Simoes, Luis Fellipe
(2022)
A Novel Approach to the Alloy Design of Low-Cost Structural Steels using Low-Mn and micro-Nb additions.
Master's Thesis, University of Pittsburgh.
(Unpublished)
Abstract
This study presents an in-depth understanding of the effects of a novel thermomechanical processing and alloy design in the microstructural and mechanical properties of A36 structural steels. The new process involves the use of high temperatures and multiple stages of deformation combined with a new alloy design using micro-additions of niobium in substitution of manganese. The reduction of Mn content is expected to reduce manufacturing issues, such as hot banding and center line segregation, while the Nb addition is expected to promote strengthening by precipitation.
Empirical calculations were used to determine the transformation temperatures (A1, A3, and Tnr) to design the optimum Recrystallization Controlled Rolling Thermomechanical Processing, where a sufficiently high temperature was employed to promote full dissolution of the added Nb and full austenite recrystallization prior to transformation. All samples studied in this project were submitted to the same processing, varying only in composition.
Using advanced characterization techniques, it was possible to observe the effects of the new alloy design in the material. Samples were observed under Optical Microscopy (OM) and showed variation in their banding behavior, with higher Mn additions leading to pearlite bands while lower Mn levels resulted in a more homogeneous structure. The bands were further investigated using Electron Probe Micro Analysis (EPMA), which indicated segregation of Mn at these regions. As expected, all materials showed similar phase balance when observed under Scanning Electron Microscopy (SEM), with a ferritic matrix and pearlitic regions. Further characterization using Electron Backscatter Diffraction (EBSD) concluded that the microstructure of all samples was similar, with recrystallized microstructures.
The precipitation behavior of these alloys was investigated using Transmission Electron Microscopy (TEM), with which it was possible to observe that the Nb additions led to precipitation of extremely fine particles organized in nanoclusters. These particles were highly effective in promoting strengthening, with the new alloy design and process increasing the yield strength from 255 MPa to 326 MPa with a 0.02 wt%Nb addition, even at a reduction of C and Mn levels. The new material showed improved microstructure, with more homogenous phase distribution and grain size.
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Details
| Item Type: |
University of Pittsburgh ETD
|
| Status: |
Unpublished |
| Creators/Authors: |
|
| ETD Committee: |
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| Date: |
10 June 2022 |
| Date Type: |
Publication |
| Defense Date: |
22 March 2022 |
| Approval Date: |
10 June 2022 |
| Submission Date: |
6 April 2022 |
| Access Restriction: |
2 year -- Restrict access to University of Pittsburgh for a period of 2 years. |
| Number of Pages: |
92 |
| Institution: |
University of Pittsburgh |
| Schools and Programs: |
Swanson School of Engineering > Materials Science and Engineering |
| Degree: |
MS - Master of Science |
| Thesis Type: |
Master's Thesis |
| Refereed: |
Yes |
| Uncontrolled Keywords: |
Precipitation, A36, Structural Steels, Segregation, Processing, Alloying, Niobium, Manganese |
| Date Deposited: |
10 Jun 2022 18:48 |
| Last Modified: |
10 Jun 2024 05:15 |
| URI: |
http://d-scholarship.pitt.edu/id/eprint/42508 |
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