Effect of quenching strategy and Nb-Mo sdditions on phase transformations and quenchability of high-strength boron steels

The application of direct quenching after hot rolling of plates is being employed in the production of ultra-high-strength hot rolled plates. When heavy gauge plates are produced, the complexity involve in achieving high cooling rates in the plate core is increased and the formation of undesirable soft phases within martensite is common. In the current paper, both direct quenching and conventional quenching (DQ and CQ) processing routes were reproduced by dilatometry tests and continuous cooling transformation (CCT) diagrams were built for four different high-strength boron steels. The results indicate that the addition of Mo and Nb-Mo suppresses the ferritic region and considerably shifts the CCT diagram to lower transformation temperatures. The combination of DQ strategy and the Mo-alloying concept provides the best option to ensure hardenability and the formation of a fully martensitic microstructure, and to avoid the presence of soft phases in the center of thick plates

Production of a non-stoichiometric Nb-Ti HSLA steel by thermomechanical processing on a steckel mill

Obtaining high levels of mechanical properties in steels is directly linked to the use of special mechanical forming processes and the addition of alloying elements during their manufacture. This work presents a study of a hot-rolled steel strip produced to achieve a yield strength above 600 MPa, using a niobium microalloyed HSLA steel with non-stoichiometric titanium (titanium/nitrogen ratio above 3.42), and rolled on a Steckel mill. A major challenge imposed by rolling on a Steckel mill is that the process is reversible, resulting in long interpass times, which facilitates recrystallization and grain growth kinetics. Rolling parameters whose aim was to obtain the maximum degree of microstructural refinement were determined by considering microstructural evolution simulations performed in MicroSim-SM (R) software and studying the alloy through physical simulations to obtain critical temperatures and determine the CCT diagram. Four ranges of coiling temperatures (525-550 degrees C/550-600 degrees C/600-650 degrees C/650-700 degrees C) were applied to evaluate their impact on microstructure, precipitation hardening, and mechanical properties, with the results showing a very refined microstructure, with the highest yield strength observed at coiling temperatures of 600-650 degrees C. This scenario is explained by the maximum precipitation of titanium carbide observed at this temperature, leading to a greater contribution of precipitation hardening provided by the presence of a large volume of small-sized precipitates. This paper shows that the combination of optimized industrial parameters based on metallurgical mechanisms and advanced modeling techniques opens up new possibilities for a robust production of high-strength steels using a Steckel mill. The microstructural base for a stable production of high-strength hot-rolled products relies on a consistent grain size refinement provided mainly by the effect of Nb together with appropriate rolling parameters, and the fine precipitation of TiC during cooling provides the additional increase to reach the requested yield strength values

Caracterización de diagramas CCT para aceros microaleados con Nb.

