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Material elements of hss roller


Description: The role of alloy elements in hss roller and their influence on the roll structure and properties are discussed in detail. On this basis, the development of cast high-speed steel roll materials and the effects of modification treatment and microalloying on the structure and properties of high-speed steel rolls are introduced, which can provide guidance for the further development of high-performance high-speed steel rolls.

Keywords: hss roller, chemical composition of high-speed steel roll, high-speed steel roll material

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Cast high-speed steel rolls, developed on the basis of ordinary high-speed tool steel by increasing the carbon and vanadium content, have the characteristics of high hardness, red hardness, hardenability and good wear resistance. Since their first successful application in Japan in 1988 , has been used for more than 20 years, and is currently widely used in steel rolling production abroad. It is not only used in hot-rolled strip mills, but also in profile rolling mills, steel pipe mills, bar and wire rod mills and cold-rolled strip mills. After using cast high-speed steel rolls, the amount of steel passed is significantly increased, the number of roll changes is significantly reduced, the amount of roll grinding is reduced, the rolling mill operation rate is improved, and fuel and power consumption are reduced, which helps to reduce steel rolling costs and improve the quality of rolled products.

Composition characteristics of high-speed steel rolls

At present, the composition of most high-speed steel rolls is based on W6Mo5Cr4V2 (M2) high-speed steel. The biggest difference from M2 high-speed steel is that it contains more carbon and vanadium. The overall fraction of carbides and the volume fraction of different types of carbides in high-speed steel rolls are closely related to their composition, while the solidification cooling method has no obvious effect on carbides. The volume fraction of carbides in high-speed steel rolls generally reaches 9% to 15%, while the volume fraction of carbides in traditional high-speed tool steels generally does not exceed 8%. The main characteristics of high speed steel roll composition are:

① It has higher carbon content and vanadium content, the purpose is to obtain high hardness MC type carbide and improve the wear resistance of the roll.

② It has a high chromium content, so that the roll contains a certain amount of M7C3 carbide, which is beneficial to improving the roughness resistance of the roll surface and reducing the rolling force.

③ The centrifugally cast high-speed steel roll contains less than 5% niobium, which can reduce segregation caused by the large density difference of alloy elements in high-speed steel. Due to different rolling steel types, rolling mill conditions and roll manufacturing methods, the composition of high-speed steel rolls also varies. The composition of commonly used high-speed steel rolls is shown in Table 1.

Table1  The composition contrast of hss rolls between domestic and abroad
Ni0.00-1.500.30-1.50< 5.000.50-2.00< 5.00
Mo2.00-8.001.00-8.00< 10.001.00-8.002.00-10.00
W0.00-8.003.00-7.00< 20.000-5.02.00-10.00
Co≤ 8.00——< 10.00——≤ 10.0
Nb≤ 5.00————0-5.0≤ 10.0

The role of alloy elements in high-speed steel rolls

High-speed steel rolls often contain alloying elements such as chromium, vanadium, tungsten, and molybdenum. Some high-speed steel rolls also contain elements such as niobium, cobalt, and nickel. The main functions of alloying elements are as follows.

2.1 Chromium

Chromium in high-speed steel can form M23C6 carbide that is completely dissolved at a lower quenching temperature, thus improving the hardenability and red hardness of high-speed steel. Therefore, high-speed steel usually contains about 4% chromium. After the chromium content exceeds 4%, the excess chromium participates in the formation of carbides precipitated during tempering, reducing the thermal stability of high-speed steel. Recent studies at home and abroad have found that in high-speed steel rolls, chromium increases the number of M7C3 carbides and reduces the number of MC carbides. For rolls with low chromium content, due to the preferential wear of the matrix and the adhesion of the rolled material to the surface of the roll, the surface of the roll tends to become rough during use, which increases 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. Increasing the 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%.

2.2 Tungsten and molybdenum

Tungsten has always been the first choice element to improve the tempering resistance and red hardness of high-speed steel. Tungsten mainly exists in the form of M6C in high-speed steel rolls, which plays a great role in improving the wear resistance of high-speed steel rolls. Undissolved M6C during quenching heating can prevent austenite grain growth at high temperatures. During high-temperature tempering, part of the tungsten disperses and precipitates in the form of W2C, causing secondary hardening.

