In order to make the
depth of penetration in the
weld bead portion uniform and obtain a
bead shape having a flat bottom without spiking and meltdown during continuous hot rolling by bonding a plurality of hot-rolled steel materials using a
laser beam, the following methods are employed: (A) a method of
butt welding wherein center gas is blown against the
welding portion symmetrically to the
optical axis of the
laser beam while side gas is being blown thereagainst from the side, the method comprising conducting
welding while the center of
laser-induced
plasma is shifted in the
welding direction from the center of the laser beam by a distance 0.2 to 0.5 times as much as a reference
plasma diameter determined from the laser output and the
beam diameter, and the type and flow rate of the center gas; or (B) a method comprising scanning the butt line with the laser beam during the laser welding at a speed of 2 to 10 m / min, and simultaneously oscillating the laser beam in the direction vertical to the butt line at a frequency of 40 to 80 Hz at an amplitude of 0.4 to 1.0 mm. Alternatively, (C) in order to prevent formation of blow holes in the
weld bead, to make the
depth of penetration in the
weld bead portion uniform, and to obtain a
bead shape having a flat bottom without spiking and excessive penetration, laser welding is conducted while a filler wire of an iron series base material, containing from 0.05 to 3% of one or at least two elements selected from aluminum,
silicon,
titanium and
manganese, is being supplied to the welding portion. Moreover, (D) in order to stably supply a filler wire to the butt portion with high supply accuracy, the filler wire W is passed through a wire supply
nozzle having a curved portion, and the filler wire is supplied toward the welding point along the welding line.