Steel component and method of producing same

Abstract

A method to achieve a yield ratio of 0.60 or more in a component from non-heat-treated steel having bainitic microstructure. The component includes a predetermined chemical composition, in mass %, in amounts such that a value of F1 obtained by Formula (1) is 0.65% or more, with the balance as Fe and inevitable impurity. Bainitic microstructure area fraction is 85% or more and Retained austenite area fraction is 5% or less. Crystal grains of the bainitic microstructure have an average diameter of 10 μm or more and 25 μm or less, an average aspect ratio of 0.5 or more, and an average ratio of crystal grain boundary length to crystal grain circumference of 60 or less.F⁢1=C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/14(1)

1 . A steel component comprising: a chemical composition containing, in mass %,
 C: 0.21% to 0.24%,   Si: 0.11% to 0.25%,   Mn: 1.81% to 1.99%,   P: 0.014% to 0.025%,   S: 0.035% to 0.060%,   Cr: 0.55% to 0.65%,   Al: 0.010% to 0.050%,   Ti: 0.005% to 0.020%,   V: 0.15% to 0.20%, and   N: 0.0090% to 0.0150%,   in amounts such that a value of F1 obtained by Formula (1) is 0.65% or more, with the balance as Fe and inevitable impurity; and a bainitic microstructure area fraction of 85% or more and a retained austenite area fraction of 5% or less, wherein   crystal grains of the bainitic microstructure have an average diameter of 10 μm or more and 25 μm or less, an average aspect ratio of 0.5 or more, and an average ratio of crystal grain boundary length to crystal grain circumference of 60 or less,   
       
         
           
             
               
                 
                   
                     
                       F 
                       ⁢ 
                       1 
                     
                     = 
                     
                       C 
                       + 
                       
                         Si 
                         / 
                         24 
                       
                       + 
                       
                         Mn 
                         / 
                         6 
                       
                       + 
                       
                         Ni 
                         / 
                         40 
                       
                       + 
                       
                         Cr 
                         / 
                         5 
                       
                       + 
                       
                         Mo 
                         / 
                         4 
                       
                       + 
                       
                         V 
                         / 
                         14 
                       
                     
                   
                 
                 
                   
                     ( 
                     1 
                     ) 
                   
                 
               
             
           
         
         where each element symbol in Formula (1) is content in mass % of the element, and elements not included are considered as 0 for F1 calculation. 
       
     
     
         2 . The steel component according to  claim 1 , wherein a ratio of a difference between the maximum value Hv 1  and the minimum value Hv 2  of Vickers hardness at 1 mm below the surface of the steel component to the maximum value Hv 1 , ((Hv 1 −Hv 2 )/Hv 1 )×100, is 10% or less. 
     
     
         3 . The steel component according to  claim 1 , wherein the chemical composition further contains, in mass %, at least one selected from the group consisting of:
 Cu: 0.25% or less,   Ni: 0.25% or less,   Mo: 0.15% or less, and   Nb: 0.030% or less.   
     
     
         4 . A method of producing a steel component, comprising: applying hot forging after heating and holding at 1150° C. or more to a steel material comprising the chemical composition according to  claim 1 , to obtain a hot forging material, and then cooling the hot forging material from 1000° C. to 800° C. at an average cooling rate of 0.7° C./s or more and 3.5° C./s or less, and from 800° C. to 550° C. at an average cooling rate of 0.5° C./s or more and 2.0° C./s or less. 
     
     
         5 . The method of producing a steel component according to  claim 4 , wherein the cooling from 800° C. to 550° C. is performed where a ratio of a difference between the maximum value V 1  and the minimum value V 2  of cooling rate distribution in the hot forging material to the maximum value V 1 , ((V 1 −V 2 )/V 1 )×100, is 25% or less. 
     
     
         6 . The steel component according to  claim 2 , wherein the chemical composition further contains, in mass %, at least one selected from the group consisting of:
 Cu: 0.25% or less,   Ni: 0.25% or less,   Mo: 0.15% or less, and   Nb: 0.030% or less.   
     
     
         7 . A method of producing a steel component, comprising: applying hot forging after heating and holding at 1150° C. or more to a steel material comprising the chemical composition according to  claim 3 , to obtain a hot forging material, and then cooling the hot forging material from 1000° C. to 800° C. at an average cooling rate of 0.7° C./s or more and 3.5° C./s or less, and from 800° C. to 550° C. at an average cooling rate of 0.5° C./s or more and 2.0° C./s or less. 
     
     
         8 . The method of producing a steel component according to  claim 7 , wherein the cooling from 800° C. to 550° C. is performed where a ratio of a difference between the maximum value V 1  and the minimum value V 2  of cooling rate distribution in the hot forging material to the maximum value V 1 , ((V 1 −V 2 )/V 1 )×100, is 25% or less.