| 摘要 |
PROBLEM TO BE SOLVED: To obtain a free-cutting non-heat treated steel having machinability equal to or above that of Pb-added non-heat treated steel without the addition of Pb and having toughness equal to or above a certain level. SOLUTION: This steel has a compsn. contg., by mass, 0.30 to 0.55% C, 0.15 to 0.40% Si, 0.80 to 1.60% Mn, 0.005 to 0.050% P, <=0.040% S, 0.005 to 0.035% Al, 0.04 to 0.15% V, Cu+Ni+Cr+Mo: <=0.45%, 0.0050 to 0.0l50% B, 0.008 to 0.018% N, and the balance Fe with inevitable impurities, in which, also, B/N lies in the range of 0.5 to 1.5. To the above steel, 0.005 to 0.20% Ti is added, the content of N is controlled to 0.010 to 0.020%, B/(N-Ti/3.42): 0.5 to 1.5 is satisfied, and also, Cu+Ni+Cr+Mo is relaxed to <=0.60%. The content of S in the steel is increased to 0.04 to 0.08%, and Ca is added by 0.001 to 0.003%.
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