The Submerged Entry Nozzle(SEN) is a refractory casing for casting that is installed at the bottom of the tundish during continuous casting and inserted below the molten steel level of the mold. It’s main function is to prevent secondary oxidation and splashing of molten steel during the injection of the tundish, and to avoid mold protection. The slag is involved in the molten steel to improve the flow state and heat flow distribution of the injection in the mold, thereby promoting the uniform growth of the shell in the mold, which is beneficial to the discharge of gas and inclusions in the steel.
Title: Cause analysis of perforation of submerged entry nozzle and material test results
Keyword: Submerged entry nozzle, tundish furnace, continuous casting, refractory casting
1.SEN Perforation accident phenomenon
- SEN online measurement after use
When the No.5 furnace of continuous pouring was produced in the steel plant, the V runner was perforated and the pouring was stopped, and the No.6 furnaceⅡrunner was perforated and stopped. The No.13 furnace for a certain steel with 450mm cross-section were produced. At the end of casting, it was found that the lower part of the nozzle I was broken. After the tundish stopped pouring and cooling, the relevant measurement was carried out, and the installation height of 5 nozzles was measured with a level meter. The test result of the level meter showed that all the nozzles were on the same level, and no abnormality was found in other nozzles and refractory materials of the tundish. See picture 1.
Picture 1 Level measurement of the tundish after stopping pouring
The SEN is a shaped product, and human factors have minimal influence on its performance. During the packaging, transportation, installation and baking process, the user, general contractor and supplier have achieved full monitoring. The nozzle is monitored in real time when it is in use. The casting curve is shown in Picture 2.
Picture 2 Casting Monitoring Curve
- Sampling from the nozzle after use
Knock out all the used SEN and splice them as they are, and measure the internal and external dimensions, as shown in Picture 3 and Table 1. It can be seen from Table 1 that the diameter of the inner hole is expanded from top to bottom to 16-18mm, the lower part of the No.1 nozzle is missing 30mm, and the outer diameter at the height of 50mm from the port is reduced by 3-15mm. The stump is shown as a smooth semicircle. Caused by slow erosion.
Picture 3 Picture of SEN after use
Table1 Inner and outer diameter of nozzle after use(mm)
Picture 4 shows the cross-sectional morphology of the nozzle after use. After use, the composite surface of the SEN(170mm from the port)ZrO2-C side material is completely oxidized and decarburized from the outside to the inside. The integrity of the nozzle changes quickly, and the effective thickness is quickly reduced from 22mm to less than 10mm. The thinnest part of the nozzle is only 2.36mm, as time increases, the occurrence of perforation accidents becomes inevitable.
No obvious oxidation occurred in the inner hole of No.5 nozzle, and the inner wall was smooth and flat, and there was no sticky steel and impurity layer like No.1 nozzle. Based on the preliminary judgment from the nozzle section, the cause of this quality accident is: No.1 nozzle body material Al2O3-C has poor anti-oxidation and anti-erosion properties.During the casting process, the inner hole gradually eroded and scoured, and zirconium was encountered at the joint of the composite layer. Oxidation of the carbon layer losers carbon and locally loses its strength, which results in perforation. The lower joint part not only has the inner hole reaming,but also the repeated erosion and erosion of steel, slag and mold slag, forming a situation of internal and external pinching, and the thickness becomes thinner and thinner, resulting in a fracture surface.
2.Test results and analysis
In order to further analyze the reasons for the damage of the nozzle, in addition to the electron microscope and other analysis of impurities and metamorphic layers, all the samples were ground to remove the surface impurities and metamorphic layers. The physical and chemical properties of the SEN after use were tested and analyzed. See Table 2 for testing items and locations.
Table2 After use, nozzle sampling and inspection site
- Chemical Composition
The comparison of the chemical composition of the nozzle is mainly carried out on the perforation part, and the MgO-C material of the wrist is not involved.
It can be seen from the chemical composition of the two nozzles in Table3 that the chemical composition of the two nozzles is similar, and the Al2O3 content of the No.5 nozzle is about 2% higher than that of the No.1 nozzle.
- Bulk density, Apparent porosity and Oxidation resistance
Oxidation resistance is one of the key properties of carbon-containing refractories, which directly determines the corrosion resistance and erosion resistance of carbon containing refractories, and is one of the decisive factors affecting the service life. Usually, metal aluminum powder, silicon powder, boron nitride and other antioxidants are added during the production of refractory materials, and the surface is coated with glaze when necessary.
Table 4 shows the volume density, porosity, and oxidation loss rate of the nozzle after use
Table4 Analysis of physical properties of bulk aluminum-carbon materials
It can be seen from Table 4 that the volume density of the two nozzles is almost the same, indicating that the process parameters such as the forming pressure of the two nozzles are basically the same. The oxidation loss rate of No.1 is higher than that of No.5 body material, indicating that No.1 SEN body material has poor oxidation resistance. In the case of the same carbon content, in terms of refractory raw materials and performance, such a difference in oxidation resistance will not occur. The obvious difference in porosity may be related to the grade of raw materials is low, the water absorption rate of particles is large, and its impurity content is high. During use, it will be gradually ablated by oxidation, leaving more pores inside the product. The above analysis is also confirmed by the chemical composition of the nozzle in Table3.
a.The performance of the Al2O3-C quality of the slag line is quite different, which is likely to cause the high temperature strength to be reduced during high temperature use, and the expansion coefficient of the two at high temperature differs greatly, and defects, cracks and the decarburization and reaming of the inner hole results in perforation in the joint part during the use of the nozzle.
b.The reason for the nozzle perforation and the lower end of the block:No.1 nozzle Al2O3-C material has a low oxidation resistance index, and the thermal expansion of the composite part is not matched, which causes part of the zirconium carbon layer to oxidize and lose carbon during actual use, and locally loses strength, thereby causing the nozzle perforation.
c.The application is the production of SEN has further increased flaw detection and quality inspection in the three major materials of continuous casting. The user unit has increased product testing and tracking during on-site use, and regularly sampled product performance to ensure stable and smooth production on-site.