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Trial of magnesia-alumina carbon bricks at the bottom of 75 t ladle

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Table of Contents

The ladle is one of the necessary production equipment in the steelmaking plant. It has the functions of transporting molten steel, refining outside the furnace, and casting molten steel. Our company’s steelmaking production is not configured with refining, so the function of the ladle is relatively simple. At present, the structural modes of ladle linings in domestic steel plants are mainly shaped brick masonry and monolithic casting of unshaped refractory materials. For ladle refractory materials, the slag line and the bottom of the ladle are the two most important parts and are the most vulnerable to damage. The bottom of the ladle bears high pressure from molten steel and is prone to sintering and slag penetration. The main forms of damage are cracking and spalling. According to this characteristic, the bottom of the package should be made of high-grade refractory materials with good resistance to slag erosion, good resistance to slag permeability and good high-temperature volume stability, such as magnesia carbon bricks, alumina-magnesia carbon bricks, magnesia-alumina carbon bricks, magnesia-chrome bricks, Magnesium calcium bricks, etc. Once a ladle wear accident occurs at the bottom of the ladle, serious consequences will occur. Our company’s steelmaking plant has experienced many accidents of lagging through the bottom. After analysis, the working layer at the bottom of the original castable material ladle was changed to bricklaying mode, and magnesia-alumina carbon bricks were selected for testing, and the results were good.

Magnesia-alumina carbon brick for bottom package

Preparation of magnesia-alumina carbon bricks

Magnesia-alumina carbon bricks are fire-free refractory products made of magnesia as the main raw material, adding high-alumina bauxite, corundum and flake graphite, and using phenolic resin as the binding agent.

Main damage forms of magnesia-alumina carbon bricks

There are four main damage forms for the working layer of the refractory lining of the ladle, namely erosion, erosion, spalling, and hydration. The damage forms vary depending on the location of the ladle. The specific conditions are listed in Table 1.

When the ladle is tapped and filled with molten steel in the converter, the high-temperature molten steel has strong mechanical erosion on the bottom, making the refractory materials in this part prone to damage due to thermal shock. It also has a greater impact on the impact area at the bottom of the bag, the nozzle slide plate and the breathable bricks higher than the bottom of the bag. In addition, the strong stirring effect of bottom-blown argon (or nitrogen blowing) also intensifies the erosion of the bottom refractory material by the molten steel; the penetration erosion of the molten steel and the chemical erosion of the liquid slag will also cause damage to the bottom refractory material. Peeling damage caused by stress in the refractory material due to rapid cooling and rapid heating mainly occurs on the bottom nozzle seat bricks and breathable seat bricks. When building the bottom of the package, the permanent layer is knotted with castables, and corundum refractory mud is used in the working layer to inlay seat bricks and breathable bricks. Before and during baking, moisture or water vapor reacts with the MgO in the bricks and hydrates, resulting in hydration damage.

Table 1 Main damage forms of working layer of ladle lining

Affected partsMain cause of damageSecondary cause of damage
slag lineerosionExfoliation, erosion, hydration
Wall coveringerosionErosion, hydration
Bottom of bagwash awayerosion, hydration
Water supply systemwash awaypeeling, erosion
Argon blowing systemPeel offwash away

Measures to reduce corrosion of refractory materials at the bottom of the package

(1) Reasonably configure various types of refractory materials for the bottom of the package based on the actual production, reduce the porosity of the refractory materials, reduce the erosion channels of slag, and improve the resistance of the refractory materials to slag permeability.

(2) Proper use of refractory materials, such as reasonable masonry and baking methods; increase the viscosity of the slag during steelmaking; and be careful not to damage the permanent layer of the ladle when unpacking. Understand the properties of the refractory materials used, formulate the conditions for use of ladles, speed up the use cycle of ladles as much as possible, and achieve “red envelope” work to avoid cold ladles caused by too long turnover periods.

(3) Timely discover the damaged parts of the refractory material at the bottom of the package and take repair measures, such as blasting or replacement.

Question raising

In 2020, two accidents of steel leakage due to the bottom of the package occurred during steelmaking production, resulting in large losses. See Table 2 for details.

