MANUFACTURE OF COLD ROLLED STEEL SHEET FOR WORKING EXCELLENT IN AGING PROPERTY AND BH PROPERTY AND HAVING FACE DISTORTING RESISTANCE AND DENT RESISTANCE

摘要

PURPOSE:To manufacture the cold rolled steel sheet by forming the structure of a steel body into a multiple layered one contg. different components of a DP steel and an IF steel with a specified compsn., subjecting this steel body to hot rolling under specified conditions, subjecting it to cold rolling and thereafter executing heat treatment under specified conditions. CONSTITUTION:As a one layer componental (A component) material, a steel contg., by weight, 0.04 to 0.2% C, 0.5 to 3% Mn, =3% Si and =0.1% P is used, and, as an other layer componental (B component) material, a steel having a compsn. contg. =0.05% C and =0.02% N and in which the amounts of C and N to be added and the total of the amounts of one or more kinds among Ti, Zr and Nb to be added lie in the relationship in the inequality is used to form a steel having a multiple layered structure of = two layers. This steel is hot-rolled at the Ar3 transformation point of both componental steels or above, is pickled, is cold-rolled, is thereafter heated to the Ac1 transformation point of the A componental material or above as well as to the Ac1, transformation point of the B componental material or below and is cooled to 400 deg.C at =20 deg.C/sec average cooling rate. By this method, the steel simultaneously satisfying both properties of face distorting resistance and dent resistance, which has hitherto been considered to be impossible, can be offered substantially in a nonaged state while it is provided with BH properties.