Abstract: With the upgrading of the rolling technology and rolling mills， higher quality requirements for rolled products，and application of semi-high-speed steel & high-speed steel work rolls ( intermediate rolls ) ， the demands for rolling performance of backup rolls become more rigorous. In this paper，the status of the backup roll manufacturing technology was described，the development trend of the materials，melting and heat treatment process were analyzed. In order to meet the demands of various mills and products，backup rolls with customized function become an inevitable choice，the higher alloy content design，electroslag remelting and induction heating process will be the critical direction in future development.
The backup roll is the core component of the rolling mill, which directly affects the stability of the rolling mill, the smooth running of the production line, production consumption and product quality.
In recent years, the continuous growth of demand for strip products has promoted the continuous improvement of rolling mill equipment, rolling technology, roll manufacturing and use technology, and the semi-high-speed steel and high-speed steel cold and hot rolling mill work rolls (intermediate rolls) have been popularized and applied. , its microstructure, hardness, strength, wear resistance, etc. have been greatly improved, and higher requirements are put forward for the performance of the backup roll. With the development and application of large-scale electroslag remelting and overall induction heating equipment, conditions have been created for the innovation of materials, smelting process, heat treatment and other technologies of large-scale forged steel backup rolls.
The use and performance characteristics of the support roller
As the core component of the rolling mill. The back-up roll not only bears all the rolling force, but also has high strength and high rigidity, and has long-term rolling contact with the work roll (intermediate roll), and the roll surface must have wear resistance, contact fatigue resistance and accident resistance. As shown in Figure 1. At the same time, the roll neck and roll body are subjected to cyclic alternating bending stress, and must have high strength, toughness and fatigue resistance. The wear resistance and accident resistance of the roll surface are the two most critical performance indicators.
The wear process of the roller surface of the back-up roller can be expressed as rolling contact-fatigue-generation of micro-cracks-crack expansion-formation of small peeling, as shown in the figure. The wear of the back-up rolls of different rolling mills is different. In the strip contact area of the front stand of the cold tandem rolling mill, the back-up rolls tend to have a “box-type” roll shape. The rear frame is affected by the bending of the work rolls, and the back-up rolls are prone to “edge drop” wear. However, due to the roughening of the work rolls in the cold rolling mill, the back-up rolls are prone to severe wear on the entire roll surface; due to the alternating action of high temperature and cooling water in the hot-rolling mill, there is an oxidation and corrosion environment, and the wear of the back-up rolls is more complicated. The failure of the backup roll is divided into types according to the type, mainly including local wear, roll surface indentation, roll peeling and broken roll. The anti-accident performance of the backup roll mainly refers to the anti-intrusion, anti-scratch, anti-contact fatigue, and anti-crack expansion capabilities, which have a lot to do with the matrix strength and fracture toughness of the material.
Development of Forged Steel Backup Roll Material
The early backup roll materials were 2% Cr (9Cr2Mo), 3% Cr series (70Cr3Mo, 50Cr3MoV, 40Cr3MoV, etc.), and 4% Cr and 5% Cr forged steel backup roll materials were developed in the 1980s. The overall development idea of back-up roll materials is to increase the content of alloys such as Cr and Mo and reduce the content of C.
At present, 5% Cr series forged steel backup rolls are widely used in large strip mills. The C content is 0.4%~0.6%, and the Cr content is 4%~5%. It has good hardenability and wear resistance. The structure is granular M and C carbides dispersed on the tempered bainite matrix. The hardness of the roll surface reaches about 70HSD, and the working layer exceeds 70 mm. Adding an appropriate amount of V can refine the grains and improve the strength and toughness of the core. In order to improve the wear resistance of backup rolls, on the basis of 5% Cr material, some roll manufacturers have developed 5% Cr improved type and 8% Cr material wear-resistant type by increasing the content of Mo, W, V and other alloys or increasing the Cr content. The structure of the forged steel back-up roll is tempered martensite and bainite matrix with M, C3, M2C, and MC type composite carbides, and its application effect is shown in the figure.
Some research results show that: the fracture toughness K, value of the back-up roll material has a corresponding relationship with the number of contact fatigue cycles, increasing the fracture toughness K, value can improve the service life of the back-up roll, reducing the number of carbides in the structure can improve the fracture toughness K, value, as the picture shows. In order to improve the anti-accident performance of backup rolls. On the basis of the 5% Cr material, some roll manufacturers reduce the content of main carbide-forming alloying elements such as Cr, Mo, V, etc., while increasing the content of Si, Mn, etc., so that the alloy enters the matrix by solid solution to achieve solid solution strengthening of the matrix. Effect. The anti-accident type forged steel backup roll was developed, the microstructure is shown in Figure 5, and the fracture toughness of the material is shown in Figure 6.
