Analysis of shell thickness irregularity in continuously cast middle carbon steel slabs using mold thermocouple data
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INTRODUCTION
SHELL thickness irregularity in continuous casting has commonly been observed for steels containing 0.10 to 0.15 wt pct carbon.[1,2] This carbon range has also been widely associated with depressed average mold heat flux,[3,4] as well as increased tendency for longitudinal and transverse surface cracking.[1,5] Further effects on shell thickness irregularity include mold heat flux,[6,7] casting speed and/or slag viscosity,[1,8–10] wide face taper,[9] and the powder pool depth above the meniscus.[11] Mold heat flux has consistently been shown to increase shell thickness irregularity. However, results for casting speed have not been conclusive. Some researchers have found irregularity to be parabolic with the product of casting speed and slag viscosity,[8] while others have found monotonic dependence of irregularity on both of these variables.[1,9] In order to study some of these effects and to investigate the nature of shell thickness irregularity, mold thermocouple readings were analyzed in the present work for middle carbon slabs. Mold thermocouple measurements were collected from the USX Gary works slab caster no. 2 in 1989 from over 3000 slabs. Since these thermocouples were originally installed for breakout detection, readings were not routinely collected but were specially downloaded to tape for this work whenever middle carbon grades were being cast. The thermocouples were ‘‘K’’ type, mounted in a plug in the mold wall. Thermocouples were buried 2.83 cm from the cold face and 0.87 cm from the hot face on average. A total of 32 thermocouples were embedded in the mold, 12 on each wide face and four on each narrow face. The positions on one wide face are diagrammed in Figure 1 (narrow face locations are published elsewhere[12]). Since these data were originally collected with regard to longitudinal surface J.P. SUNI, Staff Computer Scientist, is with the Material, Mechanics & Microstructure Center, Alcoa Technical Center, Alcoa Center, PA 15069. H. HENEIN, Professor, is with the Dept. of Minerals, Metallurgical & Petroleum Engineering, University of Alberta, Edmonton, AL, Canada T6G 2G6. Manuscript submitted July 24, 1995. METALLURGICAL AND MATERIALS TRANSACTIONS B
cracking, the narrow face thermocouple readings were discarded. The outer wide face positions were also discarded, since a significant portion of the slabs cast were not wide enough to reach these positions. This leaves six thermocouples on either wide face. Only one side of the mold was used for the present work, based on the reliability of the temperature measurements. Some thermocouples on the fixed wide face of the mold were frequently in error, so the work was concentrated on the loose side. This also corresponds to the slab top after unbending beneath the mold. For shell thickness calculations, the line of thermocouples under position 1a in Figure 1 was used. In addition to thermocouple readings, mold level and casting speed readings were collected. Mold level readings were obtained from a radiation type sensor in the mold and are
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