Einfluss der Primärumformung auf die mechanischen Eigenschaften und die Gefügeausbildung von mikrolegierten Stählen

Translated title of the contribution: Influence of hot direct rolling on the mechanical properties and the microstructural evolution of microalloyed steels

Siegfried Gelder

Research output: ThesisDoctoral Thesis

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Abstract

Thin slab direct rolling is an advanced rolling technology for near net shape production of hot rolled strips. This process, where high speed cast slabs are hot rolled to strip without intermediate cooling and reheating, gains more importance. The aim was to investigate the possibility to refine the coarse as cast austenite microstructure via a two resp. a three hit hot deformation to be fine and homogenous. The recrystallization behaviour and the evolution of the austenite grain size during hot working were examined for the steel grade S355JR microalloyed and without microalloying elements via laboratory trials. Small cast samples were direct rolled and reheated and rolled using a thermomechanical testing machine. The results show a shift of static recrystallization regime due to the direct rolling schedule towards longer times. Furthermore the microalloying elements Nb and V retard the static recrystallization regime in the temperature range below 1000°C. It was found that already after the second rolling pass of the as cast material the resulting austenite microstructure exists of fine grains. The initial large differences in grain size before rolling become negligible after three rolling passes there are no differences in the resulting microstructure to a microstructure resulting of a conventional cold charge rolling schedule. The retardation of the static recrystallization regime due to the coarse austenite microstructure via direct rolling gets less important by rolling with high temperatures and high pass strains in the first rolling passes. Hence, due to the much cheaper direct rolling schedule a similar microstructure can be achieved as via the cold charging schedule. During the start up of the ESP plant it was possible to test sheet samples produced on an industrial scale. These ESP-sheets show, like the laboratory samples, a homogenous ferritic pearlitic microstructure and meet the requested requirements in strength.
Translated title of the contributionInfluence of hot direct rolling on the mechanical properties and the microstructural evolution of microalloyed steels
Original languageGerman
QualificationDr.mont.
Supervisors/Advisors
  • Buchmayr, Bruno, Supervisor (internal)
  • Schenk, Johannes, Assessor A (internal)
Publication statusPublished - 2011

Bibliographical note

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Keywords

  • hot direct rolling
  • solidification
  • recrystallization
  • microalloyed steel

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