PURPOSE: To develop low yield ratio building steel having proof stress at a high temp. as well as excellent weldability by subjecting a low carbon steel contg. small amounts of Nb, Ti, N or the like to hot rolling, thereafter subjecting it to controlled cooling to form the structure of its base metal into a one essentially consisting of bainite and subsequently executing tempering.
CONSTITUTION: A slab having a compsn. contg., by weight, 0.05 to 0.20% C, 0.05 to 0.60% Si, 0.50 to 1.80% Mn, 0.020% or less P, 0.005% or less S, 0.004 to 0.020% Nb and 0.0020 to 0.0070% N and in which the carbon equivalent (Ceq) expressed by the formula I is regulated to 0.40% or below or furthermore contg. one or two kinds among 0.05 to 0.40% Cu, Ni, Cr and Mo, 0.005 to 0.06% V and 0.0005 to 0.0050% Ca and in which the carbon equivalent expressed by the formula II is regulated to 0.40% or below is subjected to hot rolling into steel. After that, this steel is subjected to controlled cooling to suppress the precipitation of Nb, and in such a manner that the strengthening of the transformation by solid solution Nb is activated, the structure of its base metal is formed into a one essentially consisting of bainite. After that, tempering is executed to precipitate fine Nb carbonitrides having 80% bainite structural fraction and smaller than 200 average grain size by the amt. of 1×106 pieces/mm3.
IWAI KIYOSHI
NAKATANI YOSHIYUKI
