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1 Introduction Improving the internal quality and precision of rolled products, as well as reducing production costs, are important research topics in the field of steel rolling today. The application of new rolling technologies can effectively improve the quality of rolled products, but it also places higher demands on the wear resistance, strength, and toughness of rolls. To meet this requirement, cast high-speed steel rolls were developed abroad in the late 1980s; their performance and effectiveness are significantly better than those of traditional roll materials, and high-speed steel rolls have since been widely used abroad. In recent years, research on high-speed steel rolls and ring rolls in China has also received attention. However, the manufacturing level and performance of domestically produced high-speed steel rolls and ring rolls still fall far short of those of advanced foreign products. At present, the integral casting method is primarily used for manufacturing high-speed steel ring rolls in China. The problems associated with this method include difficulties in machining the inner holes of the ring rolls, high consumption of alloys, high costs. Moreover, no heat treatment is carried out during the casting process, resulting in a coarse microstructure, low hardness uniformity, poor wear resistance, and a short service life. Therefore, it was decided to use the centrifugal composite casting method to develop high-performance high-speed steel composite roll rings, that is, to use modified high-speed steel as the outer layer of the roll ring and alloy ductile iron as the inner layer. The newly developed high-speed steel roller rings have a dense structure, high hardness, good hardness uniformity, excellent wear resistance, and low production costs. 2 Design of the chemical composition of the roller ring: The material for the outer layer of the roller ring is high-speed steel. The principle guiding the composition design is to minimize the amount of alloys (especially expensive alloys) used, while still ensuring that the high-speed steel possesses high hardness and wear resistance. W, Mo, Cr, and V are the basic elements in high-speed steel, but their densities vary greatly: W (19.3 t/m3), Mo (10.2 t/m3), Cr (7.2 t/m3), V (6.1 t/m3), Fe (7.9 t/m3). When high-speed steel roll rings are manufactured using conventional centrifugal casting methods, the alloy tends to experience segregation, which severely affects the quality of the roll rings. Kawasaki Steel Corporation in Japan studied the effect of MC-type carbide segregation resulting from centrifugal casting of high-speed steel on the wear resistance of rolls, concluding that MC-type carbides are primarily a result of the segregation of VC, which is caused by the significant difference in density between VC and the molten steel. The addition of Nb to high-speed steel results in the formation of MC-type composite carbides with high density (V, Mo, Nb-based carbides). These carbides have a density close to that of the molten steel, which helps to reduce VC content. This approach enables effective control of carbide segregation in centrifugally cast high-speed steel, thereby improving the wear resistance of high-speed steel rolls. Co is a precious element, its price being ten times that of W; its main advantage is that it enhances red hardness. Furthermore, it also improves thermal conductivity and machinability; therefore, high-speed steels containing Co are used for roller rings abroad. However, the working conditions of roller rings are different from those of cutting tools; they are not in continuous contact with the workpiece, and high-pressure water is used for cooling during operation, so the surface does not turn red. As a result, roller rings do not require as high levels of red hardness as cutting tools do. Therefore, cobalt-free high-speed steel is used in the manufacture of roller rings. The coarse as-cast structure and grain boundary network of eutectic carbides in cast high-speed steel significantly reduce the strength and toughness of the material, and these defects are difficult to eliminate through heat treatment; therefore, a RE-Mg-Ti composite modifier was used to modify the high-speed steel. RE and Mg can reduce the levels of S and O in the molten steel, increase the supercooling degree during eutectic solidification, and thereby refine the eutectic structure. In high-speed steel, RE and Mg also have the effect of refining the dendrites; as a result of the refinement of these austenite dendrites, the eutectic liquid pool that forms due to segregation between the austenite dendrites becomes smaller during the later stages of solidification, which in turn leads to the refinement of the eutectic carbides. Furthermore, TiC has a very high melting point (3150°C), and can be formed at high temperatures. During the solidification of high-speed steel, Ti reacts with C in the molten steel, resulting in the formation of a large number of dispersed TiC particles. Both TiC and austenite, as well as MC carbides, have a face-centered cubic crystal structure, and their lattice constants are similar: the lattice constant of TiC is 0.432 nm, that of MC is 0.415 nm, and that of α-gardenite is 0.357 nm; thus, the degree of lattice mismatch is very small. TiC can act as a heterogeneous core for austenite and MC, refining the austenite dendrites and promoting the extensive formation of isolated massive MC-type carbides, thereby improving the morphology and distribution of eutectic carbides. According to the findings in the literature, an