Chemical Composition Optimization and Isothermal Transformation of δ‐Transformation‐Induced Plasticity Steel for the Third‐Generation Advanced High‐Strength Steel Grade

A new low‐manganese transformation‐induced plasticity steel is designed with optimized nickel content to achieve superior strength and ductility while minimizing the use of expensive nickel. The steel is optimized using JMatPro software, then cast, and hot rolled. To assess the effect of intercritical annealing on austenite (martensite at room temperature) volume fraction and carbon content, hot‐rolled steel samples quenched from different annealing temperatures (680–1100 °C) are used. Additionally, hot‐rolled steel coupons are intercritically annealed at about 50% austenite formation temperature (740 °C) and then subjected to isothermal treatments at 300–425 °C for varying times (10–90 min). After optimizing these treatments to maximize retained austenite (RA), tensile specimens are heat‐treated first at 740 °C and then isothermally at 325 °C. Thermodynamic calculations suggest that aluminum combined with silicon may lead to the δ ferrite formation, and even minimal nickel content can stabilize a considerable amount of austenite. In the experimental studies, it is shown that lower‐temperature bainitic holding enhances austenite stability by enriching the carbon content. Optimized two‐stage heat treatments yield up to 25.8% RA, with a tensile strength of 867.2 MPa and elongation of 40.6%, achieving a strength‐elongation product of 35.2 GPa×%, surpassing the third‐generation advanced high‐strength steel grades minimum requirement of 30 GPa×%.