Effect of Mn Heterogeneous Distribution on Microstructures and Mechanical Properties of Quenching and Partitioning Steels

The ever increasing demand for safe and lightweight steel has promoted the development of advanced high-strength steel (AHSS). Recently, many AHSSs have been developed through chemical heterogeneity, resulting in microstructure refinement and mechanical property optimization. Although many efforts emphasize the construction of Mn-heterogeneous high-temperature austenite (gamma-Fe), the influence of Mn-heterogeneous distribution remains unclear. In this work, different austenitization times and temperatures are applied to Mn-partitioned pearlite, followed by the same quenching and partitioning process. The effect of Mn distribution in high-temperature austenite on the microstructural evolution and mechanical properties is systematically investigated. Results show that the Mn-heterogeneous high-temperature austenite can tailor the austenite-to-martensite transformation during quenching. The Mndepleted austenite is then readily transformed into lath martensite, and the Mn-enriched austenite is mainly retained as film roughness (RA), both of which assemble the ghost pearlite. With an increase in austenitization time and temperature, the Mn atom diffusion from the Mn-enriched austenite (originated from cementite lamellae) to the Mn-depleted one (originated from ferrite lamellae) increases, leading to the decreased chemical heterogeneity in high-temperature austenite. Thus, the fraction of ghost pearlite decreases while the fraction and size of blocky RA and coarse lath martensite increase. A wider lath martensite lowers the strength of the yield or the elastic limit of steel. The increased fraction and size of blocky RA ensure an increased uniform elongation by transformation-induced plasticity effect, whereas the transformation product (i. e., fresh martensite) is detrimental to the post-uniform elongation. Meanwhile, because the fractions of RA and martensite hardly change with austenitization condition, the ultimate tensile strength (about 1700 MPa) and total elongation (about 20%) are relatively constant. Therefore, tuning the Mn distribution in high-temperature austenite provides an effective strategy to tailor yield strength and uniform elongation while maintaining large ultimate tensile strength and total elongation.