Formability of High Strength Dual‐phase Steels

Abstract

The application of ferritic-martensitic dual-phase (DP) steels has become an increasing trend in the automotive industry due to the possibility to achieve significant weight reduction and fuel efficiency with improved crash performance while keeping the manufacturing costs at affordable levels. In order to meet the different design requirements of individual auto-body components, a wide variety of DP grades exhibiting different strength and ductility levels is currently industrially produced. Despite the numerous studies on the relationship between the mechanical properties and the microstructural characteristics of DP steels over the last decades, it is still a challenge to increase their formability at a constant strength level (or equivalently increasing the strength while maintaining a high ductility). One of the possibilities to increase strength is grain refinement. Ultrafine-grained ferritic-martensitic microstructures were produced by intercritical annealing of a cold-rolled, pre-processed dual-phase steel. Ferrite mean grain sizes in the order of -1.5 urn were obtained. The mechanical properties of these steels are studied, revealing the beneficial effect of grain refinement. Ultimate tensile strength above 800 MPa is achievable, while reaching remarkable high uniform and total elongations, which are only slightly affected by the martensite volume fraction. Moreover, the yield to tensile strength ratio can be adjusted between 0.4 and 0.5. Light and electron microscopy investigations, fracture profile and fracture surface analyses, hole expansion tests and additional ultramicrohardness measurements are used for the interpretation of the results and for the correlation of the mechanical properties and the formability characteristics with the microstructure of the steel.

Formation of Ultrafine Grained Dual Phase Steels through Rapid Heating

“…Balliger and Gladman 10) have shown that refining martensite islands in dual phase steels results in an increase in work hardening rate. Tsipouridis et al 11) revealed that the strength of dual phase steels can be improved via grain refinement while the ductility remains unaffected in a Mo-Cr alloyed steel. The yield ratio of 0.59 for a C-Mn UFG dual phase steel is comparable with the coarse grained counterparts.…”

“…However, there have been a very limited number of © 2011 ISIJ studies to develop UFG dual phase steels in plain low carbon steels. [11][12][13][14][18][19][20] Thus, the present work seeks to examine combinations of cold rolling and annealing processing steps to develop UFG dual phase structures in a plain low carbon steel with 0.17C and 0.74Mn (wt pct). A systematic study is presented to investigate the effect of initial structure and processing parameters such as heating rate and intercritical annealing time on the microstructure evolution and resulting mechanical properties.…”

AVALIAÇÃO DO DESENVOLVIMENTO DO AÇO DUAL PHASE DA CLASSE 1000MPa COM REQUISITO DE EXPANSÃO DE FURO E PROCESSADO EM DIFERENTES CONDIÇÕES

“…Outro aspecto microestrutural, que pode ter deteriorado a capacidade de expansão de furo, foi a formação mais intensa de bandeamento de martensita nesta bobina em relação às demais avaliadas. Segundo a literatura [7], durante o processo de corte por puncionamento para a confecção da borda do furo, pode ocorrer a formação de microtrincas preferencialmente em volta e ao longo dessas bandas, acelerando a propagação de trincas durante os esforços de estiramento da borda. Na figura 6 são mostradas as correlações entre os resultados de expansão de furo e a fração volumétrica de bainita, ferrita e martensita.…”

Preparation of metallic samples for electron backscatter diffraction and its influence on measured misorientation

“…The measurement of misorientation caused by mechanical polishing was conducted on a specially heat-treated coarse-grained dualphase steel (DP600) (Tsipouridis, 2006;Tsipouridis et al, 2006). The orientation of the large grains of ferrite in the steel DP600 is most likely not affected by the presence of grain and phase boundaries, as compared to the microstructures of the other more complex and fine-grained materials.…”