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Causes and control measures of tundish stopper rod breakage

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Table of Contents

Aiming at the problem of broken plug rods in the tundish of slab continuous casters No. 5 to 10 in steelmaking plants, the data of broken plug rods from different refractory manufacturers were compiled and analyzed. A detailed investigation was conducted from aspects such as the composition of molten steel, the physical and chemical indicators of the tundish plug rod, the manufacturing process, and the baking process, and corresponding optimization plans and control measures were proposed. Focus on redesigning the size of the stopper rod head and optimizing the material of the stopper rod head to control the plug rod breakage accident and reduce the accident rate.

Keywords: tundish plug rod; fracture; analysis; control

Foreword

Slab casters No. 5 to 10 of the steelmaking plant are conventional slab casters, mainly producing ordinary carbon structural steel. During the production process, plug rod breakage accidents account for a large proportion of unplanned shutdown accidents. The most prominent year was 2010, when plug rod breakage accidents accounted for 30%. Through statistical analysis of data and communication with refractory manufacturers, we found the key reasons for plug rod breakage and proposed an optimization plan, thereby improving the quality of plug rods and ensuring the smooth flow of production.

The number of stopper rod breakage accidents has decreased year by year, accounting for 15.66% in 2011 and 13.30% in 2012. In 2013, due to the change of refractory manufacturers, plug rod breakage accidents increased again, accounting for 25%. By standardizing relevant technical standards and operating requirements, there was only one unplanned shutdown accident caused by the breakage of the plug rod head in 2014.

The purpose and characteristics of the integral stopper

At present, the common aluminum carbon integral stopper rods have high aluminum carbon, aluminum zirconium carbon, magnesium carbon or other materials. Before 2011, machines No. 5 to No. 10 used aluminum carbon zirconium integral plug rods to be used in conjunction with the nozzle. The liquid level control of the crystallizer is a vortex-type automatic control device. The flow of molten steel entering the crystallizer is adjusted by controlling the position of the stopper rod head to the water port of the tundish. Argon can be blown into the tundish through the argon gas hole to prevent the water port from being blocked. It has the characteristics of good erosion resistance, strong corrosion resistance, oxidation resistance and high strength. It is suitable for continuous pouring in multiple furnaces for a long time. Its typical physical and chemical indicators are shown in Table 1.

Table 1 Typical physical and chemical indicators of stopper rods

project Overall chemical composition/w%Chemical composition of head/w%Volume density/g.cm3-1Compressive strength/MPaApparent porosity/%Thermal shock resistance (number of water cooling treatments)
Al2O3CCZrO2OverallHead
index≥65≥25≤10≥70≥2.6≥3.5≥16≤17≥5

During the production of SPHC steel, stopper rods made of this material were prone to slagging and flocculation at the rod head. Later, the material of the stopper rod head was changed to spinel. After the change to spinel, accidents of plug rod breakage occurred many times.

Overview of plug rod breakage accident

Statistics on stopper rod breakage accidents

Through statistical analysis of the quality status of the tundish plug rods of four manufacturers A, B, C and D, it was found that the steel type produced when the plug rods broke was CCSB ship plate steel only once, and the rest were Q235B steel. Moreover, manufacturer D has never had an accident in which the plug rod broke. When the plug rod of other manufacturers breaks, the life span and fracture location of the tundish are significantly different. The relevant data are shown in Table 2.

Table 2 The usage time and location of the middle package when the stopper rod of each manufacturer broke in 2010

ManufacturerAverage baking time for tundishAverage life span of tundishImpact time/minTundish remaining steel/tFracture location and number of times
Stopper head slag line
A4h 16 min10h 27 min17425.231
B3h 52 min8h 06 min10027.904
C5h16 min11h 56 min94.522.0871
D4h 15 min16h 38 min0000

Morphological characteristics of the plug rod fracture area

Manufacturer C’s stopper rods break the most, and most of the breakage locations are at the head of the stopper rods. After measuring and inspecting the accident stopper rod of manufacturer C, it was found that the head of the stopper rod broke at a distance of 5 to 10 cm from the head. After this batch of stopper rods that did not break after use were longitudinally sectioned with special equipment, obvious cracks could also be seen.

