Weld wire for fms steel and welded joint

Abstract

The present invention provides: a weld wire which is used for welding of parts of 15Mn steel or for both welding of parts of 15Mn steel and welding of 15 Mn steel and 490 MPa class steel, and with which it is possible to obtain a welded joint that does not generate solidification cracking and has a fatigue characteristic that can withstand use as a vibration suppression damper or the like by controlling the solidification mode of weld metal at a weld part; and a welded joint welded using the weld wire. The weld wire according to the present invention is used for welding of parts of 15Mn steel of which the solidification mode is FA solidification mode, and contains Cr, Ni, Mn, Mo, Si, and C as a chemical composition, the remainder consisting of Fe and unavoidable components. The content of an alloy constituent indicated by mass% is within a region which is surrounded by point a (13.09, 12.23), point b (18.55, 7.71), point c (22.79, 9.85), and point d (23.29, 21.78) in the Schaeffler diagram, and which is surrounded by straight line ab, straight line bc, broken line cd, and straight line da.

Description

FMS steel welding wire and weld joints

[0001] The present invention relates to a welding wire used for welding Fe-Mn-Si alloys (hereinafter also referred to as "FMS steel") together or for welding FMS steel to structural steel, and to a welded joint welded using this welding wire. In particular, the present invention relates to a welding wire used for welding FMS steels containing 15 mass% Mn (hereinafter also referred to as "15Mn steel") together or for welding 15Mn steel to structural steel having a tensile strength of 490 MPa or more (hereinafter also referred to as "490 MPa-class steel"), and to a welded joint welded using this welding wire. Note that in this specification, the Mn content of 15 mass% in 15Mn steel means "a range of 14.5 mass% or more and less than 15.5 mass%," taking into account significant digits. The same applies when referring to the contents of component elements other than Mn in the chemical composition of alloys such as 15Mn steel.

[0002] Recently, a vibration-damping alloy (FMS steel) has been developed that has a fatigue life approximately 10 times longer than conventional vibration-damping steel and general steel used in building structures (Patent Document 1). New vibration-damping dampers using this alloy as the core material have been applied to large architectural structures such as skyscrapers and large-scale exhibition halls. This new vibration-damping damper is expected to become even more widespread as a vibration-damping damper with a long fatigue life that can repeatedly withstand long-period seismic motion, aftershocks after major earthquakes, and subsequent earthquakes. Also, a brace-type vibration-damping damper has been developed that uses the FMS steel (15Mn steel) plate material described in Patent Document 1 and welds carbon steel to the joints of the vibration-damping damper plate to the building frame to form a cross cross section, thereby improving the strength and durability against buckling of the joints.

[0003] Regarding welding of FMS steel, for example, Patent Document 2 discloses a welding wire used for welding FMS steel containing 28 mass% Mn (hereinafter also referred to as "28Mn steel") to itself or for welding 28Mn steel to high-strength steel for construction having a tensile strength of 590 MPa or more, and a welded joint welded using this welding wire.

[0004] Furthermore, Patent Document 3 proposes conditions for a welded structure using a damping alloy such as FMS steel as a base material to exhibit excellent fatigue properties at the welded portion equivalent to those of the base material, and describes in the examples that by using 15Mn steel having a specific component composition as the base material and weld material, a welded structure can be produced that does not suffer from solidification cracking due to welding and has an excellent fatigue life.

[0005] JP 2014-129567 A JP 2015-150586 A International Publication No. 2021 / 261067

[0006] F. Yoshinaka, et al., Development of ferrous-based weldable seismic damping alloy with prolonged plastic fatigue life, Scripta Mater 197 (2021) 113815. T. Nagira, et al., Direct observation of solidification behaviors of Fe-Mn-Si alloys during TIG spot welding using synchrotron X-ray, Scripta Mater 216 (2022) 11. Matsuda Fukuhisa (ed.), Welding and Joining Technology Data Book, Industrial Technology Service Center, 2007.

[0007] According to Patent Document 3, the Fe-Mn-Cr-Ni-Si alloy described therein (which is an embodiment of FMS steel) is suitable for use as a steel material (i.e., base metal) to be welded with a welding material by satisfying a predetermined chemical composition and conditions, and is also suitable for use as a welding material for welding the steel material. However, when designing the chemical composition of a welding material, it is necessary to identify the chemical composition of the base metal and then consider factors such as the dilution rate in the case of homogeneous welding, or the welding conditions, groove shape, and dilution rate in the case of dissimilar welding, to find conditions under which the weld metal of the weld is in the FA solidification mode. Here, the FA solidification mode refers to a solidification mode in which, during the solidification of a steel material, a ferrite phase first crystallizes from the liquid phase, followed by the crystallization of an austenite phase, and solidification is completed in two phases: ferrite and austenite. When the weld metal is in the FA solidification mode, solidification cracking susceptibility can be reduced.

[0008] Therefore, an object of the present invention is to provide a welding wire that is used for welding 15Mn steel sheets together (same-material welding), which controls the solidification mode of the weld metal in the weld, prevents solidification cracking, and realizes a welded joint that has fatigue properties sufficient for use as a vibration damper, etc., and a welded joint welded using this welding wire. Another object of the present invention is to provide a welding wire that realizes the above-mentioned features not only in welding 15Mn steel sheets together but also in welding 15Mn steel and 490 MPa-class steel (dissimilar material welding), and a welded joint welded using this welding wire.

[0009] In order to solve the above-mentioned problems, one embodiment of the welding wire of the present invention is an Fe-Mn-Si alloy containing 14.5 mass% or more and less than 15.5 mass% of Mn, and is a welding wire used for welding FMS steels whose solidification mode is an FA solidification mode, and this welding wire contains, as a chemical composition, Cr, Ni, Mn, Mo, Si, and C, with the balance being Fe and unavoidable elements, and the contents of the alloy elements expressed in mass% are less than or equal to the Cr equivalent (Cr ｅｑ ) and the Ni equivalent defined by the following formula Z2 (Ni ｅｑ) in the Schaefflerian structure diagram, the area surrounded by points a (13.09, 12.23), b (18.55, 7.71), c (22.79, 9.85), and d (23.29, 21.78) (where the point coordinates are (Cr ｅｑ , Ni ｅｑ )) and falls within the area surrounded by the line ab, the line bc, the broken line cd, and the line da (here, the broken line cd is １２ (22.71, 11.03), point S １１ (23.21, 12.27), point S １０ (23.78, 13.83), point S ９ (23.90, 15.50), point S ８ (23.77, 17.12), point S ７ (23.40, 18.49), point S ６ (23.15, 20.05) and point d (23.29, 21.78) in sequence.) Formula Z1 Cr equivalent (Cr ｅｑ )=(mass%Cr)+(mass%Mo)+1.5×(mass%Si)+0.5×(mass%Nb) Formula Z2 Ni equivalent (Ni ｅｑ )=(mass%Ni)+30×(mass%C)+0.5×(mass%Mn)

[0010] It is preferable that the above-mentioned welding wire is used for welding the above-mentioned FMS steels at a dilution ratio of 20% or more and 35% or less, the structure of the weld metal is a single-phase structure of austenite or a two-phase structure of austenite and ferrite, and the chemical composition of the weld metal is in the FA solidification mode.

