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The study of influence of different heat treatment procedures on microstructure and mechanical properties of the new high-strength weldable cold-resistant steel 20G2SMRA (20Г2СМРА) is conducted. Morphology of martensite after steel quenching, as well after quenching and consequent tempering, is examined using optical metallography and electron microscopy. Regularities of varying of microstructure parameters, hardness values (HV1) and impact strength KCV were revealed in this work at the temperature –60 °С depending on heating temperature for quenching and consequent tempering. Amount of residual austenite after quenching starting from the temperatures 850, 950 and 1000 °C was determined using X-ray structural phase analysis. The rational procedure of heat treatment was established as a result of the study; it provides combination of guaranteed yield strength σ0,2 600 N/mm2 together with low-temperature impact strength KCV–60 50 J/sm2 and ductility δ5 17%. The obtained results of investigation are aimed for commercial putting into practice in the conditions of heavy plate production at 5000 rolling mill of Magnitogorsk Iron and Steel Works (MMK). 1 Nosov Magnitogorsk State Technical University (Magnitogorsk, Russia) E-mail: allakuznetsova.mgtu@mail.ru; o.nikitenko@magtu.ru © POLETSKOV P. P., KUZNETSOVA A. S., NIKITENKO O. A., ALEKSEEV D. YU., 2020 Introduction Taking into account exploration of hardly accessible northern and Arctic regions, necessity in high-strength coldresistance weldable steels rises. Such steels are aimed to be used for modernization of equipment for mining and building industries. These industries are characterized by high level of requirements in the areas of decrease of metal consumption and manufacturing expenses, as well as of increase of lifting capacity, reliability and service life of corresponding equipment. Respectively, the complex properties are required for mechanical parameters of materials used for fabrication of welding elements of a frame construction. The following properties can be mentioned here: high strength (600– 1100 N/mm2) with reservation of sufficient level of tough and ductile parameters at low temperatures (KCV–60 50 J/sm2) as well as weldability (Ceq 0.53) [1–6]. The up-to-date concept of development of the new high-strength cold-resistant steels with preset complex of properties includes microalloying of steels in combination with varying heat treatment procedures [7–11]. The world and national practice testifies that heavy plates of high-strength weldable steels with tensile strength more than 600 MPa combined with low-temperature toughness are supplied mainly in thermally improved state [12–16]. Elimination of any restrictions for dimension range for thickness more than 20 mm with minimal steel alloying degree [12] is the main advantage of quenching and high tempering technology in comparison with thermomechanical treatment, in particular for mastering of new steel rolled products with thickness 8–50 mm. At the same time optimal combination of strength and toughness can be provided in the case when steel hardness after quenching will exceed minimal allowable values, what is possible if martensite is presented in the amount not less than 90% in the structure of quenched steel [14, 17–21]. It should be mentioned that tempering temperature choice should provide reaching of the required low-temperature impact strength in combination with satisfactory parameters of strength and ductility [20]. Thereby, the aim of this work can be formulated as study of influence of heat treatment procedures (quenching and tempering) on structure and properties of the new high-strength steel with increased cold resistance. Materials and methods of investigation The samples of the newly developed steel of 20G2SMRA (20Г2СМРА) grade with chemical composition presented in the Table 1 were the material for this research [22]. Table 1. Chemical composition of the new steel 20G2SMRA Steel grade Content of the chemical element, mass. %, not more C Si Mn S P Cr+Cu Ni+Mo Al Ti B Ceq* 20G2SMRA 0.20 0.55 1.60 0.005 0.012 0.16 0.35 0.050 0.005 0.0050 0.53 * Calculation of the values of carbon equivalents (Ceq) was conducted via the formula: Ceq = = C + Mn/6 + (Cr + Mo + V)/5 + (Cu + Ni)/15; where C, Mn, Cu, Cr, Ni, Mo, V are mass parts of chemical composition elements. Metal science and metallography CIS Iron and Steel Review — Vol. 20 (2020), pp. 50–54 51 Melting, rolling and consequent heat treatment of experimental sheet samples with thickness 15 mm were conducted on the base of the Scientific and production complex “Engineering center Termodeform-MGTU” and the Center of collective usage of the Scientific and research institute “Nanosteels” at Nosov Magnitogorsk State Technical University. The critical points were determined using the method of differential scanning calorimetry (DSC) on the sensor of synchronous thermal analysis (STA) Jupiter 449 F3 (NETSCH) with speed 10 °С/min up to the temperature 1000 °С