Correlation of TRIP effect and Lüders band in a 2.3 GPa ultra-high yield strength medium Mn steel with martensite matrix

Abstract

Enhancing the ductility of ultra-high-yield-strength steel materials exceeding 2 GPa presents a significant challenge due to the intrinsic trade-off between strength and ductility. Strain-aging processes have shown potential to increase yield strength while mitigating the exhaustion of work hardening, facilitating the combination of ultra-high yield strength and exceptional ductility in Transformation-Induced Plasticity (TRIP)-assisted steels. However, the underlying mechanisms of Lüders band formation and its quantitative relationship with the TRIP effect in steels with a yield strength above 2 GPa remain to be fully elucidated. In this study, we prepared an easily formable initial medium Mn steel with martensite/austenite microstructure through low-cost composition design and a simplified process (hot rolling + tempering). The strain aging process endows this steel with a yield strength as high as 2294 MPa and a uniform elongation exceeding 10 %. The results demonstrate that with the increase in pre-strain, some block austenite gradually transforms into fresh martensite and high-density of dislocation can be obtained, which leads to a very high bake hardening response of 531–569 MPa, making significant contributions to the enhancement of yield strength. The strain mechanism of Lüders band is analyzed by digital image correlation method (DIC). When the pre-strain increases from 3 % to 4 %, the strain value of Lüders band increase, and the austenite fraction in the strain interval of Lüders zone changes little, which indicates that there is little TRIP effect in Lüders band deformation. At the same time, a large amount of austenite performs TRIP effect in the hardening stage after Lüders band deformation, which provides enough work hardening and obtains a very high uniform elongation of about 10 % at 2.3 GPa. The main mechanism in Lüders zone strain is dislocation slip, accompanied by dislocation multiplication behavior.