[0011] Another embodiment of the welding wire of the present invention, which solves the above-mentioned problems, is an Fe-Mn-Si alloy containing 14.5 mass% or more and less than 15.5 mass% of Mn, and is used for both welding of FMS steels whose solidification mode is an FA solidification mode and welding of the FMS steels to structural steel materials having a tensile strength of 490 MPa or more, and this welding wire contains, as a chemical composition, Cr, Ni, Mn, Mo, Si, and C, with the balance being Fe and unavoidable elements, and the contents of alloy elements expressed in mass% are such that the Cr equivalent (Cr ｅｑ ) and the Ni equivalent defined by the following formula Z2 (Ni ｅｑ ) in the Schaefflerian structure diagram, the area surrounded by points e (17.46, 14.82), f (22.71, 10.47), g (23.78, 13.83), and h (19.86, 17.06) (where the point coordinates are (Cr ｅｑ , Ni ｅｑ )) and falls within the area surrounded by the line ef, the broken line fg, the line gh, and the line he (here, the broken line fg is located at the point f(22.75, 10.44), the point S １２ (22.71, 11.03), point S １１ (23.21, 12.27), and point g (23.78, 13.83) in sequence.) Formula Z1 Cr equivalent (Cr ｅｑ )=(mass%Cr)+(mass%Mo)+1.5×(mass%Si)+0.5×(mass%Nb) Formula Z2 Ni equivalent (Ni ｅｑ )=(mass%Ni)+30×(mass%C)+0.5×(mass%Mn)

[0012] The above-mentioned welding wire is used both for welding the above-mentioned FMS steels together at a dilution ratio of 20% or more and 35% or less, and for welding the above-mentioned FMS steel with a structural steel material having a tensile strength of 490 MPa or more at a dilution ratio of 20% or more and 35% or less, and it is preferable that the structure of the weld metal has a single-phase austenite structure or a two-phase austenite-ferrite structure, and that the chemical composition of the weld metal has an FA solidification mode.

[0013] Additionally, in the welding wire according to both of the above-mentioned embodiments, the FMS steel has a chemical composition containing, as essential elements, 14.5 mass%≦Mn<15.5 mass%, 5 mass%≦Cr≦15 mass%, 5 mass%≦Ni<12 mass%, and 2 mass%≦Si≦6 mass%, and as optional elements or unavoidably contained impurity elements, Al, Co, Cu, Nb, Ta, V, Ti, and Mo in a total amount of 0 mass% or more and 1 mass% or less, and C, N, and B in a total amount of 0 mass% or more and 0.2 mass% or less, and the balance being Fe and unavoidable impurities, and a Gibbs free energy difference ΔG between the γ phase and the ε phase defined by the following formulas 1 to 5 and the thermodynamic parameters in Table 1. γ→ε However, -150 J / mol < ΔG γ→ε < 50 J / mol, and the ratio ([%Cr]eq / [%Ni]eq) of the Cr equivalent ([%Cr]eq) defined by the following formula 6 to the Ni equivalent ([%Ni]eq) defined by the following formula 7 satisfies the following condition: 1.33<[%Cr]eq / [%Ni]eq≦1.96.

[0014] One aspect of the welded joint of the present invention is an Fe-Mn-Si based alloy containing 14.5 mass% or more and less than 15.5 mass% Mn, characterized in that FMS steels having a solidification mode of FA solidification mode are welded together using the welding wire according to the former aspect described above.

[0015] In the above-described welded joint, it is preferable that the FMS steels are welded together using the welding wire at a dilution ratio of 20% to 35%, that the structure of the weld metal is a single-phase austenite structure or a two-phase austenite-ferrite structure, and that the chemical composition of the weld metal is in the FA solidification mode.

[0016] Furthermore, the above-mentioned welded joint preferably has a number of cycles to fracture of 3,000 or more in a low-cycle fatigue test at a strain rate of 0.4% / s and a test strain range of 2.0% (±1.0%).

[0017] Another aspect of the welded joint of the present invention is an Fe-Mn-Si alloy containing 14.5 mass% or more and less than 15.5 mass% Mn, characterized in that two FMS steels having a solidification mode of FA solidification, or the FMS steel and a structural steel material having a tensile strength of 490 MPa or more, are welded together using the welding wire of the latter aspect described above.

[0018] In the above-described welded joint, it is preferable that the FMS steels or the FMS steel and a structural steel material having a tensile strength of 490 MPa or more are welded with the above-described welding wire at a dilution ratio of 20% to 35%, that the structure of the weld metal is a single-phase austenite structure or a two-phase austenite-ferrite structure, and that the chemical composition of the weld metal is an FA solidification mode.

[0019] Furthermore, the above-mentioned welded joint preferably has a number of cycles to fracture of 550 or more in a low-cycle fatigue test at a strain rate of 0.4% / s and a test strain range of 2.0% (±1.0%).

[0020] Additionally, in the welded joints according to both of the above-mentioned embodiments, the FMS steel has a chemical composition containing, as essential elements, 14.5 mass%≦Mn<15.5 mass%, 5 mass%≦Cr≦15 mass%, 5 mass%≦Ni<12 mass%, and 2 mass%≦Si≦6 mass%, and as optional elements or unavoidably contained impurity elements, Al, Co, Cu, Nb, Ta, V, Ti, and Mo in a total amount of 0 mass% or more and 1 mass% or less, and C, N, and B in a total amount of 0 mass% or more and 0.2 mass% or less, with the balance being Fe and unavoidable impurities, and a Gibbs free energy difference ΔG between the γ phase and the ε phase defined by the following formulas 1 to 5 and the thermodynamic parameters in Table 1. γ→ε However, -150 J / mol < ΔG γ→ε < 50 J / mol, and the ratio ([%Cr]eq / [%Ni]eq) of the Cr equivalent ([%Cr]eq) defined by the following formula 6 to the Ni equivalent ([%Ni]eq) defined by the following formula 7 satisfies the following condition: 1.33<[%Cr]eq / [%Ni]eq≦1.96.

