Structural component

Abstract

A structural member comprising a steel plate, the structural member having a flat plate section (10a) and a bent section (10b), the bent section (10b) satisfying the following formulae (1), (2), and (3). 28 < = [square root of] area < = 45 (1) 0.17 < = r / t < = 6.90 (2) 0.22 < = n '(3) where [square root of] area is the square root [mu] m of the projected area of a minute defect on the inside of the bend of the bend section (10b), r is the minimum value [mm] of the radius of curvature on the inside of the bend of the bend section (10b), t is the thickness [mm] of the flat plate section (10a), and n' is the quartile range / average nano-hardness of the nano-hardness of the flat plate section (10a).

Description

Technical Field

[0001] This invention relates to a structural component. Background Technology

[0002] Previously, it was proposed to reduce the likelihood of fatigue cracks on the flanged end face by setting the arithmetic mean surface roughness to a predetermined value or less (see Patent Document 1). Additionally, a metal forming sheet was proposed that sets the arithmetic mean surface roughness of the convex side surface of the minimum thickness portion of the edge portion of the appearance surface to a predetermined value or less, thereby suppressing the generation of surface roughness on the convex surface of the edge portion (see Patent Document 2). Furthermore, to quantitatively evaluate the impact of surface roughness on fatigue strength, a scheme using the √area parameter model was proposed (see Non-Patent Document 1).

[0003] Existing technical documents Patent documents Patent Document 1: Japanese Patent No. 7036298 Patent Document 2: Japanese Patent No. 6954211 Non-patent literature Non-Patent Literature 1: "Quantitative Evaluation of the Influence of Surface Roughness on Fatigue Strength" by Keiji Murakami, Hiroji Takahashi, and Akio Yamashita, Proceedings of the Japanese Society of Mechanical Engineers (Series A), Vol. 63, No. 612 (1997), pp. 1612-1619. Summary of the Invention

[0004] The problem that the invention aims to solve For example, the application of ultra-high tensile materials to automotive chassis components offers significant advantages such as component lightweighting. On the other hand, the increased strength resulting from the application of ultra-high tensile materials tends to reduce formability. Therefore, by understanding the limitations of the material and appropriately designing the component shape, the advantages of using ultra-high tensile materials can be maximized.

[0005] In the case of chassis components such as the lower arm, by using high-strength materials and employing structures with curved sections such as M-shaped cross-sections, necessary rigidity can be ensured even with thin walls. From the viewpoint of increasing the freedom of arrangement of necessary structural components within a limited space and avoiding interference between components, it is preferable to minimize the radius of curvature on the inner side of the bend. Furthermore, in cases such as inserting a cylindrical bushing into the outer diameter of a cylindrical shape formed by flanging, it is desirable to minimize the radius of curvature on the inner side of the bend in order to maximize the fitting length within a limited space.

[0006] However, if a bend with a radius of curvature exceeding the material's limits is applied, surface roughness issues arise, such as the formation of microcracks on the inside of the bend. As a result, fatigue failure originates from this surface roughness, and sufficient fatigue durability is not achieved. Previously, ultra-high tensile materials had low forming limits, making it impossible to form bends with small radii of curvature. Furthermore, the limiting value of the radius of curvature that does not compromise fatigue durability varies depending on the material, leading to an unclear relationship between the structure of the bend and the material composition.

[0007] The technologies described in the aforementioned patent documents do not address the surface roughness of the inner side of the bend, making it difficult to suppress the reduction in fatigue durability caused by the surface roughness of the inner side of the bend.

[0008] Therefore, the object of the present invention is to provide a structural component capable of suppressing the reduction in fatigue durability caused by the surface roughness and shape of the inner side of the bend.

[0009] Methods for solving problems This disclosure is based on the above-mentioned views, and its main purpose is as follows.

