Forming Processing and Thermomechanical Treatment
Sticking in Hot Rolled Sheet of Ferritic Stainless Steel
2020 Volume 60 Issue 8 Pages 1732-1736
Details
2020 Volume 60 Issue 8 Pages 1732-1736
In hot rolling of ferritic stainless steels, prevention of sticking to rolls is very important from the viewpoint of productivity. However, the formation mechanism of the sticking has not been clarified sufficiently. Therefore, in this work, rolling experiments were carried out using a tribo-simulator. The results clarified the following points: Sticking occurs more easily on ferritic stainless steel than on high strength steel. On ferritic stainless steel, a work roll sticks with the hot-rolled sheet at the entrance of roll-bite, and the work roll then moves forward on the hot-rolled sheet. Therefore, it is thought that the surface layer of the hot-rolled sheet is fractured by large frictional shear stress, and the work roll stuck with the fractured layer advances further on the sheet, forming defects with an accumulated fractured surface layer. Lubrication with oil is effective for prevention of sticking between the work roll and the hot-rolled sheet.
It is well known that sticking defects form easily on the strip surface in hot rolling of ferritic stainless steel containing a large amount of chromium.1,2) When these defects occur on a strip, it is necessary to exchange the roll, which increases operational costs related to work rolls. Sticking defects also increase the process load for removing defects in the following descaling process, and this reduces production efficiency and strip yield. For these reasons, there is great demand for clarification of the formation mechanism of surface defects during hot rolling of ferritic stainless steel and establishment of preventive techniques.
Kato et al.3,4,5) studied the formation of surface defects on rolls during hot rolling of stainless steel using an Amsler type hot abrasion device, and reported that the surface of the stainless steel breaks due to friction with the work roll, forming micron-order flaky particles which are transferred to the roll surface. This process is then repeated, and the particles accumulate in layers and grow into surface defects. It was also reported that work rolls containing a large amount of graphite are effective in preventing surface defects. On the other hand, Toriumi and Azushima6) reported that lubrication oil containing a sulfur additive was effective for preventing surface defects based on experiments using a sliding type hot rolling lubrication simulator.7,8)
Recently, demand for ferritic stainless steels containing a large amount of chromium as an alternative to austenitic stainless steels has increased from the viewpoint of resource saving. For this reason as well, clarification of the formation mechanism and establishment of preventive methods are increasingly important for stable production and prevention of surface defects during hot rolling of ferritic stainless steel.
Some reports have investigated the causes and mechanisms of surface defects on work rolls, however no reports have examined surface defects on the strip surface. The reason is considered to be as follows: As will be described later, sticking defects of work rolls are often formed in the first half of the finishing mill, while the strip surface can be observed only after the final stand. In other words, it is not possible to observe the strip surface immediately after rolling by a work roll with sticking defects in the first half of the finishing mill. As another problem, a rolling simulator that reproduces sticking defects in steel sheets has not been developed.
Since sticking defects in strips greatly affect production efficiency and manufacturing costs, it is very important to clarify the formation mechanism and propose preventive methods. Therefore, in this study, the authors observed sticking defects on hot strips in the production line, and attempted to reproduce the sticking defects that occurred in actual strips by using a sliding type hot rolling lubrication simulator. Based on the results, we estimated the formation mechanism of sticking defects and proposed a preventive method.
First, surface defects of work rolls and hot strips in the production line were investigated. In the hot rolling of a ferritic stainless steels, the surface defect size and depth vary greatly. As an example of significant sticking defects, Fig. 1 shows a photograph of the work roll surface of the front stand of a finisher mill, and Fig. 2 shows the profile of the work roll surface. (The measurement result of the work roll replica is displayed inverted. That is, a positive value corresponds to a concavity of the work roll.) The concave part of the work roll is a stripe-like defect having a width of about 1 to 2 mm and extending in the circumferential direction, and the roll base drops off at a depth of about 20 to 30 μm. This is the same result as in the reports by Kato et al.4,5) Figure 3 shows the surface appearance of a ferritic stainless strip after rolling to the final stand and pickling. The defects in the strip are elongated streak-like defects corresponding to the work roll defects. Figure 4 shows the results of cross-sectional observation, and Fig. 5 shows the results of an EPMA analysis of the surface defects on the strip. Since the indented object has the same composition as the steel sheet, it is judged that the oxide film (scale) on the strip surface and the steel itself are indented into the strip surface. Moreover, because sticking defects in a work roll are observed in the front half of the finishing mill, the defects in the strip are considered to be elongated five to ten times by subsequent rolling. Nevertheless, the sticking defect is a deep defect with a depth of over 30 μm.
