The Research On Deformation Compatibility And Fracture Characteristics Of Dual Phase Steels

Due to the continuous yielding, good formability, high workhardening ability, high tensile strength, low yield ratio, dual phase steelshave been widely used in automotive steel. Because of different strengthof ferrite and martensite, the stress and strain partition behavior betweenthem is very inhomogeneous which results in early damage initiation andfracture of the material. Therefore, investigation of deformationcompatibility behavior and fracture characteristics of dual phase steel,grasp of the law of deformation and damage initiation is very meaningfulto the optimized design of dual phase steel and even the next generationsteel.In this paper, the microscale or nanoscale digital image correlationmark fabrication methods, strain partitioning behavior of dual phase steelscomposed of different martensite volume fraction and the fracturecharacteristics of dual phase steel of different morphology are investigated.In addition, the deformation compatibility of ferrite austenite duplexstainless steel is studied.Firstly, indium particles of0.5~1μm and indium tin alloy particles of20~100nanometer are produced with electrodeposition and magnetronsputtering methods respectively.Secondly, for dual phase steels the strain partitioning behavior issimilar and strain distribution is quite inhomogeneous at different tensilestrain levels; the inhomogeneity of strain partitioning behavior betweenferrite and martensite is more pronounced for dual phase steel with lessmartensite; deformation bands are observed in every area studyed, which are mainly distributed in ferrite regions, especially narrow ferrite bandsbetween martensite; the deformation behavior of ferrite in dual phase steelis not determined by the orientation of ferrite but the surroundings. Thestrain distribution in duplex stainless steel is more homogeneous; the straininhomogeneity is increased as deformation proceeds; the orientation ofaustenite determines its deformation behavior and generally the strainlocalization area is austenite area with preferred orientation and regionsnearby.Finally, dual phase with martensite block morphology and martensiteisland morphology of similar martensite volume fraction is produced withstep quenching and intercritical annealing methods to find that the ductilityof the former is better than the latter. According to the strain distributionand fracture characteristics, it is believed that part of cracks in phaseboundary and martensite is blunted by the intense plastic strain area offerrite in step quenching samples to make the fracture of ferrite region in aductile manner to fulfill a better ductility, while the plastic strain inintercritical annealing samples is not large enough to blunt the propagationof cracks.