Effects of retained austenite volume fraction, morphology, and carbon content on strength and ductility of nanostructured TRIP-assisted steels

Abstract

With a suite of multi-modal and multi-scale characterization techniques, the present study unambiguously proves that a substantially-improved combination of ultrahigh strength and good ductility can be achieved by tailoring the volume fraction, morphology, and carbon content of the retained austenite (RA) in a transformation-induced-plasticity (TRIP) steel with the nominal chemical composition of 0.19C-0.30Si-1.76Mn-1.52Al (weight percent, wt.%). After intercritical annealing and bainitic holding, a combination ultimate tensile strength (UTS) of 1,100 MPa and true strain of 50% has been obtained, as a result of the ultrafine RA lamellae, which are alternately arranged in the bainitic ferrite around junction regions of ferrite grains. For reference, specimens with a blocky RA, prepared without the bainitic holding, yield a low ductility (35%) and a low UTS (800 MPa). The volume fraction, morphology, and carbon content of RA have been characterized using various techniques, including magnetic probing, scanning electron microscopy (SEM), electron-backscatter-diffraction (EBSD), and transmission electron microscopy (TEM). Interrupted tensile tests, mapped using EBSD in conjunction with the kernel average misorientation (KAM) analysis, reveal that the lamellar RA is the governing microstructure component responsible for the higher mechanical stability, compared to the blocky one. By coupling these various techniques, we quantitatively demonstrate that in addition to the RA volume fraction, its morphology and carbon content are equally important in optimizing the strength and ductility of TRIP-assisted steels.

Study of the decomposition behavior of retained austenite and the partitioning of alloying elements during tempering in CMnSiAl TRIP steels

“…The mechanical property of the steels was resulted from the transformation of retained austenite (RA) to martensite, leading to an increase in the work hardening rate during plastic deformation [6][7][8][9][10][11][12][13][14]. However, high Si contents can cause formation of strong oxide layer at surface.…”

Deformation-Induced Microstructural Banding in TRIP Steels

“…Nevertheless, while a detailed characterization of microstructure constituents is not given here, it would be a useful separate study in itself, like that performed in related material. [39,40] Despite the hard phase forming an almost contiguous network around the ferrite grains, the vast majority of the deformation occurred in the relatively soft ferrite matrix phase. The ferrite elongates in the direction of the loading direction while contracting laterally and in the thickness direction, due to plastic incompressibility.…”

Activated dynamic strain aging of a TRIP590 Steel at 300 °C and low strain rate and relationship to structure

“…The decrease in ε f with decrease in temperature is associated with decrease in the stability of retained austenite. The rapid transformation of retained austenite to martensite decreases the ductility [11,18]. The decrease in ε f can be ignored compared to the increase in stress with decrease in temperature, leading toincrease in stress and strain, as shown in Table 1.…”

“…The specimens were heated to 1200°C at a heating rate of 10°C/s for 60 s. A preliminary dilatometry study indicated that the austenite-starting temperature (A c1 ) and austenite-finishing temperature (A c3 ) are 712°C and 892°C, with an error of 72°C [10]. The bainite-start temperature (B s ) and bainite-finish (B f ) temperature were 450°C and 380°C and the martensite-start temperature (M s , 370°C) at a cooling rate of 40°C/s [11]. Subsequently, samples were annealed at 820°C for 120 s in a high-temperature salt-bath furnace and quenched to a temperature slightly above Ms.…”

“…Samples were scanned at a step size of 0.02°/s in the 2θ range of 40-80°. The volume fraction of retained austenite phase (V γ ) was determined using integrated intensity of (111) γ , (200) γ and (220) γ , and ferrite peaks (110) α and (200) α [10][11][12]. The carbon concentration x c of austenite was obtained by assuming that the lattice parameter is related to the grain's chemical composition [13]:…”