Effect of the Addition of P on the Mechanical Properties of Low Alloyed TRIP Steels

Abstract

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...

The Role of New Ferrite on Retained Austenite Stabilization in Al-TRIP Steels

“…28) Then, the so-called "new" ferrite has been usually considered as something "to be avoided" and therefore many authors have used in their studies very high cooling rates (Ͼ30°C/s) or even quenching to salt baths that hinder austenite transformation during cooling. 1,2,6,[9][10][11]19,24,29,30) However, TRIP steels are usually coated and those cooling rates are much higher than the typical values close to 15°C/s found in industrial continuous galvanizing (CG) lines. 7) There is controversy in the literature about the nature and properties of new ferrite as well as its influence on the stabilization of austenite and final mechanical properties of multiphase steels.…”

“…On this matter, some authors found that Al can accelerate the bainite formation. 13,23,24) As a result, it has been suggested that TRIP steels with Al additions could be produced on a CGL without isothermal bainitic transformation section 8) or even through continuous cooling. 25) It is generally accepted that the cooling rate from the intercritical annealing (IA) to IH has to be fast enough to…”

Design of Composition in (Al/Si)-alloyed TRIP Steels

“…Phosphorus is useful for solid solution strengthening 12,13) and P-TRIP steels have shown interesting microstructural characteristics and mechanical properties. 9,14) However, it has been shown that P additions slow galvannealing kinetics 15) and can lead to cold work embrittlement 16) for low C contents. According to some authors, Al as a solute element does not segregate to the surface during intercritical annealing before galvanizing, 17) so Al additions are not expected to influence the coatability adversely.…”

“…In order to achieve the austenite stabilization some authors have used CMnSi TRIP-aided steels with Si contents above 1.5 %. [5][6][7][8] However, in selecting steels for coated applications, it is important to pay attention to the silicon content, as high Si additions are known to cause galvanizing problems 9) and it is generally accepted that a level of Si above 0.5 % will hinder coating. 10) Concretely, Si forms a very strong and adherent layer of complex manganese-silicon-oxides, which is easily rolled into the surface during hot rolling and is difficult to be removed.…”

OIM Analysis of Microstructure and Texture of a TRIP Assisted Steel after Static and Dynamic Deformation

“…As shown in the previous works these properties result from a synergic effect that combines the characteristics of ferrite, bainite and mechanically unstable retained austenite which partially transforms to martensite under strain [1,2,3,4]. Therefore, in order to obtain an accurate description of the microstructure-mechanical properties relation in TRIP-assisted steels, it is very important to know how the stress and strain (static or dynamic) distribute among the different microstructural constituents and how the microstructure responds to the external load.…”