Arc welded joint, welded structure, and manufacturing method thereof

Abstract

To provide an arc welded joint, a welded structure and a method of manufacturing them which improves a coating property and a fatigue strength of an arc welding part when a high tensile strength steel material is included as an arc welding object.SOLUTION: An arc welded joint includes a first steel material, a second steel material and an arc welding part which joins the first steel material and the second steel material. Therein, at least one side of the first steel material and the second steel material has a tensile strength of 980 MPa or more, the arc welding part has a weld bead, the weld bead has at least a stationary part, a weld slag is not attached to the surface of the weld bead and a toe of the weld bead on the stationary part has surface roughness Ra of 5.00 μm or less.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] This application discloses arc welding joints, welded structures, and methods for manufacturing them.

Background Art

[0002] Arc welding is known as a technique for joining steel materials. Problems with arc welding include that the fatigue strength and paintability tend to decrease at the arc-welded portion. In members such as automotive underbody members, the use of high-tensile steel has led to thinning, and it is further desired to improve the fatigue strength and the like of the arc-welded portion. For example, it may be possible to improve the fatigue strength of the arc-welded portion by improving welding conditions or devising the welding structure. However, in members to which high-tensile steel is applied, the required fatigue strength is also high, and it is difficult to improve it only by these methods.

[0003] As a method for improving the fatigue strength of an arc-welded portion, a compressive residual stress application treatment including shot peening is known. However, when the welding target includes high-tensile steel, this treatment may not be effective. When the welding target includes high-tensile steel, the compressive residual stress application treatment may not reach the narrow weld termination portion, or the surface roughness of the arc-welded portion may increase due to this treatment, thereby reducing the fatigue strength. This is because high-strength materials such as high-tensile steel are highly sensitive to minute defects. Therefore, even minute defects that do not cause a reduction in fatigue strength in mild steel may cause a reduction in fatigue strength in high-tensile steel.

[0004] Patent Document 1 discloses an arc welded member subjected to shot blasting treatment. Specifically, after arc welding a steel material of 1500 MPa grade, a shot blasting treatment is performed on the arc welded portion using an abrasive of φ0.3 mm. However, in the method disclosed in Patent Document 1, large indentations may occur due to the blasting treatment, and the fatigue strength may decrease due to stress concentration at the indentation portion. Further, in the method disclosed in Patent Document 1, slag may remain at the weld termination portion, and there is a risk of poor coating due to the remaining slag. Also, the blasting treatment may not reach the surface of the weld metal under the slag, and the fatigue strength may decrease.

[0005] Patent Document 2 discloses a method of hammer peening at the arc weld termination portion. The method disclosed in Patent Document 2 is a treatment only for the termination portion, and the electrocoating property of the weld bead surface is not improved. Further, when arc welding is performed on high-tensile steel, since the weld metal becomes high-strength, in the peening treatment in a mode of directly hitting with a pin, the range that can be treated at one time is narrow, and it is not suitable for treating the entire weld bead.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0007] As described above, in the prior art, there is room for improvement regarding improving the coating property and fatigue strength of the arc welded portion when the arc welding target includes high-tensile steel materials (having a tensile strength of 980 MPa or more).

Means for Solving the Problems

[0008] As one means for solving the above problems, the present application provides an arc-welded joint having a first steel material, a second steel material, and an arc-welding portion that joins the first steel material and the second steel material, wherein at least one of the first steel material and the second steel material has a tensile strength of 980 MPa or more, the arc-welding portion has a weld bead, the weld bead has at least a steady portion, no welding slag adheres to the surface of the weld bead in the steady portion, the end portion of the weld bead in the steady portion has a surface roughness Ra (equivalent to the arithmetic mean roughness Ra (JIS B 0601:2013); the same applies hereinafter) of 5.00 μm or less, an arc-welded joint is disclosed.

[0009] In the arc-welded joint of the present disclosure, the end portion of the weld bead in the steady portion may have a flank angle of 20° or more and 50° or less.

[0010] In the arc-welded joint of the present disclosure, the Vickers hardness of the weld metal constituting the arc-welding portion may be 300 HV or more.

[0011] In the arc-welded joint of the present disclosure, the weld bead may have at least a starting portion and the steady portion, and no welding slag may adhere to the surface of the weld bead in the starting portion.

[0012] In the arc-welded joint of the present disclosure, the weld bead may have at least a starting portion and the steady portion, and the end portion of the weld bead in the starting portion may have a surface roughness Ra of 5.00 μm or less.

[0013] In the arc-welded joint of the present disclosure, The welding bead may have at least a starting portion and a steady portion, and the end portion of the welding bead at the starting portion may have a flank angle of 20° or more and 50° or less.

[0014] As one of the means for solving the above problems, the present application discloses a welded structure including the arc welding joint of the present disclosure .

[0015] As one of the means for solving the above problems, the present application joins a first steel material and a second steel material via an arc welding portion, and performs a blasting treatment using an abrasive on the arc welding portion, including at least one of the first steel material and the second steel material has a tensile strength of 980 MPa or more, and the particle diameter of the abrasive is smaller than the local end radius of the arc welding portion, discloses a method for manufacturing an arc welding joint .

[0016] In the method for manufacturing an arc welding joint of the present disclosure, the arc welding portion may have a welding bead, the welding bead may have at least a steady portion, and the end portion of the welding bead at the steady portion may have a flank angle of 20° or more and 50° or less.

[0017] In the method for manufacturing an arc welding joint of the present disclosure, the first steel material and the second steel material may be arc welded using a flux-cored wire.

[0018] In the method for manufacturing an arc welding joint of the present disclosure, the blasting treatment may be a wet blasting treatment.

