Automobile member

Abstract

An automotive component, wherein the automotive component has a punched end face formed of steel, wherein the residual stress σ measured at a position on the punched end face at a distance of 1 / 6 of the plate thickness from the side surface of the die in the plate thickness direction satisfies σ / TS≥0.18 with the tensile strength TS of the steel.

Description

Technical Field

[0001] This invention relates to automotive components. Background Technology

[0002] In recent years, global fuel efficiency restrictions have increased and crash safety requirements have become more stringent, demanding a balance between lightweight vehicle bodies and improved crash safety. Furthermore, the trend towards carbon neutrality and the resulting electrification of automobiles necessitates enhanced protection performance of onboard batteries, making this a prerequisite for crash safety. Summary of the Invention

[0003] The problem the invention aims to solve

[0004] Against the backdrop of the above situation, the research predicts the advancement of high-strength steel used in automobiles. However, given that simply increasing the strength of steel is insufficient to meet the requirements of collision safety, research is also predicting the advancement of thicker steel walls. In other words, previous research focused on increasing the strength and thinning the walls of steel to balance vehicle weight reduction and improved collision safety. However, to further improve collision safety with the electrification of automobiles, there is a possibility that some components constituting the vehicle body may require higher strength and thicker walls.

[0005] On the other hand, in the stamping process of steel, the higher the tensile strength of the material and the thicker the plate, the more likely it is that unevenness will occur on the end face formed by the stamping process (hereinafter referred to as "stamped end face"), and the end face roughness will deteriorate. Components with stamped end faces with poor end face characteristics are prone to fatigue limit deterioration, but in the prior art, it is not understood what state the end face characteristics of the component after stamping process are under the condition of fatigue limit deterioration.

[0006] The present invention was made in view of the above circumstances, and its object is to provide an automotive component comprising a punched end face having excellent fatigue limit.

[0007] Solution for solving the problem

[0008] The inventors of this application conducted various experiments and studies, and found that by focusing on the residual stress state of the punched end face, and under certain conditions, the properties of the punched end face are improved, resulting in automotive components with excellent fatigue limits.

[0009] Based on this insight, one solution of the present invention to solve the above-mentioned problems is an automotive component, characterized in that the automotive component has a punched end face formed of steel, wherein the residual stress σ measured at a position on the punched end face at a distance of 1 / 6 of the plate thickness from the side surface of the die in the plate thickness direction satisfies σ / TS≥0.18 with the tensile strength TS of the steel.

[0010] The effects of the invention

[0011] It is possible to provide automotive components that include punched end faces with excellent fatigue limits. Attached Figure Description

[0012] Figure 1 This is a diagram illustrating the schematic structure of an automotive component according to one embodiment of the present invention.

[0013] Figure 2 This is an enlarged view of the punched end face used to illustrate the location for measuring residual stress.

[0014] Figure 3 This is a diagram used to illustrate the manufacturing method of automotive components.

[0015] Figure 4 This is a diagram showing the shape of the test piece in a bending fatigue test.

[0016] Figure 5 This is a graph showing the relationship between the residual stress and tensile stress index (σ / TS) and the fatigue limit.

[0017] Figure 6 This is an indicator representing the tensile strength and plate thickness (TS×t). 2 A graph showing the relationship between ( ) and fatigue limit.

[0018] Figure 7 This refers to the indicators used, such as tensile strength, uniform elongation, and plate thickness ((TS / uEL) × t). 4 A graph showing the relationship between ( ) and fatigue limit. Detailed Implementation

[0019] Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings. Furthermore, in this specification and the accompanying drawings, elements having substantially the same functional structure are omitted from repeated description by using the same reference numerals.

[0020] Figure 1 This is a diagram showing the schematic structure of the automotive component 1 according to this embodiment. The automotive component 1 (hereinafter also referred to as "component 1") is a steel sheet with a circular hole 2 formed in its central portion. The circular hole 2 is formed by a punching process, and in the following description, the inner circumferential surface of this circular hole 2 will be referred to as the punched end face 3. Furthermore, in this embodiment, the circular hole 2 is illustrated as the shape of the punched portion, but the shape of the punched portion is not limited to the circular hole 2. That is, the shape of the punched end face 3 is not limited to a circle.

