High-strength cold rolled steel sheet and galvannealed steel sheet having excellent burring property, and manufacturing method therefor

Abstract

A high strength cold rolled steel sheet having excellent burring properties includes: by weight %, 0.13-0.25% of carbon (C), 1.0-2.0% of silicon (Si), 1.5-3.0% of manganese (Mn), 0.08-1.5% of aluminum (Al)+chromium (Cr)+molybdenum (Mo), 0.1% or less of phosphorus (P), 0.01% or less of sulfur(S), 0.01% or less of nitrogen (N), and the balance of Fe and inevitable impurities; and, by area fraction, 3-25% of ferrite, 20-40% of martensite, 5-20% of residual austenite. The ferrite has an average grain size of 2 μm or less at the reference point of t/4 (wherein t refers to a steel sheet thickness), with the average ratio between lengths in the thickness direction and in the rolling direction being 1.5 or less.

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