Engineering Steel with a Bainitic Structure, Forged Parts Produced Therefrom and Method for Producing a Forged Part

Abstract

An engineering steel including (in wt.-%) up to 0.25% C, up to 0.45% Si, 0.20-2.00% Mn, up to 4.00% Cr, 0.6-3.0% Mo, 0.004-0.020% N, up to 0.40% S. 0.001-0.035% Al, 0.0005-0.0025% B, up to 0.015% Nb, up to 0.01% Ti, up to 0.10% V, up to 1.5% Ni and up to 2.0% Cu, remainder iron and unavoidable impurities, wherein the Al content % Al, the Nb content % Nb, the Ti content % Ti, the V content % V and the N content % N meet the following condition: % Al/27+% Nb/45+% Ti/48+% V/25>% N/3.75. The steel has a yield strength of at least 750 MPa, a tensile strength of at least 950 MPa and a structure having at least 80 vol.-% bainite and a total of a maximum of 20 vol.-% of retained austenite, ferrite, perlite and/or martensite.

1 . An engineering steel having a yield strength of at least 750 MPa, a tensile strength of at least 950 MPa and a structure consisting of at least 80 vol.-% of bainite and in total a maximum of 20 vol.-% of retained austenite, ferrite, perlite and/or martensite, wherein the steel comprises (in wt.-%)
 C: 0-0.25%,   Si: 0-1.5%,   Mn: 0.20-2.00%,   Cr: 0-4.00%,   Mo: 0.6-3.0%,   N: 0.004-0.020%,   S: 0-0.40%,   Al: 0.001-0.035%,   B: 0.0005-0.0025%,   Nb: 0-0.015%,   Ti: 0-0.01%,   V: 0-0.10%,   Ni: 0-1.5%,   Cu: 0-2.0%,   remainder iron and unavoidable impurities, and   the Al content % Al, the Nb content % Nb, the Ti content % Ti, the V content   % V and the N content % N of the engineering steel in each case meet the following condition:
   % Al/27+% Nb/45+% Ti/48+% V/25>% N/3.75. 
   
     
     
         2 . The engineering steel according to  claim 1 , wherein the C content is at least 0.09 wt.-%. 
     
     
         3 . The engineering steel according to  claim 1 , wherein the Al content is at least 0.004 wt.-%. 
     
     
         4 . The engineering steel according to  claim 1 , wherein the Al content is a maximum of 0.020 wt.-%. 
     
     
         5 . The engineering steel according to  claim 1 , wherein the Nb content is at least 0.003 wt.-%. 
     
     
         6 . The engineering steel according to  claim 1 , wherein the Nb content is a maximum of 0.01 wt.-%. 
     
     
         7 . The engineering steel according to  claim 1 , wherein the Ti content is at least 0.001 wt.-%. 
     
     
         8 . The engineering steel according to  claim 1 , wherein the Ti content is a maximum of 0.008 wt.-%. 
     
     
         9 . The engineering steel according to  claim 1 , wherein the V content is at least 0.02 wt.-%. 
     
     
         10 . The engineering steel according to  claim 1 , wherein the V content is a maximum of 0.075 wt.-%. 
     
     
         11 . The engineering steel according to  claim 1 , wherein the elongation at rupture A is at least 10%. 
     
     
         12 . A forged part comprising a steel in accordance with  claim 1 . 
     
     
         13 . A method for producing a forged part having a yield strength of at least 750 MPa and a tensile strength of at least 950 MPa and an at least 80 vol.-% bainitic structure, wherein the remaining maximum of 20 vol.-% of other proportions of the structure are retained austenite, ferrite, perlite and/or martensite, comprising the following process steps:
 a. providing a semi-finished product for forging, comprising an engineering steel with a composition according to  claim 1 ;   b. heating the semi-finished product for forging to a forging temperature of at least 100° C. above the Ac3 temperature of the engineering steel;   c. forging the semi-finished product for forging heated to the forging temperature into the forged part;   d. cooling the forged part from the forging temperature to a temperature of below 200° C., wherein the t8/5-time for cooling is 10-1,000 s.   
     
     
         14 . The method according to  claim 13 , wherein the forging temperature is higher than 1,150° C. 
     
     
         15 . The method according to  claim 13 , wherein following cooling the forged part undergoes tempering treatment, during which it is maintained for a duration of 0.5-2 hours at a tempering temperature of 180-375° C.