超高強度富錳鋼的中溫變形行為研究

Warm Deformation Behaviors of the Ultrahigh-Strength Mn-Rich Steels

程冠儒(Guan-Ju Cheng)

指導教授 ： 顏鴻威

國立臺灣大學/工學院/材料科學與工程學研究所/博士(2020年)

https://doi.org/10.6342/NTU202000369

摘要

先進高強度鋼隨著汽車輕量化以及工業結構用鋼的需求增加而蓬勃發展。然而，高強度鋼在常溫下成形卻有不可避免的缺點，例如：回彈現象、模具的高耗損率。因此，近年來高溫成形逐漸受到矚目，因為鋼材的強度將隨著溫度升高而降低，進而擁有良好的成形性。基於這些現況，吾人提出了利用調控顯微組織來達到高強度鋼中溫成形的構想，本研究透過兩種合金系統(Fe-11Mn-0.07C-1Al和Fe-11Mn-0.07C (in wt. %))探究超高強度富錳鋼中溫成形的特性與行為，致力發展高強度且高延性鋼中溫成形的冶金知識。
本研究首先探討冷軋富錳鋼(Fe-11Mn-0.07C-1Al (in wt. %))中溫變形與動態沃斯田鐵逆相變耦合下顯微結構的演變以及中溫延展性。本研究使用Gleeble 1500熱機模擬試驗機將冷軋鋼材退火後銜接相同溫度的中溫拉伸試驗，研究結果顯示當相變態伴隨變形時，其總伸長率可超過100%，其中，在650 °C可以獲得最佳的延伸率121%。顯微結構分析指出變形會加速沃斯田鐵逆相變，且動態逆相變與變形的速率並進時可彌補形變的沃斯田鐵，維持其等軸晶結構。此外，變形可誘發肥粒鐵的不連續再結晶，而無論是否施加形變都可觀察到肥粒鐵的連續再結晶。因此，富錳鋼中的中溫延展性可透過逆相變和肥粒鐵再結晶提升，因其皆可消弭變形過程造成的應變集中而延遲破裂。
當相變態與元素的擴散行為息息相關，溶質原子在變形與逆相變耦合下的分配行為將由三維原子探針進行實驗與探討，吾人在650 °C的變形條件下發現沃斯田鐵內部奈米尺度的成份起伏(錳和鋁)，而此現象在靜態相變態中不曾被觀察到。中溫變形時兩相之間的強度差異將使得應變集中於肥粒鐵，進而改變熱力學平衡，受應變影響之後相變的沃斯田鐵可透過成核或是成長的機制形成，具有缺錳富鋁的特性，穩定性較低，因此應變誘發的動態沃斯田鐵逆相變可造就晶粒內不同成份的奈米級結構。
基於富錳鋼的中溫變形特性與機制，本研究進一步透過合金設計優化中溫延展性，當移除材料中的鋁元素時將加速較低溫時的沃斯田鐵逆相變，可增加在550 °C (約0.37熔點溫度)時約17%的延伸率。當形變大幅度地加速相變態速率且相變態也輔助形變繼續進行，兩者相輔相成可提升550 °C的總伸長率至94%。此外，即使斷面減縮率達到92%，只要相變態與變形的速率並進仍可維持其等軸晶結構，此研究也證實沃斯田鐵逆相變對優化中溫延展性的效果大於肥粒鐵再結晶。
此外，本研究以不含鋁富錳鋼的溫軋延探討材料溫變形後的機械性能，其中，每道軋延間的回爐退火時間對機械性能沒有顯著影響，但是軋延量的多寡將直接反應在材料強度上。熱軋鋼材經過550 °C的溫軋延後，常溫下可獲得1400 MPa的降伏強度，1600MPa的抗拉強度與接近20%的延伸率，其優異的機械性能來自於溫軋延導致的超細晶結構，以及不穩定殘留沃斯田鐵的相變誘發塑性。溫軋鋼的強度與延展性都成功超越在550 °C退火的冷軋材，同時中溫軋延也大幅度地縮短了製程時間。
本論文提供富錳鋼溫成形的冶金知識，包含相變態與變形的交互作用，溶質原子行為，鋁元素的影響以及溫軋延製程，期許對高強度且高延性鋼種的熱製程設計有所助益。

