Steel plate parts

Abstract

To provide a steel plate component that exhibits improved fatigue strength and maintains that improved fatigue strength effect regardless of usage conditions or environment. [Solution] The steel plate component according to the present invention is an arc-welded joint 11 in which a steel plate 13 and a steel plate 15 are overlapped and fillet-claw-welded, and a weld toe 17a is struck using a striking pin 1, wherein the weld toe 17a has a minimum radius of curvature of 0.65 mm or less in a cross section parallel to the thickness direction of the steel plate 15, a Vickers hardness of 220 Hv or more, and the metal structure of the weld toe 17a has a crystal structure containing body-centered cubic or body-centered tetragonal crystal grains, and contains deformation twins with a thickness of 50 nm or less.

Description

Technical Field

[0001] The present invention relates to a steel plate component with improved fatigue strength.

Background Art

[0002] It is known that in a steel plate component having an arc weld joint of a steel plate, when a repeated load is applied, stress concentration occurs at the weld termination, and fatigue fracture is likely to occur. Therefore, conventionally, techniques for improving the fatigue characteristics of arc weld joints have been proposed.

[0003] For example, Patent Document 1 discloses a technique of applying an impact to the weld termination of an overlapping fillet weld joint using a striking pin whose tip radius of curvature is equal to or less than the radius of curvature of the weld termination of a thin steel plate as the base material. This technique flattens the weld ripple formed at the weld termination during welding while applying a compressive stress to the weld termination by impact. Therefore, according to this technique, it is said that stress concentration at the weld termination when a load is applied to the arc weld joint can be alleviated, the occurrence of fold flaws due to impact can be prevented, and the fatigue characteristics can be improved.

[0004] Further, Patent Document 2 discloses a technique of striking the weld termination of a weld joint while rotating the striking pin offset from its central axis with the tip radius of curvature of the striking pin being 0.05 mm or more and less than 1.00 mm. According to this technique, stress concentration is alleviated by preventing the occurrence of fold flaws due to impact and alleviating the minute uneven shape, and at the same time, a large local compressive residual stress is obtained on the surface of the processed site being struck, so that the fatigue strength is improved.

[0005] Furthermore, Non-Patent Document 1 discloses a technique of applying shot blasting to the arc weld portion of an automotive thin steel plate having a thickness of about 2 mm. According to this technique, compressive residual stress is applied by projecting steel, which is a projection material, onto the weld portion, and the fatigue strength is improved by the influence thereof.

Prior Art Documents

Patent Documents

[0006] [Patent Document 1] Patent No. 6504326 [Patent Document 2] International Publication No. WO2020 / 183783 [Non-patent literature]

[0007] [Non-Patent Document 1] Hiroki Fujimoto, Koji Akioka, and Hitoshi Tokunaga, "Improvement of post-painting corrosion resistance of arc-welded sections of thin steel sheets for automobiles by shot blasting," Nippon Steel & Sumitomo Metal Technical Report, Vol. 409, 2017, pp. 90-95. [Overview of the project] [Problems that the invention aims to solve]

[0008] Compressive residual stress applied to the weld toe using impact pins or projectiles can be released by repeated loading or heat. Therefore, in steel plate components where the weld toe of an arc-welded joint has been impacted, the compressive residual stress may be released depending on the usage conditions and environment, and the fatigue strength improvement effect may not be maintained. Furthermore, the technology described in Non-Patent Document 1 had a problem in that if the projectile material remained attached to the weld and the process proceeded to the next stage, such as painting, the paintability and rust prevention properties of the welded area would deteriorate. Therefore, there was a need for steel plate components in which fatigue strength is improved by being struck with impact pins, and in which the effect of improving fatigue strength is maintained even under usage conditions and environments in which compressive residual stress is released.

