Evaluation of Steel Cleanliness in a Steel Deoxidized Using Al
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asing need for steel grades with higher quality requirements has led to the development of complex liquid steel treatments for controlling steel cleanliness, i.e., morphology, number, size distribution, and chemical composition of inclusions, in order to ensure adequate performance.[1,2] The chemical modification of residual inclusions is a practice widely used to reduce their harmful effect on steel quality; consequently, for a successful inclusion, modification is suitable to rationalize the modification path. Solid Al2O3 inclusions are the primary deoxidation product of aluminum-deoxidized steels. Modification treatment of steel with Ca is often used to promote their partial reduction into liquid Ca aluminates. Spinel inclusions (MgO-Al2O3) can form because of the presence of Mg in the liquid steel. The Mg can be provided by the decomposition of MgO contained in the EDGAR-IVAN CASTRO-CEDEN˜O, Graduate, and MARTI´N HERRERA-TREJO and MANUEL CASTRO-ROMA´N, Professors, are with the Centro de Investigacio´n y de Estudios Avanzados, CINVESTAV Saltillo, Ave. Industria Metalu´rgica No. 1062, Parque Industrial Saltillo-Ramos Arizpe, 25900, Ramos Arizpe, Coahuila, Mexico. Contact e-mail: [email protected] FABIA´N CASTRO-URESTI, Chief of Process, is with the Process Engineering Department, Ternium Me´xico, 66450, San Nicola´s de los Garza, N.L., Mexico. MONSERRAT LO´PEZ-CORNEJO, Process Engineer, is with the Process Engineering Department, Ternium Me´xico, 72620, Xoxtla, Puebla, Mexico. Manuscript submitted on August 26, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS B
refractory and slag, and the use of deoxidizers and alloying agents containing Mg. It is accepted that both types of inclusions, Al2O3 and spinel, incur severe clogging problems and deteriorate the quality of the final product.[3,4] The modification path of Al2O3 inclusions by Ca is well known and leads to liquid inclusions. In contrast, there is uncertainty with regard to the effectiveness of Ca treatment for modifying spinel into liquid inclusion.[5,6] Recently, an extended work on LCAK steel carried out by Pretorious et al.,[5] showed that the complete modification of spinel is possible by the reduction of MgO by Ca to Mg into the steel, provided that the oxygen potential is low enough to promote the adsorption of more Mg into the steel. Optical microscopy and image analysis are generally used for obtaining steel cleanliness indicators such as number and size distribution. Furthermore, two statistical methods based on extreme value theory have been developed for practical applications to estimate the maximum inclusion size of large steel volumes using data obtained from small samples. These methods involve the description of data in the tails of the inclusion size distribution, in which standard probability distributions such as the log-normal distribution are insufficient.[7] The first method is based on the measurement of the maximum inclusion size in random inspection areas and the fitting of Gumbel distribution to these measurements.[8–10] Moreover, a
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