Peening process

Abstract

This invention provides a peening treatment method that can maintain the effect of improving the fatigue strength of steel plate parts regardless of usage conditions or environment. [Solution] The peening method according to the present invention involves striking the weld toe 17a of a welded joint 11 with a striking pin 1, wherein the weld toe 17a 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 a crystalline structure containing body-centered cubic or body-centered tetragonal crystal grains, and the weld toe 17a is struck under the striking conditions that the tip radius of the striking pin 1 is 1.3 times or less of the minimum radius of curvature of the weld toe 17a, and the kinetic energy of the striking pin 1 is 0.375 mJ or more, thereby causing deformation twins with a thickness of 50 nm or less to appear on the weld toe 17a and refining the crystal grains.

Description

Technical Field

[0001] The present invention relates to a peening treatment method for improving the fatigue strength of steel plate parts.

Background Art

[0002] In a steel plate part, it is known that in an arc weld joint where a steel plate is arc welded, when a repeated load is applied, stress concentrates at the weld termination part, and fatigue fracture is likely to occur. Therefore, conventionally, technologies for improving the fatigue characteristics of arc weld joints have been proposed.

[0003] For example, Patent Document 1 discloses a technique of applying impact to the weld termination part of an overlapping fillet weld joint having a thin steel plate as a base material using an impact pin whose tip radius of curvature is equal to or less than the radius of curvature of the weld termination part. According to this technique, while applying compressive stress to the weld termination part by impact, the weld ripple formed at the weld termination part during welding is crushed and flattened, thereby relaxing stress concentration and preventing the occurrence of folding defects due to impact, so that the fatigue characteristics can be improved.

[0004] Also, Patent Document 2 discloses a technique of setting the radius of curvature of the tip of an impact pin to be 0.05 mm or more and less than 1.00 mm, and offsetting the impact pin from its central axis and rotating it to strike the weld termination part of the weld joint. According to this technique, while preventing the occurrence of folding defects due to impact and relaxing the minute uneven shape to relieve stress concentration, a large compressive residual stress is locally obtained on the surface of the processed part 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 welded part 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 welded part, 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] In conventional technologies such as Patent Documents 1 and 2 and Non-Patent Document 1, the effect of compressive residual stress imparted by impact pins or projectiles was dominant in improving fatigue strength. However, since compressive residual stress can be released by repeated loading or heat, fatigue strength may decrease depending on the usage conditions and environment, making it impossible to maintain the effect of improving fatigue strength, and thus posing a reliability issue. Furthermore, the method 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 step, such as painting, it would degrade the paintability and rust prevention properties of the weld.

[0009] The present invention was made to solve the above-mentioned problems, and aims to provide a peening treatment method 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 peening method according to the present invention involves striking a concave portion of a steel plate part using an impact pin, The concave 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. The present invention is characterized by striking the concave portion under 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 appear in the concave portion and refining the crystal grains.

[0011] (2) In the items described in (1) above, The steel plate component has an arc-welded joint formed by welding the steel plate, The concave portion is characterized by being the weld toe of the arc welded joint. [Effects of the Invention]

[0012] According to the present invention, by inducing deformation twins with a thickness of 50 nm or less in the concave portion of a steel plate component where stress tends to concentrate, the fatigue strength of the steel plate component can be improved by imparting compressive residual stress and refining the crystal grains in the crystal structure. Furthermore, since the crystal structure of the concave portion with refined crystal grains does not change even under usage conditions or environments where the compressive residual stress is released, the effect of improving fatigue strength through crystal grain refinement can be maintained. Furthermore, according to the present invention, the yield strength and maximum tensile strength can also be improved by refining the crystal grains in the concave portion. [Brief explanation of the drawing]

[0013] [Figure 1] This figure illustrates a peening treatment method according to an embodiment of the present invention ((a) cross-sectional view in the thickness direction of an arc welded joint, (b) enlarged view of the weld toe in an arc welded joint, (c) enlarged view of the tip of a striking pin). [Figure 2]This figure illustrates deformation twinning that occurs when the weld toe of an arc welded joint is struck with a striking pin ((a) schematic diagram showing deformation twinning that occurs in the crystal grains of the metal structure at the weld toe, (b) transmission microscope image of the struck weld toe). [Figure 3] This figure shows a specific example of a concave portion of a steel plate part targeted by the peening treatment method according to the present invention ((a) cross-sectional view in the thickness direction of a buckled portion formed on the inside of the bend radius of the steel plate, (b) cross-sectional view in the thickness direction of a surface chip formed on the surface of the steel plate). [Modes for carrying out the invention]

