Estudio de la morfología bainítica y procesos de precipitación en aceros bainíticos bajos en C y aleados con Ti, Nb V y Mo

Tesis inédita de la Universidad Complutense de Madrid, Facultad de Ciencias Físicas, leída el 28-10-2022Los aceros bainíticos bajos en carbono (C) son una opción interesante en aquellas aplicaciones donde se requiere de una alta resistencia y tenacidad, una buena soldabilidad y formabilidad, y una baja temperatura de transición dúctil-frágil. En los últimos años, existe un interés en desarrollar nuevas composiciones que proporcionen una combinación de propiedades industrialmente interesantes a un precio competitivo. Las investigaciones realizadas sobre este tipo de aceros se han focalizado especialmente en el efecto que tiene una gran variedad de efectos aleantes en la microestructura final de la pieza. Sin embargo, todavía existen muchas incógnitas que resolver.Las microestructuras bainíticas bajas en C son microestructuras complejas, compuestas por dos o más fases cuya naturaleza, morfología y tamaño puede variar significativamente en función de la composición química y los parámetros del procesado. Tal complejidad ha llevado a diferentes propuestas de clasificación de morfologías bainíticas. No obstante, existen todavía ciertas discrepancias a la hora de definir alguna de estas morfologías bainíticas y su mecanismo de transformación, como es el caso de la conocida como bainita granular.,,Low carbon bainitic steels are an interesting option in those applications where high strength and toughness, good weldability and formability, and a low ductile-brittle transition temperature are required. Despite the high number of steel products offered by the steel market, the industry keeps requiring new compositions which provide a combination of industrially interesting properties at a competitive price. Research carried out on low carbon bainitic steels have focused especially on the effect of a wide variety of alloying effects on the final microstructure of the product. However, there are still many questions to answer.Low-carbon bainitic microstructures are complex microstructures, composed of two or more phases whose nature, morphology and size can vary significantly depending on the chemical composition and the processing parameters. Such complexity has led to different versions of the classification of bainitic morphologies. However, there are still discrepancies when it comes to defining some of these bainite morphologies and their transformation mechanism, as it is the case of granular bainite...Fac. de Ciencias FísicasTRUEunpu

Precipitation and grain growth modelling in Ti-Nb microalloyed steels

Peer ReviewedPostprint (author's final draft

Design and development of complex phase steels with improved combination of strength and stretch-flangeability

This study presents the design and development of a hot-rolled bainitic steel, presenting a good combination of strength and stretch-flangeability, for automotive applications. Ti, Nb, and Mo were added in the steel composition in order to control austenite grain sizes, enhance precipitation hardening, and promote the formation of bainite. This study focuses on the effect of process parameters on final microstructures and mechanical properties. These parameters are the finishing rolling temperature, which conditions the austenite microstructure before its decomposition, and the coiling temperature, which conditions the nature and morphology of the ferritic phases transformed. A preliminary study allowed to determine the austenite grain growth behavior during reheating, the recrystallization kinetics, and the continuous cooling transformation curves of the studied steel. Then, a first set of parameters was tested at a semi-industrial scale, which confirmed that the best elongation properties were obtained for homogeneous bainitic lath/granular microstructures, that can be produced by choosing a coiling temperature of 500 °C . When choosing those parameters for the final industrial trial, the microstructure obtained consisted of a homogeneous lath/granular bainite mixture that presented a Ultimate Tensile Strength of 830 MPa and a Hole Expansion Ratio exceeding 70%.Peer ReviewedPostprint (published version

ESTUDIO DE LA MORFOLOGÍA BAINÍTICA Y PROCESOS DE PRECIPITACIÓN EN ACEROS BAINÍTICOS BAJOS EN C Y ALEADOS CON Ti, Nb, V Y Mo

