The improvement of high-speed steel roll manufacturing technology mainly focuses on improving the purity and uniformity of steel, improving the strength of the roll and the metallurgical combination of the outer layer of the roll and the core of the roll. Different rolling mills and different stands of the same rolling mill have different requirements for roll performance, and the appropriate manufacturing method can be selected according to the required roll performance, roll specifications and production costs.
Features of high speed steel rolls
1) Carbide hardness is high.
The rolls used in the past mostly distributed Fe3C or M7C3 eutectic carbides on the matrix, with coarse structures and low hardness.
High-speed steel rolls generally use high-C, high-V high-speed steel. In addition, it also contains more alloying elements such as Cr, Mo, W, Nb and Co. During solidification and heat treatment, these alloying elements form primary and secondary carbides.
High chromium cast iron roll carbides are M7C3 (2500 Hv) and M23C6 (1600 Hv).
In infinite chilled cast iron rolls, the carbide is Fe3C (l300 Hv). The high-speed steel rolls mainly contain MC (3000 Hv), M7C3 (2500 Hv) and M6C (2000 Hv), which can replace ordinary rolls to improve wear resistance. In addition, with the increase of V content, the skeletal M6C carbide in the high-speed steel structure transforms into the granular VC type.
2) Good thermal stability.
High-speed steel rolls contain more elements such as W, Cr, Mo, V and Nb, and have better thermal stability. The study found that the hardness of ordinary roll material decreases significantly with the increase of temperature, while the high-speed steel roll still maintains 500 Hv at 600 °C, which will greatly improve the wear resistance of the roll. Especially when the Co element distributed in the matrix is added, this tendency is more obvious.
3) It is easy to form an oxide film during use.
During the rolling process, if a continuous, dense, uniform and well-bonded oxide film is formed on the surface of the roll, it can not only reduce the friction coefficient between the roll and the rolled material, but also improve the dimensional accuracy and surface quality of the rolled material. In the initial stage of use, ordinary rolls are more worn due to lack of oxide film protection. Oxide film can be formed quickly during the use of high-speed steel rolls. The appearance of oxide film can reduce the friction between the strip steel and the roll surface, prevent the roll from sticking to steel, and improve the life of the roll. Compared with ordinary rolls, the initial wear of high-speed steel rolls is lower, which is very beneficial for improving the wear resistance of high-speed steel rolls.
4) Good hardenability.
The high-speed steel roll has good hardenability. At room temperature, within 50mm of the working layer, the hardness drop from the roll surface to the core is less than 3HS, which can ensure that the roll has good wear resistance from the outside to the inside.
5) Excellent toughness.
Compared with ordinary infinite chilled cast iron, high chromium cast iron and semi-steel rolls, high speed steel rolls have better basic mechanical properties, tensile strength and fracture toughness than high chromium cast iron rolls and infinite chilled cast iron rolls. Close to semi-steel rolls.
6) Good thermal crack resistance.
Due to its unique structure characteristics, high-speed steel can effectively inhibit the formation and propagation of cracks. In actual use, it is found that high-speed steel rolls do have better thermal cracking resistance than high-chromium cast iron rolls. Therefore, if a general production accident occurs, the impact of the accident can be eliminated through the normal grinding amount or a small additional grinding amount, which simplifies the processing process and saves costs.
7) Good wear resistance.
Compared with semi-steel, high-chromium cast iron and high-nickel-chromium infinite chilled cast iron rolls, the rolling tonnage of high-speed steel rolls is greatly increased. The F1-F4 frame is about 10000t, and the F5 frame is about 6000t, which shows that the high-speed steel roll has good wear resistance. The increase in rolling tonnage of high-speed steel rolls has led to a significant decrease in roll consumption. Under the same rolling conditions, the roll consumption of high-speed steel rolls is 1/5 of high-chromium cast iron rolls in F1 frame, 1/7 of high-chromium cast iron rolls in F2 frame, and 1/7 of high-chromium cast iron rolls in F4 frame. 1/13 of the roll, F5 frame is 1/8 of the high nickel chromium infinite chilled cast iron roll. the
High-speed steel roll manufacturing technology, manufacturing high-speed steel roll mainly includes forging, casting, spray forming, hot isostatic pressing and other technologies. The popularization and use of forged high-speed steel rolls has progressed slowly; at present, the more common manufacturing methods of cast high-speed steel rolls in industrial production include centrifugal casting, continuous casting outer layer forming, electroslag remelting and liquid metal electroslag welding.