En el presente proyecto se han elaborado 4 diagramas CCT para el mismo acero microaleado con Nb, pero distintas condiciones de deformación. El principal objetivo es analizar la influencia de la deformación acumulada en la austenita en la cinética de transformación. Para ello se han realizado ciclos que contienen deformaciones a temperaturas superiores e inferiores a la temperatura de no recristalización del material. En primer lugar, se han ensayado las muestras en el dilatómetro, según los ciclos definidos. Se han realizado enfriamientos a velocidades de: 0,1°C/s, 0,5°C/s, 1°C/s, 2°C/s, 5°C/s, 10°C/s, 20°C/s, 50°C/s, 100°C/s y 200°C/s en los 4 ciclos. En segundo lugar, se han preparado las muestras en metalografía para analizar la microestructura obtenida. Se han definido las fases existentes a partir de las observaciones realizadas en el microscopio óptico y los ensayos de dureza Vickers realizados. A partir de los datos obtenidos en el dilatómetro, se han calculado las temperaturas de inicio y fin de la transformación. En tercer lugar, se han elaborado los diagramas CCT con los datos obtenidos en el microscopio óptico, ensayos de dureza Vickers y temperaturas de inicio y fin de la transformación. En cuarto y último lugar, se ha elaborado un modelo matemático que permite predecir las temperaturas de inicio y fin de la transformación, durezas Vickers y tamaño de grano de la ferrita, para poder simplificar la elaboración de los diagramas CCT, puesto que realizar dichos diagramas de forma experimental es más costosa que realizarlas mediante datos obtenidos de forma analítica.In the present Final Degree Project 4 CCT diagrams have been developed, for the same microstructural composition of a steel alloyed with Nb, but different deformation conditions. The aim of this Project is to analyse the influence during phase transformation of the accumulated strain in austenite. For doing so 4 cycles were defined, depending on whether the deformation was applied in a temperature superior or inferior to the temperature of nonrecrystallization. In the first place, samples were subjected to dilatometry tests. For the 4 cycles, 10 tests were made, at the cooling rates of: 0,1°C/s, 0,5°C/s, 1°C/s, 2°C/s, 5°C/s, 10°C/s, 20°C/s, 50°C/s, 100°C/s y 200°C/s . In the second place, once the samples were tested, a proper preparation should be made in order to define the microstructure in the microscope and subject the samples to harness tests. From the data obtained in the dilatometer dilation curves were defined, so as to define the transformation start and finish temperatures. In the third place, CCT diagrams were defined. Perhaps, harness data, microscope analysis and transformation start and finish temperatures were used. Eventually, in the fourth place, a mathematical model was characterized for evaluating transformation start and finish temperatures, harness values and ferrite grain size. The principal aim of this models is to simplify the elaboration of CCT diagrams, as it is a timeconsuming affair

Caracterización de diagramas CCT para aceros microaleados con Nb.

En el presente proyecto se han elaborado 4 diagramas CCT para el mismo acero microaleado con Nb, pero distintas condiciones de deformación. El principal objetivo es analizar la influencia de la deformación acumulada en la austenita en la cinética de transformación. Para ello se han realizado ciclos que contienen deformaciones a temperaturas superiores e inferiores a la temperatura de no recristalización del material. En primer lugar, se han ensayado las muestras en el dilatómetro, según los ciclos definidos. Se han realizado enfriamientos a velocidades de: 0,1°C/s, 0,5°C/s, 1°C/s, 2°C/s, 5°C/s, 10°C/s, 20°C/s, 50°C/s, 100°C/s y 200°C/s en los 4 ciclos. En segundo lugar, se han preparado las muestras en metalografía para analizar la microestructura obtenida. Se han definido las fases existentes a partir de las observaciones realizadas en el microscopio óptico y los ensayos de dureza Vickers realizados. A partir de los datos obtenidos en el dilatómetro, se han calculado las temperaturas de inicio y fin de la transformación. En tercer lugar, se han elaborado los diagramas CCT con los datos obtenidos en el microscopio óptico, ensayos de dureza Vickers y temperaturas de inicio y fin de la transformación. En cuarto y último lugar, se ha elaborado un modelo matemático que permite predecir las temperaturas de inicio y fin de la transformación, durezas Vickers y tamaño de grano de la ferrita, para poder simplificar la elaboración de los diagramas CCT, puesto que realizar dichos diagramas de forma experimental es más costosa que realizarlas mediante datos obtenidos de forma analítica.In the present Final Degree Project 4 CCT diagrams have been developed, for the same microstructural composition of a steel alloyed with Nb, but different deformation conditions. The aim of this Project is to analyse the influence during phase transformation of the accumulated strain in austenite. For doing so 4 cycles were defined, depending on whether the deformation was applied in a temperature superior or inferior to the temperature of nonrecrystallization. In the first place, samples were subjected to dilatometry tests. For the 4 cycles, 10 tests were made, at the cooling rates of: 0,1°C/s, 0,5°C/s, 1°C/s, 2°C/s, 5°C/s, 10°C/s, 20°C/s, 50°C/s, 100°C/s y 200°C/s . In the second place, once the samples were tested, a proper preparation should be made in order to define the microstructure in the microscope and subject the samples to harness tests. From the data obtained in the dilatometer dilation curves were defined, so as to define the transformation start and finish temperatures. In the third place, CCT diagrams were defined. Perhaps, harness data, microscope analysis and transformation start and finish temperatures were used. Eventually, in the fourth place, a mathematical model was characterized for evaluating transformation start and finish temperatures, harness values and ferrite grain size. The principal aim of this models is to simplify the elaboration of CCT diagrams, as it is a timeconsuming affair

