Mechanical properties of nano-structured plain low-carbon steels produced by conventional cold-rolling and annealing of martensite starting microstructure

The authors have developed a simple thermomechanical processing without intense straining to obtain ultrafine-grained bulk steel with both high strength and adequate ductility. Plain low carbon steels (JIS-SS400 and SM490) were austenitized and water-quenched to obtain martensite as starting microstructure. As-quenched sheets were conventionally cold-rolled up to a strain of 0.8, and subsequently annealed at temperatures of 673K to 973K. The SS400 sheets annealed at around 823K exhibited the multiphase microstructure composed of ultrafine grained ferrite with grain size of 50-300nm, fine cementite uniformly precipitated, and tempered martensite. Owing to this multiphase structure, the nanostructured SS400 sheets had both high strength (710MPa 0.2% offset stress and 870MPa tensile strength) and adequate ductility (9% uniform elongation and 20% total elongation). The nanostructured material impact tested at 103K showed ductile fracture even at this low temperature and the fracture surface revealed fine dimples.