Prediction of Inhomogeneous Distribution of Microalloy Precipitates in Continuous-Cast High-Strength, Low-Alloy Steel Sl
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INTRODUCTION
CARBIDE, nitride, or carbonitride precipitates formed by the microalloying elements such as, Nb, Ti, and V provide grain refinement and precipitation strengthening in high-strength, low-alloy (HSLA) steel.[1] Microalloy precipitates in continuous-cast slab can influence the microstructural changes taking place during subsequent processing such as reheating and rolling and, hence, need to be studied. Industrial reheating of HSLA steel is aimed at dissolving the Nb precipitates to encourage the fine-scale, strain-induced Nb(C,N) precipitation during hot rolling.[1,2] Pinning of austenite (c) grain boundaries by the microalloy precipitates also prevents the excessive c grain growth during soaking.[1,3] The choice of reheating temperature and time, therefore, should be based on the characterization of as-cast precipitates.[1,3,4]
SUPARNA ROY, Ph.D. Student, SUDIPTA PATRA, M.S. Student, and DEBALAY 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] S. NEOGY and A. LAIK, Scientists, are with the Materials Science Division, Bhabha Atomic Research Centre, Mumbai 400 085, India. S.K. CHOUDHARY, Researcher, is with the Research and Development, Tata Steel, Jamshedpur 831 007, Jharkhand, India. Manuscript submitted May 26, 2011. Article published online January 10, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
The nature, shape, and size of the microalloy precipitates have been widely investigated in as-cast slab as well as in thermomechanically controlled rolled (TMCR) HSLA steel plates/strips.[4–20] Both macroand microsegregation during casting may result in an inhomogeneous distribution of the precipitates.[7–11,16–20] The formation of large (>1 lm) Nb-rich dendritic precipitates[7,8,16,17] or eutectic (Nb,Ti,V)(C,N) particles[11] in the interdendritic boundaries indicated the segregation of microalloying elements, especially Nb. A higher volume fraction of Nb precipitates in pearlitic regions, which coincided with the interdendritic regions, compared with the ferritic regions, which coincided with the dendrite-center regions of TMCR steels containing 0.023 to 0.057 wt pct Nb, also has been attributed to interdendritic segregation.[18] Clustering of coarse microalloy precipitates (such as dendritic (Nb,Ti)(C,N) and TiN)) in the interdendritic region of as-cast slab can lead to slab-surface cracking during continuous casting.[9,21,22] Hence, prediction and control over precipitate size and spatial distribution of precipitates is crucial for maintaining the cast-slab quality. The effect of segregation on the stability of microalloy precipitates and on the precipitate size distributions at different regions (solute-rich and solute-depleted) of as-cast slab is not well understood. Hence, a model has been proposed here, based on the detailed characterization of cast microalloy precipitates, for predicting the spatial distribution in size and
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