[0010] (1) A structural component, which is a structural component made of steel plate, having a flat section and a bent section, The above-mentioned curved portion satisfies the following equations (1), (2) and (3). 28≤√area≤45 …(1) 0.17≤r / t≤6.90 …(2) 0.22≤n'…(3) Where √area is the square root [μm] of the projected area of ​​the minute defect on the inner side of the bend. r is the minimum radius of curvature [mm] of the inner side of the bend in the aforementioned curved section. t is the thickness of the aforementioned flat plate [mm]. n' represents the interquartile range / average nanohardness of the aforementioned plate portion.

[0011] (2) According to the structural component described in (1) above, it also satisfies the following equations (1)' and (2)'. 33≤√area≤44 …(1)' 0.17≤r / t≤3.45 …(2)'.

[0012] (3) According to the structural component described in (1) above, wherein the Vickers hardness of the plate portion is 310 or higher.

[0013] (4) According to the structural component described in (1) above, wherein the Vickers hardness of the plate portion is 340 or higher.

[0014] (5) The structural component according to any one of (1) to (4) above also satisfies the following formula (9). 33≤√area≤1.05{0.0976(r / t)) 3 -0.7185 (r / t) 2 -1.0542(r / t)+43.426}…(9).

[0015] Invention Effects According to the present invention, it is effective in providing structural components that can suppress the reduction in fatigue durability caused by the surface roughness and shape of the inner side of the bend. Attached Figure Description

[0016] Figure 1 This is a diagram showing an example of an automotive running gear as a structural component of this disclosure.

[0017] Figure 2 It means along Figure 1 A diagram of the cross section of the dotted line I-I' in the diagram.

[0018] Figure 3 This diagram shows the process of acquiring five images of the inner side of the bend and connecting the five images.

[0019] Figure 4 It is a characteristic diagram representing the method of evaluating surface roughness by converting curved surfaces into planes.

[0020] Figure 5 This is a diagram schematically representing the surface roughness of the curved inner side, as represented by property C3.

[0021] Figure 6 This is a histogram representing the nanohardness distribution of high-Si type.

[0022] Figure 7 This is a histogram representing the nanohardness distribution of low-Si type.

[0023] Figure 8 This is a characteristic graph showing the distribution of uniform elongation u-El (vertical axis) for high-Si and low-Si types and the value n' (horizontal axis) obtained by dividing the interquartile range by the average nanohardness.

[0024] Figure 9 Characteristic diagrams of √area and r / t were plotted for Invention Examples 1-10 and Comparative Examples 1-10 shown in Table 1. Detailed Implementation

[0025] Hereinafter, several embodiments of the present disclosure will be described with reference to the accompanying drawings. However, these descriptions are intended to provide simple illustrations of preferred embodiments of the present disclosure and are not intended to limit the present disclosure to such specific embodiments. The descriptions will proceed in the following order.

[0026] 1. Summary of this disclosure 2. The effect of minute defects on fatigue strength on the inner surface of the bend. 3. Structural components of this disclosure 4. Evaluation method for surface roughness on the inner side of a bend 5. Regarding high-Si and low-Si types 6. Example 1. Summary of this disclosure For example, in automotive structural components (parts constituting a structure), strength is continuously being increased year by year, and with this increasing strength, lightweighting through thinner walls is constantly being developed. To achieve lightweight yet highly functional structural components, the combination of "lightweighting based on high-strength, thin-walled components" and "structural design for effectively utilizing the strength properties of iron" becomes important. For instance, in high-strength, thin-walled chassis components (e.g., lower arm and other running gear) used in automobiles, fatigue cracking sometimes deteriorates due to the decrease in rigidity associated with thinning and the reduction in surface properties (surface roughness) associated with machining. In particular, when a structural component includes a bent portion formed by bending, the inner side of the bend is subjected to compressive deformation, resulting in a decrease in surface properties compared to before machining, and the surface roughness leads to deterioration of fatigue properties. Chassis components are important safety components, constantly subjected to vibration and load; therefore, improving fatigue durability is essential. In this embodiment, a structural component that suppresses fatigue cracking from the inner side of the bend is provided through an appropriate combination of materials and component structure (especially the surface roughness and bend shape of the inner side of the bend).

