Age-hardenable steel

Abstract

An age-hardenable steel consists of: C: 0.05 to 0.20%, Si: 0.01 to 0.50%, Mn: 1.5 to 2.5%, S: 0.005 to 0.08%, Cr: 0.03 to 0.50%, Al: 0.005 to 0.05%, V: 0.25 to 0.50%, Mo: 0 to 1.0%, Cu: 0 to 0.3%, Ni: 0 to 0.3%, Ca: 0 to 0.005%, and Bi: 0 to 0.4%, the balance being Fe and impurities. Within impurities, P £ 0.03%, Ti<0.005%, and N<0.0080%, and [C+0.3Mn+0.25Cr+0.6Mo 3 0.68], [C+0.1Si+0.2Mn+0.15Cr+0.35V+0.2Mo £ 0.85 ], and [−4.5C+Mn+Cr−3.5V−0.8Mo 3 0.00]. The hardness before aging is not more than 290 HV, a quantity of hardening by aging being not less than 25 HV, fatigue strength is not less than 350 MPa, and absorbed energy at 20° C. after aging is not less than 16 J.

1 . An age-hardenable steel, comprising a chemical composition consisting of: by mass %, C: 0.05 to 0.20%, Si: not less than 0.01% and less than 0.35%, Mn: 1.5 to 2.05%, S: 0.005 to 0.08%, Cr: 0.03 to 0.50%, Al: 0.005 to 0.05%, V: 0.25 to 0.50%, Mo: 0 to 1.0%, Cu: 0 to 0.3%, Ni: 0 to 0.3%, Ca: 0 to 0.005%, and Bi: 0 to 0.4%,
 with the balance being Fe and impurities, wherein   P, Ti, and N included in the impurities are: P: 0.03% or less, Ti: less than 0.005%, and N: less than 0.0080%, and further wherein   F1 represented by the following Formula (1) is not less than 0.68, F2 represented by the following Formula (2) is not more than 0.85, and F3 represented by the following Formula (3) is not less than 0.00;
   F1=C+0.3Mn+0.25Cr+0.6Mo  (1),
 
   F2=C+0.1Si+0.2Mn+0.15Cr+0.35V+0.2Mo  (2), and
 
   F3=−4.5C+Mn+Cr−3.5V−0.8Mo  (3),
 
   where each symbol of element in the Formulas (1) to (3) means the content of the element in mass %.   
     
     
         2 . The age-hardenable steel according to  claim 1 , wherein the chemical composition contains, by mass %, one or more kinds selected from elements shown in the following <1> to <3>;
 <1> Mo: 0.05 to 1.0%, 
 <2> Cu: 0.1 to 0.3%, and Ni: 0.1 to 0.3%, and 
 <3> Ca: 0.0005 to 0.005%, and Bi: 0.03 to 0.4%. 
 
     
     
         3 . The age-hardenable steel according to  claim 1 , wherein the steel contains bainite as a main phase, and an average block size of the bainite is 15 to 60 μm. 
     
     
         4 . The age-hardenable steel according to  claim 1 , wherein hardness is not more than 290 HV. 
     
     
         5 . The age-hardenable steel according to  claim 2 , wherein the steel contains bainite as a main phase, and an average block size of the bainite is 15 to 60 μm. 
     
     
         6 . The age-hardenable steel according to  claim 2 , wherein hardness is not more than 290 HV. 
     
     
         7 . The age-hardenable steel according to  claim 3 , wherein hardness is not more than 290 HV. 
     
     
         8 . The age-hardenable steel according to  claim 4 , wherein hardness is not more than 290 HV.