Effect of Solute Nb on Grain Growth in Fe-30 Pct Mn Steel MADHUMANTI BHATTACHARYYA, BRIAN LANGELIER, and HATEM S. ZUROB Niobium is known to segregate strongly to grain boundaries in steel. The strong interaction of Nb with grain boundaries results in an important reduction of the grain boundary mobility and strong retardation of recrystallization and grain growth. In this study, the effect of Nb on the mobility of grain boundaries in a second-generation Fe-30 pct Mn TWIP steel is investigated. Grain growth kinetics was measured in a series of Nb-containing Fe-30 pct Mn model alloys. An estimate of the grain boundary mobility was obtained for various temperatures and niobium contents. It was found that Nb slows down the mobility of random high-angle grain boundary segments that are not twin related. The effect of solute Nb on grain growth, in the presence of annealing twins, was modeled using Cahn’s theory of solute drag coupled with a recently formulated twin-inhibited grain growth model. The results are shown to be in very good agreement with the experimentally determined growth kinetics. https://doi.org/10.1007/s11661-019-05273-2 Ó The Minerals, Metals & Materials Society and ASM International 2019 I. INTRODUCTION AUSTENITE grain growth control in commercial low Mn steels has been primarily achieved through small addition ( < 0.1 pct) of microalloying elements (Nb, V and Ti). [1–8] Of all the microalloying elements, Nb is found to be the most effective at slowing down the kinetics of recovery, [9–11] recrystallization [10,12–14] and the c fi a transformation. [15–17] A good amount of work has been done to understand the solute drag effect of Nb during the recrystallization process in low carbon steel. Although the effect of Nb solute drag and Zener pinning on impeding high-angle grain boundary (HAGB) migra- tion during grain growth has been captured by Fu et al., [18] pure Nb solute drag on austenite boundaries has not been experimentally quantified to date. There are some studies performed on austenite growth in commercial low carbon–low Mn steels in the presence of Nb. [19–25] Apart from all these, the effect of Nb solute drag on austenite grain boundaries in high manganese steels has never been explored. It is well known that Nb prefers to segregate to HAGBs and impedes boundary migration. [26,27] This is why understanding the effect of solute Nb on grain growth is of paramount importance pertaining to the microstructure–property relationship. High Mn TWIP steels are an exciting class of steels that can achieve excellent combinations of strength and ductility. Development of high Mn TWIP steels by thermo-mechanically controlled rolling (TMCR) inevi- tably requires an in-depth understanding of microstruc- ture evolution during various metallurgical phenomena. [28–33] This also emphasizes the importance of understanding the grain growth process, which directly tailors the microstructure. Bhattacharyya et al. [34] reported that the unusually slow grain growth kinetics in high Mn steels is attributed to the presence of a high fraction of special boundaries in the form of annealing twins in the microstructure. In a subsequent study by the same author, it was reported that the intersection of annealing twins with random HAGBs creates low energy–low mobility segments, which are the rate controlling factors during grain growth in Fe-30Mn steel. [35] In addition to all these, the effect of solute Nb on random HAGBs during grain growth is the main focus of this study. In this work, the effect of Nb on growth kinetics in a series of Fe-30Mn steels was explored. The solute drag was quantified from the segregation profile of Nb obtained across the austenite HAGB of interest using atom probe tomography (APT). Grain boundary mobil- ity was estimated for various temperatures and niobium contents. An attempt was made to calculate the grain boundary mobility in the presence of niobium using Cahn’s solute drag model. For this calculated mobility, when used in the proposed ‘twin inhibited grain growth’ model, [35] the predicted growth kinetics showed a very good fit with the experimentally obtained growth MADHUMANTI BHATTACHARYYA, BRIAN LANGELIER, and HATEM S. ZUROB are with the Department of Materials Science and Engineering, McMaster University, 1280 Main St W., Hamilton, ON L8S 4L7, Canada. Contact e-mail: bhattam@mcmaster.ca Manuscript submitted May 22, 2018. METALLURGICAL AND MATERIALS TRANSACTIONS A