先前沃斯田鐵晶粒尺寸對於超級變韌鐵之影響與微結構組織觀察

Abstract

在鋼鐵中添加約2wt%的矽( Si )，並在過冷度大之環境下可以生成無碳化物析出之變韌鐵，此種組織主要由變韌鐵與富含碳的殘留沃斯田鐵所組成，並藉由大量的晶界與差排強化，因此具有良好的韌性與強度，稱為超級變韌鐵。不用太過複雜的製程，但需要非常長時間的恆溫熱處理。因此本實驗著重在以不改變合金成分的前提下，利用不同沃斯田鐵晶粒尺寸來進行恆溫熱處理，並加速相變態。超級變韌鐵相變態過程為成核控制，主要成核位置是在沃斯田鐵晶界上，在先前沃斯田鐵晶粒尺寸較小的試樣中，由於晶界面積增加；成核位置增多，因此恆溫相變態初期，即相變態第一階段，有較快的相變態速度。但是隨著恆溫熱處理的進行，無論沃斯田鐵尺寸大小，新生的變韌鐵會借助於先前變韌鐵的尖端或是晶界成核，因此成核位置變多，變韌鐵變態會加速並達到穩定硬度，到達穩定硬度的變態時間是相差無幾的。但是沃斯田鐵會受到變韌鐵剪切相變態的影響，會有相對較高的應變與差排密度，會減緩甚至停止變韌鐵的變態，稱為沃斯田鐵的機械穩定化。此現象在沃斯田鐵晶粒尺寸相對較小的試樣中特別明顯，並且恆溫熱處理達到穩定硬度時，此試樣會擁有較低的變韌鐵比例。利用上述之結論，本研究設計了兩段式熱處理，利用先前沃斯田鐵晶粒尺寸較大的條件，先在變態速度較快，但整體硬度較差的300℃恆溫熱處理兩小時後，降溫至變態速度較慢，但是有較佳硬度的200℃繼續恆溫熱處理，檢視硬度結果達到穩定硬度的時間明顯的縮短，但是相對的也犧牲10%左右的硬度。

Adding ~2wt% silicon to the carbon steel can produce carbide-free bainite under the proper environment. This kind of structure consists of a mixture of bainitic ferrite and carbon-enriched residual austenite. The steel is strengthening by lots of grain boundaries and dislocations; therefore it is so called “super-bainite” due to its excellent properties in tensile strength and toughness. It’s really simple to gain super-bainite through just the isothermal transformation; however it takes much heat treatment time. In this research, the method about accelerating bainite transformation just adjusts the prior austenite grain size without changing the alloy element composition design about the steel. Super-bainite transformation rate is nucleation control. The nucleation sites are mainly at the prior austenite grain boundary so that the fine prior austenite grain will increase not only the amount of nucleation sites but also the transformation rate due to the much more grain boundary area. At the beginning of isothermal transformation, the smaller prior austenite grains size the faster transformation rate. The higher bainite volume fraction will induce the faster transformation rate because the new bainite sub-unit can nucleate from the tip or grain boundary of the prior bainite sub-unit. No matter what the prior austenite grain size is, the transformation rate will tremendous speed up as heat treatment time goes by and cease while the hardness reaches a stable value. Different prior austenite grain size will take roughly the same time in order to reach their stable value of hardness. Once bainite transform more, the induced strain and also the dislocation density in austenite will increase more. These phenomena will let the austenite mechanical stabilize. Therefore, the increment of hardness is retarded by the mechanical stabilization of austenite. The smaller prior austenite grain will be induced more heavily strain, and the final hardness of the bainite transforming from the smaller prior austenite grain is lower than bainite which transforming from larger prior austenite grain. Because of the experiment result, we can introduce a new method about heat treatment. Bainite isothermal transformation at 300℃ can achieve faster transformation but lower hardness than transforming at 200℃. We can isothermal transform at 300℃for the first two hours and then at 200℃ for rest of the heat treatment time. As the result, the sacrifice about 10% hardness can accelerate the overall transformation time.