Gas shielded arc welding method and method for manufacturing welded joints

Abstract

To provide a gas shield arc-welding method that can improve a fatigue strength of a welded part, thereby enabling wall thinning and weight saving of a component.SOLUTION: In a gas shield arc-welding method which welds a steel material by melting a weld material and a part of the steel material while flowing a shield gas, the steel material has a tensile strength of 1180 MPa or more. Further, the weld material contains Cr of 6.00 mass% or more and 15.50 mass% or less, and Ni less than 9.00 mass% to total mass of the weld material.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a gas shielded arc welding method.

Background Art

[0002] In recent years, due to the increasing requirements for environmental performance, technological development related to improving fuel efficiency of automobiles and the like has been actively promoted. Methods for improving the fuel efficiency of automobiles and the like include improving the efficiency of internal combustion engines, hybridization, and electrification. Since electrification tends to increase the weight of the vehicle body due to the installation of batteries, the development of weight reduction technologies has also been promoted simultaneously. For example, attempts have been actively made to reduce the weight of the vehicle by reducing the plate thickness by using thin steel plates with higher strength than conventional steel plates.

[0003] However, even when using thin steel plates with higher strength, the fatigue strength of the welded part does not increase compared to the smooth base material. Therefore, for example, relatively thick steel plates are used for the underbody parts of automobiles and the like, and the progress of high strength and weight reduction has not been achieved, and improvement of the fatigue performance of the welded part is desired.

[0004] For example, in Patent Document 1, a gas shielded arc welding method has been proposed that is excellent in fatigue resistance and corrosion resistance (paintability) of the welded part and can perform welding at low cost when welding high-tensile steel plates, particularly steel plates with a tensile strength of 780 MPa or more. The gas shielded arc welding method described in Patent Document 1 uses a consumable electrode with controlled contents of C, Si, Mn, Cr, S, and Ni, and a shielding gas with controlled contents of at least one of CO2 and O2 and Ar.

[0005] Furthermore, Patent Document 2 discloses an arc welding method that can easily suppress hydrogen embrittlement of the weld metal and prevent cracking in high-tensile steel plates, regardless of whether it is spot welding or line welding. The arc welding method described in Patent Document 2 controls the total amount of Cr and Ni in the wire, and controls the relationship between the content of C, Si, Mn, Ni, Cr, and Mo in the welding wire and the content of C, Si, Mn, Ni, Cr, and Mo in the steel plate. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2017-225986 [Patent Document 2] Japanese Patent Publication No. 2018-187640 [Overview of the project] [Problems that the invention aims to solve]

[0007] However, even when using the welding methods described in Patent Document 1 or Patent Document 2, the fatigue strength of the welded joints of high-strength steel plates is not sufficiently high. Therefore, there is an increasing demand for welding methods that can increase the strength of components where fatigue performance is important, thereby reducing the plate thickness and thus the weight of the components.

[0008] The present invention has been made in view of the above-described circumstances, and aims to provide a gas shielded arc welding method that can improve the fatigue strength of the welded joint, thereby enabling the thinning and weight reduction of the member. [Means for solving the problem]

[0009] In other words, the above objective of the present invention is achieved by the configuration described below [1] relating to a gas shielded arc welding method. [1] A gas shielded arc welding method for welding steel materials by melting a portion of the welding material and steel materials while flowing a shielding gas, The aforementioned steel material has a tensile strength of 1180 MPa or more. The welding material is, with respect to the total mass of the welding material, Contains Cr: 6.00% by mass or more and 15.50% by mass or less, A gas shielded arc welding method characterized by having Ni content of less than 9.00% by mass.

[0010] Furthermore, preferred embodiments of the present invention relating to a gas shielded arc welding method are described in the following [2] to [4]. [2] The welding material further comprises: The gas shielded arc welding method according to [1], characterized by containing at least one of Mn and Mo.

[0011] [3] The welding material is, with respect to the total mass of the welding material, Mn: 0.69 mass% or more and 1.70 mass% or less, The gas shielded arc welding method according to [2], characterized by containing the following.

