Titanium nitride precipitation behavior in thin-slab cast high-strength low-alloy steels |
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Authors: | Michael T Nagata John G Speer David K Matlock |
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Affiliation: | (1) the Advanced Steel Processing and Products Research Center, Colorado School of Mines, 80401 Golden, CO |
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Abstract: | To assess the potential for obtaining and utilizing titanium nitride (TiN) refinement via the increased postsolidification cooling rates associated with thin-slab casting, TiN particle size distributions were evaluated
by transmission electron microscope (TEM) examination of carbon extraction replicas. Eight commercially produced thin-slab
cast TiN steels, nominally 0.05 pct C, 1.2 pct Mn, and one conventionally cast steel were received. Thin slab samples were
taken from three locations in the production process: quenched after casting before the tunnel furnace, quenched after tunnel
furnace soaking, and the as-rolled and air-cooled final product. Effects of cooling rate were evident in the results and agree
with previously documented behavior, where precipitate size decreases with increased cooling rate. Statistical differences
in particle size between specimens from steels with different chemistries were shown. These variations result from differences
in the driving force for precipitation, rates of coarsening, and differences in volume fraction due to changes in steel composition.
The interaction of composition and processing, such as soaking in the tunnel furnace and rolling, was found to be important.
For example, the hyperstoichiometric steel (excess Ti) exhibited fine TiN after casting and soaking, but dramatic coarsening
after hot rolling. This behavior was attributed to deformation enhanced particle coarsening, or incomplete precipitation after
soaking, followed by continued growth during subsequent processing. |
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