Structural component

Abstract

This structural component is formed from a steel sheet, and has a flat sheet part 10a and a bent part 10b. The bent part 10b satisfies the following formula (1), formula (2), and formula (3).　(1): 28≤√area≤45 (2): 0.17≤r / t≤6.90 (3): 0.22≤n' Wherein √area represents the square root [μm] of a projection area of a minute defect on the bending inner side of the bending part 10b, r represents the minimum value [mm] of the curvature radius on the bending inner side of the bending part 10b, t represents the sheet thickness [mm] of the flat sheet part 10a, and n' represents nano-hardness interquartile range / average nano-hardness of the flat sheet part 10a.

Description

Structural parts

[0001] The present invention relates to a structural component.

[0002] It has been proposed to make fatigue cracks less likely to occur on burred end faces by setting the arithmetic mean roughness at the burred end faces to a predetermined value or less (see Patent Document 1). It has also been proposed to provide a metal formed sheet in which the arithmetic mean surface roughness on the convex surface of the minimum thickness portion of the ridge line of the design surface is set to a predetermined value or less, thereby suppressing the occurrence of surface roughness on the convex surface of the ridge line (see Patent Document 2). It has also been proposed to apply the √area parameter model to quantitatively evaluate the effect of surface roughness on fatigue strength (see Non-Patent Document 1).

[0003] Patent No. 7036298 Patent No. 6954211

[0004] Takayoshi Murakami, Koji Takahashi, and Akio Yamashita, "Quantitative Evaluation of the Effect of Surface Roughness on Fatigue Strength," Transactions of the Japan Society of Mechanical Engineers (Series A), Vol. 63, No. 612 (1997), pp. 1612-1619

[0005] For example, the application of ultra-high tensile steel to automobile chassis components has significant benefits such as reducing the weight of parts. However, the increased strength achieved by using ultra-high tensile steel tends to reduce formability, so determining the limits of the material and appropriately designing the part shape can maximize the benefits of using ultra-high tensile steel.

[0006] In the case of chassis components such as lower arms, structures with bent sections, such as M-shaped cross sections, are adopted to ensure the necessary rigidity even when the components are thinned by using high-strength materials. From the perspective of increasing the degree of freedom when arranging the necessary structural components within a limited space and avoiding interference between components, it is desirable to make the radius of curvature on the inside of the bend as small as possible. Furthermore, even when inserting a cylindrical bushing into the outer diameter of a cylindrical shape formed by burring, it is desirable to make the radius of curvature on the inside of the bend as small as possible in order to maximize the fitting length within the limited space.

[0007] However, when bending a material with a small radius of curvature that exceeds its material limits, surface roughness such as microcracks occurs on the inside of the bend. As a result, fatigue failure occurs originating from the surface roughness, and sufficient fatigue durability cannot be achieved. Conventional ultra-high tensile steels have a low forming limit to begin with, making it impossible to form bent sections with small radii of curvature. Furthermore, the limit value of the curvature radius that does not impair fatigue durability varies depending on the material, and there was also the problem that the correlation between the structure of the bent section and the material was not clear.

[0008] None of the techniques described in the above patent documents focuses on the surface roughness on the inside of the bend, and it is difficult to suppress the decrease in fatigue durability caused by the surface roughness on the inside of the bend.

[0009] Therefore, an object of the present invention is to provide a structural component that can suppress a decrease in fatigue durability caused by the surface roughness on the inside of the bend and the shape of the inside of the bend.

[0010] The present disclosure has been made based on the above findings, and its gist is as follows.

[0011] (1) A structural component made of a steel plate, having a flat portion and a bent portion, wherein the bent portion satisfies the following formulas (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 of the projected area of ​​a microdefect on the inner side of the bent portion [μm], r is the minimum value of the radius of curvature on the inner side of the bent portion [mm], t is the thickness of the flat portion [mm], and n' is the interquartile range of nanohardness of the flat portion / average nanohardness.

[0012] (2) The structural part according to (1) above, further satisfying the following formulas (1)' and (2)': 33≦√area≦44 (1)' 0.17≦r / t≦3.45 (2)'

[0013] (3) The structural component according to (1) above, wherein the flat plate portion has a Vickers hardness of 310 or more.

