Impact of Si on Microstructure Evolution and Mechanical Properties of 22MnB5 Treated as 3rd Gen. AHSS

To optimise the strength, ductility and producibility balance of press hardening (PH) steels, 3rd gen AHSS heat treatments were applied to 22MnB5 with varying alloying additions (Si, Al, Nb, C, Mo, P). Following isothermal bainite formation (IB) and quenching & partitioning (Q&P), the microstructure was either characterised by scanning or transmission electron microscopy. Furthermore, tensile and bending properties were assessed. A particular emphasis was laid on the effect of intermediate Si contents ? 0.8% on austenite retention and its impact on different measures of ductility relevant to PH steels.

It was found, that Si not only increases the amount of retained austenite (RA) but also affects carbide morphology. After IB at 400 °C lower bainite was identified at 0.25 % Si, an intermediate microstructure at 0.5% Si and upper bainite with considerable amounts of RA at and above 0.8% Si. Similarly, Q&P resulted in multi-variant intra-lath carbides at 0.25% Si, an increased amount of inter-lath carbides at 0.5% Si and higher amounts of RA starting at 0.8% Si. Based on experimental observations, it is suggested that at low Si contents, cementite precipitates directly from martensite and bainitic ferrite, whereas at 0.8% Si, austenite is first enriched by C and then slowly decomposed by a eutectoid mechanism. It appears that cementite precipitation from RA occurs with concurrent ferrite formation as it is not accompanied by a decreasing carbon content in RA.

Independent of its C content, RA improved the balance of tensile strength and total elongation for all steels and heat treatments. Therefore, mechanisms other than TRIP may cause this improvement. In contrast, bending did not benefit from higher RA levels. On the contrary, low carbon RA, generated by one-step Q&P, appeared to deteriorate the bending angle. In total, the most promising combinations of yield strength, bending angle and total elongation were featured by two step Q&P and fully bainitic microstructures, both containing high carbon RA.