Inclusion Characterization and Formation Mechanisms in Spring Steel Deoxidized by Silicon
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TRODUCTION
HARDENED and tempered spring steel is manufactured to be suitable for working under high cyclic stresses,[1–4] especially in environments where high strength, superior wear resistance, and fatigue strength are required.[5] In the automobile industry, such steel is usually used for suspension, valve, clutch, and plate springs that require high strength and fatigue resistance.[1,6,7] It is widely acknowledged that fatigue cracking of spring steel can initiate at nonmetallic inclusions. The fatigue strength of spring steel is closely related to the properties of the inclusions.[8–14] Small and deformable inclusions are less harmful because these can reduce fatigue crack initiation and propagation during working of spring rods under elevated dynamic stresses.[15–17] Aluminum is a common deoxidizer owing to its strong affinity for oxygen; however, inclusions of Al2O3 clusters are frequently observed in Al-deoxidized steel. These SHA LYU, XIAODONG MA, HAE-GEON LEE, GEOFF WANG, JIN ZOU, and BAOJUN ZHAO are with the University of Queensland, Brisbane 4072, Australia. Contact e-mail: [email protected] ZONGZE HUANG and ZAN YAO are with the Baoshan Iron and Steel Co. Ltd., Shanghai 200122, China. ZHOUHUA JIANG is with Northeastern University, Shenyang 110819, China. Manuscript submitted April 18, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS B
cause several problems, such as clogging of submerged entry nozzles,[18–20] deterioration of surface quality,[21] and fatigue resistance of spring steel, especially when spinel-like Al2MgO4 is formed through the interaction of Al2O3 with Mg that originates, for instance, from the slag and/or MgO-C refractories.[22–25] To avoid the formation of nondeformable Al2O3-rich inclusions, ferrosilicon and ferromanganese alloys with low aluminum content are widely used for the deoxidizing of spring steel.[26,27] Different types of inclusions have been reported in Si-deoxidized spring steel. Using scanning electron microscopy and energy-dispersive X-ray spectroscopy (EDS) analysis, Lee et al.[28] reported the presence of Al2O3, SiO2, and iron oxide inclusions in spring steel. Nishijima et al.[29] explored different oxide inclusions using five microscopic methods. Suito et al.[30] investigated the equilibria between inclusions, steel, and top slag with respect to Si and Mn for tire cord steel. Stepanov et al.[31] concluded that the primary composition of inclusions in spring steel deoxidized by Si was SiO2-MnO-Al2O3-CaO-MgO, which originated from MnOSiO2-Al2O3 inclusions in the steel. Li et al.[32] investigated the preferred composition regions for MnO-Al2O3-SiO2 inclusions in spring steel deoxidized by Si. Du et al.[33] reported the presence of Al2O3-CaO-SiO2 inclusions with an Al2O3 content exceeding 60 pct in spring steel deoxidized by Si.
Among the oxide inclusions mentioned above, it is generally agreed that the sources of the MnOSiO2-(Al2O3) inclusions are either deoxidization or precipitation products.[34–37] Several studies have been conducted on Al2O3-SiO2-CaO inclusions. Korousio[3
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