| This dissertation originally designed a new type of high-performance steels suitable for automobile industry,mineral and mining industries,and construction industry where reinforcing in concrete is desired.Firstly,the heat treatment process of the newly developed high strength steels was optimized.Then,research methods such as metallographic,XRD,SEM,TEM,and tensile were utilized to systematically study the macroscopic microstructure,phase composition and conventional mechanical properties of the two tested steels.Medium-carbon dual-phase bainite/ferrite steels with different bainite and carbide volume fractions were prepared via intercritical annealing and austempering.Bainite dominated the microstructure at high intercritical temperatures,and carbides and ferrite phase dominated at low temperatures.Carbides existed in two types,undissolved alloy–cementite and vanadium carbide.The ferrite grain size was reduced to 1.22±0.7μm at790°C.Modified Crussard–Jaoul（MC–J）model was used to analyze the strain hardening process,and the highest strain hardening rate was obtained at 755°C.The high carbide fraction at 755°C increased the ferrite dislocation density and enhanced the ferrite phase strength.The high dislocation density in ferrite and the small strength difference in bainite and ferrite enhanced the strain hardening rate at 755°C.Ultrafine-grained dual-phase ferrite/martensite steel produced through intercritical annealing at 765oC,775oC and 795oC.The microstructures at all temperatures consisted of ultrafine ferrite,martensite and carbides.Carbides existed in two morphologies,（Fe Mn Cr）3C and VC.The ferrite grain size reduced to 0.83+0.3μm when the intercritical temperature was increased to 795oC.Higher phase transformation kinetics from ferrite to austenite and ferrite grains growth restriction by carbides reduced the ferrite size.The maximum yield strength of 1710+15 MPa with total elongation of11.5+0.3%was achieved at 795oC,smaller ferrite grain size,larger volume fraction of martensite and smaller（Fe Mn Cr）3C particles improved the yield strength.Despite the higher ferrite grain size and higher carbon content in martensite,the highest strain hardening rate was obtained at 765oC.Higher amount of carbides increased the strain hardening rate at 765oC.The strengthening mechanism of dual-phase steels at each intercritical temperature was studied and strength contribution from each strengthening factor was calculated.The calculated results at each temperature was agreed well with the experimental results.The effect of magnetic field on bainite microstructure and mechanical properties of 70Si3Mn Cr steel was studied.Microstructure observation showed that introducing magnetic field during the austempering process,the microstructure was significantly refined and the volume fraction of the bainitic ferrite was increased.The high nucleation rate for bainitic ferrite caused by the high magnetostatic energy should be responsible for this positive effect.As compared with samples without magnetic field treatment,the magnetic samples also show a simultaneously increased strength and ductility.The ultra-fine bainitic ferrite with higher dislocation density,in magnetic samples increased the tensile strength,and carbon-riched filmy retained austenite and small blocky retained austenite in magnetic samples have a tremendous effect on ductility.High magnetic strength in magnetic samples increases the strength of the bainitic ferrite,which acts as a stress shielding source to prevent the retained austenitic phase from higher stress and increase the plasticity.Moreover,high magnetic strength can alter the spins of the free radicals between the dislocations and obstacles from S state to T state which had weak bonding with dislocations.Hence,depinning of dislocations is facilitated which results in increased plasticity.Therefore,due to the magnetoplasicty,high magnetic properties,and ultra-fine microstructure,magnetic samples show a more excellent strength and ductility as compared with the normal samples. |
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