Formation Mechanism of Al 2 O 3 -Containing Inclusions in Al-Deoxidized Spring Steel
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INTRODUCTION
SPRING steel with excellent mechanical strength and fatigue resistance is usually used in spring wires for automobile clutches and suspensions.[1–5] Owing to the differences in thermal expansion coefficients and the mutual adhesion between the inclusion and steel matrix, local stress concentrations and minor voids would be generated around inclusions, which deteriorated the fatigue resistance and even aggravated crack initiation of coil springs during working.[6–8] Owing to the increasing demand for lightweight materials with long working life for automobile applications, nonmetallic inclusions in steel should be carefully controlled.[9] The fatigue resistance of spring steel is closely related to three inclusion properties. (1) Inclusion dimensions are crucial to the flexural fatigue limit of spring steel. Tanaka and Mura[10] concluded that fatigue strength
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 Baoshan Iron and Steel Co. Ltd., Shanghai 200122, P.R. China. ZHOUHUA JIANG is with Northeastern University, Shenyang 110819, P.R. China. Manuscript submitted January 25, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS B
and crack initiation life were reduced by increased inclusion sizes. Zhang et al.[11] investigated the effects of inclusion sizes on fatigue behaviors of spring and found that the fatigue strength was substantially improved due to the decreased average inclusion sizes. (2) Irregular morphologies with sharp edges, which were strongly influenced by primary chemical composition, could result in stress concentrations and minor cracks near inclusions. Lei et al.[12] found that the morphology of the inclusion on the fracture surface was distinguished from that of other inclusions as the fatigue crack origin. (3) The differences in thermal expansion coefficients and adhesion properties between inclusions and steel exerted enormous influence on steel properties. Laz and Hillberry[13] found that the stiffness and thermal expansion coefficients between inclusions and the surrounding matrix resulted in localized stress concentration, which easily resulted in fatigue crack formation. Kunio et al.[14] found that the poor adhesion between the matrix and aluminum oxide inclusion led to the formation of inclusion pits, which serve as a simple stress raiser. Currently, to obtain fantastic deoxidization efficiency and ultralow oxygen content in molten steel, ferroaluminum alloys are widely used for the deoxidation of spring steel.[15–18] Various nondeformable inclusions have been observed in Al-deoxidized steel, such as Al2O3 and spinel related to the deoxidization equilibrium,[19,20] which could reduce the fatigue resistance,
deteriorate the surface roughness and weldability, and act as breakage initiation sites during hot rolling.[21–24] In addition, Al2O3-CaO inclusions were frequently observed in Al-deoxidized steel.[25–27] Eid and Thomason[28] observed th
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