increase in MC will further promote the formation of flaky M2C and inhibit the formation of fishbone-shaped M6C. Ordinary alloy cast iron is used as the inner layer of the composite roll ring; due to its high brittleness, it finds it difficult to withstand large compressive stresses during use, and is prone to brittle fracture. Cast steel is used as the inner layer of the composite roll ring; although it possesses good strength and toughness, its castability is poor. Therefore, using alloy ductile iron as the inner layer of the composite roll ring overcomes the disadvantages of ordinary alloy cast iron and cast steel; moreover, since the rolling torque in the composite roll ring is transmitted through the ductile iron keyways, stress concentration in the outer layer of the composite roll ring can be reduced. Furthermore, compared with alloy cast iron and cast steel, ductile iron also possesses better machinability. The chemical composition of the designed composite roller ring is shown in Table 1. 3 Manufacturing of high-speed steel composite roller rings 3.1 Manufacturing process of high-speed steel composite roller rings The manufacturing process for high-speed steel composite roller rings is shown in Figure 1. 3.2 Melting of high-speed steel High-speed steel is melted using a 500 kg alkaline medium-frequency induction furnace. During the melting process, high-speed steel scrap, scrap steel, pig iron, ferromolybdenum, and ferrotungsten are added first; after the molten steel becomes clear, ferrochrome is added. Before it is taken out of the furnace, ferroniobium and ferrovanadium are added, and aluminum wire is used to deoxidize the molten steel. The temperature of the molten steel when it leaves the furnace is 1520–1560°C. A composite modifier containing RE, Mg, and Ti is pre-placed in the ladle; the particle size of this modifier is 8–15 mm (it must be dried before use), and the molten steel is subjected to composite modification using the injection method. 3 3 Centrifugal casting of high-speed steel composite roller rings 3 3 1 Mold preparation The mold is made of HT200; its wall thickness is 90 mm. It is preheated to a temperature above 200°C, and at this temperature, a layer of refractory coating is applied to the inner surface of the mold, with a thickness of 35–55 mm. The coating uses quartz sand and quartz powder as refractory aggregates, while water glass (with a modulus of 22–26 and a density of 150–155 g/cm3) serves as the binder. An appropriate amount of water is added to the mixture. The coating must be continuously stirred to prevent it from settling. After the coating is poured into the preheated mold, the mold begins to rotate at low speed. Once the coating is evenly distributed and thickens, the rotation speed of the mold is increased so that the coating adheres evenly to the inner surface of the mold. The rotation is stopped once no more vapor is produced from the coating. After being thoroughly dried, the mold is installed on a centrifuge for pouring; during pouring, the temperature of the mold is 160–240°C. 3 3 2 Determination of centrifugal casting process parameters for high-speed steel composite roller rings. High-speed steel composite roller rings are cast using a vertical centrifuge. The main control parameters for roll ring centrifugal casting are the rotation speed of the metal mold, the interval between the pouring of the inner and outer layers of molten metal into the mold, and the pouring temperature of the molten metal. At present, there is no formula for calculating the rotational speed based solely on theory; by introducing empirical parameters, Konstantinov developed a relatively reasonable formula for calculating the rotational speed, ensuring that casting defects such as cracking and sand adhesion do not occur. The interval between the casting of the two metals is a key factor determining the bond strength of the composite layer in high-speed steel composite roller rings. If the interval time is too short, the inner and outer layers of metal may mix with each other, resulting in a decrease in the wear resistance and mechanical strength of the composite roller ring. If the interval time is too long, the bonding strength between the inner and outer layers weakens, and separation between these layers can occur during demolding, cooling, machining, heat treatment, and use of the roller ring. From the perspective of metal solidification, when about 85% of the outer layer of metal has solidified, pouring in the inner layer of metal promptly allows the remaining 15% of the outer layer metal to mix with it and solidify in sequence, thereby achieving a higher bonding strength in the centrifugally compounded layer. Based on this analysis, the centrifugal compounding casting process for the high-speed steel composite roller ring with an inner diameter of φ180 mm and an outer diameter of φ300 mm is shown in Figure 2. The casting temperature has a significant impact on the quality of the roll ring. According to crystallographic principles, during the solidification process of a casting, the cooling time and cooling rate are determined by the structure of the casting, the temperature at which the molten steel is poured, the thermophysical properties of the metal, and the heat dissipation coefficient of the mold. Under certain conditions, the structure of the casting and the thermophysical properties of the metal remain unchanged, while the solidification rate of the molten steel and the heat dissipation conditions of the mold are influenced by human factors. Among these, the pouring temperature has the greatest impact on