The final heat treatment is an important link to make the backup roll meet the use requirements. With the continuous improvement of the requirements for the backup roll, the final heat treatment process has also changed from the previous overall heating + quenching + tempering to surface quenching + tempering of the roll body. In this process, the surface quenching of the roll body is particularly important, which directly affects the hardness of the roll body and the depth of the hardened layer, and has a great impact on the performance of the product.
The surface quenching process of the roll body should be selected according to its comprehensive performance and user requirements. The advanced surface quenching process of large forged steel backup rolls includes differential temperature quenching (differential temperature heating + spray cooling) and overall induction hardening (integral induction heating + spray cooling). The biggest difference between the two processes is that the quenching heating method is different; the similarity is that the surface of the forging is rapidly heated above the phase transition temperature while the core is kept below the phase transition temperature, and spray cooling is used. Spray cooling is mixing a certain pressure of water and compressed air in the nozzle and spraying it to the roller in the form of mist for cooling. The cooling intensity can be easily adjusted by adjusting the wind pressure and water pressure.
The surface quenching of the roll body adopts the differential temperature quenching process, that is, the support roll blanks that pass the ultrasonic flaw detection after semi-finishing are first preheated in the trolley furnace at 500-600°C to ensure that the temperature inside and outside the roll body is uniform, and then the support rolls are The roll is transferred to the differential temperature heating furnace and spray quenching equipment for differential temperature heating and quenching of the roll body. The characteristic of the differential temperature heating furnace is that the heating speed is fast, and the surface temperature of the support roll can be raised from 500 °C to above 1000 °C within 1.5 to 2 hours; the heating is uniform, and the workpiece rotates 90 ° at regular intervals in the furnace; the heating layer The depth of the heating layer can reach about 200 mm when the backup roller is kept warm in the furnace for about 4 hours. The advantage of adopting a differential temperature quenching process for surface quenching of the roll body is that when the surface layer of the roll body reaches the quenching temperature at a certain depth, the core is still at a temperature below the austenitizing temperature, and after intense spray quenching, it can ensure that the core has a small Tensile stress, thereby improving the anti-accident ability of the backup roll.
The differential temperature quenching process is an advanced and mature process in the final heat treatment process of large forged steel backup rolls in China. Cr3～Cr5 steel large-scale forged steel support roll adopts a differential temperature quenching process. The surface hardness of the support roll body is 55～71HSD, the depth of the hardened layer is ≤100mm, the uniformity of hardness is ±2HSD, the quenched structure is bainite, and the toughness is, Good strength, suitable for forged steel backup rolls required by steel mills for general hardness and depth of the hardened layer. However, it is difficult to produce high-hardness backup rolls with a martensitic structure whose roll body hardness is >72HSD.
The surface quenching of the roll body adopts the overall induction hardening process, that is, the roll body of the backup roll that has passed the semi-finishing and ultrasonic flaw detection is subjected to overall induction heating and then spray quenched. Some new large-scale forged steel backup roll production plants in China plan to adopt this process.
The overall induction hardening process has been applied in the manufacturing industry of large-scale forged steel backup rolls abroad, and it is the most advanced final heat treatment process in the world. 60～80HSD, the depth of hardened layer is ≥100mm, the uniformity of hardness is ±1.5HSD, and the quenched structure is martensite or martensite+bainite.
It can be seen that the first choice for the final heat treatment process of large-scale forged steel back-up rolls should be the overall induction quenching treatment, and the comprehensive performance of large-scale forged steel back-up rolls produced by this process is better than that of differentially quenched products. In order to narrow the gap with foreign roll manufacturing industries, it is recommended that roll manufacturers cooperate closely with relevant scientific research institutes and colleges to step up research and development of overall induction hardening treatment technology, and strive to fill this domestic gap in a relatively short period of time and apply it to rolls as soon as possible. manufacture. The reason why it is hoped that the final heat treatment of domestic large-scale forged steel backup rolls will adopt the overall induction quenching process is also based on the following reasons.
1. With the continuous improvement of plate shape requirements and the application of plate shape control technologies CVC and HC, for cold rolling continuous annealing and hot dip galvanizing four-roll leveling backup rolls, martensitic matrix structure, and roll body hardness are required to be 72-78HSD, must be achieved by overall induction hardening treatment.
2. With the wide application of semi-high-speed steel and high-speed steel materials for the work rolls of hot rolling mills, the hardness and wear resistance requirements of the backup rolls are also continuously increasing, and it is difficult to meet the requirements by using the differential temperature quenching process.
3. As the market demand continues to increase, the pressure on production costs is increasing. It is required that the roll change cycle of the backup roll be synchronized with the maintenance cycle, and the roll change cycle is longer. It is required that the wear resistance and anti-accident performance are better, and it is more suitable to use the overall induction hardening treatment process.
4. The overall induction quenching treatment is the most advanced in the final heat treatment process, and it will surely produce first-class forged steel backup rolls, which will greatly improve the performance of forged steel backup rolls in terms of wear resistance, fracture toughness, and fatigue resistance. , to narrow the gap with foreign countries.
5. The implementation of the overall induction hardening heat treatment process can fill another gap in the domestic roller body surface quenching process, which is the first in China and belongs to the advanced level in the world.