[0014] (4) The structural component according to (1) above, wherein the flat plate portion has a Vickers hardness of 340 or more.

[0015] (5) The structural part according to any one of (1) to (4) above, further satisfying 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)

[0016] The present invention has the effect of providing a structural component that can suppress a decrease in fatigue durability due to the surface roughness on the inside of the bend and the shape of the inside of the bend.

[0017] 1 is a diagram showing an automobile suspension part as an example of a structural part according to the present disclosure. It is a diagram showing a cross section taken along dashed line II' in FIG. 1. It is a diagram showing five images of the inside of a bent part acquired and connected together. It is a characteristic diagram showing a method of evaluating surface roughness by converting a curved surface into a flat surface. It is a diagram schematically showing the surface roughness of the inside of a bend represented by characteristic C3. It is a histogram showing the distribution of nanohardness of high Si type. It is a histogram showing the distribution of nanohardness of low Si type. It is a characteristic diagram showing the distribution of uniform elongation u-El (vertical axis) of high Si type and low Si type, and the value n' (horizontal axis) obtained by dividing the interquartile range by the average nanohardness. It is a characteristic diagram plotting √area and r / t for each of invention examples 1 to 10 and comparative examples 1 to 10 shown in Table 1.

[0018] Hereinafter, several embodiments according to the present disclosure will be described with reference to the drawings. However, these descriptions are intended to merely exemplify preferred embodiments of the present disclosure, and are not intended to limit the present disclosure to such specific embodiments. The description will be given in the following order: 1. Overview of the present disclosure 2. Effect of micro-defects present on the surface on the inner side of a bend on fatigue strength 3. Structural component according to the present disclosure 4. Method for evaluating surface roughness on the inner side of a bend 5. Regarding high-Si type and low-Si type 6. Examples

[0019] 1. Overview of the Disclosure For example, automotive structural parts (components that make up structures) are becoming stronger every year, and as strength increases, weight reduction through thinner wall thickness is progressing. To realize lightweight, highly functional structural parts, it is important to combine "weight reduction of parts through high-strength thin wall thickness" with "structural design that takes advantage of the strength characteristics of steel." For example, in high-strength, thin-walled chassis parts (e.g., lower arms and other suspension parts), which are structural parts for automobiles, fatigue cracks may occur due to reduced rigidity associated with thin wall thickness and reduced surface quality (surface roughness) associated with processing, resulting in deterioration of the fatigue properties of the parts. In particular, when structural parts include bent portions that are bent through bending, the inside of the bent portion is subjected to compressive deformation, which reduces surface quality compared to before processing, and the surface roughness leads to deterioration of fatigue properties. Chassis parts are important safety parts that are constantly subjected to vibration and load, making it essential to improve their fatigue durability. In this embodiment, a structural part is provided that suppresses fatigue crack initiation from the inside of the bend by appropriately combining materials and part structure (e.g., the surface roughness and bend shape of the inside of the bend).

[0020] In this disclosure, by using an ultra-high tensile steel (tensile strength of 980 MPa or more) with a better forming limit than conventional steels, deterioration of the surface roughness on the inside of the bend of a bent part of an M-shaped cross section press-formed product or the like is suppressed to produce a smooth surface, enabling forming with a smaller radius of curvature on the inside of the bend. Furthermore, the relationship between the range of the radius of curvature on the inside of the bend that satisfies fatigue durability and the material properties is clarified.

[0021] As a result of extensive research, the inventors have found that by using an ultra-high tensile steel with a better forming limit than conventional steels, fatigue cracks occurring in a bent portion can be suppressed when the parameters of the bent portion, specifically the surface roughness on the inside of the bend and the radius of curvature on the inside of the bend, satisfy certain conditions. The material properties affect the change in surface roughness before and after processing of the bent portion. Fatigue durability can be improved by controlling the appropriate surface properties of the bent portion through the material and the shape (design) of the bent portion. Compared to conventional steels, the steel with a better forming limit according to this embodiment is less likely to have increased surface roughness on the inside of the bend after processing (i.e., less unevenness is developed), improving fatigue durability. The larger the radius of curvature on the inside of the bend, the less likely the surface roughness on the inside of the bend tends to increase.

