This paper presents the results of laboratory tests and their validation, as well as field investigations, leading to a new research roll cover material.
Several experimental tests were conducted in two different configuration laboratories, simulating the rolling process of a three-roll configuration (strip simulation roll – work roll – support roll).
The relationship between the suppression of orange peel surface defect formation in strip steel and the graphite high-speed steel rolls was clearly defined. Through extensive and close collaboration between TAL and Tata Steel, an important step has been taken in the development of wear-resistant roll materials for surface-critical applications.
introduce
The hot rolling process is a critical process stage in the metallurgical industry, which directly affects the intrinsic and surface quality of the rolled product.
The formation of orange peel-like surface defects on rolled steel strip is a major challenge in achieving the desired product quality.
This paper presents the results of extensive laboratory tests, followed by validation and field investigations that enabled the development of a new roll shell material. The causes of orange peel surface defects during the rolling process of graphite high-speed steel rolls were investigated.
The collaboration between TAL and Tata Steel played a key role in achieving significant progress in the improvement of roll materials for surface critical applications. Two different laboratory test configurations were used to simulate the rolling process. These configurations included a three-roll arrangement of a roll simulating the strip, a work roll and a support roll to replicate the conditions occurring during the simulated rolling process. Various parameters were systematically varied to understand the material behavior and the key factors contributing to the formation of orange peel surface defects were identified. The test setup simulated the conditions encountered during the rolling process and provided valuable insights into the material behavior and performance.
In addition, a series of laboratory tests such as thermal fatigue tests were performed to understand and mitigate the challenges posed by graphite high-speed steel rolls. The results of the laboratory tests were verified through field investigations to ensure the suitability and reliability of the developed roll cover material in real-world scenarios. The collaboration and sharing of expertise and resources enabled a holistic approach to material development that ultimately led to success.
Experimental Procedure
The flowchart acts as a roadmap, providing a clear and structured overview of how the individual components or R&D stages are connected. Each step in the diagram corresponds to a specific action or decision and guides the entire sequence of activities. Sometimes during material development, situations arise that are not suitable for the expected results, so the path is no longer taken, as shown in Figure 1.
The process starts with identifying and defining specific requirements, outlining the criteria that the entire process must meet. Thermodynamic calculations are then performed to create numerical and theoretical foundations for subsequent steps, respectively. This is followed by a description step, where relevant properties and characteristics that are critical to the project goals are evaluated.
The process then transfers to the laboratory casting of test rolls, where the theoretical knowledge is put into practice. The results of the laboratory castings are evaluated with the help of investigations and further characterization steps in order to make adjustments if necessary. The scale-up process is the final stage to apply the results on a larger scale.
The development process continues with wear tests on the manufactured rolls to study the performance under specific conditions. Investigations of the rolls and field evaluation of the roll performance will provide real insights to validate the laboratory results and determine further development directions for the project.
Thermodynamic calculations
Thermodynamic modeling of multi-component alloys is of great importance for the development of different steel grades. Although such software is somewhat limited in the case of cast iron, some studies have addressed white cast iron casting, however, even fewer applications have been made to thermodynamic modeling of graphite cast iron.
Thermodynamic modeling of various high alloy cast irons has been performed using the database. These alloys contain carbon, silicon and carbide forming elements such as chromium, tungsten, vanadium and niobium. The microstructure consists of graphite, cementite and other carbides. The coexistence of carbides and carbon in graphite poses a challenge to the thermodynamic calculations of the CALPHAD method.
In this project, the thermodynamic modeling used the chemical compositions shown in Table 1 (all elements are given in mass%). Due to good agreement, the same phases were used to predict the alloy structure: liquid phase, face-centered cubic structure FCC_A1 (austenite, MC-carbides), cementite CEMENTITE, graphite GRAPHITE, M7C3, M6C, HCP_A3 (M2C-carbides), body-centered cubic structure BCC_A2 (ferrite) and M23C6. Equilibrium calculations were performed between 1726°C and 500°C, starting from the full liquid phase, as precipitation is unlikely to occur at this temperature due to the low diffusion rate.
Figure 2 shows the microstructure of Material 1 as a scanning electron microscope (SEM) image recorded with a backscattered electron (BSE) detector at a magnification of 300 times, as well as the measured (microstructure) values. The microstructure of this material consists of a martensitic matrix with a certain amount of retained austenite, cementite and graphite.
Equilibrium calculations for Material 1 yielded a property diagram (Figure 3) showing the formation of different phases at different temperatures.
The contours of material 1 illustrate the effect of the reduction in carbon content due to graphite formation (red line in Figure 4).
Due to the reduction in carbon content, the primary phases precipitate at higher and lower temperatures, respectively, and their fractions change, which can be estimated.