Table 2 Statistics of ladle bottom penetration and steel leakage accidents

number                 accident situationAccident cause analysisAccident losses
1At 10:54 on January 7, 2020, during the process of receiving steel in the 3* converter, the bottom of the 301* ladle broke out when 2/3 of the steel was tapped, and the tapping stopped immediately. The steel turner promptly drove the ladle truck out of the furnace bottom and drained the molten steel. Then the steelmaking plant actively organized the cleaning of the floored steel and restored the track.(1)Main reasons: The overall bottom of the ladle is seriously eroded, and there are pits in the impact area, so the monitoring of the ladle is not in place. (2)Secondary reason: The bottom of the bag eroded quickly, and the baking time was 85h, but did not reach 110h.40t of steel was lost and a section of track was burned; the furnace was shut down for 227 minutes.
 2On August 24, 2020 at 00;10, when the tapping of converter 1* was about to be completed, the bottom of the ladle 26* broke through.(1)The main reason: the impact plate at the bottom of the 26* steel ladle fell off. The thickness of the refractory material at the bottom of the ladle was about 100~120mm. The impact plate at the bottom of the ladle was improperly positioned when it was poured and knotted, and there were problems with the construction quality. (2) Secondary reason: During the use of the ladle, monitoring and confirmation work including temperature measurement of the cladding was not in place.As a result, 1* converter was shut down for 191 minutes; 1* continuous casting machine was shut down for more than 3 hours due to unplanned second start; and 60 tons of molten steel were lost.

The analysis found that the ladle bottom penetration accidents all occurred under the converter, causing serious consequences. It took a long time to stop production and deal with the accidents, seriously affecting production. Therefore, the performance requirements of the refractory materials for the bottom of the package are higher, and a plan is proposed to use magnesia-alumina carbon bricks for the working layer of the bottom of the package and a mix of castables for the permanent layer. After the test is successful, it is planned to be widely promoted and used, which will significantly reduce the baking time for overhaul and minor repairs, and reduce gas consumption.

trials

Ladle turnaround time

The ladle turnover process is: converter → nitrogen blowing behind the furnace → continuous casting and steel pouring → ladle hot repair preparation → waiting for steel tapping. The normal turnover time varies according to the steel type and continuous casting machine. Our company takes 50 ~80 min, the steel type is HRB400E, the ladle tapping temperature is 1 680 ~ 1 700 ℃, and the steel filling time is 50 ~ 60 min.

Quality requirements and technical standards

(1) In YB/T 165-2018 “Alumina-magnesia Carbon Bricks and Magnesia-Alumina Carbon Bricks”, magnesium-alumina carbon bricks are divided into 4 grades according to MgO content: MLT75, MLT65, MLT55, MLT45, and corresponding physical and chemical indicators are stipulated. The magnesia-alumina carbon brick used in this test is close to the MLT65 brand, but its basic performance indicators are slightly lower than this brand. Table 3 lists the physical and chemical performance indicators of the magnesia-alumina carbon bricks used in this sample.

The bottom working layer of the package is built with magnesia-alumina carbon bricks, and the package wall still uses the original castable form. During the test, the refractory material manufacturer needs to assign dedicated personnel to test the ladle’s masonry quality, baking quality inspection, and online operation inspection and monitoring.

Table 3 Physical and chemical performance indicators of magnesia-alumina carbon bricks

Ladle covered bottom brickGrade 35/0 (size: 350mm×150mm×100mm)Metallurgical standard MLT65 brick physical and chemical indicators
projectControl standardactual valueProject Standard
MgO/%≥65.0065.50MgO/%≥65.0
Al₂O₃/%≥13.0013.20(MgO+Al₂O₃)/%≥80.0
C/%≥9.009.20C/%≥5.0
Bulk density/(g·cm-³)≥3.023.03Bulk density/(g·cm-³)≥3.0
Apparent porosity/%≤6.05.20Apparent porosity/%≤5.5
Normal temperature compressive strength/MPa≥35.0035.70Normal temperature compressive strength/MPa≥50.0

Test process

First test

(1) Overhaul of masonry work: On the afternoon of March 23, 2021, the test ladle 30# package: the permanent layer of castables at the bottom of the package was constructed, with a thickness of 250~270 mm; on the morning of March 24, 2021, the working layer of the bottom package was constructed, The thickness of the bottom brick is 350 mm, and 230 magnesia-alumina carbon bricks are used. The masonry is dry-laying; 0.2 t of corundum filler is used, and the corundum filler is used for inlaying steel base bricks and breathable bricks. Figure 1 shows the on-site construction drawing.

Figure 1 On-site construction of steel ladle bottom overhaul

Baking time after overhaul: Baking started on March 25th (9 hours) and ended on March 27th, totaling 69 hours. After drying is completed, it will be put into operation.

