超級變韌鐵之相變態與顯微結構研究

Study on phase transformation and microstructural evolution in a super bainitic steel

張孝慈(Hsiao-Tzu Chang)

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(2011年)

https://doi.org/10.6342/NTU.2011.03334

摘要

近年來Bhadeshia等人開發出一種高碳-高矽合金鋼，此種鋼材被稱之為“超級變韌鐵”。不同於一般高強度材料之尺寸限制，超級變韌鐵利用簡單的熱處理製程產生具奈米結構之塊狀材料，而此材料之特點在於經過低溫恆溫熱處理後將具有非常高的抗拉強度與良好的韌性，未來可發展應用於防彈鋼板等需要高強度之用途。
超級變韌鐵之優異機械性質乃是來自於其奈米尺度之變韌鐵次平板與散佈其間之細化沃斯田鐵的混合組織。藉由各種合金元素的添加來促成此一特色組織結構的生成，如矽的添加可抑制雪明碳鐵的生成，有利變韌鐵組織內的薄膜狀沃斯田鐵穩定；錳、鉻、釩等合金元素的添加可以增加材料之硬化能。
本研究發現，若將此合金先進行高溫軋延後再完成後續的恆溫熱處理，會因為擴散速率較慢的合金元素偏析而在材料內產生帶狀組織。實驗結果證明，合金元素(如錳)的偏析所形成之帶狀組織會造成材料出現硬度差異，對機械性質有不好的影響。欲消除此帶狀組織，可將材料先在高溫做長時間均質化處理後再降至低溫進行預訂之恆溫相變態熱處理，則可得到強度高而均勻的組織。
本實驗的主要目的在於使用均質化後之試片進行不同的低溫恆溫熱處理條件(溫度、時間)來探討低溫變韌鐵組織的產生過程。結果顯示，恆溫相變初期組織主要是以麻田散鐵結構為主，此時具有相當高的強度；而隨持溫時間增加，變韌鐵組織開始生成而麻田散鐵的比例逐漸減少，會造成強度的下降；最後當變韌鐵含量持續增加，強度又會往上提昇，直至變韌鐵含量達到飽和。
相變溫度對於變韌鐵顯微結構亦相當敏感。變韌鐵於高溫之生長速度較快，但束狀組織卻較粗大（如300oC持溫）；而在低溫（150oC）持溫，雖可長出相當細長且具有高硬度的變韌鐵束狀組織，但其相變速度非常緩慢，甚至需要到持溫720小時才會有較明顯之相變。而使用250oC恆溫熱處理可以降低熱處理時間至約48小時即可完成變韌鐵相變態，同時亦可形成具奈米尺度的高強度超級變韌鐵結構，為較佳的變韌鐵相變態溫度。
超級變韌鐵雖然具有非常優異之機械性質，但侷限於其相變時間過長，工業上實際進行生產之效益太低。因此本研究配合先前之相變溫度，以兩階段熱處理方式來研究加速變韌鐵生成之方法。先在300oC持溫一段時間令其快速長出些許變韌鐵晶核後，再降溫至200oC持溫來生長高強度超級變韌鐵組織。研究結果顯示，此一方法的確可加速超級變韌鐵之生成，尤其是在300oC持溫4小時後降溫至200oC持溫之兩階段熱處理可加速相變態進行，同時仍可擁有最終高達600 HV以上之硬度。
由於超級變韌鐵中添加了高含量的矽以抑制碳化物析出，因此本研究同時探討高碳之超級變韌鐵熱穩定性。利用於400∼600oC回火熱處理探討碳化物析出之形貌與數量。研究結果顯示碳化物析出的數量及大小受到回火溫度與時間影響，越高溫和長時間之回火將形成較多與大的碳化物。然而，不論於任何回火條件，所分析到的碳化物均為變韌肥粒鐵中穩定之雪明碳鐵碳化物。
雖然文獻中指出超級變韌鐵具有良好的強度與破裂韌性(KIC)，但衝擊韌性卻鮮少提及，由本研究之實驗結果顯示超級變韌鐵不具有良好的衝擊韌性。此可能與變韌鐵中的碳阻止高密度差排於高速衝擊下移動有關。同時於衝擊後之TEM顯微結構中觀察到麻田散鐵的產生，此亦可能為降低衝擊韌性的原因之一。