Photograph of work roll surface after hot rolling of ferritic stainless steel strip.
Profile of work roll replica.
Photograph of ferritic stainless steel strip surface after hot rolling and pickling.
Photograph of cross section of surface defect.
Element mapping of surface defect by EPMA.
As described above, this study clarified for the first time that the linear defects formed in hot strips of ferritic stainless steel are defects in which material originating from the oxide film and steel sheet itself are intended into the strip surface.
In order to reproduce the sticking defects in the strip and discuss the formation mechanism and preventive methods, it is important to observe the sheet surface immediately after formation of the sticking defect. Although sticking defects on the work roll can be reproduced with the Amsler type abrasion device used in Kato’s study,3,4,5) this device cannot reproduce sticking defects in steel sheets, as hot-rolling of steel sheets is not possible. Therefore, in this study, we attempted to reproduce sticking defects in steel sheets by sliding rolling experiments using the hot rolling lubrication simulator developed by Azushima et al.7,8) Since this tribo-simulator can increase the sliding speed difference between the work roll and the steel sheet to the same extent as that of the front stand of the finisher mill in the production line, it was considered possible to reproduce the severe deformation generated in an actual production line.
Figure 6 shows the appearance of the tribo-simulator. The Main stand (1) has a work roll diameter of 100 mm, a maximum speed of 207 m/min, a load capacity of 200 kN, and an allowable torque of 800 Nm, and only the upper work roll is driven. The Sub-stand (2) has driven upper and lower work rolls and conveys the work piece at 8 to 32 m/min. The roll speed of the Main stand and Sub-stand can be set to a speed ratio of 6.3 to 24 by the continuously variable transmission. The distance between the Main stand and the Sub-stand is 1400 mm. The Furnace (3) is an infrared image furnace rated at 48 kW and can heat the work piece up to 1373 K. The Tensioning device (4) on the outlet side can be tensioned up to 3.5 kN by an air cylinder. It is possible to measure the torque and rolling load of the upper work roll on the Main stand.
Photograph of the tribo-simulator.
High Cr ferritic stainless steel (SUS444) and 590 MPa high strength steel (HSS) were prepared as work pieces. The high Cr ferritic stainless steel was Type 444 SUS containing 19.4 mass% Cr, 1.8 mass% Mo, and 0.4 mass% Nb. The HSS used for comparison was 590 MPa grade containing 0.2 mass% Si and 1.8 mass% Mn. The dimensions of the work pieces were 9 mm in thickness, 22 mm in width, and 3000 mm in length. The materials for both work pieces were prepared by melting ingots to the specified composition by vacuum melting, followed by hot rolling to a thin thickness. The work pieces were then prepared by laser cutting and surface grinding. High-speed steel with a surface roughness of about 0.2 μmRa was used as the work roll.
Figure 7 shows the experimental procedure.7,8) As shown in Fig. 7(a), the work piece is first set on the table and a load is applied at the Sub-stand. Then, the work piece end is clamped with the chuck part of the tension device, and a load is applied. The work piece is heated at 1073 K for 7 min using the infrared image furnace. Secondly, as shown in Fig. 7(b), the work piece with front tension is moved to the Main stand by rolling in the Sub-stand (rolling velocity of Sub-stand: V). Next, as shown in Fig. 7(c), as the heated zone of the work piece reaches the Main stand, the heated work piece is compressed by the upper roll, and the work piece is rolled at a constant sliding speed (rolling velocity of Main stand: U). Under these conditions, the rolling force P and the upper roll torque G are measured. The coefficient of friction can be calculated from P and G by using the following Eq. (1), where R is the roll radius.
| (1) |
Schematic illustration of experimental procedure.
In this experiment, the Sub-stand speed V was set to 2 m/min, the Main stand speed U was set to 12 m/min or 24 m/min, and the sliding speed ratio γ was varied between 6 and 12. Reduction was 0.3 mm. Sliding rolling was performed by supplying cooling water between the work roll and the work piece. The friction coefficient was calculated under each condition, and the longitudinal section of the work piece after rolling was observed.
The formation of sticking defects with the SUS444 and HSS work pieces was compared. The rolling conditions were V=2 m/min and U=24 m/min (γ =12). Figure 8 shows the experimental results. In the rolling test using SUS444, the test could not be completed normally because large noise occurred during hot rolling and the friction coefficient exceeded 1. Figure 9 shows a comparison of the longitudinal sections of the work pieces after rolling, and Fig. 10 shows the appearance of the SUS444 after rolling. With the SUS444, significant sticking defects with large irregularities were formed. Thus, formation of sticking defects was much easier with SUS444 than with HSS.
