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  • Evaluation of essential work of fracture in a dual phase high strength steel sheet

     Gutierrez, David; Pérez Caro, Lluís; Lara, Antoni; Casellas Padro, Daniel; Prado Pozuelo, Jose Manuel
    Revista de Metalurgia
    Date of publication: 2013-01
    Journal article

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    Fracture toughness of advanced high strength steels (AHSS), can be used to optimize crash behavior of structural components. However it cannot be readily measured in metal sheet because of the sheet thickness. In this work, the Essential Work of Fracture (EWF) methodology is proposed to evaluate the fracture toughness of metal sheets. It has been successfully applied in polymers films and some metal sheets. However, their information about the applicability of this methodology to AHSS is relatively scarce. In the present work the fracture toughness of a Dual Phase (strength of 800 MPa) and drawing steel sheets has been measured by means of the EWF. The results show that the test requirements are met and also show the clear influence of notch radii on the measured values, specially for the AHSS grade. Thus, the EWF is postulated as a methodology to evaluate the fracture toughness in AHSS sheets.

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    Estudio comparativo del conformado en caliente de un acero microaleado usando criterios termodinámicos continuos y criterios fenomenológicos  Open access

     Alcelay Larrion, Jose Ignacio; Al Omar Mesnaoui, Anas; Prado Pozuelo, Jose Manuel
    Congreso Nacional de Tratamientos Térmicos y de Superficie
    Presentation's date: 2013-04-22
    Presentation of work at congresses

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    En las últimas décadas, los mapas de procesado han sido ampliamente utilizados para diseñar, controlar y optimizar los procesos de conformado en caliente. En el presente trabajo, se presenta un estudio comparativo entre mapas de procesado elaborados usando criterios termodinámicos continuos y criterios fenomenológicos. Por ello, se llevaron a cabo ensayos de compresión en caliente en el rango de temperaturas 900 - 1150 ºC y de velocidades de deformación i: = 10-4 - 10 s-1 . El comportamiento en caliente del acero estudiado ha sido estudiado analizando las curvas de fluencia y la interacción entre el comportamiento microestructural y los parámetros de control del proceso de deformación. El estudio comparativo de los mapas de procesado muestra la diferencia entre las posiciones de los distintos dominios de deformación plástica, según el criterio de inestabilidad usado. Además, el análisis de dichos mapas indica que el acero estudiado no sufre ningún tipo de inestabilidad plástica . Sin embargo, los mapas correspondientes a la deformación ¡; = 0,6 revelan la existencia de un dominio de recristalización dinámica, considerado como el dominio más eficiente dentro de la región segura de procesado. Este dominio está centrado a 1150 ºC y 10 s·1.

    In the last decades, processing maps have been widely used to design, control and optimize hot forming processes. In this paper, we present a comparative study of processing maps constructed using phenomenological and thermodynamic continuum criteria. Therefore, hot compression tests were carried out in the temperature and strain rate ranging from 900 to 1150 ºC and 10-• to 10 s-1. The hot deformation behavior of studied steel has been investigated analyzing the flow curves and the interaction between the microstructural behavior and parameters control of the deformation process. The comparative study of processing maps show the difference between the positions of plastic deformation domains predicted by the two instability criteria. The analysis of the maps indicates that the steel does not undergo any plastic instability. However, the maps corresponding to deformation E = 0,6 reveal the existence of a domain of dynamic recrystallization, considered the most efficient domain within the safe region processing. This domain is centered at 1150 ºC and 10 s-1.

  • Simulación de un tratamiento térmico mediante elementos finitos

     Giménez, Maria; Llobet, Albert; Riera Colom, Maria Dolores; Prado Pozuelo, Jose Manuel
    Congreso Nacional de Tratamientos Térmicos y de Superficie
    Presentation's date: 2013-04-22
    Presentation of work at congresses

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  • Efecto de los contenidos de C, Si y Mn en el comportamiento a fluencia en caliente de aceros de construcción al carbono. Aplicación a la obtención de productos largos laminados.  Open access

     Varela Castro, Gonzalo
    Defense's date: 2013-06-07
    Department of Materials Science and Metallurgy, Universitat Politècnica de Catalunya
    Theses

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    El presente trabajo de investigación se enfoca en el estudio del comportamiento a fluencia y ductilidad en caliente de los denominados aceros de construcción y su dependencia con la composición química, concretamente el C, Si y Mn. La primera se ha analizado mediante compresión uniaxial isotérmica en el intervalo de temperaturas comprendido entre 900 y 1100℃ a velocidades de deformación verdadera en el intervalo de 5·10-4 y 1·10-1 s-1. Mientras que el comportamiento de ductilidad de las aleaciones se ha analizado estudiando el comportamiento y los mecanismos de fragilización operantes bajo diferentes ciclos térmicos en un amplio intervalo de temperaturas (650-1100℃) y a una velocidad de deformación constante (1·10-3 s-1), a través de ensayos de tracción uniaxial isotérmicos y técnicas de microscopía electrónica. Para ello se han refundido ocho (8) aceros por medio de la técnica de refusión por electro-escoria (ESR), con composiciones químicas comprendidas dentro de los siguientes intervalos: Fe (% bal.), 0.15-0.15-0.45%C, 0.20-0.40%Si y 0.70-1.60%Mn (% de masa). Finalmente se propone un modelo constitutivo genérico que describe el comportamiento a fluencia en caliente de aleaciones de tipo Fe-C-Si-Mn, en donde todos los parámetros característicos del mismo son una función de la composición química de la aleación. Además, según datos de literatura, se propone una expresión que muestra la variación del factor pre-exponencial del coeficiente de autodifusión del Fe en austenita con la composición química. Dado el buen acuerdo entre los resultados experimentales y calculados, el presente enfoque es susceptible de ser aplicado en la modelización constitutiva de operaciones de conformado termomecánico de metales y aleaciones metálicas.

