Method for Fabricating Steel Sheet for Press Hardening, and Parts Obtained by this Method

Abstract

The present invention provides a rolled steel sheet, for press hardening, for which the chemical composition includes, with contents expressed by weight, 0.24%≦C≦0.38%, 0.40%≦Mn≦3%, 0.10%≦Si≦0.70%, 0.015%≦Al≦0.070%, 0%≦Cr≦2%, 0.25%≦Ni≦2%, 0.015%≦Ti≦0.10%, 0%≦Nb≦0.060%, 0.0005%≦B≦0.0040%, 0.003%≦N≦0.010%, 0.0001%≦S≦0.005%, 0.0001%≦P≦0.025%, it being understood that the titanium and nitrogen content satisfy: Ti/N>3.42, and that the carbon, manganese, chromium and silicon content satisfy: 2.6   C + Mn 5.3 + Cr 13 + Si 15 ≥ 1.1  % , with the chemical composition optionally including one or more of the following elements: 0.05%≦Mo≦0.65%, 0.001%≦W≦0.30%, 0.0005%≦Ca≦0.005%, with the remainder made up of iron and inevitable impurities coming from preparation. The sheet includes a nickel content Ni surf at any point of the steel near the surface of said sheet over a depth Δ, such that Ni surf >Ni nom , where Ni nom designates the nominal nickel content of the steel, and such that Ni max designates the maximum nickel content within Δ: ( Ni max + Ni nom ) 2 × ( Δ ) ≥ 0.6 and such that: ( Ni max + Ni nom ) Δ ≥ 0.01 , with the depth Δ expressed in microns and the Ni max and Ni nom contents expressed in percentages by weight.

1 - 54 . (canceled) 
     
     
         55 : A fabrication method for a hot rolled steel sheet, comprising the steps of:
 casting an intermediate product with a chemical composition including, with contents being expressed by weight:
 0.24%≦C≦0.38%; 
 0.40%≦Mn≦3%; 
 0.10%≦Si≦0.70%; 
 0.015%≦Al≦0.070%; 
 0%≦Cr≦2%; 
 0.25%≦Ni≦2%; 
 0.015%≦Ti≦0.10%; 
 0%≦Nb≦0.060%; 
 0.0005%≦B≦0.0040%; 
 0.003%≦N≦0.010%; 
 0.0001%≦S≦0.005%; and 
 0.0001%≦P≦0.025%; 
 the titanium and nitrogen contents satisfying Ti/N>3.42; 
 the carbon, manganese, chromium and silicon contents satisfying: 
   
       
         
           
             
               
                 
                   2.6 
                    
                   C 
                 
                 + 
                 
                   Mn 
                   5.3 
                 
                 + 
                 
                   Cr 
                   13 
                 
                 + 
                 
                   Si 
                   15 
                 
               
               ≥ 
               
                 1.1 
                  
                 % 
               
             
           
         
         
           ;and a remainder of the chemical composition being made up of iron and inevitable impurities resulting from processing; 
         
         reheating the intermediate product to a temperature between 1250° C. and 1300° C. for a hold time between 20 and 45 minutes; 
         hot rolling the intermediate product until an end of rolling temperature, ERT, between 825° C. and 950° C. to obtain a heat rolled sheet; 
         coiling the hot rolled sheet at a temperature between 500° C. and 750° C. to obtain a hot rolled and coiled sheet; and 
         pickling an oxide layer formed during the preceding steps. 
       
     
     
         56 : A fabrication method for a cold rolled and annealed sheet, comprising the steps of:
 supplying a hot rolled sheet, coiled and pickled, fabricated by the method according to  claim 55 ; and   cold rolling the hot rolled, coiled and pickled sheet to obtain a cold rolled sheet; and   annealing the cold rolled sheet at a temperature between 740° C. and 820° to obtain a cold rolled and annealed sheet.   
     
     
         57 : A fabrication method for a precoated sheet, comprising the steps of:
 supplying a rolled sheet fabricated according  claim 55 ; and   performing a continuous hot-dip pre-coating, the precoating being aluminum or an aluminum or aluminum-based alloy, or zinc or a zinc or zinc-based alloy.   
     
     
         58 : A fabrication method for a precoated and pre-alloyed sheet comprising:
 supplying a rolled sheet according to  claim 55 ;   performing a continuous hot-dip precoating with an aluminum or aluminum-based alloy; and   performing a thermal pretreatment of the precoated sheet at a temperature θ 1  between 620° C. and 680° C. for a holding time t 1  between 6 and 15 hours, such that the pre-coating no longer contains free aluminum of phase τ 5  of type Fe 3 Si 2 Al 12 , and τ 6  of type Fe 2 Si 2 Al 9 , and such that an austenitic transformation does not occur in the steel substrate, the pretreatment being done in a furnace under hydrogen and nitrogen atmosphere.   
     
     
         59 : A fabrication method for a press hardened part comprising the steps of:
 supplying a sheet fabricated by a method according to  claim 55 ;   cutting the sheet to obtain a blank;   heating the blank to a temperature between 810° C. and 950° C. to obtain a fully austenitic structure in the steel;   transferring the blank inside a press;   hot stamping the blank to obtain a part;   holding the part inside the press to obtain a hardening by martensitic transformation of the austenitic structure.   
     
     
         60 : The fabrication method according to  claim 59 , wherein the steps are performed successively. 
     
     
         61 : The fabrication method according to  claim 59 , further comprising the step of deforming the blank by cold stamping after the step of cutting the sheet. 
     
     
         62 - 63 . (canceled)