Sulfide Transformation with Tellurium Treatment for Y15 Free-Cutting Steel
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ITH the increasing degree of industrial automation, free-cutting steel is widely used in automobile manufacturing, machine tool assembly, instrument production, and other fields, which puts forward higher requirements for the machinability of free-cutting steel. Y15 is a typical kind of automatic machine tool low-carbon sulfur-free-cutting steel. Sulfide plays a very important role in the processing of steel. Even in the same grade of steel, the different morphologies of sulfides will lead to differences in machinability—the lower the aspect ratio of sulfide, the better the machinability of steel. Therefore, controlling the morphology of sulfide in steel is a key point to improve the machinability of steel. There are two routes to control sulfide morphology in steelmaking process. One route is to make the precipitated sulfide fine and dispersed by using fine oxide inclusions as the heterogeneous nucleation sites and it is found that titanium, magnesium, and zirconium can usually form fine oxides when added to steel. Oikawa et al.[1] proposed that the size of MnS inclusion was
SHUO ZHANG, FENG WANG, and JIANHUA LIU are with the Institute of Engineering Technology, University of Science and Technology Beijing, Beijing 100083, China. SHUFENG YANG and JINGSHE LI are with the School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing, 100083, China. Contact e-mail: [email protected] Manuscript submitted November 7, 2018. Article published online June 24, 2019. 2284—VOLUME 50B, OCTOBER 2019
reduced by two-thirds when titanium was added to Fe-0.1C-1Mn-0.02S alloy steel. Luo et al.[2] studied that the single strip-like MnS disappeared after adding Mg into SS400 steel (w([S pct]) = 0.0066), the nucleation site of MnS changed from Al2O3 to Al2O3-MgO, and the average size of inclusions decreased. Jiang et al.[3] reported that ZrO2 became a good nucleating agent for MnS when w([Zr]) was 0.072 pct and w([S]) was 0.012 pct, which promoted MnS disperse in the steel deoxidized by Al and Zr. Another route is to modify MnS inclusions by calcium and rare-earth elements to reduce the aspect ratio of inclusions. Tomita[4] greatly reduced the average aspect ratio of sulfide in low sulfur (0.0008 pct) 0.4C-Co-Mo-Ni steel by calcium treatment. Lis[5] controlled the composition of sulfide to be (Mn, Ca)S by calcium treatment, and they were usually oxide sulfides with an oxide core and a sulfide surface layer. Zhang[6] pointed out that the effect of calcium treatment was not only related to w([S]) in steel, but also to w([O]) in steel. In Leon’ study,[7] the shape of sulfide identified as rare-earth sulfides was global in the laboratory steels containing 0.020 to 0.25 pct S. Although some previous investigations have been carried out, studies on the control of MnS morphology are still limited. The method of adding titanium and zirconium is not suitable for steels with high w(TO), and the way of adding calcium is difficult to get the best of both worlds: spheroidization of MnS and avoidance of nozzle clogging,[
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