    The present research work is focused on the flow and ductility behavior at elevated temperatures of structural steels and their dependence with the chemical composition, specifically C, Si and Mn. The hot flow behavior was studied by isothermal uniaxial compression tests that ranged from 900 to 1100℃, while applying true strain rates that ranged from 5·10-4 to 1·10-1 s-1. Ductility behavior of the alloys was analyzed by studying the embrittlement mechanisms that operate under different thermal cycles in a wide range of temperatures (650 to 1050℃) at a constant true strain rate (1·10-3 s-1) by means of isothermal uniaxial tensile tests and electron microscopy techniques. For this purpose, eight different steels were refined via an electro-slag remelting (ESR) process to obtain the following range of chemical composition: Fe (% bal.), 0.15- 0.45% C, 0.20-0.40%Si and 0.70-1.60%Mn (mass %). Finally a generic constitutive model is proposed that describes the flow stress behavior at elevated temperatures of Fe-C-Si-Mn alloys, where all the characteristic parameters are a function of the chemical composition of the alloy. This work also uses literature values to demonstrate the variation of the pre-exponential factor of the self-diffusion coefficient of Fe in austenite with the chemical composition. Given the good agreement observed between the experimental and calculated data the presented approach is likely to have an application in constitutive modeling of thermomechanical processes of metals and metallic alloys.

  • Relación microestructura y propiedades de cobre sometido a deformación plástica severa mediante presión en canal angular de sección constante  Open access

     Higuera Cobos, Oscar Fabián
    Defense's date: 2013-07-12
    Universitat Politècnica de Catalunya
    Theses

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    Muestras de cobre electrolítico (ETP) y refinado al fuego (FRHC) previamente recocidas durante 30 minutos a una temperatura de 600 ºC, fueron sometidas a presión en canal angular de sección constante (ECAP) a temperatura ambiente hasta un máximo de 16 pases (ε ~ 1 por pase) siguiendo la ruta Bc en una matriz con un ángulo interno de 90º y un ángulo exterior de 37°. La caracterización microestructural se determinó mediante la técnica de difracción de electrones retrodispersados (EBSD). Las propiedades mecánicas después de cada pase se evaluaron por ensayos de tracción, microdureza y fatiga. Además, se utilizó la calorimetría de barrido diferencial (DSC) para estimar la energía almacenada durante la deformación y la temperatura de recristalización después de cada pase por ECAP. En cuanto a las propiedades eléctricas se correlacionaron con la energía asociada a los defectos inducidos durante el proceso ECAP y con el comportamiento microestructural observado. Las características microestructurales muestran una microestructura estable para el cobre FRHC y bimodal para el cobre ETP después de cuatro pases. Se analizó el efecto de la textura inicial sobre la evolución de la textura después del proceso de ECAP para todos los materiales. Los materiales en estado de recocido presentaron una marcada anisotropía, la cual se mantiene después del proceso ECAP, observándose una preferencia hacia la fibra <110>. Las texturas obtenidas para los cobres en estudio, después de cada pasada por ECAP, presentan orientaciones predominantes con distribuciones continuas a lo largo de fibras de orientación con textura de corte simple. Desde el punto de vista mecánico, se observó que la mejora en las propiedades mecánicas de los cobres ETP y FRHC ocurre gradualmente con el número de pasadas por ECAP, obteniéndose un estado estable después de 4 pases en la sección longitudinal y después de 2 pases en la sección transversal. En cuanto al comportamiento a fatiga, se observó una mejora significativa para los dos cobres en estudio después de 8 pases por ECAP en comparación con los de cobres recocidos. Por último, los resultados de DSC mostraron que la energía almacenada en los materiales se incrementa con el grado de deformación impartida por ECAP, mientras que la temperatura de recristalización disminuye significativamente. De manera similar, se observó que la conductividad eléctrica disminuye con el incremento de la deformación. Sin embargo, esta reducción no fue muy significativa si se tiene en cuenta el grado de deformación aplicada a las muestras.

    Samples of electrolytic tough pitch (ETP) and fire refined high conductivity (FRHC) copper previously annealed for 30 minutes at a temperature of 600 °C, were subjected to equal-channel angular pressing (ECAP) for up to 16 passes (ε ~ 1 per pass) at room temperature following route Bc, using an ECAP die with an inner angle of 90° and an outer angle of 37°. The microstructural characterization was determined by electron backscattered diffraction (EBSD) technique. The mechanical properties after each pass were evaluated by tensile tests, microhardess and fatigue. Additionally, differential scanning calorimetry (DSC) was used to estimate the stored deformation energy and the recrystallization temperature after each ECAP pass, while the electrical properties were correlated with the associated energy that results from the defects induced during the ECAP process and the observed microstructural behavior. The microstructural features show a stable microstructure for the FRHC copper and a bimodal microstructure for the ETP copper after four passes. The effect of the initial texture on the evolution of texture after the ECAP process for all materials was analyzed. The annealed materials showed a marked anisotropy, which remains after the ECAP process, showing a preference for the <110> fiber. The textures obtained for the investigated coppers after each ECAP pass exhibit predominant orientations with continuous distributions along the orientation fiber with simple shear texture. From a mechanical point of view, an improvement in the mechanical properties for the ETP1 and FRHC coppers occurs gradually with the number of ECAP passes, obtaining a stable state after 4 passes in longitudinal section and after 2 passes in cross section. The fatigue behavior shows a significant improvement in the fatigue life for both the ETP2 and FRHC coppers subjected to 8 ECAP passes compared with the annealed coppers. Finally, the DSC results showed that the stored energy in materials increases with the degree of deformation imparted by ECAP, while the recrystallization temperature significantly decreases. Similarly, it was found that the electrical conductivity decreases with increasing deformation. However, this reduction was not significant when taking into account the degree of strain applied to the samples. The DSC results show that the stored energy rises on increasing ECAP deformation, while the recrystallization temperature decreases significatively. Similarly, electrical conductivity decreases up to a saturation state at increasing ECAP passes.

  • Criteria for prediction of plastic instabilities for hot working processes: Part II. Characterization of plastic flow of medium carbon microalloyed steel using phenomenological and continuum criteria

     Al Omar Mesnaoui, Anas; Alcelay Larrion, Jose Ignacio; Prado Pozuelo, Jose Manuel
    Welding international
    Date of publication: 2012
    Journal article

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    In the study of hot working processes, the knowledge of interaction between microstructural behaviour and control process parameters such as temperature, strain rate, and strain is very important. In the last decades, processing maps have been developed to design, control, and optimize the hot strain of various metallic materials. In this work, to study the hot workability of medium carbon microalloyed steel, during hot compression tests, a comparative study between two types of processing maps constructed using phenomenological and thermodynamic continuum criteria has been carried out. The analysis of the maps indicates that the studied steel does not undergo any type of plastic instability. However, the maps corresponding to the strain of ¿ = 0.6 reveal a domain of dynamic recrystallization, considered as the more efficient domain within the `safe¿ region process. This domain is centred at 1150°C and 10 s- 1. Also, the comparative study of the obtained results shows the difference between the positions of plastic strain domains predicted by the two criteria.

