Deformation-induced austenite grain rotation and transformation in TRIP-assisted steel

Abstract

Uniaxial straining experiments were performed on a rolled and annealed Si-alloyed TRIP (transformation-induced plasticity) steel sheet in order to assess the role of its microstructure on the mechanical stability of austenite grains with respect to martensitic transformation. The transformation behavior of individual metastable austenite grains was studied both at the surface and inside the bulk of the material using electron back-scattered diffraction (EBSD) and X-ray diffraction (XRD) by deforming the samples to different strain levels up to about 20%. A comparison of XRD-and EBSD-results revealed that the retained austenite grains at the surface have a stronger tendency to transform than the austenite grains in the bulk of the material.The deformation-induced changes of individual austenite grains before and after straining were monitored with EBSD. Three different types of austenite grains can be distinguished that have different transformation behaviors: austenite grains at the grain boundaries between ferrite grains, twinned austenite grains, and embedded austenite grains that are completely surrounded by a single ferrite grain. It was found that twinned austenite grains and the austenite grains present at the grain boundaries between larger ferrite grains typically transform first, i.e. are less stable, in contrast to austenite grains that are completely embedded in a larger ferrite grain. In the latter case, straining leads to rotations of the harder austenite grain within the softer ferrite matrix before the austenite transforms into martensite. The analysis suggests that austenite grain rotation behavior is also a significant contributing factor for enhancement of the ductility.

Effect of Q&P parameters on microstructure development and mechanical behaviour of Q&P steels

“…The volume fractions of RA were determined by the Cullity formula (Cullity, 1978), and the austenite lattice parameter a γ was estimated from the extrapolation function of the lattice parameter vs. cos2(θ)/sin(θ) of the (200), (220) and (311) austenite peaks. The carbon concentration X C was obtained according to the procedure proposed in van Dijk et al (2005), where the link between the lattice parameter of the retained austenite was presented as:…”

Novel ultrafine Fe(C) precipitates strengthen transformation-induced-plasticity steel

“…During deformation, the austenite grains in these steels undergo both rotations and transformations to martensite whereby the onset of necking is postponed, leading to improved ductility in these steels through the TRansformation Induced Plasticity (TRIP) effect [2]. While the tensile strength of TRIP steels is typically between 600 to 800 MPa, TRIP steels with higher tensile strengths, in excess of 800 MPa, are becoming increasingly important for automotive applications considering the high demands on weight reduction and safety requirements [3].…”

Unravelling the structural and chemical features influencing deformation-induced martensitic transformations in steels

“…EBSD enables to monitor the microstructural changes of individual grains during deformation. For instance, we recently reported an EBSD study showing that metastable austenite grains rotate within the matrix (of ferrite) during the tensile tests in TRIP steels [11], thereby contributing to the high ductility of these steels. EPMA is a very reliable analytical technique to measure the chemical composition.…”

“…In the literature several factors affecting the martensitic transformation are given: (1) the local carbon concentration in austenite, [8] (2) the grain size of the austenite grains, [9] (3) the crystallographic orientation of austenite grains with respect to the loading direction, [10] and (4) the position of the austenite grains within the ferrite matrix [11]. The above four factors were also closely assessed for these grains with the help of EBSD and EPMA analysis.…”

“…The concept of deformation-induced phase transformation has also been applied to polymers [5], brittle bulk metallic glasses [6] and very recently to titanium-based biomedical alloys [7]. In view of the technological importance there is a strong interest in understanding deformation-induced phase transformations, in particular to assess the role of various microstructural parameters such as the local chemical composition [8], grain size [9], crystal lattice orientation in relation to strain direction [10] and the location of the grain in relation to its surrounding grains [11]. An accurate understanding of all these factors and their interplay will allow fine tuning of phase transformations resulting in an enhanced control over the material properties [12].…”