Search

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. [ABSTRACT FROM AUTHOR]

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.

The dynamic recrystallization behavior of ultra-high strength boron-microalloyed steels optionally alloyed with niobium and molybdenum is analyzed in this paper. Multipass torsion tests were performed to simulate plate rolling conditions followed by direct quenching. The influence of alloy composition on the transformed microstructure was evaluated by means of EBSD, thereby characterizing the morphology of the austenite grain morphology after roughing and finishing passes. The results indicated that for Nb-microalloyed steel, partial dynamic recrystallization occurred and resulted in local clusters of fine-sized equiaxed grains dispersed within the pancaked austenitic structure. A recrystallized austenite fraction appeared and transformed into softer phase constituents after direct quenching. The addition of Mo was shown to be an effective means of suppressing dynamic recrystallization. This effect of molybdenum in addition to its established hardenability effects hence safeguards the formation of fully martensitic microstructures, particularly in direct quenching processes. Additionally, the circumstances initiating dynamic recrystallization were studied in more detail, and the interference of the various alloying elements with the observed phenomena and the potential consequences of dynamic recrystallization before quenching are discussed.

Warm forging is a promising near net shape technique, giving microstructural control and good surface quality, besides reducing or eliminating finishing steps compared to the conventional hot forging. In order to optimize said process it is essential to have a good knowledge and control of a number of metallurgical phenomena. This paper intends to clarify how phenomena such as dynamic recrystallization and phase transformation affect the behavior of operating loads and the microstructure of a 18CrNiMo7-6 steel, a material typically used in the forging of large parts, such as helicopters axes, which are subsequently surface hardened by carbonization. When this steel is alloyed with Nb, complete or partial recrystallization can be avoided, depending of process parameters. Stress-strain curves for the temperature and strain rate ranges of 700–900 °C and 0.001–10 s − 1 were obtained by compression of cylindrical samples of the steel, with and without addition of Nb. The mechanical behavior of the material, here represented by the stress-strain curves, was related to the microstructural evolution whose dependence with the macroscopic processing parameters (T, ε • ) was worked out. Further, the influence of said parameters on the efficiency of Nb as an austenite conditioner was determined. Finally, in order to enrich the analysis, process maps were constructed based on the Dynamic Material Model of the material (DMM) and the kinetics of recrystallization was described by modified Avrami's equation. [ABSTRACT FROM AUTHOR]

The dynamic recrystallization behavior of ultra-high strength boron-microalloyed steels optionally alloyed with niobium and molybdenum is analyzed in this paper. Multipass torsion tests were performed to simulate plate rolling conditions followed by direct quenching. The influence of alloy composition on the transformed microstructure was evaluated by means of EBSD, thereby characterizing the morphology of the austenite grain morphology after roughing and finishing passes. The results indicated that for Nb-microalloyed steel, partial dynamic recrystallization occurred and resulted in local clusters of fine-sized equiaxed grains dispersed within the pancaked austenitic structure. A recrystallized austenite fraction appeared and transformed into softer phase constituents after direct quenching. The addition of Mo was shown to be an effective means of suppressing dynamic recrystallization. This effect of molybdenum in addition to its established hardenability effects hence safeguards the formation of fully martensitic microstructures, particularly in direct quenching processes. Additionally, the circumstances initiating dynamic recrystallization were studied in more detail, and the interference of the various alloying elements with the observed phenomena and the potential consequences of dynamic recrystallization before quenching are discussed. [ABSTRACT FROM AUTHOR]