[0019] In the method for manufacturing an arc welded joint of the present disclosure, The Vickers hardness of the weld metal constituting the arc welded portion may be 300 HV or more.

[0020] In the method for manufacturing an arc welded joint of the present disclosure, The arc welded portion may have a weld bead, The weld bead may have at least a starting end, The end portion of the weld bead at the starting end may have a face angle of 20° or more and 50° or less.

[0021] As one of the means for solving the above problems, the present application Obtaining an arc welded joint by the manufacturing method of the present disclosure described above, A method for manufacturing a welded structure including is disclosed.

Effects of the Invention

[0022] The arc welded joint of the present disclosure is excellent in paintability and fatigue strength of the arc welded portion.

Brief Description of the Drawings

[0023]

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Mode for Carrying Out the Invention

[0024] 1. Arc Welding Joint As shown in FIGS. 1 to 3, an arc welding joint 100 according to an embodiment includes a first steel material 10, a second steel material 20, and an arc welding part 30 that joins the first steel material 10 and the second steel material 20. In the arc welding joint 100, at least one of the first steel material 10 and the second steel material 20 has a tensile strength of 980 MPa or more, the arc welding part 30 has a welding bead 31, the welding bead 31 has at least a steady part 31a, no welding slag adheres to the surface of the welding bead 31 in the steady part 31a, and the stop end 31ax of the welding bead 31 in the steady part 31a has a surface roughness Ra of 5.00 μm or less.

[0025] 1.1 Steel Material (Base Metal) In the arc welding joint 100, at least one of the first steel material 10 and the second steel material 20 has a tensile strength of 980 MPa or more. That is, in the arc welding joint 100, the tensile strength of the first steel material 10 may be 980 MPa or more and the tensile strength of the second steel material 20 may be less than 980 MPa, the tensile strength of the first steel material 10 may be less than 980 MPa and the tensile strength of the second steel material 20 may be 980 MPa or more, or the tensile strengths of both the first steel material 10 and the second steel material 20 may be 980 MPa or more. Thus, when the arc welding joint 100 includes a high-strength steel material with a tensile strength of 980 MPa or more, the problem of the fatigue strength of the arc welded portion 30 tends to become prominent. The first steel material 10 and the second steel material 20 may have tensile strengths of about the same level as each other, or may have different tensile strengths from each other. Further, in the arc welding joint 100, at least one of the first steel material 10 and the second steel material 20 may have a tensile strength of 1000 MPa or more, 1050 MPa or more, 1100 MPa or more, 1150 MPa or more, 1180 MPa or more, or 1200 MPa or more. The upper limit of the tensile strength is not particularly limited, but may be, for example, 2500 MPa or less, 2200 MPa or less, or 2000 MPa or less. Incidentally, the "tensile strength" of the steel material referred to in the present application conforms to ISO 6892-1:2009.

[0026] Regardless of the chemical compositions and metallographic structures of the first steel material 10 and the second steel material 20, the effects of the technology of the present disclosure are exhibited. That is, as long as at least one of the first steel material 10 and the second steel material 20 has a tensile strength of 980 MPa or more, the chemical compositions and metallographic structures of the respective steel materials are not particularly limited. The chemical compositions and metallographic structures of the steel materials 10 and 20 can be appropriately determined according to the use of the arc welded joint 100 and the like. The steel materials 10 and 20 contain, for example, in mass %, C: 0.01 to 0.50%, Si: 0.01 to 3.50%, Mn: 0.10 to 5.00%, P: 0.100% or less, S: 0.0300% or less, N: 0.0100% or less, O: 0 to 0.020, Al: 0 to 1.000%, B: 0 to 0.010%, Nb: 0 to 0.150%, Ti: 0 to 0.20%, Mo: 0 to 3.00%, Cr: 0 to 2.00%, V: 0 to 1.00%, Ni: 0 to 2.00%, W: 0 to 1.00%, Ta: 0 to 0.10%, Co: 0 to 3.00%, Sn: 0 to 1.00%, Sb: 0 to 0.50%, Cu: 0 to 2.00%, As: 0 to 0.050%, Mg: 0 to 0.100%, Ca: 0 to 0.100%, Zr: 0 to 0.100%, Hf: 0 to 0.100%, and REM: 0 to 0.100%, and may have a chemical composition in which the balance consists of Fe and impurities. Further, in the above chemical composition, the lower limit of the content of the optional additive element may be 0.0001% or 0.001%.

[0027] The shape of each of the first steel material 10 and the second steel material 20 is not particularly limited. For example, at least one of the first steel material 10 and the second steel material 20 may be plate-shaped (steel plate). In this case, the plate thickness is not particularly limited and may be appropriately determined according to the use of the arc welding joint 100. The plate thickness may be, for example, 0.5 mm or more, 0.8 mm or more, 1.0 mm or more, 1.2 mm or more, or 2.0 mm or more, and may also be 10.0 mm or less, 8.0 mm or less, 7.0 mm or less, 6.0 mm or less, 5.0 mm or less, 4.0 mm or less, or 3.0 mm or less. In particular, when the plate thickness is 0.6 mm or more and 4.0 mm or less, especially 0.8 mm or more and 2.9 mm or less, high effects can be expected. If the plate thickness is too thin, the plate may be deformed by the blasting treatment described later. If the plate thickness is too thick, the welding residual stress may increase, and the blasting effect on the arc welding part 30 may decrease. The plate thickness may be the same throughout the steel material, or may vary from part to part of the steel material.

