Analysis of the nonuniform slag film, mold friction, and the new cracking criterion for round billet continuous casting
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I. INTRODUCTION
MOLD powder fed onto the free surface of liquid steel melts to form a liquid slag pool and upon entering the mold/strand channel freezes against the water-cooled, copper mold, forming a solid slag film that provides the optimum level of horizontal heat transfer between the shell and mold, while the thin, liquid slag film provides liquid lubrication to the shell. It is generally considered that the liquid slag film travels with the strand and the solid slag film remains in contact with the mold and oscillates with it. These layers of mold slag provide the film with thermal resistances to the conductive heat transfer in the mold. The difference in the pressure during casting and idling periods is expressed as the frictional force, and the friction in the mold is considered to originate from two mechanisms. The motion of the mold relative to the solidified shell occurs within the liquid slag film and causes a frictional force due to the viscosity of the film. The frictional force generated through this mechanism is called “liquid lubrication.” If the relative movement occurs between the mold wall and the solid slag film, the frictional force is generated by solid-solid contact, and this is called “solid friction.” In the plant trial, load cells are instrumented in the mold housing to provide information on the overall mechanical interaction, or friction between the strand and mold. However, it cannot mark the local information, so numerical simulation is adopted. The billet surface crack is one of the primary and common defects. As for predicting the defects, many investigations have been carried out including setting various crack criterions. Almost all of the investigations involved the mechanical calculation of materials, which depends on the physical and HE-BI YIN, Graduate, and MAN YAO, Professor, are with the Department of Materials Engineering, Dalian University of Technology, Dalian 116024, China. Contact e-mail: [email protected] or [email protected] Manuscript submitted December 31, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS B
mechanical properties and the stress-strain behavior of materials. However, it is not fully understood at high temperature, and besides, the calculation is very time-consuming and difficult to use in plant. II. PREVIOUS WORK It is reported that liquid slag entering the mold/strand channel freezes to form typically 2-mm-thick solid slag and 0.1-mm liquid slag films.[1] Some studies[2–5] showed that three layers of slag film exist between the mold and strand: a glassy zone close to the mold, a crystalline layer in the center, and a liquid film in contact with the steel. Further, investigations[4,6] of the heat transfer through the slag films with laboratory experiments have been performed. Also, much various relevant calculation research[3,7–9] has been conducted, considering the conductive and radiative thermal resistances of slag film and even the thermal resistance at the mold/solid slag film interface, though they are all in static condition. Besides, Yamauchi[1
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