Low-carbon Low-alloy Steel Research Articles

Influence of alternating cold flexure on the properties of low-alloy pipe steel (strength class K56–K65)

In laboratory conditions, the cold straightening of thick (17.5 mm) low-carbon low-alloy steel sheet (yield point no less than 550 N/mm2) is simulated by alternating flexure. The influence of the number of cycles and the strain in alternating flexure on the strength of the steel is studied. For industrial production on the 5000 mill at OAO Vyksunskii Metallurgicheskii Zavod (VMZ), the influence of the cold-rolling conditions on the yield point and strength of thick (15–23 mm) steel sheet (strength class K56–K65) is demonstrated.

Lengthening kinetics of ferrite plates in high-strength low-carbon low alloy steel

The lengthening of ferrite plates in a high-strength low carbon low alloy steel, which contained 1.62 mass% Mn and small amounts of Si, Mo, Ni, Cr, and Nb, was observed in situ under high-temperature confocal laser scanning microscopy. Applying the analysis by Speich and Cohen to the growth of a lath-type plate, it was found that the measured lengthening rates are likely to fall between those calculated assuming para- and NPLE modes from the semiempirical equation proposed by Hillert, while they scattered widely presumably because the plate morphology was not a lenticular disk. In the temperature range of measurement, solute drag due to alloying element segregation at the boundary between plate tip and the matrix may not operate to an appreciable extent. The retardation of lengthening from that calculated under paraequilibrium could occur by intrinsic friction of α/γ boundary.

Formation of structure and properties in welded joints of oil line pipes

The effect of welding and heat treatment modes on mechanical properties and corrosion resistance of welded joints of oil line pipes is studied. It is shown that high-frequency welding followed by high-temperature annealing provides strength and corrosion properties in welds of pipes from low-carbon low-alloy steels at the level of the welded metal.

Changes in the structure and properties of low-carbon low-alloy pipe steels upon inoculation with REM

The influence of REM additives on the structure, mechanical properties and corrosion resistance of low-carbon low-alloy steels is studied. The effect of inoculation with rare-earth metals on the content of sulfur and oxygen in the steels and on the degree of their contamination with nonmetallic inclusions are investigated. The resistance of the steels to stress sulfide corrosion cracking, hydrogen cracking and total, local, carbon dioxide and bacterial corrosion is determined.

Complete and Incomplete Wetting of Ferrite Grain Boundaries by Austenite in the Low-Alloyed Ferritic Steel

Low-carbon low-alloyed ferritic steels are the main material for the production of high-strength pipes for the transportation of oil and gas. The formation of brittle carbide network during the lifetime of a pipeline could be a reason for a catastrophic failure. Among other reasons, it can be controlled by the morphology of grain boundary (GB) carbides. The microstructure of a low-alloyed ferritic steel containing 0.09 at.% C and small amounts of Si, Mn, Nb, Cu, Al, Ni, and Cr was studied between 300 and 900 °C. The samples were annealed very long time (700 to 4000 h) in order to produce the equilibrium morphology of phases. The (α-Fe)/(α-Fe) GBs can be either completely or incompletely wetted (covered) by the γ-Fe (austenite) above the temperature of eutectoid transition. The portion of (α-Fe)/(α-Fe) GBs completely wetted by γ-Fe is around 90% and does not change much between 750 and 900 °C. The (α-Fe)/(α-Fe) GBs can be either completely or incompletely wetted (covered) by the Fe3C (cementite) below the temperature of eutectoid transition. The portion of (α-Fe)/(α-Fe) GBs completely wetted by Fe3C changes below 680 °C between 67 and 77%. The formation of the network of brittle cementite layers between ductile ferrite grains can explain the catastrophic failure of gas- and oil-pipelines after a certain lifetime.

Microstructure in low-carbon low-alloy high-strength pipe steel

The microstructure of a broad range of low-carbon low-alloy high-strength pipe steels produced by thermomechanical treatment is studied by transmission electron microscopy. Such steels consist of a mixture of various types of ferrite matrix with high-carbon phases and structural components. The classification of the structures is refined, with separate consideration of the components forming the low-carbon ferrite matrix and the high-carbon components that appear as isolated regions and also as layers and inclusions in bainitic ferrite. A deformational dilatometer is used to determine the temperatures ranges in which the various phases and structural components are formed.