[0021] The welding wire of the present invention makes it possible to weld both 15Mn steels together and 15Mn steels to 490 MPa-class steels.

[0022] Furthermore, the welded joint of the present invention exhibits the excellent fatigue properties inherent to the 15Mn steel base material at the welded portion, making it possible to use it as a vibration damper or the like.

[0023] 1 is a Schaeffler-type structural diagram showing the solidification mode of a general stainless steel. 2 is a Schaeffler-type structural diagram showing the solidification mode of 15Mn steel. 3 is a schematic diagram of a Schaeffler-type structural diagram showing an FA solidification mode capable of suppressing solidification cracking of the weld metal. 4 is a Schaeffler-type structural diagram showing conditions capable of suppressing the occurrence of hardened structures in the weld metal. 5 is a Schaeffler-type structural diagram showing the boundary line at which σ-phase precipitation embrittlement occurs due to heat treatment during preparation of the welding material. 6 is a Schaeffler-type structural diagram relating to the chemical composition of a welding material used for welding between 15Mn steel base materials. 7 is a Schaeffler-type structural diagram relating to the chemical composition of a welding material used for welding between 15Mn steel base materials. 8 is a Schaeffler-type structural diagram relating to the chemical composition of a welding material used for welding between 15Mn steel base materials. 9 is a Schaeffler-type structural diagram relating to the chemical composition of a welding material used for welding between 15Mn steel base materials and 490 MPa class steel. 10 is a Schaeffler-type structural diagram relating to the chemical composition of a welding material used for welding between 15Mn steel base materials and 490 MPa class steel. FIG. 1 is a Schaeffler-type structural diagram relating to the chemical composition of a welding material used for welding a 15Mn steel base material and a 490 MPa-class steel. The upper part is a schematic diagram showing the specifications of a welded joint produced in an example, and the lower part is a schematic diagram explaining collection of a test piece for a low-cycle fatigue test. FIG. 2 is a diagram in which the chemical compositions of the weld metal and welding wire 1 of a welded joint 1 produced in an example are plotted on a Schaeffler-type structural diagram. FIG. 3 is a diagram in which the chemical compositions of the weld metal and welding wire 2 of a welded joint 2 produced in an example are plotted on a Schaeffler-type structural diagram. FIG. 4 is a diagram in which the chemical compositions of the weld metal and welding wire 3 of a welded joint 3 produced in an example are plotted on a Schaeffler-type structural diagram. FIG. 5 is a diagram in which the chemical compositions of the weld metal and welding wire 4 of a welded joint 4 produced in an example are plotted on a Schaeffler-type structural diagram. FIG. 6 is a diagram in which the chemical compositions of the weld metal and welding wire 1 of a welded joint 5 produced in an example are plotted on a Schaeffler-type structural diagram. Fig. 1 is a diagram in which the chemical compositions of the weld metal of a welded joint 6 produced in an example and the welding wire 2 are plotted on a Schaeffler-type structural diagram. Fig. 2 is a diagram in which the chemical compositions of the weld metal of a welded joint 7 produced in an example and the welding wire 3 are plotted on a Schaeffler-type structural diagram. Fig. 3 is a diagram in which the chemical compositions of the weld metal of a welded joint 8 produced in an example and the welding wire 4 are plotted on a Schaeffler-type structural diagram.

[0024] Hereinafter, embodiments of the present invention will be described in detail. The following description of the components may be based on a representative embodiment of the present invention, but the present invention is not limited to such an embodiment.

[0025] The characteristics of weld metal can be understood by plotting the chemical composition of the weld metal on a Schaeffler-type structural diagram (see Patent Document 2). Generally, weld metal solidifies in various solidification modes, and the solidification mode changes depending on the chemical composition of the weld metal. Figure 1 shows the solidification modes of a typical stainless steel.

[0026] On the other hand, it has been reported that the 15Mn steel targeted by the present invention exhibits a solidification mode different from that of conventional stainless steels (Non-Patent Documents 1 and 2). Figure 2 shows the solidification mode of 15Mn steel. Here, the two asterisks in Figure 2 indicate the 15Mn steel described in Patent Document 1 (having a chemical composition equivalent to "Fe-15Mn-10Cr-8Ni-4Si") (0.016C-3.94Si-15.22Mn-7.97Ni-10.10Cr-0.01Mo) (unit: mass%, balance: Fe) and the 15Mn steel described in Patent Document 3 (having a chemical composition equivalent to "Fe-15Mn-11Cr-7.5Ni-4Si") The 15Mn steel (0.028C-4.07Si-15.4Mn-7.53Ni-11.10Cr) (units: mass%, balance: Fe) and the 15Mn steel (0.028C-4.07Si-15.4Mn-7.53Ni-11.10Cr) (units: mass%, balance: Fe) have in common the chemical composition of containing 15 mass% Mn, but differ in that the solidification mode of the former is mode A (a solidification mode that begins with solidification from the liquid phase to the γ phase and ends with solidification to the γ phase), while the solidification mode of the latter is mode FA. Furthermore, compared to the solidification mode of the stainless steel shown in Figure 1, the 15Mn steel shown in Figure 2 shows that the region in which the FA solidification mode occurs is wider.

[0027] Based on the above findings, the inventors have determined that the weld metal solidifying from the melt has both microstructural conditions for a long fatigue life and FA solidification mode conditions for suppressing weld defects, that is, the solidification of the weld is in FA mode, and the chemical composition of the welding wire is selected so as to be compatible with the dilution ratios of various joints.

[0028] Furthermore, the present invention provides a welding wire that takes the following two points into consideration. First, the wire is intended for use primarily in the manufacture of vibration-damping components such as vibration dampers. The steel combinations are 15Mn steels (15Mn steel-15Mn steel) and 15Mn steels and 490 MPa-class steels (e.g., 15Mn steel-SN490 steel, 15Mn steel-SM490 steel, etc.), and the wire is intended to provide a welding material suitable for both the former same-material welding and the latter dissimilar-material welding, as well as a welding material applicable to both. Second, the weld is a fillet-welded joint, and the wire is intended to provide a welding material that can be used even with a dilution ratio ranging from 20% to 35% so that the desired weld metal characteristics are maintained (FA solidification mode) even when welding variations occur during on-site construction. By using a fillet-welded joint, the weld is more versatile and can be used in brace-type vibration dampers with enhanced durability against buckling during compression. The dilution ratio is the ratio of the amount of molten base metal to the total amount of weld metal.

