| The electroslag remelting(ESR)process is an advanced metallurgical technology with functions of remelting,secondary refining and directional solidification.Thus,fundamental understanding of complicated macro-scale transport phenomena and micro-scale solidification structure evolution in the ESR system is the key to optimize the practical parameter and control the ESR ingot quality.In the present thesis,according to the industrial ESR process for the rator steel 30Cr1MolV in an domestic special steel plant,a three-dimensional macroscopic multi-physic fields mathematical model(ESR_MPFC)was developed by coupling the ANSYS EMAG module and ANSYS FLUENT module of the ANSYS commercial software with the self-developed code.The model validation was also carried out by comparison the numerical predictions with the measurement of the magnetic flux density and liquid pool profile and empirical prediction.Finally,the numerical model was adopted to investigate the the electromagnetic,multi-phase fluid flow and heat transfer in the ESR system.Meanwhile,a multiscale mathematical model coupling the macroscopic heat transport with the mesoscopic nucleation and grain growth(ESR_CAFD)was developed to predict the grain structure evolution of solidifying ingot during the ESR process,using the two sets of grid configurations,double line interpolation method and a moving cell frame technique.And the effect of the pratical parameters on the solidification structure of ESR ingot was also investigated using the multiscale mathematical model.Based on the multi-scale numerical simulations of the ESR process,an industrial operation of the ESR process for HP/IP rotor steel 30CrMolV was performed with the optimum pratical parameters and the heat treatment pocess was also investigated to improve the menchanical properties of the final product.The main contents and achievements are concluded as follows:① Most of the Joule heating concentrates at the slag pool,because of the low electrical conductivity of slag.There are two axisymmetric inward vortexes under the conical electrode tip,where the Lorentz force and the droplets movement are dominant,and another two axisymmetric outward vortexes near the mould periphery,where the thermal buoyance is dominant,but there are only two axisymmetric outward vortexes in the liquid metal pool,where the thermal buoyance is dominant.② For the case that the slag pool depth,electrode immersion depth,fill ratio,current intensity and frequency are respectively 200mm,30mm,0.56,8000A and 50Hz,the total Joule heating in the ESR system is 434.326kW,but only 19%of the total Joule heating provides for remelting the electrode and the melt rate is 230kg/h.During ESR process,the metal droplets form at the electrode tip and uncontinuously fall through the slag pool into the metal pool.The average diameter,final velocity,temperature increasement and residence time of the metal droplets are respectively 10mm,0.35m/s,73K and 0.54s,and finally the liquid pool is shallow U-shape with the depth of 115mm.③ With the increase of current intensity,the Joule heating,current density,magnetic flux density,electromagnetic force,melt rate,slag temperature,slag flow velocity and liquid metal pool depth increase significantly.With the increase of slag pool depth,the Joule heating,melt rate,slag temperature and flow velocity increase,but the maximum values of the current density,magnetic flux density and electromagnetic force keep stable,and the liquid metal pool depth decreases.With the increase of electrode immersion depth,the Joule heating,maximum current density,maximum magnetic force,melt rate,slag temperature,slag temperature,slag flow velocity and liquid metal pool depth decrease.But the magnetic flux density keeps stable.With the increase of fill ratio,the Joule heating,maximum current density,maximum magnetic flux density,maximum electromagnetic force,melt rate,slag temperature,slag flow velocity and liquid metal pool depth decreases.④ At the beginning of the solidification,nucleation occurs almost instantaneously and lots of fine equiaxed grain structures form at the bottom of the ingot,due to the intensive cooling capability of the bottom medium.Among the fine grain structures grown from the ingot bottom,some of the grains with similar orientations with respect to the temperature gradient survive from the intensive competition grain growth and thereafter block other grains growth.As the solidification proceeded,ESR ingot grows high and the effect of the heat transfer from the lateral ingot surface on the grain growth becomes dominant.Consequently,the columnar grain grown from the lateral ingot surface significantly blocks the columnar grain grown from the ingot bottom,and the survived columnar grain grows along the axial-radial direction paralleling to the heat transfer direction.⑤ At the beginning of the solidification,the initial liquid pool is shallow and the solid/liquid interface is planar.With the proceed of solidification,the liquid pool becomes U-shape and the depth becomes higher,because the heat transfers both from the bottom and the lateral ingot surface have effect on the liquid metal pool,and finally the liquid metal pool transfers to V-shaped,because the heat transfer from the lateral ingot surface becomes dominant.⑥ With the increase of melting rate,the liquid pool deepens and the mushy zone extends.The extension of the mushy zone makes the temperature gradient decrease and facilitates the central equiaxed grain nucleation and the columnar-to-equiaxed transition(CET).With the increase of molten slag temperature,the sensible heat of the molten melt increases and the liquid pool becomes deeper,resulting in increasing the temperature gradient and suppressing the nucleation.Correspondingly,the columnar grain grown from the surface into the center of the ESR ingot becomes coarse and the thickness of the fine equiaxed grain layer at the surface and bottom of the ESR ingot decreases significantly.⑦ Based on the simulation calculation,the final ESR ingot using the appropriate parameters is with high metallurgical qualities,such as:smooth ingot surface,low harmful elements and inclusions,homogeneous solute distributions,etc.Meanwihle,the mechanical properities of the forgings is significantly improved by the new heat treatment process. |
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