What is the difference between hot and cold rolling lubricants?

January 03, 2024

What is the difference between hot and cold rolling lubricants?

Rolling is a form of pressure processing that uses a roll to roll metal billets into various specifications of plates. The important functions of the application of process lubricants are: (1) reduce the friction coefficient and friction force on the contact arc surface of the deformation zone, so that the total pressure and energy consumption of rolling are reduced with the decrease of friction, so that the pass reduction amount and the number of passes can be increased, and the rolling speed can be increased; (2) The good lubrication performance of the lubricant significantly reduces the wear of the roll, which is especially important for the hot rolling process; (3) Wet rolling parts to prevent metal sticking and improve rolling efficiency; (4) Effectively cooling the roll and material, and finally obtain the same surface properties that meet the quality requirements. Different types of metal and calender have different requirements for lubrication.

Steel rolling lubricant

At present, the steel plate rolling lubricants used at home and abroad can be divided into solid lubricants and liquid lubricants according to its state, one is water-soluble metal rolling oil, that is, a small amount of oil added to a large amount of water to become emulsified, that is, emulsified rolling oil, emulsified rolling oil has good lubricity, extreme pressure and cooling properties, therefore, the domestic is generally used emulsified rolling oil.

1) Steel hot rolling lubricant

Hot rolling is mainly used for the rolling of billets and thick steel plates, and the general temperature of rolled parts is as high as 1300℃. In the past, cooling water was used for lubrication and cooling. In recent years, hot rolling mill to high-speed, automatic development, the use of cooling water can not meet the process requirements, now the general use of hot rolling liquid. Hot rolling liquid requires good adhesion and lubrication, can be uniformly attached to the surface of the roll to form a firm lubricating film, reduce the friction in the deformation area. Water based hot rolling liquid is difficult to use in the existing hot rolling mill, and is not conducive to wastewater treatment, so most of the hot rolling liquid used at present belongs to oil-based lubricants. The Base Oil can be mineral oil, synthetic hydrocarbon and ester oil. In order to improve the lubrication performance, animal and plant oils, advanced fatty acids, fatty alcohols, synthetic esters and other oily agents are usually added, and some also add anti-rust additives.

Generally, hot rolling is to press the billet to about 30mm thick at the temperature of 1250 ~ 1350 ° C, and then at the temperature above the crystalline Ar3 transformation point (γ→γ+α transformation point) of the steel, about 900 ° C for fine rolling. Because the calendering oil attached to the press roll is in contact with the calendering material at 900 ° C, even if the contact time is only 1% s or less for a short time, the thermal decomposition problem is also serious, so the rolling lubrication fluid is required to have good heat resistance and high decomposition temperature. The heat resistant temperatures of several oils (fats) are shown in Table -1. When hot rolling steel plate, the working temperature reaches 900℃, the rolling oil containing tallow and pentaerythritol fat, zinc thiophosphate, alkyl phenol sulfonate, etc., should be used.

Table -1 Heat resistance temperature of various rolling lubricant oils

2) Steel cold rolling lubricant

Cold rolling is the process of rolling steel plate using a working roll with greater material hardness to make it undergo plastic deformation. Two types of friction are generated in the rolling process: (1) Steel plate deformation, internal friction between steel point molecules causes deformation resistance; ② Friction between strip and roll surface. Friction is accompanied by heat of friction and heat of deformation, so lubricants are applied during cold rolling to meet the requirements of lubrication and cooling, which are the two most important roles of cold rolling lubricants.

