Discussion on influencing factors of continuous casting machine mold powder test

This article introduces the main pouring process factors that affect the test effect of mold mold powder. During the test, the quality and temperature of molten steel, the stability of the pouring liquid level, the insertion depth and centering accuracy of the nozzle, the protection pouring effect, the size of the argon gas and the protection should be considered. The influence of various factors such as slag operation. Through repeated tests of mold powder, the physical and chemical indicators of mold powder and the pouring process conditions of the continuous casting machine are constantly adjusted to meet the requirements of process quality.

Keywords: protective slag performance; test; influencing factors; discussion

Mold powder is a multi-functional metallurgical auxiliary material used in continuous casting production operations. The performance of mold powder has an important relationship with the production of continuous casting processes and the quality of cast slabs. Mold powder has strong adaptability. It must be tested repeatedly before use and the physical and chemical indicators of the mold powder must be continuously adjusted to adapt to the requirements of the process and equipment. Otherwise, it is easy to cause accidents such as slab quality and steel leakage.

 Issues that should be considered when testing mold powder

Timing of mold powder test

When testing mold slag for the first time, you should choose to test in the late pouring period. At the same time, you should consider the production conditions of other continuous casting machines and the pressure of the production organization, and comprehensively consider whether the production organization conditions are met for the test; for multi-stream continuous casting machines, only one stream should be tested. to prevent unplanned pouring stops caused by accidents such as slab quality or steel leakage, which would affect the entire production organization. For the first 2 to 3 tests, the test should generally be carried out in the late pouring period and without the pressure of the production organization.

Number of mold flux tests

Mold powder has complex physical and chemical properties, such as melting temperature, alkalinity, viscosity, melting speed, melting uniformity, surface tension, vitrification rate, spreadability, etc. Each performance affects and restricts each other, and each performance must Only when the conditions are met can smooth production and slab quality be guaranteed. Therefore, the mold powder needs to undergo many repeated tests and constant adjustments to gradually adapt to the requirements of process pouring conditions. Newly developed mold powder generally needs to be tested 3 to 5 times before it can fully meet the process quality requirements. Table 1 shows the statistics of the number of experiments on slab continuous casting and profile billet continuous casting molding slag at Laigang Steelmaking Plant.

Table 1 Statistics on the number of experiments with mold powder for slab continuous casting and profiled billet continuous casting

ModelMold powder model     Test timeNumber of trials
Slab casterPeritectic steel protective slagMedium carbon steel protective slagSPHC mold powder BB12-ZT July-December 2004July-December 2004February-September 2005September-November 20054545
Profile billet continuous casting machineST-SP/BP-LOctober-December 20096

Quality of molten steel

Inclusions in molten steel are one of the important factors affecting the performance of mold powder. Mold slag liquid slag has the function of adsorbing inclusions in molten steel. After the inclusions enter the liquid slag, they form multi-phase slag and are concentrated in the steel slag interface, destroying the uniformity and fluidity of the liquid slag and preventing the liquid slag from flowing smoothly into the copper plate of the crystallizer. In the gap between the mold and the shell, a uniform slag film cannot be formed, which affects the lubrication and uniform heat transfer performance of the mold slag.

During the pouring process, if the protective pouring effect is not good, secondary oxidation will occur when the molten steel comes into contact with the air, and Al2O3-type oxidized inclusions will be generated into the molten steel, causing pollution of the molten steel. The protective slag slag will come into contact with the molten steel, adsorbing and dissolving the floating particles in the molten steel. Inclusions, when the adsorbed inclusions reach a certain content, will cause denaturation of the liquid slag, that is, changes in physical and chemical properties such as alkalinity and viscosity, which will affect the uniform melting of the protective slag, worsen the fluidity of the liquid slag, and cause poor lubrication and transmission of the shell. Problems such as uneven heat, different shell thicknesses, inclusions and cracks on the surface of the cast slab.

Especially when casting steel containing easily oxidized elements such as Ni, Nb, Ti, etc., a good protective casting effect must be ensured. Otherwise, secondary oxidation will produce a large number of inclusions into the slag, which will change the properties of the protective slag and cause a large number of casting slabs. Defects such as surface cracks or dents. Therefore, when testing mold slag, the cleanliness of the molten steel must be considered to avoid affecting the performance of the mold slag due to excessive inclusions in the molten steel. Reducing the inclusion content in molten steel is an important measure to exert the good performance of protective slag. Therefore, the converter uses sliding tapping and slag automatic control technology to reduce the amount of slag when steel is placed; soft blowing technology is used in the later stage of refining to promote the full floating of inclusions; during the pouring process, protection and steady-state pouring are implemented and large ladle lowering is adopted Automatic slag control technology is the main means to reduce the slag entrainment of molten steel in the middle ladle, control the amount of slag in the large ladle, and reduce the inclusion content of molten steel.

