| 摘要 |
PURPOSE: A manufacturing method of tensile strength 60 kg/mm¬2 class hot rolled steel sheet is provided to obtain superior stretch flangeability by properly controlling cooling patterns in the cooling section in front of down coiler and miniaturizing shape of grain boundary cementite. CONSTITUTION: In a 60 kg/mm¬2 class hot rolled steel sheet(4) manufacturing method comprising the processes of hot rolling a steel slab comprising 0.07 to 0.09 wt.% of C, 0.25 wt.% or less of Si, 1.40 to 1.60 wt.% of Mn, 0.005 to 0.06 wt.% of t-Al, 0.03 to 0.05 wt.% of Nb, 0.003 wt.% or less of S, 0.02 wt.% or less of P and a balance of Fe and other inevitably contained impurities; and coiling the water cooled steel slab after water cooling the hot rolled steel slab to a target coiling temperature in the cooling section in front of down coiler(3) equipped with a plurality of water cooling banks(2), the manufacturing method of tensile strength 60 kg/mm¬2 class hot rolled steel sheet(4) having superior stretch flangeability comprises the processes of water cooling the hot rolled steel slab to a temperature directly under Ar3 transformation point at a cooling rate of 40 to 90 deg.C on run-out table(8) just after the hot rolling; air cooling the water cooled steel slab to a temperature in which 70 to 80% of ferrite transformation is proceeded; and water cooling the air cooled steel slab to an intermediate temperature of bainite transformation initiation temperature Bs and martensite transformation initiation temperature Ms at a cooling rate of 40 to 90 deg.C so that final structure of the steel slab comprises 70 to 80% of equiaxed ferrite (F) and 20 to 30% of bainite (B).
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