Sheet Metal Part Formed from a Steel Having a High Tensile Strength and Method for Manufacturing Said Sheet Metal Part

Abstract

A sheet metal part having a tensile strength Rm≥1000 MPa and a bending angle >70° formed from a flat steel product including, in % by weight: C: 0.10-0.30%, Si: 0.5-2.0%, Mn: 0.5-2.4%, Al: 0.01-0.2%, Cr: 0.005-1.5%, P: 0.01-0.1%, and optionally one or more of Ti, Nb, V, B, Ni, Cu, Mo, and W, with Ti: 0.005-0.1%, Nb: 0.005-0.1%, V: 0.001-0.2%, B: 0.0005-0.015%, Ni: 0.05-0.4%, Cu: 0.01-0.8%, Mo: 0.01-1.0&, and W: 0.001-1.0%, and remainder iron and unavoidable impurities, wherein the structure of the sheet metal part is 40-100% by area plate-shaped bainite, 70-95% of which is made of ferrite, 2-30% of high carbon phases that are plate-shaped with the remainder made up of other components and the remainder of the structure consists of <40% by area of the total structure of non-plate-shaped bainite, of which is made of ferrite, 2-30% of high carbon phases and <5% of other components. Also, a method for manufacturing the sheet metal part.

1 . A sheet metal part having a tensile strength Rm of at least 1000 MPa and a bending angle of more than 70° that is made from a flat steel product comprising (in % by weight):
 C: 0.10-0.30%, 
 Si: 0.5-2.0%, 
 Mn: 0.5-2.4%, 
 Al: 0.01-0.2%, 
 Cr: 0.005 -1.5%, 
 P: 0.01 -0.1%, 
 as well as, in each case optionally, additionally of one or more elements from the group consisting of Nb, V, B, Ni, Cu, Mo, and W provided that 
 Ti: 0.005-0.1%, 
 Nb: 0.005-0.1%, 
 V: 0.001-0.2%, 
 B: 0.0005-0.01%, 
 Ni: 0.05-0.4%, 
 Cu: 0.01-0.8%, 
 Mo: 0.01-1.0%, 
 W: 0.001-1.0%, 
 and the remainder of iron and unavoidable impurities, wherein the unavoidable impurities comprise less than 0.05% S and less than 0.01% N,
 wherein the structure of the sheet metal part is 40-100% by area plate-shaped bainite, which is formed from 
 
 70-95% ferrite, 
 2-30% high carbon phases which are designed to be at least 70% plate-shaped with a plate length PL of at least 200 nm with a ratio of the plate length PL to the plate width PB of the plate-shaped high carbon phase PL/PB of at least 1.7 and are arranged at a distance of 50 nm to 2 μm, and 
 the remainder of less than 5% other components, 
 wherein the remaining structure of the sheet metal part which is not taken up by the plate-shaped bainite consists of up to 40% by area of the total structure of non-plate-shaped marked bainite, 
 which is formed 70-95% of ferrite, 
 2-30% of high carbon phases and 
 less than 5% of other components, 
 wherein the sum of the shares of the plate-shaped and non-plate-shaped, bainite in the structure of the sheet metal part makes up at least 60% by area, 
 wherein the remaining austenite content of the structure of the sheet metal part is 2-20% by volume, 
 and 
 wherein the remainder of the structure of the sheet metal part not taken up by the bainite components consists of one or more components selected from the group consisting of martensitic or austenitic components, proeutectoid ferrite, iron carbide, iron nitride, transition metal carbide, transition metal nitride, non-metal carbide, non-metal nitride, metal or non-metal inclusions, sulfide and other unavoidable impurities. 
 
