Control of Crystal Morphology for Mold Flux During High-Aluminum AHSS Continuous Casting Process
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URING the casting of high-Al advanced highstrength steel (AHSS), the high Al content in molten steel can easily reduce SiO2 in mold flux, resulting in the variation in the chemistry of conventional lime-silica-based mold fluxes. This variation generally causes the instability of physical and chemical properties of mold flux, such as melting point, viscosity, basicity, and so on, which has been reported as the main reasons for various problems during casting of high-Al steels such as breakout, non-uniform heat transfer across mold flux, inadequate lubrication, and poor as-cast slab surface quality.[1–4] Consequently, it is in urgent need to develop sound mold fluxes to meet the requirements JING GUO, formerly Postdoctoral Fellow with the Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Republic of Korea, is now Assistant Professor with the School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing (USTB), Beijing 100083, P.R. China. MYUNG-DUK SEO, Senior R&PD Engineer, is with Casting & Forging Business Group, Doosan Heavy Industries & Construction, Changwon 642-792, Republic of Korea. CHENG-BIN SHI, Assistant Professor, is with the State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing (USTB). JUNGWOOK CHO, Research Associate Professor, is with the Graduate Institute of Ferrous Technology, Pohang University of Science and Technology (POSTECH. Contact e-mail: [email protected] SEON-HYO KIM, Professor, is with the Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH). Manuscript submitted November 15, 2015. Article published online May 31, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS B
of high-aluminum AHSS casting. Generally speaking, there are two strategies to attempt to solve this problem at present. The first is to use the mold flux with low basicity to inhibit the reduction rate of SiO2 during the continuous casting (CC) process.[5–7] Obviously, it cannot overcome the instability of mold flux properties during CC process completely by this method. Furthermore, it also increases the difficulties to control the stable silicon content of high-Al AHSS. As a result, to develop an optimized unreactive CaO-Al2O3-based mold flux, the second strategy, is increasingly investigated by many researchers.[3,4,8–12] Blazek et al.[3] worked on lime-alumina-based mold flux for casting high-Al AHSS steel and showed that the interaction was markedly reduced and the as-cast slab quality was improved compared to that of lime-silica-based mold flux. Cho et al.[8] carried out casting trials of high-Al TRIP steel by using the newly developed lime-alumina-based mold fluxes, and their results suggested that mold heat transfer in casting process and surface quality of slab were improved, compared to that of lime-silica-based mold fluxes as well. But their experimental results also demonstrated that lubrication and flux consumption of lime-alumina-based mold
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