Study On The Temperature Field And New Technology Of Az31b Magnesium Alloy Rolling

This study is a part work of the project,"Development of high efficiency and low cost technologies of magnesium alloy plate/strip rolling" in National Key Technology R&D Program of China.The influences of main technology parameters on temperature field,rolling workability,microstructure and mechanical properties of AZ31B magnesium alloy plates during rolling were investigated.Firstly,the effect of rolling technology parameters on the temperature variation of AZ31B plates was studied by experiments;Secondly,the temperature field of the alloy during rolling was simulated by FEM;Then,the effect laws of rolling speed and reduction schedule on the rolling workability,microstructures and mechanical properties of plates were worked out;Finally,the influence of pre-side compression technology on the rolling workability,microstructures and mechanical properties of plates were studied.The results of this dissertation provided the theory support and experimental basis for the industry production of wide magnesium alloy plates and wide coiled strips.Firstly,the temperature variation during rolling with various reduction ratios,initial rolling temperatures and roller temperatures were measured.It was found that the deformation heat strongly affected the temperature of plate during rolling.The temperature variation process consisted of rolling process,when the temperature gradient along thickness direction increased sharply(about 0.02-0.1s),and subsequent temperature homogenizing process(about 1?1.5s).With increased reduction ratio,both the instantaneous temperature rise during rolling and the temperature rise at stable state increased.With increased initial rolling temperature,these two temperature rise values decreased.Heat rollers could significantly suppress the surface temperature drop and increase the temperature rise at stable state.In addition,the empirical formulas and simplified energy-based state equations were established to predict the temperature rises of plate during rolling.The relationship between heat-transfer coefficient at the roller/plate interface and mean roll pressure was established.A FEM was proposed to simulate the temperature field during rolling with different reduction ratios,initial rolling temperatures,rolling speeds,roller temperatures and initial plate thicknesses.The temperature rise of plate increased with increase in reduction ratio,decrease in initial rolling temperature and/or increase in rolling speed.More important roles were played by the rolling speed and roller temperature in temperature rises during rolling of thinner plates.The surface temperature drop reduced with lower initial rolling temperature,higher rolling speed and/or higher roller temperature.This temperature drop firstly increased and then decreased as reduction ratio increased.This temperature drop firstly decreased and then increased as initial plate thickness increased.Based on simulation results,the rolling parameters for isothermal rolling and the rolling workability window for AZ31B magnesium alloy were obtained.It was found that the facture zone,where the fracture was easily occurred,appeared at medium reduction ratios.Increasing initial rolling temperature,rolling speed and/or roller temperature could expand the reduction range of safe rolling.In addition,the reduction ratio range of safe rolling firstly expanded and then shrank with increased initial plate thickness.At low rolling speeds,the microstructures of the plates were homogeneous recrystallization in all reduction schedules.Simultaneously,as total rolling pass number increased,the grain size slightly decreased and the twin density increased so that both the strength and elongation of rolled plates increased,especially for yield strength.At high rolling speeds,as total rolling pass number decreased,homogeneous dynamic recrystallization gradually turned into localized recrystallization so that the recrystallization fraction significantly reduced and the effective grain size sharply increased.As a result,the rolling workability worsened and the elongation decreased.During 4-pass rolling,as rolling speed increased,the temperature drop was suppressed,the twin density gradually decreased,the total depth of crack reduced and the rolling workability improved.Using pre-side compression technology significantly improved the rolling workability,largely expanded the recrystallization zone,weakened basal texture and increased the elongation of rolled plates.This was mainly due to the appearance of numerous {1012}tension twins during pre-side compression which rotated the C-axis of grains to transverse direction and provided many nucleation sites for the recrystallization during heating and rolling process.Increased pre-side compression reduction could improve rolling workability when the temperature was lower than 210?.However,decreasing pre-side compression temperature and/or reduction could increase the elongation of plates.The suitable pre-side compression was recommended to conduct by a reduction of 7.6mm and at a temperature lower than 210?.