What are the main advantages of high-speed steel rolls? Start with four aspects: centrifugal casting method, continuous casting outer layer forming method, electroslag remelting method, and liquid metal electroslag dissolution method. The manufacturing process of high-speed steel rolls is studied, and the key points of process improvement are explained.
Keyword: Hss Rolls
From the perspective of steel rolling technology development, improving the quality of rolled products, improving the progress of steel rolling, and reducing the cost of steel rolling are the most mainstream development directions.
Take strip rolling as an example. Technologies such as cross rolling, roll bending technology, multi-roll mill, low temperature and large reduction rolling, continuous casting and rolling short process put forward higher requirements for roll wear resistance, strength and toughness.
Foreign countries developed cast high-speed steel rolls in the late 1980s, and used them in many fields including the finishing section, rough rolling section, steel pipe rolling mill, and high-speed wire rod rolling mill’s pre-finishing section. The advantages are quite obvious.
Analysis of Main Advantages of High Speed Steel Rolls
Compared with steel rolls in the traditional sense, high-speed steel rolls have very prominent advantages in terms of room temperature and high temperature wear resistance. At the same time, it has high hardness and good red hardness characteristics, and its thermal crack resistance and accident resistance are very outstanding.
From the perspective of microstructure, it is mainly formed by MC and M6C alloy carbide embedded on the austenite matrix. MC and M6C have very prominent hardness advantages. In addition, the austenite matrix has high thermal stability, and a large number of alloying elements participate in the reaction during the inlay process, and certain dispersed carbides can be precipitated during tempering, resulting in hardening after secondary tempering. Promote the increase of martensite decomposition temperature. Demonstrate a more definite hard edge.
Hss rolls are very adaptable, and can achieve the purpose of improving the quality of rolled products while prolonging their service life. It can be widely used in hot strip continuous rolling finish rolling stand and non-regulation rolling work of strip steel of various materials, so as to promote the further exertion of its advantages.
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 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 metal elements in the outer layer and oxidation of the interface between inner and outer materials are the keys to the success of manufacturing rolls by this method.
2.2 Continuous casting outer layer forming method
The main technical points of the continuous casting outer layer forming method are: put the roll core in the water-cooled mold, and pour the molten outer layer high-speed steel into the middle position between the carbon steel mandrel and the cold crystallization film in a vertical state. The high-speed steel roll core is fully combined with the outer metal, solidified from bottom to top, and finally the solidified part is pulled downward to form the continuous casting outer layer.
2.3 Electroslag remelting
The main technical feature of the electroslag remelting method is that the concentric water-cooled casting mold is placed on the periphery of the cylindrical high-strength alloy steel as the core material. At the same time, the consumable electrode is prepared from high-speed steel or semi-high-speed steel, and placed in the middle of the casting mold and the forged steel. After the consumable electrode is completely melted, it can play the function of the outer layer material and fully fill the space.
2.4 Liquid metal electroslag dissolution method
The main technical feature of the liquid metal electroslag dissolution method is that the mandrel, which is the core of the high-speed steel composite roll, is inserted into the crystallizer and 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 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, and at the same time, the upper molten steel is continuously injected until the predetermined composite roll length is reached.
3 Analysis of process improvement of HSS rolls
3.1 Improvement of the core strength drop problem in heat treatment
In order to effectively enhance and improve the wear resistance of the outer material of the high-speed steel roll, the key is to increase the quenching temperature during the manufacturing process of the high-speed steel roll. Under the current technical conditions, for composite high-speed steel rolls, the roll core material is mainly high-strength alloy ductile iron. Affected by the restrictive factors of the material, if the quenching temperature of the high-speed steel roll is selected above 1250°C, the structure of the roll core will be significantly coarse, and even local melting will occur, which will significantly reduce the strength of the roll core, and the roll is prone to breakage during use. The phenomenon affects the normal operation of the rolling mill equipment. The sub-temperature heat treatment process of high-speed steel rolls has been successfully developed abroad, but the process has not been reported in detail. We should develop the heat treatment process of high-speed steel rolls as soon as possible based on the domestic market. The new process should be simple and easy to implement, with low energy consumption and less pollution under the premise of canceling high-temperature quenching and ensuring high strength of the roll core. The surface induction hardening process has the characteristics of convenient operation and little influence on the strength of the roll core. It will be feasible to apply it to the cast high-speed steel composite roll. In order to ensure the depth of the hardened layer, it is recommended to adopt a double-inductor quenching process.
3.2 Improvement of the bonding strength between the core and the outer layer
In the process of manufacturing composite high-speed steel rolls by traditional centrifugal methods, most of them choose ductile iron as the core material of high-speed steel rolls, which often has the characteristics of graphite precipitation. Because the outer material high-speed steel contains certain special elements, such elements may cause the problem of whitening of cast iron. Therefore, at the part where the outer layer is in direct contact with the core of the high-speed steel roll, the problem of graphitization deterioration is very serious. In the case of continuous precipitation of carbides, the brittleness level of the bonding site is significantly increased, which may cause the outer layer of the composite high-speed steel roll to peel off during use. In addition, under the condition of using cast steel as the core material of high-speed steel rolls, due to its higher speed and elongation characteristics, its melting point is higher than that of high-speed steel. In the pouring process, it is easy to undergo a fusion reaction with the inner surface of the solidified outer high-speed steel, resulting in casting defects and affecting the strength of the joint. To solve this problem, during the implementation of the manufacturing process of high-speed steel rolls, an intermediate layer (graphite steel is preferred as the intermediate layer) can be added between the roll core material and the high-speed steel material to improve the bonding state of the roll core interface and the high-speed steel interface , and achieve the purpose of improving the bonding strength of the composite high-speed steel roll.
3.3 Improvement of casting crack problem
Combined with relevant practical work experience. In the process of casting high-speed steel rolls, due to the large content and many types of alloying elements, it may have an impact on the thermal conductivity of high-speed steel. Inhomogeneity occurs during casting cooling and increases the residual stress level of HSS rolls to some extent. In addition, alloy elements are prone to segregation during the solidification process of high-speed steel rolls. The asynchrony of phase transformation in high-speed steel materials is more obvious, and the phase transformation stress has a tendency to increase significantly. Under the influence of the above factors, there is a relatively obvious crack tendency during the manufacture of high-speed steel rolls. In order to avoid the formation of cracks from adversely affecting the overall performance of high-speed steel rolls, it is necessary to carry out timely modification of molten steel during the manufacturing process. For example, you can choose a double-layer coating that combines the heat insulation layer and the refractory layer, and you can also reasonably adjust the number of revolutions of the centrifuge during pouring, so as to minimize and eliminate casting cracks on high-speed steel rolls.