[0021] The blanking end face 3 is an end face with a shearing surface and a fracture surface. Component 1 has two surfaces 4 and 5. Based on the formation positions of the shearing surface and fracture surface on the blanking end face 3, it can be determined whether either surface 4 or 5 is the surface opposite to the punch side or the surface opposite to the die side during the blanking process. Specifically, the surface of component 1 located closer to the shearing surface is the punch side surface 4, and the surface located closer to the fracture surface is the die side surface 5.

[0022] Furthermore, for example, the inner circumferential surface of a hole formed by laser processing contains neither shear surfaces nor fracture surfaces. That is, by observing the end face formed on an automotive component and confirming the presence or absence of shear surfaces and fracture surfaces, it can be determined whether the end face was formed by punching.

[0023] The component 1 with the punched end face 3 is applied to various parts of the vehicle body that use steel components, such as the upper beam, middle pillar, floor crossbeam, lower arm of the suspension, etc.

[0024] The tensile strength of the steel used as the material for component 1 is, for example, 440 MPa or more. The tensile strength may also be 590 MPa or more, 780 MPa or more, or 980 MPa or more. Furthermore, the thickness of the steel plate is, for example, 1.5 mm or more. The plate thickness may also be 2.0 mm or more, 2.5 mm or more, or 3.0 mm or more.

[0025] (Ratio of residual stress σ to tensile strength TS)

[0026] In component 1, the residual stress σ at the punched end face 3 and the tensile strength TS of the steel satisfy σ / TS ≥ 0.18. Here, refer to... Figure 2 The location for measuring the residual stress σ on the punched end face 3 is explained. Figure 2 This is an enlarged view of the punched end face 3.

[0027] The residual stress σ is the stress value measured at position P, in the thickness direction of the blanking end face 3, at a distance of 1 / 6 of the thickness t of the blanking end face 3 from the die side surface 5. Specifically, it is the stress value measured using an X-ray diffraction apparatus in a region with a diameter of 0.5 mm centered at position P (the dashed area in the figure). Furthermore, the measurement conditions for the X-ray diffraction will be described later.

[0028] When measuring the residual stress of processed steel, it is usually measured at the center of the steel sheet thickness. However, as described above, in this embodiment, the residual stress σ is measured at a position P, which is 1 / 6 of the sheet thickness t, at a distance from the die-side surface 5 of the steel sheet. Therefore, the measurement position of the residual stress σ in this embodiment differs from the usual measurement position. The fatigue limit of the processed steel is degraded by the magnitude of the residual stress near the surface of the steel on the side where burrs are generated, i.e., the residual stress near the die-side surface. Therefore, the residual stress measured at the center of the sheet thickness cannot adequately evaluate the fatigue limit of steel with a punched end face. Thus, in this embodiment, the residual stress σ is not measured at the center of the sheet thickness of the punched end face 3, but at a position P, which is 1 / 6 of the sheet thickness t, at a distance from the die-side surface 5.

[0029] Furthermore, the residual stress measured at the center of the sheet thickness on the blanking end face 3 is different from the residual stress measured at a position P, which is 1 / 6 of the sheet thickness t at a distance from the die side surface 5. Therefore, even if the ratio of the residual stress to the tensile strength measured at the "center of the sheet thickness" on the blanking end face 3 is 0.18 or more, the ratio of the residual stress to the tensile strength measured at the "position P, which is 1 / 6 of the sheet thickness t at a distance from the die side surface 5" on the blanking end face 3 is not limited to being 0.18 or more.

[0030] The automotive component 1 of this embodiment is a component in which the ratio of residual stress σ to tensile strength TS, measured at a position P at a distance P equal to 1 / 6 of the sheet thickness t, is 0.18 or higher. This automotive component 1 becomes a component with good end-face characteristics of the punched end face 3, and is a component with excellent fatigue limit, as also shown in the embodiments described later. Furthermore, σ / TS may, for example, be 0.20 or higher, or 0.25 or higher.

