The determining role of laminar heterostructures on the mechanical properties of low-density steels

The practical application of low-density steels is hindered because of poor low-temperature toughness. To overcome this limitation, this study is focused on a new low-density steel with exceptional low-temperature toughness performance using a combination of hot rolling, intercritical quenching, and tempering. We study here the different intercritical quenching temperatures on both the microstructure and mechanical properties by EBSD, SEM, Charpy impact experiments and tensile experiments. The results showed that the presence of laminar heterostructures significantly enhanced the steel's low-temperature impact toughness, which can lead to the Charpy impact work of 61 J for the experimental steels at −40 °C. By the observation of the fracture morphology and secondary cracking of the impact specimens, it is found that this improvement was primarily attributed to the formation of obvious anisotropic distribution of randomly oriented grain boundaries between ferrite-martensite laminar heterostructures and the ferrite, which had a strong {001} texture. This led to the generation of more and longer secondary cracks, which increased the energy required for crack propagation (Ep). However, the proportion of ferrite had an optimal value, and as the intercritical quenching temperature increased, the ferrite phase gradually disappeared, resulting in an initial increase and then decrease in toughness. Once the quenching temperature was greater than the completely austenite forming temperature, the formation of a single martensite microstructure led to brittle fractures along the prior austenite grain boundaries, resulting in a significant decrease in toughness, especially at low temperatures. The underlying reasons for the observed behavior are interrogated and discussed in terms of process-structure-properties paradigm.