Search Results (64)

The isothermal bainitic transformation kinetics, microstructure, and mechanical properties of the quenched low-carbon high-strength steel have been investigated via dilatometric measurements, microstructural characterization, and mechanical tests. The results show that the pre-transformed isothermal bainite promotes martensitic transformation, increasing the martensitic transformation temperature, and enhancing the transformation rate. The microstructure of the 400 °C isothermal steel consists predominantly of lath bainite ferrite with dot/slender M-A constituents, whereas the steel treated at 450 °C contains a combination of martensite/lath bainite and granular bainite. The presence of massive M-A constituents contributes to brittle fracture as these constituents tend to promote crack initiation. Hence, the 450 °C treatment, which leads to the formation of massive M-A constituents, induces brittleness, while the finer M-A constituents formed at 400 °C exert minimal influence on the toughness and result in a more stable microstructure owing to their small size and the surrounding fine lath microstructure. The differences in microstructure and properties between the steels treated at 400 °C and 450 °C illustrate the importance of controlling the quenching cooling rate in the high-temperature bainitic transformation region during thick plate quenching processes. Full article

With the rapid development of long-distance transmission pipelines for oil and natural gas, pipeline steel is continuously evolving towards higher pressure, larger diameter, and thicker wall thickness. Many extensive studies and research have been conducted on X70 pipeline steel produced through traditional processing routes. This study focuses on Nb-V-Ti-N microalloyed steel with different nitrogen contents, systematically investigating the variations in microstructure and tensile properties after quenching and tempering processes. The results indicate that after quenching treatment, when the nitrogen content of the tested steel is 0.0020 wt%, its primary microstructure consists of granular bainitic ferrite (GBF), acicular ferrite (AF), and residual M/A (martensite/austenite) components. As the nitrogen content increases, the contents of acicular ferrite and M/A constituents gradually rise, while granular bainitic ferrite correspondingly decreases. After tempering treatment, the microstructure of the tested steel transforms into granular bainitic ferrite, acicular ferrite, and carbonitrides. Notably, with the elevation of nitrogen content, the number of high-angle grain boundaries in the microstructure significantly increases. Meanwhile, the mean equivalent diameter (MED) defined by the misorientation angle (MTA) ranging from 2 to 15° and the dislocation density (ρ) exhibit an overall decreasing trend. Both of these factors contribute significantly to yield strength, resulting in a gradual increase in yield strength (YS) as the nitrogen content rises. Additionally, the study finds that as the nitrogen content increases, the size of precipitated particles continuously enlarges, and their proportion in the microstructure gradually increases. This discovery provides important theoretical basis and practical guidance for further optimizing the microstructure and mechanical properties of X70 pipeline steel. Full article

This study addresses the characterization of the particular microstructural constituents of multiphase transformation-induced plasticity (TRIP)-aided steels belonging to the first and third generations of Advanced High Strength Steels (AHSS) to explore the possibilities of the EBSD method. Complex microstructures composed of ferrite, bainite, retained austenite and martensite were qualitatively and quantitatively assessed. Microstructural constituents with the same crystal structure were distinguished using characteristic EBSD parameters like confidence index (CI), image quality (IQ), kernel average misorientation (KAM) and specific crystallographic orientation relationships. A detailed linear analysis of the IQ parameter and misorientation angles was also performed. These tools are very helpful in linking different symmetric or asymmetric features of metallic alloys with a type of their structure and morphology details. Two types of samples were investigated: thermomechanically processed and subjected to 10% tensile strain to study the microstructural changes caused by plastic deformation. Full article