[0022] The ultra-high tensile steels with excellent forming limits according to the present disclosure 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 the two improve fatigue durability are different. The high-Si type has a relatively large surface roughness due to the presence of Si-derived scale, but its high n-value (work hardening index) related to formability makes it difficult for microcracks to develop on the inside of the bend. Therefore, the high-Si type suppresses the folding of the compressed portion on the inside of the bend during bending, thereby achieving high durability. On the other hand, the low-Si type has a relatively low n-value, but its small surface roughness derived from Si scale suppresses the development of microcracks on the inside of the bend. Therefore, the low-Si type takes advantage of its excellent surface smoothness to suppress the occurrence of fatigue cracks and achieve high durability.

[0023] As a result of extensive research, the inventors came up with the idea of ​​setting the range of the parameters for each steel type by incorporating the differences in the mechanisms of fatigue cracking between high-Si and low-Si steels. In this disclosure, we will explain the preferred range of parameters when using a high-Si ultra-high tensile steel, out of the high-Si and low-Si steels.

[0024] 2. Effect of micro defects on the inner surface of a bent part on fatigue strength The present inventors have intensively studied the effect of the square root of the projected area of ​​a micro defect on the inner surface of a bent part, √area, on fatigue strength, and as described in the above Non-Patent Document 1, have found that the fatigue limit σ Ｗ Since is expressed by the following formula (4) using √area and Vickers hardness, we focused on √area as a characteristic value that affects fatigue durability. Note that micro-defects may refer to irregularities on the surface of the press-molded body. In the case of inferior micro-defects, they may not only appear as simple irregularities but may also be observed as cracks. 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 (σ min / σ max ).

[0025] √area [μm] is calculated from the following formulas (5) and (6) based on the description in the above-mentioned Non-Patent Document 1. In formulas (5) and (6), a [μm] is the roughness height (maximum height Rz), and 2b [μm] is the roughness pitch (mean element length, wavelength Rsm of the height waveform).

[0026] ... (5) (However, when a / 2b≦0.195)

[0027] ... (6) (However, when a / 2b>0.195)

[0028] 3. Structural component according to the present disclosure In a bent portion, the larger the value of √area, the greater the surface roughness, and therefore the lower the fatigue durability of the structural component. Furthermore, the smaller the value of r / t, the ratio of the curvature radius on the inner side of the bend to the steel plate thickness, the lower the fatigue durability of the structural member. The inventors have thoroughly investigated the relationship between √area and r / t of a structural component and the fatigue characteristics of the structural component, and have found that when √area and r / t are within a specified range, fatigue durability can be improved compared to conventional structural components.

[0029] FIG. 1 is a diagram showing an automobile suspension part as an example of a structural part according to the present disclosure, and more specifically, a lower arm 10 of a suspension. FIG. 2 is a diagram showing a cross section taken along dashed line II' in FIG. 1 . Note that the structural part according to the present disclosure is not limited to use in automobiles. The lower arm 10 is subjected to press working, burring, or the like, and has a flat plate portion (top plate portion) 10a and a bent portion 10b. FIG. 2 shows an M-shaped cross section including the flat plate portion 10a and the bent portion 10b. The present inventors have discovered that fatigue properties can be improved compared to conventional structural parts by having the bent portion 10b of a structural part made of steel plate (high-Si ultra-high tensile steel) satisfy the following formulas (1), (2), and (3), and more preferably by having the bent portion 10b satisfy formulas (1)', (2)', and (3). 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: square root of the projected area of ​​micro defects on the inside of the bend of the bent portion 10b (parameter indicating the roughness of the surface after processing) [μm], r: minimum value of the radius of curvature on the inside of the bend of the bent portion 10b [mm], t: plate thickness of the flat portion 10a [mm], and n': interquartile range of nanohardness / average nanohardness of the flat portion 10a.

[0030] For example, the flat portion 10a is a portion where the thickness reduction from the thickest portion is within 10% and is not subjected to large distortion due to bending. The flat portion 10a may be a portion having the same thickness as the plate thickness of the material.

