Regular Article
Effect of Oil Film Thickness on Lubrication Property in Hot Rolling
2015 Volume 55 Issue 3 Pages 632-636
Details
2015 Volume 55 Issue 3 Pages 632-636
In hot rolling, lubrication between the work roll and hot strip plays an important role in reducing rolling force and protecting the work roll surface. However, the tribological behavior in hot rolling has not been clarified sufficiently. In this work, the effects of oil amount on the coefficient of friction in hot rolling were investigated in comparison with the case of cold rolling. The oil amount in hot rolling was measured by the amount of oil remaining on the work roll surface after rolling. The results of rolling tests clarified the following points: The coefficient of friction is reduced adequately with a small amount of oil. If the oil amount is increased, a few small oil-pits will form, but no further decrease in the coefficient of friction will be achieved. It is suggested that boundary lubrication is controlling in hot rolling, which is different from the case of cold rolling.
In hot rolling, lubrication oil is supplied to the work rolls in order to reduce electric power consumption, protect the work roll surface and improve the quality of the hot-rolled sheet.1) The role of lubrication in hot rolling is increasingly important from the viewpoints of reduction of global environmental loads and production of hot-rolled steel with an ultra-fine grain microstructure, which is achieved by heavy rolling deformation.2,3)
However, only a few reports4,5,6) have examined the behavior of lubricant oils in hot rolling, and the mechanism of lubrication has not been clarified sufficiently in comparison with cold rolling. Azushima et al.5) investigated the influence of the emulsion concentration on the friction coefficient in hot rolling, and reported that the friction coefficient decreased as the emulsion concentration increased in the range under 1%, but became constant at more than 1%. They also estimated the amount of oil in hot rolling semi-quantitatively by measurement of the remaining Ca, which is a component of lubricant oil, on the work roll surface after rolling, and concluded that the friction coefficient becomes constant when the thickness of the oil film exceeds a certain value.7)
In the present study, the influence of the oil film on lubrication in hot rolling was investigated basically in comparison with the case of cold rolling. In cold rolling, the oil film thickness is estimated8,9,10) from the rolling conditions, such as the diameter of the work rolls and the viscosity of the lubricant oil. For example, Azushima et al.10) showed that the oil film thickness introduced in the interface can be derived by the Reynolds equation, and is related to the surface gloss or number of oil-pits of cold-rolled sheets. In hot rolling, it is difficult to estimate the oil film thickness by the same method as in cold rolling because the oxidation scale on the sheet surface is fractured. For this reason, Azushima’s study was limited to a semi-quantitative estimation of the oil film thickness. It may also be noted that no studies have been reported in which oil-pits on the hot-rolled sheet surface were observed. Therefore, in this study, the amount of oil remaining on the work roll after rolling was measured directly, and the oil film introduced in the interface was then estimated from the oil amount. Furthermore, rolling experiments were also carried out in which both the specimens and the work roll were polished to a mirror surface, and the oil-pits were observed and the lubrication mechanism in hot rolling was discussed.
Table 1 shows the experimental conditions. The diameter of the work rolls was 340 mm, and the surface was plated with Cr and polished to a roughness of 0.02 μmRa or less. The specimens used in this study were Type 316 stainless steel with a thickness of 2 mm and width of 100 mm. The steel sheet surface was also polished to 0.02 μmRa or less. The reasons for using Type 316 stainless steel were that oxidation scale does not tend to form at high temperature in the case of hot rolling, and Type 316 has a α phase microstructure at both hot- and cold-rolling temperatures. Hot rolling was compared with cold rolling under the same conditions of a rolling speed of 50 mpm and rolling reduction of about 6%. In hot rolling, the specimens were reheated at 1173 K for 20 minutes in a furnace and rolled at about 973 K using a lubricant oil with a viscosity of 110 mm2/sec at 313 K. In cold rolling, the specimens were rolled at room temperature using a lubricant oil with a viscosity of 38 mm2/sec at 313 K. The rolling reduction was set low in order to prevent heat scratches even under rolling without lubricant oil.
| hot rolling | cold rolling | |
|---|---|---|
| mill | 2Hi ϕ340 mm (<0.02 μmRa) | |
| work piece size | 2 mmt × 100 mmw (<0.02 μmRa) | |
| rolling velocity | 50 mpm | |
| reduction | 6.2–6.9% | 5.4–6.1% |
| lubricant | 110 mm2/s | 38 mm2/s |
| rolling temp. | 973 K | R. T. |
| initial oil amount | 0–52900 mg/m2 | |
Figure 1 shows the experimental procedure and a schematic illustration of the method of measuring the remaining oil amount. To change the oil film thickness, the initial oil amount is controlled in the range of 0 to 52900 mg/m3. In actual works, an emulsion formed by mixing water and oil in advance is supplied to the work rolls. However, in order to control the oil amount over a wide range in this study, only lubricant oil in various amounts was dropped directly on the work rolls and spread uniformly.
Schematic illustration of experimental procedure.
