The article focuses on the effect of alloying and microstructure on formability of advanced high strength steels (AHSSs) processed via quenching and partitioning (Q&P). Three different Q&P steels with different combination of alloying elements and volume fraction of retained austenite are subjected to uniaxial tensile and Nakajima testing. Tensile mechanical properties are determined, and the forming limit diagrams (FLDs) are plotted. Microstructure of the tested samples is analyzed, and dramatic reduction of retained austenite fraction is detected. It is demonstrated that all steels are able to accumulate much higher amount of plastic strain when tested using Nakajima method. The observed phenomenon is related to the multiaxial stress state and strain gradients through the sheet thickness resulting in a fast transformation of retained austenite, as well as the ability of the tempered martensitic matrix to accumulate plastic strain. Surprisingly, a Q&P steel with the highest volume fraction of retained austenite and highest tensile ductility shows the lowest formability among studied grades. The latter observation is related to the highest sum of fractions of initial fresh martensite and stress/strain induced martensite promoting formation of microcracks. Their role and ability of tempered martensitic matrix to accumulate plastic deformation during forming of Q&P steels is discussed.

The mechanical behavior and microstructural evolution of a quenching and partitioning (Q&P) Fe-0.25C-1.5Si-3.0Mn (wt%) steel were investigated in a wide range of strain rates (10⁻⁴–10³ s⁻¹). The static tensile tests (10⁻⁴ and 10⁻² s⁻¹) were conducted using a universal testing machine, while high strain rate tests (500–1000 s⁻¹) were carried out on a split Hopkinson tensile bar system. High speed camera imaging combined with the digital image correlation (DIC) technique were employed to study homogeneity of plastic deformation. Electron backscatter diffraction (EBSD) and scanning electron microscopy were used to characterize the microstructure evolution in the deformed zone and the fracture surface, respectively. The results indicate that the yield strength of the Q&P steel in dynamic tests (500–1000 s⁻¹) is by 200 MPa higher compared to static tests (10⁻⁴ and 10⁻² s⁻¹), while the ultimate tensile strength tends to increase linearly with strain rate. The results of DIC analysis demonstrate that the homogeneity of plastic deformation is similar in static and dynamic test conditions. EBSD characterization shows that the retained austenite (RA) fraction decreases exponentially with the increase of plastic strain during both static and dynamic tensile testing. Additionally, examination of the fracture surfaces reveals the largest dimples in the statically tested specimens.

In this work, a Fe-0.25C-3.0Mn-1.5Si-0.023Al-0.015Cr (wt%) steel was subjected to the quenching and partitioning (Q&P) treatment, and its mechanical behavior and microstructure evolution during drop weight impact testing and quasi-static punch testing were thoroughly analyzed. It is shown that the 1 mm thick Q&P steel sheet can withstand 110 J impact energy without any (micro) cracking, which is well above the impact resistance of DP 1180 steel. The local true plastic strain can reach 53.4% in biaxial stretching showing excellent formability of the material. The microstructure characterization shows that the volume fraction of retained austenite decreases exponentially with increasing plastic strain under dynamic biaxial stretching. Scanning electron microscopy analysis of cracks formed after drop weight impact testing with 120 J energy reveals a ductile fracture surface consisting of elongated dimples formed by MnS inclusions which are surrounded by fine chisel-point type dimples. The potential of Q&P treated steels for application in the automotive industry is discussed.