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...

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