Evolution of Inclusions in Fe-13Cr Treated by CaO-SiO 2 -Al 2 O 3 -Based Top Slag
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INTRODUCTION
RECENTLY, martensitic stainless steels have been widely employed for various usages, including as cutlery materials, due to their high strength, high hardness, and excellent corrosion resistance. The nonmetallic inclusion size distribution, morphology, and chemistry composition have significant effects on its performance. Though inclusions formed unavoidably in the stainless steel, high hardness and high melting point inclusions must be reduced. Fe-13Cr stainless steels are deoxidized by silicon, due to the harmful effects of Al2O3 and MgO-Al2O3 inclusions on the materials, in the steelmaking process. The ferrosilicon, which is applied in silicon deoxidation, always contains a small amount of aluminum as an impurity.[1] Suzuki[2] calculated Cr13 pct silicon-aluminum complex deoxidation phase stability diagrams and pointed out that Al2O3-SiO2 system inclusions could easily form in FeSi-killed stainless steel even when [Al] content is at a quite low level. In fact, Al2O3-SiO2 system inclusions are also hard and have a high melting point. However, little attention has been paid to this type of inclusions. Much research has aimed at improving the quality of different steel grades by optimizing the composition of top slag.[3,4] Top slag plays an important
role in modifying inclusion chemistry by slag-metal-inclusion reaction and absorbing inclusions in the refining process.[3] Thus, it is important to grasp the influence of top slag on the transformation of Al2O3-SiO2 system inclusion in Fe-13Cr stainless steel. CaO-SiO2-CaF2-based slag, commonly used in FeSikilled steel production, has good deoxidation, good desulfurization, and inclusion-removing capacity due to its high basicity.[5,6] However, high-basicity slag generally has a high melting temperature and relatively low fluidity. Thus, CaF2 will be added to optimize slag properties, but also some negative influences will be presented, such as aggravated refractory erosion and environmental pollution. The contents of CaF2 and Al2O3 in reported top slags are around 15 and 5 pct, respectively.[7–10] Suitable Al2O3 content also can lower the melting temperature and reduce CaF2 usage by Al2O3, which would be a good choice. Therefore, in the current study, the CaO-SiO2-Al2O3-based slag equilibrium between the Fe-13Cr stainless steel under FeSi(Al) deoxidation was investigated to understand the effect of Al2O3 contents and basicity on the composition of Al2O3-SiO2 system inclusions.
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EXPERIMENTS
A. Materials QI WANG, Ph.D. Student, LIJUN WANG, Associate Professor, and KUO-CHIH CHOU, Professor, are with the State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, and also with the Collaborative Innovation Center of Steel Technology, University of Science and Technology Beijing, 10083, People’s Republic of China. Contact e-mail: [email protected] JUN ZHAI, Researcher, and JIANMIN LI, Senior Researcher, are with the Technical Center, Taiyuan Iron and Steel (Group) Co., Ltd, Taiyuan, Shanxi, 030003, People’s Republic of China.
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