Quantitative analysis of variant selection in ausformed lath martensite

Variant selection in lath martensite transformed from deformed austenite in a low-carbon low-alloy steel is examined quantitatively on the basis of electron backscatter diffraction analysis of martensite, in concert with a novel method for the reconstruction of austenite orientation. At a strain of 10%, variants whose (011)α plane was nearly parallel to the primary slip plane in austenite ((111)γ) were formed dominantly. At strains of 30% and 50%, variants whose (011)α plane was nearly parallel to the secondary slip plane ((−111)γ) as well as the primary slip plane were formed. Transmission electron microscopy observation of an austenite-stabilized alloy deformed under the same condition as the low-carbon low-alloy steel clarified that microband structures develop along the primary and secondary slip planes of austenite when its orientation is close to the main component of the deformation texture in austenite. A simple variant selection model is proposed in which martensite variants with habit planes nearly parallel to the primary and secondary slip planes nucleate preferentially on microband boundaries as a result of a smaller amount of activation energy and grow dominantly as a result of less inhibition from the microband boundaries.

In situ studies of nonlinear d-spacing changes in titanium and steel alloys

We traced element-partitioning behavior by measuring d-spacing changes in situ during phase transformations of titanium and steel alloys using a hybrid in situ observation system. For titanium alloy, nonlinear changes are clearly evident during heating and cooling and are related to element partitioning and microstructural evolution. For low-carbon–low-alloy steel, nonlinear changes are evident during cooling and are related to bainite and carbon-enriched austenite formation.

Diffusional and displacive transformation behaviour in low carbon-low alloy steels studied by a hybrid in situ observation system

The difference in the d -spacing change during diffusional and displacive transformation, obtained during continuous cooling processes, was characterized using a hybrid in situ observation system that we have developed. The d -spacing change in an austenite phase in the diffusional transformation was found to be discontinuous compared to that in the displacive transformation. This difference reflected the carbon partitioning behaviour, and the hybrid system enabled the tracking of transformation behaviours in both real and reciprocal lattice space.

Constitution and conditions of formation of intermediate structures with granular morphology in low-carbon low-alloy steels of bainitic class

The conditions of formation and features of intermediate structures with granular morphology are studied in isothermal decomposition of austenite in low-carbon low-alloy steels of bainitic class. The mechanisms of formation of α-phase (mesoferrite) are investigated.

Colored Metallography Study of Bainite Steel Used for High Speed Railway Crossing

Using the technique of colored metallography, the phase compositions of two medium-/low-carbon low-alloy carbide-free bainite steel used for railway crossings are obtained. The etching of the samples was carried out using the Lepera reagent and a mixed reagent of hyposulphite and picric acid respectively. The proportion of the bainite phase was calculated using an image processing system with an optical microscope and the residual austenite content was measured using XRD method. The martensite phase content can then be determined. It is found that the reagents can effectively and quantifiably analyze the phase composition of bainite steel and produce approximately the same results.

Effect of thermomechanical treatment on the cold resistance of low-carbon low-alloy welding steel

Effect of the structure and texture formed upon thermomechanical treatment (TMechT) on the cold resistance of low-carbon low-alloy welding steel is considered. The structure has been investigated by the metallographic and electron-microscopic methods; the texture has been studied by X-ray diffraction method with the aid of the analysis of the orientation distribution function (ODF). Structural factors that affect an increase in the cold resistance, including creation of a submicrocrystalline structure, have been considered. It has been established that the components of the texture result from a regular crystallographic rearrangement of the texture of hot deformation of austenite into the texture of the α phase upon the γ → α transformation. It is shown that the dependence of the cold resistance of the rolled sheet on the fraction of the cube component in the texture of the α phase has a complex nonmonotonic nature.

Technologies of Nanomodification of Low-Carbon Low Alloyed Steels

Metallurgical industry can be considered as a field most accommodated for perception of nano-technologies, which in the near future will be able to provide large scale production and high level of investments return. Specially noted should physical and mechanical properties of nano-structured steels and alloys (strength, plasticity, toughness and so on) which will cardinally excel characteristics of respective materials developed using conventional technologies. Investigations have shown that basic principles of selection of a structure up to nano-level for low-carbon low-alloy steels can be put forward, that is: 1) morphological similarity of structural components, pre-domination of globular type structures due to reduction in carbon components and rational alloying; 2) formation of fine-dispersed carbide phase of globular morphology; 3) exclusion of lengthy interphase boundaries; 4) formation of fragmented structure with boundaries close to wide-angle ones, which inherited structure of fine-grained deformed austenite.

New insights into intragranular ferrite in a low-carbon low-alloy steel

Single and multiple nucleation events on non-metallic inclusions have been observed, leading to the intragranular formation of ferrite as a function of transformation temperature. Three-dimensional shapes have also been characterized. When the ferrite forms at elevated temperatures it is in the form of isolated idiomorphs, whereas larger undercoolings are associated with the multiple nucleation of plates emanating from the inclusions. The plates grow at a fixed orientation to the parent austenite. Nanohardness tests indicate that the idiomorphs are softer when compared with the plates. The formation of intragranular ferrite laths or plates can facilitate the attainment of fine-grained microstructures.