Evaluating Strengthening and Impact Toughness Mechanisms for Ferritic and Bainitic Microstructures in Nb, Nb-Mo and Ti-Mo Microalloyed Steels

Low carbon microalloyed steels show interesting commercial possibilities by combining different “micro”-alloying elements when high strength and low temperature toughness properties are required. Depending on the elements chosen for the chemistry design, the mechanisms controlling the strengths and toughness may differ. In this paper, a detailed characterization of the microstructural features of three different microalloyed steels, Nb, Nb-Mo and Ti-Mo, is described using mainly the electron backscattered diffraction technique (EBSD) as well as transmission electron microscopy (TEM). The contribution of different strengthening mechanisms to yield strength and impact toughness is evaluated, and its relative weight is computed for different coiling temperatures. Grain refinement is shown to be the most effective mechanism for controlling both mechanical properties. As yield strength increases, the relative contribution of precipitation strengthening increases, and this factor is especially important in the Ti-Mo microalloyed steel where different combinations of interphase and random precipitation are detected depending on the coiling temperature. In addition to average grain size values, microstructural heterogeneity is considered in order to propose a new equation for predicting ductile–brittle transition temperature (DBTT). This equation considers the wide range of microstructures analyzed as well as the increase in the transition temperature related to precipitation strengthening

Analysis of Strain Partitioning in Intercritically Deformed Microstructures via Interrupted Tensile Tests

Intercritically deformed steels present combinations of different types of ferrite, such as deformed ferrite (DF) and non-deformed ferrite (NDF) grains, which are transformed during the final deformation passes and final cooling step. Recently, a grain identification and correlation technique based on EBSD has been employed together with a discretization methodology, enabling a distinction to be drawn between different ferrite populations (NDF and DF grains). This paper presents a combination of interrupted tensile tests with crystallographic characterization performed by means of Electron Backscatter Diffraction (EBSD), by analyzing the evolution of an intercritically deformed micro-alloyed steel. In addition to this, and using the nanoindentation technique, both ferrite families were characterized micromechanically and the nanohardness was quantified for each population. NDF grains are softer than DF ones, which is related to the presence of a lower fraction of low-angle grain boundaries. The interrupted tensile tests show the different behavior of low- and high-angle grain boundary evolution as well as the strain partitioning in each ferrite family. NDF population accommodates most of the deformation at initial strain intervals, since strain reaches 10%. For higher strains, NDF and DF grains behave similarly to the strain applied

Effect of Microstructure on Post-Rolling Induction Treatment in a Low C Ti-Mo Microalloyed Steel

Cost-effective advanced design concepts are becoming more common in the production of thick plates in order to meet demanding market requirements. Accordingly, precipitation strengthening mechanisms are extensively employed in thin strip products, because they enhance the final properties by using a coiling optimization strategy. Nevertheless, and specifically for thick plate production, the formation of effective precipitation during continuous cooling after hot rolling is more challenging. With the aim of gaining further knowledge about this strengthening mechanism, plate hot rolling conditions were reproduced in low carbon Ti-Mo microalloyed steel through laboratory simulation tests to generate different hot-rolled microstructures. Subsequently, a rapid heating process was applied in order to simulate induction heat treatment conditions. The results indicated that the nature of the matrix microstructure (i.e., ferrite, bainite) affects the achieved precipitation hardening, while the balance between strength and toughness depends on the hot-rolled microstructure

Exploitation of the synergetic effect of Mo and Nb on high strength quenched and tempered boron steels

