What are HSS rolls?
High-carbon high-speed steel rolls (referred to as HSS rolls) are hot roll materials with the shortest time of birth, the fastest development, and the widest application prospects. High-speed steel is also known as wind steel or front steel, which means that it can be hardened even when it is cooled in the air during quenching, and it is very sharp. It is an alloy steel with complex composition, containing carbide-forming elements such as tungsten, molybdenum, chromium, vanadium, and so on. The total amount of alloying elements is about 10%~25%.
In the field of steel rolling, high-speed steel rolls are widely used. This paper starts with the character analysis of high-speed steel rolls and analyzes the application, performance, and structure of high-speed steel rolls.
1. What are the characteristics of HSS rolls?
1.1 The hardness of carbide is higher.
The materials selected for the production of high-speed steel rolls contain relatively high carbon and vanadium elements, as well as more elements such as chromium, molybdenum, cobalt, tungsten, and niobium. After solidification and heat treatment, they will exist in the form of alloy elements or secondary carbides.
1.2 Excellent wear resistance and thermal stability.
Compared with turning rolls made of other materials, high-speed steel rolls have excellent wear resistance. Compared with high-chromium cast iron and high-nickel-chromium infinitely chilled cast iron turning rolls, the roll consumption of high-speed steel rolls is obviously lower under the same rolling conditions.
1.3 Good hardenability and toughness.
The hardenability of the high-speed steel roll is very uniform. Under the same room temperature and working conditions, if the working mesh is 50mm, the hardness drop from the surface to the core of the high-speed steel roll is mostly lower than HS3. The hardness drop of the hard cast iron roll is HSO, which makes the high-speed steel roll ensure uniform wear resistance from the inside to the outside under uniform hardenability.
High-speed steel rolls have excellent strength and toughness. Compared with other materials such as infinitely chilled cast iron and high-chromium cast iron, the tensile strength of high-speed steel rolls is much higher, and their fracture toughness values are almost equal.
1.4 Advantages of the oxide film.
During the rolling process of high-speed steel rolls, an oxide film will be formed on its surface. The oxide film has good adhesion, and is dense and uniform.
It can reduce the friction between the roll and the rolling material, which guarantees the production of high-quality and high-precision rolling material. In comparison, ordinary rolls are not protected by oxide film during use, which will cause greater loss. High-speed steel rolls will quickly produce oxide film, which can improve the wear resistance of the roll.
1.5 Strong heat resistance.
Due to the special selection of materials, high-speed steel rolls have a significant effect on inhibiting the formation and expansion of cracks, and their heat resistance is much stronger than that of high-chromium cast iron rolls, which can reduce the impact of sudden production accidents.
2. Microstructure and performance analysis of high-speed steel rolls
2.1 Organizational structure
The structure of the high-speed steel roll is related to the type, shape, volume fraction, and distribution of carbides, the properties of the martensite matrix, and the grain size. It is basically the same as the composition of M2 high-speed steel. The carbides in high-speed steel rolls contain a large number of elements such as carbon, vanadium, chromium, molybdenum, cobalt, tungsten, and silver. The number of elements will affect the type of carbides in the steel. For example, when the amount of vanadium exceeds 8%, high-speed M3C carbides will appear in steel rolls, and their hardness is low: when the carbon content in high-speed steel rolls increases from 1.7% to 2.9%, the form of carbides will change. In high-speed steel rolls, MC-type carbides include dispersed strips precipitated in hypoeutectic, skeletons precipitated in eutectic, large blocks after adding niobium, and fine carbides precipitated in hypereutectic.
The matrix structure of the high-speed steel roll is a mixture of sheet-like and lath-like martensite.
2.2 Performance Analysis
High-speed steel rolls use high-speed steel with high hardness, especially good red hardness, wear resistance, and hardenability as the working layer of the roll, and forged steel, cast steel, or nodular cast iron with toughness that meets the requirements of the roll. The core material is a high-performance roll that combines the working layer and the core by melting and casting.
The high-speed steel roll is manufactured by the CPC method. The selection of process and material makes the core and bonding layer of the high-speed steel roll have a high tensile strength in terms of performance, and the roll has excellent bending load performance.
Compared with the metallurgical, mechanical, and physical properties of high-speed steel rolls and ordinary rolls:
In the working layer of ordinary rolls, mostly M3C or M7C3 eutectic carbides are distributed on the matrix, with coarse structure and low hardness. The working layer of the high-speed steel composite roll is generally made of high-carbon and high-vanadium high-speed steel, and the matrix of the working layer is distributed with granular high-hardness MC-type carbides. And with the increase of vanadium content, the bone-like M6C of the high-speed steel structure Type carbide is converted to MC type, so the high-speed steel roll will have excellent wear resistance.
3. Application of high-speed steel rolls
High-speed steel rolls have been widely used in steel rolling production in the fields of plate, section steel, bar, and wire, not only for work rolls but also for backup rolls. The good wear resistance of high-speed steel rolls is inseparable from its characteristics which are obviously different from traditional rolls.
Since the 1980s, foreign countries have begun to try high-speed steel rolls on hot strip continuous rolling mills and achieved good results. At present, the proportion of high-speed steel rolling is continuously increasing, and some racks even use high-speed steel rolls. After using high-speed steel rolls, the rolling loss is significantly reduced, the number of roll changes is significantly reduced, the amount of roll grinding is reduced, and the mill capacity is improved. , Fuel and power consumption is reduced, which helps to reduce rolling costs and improve strip quality.
Canada Dofasco Company has been using casting high-speed steel rolls since 1993, and the proportion has been continuously increasing. At present, cast high-speed steel rolls have been used on F2, F3, and F4 frames. The average steel passing capacity of F4 frames increased from 360t/ h increased to 490t/h in November 1994. The surface quality of strip steel also increased by 20%.