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Formation of the microstructure and properties of pipe steel grade K60 (X70) are studied using physical modeling of thermomechanical processing in a DUO 300 laboratory mill and industrial experiment. Steel microstructure is investigated and temperature ranges for formation of various phases and structural components (ferrite, bainite, martensite, M/A-component) with accelerated cooling are determined. It is found that formation of martensitic regions and M/A-component as secondary structural constituents in a ferrite matrix instead of cementite-containing secondary structures facilitates an increase in steel strength and improvement of the σy /σf ratio without a significant reduction in cold resistance. The chemical composition of steel grade K60 (X70) is developed on the basis of laboratory and industrial experiments, as well as a new industrial process for producing rolled product for pipes for the Sila Sibiri gas pipeline sections crossing active tectonic disturbance zones. Sheet and pipes have a low σy /σf ratio, high values of relative and uniform elongation, impact strength and proportion of ductile component in a fracture after DWTT testing at – 20°C.

Results are provided for a study of the effect of different microalloying systems (Nb, Nb–V, V–N) on structure formation and mechanical properties of rolled product of strength class K52 produced by combined technology of melting, casting of thin slabs 90 mm thick, and direct rolling with subsequent accelerated cooling and winding on a coil. It is shown that all of the micro-alloying versions selected provide the required strength properties, while metal microalloyed solely with niobium demonstrates the best brittle fracture resistance at low temperature. It is shown that addition of niobium to steel not only refines grain size in the metal matrix, but also changes ferrite morphology increasing the proportion of acicular ferrite, which improves steel cold resistance.

Ferritic steels are developed economically alloyed with nickel (up to 7%) with increased strength and low-temperature toughness for the manufacture of liquefied natural gas (LNG) storage tanks. Results are provided for a study of the steel mechanical properties, cold resistance, and microstructure. It is shown that the achievement of a high level of cold resistance with a reduction in nickel content is possible by means of additional alloying with copper, microalloying with carbide-forming elements, and a change in heat treatment regime.

We consider domestic and foreign standards for rolled metals used in bridge building. Domestic standards contain elevated requirements to the reliability of rolled metals in terms of their fracture resistance by delamination. Foreign standards admit the application of rolled metals in broader strength ranges but do not contain obligatory requirements imposed on Z-properties and the necessity of monitoring of possible delaminations. Due to the obligatory control of reliability, bridge steels of domestic production are also extensively used in the construction of special building structures.

A study was conducted concerning the kinetics of phase transformations and structure of grade K60 (X70) pipe steels alloyed with small amounts of chromium (K60Cr) and molybdenum (K60Mo). It was shown that K60Mo steel differs by a higher structural content of M/A components. The laboratory tests and microstructure studies of the obtained samples by color etching, TEM, and XRD methods have shown that by using the improved cooling conditions after thermomechanical rolling of K60Cr steel, it becomes possible to obtain samples having different matrix microstructure (polygonal, quasi-polygonal, or bainitic ferrite) and various secondary phases (M/A or cementite), which yields different mechanical properties ranging from 467 to 633 MPa (yield point – σy ); 681 to 804 MPa (ultimate strength – σt ); 22 to 31.8% (percent elongation – δe); 8.8 to 17.4% (proportional elongation – δp); and 0.68 to 0.88 (σy /σt ratio). It was established that the presence of M/A component instead of cementite in the structure contributes to an increase in strength and decrease in σy /σt ratio without a considerable decline in cold resistance. At JSC “Vyksa Metallurgical Plant” (VMZ), K60Cr steel, which is more economical compared to K60Mo, and newly developed technology were used to produce commercial batches of rolled products and large diameter pipes, which demonstrated the specified σy /σt ratio and higher impact toughness and cold resistance values during a drop weight tear test (DWTT).

Results are given for evaluation of the effect of longitudinal deformation on the change of the mechanical properties and steel properties with a two-phase pipe metal structure previously given low-temperature heat treatment simulating application of an anticorrosion coating. A reduction in steel deformation capacity to the minimum permissible level is evaluated by a criterion δun ≥ 3% and σy(Rt0.5)/σu ≤ 0.96–0.98. As a result of testing plates subjected to longitudinal tension with plastic deformation ep = 1.5–7.0%, it is shown that exhaustion of steel deformation capacity is predicted in the transverse direction with preliminary deformation ep ≈ 5%, and in the longitudinal direction with ep ≈ 4–5%. SEM and TEM methods are used to study the effect of steel main structural characteristics whose presence facilitates retention of capacity for steel strain hardening after heating (200–220°C, 5 min): fine islands of twinned martensite, fine cementite particles and abundant precipitation of nanosize particles of Nb and V carbonitrides. During branch pipe bending deformation (ep ≤ 3.0%) in a two-phase steel structure (main components are ferrite and lath martensite) includes transformation of residual austenite from the MA-component in twinned martensite.

