The Influence Of Introducing Mullite And Fused Quartz Into The Ladle Nozzle

 The ladle nozzle serves as the channel for molten steel to flow from the ladle to the tundish and is connected to the long nozzle of the ladle, playing a role in protecting the casting and preventing secondary oxidation of the molten steel. During the use of the ladle nozzle, problems such as cross-breaking, perforation, steel clamping, and steel sticking may occur. In order to reduce the probability of cold cracks in the ladle nozzle during the operation of the ladle and lower the production cost of the ladle nozzle, using high-alumina materials, mullite, fused quartz, phenolic resin, etc. as raw materials, the influence of introducing mullite and fused quartz into the ladle nozzle on its properties was studied.

 Experiments on introducing mullite and fused quartz into the ladle nozzle show that:

① introducing mullite and fused quartz has little effect on the room-temperature flexural strength and compressive strength of the ladle nozzle;

② the flexural and compressive strength of the ladle nozzle after firing at 1000°C for 3 hours is significantly reduced after introducing mullite and fused quartz;

③ after introducing mullite and fused quartz, the bulk density at room temperature and after firing at 1000°C for 3 hours decreases, and the porosity increases. After being normally used on 130t and 180t ladles in different steel plants, the probability of cold cracks in the ladle nozzle is reduced, and the average hole expansion is 4-4.5mm, meeting the trial requirements of the steel plants.

 A steelmaking ladle is a metal container for holding high-temperature molten steel. The sliding nozzle is a control device for molten steel during the continuous casting process of a steel plant, which can accurately regulate the flow of molten steel from the ladle to the continuous casting tundish, balancing the inflow and outflow of molten steel to make continuous casting operations easier to control. It is an indispensable part of iron and steel smelting. The ladle nozzle serves as the channel for molten steel to flow from the ladle to the tundish, guiding and controlling the flow of molten steel. It is connected to the ladle long nozzle, playing a role in protecting the casting and preventing secondary oxidation of the molten steel. Problems such as cross-breaking, perforation, steel clamping, and steel sticking during the use of the ladle nozzle affect continuous casting production.

 During operation, the internal temperature of the ladle nozzle aperture reaches approximately 1600°C, while the temperature decreases toward the outer part. This causes significant tensile stress on the nozzle's exterior, making it prone to longitudinal cracks under thermal stress. Meanwhile, different cooling rates between the upper and lower parts of the nozzle during use generate thermal stress in the vertical direction, forming annular cracks. The expansion of these cracks can lead to nozzle fracture and molten steel leakage. Therefore, the ladle nozzle is required to have good erosion resistance and thermal shock stability at high temperatures, as well as certain medium-temperature and high-temperature strengths to withstand the shear forces from the sliding mechanism and protective casing.

 1.1 Raw Materials

The experimental raw materials mainly include high-alumina materials, mullite, fused quartz, phenolic resin, etc. The chemical compositions of the raw materials are shown in Table 1.

Table 1 Chemical compositions of raw materials / %

 1.2 Experimental Scheme

The experimental scheme is shown in Table 2, and the experiments are carried out in accordance with the proportions in Table 2.

Table 2 Experimental Schemes / %

 2.1 Sample Preparation Method

In the workshop, 700 kg of raw materials are proportioned according to the ratios in Table 2. First, put the aggregates into a wheel - type mixer and mix them evenly. Then, add an appropriate amount of preheated phenolic resin. After wet - mixing for a few minutes, put the uniformly mixed powder materials and continue kneading. Determine the final mixing time according to the properties such as the strength of the mud. The total mixing time is controlled at about 40 minutes. In order to ensure that the mud has good service performance, after the mud is taken out of the mill, it is placed in a material - conditioning room for conditioning. The temperature and humidity of the material - conditioning room are both controlled within the required ranges.

After the mud is conditioned in the material - conditioning room for about one day, experienced employees judge it according to the properties of the mud. Only the mud that meets the performance requirements can be used to form the nozzles for the experiment. When forming the nozzles, the mechanical pressing method is adopted, using a friction press and a forming method of light first and then heavy, to ensure that the gas in the blank is discharged as much as possible.