Las microestructuras bainíticas bajas en C son microestructuras complejas, compuestas por dos o más fases cuya naturaleza, morfología y tamaño puede variar significativamente en función de la composición química y los parámetros del procesado. Los mecanismos de endurecimiento se han centrado en incrementar la precipitación de carburos. En esta tesis se han diseñado tres aceros bainíticos bajos en C con diferentes cantidades de Molibdeno (Mo) y Vanadio (V). Con el objetivo de identificar las diferencias y similitudes entre la bainita granular y la bainita laja, se ha propuesto un nuevo tratamiento de temple y enfriamiento continuo que permite evitar la formación de cualquier transformación previa a la bainita y un posterior enfriamiento lento para estudiar de manera precisa la bainita granular y la bainita laja. El estudio detallado de la matriz ferrítica y el constituyente Martensita/austenita (M/A) presente en ambas microestructuras mediante difracción de electrones retrodispersados (EBSD), microscopía electrónica de transmisión (TEM) y microscopía electrónica de barrido (SEM) ha mostrado que ambas matrices poseen la misma jerarquía cristalográfica, por lo que ambas microestructuras se rigen por el mismo mecanismo de transformación que la bainita convencional. Por último, con el objetivo de explorar la precipitación de carburos dentro de la laja ferrítica, se ha llevado a cabo un estudio de la evolución de la distribución de los elementos aleantes dentro de las lajas de microestructuras bainíticas durante mantenimientos isotérmicos. Los resultados obtenidos mediante tomografía de sonda atómica (APT) han mostrado una evolución de los elementos aleantes dentro de la placa, especialmente los elementos formadores de carburos como el V, Mo y Nb, que tienden a agruparse con el C dentro de la laja. No obstante, los resultados obtenidos mediante TEM indican que tal agrupamiento no puede considerase un carburo debido a su falta de estructura cristalina.Ayuda Contrato Predoctoral de AEI BES-2017-080945Peer reviewe

Creep strength boosted by a high-density of stable nanoprecipitates in highchromium steels

There is a need worldwide to develop materials for advanced power plants with steam temperatures of 700°C and above that will achieve long-term creep-rupture strength and low CO2 emissions. The creep resistance of actual 9-12Cr steels is not enough to fulfil the engineering requirements above 600°C. In this paper, the authors report their advances in the improvement of creep properties of this type of steels by the microstructural optimization through nano-precipitation using two methodologies. 1) Applying a high temperature austenitization cycle followed by an ausforming step (thermomechanical treatment, TMT) to G91 steel, to increase the martensite dislocation density and, thus, the number density of MX precipitates (M = V ,Nb; X = C ,N) but at the expense of deteriorating the ductility. 2) Compositional adjustments, guided by computational thermodynamics, combined with a conventional heat treatment (no TMT), to design novel steels with a good ductility while still possessing a high number density of MX precipitates, similar to the one obtained after the TMT in G91. The microstructures have been characterized by optical, scanning and transmission electron microscopy, EBSD and atom probe tomography. The creep behaviour at 700°C has been evaluated under a load of 200 N using small punch creep tests.The authors would like to thank James Burns for assistance in performing APT sample preparation and running the APT experiments. The authors are grateful for the dilatometer tests by Phase Transformation laboratory and for the SEM microscopy by the Microscopy Lab at CENIM-CSIC. This work contributes to the Joint Programme on Nuclear Materials (JPNM) of the European Energy Research Alliance (EERA).Peer reviewe

Global variation in postoperative mortality and complications after cancer surgery: a multicentre, prospective cohort study in 82 countries

