Abstract: The heating plug for continuous casting tundish produced by a certain factory was sampled and analyzed, and the structure and composition of the plug base, rod head, anti-oxidation coating and anti-adhesion coating were analyzed.
The results show that the outer layer material of the plug head contains more granular aggregates and is denser than the structure of the core; the gradient distribution of the plug head material enhances the corrosion resistance of the material. The anti-adhesion coating of the plug head consists of Al2O3, SiO2, P2O5, Na2O, and K2O. The bonding strength between the coating and the substrate is not high and it is easy to fall off; the anti-oxidation coating of the plug consists of Na2O, Al2O3, SiO2, K2O, TiO2, and CaO. There are many alkali metal R2O phases, which make it easy to form low-melting point glaze at high temperatures. Inclusions in silicon steel may come from the anti-adhesion coating, anti-oxidation coating or corrosion products of the plug head.
The heating plug for continuous casting tundish is used to control the flow rate of molten steel from the tundish to the copper mould tube, and is mostly used in the production of high-quality steel and alloy steel. During use, the plug head is the most critical part, which determines the flow control effect and the life of the plug. The molten steel scours, wears and erodes the plug head, causing the head to deform; the vortex formed by the worn part and the upper bowl mouth of the nozzle causes the flow to become narrow and the contact area to increase. The erosion deformation in the later period of use will increase the distance from the nozzle bowl mouth, increase the drop amplitude of the plug closure, and the flow control effect will deteriorate until it fails. The erosion products often form inclusions and are brought into the copper mould tube, which ultimately affects the quality of the steel.
The stopper rod is made of aluminium carbon, aluminium zirconium carbon, etc., and its life mainly depends on the erosion at the slag line. The glaze formed by the anti-oxidation coating on the surface of the stopper rod becomes glassy and has no pores, and its anti-oxidation effect is good; if the glaze is porous during the heating process, oxygen can penetrate and oxidize with the carbon in the stopper rod. The calcium content in the molten steel of calcium-treated low-alloy high-strength steel is high, which reacts with aluminium carbon stopper rods to form low-melting silicates and calcium aluminates, which intensifies the erosion. Using magnesium carbon rod heads can improve the abnormal erosion of stopper rods and increase the number of continuous casting furnaces in the tundish by 2 to 6 furnaces.
Some anti-oxidation coatings for aluminium carbon and aluminium magnesium carbon stopper rods are prepared with boron-containing materials such as flux, clay, high-aluminium clinker, SiC, quartz and other auxiliary materials, and a glassy protective film is formed above 800 ℃. However, some studies have also believed that boron-containing oxides fill the pores of carbon-containing refractory materials by forming a liquid phase to reduce carbon oxidation, and excessive use of boron-based compounds as antioxidants will lead to a decrease in the mechanical properties of the coating. In fact, there are few systematic analyses of the composition structure of stopper rods and the anti-oxidation coatings and anti-adhesion coatings used from an application perspective. This article combines the analysis of the anatomical process of stopper rods for smelting non-oriented high-grade silicon steel to discuss its composition, structure, performance and related issues.
1. Sampling and analysis of heating plug for continuous casting tundish
Plug length 1570 mm, anti-adhesion coating part 120 mm, wrist large end diameter 135 mm, small end diameter 130 mm. The chemical analysis of the stopper rod cylindrical section is as follows:
w(Al2O3)=59.69%, w(SiO2)=10.40%, w(CaO)=0.17%, w(MgO)=0.10%, w(TFe)=1.38%, loss on ignition LOI=20.03%. The chemical analysis of the plug rod head shows the following composition:
w(Al2O3)=80.21%, w(SiO2)=0.76%, w(CaO)=0.20%, w(MgO)=0.12%, w(TFe)=2.18%, w(C)=12%, LOI=11.99%. It can be seen that the composition of the rod head is different from that of the cylindrical section.
As shown in the figure, after the plug rod is cut, the anti-adhesion coating on the outer surface of the rod head is easily scraped off after contact with water; after the scraped coating is dried, the powder method energy spectrum surface scanning analysis using FSEM scanning electron microscope shows that its composition is:
w(Al2O3)=64.60%, w(SiO2)=26.73%, w(P2O5)=6.71%, w(Na2O)=1.38%, w(K2O)=0.58%.
Judging from the composition, the coating is composed of fine powders such as alumina, mullite powder and clay, and is mixed with binders such as sodium tripolyphosphate or dihydrogen aluminium phosphate. The coating has basically no strength and softens when exposed to water. The coating should be a one-time protective coating, which is estimated to be a thermosetting material.
As shown in the figure, after the plug rod is cut longitudinally along the length direction, it can be found that the materials at the rod head are obviously different: the outermost layer has a thickness of 15 to 25 mm and contains large granular aggregates, including white granular aggregates; while the core has no obvious aggregates.
