AU2014345139A1

AU2014345139A1 - Method for producing weld joint

Info

Publication number: AU2014345139A1
Application number: AU2014345139A
Authority: AU
Inventors: Tatsuya Kumagai; Shuichi Nakamura
Original assignee: Nippon Steel and Sumitomo Metal Corp
Current assignee: Nippon Steel Corp
Priority date: 2013-11-08
Filing date: 2014-08-07
Publication date: 2015-12-17
Anticipated expiration: 2034-08-07
Also published as: CN105339132B; PH12015502625A1; CA2926569A1; CA2915026A1; KR20150136551A; MY158148A; WO2015068443A1; CA2915026C; MX352525B; AU2014345139B2; BR112015029349B1; BR112015029349A2; KR101655057B1; MX2015017087A; PH12015502625B1; CN105339132A; CA2926569C; WO2015068261A1

Abstract

This weld joint production method is a method for producing a weld joint by subjecting a steel plate having a predetermined Vickers hardness HV, plate thickness, C content, and CEN to gas-shielded arc welding by using a flux-cored wire in which a steel-made outer sheath is filled with a flux, wherein: at the time of said gas-shielded arc welding, no preheating operation is performed in cases where the temperature of the steel plate is 10°C or higher, and, in cases where the temperature of the steel plate is below 10°C, preheating operation is performed such that the temperature of the steel plate is raised to 10°C or higher; the weld metal of the weld joint has a predetermined chemical composition; the weld metal has a CEN of from 0.20 to 0.58 mass%; and the average Vickers hardness HV at a depth 1 mm below the surface of the weld metal is from 337 to 440.

Description

[Document Type] Specification [Title of the Invention] METHOD OF PRODUCING WELD JOINT [Technical Field of the Invention] [0001] The present invention relates to a method of producing a weld joint having weld metal which has high hardness and excellent abrasion resistance and does not easily cause cold cracking when a high-hardness steel plate which has excellent abrasion resistance and is used in the field of construction machines and industrial machines is welded. Priority is claimed on International Application No. PCT/JP2013/080242, filed on November 8, 2013, the content of which is incorporated herein by reference. [Related Art] [0002] In many cases, a steel plate used in a construction machine for mine excavation or civil engineering work needs to be replaced due to wear. In order to lengthen the service life of the steel plate, an abrasion resistant steel to increase the hardness of the steel plate is used. The hardness of the steel plate may vary depending on the use environment or purpose, and in general, abrasion resistant steel plates in the HB400 grade (from HB360 to HB440 in terms of Brinell hardness standard value, and from HV380 to HV469 in terms of Vickers hardness standard value), in the HB450 grade (from HB410 to HB490 in terms of Brinell hardness standard value, and from HV435 to HV533 in terms of Vickers hardness standard value), in the HB500 grade (from HB450 to HB550 in terms of Brinell hardness standard value, and from HV478 to HV585 in terms of Vickers hardness standard value), and in the HB600 grade (from HB550 to HB650 in terms of Brinell hardness - 1 .

standard value, and from HV585 to HV693 in terms of Vickers hardness standard value) are widely used. [0003] Most types of abrasion resistant steel are welded, and weld metals may also require abrasion resistance close to base metals (abrasion resistant steel). In order to increase the abrasion resistance of the weld metal, there is also a need to increase the hardness thereof. However, when the hardness of the weld metal is increased, cold cracking caused by hydrogen that infiltrates during welding is very likely to occur. Furthermore, since abrasion resistant steel having a high hardness is used as the base metal, an increase in the binding force is also a cause of the easy occurrence of cold cracking. [0004] In order to avoid such cold cracking, preheating is generally performed before welding. However, the hardness of the abrasion resistant steel is more easily reduced by heating than typical steel and thus a high preheating temperature need not be employed. It is preferable that the hardness of the weld metal be at the same level as that of the base metal. For example, in a case where the abrasion resistant steel in the HB400 grade or HB500 grade is used as the base metal, it is preferable that the hardness of the weld metal be at least HV337 (HB320) or higher, or HV380 (HB360) or higher if possible. [0005] In addition, the hardness in the vicinity of the surface is important for a weld metal zone from the viewpoint of abrasion resistance. During multi-layer welding, weld metal for a lower layer is re-heated in a subsequent pass and thus the hardness thereof is slightly reduced. However, weld metal for the uppermost layer in the case of multi-layer welding or weld metal in a case of single pass welding may have sufficient hardness in the vicinity of the surface of the weld metal. Accordingly, it is thought that a welding method of forming weld metal which has a surface hardness of HV337 or higher and HV533 or lower and sufficient abrasion resistance and does not cause cold cracking even when preheating is not performed, or a welding method of forming weld metal which has a surface hardness of HV380 or higher and HV533 or lower and sufficient abrasion resistance and does not cause cold cracking even when preheating is not performed, is extremely useful in a weld joint which uses an abrasion resistant steel having a surface hardness of HV380 or higher and HV693 or lower as the base metal. [0006] As a technique for suppressing cold cracking caused by hydrogen which occurs in high-strength weld metal, for example, methods of Patent Documents 1 to 5 are proposed. In Patent Document 1, the occurrence of cold cracking is prevented by allowing retained austenite in a steel plate used for a high-strength line pipe or the like to function as a hydrogen-trapping site. In Patent Document 2, the occurrence of cold cracking is also prevented by allowing oxides in a steel plate used for a high-strength line pipe or the like to function as a hydrogen-trapping site. [0007] Patent Document 3 discloses a technique for preventing the occurrence of cold cracking by allowing Mo carbides in steel having a tensile strength of 800 MPa to 1150 MPa to function as a trapping site. Patent Document 4 discloses a technique for improving the cold cracking resistance of steel having a tensile strength of 880 MPa to 1180 MPa by appropriately mixing Mg with the covered material of a shielded metal arc welding material and thus reducing the amount of diffusible hydrogen in weld metal immediately after welding to about 3.0 mI/100 g to 4.0 mI/100 g. Patent Document 5 discloses a technique for suppressing cold cracking by limiting the amount of hydrogen contained in a flux-cored wire for gas-shielded arc welding. The techniques are applied to base metals and weld metals having a strength of lower than 1200 MPa and are not techniques capable of improving the cold cracking properties of weld metal having a hardness of HV380 (about 1200 MPa in terms of tensile strength) and abrasion resistance. [0008] Moreover, in general, when an austenitic stainless steel welding material is used, the infiltration of hydrogen into weld metal is significantly reduced and thus sensitivity to cold cracking can also be reduced. In addition, since the material has an austenite structure, cracking due to reduced ductility is less likely to occur. However, the weld metal which uses the austenitic stainless steel welding material cannot easily increase strength, that is, hardness, and thus abrasion resistance cannot be expected. [0009] Accordingly, there is a demand for forming, in a weld joint which uses an abrasion resistant steel having a high hardness of HV380 or higher and HV693 or lower as the base metal, weld metal which has a surface hardness of HV337 or higher and HV533 or lower and excellent abrasion resistance and does not easily cause cold cracking, or weld metal which has a surface hardness of HV380 or higher and HV533 or lower and excellent abrasion resistance and does not easily cause cold cracking through gas-shielded arc welding. [Prior Art Document] [Patent Document] [0010] [Patent Document 1] Japanese Unexamined Patent Application, First Publication No. 2012-176434 [Patent Document 2] Japanese Unexamined Patent Application, First Publication No. 2012-218034 [Patent Document 3] Japanese Unexamined Patent Application, First Publication No. 2005-40816 [Patent Document 4] Japanese Unexamined Patent Application, First Publication No. H11-147196 [Patent Document 5] Japanese Unexamined Patent Application, First Publication No. 2009-255168 [Disclosure of the Invention] [Problems to be Solved by the Invention] [0011] An object of the present invention is to provide a method of producing a weld joint which uses a high-hardness steel plate having a high C content and a surface hardness of HV380 or higher and HV693 or lower as a base metal, and has weld metal which has a surface hardness of HV337 or higher and HV533 or lower and excellent abrasion resistance and does not easily cause cold cracking, or weld metal which has a surface hardness of HV380 or higher and HV533 or lower and excellent abrasion resistance and does not easily cause cold cracking. [Means for Solving the Problem] [0012] For abrasion resistant steel according to the related art, a preheating temperature during welding was important to prevent cold cracking. Accordingly, in general, welding was performed using a welding material for mild steel by setting a preheating temperature as the top priority. Therefore, the hardness of the weld metal zone was low and wear was very likely to occur. This is thought of as a problem. In the present invention, it is newly found that, when the hardness of the weld metal zone is increased on the contrary, cracking is very likely to occur not in the heat-affected zone of the base metal but in the weld metal itself Therefore, the relationship between the CEN of the weld metal and cracking is examined, and then an appropriate range of the CEN of the weld metal is obtained. [0013] Cold cracking that occurs in the weld metal during welding is affected by the strength of the weld metal, a joint-restricting force, and the amount of diffusible hydrogen in the weld metal. The inventors examined various methods to reliably suppress cold cracking using high-hardness weld metal having a surface hardness of HV337 or higher and HV533 or lower, or high-hardness weld metal having a surface hardness of HV380 or higher and HV533 or lower. As a result, it was concluded that the most reliable method is to sufficiently reduce the amount of diffusible hydrogen in the weld metal and to set a CEN specified with alloy components in the weld metal to be 0.20 mass% to 0.58 mass%. [0014] FIG. 1 shows results of a y-groove weld-cracking test specified in JIS Z 3158 performed on various welding materials which varied in steel plates and flux compositions under various conditions. Various weld metals in which the hardnesses of the weld metals vary and the amounts of diffusible hydrogen in the weld metals vary are produced, and preheating temperature limits at which the occurrence of cracking is - A suppressed are obtained. In FIG 1, the relationship between the amount of diffusible hydrogen in the weld metal and the preheating temperature limit at which the occurrence of cracking is suppressed is plotted according to the hardness levels of the weld metals. Here, as a cold-cracking test, a test based on JIS Z 3158 (method of y-groove weld-cracking test in 1993) was performed at room temperature (25 0 C), and the absence of cracking in surfaces and sections is evaluated as passing. A test for measuring the amount of diffusible hydrogen was performed according to a gas chromatography method based on JIS Z 3118 (method for measurement of amount of hydrogen evolved from steel welds in 2007). [0015] As illustrated in FIG 1, when the amount of diffusible hydrogen in the weld metal immediately after welding is lower than 1.0 ml/100 g, the preheating temperature limit for crack prevention at low temperature does not significantly depend on the hardness of the weld metal. Therefore, by allowing the amount of diffusible hydrogen to be lower than 1.0 ml/100 g, the sensitivity of the weld metal having a hardness of HV337 or higher and HV533 or lower and the weld metal having a hardness of HV380 or higher and HV533 or lower to cold cracking can be significantly reduced. [0016] However, reducing the amount of diffusible hydrogen in the weld metal immediately after welding to such a level is not easily performed in the related art. The inventors repeated various examinations, and newly found that the amount of diffusible hydrogen in weld metal can be stably reduced to a level which is not easily achieved in the related art by improving the flux composition of a flux-cored wire. - 7 - Specifically, it is found that by allowing a certain amount of fluorides including CaF 2 to be contained in the flux components, adjusting the amount of oxides, and allowing the mixing ratios of fluorides and oxides to be in predetermined ranges, the amount of diffusible hydrogen in the weld metal can be stably suppressed to be lower than 1.0 mi/100 g. [0017] The sensitivity of the weld metal to cold cracking significantly depends on the hardness of the weld metal and is also affected by alloy elements. The inventors examined the relationship between various alloy compositions and the sensitivity of cold cracking (cracking suppression preheating temperature) for weld metals having a hardness of HV337 or higher and HV533 or lower and weld metals having a hardness of HV380 or higher and HV533 or lower. As a cold-cracking test, a test based on JIS Z 3158 (method of y-groove weld-cracking test in 1993) was performed at varying preheating temperatures, and the lowest preheating temperature at which cold cracking did not occur is referred to as a preheating temperature limit for crack prevention. During welding, flux-cored weld wires of the present invention described below are used, and all of the amounts of diffusible hydrogen in the weld metals are lower than 1.0 ml/100 g. [0018] As a result, as shown in FIG. 2, it is found that when a CEN calculated by Expression 1 (refer to Welding book selections 10. "Welding of iron and steel materials" published by Sanpo Publications Incorporated. (1999), p.163) is 0.58 mass% or lower, the preheating temperature limit for crack prevention can be equal to or lower than room temperature (25 0 C), and the occurrence of cold cracking can be suppressed without preheating.

