PURPOSE: To produce a high tension steel plate having excellent low-temp. toughness and weldability by subjecting an ingot consisting of a specific compsn. successively to heating, hot rolling, water cooling, reheating and air cooling under specific conditions.
CONSTITUTION: The ingot which contains, by weight %, 0.03W0.20% C, ≤0.8% Si, 0.5W1.8% Mn, 0.03W0.05% Nb, and ≤0.10% Al, contains the respective elements so as to satisfy carbon equiv. Ceq(%)≤0.33% (where Ceq=C+Si/24+ Mn/6) and consists of the balance iron and inevitable impurities is prepd. This ingot is then heated to 950W1,050°C and is hot rolled; thereafter, the hot rolled plate is water-cooled at ≥3°C/sec and (-50×Ceq+25)°C/sec cooling rates from (Ar2 point -50)°C or above and down to ≤400°C. The steel plate is then reheated to the temp. below the Ac1 point and is air-cooled. The high tension steel plate of 60kg/fmm2 class tensile strength having the structure essentially consisting of ferrite is thereby obtd.
KAJI HARUO
YANO KAZUHIKO
KOBAYASHI YOICHIRO
