Low-carbon Low-alloy Steel Research Articles

Mechanistic understanding of banded microstructure and its effect on anisotropy of toughness in low carbon-low alloy steel

In this study, the relationship between the anisotropy of toughness and microstructure in low carbon low alloy steel treated by quenching and tempering (QT) heat treatment was investigated with the aid of scanning electron microscope, electron back-scattered diffraction and transmission electron microscope combined with energy disperse spectroscopy techniques. Results show that the impact toughness of the quenched steel plate along the longitudinal and transverse directions are little different. However, after tempering, the longitudinal impact toughness of QT steel plate is improved by 88%, while the transverse impact toughness is slightly decreased, leading to a significant anisotropy of toughness. Microstructural characterizations reveal that banded microstructure with coarse grain size exists along longitudinal direction (i.e., rolling direction) of steel plate, which is attributed to co-segregation of Mn and C and the resulting uneven recrystallization during hot rolling. After tempering, fine and dispersed carbides are precipitated in matrix microstructure, but high-density coarse carbides are formed within banded microstructure. It is suggested that the coarse grains and high-density coarse carbides significantly deteriorate the resistance against crack propagation along banded microstructure, leading to the anisotropy of toughness of QT steel plate. The findings of this study will aid to design metallurgical processes including chemical composition design, hot rolling, and heat treatments to eliminate the anisotropy of toughness of low alloy steels with high strength and toughness.

Analysis of the Effects of Welding Conditions on the Microhardness of Low-Carbon Low-Alloy Steels

Analysis of the Effects of Welding Conditions on the Microhardness of Low-Carbon Low-Alloy Steels

RESEARCH ON THE RELATIONSHIP BETWEEN THE THICKNESS AND THE STRUCTURAL CONDITION OF ROLLED METAL FROM LOW-CARBON LOW-ALLOY STEEL 10G2FB

When choosing steels for the design of high-rise and long-span buildings with increased load-bearing capacity, it is advisable to give preference to thick rolled steel from low-carbon, low-alloy steels, since it, with the same level of strength as construction steels, has a higher level of plasticity. At the same time, the problem of using thick rolled steel from low-carbon, low-alloy steels in the construction industry are the anisotropy of the rolled metal properties, which can increase with an increase in the thickness of the rolled metal. Currently, in Ukraine, controlled rolling is one of the most promising technologies of high-temperature thermomechanical processing for the production of thick rolled metal from low-carbon, low-alloy steels. At the same time, with an increase in the thickness of the rolled metal, which is produced with this technological scheme, the effect of the regulated formation of the structural state decreases due to the influence on the temperature of the surface layers of the rolled metal, the thermodynamic state of the inner layers and the inability of the rolling equipment available at domestic enterprises to deform the metal over the entire cross-sectional area. Therefore, the task of obtaining such a structural state in the thick sheet metal roll, which will ensure the reduction of the anisotropy of the properties, which will allow the use of such rolled metal in the construction industry, is urgent. Purpose of the article is to study of the structural state of low-carbon low-alloy steel 10G2FB, which was produced using the technology of controlled rolling, depending on the thickness of the rolled metal. Conclusion. The relationship between the structural state and the thickness of rolled metal from low-carbon low-alloy steel 10G2FB, which was produced by controlled rolling technology, was studied. It was established that with the increase in thickness, the percentage content of the ferrite component increases with a simultaneous decrease in the percentage content of the pearlite phase. It is shown that changes in the formation languages of structural components begin with an increase in the thickness of the rolled metal over 30 mm, which is explained by the influence of the temperature of the inner layers on the processes of forming the structural state, namely, with an increase in the thickness of the rolled metal, the thermodynamic rate of phase transformations in the middle layers of the rolled metal samples decreases. This conclusion is confirmed by two factors: firstly, an increase in the size of pearlite colonies, and secondly, a change in the morphology of the cementite framework of pearlite colonies from zigzag (thickness 16...30 mm) to ribbon (thickness 40...100 mm).

