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In-situ and correlative study of recrystallization and precipitation in cold-rolled-annealed microalloyed steels
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Kapoor, Ishwar (2019) In-situ and correlative study of recrystallization and precipitation in cold-rolled-annealed microalloyed steels. PhD thesis, University of Warwick.
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Official URL: http://webcat.warwick.ac.uk/record=b3493345~S15
Abstract
This thesis presents the research work carried out to fundamentally understand the recrystallization behaviour, evolution of precipitates and their interaction with recrystallization process in three novel cold-rolled low-carbon microalloyed steel grades (base grade containing V as microalloying element; Ti+ grade containing Ti as microalloying element in base grade; and Ti+Mn+ grade containing additional Mn in Ti+ grade) during sub-critical annealing.
Cold-rolled steel grades are annealed at different annealing conditions: 700°C 0 s, 800°C 0 s, 800°C 2 min, 800°C 5 min, 800°C 15 min, 800°C 30 min, 800°C 1 hr and 800°C 2 hrs at the heating rate of ~10°C/s inside muffle furnace. Effect of titanium and vanadium carbo nitride precipitates and manganese solute atoms on recrystallization behaviour, recrystallized texture development and recrystallization and precipitation interaction are studied using Electron Backscatter Diffraction (EBSD) and Scanning transmission electron microscopy (STEM). The average geometrically necessary dislocation (GND) density and average nanoindentation hardness values of the un-recrystallized ferrite matrix are evaluated for different conditions: cold-rolled, 700°C 0 s, 800°C 0 s, 800°C 2 min to study difference in softening behaviour among three different steel grades. Experimental results of EBSD, STEM and nanoindetation and numerical calculations of precipitate pinning and solute drag effect on recrystallization process suggest that recrystallization kinetics is highest in base grade. For Ti+ grade, fine (< 15 nm) precipitates of (Ti,V)(C/N) are significant enough to interfere with the grain boundary motion and result in sluggish recrystallization kinetics. While for base grade, precipitation is much slower than the recrystallization process. Recrystallization behaviour of Ti+ and Ti+Mn+ grades is similar for dwell time of 2 min and longer at annealing temperature of 800 oC and suggests that solute drag effect of Mn solute atoms is negligible for those conditions.
Hot-stage quasi in-situ EBSD is performed for three conditions: 700°C 0 s, 800°C 0 s, 800°C 2 min at heating rate of ~ 10°C/s to study onset and evolution of recrystallization process and texture development on sample surface of cold-rolled low-carbon steel. For the lower annealing temperature of 700°C with no dwell, almost no recrystallization is observed and microstructure resembles the as-received deformed material with the exception of occasional sub-micron sized nuclei. For the annealing conditions of 800°C 0 s and 800°C 2 min, onset and evolution of recrystallization is observed in-situ as a function of the initial as-cold rolled texture. Slower recovery rate of alpha fibre than gamma fibre is observed and confirmed by lower drop in average geometrically necessary dislocation (GND) density for un-recrystallized alpha fibres (1.1E+14 m−2 for 700°C 0 s, 1.4E+14 m−2 for 800°C 0 s and 4.5E+14 m−2 for 800°C 2 min) than for un-recrystallized gamma fibre grains (3.0E+14 m−2 for 700°C 0 s, 6.2E+14 m−2 for 800°C 0 s and 9.8E+14 m−2 for 800°C 2 min) during annealing. Strong gamma texture in recrystallized matrix is found for dwell time of 0 s and longer at annealing temperature of 800°C.
Vickers hardness and uniaxial tensile tests are performed on cold-rolled and muffle furnace annealed bulk samples to correlate microstructural observations with bulk mechanical properties and compare with automotive steel grades. Although Ti+ and Ti+Mn+ steel grades are lower on strength and ductility balance compared to DP and CP steels, cold-rolled annealed partially recrystallized Ti+ and Ti+Mn+ steel grades of high strength (750 – 880 MPa) with moderate ductility (2 – 10 %) have the potentiality to be used in light-weighting automotive body structures where high strength and moderate ductility are required.
Item Type: | Thesis (PhD) | ||||
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Subjects: | T Technology > TA Engineering (General). Civil engineering (General) T Technology > TN Mining engineering. Metallurgy |
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Library of Congress Subject Headings (LCSH): | Recrystallization (Metallurgy), Steel alloys, Lightweight construction, Motor vehicles -- Materials | ||||
Official Date: | September 2019 | ||||
Dates: |
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Institution: | University of Warwick | ||||
Theses Department: | Warwick Manufacturing Group | ||||
Thesis Type: | PhD | ||||
Publication Status: | Unpublished | ||||
Supervisor(s)/Advisor: | Janik, Vit ; Li, Zushu ; West, Geoffrey D. ; Lan, Yongjun ; Rijkenberg, Rolf Arjan | ||||
Format of File: | |||||
Extent: | xxvii, 165 leaves : illustrations (some colour) | ||||
Language: | eng |
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