Steel plate component

Abstract

A 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 lap fillet arc welded together, and which is provided with a weld toe portion 17a that is impacted using an impact pin 1, wherein the weld toe portion 17a has a minimum radius of curvature of 0.65 mm or less in a cross section parallel to the plate thickness direction of the steel plate 15 and a Vickers hardness of 220 Hv or more, and a metallographic structure of the weld toe portion 17a has a crystal structure including crystal grains in a body-centered cubic lattice or a body-centered tetragonal crystal lattice, and includes deformation twins having a thickness of 50 nm or less.

Description

Steel plate component

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

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

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

[0004] Further, Patent Document 2 discloses a technique of hitting the weld toe of a welding joint while rotating an impact pin with its tip curvature radius being 0.05 mm or more and less than 1.00 mm and offsetting the impact pin from its central axis. According to this technique, while preventing the occurrence of folding defects due to the impact, stress concentration is alleviated by alleviating the minute uneven shape, and at the same time, a large local compressive residual stress (compressive residual stress) is obtained on the surface of the processed part being hit, so that the fatigue strength is improved.

[0005] Furthermore, Non-Patent Document 1 discloses a technique for applying shot blasting to the arc welds of thin steel sheets for automobiles, with a thickness of approximately 2 mm. According to this technique, by projecting steel, which is the projectile material, onto the weld, compressive residual stress is applied, and as a result, the fatigue strength is improved.

[0006] Japanese Patent No. 6504326, International Publication No. 2020 / 183783

[0007] 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.

[0008] Compressive residual stress applied to the weld toe using impact pins or spray material can be released by repeated loading or heat. Therefore, in steel plate parts 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 the problem that if the spray material adhered to the weld and the next process such as painting was carried out, the paintability and rust prevention of the welded area would deteriorate. Therefore, there was a need for a steel plate part in which fatigue strength is improved by impact using impact pins, and in which the fatigue strength improvement effect 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 problems, and its objective is 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.

[0010] The steel plate component according to the present invention is provided with a concave portion formed by striking with a striking pin, wherein the 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, a Vickers hardness of 220 Hv or more, and the metallic structure of the concave portion has a crystalline structure containing body-centered cubic or body-centered tetragonal crystal grains, and includes deformed twins with a thickness of 50 nm or less.

[0011] The steel plate has an arc-welded joint, and the concave portion is preferably the weld toe of the arc-welded joint.

[0012] In this invention, the metal structure of the recessed portion struck with a striking 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 recessed portion and the grain refinement 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 grain refinement is maintained. In addition, in this invention, the yield strength and maximum tensile strength are also improved by the grain refinement in the struck recessed portion.

[0013] Figure 1 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, as an example of a steel plate component according to an embodiment of the present invention ((a) schematic diagram showing deformation twins that have appeared in the crystal grains of the metal structure at the weld toe, (b) transmission electron microscope photograph of the struck weld toe). Figure 2 illustrates an arc-welded joint according to an embodiment of the present invention ((a) cross-sectional view in the thickness direction of the arc-welded joint, (b) enlarged view of the weld toe in the arc-welded joint, (c) enlarged view of the tip of the striking pin). Figure 3 illustrates a specific example of a concave shape of a steel plate component according to the present invention ((a) cross-sectional view in the thickness direction of the buckling part formed on the inside of the bend R portion of the steel plate, (b) cross-sectional view in the thickness direction of the surface defect formed on the surface of the steel plate).

[0014] [Embodiment] The steel plate component according to this embodiment of the present invention is an arc-welded joint 11 formed by overlapping fillet arc welding of a steel plate 13 and a steel plate 15, as shown in Figure 2(a). The arc-welded joint 11 comprises a welded portion 17 that welds the steel plate 13 and the steel plate 15 together, and a weld toe 17a that is recessed in the thickness direction of the steel plate 15 and struck using a striking pin 1. The struck weld toe 17a corresponds to the struck recessed portion of the steel plate component according to the present invention. The arc-welded joint 11 according to this embodiment will be described below.

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

[0016] Furthermore, the microstructure of the struck weld toe 17a has a crystalline structure containing body-centered cubic or body-centered tetragonal crystal grains, and deformation twins with a thickness of 50 nm or less are present, as shown in Figure 1. Examples of microstructures with a body-centered cubic crystalline structure include martensitic steel, and examples of microstructures with a body-centered tetragonal crystalline structure include ferritic steel. The deformation twins in the microstructure of the struck weld toe 17a were formed by being struck by the striking pin 1, as shown in Figure 2(a), and their thickness is 50 nm or less, as shown in Figure 1(b). Therefore, the microstructure of the struck weld toe 17a has finer crystal grains than before it was struck.