This study explored the influence of hydrogen on the tensile properties and fracture behavior of high-strength API X70 and X80 linepipe steels with bainitic microstructures under varying hydrogen charging conditions. The X70 steel exhibited a ferritic microstructure with some pearlite, while the X80 steel showed a bainitic microstructure and fine pearlite due to the addition of molybdenum. Slow strain rate tests (SSRTs) were conducted using both electrochemical ex situ and in situ hydrogen charging methods subjected to different current densities. The SSRT results showed that in situ hydrogen-charged SSRT, performed at current densities above 1 A/m², led to more pronounced hydrogen embrittlement compared to ex situ hydrogen-charged SSRT. This occurred because hydrogen was continuously supplied during deformation, exceeding the critical concentration even in the center regions, leading to quasi-cleavage fractures marked by localized cleavage and tearing ridges. Thermal desorption analysis (TDA) confirmed that a greater amount of hydrogen was trapped at dislocations during in situ hydrogen-charged SSRT, intensifying hydrogen embrittlement, even with a shorter hydrogen charging duration. These findings highlight the importance of selecting appropriate hydrogen charging methods and understanding the hydrogen embrittlement behavior of linepipe steels. Full article

This paper comprehensively examines three structural steel grades’ microstructural features and mechanical properties, evaluating their suitability for shipbuilding applications. The steels analyzed include quench and tempered (Q and T) steel, thermomechanical controlled processed (TMCP) steel, and hot rolled (HR) steel. A microstructural characterization was performed using optical microscopy (OM), scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD). The analysis was complemented by extensive mechanical testing including assessments of hardness, tensile, and Charpy impact tests across a range of temperatures. Additionally, corrosion behavior was evaluated using the potentiodynamic polarization test. The findings revealed that Q and T grade steel exhibited the most refined microstructure, characterized by a complex mixture of ferrite, tempered martensite, upper bainite, and Fe₃C phases. In contrast, the TMCP grade steel demonstrated a balanced microstructure of polygonal ferrite and pearlite. Meanwhile, the HR grade steel contained polygonal ferrite and aligned pearlite. The tensile testing results demonstrated that the Q and T grade steel had superior hardness, yield strength (YS), and ultimate tensile strength (UTS), although it exhibited the lowest elongation % (El %). The TMCP grade steel met all ABS standards for marine steels, displaying optimal YS, UTS, and El %. Despite the superior YS of the HR grade steel, it did not meet the necessary criteria for UTS. Charpy impact tests revealed that the TMCP grade steel exhibited the highest impact energy absorption across a range of temperatures. As a result, the TMCP grade steel emerged as the optimal choice for ship construction, fulfilling all ABS requirements with a balanced combination of strength, ductility, and impact energy absorption. Additionally, the potentiodynamic polarization results revealed that the Q and T grade steel demonstrated the highest corrosion resistance. Following Q and T steel, the HR grade steel ranked second in corrosion resistance, with TMCP steel closely behind, showing only a slight difference. Full article

High-strength steels are widely used in various mechanical production and construction industries for their low cost, high strength and high toughness. Among these, bainitic steels have better comprehensive performance relative to martensite and ferrite. In this paper, from the point of view of its microscopic fine structure and mechanical properties, the high-carbon silicon-containing steel Fe-0.99C-1.37Si-0.44Mn-1.04Cr-0.03Ni was austenitized at high temperature after a brief isothermal treatment at 280 °C and is briefly reviewed. We have used EBSD, TEM and 3D-APT to observe a unique transformation in which high-carbon silicon-containing steels form nanostructured bainite with nanometer widths. Intriguingly, as the isothermal duration decreases, the beam bainite width becomes increasingly finer. When the beam bainite width falls below 50 nm, there is a sudden shift in defect type from the conventional edge-type dislocations to a defect characterized by the insertion of a semi-atomic surface in the opposite direction, which leads to different degrees of reduction in the micro- and macro-mechanical properties of high-carbon silicon-containing steels from 1754 MPa to 1667 MPa. This sudden change in the sub-structural properties is typical of the reverse Hall–Petch effect. Full article

