INTERCRITICALLY ANNEALED AND ISOTHERMALLY TRANSFORMED 0.15-PCT-C STEELS CONTAINING 1.2-PCT SI-1.5-PCT MN AND 4 PCT NI .1. TRANSFORMATION, MICROSTRUCTURE, AND ROOM-TEMPERATURE MECHANICAL-PROPERTIES

Steels containing 0.15 pct C and 1.2 pct Si-1.5 pct Mn or 4 pct Ni were intercritically annealed and isothermally transformed between 300-degrees-C and 500-degrees-C for 1 to 60 minutes. The specimens were subjected to tensile testing at room temperature, and the microstructures were evaluated by light microscopy, scanning and transmission electron microscopy (SEM and TEM, respectively), and X-ray diffraction (XRD). The microstructures consist of dispersed regions of bainite, martensite, and austenite in a matrix of ferrite, and a maximum of 11.6 pct austenite is retained after isothermal holding at 450-degrees-C in the Si-Mn steel. In specimens where austenite transforms to martensite during quenching after isothermal holding, the stress-strain curves show continuous yielding, high ultimate tensile strength (UTS), and relatively low ductility. In specimens where higher volume fractions of austenite transform to bainite during isothermal holding, the stress-strain curves show discontinuous yielding, low UTS, and high ductility.