Case hardening steel for warm forging and roughly-shaped forged material manufactured using the same

Abstract

A case hardening steel for warm forging at a forging temperature of 850° C. to 1100° C. The case hardening steel has a chemical composition including: C: 0.15 to 0.23%; Si: 0.60 to 0.95%; Mn: 0.60 to 1.20%; P: 0.035% or less; S: 0.035% or less; Cr: 1.50% or less (excluding 0%); Al: 0.050% or less; Ti: 0.01 to 0.05%; B: 0.0005 to 0.0050%; N: 0.0020 to 0.0200%; Mo: 0.20% or less as an optional element; Nb: 0.01 to 0.05% as an optional element; and Fe, and satisfies Formulae 1 and 2: Formula 1: 90≥−120*C+20.1*Si−5.3*Mn−8.5*Mo+96≥80, Formula 2: 160≥40*Si+39*Mn+10*Cr+30*Mo+84≥145.

1 . A case hardening steel having a chemical composition comprising, by mass %:
 C: 0.15 to 0.23%;   Si: 0.60 to 0.95%;   Mn: 0.60 to 1.20%;   P: 0.035% or less;   S: 0.035% or less;   Cr: 1.50% or less;   Al: 0.050% or less;   Ti: 0.01 to 0.05%;   B: 0.0005 to 0.0050%;   N: 0.0020 to 0.0200%;   Mo: 0.20% or less as an optional element;   Nb: 0.01 to 0.05% as an optional element; and   Fe, and satisfying the following Formula 1 and Formula 2:   
       
         
           
             
               
                 
                   
                     
                       
                         9 
                         ⁢ 
                         0 
                       
                       ≥ 
                       
                         
                           
                             - 
                             120 
                           
                           * 
                           C 
                         
                         + 
                         
                           20.1 
                           * 
                           Si 
                         
                         - 
                         
                           5.3 
                           * 
                           Mn 
                         
                         - 
                         
                           8.5 
                           * 
                           Mo 
                         
                         + 
                         96 
                       
                       ≥ 
                       80 
                     
                     ; 
                     and 
                   
                 
                 
                   
                     Formula 
                     ⁢ 
                         
                     1 
                   
                 
               
             
           
         
         
           
             
               
                 
                   
                     161 
                     ≥ 
                     
                       
                         40 
                         * 
                         Si 
                       
                       + 
                       
                         39 
                         * 
                         Mn 
                       
                       + 
                       
                         10 
                         * 
                         Cr 
                       
                       + 
                       
                         30 
                         * 
                         Mo 
                       
                       + 
                       84 
                     
                     ≥ 
                     141 
                   
                 
                 
                   
                     Formula 
                     ⁢ 
                         
                     2 
                   
                 
               
             
           
         
         wherein element symbols in Formulae 1 and 2 represent contents (mass %) of respective elements in mass %. 
       
     
     
         2 . A roughly-shaped forged material obtained by warm forging the case hardening steel according to  claim 1 , the roughly-shaped forged material having:
 a surface hardness of 200 HV or less;   a metal structure of which a ferrite rate is 80% to 90%; and   a ferrite hardness of 140 mHV to 160 mHV.   
     
     
         3 . A method for producing a roughly-shaped forged material, comprising:
 performing warm forging at a forging temperature of 850° C. to 1100° C. on the case hardening steel according to  claim 1 .   
     
     
         4 . The method of  claim 3 , wherein the roughly-shaped forged material has:
 a surface hardness of 200 HV or less;   a metal structure of which a ferrite rate is 80% to 90%; and   a ferrite hardness of 140 mHV to 160 mHV.