[0017] Thus, in the arc-welded joint 11 according to this embodiment, the fatigue strength is improved at the struck weld toe 17a because, in addition to the application of compressive residual stress, deformation twins with a thickness of 50 nm or less appear, resulting in refinement of the crystal grains. Furthermore, the crystal structure of the refined crystal grains at the struck weld toe 17a does not change under usage conditions (repeated loading) or environmental conditions (temperatures below approximately 550°C in steel, where the diffusion of iron atoms begins) when the compressive residual stress applied by the impact is released. Therefore, in the arc-welded joint 11, the effect of improving fatigue strength due to the refinement of crystal grains at the struck weld toe 17a is maintained even under the aforementioned usage conditions and environmental conditions. In addition, in the arc-welded joint 11, the yield strength and maximum tensile strength of the struck weld toe 17a are also improved due to the effect of crystal grain refinement.

[0018] The above description assumed that the arc-welded joint 11 formed by arc welding steel plates 13 and 15 was a steel plate component, and the struck weld toe 17a was the struck concave portion of the steel plate component. However, the present invention is not limited to steel plate components having an arc-welded joint 11. Furthermore, the concave portion of the steel plate component is the portion that was struck with a striking pin against a portion that was concave in the thickness direction of the steel plate before being struck, and it is preferable that the struck portion is one in which stress is concentrated and fatigue failure is likely to occur when a load is applied to the steel plate component.

[0019] Specific examples of steel plate parts according to the present invention include those comprising the steel plate 21 shown in Figure 3(a) or the steel plate 31 shown in Figure 3(b). In the steel plate 21 shown in Figure 3(a), the surface of the inner side 23a of the bend buckles during the bending process of the bend R portion 23, and the buckled portion 25, which has a locally smaller radius of curvature and a concave shape in the thickness direction, is struck with an impact pin. In the steel plate 31 shown in Figure 3(b), a surface chip 33, which is a defect that occurred on the surface of the steel plate during the rolling process, is 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 metal structure of the struck buckled portions 25 and surface chips 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 face-centered cubic crystal structures, which are known to be prone to twin deformation due to their low stacking fault energy. In contrast, metal structures with body-centered cubic or body-centered tetragonal crystal structures were thought to be less susceptible to twin deformation due to their high stacking fault energy.

[0024] The technologies described in Patent Documents 1 and 2 and Non-Patent Document 1 above 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 having 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, the maintenance of this improvement in fatigue strength 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 should be set to be 1.3 times or less the minimum radius of curvature r0' of the weld toe 17a before impact. If an impact pin 1 is used with a tip radius r that is too large compared to the minimum radius of curvature r0' of the weld toe 17a before impact, a winding defect may occur in the impacted 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 impact is 0.50 mm or less, and the tip radius r of the impact pin 1 is 1.3 times or less the minimum radius of curvature r0' before impact, it is possible to induce deformation twinning in the impacted 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 dent may become a stress concentration source, 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] 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 of the striking pin 1 and the striking speed v. The striking speed v can be adjusted, for example, in the case of an electrically operated striking pin 1 using an electromagnetic induction coil, by the frequency driving the striking pin 1, the driving voltage, the stroke of the striking pin 1, or the number of turns of the electromagnetic induction coil. The striking speed of the striking pin 1 can be measured, for example, by photographing the striking pin while it is being driven. The striking pin 1 strikes the weld toe 17a by reciprocating in one direction (the axial direction of the striking pin 1). Therefore, the striking speed v of the striking 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, it is desirable for the kinetic energy K of the impact pin 1 to be large. 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 were 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 the above (1) to (3), 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 a 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)). Also, as shown in Figure 3(b), when striking a surface chip 33, the tip radius of the striking pin 1 is the radius of curvature of the tip portion 1a in a cross section in the thickness direction of the plate.

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

[0040] Factors that inhibit dislocation motion include the strength, strain rate, and strain amount during static deformation. 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 strain amount, 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 strain amount. Specifically, it is desirable to adjust the parameter A = H' × K × r0' / r to be 80 or greater.

[0041] This critical value of parameter A (= 80) is for martensitic steel with a block size of 5 to 30 μm. However, when the crystal grain size ρ before impact is significantly different from that of martensitic steel, it is advisable to determine the critical value of parameter A considering its influence. This is because it is known that the so-called Hall-Petch law, where slip deformation increases in proportion to the -1 / 2 power of the crystal grain size, holds. The Hall-Petch law regarding the occurrence of deformation twins has a greater influence of crystal grain size compared to slip deformation.

[0042] Also, when the above-described content is configured as a manufacturing method of a steel plate component, it is as follows. That is, the manufacturing method of the steel plate component is to manufacture by hitting a portion with a concave shape in the thickness direction of the steel plate in the steel plate component using a hitting pin. The portion with a concave shape has a minimum curvature radius of 0.50 mm or less and a Vickers hardness of 170 Hv or more in a cross-section parallel to the thickness direction of the steel plate, and its crystal structure includes crystal grains of a body-centered cubic lattice or a body-centered tetragonal crystal. By hitting the portion with a concave shape under the hitting conditions that the tip radius of the hitting pin is 1.3 times or less of the minimum curvature radius and the kinetic energy of the hitting pin is 0.375 mJ or more, a deformation twin with a thickness of 50 nm or less is generated in the portion to refine the crystal grains. In the manufacturing method of the steel plate component, the portion with a concave shape is in the state before being hit using the hitting pin.

