The Library
Effect of internal oxidation in bendability of UHSS
Tools
Zhu, Yuyi (2019) Effect of internal oxidation in bendability of UHSS. PhD thesis, University of Warwick.
|
PDF
WRAP_Theses_Zhu_2019.pdf - Submitted Version - Requires a PDF viewer. Download (14Mb) | Preview |
Official URL: http://webcat.warwick.ac.uk/record=b3714248
Abstract
Ductile Ultra High Strength Steels (UHSS), the so-called third generation of Advanced High Strength Steels (AHSS), are increasingly used for automotive applications. The strengthening alloying additions in the dual phase UHSS, such as Mn and Si show significant impact on the oxidation behaviours. The changes in surface and subsurface morphologies due to external and internal oxidation during the steel processing have consequences for the quality and performance of the final product.
The internal oxidation zone (IOZ) initially forms during cooling of the hot rolled product will lead to surface defects of the galvanised steel sheet. This phenomenon is known to affect the formability of the steel sheet. The development of the multi-layered external oxides during hot rolling largely influence the internal oxidation.
This PhD project focuses on three critical studies upon UHSS:
• • Characterisation of scale and sub-scale development during steel processing.
• • The influence of internal oxidation on the bendability.
• • The influence of coiling environment on the internal oxidation.
In-depth analyse have been conducted on the external (scale) and internal (subscale) oxidation layer. A mixed surface characterization methods is designed, therefore, Wustite, magnetite, hematite and fayalite can be identified and localised within the coil-cooled scale structure. The subscale structures are characterised by (Secondary electron microscope) SEM and (Transmission electron microscope) TEM, the nature of the manganese and silicon-enriched precipitates are then understood. The oxide development are monitored on the coil cooled, cold rolled, annealed and galvanised steel sheets, the relationship between the steel processing conditions and surface/subsurface morphologies is determined. The effect of coil cooling temperature, coil geometries as well as the atmospheres within the tightly warped steel sheets can lead to variation in oxides development throughout the coil. A deep internal oxidation layer formed at earlier stages will lead to cracking of the oxidised grain boundaries during cold rolling. Therefore, the follow on gravanising and forming process become invalid.
Deformation experienced during the steel sheet forming processing are simulated by the in-situ bending stage and analysed by the digital image correlation (DIC) technique, based on lab-scale samples. The microscopic strain distribution and failure mechanisms established a link between the near-surface conditions and the forming limit of the final product. The oxidised grain boundaries are proved to promote crack initiation, whereas the banding structure within the dual phase steel matrix promotes crack propagation.
Industrial heat treatments are replicated in specially designed furnaces with gas analyse technique. The evolution of oxides at elevated temperature and varies coil cooing conditions are studied to elucidate the link between scale decomposition, decarburisation and internal oxidation. Micro-pores are found to form within the fayalite-wustite mixed layer, which is a sign of wustite reduction. The reduction of these wustites is assisted by the steel decarburisation as concentrated CO and CO2 signals are detected. Meanwhile, the magnetite layer thickness is reduced and oxygen is detected. The reduction of the external oxides is a continuous process, which promotes the internal oxidation consistently during the coil cooling. The shrinkage of the overall scale thickness has no direct influence on the subscale development.
Item Type: | Thesis (PhD) | ||||
---|---|---|---|---|---|
Subjects: | Q Science > QC Physics Q Science > QD Chemistry T Technology > TA Engineering (General). Civil engineering (General) T Technology > TN Mining engineering. Metallurgy T Technology > TS Manufactures |
||||
Library of Congress Subject Headings (LCSH): | Steel, High strength, Oxidation, Cooling, Metals -- Formability, Deformations (Mechanics), Wustite | ||||
Official Date: | January 2019 | ||||
Dates: |
|
||||
Institution: | University of Warwick | ||||
Theses Department: | Warwick Manufacturing Group | ||||
Thesis Type: | PhD | ||||
Publication Status: | Unpublished | ||||
Supervisor(s)/Advisor: | Shollock, Barbara A. ; Melfo, Wanda ; Proprentner, Daniela ; Auinger, Michael ; Basabe, Vladimir | ||||
Format of File: | |||||
Extent: | 189 leaves : illustrations | ||||
Language: | eng |
Request changes or add full text files to a record
Repository staff actions (login required)
View Item |
Downloads
Downloads per month over past year