Age-hardenable steel, and method for manufacturing components using age-hardenable steel

Abstract

Age hardenable steel is low in hardness after hot forging, providing a machine part with the desired fatigue strength and yield strength by aging treatment, and high in toughness after aging treatment, comprising C: 0.09 to 0.20%, Si: 0.01 to 0.40%, Mn: 1.5 to 2.5%, S: 0.001 to 0.045%, Cr: over 1.00% to 2.00%, Al: 0.001 to 0.060%, V: 0.22 to 0.55%, N: over 0.0080 to 0.0170%, and a balance of Fe and impurities, where an area rate of bainite structures is 80% or more, an effective V ratio (amount of dissolved V/total amount of V) is 0.9 or more, a P and Ti in the impurities is P: 0.03% or less and Ti: less than 0.005%, and the chemical composition is one where the following F1 is 1.00 or less and the F2 is 0.30 or more: F 1=C+0.1×Si+0.2×Mn+0.15×Cr+0.35× V F 2=−4.5×C+Mn+Cr−3.5 ×V

1 . Age hardenable steel comprising, by mass %, C: 0.09 to 0.20%, Si: 0.01 to 0.40%, Mn: 1.5 to 2.5%, S: 0.001 to 0.045%, Cr: over 1.00% to 2.00%, Al: 0.001 to 0.060%, V: 0.22 to 0.55%, N: over 0.0080 to 0.0170%, and a balance of Fe and impurities, the P and Ti in this impurities being P: 0.03% or less and Ti: less than 0.005%, wherein
 an area rate of bainite structures is 80% or more,   an effective V ratio (amount of dissolved V/total amount of V) is 0.9 or more, and   a chemical composition is one where the F1 expressed by the following formula (1) is 1.00 or less and the F2 expressed by the following formula (2) is 0.30 or more:
     F 1=C+0.1×Si+0.2×Mn+0.15×Cr+0.35 ×V   (1)
 
     F 2=−4.5×C+Mn+Cr−3.5 ×V   (2)
 
   
       where, in the above formulas (1) and (2), the element symbols mean the contents of the elements by mass %. 
     
     
         2 . Age hardenable steel comprising, by mass %, C: 0.09 to 0.20%, Si: 0.01 to 0.40%, Mn: 1.5 to 2.5%, S: 0.001 to 0.045%, Cr: over 1.00% to 2.00%, Al: 0.001 to 0.060%, V: 0.22 to 0.55%, Mo: 0.9% or less, N: over 0.0080 to 0.0170%, and a balance of Fe and impurities, the P and Ti in this impurities being P: 0.03% or less and Ti: less than 0.005%, wherein
 an area rate of bainite structures is 80% or more,   an effective V ratio (amount of dissolved V/total amount of V) is 0.9 or more, and   a chemical composition is one where the F1′ expressed by the following formula (1′) is 1.00 or less and the F2′ expressed by the following formula (2′) is 0.30 or more:
     F 1′=C+0.1×Si+0.2×Mn+0.15×Cr+0.35 ×V+ 0.2×Mo  (1′)
 
     F 2′=−4.5×C+Mn+Cr−3.5 ×V− 0.8×Mo  (2′)
 
   
       where, in the above formulas (1′) and (2′), the element symbols mean the contents of the elements by mass %. 
     
     
         3 . The age hardenable steel according to  claim 1  further comprising one or more of Cu: 0.3% or less and Ni: 0.3% or less. 
     
     
         4 . The age hardenable steel according to  claim 1 , further comprising one or more of Ca: 0.005% or less and Bi: 0.4% or less. 
     
     
         5 . A method of production of part using age hardenable steel comprising:
 a forging step of heating age hardenable steel according to  claim 1  at 1100 to 1350° C. for 0.1 to 300 minutes, then forging it so that a surface temperature after finish forging becomes 900° C. or more, then cooling it down to room temperature while making the average cooling speed in a temperature region from 800 to 400° C. a speed of 10 to 90° C./min,   a machining step machining the steel after forging, and   an aging treatment step holding the steel after machining in the temperature region from 540 to 700° C. for 30 to 1000 minutes.   
     
     
         6 . The age hardenable steel according to  claim 2  further comprising one or more of Cu: 0.3% or less and Ni: 0.3% or less. 
     
     
         7 . The age hardenable steel according to  claim 2 , further comprising one or more of Ca: 0.005% or less and Bi: 0.4% or less. 
     
     
         8 . The age hardenable steel according to  claim 3 , further comprising one or more of Ca: 0.005% or less and Bi: 0.4% or less. 
     
     
         9 . A method of production of part using age hardenable steel comprising:
 a forging step of heating age hardenable steel according to  claim 2  at 1100 to 1350° C. for 0.1 to 300 minutes, then forging it so that a surface temperature after finish forging becomes 900° C. or more, then cooling it down to room temperature while making the average cooling speed in a temperature region from 800 to 400° C. a speed of 10 to 90° C./min,   a machining step machining the steel after forging, and   an aging treatment step holding the steel after machining in the temperature region from 540 to 700° C. for 30 to 1000 minutes.   
     
     
         10 . A method of production of part using age hardenable steel comprising:
 a forging step of heating age hardenable steel according to  claim 3  at 1100 to 1350° C. for 0.1 to 300 minutes, then forging it so that a surface temperature after finish forging becomes 900° C. or more, then cooling it down to room temperature while making the average cooling speed in a temperature region from 800 to 400° C. a speed of 10 to 90° C./min,   a machining step machining the steel after forging, and   an aging treatment step holding the steel after machining in the temperature region from 540 to 700° C. for 30 to 1000 minutes.   
     
     
         11 . A method of production of part using age hardenable steel comprising:
 a forging step of heating age hardenable steel according to  claim 4  at 1100 to 1350° C. for 0.1 to 300 minutes, then forging it so that a surface temperature after finish forging becomes 900° C. or more, then cooling it down to room temperature while making the average cooling speed in a temperature region from 800 to 400° C. a speed of 10 to 90° C./min,   a machining step machining the steel after forging, and   an aging treatment step holding the steel after machining in the temperature region from 540 to 700° C. for 30 to 1000 minutes.