Formation Mechanism of CaO-SiO 2 -Al 2 O 3 -(MgO) Inclusions in Si-Mn-Killed Steel with Limited Aluminum Content During
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L rods for tire cords and saw wires are designed for higher tensile properties and drawn into ever smaller diameters. A crucial issue of these wires is the breakages during fabrication, which were mainly caused by nonmetallic inclusions as shown in many researches.[1–3] Since inclusions can act as crack initiation sites in the wires when subjected to cold drawing and cyclic stress, it would be more appreciated if they were with small size, decreased quantity, and good deformability. It is known that deformability of inclusions is affected by the melting points of themselves, which in turn closely related to their own chemistry.[4,5] Therefore, inclusions, such as alumina which is higher in melting point and KUNPENG WANG, Ph.D. Student, MIN JIANG, Assistant Professor, XINHUA WANG, Professor, and ZHANMIN CAO, Associate Professor, are with the School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, P.R. China. Contact e-mail: [email protected] YING WANG, Dr./Engineer, and HAOQIAN ZHAO, Engineer, are with the Xingtai Iron and Steel Corp., Ltd., Hebei 054027, P.R. China. Manuscript submitted September 21, 2015. Article published online November 4, 2015. 282—VOLUME 47B, FEBRUARY 2016
hardness, should be avoided. As a result, Si-Mn complex deoxidation combined with low basicity top slag refining is generally used to produce tire cord steel for improved deformation of inclusions. Commonly, inclusions in tire cord steel can be roughly classified into two categories. One type is composed of MnO-SiO2-Al2O3 system. While the other kind is CaO-SiO2-Al2O3, usually together with some MgO and sometimes also MnO.[6–8] It is generally held that best deformability of these inclusions can be expected if they contained about 20 wt pct Al2O3. Many researchers studied the effect of top slag on the composition of inclusions by either experiments or thermodynamic predictions.[6–17] To targeted this optimal Al2O3 content in inclusions, slag with basicity about 1.0 and Al2O3 about 8 wt pct was proved to be fine.[7] Under this circumstance, the optimum dissolved aluminum in steel would be about 0.0004 wt pct. As far as the formation mechanisms of these two kinds of inclusions, MnO-SiO2-Al2O3 inclusions are widely accepted as deoxidation products.[9–15] However, there are still some controversies over the origins of CaO-SiO2-Al2O3-(MgO) inclusions. Some researchers[7,8] suggested that CaO-SiO2-Al2O3-(MgO) inclusions were entrapped slag particles. Whereas other METALLURGICAL AND MATERIALS TRANSACTIONS B
studies[14,16] reported that these CaO-containing inclusions were originated from interaction of the molten steel reacted with top slag in a ladle. Nevertheless, both two points of views cannot perfectly explain the findings in practice. For instance, Al2O3 content is usually higher in CaO-SiO2-Al2O3-(MgO) inclusions than that in top slag. Moreover, CaO-SiO2-Al2O3-(MgO) inclusions always contain a small amount of MgO, whereas MnO-SiO2-Al2O3 system inclusions can be free of MgO.[16] Besides,
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