Peening method

Abstract

A peening method according to the present invention uses an impact pin 1 to strike a weld toe 17a of a weld joint 11, wherein: the weld toe 17a has a minimum curvature radius of not more than 0.50 mm in a cross-section parallel to the thickness direction of a steel sheet and has a Vickers hardness of not less than 170 Hv; the metal structure thereof has a crystal structure including body-centered cubic or body-centered tetragonal crystal grains; the tip-end radius of the impact pin 1 is not more than 1.3 times the minimum curvature radius of the weld toe 17a; and the weld toe 17a is struck under impact conditions in which the kinetic energy of the impact pin 1 is not less than 0.375 mJ so as to create deformation twinning with a thickness of not more than 50 nm in the weld toe 17a and miniaturize the crystal grains.

Description

Peening Treatment Method

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

[0002] In steel plate parts having an arc welded joint where the steel plate is arc welded, it is known that when a repeated load is applied, stress concentrates at the weld termination, and fatigue fracture is likely to occur. Therefore, conventionally, techniques for improving the fatigue characteristics of arc welded 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 weld joint using a thin steel plate as a base material with an impact pin having a tip curvature radius equal to or less than the curvature radius of the weld toe. According to this technique, while applying a compressive stress to the weld toe by the impact, the weld ripple formed at the weld toe during welding is crushed and flattened, thereby relaxing stress concentration and preventing the occurrence of folding defects due to the impact, and thus the fatigue characteristics can be improved.

[0004] Further, Patent Document 2 discloses a technique of setting the curvature radius of the tip of the 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 while hitting the weld toe of the weld joint. According to this technique, while preventing the occurrence of folding defects due to the impact and relaxing the minute uneven shape to relieve stress concentration, a large local compressive residual stress is obtained on the surface of the processed part being hit, and thus 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] Conventional technologies, such as those described in Patent Documents 1 and 2 and Non-Patent Document 1, primarily rely on the compressive residual stress imparted by impact pins and projectiles to improve 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 difficult to maintain the fatigue strength improvement effect and raising reliability issues. Furthermore, the technology described in Non-Patent Document 1 had the problem that if the projectile adhered to the weld and the next process, such as painting, was carried out, it would degrade the paintability and rust prevention properties of the weld.

[0009] The present invention was made to solve the above problems, and its objective is 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.

[0010] The peening method according to the present invention involves striking a concave portion of a steel plate part using a striking pin, wherein 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 a crystalline structure containing body-centered cubic or body-centered tetragonal crystal grains, and the concave portion is struck under striking conditions where the tip radius of the striking pin is 1.3 times or less of the minimum radius of curvature and the kinetic energy of the striking pin is 0.375 mJ or more, thereby causing deformed twins with a thickness of 50 nm or less to appear in the concave portion and refining the crystal grains.

[0011] The steel plate component has an arc-welded joint formed by welding the steel plate, and the concave portion may serve as the weld toe of the arc-welded joint.

[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 due to crystal grain refinement can be maintained. In addition, according to the present invention, yield strength and maximum tensile strength can also be improved by refining the crystal grains in the concave portion.

[0013] Figure 1 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 the striking pin). Figure 2 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 electron microscope photograph of the struck weld toe). Figure 3 illustrates a specific example of a concave shape 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 buckling part formed on the inside of the bend R portion of a steel plate, (b) cross-sectional view in the thickness direction of a surface defect formed on the surface of a steel plate).

[0014] [Background to the Invention] The inventor aimed to improve the fatigue strength of the weld toe of arc-welded joints made of steel plates. To achieve this, the inventor fabricated arc-welded joints with various changes in the striking conditions using a striking pin to strike the weld toe, and conducted fatigue tests. As a result, it was found that in weld toes struck at a higher striking speed than conventional methods, 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 sheets.

