Isothermal Holding Treatment of a Transformation-Induced Plasticity Steel for Obtaining Ultrahigh Strength and High Plasticity

The determining significance of isothermal holding on microstructure and mechanical properties of a transformation-induced plasticity steel (Fe-1.67Mn-1.32Al-0.55Si-0.47C) was studied with multiple techniques including x-ray diffraction, scanning electron microscopy and transmission electron microscopy. The objective was to design an optimal isothermal holding treatment for medium-carbon TRIP steels with ultrahigh strength and high elongation. A critical analysis of the experimental observations is presented. After isothermal holding treatment, the microstructure mainly consisted of ferrite, bainite, retained austenite, and a small amount of martensite-austenite island. The volume fraction of RA first increased from 28 to 32% with the increase of temperature from 380 to 420 °C, and then decreased dramatically to a minimum value of 23% with further increasing temperature to 450 °C. However, carbon content in RA decreased from 1.42 to 1.18% with the increase of temperature. A tensile strength of 1250 MPa and a maximum elongation of 55% were obtained at 420 °C because of the optimal combination of RA and carbon content. The highest yield strength of 660 MPa was obtained at 380 °C and the highest tensile strength of 1480 MPa was obtained at 450 °C, respectively. Less stable RA transformed to martensite, while RA with a high stability was retained during tensile straining.