Study On Equiaxed Crystal Nucleation Conditions Of Ferritic Stainless Steel Under Vibration And Chilling

Economical nickel-saving ferritic stainless steel is favored for its high thermal conductivity and good cold working performance.However,it is easy to produce wrinkles,cracks and other defects in the forming process,which limits its application scope.In order to reduce rolling defects of ferritic stainless steel,the solidification morphology of continuous casting process is generally controlled.Vibration-induced liquid metal nucleation technology can lead to the formation of solidification shell on the surface of chilled crystal nucleus generator,which hinders the grain dissociation.The formation of solidification shell is closely related to the nucleation conditions of equiaxed crystals.In order to avoid the rapid formation of solidified shell,it is particularly important to study the nucleation conditions of equiaxed crystals under vibrational chilling.In this paper,the surface roughness and wettability,surface temperature and surface material of different crystal nucleus generators were selected.The ferritic stainless steel melt was processed by vibration excitation nucleation technology to obtain ingots under different experimental conditions.The microstructure and mechanical properties of the ingot were studied by means of microstructure observation,phase analysis,stretching and fracture scanning to provide a reliable basis for the discussion of equiaxed crystal nucleation conditions under vibration chilling.The results show that the grain size decreases with the increase of surface roughness when the base and solution embryos show good wettability.While the tensile strength,yield strength and hardness of ingot show an increasing trend as a whole.When the surface roughness of the nucleation generator is the highest,the grain size of the ingot obtained by nucleation experiment reaches the minimum,which is 28.4um.Compared with the original untreated ingot,the grain size of the ingot decreases by 60%.At the same time,the tensile strength,yield strength and hardness reached the maximum,which were 595 MPa,500 MPa and 176 HV,respectively.The effect of surface roughness of nucleation generator on equiaxed nucleation conditions depends on the wettability between substrate and solution embryo.When the substrate and solution embryo show good wettability,the increase of surface roughness factor will promote the nucleation of equiaxed crystals.With the increase of the surface temperature of the nucleation generator,the average grain size of the ingot obtained after nucleation experiment decreases first and then increases,while the tensile strength,yield strength and hardness increase first and then decrease.When the surface temperature of the nucleation generator is 500?,the comprehensive properties of the ingot are the best,the average grain size is 47.66?m,and the room temperature tensile strength,yield strength and hardness are 544 MPa,443MPa and 149 HV,respectively.The effect of the surface temperature of nucleation generator on the nucleation conditions of equiaxed crystals depends on the supercooling of its surface.Although the increase of supercooling can promote the nucleation of equiaxed crystals,it is not easy for the nucleation of equiaxed crystals to occur with the increase of supercooling.Only when the nucleation rate corresponding to undercooling reaches the maximum,can the effect of promoting equiaxed crystal nucleation be optimal.The effect of surface material of nucleation generator on equiaxed crystal nucleation conditions depends on the matching relationship between surface material and metal liquid system.If there is a orientation relationship satisfying Merwe's calculation results,equiaxed crystal nucleation can be promoted.Based on the"edge-edge matching"crystallographic model,the orientation relationship of 304/430system satisfying Merwe's calculation results is deduced as follows:[111]_B/[110]_F(1 (?)0)_B/(1(?)1)_F.From the crystallographic point of view,there is a matching relationship between these systems,which can promote nucleation and refine grains.The experimental results show that under the same conditions,the average grain size of the ingot obtained by the nucleation test of 304 material is smaller than that of 430 material,and the tensile strength,yield strength and hardness of the ingot are increased.The theoretical prediction is in good agreement with the experimental results.