Abstract: In this investigation, by using continuous cooling torsion (CCT) testing, the transformation behavior of four microalloyed steels under two circumstances of austenite conditioning and non-conditioning was studied. A full scale hot-rolling schedule containing a 13-pass deformation was employed for the conditioning of the austenite. The CCT tests were then employed till temperature of ~540°C and the flow curves obtained from this process were analyzed. The initial and final microstructures of the steels were studied by optical and electron microscopes. Results show that alloying elements would decrease the transformation temperature. This effect intensifies with the gradual increase of Mo, Nb and Cu as alloying elements added to the microalloyed steels. As well, austenite conditioning increased the transformation start temperature due mainly to the promotion of polygonal ferrite formation that resulted from a pancaked austenite. The final microstructures also show that CCT alone would decrease the amount of bainite by inducing ferrite transformation in the two phase region. In addition, after the transformation begins, the deformation might result in the occurrence of dynamic recrystallization in the ferrite region. This could lead to two different ferrite grain sizes at the end of the CCT. Moreover, the Nb bearing steels show no sign of decreasing the strength level after the transformation begins in the non-conditioned situation and their microstructure is a mix of polygonal ferrite and bainite indicating an absence of probable dynamic recrystallization in this condition. In the conditioned cases, however, these steels show a rapid decrease of the strength level and their final microstructures insinuate that ferrite could have undergone a dynamic recrystallization due to deformation. Consequently, no bainite was seen in the austenite conditioned Nb bearing steels. The pancaking of austenite in the latest cases produced fully polygonal ferrite structures. [Copyright &y& Elsevier]

Abstract: The main emphasis of this study has been placed on thermomechanical (TM) processing simulations of Mn–Si transformation-induced plasticity (TRIP)-aided steel using press forging. In order to rationalize the retained austenite volume fraction in multiphase structure, three TM schedules were employed at experiment where austenite different conditioning was considered. The choice of applied strain and thermal parameters had strong impact on austenite conditioning and consequently on the kinetics of isothermal ferrite and bainite transformation. The different multiphase structure characteristics were obtained after TM, with different volume fraction of retained austenite. Modification of structural characteristics consequently influenced the mechanical behaviour of TRIP steel. The present work also reports on the results received from in situ neutron diffraction experiments focused on monitoring of retained austenite transformation during mechanical incremental straining and evaluation of the lattice strains redistribution in present phases. [Copyright &y& Elsevier]

Abstract: The main emphasis of this study has been placed on thermomechanical processing simulations performed using press forging. In order to develop a comprehensive understanding of the effect of large deformation of bulk specimens, introduced in the austenite recrystallized, nonrecrystallized and intercritical temperature regions, on the microstructure evolution and mechanical properties, the C–Si–Mn transformation induced plasticity (TRIP)-aided steel was used for experimental. The choice of applied strain and thermal parameters had strong impact on austenite transformation kinetics and obtained final multiphase structure, which clearly modified the mechanical properties of TRIP steel. To rationalize the retained austenite volume fraction employing different thermomechanical schedules, the effect of austenite conditioning on ferrite transformation was considered at the first step and the subsequent isothermal bainite transformation treatment was optimized in the second step. The microstructures of treated samples were characterized by optical metallography and scanning electron microscopy. The tensile tests were conducted to characterize the development of TRIP effect with respect to volume fraction of retained austenite in multiphase structure. The volume fraction of retained austenite was measured using X-ray and neutron diffraction methods. The specific design of thermomechanical schedule with varying of strain and temperature values affected multiphase structure development in TRIP bulk steel and yielded in good combination of high tensile strength over 900MPa and elongation over 20% nearly at all experimental schedules. The complex relationship among the volume fraction of the retained austenite, the morphology and distribution of phases present in the microstructure, and mechanical properties of TRIP steel were revealed. These findings suggested that the intercritical treatment conditions prior austenite transformation appeared to have decisive effect on steel refinement and forming of convenient multiphase structure, which should provide both, the high strength and sufficient ductility of bulky products. [Copyright &y& Elsevier]

Abstract: Steels with higher strength, ductility and improved fatigue behavior are required for light-weight structures in the transportation industry. It is shown that for martensitic steels the combination of microalloying and an optimized thermomechanical treatment (TMT) results in increased strength and improved ductility. Proper conditioning of the austenite by deformation either refines the austenitic grains or generates a dislocation substructure that is inherited to the martensite structure. In contrast to simply quenched and tempered martensite with no prior deformation, the thermomechanically processed martensite exhibits a more refined structure with refined blocks and is free of grain boundary carbides. Addition of vanadium is beneficial in controlling the austenite grain size during austenitization and for the stabilization of the austenite defect structures that are produced by deformation. It enables to use higher deformation temperatures for TMT, i.e. lower rolling forces can be applied in an industrial process. It is possible to increase the strength and ductility of conventionally heat treated Si–Cr steel by addition of vanadium or by TMT, but the highest improvement is achieved through the combination of both. In this study, an increase of more than 600MPa in the ultimate tensile strength and an improvement of 40% in the reduction area are reported. [Copyright &y& Elsevier]

