The Relationship Between Microstructure And Mechanical Properties Of Low-alloy 2000MPa Multiphase Steel

Attention has been given to low alloy multiple phase steels with prior martensite due to attractive property combinations along with appropriate cost.However,there is no systematic research associated with multiple phase transformation mechanism,especially the effect of prior martensite on subsequent microstructure evolution as well as carbon partitioning mechanism.Also,the strengthening mechanism of multiple phase steels requires further understanding and development.Thus a novel treatment process is designed and subjected to both 55Mn2SiCr(steel A)and 60Si2Mn(steel B).It includes an interrupted quench to produce target prior martensite fraction,followed by isothermal holding at different temperatures to enrich the retained austenite.Route 1 is applied to steel A to explore the effect of holding time on microstructure,bending and tensile properties.Route 2 is applied to steel B to investigate the effect of quenching temperature and isothermal holding temperature on final microstructure and mechanical properties.In this thesis,OM(Optical Microscope),SEM(Scanning Electron Microscope),TEM(Transformation Electron Microscope),EDS(Energy Dispersive Spectroscopy)and Image-pro Plus are used to investigate the microstructure evolution with different heat treatment routes.Hardness,bending and tensile tests are conducted to discuss the relationship between mechanical properties and multiple phase microstructure.Furthermore,phase transformation,prior martensite influence,carbon partitioning as well as systematic strengthening effect to obtain 2000MPa low-alloy multiple phase high strength steels are discussed.OM microstructure observations indicate that the microstructure of investigated steels after heat treatment process consists of prior martensite,bainitic ferrite and retained austenite/martensite.With prolonged holding time,bainitic ferrite gradually increases while retained austenite has a general trend to decrease,existing finer micstructure of small island or film morphology.Mechanical properties tests of steel A show that the hardness(HRC)of samples gradually decreases to 55HRC with increasing isothermal holding time.Bending and tensile strength have a similar trend over time.It raises from 30 to 120min,then declines drastically at 240min,peaking at 120min with optimal mechanical combinations of 4000.9MPa bending strength and 2030MPa tensile strength.It is found that bending and tensile fracture surfaces both exhibit brittle fracture mode.Meanwhile,XRD analysis reveals that the product of retained austenite fraction and its carbon content is highly related to the ultimate bending and tensile strength.Mechanical properties tests of steel B indicate that with increasing isothermal holding time,the tensile strength increases significantly,reaches a peak and then starts to drop.The peak value is obtained at 120min or 240min.Quchening and isothermal temperatue are also significant factors which influence the mechanical properties.It shows that the tensile properties drop drastically as the quenching temperature comes down.And the decrease of isothermal temperature generally improves tensile properties.Steel B achieves the highest tensile strength of 2106MPa at 240min when holding at 250?.The strength value is quite steady over 1600MPa when holding at 220?.Tensile fracture surfaces are typically brittle fracture mode.EDS analysis demonstrates that there is an uneven distribution of carbon inside prior martensite.Besides,carbon-depleted region and carbon-enriched region are found along prior martensite/retained austenite interface.Also,TEM observation shows the existence of ?-carbides inside prior martensite.Key mechanistic issues are described under the novel treatment process.The 2000MPa high strength can be attributed to the synergistic strengthening effect of grain refinement,solid solution,carbides precipitation,dislocation,work hardening and filmy microstructure strengthening mechanisms.