Effects of acicular ferrites on crystallography and incomplete transformation of low-carbon low-alloy steel

In this study, we investigated the effects of acicular ferrite (AF) on the crystallographic and incomplete transformation characteristics of low-carbon low-alloy steel. Intermediate annealing after austenitization induced a transformation yielding AFs, accelerating the isothermal transformation kinetics and regulating the microstructure. This process formed highly dense packet grain boundaries and improved the stability of untransformed austenite. AF formation increased the number of packet interfaces, increasing the crack propagation energy. The reduction and refinement of martensite/austenite (M/A) islands increased the crack nucleation energy. A displacive model was used to fit the transformation kinetics curve, indicating the significant effect of the nucleation rate on the isothermal transformation of the experimental steel at 450, 475, and 500 °C. The findings of this study indicate that nucleation regulation can accelerate transformation kinetics to strengthen and toughen bainitic structures.

Pulsed gas metal arc additive manufacturing of low-carbon steel: Microstructure observations and mechanical properties

Gas metal arc additive manufacturing (GMA-AM), also known as wire arc additive manufacturing (WAAM), uses an electric arc to melt a wire electrode and deposit objects layer by layer. This study focuses on creating single-pass wall structures using a low-carbon steel wire (ER70S-6) and examining the relationship between pulse frequency and weld geometry, microstructure, and mechanical properties. Microscopic observations showed a typical columnar microstructure with three distinct regions: acicular ferrite, bainite, and allotriomorphic ferrite in the first and last layers, while the mid-region exhibited homogenous polygonal ferrite grains with some pearlite at the grain boundaries. The tensile test results demonstrated a dependency of strength on the applied pulse frequency, with the highest strength (i.e., the ultimate tensile strength of 522 MPa and yield strength of 375 MPa with ductility of ∼52%) achieved in parts processed at a frequency of 100 Hz. Vickers microhardness values revealed uniform hardness in the middle region, consistent with the microstructure observation. Analyzing thermal cycles, coupled with microstructure analysis and continuous cooling transition diagrams, provided insight into how phase and microstructure evolution occurred in low-carbon low-alloy steels processed through PGMA-AM.

APPLICATION OF THE STATISTICAL TESTING METHOD FOR SIMULATION MODELLING OF COMPLEX SYSTEMS

Problem statement. The study of the multicomponent systems’ operation is possible using statistical simulation models. In contrast to traditional mathematical modeling, in order to build the specified type of models, it is not necessary to obtain a mathematical formalization of the relationship between parameters in one form or another. The main condition is the ability of the model to reproduce the phenomena being modeled, while preserving their logical and temporal sequence and physical meaning. One of the most common methods of statistical simulation modeling is the method of statistical tests - the Monte Carlo method. The basis of this method is the repeated use of a random number generator to simulate the dynamics of processes occurring in the system. Based on the obtained iterations, statistical criteria for evaluating the obtained results are calculated, which makes it possible to conduct a preliminary analysis of the physical processes under investigation and draw conclusions about the relationship between the parameters included in the model. Thus, taking into account the general principles of these building simulation models, it is advisable to use them to study a number of applied materials science problems, for example, to describe the influence of external factors on the structural state of the material. The purpose of the article. Application of statistical simulation modeling by the method of statistical tests (Monte Carlo method) for the study of physical processes that occur in complex systems. Conclusion. Using the Monte Carlo method, a simulation model of the relationship between the technological modes of welding and the parameters of the structural state of low-carbon low-alloy steel 09Г2С was obtained. The obtained simulation model is presented in the form of a matrix containing the results of statistical tests. The analysis of the obtained statistical indicators made it possible to carry out a preliminary analysis of the relationship between technological modes of welding and the corresponding structural state of low-carbon low-alloy steel 09Г2С. Preliminary evaluation of the obtained simulation model was carried out by analyzing active constraints of the type of inequalities. The conducted set of studies proved the obtained data adequacy.

