Method for Producing a Hot-Formed and Hardened Steel Component Coated with a Metallic Anti-Corrosion Coating from a Sheet Steel Product

Abstract

A method for producing a steel component with a metallic anti-corrosion coating from a sheet steel product comprising at least 0.4% by weight Mn is disclosed. The sheet steel product is annealed in a continuous furnace under an annealing atmosphere containing up to 25% by volume H 2 , 0.1% to 10% by volume NH 3 , H 2 O, N 2 , and process-related impurities as the remainder, at a dew point between −50° C. and −5° C. at a temperature of 400 to 1100° C. for 5 to 600 s. The annealed sheet steel product has a 5 to 200 μm thick nitration layer with a particle size finer than the particle size of the inner core layer. Once coated with a metallic protective layer, a blank is separated from the annealed sheet steel product, heated to an austenitising temperature of 780 to 950° C., hot-formed, and cooled so that a hardened structure forms.

1 . A method for producing a steel component that is coated with a metallic anti-corrosion coating from a sheet steel product having an Mn-content of at least 0.4% by weight, comprising the following working steps:
 providing the sheet steel product;   annealing the sheet steel product in a continuous furnace,   under an annealing atmosphere containing up to 25% by volume H 2 , 0.1 to 10% by volume NH 3 , H 2 O and N 2  as the remainder as well as process-related inevitable impurities and having a dew point ranging between −50° C. and −5° C.,   at a holding temperature of 400 to 1100° C.,   for a holding period of 5 to 600 s,   so that the sheet steel product obtained after the annealing treatment has a 5 to 200 μm thick nitration layer, which adjoins its free surface and the particle size of which is finer than the particle size of the inner core layer of the sheet steel product covered by the edge layer;   coating the annealed sheet steel product with a metallic protective layer;   separating a blank from the sheet steel product;   optionally preforming the blank;   heating the blank to an austenitising temperature of 780 to 950° C.,   hot-forming the heated blank to form the steel component,   accelerated cooling of the steel component in such a way that a hardened structure forms in the sheet steel product.   
     
     
         2 . The method according to  claim 1 , characterised in that the H 2 -content of the annealing atmosphere is at most 10% by volume. 
     
     
         3 . The method according to  claim 1 , wherein the NH 3 -content of the annealing atmosphere is at most 5% by volume. 
     
     
         4 . The method according to  claim 1 , wherein the dew point of the annealing atmosphere is −40° C. to −15° C. 
     
     
         5 . The method according to  claim 1 , wherein the holding temperature of the annealing is 680 to 840° C. 
     
     
         6 . The method according to  claim 1 , wherein the holding period of the annealing is 30 to 120 s. 
     
     
         7 . The method according to  claim 1 , wherein the characteristic particle size of the nitration layer of the annealed sheet steel product, determined in accordance with DIN EN ISO 643 before the blank is heated and hot-formed, is smaller by at least 2 than the characteristic particle size of the basic material. 
     
     
         8 . The method according to  claim 1 , wherein the coating of the sheet steel product with the metallic protective layer takes place by means of hot-dip coating, which is completed in a work sequence carried out continuously following the annealing treatment. 
     
     
         9 . The method according to  claim 8 , wherein an oxidation of the surface of the sheet steel product is carried out before the hot-dip coating. 
     
     
         10 . The method according to  claim 8 , wherein the sheet steel product is continuously diffusion-annealed after the hot-dip coating. 
     
     
         11 . The method according to  claim 1 , wherein the coating of the sheet steel product with the metallic, metallic-organic or metallic-inorganic protective layer takes place by electrolytic coating or a physical vapour or chemical vapour deposition. 
     
     
         12 . The method according to  claim 1 , wherein the metallic protective layer is a Zn, an Al, a Zn—Al, a Zn—Mg, a Zn—Ni, an Al—Mg, an Al—Si, a Zn—Al—Mg or a Zn—Al—Mg—Si coating. 
     
     
         13 . The method according to  claim 1 , wherein the austenitising temperature adjusted during the heating is 860 to 950° C. 
     
     
         14 . The method according to  claim 1 , wherein the hot-forming and the cooling of the component obtained by the hot-forming are carried out in one step. 
     
     
         15 . The method according to  claim 1 , wherein the component obtained is subjected to a blasting treatment.