| 摘要 |
PROBLEM TO BE SOLVED: To provide a free-drilling steel capable of facilitating the discharge of chips at the time of deep hole working of a steel material for use in frinctional parts, producing excellent fatigue strength, preventing the stoppage of a production line, and attaining improvement in productivity. SOLUTION: The steel has a composition consisting of, by weight ratio, 0.05-0.50% C, 0.03-1.00% Si, 0.50-2.00% Mn, 0.01-0.11% S, 0.005-0.040% Al, either or both of 0.05-0.30% Pb and 0.01-0.30% Bi, and the balance Fe with impurities. This steel has spherical or rod-like inclusions in its matrix. The spherical inclusions have oxides as nuclei and the rod-like inclusions have sulfides as nuclei, and at least a part of the outside peripheral part of each nucleus has a layer containing either or both of Pb and Bi. Further, the size of these inclusions are regulated to 9 to 50μm in width and 25 to 50μm in length, and the number of the inclusions is regulated to >=8 pieces per square millimeter at an arbitrary cross section parallel to rolling direction, in a position at a depth one-fourth the diameter of the steel.
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