Abstract: Intercritical annealing of medium-manganese steels has been shown to produce properties desired for the third generation of advanced high strength steel (AHSS) [1, 2]. These steels are typically annealed in the intercritical temperature range, where the initial microstructure transforms to a mixture of ferrite and austenite. During the intercritical annealing treatment manganese and carbon partition from ferrite to austenite, stabilizing austenite upon quenching [3, 4]. Previous work has shown significant fractions of retained austenite (20-40 pct) can be obtained using this methodology [3-6]. It has been proposed that the strength-ductility combination of medium-manganese TRIP steels may improve through the application of a second intercritical annealing heat treatment, at a higher temperature than was employed during the initial austenitization . This double soaking (DS) heat treatment has been experimentally shown to generate a material with a total elongation above 15 pct and an ultimate tensile strength above 1500 MPa . The double soaking heat treatment has also been shown to eliminate the yield point elongation (YPE) that is characteristic of many medium-manganese steels . In the current work, the addition of a tempering (DS-T) or austempering (DS-A) step to the double soaking treatment is examined. Both conditions are found to modify the yielding behavior of the medium-manganese steel and improve ductility while maintaining the high strength levels. In this investigation, the DS-T and DS-A treatments were applied to a 0.14C-7.14Mn (wt pct) medium-manganese steel. The resulting material was characterized using experimental techniques such as X-ray diffraction (XRD), uniaxial tensile testing, and scanning electron microscopy (SEM). Tensile strengths above 1600 MPa in combination with elongations greater than 18 pct indicate that the double soaking plus tempering treatment is capable of producing material with strength-ductility combinations desired for third generation AHSS.
Authors: Alexandra Glover, John Speer, and Emmanuel De Moor
Keywords: retained austenite; intercritical annealing; alloy partitioning; medium-Mn steels