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

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

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

Effect of Nb and Mo on austenite microstructural evolution during hot deformation in boron High strength steels

This work has focused on the study of hot working behavior of boron high strength steels microalloyed with different combinations of Nb and/or Mo. The role of Nb and Mo during the hot deformation of low carbon steels is well known: both mainly retard austenite recrystallization, leading to pancaked austenite microstructures before phase transformation and to refined room temperature microstructures. However, the design of rolling schedules resulting in properly conditioned microstructures, requires microstructural evolution models that take into account the effect of the different alloying elements. In this specific case, the effect that high levels of molybdenum (0.5 pct) have in the recrystallization delay was evaluated. In that respect, hot torsion tests were performed in this work to investigate the microstructural evolution during hot deformation of four boron steels, with different Nb (0.025 pct) and Mo (0.5 pct) combinations. The retardation in recrystallization kinetics was modeled in all cases and measured kinetics agree with those predicted by equations previously developed for Nb–Mo microalloyed steels with lower Mo concentrations (< 0.3 pct). The strain-induced precipitation in the Nb and Nb–Mo bearing steels was also characterized. Finally, the fractional softening evolution during multipass rolling simulations was compared with MicroSim® model predictions, showing a good agreement with experimental results

Effect of Nb and Mo on austenite microstructural evolution during hot deformation in boron High strength steels

This work has focused on the study of hot working behavior of boron high strength steels microalloyed with different combinations of Nb and/or Mo. The role of Nb and Mo during the hot deformation of low carbon steels is well known: both mainly retard austenite recrystallization, leading to pancaked austenite microstructures before phase transformation and to refined room temperature microstructures. However, the design of rolling schedules resulting in properly conditioned microstructures, requires microstructural evolution models that take into account the effect of the different alloying elements. In this specific case, the effect that high levels of molybdenum (0.5 pct) have in the recrystallization delay was evaluated. In that respect, hot torsion tests were performed in this work to investigate the microstructural evolution during hot deformation of four boron steels, with different Nb (0.025 pct) and Mo (0.5 pct) combinations. The retardation in recrystallization kinetics was modeled in all cases and measured kinetics agree with those predicted by equations previously developed for Nb–Mo microalloyed steels with lower Mo concentrations (< 0.3 pct). The strain-induced precipitation in the Nb and Nb–Mo bearing steels was also characterized. Finally, the fractional softening evolution during multipass rolling simulations was compared with MicroSim® model predictions, showing a good agreement with experimental results

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