Research On Solidification Structures Of Special Steels With Electromagnetic Stirrring

As important metal materials, special steels such as stainless steel, superalloy and silicon steel are more and more widely used in all areas of the social production and better quality is required. However, due to their own characteristics, in the traditional production process the slabs are prone to have the low rate of equiaxed grains, coarse grains, serious segregation and other defects, which bring negative effects on the improvement of product quality. As an effective means to improve the solidification structures of the slabs, the electromagnetic stirring (EMS) technology is essential in the production process of traditional steels. However, the EMS technology is not well used in the production of special steels. Many technological parameters need to be explored and the mechanism of the EMS is not clear which requires further study.This paper is financially supported by National Natural Science Foundation of China (No.50834009), and the effects of the EMS at different parameters on the solidification structures of martensitic stainless steel, superalloy and silicon steel are studied. And the mechanism of the EMS on the solidification process is discussed.First, the distribution of the electromagnetic field is measured and based on the electromagnetic theory the expression of the electromagnetic force in the molten steel agitated by the traveling magnetic field is deduced and the electromagnetic force is calculated with specific conditions. The results of the measurement of the electromagnetic field show that the magnetic induction intensity is in relation to the position, the frequency and the current. The magnetic induction intensity increases as the current increases. The influence of the frequency on the magnetic induction intensity is not obvious. The magnetic induction intensity is much stronger near the electromagnetic stirrer. As we know from the calculation, with conditions of the same stirrer and the same position the electromagnetic force increases as the current increases. When the current is constant, the electromagnetic force has a maximum value at a certain frequency. And from the calculation we know that the electromagnetic force which is perpendicular to the direction of traveling magnetic field is much smaller than the one along the direction of the traveling magnetic field. The electromagnetic force decays when the depth of the molten steel increases and the decay rate is relation to the stirrer and the frequency.The static casting experiments of martensitic stainless steel with the EMS are carried out. According to the results of the experiments, with the EMS the ratio of equiaxed grains greatly increases; the grains become finer; the carbon and the chromium in the ingots are better distributed. If the current keeps constant, the ratio of equiaxed grains has a maximum value when the frequency is between5Hz and lOHz. When the frequency is constant, the ratio of equiaxed grains increases as the current increases. By the means of XRD, we identify that the main phases of the solidified microstructure are α-Fe, M and Cr23C6and the influence of electromagnetic stirring is mainly enhancing the diffraction peak intensity. By the means of scanning electron microscope (SEM), we find that with the EMS the appearance and distribution of the carbides in the ingots obviously changed:the carbides become smaller and thinner and they distribute in the matrix dispersedly.Based on the previous experiments, the casting experiments of superalloy GH3030are carried out. And the results show that the solidification structures of the alloy are improved effectively by the EMS just as in the experiments of the martensitic stainless steel. With the EMS the dendrites are broken and the process of nucleation is accelerated. The grains become finer and the solidification structure is dense. When the EMS is imposed, the microsegregation and the macrosegregation of the chromium are inhibited. The forced convection induced by the EMS can affect the dendrites. Because of the convection the primary dendrites grow facing the direction of the flow. With the EMS the primary dendrite spacing and secondary dendrite arm spacing become smaller significantly. The solute concentration at the roots of the secondary dendrite arm becomes much higher due to the forced convection. As a result, the local melting point drops down. Consequently the dendrites remelt. When the rate of the local remelt is greater than the rate of growth of dendrites, the secondary dendrite arm will break off and the solidification structure becomes finer.Furthermore, the static casting experiments of silicon steels are carried out by using one directional solidification device and the temperature acquisition system is also used. As we know from the experiments, the EMS can promote the release of the superheat in the ingots and reduce the temperature gradient in the ingots. As the EMS is imposed the ratio of equiaxed grains greatly increases; the grains become finer; the carbon and the silicon in the ingots are better distributed. With the constant frequency, the ratio of equiaxed grains increases as the current increases. When the current keeps constant, the ratio of equiaxed grains has a maximum value when the frequency is between5Hz and lOHz. The cooling intensity and the EMS can affect the dendrites of silicon steel obviously. When the cooling intensity increases, the primary dendrite spacing gets smaller and the solidification structures become dense. Under the effect of the electromagnetic stirring, the long dendrites in the columnar grain zone break down and the primary dendrite spacing becomes smaller. At the same time, the growth of the secondary dendrite arm is inhibited. And the dendrites in the centre of the ingots are broken and coarsening. The solidification structures become dense. Because of the convection induced by the EMS, the temperature gradients and the concentration gradient in the upstream and downstream become different. As a result, the primary dendrites grow facing the direction of the flow. Meanwhile the top of the dendrites bend in the opposite direction due to the force induced by the flow. When the force is large enough, the dendrites can break off. We also find that even without the EMS the dendrites break off, too. This implies that the natural convection driven by the gravity can also induce the local remelt of the dendrites. When the EMS is imposed, the convection gets enhanced. So we can know that during the breaking down of the dendrites the mechanism of mechanical break and the mechanism of remelt both exist.