Microstructural Design Of Ultra-high Strength Steels With Ductility Enhancement By Quenching And Partitioning Process

Over the century, low alloyed cheap Fe-Mn-Si steels have been widely investigated from high-strength low-alloyed (HSLA) steels to advanced structural steels (ASS), and its development will still be focused on higher strength and better ductility steels in the future. In an international context of reducing carbon emissions to curb global warming, it is an urgent need to develop high strength and ultra-high strength steels in order to obtain lighter components, save energy and protect our environment. Therefore, we developed new ultra-high strength enhanced ductility steels by microstructural design. We proposed that this steels should have bcc martensite matrix (hard phase) and fcc austenite (soft phase) composites in microstructure. Based on this idea, we designed 0.41C-1.3Mn-1.27-Si-1.01Ni-0.56Cr steel and successful obtained the acquired steel with 1573MPa tensile strength and 20.43% total elongation by quenching and partitioning (Q&P) process.In view of the relationship between microstructure and properties, we characterize the sample by scanning electron microscopy (SEM), electron backscattering diffraction (EBSD), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The results indicate that the obtained sample has 18.7% austenite phase which is uniformly distributed between martensite lathes. The observed 50-200nm width of austenite film is much thicker than that in traditional martensite steels which enables compatible deformation between two phases. And the width of martensite laths is 100-200nm with high dislocation density inside. X-ray profile analysis shows that the micro-distortion and dislocation density in martensite is 2.60×10-3 and 8.05×1014m-2, respectively. The martensite matrix in Q&P process is softer than that in traditional quenching and tempering steels with same tempering temperature and time, which indicates that the Q&P process can accelerate the rate of reducing martensite matrix distortion and improve its ductility.For the enhanced ductility mechanism by austenite, we proposed that the good performance came from the following reasons: (1) The retained austenite helps to reduce the quenching stress in material; (2) Austenite becomes a"sponge"and absorb carbon which diffuses from supersaturated martensite. It reduces the possibility of carbides precipitation in martensite and enhanced the matrix ductility; (3) Austenite separates the martensite blocks or laths so that it can reduce the effective grain size and prevent cracks from propagating. Thus, it enhanced ductility and toughness; (4) Austenite can deform compatibly with martensite matrix during tensile testing. And it will present transformation induced plasticity (TRIP) effect at higher strain which delays the necking of sample and guarantees compatible deformation between martensite and austenite. Therefore, the ductility of the sample was improved.Based on compatible deformation between two phases and TRIP effect, we proposed modified Mileiko phenomenal model by adding an interaction term of austenite and martensite. The prediction is more consistent with present experimental results.