Analysis of the causes of plug rod breakage

According to the process conditions of No. 5 to No. 10 machines of the Second Steelmaking Plant, process technicians summarized the original factors that caused the plug rod to break, mainly including the following four aspects.

Composition of molten steel

The material of the stopper rod should be determined according to the type of steel produced by the steel plant, the method of treating the molten steel and the continuous pouring time of the tundish. For steel types treated with Ca and Si, the head of the Al-C or SiO₂-C plug rod erodes quickly, often resulting in the inability to control the steel flow. When the added amount of Ca exceeds a certain amount, the degree of damage to the Al-C stopper rod tends to increase. The main reason for analysis is that when Ca is added to a certain amount, CaO in the steel will react with Al₂O₃ to form low-melting point substances, which will increase the corrosion of Al-C refractory materials by Ca in the steel.

Installation of plug rod and water inlet

The stopper rod generally requires a certain amount of grip during installation and adjustment. However, the stopper rod is prone to deviation during the casting process, and it is very easy for the deflection of molten steel to occur. When the deflection is severe, it may cause bonding and steel leakage accidents.

Baking conditions of tundish

According to the material characteristics of the tundish lining, stopper rod, and spout, the baking curve used in machines No. 5 to 10 is: baking at low heat for 30 minutes, baking at medium heat for 30 minutes, and baking at high heat for 1 to 2 hours. However, due to changes in the production plan, the baking process adopted in actual production cannot completely follow the baking curve. In addition, the baking gas used in units 5 to 10 is converter gas, which contains large amounts of water. The gas drainage facilities are simple and the drainage effect is not ideal.

Design and production process of stopper rod

The integral stopper rod is always formed by isostatic pressing, and its shape and size depend on the capacity of the tundish. The upward lifting distance of the stopper rod determines the liquid level of the crystallizer, which can generally be observed on the scale on the side of the hoist mechanism. The upward lifting distance of the plug rod determines the size of the gap between the plug rod head and the nozzle bowl, thereby controlling the flow of molten steel into the immersed nozzle, which is related to the radius of their own arc curves. When the plug rod or nozzle is eroded, the plug rod will move downward, which is manifested as a decrease in the opening of the plug rod. Severe or uneven erosion of the plug head of the plug rod will seriously affect the flow control effect of the plug rod. After investigation, it was found that the size of the rod head provided by manufacturer C was inappropriate, accidents of rod head breakage often occurred, and the flow control effect was poor.

The basic production process flow of stopper rod is:. Raw material preparation and ingredients → mixing and granulation → trapping and drying → isostatic press 120Mpa molding → lathe turning → shuttle kiln firing → roller conveyor type fully automatic paint spraying → X-ray non-destructive detector for inspection. In terms of ingredients, manufacturer C has unreasonable control, resulting in poor integration between the plug head and the rod body, resulting in large internal stress after baking.

Control measures to avoid stopper rod breakage

Adjust the composition of molten steel and improve the deoxidation and alloying process

The steel type produced when the stopper rod broke in No. 5 to No. 10 machines was mostly Q235B steel, indicating that the composition of this steel type has a certain relationship with the stopper rod breakage.

Therefore, the deoxidation method of molten steel was studied and the process was improved. The deoxidation alloying materials used in the second steelmaking plant mainly include Si-Mn-Fe, CaBaAlSi (composite deoxidizer), CaSi wire . The order of alloy addition is: CaBaAlSi (composite deoxidizer) → Si-Mn-Fe (CaBaAlSi addition amount is 1.5~2.0 kg/t, Si-Mn-Fe addition amount is adjusted according to the residual Mn at the end point), the alloy is tapped at the beginning to 2 /3 is added. Ensure that the composition content of the argon station reaches the target range. After the molten steel enters the refining furnace, fine-tune C, Si, and Mn according to the initial refining composition. Before leaving the station, feed calcium silicate line 1.5 ~ 2.0m/t steel, and the wire feeding speed is 1.5 ~ 2 m /min. After the wire feeding is completed, the soft blowing time is ≥8 minutes, and the yellow and white slag is tapped.

After the improvement, the reaction mechanism between the molten steel and the stopper head material is weakened, which improves the corrosion resistance of the stopper head.

