A method of heat treating a steel workpiece may include two heat treatment stages. In the first stage, the steel workpiece may be heated to a first temperature greater than or equal to its A1 temperature, but less than its A3 temperature to transform the microstructure of the steel workpiece into a multiphase microstructure including grains of ferrite and grains of austenite having an average grain diameter. In the second stage, the steel workpiece may be heated to a second temperature greater than the first temperature to increase the average grain diameter of the grains of austenite. Thereafter, the steel workpiece may be cooled to ambient temperature at a rate sufficient to retain a major portion of the grains of austenite obtained during the first and second heat treatment stages. The as-heat treated steel workpiece may comprise a retained austenite phase dispersed within a ferrite matrix phase at ambient temperature.