There are certain contradictions between different performances of backup rolls, such as strength and toughness. Therefore, the development direction of backup roll materials is based on ensuring the comprehensive performance of materials. Realize functional and personalized development to meet the diverse needs of different rolling mills and different products.
Development of Roll Blank Smelting Technology for Forged Steel Backup Roll
At present, the main manufacturing process of forged steel back-up roll billet is: electric furnace smelting-ladle refining-vacuum degassing-(vacuum) ingot casting-forging-post-forging heat treatment-rough machining-ultrasonic flaw detection-forging billet. Due to the large size and heavy tonnage of the large forged steel backup roll billet, casting defects are prone to occur, and its smelting process is relatively complicated, representing the highest level of large casting and forgings. The double vacuum treatment process of vacuum degassing and vacuum pouring started in the 1990s has improved the purity of molten steel, minimized the content of H, O, N and other gases in molten steel, and reduced inclusions to a certain extent. The segregation and crystallization state of the ingot are improved, and the chemical composition, high and low magnification structure, and inclusions of the billet are effectively controlled.Compared with the traditional refining process, the electroslag remelting smelting process has the following characteristics:
(1) The purity of molten steel is high. Under the action of electroslag, large non-metallic inclusions in steel can be removed, and desulfurization and deoxidation are excellent, as shown in the figure, so the plasticity and toughness are better than other smelting methods;
(2) The degree of segregation of the ingot is small. The molten steel is solidified in the water-cooled crystallizer, and the cooling speed is fast, so the composition segregation of the steel ingot is small, and the crystallization of the steel is carried out successively from bottom to top, with uniform structure, no looseness and shrinkage cavity;
(3) The utilization rate of steel ingot is high, which can reach 85%~90%. The larger the tonnage of forgings, the greater the advantage of the utilization rate of electroslag steel ingots over ordinary steel ingots.
Due to the large size of the backup roll billet, the limitation of the electroslag remelting equipment, the large change of the slag composition, hydrogen control, deoxidation and other problems, there is no actual performance of large forged steel backup rolls manufactured by the electroslag remelting process. . In recent years, with the development and improvement of large-scale electroslag remelting technology and equipment technology, it will become possible to prepare large-scale back-up roll blanks by using electroslag remelting technology.
Development of Heat Treatment Process for Forged Steel Backup Roll
At present, the main heat treatment and processing process of forged steel backup rolls are: preliminary heat treatment of forged billet – semi-finishing – final heat treatment – finishing – ultrasonic flaw detection – finished product.
The preparatory heat treatment process is divided into high-temperature normalizing and quenching and tempering methods to ensure the mechanical properties of the roll neck and roll body, adjust the structure and prepare for the final heat treatment. Quenching and tempering heat treatment process can obtain higher strength and toughness, especially fracture toughness.
The final heat treatment is the surface quenching and tempering heat treatment of the roll body, which is the key process to determine the working layer structure, hardness distribution, stress distribution and performance of the backup roll, and is also the core of the backup roll manufacturing technology. The heating process of the final heat treatment generally adopts differential temperature heating or overall induction heating process, and the quenching cooling process generally adopts spray cooling or oil cooling process. The differential temperature heating process uses high-speed burners to spray flames to rapidly heat the surface of the roll body, so that the surface of the roll body is austenitized to a certain depth and the core remains below the temperature of the phase transition point. A deep surface heat treatment method followed by quenching, as shown in the figure.
At present, backup rolls are basically produced by differential temperature heating and quenching process. In the 1980s, Hitachi first developed and applied the power frequency overall induction heating process. As shown in the figure, Japanese cast and forged steel, Kanto Special Steel and Nippon Steel successively applied and promoted this process. It is characterized by short heating time, uniform heating temperature and deep heating layer. Compared with differentially quenched backup rolls, induction quenched backup rolls have been effectively improved in the depth of hardened layer, hardness gradient distribution, and radial residual stress distribution, as shown in Figure 9
The surface hardness of the backup roll body produced by the differential temperature furnace quenching process can only reach 70HSD, and the structure is mainly tempered bainite. However, the surface hardness of the back-up roll body produced by the overall induction hardening process can reach more than 75HSD, and the structure is mainly tempered martensite. The wear resistance is significantly improved, as shown in the figure. Integral induction hardened back-up rolls have become a requirement for the production of high-quality steel strips in the metallurgical industry.
(1) The wear resistance, contact fatigue resistance and accident resistance of the backup roll are the most critical performance indicators.
(2) The functional and personalized design of forged steel backup roll materials can meet the needs of different rolling mills and different products.
(3) Back-up roll products with excellent performance can be obtained by electroslag remelting process and overall induction hardening process, which is the development direction of back-up roll manufacturing technology and is worthy of further study.