In terms of smelting process and equipment for molten steel of steel castings, developed countries usually adopt relatively advanced process and equipment, such as high-power electric arc furnace, vacuum furnace and refining furnace (LF furnace, VOD furnace, etc.), and constantly develop various types of high-performance refractories to reduce the impact of refractories on molten steel quality in the process of molten steel smelting, so as to smelt more pure molten steel and finally improve the quality of steel castings. In terms of system, China’s casting enterprises have not only steel casting branches or workshops subordinate to the main engine production plant, but also professional steel casting plants, as well as a large number of township steel casting plants. There are great differences in process equipment and process technology level. At present, the molten steel smelting of motor shell mainly adopts two ways: medium frequency furnace steelmaking and electric arc furnace steelmaking.
There are inherent limitations of medium frequency furnace in steelmaking process:
① Unlike the converter or electric arc furnace, which can blow oxygen in the smelting process, once the five elements and other alloy elements in the liquid steel exceed the upper limit of the required composition specification in the whole smelting process, the composition of the liquid steel may be out of line, resulting in the unqualified molten steel;
② The heating mode of medium frequency furnace is magnetic field alternating mode. During the heating process, the liquid steel can form a circular flow on the horizontal liquid level, but it can not make the liquid steel flow up and down in the vertical direction, so that the harmful impurities and inclusions in the liquid steel can not float up to the maximum extent to be removed from the slag, which seriously affects the quality of liquid steel and even causes the unqualified mechanical property indexes to form waste products;
③ The heating mode of medium frequency furnace is to convert magnetic field into current, so its coil must be designed in barrel shape, so that after the melting of furnace charge, a molten pool with deep depth and small cross section can be formed. Such a small molten pool greatly reduces the contact area between steel and slag, which is not conducive to subsequent diffusion, deoxidation, desulfurization, etc. The smelting process of medium frequency furnace includes four main processes: batching → charging → melting → reduction. Among them, the most important link is batching. Whether the batching is reasonable or not will directly affect whether the qualified molten steel can be smelted. During batching, the total content of various elements contained in the raw materials in the steel must be less than the upper limit of the element content specified in the steel to be smelted. At the same time, it should be considered that some allowance should be left for the element content brought in by adding other furnace charges in the smelting process. Medium frequency furnace smelting can not adapt to the motor shell with strict requirements on chemical composition and molten steel quality. Generally, it is only used for the production of low-end motor shell.
In the process of electric arc furnace smelting, it can overcome the shortcomings of medium frequency furnace and has a series of advantages:
① It can effectively remove harmful gases and inclusions in steel;
② Deoxidation, dephosphorization, alloying, etc;
③ It is not limited by furnace charge and can use inferior furnace charge to refine high-quality steel.
④ It has strong adaptability and can be produced continuously or intermittently.
The process of EAF steelmaking is: batching → charging → melting → oxidation → reduction → tapping. Oxidation and reduction operation is the most important process and plays a key role in smelting quality. Electric arc furnace smelting process is basically adopted in the production of medium and high-end quality motor shell. However, there are also some deficiencies in the process of EAF steelmaking: uneven temperature in the furnace and large temperature difference in various parts of the molten pool; Under the action of arc, a large amount of H and N can be dissociated, which makes the gas content in steel high; There is also some segregation in the composition. These deficiencies also affect the product quality of motor shell to a certain extent, such as unqualified mechanical properties and chemical composition can not meet the process requirements. In order to solve these problems, there are out of furnace refining processes, such as argon blowing refining and argon oxygen combined refining. The refining technology improves the quality of molten steel and provides a guarantee for the production of high-quality castings.
Pouring process:
① Ladle: tilting type, bottom injection type and teapot type.
② Sedation time t: 20min ≥ t ≥ 5min.
③ See pouring temperature and time table.
| Serial number | Kind | Wall thickness (mm) | Weight | Pouring temperature | Pouring time |
| 1 | Complex thin-walled parts | ≤20 | ≤500 | 1580-1600 | ﹤30 |
| 2 | Complex thin-walled parts | ﹥20-40 | ≤3000 | 1570-1590 | ﹤60 |
| 3 | Small and medium-sized | ﹥40-80 | ≤10000 | 1560-1580 | ﹤60 |
Due to the unreasonable pouring process, insufficient level of pouring workers and improper operation in some enterprises, insufficient pouring, sand flushing, cracks, box penetration and so on often occur in the pouring of motor shell. For different types of motor casings, various casting enterprises continuously optimize the pouring process, such as supplementary pouring, adding protective gas, using computer control, and the pouring quality has been well controlled.