Description: Introduces the tubular mold/integral mold/combined mold, the performance and structural characteristics of the continuous casting mold, as well as the mold coating, taper, etc.
The type of continuous casting mold structure:
- According to the different types of continuous casting machine, the copper mould tube can be divided into two categories: straight and curved.
- According to the billet specifications and shapes, there are billets, blooms, slabs and special-shaped billet molds.
- According to the structure of the crystallizer itself, it can be divided into 3 types:
Tubular copper mould : It is made of a copper tube with a wall thickness of 6-12mm to make the required section. Outside the copper tube, a sleeve is placed to form a cooling water passage of 5-7mm to ensure the cooling water flow rate 6-10m per minute. The crystallizer is simple in structure and easy to manufacture, and is widely used in billet continuous casting machines.
Integral copper mould tube : It is made by planing a whole piece of copper ingot, and there are many small holes drilled around its inner cavity for cooling water. This kind of crystallizer has good rigidity, easy maintenance, and long service life, but it has high manufacturing cost and consumes a lot of copper, so it has not been used in recent years.
Combined copper mould tube : It is composed of 4 copper plates to form the required inner cavity. Plan a groove on a 20-50mm steel plate and connect it with a steel plate, and the cooling water passes through the groove. Both bloom and slab casters use this type of mold.
The performance that the continuous casting crystallizer should have
Mold is an important part of continuous casting machine. The molten steel is solidified and shaped in the crystallizer, forming a shell with a certain thickness and being pulled out continuously into the secondary cooling zone.
A good crystallizer should have the following properties:
- Good thermal conductivity, can make molten steel solidify quickly. Every 1kg of molten steel is poured into a slab and cooled to room temperature, the heat released is about 1340kJ/kg, and the crystallizer takes away about 5-10%, that is, 67-134kJ/kg. If the slab size is 250×1700mm, the casting speed is At lm/min, the crystallizer can take away as much as 200,000 kJ of heat per minute. And the crystallizer length is relatively short, generally no more than 1m, in such a short distance to be able to take away a lot of heat, it must have good thermal conductivity. If the thermal conductivity is poor, the slab shell coming out of the mold will become thinner. In order to prevent leakage, the casting speed has to be reduced. Therefore, good thermal conductivity of the mold is an important prerequisite for achieving high casting speed.
- Structural rigidity is better. The inner wall of the crystallizer is in contact with high-temperature metal, and the outer wall passes through cooling water, and its wall thickness is very thin (only 10-20mm), so the temperature gradient in its thickness direction is extremely large, and the thermal stress is considerable. Its structure must have a relatively Large rigidity to accommodate large thermal stress.
- Easy to disassemble and adjust. In order to quickly change the size of the billet or quickly repair the mold to increase the production capacity of the continuous casting machine, modern molds have adopted integral hoisting or online width adjustment technology.
- Long working life. The mold is accompanied by sliding friction between the billet and the inner wall of the mold under high temperature conditions, so the material of the inner wall of the mold should have good wear resistance and a higher recrystallization temperature.
- The inertial force should be small when vibrating. In order to improve the surface quality of the slab, the vibration of the crystallizer is widely used with high frequency and small amplitude, up to 400 times/min. The inertial force cannot be ignored during high frequency vibration. Excessive inertial force not only affects the strength and rigidity of the mold , which in turn affects the accuracy of the crystallizer trajectory.
What are the parts of the tubular crystallizer
The tubular crystallizer is composed of copper tubes, cooling water jackets, foot plates and foot rollers. The inner cavity is made of a tapered arc-shaped seamless copper tube, and its outer surface is covered with a steel inner water jacket to form a cooling water channel of about 5-7 mm. The partition and rubber pad are connected with the outer water jacket to form an upper and lower Two water chambers, cooling water enters the lower water chamber from the water supply pipe, flows through the water gap at a speed of 6-8m/s, enters the upper water chamber, and is discharged from the drain pipe. Copper tube crystallizers are mainly used in billet continuous casting machines.
The structural characteristics of the combined crystallizer
The combined crystallizer is composed of inner and outer arc copper plates, narrow side copper plates, cooling water tank, narrow side clamping and thick side adjustment devices and foot rollers.
In order to improve the cooling strength of the crystallizer, the inner side (inner arc, outer arc and side plate) in contact with the liquid metal are all made of copper alloy with good thermal conductivity and wear resistance. During pouring, the cast slab pulled out from the crystallizer is still a thin shell on the outside and a liquid core on the inside. In order to better support the thin shell and reduce the bulge deformation caused by the static pressure of the molten steel, in There are 2 to 3 pairs of foot rollers arranged at the lower end of the crystallizer (some also use grid structure). In order to adapt to different sizes of slabs, there are width adjustment and thickness adjustment devices. Modern slab continuous casting machines have developed online width adjustment devices, which can change the width of the slab under uninterrupted casting conditions, shorten the auxiliary time, and improve the quality of the casting machine. production capacity.
