Modeling And Process Investigation For Hot Rolling Process Of Magnesium Alloy Sheets

Wide magnesium alloy sheet is an important basic structure material with great potential for lightening vehicles.Ingot casting with the subsequent rolling process is an efficient method for large-scale manufacture of Mg alloy wide sheet at present.Magnesium alloy has poor plastic deformation capability and is excessively sensitive to deformation and temperature.Therefore,both stable rolling deformation and optimum control for structure,properties,and sheet shape of Mg alloy sheet directly depend on the accurate temperature control of work-roll and metal slab,and especially the coordination between temperature and deformation.However,there is still a lack of systematical research on thermal behavior and its interaction with deformation for the rolling process of Mg alloy.In addition,rolled Mg alloy sheet has poor and unstable mechanical properties,heavy anisotropy,low product yield,poor dimensional accuracy of sheet/plate shape,which greatly limits its utilization and causative performance.Therefore,focusing on coil rolling process of AZ31B alloy,the thermal behavior of Mg alloy slabs in both air-cooling transport process and rolling process,and its coordination with process parameters were systematically investigated in this dissertation.Meanwhile,the effective methods for improving high-reduction rolling process of AZ31B alloy slab were put forward.And,the mathematical relationships between microstructure and mechanical properties of rolled slabs were discussed.The temperature variation of AZ31B slab during the roller conveying air-cooling process was obtained by performing the air-cooling real-time temperature measurement experiments.Firstly,using the experimental temperature data,the apparent blackness which can be used to perceive the temperature stroke of magnesium sheet during the rolling process,was calculated,and apparent blackness-related model was given.Based on the apparent blackness model,the temperature control model for roller conveying air-cooling process was established.Then,the air-cooling comprehensive heat transfer coefficient was calculated by combining finite element simulation and physical experiments.This coefficient was a function associated with thickness and average temperature of Mg alloy slab.According to this coefficient function,the Biot model for air-cooling process of magnesium sheet was established to evaluate the temperature state Abstract along the thickness direction under different thickness and temperature conditions.Finally,heat transfer behavior of magnesium sheet with 1500 mm width during the air-cooling process was simulated and the variation of air-cooling time over the width of slab was investigated as well.Meanwhile,the applicability of air-cooling temperature control model to the roller conveying air-cooling process of wide slab was verified.The thermal deformation behavior of AZ31B alloy and heat transfer/friction behaviors during the cold-roll rolling were systematically studied by thermal compression experiment,and temperature tracking and rolling force testing during the rolling process.The results from isothermal compressions showed that flow stresses of AZ31B alloy decreased as an exponential function(base e)of deformation temperature but increased as a power function of strain rate.Combining the temperature structure formula,the classical Fields-Backofen model was optimized and reconstructed subsequently by implanting strain variable into strain rate-and temperature-dependent sensitivity coefficients.Meanwhile,the accuracy and applicability of the reconstruction equation were verified.Semi-empirical models related to surface temperature drop and center temperature rise in rolling deformation zone were established on the basis of the existed basic equations for engineering heat transfer and experimental temperature measurement data.After that,a criterion that can accurately evaluate the matching relation of technological parameters for near-isothermal rolling process under experimental conditions was proposed.Next,based on AZ31B constitutive equation,a finite element model for thermal-mechanical coupling analysis of hot rolling was established.By comparing simulated and experimental results(including the rolling spread in contact surface,rolling force and temperature distribution in thickness),the interfacial friction coefficient of AZ31B plate during rolling was obtained.Finally,according to the rolling theory,the influence coefficient of interfacial friction was calculated by processing experimental rolling force data and tested slab temperature data,and the prediction model of rolling force for AZ31B alloy was established further.Results showed that rolling force of AZ31B slab had a high temperature sensitivity,then according to the rolling-force monitoring data,it was feasible to use the rolling force model to perceive the change of the average plate temperature in rolling zone.The effect of pre-deformation(including pre-rolling and pre-vertical compression)on microstructure and mechanical properties of rolled AZ31B slabs were investigated through high-reduction rolling experiments with different rolling modes.Meanwhile,beneficial effect and mechanism of pre-deformation on the edge-crack defect of rolled AZ31B slabs were discussed.The results showed that high reduction rolling with pre-rolling and subsequent pre-vertical compression can not only improve the deformation and microstructure uniformity in transverse direction and thickness direction simultaneously but also refine the grains significantly by promoting the recrystallization nucleation during the deformation process.Then,the refined grains can activate the operation of non-basal slip systems,such as the<c+a>pyramidal plane,and effectively impel the continuous dynamic recrystallization.Finally,the basal texture and rolling formability of rolled AZ31B slab were improved significantly.Structure-activity relationships between the microstructure and tensile properties of rolled AZ31B slabs were preliminarily discussed through the unidirectional and cross-rolling experiments.Firstly,based on the existing models for the relationship between tensile properties and microhardness,the modified models for AZ31B alloy were proposed.By eliminating the intermediate variable of microhardness,a quantitative relation between tensile strength and yield strength(0.2%offset)was established,furthermore,it was combined with Hall-Petch relation and a relation model between tensile strength and grain size was obtained.Results showed that the proposed relation model between tensile strength and grain size had a high applicability in describing high-reduction rolled AZ31B slabs and casting AZ80 slabs.