<p> The advantages of Thin-Slab Cast Direct-Rolling (TSCDR) process include reduced
capital, energy, labour and inventory costs, as well as the ability to roll thinner strip compared
to the conventional process of thick slab casting, reheating and hot rolling. There is great
interest in utilizing this technology to produce microalloyed steels which can meet American
Petroleum Institute (API) standards. However, whereas the conventional approach can
produce APIX80, APIXlOO, and even APIX120 steels; the TSCDR process can only produce
APIX70 and APIX80. The main obstacles in the way of achieving high API grades are the
non-uniform initial as-cast microstructure and the large grains that result from grain growth at
high temperature. The production of APIX80 and higher grade steels can only be achieved
through a comprehensive research initiative that combines careful control of solidification,
homogenization, thermomechanical-processing, cooling and coiling. </p> <p> This contribution examines the solid state microstructure evolution of microalloyed
steels under simulated TSCDR conditions. The grain growth kinetics in delta-ferrite and
austenite were studied separately using two model alloys. At high temperatures and in the
absence of precipitation, the growth kinetics in both delta-ferrite and austenite appeared to
follow a simple parabolic growth law. The measured grain growth kinetics was then applied
to the problem of grain-size control during the process of TSCDR. Several strategies of
controlling and refining the grain size were examined. The kinetics of delta-ferrite to austenite phase transformation was investigated using a quenching dilatometer; the results showed that
the austenite phase formed along the original delta grain boundaries, and that the precipitation
of austenite at the delta-ferrite grain boundaries effectively pins delta grain growth. The
kinetics of the phase transformation was modeled using a local equilibrium model that
captures the partitioning of the substitutional elements during the transformation. </p> <p> A novel delta-ferrite/austenite duplex microstructure is proposed to achieve fine and
uniform high-temperature microstructure. The grain growth of the matrix phase (delta-ferrite)
is controlled by the coarsening mechanism of pinning phase (austenite). The effectiveness of
this delta/austenite duplex microstructure was validated experimentally and analyzed in
details using a physically-based model. </p> / Thesis / Doctor of Philosophy (PhD)
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/19482 |
| Date | 07 1900 |
| Creators | Zhou, Tihe |
| Contributors | Zurob, Hatem, Materials Science and Engineering |
| Source Sets | McMaster University |
| Language | English |
| Detected Language | English |
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