Study On Novel Electromagnetic Metallurgical Control Technology And Its Application In Continuous Casting Processes

Electromagnetic metallurgy technology is increasingly adopted in steel industry specially for continuous casting processes.Both the force and thermal effects produced by the electromagnetic field can be used in electromagnetic metallurgy for the regulation of temperature distribution and/or flow pattern,which then plays an important role for sequence casting,casting condition improvement and the final as-cast quality assurance.The development of any new electromagnetic technologies for quality or special steel production is an interdisciplinary research with challenge.Among them,tundish with induction heating and multi-mode electromagnetic flow control for slab casting processes are received great attention presently as hot spot issues.The semi-product casting quality is closely related to the physical phenomena such as fluid flow,heat transfer,solute transfer and solidification in the casting process,which can be controlled by electromagnetic joule heat and stirring force.Accordingly,physical modeling,numerical simulation together with industrial tests have been adopted for the study of the physical problems to meet the need of both the novel technology development and its application in actual casting production.First,in the light of inter-strand differences in conventional tundish and the need of casting operation under a given identical temperature and a constant speed,physical modelling and electromagnetic fluid dynamics research have been carried out for the development of a novel channel-typed induction heating tundish,through which a split channel structure is proposed to meet the demand.The significance of water modelling to design and optimization of induction heating tundish has been revealed.Based on study on the fluid flow and heat transfer in induction heating tundish,a 6-strand tundish has been successful developed for industry application along with a reasonable heating strategy.With this tundish,the RTD behavior for each strand has been effectively improved,and the temperature difference among each strand can be controlled within 2 to 3 degree C,which enriches the knowledge of tundish metallurgy accordingly.In order to ensure a reasonable flow field in the slab mold under different casting conditions,the mold multi-mode electromagnetic flow control technology is innovatively proposed.With this novel mold,an electromagnetic deceleration flow control is realized for higher speed casting situation,and a stirring mode for flow control for lower speed casting.The fluid flow and solidification behavior of molten steel under the two electromagnetic control modes have been studied by a magnetohydrodynamic coupling model,which was verified by a self-designed electromagnetic force measuring device.The results show that whether it is electromagnetic deceleration or electromagnetic stirring mode,the magnetic induction intensity is mainly concentrated on the subsurface of the slab casting,and the magnetic induction flux at the center is relatively small.However,the magnetic induction flux at the center of the slab by electromagnetic stirring is close to zero,while the magnetic induction intensity at the center of the slab by electromagnetic deceleration is within 100Gs.The direction of the electromagnetic force of the electromagnetic deceleration points to the center of the nozzle,but the electromagnetic force of the electromagnetic stirring is symmetrically distributed on the left and right sides of the nozzle,but being antisymmetric for the inner and outer sides of the mold.Combined with the comprehensive evaluation method for slab surface quality,suitable electromagnetic current control parameters for slab casting of IF steel with different cross-sections are obtained.For the casting speed of 1.86 m·min-1 and a cross-section of 1000 mm×230 mm,the recommended suitable deceleration current is 200 A,and for a casting speed of 0.84 m·min-1 and a cross-section of 2150 mm×230 mm,the appropriate stirring current should be 400 A.The industrial application of electromagnetic stirring technology was carried out for slab casting process with section of 2150 mmx230 mm in a steel plant.It was found that the use of electromagnetic rotary stirring was better than the non-use condition,both in terms of liquid level fluctuations and inclusions and bubble defects in the casting products.Additionally,a three-dimensional coupling model of slab continuous casting from the mold to the secondary cooling zone is established by using the segmented calculation method aimed at the local electromagnetic stirring for effective as-cast macrostructure control.The calculation results show that the maximum electromagnetic force of the traveling wave magnetic field of the roller electromagnetic stirring in the secondary cooling zone is located on the starting side of the narrow surface.With the increase of the number of stirring rollers,the effective stirring region of the molten steel inside the slab by the electromagnetic force increases,and the flow velocity of the molten steel first increases and then drops at the solidification front.The magnetic induction intensity is distributed asymmetrically and symmetrically on the inner and outer sides of the slab due to the differences in the magnetic circuit design and installation for the roller and post-roller box stirrer,respectively.The effective region of the box-typed electromagnetic stirring is larger than that of the roller electromagnetic stirring,and the molten steel in the center of the casting slab has a larger region of superheat dissipation.And the roller stirring promotes the nucleation of the molten steel in the solidification front with much greater intensity than the box stirring.Under the same stirring power and frequency(400 kW,7 Hz),the operating currents of the box and 2 pairs of roller agitators are 425 A and 500 A,respectively.Application on a ferritic stainless steel casting shows that the equiaxed crystal ratio under box-typed electromagnetic stirring is about 50%as compared 67%by the spaced-arranged roll reverse stirrers.Both of them can meet the demand of 45%equiaxed crystal ratio as a predetermined threshold value.