Cold-rolling lubricants can be divided into oil-based and water-based types according to their composition and use status. Usually mineral oil type water-soluble cold rolling liquid to light, medium mineral oil as the main component, which added additives, such as fat oil, fatty acid and other oily agents, containing sulfur, chlorine, phosphorus and other elements of extreme pressure additives, anionic and non-ionic surfactants, rust inhibitors, antioxidants, etc., in the water to form a stable emulsion, most of the emulsion particles are 1.0 ~ 1.5μm, Some particles are about 5μm. The water soluble cold rolling liquid is based on the mixture of fat oil, synthetic oil and mineral oil, in addition to emulsifiers, extreme pressure additives, rust inhibitors, antioxidants and other raw materials, most of the emulsion particles are 1 ~ 10μm. Fat water-soluble cold rolling lubricant is based on fat oil, adding extreme pressure agent, antioxidant, rust inhibitor, emulsifier, etc. This kind of oil has good adhesion to the steel plate and roll surface, small friction coefficient and good lubricity, but the residual carbon is much, and it needs to be cleaned before annealing. Water base emulsion rolling oil consists of two parts: water and oil. The content of water in the rolling oil emulsion is usually more than 95%, and the treated neutral deionized water should be used to configure the emulsion. Because the water quality has a great impact on the performance of the emulsion: first of all, the hardness of water, that is, calcium and magnesium ions often act with the anionic emulsifier in the emulsion, so that its sodium, potassium salt or soap is converted into the corresponding calcium soap and magnesium soap, so that the concentration of anionic emulsifier is reduced or the emulsification ability is lost; Secondly, the anions in the water such as chloride ions and sulfate ions have the effect of promoting metal corrosion, and when the chloride content is too high, it will cause salting out, resulting in a decrease in the concentration of the emulsion.
Surfactants have many functions in rolling lubricating oil, such as emulsification, rust prevention, lubrication, etc. It has a two-parent structure, which not only has the function of preventing the mutual exclusion of oil and water, but also has the special function of connecting oil and water without separating them. Although the rolling base oil and water can form an emulsion at high speed, the emulsion is very unstable and easy to be stratified. It is necessary to add surfactants, that is, emulsifiers, into the oil-water mixing system to form an oil-water emulsion. Here, the requirements for the chemical properties of emulsifiers are not very strict, as long as they do not interact with other additives. In the rolling process, under high pressure and high temperature, the lubrication fluid can be broken, so that the lubricating oil can spread on the surface of the rolled part, the water is dispersed or evaporated to reduce the temperature of the roll and the rolled part, the emulsification stability is too good but will affect the lubrication and cooling effect.

Due to the separation of oil and water in the rolling process, the oil is produced to spread out from the water and adhere to the surface of the rolled piece. Therefore, oil-soluble corrosion inhibitors are generally used in rolling oil. Oil soluble Corrosion Inhibitor molecules are composed of polar and non-polar parts, the special structure of the molecules can be attached to the rolled plate surface to prevent surface corrosion, but also can be used as a surfactant. Corrosion inhibitors commonly used in steel rolling oil are usually: zinc naphthenate, sodium petroleum sulfonate (also an emulsifier), barium petroleum sulfonate, benzotriazole, sorbitol monooleate, aluminum stearate, etc. Most of them are anionic surfactants.

Friction improvers are also often required to have the structural characteristics of surfactants, that is, one end is a straight chain alkyl group with more than 10 carbon atoms, and the other end is a polar group. The polarity of a polar group is an important factor affecting its properties. Commonly used friction improvers are: long chain hydroxyl acid and its derivatives, long chain amines, amides, imines and their derivatives, etc. Acids can be chemically adsorbed on metal surfaces in the form of anions, amines, phosphites and phosphate esters in the form of coordination, and salts in the form of ion pairs. Regardless of the mechanism, the result is that the Friction Modifier affects the friction properties by forming a multilayer adsorption film on the metal surface.

We know that for rolling strips of different specifications, it is not the same requirements for rolling, and people are accustomed to using saponification values to distinguish different varieties of rolling lubricants. The saponification value is a sign of the lubricity of rolling lubricant, and the rolling force of small mill decreases with the increase of saponification value. On the other hand, the lubrication of the rolling lubricant used in the form of emulsified liquid is also restricted by the ability of the lubricant to free oil from water, which is a macroscopic manifestation of the particle size distribution in the emulsion. It is generally believed that when the composition is fixed, the ductility of the more stable emulsion from water is worse than that of the unstable lubricant. The smaller the particle size of the emulsion, the more stable it is. Relatively speaking, the cooling property of the emulsion is better than the lubricating property. Unstable lubrication fluid, its particle size is large, so the amount of oil adhesion is also large, so the lubricity is better than the cooling property. The particle size of the emulsion depends on the HLB value of the emulsion system and the stirring force and temperature of the emulsion circulation system, among which the temperature factors include the emulsion temperature and the steel plate temperature, obviously the temperature is conducive to the increase of the adhesion of the emulsion phase and oil, of course, not the higher the temperature the better, usually the best temperature range is 35 ~ 45℃ when rolling steel.

Usually rolling different specifications of rolled wood, with different saponification values and different emulsion stability of rolling lubricants, such as rolling thicker than 0.5mm plate, the use of lower lubrication performance of the saponification value of about 30 ~ 50mgKOH/g mineral oil water-based rolling lubricant, the emulsion state is more stable; When rolling 0.3 ~ 0.5mm thin plate, the emulsion of water based rolling lubricant with medium lubricability of 50 ~ 130mgKOH/g or a mixture of synthetic ester and natural fat is unstable. When rolling extremely thin sheets below 0.3mm, the grease type rolling lubricant with excellent lubricities and a zonification value of 160 ~ 200mgKOH/g is used, and the state of the emulsion is extremely unstable. It is worth pointing out that with the development of science, some lubrication materials with low saponification value or no saponification value can be comparable to palm oil, and then it is meaningless to use saponification value to measure rolling lubricity.

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