The improvement of casting high-speed steel roll manufacturing technology mainly revolves around improving the purity and uniformity of steel, improving the strength of the roll and the metallurgical combination of the outer layer of the roll and the core of the roll. Different rolling mills and different stands of the same rolling mill have different requirements for roll performance, and the appropriate manufacturing method can be selected according to the required roll performance, roll specifications and production costs.
1）Centrifugal casting method.
The main feature of the centrifugal casting roll is that the liquid outer layer material and the core material are poured into the mold at a certain time interval. Centrifugal rotation time, pouring interval of molten metal in the roll core, pouring temperature, and prevention of segregation of outer layer elements and interface oxidation of inner and outer layer materials are the keys to the success of manufacturing rolls by this method.
High-speed steel contains more elements such as W, Cr, Mo and V, and the density difference between these elements and the carbides formed is large. Under ordinary centrifugal casting conditions, the element segregation of high-speed steel rolls is serious.
Kawasaki Iron and Steel Co., Ltd. of Japan analyzed that the segregation of centrifugal casting high-speed steel rolls is mainly the segregation of MC carbides, which seriously affects the wear resistance of the rolls. The study found that the MC type carbide is mainly the segregation of primary crystalline VC, which is caused by the large difference in density between VC and molten steel.
The method to prevent VC segregation is to increase the density of MC-type carbides by adding Nb element, and limit the addition of segregation elements W and Mo. Select the basic composition 2.0%C-6.0%V-7.0%Cr-2.5%Mo, add 1.0%~1.5%Nb to test. As a result, due to the formation of dense MC-type composite carbides (V, Mo and Nb-based carbides), whose density is close to that of molten steel, VC is reduced, and the carbide segregation of centrifugally cast high-speed steel rolls is effectively controlled.
However, the red hardness of the W-free low-Mo high-speed steel roll decreases significantly, and the wear resistance decreases. In addition, Nb increases the quenching temperature of the steel and reduces the temperature at which the secondary hardening peak occurs. During the casting process of high-speed steel, the MC-type carbides of the Nb-based system are coarser than the MC-type carbides of the V-based system, which makes the rolls prone to spalling during use.
Therefore, how to determine the optimal ratio of V and Nb composite addition under given process conditions. Control the generation of coarse NbC, increase the content of W and Mo in the roll, and ensure the red hardness and wear resistance of high-speed steel. At the same time, it is necessary to eliminate the segregation of high-speed steel rolls, which is an urgent problem to be solved in the production of centrifugal casting high-speed steel rolls.
In addition, during the centrifugal casting process of high-speed steel rolls, the speed of the centrifuge also has a significant impact on the performance of the rolls.
The research found that with the increase of centrifuge speed, the structure of the roll is denser, the hardness is increased, the amount of wear is reduced, and the wear resistance is continuously improved.
However, if the speed is too high, the wear resistance of the roll will not be improved significantly, but will increase the power consumption and aggravate the segregation of alloy elements. In order to improve the quality of the bonding layer of high-speed steel composite rolls, Chinese scientists have invented a multi-layer casting process for high-speed steel composite rolls. From the outside to the inside, the rolls are the high-speed steel working layer, the middle layer 1 composed of core materials, the middle layer 2, the core 1 and the core 2. After centrifugal pouring of the working layer, reduce the rotation speed to pour the middle layer 1 and the middle layer 2 respectively, then reduce the rotation speed again to 200-350 rpm to pour the core 1, turn off the power to freely decelerate and pour the core 2.
Compared with the original double-layer or triple-layer composite high-speed steel roll, the invention’s composite high-speed steel roll has a 30% reduction in the peeling rate of the working layer during the rolling process, and a 50%-60% reduction in the probability of roll breakage.
But this method manufactures the high-speed steel roll process too complicated, and there are too many types of metal melts, and the operation is cumbersome.