Improve the installation method of stopper rod

Re-formulate the installation and adjustment method of the stopper rod. Check the switch beam before adjustment, check whether the stopper rod is straight and whether the rod thread is intact. Insert the joint into the screw and tighten the screw of the rod until the joint is flush with the rod; the nibbling amount of the upper rod is 3 to 5 mm, do not nibble backwards or sideways, and the opening and closing allowance should not be less than 30 mm.

Improve the baking effect of tundish

Bake the tundish in strict accordance with the baking curve to ensure that the tundish is quickly baked to above 1000°C within 1 to 2 hours, and use an infrared thermometer gun to measure the temperature of the plug rod and water outlet to improve the baking effect.

Redesign the size of the stopper head

In the process of coordinating the design of the integral stopper rod and the immersed nozzle, the design of the integral stopper rod head and the bowl shape of the immersed nozzle is often involved. In the design, the circulation diameter of the immersed nozzle must first be determined, thereby determining the shape and opening of the bowl of the nozzle. On this basis, determine the shape and length of the overall stopper head that matches it. Based on the production conditions of the casting machine and drawing on the experience of other manufacturers, process technicians worked with Manufacturer C to redesign the plug head. The appearance of the plug head before and after the improvement is shown in Figures 1 and 2. At the same time, the overall length of the stopper rods of machines No. 7 to 9 was adjusted from the original 1590 mm to 1560 mm, which reduced the vibration of the rod body. After improving the design of the stopper rod head, the process technicians communicated with the crystallizer liquid level automatic control manufacturer and adjusted the control program to ensure the stability of the crystallizer liquid level.

Figure 1 Improved front plug head design

Figure 2 Improved plug head design

Optimize and adjust the material of the stopper head

Relevant tests show that MgO-C and ZrO₂-C have higher corrosion resistance than Al₂O₃-C. Spinel-C quality is better than MgO-C quality and ZrO₂-C quality. The spinel-C quality of the electrofusion method has higher corrosion resistance than the spinel-C quality of the sintering method. Due to the different mud materials of the rod body and rod head, the body density, porosity and strength of the plug rod are quite different, especially the thermal expansion coefficient. If they are directly combined, it will easily cause a U-turn during use. Therefore, the over-layer material is designed so that the two can be well combined. The stopper rod material was optimized and improved based on the production conditions of machines No. 5 to 10. The rod body is still made of aluminum carbon, and the stopper rod head is changed from ZrO₂-C to fused aluminum magnesium spinel. Adjusting the spinel ingredients and increasing the amount of graphite improved the thermal shock stability of the rod head (the test method was 1200°C, rapid cooling with circulating water), and the number of thermal shock stability increased from the original 3 to 4 times to 6 Second-rate.

Through the implementation of the above measures, plug rod breakage accidents have been controlled. The proportion of plug rod breakage accidents in unplanned shutdown accidents has decreased year by year, accounting for 15.66% in 2011 and 13.30% in 2012, achieving good results. .In 2013, due to the change of refractory material manufacturers, plug rod breakage accidents increased again, accounting for 25%. After investigation and research, it was found that the material of the stopper rod head used by the new refractory material manufacturer was not suitable, and the size of the upper nozzle of the tundish was different from the original design, resulting in breakage accidents after borrowing stopper rods from other manufacturers. After the related problems were solved, plug rod breakage was greatly reduced. From December 2013 to April 2013, there was only one plug rod breakage accident.

Conclusion

In addition to the quality of the refractory material, the reasons for the breakage of the plug rod in the tundish are also closely related to the length of the plug rod, the design of the rod head, the installation and adjustment process of the plug rod, and the baking effect. The production conditions of each steel plant vary greatly, and corresponding measures should be taken according to the specific conditions to avoid plug rod breakage accidents.

LMM YOTAI established in 2007. Our production technology comes from Japanese Yotai. As an experienced and international player in the refractories industry. We have succeeded in expanding both the breadth of its product range and the depth of its services. From raw material selection, refractory portofio & optimization, installation & services & recycle of used refractories on site to further reduce client’s Opex & Capex in refractory consumption per ton steel output, meanwhile improve product quality of client.

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