Flat steel product having an al-coating, process for production thereof, steel component and process for production thereof

Abstract

The invention relates to a flat steel product for hot forming, consisting of a steel substrate which consists of a steel having 0.1-3 wt % of Mn and optionally up to 0.01 wt % of B, and of an Al-based protective coating applied to the steel substrate. The iron-free mass fraction in the protective coating of Mg as additional alloy constituent adds up to less than 2.50% Mg. In addition, the iron-free mass fraction in the protective coating of Mn as additional alloy constituent adds up to more than 0.30% Mn and the iron-free mass fraction in the protective coating of Si as additional alloy constituent adds up to less than 1.80%.

1 . A flat steel product for hot forming, comprising a steel substrate comprising a steel which, as well as iron and unavoidable impurities, comprises (in wt %):
 C: 0.04-0.45 wt %,   Si: 0.02-1.2 wt %,   Mn: 0.5-2.6 wt %,   Al: 0.02-1.0 wt %,   P: ≤0.05 wt %,   S: ≤0.02 wt %,   N: ≤0.02 wt %,   Sn: ≤0.03 wt %   As: ≤0.01 wt %   Ca: ≤0.005 wt %   and of an Al-based protective coating applied to the steel substrate, where the iron-free mass fraction of the aluminum-based protective coating includes a total of up to 10% of additional alloy constituents and the rest of the iron-free mass fraction is formed from aluminum, where the iron-free mass fraction in the protective coating of Mg as additional alloy constituent adds up to less than 0.10-0.50 wt % of Mg, wherein the iron-free mass fraction in the protective coating of Mn as additional alloy constituent adds up to more than 0.30% Mn and the iron-free mass fraction in the protective coating of Si as additional alloy constituent adds up to less than 1.80%.   
     
     
         2 . The flat steel product as claimed in  claim 1 , wherein the iron-free mass fraction in the protective coating of Mn as additional alloy constituent adds up to more than 0.80% Mn. 
     
     
         3 . The flat steel product as claimed in  claim 2  wherein the iron-free mass fraction in the protective coating of Si as additional alloy constituent adds up to less than 0.80% Si. 
     
     
         4 . The flat steel product as claimed in  claim 3  wherein the iron-free mass fraction in the protective coating of Mn as additional alloy constituent adds up to less than 1.80% Mn. 
     
     
         5 . A metallurgically bonded steel component, having a thickness of the Fe seam of greater than 2.5 μm, comprising a steel substrate comprising a steel which, as well as iron and unavoidable impurities, comprises (in wt %):
 C: 0.04-0.45 wt %, 
 Si: 0.02-1.2 wt %, 
 Mn: 0.5-2.6 wt %, 
 Al: 0.02-1.0 wt %, 
 P: ≤0.05 wt %, 
 S: ≤0.02 wt %, 
 N: ≤0.02 wt %, 
 Sn: ≤0.03 wt % 
 As: ≤0.01 wt % 
 Ca: ≤0.005 wt % 
 and of an Al-based protective coating (15) applied to the steel substrate, where the iron-free mass fraction of the aluminum-based protective coating optionally includes a total of up to 10% of additional alloy constituents and the rest of the iron-free mass fraction is formed from aluminum, where the iron-free mass fraction in the protective coating of Mg as additional alloy constituent adds up to less than 0.10-0.50 wt % of Mg, wherein the iron-free mass fraction in the protective coating of Mn as additional alloy constituent adds up to more than 0.30% Mn and the iron-free mass fraction in the protective coating of Si as additional alloy constituent adds up to less than 1.80%. 
 
     
     
         6 . The steel component as claimed in  claim 5  wherein the steel component is an automobile component, comprising one of a bumper beam, a bumper reinforcement, a door reinforcement, a B pillar reinforcement, an A pillar reinforcement, a roof frame or a sill. 
     
