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.