the solidification rate; when the pouring temperature is too high, the molten steel solidifies more slowly, resulting in a coarser crystal structure. This reduces the mechanical properties and fatigue limit of the roll ring, ultimately affecting its performance in use. Practice has shown that it is reasonable to control the pouring temperature of the outer layer of molten steel at 1460–1520°C, and the pouring temperature of the inner layer of molten iron at 1350–1380°C. 3 4 Heat treatment of high-speed steel composite roller rings: Due to the significant difference in thermal expansion coefficients between the inner and outer layers of high-speed steel composite roller rings, the internal stresses generated during casting are greater than those in roller rings cast as a single piece. By formulating an appropriate heat treatment and annealing process, it is possible to eliminate these casting stresses, ensure that the roller rings do not crack during quenching, and at the same time improve their machinability. Its annealing process involves heating to 830–880°C for (3–4) hours, then cooling in the furnace to 720–760°C and holding that temperature for 2–4 hours. Following that, it is cooled in the furnace to 500°C before being taken out for air cooling. After annealing, the hardness of the roll ring is 220–250 HB, granting it good machinability. Due to the high content of alloying elements in high-speed steel, the quenching temperature is usually above 1150°C. If a quenching temperature above 1150°C is used for high-speed steel composite roller rings, the ductile iron structure in their inner layer will become significantly coarser; in some areas, it may even melt, which leads to a significant deterioration in the mechanical properties of the ductile iron in that layer and affects the safe use of the composite roller rings. Therefore, the quenching of high-speed steel composite roller rings is carried out using a power-frequency induction heating surface quenching process on a vertical quenching machine. To ensure uniform heating and cooling, the roller rings rotate at a speed of 15–25 rpm, while the speed of the induction coil movement is 10–20 mm/s. Due to the high contact stress on the roller ring, a proper distribution of residual stresses can enhance its contact fatigue strength and service life. Preheating before quenching can improve the distribution of residual stresses, and the preheating method employed is induction preheating prior to quenching. Tests have shown that increasing the preheating temperature of the core not only increases the residual compressive stress in the hardened layer, but also significantly reduces the amplitude of the tensile stress in the transition zone, which helps to extend the service life of the roll ring. To stabilize the structure and eliminate quenching stresses, the roller rings must undergo tempering after quenching. This tempering is carried out in a 45 kW box-type resistance furnace, with the tempering process being: 520–560°C × (3–4) hours × 2 times. 4 Performance and Applications of High-Speed Steel Composite Roll Rings The composite quality of the inner and outer layers of these high-speed steel composite roll rings was examined using an ultrasonic flaw detector. The results showed that the inner and outer layers were well bonded together, with no casting defects such as delamination, inclusions, or pores in the bond layer. At the same time, the surface hardness of the roller rings was also measured and compared with that of high-chromium cast iron roller rings; the results are shown in Table 2. It can be seen that the hardness of the high-speed steel roller rings is higher than that of the high-chromium cast iron roller rings, and their hardness uniformity is significantly better than that of the latter. In addition, on Y-shaped test pieces that were cast in the same furnace and subjected to the same heat treatment as the roller rings, samples measuring 10 mm × 10 mm × 55 mm were fabricated using a wire cutting machine, and their impact toughness was tested (Table 2); the impact toughness of the high-speed steel roller rings was more than twice that of the high-chromium cast iron roller rings. The developed high-speed steel composite roll rings were subjected to industrial testing on the middle frame of a wire rod rolling mill at a steel plant. The results showed that these roll rings are safe and reliable to use, and their service life is more than 5 times longer than that of high-chromium cast iron roll rings. The use of high-speed steel composite roller rings can significantly increase the operating efficiency of rolling mills, reduce the workload on workers, and improve the surface quality of the rolled products. 5 Conclusions (1) The high-speed steel composite roller rings produced by the centrifugal composite casting method, with high-speed steel as the outer layer and alloy ductile iron as the inner layer, exhibit a tight bond between the two layers; the bond zone is free from casting defects such as delamination, inclusions, and pores. (2) After induction heat treatment, the hardness of the outer layer of the high-speed steel composite roller ring increases significantly, with little impact on the properties of the inner layer. (3) High-speed steel composite roller rings are used in industrial production; they are safe and reliable to use. Their service life is more than 5 times longer than that of high-chromium cast iron roller rings, which allows for a significant increase in the operating efficiency of rolling mills and reduces the workload on workers, resulting in good economic benefits.