     
     
         2 . The sheet metal part according to  claim 1 , wherein the C content % C, the Mn content % Mn, the Mo content % Mo, the Cr content % Cr, the Ni content % Ni and the Cu content % Cu are set depending on the B content of the steel in each case in % by weight such that the activation energy Qb of the bainite formation is <45 kJ, wherein the following applies for Qb in B contents of up to 0.0005% by weight:
   Qb[kJ]=(90*% C+10*(% Mn+% Mo)+2*(% Cr+% Ni)+1*% Cu)[kJ/% by weight],   
       and the following applies for B contents of more than 0.0005% by weight:
 Qb [kJ] =(90*% C+10*(% Mn+% Mo)+2*(% Cr+% Ni)+1*% Cu+2)[kJ/% by weight]. 
 
     
     
         3 . The sheet metal part according to  claim 1 , wherein the flat steel product used to form the sheet metal contains (in % by weight) 0.13-0.25% C, 0.6-1.4% Si, 0.9-1.8% Mn, 0.01-0.1% Al and 0.15-0.75% Cr. 
     
     
         4 . The sheet metal part according to  claim 1 , wherein the flat steel product used to form the sheet metal part contains (in % by weight) 0.15-0.20% C, 0.7-1.0% Si and 1.1-1.6% Mn. 
     
     
         5 . The sheet metal part according to  claim 1 , wherein the sheet metal part is provided with a metallic corrosion protection coating. 
     
     
         6 . The sheet metal part according to  claim 5 , wherein the metallic corrosion protection coating consists of (in % by weight) 3-15% Si, 1-3.5 Fe, optionally up to 40% Zn, up to 0.5% of one or more alkaline or alkaline earth metals, optionally up to 1% Mg, with the remainder Al and unavoidable impurities. 
     
     
         7 . The sheet metal part according to  claim 1 , wherein the sheet metal part is hot stamped. 
     
     
         8 . A method for producing a sheet metal part, comprising the following steps:
 a) Provision of a cut metal sheet consisting of a steel of the following composition (in % by weight):   C: 0.10-0.30%,   Si: 0.5-2.0%,   Mn: 0.5-2.4%,   Al: 0.01-0.2%,   Cr: 0.005-1.5%,   P: 0.01-0.1%,   as well as, in each case optionally, additionally of one or more elements from the group . Nb, V, B, Ni, Cu, Mo, and W, provided that   Ni: 0.005-0.1%,   Nb: 0.005-0.1%,   V: 0.001-0.2%,   B: 0.0005-0.01%,   Ni: 0.05-0.4%,   Cu: 0.01-0.8%,   Mo: 0.01-1.0%,   W: 0.001-1.0%,   and the remainder of iron or unavoidable impurities, wherein the unavoidable impurities comprise less than 0.05% S and less than 0.01% N;   b) heating the cut sheet such that at least 30% of the volume of the cut sheet, when inserted into a forming tool intended for hot press shaping, has a temperature T_Aust above the Ac1 temperature, wherein the Ac1 temperature is determined according to the formula
   Ac1=[739−22*% C−7*% Mn+2*% Si+14*% Cr+13*% Mo+13*% Ni]° C.
 
   
       where % C=C content, % Si=Si content, % Mn=Mn content, % Cr=Cr content, % Mo=Mo content and % Ni=Ni content of the respective steel of the cut sheet;
 c) inserting the heated cut sheet into the forming tool tempered to a tool temperature T_WZ of 200-430° C., wherein the transfer time t_Trans needed to remove and insert the cut sheet is a maximum of 20 s; 
 d) hot pressing the cut sheet into the sheet metal part, wherein over the course of the hot pressing the cut sheet is cooled for a time t_WZ of 1-50 s at a cooling speed r_WZ of more than 10 K/s to a cooling stop temperature T_coolstop and optionally held there; 
 e) removing the sheet metal part cooled to the cooling stop temperature T_coolstop from the tool; 
 f1) optionally, holding the sheet metal part at a holding temperature T_Halt of 300-450° C. for a holding time t_Halt of up to 100 s; 
 f2) optionally, heating the sheet metal part to a homogenization temperature of 380-500° C. within 1-10 s; 
 f3) optionally, further reshaping of the sheet metal part; 
 g) optionally, trimming the sheet metal part; 
 h) optionally, the sheet metal part to a cooling temperature T_AB of less than 200° C. within a cooling time t_AB of 0.5-200 s. 
 
