Characterization of Ti carbosulfide precipitation in Ti microalloyed steels
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I.
INTRODUCTION
IT has
long been recognized that a variety of important properties of strip and plate steel products, such as cold formability and toughness anisotropy, can be drastically improved by Ti addition, tl] The striking effect of Ti on steel properties has been attributed to the formation of Ti carbosulfide (Ti4C2S2), which remains undeformed during hot rolling due to its high hardness at elevated temperatures. Since Ti4CzS 2 is more stable than MnS in iron-based alloys, I11the formation of MnS, which is usually deformed into undesirably long stringers during hot rolling, is restrained by the addition of Ti to the steel. Furthermore, in recent investigations carried out in Japan, |2,3] it has been shown that the formation of Ti4C2S: plays an important role in improving the deep drawability of interstitial-free (IF) steels, although the mechanisms responsible for this effect have not yet been identified. However, in spite of the importance of this phase in steel products, little information has been published to date on its solubility and precipitation behavior in the presence of Mn. TiqC2S 2 w a s first reported by Kudielka and Rohde ~41 to possess a hexagonal structure of space group D~h-P63/ mmc with lattice parameters a = 3.21 A and c = l l . 2 0 A . Later investigations of the carbosulfide phase found in a number of steel grades I5 8] and in the nickel-base superalloys f7,91 led to similar values for these quantities. The excellent agreement regarding the lattice parameters of Ti4C2S2 found in a wide range of alloy systems suggests that the limits of composition of this phase are not wide and that Ti4C2S 2 forms as a stoichiometric or near-stoichiometric compound rather than as a compound with variable stoichiometry of the type
W.J. LIU, Research Associate, formerly with the Department of Metallurgical Engineering, McGill University, is with the Centre for Metallurgical Process Engineering, the University of British Columbia, Vancouver, BC, Canada V6T lW5. S. YUE, Research Associate, and J.J. JONAS, CSIRA/NSERC Professor of Steel Processing, are with the Department of Metallurgical Engineering, McGill University, Montreal, Quebec, Canada H3A 2A7. Manuscript submitted November 14, 1988. METALLURGICAL TRANSACTIONS A
TifySl_y.
In practice, Ti4C2S2 is often confused with Ti2S , because the diffraction patterns of the two phases are nearly identical.[Sl However, in a recent paper, lSj Ball has presented evidence that the phase initially identified as Ti2S is, in fact, Ti carbosulfide, and that Ti2S does not exist in steels. Although the above controversy could be clarified by direct composition measurements, most previous investigations were carried out on extracted particles, which generally contain TiN, TiC, and other precipitates, leading to some uncertainty in the analyses. Additionally, it is difficult to detect carbon by microanalysis, so that the particle composition is frequently incompletely known. In comparison with the knowledge of the crystal structure, still less attention has been paid
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