Reducing rolling mill accidents, improving the accident resistance of roll materials, and conducting comprehensive surface wave + eddy current flaw detection on the rolls before they are put on the machine can avoid the peeling of the rolls caused by the expansion of surface cracks.
Keyword：Cold Rolled Work Rolls
Spalling of the working layer is a common failure form of cold rolling rolls, accounting for more than 50% of the normal failure of working rolls. According to the classification of the cause, the peeling of cold rolling rolls is mainly divided into peeling caused by surface cracks, peeling caused by subsurface material defects and peeling caused by contact stress. Usually, spalling caused by surface cracks, fatigue propagation bands are visible in the spalled area of the roll surface. The fatigue band track can be from a few inches long to several laps around the roll, sometimes with a bright (friction) or dull (oxidation) On the outside, the propagation direction of the main fatigue crack is generally opposite to the rotation direction of the roll, and surface cracks can usually be found by checking the peeled block or the roll surface of the non-stripped area.
The drawing design specification of the peeling roller studied in this paper is Φ535 mm X 1219.2 mm X 3523.35 mm.
The material is Cr5.
The main technical requirement is that the quenching hardness of the roll body is 94-98 HSD.
Hardened layer depth ≥ 32.5 mm. Scrap hardness ≥ 90 HSD.
Case:
The 1220mm cold rolling unit in a certain factory was running at a steady speed of 1100m/min in the coil. During the normal production process, the 4# and 5# racks had broken belt alarms. After stopping the inspection, it was found that the 3# rack entrance was seriously piled up with steel. Inspection of the rolls of the lower machine found that a large area of peeling occurred on the roll surface of the lower work roll of the 3# frame, and the peeling depth was about 20mm. Indentations and cracks are visible to the naked eye, and the edge of the lower back-up roll is obviously crushed.
The original diameter of the peeling roller is Φ535mm, and the current diameter is Φ485.9mm. The overall failure appearance of the peeling roller is shown in the figure.
Cause Analysis and Improvement of Fatigue Spalling of Work Rolls in Cold Rolling
- Morphological analysis
It can be seen from the figure that the peeling area is a “bridge” peeling morphology composed of two peeling blocks and the middle non-peeling area, which rotates clockwise along the non-peeling area A, and there is an obvious fatigue expansion track on the back of the roller.
Ultrasonic testing results show that the crack has extended from zone A along the circumferential direction for a week. Although the spalling area on the front side has not completely fallen off, due to the expansion of surface cracks, voids have occurred under the bridge. The results of ultrasonic testing and the direction and depth of crack propagation are shown in the figure.
- Hardness testing – work rolls
The original diameter of the peeling roller is Φ535mm, and the diameter is Φ485.9 mm when peeling off, and the depth of the scrapped hardened layer requires 8mm on one side. The hardness of the peeling area (as shown in the box area in Figure 3) and the non-flaking area of the roll were tested respectively. The test results showed that the hardness of the roll surface in the non-rolling area and the non-flaking area of the roll was basically 92-93 HSD, which still meets the The technical requirements of the roll drawings (94-98 HSD, scrap #90 HSD), the hardness of the roll surface (24 mm from the surface) in the peeling area is about 84 HSD, it can be seen that the depth of the peeling area of the roll is already located in the soft-hard transition area inside the roll.
- Exfoliation source analysis
From the results of ultrasonic testing in Figure 2, it can be seen that the internal fatigue cracks of the roll have extended along the circumferential direction for one week, and the crack source should be located in the A zone where the crack depth of the ultrasonic testing is 0. The opposite direction of rotation and the roll surface form an angle of approximately 90°, and fatigue propagation occurs toward the inside of the roll. When the crack penetrates the hardened layer of the roll and propagates to the soft-hard interface (about 29 mm deep from the roll surface). Due to the good toughness of the internal structure of the roll, the cracks no longer continue to expand inward but diffuse along the circumferential direction until the strength of the roll drops to the extent of spalling after one circle around the roll body, and the roll rapidly loses stability under the action of rolling stress. Thus, a large area of exfoliation occurred. After further coloring flaw detection of zone A, it was found that there was an axial crack on the roll surface of this zone, and the crack was located on the same circumference as the fatigue extension strip on the back side, so it was inferred that this crack was the source of the surface crack that caused the spalling accident .
