This paper introduces the chemical composition and performance characteristics of cast high-speed steel rolls, analyzes the causes of problems in the actual use of high-speed steel rolls, and puts forward corresponding improvement measures in combination with the actual situation on the spot, which has a good reference for the use of high-speed steel rolls significance.
Chemical composition and performance characteristics of high-speed steel rolls
Chemical Composition of Hss roller
High-speed steel is a high-alloy steel composed of a large number of W, Mo, Cr, V, Cr and other alloying elements. In order to further improve performance, people generally attach importance to increasing the carbon content of high-speed steel. In order to adapt to different rolling conditions and processes, the chemical composition of high-speed steel rolls is also different.
The main characteristics of the high-speed steel roll composition are:
1) It has higher C content and V content, the purpose is to obtain high-hardness MC-type carbides and improve the wear resistance of the roll.
2) There is a high Cr content, so that the roll contains a certain amount of Mc-type carbides, which is beneficial to improving the roughness resistance of the roll surface and reducing the rolling force.
3) Centrifugally cast high-speed steel rolls contain less than 5% Nb to reduce segregation caused by large differences in the density of alloy elements in high-speed steel.
4) Co improves the red hardness of high-speed steel. It is applied to high-speed steel rolls on hot rolling mills. Adding Co can significantly improve wear resistance. Generally, less than 10% Co is added to hot-rolled high-speed steel rolls.
Performance characteristics of high-speed steel roller:
High-speed steel is a high-alloy steel composed of a large number of alloying elements such as w, Mo, Cr, V, Co, etc. It was mostly used as tool steel and die steel in the past. The quenching temperature of high-speed steel is usually 1250°C to 1320°C. After quenching, it is tempered at 500°C to 600°C. The structure is tempered martensite + carbide + a small amount of retained austenite. After quenching and tempering, high-speed steel has a very high hardness, which can reach HRC75-88.
The carbides in the roll are mainly MC, M6C and trace amount of M2C. With the increase of V content, the volume fraction of MC-type carbides increases, and the volume fraction of fishbone-like M6C-type carbides decreases relatively.
Since high-speed steel rolls mainly contain high-hardness carbides such as MC, MC, and Mc, and the matrix structure of high-speed steel rolls is composed of stable, high-hardness martensite and good hardening and hardenability, The hardness drop of the high-speed steel roll is significantly smaller than that of other rolls, and these concessions can ensure that the roll has good wear resistance throughout the working period. From the wear comparison of high-speed steel rolls and infinite chill rolls, it can be seen that the wear of a rolling unit of high-speed steel rolls is about 0.019mm, while the wear of infinite chill rolls is about 0.092mm, and the wear of infinite chill rolls is 5 times that of high speed steel rolls.
Problems in the use of high-speed steel rolls
Plate shape problem
Due to its good wear resistance, high-speed steel rolls are ground after the machine has been used continuously for a certain number of times. Due to the hereditary and cumulative nature of roll wear, the shape of the high-speed steel rolls in the frame is difficult after a certain number of continuous use. control. The two most important quantitative indicators of plate shape are flatness and convexity. The strategy of finish rolling strip shape control is that the front stand (F1-F3) controls the crown of the strip, and the rear stand (F4-F7) controls the flatness of the strip. Practice has proved that in the case of thin strip thickness and finished product specifications Next, the shape of the plate changes significantly for the roll shape of the roll. Although the shape of the strip can be adjusted by adjusting the position of the CVC and the bending force, etc., when the change of the roll shape exceeds a certain range, the shape of the strip deteriorates sharply, seriously affecting the stability of rolling.
The effect of CVC roll wear on the shape control performance of CVC mills is shown in the figure.
Figure 1 is the crown adjustment domain of a new roller without wear. 2, 3, 4, and 5 are the convexity adjustment domains produced by superimposing the wear curves with convexities of 0.05mm, 0.1mm, 0.15mm, and 0.2mm on the CVC curve, respectively. It can be seen from Figure 4 that for 2, 3, and 4, the CVC adjustment direction is basically linear, and the fluctuation range of the adjustment domain is not large. However, when the wear crown reaches 0.2 mm, the CVC adjustment direction is no longer linear, that is, the shape control performance of CVC technology has changed, and the crown adjustment domain has also moved down greatly, which is not conducive to adjusting the crown. control. With the use of high-speed steel rolls, the wear of high-speed steel rolls increases gradually, often exceeding 0.2mm. As for the rear frame, mainly the F4 frame, the bilateral corrugation problem has always been very serious. With the increase in the number of times the F4 high-speed steel roll is used, the shape of the plate deteriorates.
The cause of the double wave in the rear rack is the cumulative effect of roll wear caused by the high-speed steel roll being used multiple times in one grinding cycle. Due to the characteristics of “high temperature and high pressure” of the front frame, the roll expansion will be very large. Under the effect of this roll shape, the longitudinal extension of different parts of the strip is successively inherited to the lower frame. At the same time, the wear ratio of the F4 frame The calculation of the model is much larger. This is because the model does not consider the number of running times in the wear calculation process, and it is calculated based on new rolls. This chain effect makes it difficult to control the shape of the rear frame.
Oxide film peeling problem
In the hot rolling process, the rolls in the stand before finishing rolling rotate once, and the temperature is cooled from 700°C to 800°C to about 60°C. Under oxidizing conditions, the roll itself will form a thin layer of oxide film, which plays a role in protecting the surface of the roll. Looking at the surface morphology of the oxide film of high-speed steel rolls under a microscope, we can see cracks distributed in a network.