[0009] The present invention was made to solve the above-mentioned problems, and aims to provide a steel plate component that can maintain the fatigue strength improvement effect even in usage conditions and environments in which compressive residual stress is released. [Means for solving the problem]

[0010] (1) The steel plate component according to the present invention has a concave portion that is struck using a striking pin, The aforementioned concave portion has a minimum radius of curvature of 0.65 mm or less in a cross-section parallel to the thickness direction of the steel plate, and a Vickers hardness of 220 Hv or more. The metal structure of the concave portion is characterized by having a crystalline structure containing body-centered cubic or body-centered tetragonal crystal grains, and containing deformation twins with a thickness of 50 nm or less.

[0011] (2) In the items described in (1) above, The aforementioned steel plates are welded together, The concave portion is characterized in that it is the weld toe of the arc welded joint. [Effects of the Invention]

[0012] In this invention, the metal structure of the concave portion struck with the impact pin contains deformation twins with a thickness of 50 nm or less. Therefore, the fatigue strength is improved by the compressive residual stress applied to the struck concave portion and the refinement of crystal grains due to the appearance of deformation twins. Furthermore, even in usage conditions and environments where the compressive residual stress is released, the effect of improving fatigue strength due to crystal grain refinement is maintained. Furthermore, in this invention, the yield strength and maximum tensile strength are also improved by refining the crystal grains in the impacted concave portion. [Brief explanation of the drawing]

[0013] [Figure 1] As an example of a steel plate component according to an embodiment of the present invention, this figure illustrates deformation twins that have appeared at the weld toe after being struck with a striking pin in an arc-welded joint where steel plates are joined by arc welding ((a) schematic diagram showing deformation twins that have appeared in the crystal grains of the metal structure at the weld toe, (b) transmission microscope image of the struck weld toe). [Figure 2] This figure illustrates an arc-welded joint according to an embodiment of the present invention ((a) cross-sectional view of the arc-welded joint in the thickness direction, (b) enlarged view of the weld toe in the arc-welded joint, (c) enlarged view of the tip of the impact pin). [Figure 3] The figure which shows the specific example of the concave-shaped part of the steel plate component which concerns on this invention ((a) The cross-sectional view in the plate thickness direction of the buckling part formed in the bending inner side of the bending R part of the steel plate, (b) The cross-sectional view in the plate thickness direction of the surface chipping formed on the surface of the steel plate).

Mode for carrying out the invention

[0014] [Embodiment] As shown in Fig. 2(a), the steel plate component according to the embodiment of the present invention is an arc welding joint 11 in which a steel plate 13 and a steel plate 15 are overlapped and fillet arc welded. And the arc welding joint 11 includes a welding part 17 that welds and joins the steel plate 13 and the steel plate 15, and a welded end part 17a that is recessed in the plate thickness direction of the steel plate 15 and is struck with a striking pin 1. Here, the struck welded end part 17a corresponds to the struck concave-shaped part of the steel plate component according to the present invention. Hereinafter, the arc welding joint 11 according to the present embodiment will be described.

[0015] In the arc welding joint 11, as shown in Fig. 2(b), the minimum radius of curvature r0 in the cross section parallel to the plate thickness direction of the steel plate 15 of the struck welded end part 17a is 0.65 mm or less, and the Vickers hardness H is 220 Hv or more. The minimum radius of curvature r0 of the struck welded end part 17a is the minimum of the radii of curvature of the welded end part 17a in the cross section parallel to the plate thickness direction of the steel plate 15. The upper limit value (= 0.65 mm) of the minimum radius of curvature r0 and the lower limit value (220 Hv) of the Vickers hardness H of the struck welded end part 17a will be described later.

