Effect of auto-tempering on the cold roll-ability of medium-Mn steel

Abstract

Recently, medium-Mn steels have attracted attention for high-strength automotive components. However, because they exhibit martensitic microstructure at room temperature, low cold roll-ability is concerned so that annealing or tempering is often performed before cold rolling. Therefore, we investigated whether cold roll-ability can be improved by adjusting coiling condition in the hot rolling process. For comparison, some specimens were water-quenched or air-cooled after hot rolling. While the water-quenched specimen revealed cracks at ∼30% cold reduction, air-cooled and coiling-simulated specimens at high reductions above ∼68%. This is because the latter had lower and wider ranged hardness values compared to the former due to partial auto-tempering occurring after coiling. The higher coiling temperature caused higher cold roll-ability probably due to the active recovery of austenite. This paper is part of a Thematic Issue on Medium Manganese Steels.

Effect of Cooling Rate on The Microstructure And Cryogenic Impact Toughness of HAZ in 9% Ni Steel

“…The reason for this result is that auto-tempered martensite and tempered martensite have better toughness after tempering compared to lath martensite [11,[27][28][29][30]33]. The higher toughness of the auto-tempered martensite and tempered martensite compared to lath martensite can be inferred from the micro-Vickers hardness results in Table 6 Therefore, auto-tempered martensite and tempered martensite, which have relatively superior toughness compared to lath martensite, can induce ductile fracture and improve Charpy impact toughness.…”

“…And while the main microstructure of 9% Ni steel is martensite, the degree of martensite tempering varies according to the cooling rate, resulting in significant changes in microstructure and mechanical properties [11]. For these reasons, it important to study the correlation between the microstructural factors and mechanical properties of CGHAZ and IC CGHAZ in 9% Ni steel according to changes in cooling rate.…”

Ultrahigh strength-ductile medium-Mn steel auto-parts combining warm stamping and quenching & partitioning

“…At 720°C, the higher fractions of both retained austenite and softer ferrite are beneficial to the coordination of the subsequent plastic deformation by absorbing the volume expansion caused by martensitic transformation [9,16], resulting in higher EI values (Figure 6). In contrast, the relatively higher faction of hard phase (martensite) appears at 800°C, which may restrain the TRIP effect and eventually deteriorate the ductility of stamped parts [1][2][3][4][5]7].…”

“…Medium Mn steels (3-10 wt-% Mn) have attracted more and more attention for ultra-high strength of over 1 GPa without compromising ductility among a new-generation of advanced high strength steels (AHSSs) [1][2][3][4][5]. These enhanced mechanical properties are mostly due to the coordinated deformation of ultrafine dual or complex phase microstructure and the enhanced transformation-induced plasticity (TRIP) effect resulting from the austenite retained with proper mechanical stability [4,5]. Hence, many studies have been performed to achieve a large volume fraction of retained austenite (V gR ) and optimise its stability by tailoring rolling and intercritical annealing processes [3][4][5].…”

Special issue on ‘Medium manganese steels’

“…Interestingly, a low amount of recrystallisation ( ∼ 10%) gives better tensile properties than near-full recrystallisation ( > 95%) particularly following cold rolling, due to the combined effect of higher intensity γ -fibre texture of ferrite and limited TRIP effect from RA in the former. The critical process step of cold rolling has been studied by Nam et al [31], finding that intensive auto-tempering of martensite just below the martensite start temperature (M s ) due to slow coil cooling facilitates cold rolling by leading to softening of the material. Martensite decomposition leads to heterogeneously distributed cementite particles and higher cold rolling reductions can be applied with higher coiling temperatures without the risk of edge cracking.…”