[0028] 1.2 Arc welding part As shown in FIG. 1, the arc welding part 30 has a welding bead 31. Further, the welding bead 31 has at least a steady part 31a. As shown in FIG. 1, in addition to the steady part 31a, the welding bead 31 may have a starting end part 31b and a terminating end part 31c. In the case where the welding bead 31 has a starting end part 31b and a terminating end part 31c, the "starting end part" refers to the range of the shorter length among the length of 15 mm from the tip on the starting end side of the welding bead 31 or 25% of the total length of the welding bead, and the "terminating end part" refers to the range of the shorter length among the length of 15 mm from the tip on the terminating end side of the welding bead 31 or 25% of the total length of the welding bead, and the "steady part" refers to the part of the welding bead 31 excluding the starting end part and the terminating end part. Note that the arc welding part 30 refers to the part changed from the original steel material by arc welding, and in addition to the above welding bead 31, it may include a base metal heat affected zone, etc., but in the technology of the present disclosure, the form of the part other than the welding bead 31 is not particularly limited.

[0029] 1.2.1 Presence or absence of adhesion of welding slag In the arc welding joint 100, no welding slag adheres to the surface of the weld bead 31 in the steady part 31a. That is, it can be said that the entire weld bead including the stop end 31ax in the steady part 31a is subjected to blasting treatment. Thereby, the arc welding part 30 becomes more excellent in fatigue strength and paintability.

[0030] In the arc welding joint 100, as described above, the weld bead 31 may have at least a starting end 31b. In this case, it is better that no welding slag adheres to the surface of the weld bead 31 at the starting end 31b. Thereby, the arc welding part 30 becomes more excellent in fatigue strength and paintability.

[0031] In the arc welding joint 100, as described above, the weld bead 31 may have at least an ending end 31c. In this case, it is better that no welding slag adheres to the surface of the weld bead 31 at the ending end 31c. Thereby, the arc welding part 30 becomes more excellent in fatigue strength and paintability.

[0032] In the present application, "no welding slag adheres to the surface of the weld bead" means that substantially no welding slag adheres to the surface of the weld bead 31, and a very small amount of welding slag adhering is allowed. For each of the steady part 31a, the starting part 31b, and the ending part 31c of the weld bead 31, by measuring the ratio A1 / A2 of the slag area A1 to the weld bead area A2 by the following method, it is possible to determine whether or not welding slag adheres to the surface of each of the steady part 31a, the starting part 31b, and the ending part 31c. Specifically, as shown in FIG. 4, a photograph is taken in a range where the weld length is 30 mm or more (when the bead is 30 mm or less, the entire length) from the direction looking down on the weld bead. The weld bead included in the photograph is divided into a starting part, a steady part, and an ending part. For each of the starting part, the steady part, and the ending part, the slag area A1 (slag part hatching in FIG. 4) on the photograph and the weld bead area A2 (total area of the part where slag adheres (slag part hatching) and the part where no slag adheres) are measured. When the ratio A1a / A2a of the slag area A1a to the weld bead area A2a in the steady part 31a is 0.1 or less, it is regarded that "no welding slag adheres to the surface of the weld bead" in the steady part 31a. The same applies to the starting part 31b and the ending part 31c. That is, when the ratio A1b / A2b of the slag area A1b to the weld bead area A2b in the starting part 31b is 0.1 or less, it is regarded that "no welding slag adheres to the surface of the weld bead" in the starting part 31b, and when the ratio A1c / A2c of the slag area A1c to the weld bead area A2c in the ending part 31c is 0.1 or less, it is regarded that "no welding slag adheres to the surface of the weld bead" in the ending part 31c. As shown in FIG. 5, welding slag tends to remain near the stop end of the weld bead. In the technology of the present disclosure, for example, as described later, by using a polishing agent with a small particle diameter as the abrasive in the blasting treatment, the welding slag near the stop end is effectively removed, whereby at least in the steady part 31a, preferably at least in the steady part 31a and the starting part 31b, more preferably in the steady part 31a, the starting part 31b, and the ending part 31c, an arc welding joint 100 in which "no welding slag adheres to the surface of the weld bead" is obtained.Figs. 4 and 5 are merely illustrative. In Fig. 4, the ratio of the slag area A1 to the weld bead area A2 exceeds 0.1 (10%), that is, "weld slag adheres to the surface of the weld bead".

[0033] 1.2.2 Surface roughness In the arc welded part 30, the part where fatigue cracks are likely to occur is the surface near the stop end. On the surface, due to the relationship between the oscillation and solidification occurring in the molten pool, a wave pattern called a ripple is generated. Further, when the blasting treatment etc. described later is performed, plastic deformation occurs on the material surface due to the collision of the abrasive, resulting in indentation, and thereby the surface roughness changes. The degree to which the fatigue strength decreases due to minute unevenness varies depending on the strength of the arc welded part 30. The higher the strength, the more sensitive it is to minute unevenness and the easier it is for the fatigue strength to decrease. That is, in the arc welded part 30 of a high-tensile steel material with a tensile strength of 980 MPa or more as described above, even minute-sized unevenness that was not a problem in the arc welded part of a low-strength steel material may cause deterioration of the fatigue strength.

[0034] From the viewpoint of avoiding the above-described problems, in the arc welded joint 100, it is important that the stop end 31ax of the weld bead 31 in the steady part 31a has a surface roughness Ra of 5.00 μm or less. The surface roughness Ra may be 4.50 μm or less, 4.00 μm or less, or 3.50 μm or less.

[0035] In the arc welded joint 100, as described above, the weld bead 31 may have at least a start end 31b. In this case, from the same viewpoint as above, the stop end 31bx of the weld bead 31 at the start end 31b may have a surface roughness Ra of 5.00 μm or less. The surface roughness Ra may be 4.50 μm or less, 4.00 μm or less, or 3.50 μm or less.

