Method for manufacturing a rail and corresponding rail

Abstract

A method for manufacturing a rail includes casting a steel to obtain a semi-product. The steel has a composition comprising 0.20%≤C≤0.60%, 1.0%≤Si≤2.0%, 0.60%≤Mn≤1.60% and 0.5≤Cr≤2.2%, optionally 0.01%≤Mo≤0.3%, 0.01%≤V≤0.30%; the remainder being Fe and impurities. The method also includes hot rolling the semi-product into a hot rolled semi-product having the shape of the rail and comprising a head, with a final rolling temperature TFRT higher than Ar3; and cooling the head to a cooling stop temperature TCS between 200° C. and 520° C. The temperature of the head over time is comprised between a upper boundary having the coordinates defined by A1 (0 second, 780° C.), B1 (50 seconds, 600° C.), and C1 (110 seconds, 520° C.) and a lower boundary having the coordinates defined by A2 (0 second, 675° C.), B2 (50 seconds, 510° C.), and C2 (110 seconds, 300° C.). The method also includes maintaining the head in a temperature range comprised between 300° C. and 520° C. during a holding time thold of at least 12 minutes, and; cooling down the hot rolled semi-product to room temperature to obtain the rail.

What is claimed is: 
     
         1 . A method for manufacturing a rail comprising a head, the method comprising successively:
 casting a steel so as to obtain a semi-product, the steel having a chemical composition comprising, by weight percent:   
       
         
           
             
               
                 
                   0.2 
                   % 
                 
                 ≤ 
                 C 
                 ≤ 
                 
                   0.6 
                   % 
                 
               
               , 
             
           
         
         
           
             
               
                 
                   
                     1. 
                     % 
                   
                 
                 ≤ 
                 Si 
                 ≤ 
                 
                   2. 
                   % 
                 
               
               , 
             
           
         
         
           
             
               
                 
                   0.6 
                   % 
                 
                 ≤ 
                 Mn 
                 ≤ 
                 
                   1.6 
                   % 
                 
               
               , 
             
           
         
         
           
             and 
           
         
         
           
             
               
                 0.5 
                 ≤ 
                 Cr 
                   
                 ≤ 
                 
                   2.2 
                   % 
                 
               
               , 
             
           
         
         and optionally one or more elements chosen among: 
       
       
         
           
             
               
                 
                   0.01 
                   % 
                 
                 ≤ 
                 Mo 
                 ≤ 
                 
                   0.3 
                   % 
                 
               
               , 
             
           
         
         
           
             
               
                 
                   0.01 
                   % 
                 
                 ≤ 
                 V 
                 ≤ 
                 
                   0.3 
                   % 
                 
               
               ; 
             
           
         
         a remainder being Fe and unavoidable impurities resulting from the smelting; 
         hot rolling the semi-product into a hot rolled semi-product having a shape of the rail and comprising a head, with a final rolling temperature T FRT  higher than Ar3; 
         cooling the head of the hot rolled semi-product from the final rolling temperature T FRT  down to a cooling stop temperature T CS  comprised between 200° C. and 520° C., such that the temperature of the head of the hot rolled semi-product over time is comprised between a upper boundary and a lower boundary, the upper boundary having coordinates of time and temperature defined by A1 (0 second, 780° C.), B1 (50 seconds, 600° C.), and C1 (110 seconds, 520° C.), the lower boundary having coordinates of time and temperature defined by A2 (0 second, 675° C.), B2 (50 seconds, 510° C.), and C2 (110 seconds, 300° C.); 
         maintaining the head of the hot rolled semi-product in a temperature range comprised between 300° C. and 520° C. during a holding time t hold  of at least 12 minutes; and 
         cooling down the hot rolled semi-product to room temperature to obtain the rail. 
       
     
     
         2 . The method according to  claim 1 , wherein a microstructure of the head of the rail consists of, in area fraction:
 49% to 67% of bainite;   14% to 25% of retained austenite, the retained austenite having an average carbon content comprised between 0.80% and 1.44%; and   13% to 34% of tempered martensite.   
     
     
         3 . The method according to  claim 2 , wherein the area fraction of bainite in the microstructure of the head consists of between 56% and 67%. 
     
     
         4 . The method according to  claim 2 , wherein the area fraction of retained austenite in the microstructure of the head consists of between 18% and 23%. 
     
     
         5 . The method according to  claim 2 , wherein the area fraction of tempered martensite in the microstructure of the head consists of between 14.5% and 22.5%. 
     
     
         6 . The method according to  claim 2 , wherein the average carbon content in the retained austenite is comprised between 1.3% and 1.44%. 
     
     
         7 . The method according to  claim 1 , wherein the cooling stop temperature T CS  is comprised between 300° C. and 520° C. 
     
     
         8 . The method according to  claim 1 , wherein the cooling stop temperature T CS  is comprised between 200° C. and 300° C., and the method further comprises, after cooling the head of the hot rolled semi-product down to the cooling stop temperature T CS  and before maintaining the head in the temperature range, heating the head of the hot rolled semi-product up to a temperature comprised between 300° C. and 520° C. 
     
     
         9 . The method according to  claim 1 , wherein the cooling of the head of the hot rolled semi-product is performed through water jets. 
     
     
         10 . The method according to  claim 1 , wherein, during the cooling of the head of the hot rolled semi-product, an entirety of the hot rolled semi-product is cooled such that the temperature of the hot rolled semi-product over time is comprised between the upper boundary and the lower boundary. 
     
     
         11 . The method according to  claim 1 , wherein, during the hot rolling of the semi-product, the semi-product is hot rolled from a hot rolling starting temperature higher than 1080° C. 
     
     
         12 . The method according to  claim 11 , wherein the hot rolling starting temperature is higher than 1180° C. 
     
     
         13 . The method according to  claim 1 , wherein the chemical composition of the steel comprises, by weight percent, 0.30%≤C≤0.60%. 
     
     
         14 . The method according to  claim 1 , wherein the chemical composition of the steel comprises, by weight percent, 1.25%≤Si≤1.6%. 
     
     
         15 . The method according to  claim 1 , wherein the chemical composition of the steel comprises, by weight percent, 1.09%≤Mn≤1.5%. 
     
     
         16 . The method according to  claim 1 , wherein the chemical composition of the steel comprises, by weight percent, 0.60%≤Mn≤1.5% and 0.5%≤Cr≤2.2%, or wherein the chemical composition of the steel comprises, by weight percent, 0.60%≤Mn≤1.60% and 1.97%≤Cr≤2.2%. 
     
     
         17 . The method according to  claim 1 , wherein the head of the rail has a tensile strength of at least 1300 MPa, a yield strength of at least 1000 MPa, a total elongation of at least 13%, and a hardness in a rolling surface of the head of at least 420 HB. 
     
     
         18 . The method according to  claim 1 , wherein the head of the rail has a tensile strength comprised between 1300 MPa and 1452 MPa. 
     
     
         19 . The method according to  claim 1 , wherein the rail is an uncoated rail.