HIGH TENSILE STRENGTH COLD ROLLED STEEL SHEET HAVING HIGH r-VALUE, EXCELLENT STRAIN AGE HARDENING CHARACTERISTIC AND COLD NON-AGING PROPERTY AND ITS PRODUCTION METHOD

摘要

PROBLEM TO BE SOLVED: To provide a high tensile strength cold rolled steel sheet having high formability and excellent strain age hardening characteristics and suitable for automobile parts requiring formability of a relatively high grade and to provide its inexpensive production method.;SOLUTION: A steel slab having a composition containing 0.025 to 0.15% C, in which the contents of Si, Mn, P and S are controlled in optimum ranges, and containing ≤0.02% Al and 0.0050 to 0.0250% N and also satisfying N/Al: ≥0.3 is heated at ≥1,000°C and is subjected to hot finish rolling where FDT is ≥800°C. After the finish of the hot finish rolling, the steel is rapidly cooled at ≥40°C/s and is coiled at ≤650°C. The steel is next cold-rolled. The steel is thereafter subjected to box annealing at the recrystallization temperature to 800°C and the subsequent continuous annealing in a two phase region. The steel is rapidly cooled to form a cold rolled steel sheet having a structure where the area ratio of ferrite with the average grain size of ≤10 μm is ≥80%, and that of martensite is ≥2%, containing solid solution N by ≥0.0010% and having an (r) value of ≥1.3, excellent strain age hardening characteristics and non-aging properties.;COPYRIGHT: (C)2002,JPO