Results are given for an experimental study of steel manufacturing technology satisfying specifications for strength class not less than K60 for connecting pieces (fittings). Laboratory melts, rolling, and heat treatment by different regimes are performed for low-alloy steels of four composition versions containing 0.05–0.08% carbon. Steels containing about 1% copper, in contrast to steels with a lower content or without addition of copper after normalizing and tempering, demonstrate a level of properties corresponding to strength class K60. Research shows that additional strengthening occurs as a result of precipitation of fine particles enriched in copper after tempering. Steel impact strength KCV–40 and relative elongation of steels alloyed with copper are at a high level due to a low carbon content and addition of nickel, and also prior grain refinement during controlled rolling. During tempering after normalizing, a more balanced set of properties is provided with tempering at 600°C. The steel developed may be used for preparing fittings of strength class K60 by hot stamping without thermal improvement or of strength class K65 after additional quenching and tempering.

A favorable effect is established for reducing carbon and manganese content in low-alloy pipe steels of strength categories X56–X65 on the increase in hydrogen-induced cracking (HIC) resistance of thick-rolled sheet prepared by a thermomechanical treatment regime. This effect is due to a reduction in segregation structural inhomogeneity in the axial area of sheets in the form of a solid band of high-carbon structures.

The effect of low-alloy steel type X70-X80 composition and thermomechanical treatment (TMT) parameters on the main processes of austenite structure formation and final intergranular ferrite-bainite structure, and also sheet and coiled rolled product properties are studied. A favorable effect is demonstrated for an increase in Nb content in steel (up to 0.09–0.10%) in limiting grain growth and a shift in the temperature range for austenite partial recrystallization (T95–T5) into a higher temperature region. Rolling in the partial recrystallization range causes formation of a varied grain structure containing extended areas of lath bainitic ferrite, as a result of which there is a reduction in low-temperature toughness and cold resistance. The effect of adding Mo, Ni, Cr, and Cu on the nature of steel X70 phase transformations is provided. Different types of Nb(C, N) and (Nb, V)(C, N) particle precipitates within austenite and ferrite are shown. Results are summarized for previous studies.

The effect of accelerated cooling parameters on microstructure and hydrogen initiated cracking (HIC) is studied for thick-walled low-alloy steels of strength categories X52–X65 used for manufacturing large diameter pipes intended for operation in hydrogen sulfide media. Optimum accelerated cooling regimes are determined providing formation of a ferrite-bainite microstructure throughout the rolled product’s thickness, exhibiting improved hydrogen cracking resistance.

Production of plates of strength category DNV SAWL 450 SFDU, intended for manufacturing pipe with a 39 mm wall thickness for submarine pipelines, is mastered under industrial conditions at the 5000 mill of Vyksa Metallurgical Plant (VMZ). The effect of chemical composition and steel manufacturing technology on features of microstructure formation (including its morphological features and excess phase precipitation) is studied. This makes it possible to formulate the optimum ferrite-bainite microstructure after TMT and provide the required strength level and cold resistance for plates, including manufacture of double length workpieces.

At the Azovstal Metallurgical Combine together with the Bardin Central Research Institute of Ferrous Metallurgy production of thick plate (3266 × 19.1 mm) is being developed for a large diameter gas pipeline of strength category X70. Plates are manufactured using controlled rolling technology with accelerated cooling (CR + AC) instead of previous controlled rolling technology with air cooling. Introduction of CR + AC promotes an increase in average values of energy for failure KV with impact tests on finished metal plate, and also a marked improvement in 3600 rolling mill productivity.

Results are presented for development and introduction into production in a new MMK 5000 mill for thick rolled sheet from low-alloy steels with increased strength properties (σy ≥ 500 N/mm2 and σy ≥ 570 N/mm2), impact strength (KCV −40 ≥ 200–270 J/cm2), and cold resistance. Features of structure formation are studied in sheet of considerable thickness in the course of controlled rolling with accelerated cooling. The effect of the amount of reduction in subsequent passes of the roughing stage for sheet on cold resistance during the FWT is revealed. Transmission electron microscopy is used to show that the optimum set of properties for high-strength sheet is achieved with formation of a fine steel ferrite-bainite microstructure whose main structural components are quasipolygonal and acicular ferrite.

The change in mechanical properties and structural state is studied for steel of strength class K65 after heating to different temperatures. The change in strength properties with heating in the temperature range for precipitation of fine particles of Nb and V carbides is compared with particle morphology. Recommendations are worked out on the basis of results obtained for improving the mechanical properties for high-strength steels with a ferrite-bainite structure.

