Difficulties in hot processing of forging steel support rollers

The support roll is the main component on the rolling mill, which plays a supporting role for the working roll to cause plastic deformation of the metal, in order to produce various specifications of rolled products.

Due to prolonged friction and rolling contact between the work roll and the support roll, the support roll bears significant shear and contact stresses, resulting in deformation strengthening and wear. Therefore, its main failure modes are peeling and wear. The tonnage of forged steel support rollers is large, the production process is complex, and waste products are easily generated. Therefore, it is difficult to produce large forged steel support rollers, and the main difficulties are manifested in the following aspects:

High roller body hardness

Due to the development of rolling technology and the application of high-speed steel work rolls, the requirements for the hardness of the support roll body are constantly increasing. At present, the surface hardness of domestically produced support rollers is usually HS55-65; The surface hardness of the support roller produced abroad is HS70-75, and the depth of the quenched layer is required to be greater than 50mm, with a hardness difference of less than HS5. In order to improve the service life of the rolling mill, it is necessary for the rolling mill to have good wear resistance, requiring high hardness but at the same time reducing the anti peeling ability of the rolling mill body. High hardness generally requires a high carbon content, and a high carbon content can form a network of carbides in the cast steel rolling mill, which is prone to peeling defects. If there is severe segregation or large non-metallic inclusions inside the rolling mill, the possibility of rolling mill peeling is greatly increased. The anti stripping of the supporting roller has become an important requirement, and accelerating the cooling rate can achieve high hardness on the surface of the roller body, while generating significant residual stress. A hardened layer depth greater than 50mm means that the cast steel rolling mill material needs to have a certain degree of hardenability. Therefore, a comprehensive analysis must be conducted on the selection of steel grades, forging processes, and heat treatment process parameters to comprehensively resolve these contradictions.

Uniformity of hardness

The method for determining the uniformity of hardness in the working layer of forged steel support rollers is to measure the hardness values at a total of 20 points along the circumference of the entire roller body in four directions and five axial positions, with a fluctuation value of less than or equal to HS2. At present, the hardness uniformity of domestically produced forged steel support rollers generally fluctuates within the range of HS3-4 hardness values, while the hardness uniformity of Japanese Hitachi produced forged steel support rollers fluctuates within HS2 hardness values. The uniformity of hardness is directly related to the quality control and technical application of various processes such as smelting, casting, forging, and heat treatment, and is not solely based on the results of forging. The uniformity of hardness solves the comprehensive problems of uniform metal metallographic structure, control of chemical element segregation in steel, and purity in various process steps.

Depth of hardened layer and residual stress

The technical standard for forged steel support rollers stipulates that the hardness difference is HS5, the depth of the quenched layer is greater than 50mm, and the residual stress is not higher than 500MPa. From these requirements, it can be seen that the material of the rolling mill must have a certain degree of hardenability and hardenability, and there must be a complete heat treatment system to ensure the performance of the rolling mill. High temperature annealing is generally used to eliminate residual stress, followed by differential heat treatment to ensure the depth and hardness of the quenched layer on the roll body, while ensuring sufficient toughness in the neck and core of the roll body.

Metallographic structure of metals

A good metallographic structure can ensure the surface hardness of the supporting roller and fluctuate within a small range. The supporting roller should have excellent metallographic structure, forming a tempered matrix of bainite or martensite. Carbides are not allowed to be concentrated or distributed in large blocks at grain boundaries, and the finer the metallographic grain size, the better. Carbides are best to exist in spherical form.

Difficulty in forging and forming

Due to its large weight and high carbon content, forged steel support rollers pose great difficulties to the forging process, and are prone to defects such as cracking and insufficient compression of the core during the forging process. In order to forge rolls that meet the requirements, a complete process system must be established to ensure the production of rolls, including the smelting of molten steel, casting of ingots, forging heating, forging forming, and heat treatment processes.