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Measurements of molten steel/flux interface phenomena in thin slab casting Shaver, Joseph W.

Abstract

Several industrial plant trials investigating meniscus behavior and defects in thin-slab casting were conducted at Nucor Steel - Indiana, USA. As some of the first experimental steps in the investigation of the Compact Strip Production (CSP) process, the trials featured mold metal level and meniscus measurements, which resulted in the emergence of a novel and inexpensive method of measuring meniscus steel velocity using nailboards. In addition, caster operating data were collected using a high frequency data acquisition system (for mold events) and the level two information system. Slab and strip samples were collected to investigate the effects of meniscus level fluctuation and fluid flow behavior upon the internal and surface quality of the steel. Metal level and meniscus measurements were made during ordinary casting operation. Nailboards were used to make the measurements. The nailboard measurements provide a meniscus shape profiles and liquid flux layer depths. An analysis of the nailboard data has yielded information pertaining to the direction and velocity of the steel in the upper recirculation zone and at the meniscus. Through the measurement of meniscus shape and the standing wave, steel flow, was determined to be influenced by casting speed, SEN (Submerged Entry Nozzle) submergence and geometry, and mold width. Knowing that present methods of measuring steel stream velocity in the continuous casting mold are costly and not ideally suited for an industrial environment, the nailboard method for measuring mold flux depths was refined. This method can measure meniscus surface velocity in a steel mill setting with reasonable accuracy and agreement with established values for flow velocity. In conjunction with the nailboard measurements, Fast Fourier Transform (FFT) analyses were performed using caster operation data collected at a frequency of 33.33 Hz. FFT analysis shows that the metal level fluctuates with a characteristic frequency similar to predictions made by mathematical and physical models of similar casting processes. In addition, significant differences between two- and four-port SENs were also observed. This project has led to some of the first in-plant measurements of meniscus flow velocities for the CSP continuous casting process. In addition to being a very cost-effective way of measuring meniscus shape, flux depths and velocity, the nailboard measurements can be made by operating personnel at very short intervals, thereby offering opportunities for greater process understanding and control. Being made of only plywood, steel nails, and aluminum wire, nailboards are easy to assemble and quite durable in the steel mill environment. Other findings, through FFT analyses, have helped to elucidate the behavior of flow controls, particularly SEN and stopper rod behavior, during casting and their effects upon meniscus behavior and the resulting product quality. It is believed that this research will further the understanding of and contribute to the elimination of mold flux-related defects. It is also expected that this work will lead to additional research in this relatively new area of steel continuous casting.

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