The correlation of structure, formed during heat treatment in steel of strength category Kh90–Kh100, with mechanical properties of sheet steel and the effect of subsequent tempering on changing them is studied. The effect of structural component morphology in the original structure of high-strength strip of strength category Kh90–Kh100 on the change in mechanical properties after tempering is demonstrated. Production regimes are proposed for providing the specified property level.

The mechanical properties of thick rolled plate for main pipelines with a ferrite-bainite microstructure in different directions over the plate plane are determined, mechanical property anisotropy is revealed, and its features are studied. It is established that the structure of bainite, consisting of ferrite platelets with fine carbide interlayers along boundaries, orientated in a specific way in the plate plane with respect to the rolling direction, provokes propagation of a brittle crack and reduces metal cold resistance considerably in directions close to the preferred orientation of microstructural components.

The desired set of characteristics can be obtained in tube steels with a ferritic-bainitic structure by varying their chemical composition, monitoring the parameters of the thermomechanical rolling operation, and properly choosing the parameters for accelerated cooling of the rolled products.

The phase composition and mechanical properties of rolled thick sheets from pipe steel 05G1MB produced by different modes of controlled rolling with accelerated cooling are studied. The effects of the temperature and deformation regime, of the final rolling temperature, and of the temperature of the start of accelerated cooling on the phase composition of the rolled sheets are considered. The temperature range in which intense reduction during rolling should be avoided in order to keep niobium in the solid solution of the γ-phase and to ensure subsequent segregation of fine niobium carbides in ferrite is determined.

The modern approach to development of chemical compositions for use in high-strength pipe steels (which calls for low carbon content (0.04–0.08%) and microalloying with niobium and small additives of Ni, Cu, Cr, and Mo) and advanced rolling processes (controllable rolling with accelerated cooling) can produce steel with fine ferrite-bainite microstructure. This type of microstructure means that it is possible to meet current requirements for strips and base metal used in large-diameter pipe — strength specifications similar to those for steel in strength classes K60 (Kh70)-K65 (Kh80), excellent cold resistance, and satisfactory weldability.

Specialists at the company Severstal’ and the Central Research Institute of Ferrous Metallurgy have developed a chemical composition and technology for making steel of strength class X56, and they have also produced a commercial batch of this steel. Production of the batch on the 5000 mill at Severstal’ entailed the use of a modern controlled-rolling technology together with accelerated cooling, which made it possible to obtain the required set of characteristics in thick plates. The Vyksa Metallurgical Plant converted the plates into 762-mm-diam. conductor pipe with a wall thickness of 38.1 mm. Analysis of the effect of the tube conversion on the mechanical characteristics of the steel showed that the changes which take place in the mechanical characteristics of the base metal during the formation of thick-walled tubes must be taken into account when specifications are developed for the flat-rolled products used to make these tubes.

A process called controlled thermomechanical rolling has been developed to make low-alloy steels used in the production of large-diameter pipes. The process, carried out on plate mills, combines thermomechanical (controlled) rolling with accelerated cooling done after the rolling operation. The results of commercial trials of the technology lead to the following conclusions: 1. Compared to conventional controlled rolling (CR), use of the additional operation of accelerated cooling (AC) makes it possible to significantly increase strength properties without lowering impact toughness or cold resistance. Accelerated cooling also makes it possible to broadly vary the properties of rolled products of a given chemical composition and thickness. 2. The accelerated cooling of plates makes it possible to replace the ferritic-pearlitic structure usually formed in steel after conventional controlled rolling with a fine-grained ferrite-bainite stucture having a diminished level of striation. 3. Compared to traditional controlled rolling, CR + AC makes it possible to attain a prescribed level of strength with lower quantities of carbon and alloying elements. That in turn improves the weldability of the steel. 4. The use of accelerated cooling reduces the load on the mill because a higher finishing temperature is used than in traditional CR. Thus, it also becomes possible to increase rolling speeds through a reduction in the number of pauses made to cool the slabs on the mill.

The effect of heat treatment on mechanical properties and microstructure of high-strength low-alloy ferrite-bainite pipe steels is studied. Specimens are heated in a laboratory furnace up to a temperature in the range 100–850°C. Dependences for the change in mechanical properties on heating temperature and the main features of steel structure are determined. TEM is used to study excess phase precipitation, including Nb and V carbonitrides, during heating. Unfavorable temperature ranges are determined for heating pipe steels with a ferrite-bainite structure. The results obtained may be used in industrial production and during development of new technology for thermomechanical treatment.