Ekoja, Gabriel Abah (2021) Three dimension cellular automata modelling of recrystallisation in steel. PhD thesis, University of Warwick.

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Abstract

A physical based 3D cellular automata (CA) model that includes the recrystallisation process has been developed for strip steels. The CA model allows the full starting austenite grain size distribution to be considered (rather than the frequently used empirical recrystallisation equations where average grain sizes are considered) and to predict the recrystallised grain size distribution and recrystallisation kinetics. A key objective of the work was to determine the cause for the temperature dependency from the previous CA model and a physically based approach to give correct recrystallised grain size predictions.

In order to be able to correctly predict both the recrystallisation kinetics and resultant grain size distribution and their appropriate dependence / independence on strain and temperature, consideration of the recrystallisation nucleation and growth to a critical nucleus size are essential and they are driven by the local stored energy conditions. This work has looked at the implementation of a boundary intensity factor to drive the recrystallisation nucleation event and subsequent growth to exceed the critical nucleus, the critical radius uses the dislocation density which is assumed to be the same at both the grain boundary and the grain interior and without the introduction of the boundary intensity factor the kinetics of recrystallisation agree poorly. The introduction of the boundary intensity factor (BIF) accurately reflects the experimentally observed inhomogeneous dislocation density in deformed samples, where higher dislocation density at the grain boundaries occurs, which results in changes in the nucleation and sub-grain growth to critical size. The width of the ‘boundary’ has been determined based on the experimental observations and the value of the boundary intensity factor for the stored energy was determined by fitting to literature experimental data for a Fe-30Ni-0.044 wt. % Nb steel, with a starting average grain size of 160 μm at a strain of 0.3, considering deformation at two temperatures (900°C and 950°C). A BIF of 2.2 was derived to match the recrystallised grain size distribution and the recrystallisation kinetics of Fe-30Ni-0.044 wt. % Nb experimental data. The recrystallised modal grain size (D50%) from the CA with a strain of 0.2 was 100 μm and 60 μm with a strain of 0.3 and both were in excellent agreement with the experimental recrystallised modal grain sizes. The recrystallisation starting time (Rs) and the recrystallisation time at 50% (R50%) are also in good agreement with the experimental results.

Item Type:	Thesis (PhD)
Subjects:	Q Science > QC Physics Q Science > QD Chemistry T Technology > TN Mining engineering. Metallurgy
Library of Congress Subject Headings (LCSH):	Cellular automata -- Mathematical models, Recrystallization (Metallurgy), Steel, Grain boundaries
Official Date:	September 2021
Dates:	Date Event September 2021 UNSPECIFIED
Institution:	University of Warwick
Theses Department:	Warwick Manufacturing Group
Thesis Type:	PhD
Publication Status:	Unpublished
Supervisor(s)/Advisor:	Davis, Claire ; Slater, Carl ; Strangwood, Martin
Sponsors:	University of Warwick ; Engineering and Physical Sciences Research Council ; Tata Steel UK
Format of File:	pdf
Extent:	213 leaves : illustrations
Language:	eng

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