Study On Hot Deformation Behavior And Microstructure Evolution Of High Silicon Austenitic Stainless Steel

The high silicon austenitic stainless steel exhibits excellent corrosion resistance in concentrated H2SO4 at high temperatures due to the formation of a composite passive film containing a high concentration of Si.However,the increase in Si content results in elemental segregation,dendritic segregation,and precipitation of additional phases,which affects the subsequent plastic forming processes and poses challenges for the practical production and processing of high silicon austenitic stainless steel.Therefore,it is necessary to investigate the thermal plasticity and deformation behavior of this material.This paper focuses on the high silicon austenitic stainless steel（XDS-2）developed by Xunda Group.Modulation of the cast structure with different silicon content by homogenization treatment.High-temperature tensile and compression tests were conducted on high silicon austenitic stainless steel with different solidification modes and microstructure characteristics used a Gleeble-3500 thermal simulation machine.The thermal plasticity and deformation behavior of high silicon austenitic stainless steel were studied through calculations using JMat Pro software,as well as various analytical techniques such as optical microscopy（OM）,electron probe microanalysis（EPMA）,scanning electron microscopy with energy dispersive X-ray spectroscopy（SEM/EDS）,electron backscatter diffraction（EBSD）,and transmission electron microscopy（TEM）.The relationship between deformation parameters and rheological stress was analyzed by stress-strain curve analysis,and the Arrhenius constitutive equation was fitted.Based on the dynamic materials model（DMM）,thermal processing maps under different strain rates were plotted.By analyzing the microstructure evolution process,the softening mechanism and crystallographic information of high silicon austenitic stainless steel during thermal deformation were elucidated.The following main conclusions were drawn in this paper.The cast high silicon austenitic stainless steels have reduced thermoplasticity compared to homogenized specimens.And an appropriate heat treatment process（1150℃×12 h）can eliminate negative effects such as precipitation and segregation,significantly improving the steel’s thermal plastic deformation ability.When the homogenization temperature of 6%Si samples is increased,the re-precipitation of a large amount of high-temperatureδ-ferrite significantly reduces the thermal plasticity,while the presence of an appropriate amount of ellipticalδ-ferrite is conducive to DRX（dynamic recrystallization）occurrence.The study established a constitutive equation for the 6%Si high silicon austenitic stainless steel after homogenization treatment,where the activation energy for hot deformation was determined to be 368.555 k J/mol.The hot working window for the high-silicon austenitic stainless steel was determined by combining the results from the hot processing map and microstructure analysis.Under the deformation conditions of 1200℃/0.01 s^-1,the precipitation ofχphase and a small amount ofδferrite can refine the grain size.However,the presence of brittle second phase can easily lead to crack initiation,and thus it is recommended to avoid processing in the region where the second phase is present.When the deformation parameters change,the 6%Si high silicon austenitic stainless steel exhibits different softening mechanisms.When the strain rate is constant,with the increase of deformation temperature,the DRX（dynamic recrystallization）of 6%Si high silicon austenitic stainless steel becomes more complete,and the softening mechanism is mainly DDRX（discontinuous dynamic recrystallization）.Between 1100～1150℃,the softening mechanism will also show CDRX（continuous dynamic recrystallization）,and the proportion of substructure grains and LAGBs（low angle grain boundaries）will increase.When the deformation temperature is constant,with the increase of strain rate,the average grain size in the structure decreases,and at≥1 s^-1,higher strain energy and adiabatic heating will lead to an increase in average grain size and an improvement in DRX degree.