Effect of Nb and Ti micro-additives and thermo-mechanical treatment of high-manganese steels with aluminium and silicon on their microstructure and mechanical properties

The r esults are based on two experimental high-manganese X98MnAlSiNbTi24-11 and X105MnAlSi24-11 steels subjected to thermo-mechanical treatment by hot-rolling on a semi-industrial processing line. The paper presents the results of diffraction and structural studies using scanning and transmission electron microscopy showing the role of Nb and Ti micro-additives in shaping high strength properties of high-manganese austenitic-ferritic steels with complex carbides. The performed investigations of two experimental steels allow to explain how the change cooling conditions after thermo-mechanical treatment of the analysed steels affects the change of their microstructure and mechanical properties. The obtained results allow assessing the impact of both the chemical composition and the applied thermo-mechanical treatment technology on the structural effects of strengthening of the newly developed steels

Influence of Isothermal Heating Time on the Disappearance of Strain Hardening in High-Manganese TRIPLEX Type Steels

The paper presents the results of the effect of isothermal heating time on the disappearance of strain hardening (the softening degree) of the studied high-manganese TRIPLEX type steels at a temperature of 900 and 1000°C. In order to determine the kinetics of recrystallization of austenite plastically deformed for selected steels, hot compression tests with draft ε = 0.2 were made. The presented results reveal that the complete recrystallization of austenite needs long isothermal heating times. In industrial conditions, such long times are not used, therefore in the initial rolling passages, the time required for half recrystallization of austenite t0.5 is often used. The total disappearance of the strain hardening, completion of the recrystallization of austenite tested high-manganese X98 and X105 TRIPLEX type steels isothermal heating time requires far more than 200 s. The increase of the deformation temperature is a factor influencing the acceleration of the disappearance of strain hardening

Carbides Analysis of the High Strength and Low Density Fe-Mn-Al-Si Steel

The work presents the results of investigations into the structure and phase composition of newly developed high manganese steels of the X98MnAlSiNbTi24-11 and X105MnAlSi24-11 type. The average density of such steels is 6.67 g/cm3, which is 15% lower as compared to typical structural steels. An analysis of phase composition and structure allows to conclude that the investigated steels feature an austenitic γ-Fe(Mn,Al,C) structure with uniformly distributed and elongated α-Fe(Mn,Al) ferrite grains on the boundaries of austenite grains and carbides. Examinations by diffraction of back scattered electrons (EBSD) allow to conclude that high-angle boundaries dominate in such steels, having significant influence on mechanical properties. Three types of carbides with differentiated chemical composition and size were identified in steel X98MnAlSiNbTi24-11 with scanning and transmission electron microscopy. κ-(Fe,Mn)3AlC carbides, having a regular, face-centered cubic lattice (fcc), were identified in austenite. Transmission electron microscopy examinations have enabled to identify M7C3-type carbide in ferrite. Nb- and Ti-based complex carbides were identified in steel X98MnAlSiNbTi24-11, both, in ferrite and austenite. (NbTi)C2 carbide precipitates were confirmed in an X-ray qualitative phase analysis. The size of the above-mentioned carbides is within several to several dozens of μm. An X-ray qualitative phase analysis has confirmed the precipitates of M7C3 carbides in both steels