關鍵字

低溫變韌鐵 ； 高碳-高矽合金鋼 ； 帶狀組織 ； 恆溫熱處理 ； 回火熱處理 ； 穿透式電子顯微鏡 ； 衝擊韌性

並列摘要

In recent years, a novel high carbon high silicon alloy has been developed by Bhadeshia et al. Different from general high strength steels, in which grain refinement is limited, a bulk material with a nano structure is achieved by simple heat treatments. This steel has excellent strength and high toughness after isothermal transformation at low temperature for several days. It can be use as a prospective armor material and is named as “super bainite”.
The super bainite microstructure consists of a mixture of the nano-scaled sub-unit bainite structure and fine austenite films. The major alloying element, silicon, suppresses precipitation of brittle cementite from the austenite, and the other alloyed elements, namely manganese, chromium, and vanadium, enhance the hardenability.
After hot rolling, the banded structure is often observed in this steel. Experimental results have shown that this banded structure, containing different phases, is caused by elemental segregation, one example being manganese, which is segregated because of its low diffusion coefficient in solid. The way to eliminate the banded structure is to homogenize the alloy at high temperature. The resulting low temperature bainite steel is uniform in both microstructure and hardness.
The main purpose of this research was to investigate the formation of low-temperature bainite under different isothermal heat treatment conditions (isothermal temperature and holding time). The results revealed that martensite is the major structure and leads to the high strength in the early stage. As the isothermal heat treatment continues, the amount of bainite rises, and it suppress the formation of martensite, which causes the strength to fall. The strength increases again when the bainite structure achieves saturation.
The transformation temperature also greatly influences the bainite structure. At higher transformation temperatures (e.g., 300oC), the bainite sheaves grow faster but thicker; the isothermal heat treatment at lower transformation temperatures (e.g., 150oC) can produce long and thin sheaves with superior hardness, but the growth rate is excessively slow. As a consequence, isothermal transformation at 250oC appears to be the proper condition for super bainite transformation. Completion of the bainite reaction requires only about 48 h, and the matrix of nano-scaled bainitic ferrite plates with carbon-enriched retained austenite can be obtained.
Although super bainite has excellent strength, the transformation time is too long for application in actual industrial production. Therefore, another goal of this work was to study two-step isothermal heat treatments, which could accelerate the bainitic transformation. First, specimens were isothermally heated at high temperature (300oC) for a very short time to nucleate the bainite sheaves and then isothermally heated at low temperature (200oC) to grow a finer bainite structure. The results showed that two-step isothermal heat treatment accelerated the formation of bainite structure. The process of first heating at 300 oC for 4 h and then cooling to 200 oC for the remainder of the time give rise to shorter transformation time and high final strength.
A high silicon content is added to suppress the precipitation of carbides in super bainitic steel; therefore, the thermal stability of high-carbon super bainite is also discussed. The specimens were tempered at 400-600oC for different holding times to observe the evolution of carbides. The results revealed that the amount and size of carbides were influenced greatly by tempering temperature and time. Larger and more massive carbides form at higher tempering temperatures or longer tempering times. However, in all experimental parameters, the observed carbides are identified as cementite, which is the stable carbide form in tempered bainitic steels.
Previous research indicated that super bainitic steel possessed excellent strength and great fracture toughness (KIC). However, the weak performance in impact toughness was observed in this research. The present study indicates that super bainitic steel has poor CVN impact toughness (about 6.5 ±0.3 J), which may be caused by the carbon pinning effect on dislocations in bainite sheaves. In addition, TEM revealed the α’-martensite structure in fracture surfaces after CVN impact tests, which might be one of the reasons for the low impact absorbed energy.

並列關鍵字

low temperature bainite ； high-carbon high-silicon alloy steel ； banded structure ； isothermal transformation ； tempering ； TEM ； fracture toughness

參考文獻

1. F. G. Caballero and H. K. D. H. Bhadeshia, Curr. Opin. Solid State Mater. Sci., Vol. 8 (2004), p.251.

2. H. K. D. H. Bhadeshia, Mater. Sci. Forum, Vol. 500-501 (2005), p.63.

5. H. K. D. H. Bhadeshia, Acta Metall., Vol. 29 (1981), p. 1117.

6. H. K. D. H. Bhadeshia, Scripta Met., Vol. 17 (1983), p.1475.

7. J. W. Christian, “Theory of Transformations in Metals and Alloys”, 2nd edn., pt. 1, Pergamon Press, Oxford.