Papers by Keyword: Softening Kinetics

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Authors: Kirill Khlopkov, Georg Paul, Thomas Pretorius
Abstract: A model describing the kinetics of static recrystallization and the kinetics of Nb and Ti precipitation has been reconsidered for modern steels. The effects of alloying and microalloying elements have been introduced into the model. The comparison of the modeling with the experimental softening shows good agreement.
Authors: Adam Grajcar, Roman Kuziak
Abstract: Two 0.17C-3Mn-1.5Al-0.2Si-0.2Mo steels with and without Nb microaddition were melted in a vacuum induction furnace. The steels are characterized by bainitic-martensitic structures with large fraction of retained austenite. To design a thermomechanical treatment for steels with required multiphase structures a knowledge of their hot deformation resistance and softening kinetics is of primary importance. The paper presents the results of the compression tests carried out at various temperatures and strain rates using the Gleeble simulator. A softening kinetics was determined in a double-hit compression test. It was found that the dynamic recrystallization was a process controlling work hardening of steels except for hot deformation conditions characterized by the highest Zener-Hollomon parameter values. Nb microalloyed steel has higher flow stresses and peak strains than the steel without Nb. A solute drag effect of niobium results also in a slower recrystallization kinetics for the Nb containing steel.
Authors: Nicolas Meyer, Yves Bréchet, Muriel Véron, Marc Mantel, Pierre Emmanuel Dubois, Oliver Geoffroy
Abstract: Softening kinetics of two 17% chromium (Cr) stainless steel grades that differ in niobium (Nb) content are compared. In the experiments, we observed that a low Nb stabilization makes recrystallization nucleation much faster and prevents incomplete recrystallization. A qualitative interpretation, based on interaction with precipitates, is proposed and explains the main features of the softening kinetics as well as the microstructures obtained. For the Nb stabilized grade, magnetic losses were measured in the deformed state and after recrystallization. Above a specific magnetizing frequency, the deformed state led to smaller losses than the recrystallized state. These results are believed to be attributed to a grain size effect.
Authors: L. Llanos, B. Pereda, B. López, J.M. Rodriguez-Ibabe
Abstract: During hot rolling, austenite recrystallization determines the grain size evolution and the extent of strain accumulation, and therefore, it can be used to tailor the microstructure and mechanical properties of the final product. However, at the moment, models describing the recrystallization kinetics of high-Mn steels are scarce and they do not take into account the effect of the alloying elements present in these steels. The aim of this work is to provide a quantitative model for the determination of the static recrystallization kinetics valid for a wide range of high-Mn steel compositions. Softening data determined for steels with different Mn (20 to 30%), Al (0 to 1.5%) and C (0.2 to 1%) levels at different strain, strain-rate and temperature conditions were analyzed. Static recrystallization of the investigated high-Mn steels follow Avrami’s law, with n Avrami exponents which are temperature dependent and lower than those determined for low C steels. A dependence of the t0.5 (time for 50% fractional softening) on the carbon content has been also observed and it was incorporated into an equation for the calculation of this parameter.
Authors: Rudolf Kawalla, Wolfhart Müller, Werner Jungnickel
Abstract: Today the numerical simulation of hot deformation processes is very advanced. But it requires mathematical models for metalphysical processes as for microstructure development, which take place during the deformation. Until now such models were developed for many steel grades and non-ferrous materials. For new steels as multi-phase steels laboratory investigations are required, in order to determine the optimal processing technologies of these materials. This applies also to the modelling. So far it is impossible, to calculate sole by mathematical solutions the manifold parameters of metalphysical processes and microstructure, for this reason laboratory trials and simulations are needed implicitly. Even for well known materials such procedures can be essential and useful. Using the multi-functional simulation system Gleeble HDS-V40 it is shown, which possibilities a physical simulation offers today. Starting with the annealing conditions, followed by microstructure development up to cooling, selected examples reflect the results of property development during hot deformation processes. The differences between conventional deformation after re-heating and deformation after direct-charging will be presented. The last-mentioned concept offers in its combination of near-netshape casting and direct charging special benefits, especially saving of energy.