And improve the red hardness of high speed steel. Satisfactory secondary hardness and thermal stability can be obtained by containing 7%~8% W in high-speed steel rolls. However, the density of tungsten and its carbides is high, as shown in Table 2. Under centrifugal casting conditions, it is prone to segregation and is concentrated in the outer layer of the roll, while the tungsten content in the inner layer of the roll is significantly reduced, causing significant fluctuations in roll performance. Therefore, most foreign centrifugally cast high-speed steel rolls do not add tungsten, or the tungsten content is controlled below 2%.

Table 2  The density of common alloy elements and carbides in high speed steel roll
Tungsten carbideW2CWCMo2CNbCVCFe3C
Density / (Kg·m3)17200158009100790057007200
Density / (Kg·m3)19300102008600790072006100

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 impact on the structural transformation and properties of high-speed steel is almost the same. The main difference is that the temperature when molybdenum causes the structural transformation is lower. Molybdenum also forms M6C carbide. Compared with M6C in tungsten-containing high-speed steel, its lattice is the same and the lattice parameters are almost the same, but the density is lower. Since the atomic weight of molybdenum is about half that of tungsten, 1% Mo can generally replace 2% W. The effects of chromium, tungsten and molybdenum elements on the wear resistance of high-speed steel rolls are shown in Figure 1. It is obvious that appropriately increasing the chromium and molybdenum content will help reduce the wear rate of the rolls and improve the wear resistance of the rolls.

Material elements of high-speed steel rolls

2.3 Vanadium

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 carbides and inhibits skeletal M6C carbides. Among various carbides in 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, and the matrix will be prone to wear preferentially. The rolled material will adhere to the roll surface, and the roll surface will be rough, which will reduce the surface quality of the rolled material and speed up the replacement of the roll. 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. Due to the low density of vanadium and the carbides it forms (see Table 2), it is easy to accumulate in the inner layer of the roll during the centrifugal casting process, reducing the wear resistance of the outer layer of the roll. Based on the analysis of the segregation mechanism of centrifugally cast high-speed steel rolls, Japan’s Kawasaki Steel Company found that adding niobium element can generate denser (V, Nb) C-type composite carbides, whose density is close to the density of molten steel. By reducing the amount of VC and limiting the addition of segregated elements tungsten and molybdenum, the carbide segregation of the centrifugally cast high-speed steel roll can be effectively controlled and the wear resistance of the roll can be improved.

2.4 Carbon

High carbon content is prone to carbide segregation and reduces the toughness of high-speed steel. Therefore, the carbon content of high-speed tool steel is usually 0.60%~1.50%. High-speed steel rolls are required to have sufficient wear resistance, and the number of carbides in the working layer of the roll must be increased. During quenching and heating, part of the carbides dissolves in austenite, thereby ensuring the hardness of martensite. During tempering, alloy carbides disperse and precipitate, causing secondary hardening; undissolved carbides play a role in preventing grain growth and wear resistance. Most of the carbon in high-speed steel rolls forms carbides with tungsten, chromium, molybdenum and vanadium, and a small amount enters the matrix. 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 significantly, 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. It is usually more reasonable to determine the carbon content of high-speed steel rolls based on the following empirical formula and combined with production actual conditions.

2.5 Other elements

In addition to adding the above elements to high-speed steel rolls, an appropriate amount of niobium is generally added when produced by centrifugal casting, 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. The addition of cobalt element reduces the strength and toughness of high-speed steel rolls. The higher the content, the more obvious the reduction. In addition, 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 too much nickel is added, more austenite will remain in the quenched structure, which will increase the number of temperings of the roll and reduce the wear resistance of the roll. The effect is better when the addition amount is controlled below 2%.

Development of high-speed steel roll materials

The development of high-speed steel roll materials is aimed at tensile strength and hardness. High-speed steel rolls are the highest quality contemporary roll materials.

