A sulfur-containing free-cutting steel for machine structural use, containing,
in weight percent, 0.10 to 0.55% of C, 0.05 to 1.00% of Si, 0.30 to 2.50% of Mn,
not more than 0.15% of P, 0.050 to 0.350% of S, more than 0.010% but not more than
0.020% of Al, 0.015 to 0.200% of Nb, 0.0015 to 0.0150% of O, and not more than
0.02% of N, and further containing, in weight percent, at least one selected from
the group consisting of 0.03 to 0.50% of V, 0.02 to 0.20% of Ti and 0.01 to 0.20%
of Zr, wherein the ratio S/O of the S content to the O content is 15 to 120, and
at least one selected from the group consisting of an oxide, a carbide, a nitride
and a carbonitride of Nb(see FIG. 1) acts as nuclei for precipitation of
MnS type inclusions. A sulfur-containing free-cutting steel for machine structural
use is obtained by adjusting the chemical composition of steel and provides a sulfur
free-cutting steel that does not contain lead but has a machinability and mechanical
properties on a par with conventional lead-containing free-cutting steel.