PURPOSE: To reduce the segregation of impurity elements such as C and Mn and to provide a steel material of substantially uniform quality and high cracking resistance by subjecting a slab of low-carbon steel contg. various additive elements to hot working and a heat treatment under specific conditions.
CONSTITUTION: A continuous cast ingot or slab of steel contg. ≤0.10% C, ≤1.0% Si, 0.2W3.0% Mn, and further 1 or ≥2 kinds of ≤0.20% V, ≤0.20% Nb, ≤1.0% Mo, ≤0.20% Cu, ≤2.0% Cr, ≤3.0% Ni, ≤0.002% B, ≤0.10% Ti, ≤0.10% Al, ≤0.01% Ca is first subjected to primary hot working of ≥20% reduction of areas at an austenite temp. region of ≤1,200°C or Ar1WAc3two phase coexistence temps. of austenite and ferrite. In succession to this or after reheating from the temp. below the primary hot working temp., the central temp. of the slab is maintained at ≥1,000°C for ≥30min. Or, after it is cooled to Ar1 or below, it is reheated to the Ac3 temp. or above or is subjected to secondary hot working.
KUBOTA TAKESHI
KONNO TAKAHARU