[0012] [4] The welding material is, with respect to the total mass of the welding material, Mo: 0.30% by mass or more and 1.05% by mass or less, A gas shielded arc welding method according to [2] or [3], characterized by containing the following. [Effects of the Invention]

[0013] According to the present invention, it is possible to improve the fatigue strength of the welded joint, thereby providing a gas shielded arc welding method that allows for thinner and lighter components. [Modes for carrying out the invention]

[0014] As a result of diligent research to solve the above problems, the inventors have found that by appropriately controlling the Cr content in the welding material, it is possible to optimally adjust the balance between the mechanical properties and residual stress of the welded part, thereby improving the fatigue strength of the welded part. The present invention has been made based on this finding.

[0015] Hereinafter, embodiments for carrying out the present invention will be described in detail. Note that the present invention is not limited to the embodiments described below, and can be arbitrarily modified and implemented without departing from the gist of the present invention. First, the gas shielded arc welding method according to the present embodiment will be described.

[0016] [Gas shielded arc welding method] The gas shielded arc welding method according to the present embodiment is a method of welding steel by melting a part of the welding material and the steel while flowing a shielding gas. In the gas shielded arc welding method according to the present embodiment, the tensile strength of the steel and the components contained in the welding material are limited. Hereinafter, the steel and the welding material will be described in more detail.

[0017] [Steel: Steel with a tensile strength of 1180 MPa or more] As described above, recently, in automotive underbody parts and the like, high strength and weight reduction have been required. Therefore, if the base material is made of high-tensile steel and a welded part having excellent fatigue strength can be obtained, the base material can be thinned, and as a result, the underbody parts can be weight-reduced. The gas shielded arc welding method according to the present embodiment is a welding method capable of improving the fatigue strength of the welded part. Therefore, by using a steel material made of ultra-high-tensile steel having a tensile strength of 1180 MPa or more as the base material to be welded, the desired strength can be ensured even when the thickness is reduced for weight reduction purposes. Note that the upper limit of the strength of the steel material is not particularly limited. The maximum strength of the thin steel plates currently generally put into practical use is about 2000 MPa. For steel plates up to this level, the welding method according to the present embodiment can be used to improve the fatigue strength of the welded part. The tensile strength of the steel plate can be obtained by the method defined in JIS Z2241.

[0018] In this embodiment, the plate thickness of the steel material is not particularly limited. However, the present invention aims to obtain high strength even when the plate thickness is reduced for weight reduction of the member. For example, even when the plate thickness is 1.0 mm or more and 2.5 mm or less, desired strength and weight reduction can be achieved.

[0019] [Welding material] As the welding material used in the gas shielded arc welding method according to this embodiment, a general welding material can be used as long as the content of each of the above components is controlled within a predetermined range. Specifically, the above welding material can be used as a consumable electrode for gas shielded arc welding. Also, the above welding material can be used as a non-consumable filler for gas tungsten arc welding.

[0020] <Cr: 6.00 mass% or more and 15.50 mass% or less> Cr is an element effective for increasing the strength of the weld metal, reducing the Ms point (martensite transformation point) to reduce the tensile residual stress, and adjusting the balance between the mechanical properties and residual stress of the welded part. Therefore, it is necessary to adjust it appropriately.

[0021] Therefore, in the steel plate with a tensile strength of 1180 MPa or more targeted by the present invention, the Cr content in the welding material is 6.00 mass% or more based on the total mass of the welding material, preferably 6.50 mass% or more, more preferably 7.00 mass% or more, and even more preferably 7.50 mass% or more. Furthermore, the Cr content in the welding material is more preferably 8.00 mass% or more, and particularly preferably 8.50 mass% or more.

[0022] On the one hand, the Cr content in the welding material is 15.50% by mass or less, preferably 15.00% by mass or less, more preferably 14.50% by mass or less, and even more preferably 14.00% by mass or less with respect to the total mass of the welding material. Furthermore, the Cr content in the welding material is preferably 13.50% by mass or less, more preferably 13.00% by mass or less, and even more preferably 12.50% by mass or less. Moreover, the Cr content in the welding material is preferably 12.00% by mass or less, more preferably 11.50% by mass or less, and even more preferably 11.00% by mass or less. Additionally, the Cr content in the welding material is preferably 10.50% by mass or less, more preferably 10.00% by mass or less, even more preferably 9.50% by mass or less, and particularly preferably 9.00% by mass or less.