In response to the demanding strength and impact resistance market requirements, plates and pipes are usually quenched and tempered (Q&T) for several applications. Regarding the production of these high strength steels, the direct quenching process offers operational and economic advantages compared to the conventional quenching route. In this study, the applicability of the direct quenching strategy is evaluated. Moreover, the addition of boron as an alloying element is a common practice in high strength steels to ensure hardenability and promote bainitic and martensitic microstructures. In some cases, the addition of boron is not enough to ensure full martensite formation, and thus, the addition of Nb and Mo can increase the efficiency of boron. This thesis, is in the frame of an industrial project developed thanks to the collaboration of the International Molybdenum Association (IMOA), Dillinger and Ceit. This thesis is focused on the study of the addition of Nb, Mo and NbMo in boron high strength steels in terms of microstructure and mechanical properties. The results extracted during this project were useful for the development of new steel grades that fulfil the most demanding market requirements. Successful results were achieved from the industrial trials performed at Dillinger. With the purpose of analysing the impact of chemical composition, the applied strategy on hot working behaviour, phase transformation and mechanical properties, several thermomechanical treatments were completed. By means of different laboratory tests, such as torsion, dilatometry and plane strain compression tests, plate hot rolling and Q&T process were simulated. This project is divided in three main tasks and each of the task is in line with the different steps involved in a real industrial process. The first task is focused on the hot working behaviour of the studied steels and multipass and double-pass torsion tests were done. Multipass torsion tests were performed in order to define the critical temperatures such as the non-recrystallization temperature (Tnr). Additionally, double-pass torsion tests were carried out to analyse the softening kinetics and to validate different approaches available regarding recrystallization kinetics. Furthermore, plate hot rolling simulations were performed in torsion, with the purpose of analysing dynamic recrystallization behaviour in more depth. The second task is focused on the phase transformation analysis. Direct quenching (DQ) and conventional quenching (CQ) processing routes were simulated by dilatometry tests and from the dilatometry curves, Continuous Cooling Transformation (CCT) diagrams were built. In the third task, the relationship between microstructure and the resulting mechanical properties were analysed. To that end, plane strain compression tests were performed for simulating quenching (Q), as well as quenching and subsequent tempering (Q&T). From the obtained samples, tensile and Charpy specimens were machined to analyse the tensile and toughness properties. Regarding tensile properties, the contribution of different strengthening mechanism to yield strength (solid solution, grain size, dislocation density, carbon in solid solution and fine precipitation) were quantified. Likewise, the impact of different microstructural parameters (grain size, solid solution, dislocation density, presence of carbides, carbon in solid solution, fine precipitation and microstructural heterogeneity) on toughness were evaluated. Furthermore, an existing equation able to predict the impact transition temperature (ITT50%) for ferrite-pearlite and bainitic microstructures was extended to tempered martensitic microstructures. Regarding microstructural characterization, the obtained microstructures in each task were characterized using advanced characterization techniques, such as optical microscopy, field emission gun scanning electron microscopy (FEG-SEM) and transmission electron microscopy (TEM). The microstructural characterization was completed by the electron backscattered diffraction (EBSD) technique, in order to quantify the crystallographic unit sizes and dislocation densities.En respuesta a los exigentes requisitos del mercado en términos de resistencia y tenacidad, los planchones gruesos y las tuberías suelen ser templadas y revenidas (Q&T) para varias aplicaciones. En cuanto a la producción de estos aceros