The choice to abandon the graphite phase is supported by calculations, and the results show that the alloy system should be treated as white cast iron. Comparison of the calculated and measured values of the phase quantities of Material 1 shows good agreement, although there are some deviations due to the suspended graphite phase. All calculations are based on these boundary conditions to predict the phase type, precipitation temperature and other relevant parameters described previously.
Laboratory casting
The tests were carried out in an induction furnace, as shown in Figure 5. 50 kg of the base material was melted (1) with a total heating time of about 3 hours. The material was heated to approximately 1500 °C to achieve a homogeneous nucleation state and then cooled to 1420 °C (2). Once this temperature was reached, the chemical composition was checked. After making necessary corrections to the chemical analysis, a thermodynamic analysis was performed to determine the liquidus temperature (= TL) of the melt. At a temperature of 200 °C above TL, the melt was poured into a ladle while the melt stream was inoculated (3). The entire amount of inoculant was added in the first third, as the turbulence of the incoming melt allowed for good diffusion and mixing of the inoculant. After the required 16 kg had been poured into the ladle and the inoculant had dissolved, the final sample was taken and the temperature was continuously monitored (4). Due to the small amount of steel being tapped (16 kg), the ladle was preheated to 700 °C to prevent rapid cooling of the melt. The melt was poured into sand moulds at a specified temperature above TL (5). Immediately after casting, the mould was covered with vermiculite to prevent rapid cooling of the mould (6). For each melt test, three 16 kg samples were cast. After cooling to room temperature, the specimens were removed from the mold. Two cuts were made using a disc cutter (7) and then the specimens were knocked out. These specimens were first ground, lapped and finally polished for analysis.
During the inspection, only a part of the sample extracted from the center was analyzed. The content of graphite and carbides was determined using an optical microscope and evaluation software. The graphite content was measured at two different distances from the casting surface, 9 and 21 mm. For each measuring point, 15 separate images were taken at 50 times magnification (1310.72×983.04μm) and analyzed by the software. As shown in Figure 6, this corresponds to an inspection area of about 4×5 mm at a depth of 9 and 21 mm below the casting surface.
During the inspection, only a part of the sample extracted from the center was analyzed. The content of graphite and carbides was determined using an optical microscope and evaluation software. The graphite content was measured at two different distances from the casting surface, 9 and 21 mm. For each measuring point, 15 separate images were taken at 50 times magnification (1310.72×983.04μm) and analyzed by the software. As shown in Figure 6, this corresponds to an inspection area of about 4×5 mm at a depth of 9 and 21 mm below the casting surface.
Expanding the pilot project
If the comparison of the microstructure of the laboratory castings with the thermodynamically calculated values is positive, a larger-scale test is planned. For this purpose, a centrifugal casting mold is prepared and only one shell is cast (see Figure 7). The resulting sleeve is then sampled, examined and compared with the results of the laboratory tests.
In addition to the measurement of graphite and carbides by optical microscopy, quantitative image analysis and SEM examinations were performed. Equipped with a scanning electron microscope. An energy dispersive X-ray (EDX) system was used, with two 200 mm2 silicon drift detectors (SDD). The experiment used an accelerating voltage of 30 kV, a beam intensity of 14, 3 frames and a detection time of 300 μs, and every pixel was detected. Phase analysis was performed with a scanning electron microscope, as shown in Figures 8 and 9. Figure 8 shows the microstructure of graphite high-speed steel. The BSE micrograph obtained by backscattered electrons highlights the tempered martensitic matrix (light gray area), surrounded by a high content of carbides (dark gray and white areas) and graphite particles (black particles).
The different gray shades of the carbides correspond to different chemical compositions, with darker phases containing more elements with lower atomic mass and lighter phases containing heavier elements.
EDX analysis performed on the same frame reported in Figure 8 confirms the presence of cementite (greenish green), graphite (dark gray), Nb-based (green) and V-based (red) MC and Mo-based (yellow) M2C/M6C carbides, as shown in Figure 9.
If all investigations and characterizations prove promising, the next step can be to move to casting a prototype roller.
Prototype Roller
Cast composite work rolls are mostly produced by centrifugal spin casting. The melt for the shell is poured into a rotating mold, and the centrifugal force distributes the melt on the inner surface of the mold. When the centrifugally cast shell solidifies, the roll core metal melt is poured into the assembled mold and forms a permanent bond between the shell and core materials, as shown in Figure 10.
Wear test
A custom designed twin roll test rig has been built and modified with a third roll to simulate the support roll commonly used in hot strip mills. In addition, a load sensing control system has been designed so that the contact force between the load roll and the test roll remains constant during the test. Test parameters including roll slip (i.e. sliding between the rolls) and the temperature of the load roll are predefined via the intuitive visual touch screen of the control unit. During each test run, the parameters are continuously recorded at a frequency of 1 Hz.