[0027] In this disclosure, by using an ultra-high tensile material (tensile strength of 980 MPa or more) with superior forming limits compared to the past, it is possible to suppress the deterioration of the surface roughness on the inner side of the bend in stamped products with M-shaped cross sections, thereby forming a smooth surface and enabling forming with a smaller radius of curvature on the inner side of the bend. Furthermore, the relationship between the range of the radius of curvature on the inner side of the bend that satisfies fatigue durability and the material properties is clarified.

[0028] The inventors conducted in-depth research and found that, using an ultra-high tensile material with superior forming limits compared to previous methods, fatigue cracking in the bending section can be suppressed when the parameters of the bending section, specifically the surface roughness of the inner side of the bending and the radius of curvature of the inner side of the bending, meet specified conditions. Material properties affect the change in surface roughness of the bending section before and after processing. By managing the appropriate surface properties of the bending section according to the material and the shape (design) of the bending section, fatigue durability can be improved. Compared to conventional steel, the steel with superior forming limits in this embodiment makes it difficult for the surface roughness of the inner side of the bending section to increase after processing (i.e., unevenness is less likely to develop), thus improving fatigue durability. The larger the radius of curvature of the inner side of the bending, the less likely the surface roughness of the inner side of the bending will increase.

[0029] The ultra-high tensile materials with excellent forming limits disclosed herein are a high-Si type with a relatively high Si content and a low-Si type with a relatively low Si content. The mechanisms by which these two types improve fatigue durability differ. The high-Si type has a larger surface roughness due to the presence of an oxide layer derived from Si, but also a high n-value (processing curing index) related to formability, thus making it difficult for microcracks to develop on the inside of the bend. Therefore, in the high-Si type, high durability is achieved by suppressing the folding of the compression portion on the inside of the bend during bending processing. On the other hand, the low-Si type has a lower n-value, but a smaller surface roughness derived from the Si oxide layer, thereby suppressing the development of microcracks on the inside of the bend. Therefore, in the low-Si type, high durability is achieved by suppressing fatigue cracking through excellent surface smoothness.

[0030] The inventors conducted in-depth research and concluded that the difference in fatigue cracking mechanisms between high-Si and low-Si types should be incorporated into the aforementioned parameters, with the parameter range set according to the steel grade. In this disclosure, appropriate parameter ranges are explained for the use of ultra-high tensile materials of the high-Si type from the low-Si type.

[0031] 2. The effect of minute defects on fatigue strength on the inner surface of the bend. The inventors have conducted in-depth research on the effect of the square root √area of ​​the projected area of ​​minute defects on the inner side of the bend on fatigue strength, as described in Non-Patent Document 1 above, the fatigue limit σ W Based on √area and Vickers hardness, it is represented by the following formula (4), therefore, √area is considered as a characteristic value that affects fatigue durability. It should be noted that minor defects can refer to unevenness on the surface of the pressed molded body. In adverse cases, minor defects are not limited to unevenness, and are sometimes observed as cracks.

[0032] σW=[1.43(HV+120){(1-R) / 2} α] / (√area) 1 / 6 …(4) Where σW: fatigue limit [MPa] HV Vickers hardness [HV kgf / mm] 2 ] α: 0.226 + HV × 10 -4 R: Stress ratio (σmin / σmax).

[0033] √area[μm] is calculated from the following equations (5) and (6) based on the description in Non-Patent Document 1 above. In addition, in equations (5) and (6), a[μm] is the height of the roughness (maximum height Rz), and 2b[μm] is the spacing of the roughness (average length of the element, wavelength of the height waveform Rsm).

[0034] [Mathematical Expression 1] …Formula (5) (Where a / 2b ≤ 0.195) [Mathematical Expression 2] …Formula (6) (Where a / 2b > 0.195) 3. Structural components according to this disclosure In the bending region, a larger √area value results in greater surface roughness, thus reducing the fatigue durability of the structural component. Furthermore, a smaller ratio (r / t) of the radius of curvature on the inner side of the bend to the thickness of the steel plate further reduces the fatigue durability of the structural member. The inventors have conducted in-depth research on the relationship between √area and r / t of the structural component and its fatigue characteristics, and found that when √area and r / t are within specified ranges, fatigue durability can be improved compared to previous methods.

