The working principle and main factors of the upward continuous casting device used to produce metal or alloy wires are analyzed, and the key factors affecting the upward continuous casting are pointed out, in order to help actual production.
Keywords: upward continuous casting; alloy melt; crystallizer; solid-liquid interface
Upward continuous casting is a continuous casting method. Its principle is to use the cooling and crystallization mechanism of molten metal to slowly and continuously extract solid metal wires, plates, etc. with a certain shape from the molten metal or alloy molten metal. The main feature of this method is that continuous wires or plates can be produced directly from molten metal or alloy melt without the need for casting, extrusion, drawing, rolling and other processing processes, which shortens the processing cycle and reduces the processing time. pollution and loss; the continuous production capacity is large, the production capacity of a single furnace can reach 500kg, and it can even produce continuous wires and plates; wires and plates with different outer diameters can be produced as needed. At present, this method is widely used in the production of oxygen-free copper wire and steel.
Structure and composition
The upward continuous casting machine is mainly composed of a melting system, a melt level tracking system, a melt cooling and crystallization system, a traction system, and a take-up system.
Melting system: heats and melts metal blocks, and provides insulation and protection for the melt. It is mainly composed of medium frequency induction power supply, melting crucible and holding furnace.
Melt level tracking system: Tracks and measures the molten metal level, and uses a lifting mechanism to keep the crystallizer in the molten cooling crystallization system synchronized with the height changes in the metal level due to continuous extraction. It is mainly composed of silicon carbide float, signal processing system and lifting mechanism.
Molten cooling and crystallization system: The molten metal entering the crystallizer is cooled and solidified to obtain a fixed-shaped wire or plate. It is mainly composed of cooling water inlet and outlet pipes, crystallization chamber, graphite shaped tube, refractory material protective jacket, cooling water system, etc.
Traction system: controlled traction of short solid metal wires or plates formed in the crystallizer to obtain continuous wires or plates.
Take-up system: Take-up the pulled wire or plate in a loop to facilitate storage.
Figure 1 is a schematic structural diagram of an upward continuous casting mold. The lower end of the crystallizer is equipped with a refractory protective sheath, graphite shaping tube, etc. A metal wire whose outer diameter is basically the same as the inner diameter of the graphite shaping tube is pre-inserted from the top of the crystallizer as a lead (cast rod), and the wire passes through the graphite shaping tube. And a small section is exposed at the bottom of the crystallizer. Pour in flowing cooling water and immerse the crystallizer in the melt. The melt level should not exceed the upper end surface of the refractory protective sheath. Fix the mold assembly on the guide rail of the traction system. Its working diagram is shown in Figure 2.
Figure 1 Schematic diagram of the structure of the upper continuous casting mold
Figure 2 Schematic diagram of upward continuous casting work
In Figure 2, when the crystallizer is immersed in the high-temperature molten metal, the exposed part of the wire that has been pre-inserted in the crystallizer and the part within the height of the crystallizer h will be melted due to heat, and h inside the crystallizer The metal wire above the height maintains its solid shape due to the continuous flow of cooling water. Therefore, an original solid-liquid interface is formed in the area of height h. The actual position of this solid-liquid interface is related to the melt temperature and It is related to the cooling intensity of the cooling water flowing in the cooling room.
Since the depth of the crystallizer immersed in the melt is H, a static pressure P will be generated at the lower port of the crystallizer, where the molten metal meets the nozzle of the graphite shaping tube. This pressure P will force the molten metal into the graphite shaping tube. inside the tube. When the lead wire begins to move upward under the action of the traction mechanism, the solid-liquid interface moves upward under the action of pressure P. After the upward part of the molten liquid is rapidly cooled, it will solidify into solid metal, and the solid-liquid interface moves upward. As the molten metal solidifies, it drops to zone h. When this process is carried out slowly and continuously, a continuous wire with a fixed shape can be obtained. The shape of the wire is related to the graphite shaping tube.
When the melt is continuously drawn out, the H height will become smaller. At this time, the melt level tracking system comes into play, and the entire crystallizer is lowered through relevant devices to keep the H height unchanged, so that the pressure P will not be significantly weakened. , which can effectively ensure the continuous progress of upward continuous casting.
Analysis of factors affecting upward continuous casting
The main factors affecting upward continuous casting include melt and external factors. The main factors of the melt itself include viscosity, temperature and purity; external factors include cooling speed and upward speed.
The viscosity of the melt will affect the rise of the solid-liquid interface in the h height area of the graphite shaping tube. If the viscosity of the melt is high, the interfacial tension between the melt and the inner wall of the graphite shaped tube will increase, and the friction will increase. The solid-liquid interface will not easily rise with the rise of the solidified material, which will cause the solidified material to separate from the melt surface. , continuous casting is interrupted when the viscosity of the melt is low, and vice versa. In general, under the action of pressure P, the viscosity of the melt is not the main reason for the failure of upward continuous casting. Only when the height H is not large enough and the pressure P becomes small, the melt viscosity will become the main reason for the failure of the upward continuous casting.
