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The refractory material of the ladle sliding nozzle is composed of upper and lower nozzle bricks, upper and lower slide bricks and joint mud.
The upper slide gate is fixed in the fixed frame of the mechanism. The lower slide gate and the lower nozzle brick are installed in the sliding frame and can be moved forward and backward through mechanical operation,
so as to achieve the purpose of controlling the amount of flow by controlling the overlap between the upper and lower injection holes. The upper and lower slide bricks are pressed tightly by the tensioning element of the mechanism,
so that the lower slide gate will not cause a gap between the upper and lower slides as much as possible during the movement, so as to prevent the occurrence of steel leakage between the slides.
Since the slide gate is repeatedly subjected to erosion, chemical erosion and strong thermal shock of molten steel during use, in order to meet the precise flow control function, the slide gate must have excellent properties such as high temperature resistance,
high strength, good corrosion resistance, good thermal shock resistance, high oxidation resistance and low creep. Specifically, it is reflected in the following aspects:
(1) It has sufficient strength at high temperature to withstand the static pressure of molten steel;
(2) The sliding surface must be very smooth, with a flatness of no more than 0.05mm, so that it has close contact and ensures that there is no steel penetration during the pouring process;
(3) It must be resistant to erosion and corrosion and have good thermal stability so that it can withstand rapid changes in temperature and the erosion and corrosion of molten steel and slag.
1. Factors affecting the service life of ladle slide gate
1.1 Thermal stress and external force
Before pouring molten steel, the temperature of the slide gate is about 200-350℃; after pouring, the temperature of the slide gate casting hole rises rapidly to more than 1500℃ within a few seconds,
and the slide gate is subjected to severe thermal shock. Under the action of thermal stress, cracks are inevitable. If multiple furnaces are used, they will inevitably be subjected to repeated thermal shocks,
which can easily cause cracks and peeling on the working surface of the slide gate. According to the Ringery thermoelastic theory, the initial thermal stress fracture coefficient R is obtained as shown in formula (1).
Once the crack occurs, it will continue to expand. The resistance coefficient RST of this crack stress follows the Hasslman fracture mechanics theory, see formula (2).
R=S(1-μ)/Eα(1)
RST=[γ(1-μ)/E0α2]1/2(2)
Where: S is the tensile strength; E is the elastic modulus; μ is the Poisson's ratio; E0 is the elastic modulus when there is no crack; α is the thermal expansion coefficient; γ is the fracture energy.
Formula (1) and formula (2) show that the greater the initial thermal stress fracture coefficient and crack stress resistance coefficient of the material, the smaller the thermal expansion coefficient and elastic modulus of the material,
the more difficult it is for cracks to occur or expand, and the better the thermal stability of the material.
1.2 The impact of scouring, erosion and oxygen burning on casting hole cleaning by high temperature molten steel and slag
During the pouring process, the scouring, erosion and oxygen burning process of molten steel and slag will cause the slide gate casting hole to expand in diameter and melt erosion,
thereby enlarging the casting hole and causing the loss of refractory materials at the sliding mark. Especially when pouring high manganese steel,
manganese in the molten steel reacts with the slide gate refractory materials, see equations (3) and (4).
MnO+SiO2=MnO·SiO2(3)
MnO+Al2O3=MnO·Al2O3(4)
According to the MnO-Al2O3-SiO2 phase diagram, MnO·SiO2 is a low melting point compound with a melting point of 1291℃, which is not resistant to erosion at the operating temperature of the slide gate;
while the melting point of MnO·Al2O3 is 1720℃, and no liquid phase is generated at the steel casting temperature, which only causes a slight increase in the MnO content in the working zone of the slide gate,
changing the original structure of the corundum in the slide gate. When pouring calcium-treated steel, SiO2 and Al2O3 in the slide plate are reduced by calcium in the molten steel, and the reactions are shown in equations (5) and (6).
2[Ca]+SiO2=2CaO+Si(5)
3[Ca]+Al2O3=3CaO+2Al(6)
The generated CaO will react with SiO2 and Al2O3 in the refractory material. According to the CaO-Al2O3-SiO2 phase diagram, the melting point of 2CaO·Al2O3·SiO2 is 1584℃,
the melting point of CaO·Al2O3 is 1600℃, the melting point of 3CaO·Al2O3 is 1539℃, the melting point of CaO·2Al2O3 is 1762℃, and the melting point of 12CaO·7Al2O3 is only 1392℃.
In addition, the coordination number of aluminum and calcium in the slide structure will be extremely irregular, and there will be a large number of structural holes.
These low-melting point compounds are very likely to become liquid phases within the pouring temperature range. The generated liquid phase will continue to flow away with the steel flow,
causing the casting hole to expand and causing the slide to be damaged. Even if the liquid phase is not generated,
the generation of these compounds will cause a slight increase in the calcium content of the slide working belt,
change the organizational structure of the slide refractory material, lead to a decrease in the performance of the slide, and affect its service life.
During the process of burning oxygen to clean the casting hole, the iron oxides in the molten steel come into direct contact with the nozzle and diffuse inward through the pores and cracks,
reacting with the corundum and mullite phases in the slide bricks to generate low-melting-point silicates, see equations (7) and (8).