[0031] Furthermore, in the stamping process of higher strength and thicker-walled steel, the end-face properties of the stamped end face 3 are prone to deterioration. However, if σ / TS ≥ 0.18, an automotive component 1 with improved end-face properties can be obtained. For example, the tensile strength TS and the plate thickness t satisfy TS × t. 2 ≥3000[MPa•mm 2 The properties of the punched end face of steel are prone to deterioration during punching, making it a material whose fatigue limit is easily degraded. However, even when using such steel, if σ / TS ≥ 0.18 is satisfied, the properties of the punched end face are improved, resulting in automotive components with excellent fatigue limits, as shown in the embodiments described later. That is, when using steel that satisfies TS × t 2For steel with a thickness of ≥3000, manufacturing automotive components in a manner that satisfies σ / TS≥0.18 is particularly useful. From the viewpoint of improving the end-face characteristics achieved by satisfying σ / TS≥0.18, TS×t 2 The preferred pressure is 3500 MPa·mm. 2 Above, more preferably 4000 [MPa•mm] 2 The above is further preferred to be 4500 [MPa•mm]. 2 ]above.

[0032] In addition to the high-strength and thick steels mentioned above, steels lacking ductility can also be cited as steels whose end-face properties are prone to deterioration in the blanking end face 3. One of the indicators for evaluating the ductility of steel is uniform elongation, which is determined by a tensile test of the material. As shown in the embodiments described later, the tensile strength TS [MPa], uniform elongation uEL [-], and plate thickness t [mm] satisfy (TS / uEL) × t 4 ≥120000[MPa•mm 4 In steels with a fatigue limit, the performance varies depending on whether σ / TS ≥ 0.18 is satisfied. Furthermore, even when (TS / uEL) × t is satisfied... 4 In steel with a strength ≥120000 [MPa•mm4], the improvement in the end-face properties of the punched end face 3 becomes significant by satisfying σ / TS≥0.18. From the perspective of enhancing the improvement in end-face properties achieved by satisfying σ / TS≥0.18, (TS / uEL)×t 4 The preferred value is 150,000 [MPa•mm] 4 More preferably, it is 200,000 [MPa•mm]. 4 The above is further preferred to be 250,000 [MPa•mm]. 4 ]above.

[0033] Furthermore, for example, steels whose combined chemical composition contains 0.02% by mass or more of Nb and Ti tend to exhibit deterioration of the blanking end face properties during blanking, making them materials prone to fatigue limit degradation. However, even when using such steels, improvements in the blanking end face properties are achieved when σ / TS ≥ 0.18, resulting in automotive components with excellent fatigue limits, as shown in the embodiments described later. In other words, it is particularly useful to manufacture automotive components using steels containing 0.02% by mass or more of Nb and Ti in a manner that satisfies σ / TS ≥ 0.18.

[0034] (Manufacturing method)

[0035] The aforementioned position P ( Figure 2 The state of residual stress varies depending on the tensile strength of the steel, the sheet thickness, the shape of the blanking portion, and the blanking conditions (hereinafter referred to as "blanking conditions"). Therefore, the conditions used to manufacture automotive components that satisfy σ / TS≥0.18 are appropriately set based on experiments, experience, etc. However, examples of blanking conditions that affect the state of residual stress include... Figure 3 The clearance setting between the punch 11 and the die 12 of the stamping die 10 shown, and the radius of curvature R of the punch shoulder. p The radius of curvature R of the die shoulder d The setting of the ratio, etc.

[0036] The aforementioned clearance is a value calculated by (the clearance G between punch 11 and die 12 / the plate thickness t of blank 20) ​​× 100 [%]. By adjusting this clearance, the state of residual stress at the aforementioned position P can be changed. Furthermore, when the shape of the blanking part is a circular hole, the clearance G is equivalent to (the diameter D of the die) × 100 [%]. d - Punch diameter D p The value is calculated as () / 2.

[0037] Additionally, by adjusting the radius of curvature R of the punch shoulder... p The radius of curvature R of the die shoulder d The change in the ratio Rp / Rd can also change the state of the residual stress at position P. For example, when R... p / R d With a value of ≥1.5, it is easy to manufacture automotive components that meet the requirement of σ / TS≥0.18.

