Free machining steel with superior hot ductility and a method of manufacturing thereof

PURPOSE: Provided is a bismuth-sulfur based free machining steel, which has superior hot ductility and does not generate surface defects such as blow hole and pin hole even at low reduction temperatures. CONSTITUTION: The method for manufacturing a free machining steel with superior hot ductility includes step of reheating a bloom comprising C 0.05 to 0.15 wt.%, Mn 0.5 to 2.0 wt.%, S 0.15 to 0.40 wt.%, P 0.01 to 0.10 wt.%, O 0.003 to 0.020 wt.%, Bi 0.02 to 0.30 wt.%, 0.01 wt.% or less of Si, 0.0009 wt.% or less of Al, N 0.0070 wt.%, B 0.0050 to 0.0150 wt.%, a balance of Fe and incidental impurities; hot rolling the bloom at 800 to 1200°C to billet; reheating the billet; and hot rolling the billet to wire rod at 800 to 1200°C, wherein the free machining steel is characterized in that portion of cross section area of MnS inclusion to total cross section area of steel is 0.5-2.0%; portion of cross section area of metallic Bi inclusion to total cross section area of steel is 0.030-0.30%; the length of each inclusion is 5 to 20 μm and the width of the each inclusion is 1 to 10 μm. 본 발명은 비스무스(Bi)-유황(S)계 쾌삭강 및 그 제조방법에 관한 것이며; 그 목적은 고온연성이 우수하여 저온의 압하온도에서도 표면결함이 발생되지 않는 쾌삭강 및 그 제조방법을 제공함에 있다. 상기 목적을 달성하기 위한 본 발명은, 중량%로, C:0.05-0.15%, Mn:0.5-2.0%, S:0.15-0.40%, P:0.01-0.10%, O:0.003-0.020%, Bi:0.02-0.30%, Si:0.01%이하, Al:0.0009%이하, N:0.0070%, B:0.0050-0.0150% 및 나머지 Fe와 기타 불가피하게 함유되는 불순물로 조성되는 고온연성이 우수한 쾌삭강 및 그 제조방법에 관한 것을 그 요지로 한다.