CEN=[C]+(0.75+0.25 xtanh(20x([C] 0.12)))x([Si]/24+[Mn]/6+[Cu]/1 5+[Ni]/20+([Cr]+[Mo]+[Nb]+[V])/5+5x [B]) ... (Expression 1) Here, elements with [] represent the amounts (mass%) of the corresponding elements. In a case where there are no added elements, [] is substituted with zero. [0019] The present invention has been made based on the findings, and the summary is as follows. [0020] (1) According to a first aspect of the invention, a method is provided of producing a weld joint by performing a gas-shielded arc welding, using a flux-cored wire filled with flux into a steel sheath, on any one of a steel plate having a Vickers hardness HV of 380 or higher and 514 or lower, a plate thickness of 20 mm to 100 mm, a C content of 0.120 mass% to 0.300 mass%, and a CEN calculated by the following Expression I of 0.20 mass% to 0.75 mass%, a steel plate having a Vickers hardness HV of higher than 514 and 565 or lower, a plate thickness of 12 mm to 100 mm, a C content of 0.120 mass% to 0.300 mass%, and a CEN calculated by the following Expression 1 of 0.20 mass% to 0.75 mass%, and a steel plate having a Vickers hardness HV of higher than 565 and 693 or lower, a plate thickness of 6 mm to 12 mm, a C content of 0.350 mass% to 0.450 mass%, and a CEN calculated by the following Expression 1 of 0.20 mass% to 0.85 mass%, the method including: (a) during the gas-shielded are welding, not performing a preheating operation in a case where a temperature of the steel plate is 10 C or higher, and in a case where the temperature of the steel plate is lower than 10 C, performing the preheating operation so that the temperature of the steel plate is 10 C or higher, - - (b) wherein the flux-cored wire contains one or more of CaF 2 , BaF 2 , SrF 2 , and MgF 2 , and when a sum of amounts thereof is a, the a with respect to a total mass of the flux-cored wire is 3.3% to 8.0% in terms of mass%, the flux-cored wire contains one or more of Ti oxides, Si oxides, Mg oxides, and Al oxides, and when a sum of amounts thereof is p, the P with respect to the total mass of the flux-cored wire is 0.10% to 1.50% in terms of mass%, a sum of amounts of CaCO 3 , BaCO 3 , SrCO 3 , and MgCO 3 with respect to the total mass of the flux-cored wire is lower than 0.60% in terms of mass%, an amount of an iron powder in the flux with respect to the total mass of the flux-cored wire is lower than 10.0% in terms of mass%, a ratio of the amount of CaF 2 to the a is 0.90 or higher, a ratio of the a to the p is 3.0 or higher and 80.0 or lower, an amount of CaO with respect to the total mass of the flux-cored wire is lower than 0.20% in terms of mass%, the flux-cored wire includes chemical components excluding metal fluorides, metal oxides, and metal carbonates, with respect to the total mass of the flux-cored wire, in terms of mass%: C: 0.010% to lower than 0.060%; Si: 0.05% to 1.80%; Mn: 0.50% to 4.00%; P: 0.050% or lower; S: 0.020% or lower; Al: 0.005% to 0.150%; Cu: 0% to 0.75%; Ni: 0% to lower than 1.00%; Cr: 0% to 3 .50%; Mo: 0% to 1.50%; Ti: 0% to 0.150%; Nb: 0% to 0.15%; V: 0% to 0.45%; B: 0% to 0.0500%; Mg: 0% to 2.0%; Ca: 0% to 2.

0 %; REM: 0% to 0.0150%; and the remainder: Fe and impurities, (c) wherein a weld metal of the weld joint includes as a chemical composition, in terms of mass%: C: 0.100% to 0.170%; Si: 0.05% to 0.80%; Mn: 0.20% to 2.50%; Al: 0.0050% to 0.1000%; P: 0.050% or lower; S: 0.020% or lower; N: 0.015% or lower; Cu: 0% to 0.50%; Ni: 0% to lower than 0.70%; Cr: 0% to 2.50

%

; Mo: 0% to 1.00%; Ti: 0% to 0.100%; Nb: 0% to 0.100%; V: 0% to 0.30%; B: 0% to 0.0100%; 0: 0% to 0.100%; Mg: 0% to 0.100%; Ca: 0% to 0.100%; REM: 0% to 0.0100%; and the remainder: Fe and impurities, a CEN of the weld metal calculated by the following Expression 1 is 0.20 mass% to 0.58 mass%, an average Vickers hardness HV of the weld metal measured at 1 mm inward from a surface of the weld metal is 337 to 440, and all of (a) to (c) are satisfied. CEN=[C]+(0.75+0.25 xtanh(20x([C] 0.12)))x([Si]/24+[Mn]/6+[Cu]/15+[Ni]/20+([Cr]+[Mo]+[Nb]+[V])/5+5x[B]) ... (Expression 1) where elements with [] represent the amounts (mass%) of the corresponding elements. [0021] (2) According to a second aspect of the invention, a method is provided of producing a weld joint by performing a gas-shielded are welding, using a flux-cored wire filled with flux into a steel sheath, on any one of a steel plate having a Vickers hardness HV of 380 or higher and 514 or lower, a plate thickness of 20 mm to 100 mm, a C content of 0.120 mass% to 0.300 mass%, and a CEN calculated by the following Expression I of 0.20 mass% to 0.75 mass%, a steel plate having a Vickers hardness HV of higher than 514 and 565 or lower, a plate thickness of 12 mm to 100 mm, a C - 191 content of 0.120 mass% to 0.300 mass%, and a CEN calculated by the following Expression 1 of 0.20 mass% to 0.75 mass%, and a steel plate having a Vickers hardness HV of higher than 565 and 693 or lower, a plate thickness of 6 mm to 12 mm, a C content of 0.350 mass% to 0.450 mass%, and a CEN calculated by the following Expression I of 0.20 mass% to 0.85 mass%, the method including: (a) during the gas-shielded arc welding, not performing a preheating operation in a case where a temperature of the steel plate is 1 OC or higher, and in a case where the temperature of the steel plate is lower than 10 C, performing the preheating operation so that the temperature of the steel plate is 10 C or higher, (b) wherein the flux-cored wire contains one or more of CaF 2 , BaF 2 , SrF 2 , and MgF 2 , and when a sum of amounts thereof is a, the a with respect to a total mass of the flux-cored wire is 3

.

3 % to 8 .0% in terms of mass%, the flux-cored wire contains one or more of Ti oxides, Si oxides, Mg oxides, and Al oxides, and when a sum of amounts thereof is P, the p with respect to the total mass of the flux-cored wire is 0.10% to 1.50% in terms of mass%, a sum of amounts of CaCO 3 , BaCO 3 , SrCO 3 , and MgCO 3 with respect to the total mass of the flux-cored wire is lower than 0.60% in terms of mass%, an amount of an iron powder in the flux with respect to the total mass of the flux-cored wire is lower than 10.0% in terms of mass%, a ratio of the amount of CaF 2 to the a is 0.90 or higher, a ratio of the a to the p is 3.0 or higher and 80.0 or lower, an amount of CaO with respect to the total mass of the flux-cored wire is lower than 0.

2 0% in terms of mass%, the flux-cored wire includes chemical components excluding metal fluorides, metal oxides, and metal carbonates, with respect to the total mass of the flux-cored wire, in terms of mass%: C: 0.060% to 0.350%; Si: 0.05% to 1.80%; Mn: 0.50% to 4.00%; P: 0.050% or lower; S: 0.020% or lower; Al: 0.005% to 0.150

%

; Cu: 0% to 0.75%; Ni: 0% to lower than 1.0 0 %; Cr: 0% to 3.50%; Mo: 0% to 1.50%; Ti: 0% to 0.150%; Nb: 0% to 0.15%; V: 0% to 0.

4 5 %; B: 0% to 0.0500%; Mg: 0% to 2

.

0 %; Ca: 0% to 2.0%; REM: 0% to 0.0150%; and the remainder: Fe and impurities, (c) wherein a weld metal of the weld joint includes as a chemical composition, in terms of mass%: C: 0.120% to 0.250%; Si: 0.05% to 0.80%; Mn: 0.20% to 2.50%; Al: 0.0050% to 0.1000%; -1,4 P: 0.050% or lower; S: 0.020% or lower; N: 0.

0 15% or lower; Cu: 0% to 0.50%; Ni: 0% to lower than 0.

7 0 %; Cr: 0% to 2.50%; Mo: 0% to 1.00%; Ti: 0% to 0.100%; Nb: 0% to 0.100%; V: 0% to 0.30%; B: 0% to 0.0100%; 0: 0% to 0.100%; Mg: 0% to 0.100%; Ca: 0% to 0.100%; REM: 0% to 0.0100%; the remainder: Fe and impurities, a CEN of the weld metal calculated by the following Expression 1 is 0.20 mass% to 0.58 mass%, an average Vickers hardness HV of the weld metal measured at 1 mm inward from a surface of the weld metal is 380 to 533, and all of (a) to (c) are satisfied. CEN=[C]+(0.75+0.25xtanh(20x([C] 0.12)))x([Si]/24+[Mn]/6+[Cu]/15+[Ni]/20+([Cr]+[Mo]+[Nb]+[V])/5+5x[B]) .. .(Expression 1) where elements with [] represent the amounts (mass%) of the corresponding - 1i1 elements. (3) According to a third aspect of the invention, a method is provided of producing a weld joint by performing a gas-shielded are welding, using a flux-cored wire filled with flux into a steel sheath, on any one of a steel plate having a Vickers hardness HV of higher than 565 and 693 or lower, a plate thickness of 12 mm to 20 mm, a C content of 0.350 mass% to 0.450 mass%, and a CEN calculated by the following Expression 2 of 0.20 mass% to 0.85 mass%, and a steel plate having a Vickers hardness HV of higher than 565 and 693 or lower, a plate thickness of greater than 20 mm to 50 mm or smaller, a C content of 0.350 mass% to 0.450 mass%, and a CEN calculated by the following Expression 2 of 0.20 mass% to 0.85 mass%, the method including: (a) during the gas-shielded arc welding, performing a preheating operation so that a temperature of the steel plate is 100 C or higher in a case where the plate thickness of the steel plate is 20 mm or smaller, and in a case where the plate thickness of the steel plate is greater than 20 mm, performing the preheating operation so that the temperature of the steel plate is 150'C or higher, (b) wherein the flux-cored wire contains one or more of CaF2, BaF 2 , SrF 2 , and MgF 2 , and when a sum of amounts thereof is a, the a with respect to a total mass of the flux-cored wire is 3.3% to 8.0% in terms of mass%, the flux-cored wire contains one or more of Ti oxides, Si oxides, Mg oxides, and Al oxides, and when a sum of amounts thereof is P, the j with respect to the total mass of the flux-cored wire is 0.10% to 1.50% in terms of mass%, a sum of amounts of CaCO 3 , BaCO 3 , SrCO 3 , and MgCO 3 with respect to the total mass of the flux-cored wire is lower than 0.

