In this highly competitive industry, advanced technology has become the new standard. As large steel producers acquire smaller operations, they are transferring knowledge, fluid requirements and operating procedures into acquired facilities. These are not last centurys steel mills. Sophisticated equipment and highly automated processes are driving demand for a well-trained, computer-literate workforce, and companies are making significant investments in training.

As a result, if a company wants to stay in business, it must become as efficient as possible. Indeed, evidence for this increased efficiency can be found by comparing the WSAs reported increase in production volume from 2009 to 2010 (up 16.2 percent, or 1.6 billion metric tons) with the fact that the utilization rate remained essentially flat for the same time period (75.2 to 75.3 percent, says Steel Times International).

With no room for error, time lost to equipment repairs can be costly, and a lubricants ability to protect and improve the performance of fluid film bearings used in rolling operations becomes even more critical.

Most steel companies produce either finished or unfinished products from scrap or iron ore. Finished products, which include steel wire, pipes, flat rolled or structural elements, are sold directly to manufacturers without additional processing. Unfinished products, like billets, bars or slabs, can be processed at secondary finishing mills into final products. Integrated steel mills transform iron ore, limestone and coke into molten steel and then roll and shape that steel into a finished form.

While the mills may produce ingots and other rough cast products, the true value of the operation is generated in the finished steel rolling operations. Equipment failure at the milling stages can shut down an entire integrated operation, which is the last thing any steel producer can afford. Choosing the right high-quality lubricant keeps equipment running smoothly and lessens the chance of lost production and expensive equipment repairs.

Water is Inevitable

Hot rolling – the process of moving heated steel through a series of rolls driven by bearings – accounts for 40 percent of global semi-finished steel production. Hot milling begins with steel being heated in a furnace to a temperature of 1,200 degrees C. When the steel leaves the furnace, it oxidizes, resulting in slag or oxide scale, which is removed using water or a fire-resistant fluid under high pressure. The steel is then sent through a series of rolling mills, also known as stages, each adding an additional degree of refinement. As the steel is compressed, oxidation takes place between each stage, and the byproducts need to be removed. The entire process involves a tremendous amount of water – up to 3,100 gallons per ton of steel (13 cubic meters/metric ton). And where water and metal meet, corrosion can be a concern.

The inside, or work rolls of a mill, are almost continuously drenched with water or a rolling solution. They are typically smaller in diameter than the back-up rolls to reduce the surface in contact with the steel (Figure 1). The situation isnt as pronounced with the outside back-up rolls, which are larger in diameter and farther from the point of fluid application, but back-up rolls still see a significant amount of water or rolling solution.

This presents a unique challenge for the lubricant in a fluid film bearing. As with all applications, the oil must first lubricate the bearings; transfer heat out of the bearings and rolls; and protect the equipment from rust, particles and the impact of oxidation of the oil. In addition, given the operating conditions, the oil must be able to separate water out very quickly.

Choosing a lubricant that can do this well is essential. If the water does not separate rapidly from the oil, it cant be removed, creating the potential for corrosion problems. Too much free water can also cause the breakdown of the hydrodynamic film required to lubricate the fluid film bearings in the mills, increasing the possibility of metal-to-metal contact and bearing failure. (If some of the water emulsifies with the oil, the opposite can occur and viscosity can actually increase – an equally dangerous situation.) In addition, too much water will interfere with the lubricants ability to transfer heat.

All of these issues can lead to bearing failure, leaving the steel manufacturer with two options: Buy a new bearing, or refurbish the existing one. Considering the size and the cost of these bearings, the second option is pursued most of the time. The bulk of this cost can come in the form of lost production. With an output of 100 metric tons per hour (common for todays mills), and the price per ton of hot rolled steel approaching the $900 mark, a stop in the line for repairs can cause the cost of lost production to mount quickly.

Demulsibility Demands

The use of high-demulsibility lubricants can minimize many of these problems. These lubricants are specially formulated to withstand the constant deluge of water, and are designed for maximum effectiveness in the milling environment.

According to Rafael Lazo, general manager, fluids department at Siemens Industry Inc. (formerly Morgan Construction Co.), the concept of high or super demulsibility oils began with a desire to understand the process of water separation in a mill environment, as experienced by his companys trademarked Morgoil bearings.

Morgoil bearings, used in milling operations, are high load, but extremely low speed, he says. Because of their large size, there is a much greater potential that water will get through the seals and mix with oil. Its recommended that the water level in bearing lubricants be maintained at or below 2 percent, but in operations that percentage number can be much higher. Once the water level moves beyond 2 percent, you need to get the water out as quick-ly as you can. Beyond 10 percent, you lose load-carrying capacity.

