Dominating Role of Film‐Like Carbon‐Enriched Austenite for the Simultaneous Improvement of Strength and Toughness in Low‐Carbon Steel

Abstract

Multiphase microstructures containing film‐like carbon‐enriched austenite and martensite matrix represent an ideal microstructure that can facilitate the breakthrough of the trade‐off between strength and toughness, as well as good low‐temperature toughness in high‐strength steels. In this study, to determine the impact of film‐like carbon‐enriched austenite on the toughness of low‐carbon high‐strength steel, quenching and low‐temperature partitioning process is applied to a newly designed alloy. The quenching step results in a mixed microstructure consisting of a martensite matrix with high dislocation density and film‐like retained austenite, which is stabilized via carbon enrichment during tempering at 200 ºC for 60 min. Thus, the produced high‐strength steel specimen shows excellent mechanical properties (yield strength = 1201 MPa and tensile strength = 1550 MPa) and superior toughness (68.8 J at −60 ºC). Its strength primarily results from dislocation strengthening, grain boundary strengthening, and solid solution strengthening, and its superior low‐temperature toughness is associated with the film‐like carbon‐enriched austenite, which relaxes local stress, dissipates the plastic energy at the microcrack tip, and delays the initiation and propagation of cracks. Finally, a schematic diagram is designed to elucidate the effect of the retained austenite stabilization on the ductile–brittle transition temperature.

Improving the Cryogenic Strength–Ductility–Toughness of Multi‐Alloy Steel by Optimizing the Feature of Retained Austenite

“…This result is related to RA stability, which affects TRIP toughening. [ 52 ] The extremely unstable RA in QIT steel could not effectively stop crack propagation and prematurely converted to martensite during cryogenic impact. However, the RA in QLIT steel was adequately stable; the RA could constantly be converted to martensite during the impact process at −196 °C, providing a substantial TRIP toughness effect.…”

Decomposition Mechanism of Prior Austenite in Low‐Temperature Zone and Its Relationship to Fracture Behavior of High‐Strength Medium‐Mn Steel

“…Furthermore, the metastable RA can transform into martensite induced by stress or strain, improving the work hardening ability of steels. [21,22] The samples were annealed at 650 °C (α þ γ regions) for 1 h and then air cooled to room temperature. The engineering stress-strain curves and work hardening rate curves of annealed samples are shown in necking in steels C and F are 0.105 and 0.128, respectively, that is, the intersection of the strain hardening rate curve and the true stress-true strain curve.…”

Micromechanics-based understanding of the stability of film-like austenite in steels

“…Hence the terms ‘inclusion’ and ‘austenite’ will be used interchangeably in the current study. Given the film-like morphology of the retained austenite [2,3,24,26–28,31], it is modelled as an elliptic cylinder having lengths of the principal half axes a1,a2,a3, respectively, with a 1 < a 2 << a 3 , while the blocky austenite [2,3,24] is modelled as a spherical inclusion with a1∼a2∼a3 as shown in Figure 1. The depiction of blocky austenite in Figure 1(b) has no preferential directions and, hence, justifies its representation by the spherical geometry as shown in Figure 1(c).…”

“…The authors have also expressed the necessity of a comprehensive study on the lines of Eshelby inclusion theory [25] to evolve a micromechanical model to explain the interaction between the retained austenite and the surrounding martensite. Other studies [2,3,24,26–28] have also experimentally confirmed that the film-like morphology of retained austenite is more stable than the blocky type, the latter transforming quickly to twinned martensite [2,3,24]. Although there have been a few studies on applying the Eshelby approach to steels containing austenite and martensite [29,30], a detailed micromechanical study on the morphological dependence of the stability of retained austenite still awaits.…”