[0035] Figure 1 This is a diagram showing an automotive running gear as an example of a structural component of this disclosure, and more specifically, a diagram showing the lower arm 10 of the suspension. Additionally, Figure 2 It means along Figure 1 A cross-sectional view of the dashed line I-I' in the diagram. Furthermore, the structural components disclosed herein are not limited to automotive applications. The lower arm 10 is subjected to stamping, flanging, etc., and has a flat plate portion (top plate portion) 10a and a curved portion 10b. Figure 2 The image shows an M-shaped cross-section comprising a flat plate portion 10a and a curved portion 10b.

[0036] The inventors have discovered that by having the bent portion 10b of a structural component made of steel plate (a high-Si type ultra-high tensile material) satisfy the following formulas (1), (2), and (3), and more preferably satisfying formulas (1)', (2)', and (3), fatigue characteristics can be improved compared to the past. 28≤√area≤45 …(1) 0.17≤r / t≤6.90 …(2) 0.22≤n'…(3) 33≤√area≤44 …(1)' 0.17≤r / t≤3.45 …(2)', Where, √area: the square root of the projected area of ​​the minute defects on the inner side of the bend of the bend 10b (a parameter representing the surface roughness after machining) [μm]. r: Minimum radius of curvature [mm] of the inner side of the bend in the curved section 10b. t: Plate thickness of flat section 10a [mm] n': Interquartile range / average nanohardness of the plate portion 10a.

[0037] As an example, the flat plate portion 10a is a portion whose wall thickness is reduced by less than 10% from the thickest part and is not subjected to large strain caused by bending processing. The flat plate portion 10a may also be a portion having the same plate thickness as the raw material.

[0038] As described above, when forming a bent portion using an ultra-high tensile material with superior forming limits compared to the past, the inventors conducted an in-depth study on the values ​​of √area and r / t described in Non-Patent Document 1, and found that by specifying the values ​​of √area and r / t within a specified range, fatigue characteristics can be improved compared to the past.

[0039] It should be noted that the curved portion of the structural component disclosed herein is formed by stamping, flanging, etc. As an example of the processing method for forming the curved portion, bending can also be performed with the inner surface of the curved portion in close contact with the mold. For example, by conducting experiments in advance to determine the conditions for bending (e.g., inner curved surface R, mold clearance, etc.), bending can be performed with the inner curved surface in close contact with the mold. As a result, the surface roughness of the inner curved surface is less likely to increase, and fatigue durability is improved. In addition, the √area value of the ground structural component is about 6.9, so it does not satisfy equation (1), and the structural component disclosed herein does not include ground parts.

[0040] 4. Evaluation method for surface roughness on the inner side of a bend When evaluating the surface roughness of the inner side of the bend in the curved portion 10b, the curved surface is converted into a plane for evaluation. Therefore, a section perpendicular to the surface of the flat plate portion 10a is fabricated (i.e., Figure 2 The cross-sectional sample of the bent portion 10b, which was mirror-finished using the cross-section shown as the measurement surface, was examined using an optical microscope. Five images of the inner side of the bent portion 10b were obtained at 500x magnification. The five images were then compared with those shown in Figure 20. Figure 3 Connect as shown. Then, using the "find edges" function of the image processing software (imageJ), extract the contours of the surface unevenness from the difference in pixel values, and output the (x, y) coordinates (in μm) using the "Analyze LineGraph" function. It should be noted that when performing coatings such as electrodeposition on structural components, the image is acquired after the coating is stripped off.

