By analyzing the composition of intermediate ladle refractory and the refractory problems in daily work, we analyzed the refractory design, use and management one by one, and issued solution measures to ensure the safe use of refractory, and at the same time put forward the direction of further efforts for the intermediate ladle refractory in the cost control of steelmaking process.
Tundish refractory composition
The inner part of the middle bag is made of high temperature resistant materials, different smelting process needs to design different bag type, different resistant material configuration, the regular use of the bag type for the “T” type single bag and the knife handle type pair of bags. The internal refractory configuration in accordance with the requirements of the use of intermediate packages and smelting of different steel specialties and designed into different materials, generally for the permanent layer of the use of casting material casting molding, magnesium working layer for the vibration of the dry material pounding molding.
Conventional intermediate package filled with steel, the capacity is generally 20% to 40% of the steel package, the body shell is welded by the steel plate assembled, lined with a combination of refractory materials.
The refractory composition of the intermediate ladle mainly includes the intermediate ladle body, intermediate ladle cover, control of liquid steel flow rate of the plug rod, steel flow through the water mouth (by the upper water mouth seat brick, the upper water mouth, lower water mouth, immersed in the mouth of the water composition). In the connection part of the ladle and the middle ladle, the ladle protection casing is used for connection, the liquid steel flows out from the bottom of the ladle, and flows into the middle ladle through the ladle protection casing, also known as the protection pouring, and some enterprises don’t use the ladle protection casing, which is commonly known as the bare pouring. In order to make the liquid steel flow down from a certain height inside the intermediate ladle to cause damage to the bottom of the ladle resistant material, set up the impact zone resistant material right below the liquid steel, use the impact plate or the flow stabilizer resistant material to accept the strong impact of the liquid steel at the initial stage. Use slider to control flow of middle package.
During the use of the tundish, the following areas are prone to accidents:
Tundish slag line
The dry material at the tundish slag line is not resistant to erosion and scouring by molten steel. Especially when production is unstable, there is a low liquid level pouring problem on site, and the slag line position changes, resulting in leakage accidents.
The main reason is that the refractory materials in the tundish are not baked properly, especially in winter, the moisture in the gas is completely removed, the baking time is short, and the strength of the dry material in the working layer of the tundish is not achieved.
Around the impact zone of the tundish
The protective plates around the impact zone of the tundish are prone to fall off and collapse, resulting in molten steel eroding the permanent layer and leaking steel.
The main reason is that when the molten steel in the ladle is initially poured, the position of the ladle and the tundish is not centered, causing the two water inlet steel flows of the ladle to be misaligned with the impact zone of the tundish, causing the molten steel to deviate to the position of the protective plate of one of the tundishes, causing the protective plate to be damaged, collapsed, and peeled off.
Tundish upper nozzle
The service life of the tundish upper nozzle is short. The main reason is that during the process of gradually increasing production on site, the zirconium core inside the nozzle is not resistant to erosion by molten steel, and the diameter is severely expanded, causing the lower nozzle to turn red and leak, and a tundish accident occurs.
On-site solution measures, Tundish baking in steel mills
Tundish baking in steel mills includes offline baking and online baking, all of which use conventional blast furnace gas burners. When baking offline (with mold), there is a baker on the tundish. When burning normally, gas enters from the middle nozzle on the upper part of the burner, and air (oxygen) enters from the side nozzle, thereby ensuring complete combustion of gas. According to the flame combustion situation, adjust the ratio of gas and air (oxygen) at any time, lengthen the burning flame, and reach the bottom of the tundish downward to bake the bottom of the tundish. The normal baking time is about 2 hours. In winter, according to the hardening degree of dry materials and the size of the flame, the baking time is appropriately extended to 3h~4h.
When baking the tundish continuous casting platform (online baking), it is carried out according to a certain baking system. Different baking systems are adopted according to the configuration of the tundish type and material. First, clean up the debris in the tundish and the debris in the nozzle. From the on-site nozzle, it is best for the flame in the tundish to descend to the nozzle.
Some companies, according to the on-site conditions, use a reverse suction device from the water outlet to suck down the flame in the bag, so that the flame and heat in the bag can fully reach the water outlet. Some companies directly install the immersion water outlet, and use the reverse suction device to suck down the flame in the bag through the upper and lower water outlets to the immersion water outlet.
For the baking of the tundish (using blast furnace gas), the key management points are as follows for reference:
(1) Before baking, clean the residue in the tundish and nozzle to ensure that the nozzle can be effectively baked.
(2) Check whether the pressure of various gas sources (gas, compressed air) of the baking machine can reach the specified pressure and whether various instruments are intact.
(3) Use an open flame to aim at the nozzle of the baking machine, or place firewood in the tundish and light it, open the main gas valve, open the air valve, open the gas valve, ignite each gas burner in sequence, and adjust the gas air volume ratio to ensure full combustion of the gas.
(4) During the baking process, the worker supervisor should always stand at the upwind position and monitor the burning of the burner at any time. If there is a flameout, turn off the gas in sequence, then close the valves, find out the cause, and start from the above steps to re-ignite.
