Achieving exceptional strength-low temperature toughness synergy in the microalloyed medium carbon steel via layered heterogeneous structure

• A heterogeneous microstructure with alternating lamellar coarse and fine grains was prepared by warm rolling and short annealing. • The coordinated dislocation motion of the multiscale layered grains produces the HDI hardening effect. • The delamination toughening mechanism acts during crack propagation. The further advancement of microalloyed medium carbon steels is often limited by low-temperature toughness instability. A microalloyed medium carbon steel with both strength and low-temperature toughness was produced through a warm rolling and short-time annealing process. A layered heterogeneous structure was introduced in the microstructure, with fine grains alternating with coarse grains. The sample rolled at 600 °C exhibited the most balanced mechanical properties, with a room temperature yield strength of 772 MPa and comparatively high impact toughness. The high impact toughness of this heterogeneous structure originates from the coordinated deformation of the grains at various scales and the delamination toughening mechanism. Interfacial dislocations and subgrain boundaries introduced during warm rolling promote the heterogeneous deformation-induced hardening effect and improve the plastic strain capacity of the steel. Nanoscale cementites can play the roles of grain boundary pinning and strain adjustment. Moreover, during crack extension, a large number of {001} < 110 > textures help expose the macroscopic disintegration surface of the grains, making it more visible. The generation of delamination cracks and the deflection of the crack tip enhance the energy absorbed during crack propagation. Low-cost heterostructured microalloyed medium carbon steels offer promising opportunities for the development of green steel.