[0029] Furthermore, in the present invention, from the viewpoint of making the welded joint suitable primarily for vibration-damping members such as vibration dampers, the target value for the low-cycle fatigue properties of the welded joint is a number of cycles to fracture of 3,000 or more in the case of same-material welding, and a number of cycles to fracture of 550 or more in the case of dissimilar-material welding, in a low-cycle fatigue test with a strain rate of 0.4% / s and a test strain range of 2.0% (±1.0%).

[0030] The chemical composition of 15Mn steel generally contains, in mass%, Mn: 14.5% or more and less than 15.5% Si: 2.0% or more and less than 6.5% Additionally, the following may be allowed to be contained: Cr: 0% or more and less than 15.0% Ni: 0% or more and less than 15.0% Al: 0% or more and less than 3.0% The balance: Fe and unavoidable impurity elements.

[0031] Here, the 15Mn steel targeted by the present invention is one in which the solidification mode shown in Fig. 2 is the FA solidification mode, and is typically the 15Mn steel described in Patent Document 3. Hereinafter, this 15Mn steel will also be referred to as "15Mn steel base material."

[0032] That is, in a typical embodiment of the present invention, the 15Mn steel base material has a chemical composition containing, as essential elements, 14.5 mass%≦Mn<15.5 mass%, 5 mass%≦Cr≦15 mass%, 5 mass%≦Ni<12 mass%, and 2 mass%≦Si≦6 mass%, and as optional elements or unavoidably contained impurity elements, Al, Co, Cu, Nb, Ta, V, Ti, and Mo in a total amount of 0 mass% or more and 1 mass% or less, and C, N, and B in a total amount of 0 mass% or more and 0.2 mass% or less, and the balance being Fe and unavoidable impurities, and a Gibbs free energy difference ΔG between the γ phase and the ε phase defined by the following equations 1 to 5 and the thermodynamic parameters in Table 1. γ→ε However, -150 J / mol < ΔG γ→ε < 50 J / mol, and the ratio ([%Cr]eq / [%Ni]eq) of the Cr equivalent ([%Cr]eq) defined by the following formula 6 to the Ni equivalent ([%Ni]eq) defined by the following formula 7 satisfies the following condition: 1.33<[%Cr]eq / [%Ni]eq≦1.96.

[0033] The chemical composition of the 490 MPa class steel used in dissimilar metal welding with the 15Mn steel base metal is exemplified by SN490 steel and SM490 steel. For example, the chemical composition of SN490B, expressed in mass%, is as follows: C: 0.18% or less (thickness 50 mm or less), 0.20% or less (thickness over 50 mm and 200 mm or less), Si: 0.55% or less, Mn: 1.65% or less, P: 0.030% or less, S: 0.015% or less, and the balance: Fe and unavoidable impurity elements.

[0034] As another example, the chemical composition of SM490A, expressed in mass%, is as follows: C: 0.20% or less (thickness 50 mm or less), 0.22% or less (thickness over 50 mm and 200 mm or less) Si: 0.55% or less Mn: 1.65% or less P: 0.035% or less S: 0.035% or less Balance: Fe and unavoidable impurity elements.

[0035] When producing a welded joint using the above-mentioned 15Mn steel base material, the welding material must satisfy the following two main requirements: (1) that solidification cracking (hot cracking) does not occur in the weld metal during welding, and (2) that hardened structures such as martensite phases are unlikely to occur in the weld metal at the welded portion after welding.

[0036] First, the chemical composition of the welding material that satisfies the above condition (1) will be explained with reference to Fig. 3. In this specification, the plot on the Schaeffler-type structural diagram showing the chemical composition of the target material is calculated using the values ​​of the Cr equivalent and Ni equivalent (Cr ｅｑ , Ni ｅｑ ) where Cr equivalent (Cr ｅｑ ) and Ni equivalent (Ni ｅｑ ) is defined by the following formulas Z1 and Z2, separately from the Cr equivalent ([%Cr]eq) (formula 6) and Ni equivalent ([%Ni]eq) (formula 7) which represent the chemical composition of the 15Mn steel base material described above.

[0037] Formula Z1 Cr equivalent (Cr ｅｑ )=(mass%Cr)+(mass%Mo)+1.5×(mass%Si)+0.5×(mass%Nb) Formula Z2 Ni equivalent (Ni ｅｑ )=(mass%Ni)+30×(mass%C)+0.5×(mass%Mn)

[0038] If the chemical composition of the welding material is (a, b), the chemical composition of the workpiece (base metal) is (c, d), and the chemical composition of the weld metal is (X, Y), the chemical composition of the weld metal can be expressed as follows using integers n and m: X = (na + mc) / (m + n), Y = (nb + md) / (m + n). In order for (X, Y) to fall within the FA solidification mode region of the 15Mn steel solidification mode shown in Figure 2, the conditions Y > αX and Y < βX must be satisfied, where α = 1 / 1.95 and β = 1 / 1.48. Therefore, the following relationships hold for the chemical composition (a, b) of the welding material: b > αa + m / n(αc - d) [1] b < βa + m / n(βc - d) [2]

[0039] In the above relational expressions [1] and [2], the values ​​of the integers n and m are determined by the dilution ratio of the weld. In the present invention, the dilution ratio of the weld is set to 20% to 35% as described above. For example, when the dilution ratio is 20%, n = 80 and m = 20, and when the dilution ratio is 35%, n = 65 and m = 35.

[0040] Next, the chemical composition of welding materials that satisfy the above condition (2) will be described with reference to Fig. 4. For convenience, the boundaries of solidification modes (A, AF-mode, FA-mode, F-mode) shown in Fig. 4 are those of the stainless steel shown in Fig. 1.

[0041] For the chemical composition (X, Y) of the weld metal to be in the austenite region (region A) or the austenite + ferrite region (region A + F), the condition Y > δX + γ must be satisfied, where δ = -0.827 and γ = 25.59. Therefore, the following relationship holds for the chemical composition (a, b) of the welding material: b > δa + 1 / n(δmc + γ(m+n) - md) [3]

[0042] As with the above-mentioned relations [1] and [2], the values ​​of the integers n and m in the above-mentioned relation [3] are determined by the dilution ratio of the weld. In the present invention, since the dilution ratio of the weld is set to 20% to 35% as described above, for example, when the dilution ratio is 20%, n = 80 and m = 20, and when the dilution ratio is 35%, n = 65 and m = 35.

