The influence of aluminium on the hot ductility of high manganese steels between 700°C and Tliq has been examined, using the hot tensile test machine. Five materials with 17 wt-% [Mn], 0.3 wt-% [C] and 0–8 wt-% [Al] were cast. To detect the cause of cracking, optical microscopy and scanning electron microscopy analysis of the fracture surfaces have been conducted and compared to thermodynamic modelling. By adding more than 3 wt-% [Al] the fully austenitic structure changed to a two-phase structure (δ-Fe, γ-Fe). In case of the one- and also two-phase structured materials two regions of lower ductility (<30% RA) are present. Complex (AlN) and Mn(S,Se) precipitates were formed with different cores in dependence of the [Al] content.
Materials made of steel are used for nearly all purposes of daily live. The steel grades are tailored for customers' requirements, e.g. hard for tools, tough for wires, or soft for packages. The development of those properties are based on the grain structure which is controlled by chemical composition of the steel melt, solidification, and thermo-mechanical forming up to the end of manufacturing line. Concerning direct hot-charge and near-net-shape casting the control of micro-structural development during solidification is important.By extended use of computer based simulations since about one decade the development of steel properties related to the requirements is accelerated in two main directions: development of existing steel grades and creation of new groups of steel alloys. In all cases, advanced modelling tools for manufacturing processes and fundamental data are necessary. In this paper the solidification aspect of a new group of steel grades based on high manganese alloys is highlighted; the micro-segregation of high alloyed manganese steel grades is in the focus.Manganese [Mn] as an steel alloying element is used to increase strength, toughness and hardenability. Further-on, manganese binds dissoluted sulphur to MnS precipitates so that effects of hot tearing are significantly diminished; on the other hand, MnS precipitates are also controlling the chip formation behaviour of free-cutting steel grades where a [Mn] ≤ 1 wt% is sufficient. Controlling the phase formation of steel on basis of iron atomic lattices dissoluted [Mn] is used to stabilise the fcc (face centred cubic, austenite phase) cell. In the same way, [Mn] suppresses the full extent of bcc (body centred cubic, ferrite phase). Modern dual or complex phase steel grades are designed of both bcc and fcc-cystals by using a convenient chemical composition and special thermo-mechanical treatment; here, [Mn] supports the stability of austenitic fractions in a ferritic matrix; [Mn] is applied up to about 2 wt%. [1] Other alloying elements to stabilise the ferritic structure are silicon [Si] or aluminum [Al], while carbon [C] or nitrogen [N] support the austenitic phases.Since several years new steel grades with austenitic matrix are designed to find steel properties, which combine high strength and excellent ductility based on iron and manganese. [2] A positive technical effect is the relatively high energy absorption in unwanted cases of material deformation, e.g. crash in automobile accidents. [3] The solidification of industrially produced and applied steel grades is carried-out in continuous casting (CC) machines where the strand thickness is typically between 60 mm and 300 mm. That means that solidification rates is of 1 mm/s in the surface region of the strand and 0.05 mm/s in the core region; [4] typical data of cooling rate at solidus temperature are in the range of 100 K/s at the cast surface and about 1 K/s in the centre. Cooling and solidification rates are controlling the solidification structure connected to microsegregation, whic...
The ductility curves of high manganese steel grades with a stepwise increasing [Al] content of 0, 1, 3, 5, and 8 wt% for two strain rates (0.01 and 0.001 s−1) and for a varying cooling rate (–3 and −7 Ks−1) have been determined to investigate their influence on the hot ductility behavior. The tests have been conducted at the hot tensile testing unit of the Department of Ferrous Metallurgy of RWTH Aachen University. This equipment is able to perform testing of semisolid samples, e.g., during solidification. After the tensile testing, the specimens are investigated using the scanning electron microscope/energy‐dispersive X‐ray spectroscopy as well as by light optical microscopy to clarify the role of the cooling and strain rates on the hot ductility of high manganese steels and on the formations of precipitates. Furthermore, thermodynamic modeling using the commercial software ThermoCalc is performed. A shifting of the decay of the ductility maximum to lower temperatures down to ≈1273 K for both an increasing strain rate and an increasing cooling rate is determined and this effect is explained through their influences on the microstructure and fracture behavior. Micrograph analyses show that (MnS) precipitates form in contact with early (AlN) precipitates.
High manganese steels are able to deform by the TRIP effect, TWIP effect and microbanding formation. These steels are quite promising materials for mechanical construction, once they show an unusual combination of high ductility and high tensile strength. The casting of these steels represents a technological challenge, because they are extremely prone to macro-and microsegregation. Segregation, on its turn, may locally impair the desired mechanical properties. Simulations by the phase-field method may be utilized to investigate microstructure formation and the development of microsegregation patterns during solidification. Nevertheless, performing reliable microstructure simulations is only possible when reliable values for the solid-liquid interface energy are available. Through utilization of the sessile-drop method, first measurements of the interface energy in the Fe-Mn-C alloy system were performed. By utilizing the obtained values for the interface energy as an input, phase-field simulations were run aiming at investigating both the effect of the value for the interface energy and of the steel composition on the dendrite morphology.
This paper presents a study comparing the microsegregation from 2-D phase-field simulations with those predicted by 1-D analytical theories and with the ones obtained from experiments. It focuses mainly on the solidification of an Fe-Mn binary alloy with high manganese content (23 wt%), which is comparable to the manganese content of highmanganese steel (HMnS) grades. The main motivation for this study comes from the strong influence of microsegregation on the local mechanical properties of these steels at room temperature. After performing simulations with the model for different realistic cooling rates, the secondary dendrite arm spacings, which are strongly related to the microsegregation phenomenon, are determined and compared with an analytical model and also with experimental results from the literature for similar alloys. Analyses of concentration of the simulated samples, which are related with real microsegregation phenomenon, are also presented in this investigation using an area-based frequency distribution. The distribution results from simulations are also compared to the 1-D analytical Brody-Flemings model, to a DICTRA simulation, and to experimental results for a similar alloy. From this investigation, it is concluded that the concentration distribution for binary alloy simulations in 2-D agrees semiquantitatively with the experimental results for ternary alloys.
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