[0036] In the arc weld joint 100, as described above, the weld bead 31 may have at least a terminal portion 31c. In this case, from the same viewpoint as above, the end portion of the weld bead 31 at the terminal portion 31c may have a surface roughness Ra of 5.00 μm or less. The surface roughness Ra may be 4.50 μm or less, 4.00 μm or less, or 3.50 μm or less.

[0037] In addition, the "surface roughness Ra" of the end portion referred to in the present application corresponds to the "arithmetic mean roughness Ra" defined in JIS B 0601:2013. The surface roughness Ra of the end portion is measured as follows. First, a cross-sectional photograph of the end portion perpendicular to the longitudinal direction of the weld bead is traced. Taking the boundary between the steel material and the arc welded portion as the origin, the x-axis is taken in the direction along the steel material surface and the y-axis is taken in the direction perpendicular to the steel material surface. In the range of x = 0 to 200 μm, the end shape is approximated by a cubic polynomial. Then, the shortest distance between the approximation formula and the traced shape points is obtained, and + is added if it is above the approximation line and - if it is below, and it is designated as yi. Also, from the origin, the distance to the intersection point of the line of each shortest distance and the approximation line is designated as xi, and when (xi, yi) is plotted, a profile of the uneven shape excluding the macroscopic undulation of the surface is obtained. The arithmetic mean roughness Ra calculated in the range of xi = 0 to 200 μm of the profile is defined as the "surface roughness of the end portion" referred to in the present application. The method for measuring the surface roughness is the same as that described in Non-Patent Document 1 below. Non-Patent Document 1: Kazuki Matsuda, Shinji Kodama, "Observation of fatigue microcracks and estimation of fatigue strength of a thin sheet arc welded part considering micro-ripples", International Journal of Fatigue, Volume 145, April 2021, 106087

[0038] 1.2.3 Frank Angle As shown in FIG. 2, at the steady portion 31a, the end portion 31ax of the weld bead 31 forms a Frank angle θ with respect to the surface of the steel material 10 or 20.a has a raised portion. When performing the blasting process described later on a narrow portion such as a stop end, abrasive may accumulate in the narrow portion, and the effect of the blasting process may not reach the surface of the stop end. That is, there is a risk that the blasting process may not be sufficiently performed at the stop end, which is the origin of the fatigue crack. Also, in the case of wet blasting, water may accumulate in the narrow portion, attenuating the kinetic energy of the abrasive, and the same problem may occur. This problem is likely to occur when the flank angle is large (when the stop end is steep). Also, when the flank angle is large, the stress concentration coefficient at the stop end increases, and thus the fatigue strength is also likely to decrease. On the other hand, when the flank angle is too small, the surface unevenness near the stop end tends to increase, stress concentration is likely to occur due to the surface unevenness, and the fatigue strength may decrease. From the above viewpoints, in the arc welding joint 100, the stop end 31ax of the weld bead 31 in the steady portion 31a preferably has a flank angle θ of 20° or more and 50° or less. a It is preferable to have. The flank angle θ a may be 21° or more, 22° or more, 23° or more, 24° or more, or 25° or more, and may be 49° or less, 48° or less, or 47° or less.

[0039] As described above, the weld bead 31 may have at least a start end 31b. In this case, as shown in FIG. 3, at the start end 31b, the stop end 31bx of the weld bead 31 has a flank angle θ with respect to the surface of the steel material 10 or 20. b has a raised portion. In the arc welding joint 100, the stop end 31bx of the weld bead 31 at the start end 31b preferably has a flank angle θ of 20° or more and 50° or less. b It may be made to have. The effect thereof is as described above. The flank angle θ b may be 21° or more, 22° or more, 23° or more, 24° or more, or 25° or more, and may be 49° or less, 48° or less, or 47° or less. The flank angle θ b may be larger than the flank angle θ a or smaller, or may be the same as the flank angle θ a but usually, the flank angle θb is larger than the Frank angle θ a is larger.

[0040] The same applies when the welding bead 31 has a termination portion 31c. That is, the termination portion of the welding bead 31 at the termination portion 31c may also have a Frank angle of 20° or more, 21° or more, 22° or more, 23° or more, 24° or more, or 25° or more, and 50° or less, 49° or less, 48° or less, or 47° or less.

[0041] Incidentally, the Frank angle θ of the termination portion is measured as follows. That is, as shown in FIG. 6, for the arc weld joint, a photograph of a cross section (a cross section perpendicular to the longitudinal direction of the welding bead) near the termination portion is acquired. In the cross section, the boundary point P1 between the welding bead 31 and the steel material 10 or 20 is specified. A straight line L1 passing through the boundary point P1 and along the surface of the steel material 10 or 20 is specified. A straight line L2 at a height H of 5% of the steel material thickness from the surface of the steel material 10 or 20 and parallel to the straight line L1 is specified. The intersection point P2 between the straight line L2 and the surface of the termination portion of the welding bead 31 is specified. A straight line L3 connecting the boundary point P1 and the intersection point P2 is specified. Of the angles formed by the straight line L1 and the straight line L3, the angle on the height H side and on the welding bead side is specified as the Frank angle θ. Incidentally, when the surface of the steel material is curved, as shown in FIG. 7, the tangent line of the steel material surface at the boundary point P1 may be specified as the straight line L1.