Subsequently, the surface metallographic and SEM inspections were carried out on the source of the crack as shown in Figure 4. The test results showed that there were no obvious metallurgical defects on both sides of the crack and the surrounding structure, as shown in Figure 5-6. There is a slight secondary tempering phenomenon in the metallographic structure at the right end of the crack, and the cause of the crack may be related to the local slight thermal shock on the roll surface.
(a) Morphology of exfoliated block after high temperature tempering (b) The appearance of the back of the exfoliated block
- Anatomical analysis
In order to further analyze the cause of roll spalling and reveal the process of crack fatigue growth, the unshed spalled blocks on the roll surface of the roll were opened by high temperature tempering, as shown in Figure 7. It can be seen that there are two fatigue strips extending from zone A on the back of the spalled block, and the fatigue extension strip II is connected with the main extension strip on the roll surface.
wire cutting according to the scribed position shown in Figure 8(a). Two test blocks as shown in Figure 8(b) were obtained, and the cross-sectional morphology of the two test blocks is shown in Figure 9. It can be seen that there are obvious crack propagation paths from the surface to the inside on the cross-section of the two test blocks. After the cracks were generated, they propagated inward along the direction of about 20°-30° from the roll surface. When it expands to the soft-hard interface layer with better toughness, it will not continue to expand radially. And began to expand along the circumferential direction,. Due to the difference in the axial position of the crack and the angle of inward propagation, two fatigue strips are formed as shown in Fig. 8(a). Among them, the crack of test block ② in Fig. 9 extended to the soft-hard junction area and continued to expand along the circumference to form the fatigue strip II in Fig. Fatigue strips in Fig. 8(a).
In order to further analyze the cause of surface cracks. Carry out wire cutting again on the test block ① according to the scribing scheme shown in Figure 10. From the surface crack source along the crack propagation track, the metallographic observation from the surface to the inside is carried out. The test results show (as shown in Figure 1112) that the surface and inside structures on both sides of the crack are tempered martensite + point granular carbides , no metallurgical defects such as carbide segregation and non-metallic inclusions were found. It can be seen that the cause of cracks has nothing to do with the original metallurgical quality of the roll, but is related to the subsequent use and maintenance.
- Discussion
Comprehensive ultrasonic testing, hardness testing and anatomical testing results can be known. The spalling of the roll belongs to the fatigue spalling failure. The crack is generated on the surface of the roll. After the crack is generated, the fatigue propagation occurs to the inside of the roll along the reverse direction of the roll rotation direction and the direction of the roll surface at an angle of approximately 90°. When it expands to the soft-hard transition zone inside the roll (about 29 mm from the roll surface), the fatigue continues to expand along the circumferential direction. Until a circle of fatigue spalling strips was formed around the roll body, and finally the roll body material was rapidly destabilized under the action of rolling stress, resulting in this large-area spalling.
The metallographic examination results of the surface metallography of the roll body and the crack section from the surface to the inside show that there are no obvious material defects on the crack source and on both sides of the crack, indicating that the crack generation has nothing to do with the metallurgical quality of the roll. Due to the existence of slight secondary tempering in the tissue around the surface crack. It can be seen that the roller had a slight thermal shock during use, or the roller had an overheating shock during the previous machine-on process before peeling off, and the amount of grinding after the machine was not enough to completely eliminate the heat-affected zone.
According to the roll grinding history, the roll was only tested by eddy current flaw detection before it was put on the machine for the last time, and no abnormal signal was found. However, a crack alarm message of about 1 mm appeared on the roll when it rolled the second coil of steel on the machine. . It can be inferred from this that the cracks should exist before the machine is put on the machine. Due to the low detection sensitivity of the eddy current test to the micro cracks and subsurface cracks, the roll is put on the machine with micro cracks. Expansion occurred in the last rolling process and finally caused peeling. The reason for the cracks was related to the thermal shock of the roll surface. Therefore, reducing rolling mill accidents, improving the anti-accident performance of roll materials, and conducting comprehensive surface wave + eddy current flaw detection on the rolls before putting them on the machine can avoid the peeling of the rolls caused by the propagation of surface cracks.
Reducing rolling mill accidents, improving the accident resistance of roll materials, and conducting comprehensive surface wave + eddy current flaw detection on the rolls before they are put on the machine can avoid the peeling of the rolls caused by the expansion of surface cracks.
