Abstract: When manufacturing a straight steel section having a joint 2 comprising a ball claw 21 and a curved claw 20 by hot-rolling a bloom vertically symmetrically to make a section blank having a flange 2A at a web 1 end (first step), vertically asymmetrically hot-rolling the section blank to adjust the size of the web and form the flange into a rough joint 2B including a projection 20A (second step), and subjecting the projection to hot bend rolling into a curved claw 20 (third step), wherein the bloom has a chemical composition comprising, in mass percentage, from 0.01 to 0.20% C, up to 0.8% Si, up to 1.8% Mn, up to 0.030% P, and up to 0.020% S; and therein the claw bending start temperature in the third step is a temperature of over Ar3 or up to Ar3-50° C., thereby achieving a depth of wrinkle flaws 10 present on the inner surface side of the curved claw of up to 0.5 mm.

Abstract: When manufacturing a straight steel section having a joint 2 comprising a ball claw 21 and a curved claw 20 by hot-rolling a bloom vertically symmetrically to make a section blank having a flange 2A at a web 1 end (first step), vertically asymmetrically hot-rolling the section blank to adjust the size of the web and form the flange into a rough joint 2B including a projection 20A (second step), and subjecting the projection to hot bend rolling into a curved claw 20 (third step), wherein the bloom has a chemical composition comprising, in mass percentage, from 0.01 to 0.20% C, up to 0.8% Si, up to 1.8% Mn, up to 0.030% P, and up to 0.020% S; and therein the claw bending start temperature in the third step is a temperature of over Ar3 or up to Ar3—50° C., thereby achieving a depth of wrinkle flaws 10 present on the inner surface side of the curved claw of up to 0.5 mm.

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: 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: In the size measurement of steel sections, a pair of two-dimensional rangefinders and a pair of one-dimensional rangefinders are arranged so as to sandwich a steel section consisting of a pair of flange portions and a web portion in a widthwise direction of the flange portion and face to each other, whereby a vertical distance ranging from the two-dimensional rangefinder to the flange portion and a vertical distance ranging from the one-dimensional rangefinder to the web portion are measured, respectively to calculate a deviation at center, a flange width and a web thickness from the above measured values. In this case, a laser rangefinder is used as each of the two-dimensional rangefinder and one-dimensional rangefinder.