The remaining oil on the work roll surface after rolling was measured.4) The remaining oil amount is equivalent to the amount that was introduced in the interface and contributed to lubrication. The remaining oil amount was measured from an area 80 mm in width and 250 mm in length on the work roll surface, which came into contact with the specimen during rolling. The oil on this area was degreased by n-hexane and Soxhlet-extracted, and then was weighed. The initial oil amount was measured from the area which did not come into contact with the specimen. The remaining oil amount of the work piece was measured from the rolled sheet surface.
The friction coefficient was calculated under each set of rolling conditions. Although formulation of the flow stress at each temperature, strain and strain rate is possible, the precision will decrease because of the large distribution of temperature and strain rate in the thickness direction. Therefore, the mean flow stress in hot rolling was assumed to be 333 MPa, which meant the friction coefficient under the no-lubrication condition was 0.35, and in cold rolling, the assumed value was 545 MPa and the friction coefficient under the no-lubrication condition was 0.20. The friction coefficients of the other lubrication conditions were then calculated based on the rolling results and the above-mentioned assumed flow stress values.
The sheet surface was observed with a laser microscope to investigate the occurrence of oil-pits.
Figure 2 shows the effect of the initial oil amount on the rolling loads in hot and cold rolling. In both cases, the rolling load decreased as the initial oil amount increased, after which the rolling load maintained the same level when the initial oil amount exceeded a certain value.
Effect of initial oil amount on rolling load.
Figure 3 shows the relationship between the initial and remaining oil amounts. In cold rolling, the remaining oil amount increased with increases in the initial oil amount, and then maintained the same level when the initial oil amount became larger than 1000 mg/m2. In cold rolling, the oil film introduced in the interface is estimated from rolling conditions such as the work roll diameter or rolling velocity, and it is well known that the introduced oil film becomes saturated when the initial oil amount is increased beyond a certain level. These experimental results also confirmed the above. On the other hand, in hot rolling, the remaining oil amount increased as the initial oil amount was increased within the range of these experimental conditions. Therefore, it was thought that the mechanism of introduction of the oil film might be different in hot rolling and cold rolling.
Relationship between initial and remained oil amount.
Figure 4 shows the relationship between the remaining oil amount and the calculated friction coefficient. In cold rolling, the calculated friction coefficient decreased monotonically as the remaining oil amount increased in the range of 0 to 600 mg/m2, and this behavior was regarded as the expansion of the fluid lubrication region. In hot rolling, the calculated friction coefficient decreased as the remaining oil amount increased from 0 to 150 mg/m2, but became constant at over 150 mg/m2.
Relationship between remained oil amount and calculated coefficient of friction.
As mentioned above, the lubricant behavior in hot rolling was different from that in cold rolling. In order to discuss this difference, observations of oil-pits on hot- and cold-rolled sheet surfaces were carried out.
Figure 5 shows a comparison of the hot- and cold-rolled sheet surfaces by 3D observation. Both the hot- and cold-rolled surfaces were approximately smooth under the condition of initial oil of 0 mg/m2. However, under large initial oil conditions, many clear oil-pits were observed in cold-rolling, but in hot rolling, only a few small oil-pits were observed.
Comparison of sheet surface by 3D observation between hot and cold rolling.
The volume of the oil-pits was compared quantitatively. Figure 6 shows sectional profiles of the hot- and cold-rolled sheet surfaces. The base lines were set so that the areas above the base line could be almost the same. The volume of oil-pits was calculated the area under the base line, and was measured in a region of 278 mm × 208 mm. Figure 7 shows the effect of the initial oil amount on the volume of oil-pits. In cold rolling, the volume of oil-pits increases with the initial oil amount and becomes constant after the initial oil amount exceeds 1000 g/m2. In contrast, in hot rolling, the volume of oil-pits hardly increases when the initial oil amount increases. Figure 8 shows the relationship between the volume of oil-pits and the calculated friction coefficient. Figure 4 shows that the volume of oil-pits does not increase in hot rolling even if the initial oil amount increases. Therefore, it is suggested that the friction coefficient does not decrease because oil-pits do not increase.
Comparison of sectional profile between hot and cold rolling.
Effect of initial oil amount on volume of oil-pits.
Relationship between volume of oil-pits and calculated coefficient of friction.
Formation of oil-pits was confirmed in hot rolling when both the work roll surface and the specimen surface were fine polished and oxidation scale did not form on the surface. However, in cold rolling, a large number of clear oil-pits about 0.5 μm in depth and 50 μm in diameter formed when the remaining oil amount was 490 mg/m2, whereas in hot rolling, only a few oil-pits about 0.2 μm in depth and 20 μm in diameter were observed even when the remaining oil amount was 1060 mg/m2, which is more than double that under the cold rolling condition.
In cold rolling, oil introduced into the roll bite will form clear oil-pits by deformation of the sheet surface. However, in hot rolling, introduced oil may form a uniform oil-film at the interface of the work roll and the sheet because low viscosity oil at a high temperature cannot deform the sheet.
