Li Discipline Inspection Commission Zhao Wei Yang Taotao
(Sinopec Lubricant Tianjin Branch)
Abstract: As hot wide-strip rolling mill equipment develops and is upgraded, the operating conditions of mill bearings become increasingly severe, placing higher demands on the water resistance, extreme-pressure performance, wear protection, and other properties of rolling mill bearing greases. With its advantages in water resistance, extreme-pressure performance, rust prevention, and mechanical stability, calcium sulfonate complex grease has attracted considerable attention in China and abroad as a grease for rolling mill bearings and has promising prospects. Application trials of domestically produced calcium sulfonate complex grease in the work-roll bearings of a large hot wide-strip rolling mill at a steel plant showed that, compared with imported lithium complex grease, the domestic calcium sulfonate complex grease offered better water resistance and extreme-pressure antiwear performance. It maintained normal bearing lubrication without abnormal wear, temperature rise, or corrosion, making it suitable for work-roll bearings in hot wide-strip rolling mills.
In recent years, with the rapid growth of domestic wide-band steel production capacity, PC (Pair Cross roll) and CVC (Continuously Variable Crown) rolling mills have been widely used in China. The rolling mill shape control ability is getting stronger and stronger, the rolling mill structure is more complex, and the stress state of the work roll bearings is harsh, which puts forward higher requirements for the extreme pressure and water resistance properties of the grease.
When the work roll of the rolling mill rotates at high speed, the internal pressure endured by the bearing seat and the erosion of the high-pressure roll cooling water will cause the bearing grease to quickly lose, resulting in the lubrication conditions required for the bearing operation being not guaranteed, the service life of the bearing being greatly shortened, and the phenomenon of bearing burning and holding the roller. At the same time, the roll cooling water will also bring iron oxide and other impurities into the bearing seat, forming abrasive wear inside the bearing, causing pitting corrosion and even fatigue spalling.
Delayed bearing maintenance can result in the bearing being scrapped. Under these conditions, fresh grease must be added before every installation in the mill, resulting in high grease consumption. As domestic steel companies pay more attention to equipment lubrication and manage it more precisely, a long-life grease with strong overall performance is urgently needed so work-roll bearings can operate through multiple cycles without replenishment. Calcium sulfonate complex grease offers broad performance benefits: its consistency remains essentially unchanged after water exposure, and it has excellent extreme-pressure and antiwear properties, low consumption and long life. It can meet the demanding conditions of rolling-mill bearings and is gradually becoming a preferred grease for work-roll bearings in hot wide-strip mills.
As domestic calcium sulfonate complex grease technology gradually matures, the water resistance, extreme pressure and wear resistance, shear stability and other properties of domestic calcium sulfonate complex grease products have reached or exceeded imported products of the same type. In recent years, the localization of grease for wide-band steel rolling mill bearings has become a trend. Domestic composite calcium sulfonate-based grease products have gradually replaced imported products and have been promoted and applied in domestic rolling mill bearings.
1 Development of composite calcium sulfonate-based lipids at home and abroad
Calcium sulfonate complex grease has excellent high-temperature properties, mechanical stability, colloidal stability, extreme pressure anti-wear, water resistance and excellent rust resistance. Its comprehensive and excellent performance has attracted widespread attention from the domestic and foreign grease industry and is known as the "new generation of high-efficiency grease".
As early as the 1950s, high-dropping-point calcium sulfonate greases using calcium sulfonate as a thickener had appeared outside China [1]. However, their low thickening capacity and very poor low-temperature pumpability restricted their wider application for a long time. It was not until the mid-1980s that Canadian scientist Ron Muir and colleagues successfully developed calcium sulfonate complex grease by combining non-Newtonian, high-base-number calcium sulfonate soap with other fatty acid salts. This improved low-temperature pumpability and reduced soap content, removing obstacles to large-scale industrial use of calcium sulfonate greases.
Research into calcium sulfonate complex grease began relatively late in China but has developed rapidly. In recent years, calcium sulfonate greases have made substantial progress domestically. In 2001, the Tianjin Lubricant Branch of China Petroleum & Chemical Corporation successfully achieved industrial-scale production of calcium sulfonate complex grease, ending foreign companies' monopoly of the domestic market. In recent years, its comprehensive and excellent performance has led to increasing use in bearings of hot wide-strip rolling mills.
2010 In 2018, the proportion of calcium sulfonate complex grease in the total amount of grease in my country has reached 0.49%. However, compared with North America, the proportion of calcium sulfonate complex grease in China is still low, less than one-tenth of that in North America [2]. It can be seen that there is still huge room for development of complex calcium sulfonate-based lipids in the country.
Performance advantages of 2 calcium sulfonate complex grease in rolling mill bearings
As a grease for rolling mill bearings, the performance index of calcium sulfonate complex grease exceeds that of traditional greases such as lithium grease, lithium complex grease, and polyurea grease. The performance comparison between domestic calcium sulfonate complex base grease and imported lithium complex base grease is shown in table 1.
Compared with traditional rolling mill bearing greases, the performance advantages of domestic complex calcium sulfonate-based grease are mainly reflected in the following aspects.
2.1 water resistance
The consistency of domestic composite calcium sulfonate-based grease changes very little when exposed to water and is not prone to softening and loss. It can maintain an appropriate consistency in the bearing and is easier to protect the bearing. It is especially suitable for use in water-rich conditions.
