| 摘要 |
PURPOSE: A method for manufacturing 60 kg grade structural steel having refined ferrite structure whose average grain size is 5 μm or less by manufacturing austenite to a grain size of 35 μm or less by static recrystallization and optimizing hot working conditions for strain induced dynamic transformation of ferrite is provided. CONSTITUTION: The method comprises a step of precipitating a precipitate by slowly cooling the cast steel after casting a steel comprising 0.02 to 0.3 wt.% of C, 1.5 wt.% or less of Si, 2.0 wt.% or less of Mn, 0.005 to 0.1 wt.% of Ti, 0.003 to 0.03 wt.% of N and a balance of Fe and inevitable impurities and satisfying the following relation: 1.2<=Ti/N<=3.4; a step of controlling grain size of austenite of the steel to 50 μm or less by reheating the slowly cooled steel in the temperature range of 1,000 to 1,250 deg.C; a step of static recrystallizing the rough-rolled steel after rough-rolling the reheated steel to a reduction ratio of 10% or more, and cooling the static recrystallized steel so that a grain size of austenite of the steel is controlled to 35 μm or less; and a step of hot rolling the cooled steel in the temperature range of Ar3 to Ar3+100 deg.C in such a way that the total reduction ratio is maintained to 50% or more as maintaining a reduction ratio per one pass to 30% or less, wherein the method further comprises a step of cooling the hot rolled steel to a cooling finishing temperature of 500 to 400 deg.C in a cooling rate of 5 deg.C/sec or more.
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