercarriage component 100 is placed on a horizontal plane.

[0026] The main body of the component 10 may include mounting portions 11, 12, and 13. If the suspension component 100 is a lower arm, the mounting portion 11 is positioned outward in the vehicle width direction (left-right direction) relative to the mounting portions 12 and 13 when the suspension component 100 is installed on the vehicle body. The mounting portion 11 includes, for example, a mounting hole 111 that penetrates the suspension component 100 in the vehicle height direction (up-down direction). A ball joint is fitted into the mounting hole 111, for example, and the suspension component 100 is connected to the vehicle's wheel via the ball joint and steering knuckle.

[0027] If the suspension component 100 is a lower arm, the mounting portions 12 and 13 are arranged in the vehicle length direction (front-to-back direction) when the suspension component 100 is installed on the vehicle body. Mounting portion 12 is positioned in front of mounting portion 13. Mounting portion 12 includes, for example, a collar 121. Mounting portion 13 includes a mounting hole 131 that penetrates the suspension component 100 in the vehicle height direction. For example, bushings are fitted to mounting portions 12 and 13, and the suspension component 100 is connected to the vehicle body via the bushings.

[0028] The component body 10 includes a bent portion 20. The component body 10 further includes a top plate 30 and vertical walls 40. The top plate 30 extends along the entire longitudinal direction of the component body 10 and is a curved plate in plan view. The vertical walls 40 rise from the side edge of the top plate 30 via the bent portion 20 in the thickness direction of the top plate 30. The vertical walls 40 may be provided continuously along the entire side edge of the top plate 30, or they may be provided on a part of the side edge of the top plate 30. The vertical walls 40 function as ribs and increase the strength and rigidity of the undercarriage component 100.

[0029] The bent portion 20 is positioned between the top plate 30 and the vertical wall 40 and is connected to both the top plate 30 and the vertical wall 40. The bent portion 20 is formed by bending (pressing) a hot-rolled steel sheet (blank material), which is the material for the undercarriage part 100. In the bending process, the imaginary line that is the center of the bend is defined as the bend line. In this embodiment, the bend line is located in the center of the bent portion 20 in the thickness direction. The bent portion 20 is formed along the bend line. The bent portion 20 has rounded edges on both the inside and outside of the bend. In other words, the bent portion 20 is a part of the undercarriage part that is positioned between the top plate 30 and the vertical wall 40, is connected to both the top plate 30 and the vertical wall 40, is concave on the inside of the bend, and is convex on the outside of the bend.

[0030] Figure 3 is a cross-sectional view of the undercarriage component 100 along the line III-III in Figure 2. Figure 3 shows a cross-section of the component body 10 when cut perpendicular to the bending line 21 of the bent portion 20. Referring to Figure 3, in the cross-section perpendicular to the bending line 21, the top plate 30 and the vertical wall 40 are flat plate-like portions. The bent portion 20 is a curved portion. As described above, in the cross-section 20CS of the bent portion 20 perpendicular to the bending line 21, the inner surface SI on the inside of the bend is concave, and the outer surface SO on the outside of the bend is convex. Preferably, the radius of curvature of the inner surface SI is constant (i.e., the inner surface SI has a single radius). Similarly, preferably, the radius of curvature of the outer surface SO is constant (i.e., the outer surface SO has a single radius). However, the inner surface SI may have multiple radii of curvature, and the outer surface SO may have multiple radii of curvature.

[0031] The bent portion 20 may be provided continuously along the entire side edge of the top plate 30, similar to the vertical wall 40 described above, or it may be provided on a part of the side edge of the top plate 30. The maximum stress occurs in the most curved portion of the component body 10 in a plan view. Therefore, the bent portion 20 and the vertical wall 40 may be formed at least on the side edge of the top plate 30 in the most curved portion in a plan view. Here, the most curved portion in a plan view is the portion with the greatest curvature of the curved portion.

[0032] In Figures 1 to 3, a lower arm was used as an example of a suspension component 100, but as mentioned above, the suspension component 100 may be a component other than a lower arm. Suspension components 100 other than the lower arm, such as suspension arms, torsion beams, and stabilizers, also include a component body 10 that includes a bent portion 20, similar to the lower arm.

[0033] [Configuration of the undercarriage parts of this embodiment] The undercarriage parts 100 of this embodiment further satisfy the following features. (Feature 1) The part body 10 is made of steel and contains 90% or more of the bcc structure in terms of area ratio. (Feature 2) The Vickers hardness of the part body 10 is 250 HV or more. (Feature 3) In a cross section perpendicular to the bending line 21 of the bent portion 20, the area ratio of crystal planes within an orientation difference of 18° from the {110} plane of the bcc structure with respect to the normal of the cross section is I ｛１１０｝Ｔ And, the area fraction I of crystal planes within an orientation difference of 18° from the {111} plane of the bcc structure with respect to the normal of the cross-section. ｛１１１｝Ｔ And, the area fraction I of crystal planes within an orientation difference of 18° from the {100} plane of the bcc structure with respect to the normal of the cross-section. ｛１００｝Ｔ The equation (1) is satisfied. 0.70 ≤ I ｛１１０｝Ｔ / (I ｛１１１｝Ｔ +I ｛１００｝Ｔ ) ≤ 1.50 (1) (Feature 4) In the cross section perpendicular to the bending line 21 of the bent portion 20, in the inner surface region of the bent portion 20, which is the region from the inner surface of the bent portion 20 to a depth of 1 / 3 of the plate thickness of the bent portion 20 in the plate thickness direction, the area ratio I of crystal planes within an orientation difference of 18° from the {100} plane of the bcc structure with respect to the normal of the cross section ｛１００｝ＳＩ In the central region of the cross-section, which is the area from 1 / 3 depth to 2 / 3 depth of the bent portion 20, the area ratio I of crystal planes within an orientation difference of 18° from the {100} plane of the bcc structure relative to the normal of the cross-section. ｛１００｝Ｃ And satisfies equation (2). ｛１００｝Ｃ -I ｛１００｝ＳＩ >0.0 (2) (Feature 5) Area ratio I of crystal planes within an orientation difference of 18° from the {110} plane of the bcc structure with respect to the normal of the cross section in the surface region on the inside of the bend of the bend of the bend 20, in a cross section perpendicular to the bending line 21 of the bend 20. ｛１１０｝ＳＩIn the outer surface region of the bent portion 20, which is the area from the outer surface of the bent portion 20 to a depth of 1 / 3 of the thickness of the bent portion in the thickness direction, the area ratio I of crystal planes within an orientation difference of 18° from the {110} plane of the bcc structure with respect to the normal to the cross-section ｛１１０｝ＳＯ And satisfies equation (3). ｛１１０｝ＳＩ -I ｛１１０｝ＳＯ >0.0 (3) The following describes each characteristic.

[0034] [(Feature 1) Regarding the crystal structure of the component body 10] The component body 10 is made of steel, and the main crystal structure of the component body 10 is a bcc (body-centered cubic) structure. Specifically, the component body 10 contains 90% or more of the bcc structure by area ratio. The preferred lower limit of the area ratio of the bcc structure in the component body 10 is 92%, more preferably 94%, more preferably 95%, and still preferably 96%. The most preferred area ratio of the bcc structure is 100%.

[0035] [Method for measuring the area ratio (%) of the bcc structural structure] The area ratio (%) of the bcc structural structure in the main body of the part 10 is determined by the following method. Figure 4 is an enlarged view of a cross section perpendicular to the bending line 21 of the bent portion 20 shown in Figure 3. Referring to Figure 4, in the cross section 20CS perpendicular to the direction of the bending line 21 of the bent portion 20 of the undercarriage part 100, the intersection point of the line segment LS extending in the thickness direction of the bent portion 20 and the outer surface SO on the outside of the bend of the bent portion 20 is defined as the vertex P of the outer surface SO. The plate thickness of the bent portion 20 is defined as t (mm). The length in the direction perpendicular to the line segment LS (width direction) with the vertex P as the center is defined as the width W. The observation field OF is defined as a roughly rectangular area where the width W is 400 μm and the length in the plate thickness direction is t (mm). A test piece having the cross section 20CS including the observation field OF is taken. After embedding the test piece in resin, the cross section 20CS is polished to a mirror finish by mechanical polishing. Subsequently, the 20CS cross section is further polished using a colloidal suspension.

