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Composition design and production process of high-speed steel composite rolls (hss rolls)

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This article discusses the composition design of high-speed steel composite rolls. From the aspects of alloy element segregation and casting cracks, the process parameters during the centrifugal casting process of the roll are analyzed; combined with the machining quality requirements of high-speed steel composite rolls, the influence of grinding parameters on the surface roughness of the roll is briefly described.

Keywords: High speed steel composite roll; high speed steel rolls

As one of the important equipment of the rolling mill, the quality of the roll has an important impact on the production cost, processing efficiency and the quality of the rolled parts. Currently, commonly used roll materials include: high-speed steel rolls, infinitely chilled cast iron rolls, semi-steel rolls and high-chromium cast iron rolls. Among them, high-speed steel rolls have a series of advantages compared with other commonly used rolls, such as high carbide hardness, good thermal stability, good hardenability, good thermal crack resistance, easy formation of oxide film during use, and long roll replacement cycle. It is widely used in hot rolling mills and cold rolling mills of ferrous metals and non-ferrous metals.

HSS ROLLS manufacturer

Material requirements for high-speed steel composite rolls

During the rolling process, on the one hand, the working layer causes plastic deformation of the rolled parts under the action of rolling force. Depending on the rolling conditions, the working layer of the roll must have high hardness, wear resistance and anti-tempering stability. . On the other hand, the roll will deform to a certain extent under the action of force. In order to ensure that the roll does not break due to the overload of local external forces, the roll also needs to have a certain degree of toughness. Most rolls at home and abroad use bimetal composite materials with the roll core and working layer made of different materials. The core is made of a material with high toughness, and the working layer is made of a material with high wear resistance. A certain preparation process is used to organically combine the two, so that the roll has both high wear resistance and good impact resistance. At present, high-speed steel composite rolls are the representative of bimetal composite materials. The working layer is made of high-speed steel, and the core material is mostly ductile cast iron.

Composition design of working layer of high-speed steel composite roll

Selection of carbon content

In order to ensure that the working layer of the roll has high wear resistance, its structure needs to contain a certain amount of carbides, generally above 15%. Carbon is the main element that forms the hard phase of carbide. The carbon content of traditional high-speed steel is usually between 0.7% and 1.5%. In high-speed steel rolls, most of the carbon exists in the form of alloy carbides, while a small amount of carbon is dissolved into the matrix. During quenching and heating, part of the carbides dissolves in austenite to ensure the hardness of martensite. During tempering, alloy carbides disperse and precipitate, causing secondary hardening. The undissolved carbides prevent grain growth and provide wear resistance. When the carbon content of the matrix is less than 0.3%, the heat treatment effect will be weakened; when it is greater than 0.6%, retained austenite and flaky martensite will increase, thereby reducing the fracture toughness of the roll. The carbon content in high-speed steel rolls, especially the ratio between carbon and carbide-forming elements, has an important impact on roll performance. Low carbon content cannot guarantee the quantity of carbides, thus reducing the volume fraction of the hard phase. High carbon content is prone to segregation of carbides and reduces the toughness of high-speed steel. In actual production, the more reasonable carbon content of high-speed steel rolls is determined according to this formula: w(C)=0.060 w(Cr)+0.063 w(Mo)+0.033 w(W)+0.235 w(V)+0.13 w (Nb), based on the above empirical formula and combined with the actual production, the carbon content of high steel rolls is determined to be 1.8%~2.8%.

Selection of chromium content

Chromium is a carbide-forming element that can form M23C6 and M7C3 carbides. For rolls with low chromium content, the matrix wears preferentially, and the rolled products tend to adhere to the surface of the roll, making the surface of the roll rough and increasing the rolling friction coefficient and rolling force. Increasing the chromium content so that the roll contains a certain amount of M7C3 carbide is beneficial to improving the roughness resistance of the roll surface and reducing the rolling force. Increased chromium content will also help improve the thermal shock resistance of high-speed steel rolls. Therefore, the chromium content in high-speed steel rolls is generally controlled at 4% to 6%.

Selection of tungsten and molybdenum content

The addition of tungsten and molybdenum alloy elements can improve the tempering resistance stability and red hardness of high-speed steel. Tungsten is the preferred element to improve the tempering resistance and red hardness of high-speed steel. It mainly exists in the form of M6C in high-speed steel and plays a great role in improving the wear resistance of high-speed steel. During high-temperature quenching, part of M6C dissolves into austenite, which can improve the hardenability of high-speed steel. The dissolution of tungsten into the matrix can produce strong solid solution strengthening effect. A low tungsten content reduces the number of carbides and affects the wear resistance of the material. However, too high a tungsten content increases the amount of ledeburite in the roll structure. The carbide particles are large and unevenly distributed, which adversely affects the thermal fatigue performance of the roll. Tungsten content is generally controlled at 5% to 8%. Molybdenum is also the main element in high-speed steel rolls. It has similar chemical properties to tungsten and its functions are similar to tungsten. The molybdenum content is controlled at 3% to 5%.

