Advanced high strength steel (AHSS) provides a lightweight material solution in response to the stringent regulation on fuel economy and greenhouse gas emissions in the automotive industry. Dual phase (DP) steels that consist of a hard martensite phase embedded in a soft ferrite matrix are the most widely used AHSS due to their simple microstructure, robust thermo-mechanical processing and attractive mechanical properties. However, DP steels are prone to deform heterogeneously with strong strain partitioning between phases. The addition of Vanadium in DP steels can form nano-precipitates of vanadium carbonitrides (V (C,N)) that strengthen the ferrite and thus reduce the strain partitioning. This study considered the influence of V (C,N) on the deformation and damage behavior of ferrite-martensite DP1300 steels at the microscopic level. The hardness of the embedded ferrite and martensite regions are determined through nano-hardness testing. In-situ uniaxial tension tests were conducted on DP steels with similar martensite volume fractions within a scanning electron microscope (SEM) chamber. Microscopic-digital image correlation (µDIC) was then employed to analyze the local strain partitioning between ferrite and martensite. Local damage events such as void formation at ferrite martensite island interfaces and in the martensite islands were observed and rationalized with the µDIC results. X-ray computed tomography (XCT) were conducted to quantitatively analyze the microstructure damage.
It was found that vanadium addition helps refine the microstructure and improve mechanical compatibility between the two phases. The overall ductility of the steel is enhanced especially in terms of post-uniform elongation and true strain to fracture. / Thesis / Master of Applied Science (MASc)
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/23030 |
| Date | January 2018 |
| Creators | Zhou, Linfeng |
| Contributors | Wilkinson, David, Engineering |
| Source Sets | McMaster University |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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