Steel for Machine Structural Use

Abstract

A steel for machine structural use is provided which is excellent in machinability, rusting characteristics, and hot ductility. The steel for machine structural use according to the present embodiment has a chemical composition which consists of, in mass %, C: 0.30 to 0.80%, 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 and O: 0.0005 to 0.0030%, with the balance being Fe and impurities, the chemical composition satisfying Formula (1). The total number of specific inclusions included in the steel 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).

1 . A steel for machine structural use having a chemical composition consisting of, in mass %:
 C: 0.30 to 0.80%,   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 to 0.70%,   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:
 Cr: 0.10 to 0.70%,   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.