[0043] However, the steel plate component according to the present invention is not limited to those manufactured by hitting a portion with a concave shape such as the welded end portion 17a etc. by the above manufacturing method. That is, for the steel plate component according to the present invention, it is sufficient that the concave portion hit using the hitting pin has a minimum curvature radius of 0.65 mm or less and a Vickers hardness of 220 Hv or more in a cross-section parallel to the thickness direction of the steel plate. In addition to these, the steel plate component according to the present invention may have a crystal structure including crystal grains of a body-centered cubic lattice or a body-centered tetragonal crystal in the metal structure of the hit concave portion and include deformation twins with a thickness of 50 nm or less.

[0044] An experiment was conducted to verify the operational effects of the present invention, and this will be described below.

[0045] In the experiment, as shown in Fig. 2, an arc welding joint 11 was fabricated in which two steel plates 13 and 15 were lap fillet welded by MAG (Metal Active Gas) welding. The steel plates 13 and 15 were hot-rolled steel plates with a tensile strength of either 540 MPa grade, 590 MPa grade or 780 MPa grade and a plate thickness of 3 mm, which were used as test materials. Also, in the fabricated arc welding joint 11, the minimum radius of curvature r0' of the weld termination part 17a before impact was 0.50 mm.

[0046] Next, using the impact pin 1 of an electric peening device, peening treatment was performed on the weld termination part 17a in the welded joint 17 where the steel plates 13 and 15 were welded together. In the peening treatment, the tip radius r and the impact speed v of the impact pin were variously changed, and the weld termination part 17a was impacted along the welding direction of the welded joint 17. The indentation density of impacting the weld termination part 17a 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 150 Hv, 170 Hv or 230 Hv by using hot-rolled steel plates with a tensile strength of 540 MPa grade, 590 MPa grade or 780 MPa grade for both of the steel plates 13 and 15. When the weld termination part 17a with a Vickers hardness H' of 150 Hv, 170 Hv or 230 Hv was impacted, the Vickers hardness H was 200 Hv, 220 H or 280 H, respectively.

[0048] Table 1 shows the Vickers hardness H of the impacted weld termination part 17a, the minimum radius of curvature r0 of the impacted weld termination part 17a, and the impact conditions by the impact pin (the tip radius r of the impact pin, the ratio to the minimum radius of curvature r0' before impact, the weight m, the impact speed v, the kinetic energy K, the parameter A). The parameter A is the one described in the above embodiment.

[0049]

[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 present 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 heat-treated at 500°C for 1 hour, and in which deformation twins with a thickness of 50 nm or less are present on the weld toe 17a.

[0051] In contrast, in Comparative Example 1, the Vickers hardness H of the struck weld toe 17a was 200 Hv, which is outside the scope of the present invention. In Comparative Example 2, the tip radius r of the striking pin 1 was 1.4 times the minimum radius of curvature r0' (= 0.50 mm) of the weld toe 17a before striking, which is also outside the scope of the present invention. In Comparative Example 3, the weld toe 17a was struck under conditions where the kinetic energy K of the striking pin 1 was unsuitable for the formation of deformation twins. Therefore, in all of the arc welded joints 11 according to Comparative Examples 1 to 3, no deformation twins were formed on the weld toe 17a.

[0052] Next, a plane bending fatigue test specimen was cut from the peened arc-welded joint 11, and a pulsating 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 applied at a repetition frequency of 10 Hz and a nominal stress of 500 MPa based on the plate thickness of the base material steel plates 13 and 15. A specimen was judged to have passed the fatigue test if it reached 300,000 load cycles without fracture occurring, and the test was terminated at 10 million cycles.

[0053] Table 1, shown above, shows the fatigue test results. Invention Examples 1 and 2 were judged to be acceptable because no fracture occurred at the weld toe 17a even after 10 million load cycles. Invention Example 2' was judged to be acceptable, although the number of cycles until fracture was 300,000, which was lower than that of Invention Example 2. The reason why the number of cycles until fracture was lower than that of Invention Example 2 is thought to be that the compressive residual stress applied to the weld toe 17a by impact was released by heat treatment, and the fatigue strength improvement effect due to compressive residual stress was no longer obtained. However, the crystal structure of the weld toe 17a in which deformation twinning occurred did not change even after heat treatment, so 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.

[0056] According to the present invention, it is possible 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.

[0057] 1 Impact pin 1a Tip 11 Arc welding joint 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 Buckled section 31 Steel plate 33 Surface chipping

Claims

1. A steel plate component having a concave portion formed by striking with a striking pin, wherein the 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, a Vickers hardness of 220 Hv or more, and 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 steel plate component according to claim 1, wherein the steel plate has an arc-welded joint, and the concave portion is the weld toe of the arc-welded joint.

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