| 摘要 |
PURPOSE: A method for manufacturing hot rolled steel sheets of which tensile strength is 780MPa is provided to increase tensile strength, elongation, and extension flangability by adding a specific amount of silicon to steel and controlling cooling condition properly after hot rolling. CONSTITUTION: The hot rolled steel sheet for automobile is manufactured by reheating a steel slab comprising C 0.06-0.10wt.%, silicon 0.5-1.0wt.%, Mn 1.5-2.0wt.%, P 0.02wt.% or less, S 0.0005wt.% or less, Al 0.010-0.050wt.%, Ti 0.050-0.10wt.%, Nb 0.020-0.040wt.%, N 60ppm or less, a balance of Fe, and other inevitable impurities; finish hot rolling the steel slab within austenite temperature zone; first water cooling to 650-700deg.C; maintaining above temperature for 5 sec in air; second water cooling to 450-500deg.C; coiling the steel sheet within the temperature range of 450-500deg.C. The obtained steel sheet by above method comprises 80-85% of ferrite and 15-20% of bainite and also its physical properties are as follows: tensile strength>=780MPa, yield strength >=680MPa, tensile strengthxelongation>=MPa·%.
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