Method of fabricating a steel forging, and a forging obtained thereby

Abstract

The invention provides a method of fabricating a steel part by forging, the method being characterized by the following steps: preparing and casting a steel having the following composition in percentages by weight: 0.06%≦C≦0.35%; 0.5%≦Mn≦2%; traces≦Si≦2%; traces≦Ni≦1.5%; traces≦Al≦0.1%; traces≦Cr≦1.5%; traces≦Mo≦0.30%; traces≦V≦0.5%; traces≦Cu≦1.5%; the remainder being iron and impurities that result from preparation; forging a blank for the part at a temperature in the range 110° C. to 1300 ° C.: cooling the blank for the part in controlled manner in still or forged air at a speed less than or equal to 3 ° C./s in the range 600° C. to 300° C., thereby imparting a bainite microstructure to the blank; machining the part; and performing a mechanical reinforcing operation on the part at locations that are to be subjected to particularly high levels of stress. The invention also provides a forging obtained in this way.

What is claimed is:  
     
         1 . A method of fabricating a steel part by forging, the method being characterized by the following steps: 
 preparing and casting a steel having the following composition in percentages by weight: 0.06%≦C≦0.35%; 0.5%≦Mn≦2%; traces≦Si≦2%; traces≦Ni≦1.5%; traces≦Al≦0.1%; traces≦Cr≦1.5%; traces≦Mo≦0.30%; traces≦V≦0.5%; traces≦Cu≦1.5%; the remainder being iron and impurities that result from preparation;    forging a blank for the part at a temperature in the range 110° C. to 1300° C.:    cooling the blank for the part in controlled manner in still or forged air at a speed less than or equal to 3° C./s in the range 600° C. to 300° C., thereby imparting a bainite microstructure to the blank;    machining the part; and    performing a mechanical reinforcing operation on the part at locations that are to be subjected to particularly high levels of stress.    
     
     
         2 . A method according to  claim 1 , wherein the steel contains 5 ppm to 50 ppm of B.  
     
     
         3 . A method according to  claim 1 , wherein the steel contains 0.005% to 0.04% of Ti.  
     
     
         4 . A method according to claims  2  and  3  taken together, wherein the steel contains 0.005% to 0.04% of Ti, and wherein the Ti content is equal to not less than 3.5 times the N content of the steel.  
     
     
         5 . A method according to  claim 1 , wherein the steel contains 0.005% to 0.06% of Nb.  
     
     
         6 . A method according to  claim 1 , wherein the steel contains 0.005% to 0.2% of S.  
     
     
         7 . A method according to  claim 6 , wherein the steel contains at least one of the following elements: Ca up to 0.007%; Te up to 0.03%; Se up to 0.05%; Bi up to 0.015%; and Pb up to 0.15%.  
     
     
         8 . A method according to  claim 1 , wherein the C content of the steel lies in the range 0.06% to 0.20%.  
     
     
         9 . A method according to  claim 8 , wherein the Mn content of the steel lies in the range 0.5% to 1.5%, and wherein the Cr content lies in the range 0.05% to 1.5%.  
     
     
         10 . A method according to  claim 8 , wherein the Cu content of the steel lies in the range 0.5% to 1.5%.  
     
     
         11 . A method according to  claim 1 , wherein the C content of the steel lies in the range 0.25% to 0.35%, the Si content lies in the range traces to 0.5%, the Mn content lies in the range 0.8% to 2%, the Cr content lies in the range 0.5% to 1.5%, the Mo content lies in the range 0.05% to 0.20%, the B content lies in the range 5 ppm to 50 mm, and the Ti content lies in the range 0.005% to 0.04%.  
     
     
         12 . A method according to  claim 1 , wherein the C content of the steel lies in the range 0.20% to 0.35%, the Si content lies in the range 0.5% to 2%, the Mn content lies in the range 0.8% to 2%, the chromium content lies in the range 0.5% to 1.5%, the molybdenum content lies in the range 0.05% to 0.20%, the boron content lies in range traces to 50 ppm, and the Ti content lies in the range 0.005% to 0.04%.  
     
     
         13 . A method according to  claim 12 , wherein annealing is performed in the range 300° C. to 500° C. for a period of 1 h to 3 h after machining or after controlled cooling in air and prior to machining.  
     
     
         14 . A method according to  claim 1 , wherein the mechanical reinforcing operation is burnishing.  
     
     
         15 . A steel forging, obtained by the method according to  claim 1 .  
     
     
         16 . A steel forging according to  claim 15 , constituting a crank shaft for an IC engine.  
     
     
         17 . A steel forging according to  claim 16 , wherein the mechanical reinforcing operation is performed on the fillets connecting the crank pins and the bearings of the crank shaft.