[0023] <Ni: less than 9.00% by mass> Ni is also a component that affects the hardness of the weld metal. If the Ni content in the weld metal is too high, the desired fatigue strength cannot be obtained. The mechanism by which the fatigue strength decreases due to excessive addition of the Ni content is not clear, but it is presumed that if the Ni content in the weld metal is too high, the Ms point becomes too low, remaining in the austenite structure, making it difficult to reduce the tensile residual stress in the welded part and likely to lower the strength of the weld metal.

[0024] Therefore, the Ni content in the welding material is less than 9.00% by mass based on the total mass of the welding material, preferably 8.50% by mass or less, more preferably 8.00% by mass or less, and even more preferably 7.50% by mass or less. Further, the Ni content in the welding material is preferably 7.00% by mass or less, more preferably 6.50% by mass or less, and even more preferably 6.00% by mass or less. Furthermore, the Ni content in the welding material is preferably 5.50% by mass or less, more preferably 5.00% by mass or less, and even more preferably 4.50% by mass or less. Furthermore, the Ni content in the welding material is preferably 4.00% by mass or less, more preferably 3.50% by mass or less, and even more preferably 3.00% by mass or less. Furthermore, the Ni content in the welding material is preferably 2.50% by mass or less, more preferably 2.00% by mass or less, and even more preferably 1.50% by mass or less. Furthermore, the Ni content in the welding material is preferably 1.00% by mass or less, more preferably 0.60% by mass or less, and even more preferably 0.50% by mass or less.

[0025] On the other hand, Ni is a component having an effect of improving the toughness of the weld metal. In the present embodiment, the lower limit of the Ni content in the welding material is not particularly limited and may be 0% by mass. However, when Ni is contained in the welding material for the purpose of improving the toughness of the weld metal and further improving the fatigue strength, the Ni content in the welding material is preferably 0.05% by mass or more, more preferably 0.30% by mass or more based on the total mass of the welding material.

[0026] <At least one of Mn and Mo> In the welding method according to the present embodiment, it is preferable that at least one of Mn and Mo is contained in the welding material. The preferable content of each component is shown below.

[0027] (Mn: 0.69% by mass or more and 1.70% by mass or less) Mn is an element effective for ensuring the hardenability of the weld metal and increasing the strength, but it is also a component that is liable to oxidize and is an element that generates slag that is difficult to peel off the bead surface. Therefore, from the viewpoint of preventing the generation of slag and improving the paintability, the Mn content in the welding material is preferably 1.70% by mass or less, more preferably 1.65% by mass or less, based on the total mass of the welding material. On the other hand, from the viewpoint of improving the strength of the weld metal, the Mn content in the welding material is preferably 0.69% by mass or more, more preferably 0.80% by mass or more, and even more preferably 1.00% by mass or more, based on the total mass of the welding material. Further, the Mn content in the welding material is preferably 1.20% by mass or more, more preferably 1.40% by mass or more, and even more preferably 1.45% by mass or more, based on the total mass of the welding material.

[0028] (Mo: 0.30% by mass or more and 1.05% by mass or less) Mo is an element useful for increasing the strength by enhancing the hardenability, but it is also a very expensive element. Further, if it is contained excessively, there is a risk of cracking at the end portion of the weld metal. Therefore, from the viewpoints of cost reduction and prevention of cracking at the end portion, the Mo content in the welding material is preferably 1.05% by mass or less, more preferably 1.00% by mass or less, and even more preferably 0.95% by mass or less, based on the total mass of the welding material. On the other hand, from the viewpoint of improving the strength of the weld metal, the Mo content in the welding material is preferably 0.30% by mass or more, more preferably 0.50% by mass or more, and even more preferably 0.80% by mass or more, based on the total mass of the welding material.

[0029] <C: 0.01% by mass or more and 0.20% by mass or less> C is an element effective for increasing the strength of the weld metal, but it is also an element harmful to hot cracking. Therefore, from the viewpoint of preventing hot cracking, the C content in the welding material is preferably 0.20% by mass or less, more preferably 0.15% by mass or less, and even more preferably 0.12% by mass or less, based on the total mass of the welding material. On the one hand, from the perspective of improving the strength of the weld metal, the C content in the welding material is preferably 0.01% by mass or more, more preferably 0.03% by mass or more, and even more preferably 0.05% by mass or more based on the total mass of the welding material.