[0036] Electron backscatter diffraction (EBSD) measurements are performed on the observation field of view OF of the 20CS cross-section after polishing. An EBSD analyzer equipped with a thermal field emission scanning electron microscope and a high-speed EBSD detector is used for the EBSD measurements. The thermal field emission scanning electron microscope is, for example, the JSM-7200F manufactured by JEOL Ltd. The EBSD detector is, for example, the EDAX Velocity manufactured by AMETEK Corporation. For the EBSD measurements, the acceleration voltage is set to 25 kV and the irradiation current level to 16. The measurement points in the observation field are set as follows.

[0037] The observation field of view (OF) is divided into hexagonal pixel units. Specifically, multiple hexagonal pixels are arranged within the observation field of view (OF) so as to completely tile the field without gaps. The distance between the centers of adjacent pixels (step size) is set to 4.0 μm. Among the pixels tiled within the observation field of view (OF), the measurement point is the center position of the pixel that is entirely contained within the observation field of view (OF). In other words, pixels that are partially outside the observation field of view (OF) are not included in the measurement.

[0038] The backscattered electron pattern (EBS pattern) obtained from EBS measurements at each measurement point is defined as the EBS pattern of the pixel containing that measurement point. Measurement points with a Confidence Index (CI value) of 0.1 or less, which indicates the certainty of the obtained EBS pattern, are not used in subsequent calculations. Furthermore, pixels containing measurement points with a CI of 0.1 or less are treated as non-existent.

[0039] The EBSD pattern of each pixel is analyzed to determine whether the structure of each pixel is bcc or fcc. Known EBSD analysis software is used for this determination. For example, the EBSD analysis software used is OIM Data Collection ver. 7, manufactured by TSL Solutions Co., Ltd.

[0040] Each pixel in the observation field of view (OF) is clustered into bcc and fcc structural structures. The total area of ​​the bcc structural pixels within the observation field of view (OF) is calculated. Based on the obtained total area of ​​the bcc structural pixels, the area percentage (%) of the bcc structural structure is calculated. The obtained area percentage (%) of the bcc structural structure is rounded to the first decimal place and expressed as an integer (%). Known EBSD analysis software is used to calculate the area percentage (%) of the bcc structural structure. For example, the product name: OIM Data Analysis ver. 7 from TSL Solutions Co., Ltd. is used as the EBSD analysis software. The area percentage of the bcc structural structure can be obtained by using the Phase Map function of this software.

[0041] [(Feature 2) Regarding the Vickers hardness of the component body 10] The Vickers hardness of the component body 10 is 250 HV or higher. In other words, the suspension component 100 has high hardness and high strength. The preferred lower limit of the Vickers hardness of the component body 10 is 252 HV, more preferably 255 HV, even more preferably 258 HV, and even more preferably 260 HV. There is no particular upper limit to the Vickers hardness of the component body 10. For example, the upper limit of the Vickers hardness of the body is 500 HV, preferably 450 HV, more preferably 400 HV, and even more preferably 350 HV.

[0042] [Method for Measuring the Vickers Hardness of the Component Body 10] The Vickers hardness of the component body 10 is measured by the following method. Test pieces are taken from five arbitrary locations on the component body 10, with the measurement surface being a cross-section parallel to the thickness direction of the component body 10. On the measurement surface of each test piece, five measurement points are selected at a depth of t / 4 from the surface of the component body 10. Here, t represents the thickness (mm) of the component body 10. A Vickers hardness test is performed at each measurement point in accordance with JIS Z 2241-1 (2020) to obtain the Vickers hardness (HV). The test force at this time is set to 0.49 N. The spacing between adjacent measurement points shall be at least three times the indentation length.

[0043] The arithmetic mean of 25 Vickers hardness values ​​obtained from five test specimens is defined as the Vickers hardness HV of the main body. The Vickers hardness is defined as an integer value obtained by rounding the arithmetic mean to the nearest tenth.

[0044] [(Feature 3) Texture of the entire cross-section of the bent portion 20] Figure 4 is an enlarged view of the cross-section perpendicular to the bending line 21 of the bent portion 20 shown in Figure 3. Referring to Figure 4, the following is defined for the cross-section 20CS perpendicular to the bending line 21 of the bent portion 20: Area ratio I ｛１１０｝Ｔ Area ratio (%) of crystal planes within an orientation difference of 18° from the {110} plane of the bcc structure relative to the normal N of the cross-section 20CS. Area ratio I ｛１１１｝Ｔ Area ratio (%) of crystal planes within an orientation difference of 18° from the {111} plane of the bcc structure relative to the normal N of the cross-section 20CS. Area ratio I ｛１００｝Ｔ Area percentage (%) of crystal planes within an orientation difference of 18° from the {100} plane of the bcc structure with respect to the normal N of the cross-section 20CS.

[0045] Here, crystal planes within an orientation difference of 18° from the {110} plane of the bcc structure relative to the normal N of the cross section 20CS are referred to as the "{110} texture". Similarly, crystal planes within an orientation difference of 18° from the {111} plane of the bcc structure relative to the normal N of the cross section 20CS are referred to as the "{111} texture". Likewise, crystal planes within an orientation difference of 18° from the {100} plane of the bcc structure relative to the normal N of the cross section 20CS are referred to as the "{100} texture".

[0046] In this embodiment, the area ratio I of the {110} texture is in the cross section 20CS of the bent portion 20. ｛１１０｝Ｔ And, {111} Texture Area Ratio I ｛１１１｝Ｔ And, the area ratio I of the {100} texture ｛１００｝Ｔ The equation (1) is satisfied. 0.70 ≤ I ｛１１０｝Ｔ / (I ｛１１１｝Ｔ +I ｛１００｝Ｔ ) ≤ 1.50 (1)

[0047] In the bcc structure, the degree of plastic anisotropy (anisotropy of the r value) differs between the {110} texture, the {111} texture, and the {100} texture. Here, as an example of the deformation direction, we will explain below using θ as the angle between the rolling direction and the tensile direction of the steel plate material that is the material of the undercarriage part 100.

[0048] In a bcc structure, the {111} texture remains almost constant even when the angle θ between the rolling direction and the tensile direction changes within the range of 0 to 90°. In other words, the anisotropy of the r value is low in the {111} texture.

[0049] On the other hand, with the {110} texture, the r value is significantly high when the angle θ between the rolling direction and the tension direction is 0°, and as the angle θ increases from 0° to 45°, the r value decreases significantly. Then, as the angle θ increases from 45° to 90°, the r value increases significantly again. In other words, with the {110} texture, the r value graph is convex downwards in the range of angle θ from 0 to 90°, and the r value is at its minimum value at angle θ of 45°.

[0050] In the {100} texture, the r value is low, almost 0, when the angle θ between the rolling and tensile directions is 0°. As the angle θ increases from 0° to 45°, the r value increases. Then, as the angle θ increases from 45° to 90°, the r value decreases again to 0. In other words, in the {100} texture, the r value graph is convex downwards in the range of angle θ from 0 to 90°, and the r value is at its maximum value at angle θ of 45°.

[0051] As described above, if the plate thickness of the bent portion 20 is locally reduced, stress will concentrate in the thinner portion when an impact is applied to the undercarriage part 100 from the outside. Therefore, cracks are more likely to occur in the thinner portion. In this embodiment, the undercarriage part 100 is made more impact resistant by suppressing the localized reduction in the plate thickness of the bent portion 20.

[0052] As described above, in cross-section 20CS, the {111} texture exhibits low r-value anisotropy. On the other hand, the r-value of the {110} texture shows a downward-convex graph in the angle θ range of 0 to 90°, while the r-value of the {100} texture shows an upward-convex graph in the angle θ range of 0 to 90°. In other words, the r-value anisotropy of the {110} texture shows the opposite trend to that of the r-value anisotropy of the {100} texture.