in argon medium with purity 99.998%, as well as using complex dilatometric analysis with assistance of the research complex Gleeble 3500 [12]. The temperature procedures of quenching and tempering were varied in the following ranges: quenching temperature was consequently increased from Ас3 + 30 °С to Тmах = 1000 °С with interval 50 °С (holding at quenching temperature 2.5 min/mm); cooling after quenching was conducted in water; tempering temperature was varied from 200 to 600 °С with interval 100 °С (duration of holding is 2.5 min/mm + 25 min). After heat treatment (quenching, quenching and tempering) the samples for mechanical testing and metallographic researches were made. To provide microanalysis, the polished sections were manufactured from the samples via the standard technique; it was provided via pressing of samples in “Transoptic” tar at automatic press Simplimet 1000 within the sample preparation line of Buеchler company. To reveal microstructure, the surface of polished sections was subjected to pickling in 4% solution of nitric acid in ethyl alcohol via dipping of polished surface in the bath with the chemical agent. Qualitative and quantitative analysis was conducted using light microscope Axio Observer (Zeiss) with magnification 50–1000 times, with assistance of software for processing of metallographic images Thixomet PRO as well as scanning electron microscope (SEM) JSM 6490 LV. To reveal the boundaries of the former austenite grain, pickling of polished sections in freshly prepared solution of picric acid was used, while (1–10)% of surface active substances were added in this solution and the chemical agent was heated to (50–70) °C [23]. Average value of austenite grain was determined manually, by linear measurements using the programs with built-in techniques adapted in correspondence with requirements of GOST 8233–56 and GOST 5639–82 standards. Pattern cutting and surface processing for extension, hardness and impact bending tests were conducted in correspondence with the requirements of GOST 7564–97 standard. The number of samples for each type of testing was varied from 3 to 6 and then the average value was calculated. Impact bending tests were conducted at the impact testing machine RKP 450 in correspondence with the requirements of GOST 9454–78 standard on transversal samples with size 10×10 mm, having V-shaped notch, after quenching and consequent tempering separately. The tested samples were cooled down to –60 °С in stabilizing cooling bed LAUDA master PL1. Hardness was measured via Vickers method in correspondence with the requirements of GOST 9450–60 standard: by indentation of a diamond pyramid having the angle 136° between the opposite planes, using hardness tester Buehler Micromet with 1 kg load. Qualitative and quantitative phase analysis of residual austenite was conducted via X-ray structural method based on comparison of lines intensity in determining phases, using X-ray diffractometer of Shimadzu company, the model XRD-7000. Results of investigation Based on the results of differential scanning calorimetry (DSC) and dilatometric analysis for the new steel grade 20G2SMRA, the values of critical points were determined (Table 2) [22]. The results of quenching temperature influence on steel structure and properties It was established on the base of optical microscopy, that the examined steel 20G2SMRA has the structure of massive (lath) martensite in all cases after quenching from temperatures from 850 °С до 1000 °С; this structure is formed via shifting diffusion-free mechanism of transversal slipping (Fig, 1, a–b). Rise of austenitization temperature leads to increase of the average size of martensite massifs and lathes, heterogeneity in their dimensional distribution is strengthening as well. The analysis conducted using SEM (Fig. 1, c, d) displayed that lath martensite (Ml) with volumetric part exceeding 70% is a dominant morphological component in the structure of α-phase of 20G2SMRA steel. Square of martensite massifs increases from 20 to 500 μm2 with rising of the heating temperature for quenching. Width of single lathes consisting a massif varies within the range from 150 to 300 nm. Total amount of lathes in a massif varies from 4–6 to 12–16. Additionally, there are single plates that can be met in the structure, with their transversal size several times exceeding width of single lathes in massive martensite and reaching 1–3 μm (see Fig. 1, d). Length of these plates makes appr. 30 μm. Such morphology can be identified as plate martensite (Mp), and its presence stipulates lowering of stresses amplitude and, consequently, decrease of the risk of quenching cracks formation. The results of researches on regularity of varying of steel microstructure parameters (real size of former ausTable 2. The measurement results for steel critical points Steel grade Examination method Temperature of critical points t, °C

The study of influence of heat treatment procedures on structure and properties of the new high-strength steel with increased cold resistance