Research on long life technology of crystallizer foot rollers

Based on the current situation of short service life and frequent replacement of the mold foot rollers of the rectangular billet continuous caster in the steelmaking plant, a systematic analysis was conducted. Through research and application in three aspects: lubrication method, material selection and arc alignment method, the life of the foot roller is greatly extended and the maintenance cost is reduced.

Keywords: crystallizer; foot roller; lubrication method; material selection; arc alignment method

The 4″ continuous caster in the steelmaking plant is a rectangular billet continuous caster with an arc radius of 11 m and produces three specifications of cast billets: 240×375, 275×380, and 32×410. In recent years, with the development of high-efficiency continuous casting With the promotion of continuous casting machines, production capacity continues to increase, and the production pace continues to accelerate, higher requirements are put forward for continuous casting machine equipment. Some equipment is in urgent need of research and modification. Among them, the most prominent problem is the short life of the crystallizer foot rollers.

There is a problem with the crystallizer foot roller.

The crystallizer foot roller is located under the crystallizer of the continuous casting machine. Together with the crystallizer, it vibrates in arc-like motion along with the vibration table to support and guide the high-temperature cast slab. In the early stages of production, the foot roller used a copper sleeve for self-lubrication, and the lubrication effect was poor. Later it was changed to rolling bearings, and the lubricating medium was GH700 high-temperature grease. Before assembly, it was injected into the foot roller assembly at one time. Although this structural form is simple to construct and assemble, it also has many flaws, which are undoubtedly exposed in continuous and fast-paced production. The main problems are:

1) Under continuous high-temperature operation, a large number of foot rollers may not rotate or get stuck. The shortened life span increases maintenance costs and offline maintenance labor intensity.

2) Oxide scale, steel slag, etc. accumulate on the surface of the non-rotating foot roller to form a hard slag pile, causing scratches on the surface of the slab and seriously affecting the quality improvement of the slab.

3) The arc alignment accuracy is low, causing the cast slab to be dented or bulging. The original arc alignment accuracy is about 2mm, which cannot meet the needs of high-quality cast blanks.

On-site survey statistics

According to on-site statistics, the service life of foot rollers is only 8 to 16 days, and some are only one or two days. Nearly 800 foot rollers of various types are consumed annually. When the surface wear is serious, it can reach 0.6 mm, causing surface quality defects of the cast slab, and the scrap rate can reach 0.011%.

Technical research ideas

1) Transform the lubrication method from self-lubrication to oil-gas lubrication. During the production process, the foot rollers get stuck or do not rotate. The main reason is that the foot roller bearings are poorly lubricated and are worn or damaged under the action of the resistance to drawing. Oil and gas lubrication is used to improve the lubrication effect.

2) Re-select the material. The existing surface material of the foot roller is not hard enough. Based on the principle of economy and practicality, materials with high hardness and strong heat resistance are selected.

3) Optimize the arc alignment method and adopt overall arc alignment to improve arc alignment accuracy, reduce foot roller wear, and improve the surface quality of the cast slab.

Technology research and application

Use oil and gas lubrication

Description of oil and gas lubrication mechanism

Oil-air lubrication is a centralized lubrication method. The working principle is that the lubricant moves forward in a wave shape along the pipe wall driven by compressed air. After being separated from the compressed air, a continuous flow of fine oil droplets is sprayed onto the lubrication point to achieve continuity of lubrication. It is called “gas-liquid two-phase fluid cooling and lubrication technology” in academia, and has incomparable advantages over traditional single-phase fluid lubrication technology. It is suitable for harsh working conditions such as the secondary cooling chamber of continuous casting machines where high temperatures and cooling water and dirt invade the lubrication points.

The oil and gas lubrication system is started before the continuous casting machine is cast, and runs continuously during the casting process to ensure that the foot roller bearings can receive fresh lubricant at all times. The system implements multiple small amounts of lubrication. The lubricating oil in the bearing seat is maintained at a constant amount per unit time, that is, the bearing can receive the same amount of fresh lubricant at all times, so that the surface of the bearing friction pair is always covered with a strong enough fresh lubricating oil film. Thereby reducing the friction and wear of the bearing and keeping the bearing in the best running-in state.

The compressed air entering the bearing seat plays three roles while overflowing:

First, the refined lubricating oil film is continuously brought into the bearing seat, so that a continuous and stable oil film is formed on the surface of the bearing friction pair;

Second, a slight positive pressure of 1×10⁴~3×10⁴ is formed inside the bearing seat relative to the outside world, thus preventing the entry of outside water, steam and iron oxide scale;

The third is to take away the heat generated by the continuous operation of the bearing and achieve continuous heat dissipation, so that the bearing operates at a lower temperature.

Implementation of oil and gas lubrication technology for crystallizer foot rollers

1) The structural form of the foot roller is modified to form an oil and gas lubrication pipeline inside the foot roller. The original foot roller assembly mainly consists of roller body, roller shaft, bearing, adjusting screw, end cover and end cover seal. The adjusting bolts are located at both ends of the roller shaft and are used to adjust the distance between the foot roller and the foot roller bracket to obtain a suitable roller spacing. The main idea is to transform the adjusting screws and rollers into oil and gas lubrication pipelines. The specific method is to drill a 06 mm shaft hole in the center of the foot roller adjustment screw (M12×80mm), tap the M10 mm thread on the head of the screw to connect the oil pipe, and the other end of the bolt is threaded with the end of the roller shaft. A shaft hole of 08 mm and a depth of 100 mm is machined from the center of the shaft end, and a roller body hole connecting the outer circumferential surface of the shaft is machined from the end of the hole. This modification overcomes the problem of the narrow installation space of the foot roller, adopts single-end shaft oil supply, uses the foot roller adjustment screw as the oil and gas supply point, seals the interface between the oil pipe and the foot roller shaft, and realizes the lubrication path to the bearing. In order: external copper pipes, bolt holes, shaft holes, roller body holes, gaps between the roller body and the roller shaft, and rolling bearings at both ends.

