Hot cracking during solidification in continuous casting is one of the quality problems due to the development of local stresses and strains in the CC strand which exceeds the material strength. Strains result from alternating motion of the steel shell due to thermal and mechanical contraction expension and ferrostatic pressure, primarily in the transition region between the geometrical strong edge and the more flexible wide side of a slab. [1] The formation of hot cracks is often related to the reduced ductility of steel in the mushy zone, caused by thin liquid layers between dendrites resulting from microsegregation. Those liquid layers between dendrite arms exist up to a high fraction of solid, just until complete solidification; [2] applied strain in the mushy zone in the solid fraction range between 0.9 and 0.99 can initiate brittle cracks. The temperature at which the solid brittle fraction is 0.9 is known as liquid impenetrable temperature (LIT) while the temperature at a solid fraction of 0.99 is the zero ductility temperature (ZDT). The region between both temperatures will be referred to as brittle temperature range. Reported critical strains are varying from 0.2 to 3.8% [3][4][5][6] for different carbon contents and strain rates, while the strains analyzed in any location of the caster are 0.3% at the most. [7] In the past it was common practice to describe the high temperature properties of steels according to tensile strength vs. temperature or reduction-of-cross-section-area (RoA) vs. temperature determined by hot tensile tests following theThe present paper gives an overview of the simultaneous research work carried out by RWTH Aachen University and ThyssenKrupp Steel Europe AG. With a combination of sophisticated simulation tools and experimental techniques it is possible to predict the relations between temperature distribution in the mould, solidification velocity, chemical steel composition and, furthermore, the mechanical properties of the steel shell. Simulation results as well as experimentally observed microstructure parameters are used as input data for hot tearing criteria. A critical choice of existing hot tearing criteria based on different approaches, like critical strain and critical strain rate, are applied and developed. The new ''damage model'' is going to replace a basic approach to determine hot cracking susceptibility in a mechanical FEM strand model for continuous slab casting of ThyssenKrupp Steel Europe AG. Critical strains for hot cracking in continuous casting were investigated by in situ tensile tests for four steel grades with carbon contents in the range of 0.036 and 0.76 wt%. Additionally to modeling, fractography of laboratory and industrial samples was carried out by SEM and EPMA and the results are discussed.94
Measurement of concentration by electron beam probe micro analysis (EPMA) gives data about the distribution of alloyed chemical elements of the solidified steel. Those experimental data are essential for mathematical prediction of the structure. Particularly, distribution coefficients which give the concentration ratio at the solid-liquid-interface are parameters for micro-segregation simulations. Depending on the structure like columnar or equiaxed dendritic the local extent of element concentrations causes a characteristic pattern. Statistical methods are used to find out characteristic values because an EPMA gives back the concentration scans of a measured line or of an area (mapping) which can consist of e.g. 6?5610 4 values for each analysed element at about 0?25 mm 2 . The length scale ratios of sample size and measurement grid have to be optimized according to the structure in the area of interest. A comparison of statistical concentration distributions was carried out from both aspects, measurement of real steel samples and mathematical approach based on micro-segregation formula. Columnar and equiaxed dendritic structures have been investigated in terms of statistical element distribution, and the resulting data are applied to modelling of solidification. Determined distribution coefficients k of [Mn] depend on the as-cast structure, the chemical steel composition, and the kind of definition of the min-and max-values of EPMA-readings.
In continuous casting, the probability of hot cracks developing strongly depends on the local solidification process and the microstructure formation. In ref. 1, an integrative model for hot cracking of the initial solid shell is developed. This paper focuses on solidification modelling, which plays an important role in the integrated approach. Solidification is simulated using a multiphase‐field model, coupled online to thermodynamic and diffusion databases and using an integrated 1D temperature solver to describe the local temperature field. Less‐complex microsegregation models are discussed for comparison. The results are compared to EDX results from strand samples of different steel grades.
In this paper an outline is given of a work carried out within a publicly supported project between the RWTH Aachen University and ThyssenKrupp Steel AG. The objective of the collaboration is to determine the hot temperature properties of different steel grades during solidification. These data will later be implemented into a FEM Simulation for continuous caster operated by the industrial partner. The calculated stress field of the slab provides an informative basis for the cracking susceptibility of the strand shell. Based on mathematical models on prediction of the solidification and microstructure parameters, the prediction of hot ductility is intended. In a first step the temperature distribution during solidification in a continuous casting mould is calculated and transferred to a microstructure simulation working on the basis of multiphase field modelling. The results are used to apply existing hot tearing criteria and determine the mechanical properties as ductility, critical strain or maximum strain rate. Additionally, combined solidification and hot tensile tests as well as microprobe analysis are carried out for evaluation of modelling results. Simulation und Modellierung der Heißduktilität unterschiedlicher Stahlsorten. Es wird ein Überblick gegeben über die Arbeit in einem von der DFG öffentlich geförderten Projekt zwischen der RWTH Aachen, ACCESS e.V. und der ThyssenKrupp Steel AG (TKS). Das Ziel der Zusammenarbeit ist die Berechnung mechanischer Kennwerte, insbesondere der Heißduktilität unterschiedlicher Stahlsorten während der Erstarrung. Die Kennwerte gehen in ein thermomechanisches FE-Modell der TKS ein und liefern somit wichtige Anhaltspunkte für die Rissempfindlichkeit der Strangschale während des Stranggießprozesses. Basierend auf mathematischen Modellen zur Berechung der Erstarrungs-und Mikrostrukturparameter, wird die Vorhersage der kritischen Parameter, die zu Rissbildung führen können, angestrebt. In einem ersten Schritt wird die Temperaturverteilung in der Kokille berechnet und an eine Mikrostruktursimulation, basierend auf einem Phasenfeld-Modell, gekoppelt. Es ist möglich, die Ergebnisse von Heißrisskriterien anzuwenden und somit mechanische Kennwerte wie Duktilität, kritische Dehnung und maximale Dehnrate zu bestimmen. Begleitend werden Erstarrungs-und Heißzugexperimente, metallographische Untersuchungen und Mikrosonden-Analysen zur Bewertung der Modellierungsergebnisse durchgeführt.
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