[0015] Therefore, in order to investigate the reason for this, the inventor observed the crystal structure of the weld toe after impact and discovered that, as shown in Figure 2, deformation twins with a thickness of 50 nm or less appeared at the weld toe, and 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 these findings, the inventor considered that by inducing deformation twins at the weld toe and refining the crystal grains, the effect of improving fatigue strength due to crystal grain refinement could be maintained even under usage conditions and environments in which the compressive residual stress was released, and investigated a specific method for doing so. The present invention is based on the results obtained from the said investigation, and its specific configuration is as follows.

[0016] [Embodiment] An embodiment of the present invention, as shown in Figure 1, involves striking the weld toe 17a of the welded portion 17 in an arc-welded joint 11 formed by overlapping steel plates 13 and 15 and fillet arc welding them together.

[0017] As shown in Figure 1(b), the weld toe 17a 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, and a Vickers hardness of 170 Hv or more. The minimum radius of curvature of the weld toe 17a is the smallest of the radii of curvature of the concave portion in a cross section parallel to the thickness direction of the steel plate 15, as shown in Figure 1(b).

[0018] Furthermore, the weld toe 17a has a crystalline structure that includes body-centered cubic or body-centered tetragonal crystal grains. Examples of crystalline structures with a body-centered cubic structure include martensitic steel, and examples of crystalline structures with a body-centered tetragonal structure include ferritic steel.

[0019] In this embodiment, the weld toe 17a is struck under the following conditions: the tip radius of the striking pin 1 is 1.3 times or less the minimum radius of curvature of the weld toe 17a, and the kinetic energy of the striking pin 1 is 0.375 mJ.

[0020] 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 1(a)), as shown in Figure 1(c). Furthermore, 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 velocity of the striking pin 1. 2 It is given by.

[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. In addition, the effect of refinement of crystal grains 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 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.

[0026] 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.

[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. This 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 also desirable that the kinetic energy of the impact pin 1 be large. Specifically, for example, if the weight m of the impact pin 1 is 3 g, it is desirable that the kinetic energy be 0.54 mJ or higher, which corresponds to an impact speed v = 0.6 m / s. 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 relative to the minimum radius of curvature r0 of the weld toe 17a, striking the weld toe 17a will cause a winding defect. Also, 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 allows for a larger 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 1 / 5 or more 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 set to the maximum speed in the direction of reciprocating motion.

[0032] The emergence of deformation twins is known to be determined by the competitive relationship between dislocation motion and slip deformation in the crystal structure. In other words, inhibiting dislocation motion can promote the emergence of deformation twins.

[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 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 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 size of 5 to 30 μm. However, if the grain size ρ before impact differs significantly from that of the martensitic steel, it is advisable to determine the critical value of parameter A while considering this difference. This is because, although the so-called Hall-Petch law, which increases proportionally to the -1 / 2 power of the grain size, is known to hold for slip deformation, the influence of grain size is greater on the Hall-Petch law 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 part 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. Figure 3(b) shows a surface defect 33 formed on the surface of a steel plate 31. An example of a surface defect 33 is a defect that occurred on the surface of the steel plate 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.

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

[0040] In this embodiment, as shown in FIG. 1, an arc welding joint 11 obtained by fillet welding two steel plates 13 and 15 by MAG (Metal Active Gas) welding was targeted. The steel plates 13 and 15 were hot-rolled steel plates with a tensile strength of any one of 540 MPa grade, 590 MPa grade or 780 MPa grade and a plate thickness of 3 mm, which were used as test materials. The minimum curvature radius r0 in the cross section parallel to the plate thickness direction of the weld termination part 17a was set to 0.50 mm, and its metal structure had a crystal structure containing crystal grains of a body-centered cubic lattice. Also, the Vickers hardness H of the weld termination part 17a (and the weld part 17) was set to 150 Hv, 170 Hv or 230 Hv. The Vickers hardness H was changed (set) to the above values by using steel plates with a tensile strength of 540 MPa grade, 590 MPa grade or 780 MPa grade for both the steel plates 13 and 15.

