奈米析出物強化雙相鋼之界面碳化物析出行為及相關機械性質研究

The Study of Interphase Precipitation Mechanism and Related Mechanical Properties in Nanometer-Sized Carbides Strengthened Dual-Phase Steels

蘇德徵(Te-Cheng Su)

指導教授 ： 楊哲人

國立臺灣大學/工學院/材料科學與工程學研究所/碩士(2013年)

https://doi.org/10.6342/NTU.2013.02416

摘要

為了要減少二氧化碳的排放，各大廠正著手研發先進輕量化高效率汽車。輕盈的汽車可以增進效能所以目前對於製造高強度、輕盈且價格合理的材料有很大的需求。
雙相鋼由於優良的成形性以及低製造成本，被視為適用於新一代汽車的主要材料。但是以往雙相鋼由於在麻田散鐵和肥粒鐵相界受力應變行為有所差異，導致擴孔時容易生成裂紋。為了解決這個問題，Fang (2003)提出只要提高降伏強度和抗拉強度的比值，也就是提高肥粒鐵的強度，就可以改善雙相鋼的擴孔性。所以此奈米析出雙相鋼計畫欲發展新一代汽車用雙相鋼捲，乃藉由麻田散鐵以及奈米析出強化的肥粒鐵之組合，增進雙相鋼的強度與並且保持其延性。此論文著重於比較鉻和鋁在肥粒鐵相變態動力學的影響以及鈮對於析出物結構和強度的影響。
在計畫的初始階段，中鋼採用熱軋延製程搭配於680°C在輸出帶上持溫的技術製造奈米析出雙相鋼計畫的原材。此熱軋延鋼板的抗拉強度超過850MPa、大約20%的伸長量以及維氏硬度270HV以上，且用原材製備TEM試片也能夠觀察到緻密的析出物結構。使用Batte和Honeycombe提出的析出物模型和實驗對於析出物平面、平面間距、析出物大小以及試片厚度等資料，評估出界面析出物對於含鉻原材的肥粒鐵能夠貢獻接近370MPa的強度。
另外，在熱處理模擬階段淬火的實驗中，藉由改變沃斯田鐵化溫度至1050°C以及搭配650°C恆溫可以得到比以斯田鐵化溫度950°C更好的維氏硬度，其原因為更高體積比的麻田散鐵以及更強的肥粒鐵，其中肥粒鐵相較於950°C沃斯田鐵化提升了40HV，在TEM下也能更容易觀察到界面析出物結構。故本報告主張沃斯田鐵化溫度能夠減緩肥粒鐵相變態速率，進而增加奈米析出碳化物成核密度。另外在沃斯田鐵化溫度1050°C搭配700oC恆溫的熱處理可以發現較寬的析出物平面間距以及稍微粗大的析出物。
為了要比較鉻、鋁和鈮對於肥粒鐵相變態動力學的影響，已繪出沃斯田鐵化溫度於1050°C、1200°C以及長時間恆溫於650°C、700°C和750°C的恆溫相變態圖。從圖中可發現含鋁鋼種的相變態速率遠快於含鉻鋼種，表現出鋁可以增進肥粒鐵相變態速率的效果。然而，含鈮鋼種表現出少量減緩肥粒鐵成核速率但增加成長速率的情況，原因可能是碳化鈮在恆溫過程中析出消耗固溶碳原子而加速肥粒鐵成長的效果，會隨著恆溫時間逐漸超過了固溶鈮在沃斯田鐵拖慢介面移動的影響。
綜合所有TEM明場像資料以及EELS的厚度量測技術，發現影響析出面間距最大的因素為恆溫溫度，而合金添加並不大會影響此項，主要的原因是高溫具有較低的相變態自由能故使間距擴大。另外在同一析出物平面上碳化物的立體平均間距則受到合金的影響，由於合金添加造成的肥粒鐵速度差異，可推知較快速移動的界面會造成平均成核間距增加。綜合以上的所有考慮，使用鉻元素添加以及選擇恆溫溫度650°C能夠獲得最緻密的界面析出物，並且最有效提升肥粒鐵的硬度。

關鍵字

先進超高強度鋼 ； 雙相鋼 ； 熱軋汽車鋼板 ； 奈米碳化物 ； 界面析出物 ； 電子顯微鏡 ； 相變曲線

並列摘要

To reduce fuel consumption and CO2 emissions, the development of advanced highly-efficient lightweight automobiles has been proceeding. The lightweight body of cars could enhance the performance of power so there is an attempt to find strong, light and affordable material.
The dual-phase (DP) steel is considered to be the potential new generation steel for automobiles owing to the high formability and low production cost. However, DP suffers from the mismatch of strain in interphase boundary between ferrite and martensite, resulting in poor hole-expansion ratio. This problem can be alleviated by reducing the difference of strength between ferrite and martensite. Therefore, the nanometer-sized precipitation strengthened dual-phase steel project attempts to develop new automobile dual phase steel strips with excellent combination of strengths and ductility by creating a microstructure of martensite and strong ferrite by means of nanometer-sized precipitation hardening. This thesis puts emphasis on the effects of Cr, Al and Nb on ferrite transformation kinetics and their effects on the structure and strength of interphase precipitation.
In the beginning of this project, the Materials Characterization Center in China Steel Corporation applied hot-rolling process with isothermal holding at 680°C for several seconds. The tensile strength of this steel can exceed 850MPa with 20% total elongation and overall hardness at least 270HV, and the dense precipitation structures were also observed in TEM. Furthermore, by using the model proposed by Batte and Honeycombe and the experimental data such as sheet plane, sheet spacing, particle size and specimen thickness, the estimated contribution of interphase precipitation can be near 370MPa in the ferrite phase of as-received Cr steel.
In addition, the mechanical properties for the dilatometer experiment simulating the step quenching technique can be improved by selecting austenization temperature at 1050°C combined with isothermal holding at 650°C than the samples austenized at 950°C. This can effectively increase the hardness with higher amount of martensite and stronger ferrite (40HV ferrite hardness increase in average) with parallel and coherent interphase precipitation structure. Therefore, it is proposed that the austenization temperature would probably affect the ferrite growth rate and further raise the carbide nucleation density at the interphase. On the other hand, relatively wide spacing and coarse precipitation size can be observed by 700°C isothermal holding.
To compare the trend of Cr, Al and Nb addition on ferrite transformation kinetics, the time-temperature-transformation curve is shown with samples austenized at 1050°C and 1200°C. From the TTT curve drawing in dilatometer experiment, the specimen with Al has faster transformation rate than Cr steel and it represents that the addition of Al increases the ferrite transformation rate. However, the small increase of the rate by niobium addition is also observed, which could be contributed to the stronger effect of carbon atoms consumption by niobium carbide than solute drag effect of free niobium in austenite.
According to the TEM bright field images with thickness measurement through EELS, it is found that main factor to control the sheet spacing of interphase precipitation is the holding temperature. The reason for the temperature affecting the spacing is the temperature driving force for ferrite transformation. On the other hand, the average intercarbide spacing in the same sheet plane is mainly affected by alloy addition, which can control the ferrite growth rate. Therefore, it is suggested that choosing isothermal holding temperature 650°C and adding hardenability element chromium can obtain the densest interphase precipitation structure, further raising the hardness of ferrite.

並列關鍵字

advanced high-strength steel (AHSS) ； dual-phase steel ； hot-rolled steel sheet for automobile ； nanometer-sized carbide ； interphase precipitation ； time-temperature-transformation (TTT) curve

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