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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 stopper 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 stopper; 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 stopper breakage accident

Statistics on stopper rod breakage accidents

Through statistical analysis of the quality status of the tundish stopper 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 stopper 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 stopper 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

Integral stopper rods are isostatically formed. Their shape and size depend on tundish capacity. The distance lifted determines the crystallizer liquid level. This is seen on the lifting mechanism scale. The distance also controls the gap between the stopper rod head and nozzle bowl, affecting molten steel flow. This relates to their arc curve radius.

Erosion causes the stopper rod to move downward, decreasing the opening. Severe or uneven erosion of the rod head affects flow control. Investigation found manufacturer C’s rod heads were unsuitable and often broke, leading to poor flow control.

The basic production process of stopper rods is as follows:

  1. Raw material preparation and ingredients
  2. Mixing and granulation
  3. Trapping and drying
  4. Isostatic press 120MPa molding
  5. Lathe turning
  6. Shuttle kiln firing
  7. Automatic paint spraying
  8. X-ray non-destructive inspection.

Manufacturer C’s ingredient control was poor. This led to poor integration between the rod head and body, causing high 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 alloy materials used in the second steelmaking plant are mainly Si-Mn-Fe, CaBaAlSi (composite deoxidizer), and CaSi wire. The order of alloy addition is: CaBaAlSi (composite deoxidizer) → Si-Mn-Fe (CaBaAlSi addition is 1.5~2.0kg/t, and Si-Mn-Fe addition is adjusted according to the residual Mn at the end point). The alloy is added to 2/3 at the beginning of steel tapping. Ensure that the content of the argon station composition 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 the calcium silicate wire 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 yellow and white slag are produced.

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

Integral stopper rod design involves coordination with submerged nozzles. Bowl shapes must align with nozzle circulation diameters. This defines nozzle openings and shapes. Then, matching stopper heads’ dimensions are set.

Drawing on industry expertise, our process engineers collaborated with Vendor C. They redesigned the stopper head. Figures 1 and 2 display pre-and post-enhancement visuals.

For machines seven through nine, stopper rod lengths were trimmed. Reduced from 1590 mm to 1560 mm, body vibrations lessened. Post-redesign, communication ensued with the mold level automation supplier. Control software adjustments ensured stable mold levels.

These modifications underscored our commitment to precision and efficiency in continuous casting operations.

Figure 1 Improved front plug head design

Figure 2 Improved plug head design

Optimize and adjust the material of the stopper head

Tests confirm superior corrosion resistance in MgO-C and ZrO₂-C materials. Spinel-C exhibits even better qualities than both. Electrofusion Spinel-C boasts higher resistance to corrosion than its sintered counterpart.

Due to varying core and head materials, differences in plug rod properties arise. This includes density, porosity, strength, and thermal expansion. Direct combination risks in-use failure. An interlayer design ensures a seamless join.

Plug rods were refined for machines five through ten. The core remains aluminum carbon-based. The head, previously ZrO₂-C, now features fused alumina-magnesia spinel. Optimized spinel composition and enhanced graphite content boost thermal shock stability. Tests at 1200°C followed by rapid cooling reveal improvements. Thermal shock endurance jumps from three to four cycles to six.

This upgrade signifies a significant leap in durability and reliability.

Through the implementation of the above measures, tundish stopper breakage accidents have been controlled. The proportion of plug rod breakage accidents in unplanned shutdowns decreased yearly: 15.66% in 2011 and 13.30% in 2012. This showed good results. In 2013, accidents increased to 25% due to a change in refractory material manufacturers. Investigation revealed the new manufacturer’s stopper rod head material was unsuitable. The upper nozzle size of the tundish differed from the original design. Using stopper rods from other manufacturers caused breakages.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

Plug rod breakage ties to its length, design, and installation. Baking quality also impacts durability. Tailored measures must address unique plant conditions. Avoidance of accidents requires specific attention.

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.

Our Product have been supplied to world’s top steel manufacturer Arcelormittal, TATA Steel, EZZ steel etc. We do OEM for Concast and Danieli for a long time

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