Zinc alloy and process for hot-dip galvannealing of steel

Abstract

The present invention relates to a hot-dip coating alloy composition suitable for galvannealing steel sheet. The addition of 0.02 to 0.11 wt. % Cr to the alloy allows for an increased Al content in the bath, while ensuring a sufficient diffusion of Fe into the Zn coating during annealing. This is particularly advantageous for use in coating lines where galvanised and the galvannealed products are manufacturing alternatively. The addition of Cr allows to lower the annealing temperature and to shorten the residence time of the steel in the annealing furnace. Galvannealing dual phase steels, which should be processed at relatively low temperatures, is therefor greatly facilitated.

1 - 6 . (canceled)  
   
   
       7 . A bath for continuous hot-dip galvannealing steel with alloyed zinc, the bath comprising zinc, 0.12 to 0.35 wt. % Al, and 0.02 to 0.11 wt. % Cr.  
   
   
       8 . The bath of  claim 7 , wherein the bath contains 0.135 to 0.29 wt. % Al and 0.05 to 0.10 wt. % Cr.  
   
   
       9 . The bath of  claim 7 , wherein the bath only contains zinc, 0.12 to 0.35 wt. % Al, 0.02 to 0.11 wt. % Cr, and unavoidable impurities.  
   
   
       10 . The bath of  claim 8 , wherein the bath only contains zinc, 0.135 to 0.29 wt. % Al, 0.05 to 0.10 wt. % Cr, and unavoidable impurities.  
   
   
       11 . A process for coating steel on an individual continuous hot-dip line, comprising, in either order, the steps of: 
 galvanising a first quantity of steel by hot-dipping in a Zn alloy bath;    galvannealing a second quantity of steel by hot-dipping in the Zn alloy bath to produce coated steel and by subjecting the coated steel to a thermal treatment in an annealing furnace,    wherein the Zn alloy contains Al and 0.05 to 0.10 wt. % Cr.    
   
   
       12 . A process for continuously galvannealing dual-phase steel comprising hot-dipping the steel in a Zn alloy bath, wherein the Zn alloy contains 0.12 to 0.35 wt. % Al and 0.02 to 0.11 wt. % Cr.  
   
   
       13 . A process of increasing the annealing reactivity in a furnace used for annealing a product after hot-dipping in a Zn alloy bath, comprising adding 0.12 to 0.35 wt. % Al and 0.02 to 0.11 wt. % Cr to the Zn alloy bath.