Mold Slag Property Measurements to Characterize CC Mold – Shell Gap Phenomena

Multi-faceted experiments were conducted to measure the properties of several mold slags, needed for fundamental characterization of heat transfer and friction in the interfacial gap between the shell and mold during the continuous casting of steel. A novel apparatus was used to measure the friction coefficient between solidified mold flux and copper at elevated temperatures. The measured softening temperature is interpreted to extrapolate the slag viscosity-temperature curves far into the low temperature – high viscosity region. Continuous-cooling transformation curves were extracted from XRD analysis of DSC test samples and thermocouple dip tests. Time-temperature transformation curves were obtained from similar analysis of melted mold powder samples that were atomized into droplets, quenched to form glass, and then partially devitrified by reheating to different temperatures for different times and quenched. Polarized light microscopy, SEM, and EDX analysis revealed distinct crystalline and glassy layers, but no severe macro-segregation in a tail-out slag film taken from an operating caster. The results from these new measurements have important implications for the prediction of interfacial gap phenomena, including mold heat transfer, friction, slag layer fracture, and steel surface quality. Introduction In the continuous casting of steel, the choice of mold slag is decisive for lubrication and heat transfer control in the mold. The composition, viscosity, solidification temperature and crystallinity are typically considered the most important properties of the slag. These properties determine how the mold powder, which is added to the top surface of molten steel will melt into a liquid layer, (called mold flux or mold slag), infiltrate into the gap between the shell and mold during continuous casting, and there control lubrication behavior, and mold heat transfer. Optimal design of the mold slag can avoid surface defects such as longitudinal, transverse and star cracks; enhance surface quality with the formation of uniform and shallow oscillation marks; prevent breakouts; and enable increased casting speed. Friction signals can be obtained by installing lubrication sensors, load cells or pressure sensors onto the mold to record the mold speed, load or pressure variation during mold oscillation. However, fundamental understanding of the meaning of these measurements and how to interpret them to solve problems is lacking. Currently mold friction measurements are evaluated mainly as a means to detect problems with the oscillation system, such as mold misalignment. If the friction signal can be better understood, then friction monitoring could be used to identify the status of mold lubrication to predict surface defects and to help prevent breakouts. Viscosity of the mold slag is highly temperature dependent. In previous work, the viscosity is often expressed as an Arrhenius-type relationship including the effect of composition . These models provide a method to design the slag composition to achieve a desired viscosity curve. However, none of them can accurately predict the viscosity near the solidification temperature. These models are only good for the low viscosity, high temperature range (<10poise) and cannot accommodate the sharp viscosity increase that occurs at lower temperature. Due to the difficulty of measuring high viscosity, slag viscosity measurements are seldom reported greater than 10Pa·s. Thus, the viscosity-temperature curve near the solidifying temperature is yet unclear for the mold slags used in continuous casting. Recent laboratory experiments show that heat transfer across the gap is significantly affected by the crystallization of the slag film while it is relatively insensitive to chemical composition . Several studies were conducted using differential thermal analysis (DTA) , single or double hot thermocouple technique (SHTT/DHTT) , Confocal Microscopy and by devitrification, to measure the fraction of crystalline phase formed after heating a previous quenched sample to a specific temperature and holding. The isothermal transformation diagrams (TTT diagram) and continuous cooling transformation diagrams (CCT diagram) of slag have been determined in controlled laboratory conditions [10, 11, 13, 15, . However, most of MS&T 2004 Conference Proceedings,(New Orleans, LA), AIST, Warrendale, PA 57 these methods are limited to relatively low cooling rates (1C/min~900C/min). The average cooling rate of the mold slag in the longitudinal (meniscus to mod exit) and transverse (mold hot face to steel shell surface) directions may be about 20~25C/sec, the local cooling rate may be as high as 50~100C/sec, especially near the meniscus where the maximum heat flux enters the mold. Thus, a method to achieve higher cooling rate is needed to study mold slag crystallization . In this work, several experiments are performed to measure new slag properties, including the friction coefficient between the slag and mold wall, the viscosity at low temperature, the glassy or crystal structure of the solidified flux, CCT and TTT curves, and the crystalline phases.