Production Practice of Tundish Sizing Nozzle

 1 Introduction

The continuous casting process is an important process in the steelmaking production. It acts as a link between the converter and the steel rolling process, that is, the qualified molten steel made by the converter is poured into qualified billets and supplied to the rolling mill in time.

Refractory materials for continuous casting play an important role in the quality of steel and the normal production and performance of continuous casting. With the development of modern high-efficiency continuous casting technology, it is necessary to further improve the performance of refractory materials for continuous casting and develop new materials. The optimization and development direction of refractory materials for continuous casting is to increase the service life of refractory materials, reduce the pollution of molten steel, and meet the requirements of longer-life continuous casting and other special functions. As a key refractory material for continuous casting, the tundish sizing nozzle plays a crucial role in continuous casting production.

The tundish sizing nozzle is widely used in the tundish non-stop bar control flow pouring system for the production of continuous casting small-section billets, which plays a role in controlling the flow of molten steel and stabilizing the drawing speed. The molten steel flows into the mold evenly and stably through the sizing nozzle, which is a necessary condition to ensure the normal operation of continuous casting.

The main raw material for the sizing nozzle is a zirconium core. The zirconium sizing nozzle is made of stabilized zirconia and natural oblique zircon after being stabilized by a special process, and then formed by high pressure molding and high temperature firing. The sizing nozzle has the characteristics of high refractoriness, good thermal shock resistance, corrosion resistance, erosion resistance, small diameter change, and long service life.

The sizing nozzle is divided into an upper nozzle and a lower nozzle. The performance of the sizing nozzle directly affects the improvement of the life of the continuous casting tundish, which is of great significance for stabilizing the number of continuous casting furnaces and ensuring the normal production organization. Domestic steel mills currently have an average life of about 40 hours for sizing nozzles. With the continuous advancement of technology, the service life of sizing nozzles of domestic steel mills has reached more than 100 hours, which not only improves the production rate of casting machines, but also increases the output. Cost reduction is of great significance.

2. Current situation analysis

The steelmaking plant has 3 billet continuous casters and 1 special-shaped billet continuous caster, mainly casting HRB400E, HRB400K, Q235B, Q345B, HPB300D, MG500, HRB5000E and other steel grades. The main process equipment parameters are shown in Table 1.

Table 1 Main process equipment parameters of continuous casting machine

Casting machine model	shape	Slab section	Arc radius	Tundish capacity	crystallizer length	Casting machine flow number	Working pulling speed
5	camber	150×150	10000	23	900	5	1.8～2.8
6	camber	430×300×85	10000	23	700	4	0.75～0.85
7	camber	150×150	8000	25	900	6	1.8～2.8
8	camber	150×150	10280	25	1000	6	1.8～2.8

In recent years, reducing iron ratio and increasing output has become a common goal pursued by all companies in the steel industry. Our plant's production capacity has been further improved. The average casting speed of the caster has increased from about 2.2m/min in 2018 to about 2.6m/mim in 2019. The normal life of the tundish is 36 hours, the life of the upper nozzle is synchronized with the life of the tundish, and the life of the lower nozzle is 6-8 hours.

The sizing nozzle is mainly affected by chemical erosion, mechanical erosion and stress spalling in use. Starting from May 2019, the continuous casting process has started to occur intermittently in the middle and late stages of the use of the tundish. The nozzle has expanded diameter, dropped blocks, and reddened the nozzle. Such problems became more and more serious in July and August. During this period, there were two incidents of steel running during the delivery nozzle, which seriously threatened normal production, and the continuous caster was forced to shut down several times in advance or to heat up the package. It not only has a great impact on the safe and stable production of the continuous casting machine, but also disrupts the original production organization plan, and the cost of tundish refractory materials has increased sharply.

Through the technical personnel's follow-up inspection and analysis of the water inlet, the water outlet and the tundish from installation to use, and offline, the reasons for the problems of the sizing nozzle are as follows;

 

2.1 The quality of the zirconium core at the upper water inlet has decreased

The sizing nozzle must have good erosion resistance, corrosion resistance and thermal stability, and is generally made of high-purity zircon, zirconia and CaO.

Zirconia exists as three different types of isoisomers at different temperatures, namely: monoclinic system and tetragonal system

And cubic crystal system. The densities of the three crystal systems are: 5.65 g/cm3, 6.10 g/cm3, and 6.27 g/cm3. The density of the three crystal systems is quite different, especially between the monoclinic system and the tetragonal system.

From the thermodynamic analysis, when the temperature is lower than 1170℃, the monoclinic phase of pure zirconia is stable. When the temperature exceeds 117 0℃, ZrO2 changes from a monoclinic phase to a tetragonal phase. When the temperature exceeds 2370℃, ZrO2 changes from a tetragonal phase to a vertical phase.

Square phase, melting until 2680-2700℃. The entire phase change process is reversible.

When changing from a monoclinic phase to a tetragonal phase, it is accompanied by a volume shrinkage of about 7%. When the reverse phase transition occurs from a high-temperature cooling process, the temperature drops by about 100°C and the volume expands by about 3-5%. This volume change is sufficient to cause microcracks or obvious cracking of the material. Therefore, an effective solution is to add stabilizers to zirconia to prevent volume changes and stress generation during the phase change process.

Therefore, the lower quality of the zirconium core at the upper water port is the main reason for the lump drop at the water port.

