WCA January 2020

Increasing the casting speed will deepen the flow depth of bubbles and inclusions, and reduce the escape time of bubbles, which is not conducive to controlling the porosity and inclusion defects. The spray heat transfer coefficient increases with the increase of casting depth, and the heat transfer between the mould and the slab increases accordingly. The numerical simulation of temperature field and flow field with variable heat transfer coefficient and average heat transfer coefficient in spray heat transfer coefficient is studied. When the heat transfer coefficient changes along the crystal wheel, the upper part of the crystallisation zone has a darker colour, indicating that the temperature of the region is higher, while the colour of the exit part near the crystallisation zone is lighter, indicating that the temperature of the region is relatively low. When the heat transfer coefficient is average, the fluctuation of free liquid surface has little effect on the surface. When the heat transfer coefficient changes along the crystallising wheel, the flow of liquid copper from the nozzle into the cavity can not be fully expanded due to the geometrical structure of the cavity. When it impacts on the wall of the crystallising wheel, there will be a recirculation and the more intense flow will generate a vortex in the upper part of the crystallising region. 4.2 Deformation simulation analysis of copper rod rolled by hot continuous rolling The stress and strain of ten-stand copper rod during hot rolling were simulated and analysed. The deformation of roughly rolled copper rod mainly concentrates on the surface and the equivalent strain at the edge and corner is relatively large, which can easily cause processing damage. The equivalent strain of roll gap in an elliptical pass system of finishing copper rod is larger than that of roll contact in circular pass system. As rolling proceeds, the equivalent strain shifts from the surface to the centre of the copper rod. Temperature is the most important process parameter in the hot rolling process. Reasonable temperature control is the key to ensure the mechanical properties of products. Evolution of the temperature field during continuous rolling of copper rod was obtained by extracting temperature changes of rolling copper rod and its rolling interface. The core and surface temperatures of rolled copper rods are quite different. Because the contact heat transfer between copper rod and roll surface is larger and the temperature drop of copper rod surface is faster, the temperature of the copper rod core increases under the action of plastic work and friction work, and the temperature of copper rod core increases faster with the increase of rolling speed and deformation. After continuous rolling, the temperature of the whole rolling piece gradually converges under the action of heat conduction [8] . The ductile damage of copper rods under large deformation, high temperature and high strain rate rolling is an important problem in the plastic forming process of copper rods.

❍ ❍ Figure 4 : Simulation of hot strip mill stand group rolling 4 Results and discussion 4.1 Effect of technological parameters on temperature field and flow field of copper billet In order to explore the influence of different temperatures on the temperature field and flow field of copper slab, the casting temperatures 1,110°C, 1,120°C, 1,130°C and 1,140°C were selected. The overall temperature of the crystallisation zone increases with the increase of casting temperature, especially at the exit and entrance. Because of the latent heat of solidification in the crystallisation process of copper liquid, the temperature growth at the outlet is non-linear. Solidification position is closely related to casting temperature. When pouring temperature is at its lowest, the solidification point of the copper liquid will affect the fluidity and increase the friction force of the contact surface and the amount of gas escaping in the liquid, and the slab is more likely to produce surface defects. With the increase of casting temperature, the solidification point gradually approaches the exit of the crystallisation zone. Analysis of the influence of different superheat on the flow field in the crystallisation zone shows that increasing the casting temperature did not significantly change the flow distribution characteristics of the flow field in the crystallisation zone, and the vortex recirculation zone remained basically unchanged, but the recirculation velocity increased. At the same time, the viscosity of liquid copper decreases; the fluidity of liquid copper enhances the impact depth of the mainstream strand, which leads to gas and inclusions in-depth. This is not conducive to the removal of upward flotation and makes gas and impurities stay in the slab to form defects. The numerical simulation of heat-flow coupling in the mould cavity crystallisation zone was carried out with different casting speeds (11.4m/min, 12.4m/min, 13.4m/ min, 14.4m/min). With the increase in casting speed, the overall temperature of the crystallisation zone increases, the length of liquid phase zone increases, and the temperature at the outlet increases significantly. Because of the difference between the material of the crystal wheel and the steel strip and the air layer on the side of the crystal wheel, the minimum temperature on the side of the crystal wheel at the outlet side is always slightly higher than that on the side of the steel strip. With the increase of billet drawing speed, the impact depth of the main stream of copper melt increases, and the swirl recirculation region becomes narrow, extending to the direction of the crystallisation wheel rotation, and the recirculation velocity also increases.

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Wire & Cable ASIA – January/February 2020