Precipitation behavior in ultra-low-carbon steels containing titanium and niobium
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TRODUCTION
IT has been assumed by many previous investigators of stabilized ultra-low-carbon (ULC) steels, or interstitial-free (IF) steels (containing C ' 0.003 wt pct),[1–6] that these steels can be considered as dilute microalloyed (highstrength low-alloy (HSLA)) steels (C ' 0.04 to 0.10 wt pct). In other words, the precipitation behaviors in these two types of steel were thought to be similar, if not identical, except for the amount of precipitates formed. While this observation is probably true for some ULC steels,[6,7,8] recent work[8–12] has shown that other precipitation is possible, depending on subtle differences in steel compositions. For example, in Ti and/or Nb microalloyed steels of typical S, C, and N contents (atomic ratio S:C:N ' 1:10:10), the major C-bearing precipitates are free-standing carbonitride (MCN) particles that are formed by the classic nucleation and growth process. On the other hand, in Ti-only ULC steels with atomic ratio S:C:N ' 1:1:1, particles of TiS and Ti4C2S2, or H phase (for its hexagonal crystal structure), were found.[8–19] The precipitation sequence has been speculated to be TiN, TiS, Ti4C2S2, and TiC, as the temperature decreases. However, this earlier work overlooked the mechanism of the formation of H phase and its importance to the stabilization of carbon. Two relevant observations were previously reported: (1) the Ti/S ratio changed from two to one within the same particle,[13] and (2) particles of TiS were found to be attached to particles of Ti4C2S2.[15] The compound Ti4C2S2 is the only reported ternary phase in the Ti-S-C system.[19,20,21] M. HUA, Assistant Research Professor, Department of Materials Science and Engineering, and Senior Research Scientist, Basic Metals Processing Research Institute, C.I. GARCIA, Associate Research Professor, Department of Materials Science and Engineering, and Associate Director, Basic Metals Processing Research Institute, and A.J. DeARDO, William Kepler Whiteford Professor, Department of Materials Science and Engineering, and Director, Basic Metals Processing Research Institute, are with the University of Pittsburgh, Pittsburgh, PA 15261. Manuscript submitted January 2, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
It has an AlCCr2 type structure, space group P63/mmc, and lattice parameters a 5 0.3210 and c 5 1.120 nm.[20] It was reported as inclusions, up to 20 mm in size, in many other types of steels and alloys.[22,23] The crystal structure of TiS in ULC steels has been described as 9R-Ti12xS (R rhombohedral, nine-lattice layers in a unit cell), with (1 2 x) 5 Ti/S ' 0.9, space group R3m, and lattice parameters a 5 0.34 and c 5 2.65 nm.[18] The MCN phase in HSLA steels is commonly reported as Fm3m with a 5 0.42 to 0.44 nm.[19] To summarize, the stabilization of carbon in ULC steel may be achieved through the formation of carbosulfides (H) and/or carbides (MC12x). The purpose of this work is to clarify (1) the mechanism by which the H phase forms and (2) the dependence of the carbon stabilization mechanism upon the composition
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