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Process Practice of Quick-Changing Technology for Sizing Nozzle of Continuous Casting Tundish

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The service life of the tundish affects the output of the continuous casting machine and the molten steel yield, and also limits the increase in the number of continuous casting furnaces. The application of quick-change technology for the sizing nozzle of the tundish solves this problem and improves the operating rate of the casting machine to a great extent.

Keywords: quick change of fixed diameter nozzle; dry vibrating packing; tundish; baking

With the rapid development of continuous casting technology and the continuous improvement of high-efficiency continuous casting machines, higher requirements have been placed on the service life of the continuous casting tundish, because its service life directly affects the improvement of the operation rate of the continuous casting machine. At present, various factories have adopted various methods to extend the service life of the tundish. Among them, the quick-changing technology of the sizing nozzle of the tundish is an effective method. The application of quick-changing technology for sizing nozzle and the improvement of related processes in the billet continuous caster of Yangjiang Hongda Steel Co., Ltd. (hereinafter referred to as Hongda Steel Plant) have achieved certain results. At the same time, some analysis and exploration were made on the prospects of using this technology.

Main process parameters

1) Equipment parameters

Continuous casting machine: 4-machine 4-stream arc billet continuous casting machine, with a capacity of 500,000 t/a, and a cross-section of 126 mm * 126 mm; tundish car: semi-gantry type, traveling speed 2 m/m in. The driving device is driven by motor gear chain; ladle rotating table: T-shaped, normal rotation speed 1r/min; rotating device is gear rotation; tundish: 4-stream semi-arc tundish, flow spacing is 1100 mm, height is 600 mm.

2) Process parameters

Steel type: HRB335, HRB400; Melting point of steel: 1 500~1 510 e; Superheat of molten steel: 15~25 e; Converter smelting cycle: 25~ 28min; Pulling speed: 2.6~3.0 m/min.

Conditions for realizing quick-change technology for tundish sizing nozzle

Equipment conditions for quick change of tundish calibrated water inlet (life span exceeds 48 hours)

1) Refractory material conditions for quick-change tundish with fixed diameter nozzle

The bottom and permanent layer of the tundish must be pounded with castables, and the thickness is generally 80~100 mm. The working layer is filled, vibrated and sintered with better dry vibrating materials. The thickness of the working layer is 150~200 mm. The straight side of the drawn steel is protected against the impact of molten steel, and the left and right sides of the steel ladle are protected against erosion by the molten steel. The impact plate can be embedded into the dry vibrating material. In order to ensure the service life and the personal safety of the steel puller, thicker dry vibrating material is used for the semi-arc working layer on the opposite side.

The molten steel impact plate must be made of high-aluminum or aluminum-carbon or magnesium-carbon materials. Generally, an impact plate is prepared at the bottom of the bag to prevent accidents. Use 1 to prepare 1, or you can use it in turns to balance.

The base brick of the intermediate bag must be made of aluminum carbon material to ensure that there will be no cracking or steel penetration within 48 hours.

The nozzle of the tundish package generally uses a calibrated nozzle with an outer layer of aluminum carbon and an inner layer of zirconium (containing more than 90% zirconium). It must be baked and preheated before use to prevent the nozzle from bursting during pouring.

The quick-change mechanism of the tundish must be inspected and repaired before making the tundish. The springs and left and right supporting plates must be carefully inspected every time and replaced regularly. The quick-change mechanism must be fully lubricated, and the quick-change hydraulic cylinder must be flexible and easy to use.

2) Process conditions for quick replacement of the sizing nozzle of the tundish

The temperature of the molten steel during quick change of the sizing nozzle of the tundish directly affects the life of the quick-change tundish, and the temperature of the molten steel in the tundish depends on the steel type and the superheat of the continuous casting steel. Hongda Steel Plant mainly uses HRB335 and HR B400. The melting point of molten steel is generally 1500~1510 knots, and the superheat is generally 20±5 knots.

When quickly changing the sizing nozzle, the temperature of the molten steel should be lower; generally it should not exceed 1535 knots, and the liquid level of the tundish should be controlled lower.

3) Proficiency in quick-change steel drawing

The faster the action and the shorter the time, the better the effect. In principle, the flow must not be interrupted. The time is generally 0. 1 ~ 0.3 s.

