Method and apparatus for producing zinc-aluminum alloy-coated steel sheet with superior workability and corrosion resistance

Abstract

Disclosed is a method for producing a zinc-aluminum-based alloy-coated steel sheet with superior workability and corrosion resistance by coating a base zinc-aluminum-based alloy-coated steel sheet in a coating bath comprising 35 to 55% by weight of zinc, 0.5 to 3% by weight of silicon, 0.005 to 1.0% by weight of chromium, 0.01 to 3.0% by weight of magnesium, 0.001 to 0.1% by weight of titanium, and the balance of aluminum and inevitable impurities. With the method, it is possible to produce a zinc-aluminum-based alloy-coated steel sheet which exhibits superior workability through control of an intermetallic compound layer (Zn—Al—Si—Cr) and formation of AlCr 2 due to Cr component present in the coating layer, reduces detachment of the coating layer and cracks of coating occurring during molding, exhibits superior corrosion resistance even after processing and has considerably superior corrosion resistance due to Mg 2 Si alloy phase and inhibition of oxide film formation.

What is claimed is: 
     
         1 . A method for producing a zinc-aluminum-based alloy-coated steel sheet with superior workability and corrosion resistance, the method comprising coating a base zinc-aluminum-based alloy-coated steel sheet in a coating bath comprising 35 to 55% by weight of zinc, 0.5 to 3% by weight of silicon, 0.005 to 1.0% by weight of chromium, 0.01 to 3.0% by weight of magnesium, 0.001 to 0.1% by weight of titanium, and the balance of aluminum and inevitable impurities. 
     
     
         2 . The method according to  claim 1 , wherein the coating bath further comprises 1 to 10% by weight of calcium, based on the total weight of magnesium. 
     
     
         3 . The method according to  claim 1 , wherein the coating layer forms a combination of a Mg 2 Si phase, an Al phase, a Zn phase, a MgZn 2  phase, an Al—Zn—Mg—Ca mixed phase, an Al—Zn—Si—Cr—Ca mixed phase and an AlCr 2  phase on a typical surface structure of a general galvalume coated steel sheet. 
     
     
         4 . A device for forming a nitrogen cloud in a circumference of a coated steel sheet ascending from a coating bath, the device being disposed between the coating bath surface to produce a zinc-aluminum-based alloy-coated steel sheet and an air knife,
 the device comprising:   lower nitrogen supply bars spaced from the surface of the coating bath by a predetermined distance, the lower nitrogen supply bars discharging nitrogen gas in a direction of the surface of the coating bath along the circumference of the coated steel sheet;   a side cover extending upward at an angle from a side of the lower nitrogen supply bars in a direction of the coated steel sheet; and   upper nitrogen supply bars formed on the upper surface of the side cover and discharging the nitrogen gas downward.