The main factors that affect the viscosity of the melt are: ① Melt temperature. If the melt temperature is high, the viscosity of the melt will decrease. However, the temperature of the melt should not be too high, generally not higher than about 200°C, the melting point of the alloy. When the melt temperature is too high, it will be difficult for the melt entering the graphite shaping tube to solidify, resulting in failure of the upper continuous casting and increased losses. ②Chemical components of the melt. Some trace elements (such as Ni, Cu, etc.) will reduce the viscosity of the melt.
Before the gas solubility of the metal or alloy melt reaches saturation, the higher the temperature, the longer the melting time or holding time, the more gas will be contained in the melt. Due to the contact between the exposed melt and the air, the metal melt will When the liquid cools and solidifies in the crystallization chamber, defects such as pores and looseness are more likely to occur, which can easily lead to the failure of upper continuous casting. When the melting temperature is too low, the viscosity of the molten metal increases, which is not conducive to the flow, causing the solid-liquid level separation in the crystallizer, which can also easily lead to the failure of upward continuous casting. Therefore, a certain amount of covering agent is usually covered on the surface of the melt to reduce the amount of air absorption of the melt, and at the same time, it can also prevent metal oxidation.
There is scum in the melt that is not easy to melt. These scum will form a thin film between the solidified metal and the melt, preventing the effective combination of the solid-liquid surface, or forming holes in the cross-section of the solidified cast rod. , inclusions, etc. 14, reduce the strength of the casting rod, causing the casting rod to be easily broken when pulling upward, causing the upward continuous casting to fail. In this case, the molten liquid should be removed for slag treatment. If necessary, a slagging agent can be used appropriately. After the slag is removed, upward continuous casting can be carried out.
The cooling rate is mainly related to the flow rate of cooling water. The larger the water flow, the faster the cooling rate. When the cooling pipe diameter, water pressure and other parameters are fixed, the flow rate of the cooling water is also fixed. At this time, the cooling rate is only affected by the initial temperature of the cooling water, but the water flow often becomes smaller due to sediments such as scale. In addition, Scale deposits have a thermal insulating effect, so scale deposits reduce the cooling rate. In order to avoid the impact of scale deposits on the cooling rate, the scale in the cooling water circulation pipeline and the crystallizer should be cleaned regularly. When conditions permit, softened water or pure water can be used for cooling.
The temperature and temperature difference between the inlet and outlet ends of the cooling water are also the main factors affecting the cooling rate. A high water temperature at the inlet end will reduce the cooling rate, while a small temperature difference between the inlet end and the outlet end means that the cooling water exchanges less heat in the crystallizer cooling chamber, which also reduces the cooling rate.
The contact state between the copper sleeve in the cooling crystallization area at the bottom of the crystallizer and the graphite shaping tube also affects the cooling rate. If the two are in close contact, it can ensure rapid heat dissipation, so that the molten metal rising to this area can solidify in time, ensuring that the upward pull can continue.
In addition, the lead-up pitch (the distance of a single upward movement of the casting core in up-lead continuous casting) also has an impact on up-lead continuous casting. If the pitch is too large, the height of the solid-liquid level that originally existed within the h height in the crystallization zone will increase. Once the molten metal below the solid-liquid level is not effectively cooled and solidified, it will not be effectively cooled and solidified during the subsequent upward movement of the solid casting core. It will separate the solid-liquid interface and cause wire breakage. If the pitch is too small, the production efficiency will be affected, so comprehensive considerations should be taken to select the appropriate pitch.
The faster the pulling speed, the shorter the cooling time for the melt entering the graphite shaping tube. Once the cooling speed cannot keep up, it is easy to separate the solid-liquid interface in the crystallization zone, causing wire breakage. Therefore, in Under certain circumstances, you can consider reducing the pull-up speed once spinning failure occurs.
In Figure 1, when the height H is not very high, the pressure P formed by the melt at the bottom of the crystallizer becomes smaller. If the upward pulling speed becomes faster, the melt entering the graphite shaping tube will rise under the action of pressure P. The speed may not be able to keep up with the pull-up speed, that is, the melt supply in the crystallizer cannot keep up. At this time, separation of the solid-liquid interface will also occur, causing wire breakage.
In upward continuous casting, how to ensure that the molten metal in the effective area above the mold can be cooled and solidified in time. Moreover, the key to continuous upward continuous casting is that the solid-liquid interface does not separate during the continuous upward process.