| Summary: | 碩士 === 國立成功大學 === 材料科學及工程學系碩博士班 === 93 === Exposing to oxidizing environment, scaling always occurs on the steel surface during heating, hot-rolling and subsequent cooling. The removal of scale by pickling is a common practice before cold rolling. The thickness, chemical composition and microstructure of scale are important factors affecting the pickling efficiency. In this study, the pickling behavior of various hot rolled steels in 5 wt % HCl solution with or without Fe+2 was investigated. Four different steels with various Si content, in the range of 0.01~1.91wt %, was used. The effect of silicon content on the scale characteristics and the subsequent pickling performance was studied. XRD, SEM and EPMA were employed to analyze the microstructure and chemical composition of the scales formed in these steels. The pickling behavior was evaluated by conducting the test either in static or flowing acidic solution.
The experimental results indicated that the upper and lower surfaces of the steel strips had different roughness. The results of XRD analyses showed the scale consisted of Fe 2 O 3 and Fe 3 O 4 . SEM examination exhibited that the thickness continuity and uniformity of the cross section of the scale varied considerably. It was also found that a continuous thin film or discrete particles containing Fe, Al, O and Si were present in the Fe / scale interface which might be Fe 2 SiO 4 , Fe 2 Al 2 O 4 or other silicates. The open circuit potential (OCP) measurements demonstrated that the exposure of the low silicon content carbon steel in the pickling solution might result in the formation of a meta-stable oxide on the steel surface. The formation of such meta-stable compound was characterized by the existence of a peak potential during OCP measurement. For the hot rolled steels, the experimental results showed the pickling efficiency decreased with increasing Si content in the steel. Furthermore, the presence of Fe+2 in HCl solution had no significant effect on the pickling performance of the steels in this study.
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