超高強度熱軋TRIP鋼之合金設計、組織控制、機械性能研究

Abstract

傳統之相變誘發塑性(Transformation-induced plasticity, TRIP)鋼的製程需經由冷軋後再退火至兩相區以利於顯微組織結構控制及溶質擴散。而熱軋製程的TRIP鋼雖然製造容易、成本與耗能較低，但其在產線上存在著無法精確控制顯微組織結構導致性能不如預期等問題。故本論文乃藉由合金設計、組織控制及機械性能等面向之研究，致力於開發新型超高強度熱軋TRIP鋼，並維持其優異之延展性。 本研究首先藉由熱力學計算鋁含量添加對於碳錳矽TRIP鋼相變態溫度之影響，目的在於提升材料相變態的溫度區間，讓完軋的同時可以產生動態析出肥粒鐵(dynamically transformed ferrite)。產生動態析出肥粒鐵有諸多好處，例如：晶粒細化、由應變量控制相比率等等。接著透過動態膨脹儀精確地模擬熱軋製程的參數，使最後得到目標顯微組織結構。本研究以光學顯微鏡及背向繞射圖譜技術分析此材料的相變行為與顯微組織結構。 吾人亦發現此合金系統在高溫變形後會有多重釋放應變能的機制，故以動態膨脹儀、光學顯微鏡、背向繞射圖譜與原子探針顯微鏡分析此合金在兩相區進行壓縮變形時產生之動態肥粒鐵相變態、肥粒鐵逆相變及沃斯田鐵再結晶之間的競合關係行為。相變晶體學被用以解釋一種全新被報導的相變行為：肥粒鐵的逆相變行程的新沃斯田鐵可作為再結晶的成核點。溶質在肥粒鐵與沃斯田鐵的晶界上的擴散行為也以原子探針顯微鏡加以分析，其結果顯示應變誘發相變態遵守亞穩平衡(para-equilibrium)，而置換型原子仍然有機會以短程擴散聚集至界面上。此外，在應變誘發相變態後，肥粒鐵與沃斯田鐵的碳濃度需以修正的相圖來解釋，此修正的相圖為沃斯田鐵加上應變後的平衡相圖。 為進一步提升熱軋鋼之強度，吾人以修正後的成分(重量百分比濃度Fe-0.21C-2.04Mn-1.04Si-1.10Al-0.064Nb-0.2Mo)進行熱軋產線實驗。兩種具有不同應力應變行為的超高強度且極具延展性的TRIP鋼得以被成功發開。兩種鋼的奈米/微米顯微組織結構、強化機制與TRIP效應以掃描式電子顯微鏡、背向繞射圖譜、穿透式電子顯微鏡與能量色散X射線譜進行分析。其結果顯示，兩種TRIP鋼的抗拉強度都超過1080 MPa，且延展性亦超過25 %。此外，吾人也針對方向性對此TRIP鋼的拉伸性能之影響加以研究，並以肥粒鐵與沃斯田鐵的織構解釋其異向性行為：以橫向拉伸的試片和以軋延方向拉伸的試片具有相當的降服強度，然而，以軋延方向拉伸的試片具有較強的相變誘發塑性行為，使其擁有較高的抗拉強度與較高的延展性。

Conventional transformation-induced plasticity (TRIP) steels rely on cold-rolling and intercritical annealing process to control the microstructure and solutes partitioning. On the other hand, although hot-rolled TRIP steels are easier to fabricate with lower cost and energy consumption, the microstructures are difficult to control, leading to undesirable mechanical properties. Therefore, we aim at developing new ultra high-strength hot-rolled TRIP steels with excellent ductility by investigating the alloy design, microstructure control and mechanical properties. The present study firstly focus on the effect of Al content on the Ae3 temperature of C-Mn-Si TRIP steels by thermodynamic calculation. Increasing the Ae3 temperature lead to the formation of dynamically transformed ferrite during rolling process, which enables several advantages, such as grain refinement and more controllable phase fraction by adjusting strain values. The thermo-mechanical treatments were precisely controlled by using DIL805A/D dynamic dilatometry to find suitable hot-rolling parameters to obtain the desirable final microstructures. The phase transformation behaviors and microstructure in the new Al-containing TRIP steels were investigated by optical metallography (OM), and electron backscattering diffraction (EBSD). It was observed that multiple softening mechanisms occurred to release the strain energy after hot deformation in the studied steel. The coopetitive relationships among dynamic ferrite transformation (DFT), reverse ferrite transformation (RFT), and austenite recrystallization were investigated by using dynamic dilatometry, OM, EBSD, and atom probe in the specimens subjected to hot compressions in the two-phase region. The transformation crystallography was used to verify the occurrence of reverse-transformation-induced recrystallization. In this microscopic mechanism, new austenite grains formed by reverse transformation can act as seed for recrystallization. The solute partitioning behaviors at the ferrite/austenite interfaces were also characterized by using atom probe tomography (APT). The results showed that the strain-induced transformation follows para-equilibrium (PE) condition, while the substitutional solutes may also segregate at the interface to form concentration spikes. In addition, the carbon concentrations in austenite and ferrite after strain-induced transformation were coincident with the modified phase diagram by adding strain into austenite phase. A modified compositions of Fe-0.21C-2.04Mn-1.04Si-1.10Al-0.064Nb-0.2Mo (in wt. %) were applied to hot-rolling processes, to further increase the strengths of the hot-rolled steels. Here, two strong and ductile TRIP steels with distinguishable stress-strain behaviors were developed. Nano/microstructures, strengthening mechanisms, and behaviors of TRIP in two steels were elucidated based on investigations of EBSD, transmission electron microscopy (TEM), and X-ray diffraction (XRD). Both developed steels had ultimate tensile strength (UTS) of over 1080 MPa and total elongation (TEL) of over 25 %. Furthermore, the effects of directionality on tensile properties were also investigated, and elucidated by the micro-textures of ferrite and austenite in the studied steel. It was observed that the specimen had isotropic yield strength (YS) when deformed along transverse direction (TD) and along rolling direction (RD). However, a stronger TRIP effect was observed in the specimen deformed along RD, leading to a higher UTS and a higher TEL.