The Phase Relations Of The Zn–Re–Al–P Quaternary System And The Influence Of Phosphorus On The Galvanized Coating

Galvannealed steel sheets are widely used in the automotive industry, due to their excellent weldability, paintability, thermostability and corrosion resistance. However, in the course of stamping, the galvannealed coating may suffer from defects, such as powdering. So it has seriously hampered the development of China's automobile industry. It has been reported that P in steel has great effects on alloying reaction between Fe and Zn during the galvannealing treatment. In order to understand the mechanism of phosphorus on the Fe-Zn alloying reaction, and intensively study the combined effect of P and Al in the hot-dip galvanizing, the following works have been carried out.To obtain Zn-rich corner of 450℃isothermal section of the Zn-Fe-Al-P quaternary system, the 450℃isothermal section of the Zn-Al-P ternary phase diagram was investigated experimentally again using scanning electron microscopy coupled with energy dispersive x-ray spectroscopy, and x-ray diffraction. The experimental results are quite different from the reported phase equilibrium relationships of the Zn-Al-P ternary system. (L-Zn+Zn₃P₂+AlP), (L-Zn+α-Al+AlP), and (Zn₃P₂+ZnP₂+AlP) three-phase regions exist in the section. The experimental results reveal that AlP is in equilibrium with L-Zn,α-Al, Zn₃P₂, and ZnP₂, respectively. P solubility in liquid Zn and inα-Al phases is limited.Based on the above results, the 450℃isothermal section of the Zn-Fe-Al-P quaternary system with Zn being fixed at 93at.% was determined by the equilibrated alloys approach.η-Zn+T+Fe₂Al₅+AlP,η-Zn+FeAl₃+Fe₂Al₅+AlP,η-Zn+δ+T+AlP,η-Zn+ζ+δ+AlP,η-Zn+FeP+Zn₃P₂+AlP, andη-Zn+ζ+FeP+AlP four-phase regions were found to exist in the section. No quaternary phase was found in all the equilibrium alloys. AlP was found to be in equilibrium with L-Zn,ζ,δ, T, Fe₂Al₅, FeAl₃, FeP and Zn₃P₂, but not with Fe₂P. The solubility of P inζ, T, Fe₂Al₅, and FeAl₃ is very limited; Zn is soluble in all known compounds.The experimental results of Zn/Fe and Zn/Fe-P liquid-solid diffusion couple indicate that P in steel can delay or inhibit the formation ofГphase. But P in steel has been found to promote the growth of theζphase, and the higher phosphorus content, the faster the growth speed of theζphase. Obviously, the role of phosphorus on the alloy layer is similar to silicon reactivity of the general galvanizing. The experimental results of pure iron hot-dip galvanizing show that total Fe-Zn alloy layer growth in the 0.15wt.%Al-Zn bath was inhibited by the initial formation of an Fe-Al intermetallic layer at the steel/coating interface in a very short time. With the increase of dipping time,δphase was found to form at first, and then a thinГphase layer formed at the steel/delta-phase interface. A small amount of phosphorus added to the substrate steel can significantly inhibit the Fe-Zn reaction by the hot-dip galvanizing.