本研究利用背向散射電子繞射術(EBSD)研究冷軋率和鋼材碳含量對極低碳鋼冷軋變形組織不均勻性的影響。鋼材冷軋率為30-90%，鋼材A含有約15 ppm固溶碳原子，而鋼材B則完全不含固溶碳原子。
在本研究的第一部分，利用EBSD觀察並分析試片TD面的冷軋顯微組織與集合組織的非均勻性。首先利用高角度邊界的密度，在介觀尺度(meso-scale)將變形組織依其非均勻性由輕微至嚴重分為A、B和C三類。在微觀尺度(micro-scale)上，利用核心平均方位差(kernel average misorientation，KAM)與晶粒平均方位差(grain average misorientation，GAM)兩種指標分析變形組織的非均勻性。依據這兩種分析方式，建立預測再結晶晶粒的空間分布與集合組織的方法。其一為HMR+KAM方法，結合高角度方位差區域 (high misorientation region，HMR)分析和KAM大於10度的像素的交集，選出可能為潛在再結晶成核(potential recrystallization nucleus，PRN) 的位置。其二為DIG+GAM方法，DIG是在變形過程中生成的被高角度晶界包圍的晶粒，本研究依據GAM值小於2度，且粒徑低於3微米的標準由DIG中篩選PRN。這些PRN的集合組織相似於完全再結晶的集合組織，而後者可以用修正型應變誘發晶界移動(strain induced boundary migration，SIBM)理論來解釋。
本研究的第二部分，利用準原位觀察(quasi in-situ)的實驗方式研究冷軋與再結晶顯微組織與集合組織的關係，首先在冷軋的狀態下先取得EBSD數據，接著再退火後，在相同的區域重新取得EBSD的數據。不同冷軋量50-90%的試片被分析(以A-5、A-7和A-9稱之)。以HMR+KAM的方式分析預測出的再結晶集合組織與空間分布在A-5和A-7試片上有很好的對應性，但是A-9試片的冷軋PRN集合組織與再結晶集合組織的吻合性較差，原因為高度軋延導致HMR分析無法完全剔除原始之高角度晶界。此外，利用DIG+GAM的方式，由A-7和A-9試片冷軋組織中篩選的PRN，其集合組織與再結晶集合組織非常相似，但是A-5試片中的PRN數量太低，且經由傳統SIBM機構成核的再結晶晶粒比例較高，因此集合組織的預測結果吻合性較差。
最後，第三部分，藉由比較冷軋70%的鋼材A與鋼材B的再結晶成核觀察結果與集合組織演化，探討了鋼鐵中固溶碳對再結晶集合組織的效應。在冷軋集合組織上，兩者並沒有太大差別，但是鋼材A的完全再結晶集合組織有較強的Goss方位而鋼材B有較強的{111}<112>。同時也利用了EBSD對再結晶不同階段的試片做詳盡的分析。特別集中觀察以下四個結晶方位：{111}<110>、{111}<112>、cube與Goss的演化。在40%再結晶時，除了鋼材A的Goss強度比較高之外，鋼材A和鋼材B的再結晶晶粒集合組織非常類似。40%再結晶之後，鋼材B的{111}<112>強度持續增加直到完全再結晶，而鋼材A的{111}<112>則沒有進一步增加。利用DIG+GAM的方法分析兩者的冷軋組織，結果顯示鋼材A有比較高的Goss的PRN；此外，在40%的再結晶階段，鋼材B中尺寸介於0.5-3微米的小顆再結晶晶粒的{111}<112>佔的比例比A料的{111}<112>小顆再結晶比例高。這些小顆的{111}<112>再結晶晶粒在40%再結晶之後持續成長增加尺寸成再結晶，最後造成鋼材B的{111}<112>的強度比較高。

