Anpassung von Mikroseigerungsmodellen durch Heißrissuntersuchungen an erstarrendem Stahl

Research output: ThesisMaster's ThesisResearch

Organisational units

Abstract

Microsegregation is in general the consequence of the different dissolution of alloying elements and residual elements in liquid and in solid steel. The result is an enrichment of alloying elements between dendrites. This enrichment is the main reason for segregation at the macroscopic scale (center segregation) or the formation of hot tear segregations (commonly also termed internal cracks) and hence a decisive factor for product quality. The relatively thin strand formed in a continuous casting machine for steel is multiply deformed by bending, straigthening or bulging. These deformations may result in the formation of hot tear segregations (HTS) along primary grain boundaries. During further processing, these kinds of defects may even lead to surface defects but more frequently to the formation of brittle phases after the rolling process. The higher the content of segregating elements the higher is the sensitivity towards the formation of HTS. Microsegregation modelling is thus the key for the understanding of HTS formation. Starting point for the present work is the critical review of liquidus formula by the comparison with results from DSC (Differential Thermal Analysis) for a wide range of steel compositions. A recently published formula could be slightly modified and improved. Microsegregation is modelled on basis of the analytical solution as proposed by Ohnaka. Equilibrium partition coefficients were determined from FactSage (FSStel2015). The calculated solidus temperature was compared with values determined from a hot tensile experiment for low carbon steel grades. The partition coefficients were again slightly adjusted. The generation of the thermophysical properties for the numerical solidification simulation demands the knowledge of phase transformation temperatures in the solid state. These values were also assessed on the basis of empirical equations and DSC measurement results. The findings were applied to calculate solidification in a continuous casting machine (1D-FV-model) and to analyse the strain in the mushy zone subsequently. The strain analysis is based on the results of parallel research work and own measurements. The basic idea of strain accumulation as main reason for HTS formation was transferred into an on-line capable HTS-index. For selected steel grades the probability of HTS formation was predicted and the influence of operating parameters was quantified. The newly developed methodology will in future be used to extend the existing models to further steel grades.

Details

Translated title of the contributionAdjustment of microsegregation models by means of hot tearing investigations on solidifying steel
Original languageGerman
QualificationDipl.-Ing.
Supervisors/Advisors
Award date16 Dec 2016
Publication statusPublished - 2016