Austenite Grain Structures in Ti- and Nb-Containing High-Strength Low-Alloy Steel During Slab Reheating
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CONTINUOUSLY cast high-strength low-alloy (HSLA) steel containing microalloying elements such as Ti, Nb, and V is usually reheated prior to thermomechanical controlled rolling (TMCR). For the success of TMCR, the reheating temperature should be sufficient to dissolve the microalloying precipitates, but it should not be too high to cause the excessive austenite (c) grain growth.[1,2] Grain growth can occur either by normal grain growth or by abnormal grain growth, which is characterized by the rapid increase of larger grains at a rate much faster than the normal grain growth.[1,3] In spite of the pinning force exerted by the microalloy precipitates on c-grain boundaries,[1] abnormal c-grain growth can still occur in microalloyed steels, accompanied by the coarsening and dissolution of microalloy precipitates.[4–6] Wide variation in prior c-grain size can result in an inhomogeneous (even bimodal) ferrite grain size distribution in TMCR rolled steel,[7–9] which may hamper the cleavage fracture toughness of the rolled plate.[10–12] It has been shown that interdendritic segregation during solidification results in an inhomogeneous distribution of Nb precipitates in continuous-cast microalloyed steel slabs (0.1C, 0.02 to 0.05Nb, 0.001 to 0.009Ti; all wt pct).[7,13] Spatial distribution of precipitates can lead to a variation in local grain boundary (Zener) pinning[7,14–17] as well as changing the degree of precipitate dissolution during reheating.[7] As a result, at
S. ROY, Postdoctoral Student, and D. CHAKRABARTI, Assistant Professor, are with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology (I.I.T.), Kharagpur, 721 302 West Bengal, India. Contact e-mail: [email protected] G.K. DEY, Head, is with the Materials Science Division, Bhabha Atomic Research Centre (BARC), Mumbai 400 085, India. Manuscript submitted November 24, 2011. METALLURGICAL AND MATERIALS TRANSACTIONS A
certain reheating temperatures, bimodal grain structures develop, with the degree of bimodality depending on the steel composition and exact reheating temperature.[5–7] In order to reduce the c-grain size variation and the associated bimodality during reheating, an increase in Ti level was recommended by the earlier studies.[18–21] The first objective of the present article is to verify the beneficial effect of Ti addition in reducing the c-grain size variation during the reheating treatment of microalloyed steels. Hence, c-grain growth was investigated here in a couple of microalloyed steels containing Ti and Nb + Ti + V. The second objective of this study is to propose a model for the prediction of c-grain size variation by considering different metallurgical aspects responsible for such variation, such as microsegregation during casting, coarsening, and dissolution of microalloy precipitates and c-grain growth during reheating. Such a model may help in selecting the appropriate reheating temperature for achieving uniform c-grain structure after the reheating treatment of a continuous cast slab, wh
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