[0038] The blanking conditions for manufacturing automotive components that satisfy σ / TS≥0.18 are not determined by a single condition. Therefore, for example, blanking tests are conducted before manufacturing automotive components as products, and appropriate blanking conditions are determined based on the results.

[0039] Specifically, first, a test piece is prepared from steel of the same specifications as the steel used in the product. This test piece is then subjected to a blanking process to impart the same blanking shape (e.g., a round hole) as the product. During this blanking process, multiple test pieces are prepared as needed, and the process is repeated multiple times under different blanking conditions. Then, based on the results of the blanking tests, the blanking conditions that satisfy σ / TS≥0.18 are extracted, taking into account factors such as cost and past experience to determine the blanking conditions for manufacturing the product. Finally, by blanking the steel used in the product under the determined blanking conditions, automotive components that satisfy σ / TS≥0.18 can be manufactured.

[0040] The above examples illustrate embodiments of the present invention, but the present invention is not limited to these examples. It is obvious to those skilled in the art that various modifications or alterations will arise within the scope of the technical concept described in the claims, and these modifications or alterations are also considered to fall within the protection scope of the present invention.

[0041] For example, the constituent elements of the above embodiments can be arbitrarily combined. Based on this arbitrary combination, the functions and effects of each combined constituent element can be obtained, and other functions and effects that are obvious to those skilled in the art as described in this specification can also be obtained.

[0042] Example

[0043] use Figure 4 A bending fatigue test was performed on a test piece 6 (tensile test piece) of the shape shown. Test piece 6 is a flat plate with a circular hole formed in the center, and the width W of the center of the test piece is narrower than the width of the ends of the test piece.

[0044] In bending fatigue tests, multiple steel samples with varying tensile strengths and plate thicknesses were prepared. These steel samples were then punched to produce multiple test pieces with a hole diameter D / width W satisfying 0.5. Furthermore, in each test piece, residual stress was measured at a position on the punched end face in the thickness direction, at a distance of 1 / 6 of the plate thickness t from the die side surface. The residual stress was measured using X-ray diffraction. The X-ray diffraction measurement conditions are as follows.

[0045] • X-ray stress measurement method: sinφ method (scanning method: parallel tilting method (φ constant method))

[0046] • Characteristic X-rays: Cr Kα

[0047] • Determine the diffraction plane: α-Fe211

[0048] • Entrance slit: Average diameter of a single collimator is 0.5 mm

[0049] • Angle of incidence (φ): 0, 15.9, 22.8, 28.3, 33.2, 37.8, 42.2, 46.5, 50.8 degrees

[0050] • Swing: ±5deg

[0051] • Diffraction angle determination method: half-value width method

[0052] • Stress constant (K): -318MPa / deg

[0053] Next, other test pieces prepared by blanking under the same conditions as those used to determine the residual stress at the blanking end face were subjected to bending fatigue tests to determine the fatigue limit. The results of the bending fatigue tests including the blanking conditions are shown in Table 1 below. Furthermore, regarding the measured residual stress σ, a positive sign indicates the tensile stress, and a negative sign indicates the compressive stress. Additionally, the relationship between the ratio of residual stress σ to tensile strength TS and the fatigue limit ratio is shown in... Figure 5 .

[0054] [Table 1]

[0055]

[0056] "Fatigue limit ratio" refers to the ratio of the fatigue limit of each test piece to the fatigue limit of a reference test piece. In Table 1, the fatigue limit ratios for No. 1 to No. 4 are calculated using the fatigue limit of No. 1 as the reference; the ratios for No. 5 to No. 9 are calculated using the fatigue limit of No. 5 as the reference; and the ratios for No. 10 to No. 16 are calculated using the fatigue limit of No. 10 as the reference. Here, the reference test pieces No. 1, No. 5, and No. 10 have sufficient fatigue limits for automotive components, and test pieces with fatigue limit ratios of 0.90 or higher relative to these No. 1, No. 5, and No. 10 test pieces also have excellent fatigue limits.