In this article, DH steel containing Nb and Nb-Cu above 1000 MPa was designed, and its phase transformation law was analyzed through thermal expansion tests. The influence of annealing temperature on the microstructure and properties of DH steel was studied using a continuous annealing simulation testing machine, SEM, and tensile testing machine. The results showed that under a continuous annealing process, the test steel is composed of ferrite, martensite, a small amount of bainite, and residual austenite. The tensile strength decreases with the increase in annealing temperature, Cu element is dissolved in the matrix which produces solid solution strengthening and results in an increase in the strength of Cu-bearing test steel. Finally, 1180 MPa grade DH steel with excellent comprehensive properties was obtained at an annealing temperature of 840 °C and an overaging temperature of 340 °C. The expansion performance of the experimental steel was studied and optimized. Under the step heating annealing process, the experimental steel is composed of tempered martensite, ferrite, and residual austenite, with smaller differences in hardness between different phases, lower average dislocation density, and better expansion performance. Cu-bearing DH steel achieved an excellent match of strength and plasticity of 1289 MPa × 19.8%, with the hole expansion rate of 21.9% and the loss rate of hole expansion rate of 10%. Full article

This study focuses on advancing the production of predominantly bainitic heavy plates to meet the API 5L X80 standard. The investigation involves a thorough evaluation of the influence of rolling parameters and austenite conditioning on both microstructural characteristics and mechanical properties. Accurate specifications for chemical composition, processing temperatures, and mean deformations were established using mathematical models and bibliographical references. Four rolling conditions were performed in a reversible single-stand mill, allowing for comprehensive comparison and critical analysis. Microstructural and mechanical characterizations were performed utilizing several techniques, including optical microscopy (OM), scanning electron microscopy (SEM), tensile tests, Charpy impact tests, and hardness tests to ensure adherence to API 5L standards. Additionally, the SEM-EBSD (electron backscattered diffraction) technique was employed for a complementary analysis. The EBSD analysis included crystallographic misorientation maps, mean kernel misorientation parameters (ϑ), low- and high-angle grains boundaries, mean equivalent diameter, and evaluation of the contribution of different strengthening mechanisms to yield strength. Results underscored the significant influence of austenite conditioning on both microstructure and mechanical properties. Considering the specificities of a reversible single-stand mill, it was concluded that, unlike the classic approach for ferritic or ferritic–pearlitic HSLA (high-strength low-alloy steel), when a product with a predominantly bainitic microstructure is required, the accumulated deformation in the austenite during the finishing rolling stage, as well as its temperature, must be meticulously controlled. It was shown that the greater the deformation and the lower the temperature, the more favorable the scenario for the undesired polygonal ferrite formation, which will deteriorate the material’s performance. Furthermore, an optimized production route was identified and adapted to the specificities of the employed rolling mill. The presented data have great importance for researchers, manufacturers, and users of API 5L X80 heavy plates. Full article

The strength–ductility mechanism of the low-carbon steels processed by laser cutting is investigated in this paper. A typical gradient-phased structure can be obtained near the laser cutting surface, which consists of a laser-remelted layer (LRL, with the microstructure of lath bainite + granular bainite) and heat-affected zone (HAZ). As the distance from the laser cutting surface increases, the content of lath martensite decreases in the HAZ, which is accompanied by a rise in the content of ferrite. Considering that the microstructures of the LRL and HAZ are completely different from the base metal (BM, ferrite + pearlite), a significant strain gradient can be inevitably generated by the remarkable microhardness differences in the gradient-phased structure. The hetero-deformation-induced strengthening and hardening will be produced, which is related to the pileups of the geometrically necessary dislocations (GNDs) that are generated to accommodate the strain gradient near interfaces. Plural phases of the HAZ can also contribute to the increment of the hetero-deformation-induced strengthening and hardening during deformation. Due to the gradient-phased structure, the low carbon steels under the process of laser cutting have a superior combination of strength and ductility as yield strength of ~487 MPa, tensile strength of ~655 MPa, and total elongation of ~32.7%. Full article