[0043] It is known that when welding conventional stainless steels containing Cr and Mn, the application of heat equivalent to postweld heat treatment (PWHT) can cause the precipitation of a σ-phase in the ingot of the welding material, resulting in cracking (see Non-Patent Document 3). Specifically, in Section 8, 8.2, Solidification Cracking, (3) Schaeffler's Microstructure and Welding Problems of Non-Patent Document 3 (Reference No. 6, Haruyoshi Suzuki: Welding Handbook, Sankaido, March 1960), a region where "sigma precipitation embrittlement occurs due to heat treatment" is illustrated. Based on this, in the present invention, for welding materials used in welding 15Mn steel base materials together (same-metal welding) and welding 15Mn steel base materials to 490 MPa-class steel (dissimilar metal welding), the boundary line where σ-phase precipitation embrittlement occurs due to heat treatment during welding material preparation is defined on a Schaeffler's microstructure diagram, as shown in Figure 5.

[0044] In FIG. 5, the boundary line is at point S １ (24.15, 28.44) to point S １７ (26.52, 7.26), and more specifically, point S １ (24.15, 28.44), point S ２ (24.40, 26.88), point S ３ (24.15, 25.33), point S ４ (23.90, 23.90), point S ５ (23.29, 21.78), point S ６ (23.15, 20.05), point S ７ (23.40, 18.49), point S ８ (23.77, 17.12), point S ９ (23.90, 15.50), point S １０ (23.78, 13.83), point S １１ (23.21, 12.27), point S １２ (22.71, 11.03), point S １３ (22.79, 9.85), point S １４ (23.46,9.09), point S １５ (24.40,8.34), point S １６ (25.46, 7.71), and point S １７The broken line is a line connecting the points (a, b) in order (26.52, 7.26). If the chemical composition (a, b) of the welding material is in the region to the right of the broken line (the region shown in dark contrast in FIG. 5), it is considered that the heat treatment during preparation of the welding material may cause σ-phase precipitation embrittlement, and the chemical composition of the welding material being outside of this region is defined as condition [4] that the chemical composition of the welding material of the present invention must satisfy.

[0045] Based on the above four conditions, i.e., the above relational expressions [1], [2], and [3], and the above condition [4], the chemical compositions (a, b) of welding materials used for welding 15Mn steel base materials together (same-metal welding) and welding 15Mn steel base material and 490 MPa-class steel (dissimilar-metal welding) are determined.

[0046] An example of the chemical composition of a 15Mn steel base metal is shown in Table 2. The chemical composition (c, d) of the material to be welded (base metal) based on this chemical composition is (17.21, 16.07).

[0047]

[0048] First, welding of 15Mn steel base materials (same-material welding) will be described with reference to Figures 6 and 7. In Figure 6, the dark-colored areas represent the areas where the chemical composition (a, b) of the welding material satisfies the above-mentioned relational expressions [1], [2], and [3] when the dilution ratio changes from 20% to 35% in the welded portion of the fillet welded joint described above. That is, the areas where the chemical composition (X, Y) of the weld metal is in the FA solidification mode and is in area A or area A+F. The line indicated by symbol (A) represents the case where the dilution ratio is 20%, and the line indicated by symbol (B) represents the case where the dilution ratio is 35%, and the line passing through the origin overlaps with (A) and (B). Applying the above-mentioned condition [4] to these areas leads to the areas indicated by dark-colored contrast in Figure 7. This area is the area surrounded by points a (13.09, 12.23), b (18.55, 7.71), c (22.79, 9.85), and d (23.29, 21.78), and is also the area surrounded by lines ab, bc, cd, and da. １２ (22.71, 11.03), point S １１(23.21, 12.27), point S １０ (23.78, 13.83), point S ９ (23.90, 15.50), point S ８ (23.77, 17.12), point S ７ (23.40, 18.49), point S ６ The broken line connects point c (23.15, 20.05) and point d (23.29, 21.78) in order. Note that points c and d are respectively connected to point S １３ and point S ５ (See FIG. 7A).

[0049] Next, welding of 15Mn steel base material and 490 MPa-class steel (dissimilar metal welding) will be described with reference to Figures 8 and 9. Because the melting points of the 15Mn steel base material and the 490 MPa-class steel are different, the melting ratio during welding varies depending on this melting point difference and the groove shape. Therefore, when the melting ratio was investigated through welding tests, it was confirmed that the melting ratio between the 15Mn steel base material and the 490 MPa-class steel was approximately constant at 3:1. Therefore, the chemical composition (c, d) of the welded material (base material) in this dissimilar metal welding is (4.83, 7.00).

[0050] In Figure 8, the region shown with dark contrast is the region where the chemical composition (a, b) of the welding material satisfies the above-mentioned relational expressions [1], [2], and [3] when the dilution rate changes from 20% to 35% in the welded portion of the above-mentioned fillet welded joint. That is, the region where the chemical composition (X, Y) of the weld metal is in the FA solidification mode and is in region A or region A+F. Note that the line indicated by symbol (A) corresponds to the case where the dilution rate is 20%, and the line indicated by symbol (B) corresponds to the case where the dilution rate is 35%. Then, by applying the above-mentioned condition [4] to this, the region shown with dark contrast in Figure 9 is derived. This area is the area surrounded by points e (17.46, 14.82), f (22.75, 10.44), g (23.78, 13.83), and h (19.86, 17.06), and is also the area surrounded by line ef, broken line fg, line gh, and line he. １２ (22.71, 11.03), point S １１The broken line connects point g (23.21, 12.27) and point g (23.78, 13.83) in order. Note that point g is the same as point S shown in FIG. １０ (See FIG. 9A).

[0051] Here, when the region surrounded by points a, b, c, and d in Fig. 7 for the same material welding is compared with the region surrounded by points e, f, g, and h in Fig. 9 for the dissimilar material welding, it can be seen that the former region encompasses the latter region. In other words, some of the welding materials used for welding 15Mn steel base materials together can also be used for welding 15Mn steel base materials and 490 MPa class steel, and furthermore, welding materials used for welding 15Mn steel base materials and 490 MPa class steel can also be used for welding 15Mn steel base materials together.