Baking of tundish and fixed diameter nozzle

Good baking of the tundish and the nozzle is a prerequisite for quick replacement of the nozzle. The tundish made of magnesia dry vibrating material is baked over low heat for 2. 5 hours (temperature is about 350 knots) and covered. Adjust the tundish to high heat and bake for more than 2 hours. The lining temperature reaches 800~1000 knots. Ensure that the temperature rises to around 1000 knots about 1 hour before quick change and bakes stably at 1050~1100 knots for 1 hour. . During the baking process, always check the inside of the tundish to ensure that the lining of the tundish is good and that there is no accumulation of slag or debris in the bag.

It is best to use a negative pressure exhaust furnace to bake the base bricks and quick-change water inlet. Start baking at low heat about 1.5 hours before the quick change. The burner of the roaster is 30~50 mm away from the water inlet of the middle package, and bake to 200~300 knots. This is to prevent the quick-change spout and bottom brick from failing to meet the baking requirements and causing explosion due to uneven temperature changes during the baking process, and ensure that the temperature is baked to above 900 knots before quick-change.

Specific operations of quick change of fixed diameter nozzle

1) Ensure the molten steel temperature before and after quick change

The temperature of the molten steel before and after the tundish is quickly changed plays an important role in the success of the quick change. The initial temperature should be higher, which is more beneficial for quick change, but this will have a negative impact on the fast-paced production of a large steelmaking plant. When the temperature is low, production accidents such as nozzle nodule and quick change failure are prone to occur. Therefore, requirements are put forward for the temperature of the molten steel in the tundish before the quick-change tundish: low alloy steel > 1530 knots, plain carbon steel > 1535 knots. The arrival temperature of the quick-change tundish steel ladle is 10 to 20 knots higher than the normal temperature limit. The time for the molten steel to wait for the casting machine should not exceed 10 minutes, and the ladle should be a red ladle with normal turnover and in good condition. According to discontinuous temperature measurement statistics, the average temperature change of the molten steel in the tundish 10 minutes before and after the quick change of the tundish is shown in Figure 1 (taking low alloy as an example), and the quick change time is from the fifth to the tenth minute. As can be seen from Figure 1, the lowest temperature of the molten steel in the tundish during quick change is 1525 knots (the lowest point), which is about 20 knots higher than the melting point temperature of low alloy steel (505 knots), which is fully consistent with the continuous casting temperature system.

Figure 1 Changes in molten steel temperature in tundish

2) Timing for quick change of fixed diameter nozzle

When the continuous casting sizing nozzle works for several hours, the nozzle becomes larger due to the erosion of the steel flow, and the continuous casting speed exceeds the critical speed (the speed of the Hongda Steelmaking Plant is set at 3. 0 m/min). To avoid leakage production accidents in continuous casting, it is necessary to quickly replace the sizing nozzle. Hongda Steelmaking Plant generally designs the sizing nozzle diameter to be 14.0 ~ 15. 0 mm, and the casting speed is controlled at 2.6 ~ 3. 0 m /min. Once the pulling speed reaches 2.9 m/min, prepare for quick change conditions (because there is no stopper rod to control the upper nozzle of the middle package, the critical pulling speed may be exceeded due to the influence of the liquid level).

3) Quick change fixed diameter water inlet

Bake the spare calibrated nozzle at the ambient temperature in the standby position for 10 to 30 minutes to ensure that the nozzle temperature reaches 200 to 300 knots. When the liquid level of the tundish drops to 300~400 mm, press the button, and the hydraulic cylinder will push the backup sizing nozzle to move along the slide to the pouring position. At the same time, the original fixed-diameter nozzle is pushed out of the pouring position, and the new nozzle starts to work. At this time, the pulling speed should be adjusted appropriately, and then press the “Hydraulic Cylinder Retract” button to retract the hydraulic cylinder and remove the hydraulic cylinder. After replacing the nozzle, the continuous casting operation proceeds as normal pouring.

Comparison of the situation before and after the application of tundish quick-change technology

Hongda Steelmaking Plant used to use insulation board tundish to organize production, and the general tundish life was 7~9 hours. In 2004, we began to trial the sizing nozzle quick-change tundish technology, and the service life of the tundish began to be extended to 16 hours. Afterwards, through technological improvements and increased operator proficiency, the life of the tundish was extended to 24h, 32h, and 40h. Later, in view of the problem that the erosion of the slag line position seriously affected the life of the tundish, the tundish was improved into a semi-arc tundish, which increased the average life of the tundish to 48 h. In 2008, the highest level reached 55 h, and it is expected to exceed 60 h with continuous improvement. . The main effects are as follows.

1) The number of continuous casting furnaces has been increased from 25 furnaces before quick change to more than 150 furnaces, which improves the operation rate of the casting machine.

