Slag Pool Depth Effectiveness of Molten Mold Flux Feeding Technology
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ODUCTION
IN the continuous casting process, an increase in production and a substantial yield have been achieved for automotive exterior sheets and high alloyed steel grades products such as Ultra-low carbon steels (ULCSs) and Twinning-Induced Plasticity (TWIP) steel. POSCO’s advanced CASting Technology (PoCAST) was developed to increase slab quality by controlling slag pool depth during continuous casting of steels.[1] The main differences between conventional casting and PoCAST casting are that PoCAST uses higher temperature of molten steel near the meniscus level and larger slag pool depth (Figure 1). PoCAST has enabled precise control of slag pool depth during continuous casting, and has enabled the first commercial production of TWIP steels for automotive applications.[2,3] In general, high content of aluminum in AHSS steel generation suppresses delayed fracture in TWIP steels[4] and increases the specific density of the steel.
SHIN YOO is with the Technical Research Laboratories, POSCO, Pohang 37859, Republic of Korea and also with the Graduate School of Engineering Mastership, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea. JUNGWOOK CHO is with the Graduate Institute of Ferrous Technology, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea. Contact e-mail: [email protected] SANG-HYUN PARK, MIN-SEOK PARK, JOONG-KIL PARK, and KI-HYEON MOON are with the Technical Research Laboratories, POSCO. Manuscript submitted December 17, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS B
However, high alloying of TWIP steels is extremely difficult during massive commercial production, especially in the continuous casting process, because aluminum reacts with silica in the mold flux. The chemical reaction between aluminum and molten steel changes the physical properties of mold flux such as viscosity, crystallization character and melting temperature[2,3] and degrades lubrication between the slab and the casting mold. 4½Al þ 3ðSiO2 Þ ¼ 3½Si þ 2ðAl2 O3 Þ
½1
Continuous casting of TWIP steels is a complicated process. Their liquidus temperature is about 100 K lower than in common low carbon steel, and they have higher irregular solidification behavior during casting, as a result of high manganese content. These characteristics cause unstable mold heat transfer and eventually affect the lubrication by the mold flux. As a result, the mold flux consumption rate increases, and the cast slabs develop low quality or even breakout. The castability of TWIP steels can be controlled by adjusting the chemical compositions of mold flux.[5–7] However, physical properties optimization of lime-alumina based mold flux is limited in the conventional powder casting system for massive production. PoCAST can increase the commercial productivity of TWIP steels.[4] Its application to composition stability of mold slag yields a successful dilution effect, because the slag-pool is deeper (10 to 50 mm) than in conventional mold-powder casting (7 to 13 mm). During casting of ultral
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