Enhanced strength-plasticity matching of lamellar 1 GPa-grade dual-phase steels via cyclic intercritical quenching

In this work, an innovative cyclic intercritical quenching process has been proposed for achieving excellent mechanical properties in terms of strength and plasticity, which results in a lamellar martensitic-ferritic dual-phase steel. The microstructural evolution under cyclic heat treatment was thoroughly investigated, and the mechanism that enables the lamellar structure to enhance its strength-plasticity matching was also explored. The experimental results revealed that the incomplete austenitization process inhibits the coarsening of the reversed austenite, and the cyclic intercritical quenching procedure finer and more evenly distributes the reversed austenite. The single intercritical quenching (C1) generated mostly massive martensite with traces of ferrite, whereas three-cyclic intercritical quenching (C3) yielded a fine and uniformly distributed lamellar structure. A comparison in terms of the mechanical properties between C3 and C1 steels indicates that uniform elongation, total elongation, and the product of strength and uniform elongation of the C3 steel increased by 85.4%, 47.3%, and 65.0%, respectively, while ultimate tensile strength was dropped by only 11.0%. In addition, the yield ratio of C3 steel is just 0.48. The slight decrease in strength is due to the fine lamellar microstructure. The high plasticity is attributed to the good deformation compatibility between lamellar ferrite and lamellar martensite with excellent plastic deformability. Therefore, cyclic mechanical heat treatment is very promising for the production of low-cost dual-phase steels.