A partial enlargement of the cut section of the plug rod is shown in Figure 2. As shown in the figure, the outer layer material after cutting in area A has a volume density of 2.85 g/cm3 and an apparent porosity of 12.8%; the core material after cutting in area B in the figure has a volume density of 2.86 g/cm3 and an apparent porosity of 14.1%. The gradient stratification of the material is estimated to enhance its anti-corrosion performance, while the straight barrel section of the rod head has no stratification.
The structure of the outermost material of the rod head shows that the large and small aggregates are composed of alumina, and the black is carbon; it is speculated that its main components are corundum and carbon, with a small amount of SiO2 and Na2O. The structure of the material matrix after magnification shows that there are more flake and fine-grained aggregates, and the local grey amorphous bonding phase at the “+” position is composed of large and small particles of alumina, and the black is C; it is speculated that its antioxidants are metallic aluminium or ultrafine alumina powder.
The temperature of molten steel in the tundish is generally between 1 470 and 1 560 ℃. In order to observe the morphological changes of the stopper rod at high temperatures, the stopper rod was heated to about 1,000 ℃ by oxygen lance combustion heating. The glaze layer formed by the anti-oxidation coating on its surface was observed to be locally accumulated in a network shape with flow. The composition of the glaze layer formed by the anti-oxidation coating was analyzed by FSEM combined with the energy spectrum. The surface scanning composition of a group of powders was:
w(Na2O)=9.51 %, w(Al2O3)=9.99 %, w(SiO2)=75.60 %, w(K2O)=2.20 %, w(TiO2)=1.05 %, w(CaO)=1.66 %. The surface scanning composition of another group of powders is:
w(Na2O)=8.17%, w(Al2O3)=9.46%, w(SiO2)=77.33%, w(K2O)=2.31%, w(TiO2)=1.20%, w(CaO)=1.52%.
From the perspective of the two groups, the content of different oxides fluctuates, which should be a problem of sampling and analysis. It is speculated that the main composition of the anti-oxidation coating is fused quartz plus a small amount of clay, sodium silicate, potassium silicate water glass and other substances. The vitrification is relatively dense and can block gas penetration.
2. Discussion on the performance, composition and influence of stopper rod application on steel quality
From the perspective of use, the stopper rod is positioned as a refractory functional device. It is not only resistant to high temperature, corrosion, wear and erosion. It first bears the function of flow control. Secondly, the products of corrosion or reaction cannot enter the molten steel to avoid the formation of inclusions that affect the performance of the steel. This is a place that was easily overlooked in the past, because the molten steel after the stopper rod controls the flow enters the copper crystallizer, which is the last link to ensure the cleanliness of the molten steel. From the perspective of refractory material suppliers, the focus of stopper rods is on its composition, raw material requirements, production process conditions, coating, baking temperature, etc., and the technical indicators controlled are mostly the performance of stopper rods; from the perspective of steel enterprise users, they rarely pay attention to the composition of the stopper rod material itself, and the focus of stopper rods is mostly on the use effect. When problems occur, they often pay attention to the product performance indicators provided by the stopper rod supplier.
Taking the stopper rod produced by a certain factory as an example, its Al2O3 mass fraction is greater than or equal to 55% in the core body and greater than or equal to 75% in the rod head; in its performance, the core body has a room temperature bulk density greater than or equal to 2.5 g/cm3, and the rod head has a room temperature bulk density greater than or equal to 2.7 g/cm3; the apparent porosity of the core body is less than or equal to 19%, and the rod head is less than or equal to 21%; the room temperature flexural strength of the core body and the rod head is greater than or equal to 6 MPa. Since the stopper rod is a special-shaped part, the analysis of its composition and performance is closely related to the sampling position and sampling method. For example, the cutting boundary of the rod head composition sample, the size of the flexural strength test sample, the sampling position, the cutting method, etc., all affect the analysis results. Therefore, from the perspective of quality supervision, the sampling frequency of the strength, volume density, and apparent porosity of the stopper rod is relatively low.
As shown in the figure, the residual sample shape and cross section after the stopper rod broke during the production process of a certain factory. Measured from the end of the stopper rod to the rod head, the break position of the stopper rod is 840~1100 mm. From the on-site process investigation, only the argon flow rate and back pressure changes of the stopper rod can be analyzed. For example, the argon flow rate is 0.06 L/min and the back pressure is 0.3 MPa. It is inferred that the argon pores of the stopper rod may be blocked during pouring, and the gas generated inside it accumulates and cannot be discharged. During pouring, the high-temperature molten steel causes the rod body to heat up rapidly, the gas expands violently, and the pressure is released at the weak point of the stopper rod, causing it to break. In the investigation of the cause of the accident, since the residual samples could not be used to prepare the standard samples required for performance testing, the only basis was the test data such as flexural strength, bulk density, and apparent porosity provided by the stopper rod supplier, which was different from the actual performance indicators of the stopper rod used on site and difficult to judge. Therefore, the composition and structure analysis of the residual samples of the stopper rod is relatively objective.