[0031] As described above, the present inventors have thoroughly studied the values ​​of √area and r / t described in Non-Patent Document 1 when a bent portion is formed using an ultra-high tensile steel having a better forming limit than conventional steels, and have found that by specifying the values ​​of √area and r / t within predetermined ranges, it is possible to improve fatigue properties compared to conventional steels.

[0032] In addition, the structural component according to the present disclosure has a bent portion formed by press molding, burring, or the like. As an example of a processing method for forming the bent portion, bending may be performed with the inner surface of the bent portion in close contact with the die. For example, by conducting tests in advance and determining the conditions for bending (e.g., inner bend R, die clearance, etc.), it is possible to perform bending with the inner surface of the bent portion in close contact with the die. This prevents the surface roughness of the inner bent portion from increasing, thereby improving fatigue durability. Note that the √area value of the ground structural component is approximately 6.9, which does not satisfy formula (1), and therefore the structural component according to the present disclosure does not include a ground product.

[0033] 4. Method for Evaluating the Surface Roughness of the Inside of a Bent Section 10b When evaluating the surface roughness of the inside of a bent section, the curved surface is converted to a flat surface to evaluate the surface roughness. To this end, a mirror-finished cross-sectional sample of the bent section 10b was prepared, with a cross section perpendicular to the surface of the flat section 10a (i.e., the cross section shown in Figure 2) as the measurement surface. Five images of the inside of the bent section 10b were acquired at 500x magnification using an optical microscope, and the five acquired images 20 were linked as shown in Figure 3. Then, using the "find edges" function of image processing software (ImageJ), the contours of the surface irregularities were extracted from the difference in pixel values, and the "Analyze Line Graph" function was used to output the results as (x, y) coordinates (units: μm). If the structural component is coated with a coating such as electrocoating, the coating must be removed before acquiring the images.

[0034] 4 is a characteristic diagram showing a method for evaluating surface roughness by converting the curved surface into a flat surface using the output coordinates (characteristic C1). Characteristic C2 (dash-dotted line) was obtained by applying a Gaussian filter with σ = 50 to characteristic C1 (dashed line) obtained by image processing, and the difference between the two was then used to obtain characteristic C3 (solid line), which is a short-period roughness component that excludes the long-period waviness of the bending shape of the cross section of the bent portion 10b (i.e., the arc shape of the inner bend R).

[0035] The Gaussian filter is not effective at the ends of characteristic C1, and because characteristics C1 and C2 deviate as shown in Figure 4, data excluding a 100 μm range from each end of characteristic C3 was used to obtain the arithmetic mean roughness Ra and mean element length Rsm. Ra is twice the average value (mean deviation) of the absolute deviation between characteristic C3 and the mean line representing the average of characteristic C3, and Rsm is the average value of the interval between peaks and valleys over one cycle, determined from the intersection of the roughness curve represented by characteristic C3 and the mean line. The minimum height defining a peak or valley was set to 2 μm, and anything below 2 μm was considered noise and part of the preceding or following peak or valley.

[0036] FIG. 5 is a diagram schematically illustrating the surface roughness on the inner side of the bend represented by characteristic C3. As shown in FIG. 5, the height a and pitch 2b are values ​​assuming that the surface roughness is a periodic asperity, and √area in Equations (5) and (6) is a value obtained assuming that the surface roughness is a periodic asperity. The cross section shown in the upper part of FIG. 5 can be replaced with the cross section shown in the lower part. As shown in FIG. 5, Ra calculated as described above corresponds to the roughness height a in Equations (5) and (6), and Rsm corresponds to the roughness pitch 2b in Equations (5) and (6). Therefore, Rsm and Ra can be calculated based on an image of the cross section of the bent portion 10b, and √area can be calculated based on Equations (2) and (3).

[0037] The maximum height Rz (the difference between the height of the highest peak and the depth of the lowest valley) may be used as the roughness height a. ｓ The measurement range is the central 500 μm range of the characteristic C3. ｓ =0.1716・(1+120 / HV) ６ = 1.05 ... (7)

[0038] 5. Regarding the high Si type and low Si type Both the high Si type and the low Si type are made of an ultra-high tensile material (DP (Dual Phase) steel) with a tensile strength of 980 MPa or more, and a Vickers hardness HV of 310 or more, preferably 340 or more. Generally, in such high strength materials, the surface roughness on the inside of the bend has a significant effect on fatigue durability, but in the high Si type and low Si type according to the present disclosure, the Si content is adjusted, and in the case of the high Si type, the n' value, which will be described later, is adjusted, thereby making it an ultra-high tensile material with a better forming limit than conventional materials.

