The microstructural failure mechanisms of two DP steel sheets cooled with different cooling rates during their heat treatment are compared in the present study. The as-cold rolled DP steel sheets were annealed at intercritical temperature and cooled down with rates of 45°C/s (quenching) and 2 °C/s (slow cooling). Uniaxial tensile tests were carried out on samples from both sheets and the microstructure of undeformed samples and the broken tensile specimens was evaluated by optical microscopy, scanning electron microscopy and electron back-scatter diffraction technique. Although the grain size did not show significant differences, the amount and size of the constituents, e.g. martensite and bainite, differ between both alloys. Concerning the mechanical properties, the quenched material showed superior strength and ductility besides a less localized deformation at higher strains. The area fraction of voids in the broken specimens was low for both steels. In the slow cooled samples the nucleation of shear bands was on the large voids and cracks were observed along these shears bands. It was concluded that the detrimental effect of void nucleation on both steels is not only attributed to their null-carry capacity but more to the stress concentration close to the voids which gives rise to strain localization in the form of shear bands.
The microstructure evolution in the deformation zone around second phase particles of IF
steel sheets subjected to tensile deformation has been investigated in order to correlate it to the
damage at microstructural scale. The experimental set-up consisted of a series of interrupted tensile
tests which were carried out at different tensile deformations up to fracture. The microstructure of
the deformed samples was investigated by EBSD analysis in which the EBSD scans were focused
on the areas containing Ti (C, N) particles of cubical shape. It was found that at tensile strains below
25%, the ferrite matrix exhibited the evolution of slip bands inside specific grains depending on
their crystallographic orientation although no special strain localization around the particles was
observed. After 35% of tensile strain, the strain concentrates around the particles and particle-matrix
decohesion was observed. The lattice rotations of the matrix surrounding the particles as well as the
selective deformation of the grains are analyzed and discussed.
Texture formation during an austempering treatment of a TRIP-assisted steel was studied by in-situ texture measurements with a high energy source (synchrotron). Samples from a cold rolled sheet were subjected to a complete heat treatment cycle for TRIP steels including reheating to the intercritical (α+γ) temperature region, isothermal soaking and bainitic holding (austempering) at 400°C for 600s. At specific points of the thermal cycle {200}γ, {220}γ {222}γ, {331}γ and {200}α, {211}α and {220}α Debye rings were recorded and the corresponding incomplete pole figures were calculated. The latter were used to derive the orientation distribution functions (ODFs) of BCC and FCC phases at specific steps of the annealing process after assuming the orthotropic sample symmetry. The acquired data for the texture evolution during the α–γ–α phase transformation showed that during the reheating for intercritical annealing the gamma phase with {011} orientation is among the first to nucleate from the recrystallized α phase during heating and the Goss and Cube orientations are among the principal gamma phase components which transform to BCC phase after cooling.
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