6 0% in terms of mass%, an amount of an iron powder in the flux with respect to the total mass of the - 1 S flux-cored wire is lower than 10.0% in terms of mass%, a ratio of the amount of CaF 2 to the a is 0.90 or higher, a ratio of the a to the P is 3.0 or higher and 80.0 or lower, an amount of CaO with respect to the total mass of the flux-cored wire is lower than 0.20% in terms of mass%, the flux-cored wire includes chemical components excluding metal fluorides, metal oxides, and metal carbonates, with respect to the total mass of the flux-cored wire, in terms of mass%: C: 0.060% to 0.350%; Si: 0.05% to 1.80%; Mn: 0.50% to 4.00%; P: 0.050% or lower; S: 0.020% or lower; Al: 0.005% to 0.150%; Cu: 0% to 0.75%; Ni: 0% to lower than 1.00%; Cr: 0% to 3.50%; Mo: 0% to 1.50%; Ti: 0% to 0.150%; Nb: 0% to 0.15%; V: 0% to 0.45%; B: 0% to 0.0500%; Mg: 0% to 2.0%; Ca: 0% to 2 .0

%

; REM: 0% to 0.0150%; -17 the remainder: Fe and impurities, (c) wherein a weld metal of the weld joint includes as a chemical composition, in terms of mass%: C: 0.120% to 0.250%; Si: 0.05% to 0.80%; Mn: 0.20% to 2.50%; Al: 0.0050% to 0.1000%; P: 0.050% or lower; S: 0.020% or lower; N: 0.015% or lower; Cu: 0% to 0.50%; Ni: 0% to lower than 0.70%; Cr: 0% to 2.50%; Mo: 0% to 1.00%; Ti: 0% to 0.100%; Nb: 0% to 0.100%; V: 0% to 0.30%; B: 0% to 0.0100%; 0: 0% to 0.100%; Mg: 0% to 0.100%; Ca: 0% to 0.100%; REM: 0% to 0.0100%; and the remainder: Fe and impurities, a CEN of the weld metal calculated by the following Expression 2 is 0.20 mass% to 0.58 mass%, an average Vickers hardness HV of the weld metal measured at 1 mm inward from a surface of the weld metal is 380 to 533, and all of (a) to (c) are satisfied. CEN=[C]+(0.75+0.25 xtanh(20x([C] 0.1 2)))x([Si]/24+[Mn]/6+[Cu]/l 5+[Ni]/20+([Cr]+[Mo]+[Nb]+[V])/5+5x[B]) ... (Expression 2) where elements with [] represent the amounts (mass%) of the corresponding elements. [0022] (4) In the method of producing a weld joint described in (1) to (3), the amount of CaO in the flux-cored wire may be 0.15% or lower in terms of mass% with respect to the total mass of the flux-cored wire. [0023] (5) In the method of producing a weld joint described in any of (1) to (4), the flux-cored wire may include the chemical components excluding the metal fluorides, metal oxides, and metal carbonates, with respect to the total mass of the flux-cored wire, in terms of mass%: Ni: 0% to 0.1%. [0024] (6) In the method of producing a weld joint described in any of (1) to (5), the flux-cored wire may include the chemical components excluding the metal fluorides, metal oxides, and metal carbonates, with respect to the total mass of the flux-cored wire, in terms of mass%: Cu: 0% to 0.50%; Cr: 0% to 1.00%; - 10 .

Mo: 0% to 0.

5 0%; Ti: 0% to 0.050%; and Nb: 0% to 0.05%. [0025] (7) In the method of producing a weld joint described in any of (1) to (6), the steel sheath of the flux-cored wire may have a slit-like gap. (8) In the method of producing a weld joint described in any of (1) to (6), the steel sheath of the flux-cored wire may not have a slit-like gap. [0026] (9) In the method of producing a weld joint described in any of (1) to (8), a perfluoropolyether oil may be applied to a surface of the flux-cored wire. [Effects of the Invention] [0027] According to the aspects of the present invention, a weld joint which uses a high-hardness steel plate having a high C content and a surface hardness of HV380 or higher and HV693 or lower as a base metal, and has weld metal which has a surface hardness of HV320 or higher and HV533 or lower and excellent abrasion resistance and does not easily cause cold cracking, or weld metal which has a surface hardness of HV380 or higher and HV533 or lower and excellent abrasion resistance and does not easily cause cold cracking can be obtained. [Brief Description of the Drawings] [0028] FIG. 1 is a diagram showing the relationship between the hardness of a base metal, the amount of diffusible hydrogen in weld metal, and a preheating temperature limit for crack prevention.

FIG. 2 is a diagram showing the relationship between a CEN and a preheating temperature limit for crack prevention in weld metal having an amount of diffusible hydrogen of lower than 1.0 mI/i 00 g among weld metals having a hardness of HV337 or higher and HV533 or lower. FIG. 3A is a view showing a cut section of a wire. FIG. 3B is a view showing a cut section of a wire. FIG. 3C is a view showing a cut section of a wire. [Embodiments of the Invention] [0029] Regarding a weld joint which uses a high-hardness steel plate as a base metal, the inventors found that when the amount of diffusible hydrogen in weld metal immediately after welding is lower than 1.0 ml/100 g as described above, a preheating temperature limit for crack prevention at low temperature does not significantly depend on the hardness of the weld metal and the sensitivity of weld metal having a hardness of HV337 or higher and HV533 or lower and weld metal having a hardness of HV380 or higher and HV533 or lower to cold cracking can be significantly reduced. [0030] Furthermore, in order to allow the amount of diffusible hydrogen in the weld metal immediately after welding to be lower than 1.0 ml/100 g, the inventors repeated examination by varying the combination of flux components of a flux-cored wire and the mixing ratios thereof As a result, it is found that fluorides including CaF 2 are particularly effective in reducing the amount of hydrogen, the amount of diffusible hydrogen in the weld metal can be significantly reduced by allowing a certain amount of fluorides to be contained in the flux components, and the amount of diffusible hydrogen can be stably - )I suppressed to be lower than 1.0 ml/100 g by adjusting the amount of oxides and allowing the mixing ratios of fluorides and oxides to be in predetermined ranges. [0031] The present invention has been made based on the examinations. Hereinafter, an aspect of a method of producing a weld joint according to an embodiment will be described. The present invention is for a weld joint which is formed by using a high hardness thick steel plate that is widely used as an abrasion resistant steel plate, has a C content of 0.12% to 0.45% in terms of mass%, and a hardness of HV380 or higher and HV693 or lower as a base metal, and performing a gas-shielded arc welding using the steel plate. In the present invention, weld metal has a chemical composition in (1) or (2) described above. Hereinafter, the reasons that the chemical composition of the weld metal is limited will be described. In the following description, "%" means "mass%" if not particularly specified. [0032] (C: 0.100% to 0.250%) C is an element which most affects the hardness of the weld metal. When the hardness of the base metal is HV380 or higher, it is preferable that the surface hardness of the weld metal be at least HV337 or higher in order to ensure a certain degree of abrasion resistance for the weld metal. For this, the C content of the weld metal needs to be 0.100% or higher. In addition, when the hardness of the base metal is HV380 or higher, it is preferable that the surface hardness of the weld metal be also HV380 or higher in order to ensure a similar degree of abrasion resistance to that of the - 99 1 base metal. In a case where the surface hardness of the weld metal needs to be HV380 or higher, the C content of the weld metal needs to be 0.120% or higher. However, when the C content is higher than 0.

2 50%, the hardness of the weld metal becomes higher than HV533 and thus the toughness of the weld metal may be reduced. Therefore, the upper limit of the C content is 0.250%. In addition, typically, the C content of the weld metal of a weld joint made by using a flux-cored wire having a C content of 0.010% to less than 0.060%, which will be described later, is 0.100% to 0.170%. In order to allow the base metal to stably obtain a hardness of H1V380 or higher, the lower limit of the C content may be 0.130% or 0.140%. In addition, in order to allow the weld metal to stably obtain toughness, the upper limit of the C content may be 0.230% or 0.210%. [0033] (Si: 0.05% to 0.80%) Si is a deoxidizing element and reduces the 0 content of the weld metal, and thus a certain amount of Si is added to the flux in order to enhance cleanliness. Therefore, the Si content in the weld metal is also 0.05% or higher. As necessary, the lower limit of the Si content may be 0.10%, 0.15%, or 0.20%. When Si is contained in a proportion of higher than 0.80%, the toughness of the weld metal may be deteriorated, and thus 0.80% is the upper limit of the Si content. In order to improve the toughness of the weld metal, the upper limit of the Si content may be 0.70%, 0.65%, 0.60%, or 0.50%. [0034] (Mn: 0.20% to 2.50%) Mn forms MnS and thus has an effect of suppressing grain boundary embrittlement due to S, and thus at least 0.20% or higher of Mn is contained in the weld metal. In addition, Mn is an element which ensures the hardenability of the weld metal and is thus effective in increasing strength. Therefore, in order to stably obtain hardness, 0.50% or higher of Mn is preferably contained. In order to enhance the hardness of the weld metal, the lower limit of the Mn content may be 0.60%, 0.70%, 0.8 0 %, or 0.90%. On the other hand, when Mn is contained in a proportion of higher than 2.50%, sensitivity to grain boundary embrittlement is increased, and thus the toughness of the weld metal is deteriorated. Therefore, 2 .50% is the upper limit of the Mn content. In order to improve the toughness of the weld metal, the upper limit of the Mn content may be limited to 2.30%, 2.10%, 1.90%, 1.70%, or 1.50%. [0035] (Al: 0.0050% to 0.1000%) Al is a deoxidizing element and like Si, reduces the 0 content of the weld metal, and thus has an effect of enhancing the cleanliness of the weld metal. Therefore, a certain amount of Al needs to be added to the flux. Typically, 0.0050% or higher Al is contained in the weld metal of the weld joint made by using the flux cored wire according to this embodiment. When the Al content is lower than 0.0050%, there is concern that the low temperature toughness of the weld metal may be degraded. On the other hand, when Al is contained in a proportion of higher than 0.1000%, Al forms nitrides or oxides and thus deteriorates the toughness of the weld metal. Therefore, 0.1000% is the upper limit of the Al content. In order to improve the toughness of the weld metal, the upper limit of the Al content may be limited to 0.0900%, 0.0800%, 0.0700%, or 0.0600%. [0036] (P: 0.050% or lower) P is an impurity element and deteriorates toughness. Therefore, the P - 'M content needs to be reduced as much as possible. However, as a range in which an adverse effect of P on toughness is acceptable, the P content of the weld metal is limited to 0.050% or lower. As necessary, the upper limit of the P content may be limited to 0.030%, 0.0250%, 0.0200%, or 0.0150%. The lower limit of the P content does not need to be limited. The lower limit of the P content is 0%. [0037] (S: 0.020% or lower) S is an impurity element, and when an excessive amount of S is present in the weld metal, both toughness and ductility are deteriorated, and thus it is preferable that the S content be excessively reduced. As a range in which an adverse effect of S on toughness and ductility is acceptable, the S content of the weld metal is limited to 0.020% or lower. As necessary, the upper limit of the S content may be limited to 0.015%, 0.010%, 0.008%, or 0.006%. The lower limit of the S content does not need to be limited. The lower limit of the S content is 0%. [0038] (N: 0.0150% or lower) N is unavoidably contained in the weld metal. However, when the N content is higher than 0.