  • On the onset of dynamic recrystallization in steels

     Varela Castro, Gonzalo; Cabrera Marrero, Jose Maria; Prado Pozuelo, Jose Manuel
    Advanced materials research
    Date of publication: 2012
    Journal article

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    The knowledge of the flow behavior of metallic all oys subjected to hot forming operations is of particular interest for designers and engineers in the practice of industrial forming processes simulations (i.e. rolling mill). Nowadays dynamic recrystallization (DRX) is recognized as one of the most relevant and meaningful mechanis ms available for the control of microstructure. This mechanism occurs during hot forming operations over a wide range of metals and alloys and it is known to be as a powerful tool which can be used to the control of the microstructure and properties of alloys. Therefore is important to kno w, particularly in low stacking fault energy (SFE) materials, the precise time for which DRX is availa ble to act. At constant strain rate such time is defined by a critical strain, . Unfortunately this critical value is not directly measurable on the flow curve; as a result different methods have been developed to derive it. Focused on steels, in the present work the state of art on the critical strai n for the initiation of DRX is summarized and a review of the different methods and expressions for determining is included. The collected data is suitable to feeding constitutive models.

  • Criteria for prediction of plastic instabilities for hot working processes. Part I, Theoretical review

     Al Omar Mesnaoui, Anas; Prado Pozuelo, Jose Manuel
    Welding international
    Date of publication: 2012
    Journal article

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    Hot working processes often induce high levels of strain at high strain rates, and impose very complex multi-axial modes of solicitation. These processes are essentially limited by the apparition and development of plastic instabilities. These may be the direct cause of rapid crack propagation, which lead to a possible final rupture. The complexity of strain modes and the simultaneous intervention of several parameters have led many researchers to develop various criteria, with different approaches, to predict the occurrence of defects and to optimize the process control parameters. The aim of the present paper is to summarize the general characteristics of some instability criteria, widely used in the literature, for the prediction of plastic instabilities during hot working. It was considered appropriate to divide the work into two parts. Part I presents the phenomenological criteria for the prediction of plastic instabilities, based on descriptive observation of the microscopic phenomena of the strain (strain hardening and strain rate sensitivity), and discusses the continuum criteria based on the principle of maximum rate of entropy production of irreversible thermodynamics applied to continuum mechanics of large plastic flow. Also, this part provides a bibliographical discussion among several authors with regard to the physical foundations of the dynamic materials model. In Part II of the work, a comparative study has been carried out to characterize the flow instability during a hot working process of a medium carbon micro-alloyed steel using phenomenological and continuum criteria.

  • Mechanical behaviour of tools for shearing Ultra High-Strength Steels: Influence of the microstructure on fracture and fatigue micro-mechanisms of tool steels and evaluation of micro-mechanical damage in tools  Open access

     Picas Anfruns, Ingrid
    Defense's date: 2012-12-10
    Department of Materials Science and Metallurgy, Universitat Politècnica de Catalunya
    Theses

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    On account of environmental and safety related requirements, the majority of the most popular automotive manufacturers convey to introduce Ultra High Strength Steels (UHSS) and Press Hardened Steels (PHS) in the vehicle body-in-white. Nevertheless, the real success of UHSS and PHS parts implementation into the upcoming automobiles is being so far restricted by their harsh cold forming and post forming operations, e.g. shearing. Their high yield stress leads to accelerated damage and premature fracture of forming and cutting tools and this stands as great handicap of their manufacturing process. Tool materials require the maximum hardness ¿ toughness relationship so as to face up to such severe mechanical solicitations. The performance of tool steels during cold forming and shearing applications depends on the mechanical properties resulting from the complex interaction between their main microstructural constituents; i.e. primary alloy carbides and tempered martensite matrix (which consists in a fine distribution of nanometer-sized alloy carbides, the so-called secondary hardening carbides, precipitated in a martensite matrix).In this regard, the present Thesis has analysed the microstructure, macro- and micro-mechanical properties, crack nucleation and growth under monotonic loads, R-curve behaviour and fatigue crack nucleation and propagation mechanisms of four different cold work tool steels: 1.2379, UNIVERSAL, K360 and HWS. The mechanical and tribological behaviour of industrial and laboratory tools made of these steels have been studied, as well as the effects of shearing process parameters on their performance and quality of sheet sheared edges. The suitability of Linear Elastic Fracture Mechanics (LEFM) based models to estimate the performance of tools has been evaluated in order to predict their service lives. Rationalisation of damage on tool steels from a micro-mechanical point of view in laboratory samples as well as in real tools has permitted to identify the aspects that a model should consider in order to successfully predict tool lives. In addition, it has permitted to propose a new approach to estimate tool performances based on the reduction of tool steels mechanical properties with the increase of the number of strokes. 1.2379, UNIVERSAL and K360 are produced following conventional ingot metallurgy routes while HWS is obtained by Powder Metallurgy (PM). As follows from their different chemical compositions and manufacturing routes, these steels show different primary alloy carbide features and distributions in the microstructure. Nucleation of cracks in tool steels is due to the failure of primary carbides by cleavage (and inclusion particles in case of PM steels). Small cracks nucleated from primary carbides in ingot cast steels are initially very shallow, but they tend to grow to semi-circular shapes as they propagate through the matrix bands. Ingot cast tool steels present R-curve behaviour, i.e. small cracks nucleated in the microstructure have lower toughness values than those determined for long cracks in standard measurement tests. The main failure mechanism of tools for punching and slitting UHSS is chipping at the cutting edge. Plastic deformation and nucleation of cracks at the surface of tools occurs as a result of the high frictional forces during sliding against the sheet. Due to the high contact pressures at the surface, carbides within a narrow range of depth beneath the surface are broken and cracks are also nucleated from surface asperities. Chipping is then explained by the nucleation and coalescence of these cracks. In presence of marked grinding grooves, failure of tools is significantly accelerated. Models based on LEFM have proved to predict with rather poor accuracy the life of tools. A new approach to predict tool performances is proposed which holds the idea that the micro-mechanical properties of the metallic matrix in tool steels decrease due to the application of repetitive load

