| 摘要 |
PURPOSE: A method for manufacturing bismuth-sulfur based free cutting steel having superior hot rolling property is provided which substantially reduces surface cracks in an ordinary rolling manner without extra facility complementation by properly adding boron to constituents of work, thereby improving hot rolling property of the work. CONSTITUTION: The method comprises the steps of controlling dissolved oxygen in molten steel contained in tundish to 5 to 20 ppm and dissolved hydrogen to 10 ppm or less; obtaining a bloom comprising 0.03 to 0.15 wt.% of carbon, 0.03 wt.% or less of silicon, 0.90 to 1.40 wt.% of manganese, 0.060 to 0.120 wt.% of phosphorus, 0.250 to 0.350 wt.% of sulfur, 0.10 to 0.20 wt.% of bismuth, 30 to 80 ppm of boron, 0.010 wt.% or less of Al, 30 to 100 ppm of nitrogen and a balance of Fe and other inevitable impurities by continuously casting the controlled molten steel; obtaining a billet by rolling the heated bloom after heating the obtained billet to a temperature of 1,200 to 1,300 deg.C; heating the obtained billet in the temperature range of 1,150 to 1,250 deg.C; rolling the heated billet into wire rod in the temperature range of 950 to 1,100 deg.C; and coiling the wire rod in an ordinary manner.
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