COLD NONAGING BH TYPE HIGH TENSILE STRENGTH COLD ROLLED STEEL SHEET FOR DRAWING AND ITS MANUFACTURE

摘要

PURPOSE:To offer a steel sheet of a cold nonaging BH type capable of obtaining good drawability through it is provided with high tensile strenght and free from the deterioration in material even if galvannealing treatment is applied. CONSTITUTION:A high tensile strength cold rolled steel sheet of =45kgf/mm2 TS contg., by weight, 0.0008 to 0.025% C, =1.0% Si, 0.1 to 2.0% Mn, =0.2% Nb as well as =5 Nb/C, 0.003 to 0.01% B, 0.005 to 0.10% Al, =0.1% P, =0.007% N and the balance substantial iron and having a composite structure of a high temp. transformation ferrite and a low temp. transformation ferritic phase having high dislocation density is obtd. A hot rolled sheet constituted of the above componental compsn. is cold-rolled at =60% draft, is thereafter annealed in the temp. arange where alpha-gamma phases are coexistent and is successively cooled at =5 to 100 deg.C/sec cooling rate.