The effects of prior austenite grain boundaries and microstructural morphology on the impact toughness of intercritically annealed medium Mn steel

Abstract

The effects of prior austenite (γ) grain boundaries and microstructural morphology on the impact toughness of an annealed Fe-7Mn-0.1C-0.5Si medium Mn steel were investigated for two different microstructure states, namely, hot-rolled and annealed (HRA) specimens and cold-rolled and annealed (CRA) specimens. Both types of specimens had a dual-phase microstructure consisting of retained austenite (γ_R) and ferrite (α) after intercritical annealing at 640 °C for 30 min. The phase fractions and the chemical composition of γ_R were almost identical in both types of specimens. However, their microstructural morphology was different. The HRA specimens had lath-shaped morphology and the CRA specimens had globular-shaped morphology. We find that both types of specimens showed transition in fracture mode from ductile and partly quasi-cleavage fracture to intergranular fracture with decreasing impact test temperature from room temperature to −196 °C. The HRA specimen had higher ductile to brittle transition temperature and lower low-temperature impact toughness compared to the CRA specimen. This was due to intergranular cracking in the HRA specimens along prior γ grain boundaries decorated by C, Mn and P. In the CRA specimen intergranular cracking occurred along the boundaries of the very fine α and α′ martensite grains. The results reveal that cold working prior to intercritical annealing promotes the elimination of the solute-decorated boundaries of coarse prior γ grains through the recrystallization of αʹ martensite prior to reverse transformation, hence improving the low-temperature impact toughness of medium Mn steel.

Introduction

During the last decades, advanced high strength steels with remarkable low-temperature toughness have been extensively studied for cryogenic applications, such as cryogenic offshore structures and storage tanks for liquefied natural gas [1], [2], [3], [4], [5], [6]. Although Fe-Ni martensitic steel has been widely used for cryogenic applications due to its excellent strength and low-temperature toughness, the high price of Ni shifts attention to less expensive alloys [1], [2], [3], [4], [5], [6]. In this regard, Fe-Mn martensitic steels with 3–10 wt% Mn, referred to as medium Mn steels [7], [8], [9], [10], [11], [12], [13], [14], have received significant attention since both, Fe-Ni and Fe-Mn alloys are capable of producing similar types of lath martensitic microstructure and room-temperature mechanical properties [15], [16], [17], [18], [19]. However, medium Mn steels exhibit lower low-temperature toughness and higher ductile to brittle transition temperature (DBTT) compared to Fe-Ni martensitic steel. For example, Fe-9Ni and Fe-8Mn (wt%) steels show a DBTT of −77 °C and −20 °C and impact energies at −196 °C of 11 J and less than 1 J, respectively [19]. The high DBTT and poor low-temperature toughness of medium Mn steels are known to be due to severe intergranular fracture [15], [16], [20].

Recently, several studies reported that retained austenite (γ_R), when prevalent after reversion treatment from α′ martensite to γ austenite [21], [22], is effective in improving the low-temperature toughness of medium Mn steel [20], [23], [24], [25]. Hu et al. [23] compared the impact toughness of hot-rolled and annealed Fe-5Mn-0.04C (wt%) steels with and without the presence of γ_R. While specimens, annealed at 600 °C for 10 min, showed no γ_R, samples that were annealed at 650 °C for 10 min, contained γ_R with a volume fraction of ∼0.15. The impact toughness of the steel containing γ_R was approximately ten times higher than that of corresponding samples without the presence of γ_R at test temperatures between −20 °C and 20 °C. This difference was attributed to the fact that metastable γ_R has high resistance to crack propagation. Similar results were also reported by Wang et al. [25]. Hu et al. [23] speculated that the γ, when reverted at the α′ martensite grain boundaries, improves interface cohesion because it promotes Mn partitioning into the newly formed γ, hence reducing the segregation of Mn to grain boundaries [20]. Indeed segregation of Mn is known to deteriorate cohesion of grain boundaries in martensitic steels [18].

Kuzmina et al. [20] reported a change in impact toughness of hot-rolled and annealed Fe-9Mn (wt%) steel as a function of both, annealing temperature (450 °C and 600 °C) and time (10 s–860 h). When the annealing time was short (e.g. 10 s) at 450 °C, the Mn segregation at the prior γ grain boundaries of α′ martensite substantially reduced the impact toughness. However, when the annealing time was long (e.g. 672 h) at 450 °C, reversion from α′ martensite to γ at grain boundaries drastically reduced the grain boundary segregation of Mn via partitioning from α′ into the newly formed γ phase, hence increasing the impact toughness. They also reported that B-containing Fe-9Mn (wt%) steel exhibited higher impact toughness compared to B-free Fe-9Mn (wt%) steel. This was attributed to the fact that B rapidly segregates with fast kinetics to pre-occupy the grain boundaries, and hence blocked the segregation of slowly diffusing Mn to the grain boundaries.

In contrast to these studies, Chen et al. [24] reported that hot-rolled and annealed Fe-5.1Mn-0.04C-1.4Ni (wt%) steel with a high volume fraction of γ_R (∼0.14) showed a lower impact toughness at test temperatures between −100 °C and 20 °C compared to similar material with a lower volume fraction of γ_R (∼0.10). This was attributed to the circumstance that the specimen with higher γ_R volume fraction had a lower mechanical stability of γ_R with lower C, Mn and Ni concentrations. While the chemical composition of γ_R in the steel with lower γ_R volume fraction was ∼0.12C, ∼10.41Mn and ∼2.91Ni (in wt%), that of γ_R in the steel with higher γ_R volume fraction was ∼0.07C, ∼7.69Mn and ∼2.29Ni (in wt%).

Although some of these previous studies on the impact toughness of medium Mn steels hence provided certain insights into the influence of volume fraction and phase stability of γ_R on the resulting impact toughness, the relationship between the underlying microstructural morphology and the toughness was not considered so far. The microstructural morphology of annealed medium Mn steels is known to change from a lath shape to a globular shape by cold rolling prior to intercritical annealing [9], [14]. When the hot-rolled steel is intercritically annealed, the reverse transformation from α′ martensite to γ occurs mainly along the martensite block boundaries prior to the recrystallization of α′ martensite, resulting in lath-shaped grain morphology [9], [10]. The absence of recrystallization of the α′ martensite matrix partly leads to the prevalence of martensitic boundaries (e.g. the boundaries of prior γ grains, packets, blocks and laths) even after intercritical annealing [26]. However, when the cold-rolled steel is annealed, recrystallization of the α′ martensite matrix and reverse transformation from α′ martensite to γ can occur simultaneously due to the high dislocation density introduced by cold rolling, resulting in a globular-shaped grain morphology [9].

Therefore, in this study we prepared medium Mn steel specimens with two different microstructural morphologies, namely, lath and globular, but with similar volume fraction and chemical composition of the reversed austenite γ_R. Using the specimens, we investigated the relationship between microstructural morphology and impact toughness at various temperatures to derive a novel pathway to the microstructural design of medium Mn steels for improving their low-temperature toughness which takes microstructure morphology into account.