[0052] The welding wire of the present invention selected as described above has a chemical composition containing Cr (chromium), Ni (nickel), Mn (manganese), Mo (molybdenum), Si (silicon), and C (carbon), with the balance consisting of Fe (iron) and unavoidable elements. Here, the content of each component element is adjusted to satisfy the above-mentioned conditions, but is limited by manufacturing constraints, etc. Specific preferred ranges, expressed in mass%, for same-material welding and dissimilar material welding are exemplified as follows:

[0053] [Wire for welding 15Mn steel base materials together (same material welding)] Cr: more than 11.5% and not more than 12.5% ​​Ni: more than 6.5% and not more than 7.5% Mn: 14.8% or more and not more than 15.2% Mo: more than 0.013% and not more than 0.015% Si: more than 3.8% and not more than 4.2% C: more than 0.01% and not more than 0.05%

[0054] [Wire for welding 15Mn steel base material and 490MPa class steel (dissimilar material welding)] Cr: more than 15.7% and not more than 16.7% Ni: more than 6.4% and not more than 7.4% Mn: 10.0% or more and not more than 11.0% Mo: more than 0.013% and not more than 0.015% Si: more than 3.8% and not more than 4.2% C: more than 0.01% and not more than 0.05%

[0055] [Preparation of Welding Wire and Weld Joint for 15Mn Steel-15Mn Steel] Welding Wires 1 and 2 having the chemical compositions shown in Table 3 below were produced as welding materials for 15Mn steel-15Mn steel. For comparison, welding Wires 3 and 4 having the chemical compositions shown in Table 3 below were also produced. All of welding Wires 1 to 4 had a wire diameter of 1.2 mm and were welding wires for gas metal arc (GMA) welding.

[0056]

[0057] Using welding wires 1 to 4, fillet welding was performed between 20 mm thick 15Mn steel base materials having the chemical compositions shown in Table 2, to produce prototype welded joints 1 to 4. The welded joints were fillet welds with a leg length of 12 mm, as shown in the upper part of Figure 10. The flange dimensions were 70 x 210 x 20 mm, and the web dimensions were 35 x 210 x 20 mm.

[0058] To examine the strength of the weld metal of the fabricated weld joints 1 to 4, dog-bone fatigue test specimens with a gage diameter of 2.8 mm were taken from the weld metal of each weld joint, and low-cycle fatigue tests were performed. Specifically, as shown in the lower part of Figure 10, two test specimens (fatigue test specimens) were taken from one bead of the weld joint. The low-cycle fatigue tests were performed until fatigue failure under the following conditions: strain rate 0.4% / s, test strain range 2.0% (±1.0%), test waveform triangular wave, and room temperature (25°C), and the fatigue life (number of cycles to failure Nf) was measured.

[0059] Table 4 shows the chemical compositions of the weld metals of weld joints 1 to 4. Figures 11 to 14 show the results of plotting the chemical compositions of the weld metals of weld joints 1 to 4 and welding wires 1 to 4 on Schaeffler-type structural diagrams, respectively. Table 5 shows the properties of the weld metals of weld joints 1 to 4, including the results of low-cycle fatigue tests (average Nf values), main structures, and solidification modes.

[0060]

[0061]

[0062] The test results are explained below.

[0063] As shown in Figure 11, point coordinates (stars) based on the chemical composition of welding wire 1 were plotted within the region surrounded by points a, b, c, and d shown in Figure 7 for the same-material welding. Furthermore, point coordinates (squares) based on the chemical composition of the weld metal of weld joint 1 were plotted within the austenite + ferrite region (A + F region) and the FA solidification mode region. In the actually fabricated weld joint 1, no solidification cracking (hot cracking) occurred in the weld metal, resulting in a good weld joint. Furthermore, the number of cycles to fracture in the low-cycle fatigue test was 3,000 or more for all test specimens, with an average of 3,500 or more. This suggests that the plastic deformation mechanism of the 15Mn steel base metal effectively functioned during the compositional deformation under repeated deformation, resulting in excellent fatigue durability in the weld metal of the same-material welding weld joint.

[0064] As shown in Figure 12, the point coordinates (stars) based on the chemical composition of welding wire 2 were plotted within the region surrounded by points a, b, c, and d shown in Figure 7 for the same-material welding. Furthermore, the point coordinates (squares) based on the chemical composition of the weld metal of weld joint 2 were plotted close to the point coordinates of welding wire 2 and within the austenite region (region A) and the FA solidification mode region. In the actually fabricated weld joint 2, no solidification cracking (hot cracking) occurred in the weld metal, resulting in a good weld joint. Furthermore, the number of cycles to fracture in the low-cycle fatigue test was 4,000 or more for all test specimens, with an average of 5,000 or more. This suggests that the plastic deformation mechanism of the 15Mn steel base metal effectively functioned during the compositional deformation under repeated deformation, resulting in excellent fatigue durability in the weld metal of the same-material welding weld joint.

[0065] As shown in Figure 13, point coordinates (stars) based on the chemical composition of welding wire 3 were plotted outside the region surrounded by points a, b, c, and d shown in Figure 7 for the same-material welding. Furthermore, point coordinates (squares) based on the chemical composition of the weld metal of weld joint 3 were plotted within the austenite + martensite region (A + M region). The chemical composition of the weld metal was within the FA solidification mode region, and no solidification cracking (hot cracking) occurred in the weld metal of weld joint 3. However, the number of cycles to fracture (average) in the low-cycle fatigue test was 161, which was significantly lower than those of weld joints 1 and 2. This is thought to be because the martensite phase formed in the weld metal prevented the plastic deformation mechanism of the 15Mn steel base material from functioning during plastic deformation under repeated deformation. As the low-cycle fatigue test progressed, the martensite phase increased, resulting in an increase in hardened structure, and sufficient fatigue durability was not obtained in the weld metal of the weld joint.

[0066] As shown in Figure 14, point coordinates (stars) based on the chemical composition of welding wire 4 were plotted outside the region surrounded by points a, b, c, and d shown in Figure 7 for the same-material welding. Furthermore, point coordinates (squares) based on the chemical composition of the weld metal of weld joint 4 were plotted within the austenite + martensite region (A + M region). The chemical composition of the weld metal was within the FA solidification mode region, and no solidification cracking (hot cracking) occurred in the weld metal of weld joint 4. However, the number of cycles to fracture (average) in the low-cycle fatigue test was 130, which was significantly lower than those of weld joints 1 and 2. This is thought to be because the martensite phase formed in the weld metal prevented the plastic deformation mechanism of the 15Mn steel base material from functioning during plastic deformation under repeated deformation. As the low-cycle fatigue test progressed, the martensite phase increased, resulting in an increase in hardened structure, and sufficient fatigue durability was not obtained in the weld metal of the weld joint.

[0067] [Preparation of Welding Wire and Welded Joint for 15Mn Steel-SN490 Steel] Next, using welding wires 1 to 4 having the chemical compositions shown in Table 3, fillet welding was performed on a 20 mm thick 15Mn steel base material having the chemical composition shown in Table 2 and a 20 mm thick SN490 steel (SN490B) having the chemical composition shown in Table 6 below, to produce welded joints 5 to 8. The welded joints were fillet welds with a leg length of 12 mm, as shown in the upper part of Figure 10. The flange dimensions were 70 x 210 x 20 mm, and the web dimensions were 35 x 210 x 20 mm. The 15Mn steel base material was used for the flange, and the SN490 steel was used for the web.

