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Annealing and Precipitation Behavior During Batch Annealing of HSLA Steels

Fang, Chao (2011) Annealing and Precipitation Behavior During Batch Annealing of HSLA Steels. Doctoral Dissertation, University of Pittsburgh.

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    Abstract

    To gain a fundamental understanding of the factors responsible for the kinetics of annealing behavior during batch annealing for high strength low alloy (HSLA) steels, studies have been conducted to analyze the influence of alloy composition, hot band state, cold rolling reduction (CR%), heating rate, soaking temperature and time, etc. on the annealing behavior of HSLA steels during batch annealing process. The recrystallization kinetics was mainly controlled by several key parameters such as stored energy, precipitation and/or solute drag, special grain boundaries, and texture, etc. The combination of Electron Back-Scattered Diffraction (EBSD) technique and the Sub-grain Method was used to construct and analyze stored energy distribution maps before and during the batch annealing process of cold rolled HSLA steels. Precipitation behavior was studied using Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS), Transmission Electron Microscopy (TEM), and High Resolution TEM (TEM). Grain boundary characteristic and texture fibers were also analyzed using EBSD scanning data. The results show that different composition and processing parameters would cause different hot band microstructure and different amounts of dislocation density, i.e., stored energy, remained in the alloys after hot rolling deformation. Higher dislocation density in the hot band steel will cause even higher dislocation density in the sheet steel after cold rolling deformation, which will cause higher recrystallization speed at early stages. If precipitates were formed during annealing, their formation would consume part of the stored energy and decreased some driving force. The new formed fine TiC precipitates would also apply certain pinning force on the grain boundaries, which dragged the moving of those boundaries, i.e., lowered the recrystallization speed further. Certain special grain boundaries like Coincident Site Lattice (CSL) boundaries have very low boundary energy and mobility. The higher volume fraction of this kind of boundaries would also slow down the recrystallization process. Texture could not be a critical factor causing the different annealing behavior in this study, since the texture fibers distributions are very similar. Two or more factors might affect the annealing process at the same time. Some factors could be more efficient than others at certain stage or stages.


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    Item Type: University of Pittsburgh ETD
    ETD Committee:
    ETD Committee TypeCommittee MemberEmailORCID
    Committee ChairDeArdo, Anthony J.deardo@pitt.edu
    Committee CoChairGarcia, C. Issaccigarcia@pitt.edu
    Committee MemberNettleship, Iannettles@pitt.edu
    Committee MemberVallejo, Luis E.vallejo@pitt.edu
    Committee MemberChoi, Shi-Hoonshihoon@sunchon.ac.kr
    Title: Annealing and Precipitation Behavior During Batch Annealing of HSLA Steels
    Status: Unpublished
    Abstract: To gain a fundamental understanding of the factors responsible for the kinetics of annealing behavior during batch annealing for high strength low alloy (HSLA) steels, studies have been conducted to analyze the influence of alloy composition, hot band state, cold rolling reduction (CR%), heating rate, soaking temperature and time, etc. on the annealing behavior of HSLA steels during batch annealing process. The recrystallization kinetics was mainly controlled by several key parameters such as stored energy, precipitation and/or solute drag, special grain boundaries, and texture, etc. The combination of Electron Back-Scattered Diffraction (EBSD) technique and the Sub-grain Method was used to construct and analyze stored energy distribution maps before and during the batch annealing process of cold rolled HSLA steels. Precipitation behavior was studied using Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS), Transmission Electron Microscopy (TEM), and High Resolution TEM (TEM). Grain boundary characteristic and texture fibers were also analyzed using EBSD scanning data. The results show that different composition and processing parameters would cause different hot band microstructure and different amounts of dislocation density, i.e., stored energy, remained in the alloys after hot rolling deformation. Higher dislocation density in the hot band steel will cause even higher dislocation density in the sheet steel after cold rolling deformation, which will cause higher recrystallization speed at early stages. If precipitates were formed during annealing, their formation would consume part of the stored energy and decreased some driving force. The new formed fine TiC precipitates would also apply certain pinning force on the grain boundaries, which dragged the moving of those boundaries, i.e., lowered the recrystallization speed further. Certain special grain boundaries like Coincident Site Lattice (CSL) boundaries have very low boundary energy and mobility. The higher volume fraction of this kind of boundaries would also slow down the recrystallization process. Texture could not be a critical factor causing the different annealing behavior in this study, since the texture fibers distributions are very similar. Two or more factors might affect the annealing process at the same time. Some factors could be more efficient than others at certain stage or stages.
    Date: 27 June 2011
    Date Type: Completion
    Defense Date: 01 March 2011
    Approval Date: 27 June 2011
    Submission Date: 04 March 2011
    Access Restriction: No restriction; The work is available for access worldwide immediately.
    Patent pending: No
    Institution: University of Pittsburgh
    Thesis Type: Doctoral Dissertation
    Refereed: Yes
    Degree: PhD - Doctor of Philosophy
    URN: etd-03042011-153053
    Uncontrolled Keywords: Annealing; Cold Rolling; Deformation; Dislocation Density; EBSD; EDS; Grain Boundary; Hardness; Hot Band; HSLA Steel; Precipitates; Precipitation; Recrystallization; SEM; Stored Energy; Sub-grain Method; TEM; Texture; TiC
    Schools and Programs: Swanson School of Engineering > Materials Science and Engineering
    Date Deposited: 10 Nov 2011 14:31
    Last Modified: 24 Feb 2012 11:40
    Other ID: http://etd.library.pitt.edu/ETD/available/etd-03042011-153053/, etd-03042011-153053

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