This article describes the technical requirements for rolling mill bearing seats and improvements in casting processes.
Keywords: rolling mill bearing; bearing seat; casting
In view of the shrinkage defects between the risers of the bearing seat casting, centralized feeding process measures were adopted to change the scattered small risers into single large risers, and MAGMA software was used for solidification simulation analysis to produce high-quality bearing seat castings. After production verification, the improved riser process not only eliminates the shrinkage and porosity defects of castings, but also stabilizes the quality of castings. The backup roller bearing seat is one of the key parts of the rolling mill. It has harsh service conditions and works in high temperature and high-intensity impact environments for a long time. The castings are required to have good comprehensive mechanical properties. Bearing seats are prone to defects such as shrinkage cavities, looseness, cracks, and pores during the casting process, thus affecting the performance of the castings. At the same time, due to the influence of many factors such as equipment capabilities, process equipment, and operating environment, thick castings often fail flaw detection due to defects such as inclusions, shrinkage porosity, and coarse grains during the manufacturing process. Therefore, it is of great significance to reduce casting defects of thick castings, reduce casting repairs, and improve the quality of castings through the improvement of casting processes.
Technical requirements for bearing seats
The rolling mill support roller bearing seat casting has an outline size of 1850mm×1708mm×854mm, a minimum wall thickness of 136mm, a maximum wall thickness of 617mm, and a casting weight of 15500kg. Casting quality requirements mainly include chemical composition, mechanical properties, non-destructive testing, etc. The casting material follows the German standard DIN17182 GS-20Mn5+N. Its chemical composition and mechanical properties are as shown in Table 1 and Table 2. Castings require ultrasonic flaw testing and magnetic particle flaw testing as a whole. Ultrasonic flaw testing is performed in accordance with the standard EN12680-level2, and magnetic particle flaw testing is performed in accordance with the standard EN1369-LM2/AM2/SM2.
| Tab.1 Chemical composition of the bearing | ||||
| C | Si≤ | Mn | P≤ | S≤ |
| 0.17～0.23 | 0.60 | 1.00～1.50 | 0.020 | 0.015 |
| Tab.2 Mechanical property of the bearing | ||||
| Heat treatment status | Rm/MPa | Re/MPa | A5(%) | Ag/J |
| Normalizing + Tempering | 480～630 | ≥260 | ≥20 | ≥50 |
Casting process design and improvement
Parting surface
According to the structural characteristics of the bearing seat, it adopts the shape of upper, middle and lower boxes. The lower box adopts the shape of real sample, the middle box adopts the composite shape of core-pulling real sample and core assembly, and the upper box adopts the shape of cover box. The lower box and the middle box use the positioning pin method, and the middle box and the upper box use the positioning line method to close the boxes to ensure the dimensional accuracy of the mold cavity.
Modeling materials
The pouring temperature of steel castings is high, and the maximum wall thickness of the bearing seat is 617mm and the minimum wall thickness is 136mm, so chromite sand with high refractoriness is used as the surface sand to prevent sand sticking defects in the castings during the solidification process. Apply alcohol-based zircon powder paint on the surface of the casting 3 to 5 times. After each brushing, polish it with sandpaper and then apply the next coat to ensure that the thickness of the paint reaches about 2mm. The use of chromite sand and zircon powder coatings not only effectively reduces sand adhesion on the surface of castings, but also significantly improves the surface quality of castings.
Gating system
The pouring temperature of molten steel is high, the fluidity is poor, and it is easy to oxidize, so it requires fast and smooth filling. In the casting process design, the pouring system is selected as an open pouring system, and the liquid steel rising speed calculation method is used to calculate the size of each unit of the casting pouring system. Among them, the cross-sectional area ratio of the ladle hole, sprue, cross runner, and inner runner A Nozzle: A straight: A horizontal: A inside = 1.0: 2.25: 2.5: 3.0. Adopting the bottom pouring + tangential pouring method, the bottom pouring can introduce the inner runner from the bottom of the casting to ensure that the molten steel rises smoothly in the cavity; at the same time, the tangential type inrunner allows the molten steel to rotate in the cavity, which ensures It helps floating sand, gas, inclusions, etc. enter the riser and reduces the occurrence of casting defects. The entire pouring system is connected by high-alumina refractory brick pipes, and is poured using a bottom pouring bag. The pouring temperature is controlled at 1550~1560°C.
Riser design
Original riser technology
Since the bearing seat is a thick casting, there are scattered hot spots at the four corners, and shrinkage cavities and shrinkage porosity defects are prone to occur during the solidification process of the casting. When designing the casting process, the riser size is designed based on the modular method and the hot section circle method. Open risers are set up at the hot sections, and a waisted riser is set up at the two large hot sections. The riser size is Y750mm. ×950mm, pouring height 1000mm; circular risers are set up at the two small hot spots, the diameter of the risers is 700mm, and the pouring height is 1000mm. The original riser process plan is shown in Figure 1.
The process plan of dispersed risers produced 4 bearing seats in the first batch, but the ultrasonic flaw detection qualification rate was low. Among them, 3 bearing seats were found to have defects between the two risers, within the range of 100 to 150mm from the parting surface. It was analyzed that the defect was shrinkage porosity, and welding was used to repair it during production. However, welding the shrinkage porosity defect was difficult, which greatly increased the production cost and also faced the risk of scrapping the casting.
Improvement of riser technology
In order to eliminate shrinkage defects between risers, improve the flaw detection qualification rate of castings, and reduce welding repairs and repairs of castings, the riser process plan of the bearing seat has been improved. A separate circular riser is set above the inner hole of the bearing seat. The size of the riser is 1100mm and the pouring height is 900mm. The inner hole is partially cast as a subsidy. The riser process plan is improved by adopting the process idea of centralized feeding instead of decentralized feeding, that is, one large riser replaces three small risers. At the same time, the weight of the risers before and after the improvement will not increase the cost, and the improvement The final riser process plan is shown in Figure 2.
Solidification Simulation
Solidification simulation can predict the location of shrinkage cavities and porosity in castings, which is beneficial to the verification and optimization of the casting process, and plays a great role in reducing production costs and improving casting quality. For the riser process plans before and after the improvement of the bearing seat, MAGMA solidification simulation software was used to conduct simulation analysis. The simulation results are shown in Figures 3 and 4. Through comparative analysis of simulation results: the scheme of improving the riser is more conducive to the centralized feeding of the riser, and the molten steel in the riser can be maintained at a high temperature for a longer time, forming an obvious temperature gradient between the riser and the casting. , realizes the sequential solidification of castings and improves the feeding effect of the riser.
Fig.3 Temperature field simulation of original riser process
Improvement effect
A total of 4 batches of 24 bearing seats were produced using the improved riser process. The castings were boxed and cleaned after being insulated in the sand mold. The risers were cut after heat treatment to eliminate casting stress. The root structure of the risers is dense. After rough machining of the bearing seat, ultrasonic flaw detection was carried out. It was found that the internal structure of the casting was dense and no shrinkage cavities or shrinkage defects were found, and the quality of the casting was stable.
Fig.4 Temperarure field simulation of improving riser process
Conclusion
For mass-produced bearing seats, through research on the casting process and using MAGMA simulation software to simulate the solidification process of the bearing seat, the process optimization of centralized feeding of large risers instead of dispersed feeding of small risers is achieved, so that the risers and castings An obvious temperature gradient is formed to achieve sequential solidification of the casting. After production verification, the process plan for improving the riser eliminated the shrinkage cavities and shrinkage defects of the castings, improved the quality of the castings, and has certain guiding significance for the casting process plans for the future production of similar castings.