[0016] Furthermore, the metal structure of the struck welded end part 17a has a crystal structure including crystal grains of a body-centered cubic lattice or a body-centered tetragonal crystal, and as shown in Fig. 1, deformation twins with a thickness of 50 nm or less are developed. Examples of the metal structure having a body-centered cubic lattice crystal structure include martensitic steel, and examples of the metal structure having a body-centered tetragonal crystal structure include ferritic steel. The deformation twins in the metal structure of the struck weld termination part 17a were developed by being struck by the striking pin 1 as shown in Fig. 2(a), and the thickness thereof is 50 nm or less as shown in Fig. 1(b). Therefore, the metal structure in the struck weld termination part 17a has finer crystal grains than before being struck.

[0017] Thus, in the arc weld joint 11 according to the present embodiment, in the struck weld termination part 17a, in addition to being applied with compressive residual stress, deformation twins with a thickness of 50 nm or less are developed and the crystal grains are refined, so that the fatigue strength is improved. And the crystal structure with refined crystal grains in the struck weld termination part 17a does not change in the use state (application of repeated load) or environment (temperature of about 550°C or lower at which diffusion of iron atoms starts in steel) in which the compressive residual stress applied by the striking is released. Therefore, in the arc weld joint 11, even in the above use state and environment, the effect of improving the fatigue strength due to the refinement of the crystal grains in the struck weld termination part 17a is maintained. Also, in the arc weld joint 11, the struck weld termination part 17a has an improved yield strength and maximum tensile strength due to the effect of refinement of crystal grains.

[0018] The above description was for the case where the arc weld joint 11 in which the steel plates 13 and 15 are arc welded is used as a steel plate component, and the struck weld termination part 17a is used as the struck concave-shaped part of the steel plate component. However, the present invention is not limited to the steel plate component having the arc weld joint 11. Further, the concave-shaped part of the steel plate component is a part struck by a striking pin against a part recessed in the plate thickness direction of the steel plate before being struck, and in particular, it is preferable that a part where stress concentration occurs and fatigue fracture is likely to occur when a load is applied to the steel plate component is struck.

[0019] As a specific example of the steel plate component according to the present invention, there is an example including the steel plate 21 shown in Fig. 3(a) or the steel plate 31 shown in Fig. 3(b). The steel plate 21 shown in Figure 3(a) is formed when the surface of the inner side 23a of the bend buckles during the bending process of the bend radius 23, and the buckled portion 25, which has a locally smaller radius of curvature and is concave in the thickness direction, is struck with an impact pin. The steel plate 31 shown in Figure 3(b) is a surface chip 33, which is a defect that occurred on the surface of the steel plate during the rolling process, and has been struck with an impact pin.

[0020] These struck buckled portions 25 and surface chips 33, like the struck weld toe 17a in the arc welded joint 11 described above, have a minimum radius of curvature of 0.65 mm or less in a cross section parallel to the thickness direction of the steel plates 21 and 31, and a Vickers hardness of 220 Hv or more. Furthermore, the metallic structure of the struck buckled portion 25 and the surface chipped portion 33 has a crystalline structure containing body-centered cubic or body-centered tetragonal crystal grains and includes deformation twins with a thickness of 50 nm or less.

[0021] In steel plate components equipped with such steel plates 21 and 31, fatigue strength is improved by the application of compressive residual stress to the struck buckled portion 25 and surface chipped portion 33, and by the refinement of the crystal grains. Furthermore, even in usage conditions and environments where the compressive residual stress in the struck buckled portion 25 and surface chipped portion 33 is released, the effect of improving fatigue strength through the refinement of the crystal grains is maintained.

[0022] Next, regarding the means by which the metal structure of the concave portion struck with a striking pin in the steel plate part according to the present invention contains deformation twins with a thickness of 50 nm or less, we will explain using the case in which the weld toe 17a of the arc welded joint 11 is struck with a striking pin 1, as shown in Figure 2.

[0023] While grain refinement due to deformation twinning has been a well-known phenomenon, most reports have focused on the microstructure of face-centered cubic crystals, which are known to be prone to twinning deformation due to their low stacking fault energy. In contrast, it was thought that microstructures with body-centered cubic or body-centered tetragonal crystal structures were less susceptible to twinning deformation due to their high stacking fault energy.

