The influence of time of holding in a melt on the morphology of zinc coating on steels with various silicon content
Keywords:zinc coating, silicon steels, coating morphology, Sandelin effect, Fe-Zn-Si system
The formation of zinc coating on steels in the process of hot galvanizing is determined by such factors as the process temperature, holding time, the chemical composition of steel, and particularly, silicon content. In the 1940s of the XX century, R.W. Sandelin described the process of the significant acceleration of the reaction between ferrum and zinc at the silicon content in steel equal to 0.06–0.10 %. There are different methods of control of silicon reactance; however, the simplest method of control of coating thickness is the proper choice of time of product holding in the melt. The paper aims at the identifying the influence of time of holding in the melt on thickness and microstructure of the coating formed on steels with different content of silicon: S235 (Si=0.02 %), S235J0 (Si=0.04 %), S235JR (Si=0.17 %), 9MnSi5 (Si=0.6 %). To quantify silicon, the authors analyzed the chemical composition of steels using the spark spectrometry technique. The study identified that the coating thickens on steel with the course of time of holding in the melt according to the parabolic law. The most intensive growth of coating thickness with the course of time is observed on reactive steels with silicon content of 0.04 % and high-silicon steels with silicon content of 0.6 %. The reactive steel showed the significant growth of variations in thickness. The authors carried out the analysis of microstructure using the TESCAN Vega SB scanning electron microscope; the analysis showed that the growth of a coating is determined by the peculiarities of ζ-phase structure. The analysis of the Fe-Zn-Si triple diagram allowed concluding that with the silicon content of 0.04 % and 0.6 % in steel, the eutectic decomposition of fluid into ζ+η+FeSi phases’ mixture progresses in the system. This process leads to the direct contact of the melt and steel base and intensifies the interdiffusion of ferrum and zinc. As a result, the ζ-phase actively produces that leads to the rapid growth of the coating thickness.
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