[0053] Considering the anisotropy of the r values ​​of the {111} texture, {110} texture, and {100} texture described above, the area ratios of the {110} texture, {111} texture, and {100} texture within the cross section 20CS are adjusted so as to sufficiently reduce the anisotropy of the r values ​​within the cross section 20CS. Specifically, in this embodiment, the area ratio I of the {110} texture in the cross section 20CS is adjusted. ｛１１０｝Ｔ , {111} Texture area ratio I ｛１１１｝Ｔ , and the area ratio I of the {100} texture ｛１００｝Ｔ This satisfies equation (1). 0.70 ≤ I ｛１１０｝Ｔ / (I ｛１１１｝Ｔ +I ｛１００｝Ｔ ) ≤ 1.50 (1)

[0054] F1 is defined by the following equation: F1 = I ｛１１０｝Ｔ / (I ｛１１１｝Ｔ +I ｛１００｝Ｔ If F1 is less than 0.70, the area ratio of the {110} texture is too low compared to the total area ratio of the {111} texture and the {100} texture. In this case, the anisotropy of the r value increases. As a result, during bending, there are areas where the plate thickness decreases excessively locally. Consequently, the impact resistance of the undercarriage parts decreases. On the other hand, if F1 exceeds 1.50, the area ratio of the {110} texture is too high compared to the total area ratio of the {111} texture and the {100} texture. In this case as well, the anisotropy of the r value increases. As a result, the impact resistance of the undercarriage parts decreases.

[0055] If F1 is between 0.70 and 1.50, the area ratio of the {110} texture is appropriate to the total area ratio of the {111} texture and the {100} texture. As a result, the anisotropy of the r value is sufficiently suppressed. Consequently, even if the steel plate (blank material) that will become the material of the undercarriage part 100 is bent during the manufacturing process of the undercarriage part 100, a local reduction in plate thickness at the formed bent portion 20 can be suppressed. As a result, excellent impact resistance can be obtained in the undercarriage part 100.

[0056] A preferred lower limit for F1 is 0.75, more preferably 0.80, and even more preferably 0.92. A preferred upper limit for F1 is 1.40, more preferably 1.35, even more preferably 1.30, even more preferably 1.25, even more preferably 1.20, even more preferably 1.16, even more preferably 1.12, and even more preferably 1.08.

[0057] [(Feature 4) Ratio of {100} texture in the surface region and central region on the inside of the bend of the bent portion 20] Referring to Figure 4, the following is defined for the cross section 20CS of the bent portion 20. Surface region 20SI: The region from the inner surface (internal surface) SI of the bent portion 20 to a depth of 1 / 3 of the plate thickness t in the plate thickness direction of the bent portion 20. Central region 20C: The region from the inner surface (internal surface) SI of the bent portion 20 to a depth of 1 / 3 to 2 / 3 of the plate thickness t in the plate thickness direction of the bent portion 20. Area ratio I ｛１００｝ＳＩ : The area percentage (%) of crystal planes within the surface region 20SI of the cross-section 20CS that are within an orientation difference of 18° from the {100} plane of the bcc structure relative to the normal N of the cross-section 20CS. In other words, the area percentage (%) of the {100} texture in the surface region 20SI. Area percentage I ｛１００｝Ｃ : The area percentage (%) of crystal planes within the central region 20C of the cross-section 20CS that are within an orientation difference of 18° from the {100} plane of the bcc structure relative to the normal N of the cross-section 20CS. In other words, the area percentage (%) of the {100} texture in the central region 20C.

[0058] In this embodiment, the area ratio I of the {100} texture in the surface region 20SI ｛１００｝ＳＩAnd the area ratio I of the {100} texture in the central region 20C ｛１００｝Ｃ The equation (2) is satisfied. ｛１００｝Ｃ -I ｛１００｝ＳＩ >0.0 (2)

[0059] In a bcc structure, the {100} plane is prone to becoming a cleavage plane. Therefore, even if F1 satisfies equation (1), if the area ratio of the {100} texture in the surface region 20SI is large, cleavage is more likely to occur in the surface region 20SI of the bent portion 20. As a result, the impact resistance of the undercarriage parts decreases.

[0060] F2 is defined by the following equation: F2 = I ｛１００｝Ｃ -I ｛１００｝ＳＩ When F2 is greater than 0.0, the area ratio of the {100} texture becomes larger in the central region 20C than in the surface region 20SI. In this case, the area ratio of the {100} texture in the surface region 20SI is sufficiently reduced. As a result, the occurrence of cleavage in the bent portion 20 is suppressed, and excellent impact resistance is obtained in the undercarriage component 100.

[0061] The preferred lower limit of F2 is 1.0, more preferably 1.5, even more preferably 2.0, even more preferably 2.5, even more preferably 3.0, even more preferably 4.0, and even more preferably 5.0. The upper limit of F2 is not particularly limited. For example, the upper limit of F2 is 15.0, for example 12.0, and for example 10.0.

[0062] [(Feature 5) Ratio of {110} texture between the inner and outer surface regions of the bent portion 20] Referring to Figure 4, the following is defined for the cross section 20CS of the bent portion 20: Surface region 20SO: Area ratio I from the outer surface (outer surface) SO of the bent portion 20 to a depth of 1 / 3 of the plate thickness t in the plate thickness direction of the bent portion 20 ｛１１０｝ＳＩ : The area percentage (%) of crystal planes within the surface region 20SI of the cross-section 20CS that are within an orientation difference of 18° from the {110} plane of the bcc structure relative to the normal N of the cross-section 20CS. In other words, the area percentage (%) of the {110} texture in the surface region 20SI. Area percentage I ｛１１０｝ＳＯ: Among the cross-sectional surface 20CS, the area ratio (%) of the crystal plane within 18° of the azimuth difference from the {110} plane of the bcc structure tissue with respect to the normal line N of the cross-sectional surface 20CS in the surface layer region 20SO. That is, the area ratio (%) of the {110} texture in the surface layer region 20SO

[0063] In the present embodiment, the area ratio I of the {110} texture in the surface layer region 20SI ｛１１０｝ＳＩ and the area ratio I of the {110} texture in the surface layer region 20SO ｛１１０｝ＳＯ satisfy the formula (3). I ｛１１０｝ＳＩ - I ｛１１０｝ＳＯ > 0.0 (3)

[0064] In the bcc structure tissue, the {110} plane suppresses the occurrence of cracks such as cleavage. The bent portion 20 is formed by bending, but in the surface layer region 20SI on the inner side of the bend of the bent portion 20, due to the influence of the residual stress and springback applied during bending, compared with the surface layer region 20SO on the outer side of the bend of the bent portion 20, the tensile residual stress is likely to be larger. Therefore, cracks are more likely to occur in the surface layer region 20SI than in the surface layer region 20SO.

[0065] Define F3 by the following formula. F3 = I ｛１１０｝ＳＩ - I ｛１１０｝ＳＯ When F3 is larger than 0.0, the area ratio I of the {110} texture in the surface layer region 20SI ｛１１０｝ＳＩ becomes larger than the area ratio I of the {110} texture in the surface layer region 20SO ｛１１０｝ＳＯ Therefore, in the surface layer region 20SI on the inner side of the bend where tensile residual stress is likely to occur, the occurrence of cracks is suppressed, and excellent impact resistance can be obtained in the peripheral component 100.

[0066] The preferable lower limit of F3 is 1.0, more preferably 1.5, still more preferably 2.0, still more preferably 2.5, still more preferably 3.0, still more preferably 4.0, still more preferably 5.0. The upper limit of F3 is not particularly limited. The upper limit of F3 is, for example, 70.0, and for example, 50.0.

[0067] [The area ratios I ｛１１０｝Ｔ , I ｛１１１｝Ｔ , I｛１００｝Ｔ , I ｛１００｝Ｃ , I ｛１００｝ＳＩ , I ｛１１０｝ＳＩ and I ｛１１０｝ＳＯ Measurement method of I: The area ratios I ｛１１０｝Ｔ , I ｛１１１｝Ｔ , I ｛１００｝Ｔ , I ｛１００｝Ｃ , I ｛１００｝ＳＩ , I ｛１１０｝ＳＩ and I ｛１１０｝ＳＯ are obtained by the following method.