[0024] The methods described in Patent Documents 1 and 2, and Non-Patent Document 1, apply compressive residual stress using impact pins or projectiles, but they were unable to induce deformation twinning and refine the crystal grains in metal structures with a body-centered cubic or body-centered tetragonal crystal structure. This is because conventional peening methods such as those described in Patent Documents 1 and 2, which use impact pins, cannot induce twinning deformation due to insufficient impact speed from the impact pins, resulting in a limited effect on crystal grain refinement. Furthermore, conventional shot blasting methods such as those described in Non-Patent Document 1 cannot secure the kinetic energy necessary to induce twinning deformation because the mass of the projectile is smaller than that of the impact pins.

[0025] The inventors aimed to improve the fatigue strength of arc-welded joints 11 and conducted fatigue tests on the arc-welded joints 11 by changing various striking conditions when striking the weld toe 17a with a striking pin 1, as shown in Figure 2(a). As a result, they found that in arc-welded joints 11 with weld toe 17a struck at a higher striking speed than conventional joints, not only was the fatigue strength improved, but the improvement in fatigue strength was maintained even in usage conditions and environments where the compressive residual stress applied by the striking was released. In particular, this maintenance of the fatigue strength improvement effect was confirmed in arc-welded joints made of high-strength steel plates.

[0026] Therefore, in order to investigate the reason for this, the inventor observed the crystal structure of the struck weld toe 17a and discovered, as shown in Figure 1, that deformation twins with a thickness of 50 nm or less had appeared and the crystal grains had been refined.

[0027] Based on the above findings, the inventors investigated a specific method for refining the crystal grains by inducing deformation twins in the weld toe 17a struck with the impact pin 1. As a result, they found that when the following conditions (1) to (3) are met, deformation twins with a thickness of 50 nm or less are introduced in the struck weld toe 17a, and the crystal grains are refined compared to the metal structure of the weld toe 17a before impact.

[0028] (1) The weld toe 17a before impact has a minimum radius of curvature r0' of 0.50 mm or less in a cross section parallel to the thickness direction of the steel plate 15, a Vickers hardness H' of 170 Hv or more, and its microstructure has a crystalline structure containing body-centered cubic or body-centered tetragonal crystal grains.

[0029] The reason why the Vickers hardness H' of the weld toe 17a before impact should be 170 Hv or higher is that a lower Vickers hardness H' reduces the probability of deformation twinning occurring due to impact. A Vickers hardness H' of the weld toe 17a before impact of 230 Hv or higher is desirable because it makes deformation twinning more likely to occur.

[0030] (2) The tip radius r of the impact pin 1 shall be set to be 1.3 times or less the minimum radius of curvature r0' of the weld toe 17a before impact. If a striking pin 1 is used whose tip radius r is too large compared to the minimum radius of curvature r0' of the weld toe 17a before striking, a winding defect may occur in the struck weld toe 17a, which can lead to a decrease in fatigue strength. If the minimum radius of curvature r0' of the weld toe 17a before striking is 0.50 mm or less, and the tip radius r of the striking pin 1 is 1.3 times or less of the minimum radius of curvature r0' before striking, it is possible to induce deformation twinning in the struck weld toe 17a.

[0031] Furthermore, the upper limit of the minimum radius of curvature r0 of the weld toe 17a after impact, which is 0.65 mm, is the value obtained when impacting with an impact pin 1 having a tip radius r that is 1.3 times the upper limit of the minimum radius of curvature r0' of the weld toe 17a before impact (= 0.50 mm).

[0032] Furthermore, it is preferable to make the tip radius r of the impact pin 1 smaller than the minimum radius of curvature r0' of the weld toe 17a before impact, as this increases the amount of strain. However, if the tip radius r is too small, the impact mark may become a source of stress concentration, potentially reducing the fatigue strength. Therefore, it is desirable that the tip radius r of the impact pin 1 be at least 1 / 5 of the minimum radius of curvature r0' of the weld toe 17a before impact.

