Component for machine structural use and method for making the same

Abstract

A component for machine structural use having improved fatigue strength. The component for machine structural use is produced by subjecting at least a part of a steel material to hardening, and the hardened structure is controlled such that the average grain diameter of former austenitic grains is 12 μm or less and the maximum grain diameter does not exceed four times the average grain diameter.

1 . A component for machine structural use, comprising a steel material at least part of which is subjected to hardening, wherein the hardened structure has an average diameter of former austenitic grains of 12 mm or less and a maximum grain diameter not exceeding four times the average grain diameter.  
   
   
       2 . The component for machine structural use according to  claim 1 , the steel material comprising a composition containing, in terms of percent by mass: 
 C: 0.3% to 1.5%.    Si: 0.05% to 3.0% and    Mn: 0.2% to 2.0%,    and satisfying a formula (1):      C 1/2 (1+0.7Si)(1+3Mn)>2.0   (1)    wherein the balance is Fe and inevitable impurities.    
   
   
       3 . The component for machine structural use according to  claim 2 , the composition further containing, in terms of percent by mass: 
 Al: 0.25% or less.    
   
   
       4 . A component for machine structural use comprising a steel material at least tart of which is subjected to hardening, wherein the hardened structure has an average diameter of former austenitic grains of 12 μm or less and a maximum grain diameter not exceeding four times the average grain diameter, the steel material comprising, a composition containing in terms of percent by mass: 
 C: 0.3% to 1.5%;    Si: 0.05% to 3.0%;    Mn: 0.2% to 2.0%;    at least one element selected from the group consisting of    Cr: 0.03% to 2.5%,    Mo: 0.05% to 1.0%,    Cu: 0.03% to 1.0%,    Ni: 0.05% to 2.5%,    Co: 0.01% to 1.0%,    V: 0.01% to 0.5% and    W: 0.005% to 1.0%,    and satisfying a formula (2):      C 1/2 (1+0.7Si)(1+3Mn)(1+2.1Cr)(1+3.0 Mo)(1+0.4 Cu)(1+0.3Ni)(1+5.0V)(1+0.5W)>2.0   (2)    wherein the balance is Fe and inevitable impurities.    
   
   
       5 . A component for machine structural use comprising a steel material at least part of which is subjected to hardening, wherein the hardened structure has an average diameter of former austenitic grains of 12 μm or less and a maximum grain diameter not exceeding four times the average grain diameter, the steel material comprising, a composition containing, in terms of percent by mass: 
 C: 0.3% to 1.5%;    Si: 0.05% to 3.0%;    Mn: 0.2% to 2.0%;    at least one element selected from the group consisting of    Cr: 0,03% to 2.5%,    Mo: 0.05% to 1.0%,    Cu: 0.03 to 1.0%,    Ni: 0.05% to 2.5%,    Co: 0.01% to 1.0%,    V: 0.01% to 0.5% and    W: 0.005% to 1.0%;    at least one element selected from the group consisting of    Ti: 0.005% to 0.1%,    Nb: 0.005% to 0.1%,    Zr: 0.005% to 0.1%,    B: 0.0003% to 0.01%,    Ta: 0.02% to 0.5%,    Hf: 0.02% to 0.5% and    Sb: 0.005% to 0.015%,    and satisfying a formula (3):      C 1/2 (1+0.7Si)(1+3Mn)(1+2.1Cr)(1+3.0 Mo)(1+0.4 Cu)(1+0.3Ni)(1+5.0V)(1+1000B)(1+0.5W)>2.0   (3)    wherein the balance is Fe and inevitable impurities.    
   
   
       6 . The component for machine structural use according to  claim 2  or  3 , the composition further containing, in terms of percent by mass, at least one element selected from the group consisting of 
 S: 0.1% or less,    Pb: 0.01% to 0.1%,    Bi: 0.01% to 0.1%,    Se: 0.003% to 0.1%,    Te: 0.003% to 0.1%,    Ca: 0.0001% to 0.01%,    Mg: 0.0001% to 0.01% and    REM: 0.0001% to 0.1%.    
   
   
       7 . A method for making a component for machine structural use, comprising subjecting at least part of a steel material to high-frequency heating at least once, the steel material containing one or both of a fine bainite structure and a fine martensite structure in a total of 10 percent by volume, wherein the high-frequency heating is conducted at a heating rate of at least 400° C./s and an ultimate temperature of 1,000° C. or less.  
   
   
       8 . The method for making the component for machine structural use according to  claim 7 , wherein the steel material is worked through: a hot-working step in which a total processing ratio at 800° C. to 1,000° C. is 80% or more; a cooling step subsequent to the hot-working step, in which the cooling rate is 0.2° C./s or more in the temperature range of 700° C. to 500° C.; and a secondary working step of either effecting 20% or more of working in a temperature range of 700° C. to less than 800° C. before the cooling step or effecting 20% or more of working in a temperature range of up to the A 1  transformation temperature subsequent to the cooling step.  
   