Authors: Georg Paul, Kirill Khlopkov
Abstract: The highly interesting properties of high manganese steels can be further improved by microalloying. The introduction of carbon-nitride precipitates improves the yield strength and the microstructural control during the production process. Due to the high manganese content in these austenitic steels significant changes in the precipitation behaviour have to be expected in comparison to conventional carbon-manganese steels. However, although crucial for steel design, this has not been systematically described before. Preliminary results showing the effect of Nb and V are presented. Namely the softening behaviour is related to the precipitation state. In summary this allows describing the precipitation-time-temperature evolution and provides the necessary background for the alloy and process design.
Authors: Adam Grajcar, Roman Kuziak
Abstract: Two 5Mn-1.5Al TRIP steels with and without Nb microaddition were developed in the present study. The steels contain bainite, martensite, interlath retained austenite and martensite- austenite islands. The paper presents the results of the compression tests carried out at various temperatures using the Gleeble simulator. To analyze the kinetics of static recrystallization in these steels, a softening kinetics were determined in a double-hit compression test. It was found that the dynamic recovery is a main thermally activated process occurring during hot deformation. The Nb microalloyed steel has higher flow stresses and peak strains than the Nb-free steel. A solute drag effect of Nb results in a slower softening kinetics of Nb containing steel. The effects of Mn on the retardation of Nb(C,N) precipitation and hot deformation characteristics are also discussed.
Authors: B. Rakshe, J. Patel, E.J. Palmiere
Abstract: The recrystallisation and precipitation kinetics of a plain carbon steel with 0.017 % Nb were studied using the double-hit deformation technique for interpass holding of 5 and 20s. The present study focuses on the effect of prestrain and deformation temperature on recrystallisation behaviour of the investigated steel. The fractional softening was calculated based on the percentage difference between the areas under the interrupted and uninterrupted deformations flow curves. The T5% and T95%, marking the beginning and end of recrystallisation, respectively, are determined as a function of strain. Quantitative microstructural studies validated the findings from the softening studies. The predicated results of recrystallisation regime are found to be in agreement with industrial observation and other experimental measurement for this steel. It can be seen that the dilute additions of Nb can influence the static recrystallisation of austenite under certain rolling condition which may lead to improved mechanical properties of steel.
Authors: Philipp Hagemann, Rudolf Kawalla, Grzegorz Korpala, Matthias Schmidtchen
Abstract: Energy-efficient production in today’s metal industry includes the usage of casting heat to allow hot deformation directly after solidification for saving time and cost of expensive reheating processes. Due to the different initial state this processing route also leads to differences in deformation behavior (strain, flow stress) and softening and precipitation kinetics compared to a reheated material. This is caused by the different microstructures in the material directly after solidification, showing a much more coarsed grain structure and a higher amount of dissolved microalloying elements. Therefore, a material directly deformed after solidification usually shows a retarded softening behavior accompanied by precipitation processes. The different behaviour of a reheated material and a material directly deformed after solidification is shown in this work. The different modes of action for micro-alloying elements in different initial states are compared at the example of steels with different chemical compositions. Differences in deformation behaviour were simulated with semi-empirical models including specific coefficients to consider the processing parameters strain, strain rate, temperature, and chemical composition. The models are capable of describing the retarded softening caused by a superimposed precipitation kinetic leading to a typical plateau. The results of these models are compared with established physical models.
Authors: John J. Jonas, Evgueni I. Poliak, Abbas Najafizadeh
Abstract: Experiments were carried out in which the dependence of the fractional softening on temperature, time and strain rate was determined in a 304H stainless steel. Three prestrain ranges were identified pertaining to three different post-deformation softening behaviors: 1) prestraining to below the DRX critical strain: strongly strain dependent softening by SRX alone with softening kinetics controlled by growth rate of the nuclei; 2) prestraining to above the DRX critical strain: SRX + MDRX softening with weaker strain dependence of the kinetics but still controlled by grain growth; 3) at a prestrain of ε* and beyond: nucleation-controlled MDRX softening with the full inhibition of SRX. The transition prestrain ε* can exceed the peak strain if the DRX grain refinement ratio g = D0/DDRX > 4. The transition to MDRX-dominated softening can be attributed to a constant value of the normalized strain hardening rate independent of the preloading temperature and strain rate. The softening data from the compression tests show that at ε*, the time for half softening t50 exhibits a minimum. These data differ somewhat from observations obtained in the torsion testing of solid bars, in which no strain dependence of t50 was detected at ε* and beyond. Whether or not the strain dependence of t50 vanishes in the MDRX range is sensitive to the test method employed to study the post-deformation softening.
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