Steel for use in machine-construction excellent in machinability

Abstract

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):

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.   
     
     
         2 . The steel according to  claim 1 , comprising:
 C: 0.2 to 1.2% (mass %, the same applies to chemical component composition hereinafter);   Si: 0.05 to 0.5%;   Mn: 0.2 to 1.8%;   P: 0.03% or less (excluding 0%); and   S: 0.03% or less (excluding 0%),   the balance being iron and unavoidable impurities.   
     
     
         3 . The steel according to  claim 2 , further comprising at least one species of element selected from the group consisting of Cr: 0.5% or less (excluding 0%), Cu: 0.5% or less (excluding 0%), Ni: 0.5% or less (excluding 0%), and Mo: 0.5% or less (excluding 0%), as other elements. 
     
     
         4 . A method for improving machined-surface characteristics of a machined cut-surface comprising:
 machining the steel according to  claim 1 .   
     
     
         5 . A method for manufacturing a forming tool comprising cutting of the steel according to  claim 1 .