Nonisothermal Austenite Grain Growth Kinetics in a Microalloyed X80 Linepipe Steel

Abstract

Nonisothermal austenite grain growth kinetics under the influence of several combinations of Nb, Ti, and Mo containing complex precipitates has been studied in a microalloyed linepipe steel. The goal of this study is the development of a grain growth model to predict the austenite grain size in the weld heat affected zone (HAZ). Electron microscopy investigations of the as-received steel proved the presence of Ti-rich, Nb-rich, and Mo-rich precipitates. The steel has then been subjected to austenitizing heat treatments to selected peak temperatures at various heating rates that are typical for thermal cycles in the HAZ. Thermal cycles have a strong effect on the final austenite grain size. Using a mean field approach, a model is proposed for the dissolution of Nb-rich precipitates. This model has been coupled to a Zener-type austenite grain growth model in the presence of pinning particles. This coupling leads to accurate prediction of the austenite grain size along the nonisothermal heating path simulating selected thermal profiles of the HAZ.

In situ measurement and modelling of austenite grain growth in a Ti/Nb microalloyed steel

“…According to the processing route of the steel, which is composed of casting, hot rolling (in the austenite region) and coiling in the temperature range 550-600°C, it is assumed that large Nb-rich particles precipitated in austenite during hot rolling [30], whereas the fine Nb-rich particles formed in ferrite during coiling [31]. This is consistent with the crystallographic orientation of the fine Nb(CN) precipitates with the ferrite matrix, as investigated by Banerjee et al [29]. The well-known Baker-Nutting relationship [42] was found, indicating that the fine Nb-rich particles had been formed during the coiling process.…”

“…during heat treatment cycles replicating those occurring in the HAZ, which may be elusive by conventional metallography. For example, in the continuous heating studies of Banerjee et al [29] the times at peak temperatures of approximately 0.5 s before starting the quench are comparable with the heating times in austenite at the highest employed heating rate of 1000°Cs À1 .…”

“…Recently, Banerjee et al [29] investigated the effect of high heating rates (up to 1000°Cs À1 ) and various austenitizing temperatures on the non-isothermal austenite grain growth kinetics in a Nb-Ti microalloyed linepipe steel. Austenite grain sizes were measured using metallographic techniques and they also studied the particle size distribution in the as-received and continuously heated steel by transmission electron microscopy (TEM).…”

“…The well-known Baker-Nutting relationship [42] was found, indicating that the fine Nb-rich particles had been formed during the coiling process. Based on the experimental observations, a phenomenological grain growth model was proposed by coupling the grain growth kinetics with the dissolution kinetics of Nb(CN) [29].…”

“…In this paper the study of austenite grain growth kinetics in the microalloyed X-80 steel grade previously investigated by Banerjee et al [29] has been extended using the laser ultrasonic technique as the grain size measurement tool. Based on the experimentally measured limiting grain sizes at various reheat temperatures an approach is proposed to evaluate the initial precipitate distribution in the asreceived material.…”