Einfluss der Rotationsparameter auf die Erstarrungsbedingungen bei mechanischem Rühren

Translated title of the contribution: Influence of the rotation parameters on the solidification conditions during mechanical stirring

Thomas Michael Holzgruber

Research output: ThesisMaster's Thesis

Abstract

Steels solidify under technical conditions, as they occur in ingot and continuous casting, near the surface directionally dendritic, i.e. with preferred growth direction, opposite to the heat flow and with the formation of pine tree-like primary crystals and near the core usually non-directionally globulitic due to the prevailing solidification conditions. This dendritic growth can be suppressed in favour of non-directional globulitic growth due to convection currents caused by stirring. In general, fluid flow also leads to a reduction in segregation and porosity due to a higher amount of globulitic solidification structure. The current state of the art in continuous casting of steel is electromagnetic stirring (EMS system), which is now widely used. In the production of very large dimensions, however, this reaches its technical limits, as extremely high electrical power must be installed to achieve a sufficient stirring effect, especially if a stirring effect is to be achieved in the core. Another option is mechanical stirring. The subject of this master's thesis is to investigate the influence of mechanical rotation conditions on the solidification structure and to evaluate whether a similar or greater positive effect can be achieved than with electromagnetic stirring. The great advantage of the mechanical stirring effect compared to the EMS system is that forced convection occurs at the solid-liquid interface at every point in the strand or block over the entire length, regardless of the size dimensions, right into the center. In the practical part, two series of trials were carried out with two different materials and a total of twelve tests. The tests differed in the type of rotation (non-rotating, continuously rotating, alternately rotating), rotation speed and period duration. Since both the tapping temperature and overheating were the same in all tests, the influence of the two rotation parameters speed and period duration could be worked out excellently. The melting tests were carried out in cooperation with the company INTECO melting and casting GmbH at the Chair of Iron and Steel Metallurgy at the University of Leoben. In the first series of tests, the material 1.4301 was examined. The results showed that the hoped-for effect of rotation on the formation of a globulitic zone failed to materialize in all five tests. This can be attributed to the fact that the austenite forms a columnar grain, irrespective of whether the primary ferrite is globulitic or columnar. The summary of the findings nevertheless allows the conclusion that alternating rotation can produce a significantly finer microstructure than in the two reference tests. The most positive effect can be achieved with the highest rotation speed and lowest period duration. In order to verify the results of the first series of tests and to be able to analyse the influence of rotation on the formation of a globulitic zone, a second series of tests was necessary. In the second series of tests, the material 1.4547 was examined. In contrast to the first series of tests, a globulitic zone with coarse grains in the center was formed in the second series of tests, as expected, even in the two reference tests. As a result of the alternating rotation, both a considerable improvement in the macrostructure and a very fine globulitic microstructure of the round plates and longitudinal samples can be observed. The greatest positive effect - high proportion of globulitic solidification, no occurrence of stirring marks and porosities, is achieved by the lowest rotation speed and highest period duration. The test with the highest speed and lowest period duration, which supposedly had the greatest positive effect in the first series of tests - has the greatest globulitic content, but stirring marks and porosity occur.
Translated title of the contributionInfluence of the rotation parameters on the solidification conditions during mechanical stirring
Original languageGerman
Awarding Institution
  • Montanuniversität
Supervisors/Advisors
  • Bernhard, Christian, Supervisor (internal)
Award date28 Jun 2024
Publication statusPublished - 2024

Bibliographical note

embargoed until 20-03-2029

Keywords

  • mechanical stirring
  • rotating ingot
  • alternating rotation
  • solidification structure

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