High chromium cast iron is generally used for casting the work rolls of the front-end stand of the finishing mill in the hot strip continuous rolling mill. It forms a layer of high-hardness and wear-resistant high-chromium oxide film on the surface of the work rolls through the contact between the rolls and the high-temperature strip steel to improve the service life of the rolls.
The process of forming this layer of the oxide film is a dynamic process, and the quality of cooling conditions and rolling conditions will directly affect the density and thickness of the oxide film. Because the whole process is difficult to grasp, it is difficult to meet the conditions for the formation of the oxide film. Therefore, when high-speed steel rolls are produced in hot-rolled strips, the oxide film will fall off, which directly affects the surface quality of the strip steel and the service life of the rolls.
1 Improvement of high-speed steel rolls
By changing the chemical composition of the roll and improving the matrix structure, the wear resistance of the roll can be improved. After more than ten years of updating and improving technology, high-speed steel rolls have been used in hot strip finishing mills. The application of this new technology in combination with the lubrication process not only improves the product quality to a certain extent but also achieves a very obvious effect in reducing production costs.
The combination of the core of the high-speed steel roll and the outer layer of the roll is a defect-free metallurgical bond, so the core of the high-speed steel composite roll has high tensile strength and the strength of the bonded layer is high, which can increase the bending load of the roll so that the plate can obtain a better shape.
Comparison of the state of high-speed steel and high-chromium iron rolls after they are off the machine
Through horizontal comparison, high-speed steel rolls exhibit good wear resistance under the same frame and the same use condition. When rolling the same steel type, the same product specification, and the same rolling amount, the different roll surfaces appear to be significantly different.
Based on the good condition of the surface of the high-speed steel roll after being off the machine, we increase the number of times the roll is used after a single grinding. This not only reduces the workload of the grinding machine but also reduces the production cost. Compared with the high-chromium steel, the mm rolling capacity of the high-speed steel has been greatly improved, which is about 26 to 28 times the original, and the cost performance has been greatly improved.
In actual use, it has been found that high-speed steel rolls do have better thermal cracking resistance than high-chromium cast iron rolls. In this way, if a general production accident occurs, the impact of the accident can be eliminated through constant grinding or a small additional grinding amount, which simplifies the processing process and saves costs.
The friction coefficient of high-speed steel rolls is 0.4 to 0.44, which is higher than that of high-chromium iron rolls on average. When the high-speed steel rolls are arranged in the F stand, the biting state can be obviously changed, but the rolling force will be increased. We can combine the use of process lubrication to reduce the rolling force of high-speed steel rolls in use.
2. Optimization and testing of high-speed steel rolls
Due to the high price of high-speed steel, we need to take it more seriously in grinding, on-machine use, off-machine and post-accident testing.
2.1 Grinding optimization of high-speed steel rolls
Because the high-speed steel roll has good wear resistance, high hardness and friction coefficient, it directly brings certain difficulties to grinding and quality control.
For example, the reduction of grinding efficiency, the vibration marks are easy to occur during grinding, the control of roll shape is difficult, and the roughness is re-determined. In this regard, the manufacturer can re-select the material of the grinding wheel, and by adjusting various parameters such as the hardness and particle size of the grinding wheel, the grinding efficiency can be greatly improved.
In terms of controlling the grinding quality, the surface quality of high-speed steel rolls can fully meet the production requirements when they are on the machine by optimizing the grinding parameters, such as controlling the feed rate and grinding current during fine grinding, using periodic variable speed control technology, and controlling the smooth grinding pass and smooth grinding current.
2.2 Use of process lubrication
The purpose of rolling with lubricating oil in the finishing mill is to reduce rolling force, reduce rolling energy consumption, reduce roll wear, reduce roll consumption, improve roll surface condition, and improve strip surface quality.
In order to improve the rolling state and improve the surface quality of the product, the process lubrication system should be invested. Process lubrication mixes oil and water, sprays the mixed oil and water on the surface of the roll, and forms a certain concentration of emulsion as a lubricant. When the work roll is cooled and closed during use, the lubricant is sprayed on the surface of the work roll at the entrance side to form a continuous, uniform and firm lubricating oil film when the roll is in contact with the strip, so as to reduce the friction coefficient between the work roll and the strip, reduce the rolling force, reduce the vibration of the rolling mill, and reduce the wear of the work roll. The oxide film of the roll falls off, the temperature of the surface of the work roll in the deformation zone is reduced, and the thermal cycle stress of the work roll is relieved, thereby suppressing the occurrence of cracks.
In actual production, the combination of process lubrication and high-speed steel rolls not only eliminates the increase in rolling force caused by the high friction coefficient of high-speed steel rolls, but their joint action can improve product quality, reduce roll consumption and energy consumption, etc., showing very obvious effects.
2.3 Detection of high-speed steel rolls
In order to ensure the normal working condition of the high-speed steel roll, it must be strictly tested. Accidents of high-speed steel rolls generally come from two aspects, one is the impact of production accidents, and the other is the impact of mechanical collisions.
When a collision occurs, mechanical cracks are sometimes generated on the surface of the roll. When a stacking accident occurs, because the high-speed steel roll has good thermal crack resistance, if the operator responds quickly and handles it in time, generally no surface cracks will occur. But sometimes due to the pile of steel
The situation where the working layer is detached and the original defect expands due to the impact. This necessitates confirmation of surface defect detection with eddy current testing after an accident. Once a crack is found, it must be ground, and the working layer is inspected by ultrasonic flaw detection to confirm that there are no defects or that the existing defects do not expand within the safe range. Another method is to use surface wave testing instead of eddy current testing.
When cracks are detected, confirm the cracks with colour penetrant inspection or magnetic particle inspection, and then use the angled probe to determine the depth, determine the appropriate grinding amount, and process the roll to ensure the normal operation of the roll.
Although high-speed steel rolls have good thermal crack resistance, they are very sensitive to mechanical cracks. Once a mechanical crack appears, it must be ground, otherwise, there will be a risk of the roll peeling off, falling blocks or even breaking the roll during production. Therefore, when the roll is mechanically collided, eddy current flaw detection or surface wave flaw detection should be carried out in time to ensure that there are no mechanical cracks and ensure the safety of production.
2.4 PC FC shape control optimization
Since the physical and chemical properties of high-speed steel rolls are different from those of high-chromium rolls or ICDP rolls, the roll shape must be adjusted. Physical and chemical properties mainly refer to thermal expansion coefficient, friction coefficient, elastic modulus and hardness.
When the rolling ends, stop the water immediately and record the water stop time, and then replace the high-speed steel roll. Measure the temperature after about 2M from the water stop time. Take the middle of the roll body as the origin of the coordinates, take points on both sides, divide the roll body into 21 equal parts for every 10M measurement point, and measure the temperature of each point. According to the measured value, the temperature distribution map of the roller body is drawn and fed back to the first level, which is optimized
Then the working rolls are hoisted to the roll room for air cooling. After air cooling to room temperature, the grinding machine measures the wear curve after the off-machine and then feeds back to PC FC to optimize its wear model.