     
     
         9 . The method according to  claim 8 , wherein the following applies for the temperature T_Aust reached in work step b):
   Ac3<T_Aust≤1250° C.,
     wherein     Ac3=[902-225*% C+19*% Si−11*% Mn−5*% Cr+13*% Mo−20*% Ni+55*% V]° C.
   
       where % C=C content, % Si=Si content, % Mn=Mn content, % Cr=Cr content, % Mo=Mo content % Ni=Ni content and % V=V content of the respective steel of the cut sheet. 
     
     
         10 . The method according to  claim 8 , wherein the cut sheet is heated to the temperature T_Aust in work step b). 
     
     
         11 . The method according to either  claim 9 , wherein for the heating in work step b) a heating time t_Aust is
   1000s/(T_Aust/° C.−Ac3/° C.+10){circumflex over ( )}2≤t_Aust≤1000 s
   
     
     
         12 . The method according to  claim 8 , wherein the cooling speed r_WZ in work step d) is more than 20 K/s. 
     
     
         13 . The method according to  claim 8 , wherein the time t_WZ in work step d) is a maximum of 20 s. 
     
     
         14 . The method according to  claim 8 , wherein the temperature T_WZ of the tool on insertion of the cut sheet depending on the cooling stop temperature T_coolstop and the thickness D of the cut sheet to be formed into the sheet metal part is determined as follows: 
       
         
           
             
               
                 T_coolstop 
                 - 
                 
                   
                     100 
                     ∘ 
                   
                    
                   
                       
                   
                    
                   
                     C 
                     . 
                   
                   * 
                   
                     
                       ( 
                       
                         D 
                         
                           1.4 
                            
                           
                               
                           
                            
                           mm 
                         
                       
                       ) 
                     
                     
                       1 
                       2 
                     
                   
                 
               
               < 
               T_WZ 
               < 
               
                 T_coolstop 
                 - 
                 
                   
                     20 
                     ∘ 
                   
                    
                   
                       
                   
                    
                   
                     C 
                     . 
                   
                   * 
                   
                     
                       ( 
                       
                         D 
                         
                           1.4 
                            
                           
                               
                           
                            
                           mm 
                         
                       
                       ) 
                     
                     
                       1 
                       2 
                     
                   
                 
               
             
           
         
       
     
     
         15 . The method according to  claim 8 , wherein the temperature T_coolstop of the sheet metal part on removal from the tool is 300-450° C. 
     
     
         16 . The method according to  claim 8 , wherein the cooling takes place in air in work step h). 
     
     
         17 . A body of a vehicle comprising a sheet metal part according to  claim 1 . 
     
     
         18 . A chassis of a vehicle comprising a sheet metal part according to  claim 1 . 
     
     
         19 . A sheet metal part according to  claim 1  that is made from a flat steel product consisting of (in % by weight):
 C: 0.10-0.30%, 
 Si: 0.5-2.0%, 
 Mn: 0.5-2.4%, 
 Al: 0.01-0.2%, 
 Cr: 0.005-1.5%, 
 P: 0.01-0.1%, 
 as well as, in each case optionally, additionally of one or more elements selected from the group consisting of Nb, V, B, Ni, Cu, Mo, and W provided that 
 Ti: 0.005-0.1%, 
 Nb: 0.005-0.1%, 
 V: 0.001-0.2%, 
 B: 0.0005-0.01%, 
 Ni: 0.05-0.4%, 
 Cu: 0.01-0.8%, 
 Mo: 0.01-1.0%, 
 W: 0.001-1.0%, 
 and the remainder of iron and unavoidable impurities comprising less than 0.05% S and less than 0.01% N.