STEEL FOR USE IN MACHINE-CONSTRUCTION EXCELLENT IN MACHINABILITY

摘要

A steel for mechanical structures has a mixed texture which is composed of a hard phase comprising at least one texture selected from pearlite, bainite and martensite and a ferrite phase, wherein the average equivalent circle diameter of ferrite grains is 7 μm or less and the (ferrite grain)-(ferrite grain) connection rate (X) which is expressed by formula (1) is 0.15 or less.;[(Ferrite grain)-(ferrite grain) connection rate (X)]=[the number (A) of (ferrite grain)-(ferrite grain) boundary surfaces]/[the number (B) of (ferrite grain)-(hard phase) boundary surfaces]  (1):

主权项

1. A steel having a mixed microstructure comprising:
a hard phase composed of at least one species selected from the group consisting of pearlite, bainite, and martensite; and a ferrite phase, wherein an average equivalent circle diameter of ferrite grains is 7 μm or less, and a [(ferrite grain)-(ferrite grain) connection rate X], as expressed by an equation (1) described hereunder, is 0.15 or less, provided that [(ferrite grain)-(ferrite grain) connection rate X]=[the number (A) of [(ferrite grain)-(ferrite grain) interface surfaces]]/[the number (B) of (ferrite grain)-(hard phase) interface surfaces] represents the equation (1), and at the time of drawing a predetermined straight line across a photograph taken by use of a scanning electron microscope, [the number (A) of [(ferrite grain)-(ferrite grain) interface surfaces]], in the equation, indicates the number of intersection points between the straight line and [the (ferrite grain)-(ferrite grain) interface surfaces], whereas [the number (B) of [(ferrite grain)-(hard phase) interface surfaces]] indicates the number of intersection points between the straight line and the [(ferrite grain)-(hard phase) interface surfaces], at the time of drawing a predetermined straight line in the same way as described above.