Age-hardening steel and method of manufacturing parts using age-hardening steel

Abstract

Age hardening steel excellent in machinability before aging treatment and excellent in fatigue characteristics, toughness, and low cycle fatigue characteristics after aging treatment, that is, age hardening steel containing predetermined amounts of C, Si, Mn, S, Cr, Al, V, Nb, Ca, and REM, limiting contents of P, Ti, and N to predetermined amounts or less, having a balance of Fe and impurities, having an area ratio of bainite structures of 70% or more, and, furthermore, having a chemical composition where F1 expressed by C+0.3×Mn+0.25×Cr is 0.68 or more, F2 expressed by C+0.1×Si+0.2×Mn+0.15×Cr+0.35×V is 0.85 or less, F3 expressed by −4.5×C+Mn+Cr−3.5×V is 0.00 or more, and F4 expressed by 10×Ca+REM is 0.012 to 0.08, is provided. Note that, the symbols of elements in the formulas showing F1 to F4 mean the contents by mass % of those elements.

1 . An age hardening steel containing, by mass %, C: 0.05 to 0.20%, Si: 0.01 to 0.50%, Mn: 1.50 to 2.50%, S: 0.005 to 0.080%, Cr: 0.03 to 1.60%, Al: 0.005 to 0.050%, V: 0.25 to 0.50%, Nb: 0.010 to 0.100%, Ca: 0.0005 to 0.0050%, and REM: 0.001 to 0.05%, limiting P to 0.030% or less, Ti to less than 0.005%, and N to less than 0.0080%, having a balance of Fe and impurities, having an area ratio of bainite structures of 70% or more and, furthermore, having a chemical composition where the following Fl expressed by formula (1) is 0.68 or more, F2 expressed by formula (2) is 0.85 or less, F3 expressed by formula (3) is 0.00 or more, and F4 expressed by formula (4) is 0.012 to 0.08:
   F1=C+0.3×Mn+0.25×Cr   (1)
     F2=C+0.1 ×Si+0.2 ×Mn+0.15 ×Cr+0.35×V   (2)
     F3=−4.5×C+Mn+Cr−3.5×V   (3)
     F4=10×Ca+REM   (4)
   where the symbols of elements in the above formula (1) to formula (4) mean the contents by mass % of those elements.   
     
     
         2 . An age hardening steel containing, by mass %, C: 0.05 to 0.20%, Si: 0.01 to 0.50%, Mn: 1.50 to 2.50%, S: 0.005 to 0.080%, Cr: 0.03 to 1.60%, Al: 0.005 to 0.050%, V: 0.25 to 0.50%, Nb: 0.010 to 0.100%, Ca: 0.0005 to 0.0050%, and REM: 0.001 to 0.05%, having an area ratio of bainite structures of 70% or more, furthermore, satisfying any one or more of the conditions of composition shown by the following <a>to <c>, limiting P to 0.030% or less, Ti to less than 0.005%, and N to less than 0.0080%, having a balance of Fe and impurities, and, furthermore, having a chemical composition where the following F1′ expressed by formula (1′) is 0.68 or more, F2′ expressed by formula (2′) is 0.85 or less, and F3′ expressed by formula (3′) is 0.00 or more, and F4 expressed by formula (4) is 0.012 to 0.08:
 <a> Mo: 0.01 to 1.0% 
 <b> One or both of Cu: 0.01 to 0.30% and Ni: 0.01% to 0.30% 
 <c> Bi: 0.01 to 0.400%
   F 1′=C+0.3 ×Mn+0.25×Cr+0.6×Mo   (1′)
 
   F2′=C+0.1×Si+0.2 ×Mn+0.15 ×Cr+0.35×V+0.2×Mo   (2′)
 
   F3′=−4.5 ×C+Mn+Cr−3 .5 ×V−0.8 ×Mo   (3′)
 
   F4=10×Ca+REM   (4)
 
 
 where the symbols of elements in the above formula (1) to formula (3′) and formula (4) mean the contents by mass % of those elements. 
 
     
     
         3 . A method of production of a part using age hardening steel comprising:
 a forging step of heating age hardening steel according to  claim 1  at 1200 to 1250° C. for 5 to 60 minutes, then forging it so that a surface temperature after final forging becomes 1100° C. or more, then, after that, cooling it by an average cooling rate in a 800 to 400° C. temperature region of 15 to 60° C./min down to room temperature,   a cutting step of cutting the forged steel, and   an aging treatment step of holding the cut steel in a 540 to 700° C. temperature region for 30 to 1000 minutes.   
     
     
         4 . A method of production of a part using age hardening steel comprising:
 a forging step of heating age hardening steel according to  claims 2  at  1200  to  1250 ° C. for 5 to 60 minutes, then forging it so that a surface temperature after final forging becomes 1100° C. or more, then, after that, cooling it by an average cooling rate in a 800 to 400° C. temperature region of 15 to 60° C./min down to room temperature,   a cutting step of cutting the forged steel, and   an aging treatment step of holding the cut steel in a 540 to 700° C. temperature region for 30 to 1000 minutes.