[0068] Using the EBSD pattern of each pixel in the observation field OF in the above [Measurement method of the area ratio (%) of the bcc structure tissue], the following analysis is performed.

[0069] Based on the crystal orientation at each measurement point of the obtained EBSD pattern, identify the region (pixel) of the crystal plane within 18° of azimuth from the {110} plane of the bcc structure tissue with respect to the normal N of the cross section 20CS (that is, the {110} texture). Obtain the total area of the identified {110} texture. Based on the total area of the {110} texture and the area of the observation field OF, the area ratio I ｛１１０｝Ｔ (%) of the {110} texture is obtained. The area ratio I ｛１１０｝Ｔ (%) of the {110} texture is taken as the value up to the first decimal place obtained by rounding the second decimal place of the obtained numerical value. Identify the region (pixel) of the crystal plane within 18° of azimuth from the {111} plane of the bcc structure tissue with respect to the normal N of the cross section 20CS (that is, the {111} texture). Obtain the total area of the identified {111} texture. Based on the total area of the {111} texture and the area of the observation field OF, the area ratio I ｛１１１｝Ｔ (%) of the {111} texture is obtained. The area ratio I ｛１１１｝Ｔ (%) of the {111} texture is taken as the value up to the first decimal place obtained by rounding the second decimal place of the obtained numerical value. Identify the region of the crystal plane within 18° of azimuth from the {100} plane of the bcc structure tissue with respect to the normal N of the cross section 20CS (that is, the {100} texture). Obtain the total area of the identified {100} texture. Based on the total area of the {100} texture and the area of the observation field OF, the area ratio I ｛１００｝ＴCalculate the percentage (%). {100} Area ratio of texture I ｛１００｝Ｔ The percentage (%) is calculated by rounding the second decimal place of the obtained value to one decimal place.

[0070] To identify the regions of each texture and calculate the total area ratio, known EBSD analysis software is used. For example, the EBSD analysis software used is OIM Data Analysis ver. 7, manufactured by TSL Solutions Co., Ltd. Using the Highlighting function of this software, an arbitrary crystal plane is selected, and the tolerance angle (the maximum allowable orientation difference from the selected orientation) is set to 18°. This allows only the regions (pixels) within an orientation difference of 18° from the arbitrary crystal plane to be extracted.

[0071] The obtained area ratio I ｛１１０｝Ｔ (%), area ratio I ｛１１１｝Ｔ (%) and area ratio I ｛１００｝Ｔ Based on the percentage, calculate F1. F1 is the value obtained by rounding the third decimal place of the result to two decimal places.

[0072] Furthermore, within the observation field OF, the area of ​​the {100} texture in the surface region 20SI is identified. Here, among the pixels tiled in the surface region 20SI, only pixels whose entirety is contained within the surface region 20SI are measured. In other words, pixels whose part extends beyond the surface region 20SI are not measured. Based on the total area of ​​the identified {100} texture and the total area of ​​the surface region 20SI, the area ratio I of the {100} texture in the surface region 20SI is determined. ｛１００｝ＳＩ Calculate the percentage (%). {100} Area ratio of texture I ｛１００｝ＳＩ The percentage (%) is calculated by rounding the second decimal place of the obtained value to one decimal place.

[0073] Within the observation field OF, the area of ​​the {100} texture in the central region 20C is identified. Here, among the pixels tiled in the central region 20C, only pixels whose entirety is contained within the central region 20C are measured. In other words, pixels whose part extends beyond the central region 20C are not measured. Based on the total area of ​​the identified {100} texture and the area of ​​the central region 20C, the area ratio I of the {100} texture in the central region 20C is determined. ｛１００｝Ｃ Calculate the percentage (%). {100} Area ratio of texture I ｛１００｝Ｃ The percentage (%) is calculated by rounding the second decimal place of the obtained value to one decimal place.

[0074] The obtained area ratio I ｛１００｝ＳＩ (%) and area ratio I ｛１００｝Ｃ Based on the percentage, calculate F2. F2 is the value obtained by rounding the second decimal place of the result to the first decimal place.

[0075] Furthermore, within the observation field OF, the area of ​​the {110} texture in the surface region 20SI is identified as described above. Based on the total area of ​​the identified {110} texture and the total area of ​​the surface region 20SI, the area ratio I of the {110} texture in the surface region 20SI is determined. ｛１１０｝ＳＩ Calculate the percentage (%). {110} Area ratio of texture I ｛１１０｝ＳＩ The percentage (%) is calculated by rounding the second decimal place of the obtained value to one decimal place.

[0076] Within the observation field OF, the area of ​​the {110} texture in the surface region 20SO is identified. Here, among the pixels tiled in the surface region 20SO, only pixels whose entirety is contained within the surface region 20SO are measured. In other words, pixels whose part extends beyond the surface region SO are not measured. Based on the total area of ​​the identified {110} texture and the total area of ​​the surface region 20SO, the area ratio I of the {110} texture in the surface region 20SO is determined. ｛１１０｝ＳＯ Calculate the percentage (%). {110} Area ratio of texture I ｛１１０｝ＳＯ The percentage (%) is calculated by rounding the second decimal place of the obtained value to one decimal place.

[0077] The obtained area ratio I ｛１１０｝ＳＩ (%) and area ratio I ｛１１０｝ＳＯ Based on the percentage, calculate F3. F3 is the value obtained by rounding the second decimal place of the result to the first decimal place.

[0078] [Effects of the undercarriage part 100] The undercarriage part 100 of this embodiment satisfies features 1 to 5. Therefore, when bending (cold pressing) is performed in the manufacturing process of the undercarriage part 100, it is possible to suppress the localized reduction of the plate thickness at the formed bent portion 20. Furthermore, it is possible to suppress the occurrence of cleavage (cracking) on ​​the surface layer on the inside of the bend of the bent portion 20. As a result, the undercarriage part 100 of this embodiment provides excellent impact resistance.

[0079] [Regarding the plate thickness t of the bent portion 20 of the main body of the part 10] In the undercarriage part 100 of this embodiment, the plate thickness t of the bent portion 20 is not particularly limited. The plate thickness t is, for example, 1.5 to 6.0 mm. From the viewpoint of further increasing the strength of the bent portion 20 of the main body of the part 10, the preferred lower limit of the plate thickness t is 1.8 mm, more preferably 2.0 mm, and even more preferably 2.2 mm. From the viewpoint of further improving the bendability of the bent portion 20 of the main body part, the preferred upper limit of the plate thickness t is 5.5 mm, more preferably 5.0 mm, even more preferably 4.5 mm, and even more preferably 4.0 mm.

[0080] Furthermore, if a coating is formed on the steel plate that is the material of the component body 10, the coating is formed on the surface of the bent portion 20 of the component body 10. In this case, the plate thickness t of the bent portion 20 means the plate thickness of the steel plate excluding the coating (the plate thickness of the steel plate constituting the bent portion 20).

[0081] [Method for measuring the plate thickness t of the bent portion 20] The plate thickness t of the bent portion 20 is measured by, for example, the following method: A profile of the cross section perpendicular to the bend line 21 of the bent portion 20 is created using a non-contact three-dimensional measuring machine. From the created profile of the bent portion 20, the plate thickness at any 10 locations in the bent portion 20 is determined. The arithmetic mean of the obtained plate thicknesses is taken as the plate thickness t (mm) of the bent portion 20. The thickness t is the value obtained by rounding the second decimal place of the calculated value to the first decimal place. For example, the non-contact three-dimensional measuring machine used is the VR-6000 manufactured by Keyence Corporation. If a coating is formed on the steel plate constituting the bent portion 20 of the part body 10, the plate thickness t of the bent portion 20 is determined by the above method using the plate thickness of the bent portion 20 after removing the coating using a well-known release agent.