[0041] Figure 4 This is a characteristic diagram representing the method of converting a curved surface into a plane to evaluate surface roughness using the output coordinates (characteristic C1). Characteristic C2 (dashed line) is obtained by applying a Gaussian filter with σ=50 to characteristic C1 (dashed line) obtained in image processing. The difference between the two is then used to obtain characteristic C3 (solid line), which represents the short-period roughness component excluding the undulations of the long-period curved shape of the cross-section of the curved portion 10b (i.e., the shape of the arc on the inner side R of the curve).

[0042] The Gaussian filter is ineffective at the end of characteristic C1, such as... Figure 4 Since characteristics C1 and C2 are separated, the arithmetic mean roughness Ra and the average feature length Rsm are obtained by using the data after removing 100 μm from both ends of characteristic C3. Ra is twice the average of the absolute deviations of the mean line representing the average of characteristic C3 from characteristic C3 (average deviation). Rsm is the average of the intervals of one period between peaks and valleys, calculated from the intersection of the roughness curve shown by characteristic C3 and the mean line. The minimum height of a peak or valley is defined as 2 μm; anything below 2 μm is considered noise and is regarded as part of the preceding or following peak or valley.

[0043] Figure 5 This is a schematic diagram illustrating the surface roughness of the curved inner side, represented by characteristic C3. (See diagram for example.) Figure 5 As shown, height a and spacing 2b are values ​​assuming the surface roughness is periodic and uneven. √area in equations (5) and (6) are values ​​obtained assuming the surface roughness is periodic and uneven. Furthermore, Figure 5 The cross-section shown in the upper section can be replaced by the cross-section shown in the lower section. For example... Figure 5As shown, Ra, calculated as above, corresponds to the roughness height a in Equations (5) and (6), and Rsm corresponds to the roughness spacing 2b in Equations (5) and (6). Therefore, Rsm and Ra can be calculated based on the image of the cross-section of the curved portion 10b, and √area can be calculated based on Equations (2) and (3).

[0044] Furthermore, the height 'a' for roughness can also be the maximum height Rz (the difference between the height of the highest peak and the depth of the lowest valley). Additionally, the roughness spacing 2b can be set as 'a' calculated according to the following equation (7). s The above refers to the average spacing between the concave and convex surfaces. Additionally, the measurement range is the 500 μm range from the center of characteristic C3.

[0045] a s =0.1716·(1+120 / HV) 6 =1.05 …(7) 5. Regarding high-Si and low-Si types Both the high-Si and low-Si types are composed of ultra-high tensile materials (DP (Dual Phase) steel) with a tensile strength of 980 MPa or higher and a Vickers hardness (HV) of 310 or higher, preferably 340 or higher. Generally, in such high-strength materials, the surface roughness on the inner side of the bend has a significant impact on fatigue durability. However, in the high-Si and low-Si types disclosed herein, by adjusting the Si content and, for the high-Si type, adjusting the n' value (described later), an ultra-high tensile material with superior forming limits compared to the past is produced.

[0046] By measuring the Vickers hardness of the structural components, it can be determined whether the structural components are made of the ultra-high tensile material with a tensile strength of 980 MPa as described in this embodiment. Figure 1 and Figure 2 Taking the lower arm 10 shown as an example, a method for measuring the hardness of a machined automotive structural component will be explained. In order to measure the hardness of the base material that is not affected by the machining process, the hardness is measured using a flat plate portion 10a that is far from the bending portion 10b and is not subjected to large strain based on the bending process.

[0047] The cross section perpendicular to the surface of the plate portion 10a (i.e., Figure 2The cross-section shown is used as the measurement surface to collect samples, and this measurement surface is prepared for hardness testing. The method for preparing the measurement surface is performed according to JIS Z 2244:2009. After grinding the measurement surface with #600 to #1500 silicon carbide sandpaper, the measurement surface is polished to a mirror finish using a liquid made by dispersing diamond powder with a particle size of 1 μm to 6 μm in a diluent such as alcohol or pure water. For the measurement surface that has been polished to a mirror finish, a hardness test is performed using the method described in JIS Z 2244:2009. It should be noted that by using the cross-section as the measurement surface, the influence of surface coatings, etc., on the measured hardness value can be suppressed.

