Different types of metal materials, different rolling methods, and different rolling mills have different requirements for roll characteristics. Therefore, there are many types of rolls. Rolling mills also vary according to the shape of the metal material after it is formed (plate, tube, rod, wire). Some rolling mills are equipped with different types of rolls, and different types of rolls are used in rolling.
The work rolls used in hot-rolled thin steel mills and in direct contact with the rolling stock will be mainly described below.
1. Roll manufacturing method
Rolls for steel rolling production can be manufactured by forging, casting, powder sintering, and other methods.
Hot-rolled work rolls are mainly cast rolls. Rolls in the past were not composite structures, but a single material. In order to improve the performance of the roll, the roll has been developed into a composite structure roll with a high degree of freedom in the material of the outer layer. The casting methods of composite rolls are mainly the centrifugal casting method and continuous casting forming composite method. The inner layer of the centrifugal casting method roll is ductile iron, and the inner layer of the continuous casting composite method roll is cast steel.
Cold-rolled work rolls are mainly forged rolls. Casting rolls and forging rolls have a variety of materials. According to the material, adopt the corresponding roll manufacturing method. After the roll is rolled and used, the surface properties of the roll will change due to wear and tear. At this time, the degraded layer on the surface of the roll can be removed by grinding or other methods and can continue to be used.
2. Performance requirements for hot strip mill rolls
The hot strip rolling unit is divided into a rough rolling section and a finishing rolling section.
The finishing section is divided into a front section and a back section. Since hot rolling is rolling performed at high temperatures, the rolls are required to have wear resistance at high temperatures. Therefore, the main material of the roll is a casting material containing a large number of carbides.
The rolling load and heat load of the rolls in the rough rolling section are large, so the material used for the rolls should be cast material containing a large number of carbides. However, starting from the finish rolling, the materials used for the rolls of each stand can be changed to materials with less carbide content.
Cast rolls utilizing carbides are generally rolls with a composite structure. The outer layer of the roll is made of hard material so that the outer layer of the roll has wear resistance and orange peel resistance. The inner layer of the roll is made of strong and tough material so that the roll will not break due to torque and pressing force.
In the rough rolling section, the reduction rate is large, the rolling load is large, and the temperature of the rolled material is high, so the rough rolling rolls use materials with high strength and good heat resistance. The finishing rolls are made of materials with good wear resistance and orange peel resistance.
In addition, the rolls used in the post-finish rolling stage must have accident-resistant materials. That is, when a rolling accident occurs in the final stage of finishing rolling, the roll will not be damaged due to deep cracks.
3. Development direction of roll steel
The front rolls of the finishing mill emphasize wear resistance and orange peel resistance. In the past, high-Cr cast iron rolls were used, but high-speed steel rolls have been used since the 1990s.
At present, the front-stage rolls of Japanese finishing mills basically use high-speed steel rolls.
At present, the use of high-speed steel rolls continues to expand, but in different countries and regions, the utilization rate of high-speed steel rolls is somewhat different.
High Cr cast iron is mainly used as cold work wear-resistant cast iron for various purposes. The carbon content of high-speed tool steel for knives is about 1%, while the carbon content of high-speed steel for casting rolls is more, about 2%.
The material of the rolls in the rough rolling section is high-speed steel, and high-hardness carbides are also used to obtain wear resistance. For this reason, the carbon content is also increased. Adding Mo and W can cause secondary hardening of the matrix when the steel is tempered, improving heat resistance and wear resistance. The practice of rolling high-speed steel and high-speed tool steel is the same.
The rolls in the post-finishing section are required to have good wear resistance and orange peel resistance. In the past, high-alloy steel was used. The wear resistance of high-speed steel is 3-5 times that of high-Cr cast iron and high-alloy steel, so the post-finishing rolls are also replaced with high-speed steel. However, the last finishing mill was not switched to high-speed steel because the rolling accidents in the last finishing mill caused crack damage to the rolls. The crack damage and accident resistance of high-speed steel rolls are not as good as those of high-alloy rolls.
In order to improve the wear resistance of high-alloy rolls, alloy elements such as V are added to the steel, and high-hardness carbide MC is used to improve wear resistance. This roll is an improved high-alloy roll.
The C content of high-alloy rolls and improved high-alloy rolls is relatively high, which is about 3%. Therefore, the carbides in improved high-alloy rolls, except MC, are mainly cementite.
In addition, the structure of the improved high-alloy roll has graphite with an area ratio of several percent, which can lubricate the thermal bonding of the steel plate when a rolling accident occurs.
In addition, about 4% Ni is added to the improved high-alloy roll steel to make the matrix form bainite or martensite and improve the wear resistance of the matrix.
The use of high-speed steel in the front stage of finish rolling improves the rolling efficiency. However, the use of improved high-alloy rolls in the latter stage has the problem of low rolling efficiency. The development of improved high-alloy rolls with better performance is currently underway.