On the systematic investigation of acicular ferrite on laboratory scale

Denise Loder

Research output: ThesisDoctoral Thesis

Abstract

Acicular ferrite nucleates intragranularly on non-metallic inclusions, forming an arrangement of fine, interlocking grains. This characteristic structure is known to improve the steel toughness significantly, what makes acicular ferrite of increasing interest to steel producers. However, the formation of acicular ferrite is still not completely clarified. Therefore, the present thesis developed a holistic and systematic methodology to study the acicular ferrite formation and its influencing factors on laboratory scale. First, the necessary steel composition to create a desired inclusion landscape is determined by thermodynamic calculations using FactSage. Second, samples with defined and homogenously distributed inclusions are produced by melting experiments in a Tammann type furnace. The methodology only uses endogenous inclusions, resulting from deoxidation and desulphurization, as nucleation sites for acicular ferrite, so that the steel cleanness is not deteriorated artificially by the addition of synthetic particles. Third, the samples are heat treated in a HT-LSCM, where the formation of acicular ferrite is observable in situ. Finally, computerized metallographic analyses are applied to evaluate the fraction of acicular ferrite, prior austenite grain size and inclusion landscape. Using this methodology, the acicular ferrite formation in different steel grades is investigated extensively. Next to the determination of the optimum austenite grain size and suitable cooling rates, the effects of carbon, manganese, titanium, boron, chromium, nickel and magnesium are analysed. In addition, the thesis focuses on the evaluation of inclusions’ potential for acicular ferrite nucleation in the different steel grades. The findings of this work provide fundamental knowledge about the acicular ferrite formation and its main influencing factors, which is essential for the production of acicular ferritic steels on industrial scale.
Translated title of the contributionÜber die systematische Untersuchung von Azikularferrit im Labormaßstab
Original languageEnglish
QualificationDr.mont.
Supervisors/Advisors
  • Michelic, Susanne Katharina, Supervisor (internal)
  • Clemens, Helmut, Assessor B (internal)
  • Bernhard, Christian, Assessor A (internal)
Publication statusPublished - 2016

Bibliographical note

embargoed until null

Keywords

  • Acicular ferrite
  • Non-metallic inclusions
  • Microstructure
  • FactSage
  • HT-LSCM

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