No abstract

Stability of retained austenite is the key issue to understand transformation-induced plasticity (TRIP) effect. In this work, both thermal stability and mechanical stability are investigated by thermo-magnetic as well as in situ conventional X-ray diffraction and micro synchrotron radiation diffraction measurements. The thermal stability in a 0.20C-1.52Mn-0.25Si-0.96Al (wt%) TRIP steel is studied in the temperature range between 5 and 300 K under a constant magnetic field of 5 T. It is found that almost all austenite transforms thermally to martensite upon cooling to 5 K and M S and M f temperatures are analyzed to be 355 and 115 K. Transformation kinetics on the fraction versus temperature relation are well described by a model based on thermodynamics. From the in situ conventional X-ray and synchrotron diffraction measurements in a 0.17C-1.46Mn-0.26Si-1.81Al (wt%) steel, the volume fraction of retained austenite is found to decrease as the strain increases according to Ludwigson and Berger relation. The diffraction measurements also show that the mechanical stability depends on the orientation of the grain with respect to the direction of the applied stress, and the austenite grains at an angle of 45°or 60°were found to be more stable than those at lower or higher angles. Both thermal and diffraction experiments show an increase in the average carbon concentration of the remaining austenite with lowering temperature or increasing stress. Thermal and mechanical stability of retained austenite is therefore attributed to the carbon distribution over different austenite grains.

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...

In‐situ deformation tests have been performed on a steel displaying the transformation‐induced plasticity (TRIP) effect, while monitoring the phase transformation by means of X‐ray diffraction. A tensile stress is applied to 0.4 mm thick samples of this steel with mass contents of 0.26 % Si, 1.5 % Mn, and 1.8 % Al in a transmission geometry for a synchrotron‐radiation beam of 25 μm · 25 μm. On the diffraction patterns every grain appears as a discrete spot. The austenite {200} reflections are analysed during this investigation. The diffraction patterns are treated like a powder pattern for five different η‐angles, with η representing the angle between the tensile direction and the normal direction of the diffracting {200} planes. The results of the analysis show that η = 0° and η = 90° are the preferential orientations for the transformation to martensite. The Ludwigson and Burger model [9] is used to gain more information about the stress dependence of the deformation induced martensite formation. The microdiffraction patterns also reveal the changes in carbon concentration in austenite at each retained austenite fraction.

No abstract