Air-hardenable bainitic steel with enhanced material characteristics

Abstract

A method of producing a forged steel part is disclosed to include providing a steel billet having a composition including 0.25-0.40 wt. % C, 1.50-3.00 wt. % Mn, 0.30-2.00 wt. % Si, 0.00-0.150 wt. % V, 0.02-0.06 wt. % Ti, 0.010-0.04 wt. % S, 0.0050-0.0150 wt. % N, 0.00-1.00 wt. % Cr, 0.00-0.30 wt. % Mo, 0.00-0.003 wt. % B, and a balance of Fe and incidental impurities. The method may further include heating the steel billet to an austenization temperature of approximately 1150 degrees C. to 1350 degrees C., hot forging the steel billet to form the steel part, and controlled air cooling the forged steel part after the hot forging. The method may still further include induction heating select portions of the forged steel part after the controlled air cooling to increase the hardness of the select portions of the forged steel part, followed by quenching and tempering before the final machining.

What is claimed is: 
     
         1 . A method of producing a forged steel part, comprising:
 providing steel billet having a composition comprising, on a weight basis:
 C: 0.25-0.40 wt. %, 
 Mn: 1.50-3.00 wt. %, 
 Si: 0.30-2.00 wt. % 
 V: 0.00-0.15 wt. %, 
 Ti: 0.02-0.06 wt. %, 
 S: 0.015-0.04 wt. %, 
 N: 0.0050-0.0150 wt. %, 
 Cr: 0.00-1.00 wt. %, 
 Mo: 0.00-0.30 wt. %, 
 B: 0.00-0.005 wt. %, and 
   a balance of Fe and incidental impurities;   heating the steel billet to an austenization temperature of approximately 1150 degrees C. to 1350 degrees C.;   hot forging the steel billet to form the steel part; and   controlled air cooling the forged steel part after the hot forging,
 wherein the controlled air cooling is performed at a rate to produce a body hardness for the steel part of approximately 35-45 Rockwell C hardness (HRC) after the controlled air cooling, a yield strength for the steel part of approximately greater than 1000 mega-pascals (MPa) after the controlled air cooling, and wherein a toughness of at least an inner portion of a body of the steel part after the controlled air cooling is approximately greater than or equal to 20 Joules at room temperature in accordance with the Charpy impact test. 
   
     
     
         2 . The method according to  claim 1 , wherein the controlled air cooling is performed by moving the forged steel part along a conveyor at ambient temperatures. 
     
     
         3 . The method according to  claim 1 , wherein the composition of the steel billet is selected such that the controlled air cooling of the forged steel part and resultant different rates of cooling of sections of the forged steel part having different thicknesses results in a microstructure throughout the entire forged steel part after the controlled air cooling of approximately greater than 50% by volume of bainitic microstructure. 
     
     
         4 . The method according to  claim 1 , wherein the composition of the steel billet is selected such that the controlled air cooling of the forged steel part and resultant different rates of cooling of sections of the forged steel part having different thicknesses results in a hardness level throughout the entire forged steel part after the controlled air cooling of approximately greater than 35-45 HRC. 
     
     
         5 . The method according to  claim 1 , further including induction heating select portions of the forged steel part after the controlled air cooling to increase hardness of the select portions of the forged steel part. 
     
     
         6 . The method according to  claim 5 , wherein the hardness of the select portions of the forged steel part after induction heating is greater than approximately 50 HRC. 
     
     
         7 . The method according to  claim 6 , further including quenching at least the induction heated portions of the forged steel part and reheating to temper the select portions of the forged steel part for enhanced toughness. 
     
     
         8 . The method according to  claim 1 , wherein the composition of the steel billet and the controlled air cooling of the forged steel part result in a microstructure of the forged steel part after the controlled air cooling of approximately greater than 85% by volume of bainitic microstructure. 
     
     
         9 . The method according to  claim 1 , wherein the composition of the steel billet and the controlled air cooling of the forged steel part result in a microstructure of the forged steel part after the controlled air cooling of approximately greater than 70% by volume of bainitic microstructure.