Cold rolling rolls (cold rolling mill rolls):
The rolls used for cold rolling are generally flat rolls and are used for rolling plates and strips. Broadly speaking, cold rolling rolls include work rolls that are in contact with the rolled piece and backup rolls that support them. Because the backup rolls are independent, the cold-rolled rolls are often called cold-rolled work rolls in a narrow sense.
Cold rolling rolls must have sufficient strength, uniform high hardness, and surface quality to withstand extremely high rolling forces, ensure sufficient wear resistance, and meet the precision requirements of rolled products. Forged steel rolls that can be quenched to high hardness are used as cold rolling rolls.
The main links in the manufacturing process of cold rolled rolls are smelting, forging, heat treatment, machining, and inspection.
The smelting method of cold-rolled roll steel must be selected according to the cleanliness requirements. Commonly used smelting equipment includes alkaline electric furnaces, acid open-hearth furnaces, and vacuum ingot casting equipment. Secondary metallurgical methods can be used to improve the cleanliness of molten steel, such as electroslag remelting or consumable electrode vacuum melting.
According to the equipment conditions of the factory, other refining methods outside the furnace can also be used for degassing, desulfurization, removal of inclusions and fine-tuning of molten steel composition. It is advisable to adopt the ingot with a polygonal wave surface and big head up.
After the steel ingot is demolded, it is sent to the forging workshop to be forged into a roll billet. Forging not only makes the roll obtain the basic shape, but also breaks the coarse as-cast structure, especially the coarse network carbide, which greatly reduces the segregation and porosity defects in the ingot, such as shrinkage cavity, porosity and microcracks. The refinement, homogenization and densification of the microstructure not only improves the mechanical properties (mainly strength and toughness), but also prepares the microstructural conditions for the subsequent severe quenching.
Forging generally adopts hydraulic press free forging process. In view of the quality advantages of small ingot forgings, it is advisable to forge one coil for one ingot. The main process control parameters are temperature, anvil shape and forging ratio. When the forging temperature is high, it is easy to crack internally; when the temperature is low, the diffusion effect is poor, and the density after forging is low. In order to forge through, sometimes the method of compacting the center after cooling the surface is used. Considering the structure of the working layer of the roll body, the final fire should give the roll body a certain amount of deformation, that is, the final forging temperature of the roll body should be considered. The V-shaped anvil should facilitate the axial flow of metal, so the V-shaped anvil is commonly used, especially the lower anvil. The size of the anvil surface affects the amount of elongation, thereby affecting the quality of the forging: the amount of elongation is small, it is not easy to forge through; the elongation is easy to crack.
Cold-rolled rolls must have a sufficient forging ratio, generally greater than 3. For this reason, after cutting off the head and tail, pressing the handle, and chamfering of the steel ingot, it needs to be pre-pierced and then stretched. In the drawing process, in addition to controlling the amount of reduction to avoid incomplete forging or cracking, the coaxiality of the centerline of the ingot and the finished product should always be maintained. The forging process should be formulated and optimized according to the composition of the incoming material (ingot), the shape of the ingot, and the size of the forging so that the structure and performance of the forging can be optimized. Very small coils can be manufactured directly by rolling billets.
Roll blanks should be annealed directly in the furnace after forging. After slow cooling, the matrix structure is flaky pearlite, and the carbides are still precipitated in the form of a network. If the forged billet is not manufactured as a commodity, it can also be cooled by normalizing to break the carbide network, and isothermal annealed to obtain a granular pearlite matrix. If necessary, hydrogen should be fully diffused in the final stage of annealing. In order to eliminate the network carbides of the forging billet and make the matrix spheroidize, it is firstly normalized and then spheroidized. Sometimes it can be simplified to high temperature normalizing and tempering. If the carbides of the forged billet are discontinuous, spheroidizing annealing can be performed directly. When normalizing large rolls, special cooling measures should be taken.
Finished rollers must be quenched and tempered. The purpose of quenching and tempering is to treat the roll core and roll neck to the required strength and hardness first, in preparation for the final quenching of the roll body. If the strength requirement is not high, it can be directly quenched without quenching and tempering. When tempering, use oil cooling or spray cooling. Spray cooling enables deep tempering of rolls.
Cold rolling of rolls is the most important heat treatment operation. In the early stage, the overall heating and quenching method was adopted after the roll neck was sheathed. With the development of heating technology, medium frequency or power frequency induction heating is mostly used. This induction heating can be single or dual frequency, integrated or mobile.
Rapid heating can also be achieved with a flame. High-pressure water is used for quenching. Cool for a certain period of time after quenching. After quenching, low-temperature tempering should be carried out in time. For rolls with low retained austenite content, the cold treatment process should be added before tempering. When the quenched cold-rolled roll has a center hole, water should also be passed into the hole to accelerate the heat dissipation of the core.
Rough machining is performed after the rolled blank is annealed. To reduce the risk of quenching and cracking, cold rolled rolls are usually designed with a center hole. At this time, attention should be paid to the surface processing quality of the center hole to avoid damage due to fatigue of the center hole during use. If the hydrogen content and residual stress are properly controlled, no central hole will be left.
The machining allowance should be considered as follows: no decarburization layer should be left on the final product; no defects that are not conducive to quenching should be left; do not leave too much machining allowance for finishing; make the amount of grinding after quenching as small as possible. Finishing and grinding before and after quenching.
Cold rolling rolls have high requirements for dimensional accuracy, concentricity, and roughness.
In addition, the quenched roll has high residual stress, and the grinding wheel and grinding parameters should be carefully selected to avoid grinding cracks. Special machine tools should be used for roll processing, and CNC machine tools should be used as much as possible.