PSI - Issue 69

Pekka Kantanen et al. / Procedia Structural Integrity 69 (2025) 53–60

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desired mechanical properties, considerations of industrial manufacturability are crucial in developing these steel grades. In medium manganese steels, the intercritical annealing treatment (IAT) process is employed to stabilize substantial fractions of metastable austenite, which enhances ductility while low-temperature transformation products such as martensite provide high strength. The amount of retained austenite (RA) is primarily controlled by the intercritical annealing time and temperature at which austenite and ferrite coexist allowing for alloying element partitioning to occur. During IAT carbon and substitutional elements such as manganese partition from martensite/ferrite to austenite. Both C and Mn are strong austenite stabilizers and an increase in their concentration can result in increasing austenite stability at room temperature [3]. Based on the microstructural characteristics such as size, morphology, and stability of RA, steels can undergo a certain level of strain-induced martensitic transformation during deformation, where the metastable austenite phase is transformed to the thermodynamically more stable α’-martensite phase due to the plastic deformation. The sufficient volume fraction of RA can transform into strain-induced martensite under applied stress and enhance the work hardening and ductility of the steels [4]. The presence of RA is particularly crucial in lightweight automotive applications where a balance between strength and ductility is desired. Furthermore, understanding the manufacturability and workshop usability of these novel steels is important. Therefore, this work, building upon our earlier study on hole expansion behavior in the same material [5], focuses on the strain-induced martensitic transformation during the punching operation. The main objective is to explore the mechanical stability of RA and the role of strain-induced martensitic transformation in this context, contributing to a broader understanding of medium manganese steels’ behavior during industrial processes. 2. Experimental 2.1. Material An experimental 0.3C-1Si-6Mn-2Al medium Mn steel was vacuum-cast into an ingot. After the vacuum-casting, pieces of ~ 40 × 80 × 90 mm were cut from the ingot and soaked in a furnace at 1200 °C for 2 h. Subsequently, the steel pieces were laboratory hot-rolled to a thickness of 4 mm with a final rolling temperature (FRT) of 920 °C followed by air cooling to room temperature (RT). Finally, the IAT were carried out in a furnace at temperatures of 650 °C and 700 °C with 10 min holding time near the target temperature to stabilize different fractions of RA within a tempered martensite/ferrite matrix. A schematic diagram of the experimental procedures, including the hot rolling schedule and furnace IAT process, is presented in Fig. 1a, along with thermocouple-measured furnace IAT conditions shown in Fig. 1b. As a result, two differently processed IAT materials, with varying RA fraction, were used in this study.

Fig. 1. (a) A schematical sketch for experimental procedure of hot rolling and furnace IAT and (b) measured furnace IAT temperature profiles.

2.2. Mechanical testing For investigation of punching-induced shear-affected zones (SAZ) in the IAT materials, 10 mm diameter holes were prepared according to the ISO 16630 hole expansion test standard. Wire electrical discharge machining (W-