Fundamental Research On The Solidification Characteristics And High-temperature Mechanical Properties Of High-Al FeCrAl Stainless Steel

High-Al FeCrAl stainless steel has important application value in the metal carrier for automobile exhaust gas purifying systems due to their high-temperature oxidation resistance.In practical production,however,the steel is faced with difficulties in solidification regulation and cracking during forging and rolling,resulting in low productivity and high cost.In view of this,the solidification characteristics and high-temperature mechanical properties of the steel are fundamentally studied in this thesis,and the main research work includes the following aspects:(1)crystal growth characteristics during solidification,(2)Solidification microstructure and inclusion characteristics,(3)High-temperature mechanical characteristics and deformation behavior and(4)Solidification microstructure regulation.The rule and mechanism of crystal grow during solidification for high-Al FeCrAl stainless steels were studied with the help of directional solidification experiments and relevant theoretical analysis.The result showed that Al content could affect both cellular and primary dendritic spacings,and the freezing range varied by Al content was considered as the main factor to affect the spacings.The critical velocity for transition from cell to dendrite was insensitive to A1 content,but high AI content could obviously decrease the critical velocity for transition from dendrite to cell.The high Al content,which shortened the mushy zone,was disadvantageous to the formation of dendrite.Based on phase identification by X-ray energy dispersive(XRD)spectrometry,the cause of coarse grains in solidification microstructure for high-Al FeCrAl stainless steels was studied.The result showed that high A1 content increased the lattice constant of matrix,which promoted atom diffusion and grain coarsening.High-temperature phase identification affirmed that the steels had no ferrite-austenite transition during solidification and lacked the grain refinement produced by the phase transition.The inclusion identification is determined by scanning electron microscopy(SEM)and X-ray energy dispersive spectrometry(EDS),and the forming characteristics of Al2O3 and AIN inclusions in high-Al FeCrAl stainless steel are further investigated by thermodynamic calculation.The result showed that Al2O3 was unlikely to form in the steels due to the high oxygen solubility limit.Affected by both high-Al and high-Cr contents,the formation of AIN was unlikely in the molten steel.However,it was theoretically possible at the solidifying front,and this had been confirmed by high-temperature in-situ observations.The rule and mechanism of high-temperature strength and ductility were studied with the help of tensile tests by a Gleeble facility.The result showed that the increase of peak stress and yield stress with increasing Al content was due to the solid-solution strengthening of A1 atoms.As the temperature increased,the ductility first decreased,then increased before decreasing again between 700? and 1200°C.According to the comprehensive analysis of transformation,precipitation and the morphology and microstructure of fracture,the drop of ductility was mainly due to the grain coarsening and microcracks caused by precipitates,while the dynamic softening could promote ductility.Considering that dynamic softening was beneficial to high-temperature ductility,the hot deformation behavior was further studied with the help of compression tests.According to the obtained true strain-stress curves,the activation energy Q and the expression of Z factor were determined by a series of linear regressions,and a constitutive equation for hot deformation was built.Based on the analysis of compressed microstucture,the characteristics of dynamic softening at 850?,950? and 1050? were analyzed respectively.The result showed that the continuous dynamic recrystallization characterized by the rotation of sub-grains could occur in the steel.The contrastive analysis of solidification microstructures before and after La alloying showed that the high-Al FeCrAl stainless steel had obvious grain refinement after La alloying.Based on the existent form of La in the solidification microstructure,the mechanism of regulation was studied.The result showed that the solidification microstructure regulation resulted from the pinning effect of La enrichment on the grain boundaries and the heterogeneous nucleation effect of La-containing inclusions.According to SEM+EDS analysis and thermodynamic calculations,the predominant stable phase fields and the projected precipitation path for La-S-0 reaction products are obtained.The tensile tests showed that high-temperature properties were improved after the regulation of La alloying.Due to the La burning loss during the practical smelting process,the Al-La alloy wire was proposed in regard to the La alloying process in high-Al FeCrAl stainless steel.The phase structures,solidification microstructures and mechanical properties of Al-La alloys with varying La content were studied,and the reasonable composition range and process temperatures were proposed as well.