PURPOSE: To obtain low yield ratio high tensile strength steel for building by specifying the chemical compsn. of steel and furthermore specifying hot rolling condition and cooling condition after the rolling.
CONSTITUTION: Steel having a compsn. contg., by weight, 0.06 to 0.15% C, 0.01 to 0.60% Si, 1.00 to 1.80% Mn, ≤0.015% P, ≤0.010% S, 0.005 to 0.10% Nb, ≤0.050% Al and 0.0010 to 0.0060% N and contg. one or two kinds of 0.005 to 0.03% Ti and 0.001 to 0.020% rare earth metals, and the balance Fe with inevitable impurities is prepd. If required, small amounts of one or more kinds among Cu, Ni, Cr, Mo and V are incorporated therein. This steel is heated to 1050 to 1300°C, and hot rolling is finished at ≥950°C. Immediately after that, it is subjected to water cooling to 750 to 600°C at ≥1°C/s cooling rate and is thereafter subjected to air cooling. In this way, the low yield ratio high tensile strength steel having ≥580MPa tensile strength can be obtd.
MATSUZAKI AKIHIRO
AMANO KENICHI