Peak, tensile strength is around 800 MPa, hardness HS 75~90. At present, the main methods of manufacturing high-speed steel rolls are centrifugal casting, continuous casting (CPC) and hot isostatic pressing. The hot isostatic pressing method currently produces small-sized rolls due to limitations in the size of the pressure sleeve. The hot isostatic pressing process uses powdery raw materials with uniform composition and relatively free adjustment. It is easy to obtain high-quality high-speed steel rolls. However, due to complex production processes and equipment, high energy consumption, and low efficiency, it cannot be used for the production of large-scale high-speed steel rolls. In practical applications of CPC high-speed steel rolls, it is found that the rolling load increases and the power consumption increases. Except for a few roll mills in Japan that use this method to manufacture rolls, it has not been promoted and applied in other countries. Nowadays, high-speed steel rolls are mainly manufactured by centrifugal casting method with high efficiency and simple process. In order to meet the requirements of centrifugal casting of high-speed steel rolls, in addition to adding niobium and reducing the content of tungsten and molybdenum. When ordinary high-speed steel rolls are used in the front-end stands of rolling mills, there are problems such as increased rolling load and inducing scale defects on the steel plate. This is because the MC-type carbides in the high-speed steel rolls form fine protrusions on the surface of the rolls. Increase the friction on the rolling surface. In order to suppress the occurrence of this phenomenon, the most effective way is to add more eutectic carbides with lower hardness to the parts of the matrix that are prone to wear to smooth the wear surface of the roll. In addition, the orange peel-like defects of high-speed steel rolls originate from the destructive effect of carbides, and increasing the carbide content can inhibit the increase in rolling load.

Research has found that Fe-Cr carbides, represented by the orthorhombic system, can not only solid-solubilize a large number of chromium atoms in the form of replacing iron atoms, but also solid-solubilize vanadium, molybdenum and tungsten. In high-speed steel, increasing the chromium content can form a large number of chromium carbides, and increasing the concentration of molybdenum with a large atomic radius can increase the binding force and density of carbides and strengthen the carbides. By selecting the ingredients (mass fraction): 2.0% ~ 4.0% C, 0.2% ~ 0.4% Si, 0.2% ~ 0.4% Mn, 5.0% ~ 20.0% Cr, 2.0% ~ 15.0% Mo, 4.0% ~ 6.0% V A hot rolling-sliding friction test was conducted with a high-speed steel roll containing 1.0% ~ 3.0% Nb. It was found that only increasing the carbon content in the reference component of the sample would increase the wear amount of the sample. If the carbon and chromium content is increased, the amount of wear will be reduced. When the carbon and chromium content of the sample is increased and the molybdenum content is increased in a balanced manner, the wear amount will be significantly reduced. When increasing the chromium and molybdenum content, carbides are strengthened and carbides are increased, significantly improving the wear resistance of high-speed steel rolls. High-chromium and molybdenum high-speed steel rolls are used on hot rolling F1 and F2 stands. They have very good roughness resistance and wear resistance. Compared with previous high-speed steel rolls, the millimeter steel thickness of the rolls has increased by 2 to 3 times.

Cheap boron is used to react with iron to generate high-hardness boride, which is beneficial to improving the wear resistance of the roll. In addition, a small amount of boron is dissolved in the iron matrix, which can improve the hardenability and hardenability of high-speed steel rolls. At the same time, greatly reducing the vanadium content in high-speed steel rolls can reduce the production cost of high-speed steel rolls and improve the grinding performance of the rolls. By reducing the carbon content of high-speed steel rolls, the thermal fatigue resistance of high-speed steel rolls can be significantly improved. The chemical composition of high-boron high-speed steel roll material is (mass fraction): 0.25%~0.80% C, 1.0%~2.5% B, 4.5%~6.5% Cr, >5.0% W, > 2.0% Mo and >1.5% V , the balance is Fe. Compared with the existing technology, this invention has the following advantages:

①The addition of tungsten, chromium, molybdenum, vanadium and other alloying elements in high-boron high-speed steel rolls is small, and does not contain expensive alloying elements such as niobium, cobalt, and nickel. Therefore, it has lower production costs than commonly used high-speed steel. Rollers reduce production costs by more than 30%.

②The high-boron high-speed steel roll material has low carbon content, and the roll has excellent thermal fatigue resistance.

③The high-boron high-speed steel roll material contains more boron, and the roll structure contains more than 12% alloy boride, making the roll material hardness reach HS 82~85.

④ High-boron high-speed steel materials are used to make rolls. Their service life is 15% to 30% longer than that of conventional high-speed steel rolls, and production costs are reduced by more than 20%.

Progress in improving the structure and performance of high-speed steel rolls

Cast high-speed steel rolls have a large number of carbides and large carbide sizes, resulting in low strength and poor toughness of the rolls, and they are prone to cracking and peeling during use. Figure 2 is a typical picture of peeling that occurs during use of high-speed steel rolls. Cracks and spalling of the rolls affect the normal production of steel rolling. Microalloying and modification treatment of cast high-speed steel rolls can improve the roll structure, improve the overall performance of the roll, and ensure the safe use of the roll.