[0014] [Background leading to the invention] The inventors aimed to improve the fatigue strength of arc-welded joints made of steel plates by arc welding. To achieve this, they fabricated arc-welded joints with various striking conditions using a striking pin, and conducted fatigue tests. As a result, they found that in arc-welded joints struck at a higher striking speed than conventional joints, not only was the fatigue strength improved, but the improvement in fatigue strength compared to the fatigue strength before striking was maintained even in usage conditions and environments where the compressive residual stress applied by the striking was released. In particular, this maintenance of fatigue strength improvement was confirmed in arc-welded joints made of high-strength steel plates.

[0015] Therefore, in order to investigate the reason for this, the inventor observed the crystal structure of the weld toe after impact and discovered, as shown in Figure 2, that deformation twins with a thickness of 50 nm or less appeared at the weld toe, and that the crystal grains were refined. Furthermore, it was found that this crystal structure with refined crystal grains did not change even under usage conditions and environments in which the compressive residual stress applied by impact was released. Based on the above findings, the inventor hypothesized that by inducing deformation twinning at the weld toe and refining the crystal grains, it might be possible to maintain the fatigue strength improvement effect due to crystal grain refinement even under usage conditions and environments where compressive residual stress is released, and investigated specific methods to achieve this. The present invention has been made based on the results obtained by the study, and its specific configuration is as follows.

[0016] [Embodiment] As an example, the peening treatment method according to an embodiment of the present invention strikes a weld stop end portion 17a of a welded portion 17 in an arc weld joint 11 obtained by overlapping a steel plate 13 and a steel plate 15 and performing fillet arc welding, as shown in FIG. 1.

[0017] As shown in FIG. 1(b), the weld stop end portion 17a has a minimum curvature radius r0 of 0.50 mm or less and a Vickers hardness of 170 Hv or more in a cross section parallel to the plate thickness direction of the steel plate 15. The minimum curvature radius of the weld stop end portion 17a is the minimum of the curvature radii of the concave portions in a cross section parallel to the plate thickness direction of the steel plate 15, as shown in FIG. 1(b).

[0018] Further, the weld stop end portion 17a has a crystal structure including crystal grains of a body-centered cubic lattice or a body-centered tetragonal crystal structure. As the metal structure having a body-centered cubic lattice crystal structure, martensitic steel can be exemplified, and as the metal structure having a body-centered tetragonal crystal structure, ferritic steel can be exemplified.

[0019] In the present embodiment, the weld stop end portion 17a is struck under the striking conditions that the tip radius of the striking pin 1 is 1.3 times or less the minimum curvature radius of the weld stop end portion 17a and the kinetic energy of the striking pin 1 is 0.375 mJ.

[0020] As shown in FIG. 1(c), the tip radius r of the striking pin 1 is the curvature radius of the tip portion 1a in a cross section orthogonal to the welding direction of the welded portion 17 (the direction perpendicular to the paper surface in FIG. 1(a)). Further, the kinetic energy K of the striking pin 1 is given by K [mJ] = (1 / 2) × m × v, where m [g] is the mass of the striking pin 1 and v [m / s] is the striking speed of the striking pin 1. , ,

[0021] , , , 2 is given.

[0021] Then, by striking the weld toe 17a under the above striking conditions, deformation twins with a thickness of 50 nm or less are formed on the weld toe 17a, as shown in Figure 2, thereby refining the crystal grains.

[0022] Thus, according to the peening treatment method of this embodiment, the fatigue strength of the arc welded joint 11 can be improved not only by applying compressive residual stress through impact, but also by inducing deformation twinning and refining the crystal grains.