[0057] As shown in Table 1 and Figure 5 As shown, the fatigue limit ratios of the test pieces that satisfy σ / TS≥0.18 are all above 0.90, and the test pieces that satisfy σ / TS≥0.18 can be said to be components with excellent fatigue limits.

[0058] Figure 6 This summarizes the results of the bending fatigue tests described above and expresses them using the tensile strength TS and plate thickness t (TS×t). 2 A graph showing the relationship between the tensile strength of the steel and the fatigue limit ratio. The greater the tensile strength of the steel and the thicker the plate, the higher the TS×t ratio. 2 The larger the value, the greater the value. Furthermore, the larger the TS×t value, the greater the potential benefit. 2 The larger the value, the easier it is for the roughness of the punched end face to increase, and the easier it is for the fatigue limit to deteriorate.

[0059] like Figure 6 As shown, TS×t 2 It is 3000 [MPa•mm 2 The test pieces shown above exhibited low fatigue limits when σ / TS was less than 0.18. On the other hand, even when TS×t... 2 It is 3000 [MPa•mm 2The test specimens with σ / TS greater than 0.18 also exhibited higher fatigue limits. That is, the method of using TS×t satisfies σ / TS ≥ 0.18. 2 It is 3000 [MPa•mm 2 This is especially useful for steel materials as described above.

[0060] Figure 7 This summarizes the results of the bending fatigue test and expresses them using the tensile strength TS, uniform elongation uEL, and plate thickness t as indicators ((TS / uEL) × t). 4 A graph showing the relationship between the tensile strength and fatigue limit ratio of steel. The greater the tensile strength of the steel, the lower the uniform elongation. Additionally, the thicker the plate, the higher the uniform elongation (TS / uEL) × t. 4 The larger the value, the greater. (TS / uEL)×t 4 The larger the value, the easier it is for the roughness of the punched end face to increase, and the easier it is for the fatigue limit to deteriorate.

[0061] like Figure 7 As shown, (TS / uEL)×t 4 It is 120000 [MPa•mm 4 The test pieces above showed low fatigue limits when σ / TS was less than 0.18. On the other hand, even if (TS / uEL) × t 4 It is 120000 [MPa•mm 4 The test pieces with σ / TS greater than 0.18 also exhibited higher fatigue limits. That is, the method satisfying σ / TS ≥ 0.18 was used when applying (TS / uEL) × t. 4 It is 120000 [MPa•mm 4 This is especially useful for steel materials as described above.

[0062] The embodiments of the present invention have been described above. The effects described in this specification are merely illustrative or exemplary, and not limiting. That is, the technology disclosed herein can achieve, together with the effects described above, other effects that are obvious to those skilled in the art based on the description of this specification, or can replace the effects described above to achieve other effects that are obvious to those skilled in the art based on the description of this specification.

[0063] Industrial availability

[0064] This invention can be applied to automotive components.

[0065] Explanation of reference numerals in the attached figures

[0066] 1. Automotive component; 2. Round hole; 3. Blanking end face; 4. Punch side surface; 5. Die side surface; 6. Test piece; 10. Stamping die; 11. Punch; 12. Die; 20. Blank; D. Hole diameter; Dp Punch diameter; D d Die diameter; G, clearance; P, location for residual stress measurement; R p The radius of curvature of the head and shoulder; R d 1. Radius of curvature of the die shoulder; t. Plate thickness; W. Width.

Claims

1. A component for automobiles, wherein, This automotive component has a stamped end face formed of steel. The residual stress σ measured at a position on the blanking end face at a distance of 1 / 6 of the plate thickness from the side surface of the die in the plate thickness direction satisfies σ / TS≥0.18 with the tensile strength TS of the steel.

2. The automotive component according to claim 1, wherein, The tensile strength TS and plate thickness t of the steel satisfy TS×t. 2 ≥3000[MPa•mm 2 ].

3. The automotive component according to claim 1 or 2, wherein, The tensile strength TS, uniform elongation uEL, and plate thickness t of the steel satisfy (TS / uEL) × t. 4 ≥120000[MPa•mm 4 ].

4. The automotive component according to any one of claims 1 to 3, wherein, The steel contains a total of 0.02% by mass or more of Nb and Ti.

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