During the application process, the corrosion products of the stopper rod may enter the steel during the corrosion process, forming fine inclusions and affecting the performance of the steel. According to research: the main inclusions in silicon steel are AlN, irregular silicate inclusions, and spherical iron oxides and sulfides; from the composition of AlN and sulfides, they have little to do with the corrosion of the stopper rod. In the non-oriented silicon steel of different grades, the inclusions of 10-20 μm are Al2O3 and SiO2 composite inclusions, but the number is small, and some are mainly CaO, MgO, Al2O3, SiO2 composite inclusions; the inclusions of 1-10 μm are two or more composite inclusions of CaO, Al2O3, SiO2 and MgO, and the number is large; the inclusions not exceeding 1 μm are MnS and MgO composite inclusions or single MnS inclusions, and the number is large. Through the scanning electron microscope, it can be seen that the inclusions in the ingot sample are CaO, Al2O3, SiO2 and MgO (SiO2 and Al2O3 are endogenous inclusions in the steel liquid, and MgO and CaO are foreign inclusions), with a size of 1.2-1.5 μm, and there are also large-sized and complex mixed inclusions, and most of the composite inclusions are brittle, with cracks on the surface.
From a similar analysis, it is found that the oxide composite inclusions of CaO, MgO, Al2O3, and SiO2 may come from slag or from the corrosion products of the stopper rod. Since the molten steel in the tundish enters the crystallizer through the gap between the stopper rod and the nozzle, the possibility of slag forming inclusions is small, except that the slag may enter the crystallizer in the later stage of casting, and the stopper rod itself, including the coating and the corrosion products of the plug head, may enter the molten steel.
Therefore, the source of the inclusions can be judged by their composition. For example, the composition of the anti-adhesion coating includes Al2O3, SiO2, P2O5, Na2O, and K2O, among which the content of Al2O3 is high and P2O5 is also high. The composition of the inclusions is similar to that of the anti-adhesion coating, which may come from the shedding of the anti-adhesion coating; the composition of the anti-oxidation coating includes Na2O, Al2O3, SiO2, K2O, TiO2, and CaO, among which the content of SiO2 and alkali metal R2O is high, and the composition of the inclusions is similar to that of the anti-oxidation coating, which may come from the melting corrosion of the anti-oxidation coating. In addition, during the smelting of certain steel grades, it is necessary to enhance the corrosion resistance of the rod head. Carbon-containing materials containing MgO are used as the outermost material of the rod head. If the inclusions are similar in composition to the carbon-containing materials containing MgO, it is judged that the source of the inclusions is related to the corrosion of the rod head.
The source of inclusions is generally traced back to the influence of materials in different parts of the process according to the composition of inclusions, such as Mg, Si, O inclusions may come from the tundish coating; Ca, Al, Si, O inclusions may come from the tundish covering agent; Ca, Mg, Al, O inclusions may come from the submerged nozzle; Ca, Al, Si, Na, O inclusions may come from the mold protection slag; Ca, Mg, Al, Si, O inclusions may come from the ladle casting residue; Cr, Mg, Al, Si, O inclusions may come from the drainage sand; Al, C inclusions may come from the tundish stopper rod; However, in the specific process practice, when encountering the problem of inclusions, when taking measures to solve the problem, when the corresponding adjustment of the corresponding equipment or materials cannot solve the problem, it is necessary to combine the specific process changes, as well as the analysis or corresponding changes of the stopper rod head and coating to solve the problem of inclusions.
3. Conclusion
1) The anatomical analysis of the stopper rod shows that the material of the stopper rod head is different from that of the main body. The outer layer of the rod head contains more granular aggregate, with a volume density of 2.85 g/cm3 and an apparent porosity of 12.8%, which is denser than the material of the core; the gradient distribution of the material is to enhance the corrosion resistance of the material.
2) The anti-adhesion coating of the stopper rod head is composed of Al2O3, SiO2, P2O5, Na2O, and K2O, among which the Al2O3 content is relatively high, the coating has a low bonding strength with the substrate, and it is easy to scrape off when it comes into contact with water. The anti-oxidation coating of the stopper rod is composed of Na2O, Al2O3, SiO2, K2O, TiO2, and CaO, among which the SiO2 and alkali metal R2O components have a relatively high content, and the alkali metal R2O phase is more, which is easy to form a low melting point glaze at high temperature.
3) The inclusions in the steel may come from the corrosion products of the anti-adhesion coating, anti-oxidation coating, and the stopper rod head, which needs to be judged from the composition analysis of the inclusions.