[0042] 1.2.4 Vickers hardness The arc welding part is composed of a weld metal in which the components of the welding wire and the steel material (base material) are mixed. Therefore, even when a soft wire is used, if the steel material has high strength and a high concentration of alloying elements, the strength of the weld metal constituting the arc welding part will be increased to a certain extent. Therefore, in the arc welding part, a relatively soft wire is often used even if the base material has a certain degree of high strength. However, when a high-strength steel material with a tensile strength of 980 MPa or more is included in the base material as in the arc welding joint 100 of the present disclosure, the difference in components between the base material and the welding wire is large, and it often becomes an under-match (base material strength > weld metal strength). Excessive under-match causes a decrease in the static strength of the joint. Therefore, for a base material of 980 MPa grade or higher, a high-strength wire may be used. When a high-strength wire is used, the weld metal constituting the arc welding part also becomes high-strength. For a high-strength arc welding part, the peening effect by blasting is difficult to be effective, and the range where the peening effect extends from the portion hit by the abrasive is narrower than that of a low-strength material. In such a case, if the blasting treatment does not reach the innermost part of the weld termination, the peening effect does not reach the position that becomes the starting point of the fatigue crack, and thus the effect of improving the fatigue strength cannot be obtained.

[0043] On the other hand, as will be described later, by devising the conditions of the blasting treatment, even if the weld metal constituting the arc welding part 30 is high-strength and hard, excellent fatigue strength and paintability can be ensured in the arc welding part 30. In this regard, in the arc welding joint 100, the Vickers hardness of the weld metal constituting the arc welding part 30 may be 260 HV or more, 270 HV or more, 280 HV or more, 290 HV or more, or 300 HV or more.

[0044] The Vickers hardness of the weld metal constituting the arc weld portion is measured as follows. That is, a cross-sectional sample is taken from the steady portion 31a of the weld bead 31, and among the surface of the weld metal in the cross section, at a position more than 0.5 mm away from the boundary between the steel material 10 or 20 and the arc weld portion 30, and at a position more than 0.5 mm away from the bead surface (that is, inside at a position more than 0.5 mm deep from the bead surface of the arc weld portion 30), the Vickers hardness is measured at 5 or more points with a load of 200 gf, and the average value excluding the maximum and minimum is defined as the "Vickers hardness of the weld metal constituting the arc weld portion".

[0045] 1.3 Weld structure of the joint The welding structure in the arc weld joint 100 is not particularly limited. For example, it may have a fillet weld structure as shown in FIG. 8, or it may have a T-joint fillet weld structure as shown in FIG. 9, or it may have other structures. In the structures shown in FIGS. 8 and 9, for example, at least in region X, the flank angle θ and the like as described above are satisfied, while at the bead termination on the side opposite to region X, the flank angle θ and the like as described above do not have to be satisfied. That is, the weld bead 31 may have different structures at the termination on one side and the termination on the side opposite thereto (the other side).

[0046] 2. Welded structure The technology of the present disclosure also has a side as a welded structure. That is, the welded structure of the present disclosure includes the above-described arc weld joint 100. By having the configuration described above, the arc weld joint 100 includes a steel material with a tensile strength of 980 MPa or more, and is excellent in the fatigue strength and paintability of the arc weld portion 30. Such an arc weld joint 100 is applicable to various welded structures. For example, various automotive members can be mentioned as welded structures that can make more effective use of the arc weld joint 100. In particular, members around the feet such as lower arms, subframes, and torsion beams are suitable.

[0047] 3. Manufacturing method of arc weld joint The above arc welding joint 100 can be manufactured, for example, by the following method. That is, the method for manufacturing the arc welding joint of the present disclosure is joining the first steel material 10 and the second steel material 20 via the arc welding portion 30, and performing a blasting treatment using an abrasive on the arc welding portion 30, including at least one of the first steel material 10 and the second steel material 20 has a tensile strength of 980 MPa or more, characterized in that the particle diameter of the abrasive is smaller than the local end radius of the arc welding portion 30.

[0048] 3.1 Arc Welding The conditions (current value, welding speed, shielding gas, etc.) for arc welding between the first steel material 10 and the second steel material 20 are not particularly limited. An example will be described later. Regarding the wire used for arc welding, either a known solid wire or a flux-cored wire can be adopted. In particular, when the first steel material 10 and the second steel material 20 are arc welded using a flux-cored wire, the end portion of the weld bead 31 is likely to have a suitable shape. That is, the flux-cored wire has a smaller heat capacity than the solid wire and is likely to have a longer arc length. Also, since the welding slag easily covers the surface of the weld metal, a smooth end shape with few irregularities is easily obtained. By adopting a flux-cored wire during arc welding in this way and performing the blasting treatment described later, the end shape is likely to become smooth, the blasting treatment is likely to reach the innermost part of the end, and the surface irregularities of the weld metal are also likely to be reduced. That is, the slag adhering to the surface of the weld bead can be appropriately removed, and it is easy to achieve the above-described surface roughness Ra and the flange angle θ, and it is easy to ensure better fatigue strength and paintability at the arc welding portion 30.

[0049] As described above, it is preferable that the fillet angle of the stop end portion of the arc welding portion is gentle. Also, as described above, the welding metal constituting the arc welding portion may be hard. Specifically, the arc welding portion 30 may have a weld bead 31, the weld bead 31 may have at least a steady portion 31a, and the stop end portion 31ax of the weld bead 31 in the steady portion 31a may have a fillet angle θ of 20° or more and 50° or less. a It may also have. Further, the weld bead 31 may have at least a start end portion 31b, and the stop end portion 31bx of the weld bead 31 at the start end portion 31b may have a fillet angle θ of 20° or more and 50° or less. b It may have. Furthermore, the Vickers hardness of the welding metal constituting the arc welding portion 30 may be 300 HV or more.

