Effect of Intercritical Deformation on Bainite Formation in Al-containing TRIP Steel

Abstract

The Influence of intercritical deformation, cooling rate and prior austenite grain size on bainite formation were investigated by dilatometry tests. Intercritical deformation (0-40 %) performed in steels with a prior austenite grain size of 15-28 mm leads to formation of more ferrite during the cooling and less bainite during the subsequent isothermal stage, and even almost no bainite is formed after 40 % strain. Fast cooling after deformation can suppress ferrite transformation. Relaxation following deformation can significantly, but not completely, reduce such effect of deformation due to the occurrence of recovery and recrystallization, particularly for the finer prior austenite grain size. When the prior austenite grain size was changed from 26.8 to 16.8 mm, bainite formation was suppressed. The mechanism for influence of deformation on bainite formation was discussed on the basis that deformation could refine the austenitic microstructure. Further, it is suggested that there is a critical size of austenite grains or subunits after deformation for the formation of bainite.KEY WORDS: TRIP steels; bainite formation; intercritical deformation; austenite grain size; cooling rate; relaxation.by relaxation for either 0 or 60 s; next, cooled to the bainitic stage (400°C) at 20 or 50°C/s and held for 10 min. Finally, they were quenched to room temperature by helium gas.The quenched samples were cut through cross-sections in the middle of deformed samples, and polished as normal, then etched with 5 % nital and rinsed with water followed by etching in a 10 % sodium metabisulphite solution. This etching is intended to better distinguish between upper bainite and ferrite for quantification of phase fractions because both of them will appear as a light color under the optical microscope after nital etching. Contrastively, such etching can reveal ferrite as light gray, bainite/martensite as black and retained austenite (if preserved) as white. 6,7) In addition, nital and saturated picric acid were also used to reveal the prior austenite grain boundaries and ferrite subgrain boundaries. Image analysis software of Leica was used to quantify the volume fractions of ferrite in the quenched specimens and austenite grain sizes achieved from the solution at 950 and 1 100°C. There are three points in the process at which austenite fractions is determined to follow transformations taking place during the whole process, as shown in Fig. 1. Intercritical austenite fraction before the deformation, i.e. f v 1 g , is derived from the optical measurement of the ferrite fractions of interruptedly quenched samples. Similarly, the austenite fraction before bainite formation, i.e. f v 2 g in Fig.1, is also calculated from the measurements on the ferrite fractions of samples after the whole processing. The remained austenite fraction just after bainite formation, i.e. f v 3 g , is calculated from the dilation signals which will be discussed later. X-ray diffractometry is used to measure the quantity of retained austenite in some quenched spe...

Cementite dissolution at 860 °C in an Fe-Cr-C steel

“…To convert the length change from the dilatometry measurement, D l/l 0 ,i nto the volume fraction change during phase transformations, many authors used alinear assumption between length and volume [33][34][35] ,w hich may be inaccurate for some cases. To improve this, the influence of chemical composition on lattice parameter is taken account, [26,36,37] which often requiresacomplicated numericalc alculation to solve the equations.…”

Drawing of CCCT diagrams by static deformation and consideration deformation effect on martensite and bainite transformation in NiCrMoV steel

“…It has been proved that deformation varies temperature and time of the transformations during continuous cooling austenite (Jandova et al, 2002;Ming-Chun and Fu-Ren, 2003;Tamura et al, 1988). Other authors have demonstrated that the type of deformation (tensile, compression, hydrostatic or torsion) (Easterling and Porter, 1992;Bhadeshia, 1992), the amount of strain (Fang et al, 2003;Murr et al, 1982), strain rate (Murr et al, 1982;Luo et al, 2003) in thermomechanical process and prior austenite grain size (Luo et al, 2003) have distinct effects on martensite transformation (MT) and bainite transformation (BT). Also it has been proved that by strain, the nucleation sites of martensite and bainite increase (Murr et al, 1982;Pope, 1972;Nichol et al, 1977;Olson and Cohen, 1975;Koyima et al, 1996;Breedis, 1965).…”

High-rate tensile properties of Si-reduced TRIP sheet steels

“…Among various alloying elements that can substitute for Si, Al has been actively investigated [9,10], as it is known to suppress carbide formation in austenite. Al, however, has little solid-solution hardening effect in ferrite, and the substitution of Si with Al significantly reduces the alloy's strength.…”