[0048] Specifically, using a micro Vickers hardness tester, at a position 1 / 4 of the plate thickness of the flat plate portion 10a adjacent to the bent portion 10b of the specimen, a load of 500 gf is applied, and 30 points are measured at intervals of more than 3 times the indentation. The average value of these measurements is taken as the hardness of the component.

[0049] High-Si steel sheets, as microstructures, consist of ferrite and tempered martensite. Ferrite is a soft layer, possessing advantages in terms of n-value and uniform elongation (u-El). The n-value is correlated with uniform elongation; high-Si steel sheets, containing soft ferrite, exhibit a large interlayer hardness difference (inter-microstructure hardness difference). On the other hand, low-Si steel sheets, as microstructures, are mainly composed of bainite (sometimes containing tempered martensite and ferrite), and compared to high-Si types, exhibit a smaller interlayer hardness difference (inter-microstructure hardness difference).

[0050] In this disclosure, high-Si and low-Si types are distinguished based on nanoindentation hardness (hereinafter also referred to as nanohardness) determined by nanoindentation testing. More specifically, as a parameter representing the processing and curing ability of raw materials, the ratio of nanohardness to the interquartile range of average nanohardness (= interquartile range of nanohardness / average nanohardness) is used to distinguish between high-Si and low-Si types.

[0051] Figure 6 This is a histogram representing the distribution of nanohardness in low-Si type nanomaterials. Additionally, Figure 7 This is a histogram representing the distribution of nanohardness in high-Si type nanomaterials. In Figure 6 and Figure 7 In the diagram, the horizontal axis represents nanometer hardness, and the vertical axis represents frequency.

[0052] The specimens for determining nanohardness were collected from the same locations as those used in determining the hardness of components based on the Vickers hardness test described above. The test surface of the specimen was the same as that used in the Vickers hardness test. The test surface was mirror-finished, and the nanohardness was measured at 1 / 4 of the plate thickness. At this point, a load of 2500 μN was applied, and 100 indentations were measured. The spacing between the indentations was ensured to be greater than 5 μm, and the nanohardness (i.e., indentation hardness) was determined based on ISO 14577-1 (Annex A). Then, based on the obtained indentation hardness from the 100 points, the interquartile range and the average nanohardness were calculated.

[0053] In high-Si type, there is soft ferrite with large differences in hardness between the microstructures, therefore... Figure 7 As shown, peaks of nanohardness appear at two locations, encompassing both hard and soft layers. In the high-Si type, the inclusion of a soft layer derived from ferrite results in excellent n-value characteristics. On the other hand, in the low-Si type, due to the small hardness difference between the microstructures, ... Figure 6 As shown, the peak of nanohardness is located at one point.

[0054] Figure 8 This is a characteristic graph showing the distribution of uniform elongation (u-El: vertical axis) and the value obtained by dividing the interquartile range by the average nanohardness (n': horizontal axis) for high-Si and low-Si types. The larger the uniform elongation value and the larger the n' value, the better the formability. Figure 8 The white circle (〇) indicates the high-Si type, and the black circle (●) indicates the low-Si type. Compared with the low-Si type, the high-Si type indicates a higher uniform elongation at the point of curing, but as mentioned above, the surface roughness is relatively large due to the presence of oxide scale from Si. The examples described later were studied, and the results showed that even with the surface roughness originating from the high-Si type, when the range of Equation (1) is 33≤√area≤44 and the range of Equation (2) is 0.17≤r / t≤3.45, the surface roughness on the inner side of the bend after processing is suppressed when the uniform elongation is 6% or more, and the fatigue characteristics are superior compared with the existing products (comparative examples described later). On the other hand, in the low-Si type, the surface roughness caused by the oxide scale derived from Si is small, thereby suppressing the development of microcracks on the inside of the bend. Therefore, by setting the range of Equation (1) to 30≤√area≤40 and the range of Equation (2) to 0.17≤r / t≤3.45, even if the value of uniform elongation is less than 6%, it is possible to meet the fatigue characteristics that are superior to those of existing products.

