Understanding the Formation and Evolution of Oxide Inclusions in Si-Deoxidized Spring Steel
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TRODUCTION
SPRING wires are extensively utilized in automotive engines and industrial suspension applications.[1,2] Because of the exposure to high-frequency dynamic loads and consistent alternating stress during working, supreme tensile, elastic and yield strengths are especially demanded for the spring steels.[3–5] In addition, spring steels are required to have superior hardenability and surface roughness.[6–8] The mechanical properties of spring steels mentioned above are greatly influenced by the cleanliness of the molten steel.[9,10] Primary contributing factors to fatigue failure are the local stress concentration caused by non-deformable inclusions, the characteristics of which include the inclusions’ diameter, morphology and elasticity and the expansion coefficient difference between inclusions and the matrix.[11,12] Large
SHA LYU, XIAODONG MA, HAE-GEON LEE, GEOFF WANG, JIN ZOU, 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 December 20, 2018. Article published online June 3, 2019. 1862—VOLUME 50B, AUGUST 2019
inclusions with poor deformability and irregular morphologies easily cause minor voids and cracks at the steel/inclusion interface during hot rolling.[13–17] Due to the impossibility of complete removal of the inclusions from molten steel, inclusions should be controlled with high deformability, spherical shapes and low liquidus temperatures. In practice, owing to the frequent presence of non-deformable and irregular Al2O3 and spinel inclusions in Al-deoxidized spring steel, which is responsible for submerged entry nozzle clogging[18–20] and aggravated surface roughness,[21] Si deoxidization and low-basicity refining slags are widely utilized for the processing route of spring steel. The large-sized Al2O3-SiO2-MnO inclusions with high liquidus temperatures, which are usually observed in Si-deoxidized spring steel, are detrimental because of their large sizes and poor deformability. Previous studies lacked agreement regarding the formation mechanisms of Al2O3-SiO2-MnO inclusions. Chen et al.[22] found these inclusions evolved in this sequence‘‘ ‘‘Al2O3-SiO2-MnO fi CaO-Al2O3-SiO2 fi Al2O3-SiO2-MnO’’. Wang et al.[23] observed CaO-SiO2-MnO-Al2O3-(MgO) inclusions were formed through the interaction of CaO-SiO2-MnO-(MgO) inclusions with molten steel. Wang et al.[24] explained SiO2 precipitation in MnOSiO2-Al2O3 inclusions based on thermodynamic analysis; they were observed as eye-like dual-phased inclusions and
METALLURGICAL AND MATERIALS TRANSACTIONS B
pure SiO2 particles after hot rolling. Also, disagreements about the formation mechanisms of CaO-SiO2 inclusions were found in previous research. Some researchers suggested these inclusions originated from modified Al2O3-SiO2-MnO inclusions, with the MnO inside reduced by the [Ca] in the molten steel.[22,25] Other res
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