0 15%, coarse AlN or BN is formed and thus toughness is reduced. As the upper limit at which the effect of N on the weld metal is acceptable, the N content is limited to 0.015% or lower. As necessary, the upper limit of the N content may be limited to 0.010%, 0.008%, or 0.006%. The lower limit of the N content does not need to be limited. The lower limit of the N content is 0%. [0039] (0: 0% to 0.100%) 0 is unavoidably contained in the weld metal. However, as a range in which - 1) 1 an adverse effect of 0 on toughness and ductility is acceptable, the 0 content of the weld metal is limited to 0.100% or lower. As necessary, the upper limit of the 0 content maybe 0.080%, 0.060%, 0.050%, or 0.040%. The lower limit of the O content does not need to be limited. The lower limit of the 0 content is 0%. [0040] (Cu: 0% to 0.50%) Cu can enhance the strength and toughness of the weld metal and thus can be contained as a selective element. However, when the Cu content is higher than 0.50%, toughness may be reduced. Therefore, the Cu content of the weld metal is 0.50% or lower. As necessary, the upper limit of the Cu content may be 0.40% or 0.

3 0%. The lower limit of the Cu content may not be limited. Therefore, the lower limit of the Cu content is 0%. On the other hand, in order to sufficiently obtain a strengthening effect, 0.10% or higher of Cu may be contained in the weld metal. As a method of including Cu in the weld metal, there is a method of adding Cu to the coating of the surface of the sheath of the wire or the flux as a single element or an alloy element, and the like. [0041] (Ni: 0% to lower than 0.70%) Ni is considered as an element effective in enhancing toughness and can be contained as a selective element. However, in a case where the C content is high, the effect of Ni is limited, and since Ni is an expensive element, the Ni content in the weld metal is lower than 0.70%. As necessary, the upper limit of the Ni content may be 0.60%, 0.40%, or 0.20%. The lower limit of the Ni content may not be limited. Therefore, the lower limit of the Ni content is 0%. On the other hand, in order to sufficiently obtain a toughness enhancing effect, 0.05% or higher of Ni may be contained in the weld metal.

[0042] (Cr: 0% to 2.50%) Cr is an element which increases hardenability and is effective in enhancing the hardness of the weld metal, and thus can be contained as a selective element. However, when Cr is excessively contained in a proportion of higher than 2.50%, toughness may be reduced. Therefore, 2.50% is the upper limit of the Cr content. As necessary, the upper limit of the Cr content may be 1.50%, 1.00%, 0.

7 0%, or 0.40%. The lower limit of the Cr content may not be limited. Therefore, the lower limit of the Cr content is 0%. On the other hand, in a case of adding Cr for the purpose of enhancing the hardness of the weld metal, in order to obtain the effect, 0.l10% or higher of Cr may be contained. [0043] (Mo: 0% to 1.00%) Mo is an element which increases hardenability and is effective in enhancing the hardness of the weld metal, and thus can be contained as a selective element. However, when Mo is excessively contained in a proportion of higher than 1.00%, toughness may be reduced. Therefore, 1.00% is the upper limit of the Mo content. As necessary, the upper limit of the Mo content may be 0.70%, 0.60%, 0.40%, or 0.20%. The lower limit of the Mo content may not be limited. Therefore, the lower limit of the Mo content is 0%. On the other hand, in a case of adding Mo for the purpose of enhancing the hardness, in order to obtain the effect, 0.05% or higher of Mo may be contained. [0044] (Ti: 0% to 0.100%) Ti is, like Al, effective as a deoxidizing element, has an effect of reducing the - '7 o content of the weld metal, and thus can be contained as a selective element. In addition, Ti is also effective in fixing solid-soluted N and relaxing an adverse effect on toughness. However, when the Ti content in the weld metal becomes higher than 0.100% and is thus excessive, a possibility of toughness deterioration due to the formation of coarse oxides and toughness deterioration due to excessive precipitation strengthening is increased. Therefore, the upper limit of the Ti content is 0.100%. As necessary, the upper limit of the Ti content may be 0.080%, 0.050%, 0.030%, or 0.020%. The lower limit of the Ti content may not be limited. Therefore, the lower limit of the Ti content is 0%. For the purpose of improving toughness, 0.010% or higher of Ti may be contained. [0045] (Nb: 0% to 0.100%) Nb is solid-soluted in the weld metal metal and has an effect of enhancing the hardness of the weld metal, and thus can be contained as a selective element. However, when Nb is contained in a proportion of higher than 0.100%, Nb is excessively contained in the weld metal, forms coarse precipitates, and thus deteriorates toughness, which is not preferable. Therefore, the upper limit of the Nb content is 0.100%. As necessary, the upper limit of the Nb content may be 0.

0 8 0 %, 0.050%, 0.030%, or 0.020%. The lower limit of the Nb content may not be limited. Therefore, the lower limit of the Nb content is 0%. For the purpose of enhancing the hardness of the weld metal, 0.010% or higher of Nb may be contained. [0046] (V: 0% to 0.30%) V is an element which increases hardenability and is effective in enhancing the hardness of the weld metal, and thus can be contained as a selective element.

However, when V is excessively contained in a proportion of higher than 0.30%, toughness may be reduced. Therefore, the upper limit of the V content is 0.

3 0%. As necessary, the upper limit of the V content maybe 0.

2 5 %, 0.20%, or 0.15%. The lower limit of the V content may not be limited. Therefore, the lower limit of the V content is 0%. For the purpose of enhancing the hardness of the weld metal, 0.01% or higher of V may be contained. [0047] (B: 0% to 0.0100%) When an appropriate amount of B is contained in the weld metal, B is bonded to solid-soluted N and forms BN, and thus has an effect of reducing an adverse effect of the solid-soluted N on toughness. In addition, B increases hardenability and contributes to the enhancement of strength, and thus can be contained as a selective element. In order to obtain this effect, 0.0003% or higher of B may be contained. On the other hand, when the B content is higher than 0.0100%, B is excessively contained in the weld metal, forms coarse BN or B compounds such as Fe 23 (C, B) 6 , and thus deteriorates toughness, which is not preferable. Therefore, the upper limit of the B content in a case of including B is 0.0100%. As necessary, the upper limit of the B content may be 0.0080%, 0.0060%, 0.0040%, or 0.0020%. The lower limit of the B content does not need to be limited, and the lower limit of the B content is 0%. [0048] (Mg: 0% to 0.100%) The lower limit of the Mg content does not need to be limited, and the lower limit of the Mg content is 0%. However, Mg is a strong deoxidizing element and thus reduces the 0 content in the weld metal, and 0.001% or higher of Mg may be contained in order to enhance the ductility and toughness of the weld metal. However, - )a when the Mg content in the weld metal is higher than 0.100%, a reduction in the toughness due to the formation of coarse oxides in the weld metal cannot be neglected. Therefore, even in a case of including Mg, the Mg content is 0.100% or lower. As necessary, the upper limit of the Mg content may be 0.0080%, 0.0060%, 0.0040%, or 0.0020%. [0049] (Ca: 0% to 0.100%) (REM: 0% to 0.0100%) The lower limits of the amounts of Ca and REM do not need to be limited, and the lower limits of the amounts of Ca and REM are 0%. However, both of Ca and REM change the structure of sulfides in the weld metal to refine the sizes of sulfides and oxides and are thus effective in enhancing ductility and toughness, and thus 0.002% or higher of Ca and 0.0002% or higher of REM may be contained. On the other hand, when Ca and REM are excessively contained, sulfides and oxides are coarsened and cause the deterioration of ductility and toughness. Therefore, in a case of including Ca and REM, the upper limits of the Ca and REM contents are respectively 0.100% and 0.0100%. [0050] In the weld metal having the above chemical composition, the remainder containing iron (Fe) as its primary component may also contain impurities that are incorporated during the production process and the like in a range in which the characteristics of the weld joint according to this embodiment are not impeded. [0051] (CEN: 0.20 mass% to 0.58 mass%) As illustrated in FIG. 2, regarding the weld metal having a hardness of HV380 - 'I A or higher and HV533 or lower, when the amount of diffusible hydrogen in the weld metal is lower than 1.0 ml/100 g, by allowing a CEN calculated by Expression I to be 0.58 mass% or lower, the preheating temperature limit for crack prevention can be 25 0 C or lower in a y-groove weld-cracking test according to JIS Z 3158, and thus welding can be performed substantially without preheating. Here, in order to reliably prevent weld cracking, the upper limit of the CEN may be 0.55 mass%, 0.53 mass%, 0.50 mass%, 0.47 mass%, or 0.45 mass%. In order to allow the hardness of the weld metal to be HV380 or higher, the lower limit of the CEN is 0.20 mass%. When the hardness of the weld metal is high, abrasion resistance is enhanced. Therefore, the lower limit of the CEN may be 0.24 mass%, 0.28 mass%, 0.30 mass%, or 0.32 mass%. (a) A base metal in which the Vickers hardness HV of the base metal is HV380 or higher and HV514 or lower (corresponding to HB360 or higher and HB475 or lower), the plate thickness of the base metal is 20 mm to 100 mm, the C content of the base metal is 0.120% to 0.300%, and the CEN calculated by Expression 1 is 0.20 mass% to 0.75 mass%. (b) A base metal in which the Vickers hardness HV of the base metal is higher than HV514 and equal to or lower than HV565 (corresponding to higher than HB475 and equal to or lower than HB530), the plate thickness of the base metal is 12 mm to 100 mm, the C content of the base metal is 0.120% to 0.300%, and the CEN calculated by Expression 1 is 0.20 mass% to 0.75 mass%. (c) A base metal in which the Vickers hardness HV of the base metal is higher than HV565 and equal to or lower than HV693 (corresponding to higher than HB530 and equal to or lower than HB650), the plate thickness of the base metal is 6 mm to 12 mm, the C content of the base metal is 0.350% to 0.450%, and the CEN calculated by - '11 - Expression 1 is 0.20 mass% to 0.85 mass%. Regarding the base metal which satisfies any one of (a) to (c) described above, in a case where the temperature of the base metal is 10 C or higher during gas-shielded arc welding, there is no need to perform a preheating operation during the welding. However, in a case where the temperature of the base metal is lower than I OC, a preheating operation needs to be performed so that the temperature of the base metal becomes 10 C or higher. That is, only in the case where the temperature of the base metal (steel plate) is lower than 10 C, the preheating operation needs to be performed so that the temperature of the base metal (steel plate) becomes 1 OC or higher. The upper limit of the temperature (preheating temperature) of the base metal does not need to be particularly determined and may be lower than 75 0 C or lower than 50'C. (d) A base metal in which the Vickers hardness HV of the base metal is higher than HV565 and equal to or lower than HV693 (corresponding to higher than HB530 and equal to or lower than HB650), the plate thickness of the base metal is 12 mm to 20 mm, the C content of the base metal is 0.350% to 0.450%, and the CEN calculated by Expression 1 is 0.20 mass% to 0.85 mass%. (e) A base metal in which the Vickers hardness HV of the base metal is higher than HV565 and equal to or lower than HV693 (corresponding to higher than HB530 and equal to or lower than HB650), the plate thickness of the base metal is 20 mm to 50 mm, the C content of the base metal is 0.350% to 0.450%, and the CEN calculated by Expression 1 is 0.20 mass% to 0.85 mass%. Regarding the base metal which satisfies (d) or (e) described above, in a case where the plate thickness of the base metal is 20 mm or smaller during gas-shielded arc welding, preheating is performed to heat the base metal to 100 C or higher. In a case where the plate thickness of the base metal is greater than 20 mm, preheating is performed to heat the base metal to 150'C or higher. The upper limit of the temperature (preheating temperature) of the base metal does not need to be particularly determined and may be lower than 175 0 C or lower than 150 0 C. In order to achieve a Vickers hardness of HV380 or higher, the CEN is allowed to be 0.20 mass%. CEN=[C]+(0.75+0.25xtanh(20x([C] 0.12)))x([Si]/24+[Mn]/6+[Cu]/15+[Ni]/20+([Cr]+[Mo]+[Nb]+[V])/5+5x[B]) ... (Expression 1) Here, elements with [] represent the amounts (mass%) of the corresponding elements. [0052] In Expression 1, regarding elements that are not contained, [] corresponding to the elements is substituted with zero. This calculation method is common to the base metal (steel plate) and the weld metal. [0053] In the present invention, an average Vickers hardness of the weld metal measured at 1 mm inward from the surface thereof is HV337 or higher and HV533 or lower, or HV380 or higher and HV533 or lower. In the present invention, the amount of diffusible hydrogen of the weld metal immediately after welding is lower than 1.0 ml/100 g. When the hardness measured at a position 1 mm inward from the surface is HV337 or higher and HV533 or lower, an abrasion resistance requirement which is necessary for the weld metal is satisfied. When the hardness is lower than HV337, abrasion resistance is insufficient. When the hardness is higher than HV533, cold cracking is likely to occur. To measure the hardness, the section of the weld metal is cut in a direction perpendicular to the welding direction and polished to acquire a sample, the Vickers hardnesses of 10 points of the sample at a position 1 mm inward from the surface of the weld metal are measured, and the average value thereof is calculated to obtain the hardness. [0054] Regarding the amount of diffusible hydrogen in the weld metal immediately after welding, as described above with reference to FIG. 1, when the amount of diffusible hydrogen is lower than 1.0 ml/100 g, the preheating temperature limit for crack prevention at low temperature does not significantly depend on the hardness of the weld metal, and the sensitivity of weld metal having a hardness of HV337 or higher and HV533 or lower and weld metal having a hardness of HV380 or higher and HV533 or lower to cold cracking can be significantly reduced. The amount of diffusible hydrogen is measured by a gas chromatography method based on JIS Z 3118 (method for measurement of amount of hydrogen evolved from steel welds in 2007). In addition, the hydrogen diffusion speed is relatively fast at room temperature, and thus the amount of diffusible hydrogen of the weld metal needs to be measured immediately after welding. Therefore, the amount of diffusible hydrogen cannot be accurately measured unless it is measured immediately after welding. [0055] In order to produce a weld joint having the weld metal described above, high hardness thick steel plates to be welded are used as the base metal, and two plates of the base metal are set on welding positions to form a groove therebetween, gas shielded are welding is performed thereon by using a flux-cored weld wire to generate weld metal between the plates of the base metal, such that a weld joint formed of the weld metal and the steel plates for the base metal on both sides of the weld metal is formed. Hereinafter, the steel plate, the flux-cored weld wire, and welding conditions used to form the weld metal will be described. [0056] As the steel plate for the base metal, a high-hardness thick steel plate having a C content of 0.120% or higher and 0.450% or lower in terms of mass% and a hardness of HV380 or higher and HV693 or lower is employed. Regarding the plate thickness of the steel plate to be used, a steel plate having a thickness of 6 mm or greater and 100 mm or smaller, generally called a thick plate, is employed. The steel plate that satisfies such conditions is widely used where abrasion resistance is necessary, such as a machine for civil engineering and construction work, and the chemical composition thereof is not particularly limited except for the C content. However, as an example, steel includes as a chemical composition: C: 0.120% to 3.000%, Si: 0.l10% to 0.55%, Mn: 0.20% to 2.00%, Al: 0.01% to 0.10%, P: 0.020% or lower, S: 0.015% or lower, Cu: 0.50% or lower, Ni: 1.00% or lower, Cr: 1.20% or lower, Mo: 0.60% or lower, Nb: 0.05% or lower, V: 0.10% or lower, and B: 0.0050% or lower. In addition, steel in which the CEN calculated by Expression I is 0.20 mass% to 0.85 mass% is employed. The upper limit of the CEN is 0.85 mass% so as not to cause weld cracking in the heat-affected zone (HAZ) of the base metal. In order to more reliably prevent weld cracking in the HAZ, the upper limit of the CEN may be 0.80 mass%, 0.75 mass%, 0.73 mass%, 0.70 mass%, 0.68 mass%, 0.65 mass%, 0.63 mass%, or 0.60 mass%. In order to allow the hardness of the base metal to be HV380 or higher, the lower limit of the CEN is 0.20 mass%. In order to increase the hardness of the base metal, the lower limit of the CEN may be 0.24 mass%, 0.28 mass%, 0.30 mass%, 0.32 mass%, 0.35 mass%, or 0.38 mass%. The CEN of a steel plate in which the hardness of the base metal is HV565 or lower does not generally exceed 0.75 mass%. Therefore, the upper limit of the CEN of the steel plate in which the hardness of the base metal is HV565 or lower is 0.75 mass%. As a method of measuring the hardness of the base metal, a method of measuring the Vickers hardnesses of five or more points at a position 1 mm inward from the surface of the section of the base metal in the plate thickness direction and obtaining the average value thereof is employed. [0057] Subsequently, regarding the flux-cored weld wire to be used, the flux components and alloy components thereof will be separately described. The amounts of the components in the description of the flux-cored weld wire represent mass% with respect to the total mass of the flux-cored weld wire. Initially, the flux components inserted into a steel sheath of the wire will be described. [0058] By including a predetermined amount of one type or two or more types of metal fluorides including CaF 2 , BaF 2 , SrF 2 , and MgF 2 and one type or two or more types of metal oxides including Ti oxides (for example, TiO 2 ), Si oxides (for example, SiO 2 ), Mg oxides (for example, MgO), and Al oxides (for example, A1 2 0 3 ) in the weld wire and by allowing the ratios of the fluorides and the oxides to be in a predetermined range, the amount of diffusible hydrogen in the weld metal is stably lower than 1.0 ml/100 g.