[0050] 3.2 Blasting After performing arc welding as described above, a blasting treatment is performed on the arc welding portion 30. By performing a blasting treatment on the arc welding portion 30, slag and scale can be removed, and furthermore, the fatigue strength is improved by the peening effect.

[0051] However, as described above, when the welding target includes high-strength steel, the arc-welded portion also becomes high-strength, and the influence range of the peening effect becomes narrow. Therefore, when the blasting treatment is performed, for example, the peening effect may not reach the stop end portion. Further, the harder the welding metal constituting the innermost part of the stop end portion (the portion where the fatigue crack starts), the more difficult it is for the peening effect to reach. Thus, when high-strength steel is adopted as the arc-welding target, the fatigue strength may not be improved even if the arc-welded portion is subjected to the blasting treatment. Also, when the effect of the blasting treatment cannot reach the stop end portion, the slag adhering to the stop end portion cannot be removed. In this regard, the paintability of the arc-welded portion is also reduced due to the remaining slag. Further, the fatigue crack occurs from the portion where the most curved stress concentration is likely to occur at the stop end portion, and when the slag remains, the crack occurs from below the slag. As described above, when the welding target includes high-strength steel, the influence range of the peening effect is narrow, and it is assumed that the effect of the blasting treatment does not reach below the slag, and the occurrence of cracks from below the slag cannot be suppressed, and it is difficult to improve the fatigue strength.

[0052] In order to solve the above problems, in the method of the present disclosure, an abrasive having a small particle size is used as the abrasive for the blasting treatment. Specifically, the particle size of the abrasive is smaller than the local stop end radius of the arc-welded portion 30. Thereby, the abrasive is likely to enter up to the stop end portion, and the deposition of the abrasive at the stop end portion and the like are also likely to be avoided, and the peening effect by the blasting treatment is likely to reach up to the stop end portion. The abrasive may contain those having a particle size smaller than the local stop end radius of the arc-welded portion 30 and those having a particle size equal to or larger than the local stop end radius, but it is better that those having a particle size smaller than the local stop end radius are as many as possible. Specifically, based on the whole abrasive used for the blasting treatment (100% by mass), those having a particle size smaller than the local stop end radius of the arc-welded portion 30 may account for 50% by mass or more, 60% by mass or more, 70% by mass or more, 80% by mass or more, 90% by mass or more, 95% by mass or more, or 99% by mass or more.

[0053] The local stop end radius of the arc welding part 30 is defined as follows. That is, as shown in FIG. 10, in the cross section near the stop end (the cross section orthogonal to the longitudinal direction of the weld bead), the boundary point P1 between the weld bead and the steel material is specified. Also, lines parallel to the straight line L1 along the steel material surface are drawn at positions 5%, 7.5%, and 10% of the steel material thickness above the steel material surface, and the points P A , P B , P C where each intersects the bead surface (weld metal surface) are specified. A straight line L0 is specified that is orthogonal to the straight line L1 along the steel material surface and passes through the boundary point P1. A circle C A with its center on the straight line L0 and passing through the boundary point P1 and the point P A , a circle C B with its center on the straight line L0 and passing through the boundary point P1 and the point P B , and a circle C C with its center on the straight line L0 and passing through the boundary point P1 and the point P C are each specified. For the three circles C A ~C C , the radius of each is obtained, and the average value is defined as the "local stop end radius".

[0054] The conditions for the blasting process may be the same as those of the conventional blasting process, except for using an abrasive having a small particle size as described above. The blasting process may be shot blasting (dry blasting) or wet blasting. Shot blasting is a technique of projecting an abrasive onto an object by air pressure or the like. Wet blasting is a technique of projecting a slurry (mud) in which an abrasive and a liquid (e.g., water) are mixed onto an object. In the method of the present disclosure, a higher effect can be expected when the blasting process is a wet blasting process. Wet blasting can use an abrasive having a smaller particle size than shot blasting by mixing a liquid (water) with the abrasive. In shot blasting, if the abrasive is too light, the kinetic energy becomes small, the speed is attenuated by air resistance, and the processing force is significantly reduced. On the other hand, such a problem is less likely to occur in wet blasting. Further, according to wet blasting, when an object is blasted, the abrasive and grinding chips can be washed away by water at the same time, so that it is easy to prevent the deposition of the abrasive and the like, and the cleanability is improved. Further, according to wet blasting, the scattering of dust is also less likely to occur.

[0055] 3.3 Supplementary Supplementary remarks are made on the blasting process at the starting position (starting end) of the arc welding part. At the starting end, the shape of the stopping end is more likely to be steeper than that of the steady part (θ in FIG. 3 b is larger than θ in FIG. 2 aSince it is more likely to become larger, stress concentration is likely to increase, and it is likely to become the origin of fatigue cracks. The appearance of the weld bead near the starting end is, for example, as shown in FIG. 11. FIG. 12 is a cross-sectional photograph taken by an optical microscope of a sample taken at the position of S (starting end) in FIG. 11. FIG. 13 is a cross-sectional photograph taken by an optical microscope of a sample taken at the position of M (steady part) in FIG. 11. From FIGS. 12 and 13, it can be seen that the starting end has a steeper end shape than the steady part. Thus, generally, the end part near the starting end is often steeper than the end part of the steady part. If the blasting conditions are selected on the premise of blasting the steady part, the blasting treatment may not be appropriately performed at the end part on the starting end side. As a result, fatigue cracks may occur from the end part near the starting end, and the fatigue strength of the joint may decrease. In order to avoid this problem, it is preferable to measure the local end radius and perform blasting treatment using an abrasive of an appropriate particle size also at the end part on the starting end side as described above.

