Abstract: A heavy-wall steel having a flange thickness of about 40 mm or more and possessing excellent strength, toughness, weldability, and seismic resistance capable of being used for structure members such as columns and beams of high-rise buildings. The heavy-wall steel has a tensile strength of about 490-690 MPa, a yield ratio of about 80% or less, and Charpy absorbed energy at 0.degree. C. of about 27 J or more at the center in terms of thickness of the flange portion in each of the rolling direction, the direction perpendicular to the rolling direction, and the plate-thickness direction.

Abstract: This invention provides bainite steel materials having a less scattering of properties in a thickness direction or between steel materials by using a chemical composition of C: not less than 0.001 wt % but less than 0.030 wt %, Si: not more than 0.60 wt %, Mn: 1.00-3.00 wt %, Nb: 0.005-0.20 wt %, B: 0.0003-0.0050 wt % and Al: not more than 0.100 wt % and rendering not less than 90% of the material into a bainite texture in steel materials such as thick steel plates, steel sheets, section steels, rod steels and the like.

Abstract: A heavy-wall steel having a flange thickness of about 40 mm or more and possessing excellent strength, toughness, weldability, and seismic resistance capable of being used for structure members such as columns and beams of high-rise buildings. The heavy-wall steel has a tensile strength of about 490-690 MPa, a yield ratio of about 80% or less, and Charpy absorbed energy at 0.degree. C. of about 27 J or more at the center in terms of thickness of the flange portion in each of the rolling direction, the direction perpendicular to the rolling direction, and the plate-thickness direction.

Abstract: An improved welding method for welding an inner seam of a longitudinally welded steel pipe along an inner groove by welding electrodes held by a boom which is supported by a tracing roller moving along the inner groove of the steel pipe. According to the invention, at least one metal plate is urged against at least one end of the steel pipe after tack welding the seam on an opposite side of the inner groove of the steel pipe to be welded. Lateral angular deformations of the end of the steel pipe are prevented on both sides of the seam. Such deformations are due to the load of the boom when the tracing roller leaves the end of the steel pipe for the metal plate. Thus, the invention provides for prevention of hot cracking at the end of the pipe.

Abstract: A four-electrode submerged arc welding process employs connections of four electrodes aligned in one row for alternating current source so that electromagnetic force Fx.sub.4 in a direction opposite to a welding direction acting upon a middle point of the arc from the rearmost electrode is within -25.ltorsim.Fx.sub.4 .ltorsim.-10 (N/m). The connections are preferably selected such that phase differences among currents applied to the three electrodes other than the frontmost or rearmost electrode are 120.degree., respectively, and a phase difference of the remaining frontmost or rearmost electrode is within 0.degree..+-.30.degree. relative to the furthermost electrode.With such connections of the four electrodes, the four-electrode submerged arc welding process remarkably prevents welding defects such as undercuts even at higher welding speeds.