The sliding plate is the core component of the ladle sliding nozzle. It is the component that directly controls the pouring of molten steel and determines the function of the sliding nozzle. If the slide plate leaks steel during the steel pouring process, it may cause the continuous casting to stop pouring, or it may cause a serious accident such as burning out the continuous casting machine. Therefore, it is of great significance to explore the causes of steel leakage accidents between slide gate plate and formulate preventive measures.
Keywords: ladle slide; steel leakage; prevention
Analysis of causes of steel leakage
Mechanical reasons for sliding nozzle
According to the formula P = N × μ × ΔX (where: P represents the surface pressure provided by the sliding mechanism to the slide gate plate, N represents the number of surface pressure springs, μ represents the spring coefficient of the surface pressure spring, ΔX represents the compression amount of the spring during operation ). It can be seen that when the movable mold frame or fixed mold frame of the mechanism is deformed or the wear amount of the surface pressure loading part exceeds the specified value, within the specified surface pressure loading stroke, the compression amount of the spring is reduced and sufficient slide gate plate surface pressure cannot be generated. However, poor connection of air cooling pipelines, insufficient air pressure, blocked pipelines, etc. result in insufficient cooling, which reduces spring performance or even fails, resulting in insufficient surface pressure. When the static pressure of molten steel is greater than the surface pressure of the slide plates, gaps will appear between the slide plates, resulting in steel leakage between the slide plates during the steel pouring process.
Reasons for slide gate plate operation and installation
(1) There are debris on the mounting surface of the sliding plate mechanism that have not been cleaned, or too much refractory mud was used when installing the water inlet. Excess mud is squeezed into the back of the slide gate plate, causing uneven pressure on the slide gate plate or the illusion that the surface pressure is sufficient. Gaps appear between the slide gate plates during the steel pouring process. (2) The surface pressure of the slide gate plate is not sufficient. (3) Failure to identify the melting damage such as hole expansion, roughening, steel clamping, etc. on the jointly used slide gate plate, resulting in excessive use of the slide gate plate.
Reasons for the quality of the slide gate plate itself
(1) The material of the slide gate plate cannot meet the steel pouring requirements of the steel type, and the harmful components in the slide gate plate exceed the standard, causing the thermochemical corrosion of the slide gate plate to intensify. (2) Cracks occur in the slide gate plate during use and expand abnormally. The molten steel causes “V”-shaped melting damage to the slide gate plate along the cracks. The iron hoop on the outer edge of the slide gate plate shifts or breaks, causing the slide gate plate to crack during use.
Reasons for ladle steel pouring operation
(1) When the spout cannot be opened automatically and oxygen needs to be burned, using oxygen to burn the slide plate before it is fully opened will cause the upper slide surface to be severely burned or even burned through. (2) When the multi-stream continuous casting machine is changed from normal multi-stream steel pouring to single-stream or few-stream steel pouring, the contact surface between the lower plate surface and the molten steel is increased compared with normal steel pouring (see Figure 1). The larger the contact surface between the slide gate plate surface and the molten steel, the faster the molten steel will corrode the slide gate plate. When erosion grooves appear on the slide gate plate surface, a thicker steel layer will be produced between the slide gate plates. At the same time, single-flow steel pouring causes the slide plate to frequently control flow, and the number of full-stroke slides in a short period of time is greatly increased compared with normal steel pouring, which intensifies the roughening of the slide plate surface and intensifies the damage to the slide plate surface, resulting in leakage of molten steel.
Figure 1 The relative positions of the upper and lower slide plates when pouring steel
Problems in molten steel smelting
Although the slide gate plate made of suitable materials was selected, due to careless control of the molten steel during the smelting process, the components in the molten steel that had a strong corrosive effect on the slide gate plate seriously exceeded the standard. The thermochemical corrosion rate of the slide gate plate was greatly accelerated, causing the slide gate plate to leak steel in a short period of time. The following is an example of the analysis of the residual slide plate (aluminum carbon) after casting calcium-treated steel to break out the steel to discuss the thermochemical corrosion of the slide plate. Observing the appearance of this remaining slide gate plate, we found that the back of the upper slide gate plate was clean and free of debris. There are two erosion grooves about 180mm long, 30mm wide and 10mm deep on the surface of the remaining lower slide gate plate. The surface of the erosion grooves is yellowish brown in color and has obvious honeycomb-shaped holes. These two erosion ditches should be the channels for the outflow of molten steel. It can be inferred that abnormal thermochemical erosion caused severe damage to the slide gate plate surface. The thermochemical erosion process is as follows. (1) Formation of decarburized layer The carbon on the surface of the slide plate is oxidized at the steel pouring temperature (1550~1600°C) to form a decarburized layer, which causes the porosity of the working surface of the slide plate to increase and the strength to decrease. When it comes into contact with molten steel, FeO, MnO, and [Ca] in the molten steel diffuse and penetrate into the slide plate through the pores. There are three main ways for the oxidation of carbon in slide gate plate: First, it is oxidized when the ladle nozzle is repaired and burns oxygen. The second is the oxidation of the skateboard by [O] in the molten steel when pouring steel. Third, due to the jet effect of high-speed steel flow during steel pouring, the air sucked in from the gap between the upper and lower slide plates oxidizes the slide plates. (2) Damage of the sliding plate by [Ca] In order to prevent Al2O3 from adhering and nodulating at the immersed nozzle of the tundish and blocking the nozzle, Ca treatment is required at the end of refining. Generally, Ca alloys, such as Ca-Fe wires and Ca-Si wires, are added to react with the Al2O3 inclusions in the steel to form low melts, thus changing the morphology of the aluminum oxide inclusions. The molten steel is discharged as the bottom-blown argon bubbles rise. However, when the Ca alloy is added in excess, that is, the amount added exceeds the amount required to react with Al2O3 in molten steel, the excess [Ca] will accelerate the erosion of the slide plate. The erosion process is as follows: Al2O3 in the slide is first reduced by [Ca] in the molten steel to generate CaO and Al, and then the generated CaO reacts with Al2O3 in the slide to form an Al2O3-CaO series low melting point compound that is washed away by the molten steel. By tracking the [Ca] content in molten steel and the corrosion degree of the sliding plate, it was found that:. When the [Ca] content (mass fraction, the same below) in molten steel is <0.003%, high melting point CaO·3Al2O3 (melting point 1850°C) and CaO·2Al2O3 (melting point 1750°C) are mainly generated, which have a weak erosion effect on the slide plate. When the [Ca] content in molten steel is 0.003% to 0.005%, some high melting point CaO·3Al2O3, CaO·2Al2O3 and some low melting point CaO·Al2O3 (melting point 1600°C) and 12CaO·7Al2O3 (melting point 1415°C) are generated. Increased erosion of slide gate plates. When the [Ca] content in molten steel is >0.005%, a large amount of 12CaO·7Al2O3 low melt and part of CaO·Al2O3 are generated, which seriously corrodes the slide plate and may cause the slide plate to break out in a short time.
Preventive measures for steel leakage accidents between ladle slides
Strictly control the end point [Ca] content of molten steel
It can be seen from the CaO-Al2O3 system phase diagram that when the mass ratio of Ca and Al in steel is >0.10, Al2O3 inclusions basically react with CaO to become 12CaO·7Al2O3 or a low-melting substance with a composition close to 12CaO·7Al2O3. Therefore, it can be easily discharged from the molten steel. Therefore, in actual production, the feeding amount of calcium wire is mainly controlled based on the mass ratio requirements of Ca and Al in steel, but the following reasons may cause [Ca] content to fluctuate: a) The amount of argon blown into the ladle during the calcium line feeding process is too large. b) The tapping amount fluctuates, and the wire feeding amount is not adjusted in time. c) Sampling was taken too early after feeding the calcium wire, the molten steel was unevenly mixed, and the [Ca] content was not representative. d) When adding steel-core aluminum for deoxidation, the argon blowing is too small, the molten steel is mixed unevenly, or the sample is taken too early, causing the sample to be unrepresentative. e) The material of the calcium wire itself fluctuates. Therefore, in order to prevent excessive [Ca], temperature measurement and sampling must be carried out before feeding the calcium line. The calcium wire feeding amount (3~4m per ton of steel) is controlled according to the steel tapping amount, molten steel temperature, [Als] content in the molten steel, calcium wire yield and the Ca and Al mass ratio required by the steel type. The amount of bottom blowing argon (50L·min-1) and the temperature of the calcium feeding line (1590~1605℃) ensure that the end point [Ca] content of the molten steel is within the control range. (Note: The reference data for steel type A is in parentheses).
Strengthen the maintenance of the sliding nozzle mechanism.
Check whether the mechanism mold frame is deformed; whether the surface pressure of the spring loaded on the slide gate plate is appropriate; replace vulnerable parts in a timely manner; refuel parts that need lubrication regularly.
Determine whether the slide gate plate can continue to be used according to the requirements of the type of steel being made.
Observe whether there are deep roughening, cracks and abnormal melting loss on the surface of the skateboard; determine whether the effective remaining stroke of the skateboard meets the requirements for reuse.
Install the slide gate plate strictly in accordance with the operating points.
Clean the debris on the mold frame, the working surface of the slide plate and the back; keep the slide plate in the fully open state when burning oxygen.
Operate carefully when pouring.
During the steel pouring process, try to reduce the number of times the slide is pulled to reduce the possibility of wear. For multi-strand steel pouring tundish, if more than 1/2 of the casting strands cannot pour steel, the continuous casting machine should stop pouring. The pulling distance of the slide should be minimized during normal flow control to protect the effective remaining stroke of the slide; slag should be prevented from the nozzle at the end of pouring to prevent unnecessary erosion of the slide.
There are many reasons for steel leakage between ladle slides. Poor control of the sliding nozzle machinery, slide installation operation, quality of the slide itself, ladle steel pouring operation, molten steel smelting, etc. may lead to steel leakage between slides. Developing corresponding preventive measures for different reasons can reduce the chance of steel leakage on the slide gate plate and improve the good sliding rate of the slide gate plate.