Patent ID: 7354487
Filing Date: 2008-04-08
Classification: C21D,C22C

Abstract:
1. A method of fabricating a steel part, the method comprising the steps of: preparing and casting a steel consisting essentially of the following composition in percentage by weight: 0.06%≦C≦0.25%; 0.5%≦Mn≦2%; traces≦Si≦3%; traces≦Ni≦4.5%; traces≦Al≦3%; traces≦Cr≦1.2%; traces≦Mo≦0.30%; traces≦V≦2%; traces≦Cu≦3.5%; 0.005%≦S≦0.2%; 5 ppm to 50 ppm of B, 0.005% to 0.04% of Ti, where the Ti content is equal to at least 3.5 times the N content of the steel; 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 0.05%; and Pb up to 0.1%, and wherein the steel satisfies at least one of the following conditions:  the remainder being iron and impurities resulting from preparation; hot deforming the cast steel at least once at a temperature in the range 1100° C. to 1300° C. in order to obtain a blank of the part; controlled cooling of the blank for the part in still air or forced air to obtain a bainite microstructure; and heating the steel to perform precipitation annealing before or after machining said blank, wherein part is obtained by machining, wherein the hot deformation is forging wherein the precipitation annealing is performed in the range of 425 C to 600 C for 1 h or more; wherein when the steel satisfies the condition regarding Cu, the precipitation annealing is performed in the range of 425° C. to 500° C. for 1 h to 10 h, and when the steel satisfies the condition regarding Ni and Al, the precipitation annealing is performed in the range of 500° C. to 550° C. for more than 1 h; wherein the controlled cooling of the blank is performed at a rate less than or equal to 3° C./s in the range of 600° C. to 300° C., wherein the part has a tensile strength Rm of 1000 MPa to 1300 MPa, and a yield strength Re of greater than or equal to 900 MPa, and wherein the bainite microstructure obtained after controlled cooling of the blank is 100% bainite.