超級變韌鐵之組織研究

Investigation on microstructure of super-bainite

林韋廷(Wei-Ting Li n)

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(2008年)

https://doi.org/10.6342/NTU.2008.01535

摘要

創新的高碳高矽變韌鐵合金鋼已被證實具有良好的強度與韌性，且不需複雜之熱處理製程以及外加應力即可在各方向具有均勻之組織與性質，藉由高體積分率之奈米尺度變韌鐵次平板達到強化之效果，稱之為”超級變韌鐵”。
在鋼鐵中添加約2wt%的矽(Si)，並在過冷度大之熱處理下可生成無碳化物析出之變韌鐵，此種組織主要由變韌肥粒鐵與富含碳的殘留沃斯田鐵組成。矽的添加主要為抑制雪明碳鐵從沃斯田鐵中析出，雪明碳鐵是硬脆容易產生應力集中之劣化相，抑制雪明碳鐵析出可提高其機械性質。
本實驗闡明兩種不同硼(B)含量之高矽高碳變韌鐵，以光學顯微鏡、穿透式電子顯微鏡做一系列奈米結構之觀察分析，以做為此類合金鋼之合金元素添加與製程設計之參考。硼是有效提高變韌鐵硬化能之元素，能加速變韌鐵生長速率(即縮短起始反應時間)，使初期變韌鐵所佔之體積分率增加。本研究兩種鋼材最終之硬度值相當接近，可高達Hv 690，值得針對其不同之動力學做更深入之探討。
溫度對於變韌鐵顯微組織有相當敏感之影響，，300℃約1天之內即長滿變韌鐵組織，生長速率相當迅速，然而其束狀組織卻顯得粗大(約200nm)，在硬度的表現相當差；在150℃持溫下，能夠長出相當細(40~50nm)且長之變韌鐵束狀組織，但在14天恆溫相變態之下仍生長不到30%之體積分率，對於工業應用較為不利。200℃持溫之變韌鐵具有Hv 690高硬度值，歸因於奈米尺度之次平板，並能在10天之內完成變韌鐵相變態，為較適合之相變態溫度。
傳統之變韌鐵在晶體結構方面與基地之沃斯田鐵具有Kurdjumov / Sachs之方位關係，然而於本研究200℃持溫之變韌鐵可觀察到已偏離KS及NW方位關係，而有較少見之Pitsch方位關係。

關鍵字

變韌鐵 ； 硼 ； 相變態 ； 熱處理 ； 奈米結構 ； 穿透式電子顯微鏡

並列摘要

New bainitic steels with high silicon and high carbon have been demonstrated to possess an excellent combination of strength and toughness. These bulk steels have isotropic microstructure and mechanical properties without any complicated heat treatment or strain induced. These steels are strengthened by high volume fraction of nanometer-scaled sub units of ferrite platelets, and have been named super-bainite.
Adding ~2wt% silicon to the carbon steel can produce carbide-free bainite under the proper heat treatment. This kind of structure consists of a mixture of bainitic ferrite and carbon-enriched residual austenite. The adding of the silicon is mainly for suppressing the brittle cementite precipitated from the austenite. The cleavage and void easily form in the boundary of cementite/ferrite, so suppressing the precipitation of cementite can improve not only the strength, but also the toughness.
This research work was attempted to elucidate the nanostructure of super-bainite in two experimental high-silicon, high-carbon steels (one with boron addition, the other without boron addition) via optical microscopy (OM), and transmission electron microscopy (TEM). It was found that the addition of boron to the high-silicon, high-carbon steel can increase the hardenability effectively and accelerate the growth rate of the bainite (i.e. shorten the start time of bainite transformation). However, these two experimental steels have nearly the same optimum hardness. The value is striking, and it is up to Hv 690. It is worthy of further study to find out the different kinetic mechanism for these two steels.
The transformation temperature has a strong impact on the transformation rate. In this work, these isothermal transformation temperatures were chosen to be 300, 200, and 150℃. It took one day to complete the bainite reaction for the isothermal transformation at 300℃; it spent 10 days to finish the bainitic reaction for the isothermal transformation at 200℃. As to the case of isothermal transformation at 150℃, it was found to produce about 30% volume percent of bainite after isothermal treatment for 14 days.
Temperature also has quite great influence in the sheaves of bainite. Under 300℃ transformation temperature, the sheaves seem to be thick (~200nm), result in a detrimental hardness. As bainite transformed at 150℃, although it can obtains quite long and thin sheaves (40~50nm), but the growth rate is too slow to apply on the industry processing. Isothermal transformation at 200℃ is apparently a proper condition in this work. The hardness Hv 690 can be obtained due to the nanometer-scaled sheaves (50~60nm) of banite at 200℃.
The ferrite platelet has the Kurdjumov and Sachs orientation relationship with austenite in the traditional bainite steels. In this novel super-bainite, ferrite was found to has large deviation from KS or NW orientation relationship with austenite at 200℃, and has Pitsch orientation relationship in this work.

並列關鍵字

bainite ； boron ； heat treatment ； phase transformation ； nanostructure ； TEM

參考文獻

44 F.B. Pickering, “Transformation and Hardenability in Steels,” Climax Molybdenum Co., (1967) p.107.

47 R.W.K. Honeycombe and F.B. Pickering, Metall. Trans., Vol. 3A (1972) p.1090.

11 William. C.Leslie in”The Physical Metallurgy of Steel” p.139.

1 H.K.D.H. Bhadeshia, Materials Science Forum, 500-501 (2005) p.63-67.

2 H.K.D.H. Bhadeshia, “Worked Examples in the Geometry of Crystals”, The Institute of Metals, (1987) p.56.