Effect of Retained Austenite Stability on Mechanical Properties of 590MPa Grade TRIP Sheet Steels

Abstract

Industrial low alloy TRIP sheet steels contain blocky and lath-shaped retained austenite. In the present study, transformation behaviour of blocky and lath-shaped retained austenite during straining was investigated to clarify its effect on mechanical properties. Two types of TRIP steels containing almost the same amount but the different morphology of retained austenite were used. A steel containing large amount of lath-shaped retained austenite exhibits superior ductility, and sustains high work-hardenability in a high strain region. On the contrast, a steel containing large amount of blocky retained austenite exhibits low ductility. The work-hardenability increased steeply to the maximum at a low strain region, and then reduced in a high strain region. The stability of the blocky austenite has been found to be poor with respected to martensite transformation. The lath-shaped retained austenite remains until a high strain region whereas the blocky retained austenite transformed into martensite in a low strain region. Carbon content was higher in the lath-shaped retained austenite than in the blocky retained austenite. Stability of retained austenite is, however, inexplicable only by the carbon content, and would be affected by the different morphology and the resulting restraint conditions.

Stability of Retained Austenite in High-Al, Low-Si TRIP-Assisted Steels Processed via Continuous Galvanizing Heat Treatments

“…All of the preceding observations correlate very well with those of Matsuda et al, [29] who showed similar transformation kinetic results for alloys in which the balance between lamellar and blocky RA was altered to favor lamellar RA, resulting in a more gradual overall RA to martensite transformation arising from the higher lamellar RA stability, a more sustained n incr curve, and overall improved properties. It should also be noted that, for a given heat treatment, the RA to martensite transformation was more gradual in the case of the 1.5Al alloy vs the 1.0Al-0.5Si alloy, consistent with the observations of Bellhouse and McDermid [17] for this alloy and of Samek et al [37] for alloys of similar composition.…”

“…This is consistent with the observations of some authors that lamellar RA has both higher chemical stability (due to its higher carbon content) and higher mechanical stability due to its low equivalent radius and the effect of being mechanically constrained by the surrounding phases, particularly the bainitic ferrite and martensite. [28,29,35,36] It should also be noted that the most gradual transformation kinetics, highest sustained n incr curves, and highest UTS 9 e u products were obtained for the 1.5Al 50 pct c120s alloy, where the ratio of blocky to lamellar RA (approximately 4) was lowest.…”

“…[6] The preceding findings are consistent with those of Bellhouse and McDermid. [17] An examination of the n incr vs strain plots for the 1.0Al and 1.5Al alloys in Figures 3(c) and (d), respectively, shows that the alloys exhibiting the highest UTS 9 e u products also exhibited a higher sustained value of n incr with increasing strain, which has been advanced as being characteristic of a more gradual transformation of RA to martensite [17,18,28,29,33] and, in particular, to the higher stability of lamellar RA vs the blocky RA. [28,29] It can also be seen that the n incr curves for the 1.5Al 50 pct c60s and 1.5Al 50 pct c120s alloys, which exhibited the flattest n incr vs strain curves, also exhibited the highest UTS 9 e u products.…”

“…[17,21,[25][26][27][28][29] Microstructural characterization was performed on the as-annealed and interrupted tensile samples using a JEOL 7000F field emission scanning electron microscope (SEM). Samples were sectioned through thickness (TT) parallel to the RD and were analyzed in the RD-TT plane.…”

“…Figure 3(c)) exhibited n incr values which peaked at low strains and decreased rapidly, consistent with the work hardening characteristics of dual phase steels and suggesting that the RA in these samples transformed rapidly upon deformation. [17,18,28,29] (a) 1.0Al steels -all phases (b) 1.5Al steels -all phases (c) 1.0Al steels -retained austenite (d) 1.5Al steels -retained austenite The evolution of the RA volume fraction as a function of heat treatment and true strain was measured from SEM images taken from the as-received and interrupted tensile samples, where particular attention was given to the morphology of the RA. The volume fractions of total RA as a function of applied strain for the 1.0Al and 1.5Al alloys for all heat treatments are presented in Figures 4(a) and (b), respectively.…”