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Abstract: ensuring stable operation and efficient production is the main problem faced by the steel rolling industry for many years. In the iron and steel industry, the requirements for long life and high reliability of bearings are constantly improving. In the iron and steel industry, the harsh environment includes iron powder, water and high temperature, and the working environment includes heavy load, impact load, high vibration value and rapid acceleration and deceleration cycle. These harsh environments are rare in other industries. NSK has been committed to developing bearing materials and bearing design to extend the service life of bearings and further improve the reliability of bearings.
Key words: Bearing for steel industry; Service life of bearing; Technical trend
1 Trend of bearings for steel equipment
Starting from the raw material plant, various bearings are used in all equipment from the upstream process to the downstream process such as steel-making process, rolling process and refining process. From standard bearings to special bearings developed for steel equipment, they are widely used.
In addition to the harsh environment of iron dust, water and heat, the bearing is also used under the severe conditions of heavy load + vibration impact, from very low speed to ultra high speed + rapid acceleration and deceleration.
The following is a representative example of various steel equipment bearings developed by NSK:
1.1 Bearings for continuous casting equipment
As mentioned above, the steel equipment is used under various cool conditions, and one of the places where it is used under very harsh conditions is the continuous casting equipment. The continuous casting equipment, as shown in Fig. 1, is an equipment that casts molten steel into the casting mold, gradually solidifies and passes through the roller, and casts under high load pressure rolling. Moreover, in order to properly solidify the high-temperature molten steel, a large amount of cooling water is often sprayed on the casting, and the environment is high temperature and wet. The bearing of the supporting roller is used at extremely low rotational speed and under high load conditions of more than 30% of the basic rated dynamic load of the bearing. Therefore, the lubricant on the contact surface between the inner and outer ring raceway surface and the roller rolling surface is difficult to enter, and the necessary lubricating oil film can hardly be formed. Moreover, the cooling water and the oxide scale falling off the surface of the casting intrude into the bearing, resulting in poor lubrication. Metal contact occurs between the raceway surface of the inner and outer rings and the moving surface of the rollers.
Fig. 1 Structure of continuous casting machine
Under high load rolling, the bearing with self-aligning performance is used under the rotation of inner ring due to the rigidity and installation of roller. The bearing adopts fixed end and movable end. The fixed end is an automatic spherical roller bearing capable of bearing radial load and axial load. In particular, the tapered roller bearing with spherical seat developed by NSK in recent years is used. Most of the free ends are full loaded cylindrical roller bearings with spherical seats and 2 sets of separable full loaded cylindrical roller bearing assemblies. (Figure 2)
Fig. 2 guide roller diagram of continuous caster
1.1.1 Spherical roller bearing
Since the spherical roller bearing rotates with the differential sliding between the inner and outer race raceway surface and the roller movement surface. The bearing is damaged earlier under such cool conditions.
Poor lubrication caused by extremely low speed, high load and water + oxide skin intrusion into the bearing, as well as two mountain shaped friction (see Fig. 3) and peeling crack damage (see Fig. 4) on the metal contact rolling surface between the outer race raceway surface and the roller moving surface that cause the differential sliding characteristic of the spherical roller bearing.
Moreover, in the end, with the roll falling off, many serious situations such as operation stop occurred. Therefore, NSK has developed new materials to ensure stable operation under such cool conditions, and commercialized the super wear-resistant spherical roller bearing (hereinafter referred to as SWR bearing) to improve the reliability of the continuous casting machine.
Fig. 3 Relationship between differential sliding and pressure distribution on outer ring raceway surface and roller rolling surface under maximum rolling body load
Figure 4 generatrix shape of outer ring raceway surface
SWR bearing is a steel added with vanadium independently developed by NSK. Due to the adoption of the most advanced carburizing technology, a large number of fine high hardness carbides are precipitated, which greatly improves the wear resistance and peel life characteristics of the bearing. Moreover, due to surface hardening, the toughness of the core is greatly improved compared with the original fully hardened SUJ2 steel. In actual use, the service life is more than 2 times longer than that of the original spherical roller bearing.
1.1.2 Tapered roller bearing with spherical seat, full loaded cylindrical roller bearing with spherical seat and full loaded separable cylindrical roller bearing assembly.