[0033] Furthermore, when striking the arc-welded joint 11 shown in Figure 2(a), the tip radius r of the striking pin 1 is the radius of curvature of the tip portion 1a in a cross section perpendicular to the welding direction of the welded portion 17 (the direction perpendicular to the plane of the paper in Figure 2(a)), as shown in Figure 2(c).

[0034] (3) The kinetic energy K of the striking pin 1 that strikes the weld toe 17a shall be 0.375 mJ or more. If the kinetic energy K of the striking pin 1 is less than 0.375 mJ, strain will not be generated in the struck weld toe 17a for deformation twinning to occur.

[0035] The kinetic energy K of the striking pin 1 can be adjusted by the weight m and striking speed v of the striking pin 1. The impact speed v can be adjusted, for example, in the case of an electrically operated impact pin 1 using an electromagnetic induction coil, by the frequency driving the impact pin 1, the driving voltage, the stroke of the impact pin 1, or the number of turns of the electromagnetic induction coil. The impact speed of the impact pin 1 can be measured, for example, by photographing the driving impact pin. The impact pin 1 strikes the weld toe 17a by reciprocating in one direction (the axial direction of the impact pin 1). Therefore, the impact speed v of the impact pin 1 is set to the maximum speed in the direction of reciprocating motion.

[0036] As mentioned above, if the Vickers hardness H' of the weld toe 17a before impact is 230 Hv or higher, a larger kinetic energy K of the impact pin 1 is desirable. Specifically, the kinetic energy K of the impact pin 1 should be 0.54 mJ or higher, corresponding to an impact speed v = 0.6 m / s, for example, if the weight m of the impact pin 1 is 3 g.

[0037] The above (1) to (3) concerning specific methods for refining crystal grains by inducing deformation twinning was applied to the weld toe 17a of the arc welded joint 11 shown in Figure 2. However, even if the concave portion of the steel plate part is a buckled portion 25 or surface chip 33 as shown in Figure 3, by striking it in a manner that satisfies (1) to (3) above, deformation twinning can be induced in the struck buckled portion 25 or surface chip 33, thereby refining the crystal grains.

[0038] As shown in Figure 3(a), when striking the buckled portion 25 formed on the inner side 23a of the bent R portion 23, the tip radius of the striking pin 1 is the radius of curvature of the tip portion 1a in a cross section perpendicular to the valley line direction of the bent R portion 23 (the direction perpendicular to the plane of the paper in Figure 3(a)). Furthermore, as shown in Figure 3(b), when striking a surface chip 33, the tip radius of the striking pin 1 is set to the radius of curvature of the tip portion 1a in the cross-section in the thickness direction of the plate.

[0039] It is known that the occurrence of deformation twinning in metallic structures is determined by the competitive relationship with slip deformation caused by dislocation movement in the crystal structure. In other words, the occurrence of deformation twinning can be promoted by inhibiting dislocation movement at the struck weld toe 17a.

[0040] Factors that inhibit dislocation motion include the strength during static deformation, the strain rate, and the amount of strain. Therefore, in order for dislocation motion to be inhibited and deformation twinning to be promoted at the struck weld toe 17a, it is desirable to appropriately adjust the Vickers hardness H'[Hv] which reflects the strength during static deformation, the kinetic energy K[mJ] of the impact pin 1 which is related to the strain rate and amount of strain, and the ratio of the minimum radius of curvature r0'[mm] of the weld toe 17a before impact to the tip radius r[mm] of the impact pin which is related to the amount of strain. Specifically, it is desirable to adjust the parameter A = H' × K × r0' / r to be 80 or greater.

