Fe-15 Mn-0.03 C (wt. %) 鋼之沃斯田鐵逆相變

Abstract

此研究中，以熱膨脹儀來進行退火及利用Koistinen-Marburger 方程式和量測晶粒大小，可以估計Ms溫度並與熱膨脹曲線量出的Ms比較，可以發現較低的退火溫度時導致晶粒尺寸較小，Ms溫度也較低，說明了經由降低晶粒尺寸，原本平衡上不穩定的沃斯田鐵可以被保留到室溫成為介穩相。 此外，此研究透過穿透式電子顯微鏡、X 光繞射、電子背向散射以及穿透式菊池繞射技術分析Fe-15 Mn-0.03 C (in wt. %) 鋼之顯微結構及機械性質，研究顯示當退火溫度較低時雖然晶粒尺寸較小，但能達到高強度及極佳的延展性，最好的拉伸性質為在600 °C 及650 °C 退火，原因為低退火溫度造成沃斯田鐵晶粒細化，在淬火至室溫時可以有效的抑制麻田散鐵相變態，因此當塑性變形時，細晶粒介穩態沃斯田鐵經由TRIP 效應轉變成ε 麻田散鐵及α'麻田散鐵，提供高強度及極佳的延展性。

In this work, the annealing is carried out by dilatometer. Applying the Koistinen-Marburger equation and calculated grain size, the Ms temperature was estimated and compared to which was measured from dilatometeric curve. It was found that annealing at a lower temperature leads to extremely small grains and low Ms temperature. This demonstrates that chemically unstable austenite can be preserved as a metastable phase at room temperature when a significant reduction in grain size is achieved. Besides, the annealed microstructure and mechanical properties of Fe-15 Mn-0.03 C (in wt. %) steel have been investigated by using transmission electron microscopy, X-ray analysis, electron backscattered diffraction and transmission Kikuchi diffraction in this work. It was found that annealing at a lower temperature leads to both high tensile strength and excellent elongation in spite of its extremely small grains. The best tensile properties is found annealing at 600 °C and 650 °C, which leading to both high tensile strength (~1.2GPa) and excellent elongation (~35%). The reason is that refinement of austenite grain size due to lowering annealing temperature could significantly inhibits martensite transformation during quenching to room temperature. Hence, during plastic deformation, tiny and metastable austenite transformed into ε martensite and α' martensite, producing high strength and excellent ductility via TRIP effects.