The damage properties in the shear-punched surface layer, such as the strain-hardening increment, strain-induced martensite fraction, and initiated micro-crack/void characteristics at the shear and break sections, were experimentally evaluated to relate to the stretch-flangeability in three types of low-carbon high-strength TRIP-aided steel with different matrix structures. In addition, the surface layer damage properties were related to the mean normal stress developed on shear-punching and microstructural properties. The shear-punched surface damage of these steels was experimentally confirmed to be produced under the mean normal stress of negative to 0 MPa. TRIP-aided bainitic ferrite (TBF) steel had the smallest surface layer damage, featuring a significantly suppressed micro-crack/void initiation. This was due to the fine bainitic ferrite lath matrix structure, a low strength ratio of the second phase to the matrix structure, and the high mechanical stability of the retained austenite. On the other hand, the surface layer damage of TRIP-aided annealed martensite (TAM) steel was suppressed next to TBF steel and was smaller than that of TRIP-aided polygonal ferrite (TPF) steel. The surface layer damage was also characterized by a large plastic strain, a large amount of strain-induced martensite transformation, and a relatively suppressed micro-crack/void formation, which resulted from an annealed martensite matrix and a large quantity of retained austenite. The excellent stretch-flangeability of TBF steel might be caused by the suppressed micro-crack/void formation and high crack propagation/void connection resistance. The next high stretch-flangeability of TAM steel was associated with a small-sized micro-crack/void initiation and high crack growth/void connection resistance. Full article

In this study, a 780 MPa grade ferritic-bainitic dual-phase steel with excellent matching of strength-plasticity and formability was developed using thermomechanical control processing. Optical microscopy, Scanning electron microscopy, and Electron Backscatter Diffraction techniques were used to characterize the microstructure comprehensively, and the effects of coiling temperature on the microstructure, the strength-plasticity, and hole-expansion ratio of the test steels were thoroughly investigated. The results showed that the test steel had an excellent combination of ferrite and bainite at the coiling temperature of 520 °C, 23.7 and 76.3%, respectively, with a hole expansion ratio of 58.5 ± 2.8%. The uniformity of the microstructure was the key to obtaining a high expansion ratio in ferrite-bainite dual-phase steels. The test steels formed granular bainite at low-temperature coiling, while polygonal ferrite was promoted at high-temperature coiling. The effect of coiling temperature on grain size is small. Dislocations were redistributed during high-temperature coiling, resulting in a decrease in dislocation density. The higher elongation and hole expansion rate at higher coiling temperatures were attributed to increased polygonal ferrite content, reduced grain size, and enhanced TRIP effect. When coiling at low temperatures, the agglomeration of polygonal ferrite or granular bainite tends to result in a non-uniform distribution of the soft and hard phases of the matrix. At the same time, the strong texture parallel to the rolling direction has a significant difference in plasticity in different directions, leading to non-uniform deformation, which is liable to stress concentration, causing crack nucleation and extension in the hole expanding process, thus reducing the hole expansion performance. Full article

The microstructural evolution of SCM435 cold heading steel at different cooling rates was investigated by means of scanning electron microscopy, TEM, XRD, and electron backscatter diffraction. The results show that the cooling rate has a significant effect on the microstructure of the experimental steel. With an acceleration in the cooling, the microstructure of the steel gradually changed from ferrite and pearlite to ferrite, pearlite, and granular bainite; finally, the pearlite disappeared, and the microstructure changed to acicular ferrite, bainite, and martensite. With an increase in the cooling rate, the morphology of the carbide underwent an evolution from sheet carbide to short-rod carbide, granular carbide, and ultimately thin-strip carbide. With the acceleration in cooling, the proportion of large-angle grain boundaries gradually decreased, and the area of small-angle grain boundaries gradually increased. When the cooling rate was 0.1 °C/s, the proportion of large-angle grain boundaries was as high as 52.8%, and the dislocation density was only 1.91 × 10¹² cm⁻². When the cooling rate was 2.0 °C/s, the proportion of large-angle grain boundaries was only 27.1%, and the dislocation density increased to 5.38 × 10¹² cm⁻². With the increase in the cooling rate, the depth of the decarbonization layer and the thickness of the scale oxide gradually decreased, the proportion of the FeO phase in the scale phase gradually decreased, and the proportion of the Fe₃O₄ phase and Fe₂O₃ phase gradually increased. The tensile strength increased monotonously with the increase in cooling rate, whereas the elongation and area reduction first decreased, then increased, and then decreased. When the cooling rate was 1.0 m/s, the short rod and granular bainite in the material structure endowed the SCM435 steel with excellent strength and toughness matching, and the tensile strength and elongation of the steel reached 895 MPa and 24%, respectively. Full article