    Per tal de satisfer les creixents demandes de seguretat i medi ambient, la major part d’empreses del sector de l’automoció han convingut en la introducció d’acers d’ultra alta resistència mecànica (UHSS) i acers d’estampació en calent (PHS) per la fabricació de peces estructurals en vehicles . Tanmateix, l’èxit de la implementació d’aquestes peces es veu afectat per les dificultats que deriven de les seves operacions de conformació i tall. L’elevat límit elàstic dels acers UHSS i PHS condueix al dany accelerat i fractura prematura de les eines, el que esdevé un impediment important a l’hora de dur a terme aquestes operacions a escala industrial.Els acers d’eina han de presentar alts valors de duresa i tenacitat per tal de resistir les elevades sol•licitacions mecàniques que reben. El seu rendiment ve determinat per les propietats micro-mecàniques que deriven de la complexa interacció entre els principals constituents microestructurals, és a dir els carburs primaris i la matriu de martensita revinguda.En aquesta Tesi s’han analitzat la microestructura, propietats macro- i micro-mecàniques, nucleació i creixement d’esquerdes a nivell monotònic, comportament de corba-R i mecanismes de nucleació i propagació d’esquerdes a fatiga en quatre acers d’eina diferents: 1.2379, UNIVERSAL, K360 i HWS. S’ha estudiat el comportament mecànic i tribològic d’eines fabricades a partir d’aquests acers treballant a escala industrial i de laboratori, així com els efectes dels paràmetres de tall en el rendiment de les eines i la qualitat dels marges tallats. La idoneïtat dels models basats en la Mecànica de la Fractura Elàstica Lineal (LEFM) a l’hora de estimar el rendiment de les eines ha estat avaluada per tal de predir la vida d’aquestes. La racionalització del dany en acers d’eina des d’un punt de vista micro-mecànic, en provetes de laboratori així com en eines reals, ha permès identificar els aspectes que un model hauria de considerar per predir de forma satisfactòria la vida d’eines. A més, també ha permès proposar una nova aproximació en aquest sentit, basada en la reducció de les propietats mecàniques de la matriu metàl•lica com a conseqüència de l’aplicació de càrregues repetitives. Els acers 1.2379, UNIVERSAL i K360 es fabriquen seguint processos de fosa i forja convencionals mentre que l’HWS s’obté per pulvimetal•lúrgia. Les diferents composicions químiques i processos de fabricació d’aquests acers expliquen que presentin microestructures amb carburs primaris de diferents característiques i distribucions. La nucleació d’esquerdes es deu a la fractura de carburs primaris per clivatge (i d’inclusions no metàl•liques en l’HWS). Les esquerdes petites nucleades a partir de carburs primaris en els acers fosos i forjats són inicialment molt poc profundes, però tendeixen a adquirir formes semi-circulars al créixer a través de la matriu metàl•lica. Els acers fosos i forjats presenten comportament de corba-R, és a dir que les esquerdes petites nucleades a la microestructura tenen una tenacitat inferior al valor determinat per esquerdes grans en els assajos estàndard. El principal mecanisme de dany observat en eines de tall d’acers UHSS és la fractura per fatiga a baix nombre de cicles. La deformació plàstica i nucleació d’esquerdes a la superfície de les eines s’explica per les elevades forces de fricció que es desenvolupen pel lliscament de la xapa. A més, les altes pressions de contacte a la superfície provoquen que els carburs primaris trenquin lleugerament per sota d’aquesta, i que d’altres esquerdes s’iniciïn a partir d’aspreses superficials. Les fractures es poden entendre doncs, per la nucleació i coalescència de les esquerdes esmentades. La presència de línies de mecanitzat afecta negativament la vida de les eines ja que accelera de forma molt significativa els processos de fractura. Els models basats en LEFM no s’han mostrat prou adients per predir la vida de les eines de tall i per això, una nova aproximació ha estat proposada.

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    Sheet failure prediction during forming of advanced high strength steels  Open access

     Gutiérrez, D.; Hernández, R.; Riera Colom, Maria Dolores; Lara, Toni; Casellas, Daniel; Prado Pozuelo, Jose Manuel
    Forming Technology Forum
    Presentation's date: 2012-06-05
    Presentation of work at congresses

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    The automotive industry has increased significantly the use of advanced high strength steels (AHSS) in many structural components of vehicles, with the aim to reduce weight and also increase safety. The high resistance of these steels is accompanied by a decrease in formability, which makes simulation a requirement to adequately foresee the formability of a stretch formed component. For sheet metal forming, the conventional diagram based in strains, Forming Limit Diagrams (FLD), is often used as a failure criterion, but this approach is not effective when working with AHSS, due to the bigger effect produced by changes on strain paths than conventional steels. This work proposes the use a Forming Limit Stress Diagram (FLSD) based on the main stresses acting on the material. These FLSD has been obtained by the simulation of stretching test. To improve the results, the tensile test curve has been extended by using an optical measuring system and a subsequent adjustment of the stress-strain curve for the material. Finally, simulations and real parts behavior have been compared and the failure predictions models have been discussed.

  • Conformabilidad de chapas de aceros metaestables

     Gutierrez, David; Mateo Garcia, Antonio Manuel; Rodríguez Calvillo, Pablo; Fargas Ribas, Gemma; Lara, Antonio; Casellas, Daniel; Prado Pozuelo, Jose Manuel
    Congreso Nacional de Materiales
    Presentation's date: 2012
    Presentation of work at congresses

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    Comportamiento termomecánico de un acero microaleado al Nb-V  Open access

     Cartaya, E. A.; Varela, G.; Cabrera Marrero, Jose Maria; Prado Pozuelo, Jose Manuel
    TRATER Press. Revista del tratamiento térmico
    Date of publication: 2012-06
    Journal article

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    Empleando ensayos de compresión simple en ca- liente y relajación de tensiones después de la de - formación en un dilatómetro modelo DIL805T, se obtuvieron las curvas de fluencia y relajación de dos aceros, el primero de ellos al C – Mn y otro HS- LA con idéntica composición química base, micro- aleado con Nb – V. Los ensayos de compresión simple fueron efectua- dos a temperaturas en el rango austenítico, desde 900 – 1.200 ºC y velocidades de deformación de 0.001, 0.01, 0.1, 1 y 10 s - 1 . Se obtuvieron las curvas de fluencia para las diferentes condiciones, permitien- do el estudio del efecto de los elementos microale- antes sobre la recristalización dinámica y la micro- estructura final; pudiendo comprobar mediante un análisis microestructural el efecto retardador en la cinética de recristalización que presenta el acero microaleado, el incremento en la deformación críti- ca y en la energía de activación. Se determina asi- mismo por medio del método de relajación de ten - siones la cinética de precipitación, elaborando los diagramas precipitación – temperatura – tiempo