de alta resistencia, el proceso de temple directo ofrece ventajas operativas y económicas en comparación con la ruta de temple convencional. En esta tesis se evalúa la aplicabilidad de la estrategia de temple desde el punto de vista de microestructura y propiedades mecánicas. Además, la adición de boro como elemento de aleación es una práctica común en aceros de alta resistencia para asegurar la templabilidad y promover microestructuras bainíticas y martensíticas. En algunos casos, la adición de boro no es suficiente para asegurar la formación de una microstructura completamente martensítica. Es por ello por lo que en determinadas ocasiones, se requiere de la adición de Nb y Mo para aumentar la eficiencia del boro. El trabajo presentado en esta tesis, es un proyecto industrial desarrollado gracias a la colaboración de la Asociación Internacional del Molibdeno (IMOA) y la acería alemana Dillinger. Esta tesis se centra en el estudio de la adición de Nb, Mo y NbMo en aceros al boro de medio carbono en términos de microestructura y propiedades mecánicas. Los resultados extraídos durante este proyecto han sido útiles para el desarrollo de nuevos grados de acero que cumplen con los requisitos más exigentes del mercado. En base a los análisis y pruebas de laboratorio realizados en el marco de la presente tesis, Dillinger realizó recientemente algunos ensayos industriales, con resultados exitosos. Con el fin de analizar el impacto de la composición química, la estrategia aplicada sobre el comportamiento de conformado en caliente, la transformación de fase y las propiedades mecánicas, se han llevado a cabo varios tratamientos termomecánicos. Mediante diferentes ensayos de laboratorio, como torsión, dilatometría y compresión plana, se ha simulado el proceso de laminación en caliente ysu posterior temple y revenido. Este proyecto se divide en tres partes principales, cada una de las partes está en línea con los diferentes pasos que involucran un proceso industrial real. La primera parte se centra en el comportamiento de conformado en caliente de los aceros estudiados y se han llevado a cabo diferentes tipos de ensayos de torsión. Se han realizado ensayos de torsión multipasada para definir las temperaturas críticas como la temperatura de no-recristalización (Tnr). Además, se han llevado a cabo ensayos de torsión de doble pasada para analizar las cinéticas de recristalización estáticas y validar diferentes ecuaciones disponibles en la literatura. La segunda parte se centra en el análisis de transformación de fase. Se han realizado ensayos de dilatometría para simular las rutas de procesamiento de enfriamiento directo (DQ) y enfriamiento convencional (CQ) se y, a partir de las curvas de dilataciónobtenidas, se han construido diagramas de transformación de enfriamiento continuo (CCT). En la tercera parte de la tesis se ha analizado la relación entre la microestructura y las propiedades mecánicas resultantes. Para ello se han realizado ensayos de compresión plana para simular el temple (Q), así como el temple y posterior revenido (Q&T). A partir de las muestras obtenidas, se han mecanizado probetas de tracción y Charpy para analizar las propiedades de resistencia y tenacidad. En cuanto a las propiedades de resistencia, se ha cuantificado la contribución de diferentes mecanismos de endurecimiento (solución sólida, tamaño de grano, densidad de dislocación, carbono en solución sólida y precipitación fina) en el límite elástico. Asimismo, se ha evaluado el impacto de diferentes parámetros microestructurales en la tenacidad (tamaño de grano, solución sólida, densidad de dislocaciones, presencia de carburos, carbono en solución sólida, precipitación fina y heterogeneidad microestructural). Además, se ha desarrollado una ecuación existente capaz de predecir la temperatura de transición frágil-dúctil (ITT50%) para microestructuras martensíticas templadas y revenidas. En cuanto a la caracterización microestructural, las microestructuras obtenidas se han caracterizado utilizando técnicas avanzadas de caracterización, como microscopía óptica, microscopía electrónica de barrido (FEG-SEM) y microscopía electrónica de transmisión (TEM). La caracterización microestructural se ha completado mediante la técnica de difracción de electrones retrodispersados (EBSD), con el fin de cuantificar los tamaños de las unidades cristalográficas y la densidad de dislocaciones