[0039] By measuring the Vickers hardness of a structural part, it can be determined whether the structural part is made of the ultra-high tensile steel with a tensile strength of 980 MPa according to this embodiment. A method for measuring the hardness of a processed structural part for an automobile will be described using the lower arm 10 shown in Figures 1 and 2 as an example. To measure the hardness of the base material that has not been affected by processing, the hardness is measured at the flat portion 10a, which is separated from the bent portion 10b and has not been subjected to significant distortion due to the bending process.

[0040] A sample is taken using a cross section perpendicular to the surface of the flat plate portion 10a (i.e., the cross section shown in FIG. 2) as the measurement surface, and the measurement surface is prepared and subjected to a hardness test. The measurement surface preparation method is performed in accordance with JIS Z 2244:2009. The measurement surface is polished using #600 to #1500 silicon carbide paper, and then the measurement surface is mirror-finished using a liquid in which diamond powder with a particle size of 1 μm to 6 μm is dispersed in a diluted solution such as alcohol or pure water. The mirror-finished measurement surface is then subjected to a hardness test using the method described in JIS Z 2244:2009. Note that using the cross section as the measurement surface can suppress the influence of surface coatings, etc. on the hardness measurement value.

[0041] Specifically, using a micro Vickers hardness tester, measurements are taken at 30 points at a position 1 / 4 of the plate thickness of the flat portion 10a adjacent to the bent portion 10b of the sample, with a load of 500 gf, at intervals of at least three times the indentation, and the average value of these measurements is taken as the hardness of the component.

[0042] High-Si steel sheets have a microstructure composed of ferrite and tempered martensite. Ferrite is a soft layer and has advantages in n-value and uniform elongation (u-El). The n-value is correlated with uniform elongation, and high-Si steel sheets contain soft ferrite, which gives them a large difference in hardness between layers (differing in hardness between structures). On the other hand, low-Si steel sheets have a microstructure composed mainly of bainite (which may contain tempered martensite and ferrite), and have a smaller difference in hardness between layers (differing in hardness between structures) compared to high-Si steel sheets.

[0043] In the present disclosure, high-Si and low-Si types are distinguished based on nanoindentation hardness (hereinafter also referred to as nanohardness) measured by a nanoindentation test. More specifically, the ratio of the interquartile range of nanohardness to the average nanohardness (= interquartile range of nanohardness / average nanohardness) is used to distinguish between high-Si and low-Si types as a parameter indicating the work hardening ability of a material.

[0044] Fig. 6 is a histogram showing the distribution of nano-hardness for low Si type. Fig. 7 is a histogram showing the distribution of nano-hardness for high Si type. In Fig. 6 and Fig. 7, the horizontal axis represents nano-hardness and the vertical axis represents frequency.

[0045] The sample for measuring nanohardness is taken from the same position as that for measuring the component hardness by the Vickers hardness test described above. The measurement surface of the sample is the same as that for the Vickers hardness test. The measurement surface is mirror-finished, and nanohardness is measured at a position 1 / 4 of the plate thickness. At this time, a load of 2500 uN is applied, and 100 measurements are performed. The distance between indentations is ensured to be 5 μm or more, and the nanohardness (i.e., indentation hardness) is determined based on ISO 14577-1 (Annex A). The interquartile range and mean nanohardness are then calculated from the obtained 100 indentation hardness values.

[0046] The high-Si type contains soft ferrite and a large difference in hardness between the structures, resulting in two nano-hardness peaks, as shown in Figure 7, and includes a hard layer and a soft layer. The high-Si type has excellent n-value characteristics due to the inclusion of a soft layer derived from ferrite. On the other hand, the low-Si type has a small difference in hardness between the structures, resulting in only one nano-hardness peak, as shown in Figure 6.