  • Aplicación de criterios de conformabilidad en productos planos de aceros de alta resistencia  Open access

     Gutiérrez Castillo, Juan David
    Defense's date: 2012-11-28
    Department of Materials Science and Metallurgy, Universitat Politècnica de Catalunya
    Theses

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    La conformabilidad de chapas metálicas suele evaluarse mediante los diagramas FLD (Forming Limit Diagrams), que informan sobre el límite de conformado de un material representado por la curva FLC (Forming Limit Curve). Estos diagramas y curvas han sido empleados durante años para estudiar la conformabilidad de un material y como criterio de fallo en la simulación por elementos finitos del conformado de componentes estructurales. La aparición de nuevos aceros con mayores prestaciones, como son los aceros AHSS (Advanced High Strength Steels), ha evidenciado las carencias de dichas curvas, debido principalmente a que son dependientes de los caminos de deformación. Por tanto, en los procesos de conformado donde existen procesos multietapas, dichas curvas no definen correctamente la conformabilidad del mismo. Para afrontar este problema, surgieron los diagramas basados en las tensiones principales FLSD (Forming Limit Stress Diagrams) y su respectiva curva límite FLSC (Forming Limit Stress Curve) que son menos sensibles a los caminos de deformación. El objetivo de la tesis es estudiar y proponer diferentes criterios de conformabilidad en productos planos (chapa metálica) de aceros de alta de resistencia que permita ser utilizado con criterio de fallo en las simulaciones por elementos finitos. Para alcanzar dicho objetivo la tesis se divide en tres bloques principales, un primer bloque dedicado a la caracterización mecánica (utilización de ensayos de tracción y compresión) y microestructural (determinación cualitativa y cuantitativa de las fases presentes). Un segundo bloque dedicado al estudio de las curvas FLC obtenidas de los ensayos de estirado Nakajima y Marciniak, donde se reflejan las diferencias de los caminos de deformación en los aceros AHSS y la diferencia según el método de cálculo para determinar el inicio de la estricción localizada. De aquí que se determinen las curvas FLC con criterio a rotura FLCF (Forming Limit Curve at Fracture) donde no se utiliza ninguna aproximación matemática para su cálculo. A partir de las curvas FLC experimentales, las leyes de endurecimiento obtenidas de los ensayos de tracción o compresión y la simulación por elementos finitos, se han determinado las curvas FLSC de los diferentes aceros estudiados. En el tercer bloque se han validado los diferentes criterios de conformabilidad mediante diferentes ensayos de conformado y la simulación por elementos finitos. Se ha utilizado el ensayo de embutición profunda para calcular el LDR (Limiting Drawing Ratio) y se ha conformado una pieza tipo montante abierto. A partir de los datos experimentales y los resultados de la simulación por elementos finitos aplicando los diferentes criterios de fallo estudiados se han verificado los criterios de conformabilidad. Para concluir, se ha estudiado la capacidad de los AHSS al ensanchamiento de orificios o abocardabilidad, definido en inglés como flangeability o stretch-flangeability. A diferencia de los aceros convencionales, se ha constatado que en un cierto rango de resistencia mecánica (800-1200 MPa), los aceros AHSS con mayor resistencia poseen una mejor abocardabilidad. Esto ha generado algunas dudas entre diferentes autores, debido a que la abocardabilidad estaba relacionada con la ductilidad y conformabilidad del material, es decir, mejores propiedades de abocardabilidad se obtenían con materiales más dúctiles y de menor resistencia. En esta sección, para explicar esta divergencia se ha utilizado el trabajo esencial de fractura (Essential Work of Fracture, EWF) que da información de la tenacidad a fractura de las chapas y de la energía necesaria para propagar una grieta. Se ha constatado que en aceros AHSS, a mayor resistencia mecánica, la energía para propagar una grieta aumenta, es decir, su valor de EWF, y que también se obtiene una mejor abocardabilidad.

    The formability of sheet metals is widely described using the FLD (Forming Limit Diagram) and FLCs (Forming Limit Curves). These diagrams and curves have been used, during recent years, to study the formability of sheet metals and as a failure criterion for finite element simulations of some forming processes of structural components. The emergence of higher performance steels, such as AHSS (Advanced High Strength Steels), has demonstrated the shortcomings of these curves, mainly because they are path dependent. Therefore, in multistep forming processes, such curves do not properly define the formability of sheet metals. To address this problem, the diagrams based on the principal stresses FLSD (Forming Limit Stress Diagrams) and the corresponding limit curve FLSC (Forming Limit Stress Curve) are used, because they are less sensitive to changes in strain paths. The aim of this thesis is to study and propose different formability criteria for flat products (sheet metal) of high strength steel that allow their use as a failure criterion in finite element simulations. To achieve this objective, the thesis is divided into three main blocks; a first section isdevoted to the mechanical characterization (using tensile and compression tests) and microstructural characterization (qualitative and quantitative determination of the current phases). The second section is dedicated to the study of FLC obtained by Nakajima and Marciniak stretch-forming tests, which reflect the differences in the strain paths for AHSS and also, the differences according to the mathematical method used to determine the onset of localized necking. Here, the forming limit has been calculated at fracture FLCF (Forming Limit Curve at Fracture) where there is no mathematical approach for their determination. For the stress criteria, FLSC of different steels have been determined using the FLC experimental curves, the hardening laws were obtained from tensile or compression tests and finite element simulations. In the third part the different formability criteria were validated using different forming process and finite element simulations. The deep drawing test and their LDR (Limiting Drawing Ratio) value and a specific workpiece were used to validate the different formability criteria. To conclude, the flangeability of AHSS has been studied. Unlike conventional steels, better flangeability can be achieved in AHSS at a certain stress range (800-1200 MPa), which also increases its mechanical properties. This has led to a disscussion among different authors, because the flangeability had been associated to the ductility and formability of the material. The belief was that a better flangeability was reached with more ductile materials and lower mechanical strength. In this third section, the Essential Work of Fracture (EWF) has been used to explain the apparent contradiction. The EWF can be considered a measure of the crack propagation resistance and is a methodology to evaluate fracture toughness in sheet metals. In AHSS, the higher mechanical properties, the better was the ability to produce a flange, and the higher was the resistance to propagate a crack, thus a connection between EWF and flangeability is presented.