Requirements for obtaining this effect are, when the total amount of CaF 2 , BaF 2 , SrF 2 , and MgF 2 being contained is a, to allow the total amount a with respect to the total mass of the flux-cored wire in terms of mass% to be 3.3% or higher and 8.0% or lower, when the total amount of the contained Ti oxides, Si oxides, Mg oxides, and Al oxides is P, to allow the total amount 3 with respect to the total mass of the flux cored wire in terms of mass% to be 0.10% or higher and 1.50% or lower, to allow the ratio of the CaF 2 content to the a to be 0.90 or higher, and to allow the ratio ([total amount a]/[total amount p]) of the total amount a to the total amount P to be 3.0 or higher and 80.0 or lower. [0059] When the total amount a of the contained metal fluorides is lower than 3.3%, the amount of diffusible hydrogen in the weld metal cannot be stably lower than 1.0 ml/100 g. In order to further reduce the amount of diffusible hydrogen in the weld metal, the lower limit of the total amount a may be 3

.

5 %, 3

.

7 %, or 3.9%. When the total amount a is higher than 8.0%, welding fumes or slag is excessively formed, and thus welding workability is significantly degraded, which is not preferable. In order to avoid the excessive generation of welding fumes or slag, the upper limit of the total amount a may be 7

.

5 %, 7 .0%, 6

.

5 %, 6.0%, or 5.7%. When the total amount p of the contained metal oxides is lower than 0.10%, the shape of welding beads may be deteriorated. When the total amount P is higher than 1.50%, toughness may be degraded. In order to enhance the shape of the welding beads, the lower limit of the total amount P may be 0.

2 0%, 0.30%, 0.

4 0%, or 0.50%. In order to improve toughness, the upper limit of the total amount p may be 1.

3 0%, 1.20%, 1.l10%, 1.00%, 0.90%, or 0.80%. Furthermore, when the ratio of the total amount a to the total amount P is lower than 3.0, the amount of diffusible hydrogen in the weld metal may not be stably lower than 1.0 ml/100 g. When the ratio thereof is higher than 80.0, welding fumes or slag is excessively generated, and thus welding workability is significantly degraded, which is not preferable. In order to further reduce the amount of diffusible hydrogen in the weld metal, the lower limit of the ratio ([total amount a]/[total amount P]) may be 3.2, 3.5, 3.7, or 4.0. In order to avoid the excessive generation of welding fumes or slag, the upper limit of the ratio ([total amount a]/[total amount p]) may be 40.0, 30.0, 20.0, 15.0, or 13.0. In a case where the ratio of the CaF 2 content to the a is lower than 0.90, the amount of diffusible hydrogen in the weld metal may not be lower than 1.0 ml/100 g. This is because CaF 2 has the greatest effect in reducing the amount of diffusible hydrogen among the metal fluorides. A situation in which the ratio of the CaF 2 content to the a is maximized means a case where no metal fluorides other than CaF 2 are contained in the flux. Therefore, the upper limit of the ratio of the CaF 2 content to the a is 1.0. [0060] Accordingly, the total amount a of the contained metal fluorides, the total amount P of the metal oxides, and the ratio of the total amount a of the metal fluorides to the total amount p of the metal oxides are limited as described above. In addition, the total amount P is the content in the flux-cored wire, and the content is obtained by also adding metal fluorides contained in a binder (water glass primarily containing SiO 2 ) used to granulate the flux and the like. [0061] To the flux-cored weld wire according to this embodiment, one type or two or more types of metal carbonates including CaCO 3 , BaCO 3 , SrCO 3 , and MgCO 3 may further be added for the purpose of enhancing an arc stabilizing effect and concentration of arc. However, when one type or two or more types of the metal carbonates are added in a proportion of 0.60% or higher, concentration of arc is too strong, and thus the amount of generated spatter is increased. Furthermore, the amount of oxygen in the weld metal is increased. Therefore, in a case of including the metal carbonates, the sum of the amounts of the metal carbonates is lower than 0.60%. The lower limit of the sum of the amounts of the metal carbonates is 0%. In order to suppress the amount of generated spatter, the upper limit thereof may be 0.500%, 0.40%, 0.20%, or 0.10%. [0062] The reason that the metal fluorides reduce the amount of diffusible hydrogen is not necessarily clear. However, it is thought that the metal fluorides are decomposed by welding arc and the generated fluorine is bonded to hydrogen and scatters in the air as HF gas, or hydrogen is fixed to the weld metal as HF as it is. [0063] In the present invention, it is preferable that CaO not be added to the flux. Therefore, the lower limit of the CaO content is 0%. However, there may be cases where CaO is contained in the raw material of the flux. In this case, the CaO content is limited to be lower than 0.20%. The CaO content is preferably 0.15% or lower or 0.10% or lower. When the CaO content is limited to be lower than 0.20%, effects according to the method of producing a weld joint according to this embodiment are obtained. CaO comes into contact with the air and changes to CaOH. Therefore, there is a possibility that CaO may increase the amount of diffusible hydrogen in the weld metal. [0064] The amounts of alloy elements in the flux-cored wire excluding the metal - '10 fluorides, metal oxides, and metal carbonates are limited as follows. [0065] (C: 0.010% to 0.350% in a case where the average Vickers hardness HV of the weld metal measured at 1 mm inward from the surface is 337 to 440, and 0.060% to 0.350% in a case where the average Vickers hardness HV of the weld metal measured at 1 mm inward from the surface is 380 to 533) When the C content in the flux-cored wire is lower than 0.010%, the C content of the weld metal becomes lower than 0.100%, and thus the hardness of the weld metal becomes lower than HV337. Therefore, the C content in the flux-cored wire is 0.010% or higher. When the C content in the flux-cored wire is lower than 0.060%, the C content of the weld metal becomes lower than 0. 120%, and thus the hardness of the weld metal becomes lower than HV380. Therefore, in order to allow the hardness of the weld metal to be HV380, the C content in the flux-cored wire is 0.060% or higher. In order to enhance the hardness of the weld metal, the lower limit of the C content may be 0.020% or 0.030%. In order to further enhance the hardness of the weld metal, the lower limit of the C content may be 0.070%, 0.080%, 0.

