Deformation-Induced Microstructural Banding in TRIP Steels

Abstract

Microstructure inhomogeneities can strongly influence the mechanical properties of advanced high-strength steels in a detrimental manner. This study of a transformation-induced plasticity (TRIP) steel investigates the effect of pre-existing contiguous grain boundary networks (CGBNs) of hard second-phases and shows how these develop into bands during tensile testing using in situ observations in conjunction with digital image correlation (DIC). The bands form by the lateral contraction of the soft ferrite matrix, which rotates and displaces the CGBNs of second-phases and the individual features within them to become aligned with the loading direction. The more extensive pre-existing CGBNs that were before the deformation already aligned with the loading direction are the most critical microstructural feature for damage initiation and propagation. They induce micro-void formation between the hard second-phases along them, which coalesce and develop into long macroscopic fissures. The hard phases, retained austenite and martensite, were not differentiated as it was found that the individual phases do not play a role in the formation of these bands. It is suggested that minimizing the presence of CGBNs of hard second-phases in the initial microstructure will increase the formability.

Industrial Trials on Preparation of Cerium‐Treated H13 Steel by Electroslag Remelting with Cerium‐Oxide Containing Slag

“…The banded segregation resulted from the dendritic segregation would have an adverse effect on the uniformity of the microstructure of the hot‐formed steel and cause stress concentration. [ 40 ] Figure 13 shows the distributions of the dominant alloying elements C, Cr, Mo, and V in the forged H13 bars. The banded distributions of Mo and V are demonstrated by the element mapping (Figure 13a) of the CA‐Slag‐H13 steel.…”

Development of a novel testing methodology for in-situ microstructural characterisation during continuous strain path change

“…Using a subset size of 25 µm and step size of 8 µm, the software tracked the motion of microstructural features on the samples during the test to produce contour plots of strains that showed the evolution of microstructural strains. This procedure of strain measurement was previously developed and demonstrated in our experimental facility [19,33]. This method is capable of a displacement accuracy of 0.01 pixels with a strain resolution of about 0.1% [18,32].…”

“…This may be done using detectors such as the secondary electron or electron back-scatter diffraction 5 (EBSD) to image and measure texture respectively. Ghadbeigi et al [18] and Celotto et al [19] deformed dual-phase 1000 (DP1000) steel and transformation induced plasticity 800 (TRIP800) steel samples uniaxially using a miniaturised loading rig. They captured SEM images of the sample surface and post-processed the images using the digital image correlation (DIC) technique to analyse the evolution of strain distribution in the ferritic and martensitic phases of their specimens.…”

Substructure Development and Damage Initiation in a Carbide-Free Bainitic Steel upon Tensile Test

“…The early transformation to martensite of these large retained austenite islands makes them hard and brittle at the same time. The appearance of brittle phases at the (prior) grain boundaries, as with the MA phase, has been reported to impair the ductility in TRIP steels [42] and the energy absorption in the Charpy test in low alloyed [43] and high strength steels [44].…”