[0056] Supplementary details of the arc welding conditions are provided. The current value of the arc welding may be, for example, 80 A or more and 250 A or less. Also, the welding speed may be 0.6 m / min or more and 1.2 m / min or less. Also, the shielding gas may be a mixed gas of argon and carbon dioxide. In this case, the ratio of carbon dioxide in the mixed gas may be 5% by volume or more and 20% by volume or less. The relationship between the welding target position and the local end radius / thickness (plate thickness) may be, for example, as shown in FIG. 14. Note that FIG. 14 shows the case where the voltage is set to ±0, +2V, and -2V from the single setting of the welding power source. Based on FIG. 14, the local end radius can also be obtained. The welding target position is, for example, as shown in FIG. 15. More specifically, in the case of fillet lap welding, the region Y shown in FIG. 16, and in the case of T-joint fillet welding, the region Y shown in FIG. 17 may be used as the welding target position.

[0057] Supplement the details of the blasting conditions. The blasting pressure may be 0.2 MPa or more and 0.4 MPa or less. Also, the projection distance may be 30 mm or more or 60 mm or more and 150 mm or less or 120 mm or less. As the abrasive, for example, those made of glass, zirconia, alumina, SUS, etc. can be used. The shape of the abrasive may be particulate, for example, bead-shaped, grid-shaped, or cut wire-shaped. As described above, the abrasive may have a predetermined particle size. Here, the "particle size" of the abrasive refers to the equivalent spherical diameter of the abrasive (the diameter of the sphere when the volume of the abrasive particles is converted into a sphere). The particle size of the abrasive may be determined according to the local stop radius of the arc weld as described above. The projection direction (projection angle) of the blast may be, for example, between 90° (directly above the steel surface) and 45° (diagonal 45° to the steel surface) as shown in FIG. 18.

[0058] 4. Method for manufacturing a welded structure The technology of the present disclosure also has an aspect as a method for manufacturing a welded structure. That is, the method for manufacturing a welded structure of the present disclosure includes obtaining an arc weld joint by the above manufacturing method. When manufacturing the welded structure, known processes may be employed except for obtaining the arc weld joint by the above procedure. Appropriate processes may be employed according to the use, etc. of the welded structure.

Examples

[0059] Hereinafter, while showing examples, the technology of the present disclosure will be described in more detail, but the technology of the present disclosure is not limited to the following examples.

[0060] 1. Preparation of base material As the base materials, three types were prepared: steel plate A made of steel with a tensile strength of 989 MPa, steel plate B made of steel with a tensile strength of 1212 MPa, and steel plate C made of steel with a tensile strength of 441 MPa. The plate thickness was 2.9 mm for all.

[0061] 2. Preparation of welding wire As welding wires for arc welding, three types were prepared: solid wire a for 490 MPa grade steel for Ar+CO2 shielding gas, solid wire b for 780 MPa grade steel, and flux-cored wire c for 780 MPa grade steel.

[0062] 3. Arc Welding and Blasting For the above-mentioned base materials, steel plates A, steel plates B, or steel materials C were joined by arc welding (fillet welding in multiple layers), and then the arc-welded parts were subjected to blasting. Table 1 shows the arc welding conditions and blasting conditions for each example. Also, the conditions common to each example are as follows.

[0063] (Arc Welding Conditions Common to Each Example) Welding current: 235 A Welding speed: 0.8 m / min Shielding gas: Ar+20%CO2 DC pulse mode

[0064] (Blasting Conditions Common to Each Example) Pressure: 0.3 MPa Projection distance: 100 mm Blasting projection direction (blasting angle, see Fig. 18): 90°

[0065] 4. Evaluation of Electrophoretic Coating Property For each of the arc-welded joints obtained as described above, first, degreasing treatment and chemical conversion treatment were performed. Next, electrophoretic coating was carried out so that the film thickness became 20 μm. Then, the welded bead part after electrophoretic coating was photographed from a direction perpendicular to the surface of the steel plate, and by analyzing the photograph, the ratio of the area of the defective part of electrophoretic coating to the area of the welded bead was measured, and this ratio was defined as the "defective coating area ratio". Note that the length of the welded bead was 120 mm. The above measurement was performed on a 90-mm-long region excluding the start end part (the region from the end on the welding start side of the welded bead to 15 mm) and the end part (the region from the end on the welding end side of the welded bead to 15 mm) of the welded bead. A welded joint with a defective coating area ratio of 5% or less was judged as qualified "○", and a welded joint with a defective coating area ratio exceeding 5% was judged as unqualified "×".

[0066] 5. Evaluation of Fatigue Characteristics For each arc welded joint obtained as described above, the fatigue characteristics of the arc welded portion were evaluated. In the evaluation of the bead steady portion, a test piece with a width of 20 mm in the welding line direction was sampled from a stable portion of the weld bead excluding the start and end portions, and in the evaluation of the start end portion, from the end portion of the weld bead on the start end side, and a plane bending fatigue test was carried out. The loading condition was displacement control with both-sided stress ratio of -1, the fracture condition was the point when the bending moment acting on the test piece became 1 N·m or less, and the fatigue limit σ W was defined as the stress amplitude [MPa] at the time when no fracture occurred after 10 7 cycles. Also, the stress reference was the maximum value of the bending stress when considering no stress concentration at the weld termination at the center of the test piece (assuming the bending moment M [N·mm], the test piece width h [mm], and the plate thickness t [mm], then 6M / ht 2 [MPa]).

