奈米級碳化物在高強度低合金鋼中之析出行為研究

Study on the Precipitation Behavior of Nano-sized Carbide in the Novel HSLA Steel

陳志遠(Chih-Yuan Chen)

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(2007年)

https://doi.org/10.6342/NTU.2007.01548

摘要

高強度低合金鋼具有高強度,凸緣延伸性,銲接性及成形性等優點因此廣泛地使用於汽車及輸油管工業上。由各種熱軋鋼材之強化方法來看，晶粒微米化及析出物尺寸奈米化是目前最有效的兩種強化機構。但在一般經過良好控制之熱軋鋼材製造過程中所產生之肥粒鐵晶粒尺寸約可達到2~3μm而不容易再降低肥粒鐵尺寸，因此在鋼材中產生數量多且尺寸達到奈米級之析出物散佈在基地中可以有效地強化鋼材,然而在一般熱機製造過程中不易產生奈米級碳化物，這是因為大部分的微合金元素會在熱浸及高溫軋延的過程中被消耗掉，使得在盤捲過程中僅有少量之微合金元素可供析出成碳化物。因此本研究之最主要目的即是利用控制合金設計及熱機製程參數兩項觀點來產生大量奈米級碳化物在高強度低合金鋼基地中產生析出。
在第一個恆溫實驗中將六種不同成份之鋼材在750℃經過不同時間(5分鐘, 10分鐘,及60分鐘)之恆溫熱處理,並觀察金相及顯微組織。實驗發現在沃斯田鐵變態成為肥粒鐵之過程中碳化物可以利用界面析出之方式大量地析出在肥粒鐵基地中。而由本實驗亦發現複合碳化物較單一成份碳化物具有較小之粗化速度,尤其是鈦、鉬兩種不同合金元素所形成之複合碳化物，這是因為鈦及鉬在肥粒鐵基地中擴散速度較鈦與鈮相差較大因此不易生長而粗化速度較慢。
在連續冷卻及階段式冷卻的實驗中發現,僅有單一成分碳化物(TiC)可以在連續冷卻實驗中發現界面析出的現象。然而在階段式冷卻實驗中三種不同成份鋼材均可以產生界面析出之碳化物分佈在基地內；而越快之冷卻速度所產生碳化物尺寸均越小這應與碳化物成核、成長的時間有關。因此由本實驗可知對高強度低合金鋼而言避免在高溫停留過久時間是產生奈米級碳化物的一個重要方法。
由鋼材在兩相區之恆溫實驗中發現不僅僅是界面析出碳化物會有效地強化肥粒鐵基地，過飽和析出碳化物亦有此功用。而界面析出物間距是由沃斯田鐵與肥粒鐵之間的界面結構所決定，對低溫恆溫而言較大之驅動力可以使得肥粒鐵與沃斯田鐵界面之非契合部分(階梯)變多使得界面析出物之間距會變小。當界面階梯之移動速度變快時會使得界面析出碳化物無法形成，造成多餘之微合金元素會殘留在肥粒鐵基地中而隨著後續之熱處理過程產生過飽和析出之現象。實驗顯示在低溫恆溫時會產生奈米級之界面析出碳化物與過飽和析出碳化物於肥粒鐵中而有效地強化基地。
而由不同種類之微合金元素鋼材恆溫實驗中發現複合型碳化物在熱力學與動力學角度來看具有較小之粗化速度。這是由於旋節分解與微合金元素擴散速度之差異造成複合型碳化物具有優越之熱穩定性存在。由於碳化鈮會在高溫沃斯田鐵析出並吸收其附近之微合金元素使得含鈮鋼材之複合型碳化物析出強化效果不佳。
高溫塑性變形之效應亦在本論文中有所討論。高溫塑性變形有加速沃斯田鐵分解之作用存在，造成階梯之移動速度加快而阻礙了界面析出碳化物析出之可能性；因此使得過飽和析出碳化物存在於大部分肥粒鐵晶粒內。然而高溫塑性變形並不能改變析出之狀態，例如析出物尺寸、數量，因為這些是恆溫溫度之函數。
熱浸溫度之實驗中發現越高之熱浸溫度可以將所有之微合金元素固溶回沃斯田鐵基地中，如此有助於將大部分之微合金元素發揮析出強化之效果。另外大量之高溫塑性變形可以產生較小之肥粒鐵晶粒這樣可以抵消高溫熱浸溫度所造成之晶粒粗大效應。
在鋼材之連續冷卻實驗中發現在熱軋完後加速冷卻之過程可以避免合金元素析出產生耗損,使得後續強化之效果更明顯。在階段冷卻實驗中發現適當之階段冷卻溫度有助於奈米級碳化物之析出，因此尋找一個適當之階段冷卻溫度對熱軋鋼片是一項重要之參數。
在合金鋼之變韌鐵變態研究中發現，合金碳化物會扮演著下列兩種角色：1.在高溫沃斯田鐵區域內所產生之合金碳化物有助於碳原子之分配排放，因此可以促進變韌鐵相變態之進一步分解，並減少塊狀沃斯田鐵之存在。2.在差排上析出之奈米級碳化物在經過長時間時效後仍具有析出強化之效果。

