Enhanced Strength and Toughness of Low‐Carbon Bainitic Steel by Refining Prior Austenite Grains and Austempering Below M_s

Abstract

Austempering is conducted below and above martensite start temperature (Ms) at the fine and coarse prior austenite grain size (PAGS), respectively, in a low‐carbon bainitic steel, and the relationship between multiphase microstructure and mechanical properties is investigated. The study indicates that refinement of bainite laths can be obtained by decreasing the PAGS and/or austempering temperature from above to below Ms. The former reduces bainite content, and increases the volume fraction of retained austenite (RA), thereby obtaining higher uniform elongation and strain hardening exponent. However, the latter effectively decreases the size and amount of blocky RA, as well as bainite content and conversely increases the ratio of volume fraction of film RA to that of blocky RA, which leads to significant improvement in yield strength and impact toughness. In addition, the variant selection is relatively weak with the decrease in the PAGS and austempering temperature, which has a beneficial effect on toughness. Furthermore, an outstanding combination of mechanical properties (1070 MPa yield strength, 1466 MPa tensile strength, total elongation of 11.68%, and V‐notch impact toughness at 20 and −40 °C of 78 and 44 J cm−2, respectively) is achieved by austempering below Ms in the finer PAGS.

Tailoring Low‐Alloyed Cryogenic Steel Strong and Tough Introduces Fine‐Grained Equiaxed Ferrite by Intercritical Subquenching

“…In particular, high-angle grain boundaries play a crucial role in absorbing crack propagation energy. [29,30] This importance arises because when a crack propagates through a high-angle grain boundary, it necessitates the formation of a large-scale step. This step significantly increases the effective surface energy in the Griffith equation, increasing the difficulty of crack propagation.…”

Isothermal decomposition of austenite in presence of martensite in advanced high strength steels: A review

“…Additionally, the remaining austenite may become enriched with carbon due to martensite tempering and bainite formation, stabilising the austenite against brittle fresh martensite formation during cooling to RT after isothermal treatment. Yao et al ( 2021) have obtained more retained austenite with finer PAGS, effectively improving the mechanical properties of a low C steel containing Mn, Si, Cr, V, Ni and Mo [108]. However, it should be noted that since finer PAGS limits the volume fraction of bainite, the remaining austenite may not become completely stabilised to avoid fresh martensite formation.…”