[0082] [Regarding the bending radius R of the bent portion 20] As described above, in a cross section perpendicular to the bending line 21 of the bent portion 20, the inner surface SI on the inside of the bend may have multiple radii of curvature. Preferably, the inner surface SI on the inside of the bend of the bent portion 20 has one radius of curvature R. Hereinafter, the radius of curvature of the inner surface SI on the inside of the bend of the bent portion 20 will be defined as the bending radius R. The bending radius R is not particularly limited. For example, the bending radius R is 8.0 mm or less. The preferred upper limit of the bending radius R is 6.0 mm, more preferably 4.0 mm, and even more preferably 3.0 mm. The preferred lower limit of the bending radius R is 1.0 mm, more preferably 1.2 mm, even more preferably 1.6 mm, and even more preferably 2.0 mm.

[0083] [Method for measuring the bending radius R of the bent portion 20] The bending radius R of the bent portion 20 is measured, for example, by the following method. The bending radius R (mm) is determined based on the cross-sectional profile perpendicular to the bending line 21 of the bent portion 20 obtained in the above-described [Method for measuring the plate thickness t of the bent portion 20]. The bending radius R is rounded to one decimal place by rounding the second decimal place of the obtained value.

[0084] [Regarding the application of the suspension component 100] The suspension component 100 of this embodiment is widely applicable to automotive applications. The suspension component 100 is not particularly limited as long as it comprises a component body 10 and a bent portion 20. Examples of suspension components 100 include upper arms, lower arms, torsion beams, stabilizers, etc.

[0085] [Regarding the steel plate material of the main body 10 of the component] The main body 10 of the undercarriage component 100 in this embodiment is made of steel plate. The chemical composition of the steel plate is not particularly limited. In other words, the steel plate may have a well-known chemical composition. The chemical composition of the steel plate may be, for example, C: 0.02 to 0.30%, Si: 0.01 to 2.00%, Mn: 0.50 to 3.00%, Al: 0.010 to 1.000%, Ti: 0.06 to 0.20%, P: 0.100% or less, S: 0.0150% or less, N: 0.0100% or less, Nb: 0 to 0.10%, V: 0 to It contains 0.40% of the following: Ca: 0-0.0060%, Sb: 0-0.080%, Mo: 0-1.00%, Cr: 0-1.00%, Ni: 0-0.40%, B: 0-0.0020%, Cu: 0-1.00%, Sn: 0-0.50%, and Zr: 0-0.050%, with the remainder being Fe and impurities. Preferably, the remainder consists of Fe and impurities. The chemical composition of the component body 10 is the same as the chemical composition of the steel sheet used as the base material.

[0086] If the carbon content is 0.02% or higher, the strength of the steel sheet will be high. Furthermore, if the carbon content is 0.30% or lower, sufficient bendability (cold press workability) of the steel sheet will be obtained. Therefore, the carbon content is, for example, between 0.02% and 0.30%.

[0087] If the Si content is 0.01% or higher, the strength of the steel sheet will increase through solid solution strengthening. Furthermore, if the Si content is 2.00% or lower, sufficient bendability of the steel sheet can be obtained. Therefore, the Si content is, for example, between 0.01% and 2.00%.

[0088] If the Mn content is 0.50% or higher, the hardenability of the steel increases, and the strength of the steel sheet increases. Furthermore, if the Mn content is 3.00% or lower, sufficient bendability of the steel sheet is obtained. Therefore, the Mn content is, for example, between 0.50% and 3.00%.

[0089] If the Al content is 0.010% or higher, the strength of the steel sheet will increase due to solid solution strengthening. Also, if the Al content is 1.000% or lower, sufficient bendability of the steel sheet can be obtained. Therefore, the Al content is, for example, between 0.010% and 1.000%. The upper limit of the Al content may be, for example, 0.100% or 0.050%.

[0090] If the Ti content is 0.06% or higher, the strength of the steel sheet increases due to precipitation strengthening. Furthermore, if the Ti content is 0.20% or lower, sufficient bendability of the steel sheet is obtained. Therefore, the Ti content is, for example, 0.06 to 0.20%.

[0091] P is an unavoidable impurity. In other words, the P content is greater than 0%. If the P content is 0.100% or less, the bendability of the steel sheet can be obtained sufficiently. Therefore, the P content is, for example, 0.100% or less. The upper limit of the P content may be, for example, 0.050% or 0.040%.

[0092] S is an unavoidable impurity. In other words, the S content is greater than 0%. If the S content is 0.0150% or less, the bendability of the steel sheet can be obtained sufficiently. Therefore, the S content is, for example, 0.0150% or less. The upper limit of the S content may be, for example, 0.0120%.

[0093] N is an unavoidable impurity. In other words, the N content is greater than 0%. If the N content is 0.0100% or less, sufficient bendability of the steel sheet can be obtained. Therefore, the N content is, for example, 0.0100% or less. The upper limit of the N content may be, for example, 0.0080%.

[0094] Nb and V are both optional elements and do not need to be present. In other words, the Nb content may be 0%, and the V content may be 0%. When present, Nb and V refine the crystal grains through a pinning effect, improving the toughness and bendability of the steel sheet. Even if only a small amount of Nb or V is present, the above effect can be obtained to some extent. Furthermore, if the Nb content is 0.10% or less and the V content is 0.40% or less, sufficient cold workability of the steel sheet can be obtained. Therefore, the Nb content is, for example, 0 to 0.10%. The V content is, for example, 0 to 0.40%. The upper limit of the Nb content may be, for example, 0.06%, and the upper limit of the V content may be, for example, 0.30%.

[0095] Ca and Sb are both optional elements and do not need to be included. In other words, the Ca content may be 0%, and the Sb content may be 0%. If included, Ca refines inclusions, improving the toughness and bendability of the steel sheet. Sb improves the uniformity of scale formed on the surface of the steel sheet, improving the toughness and bendability of the steel sheet. Even if only a small amount of Ca or Sb is included, the above effects can be obtained to some extent. Furthermore, if the Ca content is 0.0060% or less and the Sb content is 0.080% or less, sufficient bendability of the steel sheet can be obtained. Therefore, the Ca content is, for example, 0 to 0.0060%, and the Sb content is, for example, 0 to 0.080%. The upper limit of the Ca content may be, for example, 0.0040%, and the upper limit of the Sb content may be, for example, 0.040%.

[0096] Mo, Cr, Ni, B, Cu, and Sn are all optional elements and do not need to be included. In other words, the Mo content, Cr content, Ni content, B content, Cu content, and Sn content may each be 0%. When included, these elements all increase the strength of the steel sheet. The above effect can be obtained to some extent if even a small amount of any one of Mo, Cr, Ni, B, Cu, and Sn is included. Furthermore, if the Mo content is 1.00% or less, the Cr content is 1.00% or less, the Ni content is 0.40% or less, the B content is 0.0020% or less, the Cu content is 1.00% or less, and the Sn content is 0.50% or less, the bendability of the steel sheet can be obtained sufficiently. Therefore, the Mo content is, for example, 0 to 1.00%, the Cr content is, for example, 0 to 1.00%, the Ni content is, for example, 0 to 0.40%, the B content is, for example, 0 to 0.0020%, the Cu content is, for example, 0 to 1.00%, and the Sn content is, for example, 0 to 0.50%. The upper limit of the Mo content may be, for example, 0.50%, the upper limit of the Cr content may be, for example, 0.60%, the upper limit of the Ni content may be, for example, 0.30%, the upper limit of the B content may be, for example, 0.0015%, the upper limit of the Cu content may be, for example, 0.40%, and the upper limit of the Sn content may be, for example, 0.10%.

[0097] Zr is an optional element and does not need to be present. In other words, the Zr content can be 0%. When present, Zr improves the formability (bendability) of the steel sheet. Even a small amount of Zr will provide some degree of the above effect. Furthermore, if the Zr content is 0.050% or less, sufficient cold workability of the steel sheet can be obtained. Therefore, the Zr content is between 0 and 0.050%. The upper limit of the Zr content may be, for example, 0.020%.

[0098] The remainder of the chemical composition of the steel sheet contains, for example, Fe and impurities. Preferably, the remainder of the chemical composition of the steel sheet consists of Fe and impurities. Here, impurities in the chemical composition are those that are mixed in from raw materials such as ore, scrap, or the manufacturing environment during the industrial production of the steel sheet. The impurities are, for example, one or more selected from the group consisting of O, H, 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, W, and Po, and the total content of these impurities is, for example, 0.100% or less.