[0030] <Si: 0.05% by mass or more and 1.00% by mass or less> Si is an element that reduces the surface tension of the weld metal and improves the bead shape, but it is also a component that is prone to oxidation and is an element that generates slag that is difficult to peel off on the bead surface. Therefore, from the perspective of preventing the generation of slag and improving the paintability, the Si content in the welding material is preferably 1.00% by mass or less, more preferably 0.80% by mass or less, and even more preferably 0.60% by mass or less based on the total mass of the welding material. On the other hand, from the perspective of improving the bead shape, the Si content in the welding material is preferably 0.05% by mass or more, more preferably 0.10% by mass or more, and even more preferably 0.15% by mass or more based on the total mass of the welding material.

[0031] <V: 0.50% by mass or less> V is an element useful for increasing the strength by enhancing the hardenability, but if it is contained excessively, carbides will precipitate, causing the weld metal to harden and the toughness to deteriorate, which may lead to cracking. Therefore, from the perspective of preventing cracking, the V content in the welding material is preferably 0.50% by mass or less based on the total mass of the welding material, and it may even be 0% by mass.

[0032] <Nb: 0.10% by mass or less> Nb is an element useful for increasing the strength by enhancing the hardenability, but if it is contained excessively, carbides will precipitate, causing the weld metal to harden and the toughness to deteriorate, which may lead to cracking. Therefore, from the perspective of preventing cracking, the Nb content in the welding material is preferably 0.10% by mass or less based on the total mass of the welding material, and it may even be 0% by mass.

[0033] <The balance> The remainder of the welding material according to this embodiment consists of Fe and unavoidable impurities. Examples of unavoidable impurities include O, N, P, S, Al, Ti, Zr, Co, Li, Sn, Sb, Bi, B, and As. Preferably, each of the above unavoidable impurities is 0.01% by mass or less of the total mass of the welding material. Furthermore, the total amount of the above unavoidable impurities is preferably 0.10% by mass or less, and more preferably 0.08% by mass or less, of the total mass of the welding material.

[0034] Next, the shielding gas used in the gas shielded arc welding method according to this embodiment will be described.

[0035] <Shielding gas> In the gas shielded arc welding method according to this embodiment, the composition of the shielding gas is not particularly limited. However, by increasing the Ar content in the shielding gas and decreasing the CO2 content, the formation of slag caused by oxygen in CO2 can be prevented, and the coating can be reliably formed, thus preventing the occurrence of rust. Therefore, when using an Ar-CO2 mixed gas as the shielding gas, the Ar content is preferably 70% by volume or more, more preferably 80% by volume or more, and even more preferably 90% by volume or more. It is also preferable to use 100% by volume Ar gas as the shielding gas.

[0036] Furthermore, in the welding method according to this embodiment, the welding position is not particularly limited. Moreover, the type of welding is not particularly limited, and fillet welding such as lap fillet welding and T-fillet welding, or groove welding such as butt welding can be performed. Among these, fillet welding, especially lap fillet welding, is preferred as the effects of the welding method of this embodiment are more desirable. Furthermore, the diameter of the welding material used is not particularly limited, and for example, welding material with a diameter specified in welding material standards such as AWS or JIS can be used. [Examples]

[0037] The effects of the present invention will be specifically explained below with reference to examples and comparative examples, but the present invention is not limited thereto.

[0038] [Manufacturing of welding wires] Gas shielded arc welding wire (welding material) with various components as shown in Table 1 and a diameter of 1.2 mm was manufactured. In Table 1 below, "-" indicates that the level was below the detection limit.

[0039] [Gas shielded arc welding] Two steel plates were placed on top of each other, offset from one another, and gas shielded arc welding was performed on the fillet weld using the wire described above. The type of steel plate and welding conditions are shown below.

[0040] (Types of steel plates) Steel plate A: A steel plate with a tensile strength of 1180 MPa class and a thickness of 1.4 mm. Steel plate B: A steel plate with a tensile strength of 1470 MPa and a thickness of 1.5 mm.

