Link between Microstructure and Mechanical Behavior of Double Annealed Medium Mn Steel

Abstract

Double annealing of low carbon medium Mn steel was studied. The second intercritical annealing was done at 650°C within a range of holding time: 3min to 30h. Tensile properties of the steel were measured as a function of holding time and the relation between microstructure and mechanical behavior was analyzed. Furthermore, a model, based on the mixture law combined with the considerations of equivalent increment of work in each microstructural constituent during mechanical loading, was proposed. The individual mechanical behavior of each considered microstructural constituent was described with the approaches existing in the literature. The complete model shows a very good agreement with the experimental stress-strain curves and predicts well the optimum strength-ductility balance after 2h holding.

Effect of Intercritical Annealing Parameters and Starting Microstructure on the Microstructural Evolution and Mechanical Properties of a Medium-Mn Third Generation Advanced High Strength Steel

“…Figures 10b and 11) showed that the C supersaturated martensite was likely the main source for C partitioning into the intercritical austenite. This is consistent with observed results in previous research conducted on med-Mn steels with martensitic starting microstructures [10,11,20,22,34,42]. As a result of the more rapid partitioning of C from the adjacent, C supersaturated martensite and lower energy barrier for growth, the intercritical austenite was chemically stable at room temperature and did not transform to martensite during final cooling, resulting in the relatively high volume fraction of retained austenite in the final microstructure versus the equivalent heat treatments performed on the CR starting microstructures (Figure 5).…”

“…This suggests that the carbides dissolved during the IA and likely acted as a C source to chemically stabilise a significant fraction of the intercritical austenite such that it did not transform to martensite upon quenching, thereby allowing it to be retained at room temperature and result in the observed high fractions of retained austenite and low fractions of martensite (Figure 6a). This is consistent with the mechanism proposed by Arlazarov et al [34] where blocky polygonal intercritical austenite formed during IA treatment from a cold-rolled starting microstructure. Similar blocky retained austenite was also observed in annealed CR samples (Figure 7).…”

Time-evolution of microstructure and mechanical behaviour of double annealed medium Mn steel

“…At time 0 min, austenite transformation is not started yet: only prior austenite grain boundaries and some carbides are revealed. This agrees with previously performed DICTRA simulations [10] showing that the carbides, formed during heating, need some time to dissolve and to transform into austenite. Starting from 3 min, the austenite fraction and size increase.…”

“…The initial microstructure before intercritical annealing is fully martensitic without any prior deformation [18]. In the previous work, it was also shown that carbides precipitation and dissolution at higher temperatures play an important role in the austenite formation during heating and holding [10]. In this work, the objective was to characterise and to analyse the microstructural evolution during annealing at 650°C to further establish its link with the evolution of mechanical behaviour.…”

“…Recently, a lot of investigations on MMS concept were done and published [5]. Many of them studied phase transformations during heat treatments [6][7][8][9][10][11]. In the same time, other researchers were studying mechanical behaviour of MMS with the variation of annealing temperature [3,12,13] and soaking time [4,14].…”