| Summary: | 碩士 === 國立成功大學 === 材料科學及工程學系 === 104 === It is necessary to develop new free-cutting steels with good machinability in order to meet the ever-increasing demand for machining efficiency in industry. The addition of sulfur (S) can improve the machinability of steel by forming manganese sulfide (MnS) inclusions. These inclusions lower the shear strength of steel such that the cutting resistance is reduced, with MnS being the stress raiser. Since the morphology and the uniformity of the MnS inclusions critically determine the machinability of steels, the reactions involving MnS formation during solidification need to be carefully assessed, especially with regard to whether they are eutectic or monotectic reactions. Globular MnS is formed from the MnS-rich liquid (L2) through a monotectic reaction, which provides a greater benefit for machining. However, the temperature difference (“∆T”) between these two reactions is so close that doping elements may alter the solidification path of the liquid steel, and may result in a eutectic reaction.
In this study, we focus on establishing the relationships between alloying elements and solidified microstructures by utilizing both the calculation of phase diagram (CALPHAD) method and high-temperature experiments with an atmosphere-controlled high-frequency induction furnace. Based on a commercial thermodynamic database, TCFE7, we simulated the solidification path of the pure Fe-Mn-S ternary system and some alternative paths with alloying elements carbon (C) and silicon (Si) to further evaluate the effects of S content on the microstructure of MnS. Moreover, we also systematically evaluated the effects of various alloying elements on the microstructure of MnS based on their effects on changing “∆T”. These alloying elements can be categorized into three groups: C, Si, Nb, Cr, V, and Mo are eutectic-stabilizers, O, Cu and Al are monotectic-stabilizers, and Ta, Zr, Ni, N, P, W, H, Ar, B, and Co are inert dopants, which do not noticeably change the microstructure of MnS. Among these, oxygen (O) is identified as a super-strong monotectic-stabilizer, and the addition of oxygen addition can drastically enhance the monotectic-type MnS, which is desirable for free-cutting steels. The thermodynamic predictions agree closely with the results of high-temperature experiments. With the combined efforts of thermodynamic calculations and high-temperature experiments, the morphology, size, and uniformity of MnS inclusions can be optimized for the development of better free-cutting steels.
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