| Due to the poor fluidity in the mushy zone,the enriched solute ineffectively diffuse at the end of solidification,central segregations in the stainless steel,bearing steel and other special steel billets are easily observed.Meanwhile,the developed primary dendrites easily form the dendritic bridging which blocks the feeding channel.No enough liquid steel is filled at the bottom of dendritic bridging and cavity is formed.These defects are hardly eliminated during the heating and rolling process,which are harmful to the mechanical properties and corrosion resistance of the products.The rotary electromagnetic stirring(R-EMS)is one of ways to be used to solve the central quality of the billet,which enhances the heat-mass transfer processes in the molten steel and improves the quality of the billet under the forced convection.However,R-EMS mainly results in the rotary movement of the liquid steel,so that the upper liquid melt with higher temperature and lower liquid melt with lower temperature are not mixed well enough.The forced convection is incapable of promoting the solidification feeding in the direction of gravity effectively with R-EMS.Therefore,in this thesis,the billets of 1Cr13 stainless steel and GCr15 bearing steel were investigated and analyzed in one domestic iron steel company.After the metallurgical effect and deficiency of R-EMS were summarized,a type of vertical electromagnetic stirring(V-EMS)to control the central quality of the billet was proposed in continuous casting.The experiments of R-EMS and V-EMS were carried out in the laboratory,focusing on the mechanism of the dendritic growth,breaking and drifting behavior of dendrite,columnar-to-equiaxed transition,crack formation and feeding behavior during solidification.The main contents and results are as follows:(1)The quality analysis of 1Cr13 stainless steel and GCr15 bearing steel in one domestic iron steel company indicates that the forced convection induced by final R-EMS can effectively reduce the cracks and cavities of 1Cr13 billet,and reduces the segregation and porosities of GCr15 billet.However,there are still unconquerable defects in the centre of the billet,such as microcracks and porosities in the 1Cr13 billet,and porosities and carbides in the GCr15 billet.(2)A type of vertical electromagnetic stirring(V-EMS)is proposed to control the central quality of the billet.The conventional final R-EMS mainly makes the liquid steel produce rotary movement in the horizontal direction.However,V-EMS can create an upward electromagnetic force by using travelling magnetic field,generate the longitudinal loop convection,and lengthen the effective area of electromagnetic stirring along the casting direction,which makes better mixing the upper liquid steel with higher temperature and lower liquid steel with lower temperature,and effectively promote the feeding capacity of the upper liquid steel to the lower liquid steel during solidification.(3)The pouring experiments in laboratory show that the forced convection in the horizontal direction induced by R-EMS can promote columnar to equiaxed transformation of 1Cr13 and GCr15 billets,but there are still defects,such as porosities and cracks in the center of the billet.If the duration of R-EMS is too long or the stirring intensity is too strong,it is bad for the quality of the 1Cr13 billet.After applying V-EMS with the stirring current of 350 A,the solidification structures of 1Cr13 and GCr15 are obviously refined.Furthermore,the shrinkages and cracks are almost disappeared,the larger liquid carbides completely disappear and the network carbides at the grain boundary also disappear.(4)The evolution of dendrite growth and the breaking and drifting behavior of dendrite in 1Cr13 and GCr15 billets with and without EMS were obtained.The application of R-EMS can induce less dendrite fragments,which is consistent with the model of neck shrinkage and breaking.However,after applying V-EMS,there are more dendrite fragments in the centre region,and the fracture of the dendrites is consistent with the model of neck shrinkage and fusing.Furthermore,the criterions for dendrite fragmentation under R-EMS and V-EMS are derived,and the mechanism of CET is studied with EMS.(5)The formation mechanism of central star-like cracks in 1Cr13 stainless steel billet and the improvement mechanism of central cracks with EMS were analyzed.The results show that the primary dendrites form the dendritic bridging which induces the feeding channel to be blocked.No enough liquid steel is filled at the bottom of dendritic bridging and cavity is formed.In addition,the grain size in the center of the billet is irregular,and the enrichment of S and Cr at grain boundaries decrease the bonding force between the grains.At the later stage of solidification,the shrinkage stress exceeds the force between grain boundaries,and appears the intergranular cracks.The intergranular cracks propagate along the grain boundaries and connect with the cavities,and form the star-like cracks.EMS promotes the nucleation in the liquid steel,refines the solidification structure,reduces the segregation of elements,and forms more connection points resisting the solidification shrinkage stress,thereby improving the anti tearing ability of the liquid film.In addition,EMS can make the liquid steel flow better between the dendrites and feed the cracks in time,thus reducing the generation of the cracks.(6)The influence of R-EMS and V-EMS on the central shrinkage cavities and center density was analyzed.The results show that V-EMS can almost eliminate the central shrinkage cavities of the billet and obviously improve the center density of the billet,compared to applying R-EMS.Applying V-EMS generates the longitudinal loop convection,which can increase the feeding capacity,and promote the flow compensation of liquid steel between the dendrites.Furthermore,the longitudinal flow takes the upper liquid steel with high temperature enters into the lower mushy zone,and impacts or fuses the skeleton on the upper end of the interdendritic feeding area,so as to reduce the starting position of the feeding area,and further improve the critical feeding depth.In addition,the mathematical models of the feeding force of the billet with R-EMS and V-EMS are established. |
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