高強度低合金鋼之奈米級銅顆粒及合金碳化物析出強化研究

Precipitation Strengthening of Nanometer-Sized Copper Particles and Alloy Carbides in High Strength Low Alloy Steels

高芳歆(Fang-Hsin Kao)

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/博士(2009年)

https://doi.org/10.6342/NTU.2009.01312

摘要

合金鋼中添加多種少量元素可形成強硬的碳化物；然而銅元素不與其它微合金元素形成介金屬化合物，故在鋼材中形成銅析出物可以產生另一種強化的機構。近年來為了使含銅之高強度低合金鋼應用在重工業上，研究重點主要為發展強度高、韌性好且易於成型之鋼材，而時效溫度與時效時間對硬化效應之影響，是非常值得研究。另一方面，為了改善回火麻田散鐵之機械性質，於高強度低合金鋼中添加多種微合金元素可形成複合型碳化物，此碳化物相對於單一成分碳化物而言，具有較低之成長與粗化速度，因此，在麻田散鐵基地中形成複合型碳化物，可抑制麻田散鐵因長時間回火處理時而產生軟化現象。麻田散鐵經由回火處理時可產生二次析出硬化，此現象可使回火麻田散鐵在高溫約600℃中，仍具有抗潛變破裂與耐衝擊等特性，因此被廣泛使用在工程應用上，例如:渦輪葉片、傳熱管、核能工業等。
本研究利用含0.5及1.0 wt % 銅之鋼材，經由不同溫度、時間之恆溫熱處理後，肥粒鐵基地中產生許多奈米級銅析出物。經適當之熱處理，肥粒鐵基地中可同時產生界面析出物與過飽合析出物。當鋼材含銅量達1.0 wt % 時，肥粒鐵基地可產生大量的奈米級銅析出物，且經由615℃恆溫時效處理90分鐘後，可得到最大硬度值308Hv，較低的相變態溫度，肥粒鐵基地中銅元素具有較低擴散速度與較大析出驅動力，因此可產生較多奈米級銅析出物。本實驗中兩個鋼材均觀察到界面析出之現象產生；此界面析出物現象與肥粒鐵/沃斯田鐵之界面移動速度有關。當恆溫溫度降低時，界面析出物之間距亦會隨之下降。
含Ti、Ti-Mo、Ti-Nb之高強度低合金鋼材，經由長時間回火處理後，麻田散鐵基地可產生單一成分碳化物與複合型碳化物，這些碳化物可避免麻田散鐵經由回火處理後產生回火軟化現象，分別比較TiC碳化物、(Ti,Mo)C複合型碳化物和(Ti,Nb)C複合型碳化物，研究中發現(Ti,Mo)C複合型碳化物成長速度最慢，且(Ti,Nb)C 複合型碳化物最晚析出，也因如此，含Ti-Nb之高強度低合金鋼材經回火時效處理560 小時後才出現抵抗回火軟化現象，而另一方面比較Ti、Ti-Mo、Ti-Nb鋼材之微硬度值，可看發現，添加Mo於高強度低合金鋼材中，所產生的回火軟化抵抗效果最佳。
另外，研究中使用含Ti-Mo-0.5Cu 和 Ti-Mo-1.0Cu (wt %)之高強度低合金鋼材，同樣經由長時間回火處理後，麻田散鐵基地可產生不同類型之複合型碳化物，研究發現含Ti-Mo-0.5Cu (wt %)之鋼材，經由回火處理後，麻田散鐵基地可產生TiC碳化物、(Ti,Mo)C複合型碳化物和含銅之複合型碳化物，然而含Ti-Mo-1.0Cu (wt %)之鋼材，麻田散鐵基地所產生的複合型碳化物幾乎含有Ti、Mo和Cu等元素，比較這兩種含銅鋼材之微硬度值，可以發現回火處理在560小時前，其微硬度值變化非常相似，然而當回火處理至1150小時後，含1.0 wt % 銅之鋼材，抗回火軟化效果較不理想，此乃因在麻田散鐵介面上所形成之複合型碳化物粗化所致，故導致回火麻田散鐵軟化。另外，比較Ti-Mo與Ti-Mo-0.5Cu (wt %)之高強度低合金鋼材微硬度值，研究中可發現在含Ti-Mo之高強度低合金鋼材中多添加銅元素，可使抵抗回火軟化之時間延長，由此可知添加銅元素於Ti-Mo之高強度低合金鋼材中，可產生更好的回火軟化抵抗效果。

