The Research On Microscopic Deformation Behavior Of Dual Phase Steels Based On Digital Image Correlation Method

Due to the good mechanical properties and excellent formability of dual phase steels, the auto industry has witnessed its emergence and development in the past decades. Investigation of microscopic deformation behavior including deformation compatibility between martensite and ferrite, understanding of the deformation and fracture mechanisms, help to provide important reference in the optimized design of dual phase steel. Thus in the current global trend towards automobile lightening, dual phase steels can reach its full potential for further applications.In the thesis, digital image correlation method was applied to investigate the microscopic strain distribution and evolution of dual phase steels with different martensite volume fraction, dual phase steels with different martensite distribution were also studied on tensile properties, strain distribution and fracture characteristics. In addition, the strain distribution in large grained dual phase steel was also preliminarily studied.Firstly, results show that dual phase steels with different martensite volume fractions obtained via step quenching both have strain concentrated in ferrite, and mostly close to phase boundaries.The inhomogeneity of strain partitioning between ferrite and martensite is more pronounced as the whole deformation increases. Average strains of ferrite and of martensite both increase linearly with average strain of the area, the former significantly faster. And the average strain of ferrite in the sample with higher martensite volume fraction increases faster due to the higher proportion of ferrite adjacent to martensite which is more prone to strain concentration.Secondly, dual-phase steels with different martensite distribution – dispersed and continuous,with similar martensite volume fraction were produced in methods of step quenching(SQ) and intercritical annealing(IA). Compared to IA steel, SQ steel has lower strength, but longer elongation and higher fracture toughness, and the latter are attributed to larger deformation in ferrites that results in more stress relaxation of martensite during deformation. While in IA steel, the deformation in ferrites is blocked by adjacent martensites, so that a relatively small strain of ferrite cannot effectively relax the stress in martensites, which resulted in higher plastic deformation in martensite than in SQ steel; therefore, cracks preferentially initiate in martensite, and IA steel exhibits higher strength and lower plasticity.Finally, the strain distribution in large grained dual phase steels was investigated. Ferrite grain boundary was found to be strain localization spot in addition to the phase boundary. And orientation of the ferrite was not found to contribute to strain localization.