[0068]

[0069] In order to examine the strength of the weld metal of the fabricated weld joints 5 to 8, dog-bone type fatigue test specimens with a gauge diameter of 2.8 mm were taken from the weld metal of each weld joint, and low-cycle fatigue tests were performed. The method of taking the test specimens and the conditions of the low-cycle fatigue tests were the same as those for the weld joints 1 to 4 described above.

[0070] Table 7 shows the chemical compositions of the weld metals of weld joints 5 to 8. Figures 15 to 18 show the results of plotting the chemical compositions of the weld metals of weld joints 5 to 8 and welding wires 1 to 4 on Schaeffler-type structural diagrams, respectively. Table 8 also shows the properties of the weld metals of weld joints 5 to 8, including the results of low-cycle fatigue tests (average Nf values), main structures, and solidification modes.

[0071]

[0072]

[0073] The test results are explained below.

[0074] As shown in Figure 15, point coordinates (stars) based on the chemical composition of welding wire 1 were plotted within the region surrounded by points e, f, g, and h shown in Figure 9 for the dissimilar metal welding. Furthermore, point coordinates (squares) based on the chemical composition of the weld metal of weld joint 5 were plotted within the austenite + ferrite region (A + F region) and the FA solidification mode region. In the actually fabricated weld joint 5, no solidification cracking (hot cracking) occurred in the weld metal, resulting in a good weld joint. Furthermore, the number of cycles to fracture in the low-cycle fatigue test was 550 or more for all test specimens, and the average was also 550 or more. This suggests that the plastic deformation mechanism of the 15Mn steel base metal effectively functioned during the compositional deformation under repeated deformation, resulting in excellent fatigue durability in the weld metal of the dissimilar metal weld joint.

[0075] As shown in Figure 16, the point coordinates (stars) based on the chemical composition of welding wire 2 were plotted outside the region surrounded by points e, f, g, and h shown in Figure 9 for the dissimilar metal welding. Furthermore, the point coordinates (squares) based on the chemical composition of the weld metal of welded joint 6 were plotted within the austenite + martensite region (A + M region), and this plot was outside the FA solidification mode region and located within the A solidification mode region. Furthermore, in the actually produced welded joint 6, solidification cracking (hot cracking) occurred in the weld metal, so low-cycle fatigue testing could not be performed. In Table 8, this result is indicated by the symbol "-".

[0076] As shown in Figure 17, point coordinates (stars) based on the chemical composition of welding wire 3 were plotted outside the region surrounded by points e, f, g, and h shown in Figure 9 for the dissimilar metal welding. Furthermore, point coordinates (squares) based on the chemical composition of the weld metal of weld joint 7 were plotted within the martensite region (M region). The chemical composition of the weld metal was within the FA solidification mode region, and no solidification cracking (hot cracking) occurred in the weld metal of weld joint 7. However, the number of cycles to fracture (average) in the low-cycle fatigue test was 58, which was significantly lower than that of weld joint 5. This is thought to be because the martensite phase was formed in the weld metal, preventing the plastic deformation mechanism of the 15Mn steel base metal from functioning during plastic deformation under repeated deformation. As the low-cycle fatigue test progressed, the amount of martensite increased, resulting in an increase in hardened structure, and sufficient fatigue durability was not obtained in the weld metal of the weld joint.

[0077] As shown in Figure 18, point coordinates (stars) based on the chemical composition of welding wire 4 were plotted outside the region surrounded by points e, f, g, and h shown in Figure 9 for the dissimilar metal welding. Furthermore, point coordinates (squares) based on the chemical composition of the weld metal of weld joint 8 were plotted within the austenite + martensite region (A + M region). The chemical composition of the weld metal was within the FA solidification mode region, and no solidification cracking (hot cracking) occurred in the weld metal of weld joint 8. However, the number of cycles to fracture (average) in the low-cycle fatigue test was 91, which was significantly lower than that of weld joint 5. This is thought to be because the martensite phase was formed in the weld metal, preventing the plastic deformation mechanism of the 15Mn steel base metal from functioning during plastic deformation under repeated deformation. As the low-cycle fatigue test progressed, the martensite phase increased, resulting in an increase in hardened structure, and sufficient fatigue durability was not obtained in the weld metal of the weld joint.

[0078] From the above results, it was confirmed that the welding wires 1 and 2 produced in this example are suitable for use as welding materials for 15Mn steel-15Mn steel. In addition, it was confirmed that the welding wire 1 is also suitable for use as welding wire for 15Mn steel-SN490 steel. That is, it was confirmed that the welding wire 1 is a welding material that can be suitably used not only for welding 15Mn steel to 15Mn steel (same-material welding) but also for welding 15Mn steel to 490 MPa-class steel (dissimilar material welding), and that the welding wire 1 is a welding material that can control the solidification mode of the weld metal in the weld, does not cause solidification cracking, and can realize a welded joint that has fatigue properties that can withstand use as a vibration damper, etc.

Claims

1. A welding wire used for welding FMS steels, which is an Fe-Mn-Si alloy containing Mn in an amount of 14.5 mass% or more and less than 15.5 mass%, and having a solidification mode of FA solidification mode, This welding wire contains Cr, Ni, Mn, Mo, Si and C as chemical components, the balance being composed of Fe and inevitable components, and The content of the alloy components expressed in mass% is the Cr equivalent (Cr) defined by the following formula Z1 ｅｑ ), and the Ni equivalent (Ni) defined by the following formula Z2 ｅｑ ), in the Schaeffler-type structure diagram represented by points a (13.09, 12.23), point b (18.55, 7.71), point c (22.79, 9.85), point d (23.29, 21.78) (here, the point coordinates are (Cr ｅｑ , Ni ｅｑ )) and falls within the region surrounded by the straight line ab, the straight line bc, the broken line cd, and the straight line da (here, the broken line cd is composed of point c (22.79, 9.85), point S １２ (22.71, 11.03), point S １１ (23.21, 12.27), point S １０ (23.78, 13.83), point S ９ (23.90, 15.50), point S ８ (23.77, 17.12), point S ７ (23.40, 18.49), point S ６ (23.15, 20.05) and point d (23.29, 21.78) connected in sequence.)) A welding wire characterized by this. Formula Z1 Cr equivalent (Cr ｅｑ ) = (mass% Cr) + (mass% Mo) + 1.5 × (mass% Si) + 0.5 × (mass% Nb) Formula Z2 Ni equivalent (Ni ｅｑ ) = (mass% Ni) + 30 × (mass% C) + 0.5 × (mass% Mn)

