低溫沃斯回火對無碳化物析出變韌鐵顯微結構的演變及機械性質之影響

Abstract

無碳化物析出變韌鐵可以藉由在鋼鐵中添加1.5wt%的矽並經過適當的熱處理得到。此種結構主要是由束狀變韌肥粒鐵及高碳含量的殘量沃斯田鐵所組成，且具有良好的強度及韌性。些許塊狀的麻田散鐵/沃斯田鐵混和相亦存在於結構中，於降溫的階段時有可能會由沃斯田鐵分解成麻田散鐵相。此種塊狀麻田散鐵會對機械性質造成嚴重的影響。因此，沃斯田鐵的穩定性在提升鋼鐵性質中扮演一個很重要的角色。 在本篇研究中，著重於低溫的沃斯回火熱處理，我們發現由於TRIP效應的原因，含有變韌鐵組織的中碳鋼相對於低碳鋼，能在不犧牲材料伸長量的前提下，擁有更高的強度。後續則藉由XRD、EBSD及TEM技術來研究沃斯田鐵的穩定性及在材料塑性變形時的顯微結構演變。可以得知含有較高碳含量的鋼鐵具有更好的沃斯田鐵穩定性。塊狀沃斯田鐵也可以在中碳的變韌鐵組織裡面被保留到室溫。

Carbide-free bainite can be obtained by alloying over 1.5 wt% silicon (Si) in steels with appropriate heat treatments. This structure comprises of bainitic ferrite and carbon-enriched retained austenite, which possesses great combination of strength and toughness because of the TRIP effect. Some blocky martensite-austenite constituents also exist in the matrix, which might decompose into martensite at the cooling stage. Blocky martensite has detrimental effect to the mechanical properties. Therefore, the stability of retained austenite in the microstructure plays an important role in enhancing the quality of steels. In this research, low austempering temperature heat treatments were conducted. We discovered that the medium-carbon bainitic steels possess better tensile strength than low-carbon bainitic steels without sacrificing the elongation because of noticeable TRIP effect. Further investigations on the stability of austenite and the microstructure evolution during plastic deformation were also conducted by XRD, EBSD and TEM techniques. We found that the stability of austenite increases as the carbon content of steels increases. Blocky austenite can also be retained to the ambient temperature in medium-carbon bainitic steels.