© 2021 The Author(s). Published by Elsevier Ltd. This is an Open Access article under the CC BY-NC-ND 4.0 licenseBackground: 80% of individuals with cancer will require a surgical procedure, yet little comparative data exist on early outcomes in low-income and middle-income countries (LMICs). We compared postoperative outcomes in breast, colorectal, and gastric cancer surgery in hospitals worldwide, focusing on the effect of disease stage and complications on postoperative mortality. Methods: This was a multicentre, international prospective cohort study of consecutive adult patients undergoing surgery for primary breast, colorectal, or gastric cancer requiring a skin incision done under general or neuraxial anaesthesia. The primary outcome was death or major complication within 30 days of surgery. Multilevel logistic regression determined relationships within three-level nested models of patients within hospitals and countries. Hospital-level infrastructure effects were explored with three-way mediation analyses. This study was registered with ClinicalTrials.gov, NCT03471494. Findings: Between April 1, 2018, and Jan 31, 2019, we enrolled 15 958 patients from 428 hospitals in 82 countries (high income 9106 patients, 31 countries; upper-middle income 2721 patients, 23 countries; or lower-middle income 4131 patients, 28 countries). Patients in LMICs presented with more advanced disease compared with patients in high-income countries. 30-day mortality was higher for gastric cancer in low-income or lower-middle-income countries (adjusted odds ratio 3·72, 95% CI 1·70–8·16) and for colorectal cancer in low-income or lower-middle-income countries (4·59, 2·39–8·80) and upper-middle-income countries (2·06, 1·11–3·83). No difference in 30-day mortality was seen in breast cancer. The proportion of patients who died after a major complication was greatest in low-income or lower-middle-income countries (6·15, 3·26–11·59) and upper-middle-income countries (3·89, 2·08–7·29). Postoperative death after complications was partly explained by patient factors (60%) and partly by hospital or country (40%). The absence of consistently available postoperative care facilities was associated with seven to 10 more deaths per 100 major complications in LMICs. Cancer stage alone explained little of the early variation in mortality or postoperative complications. Interpretation: Higher levels of mortality after cancer surgery in LMICs was not fully explained by later presentation of disease. The capacity to rescue patients from surgical complications is a tangible opportunity for meaningful intervention. Early death after cancer surgery might be reduced by policies focusing on strengthening perioperative care systems to detect and intervene in common complications. Funding: National Institute for Health Research Global Health Research Unit

Effects of hospital facilities on patient outcomes after cancer surgery: an international, prospective, observational study

© 2022 The Author(s). Published by Elsevier Ltd. This is an Open Access article under the CC BY 4.0 licenseBackground: Early death after cancer surgery is higher in low-income and middle-income countries (LMICs) compared with in high-income countries, yet the impact of facility characteristics on early postoperative outcomes is unknown. The aim of this study was to examine the association between hospital infrastructure, resource availability, and processes on early outcomes after cancer surgery worldwide. Methods: A multimethods analysis was performed as part of the GlobalSurg 3 study—a multicentre, international, prospective cohort study of patients who had surgery for breast, colorectal, or gastric cancer. The primary outcomes were 30-day mortality and 30-day major complication rates. Potentially beneficial hospital facilities were identified by variable selection to select those associated with 30-day mortality. Adjusted outcomes were determined using generalised estimating equations to account for patient characteristics and country-income group, with population stratification by hospital. Findings: Between April 1, 2018, and April 23, 2019, facility-level data were collected for 9685 patients across 238 hospitals in 66 countries (91 hospitals in 20 high-income countries; 57 hospitals in 19 upper-middle-income countries; and 90 hospitals in 27 low-income to lower-middle-income countries). The availability of five hospital facilities was inversely associated with mortality: ultrasound, CT scanner, critical care unit, opioid analgesia, and oncologist. After adjustment for case-mix and country income group, hospitals with three or fewer of these facilities (62 hospitals, 1294 patients) had higher mortality compared with those with four or five (adjusted odds ratio [OR] 3·85 [95% CI 2·58–5·75]; p<0·0001), with excess mortality predominantly explained by a limited capacity to rescue following the development of major complications (63·0% vs 82·7%; OR 0·35 [0·23–0·53]; p<0·0001). Across LMICs, improvements in hospital facilities would prevent one to three deaths for every 100 patients undergoing surgery for cancer. Interpretation: Hospitals with higher levels of infrastructure and resources have better outcomes after cancer surgery, independent of country income. Without urgent strengthening of hospital infrastructure and resources, the reductions in cancer-associated mortality associated with improved access will not be realised. Funding: National Institute for Health and Care Research