為了要滿足工業上對於鋼鐵切削性質日益漸增的需求，開發具有良好切削性質的快削鋼材是必需的。硫化錳介在物為易切削物質之一，煉鋼過程中可藉由硫與錳元素的添加來形成硫化錳。硫化錳可以破壞鐵基體的連續性而使切屑易斷，當含硫化錳之鋼材受剪切外力作用時，其可做為應力集中源降低鋼材的切削抗力，進而增加鋼材的切削性質並降低切削刀具的磨損。硫系快削鋼的切削性能受其中硫化錳形狀、尺寸及分佈決定，因此在鋼材凝固的過程中，硫化錳的形成機制需要被謹慎的評估。硫化錳主要是鋼液經由共晶或偏晶反應所形成，而對切削性質較有利的球狀硫化錳是鋼液經偏晶反應形成的富硫化錳液態後續凝固而成的。但是這兩個反應的溫度差異(“∆T”)甚小，微量合金元素的添加即會影響硫化錳形成反應之相平衡溫度進而改變鋼液的凝固路徑使得共晶反應的硫化錳生成。
為了能夠精準地控制硫化錳的形貌，本研究使用熱力學計算方法(Calculation of phase diagram, CALPHAD) 搭配高週波感應高溫熔煉實驗。根據TCFE7商用熱力學鐵基資料庫，模擬Fe-Mn-S三元及Fe-C-Si-Mn-S五元合金系統之鋼液的凝固路徑，進一步評估硫含量對硫化錳形貌的影響。此外；根據合金元素添加對“∆T”的改變情形，有系統性的評估合金元素對硫化錳形貌的影響，並將其概分為三類：C, Si, Nb, Cr, V和Mo為共晶反應穩定劑；Al, Cu和O為偏晶反應穩定劑；Ta, Zr, Ni, N, P, W, H, Ar, B和Co為惰性添加劑。其中O為超強的偏晶反應穩定劑，可以顯著的增加球狀的硫化錳的形成。結合高溫實驗與理論熱力學計算，提出控制硫化錳析出形貌、尺寸及分布的材料設計原則，開發更具良好切削性質的快削鋼材並優化其冶煉工藝技術。

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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