Material elements of high-speed steel rolls

The influence of rare earths on the microstructure and properties of high-carbon high-speed steel rolls. The eutectic structure of unmodified high-speed steel rolls is mainly distributed in lamellar and network shapes, with a small amount of massive carbides and a coarse structure. With the addition of rare earth elements, the austenite grains and eutectic structure in the high-speed steel roll structure are significantly refined, and the lamellar carbides in the eutectic structure become shorter and thinner. Mainly because rare earths are enriched around the carbides during the solidification process, preventing the carbides from growing along the grain boundaries and making the carbides refined.

The carbide morphology changes into broken network and isolated shapes. The heat-treated structure of unmodified high-speed steel rolls is relatively coarse, with a large number of large carbides distributed along the grain boundaries. After rare earth modification treatment, when the heating temperature is 1050°C, the carbide network structure completely disappears, and most of the carbide becomes a spherical structure, which promotes a significant improvement in the performance of high-speed steel rolls, see Table 3.

Table 3  Effect of RE modification on mechanical properties of HSS roll
Deal withHardnessRed hardnessFracture toughnessImpact toughness
StateHRCHRC/ (MPa·m1/2/(J·cm-2)
Not spoiled64.560.422.087.38

As the amount of K/Na increases, the carbide distribution transforms from a continuous network distribution to a broken network distribution and an isolated distribution, and the carbides are significantly refined and distributed uniformly, which promotes the improvement of the strength and toughness of the roll. The use of titanium microalloying to treat high-speed steel rolls is also an important means to improve the structure and performance of the rolls. The author studied titanium containing > 2.0% C, 4.0%~6.0% V, 4.0%~6.0% Cr, 4.0%~7.0% W, 3.0%~ 5.0% Mo, 1.0%~3.0% Co and 1.0%~3.0 % Nb, the influence of high speed steel roll structure and properties.

With the addition of titanium, the austenite grains and eutectic structures in the high-speed steel roll structure are significantly refined, and the eutectic carbides appear to be significantly necked and disconnected. This is because titanium often exists in the form of TiC in molten steel, and both TiC and austenite have face-centered cubic lattice. aTiC=0.433 nm, aγ=0.360 nm. Calculation shows that along certain crystallographic directions, the distance difference between the two lattice atoms does not exceed 1.5%, such as [001]TiC//[110]γ-Fe and [110]TiC //[111]γ-Fe. Therefore, γ-Fe can nucleate and grow with TiC as the heterogeneous core, and titanium can increase the austenite nucleation rate, refine primary austenite, and then refine carbides. In addition, TiC can form carbides prior to MC in the high-speed steel melt, and both TiC and MC have face-centered cubic lattice. When they crystallize, the [100] crystal orientation is their optimal growth direction, so when the crystallization is completed , the crystal will be surrounded by (111) crystal plane, aMC=0.415 nm, when MC uses the (111) crystal plane of TiC as the nucleation interface, the two-dimensional lattice mismatch degree δ=4.1% can be calculated, MC and The lattice mismatch between TiC is very small, and TiC can serve as an effective heterogeneous core for MC carbide nucleation, promoting the refinement of MC carbide. The refinement of the matrix structure and the improvement of carbide morphology and distribution promote the improvement of the strength and toughness of high-speed steel rolls. In particular, the impact toughness is significantly improved, exceeding 39%, and the wear resistance and thermal fatigue resistance are also significantly improved. In centrifugally cast high-carbon high-speed steel, adding rare earth-titanium-nitrogen-potassium for composite modification treatment can significantly refine the carbides and matrix structure, improve the crack formation resistance of high-speed steel rolls, and also significantly improve the performance of high-speed steel rolls. Thermal fatigue performance.

High-boron low-alloy high-speed steel rolls have less precious elements and low production costs. In the future, more attention should be paid to the research on the solidification process, casting performance, heat treatment process, mechanical properties and wear resistance of this new material. Modification treatment, microalloying and inoculation treatment all help to improve the strength, toughness and wear resistance of high-speed steel rolls, but the stability and reliability of these processes need to be further improved. On the basis of improving the material of high-speed steel rolls, continuing to conduct in-depth research on the forming process and usage characteristics of high-speed steel rolls will also help accelerate the promotion and application of high-speed steel rolls.

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