Development of Bimodal Grain Structures in Nb-Containing High-Strength Low-Alloy Steels during Slab Reheating
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INTRODUCTION
CONTINUOUSLY-CAST high-strength low-alloy (HSLA) steels containing microalloying elements such as Ti, Nb, and V are usually reheated prior to thermomechanical controlled rolling (TMCR). Grain growth can occur during slab reheating with the driving force being primarily the tendency to reduce the material’s overall grain-boundary area. Grain growth can occur in two modes: normal grain growth and abnormal grain growth. Abnormal grain growth is characterized by the rapid increase of the largest grains at a rate much faster than that of normal grain growth.[1] Microalloying additions in HSLA steels are used to constrain grain growth by inhibiting austenite grain growth during reheating and retarding both static and dynamic recrystallization of austenite during TMCR by the fine microalloy carbide and nitride precipitates.[2] This results in a fine uniform ferrite grain-size distribution and ensures superior properties.[3–5] The most effective microalloying element to achieve grain refinement has been shown to be Nb, but it DEBALAY CHAKRABARTI, Researcher, is with the Department of Steel Metallurgy, Swinden Technology Centre, Corus Steel (Research Development and Technology), Rotherham, South Youkshire, S60 3AR United Kingdom. Contact e-mail: c.debalay@yahoo. com CLAIRE DAVIS, Reader, and MARTIN STRANGWOOD, Senior Lecturer, are with the Department of Metallurgy and Materials, The University of Birmingham, Edgbaston, Birmingham, B15 2TT United Kingdom. Manuscript submitted September 23, 2007. Article published online May 15, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A
tends to partition to interdendritic regions during solidification (microsegregation) along with carbon and nitrogen,[6,7] which can lead to inhomogeneous precipitation of C,N-rich particles on solidification.[8] Two types of segregation can be found in as-cast structures: macrosegregation, i.e., composition changes over distances comparable to the size of the casting; and microsegregation that occurs on the scale of the secondary dendritic arm spacing (SDAS).[1] The resulting inhomogeneous precipitate distribution can lead to a variation in local grain boundary (Zener) pinning,[9] as well as changing the degree and rate of particle dissolution during reheating. Localized partial dissolution and the inhomogeneous distribution of microalloy-rich precipitates may lead to abnormal or inhomogeneous grain growth during reheating.[8,10–12] During rolling, the combination of inhomogeneous prior austenite grain size and particle distributions[13,14] will affect the local recovery and recrystallization processes and, hence, the final ferrite grain size. Thus, an initial variation in prior austenite grain size can result in an inhomogeneous ferrite grain size distribution upon rolling. In recent years, a variation in grain size, particularly a bimodal ferrite grain size distribution, has been observed in some TMCR Nb- and Nb-V-containing microalloyed steel plates, with abnormally large ferrite grains present in the matrix of smaller grains, in a typically-ban
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