INTRODUCTION

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Strip cleanliness, or amount of extraneous materials (e.g., dirt, iron particles, carbon, etc.) on the steel surface, is a key performance requirement for cold rolling mills. Strip cleanliness is critical for mills supplying coating lines and the automotive industry.Numerous factors from various process steps influence final strip cleanliness. Hot mill, pickling line and annealing practices all impact cleanliness. Key factors impacting strip cleanliness during cold rolling and their relation to specific lubricant properties will be discussed. A short introduction to lubricant design.

From a fundamental point of view, there are two basic performance concerns for cold rolling lubricants. First, the lubricant usually applied as an emulsion should form an adequate film in the inlet of the roll bite (film forma). Second, intrinsic neat oil properties should display the desired properties (e.g., lower friction and/or reduced roll wear).
It is widely accepted that cold rolling involves mixed lubrication, a mix of hydrodynamic or full film lubrication and boundary lubrication.

When studying lubricant properties that impact elasto or plastohydrodynamic lubrication, three physical parameters of the lubricant are relevant. Its viscosity (m) and viscosity-pressure coefficient (a) are next to a number of process-related parameters such as speeds and loads important for thickness of lubricant film. Zhang 1 et al. proposed the film parameter MO a* as an indicator for the ability of a lubricant to form an adequate film at the roll bite inlet. Finally, the coefficient of friction of this lubricant film, often referred to as the traction coefficient, is an indicator of intrinsic lubricant properties.

In boundary lubrication, where film thickness is of the same order of magnitude as the combined surface roughness, the interaction of lubricant components with the metal surface is important. Three mechanisms can play a role: physical adsorption, chemical adsorption (e.g., absorption of fatty acids leading to an iron soap) and chemical reactions. The key physical parameter of the lubricant in this lubrication regime is the coefficient of friction of the resulting boundary layer.By studying the above-mentioned lubricant parameters in relation to the chemical structure of the lubricant, novel cold rolling lubricants with specific, custom-tailored properties can be designed.2.
Emulsifiers play a dominant role in the performance of cold rolling emulsions as they not only affect film formation and lubricant delivery, but also govern properties such as emulsion stability, iron fines handling and a clean mill housing.

Various test methods are currently used to select the appropriate emulsifier package for a certain base lubricant. Plate-out values the amount of oil (in mg/m 2 ) left on a steel strip after spraying with an emulsion are determined and can range from 100 up to 2500 mg/m 2 . ‘Iron fines free flowing’ determines the emulsion’s ability to prevent coagulation (clogging) of the iron fines generated in the mill. Film formation out of the emulsion can be studied using an optical interferometer. In long-term recirculation tests, the emulsion’s stability in time measured as the variance in particle size distribution is established.

 

on the strip In the cold rolling process, most iron fines are generated in the first two stands. The amount generated depends mainly on work roll surface roughness and the reduction schedule applied (i.e., the higher the surface roughness and the more reduction taken in the first stands, the more iron fines are generated). Coupled mills have an advantage they can run with lower surface roughness in the first stand, as the danger of bite refusal is negligible compared to standalone mills.
In view of relatively low speeds and relatively higher surface roughness in the first stands, the dominant lubrication regime will be boundary lubrication. The lubricant should fulfill a number of requirements to reduce the amount of iron fines on the strip.
Good boundary lubricant properties are essential to lower friction and avoid excessive work roll wear. Furthermore, the emulsion should have sufficient detergency to wash iron fines off the strip and keep them dispersed in the bulk emulsion. Wash-off and ‘iron fines free flowing’ are key properties that should be considered in proper selection of an emulsifier package.

Interaction of various raw materials and formulated products was studied via a reciprocating wear tester, originally developed by the Georgia Institute of Technology in Atlanta. This boundary tester (Fig. 1) determines lubricant interactions with actual strip and work roll materials. The lubricant can be studied in neat or emulsion form at temperatures up to 400°C, and the coefficient of friction can be continuously monitored over a relatively long period of time.
In Fig. 2, a number of base lubricants mineral oil, a natural ester and two synthetic esters were studied using the reciprocating wear tester. The coefficient of friction (COF) was recorded in each cycle. The coefficient of friction of mineral oil was substantially higher (0.14 to 0.15) than the coefficients of friction of the respective esters (0.10 to 0.12). Esters are much more polar than mineral oils and adhere much tighter to the metal surface. As a result, the corresponding boundary layer gives rise to a lower coefficient of friction, although details of the mechanism are not yet fully understood.
With the help of these techniques, detailed studies of individual lubricant components and interaction effects between components can be conducted to lower a formulation’s coefficient of friction.

Temperature influences boundary lubrication in a number of ways. Chemical reaction rates increase two to threefold with every 10°C increase. As a result, boundary layers are more quickly built at elevated temperatures. This is especially relevant for EP/AW additives, where the phosphor or sulfur component reacts with the metal surface forming a low-friction boundary layer.At higher temperatures, the activation energy for this reaction will be reached earlier.Coefficients of friction decrease with increasing temperature as a result of increasing molecular mobility. Lubricant molecules can absorb so much energy that they desorb from the metal surface at a critical temperature, leading to a steep coefficient of friction increase.
Several of these temperature effects are shown in Fig. 3. The coefficient of friction decreases at higher temperatures due to increased molecular mobility. Furthermore, one of the two lubricants apparently desorbs at 100°C, leading to a steep increase in friction.

Most cold mills produce a range of products with quite different metallurgical properties. In practice, the type of material rolled often impacts strip cleanliness. As described above, the interaction of lubricant components with the metal surface is highly relevant to boundary lubrication. The impact of similar rolling oil formulations on different materials was studied