2.2 Extreme pressure and anti-wear properties
Even without adding any extreme pressure agent, the sintering load PD value of domestic composite calcium sulfonate grease can reach 3920N, which can meet the extreme pressure lubrication requirements of rolling mill bearings. Under high load conditions, the colloidal structure of calcium sulfonate is easily destroyed, and the released calcium carbonate is deposited on the friction surface. The calcium carbonate of these calcites has a layered structure, low shear strength and high longitudinal load-bearing capacity, which can improve the anti-sintering ability of the grease.
2.3 Anti-rust performance
The better anti-rust performance of domestic composite calcium sulfonate-based grease is attributed to the special structure of calcium sulfonate itself. SO32- in its structure has strong polarity, is easily adsorbed on the metal surface, and can prevent oxygen and moisture from contacting the metal surface.
2.4 Mechanical stability
Compared with other greases, the domestic calcium sulfonate complex grease has good mechanical stability. After 106 shearing, its consistency (0.1mm) only changes less than 10 units. This is closely related to the structure of complex calcium sulfonate-based lipids. The calcium sulfonate complex system is composed of calcium acetate, calcium borate, calcium carbonate and other particles. Except for the fatty acid calcium fiber that is prone to partial breakage, the structure of the calcium sulfonate complex base lipid will not change and will not be damaged by long-term shearing.
Application case of 3 calcium sulfonate complex grease in hot-rolled wide-band steel mill
A large state-owned steel plant previously used imported lithium complex grease in bearings of a 1700 mm hot wide-strip mill. Problems included high grease cost, long procurement lead times, poor water resistance, rust in some bearings and short bearing life. In 2011, a 3-month trial replaced the imported lithium complex grease with domestic calcium sulfonate complex grease in one bearing set on stand F3. Throughout the trial, the bearings remained well filled with grease and showed no abnormal wear or temperature rise; lubrication remained satisfactory.
3.1 Domestic composite calcium sulfonate grease replacement before and after bearing use status comparison
The status of the 1700 mm hot-rolled wide-band steel rolling mill bearings using domestic calcium sulfonate complex base grease and imported lithium complex base grease respectively is shown in the figure 1.
As can be seen from the picture 1:
When using imported lithium complex grease, the amount of grease in the bearing is too small and the color of the grease is off-white, indicating poor adhesion; rust occurs after the bearing is disassembled, and obvious indentations appear on the outer ring of some bearings, which can easily lead to the service life of the bearing not meeting the expected requirements.
When using domestic complex calcium sulfonate-based grease, the oil groove between the bearings is filled with grease, and its color remains basically the same as the original grease, with no gray-black change; the feel, consistency and viscosity of the in-use grease remain good; there is a uniform oil film on the surface of the rolling element, the surface is bright, without pitting, rust, etc., and the bearing is in good lubrication condition.
3.2 Comparison of sampling indicators of grease in use
During the application test of 3, the bearings were regularly disassembled, and the analytical data of imported lithium complex grease and domestic calcium sulfonate complex grease taken from the same parts were compared. The results are shown in table 2.
It can be seen from the table 2:
Compared with the domestic calcium sulfonate complex-based grease, the water content of the imported lithium complex-based grease in use is at a very high level, indicating that it has strong water absorption. The presence of a large amount of water in the grease will reduce the performance of the grease, destroy the oil film formed by the grease, greatly reduce its lubrication effect, and significantly reduce the service life of the bearing.
The iron content of grease is the most critical indicator of the lubrication status of the bearing, and directly reflects the degree of wear of the bearing. The iron content of the imported lithium complex grease in use is up to 2987 mg/kg, which indicates that iron filings exist inside the bearing for a long time, which can easily cause abrasive wear and scratches on the bearing. The iron content of the domestic calcium sulfonate complex grease in use is greatly reduced, and the surfaces of the inner and outer rings and rolling elements of the bearing are bright and scratch-free, indicating that the bearing is in good lubrication condition.
After long-term use, the PD value of domestic calcium sulfonate complex grease is higher than that of imported lithium complex grease, indicating that it still has good anti-wear properties and can ensure that the bearing maintains good lubrication performance without sintering under conditions of high impact load.
Comprehensive comparison of the on-site use effects of the grease in use before and after replacement and the performance analysis test results show that the domestic composite calcium sulfonate-based grease is better applied to 1700mm hot-rolled wide-band steel rolling mill bearings. At present, this hot rolling production line has used domestic calcium sulfonate complex grease instead of imported lithium complex grease throughout the entire line, and has achieved good results.
4 Conclusion
In recent years, my country's hot rolling technology has developed rapidly, and rolling mill bearings have put forward higher requirements for the extreme pressure anti-wear properties, water resistance, rust resistance, and mechanical stability of the grease. As a long-life, environmentally friendly grease, calcium sulfonate complex grease has comprehensive and excellent performance and a small dosage. It has obvious advantages over traditional roll grease and will gradually become the mainstream grease for hot-rolled wide-band steel rolling mill bearings.
【Reference】
[1] Wang Haiyan. Preparation and properties of composite calcium sulfonate-based lipids [D]. Jinan: Shandong University, 2007.
[2] Yao Lidan, Yang Haining. Analysis of Recent Grease Production and Development Trends [C] // Proceedings of the 14th Grease Technology Symposium, 2011: 8-12.
Source: Internet
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