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URN etd-0117116-003010
Author Wei-Chih Hsu
Author's Email Address No Public.
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Department Materials and Optoelectronic Science
Year 2015
Semester 1
Degree Ph.D.
Type of Document
Language English
Title Study on heterogeneities of deformed structure and evolution of recrystallization texture of ultra-low carbon steels
Date of Defense 2016-01-26
Page Count 188
Keyword
  • ultra-low carbon steel
  • deformation structure
  • texture
  • recrystallization
  • electron backscatter diffraction
  • heterogeneity
  • Abstract The heterogeneity of deformed microstructures of an ULC steel having different cold reductions (30-90%) and two ULC steels with and without dissolved carbon, respectively, were studied in detail using the electron backscatter diffraction, EBSD, technique. The steels were hot rolled to 10 mm, annealed, and cold rolled to 1-7 mm thick. In the first part of the thesis, the cold rolled microstructure and texture were analyzed from the transverse direction (TD). The heterogeneity of deformed microstructures in a meso-scale are categorized into types A, B, and C based on the density of HABs in the original grains. The deformation heterogeneity in a micro-scale is further analyzed by considering the kernel average misorientation (KAM) and the grain average misorientation (GAM), respectively. Two methods of predicting the possible nucleation sites in recrystallization (RX) and the RX texture are proposed accordingly. The first method, HMR+KAM method selects pixels which fulfill a high KAM (>10o) criterion and are located in the high misorientation regions (HMR) are identified as the potential recrystallization nuclei (PRNs). The second method, DIG+GAM method defined areas in deformed matrix which are surrounded by high angle boundaries as the deformed induced grains (DIGs). For those DIGs having low GAM values (<2°) and being smaller than 3 m are defined as the PRNs in recrystallization. The texture of the PRNs selected by each method shows a good correlation with the texture after full RX. The latter method is explained by a modified strain induced boundary migration (SIBM) theory. 
    A quasi in-situ method was employed to study the correlation of the cold rolled (CR) and recrystallized (RX) microstructure and texture in the second part of the thesis. EBSD measurements were carried out for the CR samples prior to annealing, and again for the annealed sample of the same area. The microstructures and textures of steels subjected to 50-90% cold reductions (denoted as A-5, A-7, and A-9) were characterized accordingly. Good similarity of the spatial distribution and texture was found between the pixels selected by the HMR+KAM method and the RX grains for A-5 and A-7. By using the DIG+GAM method, the textures of PRNs and RX grains for A-7 and A-9 show good similarity.
    Finally, the effect of dissolved carbon to the formation of RX texture by comparing steels A (with dissolved carbon) and steel B (without dissolved carbon) was studied. The evolutions of RX nucleation and texture in specimens of 70% cold reduction (A-7 and B-7) were investigated. Prior to the onset of recrystallization, no significant difference in CR texture between A-7 and B-7 can be found. However, the fully recrystallized A-7 sample shows a stronger Goss texture and a weaker {111}<112> one, as compared to those of B-7. Partially recrystallized specimens with various recrystallization fractions were analyzed by EBSD. Special emphasis is given to PRN sand RX grains of four texture components: {111}<110>, {111}<112>, Goss and cube. The textures of the RX grains show great similarity at ~40% of RX for A-7 and B-7, except that A-7 exhibits a high Goss intensity from early stage of recrystallization. After 40% of RX, the intensity of the {111}<112> grains of B-7 increases steadily whereas that of A-7 keeps a relatively constant value till full RX. CR samples analyzed by the DIG+GAM method show that A-7 indeed exhibits a higher fraction of Goss PRNs. In addition, a higher area fraction of {111}<112> RX grains of sizes in the range of 0.5-3 μm were found in the deformed matrix of B-7 after 40% of RX. These small {111}<112> grains keep growth into RX grains and results in a high intensity of {111}<112> after full RX.
    Advisory Committee
  • Chih-Pu Chang - chair
  • Pei-ling Sun - co-chair
  • Jui-Chao Kuo - co-chair
  • Po-We Kao - co-chair
  • Yi-Shing Huang - co-chair
  • Liuwen Chang - advisor
  • Files
  • etd-0117116-003010.pdf
  • Indicate in-campus at 5 year and off-campus access at 5 year.
    Date of Submission 2016-02-17

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