Comparison of experimental results.
Comparison of longitudinal sections of work pieces.
Photograph of SUS444 surface after hot rolling.
The following three points were estimated as possible reasons for easy formation of sticking defects with ferritic stainless steel: (1) Ferritic stainless steel has a very thin oxide film with a lubricating effect. (2) The corundum-type Cr2O3 oxide film formed on ferritic stainless steel has poor plastic deformability. (3) Since the compositions of the work roll and the work piece are similar, adhesion is likely to occur.
Figure 11 shows the longitudinal section of the SUS444, and Fig. 12 shows the results of element mapping by EPMA of the sticking defect of the SUS444. It is observed that the steel sheet surface layer with a thickness of about 80 μm is peeled off from the base metal and falls forward. This peeling of the surface layer seems to break not in the grain boundaries but in the grains. From the EPMA analysis, the composition of the folded part was the same as the composition of the rolled sheet, and not the work roll.
Photograph of longitudinal section of SUS444.
Element mapping of SUS444 by EPMA.
Based on the above observation results, the formation mechanism of the sticking defects in which the oxide film was pushed into the steel sheet was estimated. Figure 13 shows a schematic illustration. Sticking occurs between the work roll and the work piece due to the high pressure at roll-bite. After sticking occurs, the sticking part tries to advance further on the sheet because the work roll moves forward against the steel sheet. Very large shear stress exceeding the fracture stress is generated in the steel sheet surface layer, and the surface layer peels off from the base material. As a result, the peeled layer becomes a defect that is folded in front of the steel sheet. At this time, the oxide film (scale) on the sheet surface is also entangled with the peeled layer and forms a defect consisting of the accumulated fractured surface layer and scale. The defects are then elongated by subsequent rolling, and defects like those shown in Fig. 4 are formed. Here, it should be noted that the slip speed difference of 10 m/min in this experiment can easily occur in actual production lines.
Schematic illustration of the stacking formation mechanism.
As mentioned above, we succeeded for the first time in reproducing the formation of the linear defects of steel sheets observed in hot strips of ferritic stainless steel manufactured on actual production lines. Furthermore, we also estimated the formation mechanism based those results.
Considering the defect formation mechanism proposed above, reducing the speed difference between the work roll and steel sheet and reducing the contact pressure between the roll and sheet are considered effective measures for preventing these defects, but adoption of these measures may be difficult from the viewpoint of productivity. Therefore, prevention of sticking defects in steel sheets by supplying a lubricant oil was verified.
The same type of lubricant oil containing a sulfur additive as in Toriumi and Azushima6) was used, and a 10% emulsion was supplied between the work roll and work piece. The emulsion amount was 800 mL/min. The effectiveness of lubrication using this emulsion and water lubrication in preventing sticking defects during hot sliding rolling of SUS444 was compared under rolling conditions of V=2 m/min and U=12 m/min (γ=6). Figure 14 shows the experimental results. In the case of water lubrication, large sticking defects that caused abnormal noise occurred. However, when the emulsion containing 10% lubrication oil was supplied, the rolling state became stable, μ was 0.39, and sticking defects were not formed in the steel sheet. Thus, it became clear that supplying lubricant oil is very important for reducing the friction coefficient and preventing sticking defects in hot rolling of ferritic stainless steel.
Comparison of experimental results. (Online version in color.)
Authors investigated the sticking defects that occur in steel sheets during hot rolling of ferritic stainless steel, clarified the following points, and obtained useful knowledge for improving the production efficiency and yield of ferritic stainless steel.
(1) In hot strip manufactured on actual production lines, longitudinal linear defects in which the steel itself and the oxide film (scale) are indented into the strip surface were observed.
(2) Hot-rolling experiments with a tribo-simulator successfully reproduced defects like those that formed on actual hot strips. On ferritic stainless steel, the work roll sticks with the hot-rolled sheet at the entrance of roll-bite, and the work roll then moves forward on the hot-rolled sheet. Therefore, it is thought that the surface layer of the hot-rolled sheet is fractured by large shear strain, and the work roll stuck with the fractured layer advances further on the sheet, forming a defect consisting of the accumulated fractured surface layer and scale.
(3) The formation of sticking defects in steel sheets could be reduced by supplying a lubricant oil containing a sulfur additive.