Selection of vanadium content

Vanadium has a strong affinity with carbon. In high-speed steel rolls, as the vanadium content increases, the eutectic reaction temperature of high-speed steel decreases and the formation temperature of VC increases. Vanadium is not only beneficial to the formation of VC, but also significantly promotes the formation of lamellar M2C carbide. Among the various carbides of high-speed steel rolls, VC has the highest hardness, exceeding HV3000, and it is difficult to dissolve VC during high-temperature austenitization. It exists in the remaining phase, which is beneficial to grain refinement and improved wear resistance. If the vanadium content is too high, cracks will easily appear along the grain boundaries, the matrix will easily wear first, the rolled parts will adhere to the roll surface, and the roll surface will be rough, which will reduce the surface quality of the rolled parts and speed up the replacement of the rolls. In addition, too much VC and too high hardness make roll grinding difficult. When the vanadium content in high-speed steel rolls is controlled at 4.0% to 6.0%, the use effect is better.

Selection of other element contents

In addition to adding the above elements to high-speed steel rolls, an appropriate amount of niobium is generally added when produced using the centrifugal casting method, which can reduce the segregation of VC and improve the overall performance of the rolls. In hot-rolled high-speed steel rolls, in order to improve the high-temperature wear resistance of the roll, cobalt element is usually added. However, as the cobalt content increases, the starting point of pearlite transformation of high-speed steel shifts to the left, the critical cooling rate of pearlite transformation increases, and the hardenability decreases. Its addition amount is generally controlled below 5%. In order to improve the matrix toughness of high-speed steel rolls, a small amount of nickel is sometimes added. If the nickel content is too much, the retained austenite in the quenched structure will increase, which will increase the number of roll temperings and reduce the wear resistance of the roll. The best effect is to control the addition amount below 2%.

Centrifugal casting process control of high-speed steel rolls

At present, the casting and forming methods of high-speed steel composite rolls mainly include centrifugal casting (CF), continuous casting composite casting (CPC), spray deposition forming (Osprey), hot isostatic pressing (HIP) and electroslag remelting. (ESR). Among them, the centrifugal casting method is the most widely used in actual production.

In the centrifugal composite method, molten metal is first poured into the rotating mold, formed and solidified under the action of centrifugal force to obtain the outer working layer of the roll, and then the mold is assembled and the roll neck and roll core are poured. The rolls produced by this method have the characteristics of dense outer working layer structure, few casting defects, good mechanical properties, high hardness of the surface wear-resistant layer, long service life, and high production efficiency. However, during the centrifugal casting process, the segregation of alloy elements and casting cracks are the biggest casting defects, which have a great impact on the quality of the prepared rolls.

Control process of alloy element segregation

Under ordinary centrifugal casting conditions, the alloy elements of high-speed steel in the working layer of the roll are seriously segregated. Studies have found that adding niobium can increase the density of MC carbide. The density of the generated (V, NB) C-type composite carbide is close to that of molten steel, which can control the segregation of carbides in centrifugally cast high-speed steel rolls and improve the wear resistance of the rolls.

The introduction of electromagnetic fields in centrifugal casting is also an extremely important means to reduce segregation of high-speed steel rolls. The author has conducted basic research on electromagnetic stirring of centrifugally cast high-speed steel rolls. After adding an electromagnetic field, element segregation is reduced as the magnetic field intensity increases. Compared with no magnetic field, the segregation of tungsten, molybdenum and vanadium elements in the centrifugally cast high-speed steel roll when the magnetic field intensity is 0.053T is reduced by 60.6%, 31.9% and 62.7% respectively. In addition, introducing a magnetic field during the centrifugal casting process of high-speed steel rolls can reduce the content of retained austenite in high-speed steel rolls, change the type and distribution of carbides in high-speed steel rolls, and make MC-type carbides increase in number and finer, and the distribution is more diffuse. , which increases the macroscopic hardness and effectively weakens the networking of carbides at the grain boundaries, thereby promoting the improvement of high-speed steel roll performance.

Control process of casting cracks

High-speed steel rolls are easily cracked during the centrifugal casting process, which increases the roll scrap rate. Practice has shown that pretreatment of molten steel can improve the roll’s resistance to cracking and reduce the scrap rate. The specific process is as follows 7]: When the molten steel comes out of the furnace, ferrovanadium particles are added, and the amount added accounts for 0.80% to 1.2% of the total mass of the molten steel. Before the molten steel rushes into the pouring ladle, rare earth magnesium alloy and ferrochromium nitride are pre-added into the pouring ladle. The added amounts account for 0.30% to 0.80% and 0.20% to 0.50% of the mass of the molten steel respectively. During the pouring process of molten steel, ferrovanadium and ferrotitanium particles with a particle size of 5 to 8 mm are added along with the flow. The added amounts account for 0.30% to 0.80% and 0.40% to 1.00% of the total amount of molten steel respectively. On this basis, variable-speed centrifugal casting, variable-flow pouring and variable-speed solidification cooling are used to adjust the temperature field and stress field distribution in the casting mold and roll, so that cracks in high-speed steel rolls can be eliminated.

