Bainitic microalloy steel with enhanced nitriding characteristics

Abstract

A forged, microalloyed, and nitrided steel part is disclosed to have a composition including 0.20-0.40 wt. % C, 0.50-1.60 wt. % Mn, 0.40-1.50 wt. % Cr, 0.07-0.30 wt. % Al, 0.03-0.20 wt. % V, 0.10-0.40 wt. % Si, and a balance of Fe and incidental impurities. The part may be produced by heating the steel part to austenization temperature of approximately 1100 degrees C. to 1260 degrees C., hot forging the steel part, controlled air cooling the steel part after hot forging at a rate falling approximately in the range from 1 degree C. per second to 5 degrees C. per second as the steel part cools from approximately 900 degrees C. to approximately 500 degrees C. to produce a predominantly bainitic microstructure of greater than approximately 50% bainite. The steel part may then be machined to a desired configuration, and nitrided by heating in an atmosphere containing ammonia.

What is claimed is: 
     
         1 . A forged steel part with a surface nitrided layer formed by nitriding following hot forging and controlled air cooling without heat treatment and having a composition comprising:
 C: 0.20-0.40 wt. %,   Mn: 0.50-1.60 wt. %,   Cr: 0.40-1.50 wt. %,   Al: 0.07-0.30 wt. %,   V: 0.03-0.20 wt. %,   Si: 0.10-0.40 wt. %, and   
       a balance of Fe and incidental impurities. 
     
     
         2 . The forged steel part of  claim 1 , wherein the steel part is heated to approximately 1100 degrees C. to 1260 degrees C. before being hot forged. 
     
     
         3 . The forged steel part of  claim 1 , wherein a microstructure of the forged steel part after the controlled air cooling is greater than 50% by volume bainite. 
     
     
         4 . The forged steel part of  claim 1 , wherein a microstructure of the forged steel part after the controlled air cooling is greater than 70% by volume bainite. 
     
     
         5 . The forged steel part of  claim 1 , wherein a microstructure of the forged steel part after the controlled air cooling is greater than 85% by volume bainite. 
     
     
         6 . The forged steel part of  claim 1 , wherein a microstructure of the forged steel part after the controlled air cooling is greater than 85% by volume bainite at a surface of the part and greater than 85% by volume bainite at a core of the part. 
     
     
         7 . The forged steel part of  claim 1 , wherein at least a surface of the forged steel part has a Rockwell C hardness (HRC) of at least 25 after the controlled air cooling. 
     
     
         8 . The forged steel part of  claim 1 , wherein the hardness throughout the forged steel part falls within a range between approximately 25 HRC to 30 HRC. 
     
     
         9 . The forged steel part of  claim 1 , wherein the controlled air cooling following forging is performed at a rate approximately within the range from 1 degree C. per second to 5 degrees C. per second between approximately 900 degrees C. and 500 degrees C. 
     
     
         10 . A forged steel part manufactured to have a chemical composition comprising:
 C: 0.20-0.40 wt. %,   Mn: 0.50-1.60 wt. %,   Cr: 0.40-1.50 wt. %,   Al: 0.07-0.30 wt. %,   V: 0.03-0.20 wt. %,   Si: 0.10-0.40 wt. %, and   
       a balance of Fe and incidental impurities; and the forged steel part being manufactured by hot forging, controlled air cooling after the hot forging with no further heat treatment to produce a predominantly bainitic microstructure of greater than 50% bainite throughout the forged steel part, machining, and nitriding. 
     
     
         11 . The forged steel part of  claim 10 , further including the controlled air cooling after hot forging including air cooling at a rate in the range from approximately 1 degree C. per second to 5 degrees C. per second as the forged steel part cools from a temperature of approximately 900 degrees C. to a temperature of approximately 500 degrees C. 
     
     
         12 . The forged steel part of  claim 10 , further including the nitriding enhanced by V and Al left over after the controlled air cooling reacting with nitrogen (N) diffused into a surface of the part during the nitriding. 
     
     
         13 . The forged steel part of  claim 10 , wherein a portion of the V and Al reacts with N that is dissolved into the steel to form fine precipitates of VN and AN that strengthen the steel, and a majority of the V and Al combines with N diffused into the surface of the part during the nitriding for further strengthening of the steel. 
     
     
         14 . A method of producing a forged steel part, comprising:
 subjecting a steel billet to hot forging and then to nitriding without heat treatment after the hot forging, the steel having a composition comprising, on a weight basis:   C: 0.20-0.40 wt. %,   Mn: 0.50-1.60 wt. %,   Cr: 0.40-1.50 wt. %,   Al: 0.07-0.30 wt. %,   V: 0.03-0.20 wt. %,   Si: 0.10-0.40 wt. %, and   
       a balance of Fe and incidental impurities. 
     
     
         15 . The method of  claim 14 , further including heating the steel billet to an austenization temperature of approximately 1100 degrees C. to 1260 degrees C. before the hot forging. 
     
     
         16 . The method of  claim 14 , further including:
 air cooling the forged steel part after the hot forging including controlling the rate of air cooling to fall within a range from approximately 1 degree C. per second to 5 degrees C. per second as the forged steel part cools between approximately 900 degrees C. and 500 degrees C.   
     
     
         17 . The method of  claim 16 , wherein the air cooling occurs on a conveyor belt at ambient temperature. 
     
     
         18 . The method of  claim 16 , wherein the air cooling is performed by the forced circulation of air over the forged steel part. 
     
     
         19 . The method of  claim 14 , wherein the microstructure of the forged steel part after the air cooling is greater than 50% by volume bainite. 
     
     
         20 . The method of  claim 14 , wherein the microstructure of the forged steel part after the air cooling is greater than 70% by volume bainite. 
     
     
         21 . The method of  claim 14 , wherein the microstructure of the forged steel part after the air cooling is greater than 85% by volume bainite.