The test roll consisting of the material to be tested was fixed on the upper drive shaft. The induction heated load roll, simulating the strip/slab, was fixed on the lower drive shaft. The support roll was not driven. During the wear test, the surface temperature of the heated load roll was continuously measured using a pyrometer. On this basis, the induction heater was controlled to keep the roll surface temperature constant. The contact force between the rolls was calculated according to the Hertz contact theory. During the test, the actual force was continuously measured using a load sensor and the pressure was adjusted using a cylinder acting on the upper drive unit. Therefore, the contact force between the load roll and the test roll was practically constant in each test run. The wear tests were carried out at load temperatures between 150°C and 900°C. The slip values between the test roll and the load roll were 0-12%. The nominal contact pressure between the test roll and the load roll was 1000 MPa and between the test roll and the support roll was 1250 MPa. These test parameters are based on field measurements performed at the Tata DSP plant. Since the parameters were controlled during the test, it is reasonable to assume that the parameters are constant. In order to evaluate the influence of cooling water (corrosion) on the deterioration of the work rolls, wear tests were also carried out using a rolling simulator. The most significant differences between the two mills are listed in the figure.
After the test, each roller was sectioned. One of the cross-section surfaces was polished and the wear track was studied under the test conditions.
To analyze the worn surface and the polished cross section, each test is displayed tilted as shown in the figure. In each micrograph, the boundary between the worn surface and the polished cross section is marked with an orange line. To evaluate the wear performance, the microstructure of the worn surface and the subsurface must be considered. Based on these measurements, the maximum wear depth was determined in the center of each wear track after each test run.
Tata DSP field survey and performance evaluation
The roll surfaces were analyzed using a dedicated optical microscope. The figure shows the setup used in the roll shop of the DSP mill. Micrographs of the roll surfaces were taken after cleaning (degreasing the surface) or after etching in a light 5% nitric acid to remove surface oxides formed during hot rolling.
Results and Discussion
A total of 78 variations were calculated. After this theoretical phase, 42 variations were cast in the laboratory. In parallel, 8 selected variations were cast into a sleeve in the foundry. A total of 15 prototype rollers were cast, contributing to the further development and practical application of this technology. This multifaceted approach includes both theoretical calculations and practical applications to improve understanding and implementation in various fields. This section explains the results of the wear measurement process in more detail, with the goal of providing an in-depth analysis of the wear characteristics and patterns, which in turn provides a deeper understanding of the performance and durability of the materials involved.
Collaboration in the areas of field investigations and performance evaluation is extremely important. This partnership allows us to conduct in-depth analysis of performance indicators and ensure a thorough evaluation of the real-world impact of the developed materials. The joint efforts with Tata DSP provide valuable data on how materials behave under real-world conditions, increasing the applicability and reliability of the research results. This collaborative approach emphasizes the commitment not only to theoretical innovations but also to practical solutions in the field of materials science and engineering.
Wear test
At 150°C, the main driver of the improved wear resistance is cementite; MC and M2C/M6C carbides play a smaller role. The maximum wear depth decreases with increasing total cementite content. The wear depth is highest for variant 31, which has the lowest cementite content. At relatively low contact temperatures and under poor oxidation conditions, the presence of an extended eutectic carbide network improves the load-bearing capacity of the roll surface, thereby reducing wear.
The adhesive wear mechanism was established, and the effect of carbide hardness on adhesive wear was small. At high contact temperatures (750℃ and 900℃), the formation of a protective oxide layer was more obvious, and the effect of carbide type on hardness and oxidation behavior was more important. At 150℃, the wear depth increased with the increase of MC and M2C/M6C carbide content (i.e., the increase of cementite content); at 750℃ and 900℃, the maximum wear depth decreased with the increase of MC and M2C/M6C carbide content.
At high temperatures, the hardness of the metal matrix is relatively low (compared to 150 ° C), and the preferential wear of the matrix is more obvious. The oxidation, compaction and sintering of wear debris lead to the formation of a dense oxide layer on the matrix. The carbides, especially MC and M2C/M6C, have a higher hardness than the metal matrix and oxides. However, the carbides clearly protrude from the contact surface, hindering the formation of a smooth wear surface. In this sense, it can be explained that the influence of hard carbides is greater at high temperatures, and it can be concluded that a certain content of MC and M2C/M6C is the basis for obtaining sufficient wear resistance.
