PRODUCTION OF GALVANNEALED HIGH STRENGTH COLD ROLLED STEEL SHEET FOR AUTOMOBILE USE, EXCELLENT IN FORMABILITY, HAVING BAKING FINISH HARDENABILITY, AND REDUCED IN FLUCTUATION IN BAKING FINISH HARDENABILITY

摘要

PURPOSE: To produce a galvannealed high strength cold rolled steel sheet excellent in formability, etc., by subjecting a steel of specific composition having ultralow C and N contents and containing Nb, Ti, etc., to respectively specified hot rolling, cold rolling, annealing, hot dip galvanizing, and alloying. ;CONSTITUTION: Asteel, which contains 0.0010-0.0030% C, ≤0.0030% N, ≤0.5% Si, 0.3-1.5% Mn, 0.03-0.08% P, ≤0.03% S, 0.005-0.07% acid soluble Al, ≤0.03% Nb, and 24/14N to 72/14N Ti and also contains, if necessary, 0.0001-0.0020% B or ≤1.5% Cr and in which Nb/C (by atomic quantity) is regulated to 0.7-1.3, is used. This steel is hot-rolled at ≥Ar₃, cooled rapidly within 2sec, and coiled at 650-770°C The resulting hot rolled plate is cold-rolled at 72-92% cold rolling rate. The resulting cold rolled sheet is heated in a nonoxidizing furnace having 0.85-1.2 combustion air ratio, annealed in a reducing furnace at 800-880°C for ≥20s, cooled at a rate of ≥3°C/s, galvanized at 440-460°C, and alloyed at 500-600°C. Then, skin pass rolling is applied to the steel sheet at 0.8-1.5%.;COPYRIGHT: (C)1995,JPO