Abstract: Experimental simulations of thermomechanical processing (TM) using press forging of cylindrical specimens of C–Si–Mn TRIP (transformation induced plasticity) steel were performed. The aim was to understand the effect of the hot deformation introduced in the austenite recrystallization, nonrecrystallization and intercritical temperature regions on the austenite conditioning and microstructure evolution of the studied steel. The microstructures resulting from a variety of TM processing were characterized by light and scanning electron microscopy. Volume fraction of retained austenite was evaluated by neutron diffraction. It was found that the choice of applied strain and temperature of deformation prior transformation had strong impact on transformation kinetics, final multiphase (ferrite, bainite and retained austenite) structure characteristics and mechanical properties of TRIP steel. A good combination of strength and ductility was achieved in the material with refined microstructure and sufficient volume fraction of retained austenite. [Copyright &y& Elsevier]

Abstract: In situ neutron diffraction experiment was employed for monitoring of conditioned austenite transformation to ferrite, and also for the retained austenite stability evaluation during subsequent mechanical loading. The progress in the austenite decomposition to ferrite is monitored at different transformation temperatures. The relevant information on the course of the transformation is extracted from neutron diffraction spectra. The useful information was also obtained on the retained austenite stability in the TRIP steel during mechanical testing. The in situ neutron diffraction experiments were conducted at two different diffractometers to assess the reliability of the neutron diffraction technique at monitoring the transformation of the retained austenite during room temperature tensile testing. In both experiments the neutron investigation was focused on the volume fraction quantification of the retained austenite as well as on internal stresses rising in structure phases due to the retained austenite transformation. [Copyright &y& Elsevier]

To achieve the optimum strength of Nb–Ti microalloyed steel, TMT was applied by varying the reheating temperature, the percentage reduction in the recrystallization region, the percentage reduction in the non-recrystallization region and the coiling temperature. Specimens were reheated between 900 and 1250 °C for 25 min to select the appropriate reheating temperature of 1150 °C. Then they were deformed at 1150 °C in the range of 30–50%, subsequently were deformed at 860 °C in the range of 40–70%, then cooled down to a temperature between 520 and 670 °C and finally were furnace cooled over about 24 h to ambient temperature. The prior austenite grain size, the recrystallized austenite grain size, the effective nucleation area (Sv), and the ferrite grain size were determined by optical microscopy. The mechanical properties of strips with different coiling ferrite–pearlite structure were determined by the tensile test. It was found that there is an abnormal grain growth of austenite grains in the reheating stage at 1050 °C. The increase of percentage reduction in the non-recrystallization region from 40 to 70% results in increase of Sv due to the greater amount of elongated austenite grains and deformation bands present in the interior, of the grains consequently very fine ferrite structures were formed. Decrease of the coiling temperature from 670 to 520 °C resulted in refining the ferrite grain size, thereby improving the tensile strength and yield-strength. Coiling at 550 °C results in an additional yield-strength increase due to the precipitation-strengthening effect. [Copyright &y& Elsevier]

The effects of different hot strip rolling process parameters on the phase transformation and the final properties of a microalloyed bainitic steel are investigated. Thermomechanical tests are conducted to analyze the influence on strength and toughness. Transmission electron microscopy, nanoindentation, and electron probe microanalysis are used to investigate the microstructural features and their impact on the mechanical properties. In particular, it is found that a thermomechanical treatment of the microalloyed steel leads to an increase in strength, strain hardening, and toughness. This is related to the microstructure influenced by the transformation, the carbon partitioning, and partly to strain‐induced precipitations. The pronounced strain hardening is achieved by finely dispersed martensite/austenite islands. In contrast, coarse martensite/austenite constituents reduce the toughness. Further, strain induced titanium precipitations contribute to the high yield strength. [ABSTRACT FROM AUTHOR]