[0055] like Figure 8As shown, in high-Si types, if the uniform elongation of 6% or more is replaced by n' obtained from nanohardness, it is 0.22 or more. To determine the uniform elongation, measurements based on tensile tests are required, but obtaining the uniform elongation from structural components is not easy. On the other hand, nanohardness allows for easy sampling from structural components.

[0056] Therefore, in this embodiment, the properties of high-Si and low-Si materials are evaluated using the value n' obtained by dividing the interquartile range by the average nanohardness. High-Si materials can have n' of 0.22 or higher, while low-Si materials can have n' of less than 0.22.

[0057] The high-Si type is made of DP steel with a tensile strength of 980 MPa or higher, and an n' of 0.22 or higher is obtained by making the Si content 0.9% or higher by mass%. In this embodiment, the high-Si type is characterized only by n' being 0.22 or higher; apart from this characteristic and the Si content of 0.9% or higher, the chemical composition and manufacturing method of the material are not particularly limited. Furthermore, the upper limit of the tensile strength can be applied up to approximately 1300 MPa. As an example, high-Si types can also have the following chemical composition (by mass%): C: 0.01~1.0%, Si: ≥0.9%, Mn: 0.10~5.00%, P: ≤0.100%, S: ≤0.0300%, N: ≤0.0100%, O: 0~0.020%, Al: 0~1.000%, Cr: 0~2.00%, Cu: 0~2.00%, Ni: 0~2.00%, Mo: 0~3.00%, Co: 0~3.00%, Nb: 0~0.15 The composition consists of: 0%, V: 0~1.00%, Ti: 0~1.00%, W: 0~1.00%, Sn: 0~1.00%, Sb: 0~0.50%, Ta: 0~0.10%, As: 0~0.050%, B: 0~0.0100%, Ca: 0~0.100%, Mg: 0~0.100%, Zr: 0~0.100%, Hf: 0~0.100%, Bi: 0~0.100%, Zn: 0~0.200%, REM: 0~0.100%, and the remainder being Fe and impurities. Furthermore, in the above chemical composition, the lower limit of the content of any added element can be 0.0001% or 0.001%. Alternatively, steel plates obtained by rolling steel having the above chemical composition at room temperature to 1300°C, or steel plates obtained by performing surface treatments such as heat treatment and painting after rolling, can also be used.

[0058] In the low-Si type, by making the Si content less than 0.9% by mass, the surface roughness caused by the oxide scale originating from Si can be kept small, and n' is less than 0.22. The characteristic of the low-Si type is simply that n' is less than 0.22; apart from this characteristic and the Si content being less than 0.9%, the chemical composition and manufacturing method of the material are basically not limited. The low-Si type can also have the same chemical composition as the high-Si type described above, except that the Si content is less than 0.9%. In addition, similar to the high-Si type, in the above chemical composition, the lower limit of the content of any added element can be 0.0001% or 0.001%, and steel plates obtained by rolling steel with the above chemical composition at room temperature to 1300°C, or steel plates obtained by performing surface treatments such as heat treatment and coating after rolling, can also be used.

[0059] [Example] 6. Example The present invention will now be specifically described through examples. These examples are for confirming the effects of the present invention and do not limit the scope of the invention.

[0060] A low-Si type material with a plate thickness of 2.9 mm was bent at the radius of curvature r shown in Table 1 to produce the lower arms of Comparative Examples 1-8 and Invention Examples 1-8 shown in Table 1. As shown in Table 1, Rz and Rsm were measured before and after processing using the above method, and Rz / Rsm, √area / 2b, and √area[μm] were calculated. In Invention Examples 1-8, the √area[μm] after processing satisfies Equation (1), r / t satisfies Equation (2), and n' satisfies Equation (3). On the other hand, Comparative Examples 1-8, which are existing products, do not satisfy Equation (1). Regarding √area, the surface roughness of the inner side of the bent portion 10b increases during processing, so the value after processing is not lower than the value before processing. In addition, in the measurement of Rz and Rsm before processing, the curved surface is not converted to a plane, but the equivalent value is obtained directly from the image. Figure 4 The characteristics of C3.

