| 摘要 |
PROBLEM TO BE SOLVED: To provide a steel for machine structural use exhibiting excellent machinability even when a chemical composition is normal without using Pb harmful to a human body and an expensive free-cutting element, especially, excellent in cutting surface roughness.SOLUTION: The steel for machine structural use has a mixed structure consisting of a hard phase consisting of at least one kind selected from perlite, bainite and martensite and a ferrite phase. An average circle equivalent diameter of ferrite grains is 7 μm or less, and a ferrite grain-ferrite grain coupling rate X represented by a following formula (1) is 0.15 or less, [a ferrite grain-ferrite grain coupling rate X]=[a ferrite grain-ferrite grain interface number A]/[a ferrite grain-hard phase interface number B] ... (1) |
