Study On Microstructure Regulation And Strengthening Mechanism Of Austenitic Low-Density Steel

With the strict requirements of energy saving and environmental protection,as well as lightweight and high-performance requirements of the high-end equipments,the research of lightweight,high strength and high toughness materials has become a hot topic.The addition with large amounts of C,Mn and Al elements is effective to reduce the density.Thus,C-Mn-Al steel,especially austenitic type has attracted much attention due to its combination of low density,high strength and high toughness,which have broad application prospects in transportation,aerospace,weapons,ships and other fields.In order to obtain higher strength and toughness austenitic C-Mn-Al steel with lower density,it is necessary to conduct in-depth research on the thermal mechanical process,microstructure regulation and strengthening mechanism of high-strength and toughness austenitic low-density steel.In this paper,x C-30Mn-11Al,1.0C-x Mn-11Al and1.0C-30Mn-x Al alloys were designed by adjusting the alloying element contents（C,Mn,Al）of austenitic low-density C-Mn-Al steel.The microstructure and mechanical properties of the austenitic 1.0C-30Mn-11Al steel with Nb/V/Ti microalloying design were studied after thermal deformation,solution and aging treatment.The microstructure,strength and toughness evolution of different alloy designs were studied by using multi-scale structural characterization techniques such as scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,X-ray energy spectrum（XRD）,electron backscatter diffraction（EBSD）and three-dimensional atomic probe（3DAP）,as well as tensile and impact tests.The relationship between composition design,microstructure and mechanical properties of high-strength and toughened low-density austenitic steel were analyzed,and the strengthening and deformation mechanism from liquid nitrogen temperature（LNT）to 500℃were studied.The main conclusions are as follows:（1）The quantitative relationship between the alloying element contents and density,the size ofκcarbides and ferrite contents,tensile and impact properties of C-Mn-Al low-density steel was established by quantitative analysis of the microstructure and strength and toughness of C-Mn-Al low-density steel after high temperature solid solution treatment.The density decreased⁰.1 g/cm³ with the increase of carbon and aluminum content by 1 wt.%,respectively,and the optimized alloy composition of low-density steel was obtained.It provides a basis for the design and development of austenitic C-Mn-Al low-density steel and the estimation of its density,microstructure and tensile impact properties.（2）The suitable hot working process parameters of microstructure refinement for low density steel were determined by thermal simulation experiments,the hot working diagrams were drawn,the constitutive equation of high-temperature deformation was constructed,and the recrystallization activation energy was determined as 389 k J/mol.C-Mn-Al low-density steel with fine grain,high strength and toughness was successfully obtained by continuous dynamic recrystallization during hot rolling at a lower temperature range,which provided a suitable strengthening technical route of low-density steels for industrial production.（3）It is found that the deformation structure of low-density steel is mainly plane slip by studying on the tensile deformation behavior at room temperature and mechanism of 1.0C-30Mn-11Al-0.1Nb-0.1V steel after solution treatment.Based on the structural refinement and flow stress,the dynamic strengthening and toughening mechanism of low-density steel during tensile deformation was analyzed,and the true stress-strain curve model of tensile deformation was established.The theoretical basis that Hall-Petch grain boundary strengthening constant and lattice friction stress increased with the increasing tensile strain was explained.（4）It was found that the impact toughness of 1.0C-30Mn-11Al-0.1Nb-0.1V steel is very sensitive to the size ofκcarbides.It has little effect on the toughness when theκcarbides size is smaller than 1 nm,while the impact toughness decreases significantly when the size is larger than 3 nm.During aging at 400℃,the size ofκphase less than 1nm shows high thermal stability.Theκcarbides with a size smaller than 1 nm show high thermal stability at 400℃.The size of theκcarbide does not change much after48hours aging treatment,but the yield strength increases by almost 100 MPa and the toughness does not decrease.Thus,a low-temperature long-term aging process of low-density steel for high strength and toughness is obtained.（5）The microalloying composition design of low-density steel was proposed and studied.Nano（Nb,V,Ti）C composite precipitates are dispersed in the matrix and keep stably during the high temperature solution treatment,which hindered the growth of austenite grains and refined the microstructure.At the same time,（Nb,V,Ti）C composition precipitate hinders dislocation movement,which makes the low-density steel to obtain high strength and toughness properties.（6）The deformation behavior and mechanism of low density steel at different temperatures were studied.1.0C-30Mn-11Al-0.1Nb-0.1V-0.1Ti steel has a high strength and plastic combination in the wide range temperature from LNT to 500℃.The deformation behavior and mechanism of austenitic low density steel in wide temperatures range are revealed.Stacking fault energies of the experimental specimens are higher than 50 m J·m^-2 during the wide temperature range,and the deformation mechanism is mainly plane slip.A few deformation twins are found in high local stress concentration areas such as grain boundary.