Application of new technology to reduce nitrogen addition in continuous casting machine tundish

Combined with the actual production, a new integrated technology for reducing nitrogen addition in the tundish of a continuous casting machine is introduced. This technology optimizes the structural form of the ladle nozzle bowl, improves the sealing gasket material and sealing form of the ladle nozzle bowl, and designs and invents Special hydrogen blowing cover, improved sealing method of tundish cover, design and production of special tundish covering agent, through the implementation and application of integrated technology, the nitrogen increase amount of tundish is ≤2PPm, and the tundish protective pouring effect is significantly improved.

Keywords: continuous casting; ladle nozzle; gasket; nitrogen addition amount

During the continuous casting process, the ladle long nozzle is generally used as the molten steel flow channel between the ladle and the tundish. The upper part of the ladle nozzle is connected to the ladle nozzle, and the lower part of the ladle nozzle is inserted into the molten steel in the tundish. The flow rate of molten steel in the ladle nozzle is about 1~3m/s. The fast flowing steel flow generates a “jet pump” It produces a huge suction force on the surrounding air, which will suck the surrounding air into the molten steel. If the seal between the long nozzle of the ladle and the lower nozzle of the ladle is not tight, serious secondary oxidation of the molten steel will occur. For Major steel mills are actively taking measures to solve the problem of lax sealing in this part, but the solution is not ideal. In addition, the coverage effect of the molten steel in the tundish is not good, and the amount of nitrogen added in the tundish is as high as about 10PPm, and in serious cases, it reaches more than 15PPm. After the originally clean molten steel was poured into the tundish, the secondary oxidation inclusions increased significantly, causing the molten steel to be seriously contaminated.

Basic process

Introduction to production process

The main production process paths of a company’s steelmaking plant are: molten iron, KR desulfurization, converter smelting, LF refining, RH degassing, and continuous casting. Through sampling and testing, after RH degassing, the total oxygen content of the molten steel was about 20 PPm, the nitrogen content of the molten steel was about 30 PPm, and the constant hydrogen content of the molten steel was about 1PPm. The cleanliness of the molten steel before entering the continuous casting process is high. Able to meet high-quality production requirements. After the molten steel enters the continuous casting process,

Due to the lack of thorough research on the detailed control of ladle nozzle protection pouring and tundish protection pouring, the secondary oxidation pollution of molten steel is serious, which seriously restricts the quality improvement work.

Problems existing in the current production process

The space between the ladle nozzle and the ladle long nozzle bowl is mostly sealed by gaskets and argon gas. Place an aluminum silicate fiber sealing gasket with a thickness of about 5mm between the ladle drain and the long ladle drain bowl. Then the robotic arm tightens the gap between the ladle drain and the ladle spout. However, since the ladle drain and the ladle spout are made of refractory materials, in order to prevent damage to the ladle, the gap between the ladle drain and the ladle spout needs to be tightened. The tightening force is limited. Therefore, the sealing effect of the gasket is limited. Moreover, under the high temperature of molten steel, the sealing gasket undergoes a gradual breaking process. It often happens that the sealing gasket is broken into powder before the pouring of a furnace of steel is completed. The sealing effect is lost, and the molten steel is easily oxidized. The molten steel increases nitrogen and oxygen. .

The ladle drain is poured with drainage sand. Ladle drainage sand is a bulk refractory material filled in the ladle seat brick and the ladle drain, which plays the role of isolating the molten steel from the sliding plate at the bottom of the ladle. When the ladle is poured, the sliding plate at the bottom of the ladle opens, and the drainage sand is pressed out by the static pressure of the molten steel. If a simple ladle nozzle is inserted into the ladle for pouring, since the density of the diversion sand is smaller than the density of the molten steel, the diversion sand will float on top of the molten steel after the ladle is poured. It is very easy to cause the diversion sand to not be discharged quickly from the long ladle nozzle. The nozzle is blocked or the steel flow is not smooth. Due to the poor steel flow in the nozzle, the molten steel easily overflows from the joint between the long nozzle of the ladle and the lower nozzle of the ladle. The top of the upper nozzle of the ladle is prone to steel turning, which seriously affects continuous casting. Normal casting proceeds. If pouring is started first and then the long nozzle of the ladle is connected to the lower nozzle of the ladle, the exposed molten steel will be easily oxidized by air during the pouring process, and the molten steel will increase nitrogen and oxygen seriously.

