Automotive parts

Abstract

An automotive component having a punched end face made of a steel material, wherein the residual stress σ measured at a position 1 / 6 of the plate thickness from the die-side surface in the plate thickness direction of the punched end face and the tensile strength TS of the steel material satisfy σ / TS≧0.18.

Description

[Technical Field]

[0001] The present invention relates to an automotive component. [Background technology]

[0002] In recent years, fuel economy regulations have become more stringent worldwide, and crash safety requirements have also become stricter, requiring both lighter vehicle bodies and improved crash safety. In addition, with the trend toward carbon neutrality, the electrification of automobiles is progressing, and improving the protection performance of onboard batteries has become one of the requirements for crash safety. Summary of the Invention [Problem to be solved by the invention]

[0003] Given the above circumstances, it is predicted that the strength of steel materials used in automobiles will continue to increase, but if it becomes difficult to meet the demands for collision safety by increasing the strength of steel materials alone, it is predicted that consideration will be given to increasing the thickness of steel materials. In other words, while in the past, consideration was given to increasing the strength and thinning of steel materials in order to simultaneously reduce the weight of the vehicle body and improve collision safety, in order to further improve collision safety with the electrification of automobiles, it is possible that the aim will be to increase the strength and thickness of some of the components that make up the vehicle body.

[0004] On the other hand, in the punching of steel materials, 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 surface formed by the punching process (hereinafter referred to as the "punched end surface"), and the worse the end surface roughness will tend to be. Members with punched end surfaces with poor end surface quality tend to have a deteriorated fatigue limit, but in the prior art, it was not recognized what state of the end surface quality of punched members would cause a deterioration in the fatigue limit.

[0005] The present invention has been made in view of the above circumstances, and has an object to provide an automotive component including a punched end surface having an excellent fatigue limit. [Means for solving the problem]

[0006] As a result of various tests and investigations, the inventors of the present application have focused on the residual stress state of the punched end surface and have found that when the residual stress state satisfies certain conditions, the properties of the punched end surface are improved and an automotive component with an excellent fatigue limit can be obtained.

[0007] One aspect of the present invention, which was completed based on this finding and solves the above-mentioned problems, is an automotive component having a punched end face made of a steel material, characterized in that the residual stress σ measured at a position 1 / 6 of the plate thickness from the die-side surface in the plate thickness direction of the punched end face and the tensile strength TS of the steel material satisfy σ / TS≧0.18. [Effects of the Invention]

[0008] It is possible to provide an automotive component including a punched end face having an excellent fatigue limit. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a diagram illustrating a schematic configuration of an automotive member according to an embodiment of the present invention. [Figure 2] FIG. 10 is an enlarged view of a punched end surface for explaining the measurement position of residual stress. [Figure 3] 1A to 1C are diagrams for explaining a method for manufacturing an automotive component. [Figure 4] FIG. 1 is a diagram showing the shape of a test piece for a bending fatigue test. [Figure 5] FIG. 1 is a graph showing the relationship between the index (σ / TS) using residual stress and tensile stress and the fatigue limit. [Figure 6] FIG. 1 is a diagram showing the relationship between the index (TS×t2) using tensile strength and plate thickness and the fatigue limit. [Figure 7] FIG. 1 is a graph showing the relationship between the fatigue limit and an index ((TS / uEL)×t4) using tensile strength, uniform elongation, and sheet thickness. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. In this specification and the drawings, elements having substantially the same functional configuration are designated by the same reference numerals, and redundant description will be omitted.

[0011] FIG. 1 is a diagram showing a schematic configuration of an automotive component 1 according to this embodiment. The automotive component 1 (hereinafter sometimes referred to as "component 1") is a steel plate with a circular hole 2 formed in the center. The circular hole 2 is formed by punching, and in the following description, the inner peripheral surface of this circular hole 2 will be referred to as a punched end surface 3. Note that in this embodiment, the round hole 2 is exemplified as the shape of the punched portion, but the shape of the punched portion is not limited to the round hole 2. In other words, the shape of the punched end surface 3 is not limited to a circle.

[0012] The punched end surface 3 is an end surface having a shear surface and a fracture surface. The member 1 has two surfaces 4 and 5, and from the positions of the shear surface and fracture surface on the punched end surface 3, it is possible to determine which of the two surfaces 4 and 5 faces the punch side or the die side during punching. Specifically, of the two surfaces 4 and 5 of the member 1, the surface closer to the shear surface is the punch-side surface 4, and the surface closer to the fracture surface is the die-side surface 5.

[0013] For example, the inner surface of a hole formed by laser processing does not have a sheared or fractured surface. In other words, by observing the end surface formed in an automotive part and checking whether or not there is a sheared or fractured surface, it is possible to determine whether the end surface was formed by punching.

