Improvement of Castability and Surface Quality of Continuously Cast TWIP Slabs by Molten Mold Flux Feeding Technology
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DEMAND for advanced high-strength steel (AHSS) has gradually increased. To devolop a successful automobile grade steel, for example, it is highly desirable to follow the trend of weight reduction for energy saving and environmental preservation while improving passenger safety. TWIP steel is regarded as one of the most promissing steel grades because of its larger formability and strength. Recently, POSCO has developed the TWIP cold-rolled high manganese automotive steel sheet,[1] and it has been tried on several parts successfully. Nevertheless, the considerably large amounts of alloying elements needed in TWIP steel make it difficult to carry out any successful continuous casting operations. During casting of the AHSS, the chemical reaction between mold flux and the alloying elements
JUNG-WOOK CHO, formerly Principal Researcher with the Technical Research Laboratories of POSCO, Pohang 37895 Republic of Korea, is now Associate Professor with the Graduate Institute of Ferrous Technology, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea. Contact e-mail: [email protected] SHIN YOO, Senior Researcher, MIN-SEOK PARK, Principal Researcher, and JOONG-KIL PARK and KI-HYEON MOON, Senior Principal Researchers, are with Technical Research Laboratories of POSCO. Contact e-mail: [email protected] Manuscript submitted July 25, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS B
such as aluminum[2,3] and titanium[4,5] changes the physical properties of mold slag significantly. Generally, the viscosity and crystallizing temperature of the mold slag increase drastically by the decrease of silica and increase of alumina or titania in a mold slag system, which deteriorates mold lubrication and, hence, the surface quality of cast slabs. In particular, the castability of TWIP steel is worse due to the following characteristics: (1) The liquidus temperature of TWIP steel is about 100 K (100 C) lower than that of the common low-carbon steel. If one assumes the same mold slag chemisty, the effective viscosity of mold slag during TWIP casting is usually two to three times larger than that for common carbon steel casting. Therefore, increase of viscosity due to chemical reaction is extremely harmful to the lubrication during TWIP casting. (2) The mush zone of TWIP steel is much larger than that of common carbon steel due to a larger amount of alloying elements such as manganes, carbon, and aluminum. Although TWIP steel belongs to the high-carbon grade, it shows irregular solidification behavior similar to peritectic grade steel due to the larger manganese.[1] Therefore, uneven mold heat transfer due to the unstable mold slag properties can easily yield the occurance of various surface cracks and even breakout on cast TWIP slabs.
Fig. 1—Specimen after cupping test for 1.5 pct aluminum and aluminum-free TWIP steels.
Considering these characteristics of TWIP steel, it is essential to maintain the physical properties of mold slag as stable as possible for the successful continuous casting operation
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