| 摘要 |
PURPOSE: A cold rolled steel sheet is provided which prevents deterioration of plating property and weldability due to large quantities of silicon and carbon and having 100 kgf/mm¬2 or more of strength and superior workability, and a manufacturing method of the cold rolled steel sheet is provided. CONSTITUTION: The high strength steel product having high yield ratio and high strain hardening coefficient comprises 0.05 to 0.2 wt.% of carbon, 0.3 to 0.7 wt.% of silicon, 4 to 7 wt.% of manganese, 0.03 wt.% or less of phosphorus, 0.005 wt.% or less of sulfur, 0.0015 to 0.002 wt.% of boron, 0.02 to 0.05 wt.% of molybdenum, 0.05 to 0.15 wt.% of aluminum and a balance of Fe and other inevitable impurities, wherein micro-structure of the steel product is consisted of 5 to 20 vol.% of bainite, 5 to 15 vol.% of residual austenite and a balance of ferrite. The manufacturing method of the high strength steel product having high yield ratio and high strain hardening coefficient comprises a step of reheating a steel slab comprising 0.05 to 0.2 wt.% of carbon, 0.3 to 0.7 wt.% of silicon, 4 to 7 wt.% of manganese, 0.03 wt.% or less of phosphorus, 0.005 wt.% or less of sulfur, 0.0015 to 0.002 wt.% of boron, 0.02 to 0.05 wt.% of molybdenum, 0.05 to 0.15 wt.% of aluminum and a balance of Fe and other inevitable impurities in the temperature range of 1,200 to 1,250 deg.C for 60 to 180 minutes; a step of finishing hot rolling of the reheated steel slab at a temperature of Ar3 point or more, and coiling the hot rolled steel sheet at a temperature of 560 to 680 deg.C; and a step of cold rolling the coiled hot rolled steel sheet to a reduction ratio of 40 to 45%, batch annealing the cold rolled steel sheet at a temperature of 620 to 650 deg.C, and air cooling or furnace cooling the batch annealed steel sheet.
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