[0023] Furthermore, the refined crystal structure remains unchanged even under usage conditions (repeated loading) or environmental conditions (temperatures below approximately 550°C in steel, where iron atom diffusion begins) that release compressive residual stress. Therefore, according to the peening treatment method of this embodiment, the effect of improving fatigue strength due to refinement of crystal grains can be maintained even under the aforementioned usage conditions and environmental conditions. Furthermore, the grain refinement effect can also improve the yield strength and maximum tensile strength of the weld toe 17a struck with the impact pin 1.

[0024] The reason why the peening treatment method according to this embodiment can induce deformation twinning at the weld toe 17a and refine the crystal grains will be explained below.

[0025] 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.

[0026] 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.

[0027] The peening method according to this embodiment uses a striking pin 1 to strike the weld toe 17a at a higher speed than conventional methods, i.e., with higher kinetic energy, thereby generating high deformation stress and a high strain rate in the weld toe 17a, which makes it possible to induce deformation twinning in the weld toe 17a and refine the crystal grains.

[0028] This embodiment requires that the Vickers hardness of the weld toe 17a be 170 Hv or higher. This is because if the Vickers hardness of the weld toe 17a is low, the probability of deformation twinning occurring due to impact is low. Therefore, in order to make deformation twinning more likely to occur, it is desirable that the Vickers hardness of the weld toe 17a be 230 Hv or higher. When the Vickers hardness is 230 Hv or higher, it is desirable to also increase the kinetic energy of the striking pin 1. Specifically, for example, if the weight m of the striking pin 1 is 3 g, it is desirable to have a kinetic energy of 0.54 mJ or higher, which corresponds to a striking speed v = 0.6 m / s. Furthermore, the Vickers hardness H of the weld toe 17a can be considered equal to the Vickers hardness of the weld 17.

[0029] Furthermore, in this embodiment, the tip radius r of the impact pin 1 is 1.3 times or less the minimum radius of curvature r0 of the weld toe 17a. This is because if the tip radius r of the impact pin 1 is too large compared to the minimum radius of curvature r0 of the weld toe 17a, striking the weld toe 17a will result in a tangle defect. 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, 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.

[0030] The kinetic energy K of the impact pin 1 is set to 0.375 mJ or more, as described above, and is adjusted by the weight m of the impact pin 1 and the impact speed v. If the kinetic energy K of the impact pin 1 is less than 0.375 mJ, it is not possible to generate the strain necessary to cause deformation twinning at the weld toe 17a.

[0031] 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. Since the impact pin 1 strikes the weld toe 17a by reciprocating in one direction, the impact speed of the impact pin 1 is the maximum speed in the direction of reciprocating motion.

[0032] It is known that the occurrence of deformation twinning 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.

[0033] Factors that inhibit dislocation motion include the strength, strain rate, and strain amount during static deformation. Therefore, in order to inhibit dislocation motion and promote the formation of deformation twins when the weld toe 17a is struck, it is advisable 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 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.

[0034] 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.

[0035] The above description of the peening method according to this embodiment assumed that the arc-welded joint 11 formed by arc welding steel plates 13 and 15 was a steel plate component, and the weld toe 17a of the welded portion 17 was the concave portion of the steel plate component. However, the present invention is not limited to these steel plate components and concave portions. The concave portion targeted by the present invention is a shape that is concave in the thickness direction of the steel plate, and can be a part where stress concentrates and fatigue failure is likely to occur when a load is applied to the steel plate component.

[0036] Figure 3 shows a specific example of a concave portion of a steel plate component targeted by the present invention. Figure 3(a) shows a buckled portion 25 formed when the surface of the inner side 23a of a bent R portion 23 of a steel plate 21 buckles during bending, resulting in a locally smaller radius of curvature and a concave shape in the thickness direction of the plate. Figure 3(b) shows a surface defect 33 formed on the surface of the steel sheet 31. Here, an example of a surface defect 33 is a defect that occurred on the surface of the steel sheet during the rolling process.

[0037] The crystal structure and minimum radius of curvature of the buckled portion 25 and the surface chip 33 are the same as those of the weld toe 17a of the arc welded joint 11 described above. Furthermore, the tip radius of the striking pin 1 is the radius of curvature of the tip 1a in a cross section perpendicular to the valley line direction of the bend R portion 23 (the direction perpendicular to the plane of the paper in Figure 3(a)) when striking the buckled portion 25, and the radius of curvature of the tip 1a in a cross section in the thickness direction when striking the surface chip 33.