  • Microscratch resistance of ophthalmic coatings on organic lenses

     Caro Prados, Jaume; Cuadrado Lafoz, Núria; Gonzalez, Iban; Casellas Padro, Daniel; Prado Pozuelo, Jose Manuel; Vilajoana, A; Artus, P; Peris, S; Carrilero, A; Dürsteler, J.C.
    Surface and coatings technology
    Date of publication: 2011
    Journal article

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  • Strain Influence on the Precipitation Kinetic Nb-V Microalloyed HSLA Steel

     Cartaya Ruesga, Elio Armando; Cabrera Marrero, Jose Maria; Prado Pozuelo, Jose Manuel
    Journal of iron and steel research international
    Date of publication: 2011-05
    Journal article

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  • Precipitation Kinetics on a Nb-V Microalloyed Steel

     Cartaya Ruesga, Elio Armando; Cabrera Marrero, Jose Maria; Prado Pozuelo, Jose Manuel
    Journal of iron and steel research international
    Date of publication: 2011-05
    Journal article

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  • Hot deformation behavior of a medium carbon microalloyed steel

     Mirzadeh, H.; Cabrera Marrero, Jose Maria; Prado Pozuelo, Jose Manuel; Najafizadeh, A.
    Materials science and engineering A. Structural materials properties microstructure and processing
    Date of publication: 2011-04-25
    Journal article

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    Flow regime analyses during the filling stage in powder metallurgy processes: experimental study and numerical modelling  Open access

     Cante Teran, Juan Carlos; Riera Colom, Maria Dolores; Oliver Olivella, Fco. Javier; Prado Pozuelo, Jose Manuel
    Granular matter
    Date of publication: 2011-01
    Journal article

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    Experimental and numerical studies of powder flow during the die filling stage in powder metallurgy cold compaction processes are presented. An experimental setting consisting of a horizontal pneumatically activated shoe, a vertical die and high-speed video system has been designed. The experiments show the existence of three flow regimes: continuous, transitory and discrete, which are identified in terms of the particle size, the morphology and the speed of the shoe. In the continuous regime the powder flows in a progressive manner but in the discrete one some perturbations appear as a consequence of a shear band formation that forms discrete avalanches. A numerical model, based on a ratedependent constitutive model, via a flow formulation, and in the framework of the particle finite element method (PFEM) is also proposed. For the purpose of this study, the use of the PFEM assumes that the powder can be modelled as a continuous medium. The model, provided with the corresponding characterisation of the parameters, is able to capture the two fundamental phenomena observed during the filling process: (1) the irreversibility of most of the deformation experienced by the material and (2) the quick dissipation of the potential gravitatory energy of the granular system through the inter-particle friction processes, modelled by the plastic dissipation associated with the material model. Experimental and numerical results have been compared in order to study the viability of the proposed model.

  • Modelling and simulation of stamping of high strength steels

     Riera Colom, Maria Dolores; Prado Pozuelo, Jose Manuel
    NAFEMS Awareness Seminar
    Presentation's date: 2010-04-28
    Presentation of work at congresses

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  • Effect of strain paths on formability evaluation of TRIP steels

     Prado Pozuelo, Jose Manuel; Gutiérrez, D; Lara, A; Casellas, Daniel
    Advanced materials research
    Date of publication: 2010-01
    Journal article

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  • MEJORA DEL COMPORTAMIENTO A FATIGA Y DE LA CONFORMIBILIDAD DE ACEROS METAESTABLES MEDIENTE DISEÑO

     Prado Pozuelo, Jose Manuel; Calvo Muñoz, Jessica; Riera Colom, Maria Dolores; Heredero Alari, Francisco; Llanes Pitarch, Luis Miguel; Fargas Ribas, Gemma; Salán Ballesteros, Mª Núria; Mestra Rodríguez, Alvaro Miguel; Garcia Marro, Fernando; Zapata Dederlé, Ana Cristina; Mir Carbonell, Jordi; Botero Vega, Carlos Alberto; Mateo Garcia, Antonio Manuel
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  • Influence of manganese on the hot working behaviour of construction steels

     Varela Castro, Gonzalo; Cabrera Marrero, Jose Maria; Prado Pozuelo, Jose Manuel
    Advanced materials research (online)
    Date of publication: 2010
    Journal article

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    Criterios de predicción de inestabilidades plásticas en procesos de conformado en caliente. Parte I, Revisión teórica  Open access

     Al Omar Mesnaoui, Anas; Prado Pozuelo, Jose Manuel
    Revista de Metalurgia
    Date of publication: 2010-03-01
    Journal article

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    Els processos de conformat de materials indueixen freqüentment alts graus de deformació, a grans velocitats de deformació, i imposen maneres de sol•licitació multiaxials i molt complexes. Aquests processos queden essencialment limitats per l’aparició i desenvolupament d’inestabilitats plàstiques. Aquestes poden ser la causa directa de la propagació ràpida d’una fissura, que pot donar lloc a una ruptura final. La complexitat dels modes de deformació i la intervenció simultània de diversos paràmetres han portat a diversos investigadors a desenvolupar diversos criteris,amb diferents plantejaments, per predir l’aparició de defectes i optimitzar los paràmetres de control en els processos de conformat. S’exposen, a tall de revisió, alguns criteris àmpliament utilitzats en la literatura per a la predicció de les inestabilitats plàstiques durant el conformat de materials. S’ha considerat oportú dividir la matèria del treball en dos parts: una primera, on es presenten els criteris fenomenològics de predicció de les inestabilitats plàstiques, basats en l’observació descriptiva dels fenòmens microscòpics de la deformació (enduriment per deformació i sensibilitat a la velocitat de deformació), i , on s’analitzen els criteris continus basats en els principis de màxima velocitat de producció d’entropia de la termodinàmica irreversible aplicada a la mecànica contínua de la fluència plàstica en grans deformacions. Al mateix temps, es comenta una discussió bibliogràfica entaulada entre diversos autors referida al fonament físic del Model Dinàmic de Materials. En la segona part, es fa un estudi comparatiu per a caracteritzar la inestabilitat plàstica durant un procés de conformat en calent d’un acer microaliat de mig carboni usant criteris continus i fenomenològics

