Patent ID: 7488396

Claim:
Steel superior in machinability comprised of, by wt %, C: 0.005 to 0.2%, Si: 0.001 to 0.5%, Mn: 0.2 to 3.0%, P: 0.001 to 0.2%, T.N: 0.002 to 0.02%, T.O: 0.0005 to 0.035%, restricting the amount of S to 0.25 to 0.75 wt % and the amount of B to 0.002 to 0.014 wt %, by containing amounts of S and B in a region surrounded by A, B, C, and D shown in FIG. 4 where the contents of S and B satisfy the following equation (1), and by containing sulfides with BN precipitated in MnS: ( B− 0.008) 2 /0.006 2 +( S− 0.5) 2 /0.25 2 ≦1 (1) the balance of Fe and unavoidable impurities, said steel satisfying one or both of Mn/S in the steel being 1.2 to 2.8 or an area ratio of pearlite over a grain size of 1 μm in a microstructure of the steel being not more than 5%, and having a density of MnS having a circle equivalent diameter of 0.1 to 0.5 μm at a cross-section parallel to a rolling direction of the steel material, taken from an extraction replica and observed by a transmission electron microscope, of at least 10,000/mm 2 .