Abstract
This work aimed to analyze the damage behavior of cold rolled TRIP780 steel sheet submitted to interrupted uniaxial tensile tests performed along the rolling direction. The formation of voids is investigated as a function of the straining level using digital image analysis of scanning electron micrographs to obtain the measures of void density, void area fraction, void aspect ratio and mean void size. The volume fractions of both ferrite/martensite and retained austenite constituents were obtained from X-ray diffraction measurements. An abruptly decrease of retained austenite was observed at early stages of deformation followed by a slow saturation. The resulting strain-induced martensite is responsible for improving the formability of the TRIP780 as observed by instantaneous strain-hardening exponent. In the lower strain range, growth and coalescence of existing microvoids prevailed at both in-plane directions whereas nucleation of microvoids was also observed along the loading direction. Conversely, nucleation prevailed at the transverse direction in the intermediate strain range whereas growth and coalescence were predominant aligned to the loading direction. At larger strain levels, growth and coalescence of microvoids prevailed at both directions. The microvoids were initially found around inclusions and at the interface of ferrite-martensite phases and lastly also at the ferrite matrix.
Highlights
Nowadays, advanced high-strength steels (AHSS) for the automotive industry are grouped in three generations[1] for applications demanding enhanced crashworthiness performance in frontal and lateral collisions and lightweight structures components with lower production costs aiming at substantially increasing the fuel efficiency and, reducing the greenhouse gas emissions
Bearing in mind the role of the complex characteristics of the multiphase TRIP-assisted steels in both mechanical properties and damage that control the ultimate failure in sheet metal forming, this work aimed at evaluating the volume fraction of retained austenite and void formation as a function of plastic strain of a cold rolled zinc coated TRIP780 steel sheet submitted to interrupted uniaxial tensile tests
The microstructure of the TRIP780 steel analyzed by light optical microscopy (LOM) revealed with LePera etchant, shown in Figure 2(a), with the martensite and retained austenite constituents (M/RA) in bright color whereas the ferrite (F) matrix and bainite (B) are shown in bright and dark brown, respectively
Summary
Nowadays, advanced high-strength steels (AHSS) for the automotive industry are grouped in three generations[1] for applications demanding enhanced crashworthiness performance in frontal and lateral collisions and lightweight structures components with lower production costs aiming at substantially increasing the fuel efficiency and, reducing the greenhouse gas emissions. TRIP steels sheets from the 1st generation of AHSS for automotive applications[6], according to the available commercial grades and standard nomenclature TRIP YS/ UTS (Minimum Yield Stress in MPa/Minimum Ultimate Yield Strength in MPa), are used to manufacture frame rails and rail reinforcements (TRIP 350/600), side rail and crash box (TRIP 400/700), dash panel and roof rails (TRIP 450/800) and B-pillar upper, roof rail, engine cradle, front and rear rails and seat frame (TRIP 600/980) These TRIP-assisted steels usually have a low content of allowing elements resulting in a multiphase microstructure commonly
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