Steel for Machine Structural Use

Abstract

Where, a content (mass %) of a corresponding element is substituted for each symbol of an element in Formula (1).

1 . A steel for machine structural use having a chemical composition consisting of, in mass %:
 C: 0.15 to less than 0.30%,   Si: 0.01 to 0.80%,   Mn: 0.20 to 2.00%,   P: 0.030% or less,   S: 0.010 to 0.100%,   Pb: 0.010 to 0.100%,   Al: 0.010 to 0.050%,   N: 0.015% or less,   O: 0.0005 to 0.0030%,   Cr: 0.50 to 2.00%,   Ni: 0 to 3.50%,   B: 0 to 0.0050%,   V: 0 to 0.70%,   Mo: 0 to 0.70%,   W: 0 to 0.70%,   Nb: 0 to less than 0.050%,   Cu: 0 to 0.50%,   Ti: 0 to 0.100%, and   Ca: 0 to 0.0030%,   with the balance being Fe and impurities, the chemical composition satisfying Formula (1),   wherein, in the steel, a total number of specific inclusions which are any of MnS inclusions, Pb inclusions and composite inclusions containing MnS and Pb, and which have an equivalent circular diameter of 5 μm or more is 40 per mm 2  or more;
   Mn/S≥8.0  (1)
 
   where, a content (mass %) of a corresponding element is substituted for each symbol of an element in Formula (1).   
     
     
         2 . The steel for machine structural use according to  claim 1 , wherein the chemical composition contains one or more types of element selected from a group consisting of:
 Ni: 0.02 to 3.50%,   B: 0.0005 to 0.0050%,   V: 0.05 to 0.70%,   Mo: 0.05 to 0.70%,   W: 0.05 to 0.70%,   Nb: 0.001 to less than 0.050%,   Cu: 0.05 to 0.50%, and   Ti: 0.003 to 0.100%.   
     
     
         3 . The steel for machine structural use according to  claim 1 , wherein the chemical composition contains:
 Ca: 0.0001 to 0.0030%.   
     
     
         4 . The steel for machine structural use according to  claim 1 , wherein:
 a ratio of a number of the composite inclusions with respect to the specific inclusions is 40% or more.   
     
     
         5 . The steel for machine structural use according to  claim 2 , wherein the chemical composition contains:
 Ca: 0.0001 to 0.0030%.   
     
     
         6 . The steel for machine structural use according to  claim 2 , wherein:
 a ratio of a number of the composite inclusions with respect to the specific inclusions is 40% or more.   
     
     
         7 . The steel for machine structural use according to  claim 3 , wherein:
 a ratio of a number of the composite inclusions with respect to the specific inclusions is 40% or more.   
     
     
         8 . The steel for machine structural use according to  claim 5 , wherein:
 a ratio of a number of the composite inclusions with respect to the specific inclusions is 40% or more.