Xiang Hua Liu

New Processing Technology of Twin Roll Strip Casting of AZ31B Magnesium Strip

In order to improve the poor workability and solve the problem of difficult roughing rolling of magnesium billet, the twin roll strip castor (TRSC) was used for the manufacture of magnesium thin strip directly from its molten metal. Since the cooling rates achieved by TRSC casting process are in the range of five hundred to several thousands of degrees per second and the strip is formed under the pressure, therefore, this near- rapid solidification process can leads to homogeneity of microstructures, refined grain size and increased solid solubility. The experiments were carried out on the existing equipments at State Key Laboratory of Rolling and Automation (RAL), Northeastern University. Strip samples 200~350 mm wide and 1.5~3.5 mm thick have been successfully cast in standard AZ31B alloy. The surface of the as cast magnesium strip was smooth and the edge was tidy. The microstructure analysis of the as cast magnesium strip showed that the gain size was much refined compared to that of ingot samples, and there was no segregation in the strip. The tensile strength of the strip in the cast state was about 220 MPa. The as cast magnesium strips were cold rolled directly on a two-high rolling mill, the total reduction up to 40% can be reached. X- ray diffraction result showed that the main phase in the as cast state AZ31B magnesium strip was a (Mg), the g (Mg17Al12) phase existing in the conventional wrought magnesium AZ31B sheet was vanished, therefore, the ductility of the strip increased.

Ultra-Hard Bainitic Steels Processed through Low Temperature Heat Treatment

Relative high carbon steel bearing Cr and Mo with microstructure consisting of nanoscaled bainitic laths and certain amount of retained austenite was produced through the combination of rolling and isothermal/multi-step heat treatment at low temperatures. The effect of the heat treatment temperature, time and path on the volume fraction of retained austenite and the width of bainitic lath was investigated. Nanoindentation was applied to inspect the separate hardness of the tiny bainite and retained austenite for different heat treatment parameters. The results showed that bainitic lath treatedt at 250°C was much thinner than that at 300°C and the volume fraction of retained austenite changed with different heat treatment temperatures, time and paths. The nanohardness of the baintic lath and retained austenite also changed with the processing of both carbon partitioning and displacive transformation for different heat treatment paths.

From TRB and LP Plate to Variable Gauge Rolling: Technology, Theory, Simulation and Experiment

Tailor Rolled Blank (TRB) and Longitudinal Profile plate (LP plate) are both new products for materials saving. TRB produced by cold rolling can partially substitute for Tailor Welded Blank (TWB) achieving lightweight in vehicles. LP plate produced by hot rolling can be utilized in shipbuilding, bridge manufacturing and constructions, realizing structures lightweight. This paper stressed the common point of TRB and LP plate: Variable Gauge Rolling (VGR). Basic equations of VGR were deduced, a velocity relationship which assures the shape and dimension of transition region was provided, a FEM simulation of VGR was performed, VGR experiments were carried out and TRB samples with a thickness ratio 1:2 were obtained. All above achievements have established a foundation for VGR’s industrial application.

Research on Roll Force for Variable Gauge Rolling

The production of materials saving products such as Longitude Profile plates (LP plate) and Tailor Rolled Blanks (TRB) needs to solve the problem of the calculation of variable gauge rolling (VGR) parameters. It has been proved that Karman differential equation of force equilibrium for simple rolling is a special case of VGR equations when vy＝0. Differential equations of force equilibrium for both upwards rolling and downwards rolling were solved and the rolling pressure distribution function for VGR was got in this paper. Finally, details of a set of experiments were presented and the theoretical model of rolling force was proved to be reasonable.

NUMERICAL SIMULATION OF MOLTEN POOL AND CONTROL STRATEGY OF KISS POINT IN A TWIN-ROLL STRIP CASTING PROCESS

Coupled turbulent flow, temperature fields of the twin-roll casting strip process were simulated by three-dimensional finite element method. Based on the heat balance calculation and using inverse methods between the simulations and real experiments, the relational models among casting speed, location, and coefficient of heat transfer between molten metal and rolls in different regions are given. In the simulation, the calculated surface temperatures are in good agreement with the measured values. An on-line model of kiss point is derived by simulations and the geometry of molten pool, corresponding control strategy is also proposed.

Analysis of Leveling Strategy for a Plate Mill

In order to analysis the effect of leveling strategy on the quality of plate products, the curvature integration by elastic-plastic differences was adopted to simulate leveling results by different leveling strategy. It had studied plastic deformation ratio, residual stress, residual curvature and leveling force for different leveling strategies to find the effectual strategy and the adaptability conditions were given. Additionally, static pressure leveling with the time delay strategy was analyzed, which was proved to be an effectual strategy to resolve the leveling problem for high strength thicker plate by a certain 3500mm mill plate.

FEM Simulation of Multipass Shape Rolling Processes

The updating geometric method was proposed to simulate the deformation behavior of workpiece during multipass shape rolling by using finite element method. Firstly, establish and solve the finite element model of the first pass shape rolling process, then update the geometric shape of workpiece after the first pass and delete rolls of the first pass, establish and mesh rolls of the second pass, modify the boundaries and material parameters of workpiece, and solve its deformation behavior during the second pass shape rolling. With the same steps, its deformation behavior during the following passes rolling could be solved. The method was applied to simulate the deformation behavior of a cube workpiece during six-pass H-beam rolling with split-rolling, and its shapes after every pass rolling process were obtained. Experiments on the deformation behavior of slab during multipass rolling were carried out by using pure lead in an experimental mill. The calculated results are in good agreement with the experimental ones.

Research on Behavior of Slab Surface Defects in Forward Slip Zone during V-H Rolling Process

Behavior of the transversal crack and the longitudinal crack on slab surface during V-H rolling was simulated by the FEM. The contact pressure on crack surfaces and the crack-tip stress change rules during rolling were analyzed. Results show that the contact pressure on crack surfaces decreases and the tensile stress appears at crack tip in the zone of slippage on the delivery side, which may make the cracks propagation. For the phenomenon, the stress distribution along rolling direction and along width direction in rolling are analyzed, and the influence of forward slip on the closure and growth of the surface transversal crack and the surface longitudinal crack are discussed. Results support some significant information for researching the behavior of the slab surface defects in rolling process.

A Method to Improve Model Calculation Accuracy of Process Control in Tandem Cold Mills

An adaptive learning algorithm using laser velocity meter is proposed for the process control of tandem cold mills. This method is used for on-line adaptive learning calculation of deformation resistance of strip and rolling friction models. The reverse calculation values of deformation resistance and friction coefficient as variables of non-linear equations can be obtained by introducing the measured rolling force and forward slip into the calculation model individually. Using Newton-Raphson iterative calculation to solve the non-linear equations, the reverse calculation values as actual values can be calculated. The actual forward slip obtained from the measured strip rolling speed using laser meter makes it possible that the deformation resistance of strip and friction coefficient can be uncoupled for model adaptive learning calculation. The inverse calculation and adaptive learning can improve whole control system of tandem cold mills significantly.

Development and Application of Advanced Coiling Temperature Control System in Hot Strip Mill

Run out table cooling equipment and coiling temperature control (CTC) system, especially mathematic models of a hot strip mill were introduced. Heat transfer models such as air convection model, heat radiation model and laminar cooling model, process control models such as segment tracking model, feedback control model, self-learning model and case-based reasoning model were detailed described. Since online application of the new CTC system, the laminar cooling control system has been running stably and reliably with a high precision of coiling temperature.