Ultra-high strength cold-rolled steel sheet and method for producing the same

Abstract

The ultra-high strength cold-rolled steel sheet of the present invention contains, by weight%, carbon (C): 0.23% to 0.40%, silicon (Si): 0.05% to 1.0%, manganese (Mn): 0.5% to 3.0%, vanadium (V): 0.01% to 0.12%, aluminum (Al): 0.01% to 0.3%, chromium (Cr): more than 0% to 0.5%, titanium (Ti): more than 0% to 0.1%, phosphorus (P): more than 0% to 0.02%, sulfur (S): more than 0% to 0.01%, boron (B): 0.001% to 0.005%, and the balance is an ultra-high strength cold-rolled steel sheet containing iron (Fe) and other inevitable impurities. The final microstructure contains tempered martensite with a volume fraction of 90% or more. The average separation distance between precipitates in the tempered martensite is 300 nm or more, the average size of the precipitates is 200 nm or less, and based on the area of 20 μm 2 in the final microstructure, the number of precipitates with an average size of 40 nm or less is 25 or more.

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