The crystallographic orientation relationships that are active during the transformation of austenite to bainite are studied for two TRIP steels by means of Electron BackScatter Diffraction (EBSD). A detailed evaluation of about 360 retained austenite grains and their BCC neighbours was performed. Three relationships were considered, namely Kurdjumov-Sachs, Nishiyama-Wassermann and Pitsch. It was found that the majority of the austenite grains had at least one neighbour that could be related with one of the three orientation relationships. The Kurdjumov-Sachs relationship appeared to be dominant and no strong indication for variant selection could be retrieved from the studied data. It was, however, also demonstrated that some precautions need to be made since a clear distinction between the evaluation of a small region of the microstructure and conclusions made for the complete material is necessary.KEY WORDS: TRIP steels; EBSD; phase transformation; crystallographic orientation relationships. 1601© 2009 ISIJ KS variants. The number of variants for the other orientation relationships can be deduced in a similar way. A g-a orientation relationship actually represents a misorientation between two crystallographic orientations. Therefore, this misorientation is most frequently described by means of an axis/angle pair ͗d͘w. Consequently, the last column of Table 1 gives the axis/angle representation with the minimum misorientation angle of the different orientation relationships.The Bain orientation relationship is the simplest orientation relationship, but it is never observed in steels. Therefore, this orientation relationship serves as a first approximation or a reference point when the transformation of austenite into a BCC phase is studied. Mostly, the Kurdjumov-Sachs and the Nishiyama-Wassermann relationships are used when the orientation relations between FCC and BCC phases are studied. Although there has been a lot of recent research [10][11][12][13][14][15][16][17][18][19] concerning which of these orientation relationships prevails, there is still considerable debate on the relative importance of these orientation relationships because the angular differences between the different orientation relationships is small which makes the experimental verification difficult. Recently, also other orientation relationships 5,6) were taken into account during the study of the crystallographic aspects of the FCC-BCC phase transformation. 10,20) An overview of some features of the different rational orientation relations is given in Fig. 2. Figure 2 illustrates how the different variants of the different orientation relationships would appear around one Bain variant on a {100} BCC pole figure. When the three Bain variants are shown on a pole figure, each of them is surrounded by eight KS and four NW variants, respectively. The crystallographic misorientation between two neighbouring KS variants is 10.53°, whilst the misorientation between Bain and a KS variant is 11.06°and the misorientation between Bain and a NW vari...

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IntroductionCommon TRIP-aided steels contain roughly 0.15 mass% C, 1.5 mass% Si and 1.5 mass% Mn. The high Si content in conventional CMnSi TRIP-aided steels is known to give rise to galvanizing problems and is also known to cause low ductility levels in the as-cast condition. Si is also known to increase the ductile-to-brittle transition temperature of ferritic steels. These are the main reasons to keep the Si content of TRIP-aided steels as low as possible. It is known that Al, in addition to being a strong ferrite stabilizer, retards the tempering reaction and inhibits the formation of cementite. [1][2][3][4][5] These two features have resulted in the development of low alloy CMnSiAl TRIP steels in which the Si is partially replaced by Al. Al is, however, not an effective solid solution strengthening element and it has been reported to give rise to poor surface quality in casting. Because of these disadvantages, the potential of P-added TRIP-aided steels with reduced Si and Al contents was evaluated.The C activity coefficient calculations in ferrite and cementite are shown in Fig. 1; the data reveal that P has the same effect as Si in delaying the precipitation of cementite and promoting the retention of metastable austenite. However, because only limited additions of P can be used, to avoid segregation phenomena, Si and/or Al are still required in a sufficient amount to obtain the TRIP-effect. In a-Fe, the alloying with P results in particularly high strength levels. 6,7) This solid solution strengthening effect of phosphorous can, in addition, be used to reduce the carbon content, which considerably improves the weldability of these CMnAlSiP TRIP-aided steels.Several alloying elements have been reported in literature 1,2,4,[8][9][10] to be useful in order to fully or partially replace Si in conventional CMnSi TRIP-aided steels. With respect to the Zn-coatability of P-added steels, Hertveldt et al. 11) reported that the galvannealing kinetics of P-and Mnadded interstitial free high strength steels, processed at low dew point, were slower than those of a standard interstitial free deep drawing steel. These authors also showed that an- (Received on March 20, 2006; accepted on May 5, 2006 ) Common TRIP (TRansformation Induced Plasticity)-aided steels contain roughly 0.15 mass% C, 1.5 mass% Si and 1.5 mass% Mn. The high Si contents in conventional CMnSi TRIP-aided steels are known to cause low ductility levels in the as-cast condition and give rise to galvanizing problems which is an essential challenge limiting their use in automotive applications. Partial substitution of the Si by Al leads to improved galvanising properties, but a loss in strength. The effects of substituting the Si and Al partially by P was therefore studied in detail with a special attention to the processing conditions in the hot dip galvanizing and continuous annealing processes. The addition of P was found to result in a higher amount of retained austenite which was more resistant to decomposition at longer austempering times compar...

TRIP‐aided steels offer an excellent combination of strength and formability, which makes them particularly interesting for use in automotive applications. Recent investigations have shown that while the typical high CMnSi TRIP‐aided steel composition offers good mechanical properties, alloying with other elements or a modification of the processing are required to make this steel readily galvanizable without loss of the TRIP properties. Al‐alloying seems especially promising to realize this goal and P could also be an alternative. Due to the very specific thermal processing needed to obtain a TRIP microstructure, it is important to know the influence of these alloying elements on the re‐austenitization kinetics during the annealing. This paper aims at identifying the differences in the influence of Si, Al and P on the intercritical annealing of TRIP‐aided steels. The equilibrium thermodynamics calculations and diffusion‐controlled transformation simulations were used in order to predict the transformation behaviour, and experimental verification was done based on dilatometric experiments.