The structural which is superior in fire resistance it is low production manner of the yield ratio melting galvanize hot rolling steel sheet and the structural steel charge null which uses its

摘要

PURPOSE:To obtain the hot rolled steel sheet having excellent high temp. characteristics, in which fireproofing can be reduced or eliminated and having steel components similar to those in a common steel by specifying the chemical components in the steel sheet and specifying its hot rolling conditions and hot dip galvanizing conditions. CONSTITUTION:A steel slab constituted of, by weight, 0.02 to 0.1% C, =0.5% Si, 0.2 to 1.0% Mn, 0.005 to 0.04% Nb, 0.3 to 0.7% Mo, =0.05% Al, =0.006% N and the balance Fe is hot-rolled directly at a high temp. as it is, or it is heated to 1100 to 1300 deg.C and is thereafter hot-rolled at 850 to 950 deg.C finishing end temp. In succession, the slab is cooled at 5 to 30 deg.C/s average cooling rate and is coiled at 400 to 650 deg.C; continuously, in the stage of passing the steel sheet through a continuous hot dip galvanizing line, it is heated to 500 to 700 deg.C in a reducing atmosphere, is thereafter cooled, is immersed into a hot dip galvanizing bath and is galvanized.