Mechanical structure steel for cold-working and manufacturing method therefor

Abstract

where [C (%)] in the formula (1) indicates the C content in percent by mass.

1 : A mechanical structure steel for cold-working, comprising, in percent by mass:
 C: 0.07% or more and less than 0.3%,   Si: 0.05 to 0.5%,   Mn: 0.2 to 1.7%,   P: more than 0% and 0.03% or less,   S: 0.001 to 0.05%,   Al: 0.01 to 0.1%,   N: 0 to 0.015%, and   iron and inevitable impurities,   wherein:   the steel has a metal microstructure comprising proeutectoid ferrite and pearlite, a total area ratio of the proeutectoid ferrite and the pearlite with respect to the entire microstructure is 90% or more, while an area ratio Af of the proeutectoid ferrite with respect to the entire microstructure satisfies a relationship of Af≥A where an A value is represented by formula (1):
     A =(103−128×[ C (%)])×0.80(%)  (1)
 
   
       where [C (%)] in the formula (1) indicates the C content in percent by mass;
 an average circle equivalent diameter of a bcc-Fe crystal grain ranges from 15 to 30 μm; and 
 a pearlite lamellar spacing is 0.20 μm or less on average. 
 
     
     
         2 : The mechanical structure steel for cold-working according to  claim 1 , further comprising, in percent by mass, one or more elements selected from the group consisting of:
 Cr: more than 0% and 0.5% or less,   Cu: more than 0% and 0.25% or less,   Ni: more than 0% and 0.25% or less,   Mo: more than 0% and 0.25% or less, and   B: more than 0% and 0.01% or less,   
       the mechanical structure steel satisfying formula (X) below:
   [Cr %]+[Cu %]+[Ni %]+[Mo %]≤0.75  (X)
 
 
       where [Cr %], [Cu %], [Ni %], and [Mo %] indicate contents of Cr, Cu, Ni, and Mo in percent by mass, respectively. 
     
     
         3 : The mechanical structure steel for cold-working according to  claim 1 , further comprising, in percent by mass:
 Ti: more than 00% and 0.1% or less.   
     
     
         4 : A method for manufacturing the mechanical structure steel for cold-working according to  claim 1 , the method comprising:
 finish rolling the steel at a temperature of 950° C. or higher and 1,150° C. or lower; and then   in the following order, first cooling the steel to a first-cooling end temperature of 700 to 750° C. at an average cooling rate of 3° C./sec. or less, and second cooling the steel in a temperature range from the first-cooling end temperature to at least 600° C. at an average cooling rate of 5 to 30° C./sec.   
     
     
         5 : The mechanical structure steel for cold-working according to  claim 2 , further comprising, in percent by mass:
 Ti: more than 0% and 0.1% or less.   
     
     
         6 : A method for manufacturing the mechanical structure steel for cold-working according to  claim 2 , the method comprising:
 finish rolling the steel at a temperature of 950° C. or higher and 1,150° C. or lower; and then   in the following order, first cooling the steel to a first-cooling end temperature of 700 to 750° C. at an average cooling rate of 3° C./sec. or less, and second cooling the steel in a temperature range from the first-cooling end temperature to at least 600° C. at an average cooling rate of 5 to 30° C./sec.   
     
     
         7 : A method for manufacturing the mechanical structure steel for cold-working according to  claim 3 , the method comprising:
 finish rolling the steel at a temperature of 950° C. or higher and 1,150° C. or lower; and then   in the following order, first cooling the steel to a first-cooling end temperature of 700 to 750° C. at an average cooling rate of 3° C./sec. or less, and second cooling the steel in a temperature range from the first-cooling end temperature to at least 600° C. at an average cooling rate of 5 to 30° C./sec.   
     
     
         8 : A method for manufacturing the mechanical structure steel for cold-working according to  claim 4 , the method comprising:
 finish rolling the steel at a temperature of 950° C. or higher and 1,150° C. or lower; and then   in the following order, first cooling the steel to a first-cooling end temperature of 700 to 750° C. at an average cooling rate of 3° C./sec. or less, and second cooling the steel in a temperature range from the first-cooling end temperature to at least 600° C. at an average cooling rate of 5 to 30° C./sec.