0 9 0 %, 0.100%, or 0.110%. When the C content in the flux-cored wire is higher than 0.350%, the C content of the weld metal becomes higher than 0.250%. Therefore, the C content in the flux-cored wire is 0.350% or lower. In order to improve the cold cracking resistance of the weld metal, the upper limit of the C content may be 0.300%, 0.250%, 0.180%, 0.170%, or 0.160%. [0066] (Si: 0.05% to 1.80%) When the Si content in the flux-cored wire is lower than 0.05%, the Si content of the weld metal becomes lower than 0.05%. Therefore, the Si content in the flux - JA( cored wire is 0.05% or higher. In order to reduce the 0 content in the weld metal, the lower limit of the Si content may be 0.10%, 0.20%, 0.30%, or 0.40%. When the Si content in the flux-cored wire is higher than 1.80%, the Si content of the weld metal becomes higher than 0.80% even when oxidative consumption is considered. Therefore, the Si content in the flux-cored wire is 1.80% or lower. In order to improve the toughness of the weld metal, the upper limit of the Si content may be 1.50%, 1.20%, 1.00%, 0.80%, or 0.60%. [0067] (Mn: 0.50% to 4.00%) When the Mn content in the flux-cored wire is lower than 0.50%, the Mn content of the weld metal becomes lower than 0.20%. Therefore, the Mn content in the flux-cored wire is 0.50% or higher. In order to enhance the hardness of the weld metal, the lower limit of the Mn content may be 0.70%, 0.80%, 0.90%, 1.00%, or 1.10%. When the Mn content in the flux-cored wire is higher than 4.00%, the Mn content of the weld metal becomes higher than 2.50% even when oxidative consumption is considered. Therefore, the Mn content in the flux-cored wire is 4.00% or lower. In order to improve the toughness of the weld metal, the upper limit of the Mn content may be 3.00%, 2 .50%, 2.20%, 2.00%, or 1.

8 0 %. [0068] (P: 0.050% or lower) When the P content in the flux-cored wire is higher than 0.050%, the P content of the weld metal may become higher than 0.050%. Therefore, the P content in the flux-cored wire is 0.050% or lower. As necessary, the upper limit of the P content maybe limited to 0.030%, 0.025%, 0.020%, or 0.015%. The lower limit of the P content does not need to be limited. The lower limit of the P content is 0%. - 41 - [0069] (S: 0.020% or lower) When the S content in the flux-cored wire is higher than 0.020%, the S content of the weld metal may become higher than 0.020%. Therefore, the S content in the flux-cored wire is 0.020% or lower. As necessary, the upper limit of the S content may be limited to 0.015%, 0.010%, 0.008%, or 0.006%. The lower limit of the S content does not need to be limited. The lower limit of the S content is 0%. [0070] (Al: 0.005% to 0.150%) When the Al content in the flux-cored wire is lower than 0.005%, the Al content of the weld metal becomes lower than 0.005%. Therefore, the Al content in the flux-cored wire is 0.005% or higher. In order to further reduce the 0 content in the weld metal, the lower limit of the Al content may be 0.007%, 0.010%, or 0.012%. When the Al content in the flux-cored wire is higher than 0.150%, the Al content of the weld metal may become higher than 0.100%. Therefore, the Al content in the flux cored wire is 0.150% or lower. In order to improve the toughness of the weld metal, the upper limit of the Al content may be limited to 0.090%, 0.070%, 0.050%, or 0.040%. [0071] (Cu: 0% to equal to or lower than 0.75%) When the Cu content in the flux-cored wire is higher than 0.75%, the Cu content of the weld metal becomes higher than 0.50%. Therefore, the Cu content in the flux-cored wire is 0.

7 5 % or lower. In order to further reduce the Cu content of the weld metal, the Cu content may be 0.50% or lower. As necessary, the upper limit of the Cu content may be 0.

4 0% or 0.

3 0%. The lower limit of the Cu content may not - Jt) be limited. Therefore, the lower limit of the Cu content is 0%. On the other hand, in order to enhance the hardness of the weld metal, 0.10% or higher of Cu may be contained in the weld metal. [0072] (Ni: 0% to lower than 1.00%) When the Ni content in the flux-cored wire is 1.00% or higher, the Ni content of the weld metal becomes 0.70% or higher, and the alloy cost of the wire is increased. Therefore, the Ni content in the flux-cored wire is lower than 1.00%. In order to prevent solidification cracking of the weld metal, the upper limit of the Ni content may be 0.50%, 0.40%, 0.30%, 0.20%, or 0.10%. The lower limit of the Ni content may not be limited. Therefore, the lower limit of the Ni content is 0%. [0073] (Cr: 0% to 3.50%) When the Cr content in the flux-cored wire is higher than 3.50%, the Cr content of the weld metal becomes higher than 2.50%. Therefore, the Cr content in the flux-cored wire is 3

.

5 0% or lower. As necessary, the upper limit of the Cr content may be 1.50%, 1.00%, 0.50%, or 0.10%. The lower limit of the Cr content may not be limited. Therefore, the lower limit of the Cr content is 0%. On the other hand, in a case of adding Cr for the purpose of enhancing the hardness of the weld metal, in order to obtain the effect, 0.05% or higher of Cr may be contained. [0074] (Mo: 0% to 1.50%) When the Mo content in the flux-cored wire is higher than 1.50%, the Mo content of the weld metal becomes higher than 1.00%. Therefore, the Mo content in the flux-cored wire is 1.50% or lower. In order to enhance toughness, the upper limit - 41l of the Mo content may be 0.

7 0%, 0.5 0 %, 0.30%, or 0.

2 0%. The lower limit of the Mo content may not be limited. Therefore, the lower limit of the Mo content is 0%. On the other hand, in a case of adding Mo for the purpose of enhancing the hardness of the weld metal, in order to obtain the effect, 0.05% or higher of Mo may be contained. [0075] (Ti: 0% to 0.150%) When the Ti content in the flux-cored wire is higher than 0.150%, the Ti content of the weld metal becomes higher than 0.100%. Therefore, the Ti content in the flux-cored wire is 0.150% or lower. In order to enhance toughness, the upper limit of the Ti content may be 0.100%, 0.080%, or 0.050%. The lower limit of the Ti content may not be limited. Therefore, the lower limit of the Ti content is 0%. For the purpose of enhancing toughness, 0.010% or higher of Ti may be contained. [0076] (Nb: 0% to 0.15%) When the Nb content in the flux-cored wire is higher than 0.15%, the Nb content of the weld metal becomes higher than 0.10%. Therefore, the Nb content in the flux-cored wire is 0.15% or lower. In order to enhance toughness, the upper limit of the Nb content may be 0.10%, 0.08%, or 0.05%. The lower limit of the Nb content may not be limited. Therefore, the lower limit of the Nb content is 0%. For the purpose of enhancing the hardness of the weld metal, 0.0o1% or higher of Nb may be contained. [0077] (V: 0% to 0.45%) When the V content in the flux-cored wire is higher than 0.45%, the V content of the weld metal becomes higher than 0.30%. Therefore, the V content in the flux - ztd cored wire is 0.45% or lower. In order to enhance toughness, the upper limit of the V content may be 0.25%, 0.20%, or 0.15%. The lower limit of the V content may not be limited. Therefore, the lower limit of the V content is 0%. For the purpose of enhancing the hardness of the weld metal, 0.01% or higher of V may be contained. [0078] (B: 0% to 0.0500%) When the B content in the flux-cored wire is higher than 0.0500%, the B content of the weld metal becomes higher than 0.0100%. Therefore, the B content in the flux-cored wire is 0.0500% or lower. In order to enhance toughness, the upper limit of the B content may be 0.0400%, 0.0200%, 0.0100%, or 0.0050%. The lower limit of the B content does not need to be limited, and the lower limit of the B content is 0%. [0079] (Mg: 0% to 2.0%) When the Mg content in the flux-cored wire is higher than 2.0%, the Mg content of the weld metal becomes higher than 0.10%. Therefore, the Mg content in the flux-cored wire is 2.0% or lower. In order to enhance the toughness and ductility of the weld metal, the upper limit of the Mg content may be 1. 5%, 1.0%, 0.

4 %, or 0.2%. The lower limit of the Mg content does not need to be limited, and the lower limit of the Mg content is 0%. [0080] (Ca: 0% to 2

.

0 %) When the Ca content in the flux-cored wire is higher than 2.0%, the Ca content of the weld metal becomes higher than 0.10%. Therefore, the Ca content in the flux-cored wire is 2 .0% or lower. In order to enhance the toughness and ductility - 11 1 of the weld metal, the upper limit of the Ca content may be 1.

5 %, 1.0%, 0.50%, or 0.

