Achieving Fine-Grained Microstructure and Superior Mechanical Property in a Plain Low-Carbon Steel Using Heavy Cold Rolling Combined with Short-Time Heat Treatment

A new thermomechanical process consisting of heavy cold rolling (HCR) and short-time heat treatment (STH) is developed to fabricate fine-grained martensite microstructure in a low-cost plain low-carbon steel. To achieve the optimal mechanical properties after STH, three different ferrite–pearlite (F–P) dual-phase microstructures are prepared via hot rolling (HR), HR and austenitizing, and HR and HCR. The microstructure evolution and the comprehensive mechanical properties of the alloy during STH are then investigated. We find that the volume fractions of transformed martensite after STH increase with decreasing grain sizes of the pre-STH F–P dual phases. The rapid heating and short-time holding of STH promote grain nucleation and inhibit grain growth, resulting in microstructure refinement. The formation of martensites with different morphologies and different carbon concentrations in the HR and HCR + STH alloy is identified, owing to the inhomogeneous carbon distribution by STH. Tensile experiments demonstrate that STH greatly improves the comprehension mechanical properties of the alloy. Excellent mechanical properties, with a yield strength of 1224 MPa, a tensile strength of 1583 MPa, a uniform elongation of 4.0% and a total elongation of 7.3% are achieved in the HR and HCR + STH alloy. These excellent mechanical properties are principally attributed to the microstructure refinement and martensite formation induced by STH, with a yield strength improvement of 134% and a tensile strength improvement of 150% relative to the HR alloy.