Effect of Titania Absorption into a Mold Flux on the Heat Transfer Between the Mold and the Slab in the Continuous Casti
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DURING the continuous casting of stabilized stainless steel, mold slag samples from operating casters show a steady-state increase in the titania content of 3 to 4 pct for ferritic stainless steel 409 (0Cr11Ti) and of approximately 6 pct for austenitic stainless steel 321 (1Cr18Ni9Ti). The increase of titania concentration is a result of the pickup of titania inclusions from the liquid steel as well as the pickup of the product of reactions between the titanium in liquid steel and the oxides in the molten slag. The increased titania content exerts an impact on the properties of the mold flux (i.e., viscosity, crystallization ability, etc.).[1,2] Studies of the effect of titania on viscosity and the solidification temperature have been done extensively worldwide[2–4]; however, the influence on the heat transfer behavior between a mold and slab is unreported so far. Adjusting the horizontal heat transfer is one of the important functions achieved by the mold flux in the mold, and the heat transfer has a critical influence on both the process stability and the surface quality of the cast product, such as longitudinal cracks, depth of oscillation marks, sticker breakouts, and depressions.[5–8] For continuous casting operation, different steels with different solidification characteristics, high-temperature strengths, and ductility require different heat transfer properties. For example, during the initial solidification of the steel grade 409, only the d-phase is created; no phase transform takes place, and solidification shrinkage is even and small. So the stress caused by the ZHANQUAN HAO, Ph.D. Candidate, and WEIQING CHEN, Professor, are with the School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Haidian District, Beijing 100083, P.R. China; Contact e-mail: zhanquanhao@ yahoo.com.cn CARSTEN LIPPOLD, Technical Director, is with Qingdao Stollberg & Samil Co. Ltd, Jiaozhou 266300, P.R. China. Manuscript submitted July 8, 2009. Article published online April 20, 2010. METALLURGICAL AND MATERIALS TRANSACTIONS B
solidification shrinkage is not the main consideration, and properties of good high-temperature ductility but low high-temperature strength indicate that the slab is deformed easily, even when suffering low stress.[9–12] Therefore, to decrease the friction on the slab surface as well as to form a thicker solidified shell that can endure more tension, a high heat transfer is more acceptable in the casting of steel grade 409. However, the dilatability of steel grade 321 is higher than that of carbon steel grades. A higher dilatability causes a higher expansion stress to act on the slab and causes a higher tendency to form a nonuniform shell during casting. Depression defects are easily formed. Therefore, a low heat transfer and soft mold cooling are essential in the casting of steel grade 321.[10,12] In this article, we report on the trend and the extent of heat transfer changes caused by increasing titania concentrations in the mold flux during the casting of Ti-containing steel grades
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