Microstructure Design And Strengthening Mechanisms Of 0.1C5Mn3Al Laminated Dual Phase Steel

In view of the demand for energy conservation,emission reduction and high safety,aluminum containing medium manganese steel has become one of the research hotspots of iron and steel materials in recent years.In this paper,a recently developed0.1C5Mn3Al laminated dual phase steel was used.The effects of different preparation processes including room temperature cold forging(CF),high temperature hot rolling(HR)or medium temperature drawing(WD)and subsequent heat treatment on the structure and properties of 0.1C5Mn3Al laminated dual phase steel were systematically investigated.The microstructural evolution during deformation and annealing were analyzed by scanning electron microscope(SEM),electron backscatter diffraction(EBSD),transmission electron microscope(TEM).The mechanical properties were obtained by tensile testing and hardness testing.The underlying deformation mechanism and the stress-strain distribution of the material during deformation were characterised and analysed by nanoindentation,in-situ compression in SEM,in-situ tensile test in SEM with digital image correlation(DIC)and in-situ tensile test in TEM.The main results are as following:(1)The as-received sample was cold-forged by von Mises strain of 1.05(engineering strain of 60%),followed by subsequent annealing treatment.The results show that laminated ferrite and martensite phases alternatively arranged after cold forging with refined layer thickness/spacing.And both phases were severely deformed.The strength increased while the elongation decreased significantly.Subsequent annealing could not result in better mechanical properties,for example,the ultimate tensile strength(UTS)of the 600 ℃-5 min annealed sample was 910 MPa,which was comparable to that of AR sample,but the uniform elongation(UE)and total elongation(TE)were only 9%and 11%,respectively.The results of nanoindentaion showed that the AR sample had better elongation due to that the stress accumulated in the martensitic phase can be released through the interface into the adjacent ferrite phase,which relieved the stress concentration.(2)Ferrite and martensite laminated dual phase steel with von Mises strains of 2.0,2.5,2.9 and 4.5(engineering strain of 82%,89%,92%and 98%)were successfully prepared by a high tempreture hot rolling(final rolling temperature of 950 ℃).It was found that as the strain increased,the microstructures of the samples gradually refined and the tensile strength gradually increased with little change in elongation.The sample with von Mises strain of 4.5 had a martensite lath size of about 240 nm and ferrite subgrain size of about 540 nm.The yield strength(YS)and UTS are 965 MPa and 1277MPa,respectively,with the TE of 11.8%.The deformation mechanism was systematically evaluated by in-situ compression in SEM,in-situ DIC in SEM and in-situ tensile in TEM.It is found that after high strain and high temperature hot rolling,the ultrafine lath martensite transformed from the deformed austenite phase transformation during the cooling process has a strong deformation capacity,so that the stress concentration was effectively relieved during deformation.Furthermore,the strain distribution was more uniform with smaller phase thickness,and a combination of high strength and good ductility was thus obtained.(3)Warm drawing was applied to the laminated dual phase steel(held at 600 ℃ before drawing).The lamellar structure was characterized by layered ferrite with a large number of carbides which were formed during the holding process before drawing and reprecipitation or fragmentation during warm drawing.With the increase of the drawing strain,the layers thicknesses were effectively refined and the tensile strength of the samples gradually increased,but the elongation decreased rapidly.The warm drawing samples with von Mises strain of 1.96(engineering strain of 85%)has a layer thickness of 1.8μm and the UTS of 1388 MPa and it is up to now the highest strength of the0.1C5Mn3Al laminated dual phase steel.The selected warm drawn samples were annealed and the elongation was improved,and all of the stress-strain curves of the annealed samples shown obvious discontinuous yielding phenomenon.To sum up,by studying the microstructure and mechanical properties of0.1C5Mn3Al laminated dual phase steel prepared by cold forging,hot rolling and warm drawing,it is found that with the increase of strain,the layer thicknesses of the phases gradually decreases,the microstructure is refined and the strength gradually increases.For cold-forged and warm-drawn samples,the elongation decreases rapidly with the increase of strain.Annealing processes can further tune the microstructure and obtain a certain strength-ductility matching,while for hot rolled samples,a better strength-ductility combination can be obtained only by the increase of strain.The strengthening mechanism of 0.1C5Mn3Al laminated dual phase steel was studied by means of in-situ mechanical testing.The results show that the martensite produced by high temperature hot rolling has good deformation ability after being refined to sub-micron to nanometer scale,and the refined layer structure makes the stress distribution more uniform and the stress concentration alleviated,thus,excellent strength and ductility matching can be obtained.