Formation Mechanism of SiO 2 -Type Inclusions in Si-Mn-Killed Steel Wires Containing Limited Aluminum Content
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INTRODUCTION
INCLUSIONS higher in hardness and melting point are harmful to high-strength steel rods like tire cords and saw wires because inclusions often act as breakage initiation sites when subjected to cold drawing. However, the complete removal of harmful inclusions from molten steel during refining is almost impossible. It would be much more desirable for the remained inclusions to be small in size and to demonstrate improved deformability. For these reasons, Si-Mn complex deoxidation combined with low-basicity top slag refining are generally used to these steels for improved deformation of the residual inclusions.[1–7] Unfortunately, the CaO-SiO2-Al2O3 system top slag of low basicity together with low aluminum and relatively KUNPENG WANG, Ph.D. Student, MIN JIANG, Assistant Professor, XIN-HUA 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 and HAOQIAN ZHAO, Engineers, are with the Xingtai Iron & Steel Corp., Ltd., Hebei 054027, P.R. China. Manuscript submitted May 25, 2015. Article published online July 9, 2015. 2198—VOLUME 46B, OCTOBER 2015
high silicon content in steel can cause the formation of SiO2, which is generally undesirable in steel wires because it is hard.[8,9] It has been proven that there is a relation between the SiO2 index (SiO2 index represents the number of SiO2 type inclusions per one gram of steel) and wire breakage frequency.[10] Feichtinger et al.[11] suggested that there were two types of SiO2 inclusions in final wire products: One was the deoxidation product and the other was the precipitates during casting. They carried out an impressive study and established a mathematical model on the removal of SiO2 inclusions by the absorption of the top slag based on the in situ observation under confocal laser scanning microscopy (SEM). Recently, several researchers[12–15] focused on determining the composition and structural changes of nonmetallic inclusions in solid steel with Si-Mn deoxidation. For example, Kim et al.[15] studied the diffusion behavior of Mn and Si at the interface between the Fe-Mn-Si alloy and the MnO-SiO2(FeO) oxide by diffusion couple experiments. Fine oxide particles were observed near the interface, and all of these oxide particles were smaller than 2.5 lm. However, up to now, there is still a lack of systematical research on the origin and formation mechanism of SiO2 in Si-Mn killed rod steel, that is, an evaluation on its evolution in the long process during industrial production. METALLURGICAL AND MATERIALS TRANSACTIONS B
In the current study, industrial experiments were performed to investigate the composition and morphology of inclusions in the whole process including steelmaking process and hot-rolling process. Based on the experiment results, the formation mechanism of SiO2 type inclusions in wire rod was put forward.
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EXPERIMENTS
A. Experimental Procedure and Sampl
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