| Super austenitic stainless steel(SASS),which has excellent corrosion resistances and comprehensive mechanical properties,is widely utilized in petroleum and petrochemical,nuclear and thermal power,shipbuilding,marine engineering and other service conditions.With the development of high-end equipment manufacturing industry in China,the domestic demand for SASS continuous to climb.It is important to master the advanced production technology of SASS.In recent years,remarkable achievements in the research and development,manufacturing,processing and application of SASS have been achieved in China.However,there are still many problems in the production processes,such as serious center macro-segregation of alloying elements,frequent delamination cracking during hot rolling deformation,high requirements in heat treatment processes,long production process and sensitive precipitation behavior.Therefore,in this paper,the solidification structures,alloying elements segregation,the redistribution of alloying elements during homogenization treatments and hot deformation,flow behavior and microstructure evolution in hot deformation processes,precipitation behavior during isothermal aging treatments were systematically investigated.The main research results of this paper are as follows:(1)Solidification and alloying elements segregation behaviors in SASS ingot were analyzed.Solidification structures of ingot consisted of coarse and unevenly distributed austenite dendrites and eutectics,which existed in interdendritic regions and enriched in elements Cr and Mo.Serious center macro-segregation and dendritic segregation of solute elements Mo and Cr were detected in ingot,in which Mo segregation was the most serious.During the solidification of molten steel,the solute atoms in the primary solid phase are gradually rejected into the liquid phase.As the interface of solid phase and liquid phase moves forward,solute atoms were continuously enriched in the center unfrozen liquid phase and form center macro-segregation.When the austenite dendrites form network structures,the diffusion of solute atoms in interdendritic liquid phase is inhibited to some extent,leading to the formation of eutectics enriched in alloying elements Cr and Mo in the last solidified liquid in interdendritic regions.(2)The solidification and alloying elements segregation behaviors of SASS cast strip under sub-rapid solidification condition were analyzed and clarified.Cast strip was prepared by twin-roll strip casting(TRSC)which belongs to typical sub-rapid solidification process,in which the solidification cooling rate exceeds 102?·s-1.The solidification structures of cast strip were composed of fine and evenly distributed austenite dendrites,while no eutectics were observed in interdendritic regions.Center macro-segregation of alloying elements Cr and Mo was inhibited,but dendritic segregation still existed.TRSC process allows the molten steel to transform into solid phase in a very short time.Due to the sub-rapid solidification rate,solute atoms in the interface front of solid phase and liquid phase cannot be sufficiently diffused before the liquid phase solidifies,leading to the diffusion and enrichment of solute atoms toward the interdendritic liquid phase and center liquid phase were prevented.Therefore,the generation of eutectics in interdendritic regions and center macro-segregation of alloying elements were eliminated.(3)The effect of cooling rate on SASS solidification behavior and solidification structure was quantitatively studied.As the cooling rate increases,the crystallization temperature gradually decreased,the dendrite arm spacing gradually narrowed,the volume fraction of eutectics gradually reduced and the segregation coefficients of alloying elements in dendrites were close to the ideal equilibrium state.Based on above experimental results,the expressions about the crystallization temperature(TL?S),the secondary dendrite arm spacing(?2)and the volume fraction of eutectics(V)depend on the cooling rate(v)were established as TL?S=190×exp(-v/25)+1220,?2=39×v-0.22 and V=3.12291×v-0.13511,respectively,which could accurately predict the cooling rate and the volume fraction of eutectics in the solidification processes of ingot and cast strip.(4)Redistribution behavior of alloying elements in solidification structures of SASS ingot and cast strip during homogenization treatments was studied and clarified.With increasing the homogenization temperature or prolonging homogenization time,the dendrite micro structures gradually melted and alloying elements gradually diffused evenly.When ingot was homogenized at 1200? for 480 min,eutectics existed in the interdendritic regions were all dissolved into the matrix and the segregation coefficients were lower than 1.2,indicating that the requirements for homogenization treatments were achieved.When cast strip was homogenized at 1150? for 60 min or homogenized at 1200? for 30 min,the original columnar grains in solidification structures were totally transformed into equiaxed grains and the segregation coefficients were lower than 1.2,indicating that the requirements of industrial production and processing have been satisfied.Therefore,it can be concluded that the cast strip has obvious advantages in completing homogenization treatment.(5)The formation mechanisms of microstructural bandings(MSBs)from continuous casting billet to hot-rolled plates were revealed.SASS continuous casting billet was hot forged and cut into surface forging billet(SFB)and center forging blank(CFB),and then the same hot rolling deformation processes were carried out to prepare SFB hot-rolled plate and CFB hot-rolled plate.It is found that the relative enrichment zones and the relative depletion zones of alloying elements were formed when the forging billet was not completely homogenized after homogenization treatments.After hot deformation,the alloying elements relative enrichment zones and relatively barren zones would be crushed,elongated and preserved in the hot-rolled plates.This would result in uneven distribution of alloying elements and heterogeneity of micro structures,which leads to the formation of MSBs.A small amount of MSBs without precipitates were formed in the SFB hot-rolled due to the low degree of segregation in SFB,while high density of MSBs with plentiful precipitates were formed in the CFB hot-rolled plate.Those precipitates in MSBs were composed of large continuous sigma phase formed through direct transformation from undissolved eutectics and fine sigma phase formed through uneven distribution of alloying elements and strain induced precipitation during hot rolling deformation.(6)Hot deformation behavior of SASS at elevated temperatures was systematically investigated.It is observed that the adiabatic temperature was increased with the increase of strain rate,the decrease of deformation temperature and the increase of true strain,which makes the isothermal deformation transform into non-isothermal deformation and leads to the experimental flow curves cannot reflect the real flow behavior under isothermal deformation conditions.Based on the double corrections of adiabatic heating and friction for the true stress-true strain curves,a strain-dependent Arrhenius-type constitutive equation was established,which has good prediction accuracy.In addition,it is found that the volume fraction of DRX grains was accelerated with decreasing deformation temperatures lower than 1050? and increasing strain rates larger than 1 s-1.This is because high deformation storage generated at high strain rates would decrease the recrystallization temperature and adiabatic heating would increase the deformation temperature and accelerate the nucleation and growth of DRX grains.(7)Precipitation rules of SASS conventional and cast strip hot-rolled plates during isothermal aging treatments were measured.The main precipitates in conventional hot-rolled plate during aging treatments were sigma phase and Laves phase.The main precipitation temperature range for sigma phase was 625-1 050?,while the main precipitation temperature range for Laves phase was 625-825?.Precipitation kinetics curve of conventional hot-rolled plate basically exhibited C-shape and the nose temperature was about 925? and the corresponding incubation time was within 2 min.The type of precipitates and precipitation temperature ranges in cast strip hot-rolled plate during aging treatments were consistent with those in conventional hot-rolled plate.However,the nose temperature of cast strip hot-rolled plate was about 950? and the corresponding incubation time was within 4 min,indicating that the precipitation behavior in cast strip hot-rolled plate was delayed than that in conventional hot-rolled plate. |
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