[0014] The member 1 having the punched end surface 3 is applied to various parts of the vehicle body where steel parts are used, such as roof side rails, center pillars, floor cross members, and lower arms of the suspension.

[0015] The tensile strength of the steel material used to form the member 1 is, for example, 440 MPa or more. The tensile strength may be 590 MPa or more, 780 MPa or more, or 980 MPa or more. The thickness of the steel material is, for example, 1.5 mm or more. The thickness may be 2.0 mm or more, 2.5 mm or more, or 3.0 mm or more.

[0016] (ratio of residual stress σ to tensile strength TS) In the member 1, the residual stress σ at the punched end surface 3 and the tensile strength TS of the steel material satisfy σ / TS≧0.18. Here, the measurement position of the residual stress σ at the punched end surface 3 will be described with reference to Fig. 2. Fig. 2 is an enlarged view of the punched end surface 3.

[0017] The residual stress σ is a stress value measured at a position P that is 1 / 6 of the thickness t of the punched end face 3 from the die-side surface 5 in the thickness direction of the punched end face 3. Specifically, it is a stress value measured using an X-ray diffraction device in a region with a diameter of 0.5 mm (the region indicated by the dashed line in the figure) centered on position P. The measurement conditions for the X-ray diffraction measurement will be described later.

[0018] When measuring the residual stress of a processed steel material, the residual stress is generally measured at the center of the steel material's thickness. However, as described above, the residual stress σ in this embodiment is measured at a position P that is 1 / 6 of the steel material's thickness t from the die-side surface 5. Therefore, the measurement position of the residual stress σ in this embodiment differs from the general measurement position of residual stress. The fatigue limit of a processed steel material is affected by the magnitude of the residual stress near the surface on which burrs are generated, i.e., the residual stress near the die-side surface. Therefore, the residual stress measured at the center of the thickness cannot adequately evaluate the fatigue limit of a steel material having a punched edge. Therefore, in this embodiment, the residual stress σ is measured at a position P that is 1 / 6 of the steel material's thickness t from the die-side surface 5, rather than at the center of the steel material's thickness at the punched edge 3.

[0019] The residual stress measured at the center of the sheet thickness at the punched end face 3 and the residual stress measured at a position P that is 1 / 6 of the sheet thickness t from the die-side surface 5 have different values. Therefore, even if the ratio of the residual stress to the tensile strength measured at the "center of the sheet thickness" at the punched end face 3 is 0.18 or more, this does not necessarily mean that the ratio of the residual stress to the tensile strength measured at "the position P that is 1 / 6 of the sheet thickness t from the die-side surface 5" at the punched end face 3 will be 0.18 or more.

[0020] The automotive component 1 according to this embodiment is a component having a ratio σ / TS of the residual stress σ to the tensile strength TS measured at a position P that is 1 / 6 of the plate thickness t from the die-side surface 5 of 0.18 or more. This automotive component 1 has good end face properties on the punched end face 3 and has an excellent fatigue limit, as will be shown in the examples described later. Note that σ / TS may be, for example, 0.20 or more, or 0.25 or more.

[0021] Furthermore, in punching a higher strength and thicker steel material, the quality of the punched edge 3 tends to deteriorate. However, if σ / TS≧0.18 is satisfied, an automotive component 1 with improved quality of the edge can be obtained. For example, when the tensile strength TS and the plate thickness t are TS×t 2 ≧3000 [MPa·mm 2 Steel materials that satisfy the above condition are likely to have poorer properties at the punched edge during punching, and are therefore prone to a deterioration in fatigue limit. However, even when such steel materials are used, if σ / TS≧0.18 is satisfied, the properties of the punched edge are improved, and automotive components with excellent fatigue limits can be obtained, as will be shown in the examples below. That is, TS×t 2 When using steel materials that satisfy σ / TS ≥ 3000, it is particularly useful to manufacture automotive components so that σ / TS ≥ 0.18 is satisfied. From the viewpoint of enhancing the effect of improving the end surface properties by satisfying σ / TS ≥ 0.18, TS × t 2 is preferably 3500 [MPa·mm 2 ] or more, more preferably 4000 [MPa·mm 2 ] or more, and more preferably 4500 [MPa mm 2 ]That's all.

[0022] Steel materials that are prone to deterioration of the edge quality of the punched edge 3 include the high-strength and thick steel materials mentioned above, as well as steel materials with poor ductility. One index for evaluating the ductility of steel materials is uniform elongation, which is measured by a tensile test of the material. As will be shown in the examples below, the tensile strength TS [MPa], uniform elongation uEL [-], and thickness t [mm] are expressed as (TS / uEL) × t 4 ≧120000 [MPa·mm 4 In steel materials that satisfy the condition σ / TS≧0.18, the fatigue limit performance differs depending on whether or not it satisfies the condition (TS / uEL)×t 4 ≧120000 [MPa·mm 4 In steel materials that satisfy the above condition, satisfying σ / TS≧0.18 significantly improves the end surface quality of the punched end surface 3. From the viewpoint of enhancing the effect of improving the end surface quality by satisfying σ / TS≧0.18, (TS / uEL)×t 4 is preferably 150,000 [MPa mm 4 ] or more, more preferably 200,000 [MPa·mm 4 ] or more, and more preferably 250,000 [MPa mm 4 ]That's all.