Minor repair: At 11:47 on April 7, because the steel ladle seat brick was too short, it was taken off the line after being used 145 times for minor repairs. During the minor repair, only the seat brick was replaced, and the castable was inlaid filler. During minor repairs, it was observed that the thickest part of the bottom brick is about 300 mm and the thinnest part is about 170~180 mm. This is due to severe erosion and peeling around the steel package seat brick.

Baking time after minor repairs: Baking started on April 11th (9 hours) and ended on April 13th, totaling 57 hours. It went online after minor repairs at 22:16 on April 14. At 0:07 on April 18, the package wall was eroded too thin and the temperature of the package wall was too high. It was forced to go offline. The package age was 207 times.

Second test

Overhaul of masonry construction: At 8:00 on April 25, test ladle 30# was laid directly after cleaning the remaining bricks at the bottom of the ladle, and the permanent layer at the bottom of the ladle was not moved.

Baking time after overhaul: Baking will start at 10:00 on April 26 and end at 9:00 on April 29. The total baking time is 79 hours. After drying, the package will be prepared and run online.

Minor repair: At 8:00 on May 26, the seat brick was short and needed to be replaced. After being used 136 times, it was offline for minor repair. During the minor repair, the seat brick was removed. The thickest part of the bottom brick was about 250 mm, and the thinnest part was about 150~ 180mm.

Baking time after minor repair: Start baking at 11:00 on May 26, end at 9:00 on May 28, total baking time is 56 hours, go online after minor repair at 9:00 on May 30. At 22:58 on June 2, the package was taken offline due to a pit at the bottom, and the package age was 197 times.

test results

Ladle age

The first test package of 30# ladle brick bottom was used 145 times for minor repairs and was used for 207 times. It was able to meet production needs and saved 75 hours of baking time. The second test package with 30# ladle brick bottom was used 136 times for minor repairs and was used for 197 times. It was able to meet production needs and saved 63 hours of baking time. A comparison of the results of the two tests is listed in Table 4. The conditions of the bottom of the ladle during the test are shown in Figures 2 and 3 respectively. The magnesia-alumina carbon bricks remaining in the ladle after the first test are shown in Figure 4.

Table 4 Comparison of cold repair of two refractory material bottoms

projectCasting material bag bottomBrick bottom
Time spent on overhauling bottom masonryOne casting moldingAfter the permanent layer is poured, it is cured and then laid. It takes about 20 hours at most.
Overhaul baking time120h1st time 69 hours/2nd time 79 hours
Including age during minor repairs110~120 times145 times for the 1st time/136 times for the 2nd time
Minor fixes to baking time80h1st time 56 hours/2nd time 57 hours
Total baking time savedCasting material bag bottom1st time 75 hours/2nd time 63 hours

Figure 2 The bottom condition of the bag when the bag age is 121 times in the first test

Figure 3 The bottom condition of the bag when the bag age is 141 times in the second test

Figure 4 Magnesia-alumina carbon bricks remaining from the ladle after the first test

Matching of the service life of bottom refractory materials, breathable bricks and seat bricks

In terms of the structure and maintenance process of the refractory material at the bottom of the ladle, two breathable bricks and a seat brick are used at the bottom of the ladle. During the smelting process of molten steel, the breathable bricks are used alternately to achieve synchronization of the service life of the breathable bricks at the bottom and the refractory material at the bottom. ; However, the service life of the two nozzle seat bricks can be synchronized with the service life of the bottom refractory material, so the nozzle seat bricks need to be replaced for minor repairs. The length of the magnesia-alumina carbon bricks in the impact area at the bottom of the ladle should be slightly higher than the plane of the bottom of the ladle, so that the service life of the refractory material in the impact area at the bottom of the ladle can be synchronized with the entire ladle cycle.

Through experiments, it was found that the service life of the ladle seat bricks and the working layer of ladle wall castables needs to be improved. In particular, the use of ladle wall castables should be increased to more than 200 times to generally increase the ladle age.

Conclusion

(1) The working layer at the bottom of the ladle is constructed with magnesium-alumina carbon bricks and the permanent layer is constructed with a mix of castables. Through experiments, it was found that the performance of the bottom working layer was improved, indicating that the magnesia-alumina carbon bricks have superior corrosion resistance and erosion resistance.

(2) The original castables are still used as the wall refractory material in this test. If higher quality wall refractory materials are used together, the package age will be increased even more.

(3) The working layer of the brick-laying bottom package adopts the form of dry laying, which requires a shorter baking time. Compared with the castable bottom package, the baking time is reduced by about 60 to 80 hours, and the gas saving effect is obvious.

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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