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Thermo-mechanical controlled processing (TMCP) is employed to obtain the required level of mechanical properties of contemporary HSLA steel plates utilized for gas and oil pipeline production. The strength and crack resistance of pipeline steels are mainly determined by its microstructure and crystallographic texture. In this study, the influence of the structural and textural states of industrially produced API-5L X70-X80 pipeline steels on tensile mechanical properties was analyzed. TMCP routes with different hot rolling temperatures and cooling rates were employed. The texture of steel was assessed using the Taylor factor, which was calculated based on electron backscatter diffraction (EBSD). The decrease in rolling temperature resulted in the sharper texture characterized by {001} planes banding (cleavage planes in the bcc lattice) parallel to rolling direction. The tensile deformation behavior at the stage of necking was determined by the crystallographic and morphological texture of the material and demonstrated significant anisotropy. Rupture of all investigated samples was accompanied by the development of splitting on the fracture surface. The splitting was localized in the rolling plane similar to the splitting in standard Charpy tests of pipeline steels. © 2022 by the authors. Licensee MDPI, Basel, Switzerland. Funding: This research was funded by Russian Science Foundation, grant number 22-29-00411.

An original laboratory bench, which allows for modeling technological rates of cooling of hot-rolled sheets in the process of controlled thermomechanical processing (TMCP) has been designed. Samples cut from an industrial sheet of 06G2MB type pipe steel used in production of large diameter pipes with strength grade X80 have been used. Time dependencies of the actual temperature at initial cooling rates of 100 – 500 K / s have been obtained. All processing modes resulted in almost identical structures mainly formed due to the bainitic transformation. The dispersion of structures decreased with the increase of the cooling rate. Thermotechnical calculations were carried out assuming that the temperature equalization over the sample thickness was instant. The times of transformation amounted to 1 – 9 s. Most of the transformation at all cooling modes occurred in conditions close to the isothermal one. The athermal nature of the bainitic transformation in TMCP was recorded. Within the observed interval of cooling rates, the temperature at the starting point of the bainitic transformation was 660 – 730°C. The heat effect approximately was equal to 120 kJ / kg, which is twice larger than the thermal effect of the martensite transformation for low-carbon steels. This fact suggests that the kinetics of the bainitic transformation is largely determined by the energy of the slowest process, namely, the re-arrangement of carbon atoms in the austenite occurring parallelly to the shear transformation of FCC lattice into BCC one. It is assumed that energy contribution of the redistribution process of carbon atoms (if their amount is small) to the thermal effect of the bainitic transformation is comparable, at least, to the energy effect of the shear lattice rearrangement. © 2018, Institute for Metals Superplasticity Problems of Russian Academy of Sciences. All rights reserved. Благодарности/Acknowledgements. Работа выпол-нена на оборудовании ОАО «Аусферр». Работа выпол-нена в рамках государственного задания ФАНО России, тема «Структура» (”Structure“), номер госрегистрации 0120146333. Авторы выражают признательность за со-действие программе поддержки ведущих университетов РФ в целях повышения их конкурентоспособности № 211 Правительства РФ №02.А03.21.0006./The study was performed on the equipment from OAO “Ausferr”. The study was performed within the framework of the state assignment from the Federal Agency for Scientific Organizations of Russian Federation, theme “Structure”, registration number 0120146333. The authors express their gratitude

The kinetics of phase transformations at cooling of carbon steel in dependence on the temperature of preliminary annealing T$_{an}$ is studied. It is shown that the cooling from T$_{an}$ > A$_3$ (i. e. above the temperature of ferrite start) with the rate 90 - 100 K/s results in structural state which essentially dependent on T$_{an}$; at 750$^0$C < T$_{an}$ < 830$^0$C the transformation is of perlite type whereas at T$_{an}$ > 830$^0$C the martensitic structure arises. Our results evidence the formation of a special structural state in a certain range of temperatures near and above the boundary of two phase region which is characterized by a substantially nanoscale heterogeneity in carbon distribution, lattice distortions, and magnetic short-range order., 6 pages, 4 figures

Energetics of the fcc ($\gamma$) - bcc ($\alpha$) lattice transformation by the Bain tetragonal deformation is calculated for both magnetically ordered and paramagnetic (disordered local moment) states of iron. The first-principle computational results manifest a relevance of the magnetic order in a scenario of the $\gamma$ - $\alpha$ transition and reveal a special role of the Curie temperature of $\alpha$-Fe, $T_C$, where a character of the transformation is changed. At a cooling down to the temperatures $T < T_C$ one can expect that the transformation is developed as a lattice instability whereas for $T > T_C$ it follows a standard mechanism of creation and growth of an embryo of the new phase. It explains a closeness of $T_C$ to the temperature of start of the martensitic transformation, $M_s$., Comment: 4 pages, 3 figures, submitted in Phys. Rev. Letters

Energetics of the fcc ($��$) - bcc ($��$) lattice transformation by the Bain tetragonal deformation is calculated for both magnetically ordered and paramagnetic (disordered local moment) states of iron. The first-principle computational results manifest a relevance of the magnetic order in a scenario of the $��$ - $��$ transition and reveal a special role of the Curie temperature of $��$-Fe, $T_C$, where a character of the transformation is changed. At a cooling down to the temperatures $T < T_C$ one can expect that the transformation is developed as a lattice instability whereas for $T > T_C$ it follows a standard mechanism of creation and growth of an embryo of the new phase. It explains a closeness of $T_C$ to the temperature of start of the martensitic transformation, $M_s$., 4 pages, 3 figures, submitted in Phys. Rev. Letters