USING THE METHOD OF CONFIRMATORY FACTOR ANALYSIS FOR THE SIMULATION OF TECHNICAL SYSTEMS

Problem statement. Analyzing the dynamics of the functioning of complex technical systems, which are characterized by a certain architecture and the interaction of system components, in most cases, there are certain difficulties associated with the description of system-wide issues. Among such issues, first of all, it is possible to include problems related to the direct mathematical formalization of the overall system structure, the organization of relationships between its elements, the interaction of system elements with its external environment, control of the activities of its elements, etc. In such cases, most often researchers use the procedure of mathematical modeling. At the same time, there is a fairly large range of phenomena, in the analysis of which it is possible to use the traditional apparatus of mathematical modeling. In such situations, it is possible to apply modeling techniques that represent the model in the form of an algorithmic program for an electronic computer, the so-called simulation modeling. The purpose of the article. Application of simulated physical and mathematical modeling by the method of confirmatory factor analysis for the study of complex technical processes. Conclusion . The relationship between the parameters of the laser welding mode and the morphological criteria of the structural state of low-carbon low-alloy steel 09Г2С was investigated using the mathematical apparatus of simulation modeling, namely confirmatory factor analysis. The confirmatory factor method was implemented in the form of a path diagram, namely a graphical interpretation of the relationship between the structural condition criteria and laser welding parameters. The percentage content and geometric dimensions of the main structural components of ferrite and pearlite were used as criteria for the morphology of the structural state, and the geometric dimensions of the welded joint sections were used as welding parameters. The adequacy of the obtained model was confirmed by constructing a probability plot of normalized residuals according to the quasi-Newtonian method of residuals.

Effect of Controlled Thermomechanical Processing Routes on the Structural and Textural States of Low-Carbon Low-Alloy Steel

Effect of Controlled Thermomechanical Processing Routes on the Structural and Textural States of Low-Carbon Low-Alloy Steel

Achieving High Plasticity and High Toughness of Low-Carbon Low-Alloy Steel through Intercritical Heat Treatment

Medium manganese steel has excellent comprehensive properties due to the TRIP effect of retained austenite, but its welding performance is unsatisfactory for its high alloy content. This study obtained retained austenite in low-carbon low-alloy steel with low contents of silicon and manganese elements through intercritical heat treatment. The influence of intercritical quenching temperature on the content and characteristics of the retained austenite, as well as the functional mechanism of the retained austenite during low-temperature impact, was studied. The results showed that the content of the retained austenite increased from 12% to 17%, and its distribution extended from grain boundaries to martensite lath boundaries, with increasing intercritical quenching temperature. The retained austenite on the grain boundaries was in blocks, and that on the martensitic lath boundaries formed slender domains. The stability of the retained austenite was achieved through the enrichment of C and Mn during intercritical heat treatment. The contribution of retained austenite to low-temperature mechanical properties was closely related to its stability. The retained austenite with poor stability underwent martensite transformation at low temperatures, and the high-carbon martensite was a brittle phase that became the nucleation site of cracks or the path of crack growth during impact. Stable retained austenite passivated crack tips and hindered crack propagation during impacts, which improved the impact performance of the steel.

Comparison of the Effect of Microadditions of Niobium, Titanium and Vanadium on Formation of Microstructure of Low-Carbon Low-Alloy Steels

Comparison of the Effect of Microadditions of Niobium, Titanium and Vanadium on Formation of Microstructure of Low-Carbon Low-Alloy Steels

Influence of technological heating used in the manufacture of parts on the initial structure of 10G2FBYu steel after controlled rolling

Modern pipe steels for the main pipelines belong are of the high-strength class of X70 and higher. The steels must simultaneously still have sufficient viscosity at low operating temperatures, hardenability and good weldability. This set of properties is achieved by combining the optimal material composition and the method of manufacturing a sheet billet using controlled rolling. Subsequent operations for the manufacture of pipes, connecting parts of oil and gas pipelines (welding, heat treatment, etc.), accompanied by inevitable high-temperature heating, can significantly affect the initial structure of the sheet and the level of its properties. In this case, the possible growth of austenite and ferrite grains, as well as the loss of hot work hardening, should negative effect the mechanical properties of pipe steel. However, taking into account the peculiarities of the chemical composition and structure of sheet pipe steels subjected to controlled rolling, containing hardly soluble fine particles of carbides and carbonitrides based on vanadium, niobium, titanium, which inhibit recrystallization, one can expect the preservation of a fine-grained structure and a part of hot work hardening with a rational scheme for manufacturing parts. The article presents data on the study of the structure and hardness of low-carbon low-alloy steel 10G2FBYu of strength class K60 after repeated normalization from temperatures of 850, 880, 910 and 940°C. Structural studies performed using metallographic and electron microscopes Carl Zeiss Axio Observer and JEOL JSM-7001F showed that after additional heating at the indicated temperatures, the grain sizes increase, but not significantly. The hardness of steel is reduced by 20% or less, depending on the normalization mode

