Patent ID: 8137484

Claim:
A method of production of steel superior in machinability, said method of production of steel characterized by casting molten steel consisting 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%, S: 0.3 to 1.0%, T.N: 0.002 to 0.02%, T.O: 0.0005 to 0.035%, optionally, one or more of: V: 0.05 to 1.0%, Nb: 0.005 to 0.2%, Cr: 0.01 to 2.0%, Mo: 0.05 to 1.0%, W: 0.5 to 1.0%, Ni: 0.05 to 2.0%, Cu: 0.01 to 2.0%, Sn: 0.005 to 2.0%, Zn: 0.0005 to 0.5%, Ti: 0.0005 to 0.1%, Ca: 0.0002 to 0.005%, Zr: 0.0005 to 0.1%, Mg: 0.0003 to 0.005%, Bi: 0.005 to 0.5%, Pb: 0.01 to 0.5%, and Al: ≦0.015%, and a balance of Fe and unavoidable impurities, then cooling the molten cast steel at a cooling rate of 10 to 100° C./min at the time of cooling from the liquid phase line temperature to the solid phase line temperature in a billet thickness Q part, hot rolling, then cooling from a temperature of at least the A 3 point to 550° C. or less at a cooling rate of at least 0.5° C./sec to provide an area ratio of pearlite grains of 1 μm or more of not more than 5%, wherein the balance of the area ratio is ferrite-pearlite or ferrite.