During the casting process of molten steel in the tundish, a covering agent needs to be spread on the molten steel surface in the tundish. The covering agent can isolate the air and absorb floating inclusions from the molten steel. However, the covering agent Contact with air for heat dissipation will cause crusting on the slag surface. The surface of the coating agent after crusting is prone to cracks due to uneven shrinkage. Air can easily transfer oxygen to the inside of the molten steel through the crack channels on the surface of the coating agent, and the molten steel can easily increase nitrogen and oxygen.

 Application of key technologies for nitrogen control

Optimize and make large bag water inlet sealing gasket

As we all know, the sealing effect of the ladle nozzle bowl is the key control point for protective pouring in the continuous casting process. To this end, on the premise of retaining the original argon gas seal and single-layer gasket seal, another two-layer sealing structure was installed inside the gasket, and a three-layer sealing structure was used at the junction of the long ladle nozzle and the ladle lower nozzle. The outermost layer of the sealing structure is the traditional first sealing cushion layer, the middle layer is the second sealing cushion layer, and the innermost layer is cement sealing (Figure 1-2).

Figure 1 Clay pad

Figure 2 Combination of clay mat and fiber mat

The first sealing gasket layer and the second sealing gasket layer are both made of aluminum silicate fiber. However, in the process of producing the gasket, the first gasket layer is pressed with a larger pressing force, and the second gasket layer is pressed with a smaller pressing force. Make the second gasket layer 2 to 3 times softer than the first gasket layer. Since the second sealing gasket layer is soft and has a large deformation rate, a large tightening force can be exerted between the ladle nozzle and the ladle long nozzle to improve the sealing effect. Moreover, the superposition of the second sealing gasket and the first sealing gasket increases the total thickness of the sealing gasket, and the sealing gasket can be completely broken until the furnace steel is poured, thereby improving the sealing effect.

The clay sealing gasket is a clay-like, easily deformable refractory material at room temperature. The clay sealing gasket is soft. Under the action of the tight force between the ladle lower nozzle and the ladle long nozzle, the sealing clay will flow and deform to block the sealing gap. It can flow to the gap between the first sealing gasket and the second sealing gasket, or it can wrap the ladle drain port to improve the sealing effect. At the same time, during the continuous casting process, the heat emitted by the molten steel bakes the clay, causing the magnesia and corundum powder in the clay to undergo a sintering reaction to form magnesia-aluminum spinel. The generated magnesium-aluminum spinel is a dense sintered layer. The magnesia-aluminum spinel firmly connects the ladle nozzle and the second sealing gasket, and during the reaction of magnesia and corundum powder to form magnesia-aluminum spinel, there is a volume expansion of 5% to 8%. It can further block the sealing gap and improve the tightness of the bond between the second sealing gaskets at the ladle drain outlet. After each heat of pouring, replace the new sealing gasket to ensure the sealing effect of the bowl mouth.

Making and using the bell-shaped trumpet spout

Compared with the original straight cylindrical structure of the big bag spout, the outstanding advantages of the bell-shaped trumpet shape spout (as shown in Figure 3) are: The larger inner diameter at the lower part of the ladle nozzle leaves enough space for the flow of molten steel and drainage sand. During the pouring process of the ladle nozzle, even part of the drainage sand cannot be discharged from the ladle long nozzle due to buoyancy in a short period of time. It still leaves enough space for the molten steel to run, and the top of the long nozzle of the large ladle will not turn over due to the obstruction of the flow of molten steel. Since the phenomenon of steel turning will not occur, the pouring of molten steel can be started by connecting the long nozzle of the ladle with the lower nozzle of the ladle and inserting it into the steel liquid level of the tundish before starting the pouring. There will be no air oxidation caused by the exposure of molten steel during pouring.

