Research On Microstructure Evolution Of Micro-laminated Dual Phase Aluminum Containing Steel And Its Effects On The Mechanism Of Mechanical Properties

In these recent years,the low density aluminum containing steel became one of the hot research topics in structural steel research field.A systematic research on the low carbon medium manganese medium aluminum alloy system was made in this study.A series of newly developed structural steels with the special micro-laminated dual-phase microstructure were successfully prepared through alloy composition design and hot rolling process design in the dual phase region at high temperature.This kind of steel exhibits ultrahigh impact toughness similar to the laminated metal composite.It also exhibits good tensile properties,including high strength and good plasticity.It provides a new research way to develop new structural steel with high strength and high toughness.Firstly the low carbon medium manganese medium aluminum alloy system was evaluated by using the Thermal-Calc thermodynamic calculation software to draw the Schaeffler phase diagram,which is specially used for low carbon medium manganese medium aluminum alloy system.A series of micro-laminated hot rolled ferrite and martensite dual-phase steels were prepared successfully through alloy composition design and simple hot rolling process design in dual phase region at high temperature.The newly developed structural steel with the special micro-laminated dual-phase microstructure exhibits high strength and ultrahigh impact toughness(L-S direction).Excellent mechanical properties,for example 1200MPa tensile strength and 400J impact toughness or 1450MPa tensile strength and 200J impact toughness,can be obtained in this kind of steel,which exceeds most of other kinds of structural steels.The effects of alloy composition and hot rolling process on the micro-laminated microstructure and the mechanical properties were evaluated systematically by a variety of microstructure description methods(OM,SEM,TEM,EBSD,etc.)and mechanical property test methods(micro-hardness,tensile,impact toughness,etc.).Microstructure variables and parameters were quantified to establish the intrinsic relationship between the micro structure and mechanical properties.The high strength and ultrahigh impact toughness mechanisms of the micro-laminated ferrite + martensite hot rolled dual-phase steel were discussed in this paper on the basis of microstructure observation,mechanical property test and fracture and crack analysis.It is believed that the micro-laminated microstructure promotes the strengthening of high volume fraction hard martensite phase by weak strain partitioning,thus increase the comprehensive strength of the steel.The soft ferrite lamina and the hard martensite lamina along the thickness direction have strong prevention and guidance effects on crack propagation,which increases crack surface area and the plastic deformation zone with more tensile and bending plastic deformation,this further enhances the energy absorption capacity of the steel under impact load or bending load.With the deep understanding of microstructure evolution characteristic and the mechanical property mechanism,the microstructure and mechanical properties of this kind of steel were further improved.Meta-stable austenite phase was introduced into these micro-laminated dual-phase steels through annealing process at intermediate temperature.The large volume fraction meta-stable austenite phase further enhanced the tensile properties of this kind of steel through strong TRIP effect.The softened micro-laminated microstructure was kept during the annealing process,which further enhanced the impact toughness.Excellent comprehensive mechanical properties,including 45 GPa%tensile property,400J impact toughness of L-T sample and 140J impact toughness of L-S sample,were obtained in the annealed steel.According to the micro area composition analysis,the first element partition behavior during the hot rolling process at high temperature and the second element partition behavior in the decomposition of martensite during the annealing process contributed to the fast formation of large volume fraction meta-stable austenite phase.