Recently, relevant institutions have carried out in-depth research on the segregation mechanism of high-speed steel rolls, and found that the main reason for the segregation of centrifugally cast high-speed steel rolls is the presence of atomic clusters in high-speed steel that have a different density from the molten metal. Under the action of centrifugal force, atomic clusters with high density move to the roll surface, and atomic clusters with low density move to the center of the roll. The main reason for the formation of atomic clusters is that the physical and chemical properties of various elements in high-speed steel rolls are different. The main factors affecting the segregation of centrifugal casting high-speed steel rolls are: the number of revolutions of the centrifuge, the cooling rate of molten metal solidification and the properties of atomic clusters. The higher the rotational speed, the smaller the solidification cooling rate and the more severe the segregation, otherwise the segregation is reduced. Atom clusters with the same volume and density are in the form of agglomerates or strips, which have a larger spatial extension than when they are spherical, which is beneficial to reduce segregation.
On this basis, the influence of electromagnetic field on the segregation of centrifugally cast high-speed steel rolls was studied. It is found that the electromagnetic field generates electromagnetic force in the metal solution, and its tangential component is opposite to the movement direction of the solution, forcing the solution in front of the solid-liquid interface to flow, causing the change of the element distribution coefficient k, which is beneficial to reducing element segregation in high-speed steel rolls. The action of electromagnetic force will also promote the strong scouring effect of the metal melt on the solid-liquid interface and dendrite ends, causing crystals to fall off from the mold wall and dendrites to break, and promote the formation of equiaxed crystals, which is conducive to the reduction of macro segregation.
In the electromagnetic centrifugal casting of high-speed steel rolls, the action of the magnetic field also leads to the dissimilarity of the motion state between the first precipitated phase, the atomic cluster and the metal solution due to the difference in conductivity and density. This effect also has an important influence on the solute redistribution and final composition segregation during the solidification process of high-speed steel rolls.
2) Continuous casting outer layer forming method (CPC).
In order to overcome the segregation defects of centrifugal casting high-speed steel rolls, Nippon Steel Corporation of Japan has developed the CPC method for manufacturing high-speed steel rolls, which has the characteristics of energy saving and good roll performance.
The basic principle is to cast molten steel made of the outer layer of the roll into the gap between the vertically erected mandrel and the water-cooled mold. While the molten steel is gradually melting with the mandrel, it is solidified sequentially and pulled downward intermittently. Made into composite rolls.
In order to completely weld the cast outer material and the mandrel, heat is supplied to the molten steel and the mandrel by electromagnetic induction heating. The high-speed steel roll produced by the CPC method has a fine, uniform structure and few inclusions, almost no defects such as shrinkage cavity and porosity, and its overall performance is significantly better than that of ordinary centrifugally cast high-speed steel rolls. It not only overcomes the segregation defects of the centrifugal casting roll, but the core of the roll can be made of high-strength forged steel, and the core has a higher strength, which is also impossible for the centrifugal casting method. At present, the CPC method in foreign countries has been industrialized. A set of CPC devices designed by Nippon Steel Corporation of Japan has the following capabilities: roll body diameter: 250~850mm; roll body length: ≤3000mm; outer layer thickness: ≤100mm; roll length : ≤5700mm; Roll weight: ≤15000Kg.
Chinese scientists have also invented a continuous casting device for composite high-speed steel rolls, which consists of an operating platform, a molten steel casting system, a crucible, a separation ring, a water-cooled crystallizer, a vibrator, a positioning guide device, an induction heating device, and a casting system.
Its main technical features are reflected in the fact that the water-cooled crystallizer and the crucible are on the operating platform, the two vibrators are under the operating platform, the water-cooling crystallizer is connected to the crucible through a separation ring, and the molten steel casting system is arranged on the side above the crucible. The roller core induction heating device and positioning guide device are located above the crucible, the billet drawing system is located under the operating platform, and the dummy plate is facing the lower end of the crystallizer. At present, this technology has been applied to produce composite high-speed steel rolls with high W and V content, light segregation, and good wear resistance, which can reduce the frequency of roll changing during rolling, greatly improve the operating rate of the rolling mill, reduce production costs, and improve economic benefits .