About 40 nozzles are formed for each of the two types of mud. Make corresponding marks on the nozzle blanks respectively. The ordinary nozzle is marked as M1, and the nozzle with mullite and fused quartz introduced is marked as M2. The nozzles that are inspected to be qualified and have no appearance quality problems are placed on a kiln car and pushed into a drying kiln for drying, and dried at 200 °C according to the set heating curve. After drying and cooling out of the kiln, take one nozzle of each type for cutting. Cut it into a standard sample block of 40 mm × 40 mm × 40 mm according to the sampling and sample - making regulations of GB/T10325 and GB/T7321. The sample block that needs to be tested for performance indicators after being fired at 1000 °C for 3 hours is subjected to corresponding heat treatment and cooled with the furnace.

 2.2 Performance Testing

For the standard sample blocks after cutting and drying, test the room - temperature flexural strength according to the relevant regulations of GB/T3001-2017, test the room - temperature compressive strength according to the relevant regulations of GB/T5072-2008, and test the bulk density and apparent porosity according to the relevant regulations of GB/T2997-2015. The data of each item tested for the sample blocks after different temperature treatments are shown in Table 3.

Table 3 Comparison of Test Results of Nozzle Brick Blank Samples with Two Different Materials

 2.3 Results and Discussion

From the above experimental data, it can be concluded that: On the basis of the original formula of the ladle nozzle, introducing mullite and fused quartz to replace 30% of high - alumina materials for producing the ladle nozzle has little impact on its room - temperature flexural strength and compressive strength; however, the flexural and compressive strength of the ladle nozzle after being fired at 1000 °C for 3 hours is significantly reduced after introducing mullite and fused quartz; after introducing mullite and fused quartz, the bulk density at room temperature and after being fired at 1000 °C for 3 hours decreases, and the porosity increases.

 2.4 Trial Use in Steel Plant

The two types of ladle nozzle brick blanks with different materials produced in this experiment are inspected and produced in accordance with the subsequent relevant process requirements. The produced qualified nozzles are trialed in a steel plant in Tangshan. The steel - making environment at this site is: 130t ladle, the main steel grades produced are Q195L and Q235B, with full - process refining, the steel - receiving temperature of the ladle is about 1650 °C, and the steel - casting time is about 30 minutes. The currently used nozzle is used twice, the diameter expansion after use is not obvious, the inner diameter is relatively smooth, but there are cold cracks in some cases. The two types of ladle nozzles with different materials are applied to this site, and the post - use effects are not much different. The data results after use are shown in Table 4.

Table 4 Nozzle Trial Results

Through the comparison of on - site trial data, the use effects of these two types of nozzles with different materials during the use process have little difference. As the number of uses increases, the probability of cracking at the aperture decreases. The photos of the nozzles after use are shown in Figure 1.

Figure 1 Photos of the experimental nozzles after on-site use

Subsequently, nozzles of other brick types were produced according to this experimental formula, and an expanded experiment was carried out in another steel plant in Tangshan. The on-site steelmaking environment is as follows: the ladle capacity is 180t, the steel grades produced are Q195L, SPHC, ZFQ235B, ZFQ355B, and Q235B, with 85% refining. The steel-receiving temperature of the ladle is about 1630°C, and the steel pouring time is about 25 - 40 minutes. The two types of ladle nozzles with different materials were applied to this site, and the post-use effects were not much different. The post-use data results are shown in Table 5. The on-site use situation is relatively stable, reducing the probability of cold cracks. At present, it has been popularized and applied in this site. The photos of the nozzles after use are shown in Figure 2.

Table 5 Trial Results of Nozzles in Another Steel Plant

Figure 2: Photos of the experimental nozzles after on-site use in another steel plant

From the results of the actual application in the above two steel plants, it can be seen that: The ladle nozzle with mullite and fused quartz introduced has no impact on the normal use of the ladle nozzle in the steel plant, can meet the use requirements of the steel plant, and improves the thermal shock resistance of the ladle nozzle, reduces the probability of cold cracks, improves the safety factor of the product, and reduces the raw material cost, thereby increasing the economic benefits of the product.

Introducing mullite and fused quartz into the ladle nozzle has little effect on its room-temperature flexural strength and compressive strength. However, the flexural and compressive strengths after firing at 1000°C for 3 hours are significantly reduced. The bulk density at room temperature and after firing at 1000°C for 3 hours decreases, while the porosity increases. This improves the thermal shock stability of the ladle nozzle, reduces the probability of cold cracks in the ladle nozzle after use, enhances the operational safety factor, lowers raw material costs, and improves the economic benefits of the product. After being normally used in 130t and 180t ladles at different steel plants, the probability of cold cracks in the ladle nozzle is reduced, with an average hole expansion of 4-4.5mm, meeting the trial requirements of steel plants and can be continuously promoted and applied in suitable sites.