關鍵字

中溫延展性 ； 沃斯田鐵逆相變 ； 超細晶 ； 複相鋼

並列摘要

The need to lighten vehicles or structures in industry promotes the development of advanced high-strength steels. However, other risks, such as spring-back and forming costs, arise from forming these strong steels at ambient temperature. Recently, high temperature forming of high strength steels has attracted more attention because the formability can be improved at elevated temperature. This inspired our innovation to combine the forming process and microstructure control at medium temperature. Hence, this study examined the principles of physical metallurgy for warm deformation of ultrahigh-strength Mn-rich steels. Two kinds of steel are employed, one is Fe-11Mn-0.07C-1Al (in wt. %) and the other eliminates the content of aluminium. This thesis presents knowledge for the production of strong but ductile steel by warm forming.
This study first demonstrated the microstructural evolution and the warm ductility when deformation was synchronized with austenite reversion in a cold-rolled steel (Fe-11Mn-0.07C-1Al (in wt. %)). The cold-rolled steel was warm-deformed under tension by a Gleeble 1500 after isothermal annealing. It was found that deformation at inter-critical temperatures enhanced the total elongation to over 100 % due to the accompanying phase transformation. It reached the highest ductility at 650 °C with a total elongation of 121%. The microstructural evolution showed that deformation accelerated the austenite reversion rate, and that the transformation was able to maintain the equiaxed austenite grains when it kept pace with the deformation rate. In addition, deformation triggered the discontinuous recrystallization of ferrite, while continuous recrystallization of ferrite was observed whether or not deformation was applied. As a result, the warm ductility benefited from the reversed transformation and the ferrite recrystallization, which annihilated the localized strain to delay fracture.
Solute behaviors during the reversed transformation–deformation coupled condition were studied in-depth by atom probe tomography (APT), while it was highly related to the process of phase transformation. It was found that concentrations of Mn and Al fluctuated in austenite at the nanoscale during deformation at 650 °C, which was never observed in static reversed transformation. It was proposed that stored energy in ferrite can modify the thermodynamic equilibrium, and the reversed transformation mechanisms were addressed based on growth mode and nucleation mode. The austenitic zones developed with less Mn and higher Al contents, which were the less stable chemical zones in austenite, leading to the nanostructures in dynamic strain-induced austenite.
Based on the deformation behaviors and mechanisms we revealed, warm ductility was further optimized through alloy design. It was found that accelerating austenite reversion by removing aluminum increased ductility by about 17% at 550 °C (0.37 Tm of the steel). It was found that deformation accelerated the transformation rate significantly and that the process of deformation was also assisted by transformation, leading to total elongation of 94% at 550 °C. Furthermore, the phase transformation was able to maintain the equiaxed microstructure in the region with 92% area reduction. The present work evidences the greater importance of austenite reversion than of ferrite recrystallization in improving warm ductility.
Moreover, warm rolling was employed on the hot-rolled Al-free steel to investigate the mechanical properties of the steels after warm deformation. The annealing intervals between each rolling pass did not affect the properties, but decreasing the rolling amount decreased the YS and UTS. After rolling of the hot-rolled steel at 550 °C, extraordinary mechanical properties, namely, 1400 MPa of yield stress (YS), 1600 MPa of ultimate tensile stress (UTS), and total elongation approaching 20%, were achieved at ambient temperature. Ultra-fine structures induced by warm-rolling led to the ultrahigh strength, and transformation-induced plasticity from metastable retained austenite improved the UTS and elongation by enhancing the work hardening rate. These structures successfully defeated the properties obtained from the cold-rolled steel annealed at 550 °C, and the warm rolling process also shortened the processing time significantly.
This dissertation provides a comprehensive look at research on warm forming in Mn-rich steels, including the effects of phase transformation, solute behaviors, the influences of aluminum addition, and the warm-rolling process. Overall, this research could provide metallurgical information useful in designing strong but ductile steels for warm forming.

並列關鍵字

warm ductility ； austenite reversion ； ultrafine grain ； duplex steel

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