Almatani, Rami A.
(2022)
Rational Design of Microalloyed High Strength-Low Alloy (HSLA) Plate and Strip Steels.
Doctoral Dissertation, University of Pittsburgh.
(Unpublished)
Abstract
The central purpose of this investigation was to understand the practical limits of Nb microalloying in controlled rolled low carbon plate and strip steels. More specifically, the influence of the high Nb addition exhibited in the so-called high-temperature processing (HTP) steels (i.e., 0.08 to 0.1 wt. pct. Nb) on the austenite recrystallization behavior at high deformation temperatures and the subsequent final transformation products.
Six candidate steels, each of the same nominal base composition, with 0.03/0.06 wt. pct. C and 0/0.04/0.08 wt. pct. Nb were designed and melted based on an established solubility equation and a modified Zener pinning force model (i.e., subgrain boundary model). Extensive experimental studies on the progress of the microstructure through reheating and subsequent rough rolling, finish rolling and accelerated cooling simulations were conducted. Of these conducted experiments, double-hit tests were critical to better understanding the extent of austenite recrystallization in the temperature range of roughing and finishing passes of a 5-m wide reversing plate rolling mill, namely, between 1150 °C and 900 °C.
Double-hit test results indicated that as the Nb content is increased, the extent of austenite recrystallization is decreased. Additionally, there exists a plateau in the fractional softening curves of the high-Nb steels (0.08 wt. pct. Nb), which indicated that strain-induced precipitates of Nb, carbides or carbonitrides are taking place, hence, hindering further austenite recrystallization.
Roughing passes simulation revealed that doubling the amount of Nb in the steel resulted in grain refinement but mixed grain structure developed. The retardation of austenite recrystallization could be raised to higher temperatures in HTP steels (near 1050 °C). Hence, the austenite of the high-Nb steels can enter subsequent deformation in the unrecrystallized or partially- recrystallized state instead of full recrystallization and grain refined as expected. The roughing schedule plays a role in the final mechanical properties, and subsequently, a combination of fine effective grain size, reduced area fraction of martensite-austenite (MA) microconstituents, and adequate dislocation densities could be inherited in the final microstructure from a uniformly conditioned austenite.
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Details
| Item Type: |
University of Pittsburgh ETD
|
| Status: |
Unpublished |
| Creators/Authors: |
|
| ETD Committee: |
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| Date: |
6 September 2022 |
| Date Type: |
Publication |
| Defense Date: |
19 May 2022 |
| Approval Date: |
6 September 2022 |
| Submission Date: |
6 July 2022 |
| Access Restriction: |
1 year -- Restrict access to University of Pittsburgh for a period of 1 year. |
| Number of Pages: |
204 |
| 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: |
Microalloyed steels; Nb; Roughing; Zenner Pinning force |
| Date Deposited: |
06 Sep 2022 16:18 |
| Last Modified: |
06 Sep 2022 16:18 |
| URI: |
http://d-scholarship.pitt.edu/id/eprint/43265 |
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