Interaction between Microalloying Additions and Phase Transformation during Intercritical Deformation in Low Carbon Steels

Heavy gauge line pipe and structural steel plate materials are often rolled in the two-phase region for strength reasons. However, strength and toughness show opposite trends, and the exact effect of each rolling process parameter remains unclear. Even though intercritical rolling has been widely studied, the specific mechanisms that act when different microalloying elements are added remain unclear. To investigate this further, laboratory thermomechanical simulations reproducing intercritical rolling conditions were performed in plain low carbon and NbV-microalloyed steels. Based on a previously developed procedure using electron backscattered diffraction (EBSD), the discretization between intercritically deformed ferrite and new ferrite grains formed after deformation was extended to microalloyed steels. The austenite conditioning before intercritical deformation in the Nb-bearing steel affects the balance of final precipitates by modifying the size distributions and origin of the Nb (C, N). This fact could modify the substructure in the intercritically deformed grains. A simple transformation model is proposed to predict average grain sizes under intercritical deformation conditions

Exploitation of the synergetic effect of Mo and Nb on high strength quenched and tempered boron steels

In response to the demanding strength and impact resistance market requirements, plates and pipes are usually quenched and tempered (Q&T) for several applications. Regarding the production of these high strength steels, the direct quenching process offers operational and economic advantages compared to the conventional quenching route. In this study, the applicability of the direct quenching strategy is evaluated. Moreover, the addition of boron as an alloying element is a common practice in high strength steels to ensure hardenability and promote bainitic and martensitic microstructures. In some cases, the addition of boron is not enough to ensure full martensite formation, and thus, the addition of Nb and Mo can increase the efficiency of boron. This thesis, is in the frame of an industrial project developed thanks to the collaboration of the International Molybdenum Association (IMOA), Dillinger and Ceit. This thesis is focused on the study of the addition of Nb, Mo and NbMo in boron high strength steels in terms of microstructure and mechanical properties. The results extracted during this project were useful for the development of new steel grades that fulfil the most demanding market requirements. Successful results were achieved from the industrial trials performed at Dillinger. With the purpose of analysing the impact of chemical composition, the applied strategy on hot working behaviour, phase transformation and mechanical properties, several thermomechanical treatments were completed. By means of different laboratory tests, such as torsion, dilatometry and plane strain compression tests, plate hot rolling and Q&T process were simulated. This project is divided in three main tasks and each of the task is in line with the different steps involved in a real industrial process. The first task is focused on the hot working behaviour of the studied steels and multipass and double-pass torsion tests were done. Multipass torsion tests were performed in order to define the critical temperatures such as the non-recrystallization temperature (Tnr). Additionally, double-pass torsion tests were carried out to analyse the softening kinetics and to validate different approaches available regarding recrystallization kinetics. Furthermore, plate hot rolling simulations were performed in torsion, with the purpose of analysing dynamic recrystallization behaviour in more depth. The second task is focused on the phase transformation analysis. Direct quenching (DQ) and conventional quenching (CQ) processing routes were simulated by dilatometry tests and from the dilatometry curves, Continuous Cooling Transformation (CCT) diagrams were built. In the third task, the relationship between microstructure and the resulting mechanical properties were analysed. To that end, plane strain compression tests were performed for simulating quenching (Q), as well as quenching and subsequent tempering (Q&T). From the obtained samples, tensile and Charpy specimens were machined to analyse the tensile and toughness properties. Regarding tensile properties, the contribution of different strengthening mechanism to yield strength (solid solution, grain size, dislocation density, carbon in solid solution and fine precipitation) were quantified. Likewise, the impact of different microstructural parameters (grain size, solid solution, dislocation density, presence of carbides, carbon in solid solution, fine precipitation and microstructural heterogeneity) on toughness were evaluated. Furthermore, an existing equation able to predict the impact transition temperature (ITT50%) for ferrite-pearlite and bainitic microstructures was extended to tempered martensitic