  • Access to the full text
    Criterios de predicción de inestabilidades plásticas en procesos de conformado en caliente. Parte II, Caracterización de la fluencia plástica de un acero microaleado de medio carbono usando criterios continuos y fenomenológicos  Open access

     Al Omar Mesnaoui, Anas; Alcelay Larrion, Jose Ignacio; Prado Pozuelo, Jose Manuel
    Revista de Metalurgia
    Date of publication: 2010-03-01
    Journal article

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    En l'estudi dels processos de conformat en calent és molt important conèixer la interacció entre el comportament microestructural i els paràmetres de control del procés de deformació, tals com temperatura, velocitat de deformació, deformació, etc. En les últimes dècades, els mapes de processat han estat elaborats per dissenyar, controlar i optimitzar la deformació en calent de diversos materials metàl•lics. Amb el fi d'estudiar la conformabilitat en calent d'un acer microaliat de mig carboni, durant assajos de compressió en calent, es presenta un estudi comparatiu entre dos tipus de mapes de processat construïts usant criteris termodinàmics continus i criteris fenomenològics. L'anàlisi dels mapes de processat indica que l'acer estudiat no pateix cap tipus d'inestabilitat plàstica. Tot i així, els mapes corresponents a la deformació e = 0,6 revelen l'existència d'un domini de recristalització dinàmica, considerat com el domini més eficient dins de la regió segura de processat. Aquest domini està centrat a 1.150 °C i 10 s–1. L'estudi comparatiu dels resultats obtinguts mostra la diferència entre les posicions dels diferents dominis de deformació plàstica segons el criteri d'inestabilitat usat.

  • The elastic behaviour of non-sintered metal powder compacts

     Riera Colom, Maria Dolores; Prado Pozuelo, Jose Manuel
    Proceedings of the Institution of Mechanical Engineers. Part E, journal of process mechanical engineering
    Date of publication: 2010
    Journal article

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  • The influence of deformation on microstructure evolution of low alloy TRIP steel

     Suarez Fernandez, Lucia; Benito Paramo, Jose Antonio; Rodríguez Calvillo, Pablo; Houbaert, Yvan; Casellas Padro, Daniel; Petrov, Roumen H.; Prado Pozuelo, Jose Manuel
    Materials science forum
    Date of publication: 2010
    Journal article

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  • AUMENTO DE LA DUCTIBILIDAD EN FRIO DE MATERIALES METÁLICOS CON TAMAÑO DE GRANO NANOCRISTALINO Y ULTRAFINO

     Riera Colom, Maria Dolores; Prado Pozuelo, Jose Manuel; Benito Paramo, Jose Antonio; Llumà Fuentes, Jordi; Jorba Peiró, Jordi; Cabrera Marrero, Jose Maria
    Participation in a competitive project

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  • PROCESAMIENTO, PROPIEDADES Y CARACTERÍSTICAS DE ACEROS DESTINADOS AL SECTOR ELECTRICO

     Suarez Fernandez, Lucia; Calvo Muñoz, Jessica; Rodríguez Calvillo, Pablo; Benito Paramo, Jose Antonio; Riera Colom, Maria Dolores; Prado Pozuelo, Jose Manuel; Cabrera Marrero, Jose Maria
    Participation in a competitive project

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  • Ductility of Bulk Nanocrystalline and Ultrafine Grain Iron and Steel

     Benito Paramo, Jose Antonio; Tejedor Busquets, Robert; Rodríguez Baracaldo, R.; Cabrera Marrero, Jose Maria; Prado Pozuelo, Jose Manuel
    Materials science forum
    Date of publication: 2009-11
    Journal article

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  • Strain rate sensitivity of nanocrystalline and ultrafine-grained steel obtained by mechanical attrition

     Rodriguez-Baracaldo, R; Benito Paramo, Jose Antonio; Caro, J; Cabrera Marrero, Jose Maria; Prado Pozuelo, Jose Manuel
    Materials science and engineering A. Structural materials properties microstructure and processing
    Date of publication: 2008-06
    Journal article

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  • Análisis de la fractura de un acero de bajo contenido en carbono con grano nanométrico y ultrafino obtenido por molienda mecánica

     Tejedor, R; Rodríguez, R Baracaldo; Benito Paramo, Jose Antonio; Cabrera Marrero, Jose Maria; Prado Pozuelo, Jose Manuel
    Anales de mecánica de la fractura
    Date of publication: 2008-03
    Journal article

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  • Study of the plastic deformation in nanocrystalline and ultrafine iron and carbon steels

     Benito Paramo, Jose Antonio; Tejedor, R; Rodríguez-Baracaldo, R; Cabrera Marrero, Jose Maria; Prado Pozuelo, Jose Manuel
    Materials science forum
    Date of publication: 2008-07
    Journal article

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  • Role of Cu2O precipitates during hot compression of 99.9% pure coppers with different oxygen contents

     García, V G; Cabrera Marrero, Jose Maria; Prado Pozuelo, Jose Manuel
    Materials science and engineering A. Structural materials properties microstructure and processing
    Date of publication: 2008-08
    Journal article

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  • Microstructural evolution and mechanical response of nanocrystalline and ultrafine-grained steel obtained by mechanical milling

     Rodríguez-Baracaldo, R R Tejedor; Benito Paramo, Jose Antonio; Cabrera Marrero, Jose Maria; Prado Pozuelo, Jose Manuel
    Materials science and engineering A. Structural materials properties microstructure and processing
    Date of publication: 2008-09
    Journal article

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  • Influence of the carbon content on the strain rate sensitivity of nanocrystalline steels

     Tejedor Busquets, Robert; Rodriguez-Baracaldo, R; Benito Paramo, Jose Antonio; Caro, J; Cabrera Marrero, Jose Maria; Prado Pozuelo, Jose Manuel
    Scripta materialia
    Date of publication: 2008-09
    Journal article

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  • Effect of initial microstructure, frequency and temperature on the low cycle fatigue behaviour of the soldering alloys 96.5Sn¿3.5Ag and 63Sn¿37Pb

     Boulaajaj, A; Cabrera Marrero, Jose Maria; Prado Pozuelo, Jose Manuel
    Engineering failure analysis
    Date of publication: 2008-04
    Journal article

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  • Caracterización y desarrollo de un material tixotrópico base SnPb mediante compocasting