Experimental tests at 900 °C showed slightly better wear behavior. The roughness of the worn surface increases significantly with increasing MC and M2C/M6C contents. As a result, in grades containing only MC and M2C/M6C, the formation of sharp surface protrusions prevents the formation of a smooth contact surface and leads to a stronger deterioration of the surface finish at high temperatures. This is a very decisive aspect for rolls installed in the final finishing stand of a hot rolling train. This effect is particularly important for MC, which shows a higher oxidation tendency compared to M2C/M6C. Cementite shows a lower oxidation tendency compared to MC and M2C/M6C, but the lower hardness of this carbide reduces the surface protrusions and makes the worn surface profile smoother. After a running-in period, the differences in wear depth between the steel grades can be clearly seen. At high temperatures, the V19 alloy, which contains only small amounts of MC and M2C/M6C, has a lower wear resistance than the V17 and V31 alloys, but a better surface finish (lower surface roughness). Therefore, low ratio fractions of MC and M2C/M6C and high ratio fraction of cementite are generally beneficial to the surface finish of the roll.
A balance needs to be struck between MC, M2C/M6C and cementite to achieve less wear and avoid unwanted oxidation behavior during hot rolling. Often, micro-chips and micro-cracks must also be considered, where the cementite structure shows its disadvantages.
Conducting field surveys at Tata DSP
Finishing rolling F3 stand:
The main drivers of roll degradation in the F3 stand of the finishing mill are thermal fatigue, contact fatigue and corrosion. Wear cannot be assessed by surface inspection and therefore cannot be excluded from the wear mechanisms. Two versions of the F3 stand, V31 and V19, were analyzed and compared. Although the rolled steel grade and rolled strip length differed significantly between the two materials, the following observations were made. V31 has a very rough surface, is highly oxidized and seems to be sensitive to oxidation of the rolls and corrosion from the cooling water. V19, although analyzed after a very short rolled strip length (worn after 8 km strip length) and rolling electrical steel (very heavily worn compared to mild steel), showed a very smooth surface and some oxide particles from the strip were observed on the roll surface. This material has a low degree of surface oxidation and carbides can still be clearly seen.
Finishing rolling F4 stand:
Roll degradation in the F4 stand followed the same mechanisms but to a lesser extent compared to the F3 stand. Rolls from two rolling cycles were analyzed and the V19 had relatively similar surface degradation compared to the reference version, albeit slightly rougher. However, contact fatigue and thermal fatigue were slightly lower on the V19. V17 and rolls from a second roll supplier performed similarly but had a higher degree of surface degradation compared to the reference or V19. This is most likely related to the higher sensitivity of these roll materials to oxidation/corrosion.
Finishing rolling F5 stand:
On the finishing stand F5, 11 rolls were analyzed. The roll deterioration mechanisms highlighted were contact fatigue and corrosion (wear could not be assessed). In contrast to the previous stands, no thermal fatigue phenomena were observed on the rolls from the F5 stand. Version 19 maintained a very smooth surface and a (very) thin oxide layer. This roll material showed almost no deterioration. V19 showed similar trends to the special version, but with a slightly higher degree of deterioration and oxidation of the rolls.
Performance Evaluation
These results provide insights into the potential impact of different types and amounts of carbides on the roll surface on the surface quality. Increasing the volume fraction of MC and M2C/M6C carbides in the microstructure helps improve wear resistance and thus the rolling performance of the roll. However, this also leads to higher surface roughness after the roll reaches the end of its service life. This is due to the different oxidation levels, higher hardness and better wear resistance of MC and M2C/M6C carbides compared to matrix and cementite carbides. As MC carbides are embedded in the matrix, the roll surface profile after the rolling cycle has more spikes, which causes the formation of an orange peel defect effect on the worn surface. Figure 14 shows the progress of the orange peel defect effect during the three-year project. The severity of the orange peel defect, measured by the Parsytec strip surface inspection system camera, is shown in the figure for values greater than 2 (≥2) on the SH scale. From January 2021, various measures were implemented and new customized grades were introduced at the DSP plant. The launch of V19 is considered a breakthrough. This graphite HSS variant was tested in spring 2021 and showed low orange peel defect formation. In late summer 2021, mainly standard roll versions were used, which led to an increase in orange peel defects in the strip.
In contrast to the results for V19, V31, although labeled as “highly wear resistant”, showed unpleasant orange peel effects and did not meet the critical requirements for the strip surface. The results for the different steel grades are shown in Figure 15.
Figure 15 shows the wear, with the measurement length plotted along the x-axis (corresponding to the z-axis of the grinder). The profile measurement starts at 100 mm from the edge of the work roll. Considering a work roll width of 1680 mm, the effective measurement width is 1480 mm (calculated as 1680-2×100), and the interval measurement value is 5 mm.
The total wear is summarized by dividing by the kilometers of rolled strip length and then normalized to allow comparison with the wear of 10 kilometers of rolled strip length for each version grade. It can be clearly seen from Figure 15 that V19 has the highest macroscopic wear, while V31 performs excellently and has the best wear resistance among the three versions.