3) Electroslag remelting (ESR).
Electroslag remelting is a remelting process widely used in the production of high-quality steel. Initially, this process was mainly used for deoxidation and desulfurization, coupled with the effect of controlling solidification, which greatly improved the distribution characteristics of non-metallic inclusions.
Its main advantages are as follows: a) refine grains; b) reduce non-metallic impurities and inclusions in steel; c) improve hot workability of steel; d) reduce macro-segregation and micro-segregation of the structure. Improves carbide distribution in tool and die steels.
In recent years, the rotary electroslag casting method developed on the basis of the ordinary electroslag remelting method has been used to manufacture high-speed steel composite rolls. The basic principle of the rotary electroslag casting method is to place a concentric water-cooled casting mold around a cylindrical high-strength alloy steel as the core material, and place a consumable electrode made of high-speed steel or semi-high-speed steel between the forged steel and the casting mold , while the electroslag remelting of the consumable electrode, the forged steel shaft and the crystallizer rotate synchronously, and the gap between the two is continuously filled with remelted high-speed steel or semi-high-speed steel molten steel, and the molten steel melts the surface of the forged steel shaft , to form a metallurgical bond with the forged steel shaft after the molten steel solidifies.
With the continuous injection and solidification of molten steel, the crystallizer moves upwards, and finally forms a composite roll with the outer layer made of high-speed steel, and the roll core and roll neck made of forged steel. Because the outer material is refined by electroslag, it has high cleanliness and can meet the requirements of cold rolling, so it can be used as a cold rolling work roll.
Hitachi, Japan produced cold-rolled semi-high-speed steel rolls with a size of φ425 mm × 1880 mm by ESR method. The biggest problem with the ESR method is that it is expensive and difficult to manufacture larger rolls. In addition, the electroslag material contains more CaF, which will escape harmful gases such as HF, SiF4, and SF6, which will endanger the health of workers and cause environmental pollution.
The semi-high-speed steel roll blank produced by ESR method is processed into a roll after light forging, quenching at 1060°C and tempering at 500°C. The surface hardness is 97 HS, and it has good wear resistance and accident resistance.
Chinese scientists have also invented the method of electroslag casting to manufacture composite rolls. The pre-manufactured roll core is used as the inner mold for electroslag casting, and the roll outer layer material to be compounded is used as the consumable electrode for electroslag casting. The inner diameter of the outer crystallizer of the device and the diameter of the molten metal outlet of the connected double U-shaped crucible are equal to the outer diameter of the compound roll to be compounded; the melting rate of the consumable electrode is controlled at 100-1000 kg/h. Consumable electrodes can be one or more than two. The method can make the composite roll obtain a narrower transition layer and good composite performance.
4) Electroslag welding of liquid metal (ESSLM).
In 1996, the Ukrainian ELMET Roll Company developed a new process for manufacturing high-speed steel composite rolls by liquid metal electroslag welding, which is a new roll manufacturing method based on the CPC method and electroslag purification. When the roll is manufactured by the ESSLM method, its outer layer is solidified in a specially designed conductive water-cooled copper crystallizer. The crystallizer not only solidifies the poured outer layer of molten steel, but also serves as a non-consumable electrode in the electroslag process. At the beginning of the compounding process, the mandrel, which is the core of the high-speed steel roll, is inserted into the mold and coaxial with it. The gap between the outer surface of the shaft and the inner surface of the mold determines the thickness of the outer layer of the roll. Then the slag liquid melted in another melting device is poured into the gap between the crystallizer and the mandrel, the slag liquid forms a slag pool, and its heat preheats the surface of the mandrel. Then pour the outer layer of high-speed steel, which can be poured continuously or according to a preset program. The molten steel floats the slag up and is refined by the slag as it passes through the slag pool. The molten steel fuses with the preheated mandrel surface and solidifies due to the cooling of the mold to form a composite layer. The solidified part is continuously pulled out from the crystallizer by means of a moving device (or the mold is moved up), and at the same time, the upper molten steel is continuously injected until the predetermined length of the roll is reached. At present, the ESSLM method has been industrialized abroad, and the outer layer of the high-speed steel composite roll produced by the ESSLM process is dense. No shrinkage cavity, crack, porosity and other defects, the outer metal and the mandrel are well fused, and the main alloy elements, hardness and microstructure are evenly distributed in the height direction and cross-section.