[0023] For example, steel containing 0.02% by mass or more of Nb and Ti as its chemical components tends to deteriorate the properties of the punched edge during punching, and the fatigue limit tends to deteriorate. However, even when such steel is used, if σ / TS≧0.18 is satisfied, the properties of the punched edge are improved, and automotive components with an excellent fatigue limit can be obtained, as shown in the examples described below. In other words, when using steel containing 0.02% by mass or more of Nb and Ti in total, it is particularly useful to manufacture automotive components so that σ / TS≧0.18 is satisfied.

[0024] (Manufacturing method) The state of residual stress at the position P (Fig. 2) mentioned above changes depending on the tensile strength and thickness of the steel material, the shape of the punched part, and the punching conditions (hereinafter referred to as "punching conditions"), so the conditions for manufacturing automotive parts that satisfy σ / TS≧0.18 are set appropriately based on tests, experience, etc. However, examples of punching conditions that affect the state of residual stress include the clearance setting between the punch 11 and die 12 of the press die 10 as shown in Fig. 3, and the radius of curvature R of the punch shoulder. p and the radius of curvature of the die shoulder R d There are settings for the ratio etc.

[0025] The above clearance is a value calculated by (gap G between punch 11 and die 12 / thickness t of blank 20) ​​x 100 [%]. By adjusting this clearance, the state of residual stress at the position P mentioned above can be changed. Note that when the shape of the punched portion is a round hole, the gap G is (die diameter D d -Punch diameter D p ) / 2.

[0026] In addition, the radius of curvature of the punch shoulder R p and the radius of curvature of the die shoulder R d Ratio of p / R d By changing R, it is possible to change the state of residual stress at the position P. For example, p / R d When σ / TS≧1.5 is satisfied, it becomes easier to manufacture automotive parts that satisfy σ / TS≧0.18.

[0027] Since there is no single set of punching conditions for manufacturing automotive parts that satisfy σ / TS≧0.18, appropriate punching conditions are determined, for example, based on the results of a punching test conducted before manufacturing the automotive part as a product.

[0028] Specifically, a test piece made of steel of the same specifications as the steel used in the product is first prepared, and the test piece is then punched to create the same punched shape (e.g., a round hole) as the punched shape of the product. This punching is performed multiple times under different punching conditions, using multiple test pieces as needed. Then, from the results of the punching test, punching conditions that satisfy σ / TS ≥ 0.18 are extracted, and the punching conditions for product manufacturing are determined, taking into consideration factors such as cost and past experience. Then, by punching the steel for the product under the determined punching conditions, automotive components that satisfy σ / TS ≥ 0.18 can be manufactured.

[0029] While the embodiments of the present invention have been described above, the present invention is not limited to these examples. It is clear that a person skilled in the art can conceive of various modifications or alterations within the scope of the technical ideas described in the claims, and it is understood that these modifications also fall within the technical scope of the present invention.

[0030] For example, the components of the above-described embodiments can be combined in any manner, and such combinations will naturally provide the functions and advantages of the individual components involved in the combination, as well as other functions and advantages that will be apparent to those skilled in the art from the description herein. [Example]

[0031] A bending fatigue test was carried out using a test piece 6 (tensile test piece) having the shape shown in Fig. 4. The test piece 6 was a flat plate with a round hole formed in the center, and the width W of the center of the test piece was narrower than the width of the ends of the test piece.

[0032] For the bending fatigue test, several steel materials with different tensile strengths and thicknesses were prepared and punched to create several test pieces with a hole diameter D / width W ratio of 0.5. Furthermore, for each test piece, residual stress was measured in the thickness direction of the punched end face at a position 1 / 6 of the plate thickness t from the die-side surface of the punched end face. Residual stress was measured using a method that utilizes X-ray diffraction. The measurement conditions for the X-ray diffraction measurement were as follows: ·X-ray stress measurement method: sinφ method (scanning method: parallel tilt method (constant φ method)) ·Characteristic X-ray: Cr Kα ·Measurement diffraction surface: α-Fe211 Entrance slit: Single collimator, average diameter 0.5 mm ·Incidence angle (φ): 0, 15.9, 22.8, 28.3, 33.2, 37.8, 42.2, 46.5, 50.8 degrees Oscillation: ±5 deg Diffraction angle determination method: Half-value width method Stress constant (K): -318 MPa / deg

[0033] Next, a bending fatigue test was conducted using other test pieces prepared by punching under the same conditions as those used to measure the residual stress at the punched end face, and the fatigue limit was measured. Table 1 below shows the results of the bending fatigue test, including the punching conditions. Note that for the measured residual stress σ, a positive stress value indicates a tensile stress value, and a negative stress value indicates a compressive stress value. Figure 5 also shows the relationship between the ratio of residual stress σ to tensile strength TS and the fatigue limit ratio.

