界面析出型冷軋雙相鋼之合金設計、組織演化及機械性能研究

Abstract

鋼鐵中的界面析出物係指以陣列形式排列於肥粒鐵相中之奈米等級碳化物，界面析出物成核於鋼鐵沃斯田鐵相變至肥粒鐵過程中的相界面，其優異之析出強化貢獻為肥粒鐵系鋼種帶來不少新的可能。然而，至今為止，幾乎所有的界面析出型鋼種與界面析出物研究均聚焦於熱軋製程鋼板上，尚未有研究或產品探討將其導入冷軋製程鋼種之可能性，由於冷軋鋼種為汽車用鋼之大宗，本研究於是旨在設計出當代第一款界面析出型冷軋雙相鋼以提升現有鋼種性能，並對其合金設計、顯微組織演化與機械性能進行深入探討。 本研究於設計過程發現，現行冷軋產線之退火溫度上限(920 °C)應是導致界面析出物不易於冷軋鋼種中達成之主因，而若一味添加元素以滿足界面析出鋼需在退火溫度限制下完全沃斯田鐵化之要求，亦可能因添加不當，合金將無法誘發後續之肥粒鐵相變態及界面析出。於是，本團隊首先從製程及顯微組織層面考量，使用熱力學相圖計算輔助材料設計，並洞悉相變態之區域平衡理論，設計出一款理論上可達成界面析出型冷軋雙相鋼之合金成分: Fe-0.12C-2.0Mn-0.12V合金系統，簡稱1V鋼。 實驗部分首先及對設計之1V鋼進行驗證，檢驗其是否可實現界面析出型冷軋雙相鋼。於驗證過程中，吾人發現1V鋼可於870 °C持溫5分鐘即達到完全沃斯田鐵化，並在後續降至630–700 °C持溫10分鐘後達成雙相結構，證實了合金設計之成功。然而，於後續以穿透式電子顯微鏡觀察界面析物的過程，吾人亦發現1V鋼中的界面析出物數量稀少，且有分布不均、集中於一側晶界的現象，在後續與未添加析出元素之對照組相比，其機械性能對比並無顯著之提升。 為獲得更好的界面析出行為，吾人以兩種策略對1V鋼進行調整。首先，導入鉬元素希望藉由降低界面移動速率(1VMo鋼)使界面析出更容易發生；再者，添加更足量的釩以提升析出驅動能(2VMo鋼)。在成分調整後，兩種新合金仍能形成冷軋雙相鋼。藉由肥粒鐵硬度之測量，以及穿透式電子顯微鏡的觀察，皆顯示2VMo鋼中可以獲得最大量之界面析出物，以使肥粒鐵強度提升至201 HV。後續，吾人使用熱膨脹儀及三維原子探針深入解析其中原理，發現鉬之添加可以大幅降低界面移動速度，並在相界面處產生釩與鉬之複合偏析，而進一步添加足量的釩可使複合偏析量提升，提供更足量之析出驅動力，這兩種策略的協同作用是提升界面析出量的關鍵。 初步完成設計驗證工作後，吾人藉由串連各先進表徵技術，對擁有不同晶體方位關係之沃斯田鐵/肥粒鐵界面進行深入觀察研究，找尋相界面與界面析出之關聯，以更了解此設計鋼種中的界面析出行為。藉由三維原子探針數據的團簇分析與界面偏析定量，吾人發現當非共格界面方位越偏離理想K–S方位時，界面移動速率、界面偏析量及界面析出物大小將會上升，然而界面析出數量密度會下降，界面移動速率與偏析量同時上升之情形不符合現今主流以溶質擴散來解釋界面偏析產生的機制，吾人認為在本研究中產生界面偏析之機制更符合界面移動時將溶質掃入之機制。 此外，本研究亦使用電子微探儀及三維原子探針重構技術進行元素定量，以探討鋼鐵中微偏析帶對界面析出之影響，在定量結果中，發現肥粒鐵主要成核成長於微觀偏析缺乏帶上，由於此處缺乏所有溶質元素，大部分新形成的肥粒鐵將擁有較快的界面移動速度以及較少的析出驅動力，由定量結果我們可以證明微觀偏析帶對界面析出行為有害，應設法去除或避免他才能在冷軋雙相鋼中創造更好的界面析出行為。

