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Structure and phase composition of newly developed high manganese X98MnAlSiNbTi24-11 steel of TRIPLEX type DOI:

  The work presents the results of investigations of the structure and phase composition of newly developed high manganese X98MnAlSiNbTi24-11 steel of TRIPLEX type. The average density of such steel is 6.67 g/cm3, which is less than for typical structural steels by even 15%. A preliminary analysis of phase composition and structure allows to find an austenitic γ-Fe(Mn, Al, C) structure in the investigated steel with uniformly distributed ferritic α-Fe(Mn, Al) areas elongated towards the boundaries of austenite grains and numerous carbides with differentiated chemical composition and varied size. Nb- and Ti-based complex carbides are dominant in the steel. The investigations of the chemical composition of the carbides revealed in the matrix allow to identify with high probability dispersive κ-(Fe, Mn)3AlC carbides with the nanometric size of approx. 10÷160 nm, which has to be yet confirmed with electron transmission microscopy methods. Fe, Mn and Al as well as small amounts of Nb, Ti and Si are contained in such carbides. The occurrence of aluminium carbonitrides with a fraction of Nb and Ti was also revealed. The size of the above Nb and Ti carbides revealed in solid specimens in the matrix of the studied steel is between approx. 10 nm to 15 μm. X-ray diffraction examinations of carbide isolates prepared by the method of chemical dissolution in HCl showed the existence of NbTiC2 carbides in the studied steel. The diffraction examinations of solid specimens revealed, apart from austenite and ferrite, also the existence of TiC carbides and such initially classified as Mn3.6C0.4 type. Key words: high manganese steels, microstructure, phase composition, NbTiC2, TiC, κ-(Fe, Mn)3AlC.1. INTRODUCTION High manganese steels have become well established in the area of research over high-strength steels which at the same time maintain high plastic properties. Multiple new steel grades containing high contents of manganese (≥[...]

Structure and mechanical properties of newly-developed high-strength TRIPLEX type steels DOI:10.15199/28.2018.5.4

  1. INTRODUCTION High-manganese steels belong to the high-strength AHSS group steels (Advanced High Strength Steels) of the second generation, are characterised by a good combination of strength and plastic properties. Fe-Mn-Al-C steels (TRIPLEX type) belong to the group of high manganese steels of different participating in the structure of the three phases: austenite, ferrite and carbides, including those responsible for very good mechanical properties of these dispersion steels of κ-(Fe, Mn)3AlC carbides [1÷15]. Nano-sized carbides type M3C — (Fe, Mn)3AlC (so-called κ carbides) precipitation in this type of steels is affected by the addition of Al greater than 5% [1÷4]. These carbides shape the strength properties of this group of steels. The mechanical properties of these steels are determined to a large extent by the location, dimensions, coherence with the matrix as well as the morphology of κ-(Fe, Mn)3AlC carbides. κ carbides can also cause the appearance of the brittleness of steel during plastic deformation at room temperature when they are formed at grain boundaries as large precipitates [1÷10, 14÷17]. Due to the relatively high addition of Al ~11%, as well as Mn (18÷28%) and C (0.5÷1.2%), metallurgy, processing, microstructure and deformation mechanisms of these steels are significantly different from those in traditional structural steels. These steels are characterised by 15% lower density compared to conventional structural steels (Fig. 1), making them very attractive from the point of view of their use in structural elements. Addition of Al to steel with a high content of Mn affects two important effects: increasing the stacking fault energy and the precipitation of κ-(Fe, Mn)3AlC carbide. The dominant mechanisms in Fe-Mn-Al-C steels include: microband induced plasticity (MBIP), dynamic slip band refinement (DSBR), shear band induced plasticity (SIP), transformation induced plasticit[...]

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