[0038] Then, by striking the buckled portion 25 or surface chip 33 with the striking pin 1 under the aforementioned striking conditions (tip radius and kinetic energy of the striking pin), deformation twins with a thickness of 50 nm or less can be generated, thereby refining the crystal grains. This improves the fatigue strength of steel plate parts having buckled portions 25 or surface chips 33, and maintains the effect of improving fatigue strength through crystal grain refinement even in usage conditions and environments where compressive residual stress is released. [Examples]

[0039] Experiments were conducted to confirm the effects of the present invention, and these are described below.

[0040] In this embodiment, as shown in Figure 1, the focus was on an arc-welded joint 11 formed by overlapping and 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. The weld toe 17a has a minimum radius of curvature r0 of 0.50 mm in a cross-section parallel to the thickness direction of the steel plate 15, and its microstructure has a crystalline structure containing body-centered cubic grains. Furthermore, the Vickers hardness H of the weld toe 17a (and weld 17) was set to 150Hv, 170Hv, or 230Hv. The Vickers hardness H was changed (set) to the above values ​​by using steel plates with tensile strengths of 540MPa, 590MPa, or 780MPa for both steel plates 13 and 15.

[0041] The experiment involved using an electric peening device to perform peening treatment on the weld toe 17a along the welding direction of the weld, while varying the Vickers hardness H of the weld toe 17a, the tip radius r of the impact pin 1, and the impact speed v of the impact pin 1, as shown in Table 1 below. In the peening treatment, the indentation density when striking the weld toe 17a along the welding direction was set to 10 indentations / mm, and the impact speed of the impact pin 1 was adjusted by changing the drive voltage of the electric peening device.

[0042] [Table 1]

[0043] In Table 1, Invention Examples 1 and 2 are in which the Vickers hardness H of the weld toe 17a, the minimum radius of curvature r0 of the weld toe 17a, the tip radius r of the impact pin 1, the weight m, and the kinetic energy K are all within the scope of the present invention. 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. In contrast, Comparative Examples 1 to 3 have Vickers hardness H of the weld toe 17a, tip radius r of the impact pin, or kinetic energy K of the impact pin 1 outside the range of the present invention.

[0044] After peening, the metal structure at the weld toe 17a was observed using a transmission electron microscope to confirm the presence or absence of deformation twins.

[0045] As shown in Table 1, deformation twins with a thickness of 50 nm or less were observed at the weld toe 17a in Invention Example 1 and Invention Example 2. The reason why the value of parameter A was set to a different value in Invention Example 1 and Invention Example 2 is that as the Vickers hardness H of the weld toe 17a increases, the stress accumulated at the grain boundaries, which is the driving force for the appearance of deformation twins, increases. Furthermore, in Invention Example 2', where heat treatment was performed after peening, deformation twins were also observed at the weld toe 17a. From the results of Invention Example 2 and Invention Example 2', it was confirmed that the deformation twins that appeared at the weld toe 17a due to impact do not change with heat treatment. In contrast, no deformation twinning was observed at the weld toe 17a in Comparative Examples 1 to 3.

[0046] 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.

[0047] 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 by impact was released by the heat treatment, and the fatigue strength improvement effect due to compressive residual stress was no longer obtained. However, since the crystal structure in which deformation twinning occurred did not change even after heat treatment, the fatigue strength improvement effect due to grain refinement was maintained.

[0048] 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.

[0049] In summary, the present invention demonstrates that it is possible to refine the crystal grains by inducing deformation twins with a thickness of 50 nm or less at the weld toe of an arc-welded joint. Furthermore, the present invention demonstrates that even when heat treatment is performed at a temperature at which compressive residual stress is released, no change is observed in the crystal structure in which deformation twins have been expressed, and the effect of improving fatigue strength through crystal grain refinement can be maintained. [Explanation of symbols]

[0050] 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 peening method for striking a concave portion of a steel plate part using a striking pin, The concave 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. A peening method characterized by striking the concave portion under 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 appear in the concave portion and refining the crystal grains.

2. The steel plate component has an arc-welded joint formed by welding the steel plate, The peening method according to claim 1, characterized in that the concave portion is the weld toe of the arc welded joint.

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