| 摘要 |
PROBLEM TO BE SOLVED: To obtain excellent machinability, cold forgeability, and hardenability by obtaining a composition consisting of specific elements and the balance Fe with inevitable impurities, further regulating contained C so that it is composed essentially of graphite and cementite, and also specifying the graphitization rate determined by a prescribed definition. SOLUTION: The steel product for machine structure has a composition consisting of, by mass, 0.1-1.5% C, >0.15-<0.50% is Si, 0.05-0.3% Mn, 0.005-0.1% Al, 0.0003-0.0150% B, 0.0015-0.01505% N, <=0.020% P, <=0.035% S, and <=0.0030% O and also has a structure in which the graphitization rate defined by an equation is regulated to 5-60%. In addition to the composition, it is preferable to further incorporate one or >=2 kinds selected from 0.1-3.0% Ni, 0.1-3.0% Cu, and 0.1-3.0% Co by mass. Further, it is preferable to incorporate either or both of 0.05-0.5% V and 0.005-0.05% Nb. Further, it is also preferable to incorporate 0.1-1.0% Mo.
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