[0047] Figure 8 is a characteristic diagram 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 and the larger the value of n', the better the formability. In Figure 8, open circles (◯) indicate high-Si types, and filled circles (●) indicate low-Si types. High-Si types have higher uniform elongation, which indicates work hardening capacity, than low-Si types, but as mentioned above, the surface roughness is relatively large due to the presence of Si-derived scale. As a result of examining the examples described below, it was found that even with surface roughness derived from high-Si types, if the range of formula (1) is 33≦√area≦44 and the range of formula (2) is 0.17≦r / t≦3.45, and the uniform elongation value is 6% or more, the surface roughness on the inside of the bend after processing is suppressed, and the fatigue properties are superior to those of conventional products (comparative examples described below). On the other hand, in the low-Si type, the development of microcracks on the inside of the bend is suppressed due to the small surface roughness caused by the Si-derived scale. Therefore, by setting the range of formula (1) to 30≦√area≦40 and the range of formula (2) to 0.17≦r / t≦3.45, it was possible to achieve fatigue properties superior to those of conventional products even when the uniform elongation value was less than 6%.

[0048] As shown in Figure 8, for high-Si materials, if the uniform elongation of 6% or more is replaced with n' obtained from nanohardness, it becomes 0.22 or more. Measurement by tensile testing is required to determine uniform elongation, but it is not easy to determine uniform elongation from structural components. On the other hand, nanohardness allows for easy sampling from structural components.

[0049] Therefore, in this embodiment, the characteristics of high-Si and low-Si materials are evaluated by the value n' obtained by dividing the interquartile range by the average nanohardness. For high-Si materials, n' is 0.22 or more, and for low-Si materials, n' is less than 0.22.

[0050] The high-Si type is made of DP steel with a tensile strength of 980 MPa or more, and by setting the Si content to 0.9% or more by mass, an n' of 0.22 or more is obtained. In this embodiment, the high-Si type is characterized solely by an n' of 0.22 or more. Other than this feature and the Si content of 0.9% or more, the chemical composition of the material and the manufacturing method are not fundamentally limited. Note that the upper limit of tensile strength can be applied up to about 1300 MPa. As an example, the high Si type has the following chemical composition by mass: C: 0.01 to 1.0%, Si: 0.9% or more, Mn: 0.10 to 5.00%, P: 0.100% or less, S: 0.0300% or less, N: 0.0100% or less, O: 0 to 0.020%, Al: 0 to 1.000%, Cr: 0 to 2.00%, Cu: 0 to 2.00%, Ni: 0 to 2.00%, Mo: 0 to 3.00%, Co: 0 to 3.00%, Nb: 0 to 0.150%, V: 0 to 1.00%, T The alloy may have a chemical composition consisting of: 0-1.00% I, 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, and the balance: Fe and impurities. In the above chemical composition, the lower limit of the content of optional additional elements may be 0.0001% or 0.001%. Alternatively, a steel sheet obtained by rolling steel having the above chemical composition at room temperature to 1300°C, or a steel sheet obtained by rolling and then subjecting it to surface treatment such as heat treatment or painting, may be used.

[0051] In the low-Si type, by setting the Si content to less than 0.9% by mass, surface roughness due to Si-derived scale can be kept small, and n' is less than 0.22. The low-Si type is characterized solely by n' being less than 0.22. Apart from this feature and the Si content being less than 0.9%, the chemical components of the material and the manufacturing method are not fundamentally limited. The low-Si type may have the same chemical composition as the high-Si type described above, except that the Si content is less than 0.9%. Furthermore, as with the high-Si type, the lower limit of the content of optional added elements in the above chemical composition may be 0.0001% or 0.001%, and a steel sheet obtained by rolling steel having the above chemical composition at room temperature to 1300°C, or a steel sheet obtained by rolling and then performing surface treatment such as heat treatment or painting, may be used.

[0052] 6. Examples Hereinafter, aspects of the present invention will be described in detail with reference to examples. These examples are provided to confirm the effects of the present invention, and are not intended to limit the scope of the present invention.