關鍵字

高強度低合金鋼 ； 界面析出物 ； 過飽和析出物 ； 單一成份碳化物 ； 複合碳化物 ； 奈米級析出物 ； 熱穩定性 ； 汽車鋼板 ； 高解析像電子顯微鏡

並列摘要

Abstract
High strength low alloy (HSLA) steel usually possess many advantages: such as good strength, better stretch flange, good weld-ability and easy formability. Therefore, this type of steel is commonly used in the automobile and pipeline industry. From the various strengthening mechanisms; grain size minimization and generation large amounts of precipitates in the matrix can strengthen steel effectively. However, it is very difficult to obtain grain size less than 2~3μm in the thermomechanical process (TMCP). Nano-sized carbides in the ferrite matrix can lead to very greatly strengthening effect; nevertheless, it is very difficult to obtain these nano-sized carbides in the steel due to complex manufacturing process. In fact, most of microalloy elements exhaust in the soaking and hot deformation process, which result in less microalloy carbide precipitation in the coiling. Therefore, studying the various microalloy additions and thermomechanical process parameters is the main objective of present study, which can generate large amounts of precipitates in the steel.
In this work, isothermal aging experiments at 750℃ for 5min, 10min, and 60min were carried out for six different steels. The isothermal aging used in the present study corresponds to coiling process in the hot rolled steel strip. The corresponding metallographs and transmission micrographs for the different aging times of six steel have been investigated. Interface precipitation carbides during γ→α transformation can generate large amounts of carbides in the steel. The result also indicates that the [(TiMo)C] has slower coarsening rate compared with other carbides during isothermal aging experiment, which is beneficial for strengthening steel effectively. This is because the larger difference in diffusivity in Ti/Mo compared to Ti/Nb indicates that it takes more time to grow [(TiMo)C] than to grow [(TiNb)C].
In continuous and interrupted cooling experiments, three different composition steels were conducted for studying the precipitation behavior of various carbides. Single component carbide (TiC) has larger coarsening rate compared to other carbides; hence, it can be seen that the interface precipitation of titanium carbide can occur in the continuous experiment. However, Complex carbide, such as [(TiMo)C] and [(TiNb)C], cannot happen in the continuous cooling. On the other hand, all steels can generate interface precipitates in the interrupted cooling experiment, and more quickly cooling rates lead to smaller carbides in the ferrite matrix. Therefore, this result suggests that it can generate nano-sized carbides in the HSLA steel if it avoids staying at high temperature for longer times.
From isothermal aging at two phase region, it finds out that not only interface precipitation but also supersaturated precipitation occurring in the ferrite region, which can harden ferrite matrix effectively. The spacing of intersheet for interface precipitation mainly determines by the characteristic of austenite and ferrite boundary. For low isothermal forming temperature, the large driving force induces more incoherent parts (ledge) which can get smaller intersheet. The supersaturated precipitates form when the migration rate of ledge becomes fast, which remain some microalloy elements in the ferrite matrix and precipitate in the subsequently treatment. It reveals that much of nano-sized carbides of interface precipitation and supersaturated precipitation generating in the ferrite matrix when the isothermal temperature is low.
Comparing the effect of different categories of microalloy addition in the same isothermal aging experiment, it finds that complex carbide has a low coarsening rate due to both thermodynamic and dynamic reasons. The spinodal decomposition of complex carbide and discrepancy of diffusivity for various microalloy elements occurring in the two phase region aging makes the excellent thermal stability for complex carbide. Niobium carbide will form in the high temperature region, which will absorb surrounding microalloy elements and reduce the amount of precipitation carbide in the low temperature, which reduces the precipitation strengthening effect for the Nb-bearing steel.
The effect of high temperature deformation also discusses in the present thesis. The acceleration of austenite decomposition occurring in the high temperature deformation process can results higher migration rate of ledge, which inhibits the interface precipitation occurring. It is thus more supersaturated precipitation carbides can happen in mostly of ferrite grains. However the high temperature deformation cannot alter the precipitation status such as size and quantity, which also be determined by the isothermal aging temperature.
The effect of soaking temperature on the precipitation behavior in the HSLA steel also discusses in the present thesis. It should be emphasized that higher soaking temperature can dissolve all microalloy elements in the austenite and bring their strengthening effect into full play. Large amounts of deformation can lead to smaller ferrite grains which can balance the high temperature soaking effect.
In the following continuous cooling and interrupted cooling experiment, it finds that accelerated cooling can remain most of microalloy elements in the ferrite which would be benefit for getting high hardness. In the interrupted cooling experiment, the suitable interrupted cooling temperature is benefit for getting much of nano-sized carbides in the ferrite matrix. Therefore to discovery a proper interrupted cooling temperature is very important parameter for hot rolled strip.
Isothermal aging in the bainite phase, the microalloy elements have two effects:
1.Carbide forming in the high temperature region can promote partition of carbon atoms which can promote the bainite phase transformation further and reduce the existence of blocky austenite.
2. The nano-sized carbide nucleating at dislocation can sustain their tiny size and get

並列關鍵字

HSLA steel ； Interface precipitation ； Supersaturated precipitation ； Single component carbide ； Complex carbide ； Thermal stability ； Automobile steel ； Nano-sized carbide ； and HRTEM

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