關鍵字

高強度低合金鋼 ； 過飽和析出 ； 介面析出 ； 銅析出物 ； 複合型碳化物 ； 恆溫時效處理 ； 回火麻田散鐵

並列摘要

Most microalloyed steels contain small amounts of one or more strong carbide forming element. It is known that copper does not form intermetallic compounds with other microalloying elements, so copper precipitates can also contribute a strengthening mechanism. There has recently been considerable interest in the use of low carbon, copper containing, high strength, low alloy (HSLA) steels for application in heavy engineering, where strength, toughness and deformability are the most important requirements. The hardening effect depends both on the aging temperature and aging time; hence, it is worthy of study. On the other hand, in order to modify the mechanical properties of the tempering martensitic steels, HSLA steels contain multi-microalloy elements that form complex carbides with slower growth or coarsening rates compared to single component carbides. Therefore, the complex carbides in the martensite matrix are effective in temper-softening resistance during long duration tempering. Secondary precipitation hardening in martensitic steels, which occurs during tempering treatment, is widely used in many engineering applications, such as turbine blades, heat transfer tubes, and the nuclear industry; steels in these applications must have superior creep rupture strength and impact properties at high temperatures of about 600℃.
In the present work, nano-sized cooper precipitates in the ferrite matrix were formed in low carbon steels containing 0.5 and 1.0 copper (wt %) during isothermal aging at various temperatures and times. The corresponding transmission microscopy and microhardness for the different aging temperatures and times of these two copper-containing steels have been investigated. It can be found that both interface precipitation and supersaturated precipitation are present in the ferrite grain under certain isothermal treatment conditions.
The precipitation of enriched-copper particles can take place in the ferrite matrix in 1 wt % copper containing steel, and the maximum hardness can be obtained in this steel by aging at 615℃ for 90 min. A lower transformation temperature can cause a great amount of nano-sized precipitation in ferrite due to the lower diffusivity of microalloying elements and large driving force for precipitation. Interphase precipitation is also observed in these two steels. This phenomenon depends on the austenite/ferrite boundary migration rate being reduced. It is also found that the sheet spacing of interphase precipitation decreases with aging temperature; however, the interface precipitation displays the lightness and non-uniformity that cause scattered mechanical properties.
HSLA steels contain Ti, Ti-Mo and Ti-Nb elements that form complex carbides in the martensite matrix during long-term tempering, leading to temper-softening resistance. Comparing the TiC carbides, (Ti,Mo)C complex carbides and (Ti,Nb)C complex carbides, (Ti,Mo)C complex carbides have a lower growing rate. However, (Ti,Nb)C complex carbides appear late, even with tempering at 600℃ for 560 h, which is why the temper-softening resistance appears after 560 h in Ti- Nb steel. This phenomenon differs greatly from those of Ti steel and Ti-Mo steel. From the hardness curves of Ti steel, Ti-Mo steel and Ti-Nb steel, it is clear that Mo addition results in the best temper-softening resistance when combined with Ti.
HSLA steels containing Ti-Mo-0.5Cu and Ti-Mo-1.0Cu (wt %) elements form different types of complex carbides in the martensite matrix during long-term tempering. In Ti-Mo-0.5Cu (wt %) steel, TiC carbides, (Ti,Mo)C complex carbides and Cu rich complex carbides can be observed; however, when the precipitates form in Ti-Mo-1.0Cu (wt %) steel, the Ti, Mo and Cu elements tend to combine with each other. The abilities of temper-softening resistance in these two steels are similar before tempering for 560 h. However, the addition of 1.0 wt % copper is unfavorable to further tempering for 1150 h because the complex carbides that coarsen rapidly in the grain boundary lead to temper-softening. From comparing the hardness curve of Ti-Mo steel and Ti-Mo-0.5Cu (wt %) steel, it is known that Ti-Mo steel can produce effective temper-softening resistance during long-term tempering; however, Ti-Mo steel with a further 0.5 wt % copper addition has extended temper-softening resistance. Therefore, Cu addition is favorable for temper-softening resistance when combined with Ti and Mo.

並列關鍵字

HSLA steel ； Supersaturated precipitation ； Interface precipitation, Copper precipitation ； Complex carbides ； Isothermal aging ； Tempered martensite

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