PRODUCTION OF 70KG/MM2 CLASS STRONG AND TOUGH STEEL BAR WITHOUT HARDENING AND TEMPERING

摘要

PURPOSE:To obtain a strong and tough steel bar having excellent tensile strength and toughness by heating the steel bar contg. C, Si, Mn, P, S, Cr, acid soluble Al and N respectively in a prescribed ratio to the Ac1 transformation point or above and a specific temp. or below then allowing said steel bar to cool. CONSTITUTION:A steel bar contg., by weight %, 0.25-0.40% C, 0.20-0.45% Si, 1.20-1.80% Mn, =0.040% P, =0.030% S, 0.10-0.40% Cr, 0.015-0.050% acid soluble Al and =0.0060% N, and consisting of the balance Fe and unavoidable impurities is prepd. Such steel bar is heat-treated to the Ac1 transformation point or above and =980 deg.C to eliminate the influence of the previous processing of the steel material and to fine the crystal grains. The heat-treated steel bar is preferably cooled at approximately 15-50 deg.C/sec air cooling rate in a temp. range from the normalizing temp. down to 500 deg.C. As a result, the highly strong and tough steel bar having =70kgf/mm.2 tensile strength and =6kg-m toughness in V-notch Charpy impact absorption energy of the base material is obtd.