Thermomechanically Controlled Processed Low Carbon Ultra Strength Steel: Microstructure and Mechanical Properties

Abstract

A low carbon, microalloyed ultra high strength steel has been thermomechanically processed on a laboratory scale unit. Evolution of microstructure and mechanical properties in accordance with phase transformation phenomenon has been studied at different finish rolling temperatures(FRT). A mixture of granular bainite and bainitic ferrite along with the precipitation of microalloying carbide particles are the characteristic microstructural features of the investigated steels. High hardness value in the range of 449-465 HV and yield strength (1110-1143 MPa) along with high tensile strength (1591-1688 MPa) has been achieved along with moderate ductility (12-14%) at the selected range of FRT. Moderate impact toughness values have been obtained 28-32J at room temperature which is found to decrease marginally at the sub-zero temperature (-40°C). The fractographic feature at 750°C FRT tested at room temperature is predominantly dimple fracture in nature which is initiated by the oxide inclusion. However, the fracture surface at -40°C is quasi-cleavage in nature, comprising shallow dimples, intergranular cracks along with the river patterns.