Interphase precipitation in steel refers to nanoscale carbides arranged in an array within the ferrite phase. Interphase precipitation nucleates at the austenite/ferrite interface during the austenite to ferrite transformation, and their excellent precipitation hardening contributes to new possibilities for ferritic-based steels. However, to date, almost all research and product development on interphase precipitation have focused on hot-rolled steel strips, and there has been no investigation or exploration of their potential in cold-rolled processed steels. Since cold-rolled processed steels are mainly used in the automobile industry, this study thus aims to design the first contemporary interphase precipitation strengthened cold-rolled Dual Phase (DP) steel and thoroughly investigate its alloy design, microstructural evolution, and mechanical properties. During the design of this research, we found out the main reason causes the difficulties in implementing interphase precipitation in the cold-rolled steel strips may be the ceiling annealing temperature (920 °C) of the continuous cold-rolled and annealing line. If we blindly added too many alloying elements to let the interphase precipitation strengthened steel reach full austenitization under the annealing temperature limit, the subsequent austenite to ferrite transformation might not be triggered and no interphase precipitation will form. Hence, this research first considered the possible processing route and the needed microstructure then applied CALPHAD-assisted design and taking into account the local equilibrium mechanism. Finally, an alloy composition that may have a chance to form interphase precipitation in the cold-rolled DP steel process was designed, which is the Fe-0.12C-2.0Mn-0.12V alloy system and is denoted as 1V steel After alloy design, we initially validate through the designed 1V steel, checking its potential to become an interphase precipitation strengthened cold-rolled DP steel. During the validation, we found out that the designed alloy can be fully austenitized after holding at 870°C for 5 minutes, and can produce a dual phase microstructure after subsequent holding at 630–700°C for 10 minutes. These results demonstrated the success of alloy design. However, during the observation under transmission electron microscopy, we discovered that the quantity of interphase precipitation within the ferrite phase is scarce, unevenly distributed, and concentrated near one side of the ferrite grain boundary. As compared with the reference steel without interphase precipitation, the enhancement of mechanical properties in 1V steel is not profound. To achieve better interphase precipitation behavior, we utilized two strategies for tuning the 1V steel. First, slow down the interface moving velocity by molybdenum addition (1VMo steel), which makes the nucleation of interphase precipitation easier at the interface. Second, adding an ampler amount of vanadium to enhance the precipitation driving force (2VMo steel). After composition tuning, both new alloys can still form a dual phase microstructure. According to the ferrite hardness results and throughout the transmission electron microscopy observation, 2VMo exhibits more interphase precipitation formation, leading to a 201 HV ferrite hardness. Subsequently, dilatometer and 3D atom probe tomography techniques were applied to understand the theory behind them. The Mo addition drastically decreased the interface velocity, exerting a V+Mo co-segregation at the interface. The ampler amount of V addition can enhance the interfacial segregation content, providing more precipitation driving force. The synergy effect of both strategies is the key to enhancing the amount of interphase precipitation. After the work of validation, we then connected the use of various advanced characterization techniques, conducting an in-depth investigation on the austenite/ferrite interface with different crystallography orientation relationships to find out the correlation between the interface and the designed steel’s interphase precipitation behavior. From the cluster analysis and interfacial segregation quantification of 3D atom probe data, we found out that as an incoherent interface deviates more from the ideal K–S orientation relationship, the interface velocity, interfacial segregation, and interphase precipitation size will increase, while the number density of interphase precipitation will decrease. The simultaneous increment in interface velocity and interfacial segregation do not meet the mainstream nowadays, which considers the formation of interfacial segregation by bulk solute diffusion. We believe that the formation of interfacial segregation in this dissertation better aligns with the swept-in mechanism during interface movement. Moreover, the present dissertation also utilized electron probe micro-analyzer and 3D atom probe tomography techniques on chemical quantification to discuss the impact of micro-segregation bands on interphase precipitation behavior in steels. In the quantification results, ferrite was found to predominantly nucleate and grow on the micro-depletion bands, which lacks in all solute elements. Therefore, most ferrite grains grow too rapidly and possess insufficient precipitation driving force. The quantification results demonstrated that micro-segregation bands are harmful to the interphase precipitation behavior. It should be eliminated or avoided for better interphase precipitation behavior in the cold-rolled dual phase steel.