[0041] In the experiment, using an electric peening device, as shown in Table 1 below, the Vickers hardness H of the weld termination part 17a, the tip radius r of the impact pin 1, and the impact speed v of the impact pin 1 were variously changed, and a peening treatment was performed to impact the weld termination part 17a along the welding direction of the weld part 17. In the peening treatment, the indentation density for impacting the weld termination part 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 driving voltage of the electric peening device.

[0042]

[0043] In Table 1, Invention Example 1 and Invention Example 2 are those in which the conditions of the Vickers hardness H of the weld termination part 17a, the minimum curvature radius r0 of the weld termination part 17a, the tip radius r of the impact pin 1, the weight m, and the kinetic energy K are within the scope of the present invention. Also, Invention Example 2' is an arc welding joint 11 manufactured and subjected to peening treatment under the same conditions as Invention Example 2, which was heat-treated at 500 °C for 1 hour. On the other hand, Comparative Examples 1 to Comparative Examples 3 are those in which the Vickers hardness H of the weld termination part 17a, the tip radius r of the impact pin, or the kinetic energy K of the impact pin 1 is outside the scope of the present invention.

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

[0045] As shown in Table 1, in Invention Example 1 and Invention Example 2, deformation twins with a thickness of 50 nm or less were observed at the welded stop end portion 17a. The values of parameter A were set to different values in Invention Example 1 and Invention Example 2 because the stress accumulated at the grain boundaries that serve as the driving force for the expression of deformation twins increases as the Vickers hardness H of the welded stop end portion 17a increases. Also, in Invention Example 2' where heat treatment was performed after the peening treatment, deformation twins were observed at the welded stop end portion 17a. From the results of Invention Example 2 and Invention Example 2', it was confirmed that the deformation twins expressed at the welded stop end portion 17a by impact do not change due to heat treatment. In contrast, in Comparative Examples 1 to 3, no deformation twins were observed at the welded stop end portion 17a.

[0046] Subsequently, a plane bending fatigue test piece was cut out from the arc welded joint 11 subjected to peening treatment, and a pulsating fatigue test was conducted under the condition that the welded stop end portion 17a was on the tensile load side. The fatigue test was carried out at room temperature, and the repeated load was a nominal stress of 500 MPa based on the thickness of the steel plates 13 and 15, which are the base materials, at a repetition frequency of 10 Hz. Then, in the fatigue test, those that reached 300,000 repetitions of the load without fracture occurring in the plane bending fatigue test piece were judged as qualified, and the test was terminated at 10 million repetitions.

[0047] The fatigue test results are shown in Table 1 above. In Invention Examples 1 and 2, no fracture occurred at the welded stop end portion 17a even after 10 million repetitions of the load, and they were judged as qualified. Also, in Invention Example 2', the number of repetitions until fracture was 300,000 times, which was lower than that of Invention Example 2, but it was judged as qualified. The reason why the number of repetitions until fracture decreased compared to Invention Example 2 is considered to be that the compressive residual stress applied by impact was released by heat treatment, and the effect of improving the fatigue strength by the compressive residual stress could not be obtained. However, since the crystal structure in which deformation twins were expressed did not change even by heat treatment, the effect of improving the fatigue strength due to the refinement of crystal grains could be 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.

[0050] According to the present invention, it is possible to provide a peening treatment method that can maintain the effect of improving fatigue strength even in usage conditions and environments in which compressive residual stress is released.

[0051] 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 peening method for striking a concave portion of a steel plate part using a striking pin, wherein 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 a crystalline structure having a body-centered cubic lattice or body-centered tetragonal crystal grains, and the concave portion is struck under striking conditions such that the tip radius of the striking pin is 1.3 times or less of the minimum radius of curvature and the kinetic energy of the striking pin is 0.375 mJ or more, thereby causing deformation twins of 50 nm or less in thickness in the concave portion and refining the crystal grains.

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

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