HOT ROLLED STEEL FOR SHEAR REINFORCING BAR, EXCELLENT IN WELDABILITY AND HAVING HIGH STRENGTH AND HIGH DUCTILITY, AND METHOD FOR MANUFACTURE THEREOF

摘要

PROBLEM TO BE SOLVED: To provide a hot rolled steel for shear reinforcing bar, having material characteristics of ≥685 MPa yield stress and ≥8% elongation and excellent in weldability, and provide its manufacturing method. ;SOLUTION: The hot rolled steel for shear reinforcing bar, excellent in weldability and having high strength and high ductility, is composed of a steel having a composition consisting of, by mass, 0.08-0.20% C, 0.30-1.00% Si, 1.0-2.0% Mn, 0.005-0.10% V, 0.05-0.20% Ti, 0.0005-0.0050% B and the balance Fe with inevitable impurities. Further, the fraction of bainite and that of retained austenite in a microstructure are ≥95% and ≤2%, respectively, and the thickness of surface-layer scale is ≤50 μm. The steel can be manufactured by heating a steel having the above steel components to 1100-1280°C, subjecting the steel to hot rolling and to finish rolling at 800-1050°C, and applying immersion cooling to the rolled steel from ≥800°C in hot water of ≥90°C.;COPYRIGHT: (C)2001,JPO