microstructures. Regarding microstructural characterization, the obtained microstructures in each task were characterized using advanced characterization techniques, such as optical microscopy, field emission gun scanning electron microscopy (FEG-SEM) and transmission electron microscopy (TEM). The microstructural characterization was completed by the electron backscattered diffraction (EBSD) technique, in order to quantify the crystallographic unit sizes and dislocation densities.En respuesta a los exigentes requisitos del mercado en términos de resistencia y tenacidad, los planchones gruesos y las tuberías suelen ser templadas y revenidas (Q&T) para varias aplicaciones. En cuanto a la producción de estos aceros de alta resistencia, el proceso de temple directo ofrece ventajas operativas y económicas en comparación con la ruta de temple convencional. En esta tesis se evalúa la aplicabilidad de la estrategia de temple desde el punto de vista de microestructura y propiedades mecánicas. Además, la adición de boro como elemento de aleación es una práctica común en aceros de alta resistencia para asegurar la templabilidad y promover microestructuras bainíticas y martensíticas. En algunos casos, la adición de boro no es suficiente para asegurar la formación de una microstructura completamente martensítica. Es por ello por lo que en determinadas ocasiones, se requiere de la adición de Nb y Mo para aumentar la eficiencia del boro. El trabajo presentado en esta tesis, es un proyecto industrial desarrollado gracias a la colaboración de la Asociación Internacional del Molibdeno (IMOA) y la acería alemana Dillinger. Esta tesis se centra en el estudio de la adición de Nb, Mo y NbMo en aceros al boro de medio carbono en términos de microestructura y propiedades mecánicas. Los resultados extraídos durante este proyecto han sido útiles para el desarrollo de nuevos grados de acero que cumplen con los requisitos más exigentes del mercado. En base a los análisis y pruebas de laboratorio realizados en el marco de la presente tesis, Dillinger realizó recientemente algunos ensayos industriales, con resultados exitosos. Con el fin de analizar el impacto de la composición química, la estrategia aplicada sobre el comportamiento de conformado en caliente, la transformación de fase y las propiedades mecánicas, se han llevado a cabo varios tratamientos termomecánicos. Mediante diferentes ensayos de laboratorio, como torsión, dilatometría y compresión plana, se ha simulado el proceso de laminación en caliente ysu posterior temple y revenido. Este proyecto se divide en tres partes principales, cada una de las partes está en línea con los diferentes pasos que involucran un proceso industrial real. La primera parte se centra en el comportamiento de conformado en caliente de los aceros estudiados y se han llevado a cabo diferentes tipos de ensayos de torsión. Se han realizado ensayos de torsión multipasada para definir las temperaturas críticas como la temperatura de no-recristalización (Tnr). Además, se han llevado a cabo ensayos de torsión de doble pasada para analizar las cinéticas de recristalización estáticas y validar diferentes ecuaciones disponibles en la literatura. La segunda parte se centra en el análisis de transformación de fase. Se han realizado ensayos de dilatometría para simular las rutas de procesamiento de enfriamiento directo (DQ) y enfriamiento convencional (CQ) se y, a partir de las curvas de dilataciónobtenidas, se han construido diagramas de transformación de enfriamiento continuo (CCT). En la tercera parte de la tesis se ha analizado la relación entre la microestructura y las propiedades mecánicas resultantes. Para ello se han realizado ensayos de compresión plana para simular el temple (Q), así como el temple y posterior revenido (Q&T). A partir de las muestras obtenidas, se han mecanizado probetas de tracción y Charpy para analizar las propiedades de resistencia y tenacidad. En cuanto a las propiedades de resistencia, se ha cuantificado la contribución de diferentes mecanismos de endurecimiento (solución sólida, tamaño de grano, densidad de dislocación, carbono en solución sólida y precipitación fina) en el límite elástico. Asimismo, se ha evaluado el impacto de diferentes parámetros microestructurales en la tenacidad (tamaño de grano, solución sólida, densidad de dislocaciones, presencia de carburos, carbono en solución sólida, precipitación fina y heterogeneidad microestructural). Además, se ha desarrollado una ecuación existente capaz de predecir la temperatura de transición frágil-dúctil (ITT50%) para microestructuras martensíticas templadas y revenidas. En cuanto a la caracterización microestructural, las microestructuras obtenidas se han caracterizado utilizando técnicas avanzadas de caracterización, como microscopía óptica, microscopía electrónica de barrido (FEG-SEM) y microscopía electrónica de transmisión (TEM). La caracterización microestructural se ha completado mediante la técnica de difracción de electrones retrodispersados (EBSD), con el fin de cuantificar los tamaños de las unidades cristalográficas y la densidad de dislocaciones