     
         7 . A method of producing a flat steel product having the characteristics as claimed in  claim 4 , comprising the following steps:
 providing a steel substrate composed of a steel which, as well as iron and unavoidable impurities, consists (in wt %) of   C: 0.04-0.45 wt %,   Si: 0.02-1.2 wt %,   Mn: 0.5-2.6 wt %,   Al: 0.02-1.0 wt %,   P: ≤0.05 wt %,   S: ≤0.02 wt %,   N: ≤0.02 wt %,   Sn: ≤0.03 wt %   As: ≤0.01 wt %   Ca: ≤0.005 wt %   coating the steel substrate with an Al-based protective coating, where the iron-free mass fraction of the aluminum-based protective coating includes a total of up to 10% of additional alloy constituents and the remaining iron-free mass fraction is formed from aluminum, where the iron-free mass fraction in the protective coating of Mg as additional alloy constituent adds up to less than 0.10-0.50 wt % of Mg and where the iron-free mass fraction in the protective coating of Mn as additional alloy constituent adds up to more than 0.30% Mn and the iron-free mass fraction in the protective coating of Si as additional alloy constituent adds up to less than 1.80%.   
     
     
         8 . The method as claimed in  claim 7 , wherein the protective coating is applied to the steel substrate by hot dip coating. 
     
     
         9 . The method as claimed in  claim 8 , wherein the flat steel product is prealloyed immediately after coating by keeping it at a prealloying temperature of 500° C.-600° C. for a prealloying time of 15-30 seconds. 
     
     
         10 . A method of producing a steel component having the characteristics as claimed in  claim 6 , comprising the following steps
 producing a flat steel product by employing a method specified in  claim 8 ;   annealing the flat steel product in a furnace preheated to a temperature T for an annealing time t within a polygon formed by the points ABCD for flat steel products having a thickness between 0.7 mm and 1.5 mm, or in a furnace preheated to a temperature T for an annealing time t within a polygon formed by the points EFGH for flat steel products having a thickness between 1.5 mm and 3.0 mm, so as to form an Fe seam having a thickness greater than 2.5 μm, where the points ABCD/EFGH are as follows:   
       
         
           
                 
                 
                 
               
                     
                 
                   Point 
                   Temperature [° C.] 
                   Annealing time t [min] 
                 
                     
                 
                     
                 
                 
                 
                 
               
                   A 
                   930 
                   1.5 
                 
                   B 
                   930 
                   7 
                 
                   C 
                   880 
                   12 
                 
                   D 
                   880 
                   2.5 
                 
                   E 
                   940 
                   2.5 
                 
                   F 
                   940 
                   9 
                 
                   G 
                   900 
                   13 
                 
                   H 
                   900 
                   4 
                 
                     
                 
             
                
                
                
               
               
                
               
            
             
                
                
                
                
                
                
                
                
                
               
            
           
         
         hot forming the flat steel product to give the steel component. 
       
     
     
         11 . The method as claimed in  claim 10 , wherein in that the flat steel product is taken from the furnace after the annealing time t at a heating temperature, where the heating temperature is sufficiently high that the flat steel product at the start of forming has a hot forming temperature at which the microstructure of the steel substrate has been fully or partly converted to austenitic microstructure, and in that the flat steel product is quenched after the forming or in the course of forming, such that hard microstructure is formed in the microstructure of the steel substrate of the flat steel product. 
     
     
         12 . The method as claimed in  claim 11 , wherein the heating temperature is between 880° C. to 950° C. 
     
     
         13 . The flat steel product of  claim 1  wherein the steel substrate further comprises:
 one or more of the elements “Cr, B, Mo, Ni, Cu, Nb, Ti, V” in the following contents: 
 Cr: 0.08-1.0 wt %, 
 B: 0.001-0.005 wt % 
 Mo: ≤0.5 wt % 
 Ni: ≤0.5 wt % 
 Cu ≤0.2 wt % 
 Nb: 0.02-0.08 wt %, 
 Ti: 0.01-0.08 wt % 
 V: ≤0.1 wt %. 
 
     
     
         14 . The steel component of  claim 5  wherein the steel substrate further comprises:
 one or more of the elements “Cr, B, Mo, Ni, Cu, Nb, Ti, V” in the following contents: 
 Cr: 0.08-1.0 wt %, 
 B: 0.001-0.005 wt % 
 Mo: ≤0.5 wt % 
 Ni: ≤0.5 wt % 
 Cu ≤0.2 wt % 
 Nb: 0.02-0.08 wt %, 
 Ti: 0.01-0.08 wt % 
 V: ≤0.1 wt %.