Study On Microstructal Mechanisms Of Hot Deformation And Recrystallization In Single-phase And Duplex Alloys

In the modern steel industry,hot processing processing is the main production method of steel materials.Dynamic softening behaviors,such as dynamic recovery(DRV)and dynamic recrystallization(DRX),are very important phenomena that occur during the hot processing of steels.The occurrence of DRX does not only affect the flow stress of the material as well as the load control of the equipment during the hot deformation,but also significantly change the microstructure characteristics and crystallographic textures of the material,which directly affects the microstructure and mechanical properties of the final product.Therefore,it is of great importance to accurately understand the DRX mechanisms of steels during the hot processing,which is significante for microstructure control and optimization of processing technology of new steel.A Ni-30%Fe austenitic model alloy and a 2205 duplex stainless steel(DSS)were used in this thesis.Hot compression tests were performed on a Gleeble-3500 thermo-mechanical simulator.The microstructures were analyzed by the scanning electron microscope(SEM),the electron backscattered diffraction(EBSD),the transmission electron eicroscope(TEM)and the scanning transmission electron microscope(STEM).The DRX behaviors and its microcosmic mechanisms of the single-phase austenite alloy and the duplex-phase alloy were systematically analyzed through detailed characterizations of the microstructure evolution during hot deformation in the Ni-30%Fe austenitic model alloy and the 2205 duplex stainless steel.The main research contents and conclusions of the thesis are summarized as follows:(1)The microstructural mechanism of austenite DRX and its correlations with the deformation condition was studied.The influce of deformation temperatures and strain rates on austenite DRX mechanisms were analyzed in detail.The results show that the change in deformation temperatures and strain rates would significantly affect hot deformation behaviors and DRX mechanisms of austenitic steels.During high temperature deformation,the main DRX mechanism in austenite was the discontinuous dynamic recrystallization(DDRX)mechanism.As the deformation temperature decreases,the DRX mechanism of austenite will gradually change from DDRX mechanism to continuous dynamic recrystallization(CDRX)mechanism.On the other hand,the effect of strain rate on the DRX behavior of austenite was relatively complicated.At the low strain rate condition,the DRX fraction and the grain size would gradually decrease with the increase of the strain rate.but at the strain rate conditions(i.e.,?>1 s-1),the DRX fraction and the grain size would gradually increase with the increase of strain rate.(2)The roles of deformation substructures in austenite in the recrystallization tranformation during the subsequent deformation or the annealing processes were studied by the strain rates transient deformations and the a two-pass compression deformation,respectively.The results show that the DRX mechanism of austenite were not only related to the deformation conditions,but also affected by the deformation history.During the deformation processes with the strain rates jump,the deformation substructure formed during the former deformation stagewould resulte in a transition between the CDRX mechanism and the DDRX mechanism.The deformation substructures would also significantly affect the softening mechanisms of austenite during further annealing after hot deformation.This leads to both substructure recovery and post-dynamic recrystallization might become the main post-dynamic softening mechanisms of austenite during the annealing process after hot deformation.(3)The evolution of twin and sub-boundaries in austenite during DRX is clarified by the detailed analysis of the characteristics of twin and sub-boundaries in the Ni-30%Fe austenitic model alloy during the deformation.The results show that during the initial stage of deformation,the DRX nucleation mainly takes place by the formation of twins or subboundaries behind original grain boundaries and triple junctions.The pre-existing ?3 twin boundaries in austenite interact with dislocations and gradually transform into ordinary high-angle grain boundaries during the hot deforamtion,which increase potential sites for DRX nucleation.The formation and development of subboundaries within deformed matrix might also promote the formation of new grains through CDRX in austenite.In addition,the subboundaries formed within the growing DRX grains also accelerate bulging of high-angle grain boundaries,which will inhibit the continued growth of grains and promote the occurrence of a new round of DRX nucleation.(4)The microstructural behavior of austenite and ferrite in DSS under different deformation conditions were analyzed in detail.The dynamic softening behaviors of ferrite and austenite and their mutual influence during hot deformationare discussed.The results show that,during the hot deformation of DSS,the strain is preferentially accumulated in the ferrite phase,resulting in advanced ration of dynamic softening in ferrite than that in austenite.The dynamic softening mechanisms of ferrite in DSS obviously change under different deformation conditions.At low strain rate,the ferrite is dynamicly softentened by the sub-grain coarsening.At intermediate strain rate condition,the dynamic softening mechanism changes to the CDRX.At the high strain rate conditions,the dynamic softening mechanism of ferrite changes from CDRX to DDRX.Increasing the deformation temperature was found to be conducive to the occurrence of DDRX.Meanwhile,at the low strain rate,the DRX nuclei of austenite in DSS are mainly formed by the development of subboudaries,which is analogous to the way of CDRX.At high strain rate,rapid deformation process leads to high-density dislocations formed surrounding the twin boundaries,promoting the pre-existing twins in austenite transformed towards ordinary high-angle grain boundaries.This will promote the DRX nucleation of austenite in the way of SIBM.At the intermediate strain rate,however,on the one hand,only short time is available for the DRX,leading to insufficient development of sub-boundaries.On the other hand,large strain gradients are also absent to motivate migrations of high-angle grain boundaries,which can hinder the DRX nucleation of austenite through SIBM.As a result,the DRX process is significantly inhibited.