HSS steel means that it is hardness will be higher when cooled in the air after quenching. Nowadays, high-carbon high-speed steel roll (HSS roll) is the hot-roll material with the shortest appearance, the fastest development, and the widest application prospect.
It is complex alloy steel with carbide-forming elements such as tungsten, molybdenum, chromium, and vanadium. The total amount of alloy elements reaches about 10%~25%. High-speed steel rolls are widely used in steel rolling in the fields of plates, sections, bars, and wires, not only for working rolls but also for supporting rolls. The good wear resistance of high-speed steel rolls is the most distinguished point from the obvious difference from traditional rolls.
The hardness of HSS rolls is HSD82～88. The product has good wear resistance, wear uniformity, impact resistance, no spalling phenomenon, good thermal fatigue resistance, and easy processing. The average steel quantity of single grooves can reach 3~5 times of high NiCr rolls. The total steel volume can be guaranteed to be 4 times on average and can be used continuously for 1~2 levels of slot change, which effectively improves the continuous running time of the mill.
Six Advantages of HSS Rolls
1. Cost reduction and efficiency improvement
High-speed steel roll material contains high vanadium, tungsten, chromium, molybdenum, niobium, and other alloy elements. The type of carbide in the roll organization is mainly MC type and M, C type, high carbide hardness, good wear resistance. Ccompared with the traditional ductile iron rolls and high nickel chromium rolls, single slot (times) high steel throughput, saving the time to change rolls improve the mill operating rate, reduce roll consumption, reduce production cost and improve the overall efficiency of the plant. Ps: M=Nb,Ti,V,Mo,W
2. High Hardness and wear resistance
HSS rolls have good thermal stability, and the roll surface has high hardness and good wear resistance under rolling temperatures.
3. Good hardenability
HSS rolls have good hardenability and almost no drop in hardness from the surface of the roll body to the inside of the working layer, thus ensuring that the rolls are. The role has the same good wear resistance from outside to inside.
4. Formation of oxide film at high temperature
During the use of HSS rolls, under good cooling conditions, a thin and dense oxide film is formed on the surface of the roll body. And this uniform, thin, and dense oxide film can exist for a long time without shedding, which makes the wear resistance of HSS rolls significantly improved.
5. The large expansion coefficient to expand the lifespans
HSS roll material expansion coefficient is large, has good thermal conductivity, and rolls in the rolling process with the rolling groove micro loss at the same time. It is due to the expansion of the HSS material itself and making the hole shape constantly smaller. Therefore, HSS rolls in the rolling process groove change are small, and the consistency of the hole size to maintain for a long time, especially in the role.
6. The high strength roll neck
Since the core of centrifugal cast HSS rolls is made of alloy ductile iron material, the strength of the roll neck is high.
Each Alloy Composition on HSS Rolls
The carbon content of HSS rolls is higher compared with ordinary HSS, generally, 1.4% ~ 3.5% depending on the composition ratio. Carbon is mainly used with strong carbide-forming elements to produce MC, M2C, and other high hardness and high thermal stability carbides, to improve wear resistance and red hardness. Carbon in the melting can also avoid excessive oxidation of iron, but if the carbon content is too high carbide segregation, reduces the use of roll performance. Therefore, the carbon content should be determined according to the content of strong carbide-forming elements.
Silicon has a certain deoxidation effect, but the silicon content is too high will form coarse M, C3 carbide, and ferrite, reducing the organization of toughness. The toughness and hardness of the tissue will be reduced. Therefore, the appropriate silicon content should be 0.2% ~ 1. 0%.
Manganese can reduce the martensite transformation temperature, but if the content is too high will increase the amount of residual austenite. The content can be set at 0.3% ~ 1.0%.
The hardenability of high-speed steel rolls mainly depends on chromium, chromium first formed carbides MzC and M23C6. These carbides are basically dissolved in the matrix during quenching and heating, increasing the stability of the matrix, so high-speed steel rolls produce highly hardenability. Chromium can also regulate the carbon balance of the matrix and can improve antioxidant decarburization and corrosion resistance. However, too much chromium will form unstable carbides during tempering, reducing thermal stability and red hardness.
Nickel ( Ni)
Nickel can improve the mechanical properties of HSS rolls through increasing the hardenability but increasing the amount of residual austenite. However, the excess residual austenite will eliminate when in a suitable composition design and production process. Therefore, the addition of less than 2% nickel improves the mechanical properties of HSS rolls.
Molybdenum acts similarly to tungsten, forming Mo2C, and 1% of molybdenum can replace about 1.8% of tungsten. The density of molybdenum is lower than tungsten, to a certain extent, can eliminate the phenomenon of segregation. Molybdenum in high-speed steel rolls can improve the unevenness of the primary carbide and reduce brittleness while tempering solid solution of molybdenum. It can prevent carbide precipitation along the grain boundary, improve strength and toughness.
Vanadium forms the carbide VC, which precipitates diffusely during tempering to form secondary hardenings and is hard, fine-grained, and not easily broken. It mainly in the form of spheres or sub-spheres diffusely distributed in the matrix to improve impact toughness, hardness, and wear resistance. Vanadium can also effectively remove impurities from the steel, but high vanadium content will increase the oxidation of the steel. High vanadium content increases the oxidation of the steel and greatly increases the cost of the material. Vanadium content should be controlled from 2.5% to 4.0%.
HSS Roll Heat Treatment Technology
Annealing can reduce the hardness and improve the cutting performance, the steel with high carbon content often too hard, not easy to cut. However, annealing treatment can reduce the hardness and improve the cutting performance of steel.
Annealing can refine the grain, improve the organization and performance. After casting, forging, welding forming billet, the organization of the coarse grain often exists in the superheated organization and other defects. Through annealing treatment, in the process of heating the steel austenitization, control the heating temperature and preservation of time. In the subsequent cooling process transformed into fine pearlite. So it will eliminate the coarse grain of superheated organization, refining the grain, improve the performance, and for the final heat treatment to prepare the organization.
Annealing can eliminate internal stress, stabilize the size of the workpiece and prevent deformation. Workpiece hydrocarbon casting, forging, welding or cold processing, often produce internal stress, such as not timely elimination. They will cause deformation or cracking of the workpiece. After annealing heating and insulation, through the atomic diffusion of internal stress relaxation and elimination.
Annealing can improve plasticity and facilitate cold deformation processing. Cold deformation makes the metal work hardening, plasticity decreases, brittleness increases, can not continue to cold deformation. Annealing through recrystallization can eliminate the work hardening, improve plasticity, continue to deformation, such as cold drawing, cold rolling, sheet stamping, etc.
Heat treatment is to heat the metal material to a certain temperature, hold the temperature for a certain period of time, and then cool down to room temperature or lower at a certain rate, so as to improve the material organization and structure to obtain a material with excellent performance, generally refers to the treatment of metal materials, especially steel. Commonly used classification methods are normalizing, annealing, quenching, tempering and surface hardening and so on.
Heat treatment is the key to product quality, improper heat treatment will cause shading and chipping, also affect the chemical reaction of carbide and thus affect the wear resistance of roller ring. The following points are the summary of the necessity of annealing for HSS rolls.