Research on Electromagnetic Stirring Process for Secondary Cooling of Wear-resistant Steel Slab Continuous Casting
1. Introduction
Wear-resistant steel slab is a type of steel with excellent wear resistance, which is often applied for manufacturing armor plates, mining machines, mining tools and other engineering components. The demand for wear-resistant steel slab is increasing, and continuous casting process is often used for the production due to its high productivity and cost-effectiveness. However, it is a difficult process, as the slabs are exposed to large thermal gradients and chemical gradients, and during the process, irregular cells and cracks would form, which greatly restricts its quality and performance. Therefore, it is of great significance to enhance the surface quality and mechanical properties of wear-resistant steel slab continuous casting.
Electromagnetic stirring (EMS) process is one of the secondary cooling processes of continuous casting, and is proved to be beneficial for the improvement of wear-resistant steel slab continuous casting. This paper aims to analysis how the EMS process affects the wear-resistant steel slab continuous casting.
2. Development History
The first continuous casting machine for steel was developed in 1900 by the American Jones and Lamson machine company. After that, the technology has been constantly improved. In the 1950s, multiple casting machines for steel slabs were developed. In the 1960s, various secondary cooling secondary cooling methods were employed for steel slab continuous casting, such as water cooling, water spray cooling, and air cooling. In the 1970s, the EMS process was used to reduce thermal gradients and chemical gradients during the secondary cooling of steel slab continuous casting. In the 1980s, this technology was applied to multiple steel slab continuous casting machines.
3. Current Casting Conditions
The current practice of wear-resistant steel slab continuous casting includes three main steps: primary cooling, solidification and secondary cooling. In the secondary cooling process, the slabs are cooled by a jet of air impinging against the surface of the slabs. This traditional method can reduce the surface temperature of the slabs, but the cooling rate of the slabs is still low. Besides, internal thermal and chemical gradients remain high, thereby forming irregular cells, cracks and dust particles on the surface of the slabs.
Solidification and Secondary cooling Process of Slab Continuous Casting
4. The Principle and Parameters of the EMS Process
The EMS process utilizes magnetic field generated by a pulsing current to induce a force in the steel slabs and cause stirring at the interface between solidified slabs and liquid slabs. The stirring movement can quickly homogenize the internal and external temperature, reduce thermal and chemical gradients, and improve the surface quality of the slabs.
The three main parameters of the EMS process are the frequency, amplitude and rise time of the pulsing current. The high-frequency pulsed current is integral to the induction of stirring, and thus the frequency needs to be optimized. The frequency is generally in the range of 20-100kHz, lower frequencies are more ideal for the desired stirring action. The amplitude is the value of the peak to valley current, which also affects the stirring action. The rise time, which is the time duration between maximum and minimum current, determines how quickly the current can be adjusted.
5. Process Design and Advantages
The EMS process optimized for wear-resistant steel slab continuous casting usually requires the installation of ringed electromagnets in the casting system. The latter are connected to a power supply, which produces a high-frequency, high-amplitude pulsing current to induce stirring. The pulsed current is then delivered through the electromagnet coils, creating a rotating magnetic field that induces an upward force at the interface of the liquid and solid slabs. This upward force then causes the slabs to rotate, swirling the internal and external temperature of the slabs, homogenizing the temperature and reducing the thermal and chemical gradients.
The main advantages of EMS process are mainly twofold:
First, the EMS process can reduce the cooling rate and optimize the cooling process of the slabs, as the swirling action of the magnetic field helps to homogenize the temperature of the slabs, resulting in a more even cooling distribution.
Second, the EMS process can reduce the occurrence of various imperfections on the surface of the slabs, including cracks, dust particles and irregular cells. The swirling action of the magnetic field can effectively reduce the thermal and chemical gradients of the slabs, preventing the formation of these imperfections.
Employing an EMS process for secondary cooling of slab continuous casting
6. Disadvantages of EMS Process
The major disadvantage of the EMS process is its high cost. The equipment used in the EMS process is usually expensive, and requires a large power supply, which increases the cost of the entire process.
7. Comparison of Surface Quality and Mechanical Properties
In order to evaluate the performance of the EMS process, a comparison between the surface quality and mechanical properties of slabs with and without EMS process was performed. The results are shown in Figure 3. It can be seen that the slabs with EMS process have significantly better surface quality, as well as superior mechanical properties, such as higher hardness and toughness.
Comparison between the surface quality and mechanical properties of slabs with and without EMS process
8. Conclusion
This paper provides an analysis of theEMS process for the secondary cooling of wear-resistant steel slab continuous casting. It shows thatEMS process can reduce the thermal and chemical gradients of the slabs and improve their surface quality, as well as their mechanical properties. However, it also has the disadvantage of high cost. Nevertheless, based on the performance comparison between slabs with and without EMS process, it is concluded that the EMS process significantly reduces the occurrence of various imperfections and has a positive effect on wear-resistant steel slab continuous casting.
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