Coated steel sheet and high strength press hardened steel part and method of manufacturing the same

Abstract

A coated steel sheet and press hardened steel part having a composition including, by weight percent: C 0.15-0.25%, Mn 0.5-1.8%, Si 0.1-1.25%, Al 0.01-0.1%, Cr 0.1-1.0%, Ti 0.01-0.1%, B 0.001-0.004%, P≤0.020%, S≤0.010%, N≤0.010% the remainder of the composition being iron and unavoidable impurities resulting from the smelting. The press hardened steel part includes a bulk having a microstructure including, in surface fraction, more than 95% of martensite and less than 5% of bainite, a coating layer at the surface of the steel part, a ferritic interdiffusion layer between the coating layer and the bulk, and a ratio between the ferritic grain width in the interdiffusion layer GWint over prior austenite grain size in the bulk PAGSbulk, satisfying following equation (GWint/PAGSbulk)−1≥30%.

What is claimed is: 
     
         1 . A coated steel sheet made of a steel having a composition comprising, by weight percent:
 C: 0.15-0.25%   Mn: 0.5-1.8%   Si: 0.1-1.25%   Al: 0.01-0.1%   Cr: 0.1-1.0%   Ti: 0.01-0.1%   B: 0.001-0.004%   P≤0.020%   S≤0.010%   N≤0.010%   and comprising optionally one or more of the following elements, by weight percent:   Mo≤0.40%   Nb≤0.08%   Ca≤0.1%   a remainder of the composition being iron and unavoidable impurities resulting from processing;   the coated steel sheet comprising from a bulk to the surface of the coated steel sheet:   the bulk with a microstructure comprising, in surface fraction, from 60% to 90% of ferrite,   a rest being martensite-austenite islands, pearlite or bainite,   a decarburized layer topping the bulk and comprising in upper part a ferrite layer having a thickness from 25 μm to 80 μm; and   a coating layer made of aluminum or aluminum alloy.   
     
     
         2 . A method for producing a coated steel sheet, the method comprising the following successive steps:
 casting a steel to obtain a slab, the steel having a composition comprising, by weight percent:
 C: 0.15-0.25% 
 Mn: 0.5-1.8% 
 Si: 0.1-1.25% 
 Al: 0.01-0.1% 
 Cr: 0.1-1.0% 
 Ti: 0.01-0.1% 
 B: 0.001-0.004% 
 P≤0.020% 
 S≤0.010% 
 N≤0.010% 
   and comprising optionally one or more of the following elements, by weight percent:
 Mo≤0.40% 
 Nb≤0.08% 
 Ca≤0.1% 
   a remainder of the composition being iron and unavoidable impurities resulting from processing;   reheating the slab at a temperature T reheat  of 1100° C. to 1300° C.;   hot rolling the reheated slab at a finish hot rolling temperature of 800° C. to 950° C.;   coiling the hot rolled steel sheet at a coiling temperature T coil lower than 670° C. to obtain a coiled steel sheet;   optionally pickling the coiled steel sheet;   cold rolling the coiled steel sheet to obtain a cold rolled steel sheet with a cold-rolling reduction ratio from 20% to 80%;   heating the hot rolled steel sheet or the cold rolled steel sheet to an annealing temperature T A  of 700° C. to 850° C. and maintaining the steel sheet at the temperature T A  for a holding time t A  of 10 s to 1200 s, to obtain an annealed steel sheet, the atmosphere comprising from 0% to 15% of H2 and having a dew point Top1 strictly higher than −10° C. and below or equal to +20° C.;   cooling the annealed steel sheet to a temperature range from 560° C. to 700° C.;   coating the annealed steel sheet with aluminum or with an aluminum alloy coating; and   cooling the coated steel sheet to room temperature.   
     
     
         3 . A press hardened steel part, the steel part having a composition comprising, by weight percent:
 C: 0.15-0.25%   Mn: 0.5-1.8%   Si: 0.1-1.25%   Al: 0.01-0.1%   Cr: 0.1-1.0%   Ti: 0.01-0.1%   B: 0.001-0.004%   P≤0.020%   S≤0.010%   N≤0.010%   and comprising optionally one or more of the following elements, by weight percent:   Mo≤0.40%   Nb≤0.08%   Ca≤0.1%   the remainder of the composition being iron and unavoidable impurities resulting from processing;   the steel part comprising successively from the bulk to the surface of the steel part:   a bulk,   a ferritic interdiffusion layer,   a coating layer based on aluminum,   wherein a ratio between the ferritic grain width in the interdiffusion layer GW int  over prior austenite grain size in the bulk PAGS bulk , satisfies the following equation:   
       
         
           
             
               
                 
                   ( 
                   
                     
                       
                         GW 
                         int 
                       
                       / 
                       PAG 
                     
                     ⁢ 
                     
                       S 
                       bulk 
                     
                   
                   ) 
                 
                 - 
                 1 
               
               ≥ 
               
                 30 
                 ⁢ 
                 
                   % 
                   . 
                 
               
             
           
         
       
     
     
         4 . The press hardened steel part as recited in  claim 3  wherein the press hardened steel part includes a layer of martensite with a carbon gradient between the bulk and the ferritic interdiffusion layer. 
     
     
         5 . The press hardened steel part as recited in  claim 3  wherein the press hardened steel part has a tensile strength TS above or equal to 1350 MPa and a bending angle higher than 70°. 
     
     
         6 . The press hardened steel part as recited in  claim 5  wherein the press hardened steel part has a yield strength YS above or equal to 1000 MPa. 
     
     
         7 . A process for manufacturing the press hardened steel part as recited in  claim 5 , the process comprising the following successive steps:
 providing a steel sheet according to  claim 1 ;   cutting the steel sheet to a predetermined shape, so as to obtain a steel blank;   heating the steel blank to a temperature of 880° C. to 950° C. for 108 to 900 s to obtain a heated steel blank;   transferring the heated steel blank to a forming press;   hot-forming the heated blank in the forming press to obtain a formed part; and   die-quenching the formed part.