3 %. The lower limit of the Ca content does not need to be limited, and the lower limit of the Ca content is 0%. [0081] (REM: 0% to 0.0150%) When the REM content in the flux-cored wire is higher than 0.0150%, the REM content of the weld metal becomes higher than 0.0100%. Therefore, the REM content in the flux-cored wire is 0.0150% or lower. In order to enhance the toughness and ductility of the weld metal, the upper limit of the REM content may be 0.0o100%, 0.0050%, or 0.0030%. The lower limit of the REM content does not need to be limited, and the lower limit of the REM content is 0%. [0082] The reason that the chemical composition of the flux-cored wire according to this embodiment is limited has been described above. Regarding the other chemical composition of the alloys of the remainder, the remainder primarily containing Fe may also contain impurities that are incorporated during the production process and the like in a range in which the characteristics of the weld joint according to this embodiment are not impeded. The Fe component contains Fe in the steel sheath, and Fe in iron powder and alloy components added to the flux. The iron powder content in the flux is lower than 10.0% in terms of mass% with respect to the total mass of the flux-cored wire. When the iron powder content is increased, there may be a case where the amount of oxygen is also increased. As necessary, the iron powder content may be lower than 5.0% or lower than 1.0%. Since the iron powder does not need to be contained, the lower limit of the iron powder content is 0 %. [0083] Subsequently, the morphology of the flux-cored wire will be described. The flux-cored wire is primarily divided into a seamless wire (that is, a wire in which the seams of the steel sheath are welded to each other) in which slit-like seams are not formed in the steel sheath, and a seamed wire in which the seams of the steel sheath have a slit-like gap. The present invention may employ any sectional structure. However, in order to suppress the cold cracking of the weld metal, a wire without slit-like seams (seamless wire) is preferable. [0084] Hydrogen infiltrated into the weld zone during welding is diffused into the weld metal and the steel side, is accumulated to a stress concentration zone, and acts as a cause of the occurrence of cold cracking. As the hydrogen source, moisture held in the welding material, moisture incorporated from the air, rust or scales adhered to the surface of the steel, and the like are mentioned. However, during welding in which the cleanliness of the weld zone and shielding gas conditions are sufficiently managed, hydrogen contained in the wire primarily in the form of moisture becomes the main cause of diffusible hydrogen that is present in the weld joint. [0085] Therefore, it is preferable that a (seamless) pipe without slit-like seams be used as the steel sheath to suppress the infiltration of hydrogen in the air from the steel sheath to the flux until the wire is used after being produced. In a case where a (seamed) pipe with slit-like seams is used as the steel sheath, moisture in the air easily infiltrates into the flux from the slit-like seams (seamed portion) of the sheath. Therefore, when such a pipe is used as it is, the infiltration of the hydrogen source such as moisture cannot be prevented. Therefore, in a case where a time period from production to use is long, it is preferable that the entire wire be vacuum-packed or be - A'7 stored in a container that can be maintained in a dry state. In addition, in order to enhance the transportation performance of the wire, there may be a case where lubricating oil is applied to the surface of the wire. From the viewpoint of reducing the amount of diffusible hydrogen, as the lubricating oil applied to the surface of the wire, oil that does not contain hydrogen such as perfluoropolyether (PFPE) oil is preferable. [0086] The flux-cored wire used in the present invention can be produced in the same production process as that of a typical method of producing a flux-cored wire. That is, first, a steel strip which is to become the sheath, and a flux in which metal fluorides, alloy components, metal oxides, metal carbonates, and an arc stabilizer are mixed to have predetermined contents are prepared. While the steel strip is transported in the longitudinal direction thereof, the steel strip is formed into an open pipe (U-shape) by a forming roll to be used as the steel sheath, the flux is supplied from the opening of the open pipe during the formation, and the edge faces of the opening that oppose each other are subjected to butt seam welding. A seamless pipe obtained by the welding is drawn, and is subjected to annealing during the drawing or after the completion of the drawing process, thereby obtaining a (seamless) wire having a desired wire diameter without slit-like seams. In addition, a (seamed) wire having slit-like seams is obtained by supplying a flux from the opening of the open pipe to be formed as a seamed pipe that is not subjected to seam welding, and drawing the pipe. A cut section of the wire without slit-like gaps, which is made by butt seam welding, is illustrated in FIG. 3A. When the section is polished and etched, welding traces are observed. However, when the section is not etched, welding traces are not observed. Therefore, the section may be called "seamless". On p.111 of "New - JRS - Edition of Introduction to Welding and Joining Techniques" (2008) edited by "the Japan Welding Society" and published by Sanpo Publications Incorporated, a seamless type is described. As illustrated in FIG. 3B, when brazing is performed after butting is performed, or as illustrated in FIG. 3C, when brazing is performed after caulking is performed, wires without slit-like gaps can also be obtained. In FIGS. 3B and 3C, the wires that are not subjected to brazing and are used as they are become wires having slit-like gaps. [0087] In the present invention, gas-shielded are welding as multi-layer welding is performed on the steel plate by using the flux-cored wire that satisfies the above described conditions to form weld metal that satisfies the above-described conditions, thereby accomplishing the object. The gas-shielded arc welding method is not particularly limited, and a typically used method can be employed. For example, as the shielding gas, as well as 100% CO 2 gas, a mixed gas of 3 vol% to 20 vol% of CO 2 gas and Ar gas, or the like can be used. The flow rate of shielding gas may be under typical conditions, that is, about 15 L/min to 30 L/min. In addition, regarding welding conditions such as current, voltage, and the like, for example, a current of 200 Ato 350 A, a voltage of 25 V to 35 V, and the like may be employed. The welding rate may be controlled to allow a weld heat input to be 10 kJ/cm to 50 kJ/cm. [0088] The shape of the produced weld joint is determined depending on the application or the like and is not particularly limited. Weld joints in which a groove is formed, such as a typical butt joint, a corner joint, and a T joint may be applied. Therefore, the shape of the steel plate to be welded may be formed so that at least a - 40 portion thereof where the weld joint is formed is a plate shape, and the shape may not entirely have the plate shape. For example, shaped steel may also be included. In addition, the steel plate is not limited to various steel plates, and a single steel plate may be formed into a predetermined shape such as a pipe shape. However, a butt weld joint may also be employed. [Examples] [0089] Next, the applicability and effects of the weld joint according to this embodiment will be described with reference to Examples. Steel plates having components shown in Table 1 were used as base metals. In addition, as backing metals for welding, the same steel plates as the base metals were used. While a steel strip was transported in the longitudinal direction thereof, the steel strip was formed into an open pipe by a forming roll, a flux was supplied from the opening of the open pipe during the formation, and the edge faces of the opening that opposed each other were subjected to butt seam welding, thereby forming a pipe without slit-like seams. During drawing work of a wire of the formed pipe, annealing was performed, thereby producing a flux-cored wire having a final wire diameter of 41.2mm. In addition, some of the steel plates were formed into pipes having slit-like seams that were not subjected to seam welding, and the pipes were drawn, thereby producing flux-cored wires having a wire diameter of $1.2 mm. In the case of the wire having slit-like gaps, the entire wire was vacuum-packed and stored in a container so as to be maintained in a dry state, until welding is performed. The chemical components of the produced flux-cored wire were analyzed as follows. First, the filling flux was extracted from the flux-cored wire, and the fluxcored wire was separated into the steel sheath and the flux. The chemical components of the steel sheath were obtained by measuring the content of each of metal components through chemical analysis. The chemical components of the flux were performed in the following order. First, the constituent materials and components of the flux were subjected to quantitative evaluation by X-ray diffractometry and fluorescent X-ray spectroscopy. Thereafter, the flux was separated into a slag content and an alloy content by using a separation method such as flotation or magnetic separation, and the chemical components thereof were analyzed by performing chemical analysis, gas analysis, or the like. The chemical compositions of the produced flux-cored wires are shown in Tables 2-1-1 to 2-2, and Tables 3-1-1 to 3-2. [0090] The base metals were allowed to abut each other with a root gap of 16 mm and a groove angle of 200 by using the flux-cored wire, and were welded by using the backing metal under the welding conditions shown in Tables 4-1-1 to 4-2-3. On the surfaces of the groove surface of the base metal and the backing metal, buttering of two or more layers and an excess weld metal height of 3 mm or higher was performed by using the tested flux-cored wire. Here, as Ti oxides, Si oxides, Mg oxides, and Al oxides, TiO 2 , SiO 2 , MgO, and A1 2 0 3 were respectively used. In Tables 2-2 to 2-4, the metal carbonates include CaCO 3 , BaCO 3 , SrCO 3 , and MgCO 3 . [0091] The analysis results of the chemical compositions of the obtained weld metals are shown in Tables 5-1-1, 5-1-2, 5-2-1, 5-2-2, 5-2-4, and 5-2-5. A sample of a polished section of the weld metal, which is perpendicular to the welding direction, - ;I was acquired, and the Vickers hardnesses of 10 points of the sample at a position 1 mm inward from the surface of the weld metal were measured, and were converted into Brinell hardnesses using the hardness conversion table from SAE J417 (1983). In addition, a No. 4 Charpy test piece (2 mm V-notch) based on JIS Z3111 (2005) was acquired, and the Charpy absorbed energy of the weld metal at -40'C was measured. A -40'C absorbed energy of 27 J or higher was evaluated as passing. The obtained results of the hardnesses and the Charpy test are shown in Tables 5-1-3, 5-2-3, and 5-2-6. [0092] In addition, a cold-cracking test and a diffusible hydrogen amount-measuring test were performed on each of the weld joints obtained under the same welding conditions. As the cold-cracking test, a test based on JIS Z 3158 (method of y-groove weld-cracking test in 1993) was performed at room temperature (25 0 C), and the absence of cracking in surfaces and sections was evaluated as passing. The diffusible hydrogen amount-measuring test was performed according to a gas chromatography method based on JIS Z 3118 (method for measurement of amount of hydrogen evolved from steel welds in 2007). An amount of diffusible hydrogen of lower than 1.0 ml/100 g was evaluated as passing. The results are shown in Tables 5-1-3, 5-2-3, and 5-2-6. [0093] During welding, a significant level of the generation of fumes or slag was evaluated as poor welding workability. A low level of the generation of fumes or slag was evaluated as good welding workability. The results are shown in Tables 5-1-3, 5 2-3, and 5-2-6. [0094] As shown in the test results of Table 5-1-3, the weld metals of Examples I to 54 which are examples of the present invention were excellent in all of hardness, toughness, cold cracking resistance, and welding workability and thus passed the tests. On the other hand, as shown in the test results of Tables 5-2-3 to 5-2-6, the weld metals of Comparative Examples 101 to 165 did not satisfy the requirements specified in the present invention and at least one of hardness, toughness, cold cracking resistance, and welding workability did not pass the tests. The underlined numbers in Comparative Examples of Tables 5-2-1 to 5-2-6 represent outside of the ranges of the present invention. [0095] [Table 1] [0096] [Table 2-1-1] [0097] [Table 2-1-2] [0098] [Table 2-2] [0099] [Table 3-1-1] [0100] [Table 3-1-2] [0101] [Table 3-2] [0102] [Table 4-1-1] [01031 [Table 4-1-2] [0104] [Table 4-2-1] [0105] [Table 4-2-2] [0106] [Table 4-2-3] [0107] [Table 5-1-1] [0108] [Table 5-1-2] [0109] [Table 5-1-3] [0110] [Table 5-2-1] [0111] [Table 5-2-2] [0112] [Table 5-2-3] [0113] [Table 5-2-4] [0114] [Table 5-2-5] [0115] [Table 5-2-6] [Industrial Applicability] [0116] According to the present invention, in a weld joint which uses a high-hardness steel plate having a high C content and a surface hardness of HV380 or higher and HV693 or lower as a base metal, weld metal which has a surface hardness of HV337 or higher and HV533 or lower and excellent abrasion resistance or weld metal which has a surface hardness of HV380 or higher and HV533 or lower and excellent abrasion resistance can be obtained without the occurrence of cold cracking even when preheating is not performed. Therefore, welding efficiency can be significantly enhanced, and thus such a weld joint is extremely valuable in the industrial field.

Claims

1. A method of producing a weld joint by performing a gas-shielded arc welding, using a flux-cored wire filled with flux into a steel sheath, on any one of a steel plate having a Vickers hardness HV of 380 or higher and 514 or lower, a plate thickness of 20 mm to 100 mm, a C content of 0.120 mass% to 0.300 mass%, and a CEN calculated by the following Expression 1 of 0.20 mass% to 0.75 mass%, a steel plate having a Vickers hardness HV of higher than 514 and 565 or lower, a plate thickness of 12 mm to 100 mm, a C content of 0. 120 mass% to 0.300 mass%, and a CEN calculated by the following Expression I of 0.20 mass% to 0.75 mass%, and a steel plate having a Vickers hardness HV of higher than 565 and 693 or lower, a plate thickness of 6 mm to 12 mm, a C content of 0.350 mass% to 0.450 mass%, and a CEN calculated by the following Expression 1 of 0.20 mass% to 0.85 mass%, the method comprising: (a) during the gas-shielded arc welding, not performing a preheating operation in a case where a temperature of the steel plate is 10 C or higher, and in a case where the temperature of the steel plate is lower than 10 C, performing the preheating operation so that the temperature of the steel plate is 10 C or higher, (b) wherein the flux-cored wire contains one or more of CaF 2 , BaF 2 , SrF 2 , and MgF 2 , and when a sum of amounts thereof is a, the a with respect to a total mass of the flux-cored wire is 3.3% to 8.0% in terms of mass%, the flux-cored wire contains one or more of Ti oxides, Si oxides, Mg oxides, and Al oxides, and when a sum of amounts thereof is p, the P with respect to the total mass of the flux-cored wire is 0.10% to 1.50% in terms of mass%, a sum of amounts of CaCO 3 , BaCO 3 , SrCO 3 , and MgCO 3 with respect to the total mass of the flux-cored wire is lower than 0. 6 0% in terms of mass%, - SA - an amount of an iron powder in the flux with respect to the total mass of the flux-cored wire is lower than 10.0% in terms of mass%, a ratio of the amount of CaF 2 to the a is 0.90 or higher, a ratio of the a to the p is 3.0 or higher and 80.0 or lower, an amount of CaO with respect to the total mass of the flux-cored wire is lower than 0.20% in terms of mass%, the flux-cored wire include as a chemical composition excluding metal fluorides, metal oxides, and metal carbonates, with respect to the total mass of the flux cored wire, in terms of mass%: C: 0.010% to lower than 0.060%; Si: 0.05% to 1.80%; Mn: 0.50% to 4.00%; P: 0.050% or lower; S: 0.020% or lower; Al: 0.005% to 0.150%; Cu: 0% to 0.75%; Ni: 0% to lower than 1.00%; Cr: 0% to 3.50%; Mo: 0% to 1.50%; Ti: 0% to 0.150%; Nb: 0% to 0.15%; V: 0% to 0.45%; B: 0% to 0.0500%; Mg: 0% to 2.0%; Ca: 0% to 2. 0 %; - ; S7 - REM: 0% to 0.0150%; and the remainder: Fe and impurities, (c) wherein a weld metal of the weld joint includes as a chemical composition, in terms of mass%: C: 0.100% to 0.170%; Si: 0.05% to 0.80%; Mn: 0.20% to 2.50%; Al: 0.0050% to 0.1000%; P: 0.050% or lower; S: 0.020% or lower; N: 0.015% or lower; Cu: 0% to 0.50%; Ni: 0% to lower than 0.70%; Cr: 0% to 2.50%; Mo: 0% to 1.00%; Ti: 0% to 0.100%; Nb: 0% to 0.100%; V: 0% to 0.30%; B: 0% to 0.0100%; 0: 0% to 0.100%; Mg: 0% to 0.100%; Ca: 0% to 0.100%; REM: 0% to 0.0100%; and the remainder: Fe and impurities, a CEN of the weld metal calculated by the following Expression 1 is 0.20 mass% to 0.58 mass%, an average Vickers hardness HV of the weld metal measured at 1 mm inward from a surface of the weld metal is 337 to 440, and all of (a) to (c) are satisfied. CEN=[C]+(0.75+0.25xtanh(20x([C] 0.12)))x([Si]/24+[Mn]/6+[Cu]/15+[Ni]/20+([Cr]+[Mo]+[Nb]+[V])/5+5x[B]) ... (Expression 1) where elements with [] represent the amounts (mass%) of the corresponding elements.

