Abstract
A novel medium manganese (MMn) steel with additions of Cr (18%), Ni (5%), V (1%), and N (0.3%) was developed in order to provide an enhanced corrosion resistance along with a superior strength–ductility balance. The laboratory melted ingots were hot rolled, cold rolled, and finally annealed at 1000 °C for 3 min. The recrystallized single-phase austenitic microstructure consisted of ultrafine grains (~1.3 µm) with a substantial amount of Cr- and V-based precipitates in a bimodal particle size distribution (100–400 nm and <20 nm). The properties of the newly developed austenitic MMn steel X20CrNiMnVN18-5-10 were compared with the standard austenitic stainless steel X5CrNi18-8 and with the austenitic twinning-induced plasticity (TWIP) steel X60MnAl17-1. With a total elongation of 45%, the MMn steel showed an increase in yield strength by 300 MPa and in tensile strength by 150 MPa in comparison to both benchmark steels. No deformation twins were observed even after fracture for the MMn steel, which emphasizes the role of the grain size and precipitation-induced change in the austenite stability in controlling the deformation mechanism. The potentio-dynamic polarization measurements in 5% NaCl revealed a very low current density value of 7.2 × 10−4 mA/cm2 compared to that of TWIP steel X60MnAl17-1 of 8.2 × 10−3 mA/cm2, but it was relatively higher than that of stainless steel X5CrNi18-8 of 2.0 × 10−4 mA/cm2. This work demonstrates that the enhanced mechanical properties of the developed MMn steel are tailored by maintaining an ultrafine grain microstructure with a significant amount of nanoprecipitates, while the high corrosion resistance in 5% NaCl solution is attributed to the high Cr and N contents as well as to the ultrafine grain size.
Highlights
High manganese steels (HMnSs) with the twinning-induced plasticity (TWIP) effect exhibit an excellent combination of high ultimate tensile strength, large uniform elongation, and high workMetals 2019, 9, 705; doi:10.3390/met9060705 www.mdpi.com/journal/metalsMetals 2019, 9, 705 hardening capacity, which makes them a competitive candidate for a wide range of uses in automotive industry, liquefied natural gas (LNG)-shipbuilding, and the oil and gas industry [1,2]
A processing route consisting of cold rolling followed by a recovery-annealing heat treatment, during which the dislocation density is reduced while previously introduced deformation twins are thermally stable, has been proposed as a solution for the low yield strength of TWIP steels [3,4]
Figure shows the fraction of equilibrium phases based on the actual chemical composition of the shows the fraction of equilibrium phases based on the actual chemical composition of the medium manganese (MMn) steel
Summary
High manganese steels (HMnSs) with the twinning-induced plasticity (TWIP) effect exhibit an excellent combination of high ultimate tensile strength, large uniform elongation, and high workMetals 2019, 9, 705; doi:10.3390/met9060705 www.mdpi.com/journal/metalsMetals 2019, 9, 705 hardening capacity, which makes them a competitive candidate for a wide range of uses in automotive industry, liquefied natural gas (LNG)-shipbuilding, and the oil and gas industry [1,2]. High manganese steels (HMnSs) with the twinning-induced plasticity (TWIP) effect exhibit an excellent combination of high ultimate tensile strength, large uniform elongation, and high work. The use of microalloying elements has been approached under consideration of the stacking fault energy (SFE), which determines the austenite stability and controls the activation of the secondary deformation mechanism, such as the TWIP effect or deformation induced phase transformation (TRIP—transformation-induced plasticity) in addition to dislocation slip [5]. Due to the high dissolution rate of Mn, TWIP steels exhibit a relatively poor wet corrosion resistance [6,7]. In this regard, different single or combined additions of
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