Achieving high strength and low yield ratio by constructing the network martensite-ferrite heterogeneous in low carbon steels

In this research, focusing on low-carbon steel, a martensite-ferrite heterogeneous structure dual-phase (MFDP) steel with a network morphology where ferrite is surrounded by martensite was obtained via cyclic annealing and subcritical quenching heat treatment processes. With the initial microstructure of ferrite and lamellar pearlite, a spherical pearlite and martensitic structure surrounding the ferrite was first obtained by applying the cyclic annealing process near the Ac₁ temperature. Subsequently, the annealed structure was subjected to subcritical quenching heat treatment, thereby establishing a network-like martensite-ferrite dual-phase heterogeneous structure and named N-760 °C and N-780 °C. In comparison with the ferrite-martensite dual-phase steel where ferrite envelopes martensite, N-780 °C witnessed a marked increase in tensile strength and uniform elongation, while the yield ratio dropped by 20 %. Through cyclic loading and unloading tensile tests, it was found that the N-760 °C showed a more obvious heterogeneous deformation-induced (HDI) strengthening effect. The results from electron backscattering and transmission electron microscopy indicate that, in the N-760 °C, a small quantity of dislocations is produced in the ferrite due to the martensitic phase transformation prior to the tensile test. During the tensile process, as the strain increases, the ferrite undergoes significant deformation, and the intragranular dislocations re-arrange to form dislocation cells and deformation-induced grain boundaries (SIBs). Meanwhile, geometrically necessary dislocations (GNDs) accumulate at the ferrite/martensite interface. Therefore, the non-coordinated deformation between the mesh-like dual-phase microstructure offers additional HDI strengthening for MFDP steel.