2FeO+SiO2=2FeO·SiO2(7)
FeO+Al2O3=FeO·Al2O3(8)
The melting point of FeO·Al2O3 is 1780℃, and a spinel reaction layer is formed around the corundum particles. However, the melting point of fayalite 2FeO·SiO2 is only 1205℃.
It exists in liquid state at the steel pouring temperature and continuously flows away with the steel flow, causing the casting hole to expand and leading to damage of the slide gate.
1.3 Effects of sliding resistance, oxidation, and slag metal penetration
When the slide plate is working, the friction between the upper and lower slide gate will cause wear. To ensure that there is no leakage, the contact surface of the slide gate must be very tight,
and a specific surface pressure of 0.5 to 1.0 N/mm2 is required between the slide gate. With the erosion and infiltration of molten steel and slag metal, the friction in the sliding mark area increases, causing the sliding mark to become rough.
When the slide gate adjusts the steel flow, turbulence is easily generated, which will also increase wear, resulting in steel infiltration between the slide gate. In addition, at a temperature of not less than 500°C,
the oxidation of carbon reduces the strength of the material, makes the surface rough, and intensifies the friction. In severe cases, steel leakage may also occur.
2. Measures to improve the service life of ladle slide gate
The corrosion of the ladle slide gate seriously affects the realization of the multi-furnace continuous casting process. In response to the occurrence of the above phenomenon,
the technicians proposed to optimize the material of the slide gate to improve the performance of the slide and adjust the casting process to ensure the multi-furnace casting of the slide brick. It is mainly reflected in the following aspects.
2.1 New Directions
A variety of metal powders and special nano-additives are introduced, as well as the carbon source is graphitized to improve the bonding system of the slide gate,
so that this metal-ceramic bonded non-fired and non-immersed slide gate has higher low-temperature strength, medium-temperature strength and high-temperature strength.
2.2 Optimize the performance of Slide Gate
Using dense fused corundum as the main raw material, reducing the amount of SiO2 introduced into the raw material,
improving the purity and corrosion resistance of the slide gate brick; adding appropriate carbon-containing raw materials to improve the thermal shock stability of the slide gate brick;
adding reinforcing agents to the matrix,
forming a ceramic bonding phase of SiC whiskers during the firing process to improve the thermal strength of the slide gate brick, while preventing the expansion of cracks to further enhance the thermal shock stability;
requiring suppliers to improve the processing flatness and finish of the sliding surface,
and applying lubricants on the sliding surface to reduce resistance during sliding and prevent oxygen absorption and oxidation during pouring.
2.3 Usage Requirements
The customer requires that there should be no refractory material falling off or sticking on the surface of the slide gate used in 2 to 4 furnaces, no steel clamping between the slide gate,
no obvious cracks on the slide surface, no "V"-shaped melting loss or enlarged aperture of the steel casting hole, and focus on observing whether there is steel slag intrusion in the cracks around the aperture,
whether the cracks are open, and whether the crack surface is accompanied by oxidation and particle peeling. If the above phenomena exist, the slide gate should be replaced immediately;
if small cracks are found and the surface around the cracks is flat, the cracks are normal cooling cracks and the slide gate can continue to be used;
sometimes small cross cracks can be seen on the upper plate surface in the closed position of the lower slide gate, which is normal and the slide plate can continue to be used; if obvious horizontal and vertical cross cracks are found in the direction of the slide,
the slide gate should be replaced. When a "step" appears at the joint of the upper slide gate and the upper water inlet after use, it is necessary to use aluminum-chromium fire clay to bore the hole.
Before boring, the residual material in the water inlet eye must be blown clean with an air duct, and then the fire clay is used to bore the hole to eliminate the "step". When boring, the mud material must be evenly smoothly,
firmly, and of appropriate thickness. After boring, the hole diameter of the upper water inlet brick should not be less than 85mm. In production practice,
it is strictly forbidden to use slide gate with water inlet hole inner diameter erosion and expansion greater than 10mm. If the width of the water inlet crack is greater than 1mm,
or the crack width is not greater than 1mm but there are two or more cracks, or ring cracks are found, the water inlet must be replaced.
3. Results and analysis
After the technical staff's research, the service life of the ladle slide gate has been significantly improved on the premise of ensuring safe use. The service life of the ladle slide gate has increased from an average of 2.8 furnaces to the current 3.6 furnaces.
The increase in the number of furnaces used for slide gate not only reduces production costs,
It also reduces the labor intensity of ladle hot repair workers. After using the slide gate for 4 furnaces, the surface of the slide gate and the steel pouring holes are evenly eroded without obvious cracks. After using the slide gate for 4 furnaces, the surface of the slide gate is flat and smooth,
and the slideway has no obvious roughening. phenomenon, there is no diameter expansion or corrosion of the pouring hole, and there are no obvious large cracks around the hole.
4. Conclusion
By optimizing the ladle slide gate material, the oxidation resistance and high temperature strength of the slide gate bricks are improved, the corrosion resistance of the slide gate bricks is ensured, and the foundation for the continuous use of the slide is laid.
Strictly controlling the on-site quality inspection reduces the time for replacing the ladle slide, further ensuring the service life of ladle slide gate, which can be increased from the original average of 2.8 furnace campaigns to 3.6 furnace campaigns,
reducing the labor intensity of the ladle hot repair workers, speeding up the turnover of the ladle, reducing production costs, and better meeting the requirements of energy saving, emission reduction, and consumption reduction in the production process.
