A welding system that, using a junction at which a standing plate and a bottom plate meet as a welding line, makes an electrode weave centered on the welding line and thereby welds along the welding line. In this welding system, control is performed such that the arc voltage at a standing-plate-side weaving edge is equal to or less than the arc voltage at a welding-line center position, such that the arc voltage at a bottom-plate-side weaving edge becomes equal to or greater than the arc voltage at the welding-line center position, and such that the arc voltage at the standing-plate-side weaving edge becomes lower than the arc voltage at the bottom-plate-side weaving edge. The system can suppress occurrence of inferior bead appearance and of welding defects in horizontal fillet welding.

A welding system that, using a junction at which a standing plate and a bottom plate meet as a welding line, makes an electrode weave centered on the welding line and thereby welds along the welding line. In this welding system, control is performed such that the arc voltage at a standing-plate-side weaving edge is equal to or less than the arc voltage at a welding-line center position, such that the arc voltage at a bottom-plate-side weaving edge becomes equal to or greater than the arc voltage at the welding-line center position, and such that the arc voltage at the standing-plate-side weaving edge becomes lower than the arc voltage at the bottom-plate-side weaving edge. The system can suppress occurrence of inferior bead appearance and of welding defects in horizontal fillet welding.

Provided are a gas-shielded arc welding method, a welded joint, and a method for producing the welded joint. In gas-shielded arc welding including one-side butt welding of steel plates in the present invention, a ceramic-made backing material is attached to a bottom surface of a groove, and root pass is performed in one pass using a welding current I of 200 to 450 A and a welding voltage V of 25 to 50 V while a welding heat input Q is controlled within the range defined by formula 1 below. 0.4? G?1 ?Q ?0.6? G+1 (formula 1)
Here, G in formula 1 is a root gap (mm).

A gas-shielded arc welding method includes welding a steel plate having a tensile strength of 780 MPa or more while feeding a consumable electrode via a welding torch and flowing a shielding gas. The consumable electrode includes, in mass %, C: 0 to 0.20%, Si: 0 to 0.50%, Mn: 0 to 0.50%, Cr: 1.00% to 9.00%, S: 0.0020% to 0.0600%, and Ni: 0 to 0.50%. The shielding gas includes, in vol. %, at least one of CO.sub.2 and O.sub.2: 1% to 15% in total, with the remainder being Ar and unavoidable impurities. Welding is performed under the condition satisfying the relationship of 1{0.05[CO.sub.2+O.sub.2]}+[Cr]8.3, and [Cr] represents the content of Cr in the consumable electrode, and [CO.sub.2+O.sub.2] represents a total content of at least one of CO.sub.2 and O.sub.2 in the shielding gas.

A gas-shielded arc welding method includes welding a steel plate having a tensile strength of 780 MPa or more while feeding a consumable electrode via a welding torch and flowing a shielding gas. The consumable electrode includes, in mass %, C: 0 to 0.20%, Si: 0 to 0.50%, Mn: 0 to 0.50%, Cr: 1.00% to 9.00%, S: 0.0020% to 0.0600%, and Ni: 0 to 0.50%. The shielding gas includes, in vol. %, at least one of CO.sub.2 and O.sub.2: 1% to 15% in total, with the remainder being Ar and unavoidable impurities. Welding is performed under the condition satisfying the relationship of 1{0.05[CO.sub.2+O.sub.2]}+[Cr]8.3, and [Cr] represents the content of Cr in the consumable electrode, and [CO.sub.2+O.sub.2] represents a total content of at least one of CO.sub.2 and O.sub.2 in the shielding gas.

A method for preparing a metal composite plate by rolling includes the following steps: 1) rolling composite surfaces of a base plate and a cladding plate, respectively, to obtain the base plate corrugation and the cladding plate corrugation for mating with each other; 2) cleaning the composite surfaces of the base plate and the cladding plate to expose the metal matrixes of the base plate and the cladding plate; 3) laminating the base plate and the cladding plate sequentially so that the base plate corrugation on the base plate and cladding plate corrugation on the cladding plate mate with each other, compacting, and performing welding sealing treatment to the base plate and the cladding plate to obtain a composite plate slab; and 4) rolling the composite plate slab after inspection by using a compositing machine to a desirable thickness, to obtain a metal composite plate.

A method for preparing a metal composite plate by rolling includes the following steps: 1) rolling composite surfaces of a base plate and a cladding plate, respectively, to obtain the base plate corrugation and the cladding plate corrugation for mating with each other; 2) cleaning the composite surfaces of the base plate and the cladding plate to expose the metal matrixes of the base plate and the cladding plate; 3) laminating the base plate and the cladding plate sequentially so that the base plate corrugation on the base plate and cladding plate corrugation on the cladding plate mate with each other, compacting, and performing welding sealing treatment to the base plate and the cladding plate to obtain a composite plate slab; and 4) rolling the composite plate slab after inspection by using a compositing machine to a desirable thickness, to obtain a metal composite plate.

An arc welding device includes memory in which a combination of a short circuit frequency, a peak current, and a peak current period is stored and determinator that determines a peak current and a peak current period based on a set short circuit frequency and the combination stored in memory. A welding output part performs welding output based on the peak current and the peak current period determined by determinator.

A steel plate has a chemical composition containing, by mass %, C: 0.03% or more and 0.08% or less, Si: 0.01% or more and 1.0% or less, Mn: 1.2% or more and 3.0% or less, P: 0.015% or less, S: 0.005% or less, Al: 0.08% or less, Nb: 0.005% or more and 0.07% or less, Ti: 0.005% or more and 0.025% or less, N: 0.010% or less, O: 0.005% or less and the balance being Fe and inevitable impurities, a structure being a dual-phase structure consisting of a bainite phase and island martensite, wherein the area fraction of the island martensite is 3% to 15%, the equivalent circle diameter of the island martensite is 3.0 m or less, and the remainder of the structure is a bainite phase.

A steel plate has a chemical composition containing, by mass %, C: 0.03% or more and 0.08% or less, Si: 0.01% or more and 1.0% or less, Mn: 1.2% or more and 3.0% or less, P: 0.015% or less, S: 0.005% or less, Al: 0.08% or less, Nb: 0.005% or more and 0.07% or less, Ti: 0.005% or more and 0.025% or less, N: 0.010% or less, O: 0.005% or less and the balance being Fe and inevitable impurities, a structure being a dual-phase structure consisting of a bainite phase and island martensite, wherein the area fraction of the island martensite is 3% to 15%, the equivalent circle diameter of the island martensite is 3.0 m or less, and the remainder of the structure is a bainite phase.