Structure-mechanical property relationship in low carbon microalloyed steel plate processed using controlled rolling and two-stage continuous cooling

Controlled rolling followed by two-stage continuous cooling was carried out in-house to study the microstructure and mechanical properties of a low carbon microalloyed steel plate of medium thickness gauge. The objective of the study was to develop a process of obtaining excellent mechanical properties (strength, toughness, and ductility) in a microalloyed steel. The process is industrially viable because it does not require high degree of cooling and large reduction during thermo-mechanical processing. The study demonstrates that it is possible to obtain yield strength, tensile strength, elongation, and percentage reduction in area 585 MPa, 680 MPa, 29.5%, and 55%, respectively, with total deformation reduction of 50% and small first stage cooling rate. The total impact energy at −20 °C was 140 J. The microstructure consists of polygonal ferrite and acicular ferrite. The strain-induced 20-30 nm precipitates act as nucleation sites for acicular ferrite. The acicular ferrite contributes to high strength and good toughness, while polygonal ferrite provides excellent elongation.