Microalloying Precipitation during Hot Rolling of Seamless Tubes in a Continuous Mandrel Mill Ricardo N. Carvalho,* Marcelo A. C. Ferreira, Marília M. Lima, Dagoberto B. Santos, and Ronaldo Barbosa Conventional controlled rolling is widely used in a hot strip mill and requires the occurrence of strain-induced precipitation before that any recrystallization process takes place. On the other hand, microalloying elements dissolved in the austenite can have a strong effect on hardenability. These aspects show how important is the knowledge on the precipitation evolution and the remaining thermodynamic driving force in each step of hot rolling. This is particularly true for seamless tube production that have some specific characteristics such as: longer transfer times between roughing and finishing, the possibility of promoting one transformation cycle before finishing, lower deformation at finishing steps, no accelerated cooling, etc. In this work, hot torsion simulation was applied on a real case of seamless tube production and the samples were investigated by transmission electron microscopy. The particles size distributions were accessed by high angle annular dark field, and typical precipitates were characterized by EDX, PEELS, and EFTEM. Thermo-Calc software was used to evaluate the precipitation and the amounts of microalloying elements that remains dissolved in austenite. 1. . Introduction At the end of last century, an intensive research effort was dedicated to the development of thermomechanical processing. Conventional controlled rolling, nowadays widely employed in a hot strip mill, hot strip mill (HSM), requires the occurrence of strain-induced precipitation before that any recrystallization process takes place. [1] On the other hand, microalloying elements dissolved in the austenite can have a strong effect on hardenability [2] and techniques such as high-temperature processing (HTP), were developed applying this alloying concept in the production of plates and strips for welded pipes. [3] Both interphase precipitation and general precipitation in the ferrite are also important strengthening mecha- nisms for steels. [4–13] Recent comprehensive reviews on thermomechanical processing and microalloyed steels can be found in the literature. [14,15] Just a small part of this effort was dedicated to long products such as hot rolling of seamless tubes. [16,17] Alloy and process design for this kind of process has to consider some key features. The first one is the presence of intermediate reheating before finishing, that increases the transfer time between roughing and finishing steps to about 20 times higher than those observed in a HSM. Other key feature is the low flexibility concerning deformation schedules that implies in low deformations at finishing steps, especially for large sizes which has the final diameter close to that one after roughing step. Besides, after finishing steps, tube mills do not count with acceler- ated cooling to decrease the transformation temperature and other operations such as coiling that are important to promote fine precipitation in the ferrite. All those differences make more difficult the use of rolling techniques such as conventional controlled rolling in tube mills. [6,18] Considering those limitations, this work has as goal to investigate the microalloying precipitation along the hot rolling of seamless tube. Samples were obtained by hot torsion simulation of a real case of seamless tube production were quenched after selected steps and prepared for TEM investiga- tion. The particles size distributions were accessed by high angle annular dark field, and typical precipitates were characterized by energy dispersive X-ray spectroscopy (EDX), parallel electron energy loss spectroscopy (PEELS), and rnergy filtering trans- mission electron microscopy (EFTEM). The expected precip- itates and the amount of microalloying elements dissolved in austenite matrix were assessed by ThermoCalc software. 2. Experimental Section The steel used in this work is presented in Table 1. It is a V-N steel, typically used for long products and, in the case of seamless tubes, intended to mechanical and trailer axles markets. As these applications require a combination of high strength, high toughness, and good weldability, the grain refinement and precipitation are the main hardening mechanisms. The higher contents of V and N present in this steel aims to improve the Dr. R. N. Carvalho, M. A. C. Ferreira, Dr. M. M. Lima Vallourec Research Center Belo Horizonte  Vallourec SoluS c ~ oes Tubulares do Brasil Av. Olinto Meireles, 65, 30640-010, Belo Horizonte, MG, Brazil E-mail: ricardo.nolasco@vallourec.com Dr. D. B. Santos, Dr. R. Barbosa Department of Metallurgical and Materials Engineering  Federal University of Minas Gerais Av. Antonio Carlos, 6627, 31270-901, Belo Horizonte, MG, Brazil The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/srin.201800330. DOI: 10.1002/srin.201800330 Microalloying Precipitation www.steel-research.de FULL PAPER steel research int. 2019, 90, 1800330 © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1800330 (1 of 10)