2. A method of producing a weld joint by performing a gas-shielded are welding, using a flux-cored wire filled with flux into a steel sheath, on any one of a steel plate having a Vickers hardness HV of 380 or higher and 514 or lower, a plate thickness of 20 mm to 100 mm, a C content of 0.120 mass% to 0.300 mass%, and a CEN calculated by the following Expression 1 of 0.20 mass% to 0.75 mass%, a steel plate having a Vickers hardness HV of higher than 514 and 565 or lower, a plate thickness of 12 mm to 100 mm, a C content of 0.120 mass% to 0.300 mass%, and a CEN calculated by the following Expression 1 of 0.20 mass% to 0.75 mass%, and a steel plate having a Vickers hardness HV of higher than 565 and 693 or lower, a plate thickness of 6 mm to 12 mm, a C content of 0.350 mass% to 0.450 mass%, and a CEN calculated by the following Expression 1 of 0.20 mass% to 0.85 mass%, the method comprising: (a) during the gas-shielded arc welding, not performing a preheating operation in a case where a temperature of the steel plate is 10 C or higher, and in a case where the temperature of the steel plate is lower than 10 C, performing the preheating - 1;o - operation so that the temperature of the steel plate is 10 C or higher, (b) wherein the flux-cored wire contains one or more of CaF 2 , BaF 2 , SrF 2 , and MgF 2 , and when a sum of amounts thereof is a, the a with respect to a total mass of the flux-cored wire is 3 . 3 % to 8 .0% in terms of mass%, the flux-cored wire contains one or more of Ti oxides, Si oxides, Mg oxides, and Al oxides, and when a sum of amounts thereof is P, the f with respect to the total mass of the flux-cored wire is 0.10% to 1.50% in terms of mass%, a sum of amounts of CaCO 3 , BaCO 3 , SrCO 3 , and MgCO 3 with respect to the total mass of the flux-cored wire is lower than 0. 6 0% in terms of mass%, an amount of an iron powder in the flux with respect to the total mass of the flux-cored wire is lower than 10.0% in terms of mass%, a ratio of the amount of CaF 2 to the a is 0.90 or higher, a ratio of the a to the p is 3.0 or higher and 80.0 or lower, an amount of CaO with respect to the total mass of the flux-cored wire is lower than 0.20% in terms of mass%, the flux-cored wire includes chemical components excluding metal fluorides, metal oxides, and metal carbonates, with respect to the total mass of the flux-cored wire, in terms of mass%: C: 0.060% to 0.350%; Si: 0.05% to 1.80%; Mn: 0.50% to 4.00%; P: 0.050% or lower; S: 0.020% or lower; Al: 0.005% to 0.150 % ; Cu: 0% to 0.75%; -An) Ni: 0% to lower than 1.00%; Cr: 0% to 3 .50%; Mo: 0% to 1.50%; Ti: 0% to 0.150%; Nb: 0% to 0.15%; V: 0% to 0.45%; B: 0% to 0.0500%; Mg: 0% to 2.0%; Ca: 0% to 2.0%; REM: 0% to 0.0150%; and the remainder: Fe and impurities, (c) wherein a weld metal of the weld joint includes as a chemical composition, in terms of mass%: C: 0.120% to 0.250%; Si: 0.05% to 0.80%; Mn: 0.20% to 2.50%; Al: 0.0050% to 0.1000%; P: 0.050% or lower; S: 0.020% or lower; N: 0. 0 15% or lower; Cu: 0% to 0.50%; Ni: 0% to lower than 0. 7 0 %; Cr: 0% to 2 .50%; Mo: 0% to 1.00%; Ti: 0% to 0.100%; Nb: 0% to 0.100%; V: 0% to 0.30%; B: 0% to 0.0100%; 0: 0% to 0.100%; Mg: 0% to 0.100%; Ca: 0% to 0.100%; REM: 0% to 0.0100%; the remainder: Fe and impurities, a CEN of the weld metal calculated by the following Expression 1 is 0.20 mass% to 0.58 mass%, an average Vickers hardness HV of the weld metal measured at 1 mm inward from a surface of the weld metal is 380 to 533, and all of (a) to (c) are satisfied. CEN=[C]+(0.75+0.25 xtanh(20x([C] 0.12)))x([Si]/24+[Mn]/6+[Cu]/15+[Ni]/20+([Cr]+[Mo]+[Nb]+[V])/5+5x[B]) ... (Expression 1) where elements with [] represent the amounts (mass%) of the corresponding elements.

3. A method of producing a weld joint by performing a gas-shielded arc welding, using a flux-cored wire filled with flux into a steel sheath, on any one of a steel plate having a Vickers hardness HV of higher than 565 and 693 or lower, a plate thickness of 12 mm to 20 mm, a C content of 0.350 mass% to 0.450 mass%, and a CEN calculated by the following Expression 2 of 0.20 mass% to 0.85 mass%, and a steel plate having a Vickers hardness HV of higher than 565 and 693 or lower, a plate - A 1 - thickness of greater than 20 mm to 50 mm or smaller, a C content of 0.350 mass% to 0.450 mass%, and a CEN calculated by the following Expression 2 of 0.20 mass% to 0.85 mass%, the method comprising: (a) during the gas-shielded arc welding, performing a preheating operation so that a temperature of the steel plate is 1 00 0 C or higher in a case where the plate thickness of the steel plate is 20 mm or smaller, and in a case where the plate thickness of the steel plate is greater than 20 mm, performing the preheating operation so that the temperature of the steel plate is 150'C or higher, (b) wherein the flux-cored wire contains one or more of CaF 2 , BaF 2 , SrF2, and MgF 2 , and when a sum of amounts thereof is a, the a with respect to a total mass of the flux-cored wire is 3.3% to 8.0% in terms of mass%, the flux-cored wire contains one or more of Ti oxides, Si oxides, Mg oxides, and Al oxides, and when a sum of amounts thereof is P, the P with respect to the total mass of the flux-cored wire is 0.10% to 1.50% in terms of mass%, a sum of amounts of CaCO 3 , BaCO 3 , SrCO 3 , and MgCO 3 with respect to the total mass of the flux-cored wire is lower than 0.60% in terms of mass%, an amount of an iron powder in the flux with respect to the total mass of the flux-cored wire is lower than 10.0% in terms of mass%, a ratio of the amount of CaF 2 to the a is 0.90 or higher, a ratio of the a to the P is 3.0 or higher and 80.0 or lower, an amount of CaO with respect to the total mass of the flux-cored wire is lower than 0. 2 0% in terms of mass%, the flux-cored wire includes chemical components excluding metal fluorides, metal oxides, and metal carbonates, with respect to the total mass of the flux-cored wire, in terms of mass%: C: 0.060% to 0.350%; Si: 0.05% to 1.80%; Mn: 0.50% to 4.00%; P: 0.050% or lower; S: 0.020% or lower; Al: 0.005% to 0.150%; Cu: 0% to 0.75%; Ni: 0% to lower than 1.00%; Cr: 0% to 3 .5 0 %; Mo: 0% to 1.50%; Ti: 0% to 0.150%; Nb: 0% to 0.15%; V: 0% to 0.45%; B: 0% to 0.0500%; Mg: 0% to 2.0%; Ca: 0% to 2.0%; REM: 0% to 0.0 150%; the remainder: Fe and impurities, (c) wherein a weld metal of the weld joint includes as a chemical composition, in terms of mass%: C: 0.120% to 0.250%; Si: 0.05% to 0.80%; Mn: 0.20% to 2.50%; Al: 0.0050% to 0.I1000%; P: 0.050% or lower; S: 0.020% or lower; N: 0.015% or lower; Cu: 0% to 0.50%; Ni: 0% to lower than 0.70%; Cr: 0% to 2.50 % ; Mo: 0% to 1.00%; Ti: 0% to 0.100%; Nb: 0% to 0.100%; V: 0% to 0.30%; B: 0% to 0.0100%; 0: 0% to 0.100%; Mg: 0% to 0.100%; Ca: 0% to 0.100%; REM: 0% to 0.0100%; and the remainder: Fe and impurities, a CEN of the weld metal calculated by the following Expression 2 is 0.20 mass% to 0.58 mass%, an average Vickers hardness HV of the weld metal measured at 1 mm inward from a surface of the weld metal is 380 to 533, and all of (a) to (c) are satisfied. CEN=[C]+(0.75+0.25 xtanh(20x([C] 0.1 2)))x([Si]/24+[Mn]/6+[Cu]/l 5+[Ni]/20+([Cr]+[Mo]+[Nb]+[V])/5+5x[B]) ... (Expression 2) where elements with [] represent the amounts (mass%) of the corresponding elements.

4. The method of producing a weld joint according to any one of Claims 1 to 3, wherein the amount of CaO in the flux-cored wire is 0.15% or lower in terms of mass% with respect to the total mass of the flux-cored wire.

5. The method of producing a weld joint according to any one of Claims I to 4, wherein the flux-cored wire includes the chemical components excluding the metal fluorides, metal oxides, and metal carbonates, with respect to the total mass of the flux-cored wire, in terms of mass%: Ni: 0% to 0.1%.

6. The method of producing a weld joint according to any one of Claims 1 to 5, wherein the flux-cored wire includes the chemical components excluding the metal fluorides, metal oxides, and metal carbonates, with respect to the total mass of the flux-cored wire, in terms of mass%: Cu: 0% to 0.50%; Cr: 0% to 1.00%; Mo: 0% to 0.50%; Ti: 0% to 0.050%; and Nb: 0% to 0.05%.

7. The method of producing a weld joint according to any one of Claims 1 to 6, wherein the steel sheath of the flux-cored wire does not have a slit-like gap.

8. The method of producing a weld joint according to any one of Claims 1 to 6, wherein the steel sheath of the flux-cored wire has a slit-like gap.

9. The method of producing a weld joint according to any one of Claims 1 to 8, wherein a perfluoropolyether oil is applied to a surface of the flux-cored wire. - A 7 -