Steel section having a thickness of at least 100mm and method of manufacturing the same

Abstract

A steel section has a web central portion connected on each side to a flange portion having a thickness of at least 100 mm. The steel section microstructure includes at least one kind of vanadium precipitates possibly comprising also one or more metal chosen among chromium, manganese and iron, the precipitates being chosen among nitrides, carbides, carbo-nitrides or any combination of them, more than 70% of such precipitates having a mean diameter below 6 nm. It also deals with a manufacturing method thereof.

1 - 14 . (canceled) 
     
     
         15 . A steel section comprising a web central portion connected on each side to flange portions having a thickness of at least 100 mm, the steel section having a composition comprising, in weight percentage:
 C: 0.06-0.16%   Mn: 1.10-2.00%   Si: 0.10-0.40%   Cu: 0.001-0.50%   Ni: 0.001-0.30%   Cr: 0.001-0.50%   Mo: 0.001-0.20%   V: 0.06-0.12%   N: 0.0050%-0.0200%   Al≤0.040%   P≤0.040%   S≤0.030%   
       and optionally including at least one of the following elements, in weight percentage:
 Ti<0.005% 
 Nb≤0.05% 
 
       a reminder being iron and impurities resulting from processing;
 a microstructure of the steel structure including vanadium precipitates, the vanadium precipitates optionally including at least one metal chosen from the group consisting of chromium, manganese and iron, the vanadium precipitates being chosen among nitrides, carbides, carbo-nitrides or any combination of them, more than 70% of the vanadium precipitates having a mean diameter below 6 nm. 
 
     
     
         16 . The steel section as recited in  claim 15  wherein such section composition is such that the following relationship is fulfilled:
   0.4≤CEV≤0.6 with CEV=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15
 
 
     
     
         17 . The steel section as recited in  claim 15  wherein the ratio of vanadium to nitrogen amounts is between 2.5 and 7. 
     
     
         18 . The steel section as recited in  claim 15  wherein the microstructure of the flanges portions includes, from surface to core, a hardened zone including tempered martensite and optionally bainite and a core zone including ferrite and pearlite. 
     
     
         19 . The steel section as recited in  claim 15  wherein such steel section includes portions having a mean density of the vanadium precipitates of at least 500 precipitates per mm 2 . 
     
     
         20 . The steel section as recited in  claim 15  wherein at least part of the vanadium precipitates is arranged in regularly spaced bands. 
     
     
         21 . The steel section as recited in  claim 20  wherein more than 80% of such regularly spaced precipitates have a mean diameter below 3 nm. 
     
     
         22 . The steel section as recited in  claim 21  wherein such regularly spaced precipitates include at least vanadium and chromium. 
     
     
         23 . The steel section as recited in  claim 15  wherein at least part of such precipitates is randomly distributed in a ferrite phase located in a core zone of the steel section. 
     
     
         24 . The steel section as recited in  claim 23  wherein more than 80% of the randomly distributed vanadium precipitates have a mean diameter between 3.5 and 6 nm. 
     
     
         25 . The steel section as recited in  claim 24  wherein the randomly distributed vanadium precipitates include at least vanadium, chromium and iron. 
     
     
         26 . The steel section as recited in  claim 15  wherein the vanadium precipitates are located in a core zone of the steel section. 
     
     
         27 . The steel section as recited in  claim 15  wherein the flange portions have a thickness of at most 140 mm. 
     
     
         28 . A method of manufacturing of a steel section comprising the following steps:
 feeding a steel semi-product comprising, in weight percentage:
 C: 0.06-0.16% 
 Mn: 1.10-2.00% 
 Si: 0.10-0.40% 
 Cu: 0.001-0.50% 
 Ni: 0.001-0.30% 
 Cr: 0.001-0.50% 
 Mo: 0.001-0.20% 
 V: 0.06-0.12% 
 N: 0.0050%-0.0200% 
 Al≤0.040% 
 P≤0.040% 
 S≤0.030% 
    and optionally including at least one of the following elements, in weight percentage:
 Ti≤0.005%
 Nb≤0.05% 
 
    a reminder being iron and impurities resulting from processing;   reheating the steel semi-product at a temperature above 1000° C. and hot rolling the steel semi-product with a final rolling temperature of at least 850° C., to obtain a hot rolled steel section; and   cooling the hot rolled steel section so as to produce martensitic or bainitic quenching of the surface layer of all or part of the product, a non-quenched portion of the rolled product remaining at a temperature high enough to enable a self-tempering of the quenched surface layer of martensite or bainite and to transform the austenite into ferrite and carbides in a core part of the section during the subsequent cooling, a maximum temperature of the tempered surface of the product after quenching being 450 to 650° C.