Temperature of molten steel in tundish

If the temperature of the molten steel is too high, the mold slag melts quickly, the liquid slag layer is thick, and the powder slag layer is thin. The red slag is easily exposed, causing the mold slag to agglomerate. The temperature of the molten steel is low, and the heat provided for melting the mold slag is insufficient. The melting effect of the mold slag is not good, the melting speed is slow, and there is less liquid slag, which affects the lubrication and heat transfer functions of the mold slag. Therefore, when testing the mold slag, it is necessary to control the appropriate temperature of the molten steel in the tundish to create good conditions for the melting of the mold slag.

Liquid level stability

The stability of the molten steel level in the mold plays a key role in the good performance of the protective slag. Stable pouring speed and tundish liquid level can reduce the turbulent flow of molten steel in the tundish from bringing a large number of inclusions into the crystallizer, and prevent the liquid slag from adsorbing more inclusions and causing denaturation. The stable pouring speed and billet pulling speed can reduce the fluctuation of the mold liquid level, maintain the stable three-layer structure of the mold slag and the uniformity of the liquid slag melting.

On the contrary, if the pouring speed and billet drawing speed are unstable, the liquid level of the mold will fluctuate, and even the steel turning around the nozzle will occur. In this case, the stable structure of the mold slag will be destroyed, the melting will be uneven, and the liquid slag will become slag. The film thickness is uneven and the thermal insulation effect is poor; the molten steel level in the mold is unstable, which will produce a large number of slag strips in the protective slag, hindering the inflow of liquid slag, resulting in poor lubrication of the shell. Therefore, when testing mold flux, it is necessary to maintain a stable molten steel level and a stable drawing speed.

Nozzle insertion depth

The insertion depth of the immersed nozzle generally refers to the depth of the nozzle inserted below the molten steel surface of the mold. The appropriate insertion depth of the nozzle is one of the necessary conditions for the mold powder to have a stable three-layer structure, uniform melting, and uniform flow.

The insertion depth of the nozzle is deep, and the floating time and distance of the high-temperature steel water are long. The heat cannot be transferred to the crystallizer liquid level quickly, and sufficient heat and melting heat energy cannot be brought to the melting of the mold slag in time, resulting in poor melting of the mold slag and less liquid slag.

The insertion depth of the nozzle is too shallow, and the molten steel floats up quickly, causing the molten steel surface in the mold to churn and the liquid level to fluctuate greatly, destroying the stable three-layer structure of the mold slag, directly affecting the melting and lubrication of the mold slag, and also prone to agglomeration, Problems such as excessive slag strips affect the performance of the protective slag, increase slag entrainment by molten steel, and even lead to slag entrainment and steel leakage accidents. Therefore, when testing the mold slag, it is necessary to control the depth of the nozzle insertion, so that the molten steel level in the mold cannot turn over, and the liquid level in the mold must not be too stable. The liquid level in the mold should be kept slightly active, so as to promote The mold slag melts evenly, improves the fluidity of the mold slag, and exerts a good lubrication effect of the mold slag. In actual production, the insertion depth of the nozzle of the slab continuous caster is generally ≤160 mm and ≥120 mm; the insertion depth of the nozzle of the profile blank and small-section continuous caster is generally ≤80 mm and ≥50 mm.

Nozzle alignment

The centering accuracy of the nozzle is a key technological requirement for continuous casting production. The quality of the centering directly affects the performance of the mold powder, the quality of the cast slab and whether the production is stable and smooth.

Poor centering of the nozzle will cause injection flow deviation. The molten steel flow and liquid level fluctuations in different areas of the mold will be uneven. The liquid level in some areas is too active, and in some areas the liquid level is inactive. Mold slag melting requires Thermal energy and the three-layer structure of the mold slag will become unstable, resulting in uneven melting of the mold slag, unstable consumption, and uneven slag film thickness, which worsens the normal function and performance of the mold slag.

