Hot-dip galvanised steel sheet

Abstract

The present disclosure relates to a hot-dip-coated steel sheet having a Zn—Mg—Al coating which includes aluminum at between 0.1 and 8.0 wt %, magnesium at between 0.1 and 8.0 wt %, the balance being zinc and unavoidable impurities, wherein the coating comprises zinc grains and further phases of magnesium and/or aluminum and also eutectic structures including at least intermetallic zinc-magnesium phases, wherein a native oxide layer is formed on the coating. In accordance with the present disclosure, the coating beneath the native oxide layer has an area fraction of at least 35% in which there is an average nanohardness of at least 4 GPa.

1 . A hot-dip-coated steel sheet having a Zn—Mg—Al coating which includes aluminum at between 0.1 and 8.0 wt %, magnesium at between 0.1 and 8.0 wt %, the balance being zinc and unavoidable impurities, wherein the coating comprises zinc grains and further phases of magnesium and/or aluminum and also eutectic structures including at least intermetallic zinc-magnesium phases, wherein the coating comprises a native oxide layer being formed on the coating, and a coating beneath the native oxide layer having an area fraction of at least 35% in which there is an average nanohardness of at least 4 GPa. 
     
     
         2 . The steel sheet as claimed in  claim 1 , wherein the coating in a depth of 20 nm beneath the native oxide layer has an area fraction of at least 35% in which there is an average nanohardness of at least 3 GPa. 
     
     
         3 . The steel sheet as claimed in  claim 1 , wherein the coating in a depth of 40 nm beneath the native oxide layer has an area fraction of at least 35% in which there is an average nanohardness of at least 2.5 GPa. 
     
     
         4 . The steel sheet as claimed in  claim 2  wherein the coating in a depth of 70 nm beneath the native oxide layer has an area fraction of at least 35% in which there is an average nanohardness of at least 2 GPa. 
     
     
         5 . The steel sheet as claimed in  claim 4  wherein the coating includes aluminum and magnesium at in each case at least 0.5 wt %. 
     
     
         6 . The steel sheet as claimed in  claim 5  wherein aluminum and magnesium in the coating are limited to in each case not more than 3.5 wt %. 
     
     
         7 . The steel sheet as claimed in  claim 1  wherein the coating has a thickness of between 2 and 20 μm. 
     
     
         8 . The steel sheet as claimed in  claim 7  wherein the coating bears an impressed deterministic or stochastic surface structure. 
     
     
         9 . The steel sheet as claimed in  claim 1  wherein the hard regions of the surface of the coating that are exposed after treatment with an inorganic acid have a developed boundary area ratio Sdr of at least 5.5%, based on an AFM scan region of 5×5 μm 2 . 
     
     
         10 . The steel sheet as claimed in  claim 1  wherein the steel sheet has a surface-coveringly homogeneous phosphate layer with zinc phosphate crystals of up to 3 μm in size. 
     
     
         11 . The steel sheet as claimed in  claim 3  wherein the coating in a depth of 70 nm beneath the native oxide layer has an area fraction of at least 35% in which there is an average nanohardness of at least 2 GPa. 
     
     
         12 . The steel sheet as claimed in  claim 11  wherein the coating includes aluminum and magnesium at in each case at least 0.5 wt %. 
     
     
         13 . The steel sheet as claimed in  claim 12  wherein aluminum and magnesium in the coating are limited to in each case not more than 3.5 wt %.