 

2.2 The mud gap between the zirconium core and the outer skin penetrates the steel

The upper nozzle of the tundish used in our factory is an inlaid sizing nozzle, the body is made of high aluminum, and the inner core of the nozzle is made of zircon and zirconia composite. The nozzle zirconium core and the nozzle body are made separately, and then the two are glued together with refractory mud. The nozzle has low cost and good thermal stability.

After follow-up inspection of the off-line tundish, it was found that the redness of the upper nozzle is also directly related to the penetration of steel between the zirconium core and the outer skin. Sludge penetration of steel causes the body of the upper and lower nozzle working face to be corroded by molten steel, and cold steel is entrained, which is very easy to cause steel running accidents during the replacement of the nozzle; on the other hand, the mud joints penetrate the steel to cause the upper nozzle to become red, and it is easy to crack accident.

If the zirconium core of the nozzle and the nozzle body are not well bonded, the nozzle core will fall off and the steel will be penetrated by the mud gap after long time use. Its safety depends on the properties of the mud and the bonding process.

 

2.3 Increased pulling speed increases erosion of mud joints in the upper water mouth

The erosion of the zirconium core by molten steel is very serious. Due to the influence of the earth's gravity, the molten steel flows downward in a spiral vortex in the upper nozzle of the tundish, and the flow velocity can reach 14m/s. Due to the high specific gravity, high temperature, and fast flow rate of molten steel, severe heat scouring of the upper nozzle occurs. Since the inlaid top nozzle is a weak link in the mud joints, it is easily washed away by the high-speed rotating molten steel, causing the steel to penetrate to the top and bottom nozzle working surface.

Tracking found that when the continuous casting speed is low, the phenomenon of steel penetration in the mud seam basically does not occur, and there are more accidents of steel penetration in the mud seam when the drawing speed is increased.

 

2.4 Misalignment of the upper and lower nozzles

The service life of the drain is about 6-8 hours. During the quick change process, the drain is not in place due to various reasons, and the upper and lower nozzles are misaligned, which causes the high aluminum refractory material outside the zirconium core of the lower slider to be corroded by molten steel to produce pits After the pit is enlarged, the outer side of the zirconium core of the upper nozzle is eroded, resulting in serious steel strip on the upper and lower nozzle working face. The lighter ones cause the water outlet to become red and block the flow, and the more serious ones cause steel wear accidents in the process of replacing the water outlet.

 

2.5 Uneven working surface of upper and lower nozzles

Uneven working faces of the upper and lower runners will cause cold steel to be clamped on the working faces of the upper and lower runners during production, and the working faces will be scratched during the replacement of the runners, resulting in accidents such as steel running on the working faces.

 

3. Take measures

3.1 Improve the quality of the zirconium core at the water inlet

In order to improve the quality of the zirconium cores in Sheung Shui, on the one hand, manufacturers are required to adjust the formula and improve the production process, and on the other hand, find other manufacturers to conduct tests. After testing the water inlets of different manufacturers and different formulations, a new formula and manufacturer were finally determined. The expansion of the zirconium core and the phenomenon of zirconium core lumps in the water inlet were fundamentally controlled.

 

3.2 Change the block

After continuous tracking by the technicians, due to the direct erosion of the steel flow to the upper nozzle, it was decided to change the nozzle block, from the open upper mouth to the upper mouth capped type to protect the upper nozzle mud joints Not directly washed by molten steel.

3.3 Improve the quality of mud joints inlaid with zirconium cores

Improve the quality of refractory mud for mud joints and use zirconium fire mud that is more resistant to erosion. Improve the inlay process and minimize the mud gap to less than 0.5mm.

 

3.4 Ensure that the nozzle is centered

Train workers who install quick-change institutions to achieve standardized operations.

Before installing the upper and lower nozzles, pick out and return the upper and lower nozzles with obvious eccentricity.

Before installing the quick-change mechanism each time, measure the pressure of the metal parts, the nozzle bracket, and the spring to ensure that the cylinder stroke error is less than 1mm, the spring pressure is within the specified range, and the slides with obvious wear are replaced in time. Before each tundish baking, the captain will check the quick-change mechanism and measure the stroke of the quick-change cylinder to ensure that the nozzle is centered during production.

 

3.5 Return of nozzles with poor working surface flatness

After the arrival of each batch of nozzles, randomly sample the smoothness of the nozzle working surface, and return the nozzles with poor flatness.

3.6 Improved slider zirconium core

The zirconium core of the slider is thickened, especially the area of ​​the zirconium core of the working surface part of the slider is enlarged to ensure that the high-aluminum skin is not affected by the erosion of molten steel in normal use when there is a centering error in the nozzle.

 

4 Conclusion

After nearly half a year of follow-up, improvement, and testing, accidents such as nozzle blockage and redness have been thoroughly controlled. The disorder of production organization caused by the nozzle problem of the tundish was solved.

With the rapid development of continuous casting technology, there has been a billet caster capable of reaching above 5m/min or even 6-7 m/min, and the continuous casting time reached more than 100 hours. The development of these technologies is inseparable from the support of new refractory materials. All in all, refractory materials for continuous casting are developing in the direction of multi-function, high performance and long life. The development of new refractory materials for continuous casting should be accelerated, and the upgrading of products should be accelerated.