Microstructural Mechanisms Of Metastable Austenite Control In High Strength-ductility Medium-Mn Steels

Demands on both improved mechanical properties and energy conservation in the automotive industry drive the lightweight material evolution,and the result has been a spectrum of new generation advanced high strength steels(AHSS)engineered to meet specific structural use requirements.The medium Mn steels(MMS),contained 4 to 12 wt.%Mn,have emerged to meet these demands and attracted growing attention.It was confirmed that the mechanical behavior of MMS is strongly governed by the transformation-induced plasticity(TRIP)effect of metastable retained austenite,which is tailored by the temperature and duration of intercritical annealing.Thus,most works have focused on optimizing these intercritical annealing parameters.To obtain appropriate mechanical stability and sufficient content of retained austenite,MMS is usually subjected to complex thermo-mechanical treatment and lengthy intercritical annealing that has a large negative footprint in low efficiency and enhanced production cost.In addition,tempering precipitation of cementite during the heating period is considered inevitable,which is also expected to have a significant impact on the austenite reversion of MMS during intercritical annealing.The micro structures and chemical heterogeneity were analyzed by the scanning electron microscope,the electron backscattered diffraction and the scanning transmission electron microscope.In the present work,the precipitation and dissolution behavior of cementite and the interaction between chemical heterogeneity and phase transformation behavior of medium Mn steel were studied.The main research contents and conclusions of the thesis are summarized as follows:The influence mechanism of precipitation and dissolution of cementite on the austenite reversion has been systematically studied in hot-rolled and cold-rolled medium Mn steels.The results showed that,the competition between cementite precipitation and austenite reversion leads to the long annealing period of normal austenite reverted transformation(ART)process.The retained austenite obtained by long-time annealing treatment has the same mechanical stability due to its similar morphology,grain size and chemical composition,and is considered to show similar TRIP effect and excessive work hardening during deformation.An approach of optimizing the intercritical annealing path in a 0.2C-5Mn medium-Mn steel is presented by introducing precursor micro structure prior to normal ART annealing.The steel is firstly pre-annealed at different intercritical temperatures to form designed precursor microstructures.Then they are employed for subsequent conventional ART annealing processing.It is found that pre-annealing at high intercritical temperatures can promote the precipitation and dissolution of the cementite in the steel and re-distribute the C and Mn in the microstructures.The produced microstructural precursors show excellent merits in accelerating the austenite reversions in subsequent normal ART processing and assisting the RA formation.Tensile test reveals that the excellent strength-elongation balance can be achieved in the heat-treated samples using different microstructural precursors,which suggests the potential applicability in producing the medium Mn steels with shortened processing period.In addition,a promising strategy is also proposed to utilize chemical heterogeneity for designing medium Mn steels with the purpose of tuning the austenite reversion and stability.Four types of chemical heterogeneity structures are introduced into the precursor by flash annealing.In the subsequent annealing process,the Mn concentration can be accumulated again in austenite grains through the Mn partitioning between ferrite and austenite,which accumulated firstly in the flash annealing process,resulting in 57.6%of austenite retained after quenching.The Mn chemical heterogeneity can promote the explosive nucleation and growth of austenite,and plays a notable role in enhancing thermal stability of austenite.While the high-density dislocations in fresh martensite the kinetics of Mn partitioning and austenite reversion.An ultra-fine heterogeneous micro structure with lath-shaped and granular-shaped retained austenite are developed in cold-rolled medium Mn steels through a two-step intercritical annealing strategy.The role of these heterogeneous structures in the mechanical properties is revealed.This microstructure is able to provide active TRIP effect over a broad strain range owing to dispersive austenite stabilities.Excellent mechanical properties of significant strength enhancement with negligible ductility loss can be achieved.Meanwhile,the Lüders band is also eliminated.It is proved that excellent mechanical properties with the UTS up to 1206 MPa and TE of 30.6%have been achieved.The optimization of heterogeneous microstructure and mechanical stability of γR may provide an alternative way to the development of cold-rolled MMSs with high strength-ductility and reduced YPE.A new process of intercritical stabilization is proposed to improve the stability of austenite.The kinetics of cementite precipitation and austenite reversion are accelerated simultaneously in the early stage of elevated temperature annealing,and a core-shell structure with Mn chemical heterogeneity is obtained in austenite due to the stasis of the austenite to ferrite transformation in Al-containing medium Mn steels.The effects of refined martensite microstructure on austenite reversion and mechanical properties were also investigated for two different initial martensite micro structure states.It is interestingly found that the kinetics of cementite dissolution and austenite reversion was accelerated simultaneously,and the corresponding dual-phase microstructure consisting of ferrite and retained austenite grains is obtained.Meanwhile,the thermal stability of reversed austenite during intercritical annealing is significantly improved by the chemical heterogeneity at austenite boundaries,which has been proven by both STEM-EDXS measurements and DICTRA simulations.This kind of chemical heterogeneity on the deliberately introduced austenite boundaries finally led to appropriate austenite mechanical stability and thus sustainable martensite transformation during tensile deformation,improving strength and ductility simultaneously.