[0099] Furthermore, a coating may be formed on part or all of the surface of the steel plate that constitutes the component body 10. The coating may be, for example, a paint coating containing an epoxy resin. Such a paint coating is applied to the surface of the steel plate by, for example, cationic electrodeposition coating. When a coating is formed on the steel plate, the corrosion resistance of the component body 10 is increased. A coating does not have to be formed on the surface of the steel plate.

[0100] [Method for Measuring Chemical Composition] The chemical composition of steel sheets can be measured using a well-known component analysis method in accordance with JIS G0321:2017. Specifically, chips are collected from the steel sheet using a drill. The collected chips are dissolved in acid to obtain a solution. Elemental analysis of the chemical composition is performed on the solution using ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrometry). The C, S, and O content are determined by the well-known high-frequency combustion method (combustion-infrared absorption method). The N content is determined using the well-known inert gas melting-thermal conductivity method.

[0101] [Manufacturing Method for Underbody Parts 100] An example of a manufacturing method for the underbody parts 100 of this embodiment will be described. The underbody parts 100 of this embodiment may be manufactured by other manufacturing methods other than those described below. However, the manufacturing method described below is a preferred example of a manufacturing method for the underbody parts 100 of this embodiment.

[0102] In the manufacturing method of the undercarriage parts 100 of this embodiment, hot-rolled steel sheets are manufactured, and the manufactured hot-rolled steel sheets are subjected to bending (cold pressing) to manufacture the undercarriage parts 100. Specifically, an example of the manufacturing method of the undercarriage parts 100 includes the following steps: (Step 1) Rough rolling step (Step 2) Finish rolling step (Step 3) Cold pressing step Each step will be described below.

[0103] [(Process 1) Rough Rolling Process] In the rough rolling process, a rough rolling mill (not a tandem type) is used to perform hot rolling (rough rolling) on ​​the prepared material (slab) to produce an intermediate steel plate (rough bar). The heating temperature of the material in the rough rolling process is, for example, 1150 to 1300°C.

[0104] [(Process 2) Finish Rolling Process] In the finish rolling process, the intermediate steel sheet is further hot-rolled (finish-rolled) using a finish rolling mill to produce a steel sheet (hot-rolled steel sheet). The finish rolling mill is a tandem-type rolling mill with multiple rolling stands ST arranged in a row. １ ~ST ｊ (where j is a natural number) is included. Each rolling stand ST ｊ It is equipped with a pair of work rolls. Multiple rolling stands ST of the finishing rolling mill. １ ~ST ｊ Of these, the rolling stand that ultimately reduces the steel plate is called the rolling stand ST. ｎ Let n be a natural number less than or equal to j. Rolling stand ST ｎ The rolling stand ST is located at the downstream end of the finishing rolling mill. ｊ Alternatively, the rolling stand ST ｊ A rolling stand ST located further upstream may also be used.

[0105] The finish rolling temperature is not particularly limited, but for example, it is 850 to 950°C. Here, the finish rolling temperature refers to the temperature at which the steel sheet is finally rolled down at the rolling stand ST. ｎ This refers to the surface temperature (°C) of the steel sheet at the exit of the rolling mill. The finishing rolling temperature is measured at the rolling stand ST. ｎ The temperature is measured by a thermometer (such as an infrared thermometer) placed on the outlet side.

[0106] The steel plate after finish rolling is placed in the rolling stand ST, which is the last place the steel plate was rolled down.ｎ The hot-rolled steel sheet is cooled to the winding temperature by a water cooling device located on the exit side. The winding temperature is not particularly limited, but for example, it is 350 to 550°C. Hot-rolled steel sheets are manufactured through the above process. In the manufactured hot-rolled steel sheets, the area ratio of the bcc structure is 90% or more, and the Vickers hardness of the steel sheet is 250 HV or more.

[0107] The following conditions must be met in the finishing rolling process: (Condition 1) Rolling stand ST, which is the last machine to roll down the steel sheet in the finishing rolling mill. ｎ Upstream of the rolling stand ST ｎ Rolling stand ST located next to it ｎ－１ The amount of heat generated by friction Q ｎ－１ (kJ) shall be 100 to 300 kJ. (Condition 2) Rolling stand ST which is the last to reduce the steel plate in the finishing rolling mill. ｎ Amount of frictional heat generated Q ｎ Let (kJ) be between 20 and 100 kJ. Note that the frictional heat generation Q ｎ－１ (kJ) and Q ｎ (kJ) represents the amount of frictional heat generated per meter of steel plate width. Although the width of the steel plate fluctuates slightly during hot rolling, such fluctuations in the width of the steel plate do not affect the frictional heat generated Q mentioned above. ｎ－１ and Q ｎ It has almost no effect. Conditions 1 and 2 are explained below.

[0108] [(Condition 1) Amount of heat generated by friction Q] ｎ－１ [Regarding] The rolling stand ST, which is the final rolling machine used to reduce the thickness of the steel plate. ｎ Upstream of the rolling stand ST ｎ Rolling stand ST located next to it ｎ－１ The amount of heat generated by friction Q ｎ－１ (kJ) is defined by the following equation: Q ｎ－１ = μ ｎ－１ P ｎ－１ ・V ｎ－１ ・t ｎ－１ Here, the coefficient of friction μ in the equation ｎ－１ This is steel plate and rolling stand ST ｎ－１ This is a dimensionless quantity that indicates the magnitude of friction with the roll. Rolling load P ｎ－１V is the vertical force (kN) applied when rolling down a steel plate with a roller. ｎ－１ is the speed (m / sec) at which the steel plate is pulled into the roll. Contact time t ｎ－１ t is the time (in seconds) that the steel plate and the roll are in contact. ｎ－１ This can be calculated based on the roll diameter, the thickness of the steel plate, and the rolling speed.

[0109] coefficient of friction μ ｎ－１ This is determined by inverse analysis based on the well-known OROWAN non-uniform rolling theory (two-dimensional plane strain model). Specifically, it is as follows: First, the rolling stand ST ｎ－１ Measured advance rate S ｅｘｐ The value is obtained using the following method: Rolling stand ST ｎ－１ Markings (indentations, etc.) to be transferred to the steel sheet are pre-formed on the outer surface (roll surface) of the work roll. The spacing L of the markings transferred to the steel sheet during finish rolling. ｍａｒｋ Measure the measured marking interval L. ｍａｒｋ And the circumferential length L of the work roll ｒｏｌｌ Based on this, the measured advance rate S ｅｘｐ S is calculated using the following formula. ｅｘｐ = (L ｍａｒｋ -L ｒｏｌｌ ) / L ｒｏｌｌ ×100 (%) Also, Rolling stand ST ｎ－１ Measured rolling load P ｅｘｐ (kN) to the rolling stand ST ｎ－１ The measurement is taken using a load cell installed in the stand housing. The obtained measured advance rate S ｅｘｐ and the measured rolling load P ｅｘｐ Using and as target values, the contact arc equation of the OROWAN two-dimensional plane strain model is used to integrate the normal stress distribution on the contact arc under plane strain conditions, and the calculation advance rate S is calculated. ｃａｌ and calculated rolling load P ｃａｌ We obtain the calculation advance rate S. ｃａｌ and calculated rolling load P ｃａｌ However, the measured advance rate S ｅｘｐ and measured rolling load P ｅｘｐ The coefficient of friction μ ｎ－１ Optimize the coefficient of friction. The optimized coefficient of friction is the coefficient of friction μ.ｎ－１ Let's assume that.

[0110] Frictional heat generation Q ｎ－１ If it is less than 100 kJ, the amount of {110} texture will be reduced in the manufactured suspension part 100. Therefore, in the bent portion 20 of the manufactured suspension part 100, the area ratio of the {110} texture will be excessively small compared to the total area ratio of the {111} texture and the {100} texture. As a result, F1 may be less than 0.70. Frictional heat generation Q ｎ－１ If it exceeds 300 kJ, the {110} texture will increase excessively in the manufactured suspension part 100. As a result, in the bent portion 20 of the manufactured suspension part 100, the area ratio of the {110} texture will be excessively large compared to the total area ratio of the {111} texture and the {100} texture. Consequently, F1 may exceed 1.50. Therefore, the frictional heat generation Q ｎ－１ Let this be between 100 and 300 kJ.