[0041] This critical value (=80) for parameter A is for martensitic steel with a block grain size of 5 to 30 μm. However, if the grain size ρ before impact differs significantly from the grain size of the martensitic steel, it is advisable to determine the critical value of parameter A while taking that influence into consideration. This is because, although the so-called Hall-Petch rule, which states that slip deformation increases in proportion to the -1 / 2 power of the grain size, is known to hold true, the influence of grain size is greater on the Hall-Petch rule concerning the appearance of deformation twins compared to slip deformation.

[0042] Furthermore, if the above-mentioned method is implemented as a method for manufacturing steel plate parts, it will be as follows. In other words, the manufacturing method for steel plate parts is This method involves using an impact pin to strike a recessed portion of a steel plate component in the thickness direction of the steel plate. The recessed portion has a minimum radius of curvature of 0.50 mm or less in a cross-section parallel to the thickness direction of the steel plate, a Vickers hardness of 170 Hv or more, and its microstructure has a crystalline structure containing body-centered cubic or body-centered tetragonal crystal grains. This method involves striking a recessed area with striking conditions where the tip radius of the striking pin is 1.3 times or less the minimum radius of curvature, and the kinetic energy of the striking pin is 0.375 mJ or more, thereby causing deformation twins with a thickness of 50 nm or less to form in that area, and thus refining the crystal grains. In the manufacturing method of the steel plate part, the recessed portion represents the state before it is struck with the impact pin.

[0043] However, the steel plate component according to the present invention is not limited to those manufactured by striking a recessed portion such as the weld toe 17a using the above manufacturing method. That is, the steel plate component according to the present invention is sufficient if the recessed portion struck with the striking pin has a minimum radius of curvature of 0.65 mm or less in a cross section parallel to the thickness direction of the steel plate, and a Vickers hardness of 220 Hv or more. In addition to these, the steel plate component according to the present invention is sufficient if the metal structure of the struck recessed portion has a crystal structure containing body-centered cubic or body-centered tetragonal crystal grains, and contains deformation twins with a thickness of 50 nm or less. [Examples]

[0044] Experiments were conducted to verify the effects of the present invention, and these are described below.

[0045] In the experiment, as shown in Figure 2, an arc-welded joint 11 was fabricated by overlapping fillet welding two steel plates 13 and 15 using MAG welding. Steel plates 13 and 15 were hot-rolled steel plates with a tensile strength of 540 MPa, 590 MPa, or 780 MPa, and a thickness of 3 mm. Furthermore, in the fabricated arc-welded joint 11, the minimum radius of curvature r0' of the weld toe 17a before impact was 0.50 mm.

[0046] Next, the weld toe 17a of the weld joint 17, which welds steel plates 13 and 15 together, was peened using the impact pin 1 of an electric peening device. During the peening process, the tip radius r and impact speed v of the impact pin were varied, and the weld toe 17a was struck along the welding direction of the weld joint 17. Here, the indentation density of the weld toe 17a struck along the welding direction was set to 10 indentations / mm, and the impact speed v of the impact pin 1 was adjusted by changing the drive voltage of the electric peening device.

[0047] The Vickers hardness H' of the welded joint 17 was changed (set) to 150Hv, 170Hv, or 230Hv by using hot-rolled steel plates with tensile strengths of 540MPa, 590MPa, or 780MPa for both steel plates 13 and 15. When the weld toe 17a with a Vickers hardness H' of 150Hv, 170Hv, or 230Hv was struck, the Vickers hardness H was 200Hv, 220H, or 280Hv, respectively.

[0048] Table 1 shows the Vickers hardness H of the struck weld toe 17a, the minimum radius of curvature r0 of the struck weld toe 17a, and the striking conditions with the striking pin (tip radius r of the striking pin, ratio to the minimum radius of curvature r0' before striking, weight m, striking speed v, kinetic energy K, parameter A). Parameter A is the same as described in the above embodiment.