The heterogeneity of deformed microstructures of an ULC steel having different cold reductions (30-90%) and two ULC steels with and without dissolved carbon, respectively, were studied in detail using the electron backscatter diffraction, EBSD, technique. The steels were hot rolled to 10 mm, annealed, and cold rolled to 1-7 mm thick. In the first part of the thesis, the cold rolled microstructure and texture were analyzed from the transverse direction (TD). The heterogeneity of deformed microstructures in a meso-scale are categorized into types A, B, and C based on the density of HABs in the original grains. The deformation heterogeneity in a micro-scale is further analyzed by considering the kernel average misorientation (KAM) and the grain average misorientation (GAM), respectively. Two methods of predicting the possible nucleation sites in recrystallization (RX) and the RX texture are proposed accordingly. The first method, HMR+KAM method selects pixels which fulfill a high KAM (>10o) criterion and are located in the high misorientation regions (HMR) are identified as the potential recrystallization nuclei (PRNs). The second method, DIG+GAM method defined areas in deformed matrix which are surrounded by high angle boundaries as the deformed induced grains (DIGs). For those DIGs having low GAM values (<2°) and being smaller than 3 m are defined as the PRNs in recrystallization. The texture of the PRNs selected by each method shows a good correlation with the texture after full RX. The latter method is explained by a modified strain induced boundary migration (SIBM) theory.
A quasi in-situ method was employed to study the correlation of the cold rolled (CR) and recrystallized (RX) microstructure and texture in the second part of the thesis. EBSD measurements were carried out for the CR samples prior to annealing, and again for the annealed sample of the same area. The microstructures and textures of steels subjected to 50-90% cold reductions (denoted as A-5, A-7, and A-9) were characterized accordingly. Good similarity of the spatial distribution and texture was found between the pixels selected by the HMR+KAM method and the RX grains for A-5 and A-7. By using the DIG+GAM method, the textures of PRNs and RX grains for A-7 and A-9 show good similarity.
Finally, the effect of dissolved carbon to the formation of RX texture by comparing steels A (with dissolved carbon) and steel B (without dissolved carbon) was studied. The evolutions of RX nucleation and texture in specimens of 70% cold reduction (A-7 and B-7) were investigated. Prior to the onset of recrystallization, no significant difference in CR texture between A-7 and B-7 can be found. However, the fully recrystallized A-7 sample shows a stronger Goss texture and a weaker {111}<112> one, as compared to those of B-7. Partially recrystallized specimens with various recrystallization fractions were analyzed by EBSD. Special emphasis is given to PRN sand RX grains of four texture components: {111}<110>, {111}<112>, Goss and cube. The textures of the RX grains show great similarity at ~40% of RX for A-7 and B-7, except that A-7 exhibits a high Goss intensity from early stage of recrystallization. After 40% of RX, the intensity of the {111}<112> grains of B-7 increases steadily whereas that of A-7 keeps a relatively constant value till full RX. CR samples analyzed by the DIG+GAM method show that A-7 indeed exhibits a higher fraction of Goss PRNs. In addition, a higher area fraction of {111}<112> RX grains of sizes in the range of 0.5-3 μm were found in the deformed matrix of B-7 after 40% of RX. These small {111}<112> grains keep growth into RX grains and results in a high intensity of {111}<112> after full RX.

摘要 iv
Abstract vi
Abbreviations xii
Chapter I Introduction 1
Chapter II Literature review 6
2.1 Textures of low carbon steels 6
2.2.1 Cold rolling (CR) texture 7
2.2.2 Recrystallization (RX) texture 7
2.2 The effect of dissolved carbon on textures of low carbon steel 9
2.3 Textural related properties of low carbon steels 12
2.3.1 r-value 12
2.3.2 Magnetic flux density 13
2.4 Deformation structures of metals 14
2.4.1 Deformation structures of FCC metals 15
2.4.2 Deformation structure of low carbon steels 17
2.5 Recovery and recrystallization behavior of low carbon steels 20
2.5.1 Recovery 21
2.5.2 Recrystallization 22
2.5.3 Oriented nucleation and growth 26
2.6 Applications of EBSD technique on characterization of deformation and recrystallization structure and related studies 29
2.6.1 Characterization of deformation structure using EBSD 31
2.6.2 Texture evolution in the recrystallization of cold-rolled steel 33
2.6.3 Formation of Goss texture in recrystallization of cold-rolled steel 34
Chapte III Experimental Procedures 36
3.1 Materials and processing 36
3.2 Microstructure of the materials prior to cold-rolling 37
3.3 Determination of the recrystallization temperature 38
3.4 Macrotexture analysis by using X-ray diffraction 38
3.5 Microtexture analysis by using EBSD 39
3.6 Quasi in-situ EBSD observations of the evolution of recrystallization texture 39
3.7 Recrystallized (RX) grains identification in EBSD 41
Chapter IV Heterogeneity of deformed microstructure and its relation with recrystallization nucleation 42
4.1 Heterogeneity of deformed microstructures in a meso-scale 43
4.2 Heterogeneity of deformed microstructures in a micro-scale 48
4.2.1 Application of the KAM method on heterogeneity analysis 48
4.2.2 Application of the HMR+KAM method on heterogeneity analysis 51
4.3 A modified SIBM theory for recrystallization nucleation in highly deformed matrix and DIG+GAM method 54
4.4 Summary 60
Chapter V Heterogeneities in deformed structure and its effect on the formation of recrystallization texture 63
5.1 Characteristics of the deformed structure after different CR reductions 64
5.2 A direct correlation between recrystallization nucleation and heterogeneities in CR structure 67
5.3 Prediction of RX texture from the CR structure by using the HMR+KAM method 69
5.4 Correlation between the texture of PRNs in CR state and the RX texture 71
5.5 Summary 73
Chapter VI Effect of dissolved carbon on oriented nucleation and texture evolution in recrystallization of 70% cold-rolled steels 75
6.1 Recrystallization texture influenced by carbon dissolved in ferrite 75
6.2 Microstructural evolution in recrystallization 76
6.3 Texture evolution in RX grains 80
6.3.1 Area fractions of major texture components in RX grains 81
6.3.2 Recrystallized grain size 83
6.3.3 Number of recrystallized grains 84
6.4 The DIG+GAM method on the steel A and B in 70% cold reduction 86
6.5 Further discussions 88
6.6 Summary 91
Chapter VII Conclusions 93
References 98
Tables 107
Figures 111
Appendix 165

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