The quality and complexity demands of manufactured parts in sectors such as automotive and aeronautics lead to narrower process windows. This affects the repeatability and stability of the process, where material properties and process variations have a major impact. In bending processes, the bending angle is affected by variability in mechanical and microstructural properties, especially in high-strength materials. To address this, mechanical and microstructural characterization is crucial. This study conducted mechanical and microstructural characterization on five high-strength steels from different suppliers: three DP980 and two CP980. These materials are currently used by an industrial company in the automotive sector to manufacture a real product by means of U-bending, where a real issue of variability exists. Tensile tests were performed to quantify mechanical fluctuations. Microstructural analysis was also performed to determine the grain size and volume fractions of martensite and ferrite in the case of DP980, and ferrite, bainite, and retained austenite in the case of CP980. The largest variations were found for the hardening exponent, mean grain size, and elongation. To analyze their variability in an industrial process, U-bending tests were carried out using the five materials and the bending angle after the springback was measured. A total of 250 pieces were bent for the different materials and press strokes. Variations up to 1.25° in bending angle were found between the five batches for the same press stroke. A quantitative correlation analysis was performed to estimate the influence of the different parameters on the bending angle, where sheet thickness and tensile strength were shown to be two of the most influential parameters. Knowing this influence based on the variability of the properties, a control approach can be developed to reduce defects. Full article

By utilizing hydrogen as an eco-friendly energy source, many metals are exposed to gaseous (pressurized) hydrogen. High-strength steels with an ultimate tensile strength of 800 MPa and above are especially susceptible to hydrogen-induced fracturing, also referred to as hydrogen embrittlement (HE). Both the microstructure and phase fractions within the steel, as well as lattice distortion, carbide precipitation, residual stress, etc., significantly affect the susceptibility to HE. Among others, one important cause for this observation is found in the locally varying hydrogen solubility within different microstructural phases such as martensite, bainite, pearlite, and ferrite. Both a thorough understanding of the HE mechanisms and taking countermeasures in the form of alloying design require an accurate analysis of local diffusive hydrogen concentrations within the material. Thermal analysis methods such as Thermal Desorption Mass Spectrometry only display an integral hydrogen concentration throughout the whole sample volume. To analyze the local diffusive hydrogen concentration, novel measuring techniques with a high special resolution must therefore be utilized. The current research presents first-of-its-kind hydrogen analyses by means of the electrochemical microcapillary cell. Using a 10 µm tip opening diameter allows for conducting local diffusive hydrogen measurements within individual grains of multi-phase carbon steel C60E (1.1221). The results confirm that hydrogen is distributed heterogeneously within multi-phase steels. Considering the individual phase fractions and the respective local diffusive hydrogen concentrations, a total diffusive hydrogen concentration can be calculated. The obtained value is in good agreement with reference thermal hydrogen analyses. Our results suggest that electrochemical microcapillary cell measurements offer great potential for further studies, which will provide a better understanding of HE and local hydrogen accumulation. Full article

The tensile properties and work-hardening behavior of austempered bainitic steels below martensite start temperature (Ms) were investigated and compared with those of bainitic steel austempered above Ms. The results show that the tensile strength and yield strength increased from 1096 MPa and 734 MPa to 1203 MPa and 951 MPa, respectively, when the austempering temperature was decreased from 400 °C to 300 °C. However, the total elongation decreased from 23% to 16%. The martensite-retained austenite blocks and bainitic ferrite laths are significantly refined. With a decrease in the austempering temperature, the volume fraction of retained austenite decreased from 15.4 vol% to 6.2 vol%. The carbon content in retained austenite increased from 1.12 wt% to 1.69 wt%. All tensile specimens exhibited three stages of deformation in the differential Crussard−Jaoul (C−J) models. The difference in ductility is mainly attributed to the transformation of the retained austenite blocks into strain-induced martensite during deformation. The initial content of retained austenite is the main factor affecting the ductility of bainitic steels. Therefore, the work-hardening ability of austempered bainitic steel above Ms is higher than that of bainitic steel below Ms. Full article