[0034] [Table 1]

[0035] The "fatigue limit ratio" is the ratio of the fatigue limit of each test specimen to the fatigue limit of a reference test specimen. The fatigue limit ratios of Nos. 1 to 4 in Table 1 were calculated based on the fatigue limit of No. 1 as the reference, the fatigue limit ratios of Nos. 5 to 9 were calculated based on the fatigue limit of No. 5 as the reference, and the fatigue limit ratios of Nos. 10 to 16 were calculated based on the fatigue limit of No. 10 as the reference. The reference test specimens Nos. 1, 5, and 10 have fatigue limits sufficient for automotive components, and test specimens whose fatigue limit ratios to Nos. 1, 5, and 10 are 0.90 or more also have excellent fatigue limits.

[0036] As shown in Table 1 and Fig. 5, the test specimens that satisfied σ / TS≧0.18 all had fatigue limit ratios of 0.90 or more, and can be said to be members with excellent fatigue limits.

[0037] Figure 6 summarizes the results of the bending fatigue test and shows the index (TS × t) using the tensile strength TS and plate thickness t. 2 ) and the fatigue limit ratio. The higher the tensile strength of the steel material and the thicker the plate, the greater the fatigue limit ratio. 2 becomes a large value. And, TS×t 2 The larger the value of the material, the more likely it is that the roughness of the punched end surface increases and the more likely it is that the fatigue limit deteriorates.

[0038] As shown in Figure 6, TS × t 2 is 3000 [MPa·mm 2 ] or more, the fatigue limit is low when σ / TS is less than 0.18. 2 is 3000 [MPa·mm 2 ] or more, a test piece with σ / TS of 0.18 or more has a high fatigue limit. 2 is 3000 [MPa·mm 2 This is particularly useful when using steel materials with a strength of 1000 MPa or more.

[0039] Figure 7 summarizes the results of the bending fatigue test described above and shows the index ((TS / uEL) × t) using the tensile strength TS, uniform elongation uEL, and plate thickness t. 4 The higher the tensile strength of the steel, the lower the uniform elongation, and the thicker the plate, the greater the relationship between (TS / uEL)×t 4 (TS / uEL)×t 4 The larger the value of the material, the more likely it is that the roughness of the punched end surface increases and the more likely it is that the fatigue limit deteriorates.

[0040] As shown in Figure 7, (TS / uEL) × t 4 is 120,000 [MPa·mm 4 ] or more, the fatigue limit is low when σ / TS is less than 0.18. 4is 120,000 [MPa·mm 4 ] or more, a test piece with σ / TS of 0.18 or more has a high fatigue limit. In other words, satisfying σ / TS ≥ 0.18 means that (TS / uEL) × t 4 is 120,000 [MPa·mm 4 This is particularly useful when using steel materials with a strength of 1000 MPa or more.

[0041] The above describes the embodiments of the present invention. The effects described herein are merely descriptive or exemplary and are not limiting. In other words, the technology disclosed herein may achieve other effects that will be apparent to those skilled in the art from the description of this specification, in addition to or in place of the above effects. [Industrial Applicability]

[0042] The present invention can be applied to automobile components. [Explanation of symbols]

[0043] 1 Automotive parts 2 round holes 3 Punched end face 4 Punch side surface 5 Die side surface 6 Test pieces 10 Press mold 11 Punch 12 Die 20 blank D hole diameter D p Punch diameter D d Die Diameter G gap P Residual stress measurement position R p Punch shoulder curvature radius R d Die shoulder curvature radius t Plate thickness W width

Claims

1. An automotive component having a punched end face made of steel, An automotive component wherein the residual stress σ measured at a position 1 / 6 of the plate thickness from the die-side surface in the plate thickness direction of the punched end face and the tensile strength TS of the steel material satisfy σ / TS ≥ 0.

18.

2. The tensile strength TS and plate thickness t of the aforementioned steel material are TS × t 2 ≧3000 [MPa・mm 2 The automotive component according to claim 1, satisfying the following conditions.

3. The tensile strength TS, uniform elongation uEL, and plate thickness t of the aforementioned steel material are given by (TS / uEL) × t 4 ≧120000 [MPa・mm 4 An automotive component according to claim 1 or 2, satisfying the following conditions.

4. The aforementioned steel material contains 0.02% by mass or more of Nb and Ti in total, as described in claim 1 or 2 for an automobile component.

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