| Super austenitic stainless steel(SASS)is a kind of low carbon high alloy steel with a high content of Cr,Ni,Mo and N.This steel holds excellent corrosion resistance(PREN?40)and has been developed for industries with harsh conditions.However,a series of problems need to be solved,such as coarse solidification microstructure,serious segregation and precipitation of brittle phases,which would increase the difficulty of steel production.This research explores potential ways to refine the grain structure,reduce the element segregation and further control the precipitation of brittle phases in S31254 SASS by using tiny Ce addition.REs(rare earths)are effective chemical grain refiners commonly used in low alloy steel.In view of this,the solidification characteristics of Ce-containing SASS are fundamentally studied in this thesis.Firstly,the solidification structure and element segregation in S31254 SASS casting slab were investigated.The solidification structure is mainly bulky columnar grains(85.3%)and only has 10%equiaxed grains.Large amounts of ?phase were observed in the CET position.? phase in solidification structure is precipitated through a divorced eutectic reaction.The coarsest dendritic microstructure and severest Mo microsegregation(SR=6.43)occur in the center of billet.The fraction of ? phase in the same area could reach 6.1%.Therefore,fine equiaxed grain structure is desired to be obtained.Thermodynamic calculation of phase transition during solidification in SASS was carried out by Thermo-Calc software(TCFE9 database).The equilibrium solidification path is L?L+? and non-equilibrium solidification path is L?L+??L+?+??L+?+?+?.? phase is precipitated due to the segregation of Mo.The critical Mo contents in the liquid steel for ? phase and ? phase precipitation is 8.5wt%and 11.3 wt%,respectively.An addition of Ce could promote ? phase precipitation during solidification process.Ce addition could expand the range of Mo content for ? phase precipitation.After 0.1 wt%Ce addition,the content of Mo for ? precipitation is reduced by 0.24 wt%and the precipitation temperature is increased by 1.6 ?.According to the Gulliver-Scheil model,0.016 wt%Ce addition could obtain a high content of Ce(1.37 wt%)in residual liquid.Insitu observation was used to investigate the influences of 0.016 wt%Ce addition on the solidification phenomena of SASS.Under the cooling rate of 50?/min,Ce addition could increase nucleation density from 68.8/mm2 to 280.0/mm2 and limit the growth velocity from?20 ?m/s to?2 ?m/s.The kinetics of solidification process was analysed by Johnson—Mehl—Avrami—Kologoromov theory.Cerium addition in steel could change the nucleation mechanism from site saturation to site saturation plus avrami,lower the activation energy by 139.86 kJ/mol and reduce the value of k from 3.91×10-3 to 1.79×10-5.It is the heterogeneous nucleation effect of Ce-containing inclusions(Ce2O3-Al2O3-SiO2),which governs the austenite nucleation.Cerium atoms would enrich in the sample surface and solid-liquid interface,which influences the inhibition of austenite growth.When the cooling rate increases to 100?/min,high nucleation density(440.3/mm2)with higher growth velocity(22 ?m/s)makes austenite grains easily grow together,and this would reduce the ability of grain refinement.The appropriate cooling rate for 0.016 wt%Ce addition SASS is around 50?/min.The influence of 0.016 wt%Ce addition on solidification segregation and phase transformation were investigated by directional solidification and thermal analysis.The solidification sequence follows the pattern:(1)L?? and(2)L??+secondary phase.Ce addition could promote the precipitation of secondary phase and reduce the ?T by 3.2?.Ce atoms can be enriched in the interdendritic,which substantially increases the constitutional undercooling degree and promotes the precipitation of ? phase in preference to ? phase.However,the effect is suitable for medium cooling rate(<78?/min).The preferential precipitation of? near the end of solidification is the main reason to improve Mo segregation.This phase transition could shorten the homogenization time and reduce the hardness in interdendritic,which is significant for hot working.The influence of 0.005 wt%Ce addition on the solidification structure of SASS ingots was investigated by vacuum induction melting experiment.Ce addition in steel is mainly used to form complex Ce-containing inclusions that could be an effective core for austenite heterogeneous nucleation.As for Ce-bearing ingot under medial cooling rate(37.5?/min-69?/min),the region of equiaxed crystals is increased from 18.38%to 50.81%,and the central micro grain density is increased from 2.06×108/m2 to 3.00×108/m2.In addition,the precipitation of ? phase is significantly reduced from 5.6%to 2.5%.? phase is small in size and quite uniformly dispersed in terms of distribution in Ce bearing ingot,which is significant for homogenization. |
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