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

Abstract

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.