[0111] [(Condition 2) Amount of frictional heat generated Q] ｎ [Regarding] The rolling stand ST, which is the final rolling machine used to reduce the thickness of the steel plate. ｎ The amount of heat generated by friction Q ｎ This is defined by the following equation: Q ｎ = μ ｎ P ｎ ・V ｎ ・t ｎ Here, the coefficient of friction μ in the equation ｎ This is steel plate and rolling stand ST ｎ This is a dimensionless quantity that indicates the magnitude of friction with the roll. Rolling load P ｎ V is the vertical force (kN) applied when rolling down a steel plate with a roller. ｎ is the speed (m / sec) at which the steel plate is pulled into the roll. Contact time t ｎ t is the time (in seconds) that the steel plate and the roll are in contact. ｎ This can be calculated based on the roll diameter, the thickness of the steel plate, and the rolling speed. (Friction coefficient μ) ｎ This is the friction coefficient μ mentioned above. ｎ－１It is determined by inverse analysis based on the well-known OROWAN non-uniform rolling theory (two-dimensional plane strain model), using a method similar to that used for the previous determination.

[0112] Frictional heat generation Q ｎ If the friction heat generation is less than 20 kJ, the amount of {110} texture will be reduced in the manufactured suspension part 100. As a result, the area ratio of the {110} texture in the bent portion 20 of the manufactured suspension part 100 will be excessively small compared to the total area ratio of the {111} texture and the {100} texture. Consequently, F1 may be less than 0.70. Friction heat generation Q ｎ If it exceeds 100 kJ, the {110} texture will increase excessively in the manufactured suspension part 100. As a result, in the bent portion 20 of the manufactured suspension part 100, the area ratio of the {110} texture will be excessively large compared to the total area ratio of the {111} texture and the {100} texture. Consequently, F1 may exceed 1.50. Therefore, the frictional heat generation Q ｎ Let's assume it's between 20 and 100 kJ.

[0113] [(Process 3) Cold Pressing Process] In the cold pressing process, the manufactured steel sheet (hot-rolled steel sheet) is subjected to bending (cold pressing) to produce the undercarriage part 100. First, in the cold pressing process, the manufactured steel sheet (hot-rolled steel sheet) is punched out into a predetermined shape to produce a blank material. The blank material corresponds to the shape of the undercarriage part 100 before bending and is in the form of a plate. The manufactured blank material is subjected to bending (cold pressing) to produce the undercarriage part 100 of the desired shape.

[0114] In the cold pressing process, the following conditions are met: (Condition 3) After applying lubricant to the surface of the steel sheet that comes into contact with the die for cold pressing, cold pressing is performed. If lubricant is not applied to the surface of the steel sheet before cold pressing, the coefficient of friction between the die and the surface of the steel sheet becomes excessively large. In this case, excessive crystal rotation of the surface occurs during processing. Therefore, even if F1 satisfies equation (1), F2 may not satisfy equation (2), or F3 may not satisfy equation (3), or F2 may not satisfy equation (2) and F3 may not satisfy equation (3).

[0115] By applying lubricating oil to the surface of the steel sheet before cold pressing, or to the part of the die that contacts the surface of the steel sheet, the coefficient of friction between the die and the surface of the steel sheet can be appropriately suppressed. As a result, excessive crystal rotation of the surface during processing is suppressed. Consequently, F1 satisfies equation (1), F2 satisfies equation (2), and F3 satisfies equation (3). The lubricating oil is, for example, a water-insoluble lubricating oil containing mineral oil.

[0116] The undercarriage components for automobiles according to this embodiment are manufactured through the above manufacturing process. As stated above, the undercarriage components for automobiles according to this embodiment may be manufactured by methods other than the above manufacturing method.

[0117] Hot-rolled steel sheets having the chemical compositions shown in Table 1 (Table 1A and Table 1B) were prepared.

[0118]

[0119]

[0120] Specifically, slabs with the chemical compositions shown in Table 1 were prepared. Using these slabs, a rough rolling process was carried out to produce intermediate steel plates (rough bars). The heating temperature of the slabs during the rough rolling process was set to 1150 to 1300°C.

[0121] A finish rolling process was performed on the manufactured intermediate steel sheet to produce a hot-rolled steel sheet. The thickness of the hot-rolled steel sheet was 2.6 mm. Rolling stand ST during the finish rolling process. ｎ－１ The amount of heat generated by friction Q ｎ－１ (kJ), and rolling stand ST ｎ The amount of heat generated by friction Q ｎ Table 2 shows the (kJ) values. The finishing rolling temperature in the finishing rolling process was 850 to 950°C. In addition, the hot-rolled steel sheets were water-cooled after rolling until the steel sheet temperature reached the coiling temperature of 450 to 550°C. Steel sheets (hot-rolled steel sheets) were manufactured through the above process.

[0122]

[0123] The chemical composition of the manufactured hot-rolled steel sheet was analyzed based on the method described in [Method for Measuring Chemical Composition] above. The results showed that the chemical composition of the hot-rolled steel sheet was as shown in Table 1.

[0124] As shown in Figure 5, strip-shaped blank materials 220 measuring 30 mm x 100 mm x 2.6 mm thick were manufactured from the hot-rolled steel sheet 150 by punching. The angle A (°) (see Figure 5) between the bending line 21 of the blank material 220 and the rolling direction RD of the steel sheet 150 when the blank material was punched out was as shown in Table 2.

[0125] A simulated undercarriage part 100 was manufactured by bending (cold pressing) the manufactured blank material in accordance with JIS Z 2248:2022 6.5 "Bending apparatus with V-block and push fitting (V-block method)". The angle AN of the tapered surface of the V-block 200 in Figure 6 was set to 90°. The radius r of the tip of the push fitting 210 was the same as the plate thickness t0 of the blank material 220. As shown in Figure 7, the blank material 220 was bent by pushing the push fitting 210 downwards to manufacture a V-shaped simulated undercarriage part 100. In tests 1 to 15, lubricating oil was applied to the blank material before bending (indicated as "ON" in the "Lubricating Oil" column in Table 2). A water-insoluble lubricating oil containing mineral oil was used. In tests 16 to 18, bending was performed without applying lubricant to the blank material, V-block 200, and pressing tool 210 (indicated as "OFF" in the "Lubricant" column in Table 2).

[0126] The following evaluation tests were conducted using the manufactured simulated undercarriage parts 100: (Test 1) Measurement of the area ratio of the BCC crystal structure (Test 2) Vickers hardness measurement test (Test 3) Measurement of the crystal surface area ratio (Test 4) Measurement of the plate thickness reduction rate (Test 5) Impact resistance evaluation test. Tests 1 to 5 will be described below.

[0127] [(Test 1) Measurement Test of Area Ratio of BCC Crystal Structure] Based on the method described in [Method for Measuring Area Ratio (%) of BCC Structural Structure] above, the area ratio (%) of the BCC structural structure in the part body 10 of the simulated undercarriage part 100 was determined. The results obtained are shown in the "BCC Area Ratio (%)" column of Table 3.

[0128]

[0129] [(Test 2) Vickers Hardness Measurement Test] Based on the method described in [Method for Measuring the Vickers Hardness of the Part Body 10] above, the Vickers hardness (HV) of the part body of the simulated undercarriage part was determined. The results obtained are shown in the "Vickers Hardness (HV)" column of Table 3.