Main structural factors of strengthening of low-carbon low-alloy pipe steels after controlled rolling

The mechanical properties of a large group of commercial heats of three grades of pipe steel (17G1S-U, 10G2FB, and 12G2SB) are studied after controlled rolling. The interrelation of alloying, structure, and mechanical properties of the steels subjected to controlled rolling under the conditions of the Magnitogorsk Iron and Steel Works is studied, and the main structural factors responsible for strengthening of low-alloy steels are determined.

Ultrafine Grained Low Carbon Steels Processed by Severe Plastic Deformation

The structure, mechanical and functional properties of ultrafine-grained low-carbon steels have been studied after severe plastic deformation (SPD) by high pressure torsion (HPT) and equalchannel angular pressing (ECAP). It is revealed that HPT of low carbon steels at a temperature below 0.3 Tm leads to the formation of nanocrystalline structure with a grain size of <100 nm or a mixture of oriented substructure and nanograins. ECAP under similar conditions leads to the formation of submicrocrystalline structure with a grain size of 200-300 nm. The initial martensitic state compared with the initial ferritic-pearlitic state of the low-carbon steels results in formation of finer structure after SPD and less intense grain growth upon heating, i.e., results in a higher thermal stability. Low-carbon low-alloy steels after ECAP are characterized by high strength (UTS > 1000 MPa) and plasticity (EL = 10-15%). The high-strength state after ECAP is retained upon tensile test testing up to a temperature of 500°C. The submicrocrystalline low-carbon steels after ECAP processing and subsequent heating is characterized by an increased impact toughness at test temperatures down to -40°C.

Chemical composition and structure of low-carbon low-alloy pipe steel following controlled rolling

We study the patterns of variations in chemical composition for a large number of commercial melts of three grades of pipe steel: 17G1S-U, 10G2FB, and 12G2Sb via statistical clustering of experimental data. A comparative structural study was performed; the effects of various steel alloy components on the amount, distribution, and morphology of the pearlite component; the size and shape of the ferrite grains; and the development of dynamic recrystallization processes.

Effect of deformation and cooling rate on the transformation behavior and microstructure of X70 steels

The effects of the deformation in the non-recrystallization region of austenite and the cooling rate on the transformation behavior and microstructure of low-carbon low-alloy steel for pipeline application were studied on the thermal-mechanical simulator Gleeble-1500. It was shown that an increase in deformation amount can greatly increase the nucleation site of ferrite when deformed in the non-recrystallization region of austenite, and an increase in nucleation ratio can greatly refine grains. When the cooling rate is accelerated, the driving force of nucleation is increased and the nucleation rate also improves. Ultra-refine grains can be obtained by controlled rolling. The high density of ferrite nucleus, which forms along the austenite grain boundary, twin interface, and deformation band are introduced in the matrix of austenite by the control of hot rolling, after which the microstructure can be refined. It was found that the acicular ferrite has a very fine sub-structure, high dislocation density, and a thin slab with ultra-fine grains. Small M/A islands and cementite are precipitated on the matrix of the slabs by the analysis technique of TEM and SEM.

Effect of prestraining on the behavior of hydrogen in structural steel

We study the influence of cold working on the behavior of hydrogen in 0.25C-1Cr-1Mn-1Si-1Ni high-strength low-carbon low-alloy steel after various types of thermal treatment. The results of measuring the permeation of hydrogen are used to determine the lattice diffusion coefficient, the efficiency of trapping of hydrogen, and the formation of internal cracks for specimens deformed by shot peening, cyclic tension-compression, and uniform tension. The mode of deformation does not affect the value of the lattice diffusion coefficient for the bainite structure. All types of deformation increase the efficiency of traps corresponding to the deformation of the bainite lattice by cold working. The type of deformation specifies the morphology of internal hydrogen-induced cracks along the local slip strips in the metal.

TEM Study of Bainitic Low-Carbon HSLA Steel: The Orientation Relationships of Cementite

Abstract Two different bainitic structures can be present in steel depending on the heat treatment to which the samples are subjected. The two different types of bainitic structures exhibit a different orientation relationship between the cementite and the ferrite matrix. The Pitsch orientation relationship is observed in upper bainite while the Bagaryatski orientation relationship is observed in lower bainite [1]. Different heat treatment samples of low-carbon High Strength Low Alloy (HSLA) steel were studied using TEM observations in order to ascertain the orientation relationship between ferrite and carbide in the different bainitic structures and to determine whether this relationship may indicate the type of bainitic structure.