The tapered roller bearing developed by NSK, which has no differential sliding on the raceway surface, bears high radial load and has centering performance, has been put on the market as the fixed end of the continuous casting machine.
The movable end bearing, due to the radiant heat from the high-temperature casting, avoids shaft extension during the rotation between the roller rolling surface and the inner ring raceway surface. Since full loaded rollers are used, high load capacity cannot be carried out. Therefore, the commercialization of full loaded cylindrical roller bearing with spherical seat and self-aligning performance is being carried out. In addition, two separable cylindrical roller bearing assemblies with full rollers were launched in the market, and the reliability of these bearings was highly praised.
2.2 Journal bearing for rolling mill
Like the continuous casting machine, the service conditions of the journal bearing for rolling mill are very harsh. The journal bearing for rolling mill is mainly used in heavy load, vibration, impact, cooling water or rolling water + debris mixing. Originally, in order to prevent sudden accidents caused by poor lubrication, a large amount of lubricant was supplied. However, in recent years, the environmental problem has become increasingly important. Reducing the use of grease in rolling mills has become one of the important topics for iron and steel plants.
In order to meet the general requirements of high reliability and long service life of bearings, NSK has developed roll journal bearings for rolling mill from two aspects of material and design.
(1) Type - Super ~ bearing (super-it bearing)
When foreign matters such as iron powder and dust are mixed into the bearing, it can be concluded that foreign matters are mixed between the inner and outer race raceway surface and the roller rolling surface -. + indentation formation · stress concentration occurs at the indentation edge -. + fracture occurs at the indentation edge -. + Crack Propagation -_+ Exfoliation (see Fig. 5). The countermeasure is that NSK adds chromium and molybdenum to the new steel grade (SAC material). Followed by carburizing heat treatment. Super-TF bearing was developed by controlling retained austenite properly. As a result, when foreign matters are mixed in, the stress concentration at the edge of the indentation is alleviated, and a long life is achieved.
(2) Type - water ~ bearing (water - it bearing)
If the bearing is used in the environment of water intrusion, the raceway surface will cause small friction and peel off from the surface starting from hard oxide inclusions (see Fig. 6). NSK has studied the mechanism of peeling off. The countermeasure is to suppress the generation of cracks through materials with high cleanliness, especially to optimize the chemical composition and control the crack propagation, so as to achieve a long life under water intrusion.
(1)Indentation due to foreign matter intrusion
(2)When the roller passes through the indentation, the stress on the indentation edge is concentrated and peeled off
(3)Spalling occurs repeatedly under the load of the roller, and spalling expands
Fig. 5 process of surface peeling
Fig. 6 mechanism of spalling under water intrusion
(3) Extra capacity @ Four row tapered roller bearing for sealing roller neck
Due to the adoption of oil seal with special shape design and cage processed by new processing method, the basic rated dynamic load of extra capacity seal roller journal bearing is increased by about 30% compared with the original bearing.
In addition, in order to prevent water from entering the bearing and causing negative pressure, the newly developed special seal can control water intrusion below I / 4. Deterioration of the lubricant can be prevented, and as a result, not only the service life but also the maintenance period can be prolonged. (see Figure 7)
Fig. 7 characteristics of roller journal bearing with extra large load capacity
3 Conclusion
This paper introduces the bearings for continuous caster and rolling mill. The bearings used in steel equipment are all used under very harsh conditions. Therefore, NSK has carried out countermeasures, research and development from the aspects of bearing design, bearing type, bearing material, lubrication method and lubricant. In order to improve production, the bearing will have a long life and high reliability. In the past, the goal of the steel industry was high value-added products. In the future, it will move towards environmental protection and reduce maintenance costs.
More about KYOCM Spherical Roller Bearing：
A spherical roller bearing is a rolling-element bearing that permits rotation with low friction, and permits angular misalignment. Typically these bearings support a rotating shaft in the bore of the inner ring that may be misaligned in respect to the outer ring. The misalignment is possible due to the spherical internal shape of the outer ring and spherical rollers.[1] Despite what their name may imply, spherical roller bearings are not truly spherical in shape. The rolling elements of spherical roller bearings are mainly cylindrical in shape, but have a (barrel like) profile that makes them appear like cylinders that have been slightly over-inflated.
http://www.kyocm.com/products/Spherical-Roller-Bearing/739.html
2022-09-05