4.2. Comparison of Lubrication Mechanisms in Hot Rolling and Cold RollingIn order to discuss the lubrication mechanism in hot rolling, the lubrication mechanisms in hot and cold rolling were compared. In hot rolling, Azushima et al. reported that the friction coefficient decreases with increases in the oil film introduced in the interface between the work roll and the sheet in the region where the introduced oil film is lower than a certain value, and then becomes constant in the region exceeding that value. In the former range, the contact interface consists of a region covered with the lubrication film and a region without the lubrication film, and in the latter range, all of the contact interface is covered with the lubrication film. Against this model, in the present study, the behavior of the lubricant oil in the interface between the work roll and the sheet was discussed based on measurements of the remaining oil amount on the work roll after hot rolling and observations of the hot-rolled sheet surface.
Figure 9 shows the relationship between the oil film thickness and the calculated friction coefficient. The mean oil film thickness was determined by the sum of the remaining oil amounts of both the work roll and the sheet surface after hot rolling. In cold rolling, the remaining oil amount of the work roll and that of the sheet surface were approximately the same. In hot rolling, the remaining oil amount on the sheet surface was about 50 mg/m2, and it is thought that the oil burned.
Relationship between oil film thickness and calculated coefficient of friction.
The lubrication mechanism in cold rolling is mixed lubrication consisting of boundary lubrication and fluid lubrication. The friction coefficient decreases and oil-pits are formed as an increasing amount of oil film is introduced in the interface. In this study, the same results were confirmed, and the friction coefficient decreased monotonically in the range to an oil film of 1.0 μm. On the other hand, in hot rolling in which the rolling conditions such as the work roll roughness, rolling reduction and thickness of the sheet are the same as in cold rolling, the friction coefficient decreased only in the region to 0.3 μm and became constant even when oil film exceeded 0.3 μm. This corresponds to the observation results that few oil-pits were formed even when the oil film increased. In hot rolling, the friction coefficient does not seem to decrease by fluid lubrication, even if an oil film with a thickness of more than 1.0 μm exists in the interface between the work roll and the sheet.
As mentioned above, it is suggested that the lubrication mechanisms are completely different in hot rolling and cold rolling. Figure 10 shows a schematic illustration of the lubrication models and the states of the oil in the interface in hot and cold rolling.
Schematic illustration of lubricant model on hot and cold rolling.
In the case of hot rolling, in the region where the oil film is thin, as shown in Fig. 10(a), it is thought that there is both a dry lubrication area, in which the work roll is in direct contact with the work piece, and a boundary lubrication area, in which the work roll contacts the work piece through the oil film. Therefore, the friction coefficient μ is given by Eq. (1).
| (1) |
However, if we regard hot rolling in the same way as cold rolling, in the region where the oil film is thick, as shown in Fig. 10(b), it is thought that the dry lubrication area disappears and both a boundary lubrication area and a fluid lubrication area coexist; in this case, the friction coefficient μ is given by Eq. (2).
| (2) |
In cold rolling, the oil film thickness increases, and the fluid lubrication area with a small friction coefficient spreads, and as a result, the friction coefficient decreases. However, in hot rolling, fluid lubrication may not occur even if the oil film thickness increases. The reason for this phenomenon is presumed to be that the oil viscosity becomes extremely low in the high temperature interface between the work roll and the sheet, and then the oil film exists as a pseudo-solid without lubricity. Therefore, β equals 0 and μ equals μb regardless of the thickness of the oil film.
The features of the lubrication mechanisms in hot and cold rolling are as follows. In cold rolling, the oil film thickness increases and the fluid lubrication ratio becomes larger, so the friction coefficient decreases and oil-pits are formed. On the other hand, in hot rolling, the friction coefficient decreases sufficiently if direct contact between the work roll and the sheet is protected with even a thin oil film. Furthermore, even if the oil film thickness increases, oil-pits are not formed and the friction coefficient does not decrease. As the reason for this, it is suggested that the oil viscosity becomes extremely low in the high temperature interface, and as a result, the oil film no longer behaves as a fluid. That is to say, the oil film without fluidity does not show fluid lubrication behavior and shows boundary lubrication-like behavior.
In order to clarify the lubrication mechanism in hot rolling, the remaining oil amount on the work roll was measured and oil-pits on the sheet surface were observed in rolling experiments with work rolls and specimens having a roughness of less than 0.02 μmRa. The following knowledge was obtained by the comparison of hot- and cold-rolling lubrication.
(1) In cold rolling, the friction coefficient decreased as the remaining oil amount increased. In contrast, in hot rolling, the friction coefficient decreased sufficiently when the remaining oil amount was 150 mg/m2 and became constant at higher levels. This confirmed that different lubrication mechanisms exist in hot and cold rolling.
(2) In cold rolling, clear oil-pits increased as the remaining oil amount increased. However, in hot rolling, only a few small oil-pits were observed when the remaining oil amount exceeded 1000 mg/m2.
(3) The difference in the formation mechanism of oil-pits was considered to be a cause of the difference in the lubrication mechanisms in hot and cold rolling. In particular, in hot rolling, the oil viscosity is presumed to decrease remarkably in the high temperature interface.