(5) The blast furnace gas baking time is controlled to be more than 5 hours. The specific baking standards are as follows:
Blast furnace gas baking can be divided into three stages:
In the first stage, low-temperature baking (<800 ℃) is more than 120 minutes (the time can be extended as needed).
In the second stage, the temperature is raised from low temperature to medium temperature (800-1 000 ℃) for more than 60 minutes.
In the third stage, high-temperature baking (1000-1050 ℃) is more than 90 minutes, and then the steel can be poured.
(6) Precautions during baking:
①In principle, the high-fire baking time should not exceed 6 hours, because at this time the dry material has low strength and large thermal expansion, which may cause the package to collapse. According to the actual situation on site, after high-fire baking, it is not allowed to switch to low-fire and bake repeatedly, which is easy to cause cracks and embrittlement of the dry material.
②When the temperature rises too quickly from low to medium fire, the gas in the package wall cannot be effectively removed, which will cause large-area cracks and severe separation of the package wall. The medium package with severe separation may cause the package to collapse and the overall quality and strength of the refractory material to decrease during use.
③When baking with high fire, the package wall has a slight bone separation problem (less than 20 mm) which does not affect normal use. At the same time, the ratio of air to gas should be strictly controlled when baking the package.
Tundish nozzle modification
The problem of short service life of the tundish upper nozzle is mainly due to the poor impact resistance of the aluminum-carbon material outside the zirconium core of the tundish upper nozzle to the molten steel. After the continuous casting speed is increased (the diameters of the upper and lower nozzles are expanded at the same time), the aluminum-carbon refractory material of the zirconium core of the upper nozzle and the outer part of the zirconium core is affected by the increase in the flow rate of the molten steel, and the corrosion resistance decreases; the on-site technicians combined with the refractory manufacturer to improve the aluminum-carbon material outside the zirconium core of the tundish upper nozzle, mainly to improve the material, improve the compressive strength, and improve the impact resistance of the aluminum-carbon material outside the zirconium core nozzle; at the same time, the zirconium core inside the nozzle was modified, and the outer diameter of the zirconium core inside the upper and lower nozzles was expanded by 10mm~20mm.
Through modification and practice, the service life of the tundish upper nozzle has been improved, the lower nozzle will no longer appear red, reducing safety hazards, and ensuring safe operation and continuous operation of production.
Strengthening daily management of intermediate packages
The daily maintenance and management of the tundish directly determines the service life and quality of the molten steel. In steel enterprises, the tundish is outsourced as a whole and dual management is implemented, namely, the management of the continuous casting operation area of the steel plant within the enterprise and the management of the outsourcing unit. From the daily tundish refractory quality, on-site construction quality, offline and online baking quality to the monitoring and control of the use process, it is necessary to meet the specifications and standards. It is mainly reflected in the following aspects:
(1) Select appropriate tundish refractory materials according to the working conditions of the tundish. For smelting ordinary steel grades, long-life refractory materials can be selected to increase the service life of the tundish and reduce process costs; for smelting special steel grades, special refractory materials can be used, and economic tundish age can be selected to improve the cleanliness of molten steel and improve the quality of molten steel.
(2) Cleaning and management of debris inside the tundish to ensure that the tundish is clean and tidy without debris; for cold shrinkage cracks generated during offline baking, the coating material construction method can be used for rectification.
(3) Improvement of refractory materials in the impact area of the tundish. For the tundish with a knife-handle type, an impact plate is generally used at the bottom of the ladle and a guard plate is used on the side to form a pouring area. This area is subject to the greatest impact of the molten steel when the tundish is poured. The material is generally selected from corundum mullite, silicon carbide, etc.; for the single-bag “T” type ladle, a flow stabilizer is generally used, and the material is generally selected from high-aluminum mullite.
(4) The tundish needs to be monitored and managed throughout the process of use:
① For the steel flow when the ladle is poured, the ladle turntable must be adjusted at any time to ensure the steel flow is centered, to avoid the molten steel from scouring the refractory in the impact zone, causing the guard plate to erode too quickly and reduce its life.
② For abnormal conditions that occur during the use of the tundish, such as the burning eye operation when the ladle does not open automatically, according to the matching relationship between the nozzle and the pulling speed in the relevant data, combined with the on-site production situation, the nozzle and the pulling speed are adjusted in time to ensure the stability of the tundish liquid level, temperature and quality.
③ Make good use cards and records of the tundish to facilitate analysis and summary after the tundish is offline.
④ For the use of stoppers, slag walls, slag weirs, etc. in the tundish, analyze and manage according to different situations.
For the steelmaking process in steel enterprises, the refractory consumption in the tundish pouring steel process in the continuous casting system is relatively high, and it is necessary to consider increasing the number of multi-furnace continuous castings, adopt new technologies, promote competition among multiple companies, and select refractory manufacturers with the best cost-effectiveness. At the same time, further study the erosion mechanism of refractory in the tundish pouring steel system, explore the erosion laws of its various parts, improve the use conditions of continuous casting refractory, and continuously increase the service life of continuous casting refractory. For tundishes used for high value-added or high-quality special steels, full consideration should be given to refractory with the best cost-effectiveness and economic tundish age to reduce refractory safety accidents and use costs, and improve the quality of steel products.