[0053] A 2.9 mm thick low-Si material was bent with the curvature radius r shown in Table 1 to produce lower arms for Comparative Examples 1 to 8 and Inventive Examples 1 to 8. As shown in Table 1, Rz and Rsm were measured before and after processing using the method described above, and Rz / Rsm, √area / 2b, and √area [μm] were calculated. For Inventive Examples 1 to 8, √area [μm] after processing satisfies formula (1), r / t satisfies formula (2), and n' satisfies formula (3). On the other hand, Comparative Examples 1 to 8, which are conventional products, do not satisfy formula (1). Regarding √area, because the surface roughness of the inner bend of the bent portion 10b increases during processing, the value after processing never falls below the value before processing. Note that when measuring Rz and Rsm before processing, a characteristic corresponding to characteristic C3 in Figure 4 can be directly obtained from the image without converting the curved surface to a flat surface.

[0054] A fatigue test was conducted on each of the lower arms of the comparative example and the invention example, in which a load was applied in the longitudinal direction of the vehicle. Specifically, a load was applied in the longitudinal direction of the vehicle to the bent portion of each of the lower arms of the comparative examples 1 to 8 and the invention examples 1 to 8, which differ in the roughness parameter √area after processing, and the 200,000 cycles strength σf was measured. The 200,000 cycles strength is the load stress generated in the lower arm after 200,000 cycles before fatigue failure occurs.

[0055] For pairs of comparative examples and inventive examples with the same ratio r / t of the radius of curvature inside the bend to the plate thickness, the time strength σf1 of the comparative example was compared with the time strength σf2 of the inventive example to evaluate the fatigue durability of the inventive examples. Specifically, for each inventive example, based on the results of the comparison with the comparative example, if the value of σf2 / σf1 exceeded 1.2, it was evaluated as "Good", and if the value of σf2 / σf1 exceeded 1.4, it was evaluated as "Excellent".

[0056]

[0057] FIG. 9 is a characteristic diagram plotting √area and r / t for each of Inventive Examples 1 to 8 and Comparative Examples 1 to 8 listed in Table 1. In FIG. 9, open circles (◯) represent Inventive Examples, and filled circles (●) represent Comparative Examples. As shown in FIG. 9, the smaller the r / t, i.e., the smaller the r, the greater the √area value, and the rougher the surface roughness on the inner side of the bend becomes, resulting in a decrease in fatigue durability. However, each of Inventive Examples 1 to 8 has a lower √area value than Comparative Examples 1 to 8 with the same r / t. Compared to Comparative Examples 1 to 8, 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 indicate that the fatigue properties of Inventive Examples 1 to 8 are superior to those of Comparative Examples 1 to 8. Furthermore, the results in Table 1 indicate that the larger the r and the smaller the √area value, the better the fatigue properties are compared to Comparative Examples with the same r.

[0058] In FIG. 9, characteristic C4, which is an approximation of the curve connecting the plots of Examples 1 to 8, can be expressed by the following equation (8): √area=0.0976(r / t) ３ -0.7185 (r / t) ２ -1.0542(r / t)+43.426...(8)

[0059] Therefore, preferably, the structural member according to the present disclosure satisfies the following formula (9): Note that in formula (9), a 5% margin is added to the √area in formula (8) to set the upper limit value.

[0060] 33≦√area≦1.05{0.0976(r / t)) ３ -0.7185 (r / t) ２ -1.0542(r / t)+43.426}...(9)

[0061] 10 Lower arm 10a Flat plate portion 10b Bent portion 20 Image

Claims

1. A structural component made of a steel plate, comprising a flat portion and a bent portion, the bent portion satisfying the following formulas (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 of the projected area of ​​a micro defect on the inner side of the bent portion [μm], r is the minimum value of the radius of curvature on the inner side of the bent portion [mm], t is the thickness of the flat portion [mm], and n' is the interquartile range of nanohardness of the flat portion / average nanohardness.

2. The structural part according to claim 1, further satisfying the following formula (1)' and formula (2)': 33≦√area≦44 (1)' 0.17≦r / t≦3.45 (2)' 3. The structural component according to claim 1, wherein the flat portion has a Vickers hardness of 310 or more.

4. The structural component according to claim 1, wherein the flat portion has a Vickers hardness of 340 or more.

5. The structural part according to any one of claims 1 to 4, further satisfying 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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