2. A welding wire used for both welding of FMS steels, which are Fe-Mn-Si alloys containing Mn in an amount of 14.5 mass% or more and less than 15.5 mass% and having a solidification mode of FA solidification mode, and welding of the FMS steel and a structural steel material having a tensile strength of 490 MPa or more, This welding wire contains Cr, Ni, Mn, Mo, Si and C as chemical components, the balance being composed of Fe and inevitable components, and The content of alloy components expressed in mass% is the Cr equivalent (Cr) defined by the following formula Z1 ｅｑ ), and the Ni equivalent (Ni) defined by the following formula Z2 ｅｑ ) in the Schaeffler type structure diagram. The region surrounded by points e (17.46, 14.82), f (22.71, 10.47), g (23.78, 13.83), and h (19.86, 17.06) (where the point coordinates are (Cr ｅｑ , Ni ｅｑ )), and it enters the region surrounded by the straight line ef, the broken line fg, the straight line gh, and the straight line he (where the broken line fg is the broken line connecting points f (22.75, 10.44), S １２ (22.71, 11.03), S １１ (23.21, 12.27), and point g (23.78, 13.83) in sequence).) A welding wire characterized by this. Formula Z1 Cr equivalent (Cr ｅｑ ) = (mass% Cr) + (mass% Mo) + 1.5 × (mass% Si) + 0.5 × (mass% Nb) Formula Z2 Ni equivalent (Ni ｅｑ ) = (mass% Ni) + 30 × (mass% C) + 0.5 × (mass% Mn)

3. The welding wire according to claim 1, which is used for welding at a dilution rate of 20% or more and 35% or less between the FMS steels, the structure of the weld metal being a single-phase structure of austenite or a two-phase structure of austenite and ferrite, and the chemical composition of the weld metal being in the FA solidification mode.

4. The welding wire according to claim 2, which is used for both welding at a dilution rate of 20% or more and 35% or less between the FMS steels and welding at a dilution rate of 20% or more and 35% or less between the FMS steel and a structural steel material having a tensile strength of 490 MPa or more, the structure of the weld metal being a single-phase structure of austenite or a two-phase structure of austenite and ferrite, and the chemical composition of the weld metal being in the FA solidification mode.

5. The FMS steel has a chemical composition of As essential elements, it contains 14.5 mass% ≤ Mn < 15.5 mass%, 5 mass% ≤ Cr ≤ 15 mass%, 5 mass% ≤ Ni < 12 mass%, 2 mass% ≤ Si ≤ 6 mass%, As optional elements, or as impurity elements inevitably contained, it contains Al, Co, Cu, Nb, Ta, V, Ti, Mo in a total amount of 0 mass% or more and 1 mass% or less, and C, N, B in a total amount of 0 mass% or more and 0.2 mass% or less, The balance consists of Fe and inevitable impurities, The Gibbs free energy difference ΔG between the γ-phase and the ε-phase defined by the following expressions 1 to 5 and the thermodynamic parameters in Table 1 γ→ε is -150 J / mol < ΔG γ→ε < 50 J / mol Under the conditions of, and The ratio ([%Cr]eq / [%Ni]eq) of the Cr equivalent ([%Cr]eq) defined by the following formula 6 and the Ni equivalent ([%Ni]eq) defined by the following formula 7 is 1.33 < [%Cr]eq / [%Ni]eq ≤ 1.96 The welding wire according to any one of claims 1 to 4, which satisfies the given conditions. 【Number 1】 【Number 2】 【Number 3】 【Number 4】

6. An FMS steel joint, characterized in that FMS steels, which are Fe-Mn-Si alloys containing 14.5 mass% or more and less than 15.5 mass% of Mn and have an FA solidification mode, are welded by the welding wire according to claim 1.

7. An FMS steel joint, characterized in that FMS steels, which are Fe-Mn-Si alloys containing 14.5 mass% or more and less than 15.5 mass% of Mn and have an FA solidification mode, or the FMS steel and a structural steel having a tensile strength of 490 MPa or more are welded by the welding wire according to claim 2.

8. The welded joint according to claim 6, characterized in that the FMS steels are welded by the welding wire with a dilution rate of 20% or more and 35% or less, the structure of the weld metal is a single-phase structure of austenite or a two-phase structure of austenite and ferrite, and the chemical composition of the weld metal is in the FA solidification mode.

9. The welded joint according to claim 7, characterized in that the FMS steels, or the FMS steel and a structural steel having a tensile strength of 490 MPa or more are welded by the welding wire with a dilution rate of 20% or more and 35% or less, the structure of the weld metal is a single-phase structure of austenite or a two-phase structure of austenite and ferrite, and the chemical composition of the weld metal is in the FA solidification mode.

10. The welded joint according to claim 6, characterized in that in a low-cycle fatigue test with a strain rate of 0.4% / s and a test strain range of 2.0% (±1.0%), the number of fracture cycles is 3000 or more.

11. The welded joint according to claim 7, characterized in that in a low-cycle fatigue test with a strain rate of 0.4% / s and a test strain range of 2.0% (±1.0%), the number of fracture cycles is 550 or more.

12. The FMS steel has a chemical composition of As essential elements, it contains 14.5 mass% ≤ Mn < 15.5 mass%, 5 mass% ≤ Cr ≤ 15 mass%, 5 mass% ≤ Ni < 12 mass%, 2 mass% ≤ Si ≤ 6 mass%, As optional elements, or as unavoidably contained impurity elements, it contains a total of 0 mass% or more and 1 mass% or less of Al, Co, Cu, Nb, Ta, V, Ti, Mo, and a total of 0 mass% or more and 0.2 mass% or less of C, N, B. It consists of the remaining Fe and inevitable impurities, The Gibbs free energy difference ΔG between the γ-phase and the ε-phase defined by the following expressions 1 to 5 and the thermodynamic parameters in Table 1 γ→ε is −150 J / mol < ΔG γ→ε < 50 J / mol under the conditions that, and the ratio ([%Cr]eq / [%Ni]eq) of the Cr equivalent ([%Cr]eq) defined by the following formula 6 and the Ni equivalent ([%Ni]eq) defined by the following formula 7 is 1.33 < [%Cr]eq / [%Ni]eq ≤ 1.96 The welded joint according to any one of claims 6 to 11, which satisfies the above conditions. 【Number 5】 【Number 6】 【Number 7】 【Number 8】

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