淬火暨沃斯田鐵逆相變微合金中錳鋼之研究

Abstract

本研究係著重於觀察淬火暨沃斯田鐵逆相變微合金中錳鋼之顯微結構與機械性質，以及討論與比較鈦、鈮、鉬、釩的微合金碳化物析出行為與其所扮演之角色。此研究發現微合金碳化物的形成可藉由兩種形式達成材料強度的強化。在沃斯田鐵化中，析出粗大的碳化物可以藉由Zener Pinning Effect幫助沃斯田鐵晶粒細化，並產生較細小的麻田散鐵組織以及隨後產生均勻分布的殘餘沃斯田鐵。而在沃斯田鐵逆相變中，大量析出細小的碳化物可以有效地提供析出強化。此目的在於彌補高溫退火造成的軟化以及增加材料強度。此外，藉由良好地調控沃斯田鐵逆相變的溫度，即可產生最大量的殘留沃斯田鐵以及提供最佳的延伸率。基於本研究提出的結論，我們設計了一新穎的鈮鉬釩添加的鋼種且結果顯示其有最佳的機械性質(降伏強度> 700 MPa；抗拉強度> 1000 MPa；均勻延伸率> 14 %；破壞延伸率> 20 %)。本研究係展現透過合金設計以及製程調控使先進超高強度鋼達到優異的機械性質。

The current work investigates micro/nanostructure and mechanical behaviors of microalloyed medium-Manganese steels processed by quenching and austenite reversion (Q&AR). Precipitation behaviors of carbides due to complex additions of Ti, V, Nb, and Mo and their roles were compared and discussed in this work. It was found that formation of microalloyed carbide can contribute to materials’ strength in two ways. During austenization, coarser carbides enable significant grain refinement by the Zener pinning effect, leading to finer martensite laths and uniform distribution of reversed austenite in latter processes. During austenite reversion, precipitation of extremely fine carbides enables effective precipitation strengthening. All these benefits compensate the softening due to inter-critical annealing and increase materials yield strength. Moreover, with well-controlled temperature of austenite reversion, optimized ductility can be obtained when volume fraction of retained austenite is maximized. Based on established principles, a Nb-Mo-V-adding steel was designed for optimized mechanical properties: yield strength > 700 MPa, ultimate tensile strength > 1000 MPa, uniform elongation > 14 %, total elongation > 20 %. This thesis demonstrates the novel process and the combination of alloy design to create advanced high-strength steels with excellent properties.