臺灣博碩士論文加值系統

高碳鉻軸承鋼SUJ2在製造過程中，由於鋼鐵的非平衡凝固造成碳化物帶的產生，對產品品質造成負面影響。因此，本研究使用熱力學計算方法 (Calculation of Phase Diagram, CALPHAD) 搭配高週波感應高溫熔煉實驗，探討碳化物帶的生成機制與消除碳化物帶的方法。
利用熱力學計算軟體模擬鋼鐵非平衡凝固，結果顯示鉻的偏析加強了雪明碳鐵的生成，其原因為鉻在雪明碳鐵中的溶解度大於沃斯田鐵和肥粒鐵，而鉻在鋼鐵中為置換型原子，故熱處理很難有效減緩鉻偏析的情況，因此，本研究計算並分析不同元素的添加對於鉻偏析的影響，並從中挑選出鋯、鈦、鉭三種元素做更進一步的探討，計算結果顯示鋯和鉭的添加可以降低鉻在鋼液中的最大溶解度，而鉭和鈦的添加可以降低凝固過程中雪明碳鐵的生成量，而實驗觀察結果顯示有摻雜鋯的SUJ2軸承鋼可以最有效地降低鋯的偏析與碳化物的生成，此外，根據EPMA區域掃描分析得出結論，在連續鑄造中，將SUJ2軸承鋼添加可降低鉻在鋼液中的最大溶解度的元素，對於抑制雪明碳鐵帶具有顯著的影響。

In the mass production of steel, in the case of high-carbon and high-chromium steel, SUJ2, carbide band will form as a result of non-equilibrium cooling during steel production. Using combined investigation of CALPHAD (phase diagram calculation) and experiments, the cause of carbide band formation is analyzed and the solution of the problem is investigated. By CALPHAD analysis, mainly using non-equilibrium cooling simulations, it is found that chromium (Cr) segregation strengthen cementite presence in the steel, since chromium is dissolved in cementite in larger concentration than in austenite or ferrite. Combined with Cr status as substitutional atom in steel, heat treatment is almost useless to reduce Cr segregation and therefore eliminate carbide presence, as confirmed by the composition (EPMA) mapping of as-rolled SUJ2 specimen. By CALPHAD calculation, several common steel alloying elements are calculated, and from calculation result zirconium, titanium, and tantalum are chosen for further investigation. Zirconium and tantalum reduce maximum chromium concentration in liquid steel, meanwhile tantalum and titanium reduce amount of cementite formed during solidification. After experimental observations, it is found that the zirconium doped SUJ2 specimen yield best reduction of chromium segregation and carbide presence. From EPMA mapping, it is concluded that addition of element that reduce maximum chromium in liquid phase will have a profound effect on cementite band reduction in SUJ2 production through continuous casting.

ABSTRACT ii
ACKNOWLEDGEMENTS iv
CONTENTS v
LIST OF FIGURES vii
LIST OF TABLES 1
CHAPTER 1. INTRODUCTION 2
1.1. Introduction 2
1.2. Goals 3
CHAPTER 2. LITERATURE REVIEW 4
2.1 Phase diagrams and phase equilibria 4
2.1.1 Gibbs’ Phase Rule 4
2.1.2 Lever rule 6
2.1.3 Metastable Phase Diagram 8
2.2 SUJ2 Bearing Steel 10
2.3 Industrial Production Method of SUJ2 Steel 12
2.4 Segregation Problem in Steel 13
2.5 Influence of Steel Alloyings 24
2.6 Calculation of Phase Diagram (CALPHAD) 27
2.6.1. Scheil Solidification 28
2.7. Recent Developments 29
CHAPTER 3. EXPERIMENTAL PROCEDURES 32
3.1. Schematic Diagram of Research 32
3.2. Calculation of Phase Diagram (CALPHAD) 33
3.2 Experimental Observation 36
3.3. Experimental apparatus 39
3.2.1 Electron Probe Micro Analysis (EPMA) 39
3.2.2. Induction Furnace 40
3.2.3. Box Furnace 41
CHAPTER 4. RESULTS AND DISCUSSION 43
4.1 Liquidus Projection of the Fe-Cr-C Ternary System and the Superimposed Solidification Path of SUJ2 Steels 43
4.2 CALPHAD-type Thermodynamic Analyses on the Alloying Effects 47
4.3. Experimental Results 70
4.3 Discussion 83
CHAPTER 5. CONCLUSIONS 89
CHAPTER 6 REFERENCES 90

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