Before testing the mold flux, attention should be paid to maintaining good nozzle alignment. For slabs, the centering accuracy of the nozzle in the width direction should be controlled within 10 mm, and in the thickness direction, the centering accuracy should be controlled within 5 mm; for small-section cast slabs, the centering accuracy should be controlled Within 3 mm. In addition, it is necessary to ensure that the nozzle can be inserted vertically into the liquid surface to avoid the phenomenon that the upper part of the nozzle is well centered and the lower part is poorly centered.

Argon size

The influence of argon gas on the performance of mold slag is mainly due to the issues that should be paid attention to when blowing argon through the plug hole and argon sealing of the immersed nozzle. Stopper rod blowing uses argon gas to float rapidly in the molten steel in the mold to drive the molten steel to flow upward, causing the high-temperature molten steel to float quickly, promoting heat transfer in the low-temperature area, improving the melting of the mold slag and the flow of the liquid slag, improving the lubrication effect, and reducing the Adhesion problem. A reasonable argon gas flow rate plays an important role in improving the flow field of the molten steel in the mold. An unreasonable argon gas flow rate will destroy the performance of the mold powder.

When the argon gas flow rate is too large, a large number of argon bubbles in the molten steel continue to accumulate and float rapidly, causing the crystallizer liquid level to be too active, and even causing steel turning and mold slag turning, destroying the stable three-layer structure of the mold slag and causing powdery slag. The layer, sintered layer, and liquid slag layer are mixed together, and a large number of slag strips are also produced, which seriously affects the performance of the mold slag.

When the argon gas flow rate is too small, the number of argon bubbles in the molten steel is small, and the argon bubbles floating in the molten steel bring less heat to the steel slag interface. Especially when the slab is cast at low casting speed, the injection flow flows slowly and reaches the narrow surface of the crystallizer. There are fewer argon bubbles, causing problems such as inactive slag surface on the narrow surface of the crystallizer, less liquid slag, and poor lubrication.

During the test process of the mold slag, the argon gas flow rate was reasonably adjusted according to factors such as the pouring speed and the size of the pouring section, so that the liquid level of the crystallizer reached the optimal state, which ensured the stable melting of the mold slag, the uniform flow of the liquid slag, and the good performance of the mold slag. Functions such as lubrication and heat conduction create good conditions.

Use and management of protective powder

The performance of mold powder is closely related to the way it is used. Even if the quality of the mold powder is very good, if the method of using the mold powder is incorrect, it will also directly affect the performance of the mold powder.

1) Mold slag thickness: When adding mold slag, add it frequently, sparingly, and evenly. Control the appropriate thickness of the mold slag so that the mold slag has a good heat preservation effect and the liquid slag melts evenly; avoid slag caused by adding too much mold slag. The strips are not easy to find, and at the same time, it is necessary to avoid the protective slag being too thin and exposing the red slag.

2) Slag adding sequence: For large-section crystallizers, the stability of the liquid level is difficult to control. In some areas, the liquid level is active and the mold powder is consumed quickly. In some areas, the liquid level is dead and the mold powder is consumed slowly. In view of this situation, Every time you add mold slag, you should first add the area where the mold slag is consumed quickly to avoid excessively thin mold slag and exposed red slag.

3) Picking out slag strips: The principle of picking out slag strips is that large slag strips must be picked out in time, and small slag strips do not need to be picked up. Pick them gently and slowly. When picking up large slag strips, use a slag picking rod to gently push the slag strips away from the copper plate, and then pick them out, or directly pull the slag strips upward. It is strictly forbidden to press the slag strips downward to prevent the slag strips from blocking the liquid slag inflow channel; the slag picking rod Do not insert into liquid slag and molten steel to avoid damaging the uniform melting of the protective slag.

4) Mold slag management: Mold slag should be stored dry to prevent the mold slag from getting damp. Before the test, samples should be taken to check whether the physical and chemical performance indicators of the mold slag meet the requirements.

Conclusion

Mold powder is a multi-functional metallurgical auxiliary material with complex use environment and strong adaptability. Whether the performance of newly developed mold powder can meet the process requirements cannot be determined before use. Tests must be conducted to verify its usability. Only by testing the mold flux under good molten steel quality and pouring conditions can the performance of the mold flux be correctly evaluated. Therefore, when testing mold slag, it is necessary to consider the influence of various factors such as the quality and temperature of molten steel, the stability of the pouring liquid level, the insertion depth and centering accuracy of the nozzle, the protective pouring effect, the size of the argon gas, and the operation of the mold slag to avoid process defects. Unfavorable conditions affect the performance of the mold powder.

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