Free cutting steel containing controlled inclusions and the method of making the same

摘要

Mechanical anisotropy of a free cutting steel having excellent machinability can be effectively decreased by controlling inclusions so that the steel may contain inclusion A which softens at a temperature below 1000 DEG C. and inclusion B which has a melting point above 1300 DEG C. but exhibits plasticity at a temperature between 900 DEG and 1300 DEG C., that the inclusion A and the inclusion B may exist in a mutually adhered form, and that the areal percentage of the inclusion A may be at least 1% of areal percentage of the inclusion B. Typical compositions of the inclusion A are: Pb, Bi, MnS-MnTe, SiO2-K2O, SiO2-Na2O, SiO2-K2O-Al2O3, SiO2-Na2O-Al2O3 and SiO2-Na2O-CaO-MnO; and typical compositions of the inclusion B are: MnS, MnSe and Mn(S,Se).