[0061] Fatigue tests were conducted on the lower arms of the comparative examples and the invention examples to test the loads in the longitudinal direction of the vehicle. Specifically, for the lower arms of Comparative Examples 1-8 and Invention Examples 1-8 with different machined roughness parameters such as √area, the load in the longitudinal direction of the vehicle was tested on the bending portion, and the time-stress σf was measured for 200,000 cycles. It should be noted that the 200,000-cycle time-stress refers to the load stress generated in the lower arm during the 200,000 repetitions before fatigue failure occurs.

[0062] Then, for comparative examples and inventive examples with the same ratio r / t of the inner radius of curvature to the plate thickness, the time strength σf1 of the comparative examples and the time strength σf2 of the inventive examples are compared to evaluate the fatigue durability of the inventive examples. Specifically, for each inventive example, based on the results of the comparison with the comparative examples, cases where the value of σf2 / σf1 exceeds 1.2 are evaluated as "0", and cases where the value of σf2 / σf1 exceeds 1.4 are evaluated as "◎".

[0063] Figure 9 Characteristic diagrams of √area and r / t were plotted for Invention Examples 1-8 and Comparative Examples 1-8 shown in Table 1. Figure 9 The white circles (〇) in the diagram indicate invention examples, and the black circles (●) indicate comparative examples. For example... Figure 9 As shown, the smaller the r / t, i.e., the smaller the r, the greater the value of √area, and the rougher the surface roughness on the inner side of the bend, thus reducing fatigue durability. However, compared with Comparative Examples 1-8 with the same r / t, the values ​​of √area in Invention Examples 1-8 are lower, and the surface roughness on the inner side of the bend is less likely to deteriorate, resulting in a smoother surface. Therefore, the results in Table 1 show that the fatigue characteristics of Invention Examples 1-8 are superior to those of Comparative Examples 1-8. Furthermore, the results in Table 1 show that the larger the r and the smaller the value of √area, the better the fatigue characteristics compared to the comparative examples with the same r.

[0064] exist Figure 9 In the present invention, the characteristic C4 that the line connecting each drawing point of the invention examples 1 to 8 is approximated as a curve can be expressed by the following formula (8).

[0065] √area=0.0976(r / t)) 3 -0.7185 (r / t) 2 -1.0542(r / t)+43.426…(8) Therefore, preferably, the structural member according to this disclosure satisfies the following expression (9). In addition, in equation (9), the √area in equation (8) takes a 5% margin as the upper limit value.

[0066] 33≤√area≤1.05{0.0976(r / t)) 3 -0.7185 (r / t) 2 -1.0542(r / t)+43.426}…(9) Explanation of reference numerals in the attached figures 10. Lower arm 10a Flat Plate Section 10b Bend 20 images

Claims

1. A structural component made of steel plate, having a flat section and a bent section, The curved portion satisfies the following equations (1), (2) and (3). 28≤√area≤45 …(1) 0.17≤r / t≤6.90 …（2） 0.22≤n’…（3） Where √area is the square root [μm] of the projected area of ​​the minute defect on the inner side of the bend. r is the minimum radius of curvature [mm] of the inner side of the bend. t is the thickness of the flat plate [mm]. n' represents the interquartile range / average nanohardness of the plate portion.

2. The structural component according to claim 1, further satisfies the following equations (1)' and (2)', 33≤√area≤44 …(1)' 0.17≤r / t≤3.45 …（2）’。 3. The structural component according to claim 1, wherein, The Vickers hardness of the flat plate portion is 310 or higher.

4. The structural component according to claim 1, wherein, The Vickers hardness of the flat plate portion is 340 or higher.

5. The structural component according to any one of claims 1 to 4, further satisfies the following formula (4). 33≤√area≤1.05{0.0976(r / t)) 3 -0.7185(r / t) 2 -1.0542(r / t)+43.426} …(4)。

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