Effect of Alloying Element on Mechanical Properties of High Strength Austenitic Steel

A high strength austenitic steel is expected as a structural material for cryogenic use because fcc material does not cause a cleavage fracture despite high strength. High manganese steel which is a strong candidate material of the cryogenic high strength austenitic steel was originally famous for the Hadfield steel and widely applicable in actual use. In general, an excellent cryogenic toughness of the high manganese steels is achieved by obtaining stable fcc microstructure with an adequate amount manganese which is a typical austenite former alloy. However, as addition of manganese is not effective for increasing strength, other strengthening alloying elements like carbon and chromium need to be added. In this study, an effect of alloying elements on strength and cryogenic toughness of the high manganese austenitic steel is studied.

Effect of CaO–SiO2–Al2O3–MgO top slag on solute elements and non-metallic inclusions in Fe-xMn(x = 10, 20 mass pct) steel

Medium/high manganese steels have broad application prospects in automotive industry, cryogenic material, etc. because of excellent properties. Precise control on steel composition and improvement of cleanliness are very important for commercial production of these steel grades. In this study, the effect of CaO–SiO2–Al2O3–MgO slag on solute elements and inclusions of Fe-xMn(x = 10, 20 mass pct) steel was studied and discussed. After slag/steel reaction, the concentration of Mn and S in steel reduced, while Si increased. Most MnO type inclusions, which were the main inclusions in master high manganese steel, transformed to MnO–SiO2 type and MnO–Al2O3–MgO type, with MnO–SiO2 sharing the majority. Thermodynamic analysis indicates that the change of solute elements and inclusions was mainly the result of reaction SiO2(s) + 2[Mn] = 2MnO(s) + [Si] between molten steel and top slag as well as slag desulphurization. Increase of oxygen potential of the reaction system would restrain the reaction. Because of the inclusion absorption by top slag, large sized inclusions decreased and steel cleanliness improved greatly after CaO–SiO2–Al2O3–MgO slag was added.

Mechanical Properties and Microstructural Aspects of Two High-Manganese Steels with TWIP/TRIP Effects: A Comparative Study

This article is focused on the mechanical behavior and its relationship with the microstructural changes observed in two high-manganese steels presenting twinning-induced plasticity (TWIP) and transformation-induced plasticity (TRIP), namely Steel B and Steel C, respectively. Chemical compositions were similar in manganese, but carbon content of Steel B approximately doubles Steel C, which directly impacted on the stacking fault energy (SFE), microstructure and mechanical response of each alloy. Characterization of as-cast condition by optical microscope revealed a fully austenitic microstructure in Steel B and a mixed microstructure in Steel C consisting of austenite grains and thermal-induced (εt) martensite platelets. Same phases were observed after the thermo-mechanical treatment and tensile tests, corroborated by means of X-Ray Diffraction (XRD), which confirms no phase transformation in Steel B and TRIP effect in Steel C, due to the strain-induced γFCC→εHCP transformation that results in an increase in the ε-martensite volume fraction. Higher values of ultimate tensile strength, yield stress, ductility and impact toughness were obtained for Steel B. Significant microstructural changes were revealed in tensile specimens as a consequence of the operating hardening mechanisms. Scanning Electron Microscopy (SEM) observations on the tensile and impact test specimens showed differences in fracture micro-mechanisms.