[0049] [Table 1]

[0050] In Table 1, Invention Example 1 and Invention Example 2 are arc welded joints 11 in which the Vickers hardness H of the struck weld toe 17a is 220 Hv or 280 Hv, and deformation twins with a thickness of 50 nm or less are exhibited on the struck weld toe 17a. Invention Example 2' is an arc-welded joint 11 that was manufactured and peened under the same conditions as Invention Example 2, and then subjected to heat treatment at 500°C for 1 hour, resulting in an arc-welded joint 11 in which deformation twins with a thickness of 50 nm or less have appeared at the weld toe 17a.

[0051] In contrast, Comparative Example 1 has a Vickers hardness H of 200 Hv at the struck weld toe 17a, which is outside the scope of the present invention. In Comparative Example 2, the tip radius r of the impact pin 1 is 1.4 times the minimum radius of curvature r0' (=0.50 mm) of the weld toe 17a before impact, which falls outside the scope of the present invention. Comparative Example 3 involves striking the weld toe 17a under conditions where the kinetic energy K of the impact pin 1 is unsuitable for the formation of deformation twins. Therefore, in all of the arc-welded joints 11 according to Comparative Examples 1 to 3, deformation twinning did not occur at the weld toe 17a.

[0052] Next, a planar bending fatigue test specimen was cut from the peened arc-welded joint 11, and a unidirectional fatigue test was performed under the condition that the weld toe 17a was on the tensile load side. The fatigue test was performed at room temperature, and the repeated load was set to a nominal stress of 500 MPa based on the plate thickness of the base material steel plates 13 and 15, with a repetition frequency of 10 Hz. A specimen was judged to have passed if it reached 300,000 load cycles without fracture in the planar bending fatigue test specimen, and the test was terminated at 10 million cycles.

[0053] The fatigue test results are shown in Table 1 above. Inventions 1 and 2 were deemed acceptable because no fracture occurred at the weld toe 17a even after 10 million load cycles. Furthermore, Invention Example 2' was judged acceptable despite a lower number of cycles to fracture (300,000) compared to Invention Example 2. The reason for the lower number of cycles to fracture compared to Invention Example 2 is thought to be that the compressive residual stress applied to the weld toe 17a by impact was released by the heat treatment, and the fatigue strength improvement effect due to the compressive residual stress was no longer obtained. However, since the crystal structure of the weld toe 17a where deformation twinning occurred did not change even after heat treatment, the fatigue strength improvement effect due to grain refinement was maintained.

[0054] Comparative Example 1, Comparative Example 2, and Comparative Example 3 all failed to pass testing, with the number of cycles required to break being 130,000, 110,000, and 70,000 cycles, respectively.

[0055] In summary, the present invention demonstrates that the fatigue strength of an arc-welded joint struck with a striking pin is improved by the introduction of deformation twins with a thickness of 50 nm or less in the metal structure of the weld toe, thereby imparting compressive residual stress and refining the crystal grains. Furthermore, it was shown that the effect of improving fatigue strength through crystal grain refinement is maintained even when heat treatment is performed at a temperature at which the compressive residual stress applied to the weld toe 17a is released. [Explanation of Symbols]

[0056] 1 striking pin 1a Tip 11 Arc Welded Joints 13 Steel plate 15 Steel plate 17 Welded section 17a Weld toe 21 Steel plate 23. Bending radius section 23a Inside of the bend 25 Buckling part 31 Steel plate 33. Surface chipping

Claims

1. A steel plate component having a concave shape formed by striking with a striking pin, The aforementioned concave portion has a minimum radius of curvature of 0.65 mm or less in a cross-section parallel to the thickness direction of the steel plate, and a Vickers hardness of 220 Hv or more. The steel plate component is characterized in that the metal structure of the concave portion has a crystalline structure containing body-centered cubic or body-centered tetragonal crystal grains and contains deformation twins with a thickness of 50 nm or less.

2. The aforementioned steel plates are welded together, The steel plate component according to claim 1, characterized in that the concave portion is the weld toe of the arc welded joint.

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