     MERIZALDE RODRÍGUEZ, CARLOS FEDERICO
    Defense's date: 2008-05-16
    Department of Materials Science and Metallurgy, Universitat Politècnica de Catalunya
    Theses

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  • Comportamiento a deformación plástica estática y cíclica de las aleaciones soldantes 63Sn-37Pby 96,5Sn-3,5Ag

     BOULAAJAJ, AHMED
    Defense's date: 2008-02-06
    Department of Materials Science and Metallurgy, Universitat Politècnica de Catalunya
    Theses

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  • Conformado en caliente de chapa de acero

     Riera Colom, Maria Dolores; Hernandez Rossi, Ricardo; Prado Pozuelo, Jose Manuel
    Congreso Nacional de Tratamientos Térmicos y de Superficie
    Presentation's date: 2008-03
    Presentation of work at congresses

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    De Bolonia (1088) a Bolonia (1988-2010). De la enseñanza basada en el conocimiento a la enseñanza basada en las habilidades  Open access

     Riera Colom, Maria Dolores; Prado Pozuelo, Jose Manuel
    Taller Iberoamericano de Educación en Ciencia e Ingeniería de Materiales
    Presentation's date: 2008-12-02
    Presentation of work at congresses

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    La humanidad está viviendo un período histórico de profundas transformaciones. La sociedad avanza a un ritmo muy superior al de sus propias estructuras, y la universidad reacciona por detrás de los acontecimientos. Se dice que la universidad está en crisis y no sólo de tipo financiero. En este trabajo se expone una breve historia de la universidad; se exponen sucintamente los distintos modelos a partir de los cuales se crean las universidades modernas y se plantean algunos de los aspectos fundamentales de la situación de la universidad actual debidos a la democratización y al cambio profundo, rápido y continuo del mercado laboral y, en general, de la sociedad. El proceso de Bolonia, a través del cual se define un Área de educación superior en Europa, está cargado de buenas intenciones y mejores palabras; pero, ¿serán todos los estamentos involucrados capaces de llevar a buen puerto una transformación tan intensa de la universidad?

  • Fracture toughness characterization of advanced high strength steels by means of the essential work of fracture

     Llobet Lorenzo, Alberto; Casellas Padro, Daniel; Riera Colom, Maria Dolores; Prado Pozuelo, Jose Manuel
    International Conference on New Developments in Advanced High Strength Sheet Steels
    Presentation's date: 2008
    Presentation of work at congresses

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    Fracture toughness cannot be readily measured in metal sheet because the small thickness in which they are used (1-3 mm). In this sense the essential work of fracture (EWF) has been successfully applied to determine fracture toughness in polymers films and many metal sheets. However, their information about the applicability of this methodology to AHSS is relatively scarce. In the present work the fracture toughness of some advanced high strength steel Dual Phase and drawing steel sheets has been measured by means of the EWF. The results show that AHSS steels with yield strength up to 800 MPa meet the requirements of the method, and therefore the values obtained for the EWF are valid. Thus, the EWF is postulated as methodology to evaluate the fracture toughness and can be used as a mechanical parameter to characterize the fracture behavior during crash of AHSS steel sheets

  • Plastic deformation of a nanostructured and ultra-fine grained Fe-1%Cr with a bimodal grain size distribution

     Tejedor Busquets, Robert; Rodríguez Baracaldo, R.; Benito Paramo, Jose Antonio; Cabrera Marrero, Jose Maria; Prado Pozuelo, Jose Manuel
    International journal of material forming
    Date of publication: 2008-04-01
    Journal article

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  • Role of Cu2O during hot compression of 99.9% pure copper

     Garcia Fernandez, Victor Gerardo; Cabrera Marrero, Jose Maria; Prado Pozuelo, Jose Manuel
    Materials Science and Engineering: A
    Date of publication: 2008-08-15
    Journal article

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    Several authors have noticed that relatively pure coppers with different oxygen levels present a higher hot flow stress as the oxygen content increases. This work attempts to demonstrate that a large contribution to the observed increase of stress is due to precipitation of fine Cu2O particles during hot working. Three fire-refined 99.9% pure coppers with different oxygen levels (26 ppm, 46 ppm, and 62 ppm) were hot compressed at temperatures 600¿950 °C and at a strain rate range 0.001¿0.3 s-1. A comparison of transmission electron micrographs prepared from coppers compressed at 750 °C and 900 °C was performed to characterize the possible precipitates. Fine Cu2O precipitates were found only in the two coppers with higher oxygen and at the lower temperature, where stress differences were larger. Precipitation¿hardening theories were adapted for higher temperature and used to iteratively determine if the precipitate sizes and calculated volume fractions could be indeed causing an increase of stress

  • Efecto de tierras raras sobre la resistencia en caliente de aceros de bajo carbono

     Cabrera Marrero, Jose Maria; Prado Pozuelo, Jose Manuel; Mejía, I
    Date of publication: 2008-07
    Book chapter

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  • Hot flow description of ARMCO iron

     Tafzi El Hadri, Khalil; Cabrera Marrero, Jose Maria; Prado Pozuelo, Jose Manuel
    Date of publication: 2008-07
    Book chapter

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  • Application of the EBSD technique to the study of dynamic recrystallization

     Cabrera Marrero, Jose Maria; Prado Pozuelo, Jose Manuel; Wahabi, M; Gavard, L; Montheillet, F
    Date of publication: 2008-07
    Book chapter

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  • Effect of the chemical composition on the work hardening and dynamic recovery of some metallic alloys

     Cabrera Marrero, Jose Maria; Prado Pozuelo, Jose Manuel
    Date of publication: 2008-07
    Book chapter

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  • Model input data - Elastic properties

     Riera Colom, Maria Dolores; Prado Pozuelo, Jose Manuel; Doremus, Pierre
    Date of publication: 2008
    Book chapter

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  • Mechanical behaviour of Sn-Pb and Sn-Ag soldering alloys for printed circuit boards of the automotive industry

     Cabrera Marrero, Jose Maria; Prado Pozuelo, Jose Manuel; Subirats, A; Cubero, J A
    Date of publication: 2008-07
    Book chapter

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  • Mechanical response of nanocrystalline steel obtained by mechanical attrition

     Rodriguez-Baracaldo, R; Benito Paramo, Jose Antonio; Cabrera Marrero, Jose Maria; Prado Pozuelo, Jose Manuel
    Journal of materials science
    Date of publication: 2007-03
    Journal article

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