   
       9 . The method for making the component for machine structural use according to  claim 7  or  8 , wherein a retention time at 800° C. or more in the high-frequency heating is 5 seconds or less at a time.  
   
   
       10 . The method for making the component for machine structural use according to  claim 7  or  8 , wherein the steel material comprising, in terms of percent by mass, 
 C: 0.3% to 1.5%;    Si: 0.05% to 3.0% and    Mn: 0.2% to 2.0%,    and satisfying a formula (1):      C 1/2 (1+0.7Si)(1+3Mn)>2.0   (1)    wherein the balance is Fe and inevitable impurities.    
   
   
       11 . The method for making the component for machine structural use according to  claim 10 , the steel material further comprising: 
 Al: 0.25 mass % or less.    
   
   
       12 . The method for making the component for machine structural use according to  claim 10 , wherein the steel material comprising, in terms of percent by mass, 
 C: 0.3% to 1.5%;    Si: O.005% to 3.0%;    Mn: 0.2% to 2.0%;    at least one element selected from the group consisting of    Cr: 0.03% to 2.5%,    Mo: 0.05% to 1.0%,    Cu: 0.03% to 1.0%,    Ni: 0.05% to 2.5%,    Co: 0.01% to 1.0%,    V: 0.01% to 0.5% and    W: 0.005% to 1.0%,    and satisfying a formula (2):      C 1/2 (1+0.7Si)(1+3Mn)(1+2.1Cr)(1+3.0 Mo)(1+0.4 Cu)(1+0.3Ni)(1+5.0V)(1+0.5W)>2.0   (2)    wherein the balance is Fe and inevitable imurities.    
   
   
       13 . The method for making the component for machine structural use according to  claim 10 , wherein the steel material comprising, in terms of percent by mass, C: 0.3% to 1.5%; 
 Si: 0.05% to 3.0%;    Mn: 0.2% to 2.0%;    at least one element selected from the group consisting of    Cr; 0.03% to 2.5%,    Mo: 0.05% to 1.0%,    Cu: 0.03% to 1.0%,    Ni: 0.05% to 2.5%,    Co: 0.01% to 1.0%,    V: 0.01% to 0.5% and    W: 0.005% to 1.0%;    at least one element selected from the group consisting of    Ti: 0.005% to 0.1%,    Nb: 0.005% to 0.1%,    Zr: 0.005% to 0.1%,    B: 0.0003% to 0.01%,    Ta: 0.02% to 0.5%,    Hf: 0.02% to 0.5% and    Sb: 0.005% to 0.015%,    and satisfying a formula (3):      C 1/2 (1+0.7Si)(1+3Mn)(1+2.1Cr)(1+3.0 Mo)(1+0.4 Cu)(1+0.3Ni)(1+5.0V)(1+1000B)(1+0.5W)>2.0   (3)    wherein the balance is Fe and inevitable impurities.    
   
   
       14 . The method for making the component for machine structural use according to  claim 10 , wherein the steel material further comprising, in terms of percent by mass, at least one element selected from the groud consisting of 
 S: 0.1% or less,    Pb: 0.01% to 0.1%,    Bi: 0.01% to 0.1%,    Se: 0.003% to 0.1%,    Te: 0.003% to 0.1%,    Ca: 0.0001% to 0.01%,    Mg: 0.0001% to 0.01% and    REM: 0.0001% to 0.1%.    
   
   
       15 . (canceled)  
   
   
       16 . The component for machine structural use according to  claim 4 , the composition further containing, in terms of percent by mass, at least one element selected from among the group consisting of 
 S: 0.1% or less    Pb: 0.01% to 0.1%    Bi: 0.01% to 0.1%,    Se: 0.003% to 0.1%,    Te: 0.003% to 0.1%,    Ca: 0.0001% to 0.01%,    Mg: 0.0001% to 0.01% and    REM: 0.0001% to 0.1%.    
   
   
       17 . The component for machine structural use according to  claim 5 , the composition further containing, in terms of percent by mass, at least one element selected from the group consisting of 
 S: 0.1% or less,    Pb: 0.01% to 0.1%,    Bi: 0.01% to 0.1%,    Se: 0.003% to 0.1%,    Te: 0.003% to 0.1%,    Ca: 0.0001% to 0.01%,    Mg: 0.0001% to 0.01% and    REM: 0.0001% to 0.1%.    
   
   
       18 . (canceled)  
   
   
       19 . The component for machine structural use according to  claim 4 , wherein the composition further comprises 0.25% by mass or less of Al.  
   
   
       20 . The component for machine structural use according to  claim 5 , wherein the composition further comprises 0.25% by mass or less of Al.  
   
   
       21 . The method for making the component for machine structural use according to  claim 12 , wherein the steel further comprises 0.25% by amss or less of Al.  
   
   
       22 . The method for making the component for machine structural use according to  claim 13 , wherein the steel further comprises 0.25% by mass or less of Al.