Influence of Heat Treatments on the Microstructural Evolution and Resultant Mechanical Properties in a Low Carbon Medium Mn Heavy Steel Plate

Abstract

In this study, the microstructural evolution and resultant mechanical properties in a low carbon medium Mn heavy steel plate were investigated in detail. The results show that the introduction of medium manganese alloy design in the heavy steel plate has been shown to achieve the outstanding combination of strength, ductility, low-temperature impact toughness, and strain hardening capacity. It has been found that the austenite phase mainly displays at martensitic lath boundaries and shows lath shape for the heat treating at 873 K (600°C) for 1 to 10 hours or 893 K (620°C) for 2 hours, and not all the austenite phase obeys the K-S or N-W orientation relationship with respect to abutting martensitic lath. Although the microstructure in the steel after heat treating at 873 K (600°C) for 1 to 10 hours is similar to each other, the resultant mechanical properties are very different because the volume fraction and stability of retained austenite vary with the heat treatments. The best low-temperature impact toughness is achieved after heat treating at 873 K (600°C) for 2 hours due to the formation of a considerable volume fraction of retained austenite with relatively high stability, but the strain hardening capacity and ductility are disappointing because of insufficient TRIP effect. Based on enhancing TRIP effect, the two methods have been suggested. One is to increase the isothermal holding temperature to 893 K (620°C), and the other one is to prolong the isothermal holding time to 10 hours at 873 K (600°C). The two methods can significantly increase strain hardening capacity and ductility nearly without harming low-temperature impact toughness. In addition, the stability of retained austenite has been discussed by the quantitative analysis and it has been demonstrated that the stability of retained austenite is related to the chemical composition, size, and morphology. Moreover, the isothermal holding temperature has a great effect on the stability of retained austenite, while the effect of the isothermal holding time is relatively poor.

Strong Interactions between Austenite and the Matrix of Medium-Mn Steel during Intercritical Annealing

“…Based on the microstructure in Figure 5 , an obvious growth of austenite was observed with an increase in the annealing temperature. It is worth noting that when the temperature reached 725 °C, a large number of austenite grains transformed into martensite (M) during the cooling process ( Figure 5 d), which was caused by an increase in austenite grain size and a decrease in the element content in austenite [ 17 , 18 ].…”

Austenite Reversion Tempering-Annealing of 4 wt.% Manganese Steels for Automotive Forging Application

“…The austenite was always found at interfaces, primarily at prior austenite grain boundaries or at martensite lath boundaries. Due to the enrichment of manganese at these boundaries [21,24], austenite will form earlier in these regions during the heat treatment. When the nucleation of austenite starts, the newly formed austenite acts as a diffusion sink for manganese and carbon at the surrounding grain boundary, as the main driving force for manganese is the formation of austenite close to the thermodynamic equilibrium [25].…”

Improvement of Strength-Toughness-Hardness Balance in Large Cross-Section 718H Pre-Hardened Mold Steel

“…The mechanism for enhancing the ductility after A-T treatment may be summarized as the retained austenite in matrix after tempering. Usually, the film-like and massive retained austenite is commonly observed at austenite grain boundaries [ 38 , 39 , 40 , 41 ]. In addition, the nucleation sites of retained austenite might tend to be changed from austenite grain boundaries to lath boundaries due to the decrease in transformation resistance [ 42 ].…”