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Thermal Simulating of the Continuous Casting Solidiifcation Process for2205Duplex Stainless Steel

Author: ChenXiangRu
Tutor: DiQiJie
School: Shanghai University
Course: Iron and steel metallurgy
Keywords: Duplex stainless steel Continuous casting Experimental simulation Forced convection Heterogeneous core
CLC: TF777
Type: PhD thesis
Year: 2013
Downloads: 189
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Abstract

Since the typical Grade2205（Fe-22.5Cr-5Ni-3Mo-0.17N） was confirmed in1991, duplex stainless steel has been widely used for nearly20years, which is broadapplied in various industry fields requiring high corrosion resistance, such as oil andgas pipeline, chemical engineering, paper manufacturing, architecture，ocean platform，energy and heat exchanger，fresh water treatment and so on. However，due to the poorthermal conductivity，high liquid viscosity，complicated solidification microstructureand containing many alloy elements for stainless steel，several problems such ascoarse microstructure, crack and nozzle clogging easily tend to exist in continuouscasting process, resulting in the manufacturing defects, such as the formation ofinclusions, which reduces the production effectiveness.According to the analysis on the characteristics of the DSS and the problemsexist in the production of the continuous casting slab, traditional directionalsolidification, the simulation equipment for continuous casting slab andheterogeneous nucleation technology are adopted in this article, studying thesolidification characteristic of2205DSS, the influence of the parameters of thecontinuous casting processing on solidification microstructure and the solidificationmicrostructure refinement of heterogeneous nucleation technology.According to the formula of phase selection and the results of directionalsolidification experiment, we can conclude that the solidification mode of2205DSSis F type, prior forming phase is ferrite, on the basement of which forming theaustenite, and the phase transformation process is as follow: L→L+δ→δ→δ+γ.According to the LKT model and the results of the directional solidificationexperiment, the critic velocity for planar-cell transition of2205DSS is Vcs=2μm/s, thecritic velocity for cell-dendrite transition is Vcd=5μm/s. When the growth velocity is10¹00μm/s, the interface morphology is typical dendrite; while the growth velocityis500¹000μm/s, the interface morphology is fine dendrite.The WDS analysis on the main solutes such as Cr, Ni, Mo and Si on the S/Linterface shows that Cr is an element of K＞1，while the K value of other elementssuch as Ni，Mo，Mn and Si is below1. The growth temperature of austenite gradually increases with the increasing growth velocity. For2205DSS，increasing the growthvelocity from2μm/s to100μm/s，the growth temperature of austenite increases from1096℃to1270℃respectively.Compared the simulated samples to the continuous casting slab， theirmacrostructure and microstructure can be demonstrated to be very similar，howeverthe grain size of simulated samples in equiaxed grain zone is much coarser than thatof continuous casting slab. Adjusting the superheat and cooling strength have littleinfluence on the CET transition. According to the analysis on the dynamictemperature gradient of the samples under different experiment parameters，we canfind that within the60mm distance from water-cooled side upward of the sample, thetemperature gradient changes a lot in response to the adjustment of the experimentparameters, while beyond that zone, the temperature gradient almost doesn’t change.This is due to the poor heat conductivity of DSS, the heat resistance of the unmeltedpart of the sample is much larger than that of heat-transfer of cooling-waterconvection, and the heat conductivity of the sample becomes the main heat resistance,thus alleviating the influence the adjusting of processing parameters bring to CETtransition.Mechanical stirring can effectively increase the ratio of equiaxed grain in DSSand refine the solidification microstructure. Increasing the stirring rate，the length ofcolumn zone reduces from8.2cm to4.6cm and the mean grain size of equiaxed grainreduces from9.5mm to1.2mm. Meanwhile，with the refinement of the grains，themicrostructure of second phase in the DSS are also obviously refined. With therotation rate increasing, the morphology of austenite changes from coarse acicular andwidmanstatten structure to refined particles and islands structure. According to theanalysis on the distribution of the elements between the two phases, it is obvious thatthe distribution difference of the elements between the two phases on the grainboundary is larger than that in the grain, but the distribution state of the elements onthe grain boundary or in the grain of the column grain zone is similar to that ofequiaxed grain zone. In addition, stirring has no significant effects on the distributioncoefficient of the elements between the two phases.The theoretical calculation shows that the Ti, N and O all form the segregation, resulting in the formation of the supersaturation degree during the solidificationprocess, and the supersaturation degree of Ti₂O₃is significantly larger than that of TiN,which helps that the precipitation of Ti₂O₃has priority to that of TiN; Due to the highmatching ability, high-bond supersaturation between the composite nuclei basementand TiN,it can give rise to the nucleation rate; High amount of nuclei can obviouslyincrease the critical temperature gradient G, leading to the tendency to attain highequiaxed crystal ratio.According to the analysis on the observation of the precipitation in the simulatedsamples, it shows that the acting of Ti₂O₃-TiN composite nuclei as nuclei ofheterogeneous nucleation plays a very important role in the solidification structurerefinement of the DSS, which can be realized by the addition of proper amount of Tipowders, thus increasing the equiaxed crystal ratio of the casting slab, refining thesolidification and second phase structure. Moreover, on the basis of the compositenuclei, this technology adopts low Ti content to attain high equiaxed crystal ratio,which can effectively avoid the problems, like nozzle clogging and surface inclusions,existing in the continuous casting process of the steels containing Ti.

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