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Effect of Rolling Parameters on Surface Strain Variation in Hot Strip Rolling

김경석 (Kyung Seok Kim, 포항공과대학교)

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The present thesis investigates the effect of rolling parameters on the surface strain variation in a hot strip rolling process. Although the mechanical and metallurgical advantages of lubrication in hot strip rolling are well proven, owing to the difficulties in process control, most lubrication te...
The present thesis investigates the effect of rolling parameters on the surface strain variation in a hot strip rolling process. Although the mechanical and metallurgical advantages of lubrication in hot strip rolling are well proven, owing to the difficulties in process control, most lubrication technologies have not been commercialized, and several steel companies have conducted a number of studies on the development of alternative technologies. This study aims at finding alternative process methods and conditions for lubrication in hot strip rolling. Chapter 1 provides a general and extensive literature review of the influence of lubrication on the mechanical behavior and microstructure of a material in hot strip rolling. It also describes the main issues with currently implemented lubrication technologies in the hot rolling process. Chapter 2 introduces basic rolling theory in order to explain the influence of lubrication on the stress and strain development at the roll bite. The hot strip rolling process equipment and methodology used for the present work is also described. The requirements for the simulations are that the material should be subject to the boundary conditions that are the same as those for the actual process. These conditions led to the selection of a particular stand for this study. In Chapter 3, the 430J1L ferritic stainless steel (FSS) sample, which is investigated in this study, is introduced. High-temperature uniaxial compression tests are performed and the constitutive equations for the flow stress are determined. Based on the process conditions of the actual hot strip rolling mill used in this work, a 3D coupled thermo-mechanical finite element (FE) model is validated by comparing the numerical results obtained with the actual mill log data. The range of suitable rolling parameters is then determined in order to conduct a parametric study. In these simulations, a number of parameters, such as the friction coefficient, conduction heat transfer coefficient, and internal temperature gradient, were selected on the basis of results published in the literature. In order to assess the influence of each process parameter, a relevant variable called the relative variation of the surface strain (RVSS), which characterizes the changes in the surface strain under a given parametric study as compared to those in the current process condition, was proposed. In Chapter 4, based on the validated 3D FE model, a comprehensive numerical analysis was performed to determine the exact mechanisms of lubrication during hot strip rolling through a detailed analysis of the evolutions of the stress and strain at the roll bite. A parametric study was conducted, during which one variable (tension, surface temperature, conduction heat transfer coefficient, or reduction ratio) was varied while the others remained fixed in order to assess their individual influence on the evolution of the surface strain during hot rolling. From these results, a simple mathematical model for predicting the variation in the surface strain in a single rolling stand was developed to calculate, in a simple and time-efficient manner, the optimized parameters of a process condition that can be used as an alternative to lubrication. Chapter 5 describes the remaining work in the future. The current work reveals not only the exact deformation mechanisms due to lubrication in terms of strain development but also provides various correlations between the variation of the surface strain and the rolling process parameters; however, it has not been experimentally verified that the current simulation results are practically effective for the final product. Further research for the experimental validation of this study and an integrated design that can be applied to an actual rolling mill were then presented. Chapter 6 summarizes the present work and suggests further topics for investigation that are relevant to commercialization.
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ABSTRACT I
CONTENTS V
LIST OF TABLES IX
...
ABSTRACT I
CONTENTS V
LIST OF TABLES IX
LIST OF FIGURES X
ABBREVIATIONS XIV

CHAPTER 1. Introduction 1
1. Introduction 2
1.1. Mechanical effects of lubrication in hot strip rolling process 3
1.2. Metallurgical effects of lubrication in hot strip rolling process 6
1.3. Issues in lubrication process control 12
1.3.1. Cambering and steering problems 12
1.3.2. Lubricant Control 14
1.3.3. Structural Constraint 16
1.4. Objective and outline of the thesis 18

CHAPTER 2. FUNDAMENTALS OF HOT STRIP ROLLING MILLS 20
2. Fundamentals of hot strip rolling mills 21
2.1. Basic rolling theory 22
2.2. Hot strip rolling mill 26

CHAPTER 3. 3D coupled thermo-mechanical FE modeling 28
3. 3D coupled thermos-mechanical FE modeling 29
3.1. Basic material properties 30
3.2. FE modeling description 35
3.3. 40
3.4. FE model verification 45
3.5. Parametric study conditions 47
3.6. Summary 51

CHAPTER 4. results and discussion about the FE Simulation 53
4. Results and discussion about FE Simulation 54
4.1. Effect of lubrication 55
4.2. Factors affecting the surface strain variation 63
4.2.1. Influence of tension 63
4.2.2. Effect of surface temperature 65
4.2.3. Effect of conductive heat transfer coefficient 68
4.2.4. Effect of reduction ratio 71
4.3. Optimum process condition by using simple strain prediction model 75
4.4. Conclusions 79

CHAPTER 5. FUTURE WORKS 81
5. Future works 82
5.1. Experimental validation 83
5.1.1. Experimental validation of lubrication in hot rolling 83
5.1.2. Experimental validation of alternative technologies replacing lubrication 85
5.1.3. Integrated analysis for all the stands of a mill and the development of new process conditions for commercial applications 90
5.1.4. Application to new steel grades 90

CHAPTER 6. CONCLUDING REMARKS 91
6. Concluding remarks 92
6.1. Concluding Remarks 93

REFERENCES 96
요약문 (SUMMARY IN KOREAN) 101
ACKNOWLEDGEMENTS 104
CURRICULUM VITAE 106
PUBLICATIONS 107