HIGH TENSILE COLD-ROLLED STEEL SHEET EXCELLENT IN DUCTILITY AND IN STRAIN AGING HARDENING PROPERTIES, AND METHOD FOR PRODUCING THE SAME

摘要

A high tensile cold-rolled steel sheet which has a chemical composition, in mass %, wherein C: 0.05 % to 0.30, Si: 0.4 to 2.0 %, Mn: 0.7 to 3.0 %, P: 0.08 % or less, Al: 0.02 % or less, and N: 0.0050 to 0.0250 %, and N /Al is 0.3 or more, and a specific composite structure, and which exhibits excellent ductility and impact resistance and excellent strain aging hardening properties represented by a DELTA TS of 50 Mpa or more. The steel sheet is produced by a method which comprises preparing a cold-rolled thin steel sheet having the above composition, heating the cold-rolled thin steel sheet to a temperature between (Ac1 transformation point) and (Ac3 transformation point + 50 DEG C, and then cooling it at a cooling rate of 5 to 150 DEG C at least in the range of from 600 DEG C to 500 DEG C and holding it in a temperature range of 350 DEG C to 500 DEG C.