2) The casting machine productivity is increased and the number of casting machine shutdowns is reduced. The average number of monthly shutdowns is 1/6 of that before quick change. Based on the calculation of 90 shutdowns per month before quick change, each shutdown time of 50 minutes, pulling speed of 2. 6 m/min, and unit weight of 0. 121 t/m, the average monthly increase in output reached 4 719t

3) Reduce joint scrap steel and increase the molten steel yield from 97.6% before quick change to 98.92%.

4) Reduce the secondary oxidation rate of exposed molten steel when pouring is started or stopped, and the billet pass rate is also improved accordingly.

5) Eliminate production accidents caused by pouring low-temperature molten steel. After the quick-change technology of the sizing nozzle is applied, it can be decided whether to quickly change the sizing nozzle according to the temperature of the molten steel to be poured and the production rhythm. If there is an abnormal situation, the molten steel can be quickly replaced by one furnace in advance or postponed, which greatly improves the safety factor of production.

6) Reduces the consumption of raw materials, such as tundish refractory materials, cast iron particles, springs, paper ropes, asbestos, etc., making a certain contribution to energy saving and consumption reduction.

7) Improved the yield rate of steel products. In the past, the yield rate of steel rolling did not exceed 97. 20%. Later, through continuous casting of red billets in hot-feed heating furnaces or direct rolling of red billets, the yield rate of steel products increased to 97. 68%.

Analysis and Summary

Although there is a gap compared with domestic counterparts in terms of quick-changing technology for tundish sizing nozzle (for example, the tundish life of Anyang Steel Plant exceeds 60 hours). However, Hongda Steelmaking Plant has been insisting on using continuous casting tundish quick-change technology in recent years, and has achieved certain progress and results.

Tundish material and equipment

1) The material of the tundish must be made of good raw materials. For example, the magnesium oxide content of the dry vibrating material should exceed 80%, and the base brick of the package should be made of high-alumina magnesium carbon m (MgO + Al2O 3 ) > 80%. The requirements for the nozzle of the tundish should be even higher. The inner layer should be made of zirconium m (ZrO2) > 90%, and the outer layer should be made of high aluminum magnesium carbon m (MgO + Al2O3) > 80%. The permanent layer at the slag line should be pounded with good castables, and corrosion-resistant refractory materials such as magnesia carbon bricks and aluminum carbon bricks can even be laid. The working layer at the slag line is embedded with a molten steel impact plate in the dry vibrating material (Hongda Steelmaking Plant has achieved good results through tests).

2) It is best to improve the tundish from the “mouth” shape to a semi-arc tundish or a double arc-shaped tundish, which is beneficial to the erosion resistance of the working layer of the slag line. In order to prevent the impact of molten steel from penetrating the bottom of the tundish package, a box bottom is made and built with magnesia carbon bricks and aluminum carbon bricks to prevent the bottom of the package from being penetrated.

Process operations of tundish

1) Influence of steel type. For example, HR B335 and HR B400 killed steel are less oxidizing than Q195~235 boiling steel, and have less corrosion on the tundish lining.

2) The influence of molten steel temperature. The temperature and stability of the molten steel in the tundish have a greater impact on the service life of the tundish. A higher superheat of molten steel will have a shorter service life than a tundish with a lower superheat of molten steel. In the past two years, Hongda Steelmaking Plant has implemented “low-temperature fast drawing”, stabilized the temperature of molten steel, and established a molten steel treatment station. The molten steel in the upper continuous casting ladle is tempered (controlled at 1580 ~ 1590 e), and the continuous casting steel pressing time is controlled within 10 minutes to stabilize the temperature of the molten steel in the tundish at 1525 e.

3) The influence of the height of the molten steel level in the tundish. The height is generally controlled at 400~500 mm, and the position of the slag line is relatively fixed for easy observation.

4) The influence of the level of production organization. The level of production organization of a steelmaking plant largely reflects the average life of the tundish. Hongda Steelmaking Plant takes “continuous casting as the center and the furnace as the basis” to better grasp the smelting cycle in front of the furnace and provide stable and qualified molten steel for continuous casting. At the same time, strict inspection and maintenance of equipment are strictly carried out, creating conditions for the life of the continuous casting tundish to be increased to 50 hours.

5) Stable operation, less and quick replacement of sizing nozzles. It is necessary to configure the sizing nozzle according to the production situation, and strive to replace the sizing nozzle every 8 hours.

6) Discharge slag frequently. Reduce the thickness of the intermediate slag layer as much as possible, and discharge slag at least once per shift.

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