Grinding process control of high-speed steel rolls

The surface quality of high-speed steel rolls determines the quality of finished products. During the grinding process of high-speed steel, there are defects such as scratches, tool marks, naps, and burns on the surface of the rolls. Roll grinding accuracy and surface quality not only rely on the working accuracy of sophisticated roll grinders, but also depend on the selection of matching grinding wheels, coolants and grinding process parameters for specific processed rolls. Reasonable selection of grinding parameters can avoid defects and ensure roll quality.

Selection of grinding wheels

The grinding wheel is an important tool for roll grinding. The selection of the grinding wheel is mainly based on the material, hardness and surface roughness requirements of the roll, as well as the abrasive, particle size, hardness, bonding agent, etc. of the grinding wheel.

Selection of abrasives, particle size and adhesives

Grinding wheels are composed of abrasives and adhesives. The selection of abrasives should consider the hardness, tensile strength of the workpiece material and the chemical reactions that may occur during the grinding process. The harder the workpiece material is, the harder the abrasive should be. Generally speaking, the hardness of the abrasive should be more than 2 to 4 times higher than the hardness of the workpiece material. If the hardness of the abrasive is low, it will quickly passivate and lose its cutting ability during high-speed cutting, making the grinding wheel durability too low, thus affecting the cutting efficiency, and the processing quality cannot be guaranteed. When grinding materials with higher tensile strength, corundum abrasives with higher toughness should be used. In addition, four abrasives are commonly used for ultra-precision grinding of alloy steel: white corundum, chrome corundum, microcrystalline corundum and single crystal corundum. Among them, single crystal corundum is the best type of corundum abrasive currently used.

Generally speaking, the size of the abrasive particle size is expressed by the mesh number of the filter screen used in the abrasive processing process. The smaller the number, the coarser the particle size. The larger the number, the finer the particle size. In most cases, a coarse-grained (grade 20-40) grinding wheel is used for rough grinding, a fine-grained (grade 60-100) grinding wheel is used for fine grinding, and a grinding wheel (grade 80-120) is used for precision grinding to polish the metal surface. , when super fine grinding or mirror grinding, generally use (W63 ~ W35) grinding wheels.

The bonding agents of grinding wheels include ceramics, silica gel, rubber, shellac and resin. When mirror grinding the roll body, resin adhesive is used. For other types of grinding (because ceramic bonded grinding wheels have the characteristics of good rigidity, brittleness, durability, high temperature resistance, good shape retention, strong physical and chemical stability, and wide adaptability to the cooling system), ceramic grinding wheels are used.

Grinding wheel hardness

The hardness of the grinding wheel does not refer to the hardness of the abrasive material, but the bonding strength between the abrasive and the binder, or the resistance of the abrasive to break away from the binder. Grinding wheel hardness values increase in alphabetical order. If the grinding wheel wears too fast, it means that the grinding wheel is too soft. If the grinding wheel is obviously blocked when grinding the roll, the passivated sand grains are not easy to fall off, and there are defects such as burns and naps on the roller surface, which means that the grinding wheel is too hard. Generally speaking, the higher the roll surface hardness, the softer the grinding wheel hardness should be. Harder grinding wheels should be selected for fine grinding and form grinding to maintain the necessary shape accuracy of the grinding wheel.

 Selection of grinding parameters

(1) Grinding wheel and roll speed

When grinding rolls, the grinding wheel rotates at a high speed, which can easily cause vibrations in the processing system and cause defects such as polygons, spirals, and burns on the roll surface. Therefore, during the grinding process, the grinding wheel speed is controlled at 25m/s~35m/s during rough and fine grinding; the grinding wheel speed is controlled at 15m/s~20m/s during super-fine grinding and mirror grinding.

When the roll speed is too low, surface burns and spiral knife marks are likely to occur due to slow heat dissipation; when the roll speed is too high, roll vibration is likely to occur and the depth of surface ripples will be deepened. During fine grinding, the roller speed can be increased appropriately and the longitudinal feed of the grinding wheel can be reduced to improve the surface finish. Generally speaking, the roller speed during rough grinding is 20r/min~40r/min, the roller speed during fine grinding is 15r/min~28r/min, and the roller speed during polishing grinding is 10r/min~15r/min.

(2)Grinding wheel longitudinal feed

The longitudinal feed of the grinding wheel directly affects the surface roughness of the roll. When the longitudinal feed is increased, the grinding force and grinding heat increase, and spiral marks are easily produced on the surface of the roll. The longitudinal feed amount is generally 80~400mm/min.

Conclusion

To ensure the quality of high-speed steel composite rolls, we need to start from every aspect of the component design, casting process, heat treatment process and machining process. According to the characteristics of the rolled products and the working environment characteristics of the rolls, we need to formulate process parameters that are suitable for them. Thereby optimizing process design and reducing production and manufacturing costs, and realizing the promotion and application of high-speed steel composite rolls.

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The world No. 1 in crude steel production steel plant, over 70% of the HSS for bar and wire are from us.

LMM high-speed steel rolls have higher steel passing per groove (times) due to their good wear resistance, which saves roll changing time, improves rolling mill operation rate, reduces roll consumption, and improve the overall efficiency of the factory.
Generally, the amount of steel passing in a single groove (times) is 3 to 5 times that of cast iron rolls.

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