Prior to the availability of high-demulsibility lubricants, steel manufacturers often had to use two operating tanks. As one became contaminated with water, operators switched to the second tank to allow the water to separate out of the first tank, a process that could take 24 hours. High-demulsibility oil allows the water to separate in the tank while the machine is operational, eliminating the time, cost and loss of efficiency that resulted from the old two-tank method (Figure 2).

Bearing manufacturers have established specifications describing the general requirements for the lubricants used with their equipment. Morgoil bearings, among the most widely used in the steel industry, now are manufactured by Siemens VAI and installed by SMS Siemag. There are two Morgoil specifications for the back-up roll bearing lubricant: the standard (Morgoil Lubricant Specification SN-180 Part 3), which applies to Morgoil KL bearings; and the advanced (Morgoil Lubricant Specification SN-180 Part 4), used for Morgoil KLX bearings.

The main point of differentiation between the two specifications is that the advanced Morgoil systems are designed for use with high-demulsibility lubricants. Lubricants meeting the advanced specification must use a high-grade mineral oil; have a high resistance to oxidation and formation of sludge when subjected to rolling mill service; and be able to separate rapidly from water, air and other contaminants at normal operating temperatures. Both specifications require high demulsibility be demonstrated in two standard tests, ASTM D1401 and ASTM D2711. However, the advanced specification adds a third and critical element, the UEC Dynamic Demulsibility Endurance Test.

Defining Whats Super

Seeking ways to remove water from bearing lubricants quickly and effectively, the Siemens business unit worked with UEC Technologies, a subsidiary of United States Steel, to develop the Dynamic Demulsibility Endurance Test.

The test defines the condemning parameters to accept oils as super demulsibility, says Lazo. If the oil passes the test, it can be considered super demulsibility. The advantage of the test, and of the process itself, is the ability to find oils that will allow us to reduce the size of lubrication systems, resulting in reduced power use and less potential to contaminate large volumes of oil, and better performance of the mill equipment.

The super demulsibility concept is also applied in Morgan high-speed mills, but the percentages are much lower than Morgoil. A maximum of 0.2 percent is allowed in high-speed equipment such as the No-Twist Mill and the reducing/sizing mill technology.

The Dynamic Demulsibility Endurance Test measures the ability of lubricants to dynamically separate from water while under controlled operating conditions of flow rate, residence time, temperature and water introduction. The candidate lubricants ability to perform well in this test is the most important indication of an oils expected performance in actual use, per Morgoil Lubricant Specification SN-180 Part 4.

Base Stock Challenges

The UEC Dynamic Demulsibility Endurance Test offers a challenge for additive formulators. Because lubricants used for this application tend to be heavier oils (ISO viscosity grades 320 to 1000), the use of bright stock is often required. The properties and quality of bright stocks can differ widely however, depending on the region and refinery in which they are produced. This variability means that an additive package that works well in one base oil may be much less successful in another.

This is an issue in todays marketplace, in which global lubricant suppliers use multiple base stocks from different suppliers to blend their products, yet still expect those products to align with brand standards and deliver the same performance regardless of the base stock source.

High-demulsibility lubricants for hot rolled milling applications are most likely to be formulated with base stocks produced in close proximity to that of the steel production. The top four countries for hot rolled steel production are China, Japan, the United States and Russia, with growth in China rapidly outpacing that of the rest of the world. Additive packages formulated for the base oils in one region may not deliver acceptable results in base oils from another location.

Technology is available today to meet the requirements of the UEC Dynamic Demulsibility Test across a variety of base oils, but in some cases may require a top treat of demulsifier. The selection and treat rate of this demulsifier is dependent on the base oils used and the additive chemistry present. Although it can sometimes be difficult, it is possible to meet the challenge of regionally available base oils.

For the foreseeable future, the steel industry will remain a highly competitive market where companies will continue to raise the bar by investing heavily in new technologies and the development of higher performing products. Sophisticated equipment for hot rolling applications will demand a high level of performance from the lubricant, especially in terms of demulsibility.

The lubricant and additive industries are focused on developing oils capable of meeting the requirements of industry specifications, like Morgoils, in a range of different base oils. Even though the additive technology exists, formulating with specific base oils to meet these specifications can be a difficult process that requires adapted chemistry to respond to different base oil characteristics.

As the steel industry moves forward, it will continue to challenge lubricant and additive producers to deliver the advanced lubricant technology needed to keep todays steel mills rolling.