[0067] Since there is an influence of plate thickness on the fatigue limit in bending loading, when the fatigue limit σ W [MPa] that satisfies the following relationship of formula (1) considering the influence of plate thickness was obtained, it was judged that a good fatigue limit "○" was obtained. When formula (1) was not satisfied, it was judged as "×". Furthermore, when formula (1) was satisfied and formula (2) was also satisfied, it was judged that a more excellent fatigue limit "◎" was obtained. 221 + 56 / t ≤ σ W < 232 + 51 / t …(1) 232 + 51 / t ≤ σ W …(2)

[0068] 6. Evaluation Results Table 1 below shows the arc welding conditions, blast treatment conditions, evaluation results of electrodeposition paintability, and evaluation results of fatigue characteristics for each example.

[0069]

Table 1

[0070] Summarizing the results shown in Table 1, it is as follows.

[0071] For No. 4 and 10, the particle size of the abrasive was larger than the local end radius, and slag remained at the end part. As a result, the paintability deteriorated. Also, the blasting treatment was not sufficiently performed at the lower part of the slag, and sufficient fatigue characteristics could not be obtained.

[0072] For No. 9, the surface roughness Ra of the end part of the weld bead in the steady part exceeded 5 μm. As a result, stress concentration due to minute unevenness occurred in the fatigue test, and sufficient fatigue characteristics could not be obtained.

[0073] For No. 12, the particle size of the abrasive was larger than the local end radius, and slag remained at the end part. As a result, the paintability deteriorated. On the other hand, since the Vickers hardness of the weld metal constituting the arc welded part was less than 300 HV and was too soft, the blasting treatment effect applied to the peripheral part of the remaining slag reached the lower part of the slag, and good fatigue characteristics were obtained.

[0074] For No. 14, the particle size of the abrasive was larger than the local end radius, and slag remained at the end part. As a result, the paintability deteriorated. On the other hand, since the base metal steel plate had low strength and as a result the Vickers hardness of the weld metal constituting the arc welded part became less than 300 HV and was soft, the blasting treatment effect applied to the peripheral part of the remaining slag reached the lower part of the slag, and good fatigue characteristics were obtained.

[0075] On the other hand, for Nos. 1 to 3, 5 to 8, and 13, there is no welding slag adhering to the surface of the weld bead in the steady part, and the termination part of the weld bead in the steady part has a surface roughness of 5.00 μm or less. As a result, they had excellent paintability and fatigue characteristics. Also, for No. 11, there is no welding slag adhering to the surface of the weld bead at the starting part, and the termination part of the weld bead at the starting part has a surface roughness of 5.00 μm or less. As a result, it had excellent paintability and fatigue characteristics. Incidentally, for No. 11, there was also no welding slag adhering to the surface of the weld bead in the steady part, and the termination part of the weld bead had a surface roughness of 5.00 μm or less.

Explanation of Reference Numerals

[0076] 10 First steel material 20 Second steel material 30 Arc welding part 31 Weld bead 31a Steady part 31ax Termination part 31b Starting part 31bx Termination part 31c Ending part 100 Arc welded joint

Claims

1. An arc welding joint, comprising a first steel material, a second steel material, and an arc welding portion for joining the first steel material and the second steel material, wherein at least one of the first steel material and the second steel material has a tensile strength of 980 MPa or more, the arc welding portion has a weld bead, the weld bead has at least a steady portion, no welding slag adheres to the surface of the weld bead in the steady portion, the end portion of the weld bead in the steady portion has a surface roughness Ra of 5.00 μm or less, an arc welding joint.

2. The end portion of the weld bead in the steady portion has a flank angle of 20° or more and 50° or less, the arc welding joint according to Claim 1.

3. The Vickers hardness of the weld metal constituting the arc welding portion is 300 HV or more, the arc welding joint according to Claim 1 or 2.

4. The weld bead has at least a starting portion and the steady portion, no welding slag adheres to the surface of the weld bead in the starting portion, the arc welding joint according to any one of Claims 1 to 3.

5. The weld bead has at least a starting portion and the steady portion, the end portion of the weld bead in the starting portion has a surface roughness Ra of 5.00 μm or less, the arc welding joint according to any one of Claims 1 to 4.

6. The weld bead has at least a starting portion and the steady portion, the end portion of the weld bead in the starting portion has a flank angle of 20° or more and 50° or less, the arc welding joint according to any one of Claims 1 to 5.

7. A welded structure comprising the arc welding joint according to any one of Claims 1 to 6. A welded structure.

8. Joining a first steel material and a second steel material via an arc welding portion, and performing a blasting treatment using an abrasive on the arc welding portion, including, wherein at least one of the first steel material and the second steel material has a tensile strength of 980 MPa or more, the particle diameter of the abrasive is smaller than the local stop radius of the arc welding portion, a method for manufacturing an arc welding joint.

9. The arc welding portion has a weld bead, the weld bead has at least a steady portion, the end portion of the weld bead in the steady portion has a flank angle of 20° or more and 50° or less, the method for manufacturing an arc welding joint according to Claim 8.

10. Arc-welding the first steel material and the second steel material using a flux-cored wire The method for manufacturing an arc-welded joint according to claim 8 or 9

11. The blasting treatment is wet blasting treatment The method for manufacturing an arc-welded joint according to any one of claims 8 to 10

12. The Vickers hardness of the weld metal constituting the arc-welded portion is 300 HV or more The method for manufacturing an arc-welded joint according to any one of claims 8 to 11

13. The arc-welded portion has a weld bead The weld bead has at least a starting end The end portion of the weld bead at the starting end has a flange angle of 20° or more and 50° or less The method for manufacturing an arc-welded joint according to any one of claims 8 to 12

14. Obtaining an arc-welded joint by the manufacturing method according to any one of claims 8 to 13 A method for manufacturing a welded structure, comprising

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