PRODUCTION OF LOW YIELD RATIO HIGH TENSILE STRENGTH STEEL PIPE

PURPOSE:To obtain a low yield ratio high tensile strength steel pipe suitable as a building material by specifying the chemical compsn. of steel and furthermore specifying the draft in hot rolling and heat treating conditions. CONSTITUTION:The compsn. of steel is constituted of, by weight, 0.02 to 0.2O% C, 0.02 to 0.50% Si and 0.50 to 2.00% Mn and furthermore one or >= two kinds selected from the group constituted of 0.10 to 1.5% Cu, 0.10 to 0.50% Ni, 0.005 to 0.10% Nb and 0.005 to 0.10% V, and the balance Fe with inevitable impurities, and its carbon equivalent Ceq shown by the formula is regulated to 0.38 to 0.45. This steel is subjected to hot rolling so as to regulate the working ratio per pass in the temp. range of >=900 deg.C into