The inner diameter of the inner cavity of the upper part of the long nozzle of the big bag is 90 to 110 mm. The inner diameter of the lower cavity of the large bale long nozzle is 112.5 to 160 mm. The wall thickness of the long ladle nozzle is 40~60mm, and a first argon blowing nozzle is arranged on the upper part of the long ladle nozzle. The argon gas source supplies argon gas to the first argon blowing nozzle through the argon blowing pipe and valve to provide argon sealing protection for the long nozzle of the large bale. It can keep the joint between the top of the long ladle nozzle and the ladle nozzle in an inert gas atmosphere to prevent the molten steel from being oxidized. At the joint between the top of the ladle nozzle and the ladle drain, there is a layer of sealing iron sheet on the upper part of the ladle nozzle to prevent the argon sealing gas from leaking downward. After each batch of pouring, the damaged sealing gasket and cold steel at the bowl mouth of the long nozzle of the large package need to be cleaned to ensure that the bowl mouth step is intact and not damaged, and the new sealing gasket is placed into the bowl mouth to ensure a complete fit.

Figure 3 Bell-shaped horn long nozzle

Develop and use high alkalinity and low melting point covering agents

Tundish covering agent is an important measure and means to effectively isolate the contact between air and molten steel in the tundish. However, when major steel plants use covering agents, they mainly have problems such as serious encrustation and poor inclusion adsorption effect. The surface of the encrusted coating agent is prone to cracks due to uneven shrinkage. Air can easily transfer oxygen to the interior of the molten steel through the crack channels on the surface of the coating agent, and the molten steel can easily increase nitrogen and oxygen. To this end, combined with the actual production conditions, a special covering agent and covering method were developed. The covering agent can remain completely liquid at the temperature of molten steel. The composition of the covering agent is: Al2O3: 41~51%, CaO: 37%~47%, CaF2: 8%~10%, MgO≤3%, SiO2≤3%. The melting point of the covering agent is 1100~1200℃, the usage amount of the covering agent is 0.5~1t/pouring time, a layer of low melting point covering agent is spread on the surface of the tundish, and a layer of carbonized rice husk is spread on the upper layer of the covering agent. It is used to insulate the covering agent. The dosage of covering agent is 0.3~0.5t/pouring time.

WAl2O3/WCaO=0.87~1.38 in the covering agent, and AlL2O3 and CaO in the covering agent mainly exist in the form of 7Al2O3·12CaO. The melting point of 7Al2O3·12CaO is 1395℃, and the covering agent also contains 8% to 10% CaF2. CaF2 can further reduce the melting point of the covering agent to 1100~1200℃. Carbonized rice husk is a commonly used thermal insulation material in steelmaking production. It can prevent the covering agent from transferring heat to the air, keep the covering agent in a liquid state during the pouring process, and improve the anti-oxidation effect.

Design and manufacture the entire argon blowing tundish cover

When welding the tundish cover, embed one φ30mm stainless steel pipe in each of the front and rear covers. Refractory materials are used for knotting (as shown in Figures 4-5). When in use, argon gas is connected to the outer metal hose and the flow is controlled according to 100 L/min to form an argon atmosphere in the tundish to reduce oxidation problems.

Figure 4 Pre-blown fluorine cover welding production

Figure 5 The pre-blown fluorine cover is put into use 

Technical characteristics and application effects

Through the comprehensive application of new technologies, the fully protected pouring effect of the continuous casting process has been significantly improved. About 500 heats were sampled for gas sample detection and analysis. The qualified rate of nitrogen addition to molten steel was ≤2 PPm, which was over 99.95%. This reduced aluminum loss in molten steel and laid a solid foundation for building a clean steel production platform.

This technology can be used for reference whether it is producing special steel, stainless steel or other alloy steel, and has promotion and application value in all continuous casting machines.

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