MY171981A - High-strength, cold rolled steel sheet suitable for chemical conversion treatment, and method for manufacturing the same - Google Patents

Abstract

High-strength, cold rolled steel sheet with improved chemical conversion treatment property, which can be suitably used in framework parts of automobiles, has a composition including by mass%, C: 0.05% to 0.1 %; Si: 0.05% to 0.45%; Mn: 2.5% to 3.5%; Al: 0.01 % to 0.08%; P: 0.05% or less; S: 0.0050% or less; N: 0.01% or less; Nb: 0.02% to 0.1%; Ti: 0.001% to 0.05%; the balance being Fe and unavoidable impurities. The Si/Mn ratio is 0.02 to 0.15. The steel sheet has a microstructure including 50% to 80% of ferrite phase and 20% to 50% of martensite phase in an area ratio relative to the whole microstructure. The ferrite phase and the martensite phase each has an average grain size of 0.5 ?m to 3.0 ?m, and the ratio of the average grain size of the ferrite phase to that of the martensite phase (average grain size of ferrite phase/average grain size of martensite phase) is 0.5 to 5.0.