Steel for surface-treated mechanical parts with high characteristics, and mechanical parts in this steel and manufacturing method thereof

Abstract

Steel for which the composition is, in weight percentages: 0.35%≦C≦0.50%; 0.30%≦Mn≦1.50%; trace amounts ≦Cr≦1.50%; 0.05%≦Mo≦0.50%; 0.15≦Si≦1.20%; trace amounts ≦Ni≦1.0%; trace amounts ≦Cu≦1.0%; trace amounts ≦V≦0.35%; trace amounts ≦Al≦0.10%; trace amounts ≦B≦0.005%; trace amounts ≦Ti≦0.10%; trace amounts ≦Nb≦0.10%; trace amounts ≦S≦0.15%; trace amounts ≦Ca≦0.010%; trace amounts ≦Te≦0.030%; trace amounts ≦Se≦0.050%; trace amounts ≦Bi≦0.050%; trace amounts ≦Pb≦0.100%; trace amounts ≦N≦0.020%; the remainder being iron and impurities resulting from the elaboration, and the C, Mn and Cr contents being such that 830-270 C %-90 Mn %-70 Cr %)≦620.

1 . A method for manufacturing a mechanical steel part, including the following steps:
 a steel for surface-treated mechanical parts with high characteristics is cost and solidified, the composition of which is, in weight percentages:   0.35%≦C≦0.50%;   0.30%≦Mn≦1.50%;   trace amounts ≦Cr≦1.50%;   0.05%≦Mo≦0.50%;   0.15≦Si≦1.20%;   trace amounts ≦Ni≦1.0%;   trace amounts ≦Cu≦1.0%;   trace amounts ≦V≦0.35%;   trace amounts ≦Al≦0.10%;   trace amounts ≦B≦0.005%;   trace amounts ≦Ti≦0.10%;   trace amounts ≦Nb≦0.10%;   trace amounts ≦S≦0.15%;   trace amounts ≦Ca≦0.010%;   trace amounts ≦Te≦0.030%;   trace amounts ≦Se≦0.050%;   trace amounts ≦Bi≦0.050%;   trace amounts ≦Pb≦0.100%;   trace amounts ≦N≦0.020%;   
       the remainder being iron and impurities resulting from the elaboration, and the C, Mn and Cr contents being such that 830-270 C %-90 Mn %-70 Cr %) 620;
 hot-forming of said solidified steel is carried out in the austenitic domain in order to obtain a hot-formed semi-finished product; 
 the hot-formed semi-finished product is cooled at a rate giving it a bainitic structure containing a total of at most 20% of martensite and/or perlite and/or ferrite; 
 it is proceeded with a partial heat treatment with the purpose of locally obtaining a weakly or non-annealed martensitic microstructure. 
 
     
     
         2 . The method according to  claim 1 , wherein the steel contains trace amounts ≦Ni≦0.5%. 
     
     
         3 . The method according to  claim 1 , wherein the steel contains 0.15%≦Mo≦0.30%. 
     
     
         4 . The method according to  claim 1 , wherein the steel contains 0.005%≦Al≦0.10%. 
     
     
         5 . The method according to  claim 1 , wherein the steel contains 0.0005%≦B≦0.005%. 
     
     
         6 . The method according to  claim 1 , wherein the steel contains 0.005%≦Ti≦0.03%. 
     
     
         7 . The method according to  claim 6 , wherein the steel contains 0.0005%≦B≦0.005%, trace amounts ≦N≦0.0080%, and ≦Ti≦3.5 N. 
     
     
         8 . The method according to  claim 1 , wherein 0.005%≦S≦0.15%. 
     
     
         9 . The method according to  claim 1 , wherein the partial heat treatment is a surface treatment. 
     
     
         10 . The method according to  claim 1 , wherein said partial heat treatment is a surface treatment by induction followed by local quenching. 
     
     
         11 . The method according to  claim 1 , wherein the cooling of the hot-formed semi-finished product is followed by annealing at 200-400° C. for 30 mins to 8 hrs. 
     
     
         12 . The method according to  claim 1 , wherein the partial heat treatment is followed by annealing between 150 and 350° C. 
     
     
         13 . The method according to  claim 1 , wherein the cooling following the hot-forming of the semi-finished product takes place in calm air. 
     
     
         14 . The method according to  claim 1 , wherein the cooling following the hot-forming of the semi-finished product takes place in pulsed air. 
     
     
         15 . The method according to  claim 1 , wherein hot-forming of the solidified steel in the austenitic domain is performed by hot-forging or hot-rolling of the solidified steel. 
     
     
         16 . The method according to  claim 1 , wherein it is proceeded with one or several machining operations in order to give the part its intended dimensions. 
     
     
         17 . The method according to  claim 16 , wherein, before or after the machining operation(s), it is proceeded with annealing carried out at 200-400° C. for 1-2 hours. 
     
     
         18 . The method according to  claim 1 , wherein it is proceeded with rectification in order to give the part its final dimensions. 
     
     
         19 . A mechanical steel part which has been obtained with a method including the following steps:
 a steel for surface-treated mechanical parts with high characteristics is cost and solidified, the composition of which is, in weight percentages:   0.35%≦C≦0.50%;   0.30%≦Mn≦1.50%;   trace amounts ≦Cr≦1.50%;   0.05%≦Mo≦0.50%;   0.15≦Si≦1.20%;   trace amounts ≦Ni≦1.0%;   trace amounts ≦Cu≦1.0%;   trace amounts ≦V≦0.35%;   trace amounts ≦Al≦0.10%;   trace amounts ≦B≦0.005%;   trace amounts ≦Ti≦0.10%;   trace amounts ≦Nb≦0.10%;   trace amounts ≦S≦0.15%;   trace amounts ≦Ca≦0.010%;   trace amounts ≦Te≦0.030%;   trace amounts ≦Se≦0.050%;   trace amounts ≦Bi≦0.050%;   trace amounts ≦Pb≦0.100%;   trace amounts ≦N≦0.020%;   
       the remainder being iron and impurities resulting from the elaboration, and the C, Mn and Cr contents being such that 830-270 C %-90 Mn %-70 Cr %)≦620;
 hot-forming of said solidified steel is carried out in the austenitic domain in order to obtain a hot-formed semi-finished product; 
 the hot-formed semi-finished product is cooled at a rate giving it a bainitic structure containing a total of at most 20% of martensite and/or perlite and/or ferrite; 
 it is proceeded with a partial heat treatment with the purpose of locally obtaining a weakly or non-annealed martensitic microstructure. 
 
     
     
         20 . The mechanical part according to  claim 19 , being a rack machined from a bar which has been rolled then cut into slugs. 
     
     
         21 . The mechanical part according to  claim 19 , being a forged crankshaft whose grooves and/or crankpins and/or bearings have been reinforced by means of said partial heat treatment.