Spray Formed (Osprey) High Speed Steel Rolls
Osprey technology is a near-final forming technology developed on the basis of powder metallurgy inert gas atomization powder making. It makes full use of the refined liquid metal and uses high-pressure inert gas to atomize the alloy liquid flow into fine molten droplets. , the droplets fly under the action of high-speed airflow and are cooled by the atomizing gas. These droplets are deposited on a receiver with a certain shape before they are completely solidified, and a deposited blank with a certain shape can be obtained by controlling the movement of the receiver.
The material manufactured by Osprey technology has the following technical characteristics: ① no macro segregation; ② isotropic and uniformly dispersed structure; ③ dispersed and precipitated initial grains; ④ low oxygen content; ⑤ improved hot processing performance.
Because of the above-mentioned characteristics of Osprey technology, the use of it to manufacture high-speed steel rolls has attracted people’s attention.
Domestic scholars have studied the structure and performance of spray-formed high-speed steel rolls. The main components (wt.%) of high-speed steel roll materials are: 0.9-1.1C, 0.35-0.45Si, 0.70-0.80Mn, 7.90-8.20Cr, 1.45-1.55Mo, 1.5-1.7V, 0.45-0.55W. No obvious carbide was observed in the optical microstructure of the spray-formed high-speed steel roll, but there was a certain amount of retained austenite in the structure. Discontinuously distributed carbide particles were observed on the grain boundaries under a scanning electron microscope. In addition, the crystal grains of the sprayed morphology samples were obviously refined.
High-speed steel rolls produced by the British National Roll Manufacturing Company with Osprey technology. The size of the roll body reaches φ400 mm×1000 mm, and its structure is much finer than that of forging, and the coarse eutectic carbide is completely eliminated. The combination of the boundary between the roll core and the spray layer is a good metallurgical bond, the fatigue performance of the roll is improved, and the service life is extended. Currently, the development and manufacture of high-speed steel composite roll equipment with a diameter of φ800 mm×2000 mm is being considered.
The BABCOCK company and WILCOX company of the United States and the national roll company are also planning the same project to study the manufacture of high-speed steel composite rolls with Osprey technology.
Japan’s Sumitomo Heavy Industries Co., Ltd. has used a spray forming equipment with a receiver horizontal reciprocating device to produce high-speed steel roll sleeves and high-speed steel/carbon steel composite rolls with a maximum diameter of 800 mm and a weight of 1 ton since 1989. Used for various profile rolling. In actual use, the roll produced by this process can increase the service life by 1.6-3.0 times than the general powder metallurgy high-speed steel roll, and more than 3.6 times than the traditional casting high-speed steel roll, which has a huge market prospect.
Hot isostatic pressing (HIP) high speed steel roll (H2)
Compared with steel produced by traditional methods, the high-speed steel produced by applying the combined process of spray granulation and hot isostatic pressing in powder metallurgy technology has many advantages, such as excellent machinability, toughness, hardness and shape stability after heat treatment. Compared with cast high-speed steel rolls with the same composition, the high-speed steel rolls produced by the HIP process have finer and more uniform carbides, and the morphology and distribution of carbides play a decisive role in the thermal fatigue performance, spalling resistance and toughness of the rolls , so the overall performance of HIP high-speed steel rolls is significantly better than that of cast rolls.
In addition, in order to further improve wear resistance, HIP high-speed steel rolls can adopt higher carbon content and alloy content, and still maintain good carbide morphology. When producing high-speed steel rolls with the HIP process, the roll core is generally made of cast or forged steel materials. The high-speed steel powder used for the outer layer of the roll body is filled outside the core of the roll, and then sintered into a roll at a high temperature above 1000°C and a pressure above 100 MPa after vacuuming. Since the HIP process equipment needs to withstand high pressure, limited by the equipment, the HIP process can only produce small-diameter high-speed steel rolls.