Effect of the addition of P on the mechanical properties of low alloyed TRIP steels

Common TRIP (TRansformation Induced Plasticity)-aided steels contain roughly 0.15mass% 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 compared to non-P alloyed TRIP steel. A synergetic effect of Si and P was observed as the increase in tensile strength per mass% Si was five times larger than expected for solid solution strengthening. The volume fraction of retained austenite was found to depend very much on the annealing cycle, with long austempering times resulting in lower amounts of retained austenite (6 to 12 vol%), while short austempering times resulted in higher retained austenite contents (12 to 20 vol%).