1. Analysis of the main advantages of high-speed steel rolls
Compared with steel rolls in the traditional sense, high-speed steel rolls have outstanding advantages in terms of wear resistance at room temperature and high temperature. At the same time, it has high hardness and good red hardness characteristics, and its thermal crack resistance and accident resistance are outstanding. From the perspective of microstructure, it is mainly formed by MC and M6C alloy carbides embedded in the austenite matrix. MC and M6C have outstanding hardness advantages. In addition, the austenite matrix has high thermal stability, a large number of alloying elements participate in the reaction during the mosaic process, and a certain amount of dispersed carbides can be precipitated during tempering. The hardening phenomenon after secondary tempering occurs, promotes the increase of martensite decomposition temperature and shows more precise hardness advantages. High-speed steel rolls are very adaptable and can improve the quality of rolled products while extending their service life. They can be widely used in hot strip continuous rolling finishing stands and unregulated rolling of strips of various materials. , to promote further development of its advantages.
2. Analysis of high-speed steel roll manufacturing technology
2.1 Centrifugal casting method
The main feature of the centrifugal casting method is that the liquid outer layer material and core material are poured into the mould at certain intervals. The key to the success or failure of this method is the centrifugal rotation time, the pouring interval of the roller core molten metal, the pouring temperature, and the prevention of segregation of metal elements in the outer layer and oxidation of the interface between the inner and outer layer materials.
2.2 Continuous pouring outer layer forming method
The main technical point of the continuous pouring outer layer forming method is to put a roller core into the water-cooled casting mould and pour the molten outer layer of high-speed steel molten steel into the middle position of the carbon steel mandrel and the cold crystallized film in a vertical state, so that the high-speed steel is fully combined with the outer metal, solidifying from bottom to top, and finally the solidified part is pulled downward to form a continuous casting outer layer.
2.3 Electroslag remelting method
The main technical features of the electro-slag remelting method are:
The concentric water-cooled casting mould is placed around the cylindrical high-strength alloy steel as the core material. At the same time, a consumable electrode is prepared from high-speed steel or semi-high-speed steel and placed in the middle of the casting mould and the forged steel. After the consumable electrode is completely melted, it can give full play to the function of the outer material and fully fill the space.
2.4 Liquid metal electro-slag dissolution method
The main technical characteristics of the liquid metal electro-slag dissolution method are
Insert the mandrel as the core of the high-speed steel composite roll into the crystallizer and make it coaxial with it. The gap between the outer surface of the shaft and the inner surface of the crystallizer determines the thickness of the outer layer of the composite roller. 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 molten steel, either continuously or according to a preset program. The molten steel floats the molten slag up and is refined by the slag as it passes through the slag pool. The molten steel fuses with the surface of the preheated mandrel and solidifies due to the cooling of the crystallizer to form a composite layer. With the help of a moving device, the solidified part is continuously pulled out from the crystallizer, while the upper molten steel is continuously injected until the predetermined composite roller length is reached.
3. Analysis of high-speed steel roll process improvement
3.1 Improvement of the problem of decrease in roller core strength during heat treatment
In order to effectively enhance and improve the wear resistance of the outer layer material of high-speed steel rolls, the key is to increase the quenching temperature during the manufacturing process of high-speed steel rolls. Under the current technical conditions, for composite high-speed steel rolls, the core material of the roll is mainly high-strength alloy ductile iron. Affected by the limiting factors of the material, if the quenching temperature of the high-speed steel roll is selected above 1250°C, the roll core structure will be significantly coarser, and even local melting will occur, which will significantly reduce the strength of the roll core, and the roll breakage will easily occur during use. Affect the normal operation of rolling mill equipment. We should develop a heat treatment process for high-speed steel rolls as soon as possible. The new process should be simple and easy to implement, with low energy consumption and less pollution, while eliminating high-temperature quenching and ensuring the high strength of the roll core.
The surface induction quenching process is easy to operate and has little impact on the strength of the roll core. It will be feasible to apply it in casting high-speed steel composite rolls. In order to ensure the depth of the hardened layer, it is recommended to use a double-inductor quenching process.
3.2 Improvement of the bonding strength between the roller core and the outer layer
In the process of manufacturing composite high-speed steel rolls using the traditional centrifugal method, ductile iron is mostly chosen as the core material of high-speed steel rolls, which often has the characteristics of precipitated graphite. Since the outer layer material of high-speed steel contains certain special elements, such elements may cause the problem of whitening cast iron. Therefore, the problem of graphitization deterioration is very serious in the areas where the outer layer is in direct contact with the high-speed steel roll core. As carbides continue to precipitate, the brittleness level of the bonded parts increases significantly, which may cause the outer layer of the composite high-speed steel roll to peel off during use.
In addition, when cast steel is used as the core material of high-speed steel rolls, due to its high speed and elongation characteristics, its melting point is higher than that of high-speed steel, and it is easy to mix with the solidified material during the pouring process. The fusion reaction occurs on the inner surface of the outer high-speed steel, causing casting defects and affecting the strength of the joint. To address this problem, during the implementation of the manufacturing process of high-speed steel rollers, an intermediate layer can be added between the roller core material and the high-speed steel material (graphite steel is preferred as the intermediate layer) to improve the bonding state of the roller core interface and the high-speed steel interface. , and achieve the purpose of improving the bonding strength of composite high-speed steel rolls.
3.3 Improvement of casting crack problem
Based on relevant practical work experience, the process of casting high-speed steel rolls, due to the large content and many types of alloy elements, may have an impact on the thermal conductivity of high-speed steel, which occurs during the casting cooling process. The problem of unevenness and to a certain extent increases the residual stress level of high-speed steel rolls.
In addition, alloying elements are prone to segregation problems during the solidification process of high-speed steel rolls. The phase transition in high-speed steel materials is more obvious and the phase transformation stress has a significant increase trend.
Under the influence of the above factors, high-speed steel rolls have a relatively obvious tendency to crack during manufacturing. In order to prevent the formation of cracks from adversely affecting the overall performance of high-speed steel rolls, it is necessary to promptly modify the molten steel during the manufacturing process. For example, you can choose a double-layer coating that combines a heat insulation layer with a refractory layer, or you can Reasonably adjust the speed of the centrifuge during pouring to minimize and eliminate casting cracks in high-speed steel rolls.