COLD ROLLED STEEL SHEET FOR AUTOMOBILE HAVING LOW CORROSION RATE AND HIGH FORMABILITY

摘要

PURPOSE:To reduce the corrosion rate of a steel sheet and to improve its workability and coating suitability by using a high purity steel in which the content of C and N is extremely lowered and the content of S and Mn is reduced to the utmost limit as a base and adding trace amounts of Ti and Nb for formability thereto into a specified compsn. CONSTITUTION:This cold rolled steel contains, by weight, =0.0030% C, =0.0025% N, =0.02% Si, 0.03 to 0.15% Mn, 0.018 to 0.045% P, =0.005% S, 0.005 to 0.035% Al, 0.005 to 0.025% Nb, =0.02% Ti and the balance Fe. This steel has an extremely slow corrosion rate, excellent workability and good coating suitability. In this steel, C and N are limited because they deteriorate its-r value and ductility and from the need of covering its hardening caused by P. Si is limited because it deteriorates its chemical convertibility and coating suitability. As for S and Mn, because they are formed into MnS and promote its corrosion and Mn deteriorate its-r value and ductility, their content is regulated to the upper limit or below, and in consideration of the prevention of the embrittlement of FeS, their content is regulated to the lower limit or above. As for P, because it reduces its corrosion rate, its content is regulated to the lower limit or above and to the upper limit or below for maintaining its formability.