With the progress of the rolling process, under the interaction of thermal fatigue and rotational fatigue, the crack size grows and expands along the MC and MC carbides to the inside of the roll. In addition to cracks perpendicular to the surface of the roll, cracks parallel to the surface will also occur. At this time, the formed roll oxide film plays a role in protecting the roll surface. However, if the cracks parallel to the surface of the roll merge with the cracks perpendicular to the surface of the roll, under the action of strong shear stress between the roll and the rolled piece, the oxide film will peel off along with the surface of the roll, and banding defects will occur.
Sticking steel problem
When rolling thin strip steel, due to the influence of rolling stability, the phenomenon of “flicking” will occur when throwing steel, so that the broken strip steel fragments will stick to the roll, but this phenomenon has never occurred with ordinary rolls. There are two reasons for this problem:
1) The reason for the microstructure and composition of the roll: the high-speed steel roll has low carbon content and is closest to the composition of the strip steel, making it easier to adhere to the strip steel. The infinite chilled steel roll contains 2% to 5% in its microstructure Free graphite makes it difficult to stick to the strip, while high-speed steel rolls do not have free graphite;
2) Oxidation film of the roll: the oxide film is formed in the contact between the high-speed steel roll and the strip steel in the front stand, which hinders the bonding between the strip steel and the roll, and the speed of the roll in the front stand is relatively low, and the contact time between the roll and the strip steel is long , so the oxidation time is long, and the high speed of the rolls in the rear stand makes the oxidation time short, and the oxide film produced is thin, which cannot effectively prevent the bonding of the rolls and the strip. If the “sticking steel” phenomenon occurs, if the roll is not replaced in time, it is easy to cause the crack to expand, and eventually an accident will occur.
According to the analysis of the above reasons, considering the characteristics and usage characteristics of high-speed steel rolls, it is mainly to solve the wear and tear of high-speed steel rolls and protect the oxide film of high-speed steel rolls. Based on the above two points, the following improvement measures are proposed.
Wear Calculation Model Optimization
The initial design of the plate shape wear calculation model does not consider the wear of the high-speed steel roll when it is used. There is no wear before use, but in fact, according to the field measurement, although the high-speed steel roll wears less during the rolling process, the wear still exists, especially after rolling some high-hardness steel grades. This kind of wear will have a great impact on the CVC position and the bending force setting, especially when the high-speed steel roll is used many times.
To solve this problem, we save the wear crown calculation value of the high-speed steel roll when it is off the machine. When the set of rolls is used again, the last calculated wear value is used as the initial value for calculation. After optimization, the calculated value of roll wear crown is more consistent with the actual value, ensuring the setting accuracy.
High speed steel roll oxide film protection
The oxide film produced on the surface of the roll on the strip hot rolling mill can reduce the friction between the strip and the roll surface, prevent the roll from sticking to steel, and improve the life of the roll. There is no oxide film protection in the initial stage of roll use, so the initial wear is relatively large in a rolling cycle. The high-speed steel roll surface can generate oxide film faster than conventional rolls, which can significantly reduce initial wear and improve roll life. It is very beneficial, so it is extremely important to protect the oxide film on the surface of the roll.
Reasonable load distribution
Reasonable distribution of load is crucial to the protection of roll oxide film. The direct reason for the destruction of the oxide film is caused by excessive rolling force, so the load of the key frame should be properly reduced. If it is found that there are scale defects in the strip rolled by a certain rack during production, the load of this rack can be properly distributed to other racks, or the load can be properly adjusted to the rough rolling rack to reduce the time before the slab enters the finishing mill. The thickness of the oxide film on the roll surface of the work roll is larger in order to maintain the load distribution.
Roll temperature control
The control of the roll temperature is actually the control of the roll cooling water, especially the control of the F2 and F3 stand roll cooling water. If the amount of cooling water for the roll is too small, the oxide film formed by the high surface temperature of the roll will be too thick, brittle and easy to peel off; if the amount of cooling water for the roll is too large, the oxide film on the roll surface will be too thin, and the roll surface will be rough It leads to the occurrence of scale defects on the surface of hot-rolled strip steel. For this reason, in actual production, the amount of cooling water for the rolls of the stand should be properly adjusted so that the temperature of the rolls can be controlled within a certain range, and a more obvious effect can be received. For example, when the roll is hot, the cooling water of the roll is turned on 80% and then 100%, so that the oxide film can be fully established.
Use of rolling lubricants
The widespread use of high-speed steel rolls is an obvious advance in rolling tools. However, high-speed steel rolls also have obvious disadvantages, that is, the friction coefficient is large, and the crown of the roll surface is difficult to control. Due to the large friction coefficient, the rolling force is large. Using high-speed steel rolls will increase the rolling force by 10% to 20% under the same conditions. To reduce the rolling force and ensure the equipment load, hot rolling lubrication is the most effective Means, that is, the use of high-speed steel rolls, more need for hot rolling lubrication.
Because high-speed steel rolls are used many times in one grinding cycle, the surface oxide film will gradually deteriorate. For different steel types, the surface quality requirements are different, such as the outer panels of automobiles, which have the highest requirements. Therefore, it is extremely important to rationally equip the rolls in consideration of the surface quality of the strip, the reduction of the roll consumption, and the reduction of the grinding amount of the high-speed steel rolls. For the audit of the rolling unit, it is the basic idea to equip the rolls with the corresponding roll surface grade according to the requirements of the surface quality.
1) High-speed steel rolls are characterized by good wear resistance, which is almost 5 times that of infinite chilled rolls.
2) The high-speed steel roll has a great influence on the shape of the plate when it is used for many times. Optimizing the calculation model of roll wear is an effective solution.
3) Optimizing load distribution, using rolling lubricating oil, and controlling roll temperature are more effective methods for establishing and maintaining the oxide film of high-speed steel rolls.
4) The surface quality of the finished product and the reduction of roll consumption are the focus of the future work of high-speed steel rolls.