The assembly after modification is shown in Figure 1. The double-dotted line in Figure 1 shows the direction of oil, gas and lubricating oil.

1-Roller body; 2-Bearing; 3-Bearing end cover; 4-Roller shaft; 5-Adjusting screw

Figure 1 Foot roller oil and gas lubrication modification assembly

Roller body material selection

Analysis of foot roller wear

As the molten steel forms a billet from the green shell in the crystallizer, it continuously passes through the foot rollers of the crystallizer. Through research and analysis of the foot roller material and heat treatment process, the original roller body material is alloy structural steel 35CrMo, the surface is high-frequency quenching and tempering treatment HB210-240, and the hardness is HRC45~48. At high temperatures of 1100 to 1300°C, the surface hardness of the foot roller drops sharply, the wear resistance is greatly reduced, and the wear is accelerated.

Roller material selection

The roller body is made of low-nickel-chromium-molybdenum alloy, and the surface is injected with tungsten carbide. Tungsten carbide has good red hardness, that is, it can maintain high wear resistance and hardness under high temperature conditions. The hardness is HRC58~60, which prevents the roller body from wearing too fast and cracking.

Before and after material selection, comparative tests under the same working conditions found that the wear amount of the roller body was reduced from a maximum of 0.6 mm to 0.2 mm.

Optimize the arc alignment method

Improvement of mold arc alignment method

A regular arc alignment system is adopted to improve the arc alignment accuracy of the mold copper plate and foot rollers. That is, the arc alignment of the mold copper tube (copper plate) is approximately every 10,000 tons of working steel, and the arc alignment of the foot rollers is performed once again. Each time the arc is aligned, the wear amount of the lower edge of the copper plate is fully considered to determine the distance between the rollers L. The value principle is based on the formula: L=L₁+(L₂~L₁)/2, see Figure 2.

L—the actual value of the distance between full rollers; L1—the spacing between the lower openings of new copper pipes (copper plates); L3—the spacing between the lower openings of copper pipes (copper plates) with a steel capacity exceeding 10,000 tons

Figure 2 Schematic diagram of the distance between foot rollers

Implementation of overall process arc

In the past, split arc alignment was mostly used, that is, the mold, sector segments, and vibration table after offline maintenance were respectively hoisted to the casting line for assembly and alignment. This method resulted in long maintenance time and poor arc alignment accuracy. After research, the overall process for arc alignment is adopted, that is, the repaired crystallizer, sector segments, and vibration table are assembled offline on the arc alignment table. When overhauling and changing squares, the whole machine can be lifted to the casting line, which saves maintenance time and greatly improves arc alignment accuracy. The maximum arc alignment gap is 0.2mm, which is smaller than the standard gap of 0.5mm. The two key parameters of the alignment table: the position of the positioning pin of the vibration table and the specified position of the template, should be checked and adjusted every six months. The whole assembly is done offline on the aligning table, see Figure 3.

1-Model upper bracket; 2-Model lower bracket; 3-Arc template; 4-Locating pin

Figure 3 Offline arc alignment diagram

Effect verification: High-quality arc alignment accuracy not only improves the centering effect of the foot roller. Moreover, while reducing the tension and impact force of the counter-foot roller during the operation of the slab and dummy bar, it also reduces the severe local wear of the counter-foot roller.

Effect tracking survey

In January 2009, before and after the research on the long life of foot rollers, technical staff conducted a follow-up investigation on the site and obtained the following data:

1) The consumption quantity of foot rollers is shown in Table 1.

Table 1 Number of foot rollers consumed from 2005 to 2009

Time/Year20052006200720082009
Foot roller consumption quantity81079077475892

It can be seen from Table 1 that before 2008, the annual consumption of foot rollers was relatively large; in 2009, the annual consumption was small.

2) The wear amount of the foot roller is shown in Table 2.​

Table 2 Wear conditions of the foot rollers of each crystallizer in April 2009

Crystallizer serial number1#2#3#5#7#8#9#
Foot roller wear/mm0.180.150.20.10.120.160.2

It can be seen from Table 2 that the maximum wear amount of the foot roller surface after one pouring (9 days) is 0.2 mm, which is less than the original 0.6 mm.

Conclusion

Through the implementation of these three measures, it can be concluded that the application of oil and gas lubrication technology has greatly improved the rotation effect of the foot roller. The application of carburized tungsten material improves the hardness and wear resistance of the foot roller. The application of the overall arc alignment technology improves the arc alignment accuracy of the equipment, reduces the impact force and the expansion force of the molten steel on the foot rollers, and greatly extends the service life of the mold foot rollers. The average lifespan reaches 93 days, which reduces maintenance costs and worker maintenance labor. This technology has high promotion value.

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