Explaining the Abnormal Dilatation Behavior During the Austenite Formation in a Microstructure of a Low-Carbon Low-Alloy Steel Containing Retained Austenite

Explaining the Abnormal Dilatation Behavior During the Austenite Formation in a Microstructure of a Low-Carbon Low-Alloy Steel Containing Retained Austenite

Structure forming processes in economically alloyed shipbuilding steel of 890 MPa yield strength level with a bainite-martensite structure when microalloyed with vanadium

The kinetics of growth of austenite grains during heating, the features of the processes of dynamic and static recrystallization occurring under various temperature-deformation regimes of hot plastic deformation have been studied. Phase transformations have been studied during continuous cooling of hot-worked austenite in a low-carbon low-alloy steel with a guaranteed yield strength of 890 MPa. As a result, the boundary temperature-deformation conditions for the formation of a finely dispersed bainite-martensite structure were established, on the basis of which technological modes for the production of thick-plate rolled products in industrial conditions were developed. The structure and properties of rolled sheets 35 mm thick from shipbuilding sparingly alloyed steel of strength level 890 are presented.

Effect of tempering and partitioning (T&P) treatment on microstructure and mechanical properties of a low-carbon low-alloy quenched and dynamically partitioned (Q-DP) steel

We performed a direct quenching and dynamical partitioning (Q-DP) process, following intercritical annealing, to a low-carbon low-alloy steel. Performing an additional tempering treatment on the Q-DP steel, we investigated the carbon partitioning behavior upon tempering. We emphatically analyzed the effect of tempering and partitioning (T&P) treatment on the microstructure evolution and mechanical properties of the Q-DP steel, especially on carbon partition, retained austenite stabilization, TRIP effect and work hardening. Results show that the synergetic partition of carbon and Mn from ferrite to austenite during intercritical annealing and the dynamical partition of carbon upon quenching jointly achieve the stabilization of retained austenite in the Q-DP sample. The actual volume fraction of retained austenite is larger than the theoretical calculating value, which is attributed to the dynamical partition of carbon. The T&P treatment at 300 °C for 15 min does not result in obvious decomposition of retained austenite, but leads to apparent carbon partition from martensite to retained austenite. The carbon partition upon tempering enhances the mechanical stability of retained austenite, thus weakening the TRIP effect during tensile deformation and promoting the TRIP effect to occur at higher true strain level. The much higher work hardening rate at the late deformation stage near necking, for the quenched tempered and partitioned (Q-T&P) sample, leads to a higher uniform elongation, compared with the Q-DP sample. The existence of carbon-enriched stable retained austenite after necking happens finally enhances the post necking elongation of the steel.

Changes in the structure and properties of the thick sheets of bainitic/martensitic steel at a cross-section

To expand the grade of rolled low-carbon low-alloy steels chrome-nickel-molybdenum composition. The analysis of changes the mechanical properties and structure in the thickness of rolled sheets of 15, 50 and 60 mm produced using quenching with furnace heating and high tempering was performed.

Effects of Multiple Defects on Welded Joint Behaviour under the Uniaxial Tensile Loading: Fem and Experimental Approach

The research represents the ongoing investigation of the welded joints behavior made of low-carbon low-alloyed steel in the presence of different multiple defects. Following the initial experimental and numerical analyses performed on low-grade steel, a set of experiments were performed with specimens made of steel EN 1.0044 (commercial designation S275JR), along with development of new numerical models. Four combinations were made, including defects like undercuts, excess weld metal, misalignment, weld face sagging and incomplete root penetration, considering that these defects are often encountered in practice, and can appear simultaneously. The finite element method (FEM) was used to simulate the experiments. Tensile properties of the HAZ and weld metal were calculated using strain measurements by the digital image correlation (DIC) method. The finite element method (FEM) results were in good agreement with the experimental ones.

