Comparative Investigation of Phase Transformation Behavior as a Function of Annealing Temperature and Cooling Rate of Two Medium‐Mn Steels

Abstract

Transformation-induced plasticity (TRIP)-assisted medium-Mn steels with a ferritic matrix containing considerable amounts of retained austenite are a promising candidate to fulfill the requirements for the third-generation of advanced high-strength steels (AHSS), which is currently under development. The influence of the intercritical annealing temperature and cooling rate on the final microstructure of a 0.1C3.5Mn and 0.1C5Mn steel, respectively, was elaborately investigated. Dilatometric experiments were carried out and additionally supported by microstructural observations. During soaking in the two-phase ferriteaustenite region and subsequent slow cooling the C and Mn concentration in the austenite increased and resulted in its chemical stabilization. The variation of the annealing temperature and cooling rate altered the amounts of ferrite, retained austenite, bainite, pearlite, and martensite in the final microstructure. Furthermore, two thermodynamical models for the prediction of the maximum retained austenite content and the optimal annealing temperature have been thoroughly evaluated in this work. It can be stated that the experimental data revealed a shift of the maximum retained austenite to higher annealing temperatures compared to the model calculations. As diffusional transformations were not considered in the applied models, slow cooling rates led to pronounced deviations between calculation and experiment, thereby showing the need for model adaptions.

Effect of Manganese on the Structure-Properties Relationship of Cold Rolled AHSS Treated by a Quenching and Partitioning Process

“…Medium Mn steels with a typical chemical composition of 0.05-0.2 wt.% C and 3-10 wt.% Mn have a microstructure consisting of an ultrafine-grained ferritic (α) matrix and volume fractions of retained austenite (RA) up to 40 vol.%. Therefore, they are characterized by an excellent combination of strength and ductility, achieving ultimate tensile strengths (UTS) > 800 MPa combined with total elongations (TE) of up to 40% [6][7][8].…”

Dilatometric Study of Phase Transformations in 5 Mn Steel Subjected to Different Heat Treatments

“…When the reverse austenite transformation took place, the morphology of martensite is inherited by austenite and ferrite [23,24]. Moreover, this lath morphology of austenite influences positively its stability [25,26]. Figure 12.…”

Deformation Behavior of a Double Soaked Medium Manganese Steel with Varied Martensite Strength

“…With typical retained austenite volume fractions between 5 and 30 vol %, the improved mechanical properties of these steels can be attributed in part to the strain-induced transformation of austenite to martensite, which has been shown to help maintain a high rate of work hardening during deformation [4,[7][8][9][10][11]. In intercritically annealed medium manganese steels, the resistance of austenite to strain-induced transformation has been shown to be highly dependent upon intercritical annealing heat treatment parameters [5,6,12]. This is because the intercritical annealing temperature within the ferrite plus austenite phase field determines the equilibrium austenite volume fraction and the associated C and Mn enrichment.…”

“…These intercritically annealed medium manganese steels, which contain primarily retained austenite and ferrite, may be strengthened through the substitution of ferrite with athermal martensite. Several studies of medium manganese steels which contain small volume fractions of athermal martensite have shown, in some conditions, to promote continuous yielding and improve the overall strength-ductility combination of the material [5,6,[12][13][14]. Due to the nature of the single intercritical annealing heat treatment used in these studies, the introduction of martensite was only possible through the incomplete stabilization of austenite during intercritical annealing.…”