The high temperature properties of steels depend on the solidification parameters and the formation parameters of manganese sulphide precipitates. Therefore, the occurrence of MnS precipitations in relation to primary and secondary microstructures was studied for different steel grades with a primary delta‐ferritic solidification or a primary austenitic solidification. The liquidus and solidus temperatures as well as the δ‐γ‐transformation temperature were calculated thermodynamically and measured by a DTA analysis in order to describe the solidification and transformation temperature range. The MnS formation temperature was calculated thermodynamically and compared to the results of SEM/EDX investigations on fracture surfaces of hot tensile specimens torn at different temperatures after in situ melting and controlled solidification. A special focus of these investigations was the location of MnS precipitates in relation to the primary and secondary grain boundaries. To explain the results, calculations were carried out taking into account the supersaturation of manganese and sulphur during the solidification in residual melt on the primary grain boundaries.

The austenite conditioning during heat treatment determines the mechanical properties of case hardening steels. New developments in carburising process parameters with temperatures up to 1150degreesC require a precise austenite grain size controls The objective of this investigation is the determination of the influence of microalloying with Niobium and Titanium with regard to transformation and precipitation behaviour as well as microstructure stability and the mechanical properties. A standard case hardening steel grade (16MnCr5) was selected together with three microalloyed steels of the same type. Calculations of precipitates in thermodynamic equilibrium have been performed using Thermo-Calc. Heat treatments in the temperature range of 900degreesC to 1150degreesC were carried out to determine grain stability. The evaluation and documentation of the grain size as a function of heat treatment temperature were made using statistical methods and quantitative metallography.

Phase transformations play a significant role during the technical heat treatment of steel. For instance a controlled transformation in an annealing process allows for the production of multiphase steels. In this investigation the procutectoid ferrite precipitation will be looked into. The transformation kinetics can be described as diffusion controlled,yet another type of kinetics is also thinkable Which Would be, controlled by interface mobility. Isothermal transformation dilatometry has been performed oil high carbon steel grade samples. In a detailed metallographic analysis the samples were examined according to probable nucleation sites of proeutectoid ferrite. The simulations in this work have been performed using the software, MICRESS. MICRESS is based oil the multicomponent multiphase-field model. Besides diffusion it also takes finite interface nobilities into account. It allows for multi-dimensional calculations, which illustrates the ferrite precipitation in real microstructures. For comparison reasons, simulations in a one-dimensional domain have been performed using the commercial program DICTRA.

This work is a review of research results, which have been determined in recent years in order to reveal the effects of the chemical compositions on the high temperature properties of structural steels. Special emphasis has been laid on the solidification structure, phase reactions and precipitates. For this comparison exemplary different structural steel grades have been chosen. The effect of the chemical composition on the solidification structure is shown by increasing Ni mass contents up to 10%, the influence on the phase transformation is illustrated on the basis of a steel with a Mn mass content of 1.6% and varying carbon contents. The influence of precipitates has been investigated both on the basis of the Mn/S-ratio and by microalloyed structural steels with different Nb, V and Ti additions. For all these steel grades the laboratory testing conditions were the same. The high temperature properties of steels can be investigated by high temperature tensile tests; the range of good ductility is determined by the measurement of the reduction of area, e.g. RoA > 50% or > 70%. The upper limit of the ductility range is the temperature of zero ductility, T-ZD%. The lower limit of the good ductility range is marked by the transition to the ductility minimum II. The experiments for a series of different structural steels show that the hot ductility properties are affected by the metallurgical phenomena mentioned before.

Phase transformation is a powerful tool to change the properties of steels. Of the known transformations especially the gamma-alpha -transformation is utilised. It occurs in a temperature range relevant for heat treating and hot deformation processes. In this paper an approach is presented in which the gamma-alpha -transformation is simulated with Micress.: This software applies the multicomponent multiphase-field model, which is based on the reduction of total free enthalpy. Two different steels have been selected for the simulations, an ULC and an IF steel. Dilatometric tests serve as a basis for the simulations. These tests have shown that the transformation behaviours of the two steel grades are governed by two different kinetics. The transformation kinetics of the IF grade is influenced by the microalloying concept applied, resulting in a very slow start of the transformation. This has also been incorporated in the simulations by choosing two different grain boundary mobilities, one main parameter of the simulation. The simulation results of the ULC grade show the huge influence of nucleation undercooling as another one of the main parameters. Both simulation results are satisfying. They show that the phase-field method offers a strong simulation tool in the area of phase transformation.