溫度與應變速率對雙相不銹鋼熱作性之影響

The Effects of Temperature and Strain Rate on the Hot Workability of Duplex Stainless Steel

洪明泰(Ming-Tai Hong)

指導教授 ： 蔡劭璞

國立臺灣大學/工學院/材料科學與工程學系/碩士(2024年)

https://doi.org/10.6342/NTU202403466

摘要

本研究探討了溫度和應變速率對雙相不銹鋼熱作性的影響，重點分析裂縫形成、動態再結晶（DRX）和織構演變。對破斷面的統計分析顯示，隨著溫度升高，裂縫面積比例下降。根據文獻，雙相不銹鋼在急速變形過程中存在應變不匹配現象，與多晶材料的Taylor模型相矛盾，也是導致裂縫成核成長的主要原因。進一步的裂縫成核位置分析顯示，裂縫傾向於在肥粒鐵與沃斯田鐵界面形成而非肥粒鐵與碳化物界面，且溫度影響主要裂縫起始位置由何種界面主導，結論是在大部分溫度範圍由肥粒鐵與沃斯田鐵相界主導裂縫成核。

利用GOS (Grain Orientation Spread)值曲線量化殘留應變，為分析雙相不銹鋼等複合型材料的應變分布提供了一種新工具。低溫下，DDRX (Discontinuous Dynamic Recrystallization)由相界處的異質成核主導，而高溫下則加強了CDRX (Continuous Dynamic Recrystallization)和動態回復。肥粒鐵因其較高的疊差能表現出顯著的動態回復，而沃斯田鐵因疊差能較低顯示出較高的殘留應變與相對疲弱的動態回復效益，有趣的是透過GOS值曲線發現沃斯田鐵的應變量達到臨界值時會啟動特定的動態回復機制使應變量大幅下降。

從EBSD數據中獨立分析動態再結晶晶粒，探討溫度和應變速率對軟化機制的影響。結果顯示，隨著應變速率增加，肥粒鐵和沃斯田鐵的動態再結晶比例顯著增加，這主要是由於高應變速率下應變量或差排密度的增加促進了動態再結晶過程，特別是在1050°C至1100°C之間，變化尤為明顯。溫度升高使肥粒鐵形成織構，表明從低溫時的不連續動態再結晶（DDRX）轉變為高溫時的連續動態再結晶（CDRX），形成<110>的纖維織構。對於沃斯田鐵而言，需要在更高的溫度和應變速率下，行為才會由不連續動態再結晶轉為連續動態再結晶主導，形成<111>和<001>纖維織構。這些發現強調了調整應變速率和溫度以優化雙相不銹鋼熱作性和微觀結構特性的必要性。

關鍵字

雙相不銹鋼 ； 熱作性 ； 不連續與連續動態再結晶 ； 織構演變

並列摘要

This study investigates the effects of temperature and strain rate on the hot workability of duplex stainless steel, focusing on crack formation, dynamic recrystallization (DRX), and texture evolution. Statistical analysis of the fractography shows a decrease in crack area proportion with increasing temperature. This indicates the presence of strain incompatibility in duplex stainless steel during severe deformation, contrary to the Taylor model of polycrystalline materials. Further analysis of crack nucleation sites reveals that cracks tend to form at the ferrite-austenite interfaces rather than at ferrite-carbide interfaces. Temperature primarily influences the dominant interface for crack initiation, with ferrite-austenite interfaces being the main nucleation sites across most temperature ranges.

Using GOS value curves to quantify residual strain provides a new method for analyzing strain distribution in composite materials such as duplex stainless steel. At low temperatures, DDRX is dominated by heterogeneous nucleation at phase boundaries, while at high temperatures and high strain rates, CDRX and dynamic recovery are enhanced. Ferrite, with its high stacking fault energy, exhibits significant dynamic recovery, whereas austenite, with its lower stacking fault energy, shows higher residual strain and relatively weaker dynamic recovery efficiency. Interestingly, GOS value curves reveal that when the strain in austenite reaches a critical value, specific dynamic recovery mechanisms are activated, leading to a significant reduction in strain.

Independent analysis of DRX grains from EBSD data explores the effects of temperature and strain rate on softening mechanisms. Results indicate that with increasing strain rate, the proportion of DRX in ferrite and austenite significantly increases. This is mainly due to the increased strain or dislocation density at high strain rates, promoting the DRX process, especially between 1050°C and 1100°C. Elevated temperatures cause ferrite to form textures, indicating a transition from low-temperature DDRX to high-temperature CDRX, forming <110> fiber textures. On the other hand, for austenite, behavior shifts from DDRX to CDRX dominance at higher temperatures and strain rates, forming <111> and <001> fiber textures. These findings underscore the need to adjust strain rate and temperature to optimize the hot workability and microstructural characteristics of duplex stainless steel.

並列關鍵字

duplex stainless steel ； hot workability ； discontinuous and continuous dynamic recrystallization ； texture evolution

參考文獻

[1] K.H. Lo, C.H. Shek, J.K.L. Lai, Recent developments in stainless steels, Materials Science & Engineering R-Reports 65(4-6) (2009) 39-104.

[2] V. Shamanth, K.S. Ravishankar, K. Hemanth, Duplex Stainless Steels: Effect of Reversion Heat Treatment, in: D. Zoia (Ed.), Stainless Steels and Alloys, IntechOpen, Rijeka, 2019, p. Ch. 5.

[3] S. Tsuge, Recent Advances in Stainless Steel, in: F.G. Caballero (Ed.), Encyclopedia of Materials: Metals and Alloys, Elsevier, Oxford, 2022, pp. 200-207.

[4] W.D. Callister, D.G. Rethwisch, Materials Science and Engineering, Wiley2014.

[5] G. Chail, P. Kangas, Super and hyper duplex stainless steels: structures, properties and applications, Procedia Structural Integrity 2 (2016) 1755-1762.