碳含量及二階段相變態對無碳化物變韌鐵的顯微結構及機械性質之影響

Abstract

在鋼鐵中添加約1.5-2.0 wt%的矽並且設計適當的熱處理可以生成無碳化物析出變韌鐵，此種結構主要是由束狀變韌鐵以及富高碳含量的殘留沃斯田鐵和些許的塊狀麻田散鐵所組成，具此種結構的鋼具有良好的強度以及韌性，但此種鋼隨著碳含量的上升，其反應速率也會跟著下降，進而使整體反應時間拉長，所以添加的元素對於其影響十分重要。另外，透過熱處理的設計也可以達到降低反應時間的效果，例如二階段式熱處理。本篇研究著重於探討改變碳含量及使用二階段式熱處理對變韌鐵之微結構及機械性質的影響。當碳含量增加，整體反應溫度會下降，使反應時間較長且組織會有細化的效果，而殘留沃斯田鐵的量也會因為基底和從過飽合變韌鐵中排出的碳增加而得到提升，進而使整體機械性質得到提升。透過EBSD和TEM的分析也可以發現層狀沃斯田鐵因含有較多比例的碳，所以降到室溫時能穩定存在。將變韌鐵組織進行慢速升溫時會發現，層狀沃斯田鐵會分解成碳化物加上肥粒鐵。另外，透過二階段式熱處理，在相對高溫下放一小段時間長出些許變韌鐵，再降到低溫完成反應可以有效縮短低溫變韌鐵成核期所需要的時間，達到加速反應速率的效果，且因為有兩個階段的熱處理，最終結構中也會有粗細不同的束狀變韌鐵，二階段式熱處理過後的材料強度及硬度會略微下降，其原因也是和具有一部份較粗的變韌鐵有關。

By alloying 1.5-2.0 wt% silicon in the steels and designing appropriate heat treatments, carbide-free bainite can be produced. This structure is consisted of plate-liked bainite and high carbon content retained austenite plus some blocky martensite. The steels with such structure include outstanding strength and toughness. However, as carbon content increases, the reaction rate reduces and the time for completion bainite transformation last longer. Therefore, alloying elements play an important role in affecting the properties of bainitic steels. In addition, by modifying the heat treatments can also reach the goal in decreasing the reaction time, such as two-step isothermal transformation. In this research, we are focusing on investigating the microstructure and mechanical properties in bainite by changing the amount of carbon and after undergoing two-step isothermal transformation. When the carbon content raises, overall reaction temperature will be lower, which causes the reaction time increase and the structure refinement. The quantity of retained austenite will also increase since the carbon rejected by bainite raised. The effects mention above lead to the mechanical properties’ enhancement. TEM and EBSD analysis indicate that only high carbon film-liked austenite will appear at ambient temperature. Moreover, by utilizing two-step isothermal transformation, which we put the specimen at a slightly higher temperature for a while to form some bainite, and then cool to lower temperature to complete the reaction can effectively shorten the time for nucleation and leads to the acceleration of low temperature bainite reaction. Furthermore, the contribution of this heat treatment will form different size of bainite and hence results in the reduction of strength and hardness.