Titanium nitride precipitation behavior in thin-slab cast high-strength low-alloy steels
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I. INTRODUCTION
THIN-SLAB casting is being used increasingly as a means of producing sheet steels because of its reduced capital costs. While initial difficulties due to differences in production methods have been overcome and a viable product is being produced, much remains unknown about this relatively new technology. Some metallurgical factors associated with thin-slab casting, when compared to conventional casting, include faster postsolidification cooling rates, less time at very high temperatures, predominance of direct charging, and lower slab soaking temperatures. These intrinsic differences may provide an opportunity to control microstructure and properties in new ways through processing and chemistry. One aspect of special interest is the precipitation behavior of microalloy carbides and nitrides. For example, there may be potential for obtaining and utilizing titanium nitride (TiN) refinement via the increased postsolidification cooling rates[1] associated with thin-slab casting. The differences between conventional casting and thin-slab casting may conceivably change the precipitation and coarsening behavior of TiN precipitates leading to a finer precipitate dispersion. Smaller precipitates are beneficial in retarding austenite grain growth, according to the Gladman equation: R⫽
再 冎
r 3 2 ⫺ 6f 2 z
[1]
where R is the grain radius for which particle pinning remains effective, r is the radius of the pinning particles, f is the
MICHAEL T. NAGATA, former graduate (M.S.) student, JOHN G. SPEER, and DAVID K. MATLOCK, Professors, are with the Advanced Steel Processing and Products Research Center, Colorado School of Mines, Golden, CO 80401. Contact e-mail: [email protected] Manuscript submitted January 25, 2002. METALLURGICAL AND MATERIALS TRANSACTIONS A
volume fraction of the pinning particles, and z is a grain size heterogeneity factor.[2] Changes in chemical composition (e.g. Ti or N levels), liquidus/solidus temperature, and cooling rate may influence the amount of TiN that may be precipitated in the solid and the temperatures over which precipitation occurs. Consequently, this study was designed to improve the fundamental understanding of TiN technology in steels produced via thinslab casting. Titanium nitride particle size distributions and coarsening kinetics are examined in 0.05C, 1.2Mn steels with different Ti:N ratios to characterize their response to thin-slab casting. Some processing effects on precipitate evolution are also considered. II. EXPERIMENTAL PROCEDURE Eight commercial thin-slab cast steels were obtained for examination, along with one 25.4-cm-thick, conventionally cast (thick-slab) titanium-bearing steel for comparison. Systematic variations in titanium and nitrogen concentrations were intended in the thin-slab samples, and the resulting chemical compositions are summarized in Table I. The TiN ratios are provided in Table II. In this article, each steel is identified by its number indicated in Table I. The thin slabs were direct charged into a tunnel furnace after casting and allo
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