Why is the crystallizer made of copper alloy
The inner layer of the crystallizer is a key part for heat exchange and shaping of the molten steel when the molten steel solidifies, so it is required to be made of a material with good thermal conductivity. The copper plate has good thermal conductivity, but its strength and hardness are low, especially at high temperatures, the strength is lower, so its life is shorter. In order to improve the service life, copper alloys are commonly used, such as: copper-silver alloy, copper-chromium-zirconium-arsenic alloy, copper-magnesium-zirconium alloy, etc.
The composition of the copper-silver alloy is: Cu99.5%, AgO.07-0.1%. The purpose of adding silver is to increase the recrystallization temperature of the copper plate. When the silver content is 0.08-0.1%, the recrystallizer temperature is 318-326°C (50°C higher than that of ordinary copper plates), which is higher than its working temperature. Under cooling conditions, the working temperature of the inner wall of the crystallizer is 250-320°C, which can prevent recrystallization.
On-line width adjustment of crystallizer and its adjustment method
Under the condition of non-stop casting, changing the width of the casting slab is called online crystallizer width adjustment. Its advantages are:
- Continuous casting of slabs with different widths and sizes shortens the downtime and improves the production capacity of the casting machine;
- It can reduce the loss of cutting the head and tail of the billet and improve the yield;
- Molten steel with similar composition can be poured without shutting down. In recent years, the online width adjustment technology of crystallizer has developed rapidly.
Crystallizer copper plating
When the mold is working, it is in direct contact with the high-temperature billet, and the two are often in a state of sliding friction. In order to improve the thermal conductivity of the mold, its inner wall is generally made of copper alloy, but its hardness is low. Therefore, in order to improve the mold In order to improve the life of the copper alloy plate on the inner wall, the method of plating is adopted.
- Single coating is Cr-plated on the surface of the copper plate on the inner wall of the crystallizer. The expansion coefficients are 0.165×10-4 and 0.084×10-4 (1/℃), so the coating will start to peel off after working for a period of time.
The linear expansion coefficients of Ni and Cu are very close. Plating Ni on the copper plate can improve the service life compared with Cr plating, and the thickness of the Ni plating layer can be increased, up to 5mm.
- Composite coating In order to improve the service life of the copper plate, Ni, Ni alloy and Cr3 composite coating are used. The first layer of Ni plating, because its linear expansion coefficient is (0.167×10-41/°C), which is similar to Cu, can prevent thermal expansion and peeling off. The second layer is Ni-P alloy layer, which has higher hardness at high temperature. The third layer is Cr layer, which can increase the finish and reduce frictional resistance. The service life of this composite coating can be increased by 5 to 7 times than that of Ni plating alone.
Ni—W—Fe coating, due to the addition of W and Fe, improves the strength and hardness of the coating. Its Vickers hardness Hv=300~780, its linear expansion coefficient is roughly equal to that of Cu and Ni, and its high temperature strength stability is good. It is suitable for high temperature It is adopted for pulling speed continuous casting machine (1.4～2.0m/min).
Crystallizer reverse taper
The dimensions of the longitudinal section of the inner cavity of the crystallizer are made to be larger at the top and smaller at the bottom, forming a taper. Because the top is large and the bottom is small, it is called an inverted taper.
In the mold, the molten steel forms a billet shell of a certain shape due to cooling. As the billet continues to move down, the temperature also decreases and shrinks. If the mold does not have an inverted taper, it will form between the billet shell and the mold. The gap is called the air gap. Due to the existence of the air gap, the cooling effect is reduced, and at the same time, because the billet shell is separated from the inner wall of the crystallizer prematurely, the billet shell will be bulging and deformed under the action of the hydrostatic pressure. Therefore, the above situation can be avoided by making the crystallizer into an inverted taper, but the size of the taper should be compatible with the cooling shrinkage of the slab.
If the back taper is too small, an air gap will be formed, and if the back taper is too large, the drawing resistance will be increased. According to experience, the back taper is generally 0.5-0.8%. For example, for a slab continuous casting machine in a factory in my country, the reverse taper is 0.63-0.65%.
Crystallizer cooling water system
In the crystallizer cooling water circulation system, in order to remove impurities mixed into the cooling water, a self-washing filter should be installed on each continuous casting machine and installed in the valve room near the continuous casting machine for centralized control and manual operation.
The cooling water circuits of the crystallizer are all collected into the machine’s indirect cooling water main pipe and flowed back to the water treatment system.
In order to facilitate the loading and unloading of water pipes, expansion joints are used to connect the cold water pipes of the crystallizer and the pipeline water pipes, so as to adapt to the up and down vibration of the crystallizer.
In order to ensure that the cooling water can fill the cooling water tank and each pipeline, an air removal pipe and cock should be installed at the highest point to discharge the air in the pipeline. The water quality of cooling water should have strict requirements.