Wu, Yingjie
(2021)
Processing-Microstructure-Property Relationship in the High Strength Dual-Phase Steels.
Doctoral Dissertation, University of Pittsburgh.
(Unpublished)
Abstract
The goal of this thesis was to study how to further increase the ultimate tensile strength (UTS) of dual-phase (DP) steels, while not severely sacrificing global ductility (total elongation (TE)) and local ductility (hole expansion ratio (HER)). Candidate steels with 0.15 wt.% C, 0.04/0.4/0.8 wt.% Al and 0.06/0.12 wt.% V were designed and melted. The pre-annealing conditions studied included two coiling temperatures of 580°C and 677°C and one cold reduction of 60%. Two annealing continuous galvanizing line (CGL) simulations (the conventional standard galvanizing (GI) treatment and a new process called the supercooling process (SC)) were selected for this study. The results indicated that the highest UTS of 1181.4 MPa can be obtained for the steel condition of 0.04 wt.% Al, 0.12 wt.% V, a low coiling temperature of 580ºC and the GI anneal with the microstructures characterized by a large amount of fresh martensite, and fine-grained ferrite which itself is strengthened by fine vanadium carbides precipitates.
Three methods, based on electron backscattered diffraction (EBSD) technologies, i.e., the sub-grain, image quality and kernel average misorientation techniques, were used to evaluate the stored energy for DP steels with different pre-annealing conditions. The results indicated that the steels with the combination of low coiling temperature and cold reduction had the highest stored energy, providing more initial driving force for ferrite recrystallization and austenite formation during the subsequent intercritical annealing.
Finally, with the purpose of better understanding improvements in the hole expansion performances of high-strength DP steels, HER values of different steel conditions were determined, and the defects formed during hole punching and their growth during hole expansion were examined. The results implied that coiling temperatures and CGL simulations played a significant role in influencing HER values. Also, micro-voids or micro-cracks were introduced in the initial hole inner surfaces and sheared edges during hole punching. Punching-induced pre-strains were not uniform, since they varied with the distanced from the sheared edges. These damages caused by hole punching later strongly influenced performance during hole expansion. Microstructures beneficial to HER retard the growth of the defects accompanying hole punching, leading to high values of HER.
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Details
| Item Type: |
University of Pittsburgh ETD
|
| Status: |
Unpublished |
| Creators/Authors: |
|
| ETD Committee: |
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| Date: |
13 June 2021 |
| Date Type: |
Publication |
| Defense Date: |
2 December 2020 |
| Approval Date: |
13 June 2021 |
| Submission Date: |
11 November 2020 |
| Access Restriction: |
No restriction; Release the ETD for access worldwide immediately. |
| Number of Pages: |
196 |
| Institution: |
University of Pittsburgh |
| Schools and Programs: |
Swanson School of Engineering > Materials Science and Engineering |
| Degree: |
PhD - Doctor of Philosophy |
| Thesis Type: |
Doctoral Dissertation |
| Refereed: |
Yes |
| Uncontrolled Keywords: |
Dual-Phase Steels; Vanadium-Bearing; Ultra-High Strength; Stored Energy; Austenite Formation; Hole Expansion |
| Date Deposited: |
13 Jun 2021 18:48 |
| Last Modified: |
17 Jun 2021 02:05 |
| URI: |
http://d-scholarship.pitt.edu/id/eprint/39702 |
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