Dependence of the microstructure and mechanical properties of cold-resistant steel sheets with a guaranteed yield strength of 420 MPa on the thermomechanical processing

For the production of sheets with a thickness of 5 and 15 mm from cold-resistant steel with a guaranteed yield strength of 420 MPa, continuous rolling was developed and tested in laboratory conditions. The present study considers the influence of the parameters of thermal deformation treatment (temperature of the end of rolling and cooling rate) on the formation of the final microstructure. The mechanical properties of the sheets were analyzed taking into account the formed structural features. Technological recommendations for hot rolling of low-carbon low-alloy steel sheets with a guaranteed yield strength of 420 MPa are proposed.

Microstructure evolution and compressive properties of a low carbon-low alloy steel processed by warm rolling and subsequent annealing

A low carbon-low alloy steel was processed by warm rolling with reductions range from ~30% to ~70% followed by annealing at 450°C. Then, the microstructural evolution was characterized by Field Emission Scanning Electron Microscopy (FE-SEM), Electron Backscatter Diffraction (EBSD), Transmission Electron Microscopy (TEM), X-ray Diffraction (XRD) and compressive testing under strain rates of 1.0 × 10−3–2.0 × 103 s−1 was carried out. Microscopy analyses showed that ultrafine-grained structures with high-density dislocations and more and finer M3C carbides by comparison with the tempered steel were achieved after warm rolling. Subsequent annealing promoted the further precipitation of finer carbides and led to dislocation recovery as well as a slight coarsening of grains. Compressive testing results indicated that the yield strengths of the warm rolled steels at different strain rates were significantly increased by ~40–70% compared with the as-received sample, which was mainly attributed to a combination of dislocation strengthening, grain boundary strengthening and precipitation strengthening. After annealing, the yield strength decreased slightly due to a dislocation recovery and a slight increment of the grain sizes. In addition, the influence of microstructure evolutions including dislocation densities, grain sizes and carbide precipitations during warm rolling and subsequent annealing on the strain rate dependence of strength for steels was also analyzed.

Effect of ultra-fast heating on microstructure and mechanical properties of cold-rolled low-carbon low-alloy Q&P steels with different austenitizing temperature

We performed ultra-fast heating (150 °C/s) quenching and partitioning (Q&P) processes to a cold-rolled low-carbon low-alloy steel. Adjusting the annealing temperature, we obtained two types of ferrite-containing Q&P steels with different matrix structure, i.e., martensitic matrix and ferritic matrix. The effect of ultra-fast heating on microstructure evolution and mechanical properties was analyzed, compared with the samples treated with conventional heating rate (10 °C/s). We also investigated the strain partition and fracture behaviors, basing on in-situ tensile tests under SEM. Results show that ultra-fast heating can significantly increase the A c1 and A c3 temperatures, and extend the temperature span of intercritical zone. It can also delay the complete recrystallization of deformed ferrite and promote the austenite nucleation, which leads to the structure refinement of the sample with martensitic matrix, improving both the strength and ductility. The increase of austenite nucleation rate and the incomplete recrystallization of deformed ferrite jointly promote the austenite transformation, which results in a distinct increase of martensite islands fraction in the sample with ferritic matrix, leading to its obvious increase of tensile strength and decrease of ductility. Strain partition is an important factor affecting the TRIP effect in ferrite-containing Q&P steels. For the sample with a martensitic matrix, retained austenite mainly possesses a film-like morphology and exists between martensite laths. The relatively high mechanical stability of retained austenite films and low local strain in martensitic regions result in the weak TRIP effect. The retained austenite in the sample with a ferritic matrix mainly distributes among ferrite grains and exhibits a blocky morphology. The relatively low mechanical stability of blocky retained austenite and high local strain in ferrite jointly enhance the TRIP effect. • Ultra-fast heating delays ferrite recrystallization. • Ultra-fast heating restrains martensite tempering. • Ultra-fast heating results in grain refinement of martensitic matrix. • Strain partition is a crucial factor controlling TRIP effect. • Main ductility origin is compatible deformation of matrix structures.

Crucial Microstructural Features to Determine the Mechanical Properties of Welded Joints in a Cu-Containing Low-Carbon Low-Alloy Steel After Postweld Heat Treatment

Crucial Microstructural Features to Determine the Mechanical Properties of Welded Joints in a Cu-Containing Low-Carbon Low-Alloy Steel After Postweld Heat Treatment