| The steel is used widely in industry, but it would be corroded often in some degree when it meets air, water, soil and other especial substance. In order to avoid corrosion, the steel is aften plated with protective layer to be insulated with corrosive substance. The most important use of Zinc is to mitigate corrosion by coating mainly, because it has excellent corrosion resisting property in nature environment. Electic plating method is used early, but the cost is very high and the hot galvanizing is replacing it gradually. Hot galvanizing is a method acquiring coating by immersing steel in the melting metal.Zinc is a more active element than Fe which could be oxidated more easily. Zinc is unconveted in dry air. But in moist air its surface would generate dense zinc carbonate film which could protect internal zinc from corrosion sequentially prolong steel service life.Electrode voltage of Zn-Fe alloy coating (-0.59V～-0.66V) is between the Zinc and Fe, so the corrosion rate of Zn-Fe alloy coating is more slowly than pure zinc. Galvannealed coatings are produced by a continuous hot dipping process followed by an in-line annealing cycle, during which alloying takes place and the different Fe-Zn phases are formed.Inadditon, it has excellent spot weldability and paintability,so Zn-Fe alloy coating is widely used gradually. But powdering and flaking of the coating often happen when the steel band is punched. Powdering and flaking make the coating’s corrosion resisting property and appearance after painting worse and zinc powder hich comes from powdering and flaking adheres to the die would affect its life. So how to improve the powdering resisting property of coating is the research hotspot all along for galvannealed coating. The powdering resisting property of coating is correlative with the phase structure of galvannealed coating.In order to research the phase structure of galvannealed coatings made by some steel plant and improve its powdering and flaking resisting property, systemic research are made as follows:1) The fracture evolution of galvannealed coating is observed by dynamic tensility experiment in-situ SEM observation for the first time. It is thought that flaking takes place at the interface of substrate/Γ phase by SEM and EDX analysis. The Fracture of Γ phase is serrated and its thickness is hundreds of nano-metres.δphase is main in the coating.2) The powdering resisting property of coatings and adhesive power is analysed and it is found that the performance of early galvannealed coating of galvannealed steel sheet(mainly DX54D+ZF) made in some steel plant is instable, the quantity of powdering is a little more than other products.It is found by Glow Discharge Spectrometer that Γ phase in the coating of DX54D+ZF is more than in other products coating.The surface topography of DX54D+ZF is simple columnar Zn-Fe Alloy phases in SEM and its surface is more brittle than others,no indentation by skin rolling could be found, there are many early cracks before deforming.Improved methods are brought forward.3)Simulation annealing experiments are made to DX54D+ZF at different annealing temperatures and different annealing times and the powdering resisting property of coatings with different processing parameters is discovered by OM, SEM,GDS and XRD:The quantity of powdering is little in a long annealing time at the temperature of450℃. The quantity of powdering dose not change within120s at the temperature of500℃.The quantity of powdering increases quickly in a short time at the temperature of550℃.4) Annealing experiments for simulating DX56D+ZF production are made at different annealing temperature and different annealing time and the powdering resisting property of coatings with different processing parameters is discovered by OM, SEM,GDS and XRD:The quantity of powdering is little for a long annealing time at the temperature of480℃. If the annealing time is no more than25s, Al is aggregate at the interface of coating/substrate and it is advantageous to inhibit brittle phase forming. When the annealing temperature is higher and the annealing time is longer, the brittle phases appears and its thickness become more and more thick, the powdering resisting property of coatings is descending.5) Annealing experiments for simulating H180BD+ZF production are made at different annealing temperature and different annealing time and the powdering resisting property of coatings with different processing parameters is discovered by OM, SEM,GDS and XRD:The quantity of powdering is little for a less annealing time than20s at the temperature of450℃or480℃and the effect is better at the temperature of450℃. The quantity of powdering increases quickly when the annealing time is more than25s at the temperature of480℃, but at the same annealing time the quantity of powdering is descending a little at the temperature450℃and the quantity of powdering is increased slowly until the annealing time more than30s. The powdering resisting property of coatings increases fast at the temperature of510℃or540℃,even in a short annealing time.6) In the beginning of annealing, Al enrichs interface of coating/substrate for all coatings of galvannealed steel sheets. With the time prolonged, Al diffuses to the surface of coating continuously and it would evenly distributed to the whole coating in the end.The process is more notable when the annealing temperature is higher.7)The surface appearance of coatings would change remarkably when the annealing temperature is higher, but when the galvannealed coatings has formed, the surface appearance of coating would not change remarkably if the annealing time is increased. |
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