PSI - Issue 40

S.V. Danilov et al. / Procedia Structural Integrity 40 (2022) 112–117 S.V. Danilov at al. / Structural Integrity Procedia 00 (2022) 000 – 000

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2. Materials and methods The study was conducted on the specimens of 06Mn2MoNb low-carbon, low-alloy pipe steel (~ 0.05 wt. % C, ≤ 2.0 wt.% Mn, ~ 0.2 wt.% Mo, ~ 0.05 wt.% Nb, balance Fe and unavoidable impurities) designed for production of X70, X80 large diameter pipes. The specimens corresponded to the thickness of the plates 26 – 27 mm – that had been processed at 5000 Mill according to three modes with different finish-rolling temperatures (I – 920 °С, II – 844 °С, III – 760 °С). The specimens for mechanical testing were cut out transversely to the rolling direction (RD) from the central areas of the plates, 11 – 13 mm from their surfaces. Standardized fivefold cylindrical specimens with 5 mm reduced section were tested according to ASTM E8/E8M-21. Tensile testing was carried out on Instron 3382 universal testing machine at 5 mm/s test speed at room temperature. Stain curves were analyzed according to the method described in Erpalov and Khotinov (2020). Metallographic samples were prepared across the thickness of each specimen. Sample surface preparation for EBSD (see Carneiro and Simões (2020)) was carried out on a Struers LaboPol-5 grinding and polishing machine with a Struers LaboForce-1 device for a semi-automatic preparation of 1 – 3 samples. An electrolytic polishing of the samples in a solution of 15 % НClO 4 (perchloric acid), 85 % СН 3 СООН ( acetic acid) was preformed after the grinding at 21 V voltage. Electron microscopy study of the structure was carried out on a Tescan Mira 3 microscope with an auto-emission cathode with an accelerating voltage of 20 kV. EBSD HKL Inca system with Oxford Instruments analyzer was used to determine orientations of individual grains (crystallites). Step size was 0.1 µm. Orientation estimation inaccuracy did not exceed ± 1° (± 0.6° on average). A coordinate system (X, Y, Z) with its axes coherent with the rollin g direction (X║RD), the normal to the rolling plane (Y║ND) and with the direction perpendicular to them (Z║TD) was used in both the structural studies and the textural analysis. Z axis also coincided with the normal to the surface of the metallographic samples. The three chosen directions formed the vector right-handed triplet. 3. Results and discussion Metallographic study of the specimens by means of scanning electron microscopy (Fig. 1) revealed that the microstructure of the specimens processed according to mode I (Fig. 1, a) predominantly consisted of grains with non-equilibrium shapes, while microstructures after mode II and III (Fig. 1, b, c) were represented by different-sized grains (presumably bainite) extended in the rolling direction. These grains were more disperse than those of mode I and had dissected boundaries. Some of the grains were consolidated into areas with smoothed boundaries. These areas extended in the rolling direction and evidently represented the austenite grains that had been deformed during the hot rolling. Specimen texture (Fig. 2) determined by means of EBSD was comprised by a range of dispersed, and therefore overlapping, components: {001}<110>, {113}<110>, {112}<110> {111}<110>, {111}<112>, {221}<012>, {223}. This coincided with the TMCP structure presented in Lobanov et al. (2019) or Yang et al. (2014) or Lobanov et al. (2019) or Hutchinson et al. (1998). {001}<110> orientation intensified considerably in the direction from the surface towards the center. Some mechanical properties (YS, UTS, elongation) of all specimens after mode I were significantly different from those after mode II and III (Table 1) in terms of their deformation behavior. Higher strength and somewhat lower ductility properties of the specimens II and III were associated with their microstructure being significantly more disperse, which had been caused by TMCP parameters (finish hot rolling temperature). Specimen deformation behavior at the moment of time preceding to fracture appears to be of higher significance. Specifically, the cross-sections of all specimen reduced sections obtained ellipsoid shape, while larger ellipse axis lied almost precisely on the rolling plane (Fig. 3, a – c). Cross-sections of the higher strength specimen reduced sections (after modes II and III) obtained significantly more pronounced ellipsoid shapes. It should be noted that the fracture of all specimens occurred at relatively similar loads (BS, Table 1).