[0041] (Welding conditions) Welding current: 110~120A Welding voltage: 23~25V Welding speed: 90 cm / min Shielding gas: 90% Ar-10% CO2

[0042] [Fatigue Test] From the obtained welded joint, a test specimen was prepared by machining so that the weld bead toe on the lower plate side was in the center of the test specimen. A planar bending fatigue test was then performed using a PBF30 testing machine manufactured by Tokyo Koki Co., Ltd. under the test conditions shown below.

[0043] (Test conditions) Stress ratio R: -1 Frequency: 25Hz Stopping condition: When torque drops by 50% Maximum number of repetitions: 2 × 10 6 times

[0044] [Table 1]

[0045] Next, the number of cycles (fatigue life) was compared for test specimens welded to the two types of steel plates described above using wires of various compositions, with the stress amplitude fixed at 400 MPa. The fatigue life was compared between test specimens using steel A and between test specimens using steel B. For the test specimens using steel plate A, which has a strength of 1180 MPa, the fatigue life of Comparative Example No. a1, which used wire W1, was set to 1.00, and the fatigue life was expressed as the ratio of the number of cycles of Examples No. A1 to A6, which used other wires, to the number of cycles of Comparative Example No. a1. Similarly, for the test specimens using steel plate B, which has a strength of 1470 MPa, the fatigue life of Comparative Example No. b1, which used wire W1, was set to 1.00, and the fatigue life was expressed as the ratio of the number of cycles of Examples No. B1 and B2, which used other wires, to the number of cycles of Comparative Example No. b1. The evaluation results are shown in Table 2 below.

[0046] [Table 2]

[0047] As shown in Table 2 above, when using steel plates with a tensile strength of 1180 MPa, Examples A1 to A6, in which the Cr and Ni content in the welding material was within the range of the present invention, achieved superior fatigue life compared to Comparative Example No. a1, in which the Cr content was outside the range of the present invention. Furthermore, when using steel plates with a tensile strength of 1470 MPa, Examples B1 and B2, in which the Cr and Ni content in the welding material was within the range of the present invention, achieved superior fatigue life compared to Comparative Example No. b1, in which the Cr content was outside the range of the present invention. These results demonstrate that the gas shielded arc welding method according to the present invention is extremely effective as a welding method for steel materials having a tensile strength of 1180 MPa or higher.

Claims

1. A gas shielded arc welding method in which a steel material is welded by melting a portion of the welding material and the steel material while flowing a shielding gas, The aforementioned steel material has a tensile strength of 1180 MPa or more. The plate thickness of the aforementioned steel material is 2.5 mm or less. The welding material is, with respect to the total mass of the welding material, Cr: 6.00% by mass or more and 15.50% by mass or less, Mn: 0.69% by mass or more and 1.70% by mass or less, Mo: 0.30% by mass or more and 1.05% by mass or less, C: 0.01% by mass or more and 0.20% by mass or less, and Si: 0.05% by mass or more and 1.00% by mass or less It contains, Ni: Less than 9.00% by mass, V: 0.50% by mass or less, and A gas shielded arc welding method characterized in that Nb is 0.10% by mass or less, with the remainder being Fe and unavoidable impurities.

2. The welding material is, with respect to the total mass of the welding material, The gas shielded arc welding method according to claim 1, comprising Cr: 6.00% by mass or more and 9.00% by mass or less.

3. A method for manufacturing a welded joint obtained by gas shielded arc welding, in which a welding material and a part of the steel material are melted while a shielding gas is flowed through the steel material, The aforementioned steel material has a tensile strength of 1180 MPa or more. The plate thickness of the aforementioned steel material is 2.5 mm or less. The welding material is, with respect to the total mass of the welding material, Cr: 6.00% by mass or more and 15.50% by mass or less, Mn: 0.69% by mass or more and 1.70% by mass or less, Mo: 0.30% by mass or more and 1.05% by mass or less, C: 0.01% by mass or more and 0.20% by mass or less, and Si: 0.05% by mass or more and 1.00% by mass or less It contains, Ni: Less than 9.00% by mass, V: 0.50% by mass or less, and A method for manufacturing a welded joint, characterized in that Nb is 0.10% by mass or less, and the remainder is Fe and unavoidable impurities.

4. The welding material is, with respect to the total mass of the welding material, A method for manufacturing a welded joint according to claim 3, comprising Cr: 6.00% by mass or more and 9.00% by mass or less.

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