Process for manufacturing a galvannealed steel sheet by dff regulation

Abstract

The invention deals with a process for manufacturing a hot-dip galvannealed steel sheet having a TRIP microstructure, and comprising, by % by weight, 0.01≦C≦0.22%, 0.50≦Mn≦2.0%, 0.5<Si≦2.0%, 0.005≦Al≦2.0%, Mo<0.01%, Cr≦1.0%, P<0.02%, Ti≦0.20%, V≦0.40%, Ni≦1.0%, Nb≦0.20%, the balance of the composition being iron and unavoidable impurities resulting from the smelting, said process comprising the steps consisting in:—oxidizing said steel sheet in order to form a layer of iron oxide on the surface of the steel sheet, and to form an. internal oxide of at least one type of oxide selected from the group consisting of Si oxide, Mn oxide, Al oxide, complex oxide comprising Si and Mn, complex oxide comprising Si and Al complex oxide comprising Al and Mn, and complex comprising Si, Mn and Al,—reducing said oxidized steel sheet in order to reduce the layer of iron oxide,—hot-dip galvanising said reduced steel sheet to form a zinc-based coated steel sheet, and—subjecting said zinc-based coated steel sheet to an alloying treatment to form a galvannealed steel sheet.

1 . Process for manufacturing a hot-dip galvannealed steel sheet having a TRIP microstructure comprising ferrite, residual austenite and optionally martensite and/or bainite, said process comprising the steps consisting in:
 providing a steel sheet whose composition comprises, by weight:
   0.01≦C≦0.22% 
   0.50≦Mn≦2.0% 
   0.5<Si≦2.0% 
   0.0055≦Al≦2.0 
   Mo<0.01% 
   Cr≦1.0% 
   P<0.02% 
   Ti≦0.20% 
   V≦0.40% 
   Ni≦1.0% 
   Nb≦0.20%, 
   the balance of the composition being iron and unavoidable impurities resulting from the smelting,   oxidizing said steel sheet in order to form a layer of iron oxide on the surface of the steel sheet, and to form an internal oxide of at least one type of oxide selected from the group consisting of Si oxide, Mn oxide, Al oxide, complex oxide comprising Si and Mn, complex oxide comprising Si and Al, complex oxide comprising Al and Mn, and complex comprising Si, Mn and Al,   reducing said oxidized steel sheet in order to reduce the layer of iron oxide,   hot-dip galvanising said reduced steel sheet to form a zinc-based coated steel sheet, and   subjecting said zinc-based coated steel sheet to an alloying treatment to form a galvannealed steel sheet.   
     
     
         2 . Process according to  claim 1 , wherein said steel sheet comprises, in % by weight, P<0.010%. 
     
     
         3 . Process according to  claim 1  or  2 , wherein said steel sheet comprises, in % by weight, Mo≦0.006%. 
     
     
         4 . Process according to any one of  claims 1  to  3 , wherein the oxidation of the steel sheet is performed by heating it from ambient temperature to temperature T1, in a direct flame furnace where the atmosphere comprises air and fuel with an air-to-fuel ratio between 1.0 and 1.2. 
     
     
         5 . Process according to  claim 4 , wherein said temperature T1 is between 680 to 800° C. 
     
     
         6 . Process according to any one of  claims 1  to  5 , wherein the reduction of said oxidized steel sheet consists of a heat treatment performed in an atmosphere comprising more than 15% by volume of hydrogen, the balance being nitrogen and unavoidable impurities, said heat treatment comprising a heating phase from the temperature T1 to a soaking temperature T2, a soaking phase at said soaking temperature T2 for a soaking time t2, and a cooling phase from said soaking temperature T2 to a cooling temperature T3. 
     
     
         7 . Process according to  claims 6 , wherein said soaking temperature T2 is between 770 and 850° C. 
     
     
         8 . Process according to  claim 6  or  7 , wherein said soaking time t2 is between 20 and 180 s. 
     
     
         9 . Process according to any one of  claims 6  to  8 , wherein said cooling temperature T3 is between 460 to 510° C. 
     
     
         10 . Process according to any one of  claims 5  to  9 , wherein said reduction is performed in a radiant tube furnace or in a resistance furnace. 
     
     
         11 . Process according to any one of  claims 1  to  10 , wherein the hot-dip galvanizing is performed by hot-dipping said reduced steel sheet in a molten bath comprising 0.08 to 0.135% by weight of aluminium, the balance being zinc and unavoidable impurities. 
     
     
         12 . Process according to  claim 11 , wherein the temperature of said molten bath is between 450 and 500° C. 
     
     
         13 . Process according to any one of  claims 1  to  12 , wherein said alloying treatment is performed by heating said zinc-based coated steel sheet at a temperature T4 between 460 and 510° C. for a soaking time t4 between 10 and 30 s.