[0130] [(Test 3) Crystal plane area ratio measurement test] The above-mentioned [area ratio I at the cross-section 20CS of the bent portion 20] ｛１１０｝Ｔ , I ｛１１１｝Ｔ , I ｛１００｝Ｔ , I ｛１００｝Ｃ , I ｛１００｝ＳＩ , I ｛１１０｝ＳＩ and I ｛１１０｝ＳＯ Based on the method described in [Measurement Method], the area ratio I of the {110} texture ｛１１０｝Ｔ , {111} Texture area ratio I ｛１１１｝Ｔ , {100} Texture area ratio I ｛１００｝Ｔ , the area ratio I of the {100} texture in the surface region 20SI ｛１００｝ＳＩ , the area ratio I of the {100} texture in the central region 20C ｛１００｝Ｃ , the area ratio I of the {110} texture in the surface region 20SI ｛１１０｝ＳＩ , and the area ratio I of the {110} texture in the surface region 20SO ｛１１０｝ＳＯ We calculated the result. The result is shown in Table 3, "I ｛１１０｝Ｔ (%)” column, “I ｛１１１｝Ｔ (%)” column, “I ｛１００｝Ｔ (%)” column, “I ｛１００｝ＳＩ (%)” column, “I ｛１００｝Ｃ (%)” column, “I ｛１１０｝ＳＩ (%) column and "I ｛１１０｝ＳＯ These are shown in the "(%)" column. Furthermore, F1, F2, and F3 are shown in the "F1", "F2", and "F3" columns of Table 3.

[0131] [(Test 4) Thickness Reduction Rate Measurement Test] The shape profile of the cross section 20CS perpendicular to the bending line 21 of the bent portion 20 of the manufactured simulated undercarriage part 100 was measured. A product name: VR-6000 manufactured by Keyence Corporation was used for the measurement. From the obtained shape profile, the minimum thickness tmin (mm) of the thickness in the shape profile was determined. Using the obtained thickness tmin (mm), the thickness reduction rate (%) was calculated using the following formula: Thickness reduction rate = (t0 - tmin) / t0 × 100 Here, t0 is the thickness t0 (mm) of the blank material. The obtained results are shown in the "Thickness Reduction Rate (%)" column of Table 3.

[0132] [(Test 5) Impact Resistance Evaluation Test] The simulated undercarriage parts 100 for each test number were cooled to -40°C. As shown in Figure 8, the simulated undercarriage parts 100 cooled to -40°C were placed on a horizontal test stand 300 so that the outer bend of the bent portion 20 faced upward. Furthermore, a 120kg rectangular parallelepiped (200mm × 160mm × 400mm) weight 310 was placed at a position D = 18m vertically from the surface of the test stand 300. The lower surface of the weight 310 was 200mm × 160mm, and the weight 310 was positioned so that the center of the width of the weight 310 was directly above the top of the outer bend of the bent portion 20 of the simulated undercarriage part 100, and so that when the weight 310 was viewed from above, the entire bend of the simulated undercarriage part 100 was located within the lower surface of the weight 310.

[0133] After placing the simulated undercarriage component 100 and the weight 310 in the above positions, the weight 310 was allowed to free-fall and collide with the bent portion 20 of the simulated undercarriage component 100. The surface of the simulated undercarriage component 100 was visually inspected for cracks after the collision. If any areas were suspected of cracking, the suspected areas were examined with a 10x magnification loupe to confirm the presence or absence of cracks. If no cracks were observed on the surface of the simulated undercarriage component, it was determined that excellent impact resistance had been achieved (indicated as "pass" in the "Impact Resistance" column of Table 3). On the other hand, if even one crack was found, it was determined that sufficient impact resistance had not been achieved (indicated as "fail" in the "Impact Resistance" column of Table 3).

[0134] [Evaluation Results] The evaluation results are shown in Table 3. In test numbers 1 to 11, conditions 1 to 3 during the manufacturing process were appropriate. Therefore, the manufactured undercarriage parts met features 1 to 5. As a result, the plate thickness reduction rate for the undercarriage parts in these test numbers was less than 25.0%. Consequently, they exhibited excellent impact resistance.

[0135] On the other hand, in test number 12, the amount of frictional heat generated Q ｎ－１ The values ​​were too low. As a result, F1 was less than 0.70, F2 was 0.0 or less, and F3 was 0.0 or less. Consequently, the plate thickness reduction rate was 25.0% or more. As a result, sufficient impact resistance could not be obtained.

[0136] In test number 13, the amount of heat generated by friction Q ｎ－１ The pressure was too high. As a result, F1 exceeded 1.50. Consequently, the plate thickness reduction rate was 25.0% or more. Consequently, sufficient impact resistance could not be obtained.

[0137] In test number 14, the amount of heat generated by friction Q ｎ The values ​​were too low. As a result, F1 was less than 0.70, F2 was 0.0 or less, and F3 was 0.0 or less. Consequently, the plate thickness reduction rate was 25.0% or more. As a result, sufficient impact resistance could not be obtained.

[0138] In test number 15, the amount of heat generated by friction Q ｎ The pressure was too high. As a result, F1 exceeded 1.50. Consequently, the plate thickness reduction rate was 25.0% or more. Consequently, sufficient impact resistance could not be obtained.

[0139] In tests 16-18, no lubricant was applied to the blank material before cold pressing. Therefore, in tests 16-18, either equation (2) or equation (3) was not satisfied, or neither equation (2) nor equation (3) was satisfied. As a result, sufficient impact resistance was not obtained.

[0140] The embodiments of this disclosure have been described above. However, the embodiments described above are merely examples for implementing this disclosure. Therefore, this disclosure is not limited to the embodiments described above, and the embodiments described above can be modified as appropriate without departing from the spirit of this disclosure.

[0141] 10 Main part 20 Bent section 20CS Cross section 20SI, 20SO Surface area 20C Central area 21 Bent line 30 Top plate 40 Vertical wall 100 Underbody parts

Claims

1. An underbody part for an automobile, comprising a part body including a bent portion, wherein the part body is made of steel, contains a bcc structure texture with an area ratio of 90% or more, the Vickers hardness of the part body is 250 HV or more, and in a cross section perpendicular to the bending line of the bent portion, the area ratio I of crystal planes within 18° of azimuth from the {110} plane of the bcc structure texture with respect to the normal line of the cross section ｛１１０｝Ｔ and the area ratio I of crystal planes within 18° of azimuth from the {111} plane of the bcc structure texture with respect to the normal line ｛１１１｝Ｔ and the area ratio I of crystal planes within 18° of azimuth from the {100} plane of the bcc structure texture with respect to the normal line ｛１００｝Ｔ satisfy formula (1), and in the surface layer region on the inner side of the bend, which is the region from the surface on the inner side of the bend of the bent portion to a depth of 1 / 3 of the plate thickness of the bent portion in the plate thickness direction of the cross section, the area ratio I of crystal planes within 18° of azimuth from the {100} plane of the bcc structure texture with respect to the normal line ｛１００｝ＳＩ and in the central region, which is the region from a depth of 1 / 3 of the plate thickness to a depth of 2 / 3 of the plate thickness of the cross section, the area ratio I of crystal planes within 18° of azimuth from the {100} plane of the bcc structure texture with respect to the normal line ｛１００｝Ｃ satisfy formula (2), and in the surface layer region on the inner side of the bend of the cross section, the area ratio I of crystal planes within 18° of azimuth from the {110} plane of the bcc structure texture with respect to the normal line ｛１１０｝ＳＩ and the area ratio I of crystal planes within 18° of azimuth from the {110} plane of the bcc structure texture with respect to the normal line in the surface layer region on the outer side of the bend, which is the region from the surface on the outer side of the bend of the bent portion to a depth of 1 / 3 of the plate thickness of the bent portion in the plate thickness direction of the cross section ｛１１０｝ＳＯ satisfy formula (3), the underbody part. 0.70 ≦ I ｛１１０｝Ｔ / (I ｛１１１｝Ｔ + I ｛１００｝Ｔ ) ≦ 1.50 (1) I ｛１００｝Ｃ - I ｛１００｝ＳＩ > 0.0 (2) I ｛１１０｝ＳＩ - I ｛１１０｝ＳＯ > 0.0 (3) 2. An undercarriage component according to claim 1, wherein the component body comprises a top plate and a vertical wall that rises in the thickness direction of the top plate, and the bent portion is positioned between the top plate and the vertical wall and connected to the top plate and the vertical wall.

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