Effect of Acid-Soluble Aluminum on the Evolution of Non-metallic Inclusions in Spring Steel
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INTRODUCTION
60SI2MN-CR is an alloyed spring steel used to produce quality springs for various applications, especially in the vehicle industry, in which the service performances, such as reliability, stability, and longevity, need to be ensured through the strict control of steel cleanliness and microstructure. It has been shown that brittle Al2O3 inclusions from Al deoxidation have a very harmful effect on the fatigue life of this steel,[1,2] and thus Si and Mn are usually adopted for the steel deoxidizing.[3–6] In this case, although no Al ingot or Al wire has been added into the molten steel, there are frequently trace amounts of acid-soluble Al ([Al]s) and Al2O3-type inclusions in the molten steel from the residual Al in Si-Fe and Mn-Fe alloys. If we define the process for Si and Mn to deoxidize as a low aluminum process (LAP), the total oxygen (TO) content of the steel in this process is usually higher due to the weaker deoxidation capacity, which will cause the cleanliness of the billet casting to decrease and the number of other oxide inclusions to increase. These will similarly affect the fatigue life of a spring. For this reason, the process of using a small amount of Al combined with Si-Mn to deoxidize begins to attract attention in the industry and is being experimentally adopted in some steel plants in China. In this paper, this process is called the high aluminum process YONG WANG, HAIYAN TANG, TUO WU, GUANGHUI WU, and JINGSHE LI are with the School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Room 412, Yejin Building, USTB, 30 Xueyuan Road, Haidian District, Beijing 400044, China. Contact e-mail: [email protected] Manuscript submitted September 11, 2016. Article published online January 27, 2017. METALLURGICAL AND MATERIALS TRANSACTIONS B
(HAP). Under the circumstance of using Al, to avoid nozzle clogging during casting and the effect of Al2O3 inclusions on the fatigue life of the spring steel, Ca-Si wire has to be fed into molten steel to modify Al2O3 into low melting point inclusions. In this case, CaS inclusions may generate under a higher S content in molten steel and/or excessive Ca-Si wire, which will also cause nozzle clogging and the decrease of the quality of the rolling product if the CaS inclusions are large in number or size. The required amount of Ca-Si wire is closely related to the [Al]s and S contents in the molten steel. For the above considerations, a proper [Al]s is of importance for the control of the TO of the spring steel and the smooth casting of the molten steel, and for the improvement of the surface and internal qualities of its billet and rolling products.[7–9] For the effect of [Al]s on molten steel, Sasai and Mizukami[10] and Min-Kyu et al.[11] proposed that less [Al]s content increased the oxygen content and the number of inclusions in the steel, and excessive [Al]s caused reoxidation during continuous casting, and the formation of Al2O3 and AlN inclusions. Tomioka et al.[12] studied the reaction thermodynamics betwe
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