Enhancement of Mechanical Properties through Subzero Treatment in Medium Mn Lightweight Steels

Abstract

Recently, as global warming, emission gas and fine dust have become an issue, the demand for next-generation advanced high-strength steels satisfying high strength and low density is increasing. The most efficient way is to replace materials that have been conventionally used with lighter ones. Al and Mg alloys are being developed for lightweight, but applications are limited not due to their high costs but also due to their formability and low mechanical properties. Lightweight steel is promising material that offers a combination of high mechanical properties and weight reduction (1.5% weight reduction per 1 wt.% Al addition). The high mechanical properties of lightweight steel mainly depend on austenite fraction or deformation mechanism. The stacking fault energy of austenite is changed according to the addition of Mn, Al, and C, resulting in a various deformation mechanism such as transformation-induced plasticity (TRIP), twinning induced plasticity (TWIP) and microband-induced plasticity (MBIP). However, these properties are difficult to achieve high yield strength. Passenger compartments require high yield ratio rather than energy absorption because deformation must be minimized. In addition, due to TRIP behavior during deformation, the deformation is concentrated at ferrite/austenite boundary, resulting in a crack initiation, which cause a ductility reduction in the duplex lightweight steel. Subzero treatment has been widely accepted in various materials such as stainless steels, ball steels, and ledeburitic tool steels. Subzero treatment is mainly used for the purpose to enhance mechanical properties such as yield strength, hardness, and wear resistance of steels through martensitic transformation of retained austenite. However, most mechanisms for increasing strength lead to ductility loss, which is referred as the strength-ductility trade off. It has been a long-standing dilemma in materials science. Firstly, in the present study, Fe-0.3C-9Mn-5Al-1Si (ferrite + austenite + bainite) triplex lightweight steel showing the operation of TRIP behavior was developed. Subzero-treatment was carried out according to temperature to enhance yield strength. As subzero-treatment temperature decreases, yield strength and tensile strength are improved without ductility reduction. Detailed microstructure evolution, tensile properties, and deformation mechanisms according to subzero-treatment temperature were investigated by interrupted electron back-scatter diffraction (EBSD) and transmission electron microscopy (TEM) analyses. Secondly, Fe-0.3C-9Mn-5Al and Fe-0.3C-9Mn-5Al-0.005B (ferrite + austenite + martensite) triplex lightweight steels showing the operation of TRIP behavior were developed. Water quenched steel after two hours annealing at 1100 C was compared with subzero treated steel. Both steels were tempered after water quenching and subzero treatment, and the strength increased sharply compared to non-tempered steels because nano lath austenite was reverted in martensite during tempering. The yield strength, tensile strength, and elongation of the subzero-treated steel all increase than the properties of the water quenched steel. Crack is initiated at ferrite in contact with metastable retained austenite because dislocations are generated at ferrite by TRIP of retained austenite. The strength ductility trade-off is overcome through the reduction of crack initiation site during subzero-treatment and subsequent tempering. In addition, the addition of boron enhances the mechanical properties by improving the bonding strength of the crack initiation site. The cause of austenite reversion during tempering and overcoming strength ductility trade-off after subzero treatment, and effects of boron were investigated through interrupted electron back-scatter diffraction (EBSD), atom probe tomography (APT), and transmission electron microscopy (TEM) analyses.