Gonzalez-Ojeda, Roberto
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Gonzalez-Ojeda, Roberto
Official Name
Gonzalez Ojeda, Roberto
Alternative Name
robglez
González-Ojeda, R.
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ORCID
Scopus Author ID
56314856300
Researcher ID
DTL-2716-2022

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Superplasticity of ultrafine grained low-carbon HSLA steels

2012 , Verdeja González, José Ignacio , Quintana, María José , García García, José Ovidio , Verdeja González, Luis Felipe , Gonzalez-Ojeda, Roberto , Fernández Fernández, Sara

Steels with ultrafine grained structure may present superplastic behavior at specific temperatures and strain rates that allow the grain boundary sliding mechanisms to be activated. The work presents high temperature tension tests in a low carbon, low alloy steel obtained by advanced thermomechanical controlled rolling processes, showing at 800°C elongations as high as 200%. The microstructure of the steel was analyzed in order to identify ferrite and pearlite grain boundaries, and their interaction after the specimens were deformed, showing intergranular decohesions, restored ferrite grains and elimination of banded structure, which are evidence of superplastic mechanisms in this material which is, in fact, ultrafine grained as demonstrated by quantitative metallographic techniques and grain size distribution analysis.

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La temperatura de la intercara arrabio/refractario como variable determinante de los mecanismos de corrosión del crisol del horno alto

1998 , Verdeja González, Luis Felipe , Pusek, P., Alfonso , Fernández, Ángel Alejandro , Gonzalez-Ojeda, Roberto

El desgaste de materiales en el horno alto puede realizarse bajo la consideración de mecanismos estrictamente abrasivos o con el predominio de las componentes químicas y erosivas resultantes de la circulación de fluidos sobre el refractario. No obstante, aunque en determinadas situaciones, la abrasión o las componentes químicas pueden resultar los mecanismos controlantes, es necesario considerar que todos ellos (abrasión, erosión, desgaste químico, adhesión y desgaste térmico-choque térmico) "cooperan" en la degradación del refractario del horno, Para cualquiera de los mecanismos que intervenga en el desgaste de los materiales, resulta conveniente conocer el valor de la temperatura a lo largo de las intercaras del refractario con los sólidos, líquidos o gases presentes en el horno. En la ponencia será desarrollada de forma específica, la importancia que tiene la temperatura en la intercara refractario-arrabio sobre el desgaste de los materiales del crisol. Se discutirán las diferentes hipótesis de transporte (energía, cantidad de movimiento y materia) más adecuadas para la obtención de la temperatura de intercara.

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Structural Ultrafine Grained Steels Obtained by Advanced Controlled Rolling

2013 , Gonzalez-Ojeda, Roberto , García García, José Ovidio , Barbés Fernández, Miguel Ángel , Quintana, María José , Verdeja González, Luis Felipe , Verdeja González, José Ignacio

Steels with ultrafine grains (lower than 5 μpa), which usually known as ultrafine ferrite or ultrafine grained materials, are presently the object of intense research, because of the improvement in resistance and fracture toughness they may reach compared to conventional steels (with grain sizes above this value). It is shown that the forenamed steels designated in the Euronorm EN 10149-2, which are manufactured by advanced techniques of controlled rolling and mainly used in automotive industry, have an ultrafine grain size in the range of 2. 5 to 3. 5 μm, and with elastic yield stresses higher than 400 MPa. Based on the Morrison-Miller criterion, it is shown that values of the strain-hardening coefficient lower than 0. 08 would make the industrial application of these steels unfeasible.

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Thermal modelling of a torpedo-car

2005 , Verdeja González, Luis Felipe , Barbés Fernández, Miguel Ángel , Gonzalez-Ojeda, Roberto , Castillo, G. A. , Colás, R.

A two-dimensional finite element model for computing the temperature distribution in a torpedo-car holding pig iron is described in this work. The model determines the temperature gradients in steady and transient conditions whiting the different parts that constitute the systems, which are considered to be the steel casing, refractory lining, liquid iron, slag and air. Heat transfer within the main fluid phases (iron and air) is computed assuming an apparent thermal conductivity term incorporating the contribution from convention and radiation, and it is affected by the dimensions of the vessel. Thermal gradients within the constituents of the torpedo-car are used to calculate heat losses during operation. It was found that the model required the incorporate of a region within the iron-refractory interface to reproduce thermographic data recorded during operation; the heat transfer coefficient of this interface was found to be equal to 30 Wm''-2K''-1.

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Dual-phase ultrafine grained steels produced by controlled rolling processes

2011 , Quintana, María José , Gonzalez-Ojeda, Roberto , Verdeja González, Luis Felipe , Verdeja González, José Ignacio

Double-phase steels are an excellent alternative in the production of automotive parts that require high mechanical resistance, high impact strength and elevated elongation. These materials are produced using low-alloy steels as a basis, reducing costs and resulting in a combination of martensite and ferrite structures with ultrafine grain sizes. These characteristics are accomplished through a strict control of rolling conditions: strain rate, cooling rate and direct quenching. This work presents the results of tension testing of two types of double phased steels, along with microstructural characterization, in order to understand the effect of the advanced thermomechanical controlled rolling processes on the formation of the microstructure and the resulting mechanical properties.

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Superplastic HSLA Steels: Microstructure and Failure

2013 , Fernández, Sara , Quintana, María José , García García, José Ovidio , Verdeja González, Luis Felipe , Gonzalez-Ojeda, Roberto , Verdeja González, José Ignacio

Certain materials can show superplasticity when traction tested at temperatures higher than 50% of their melting point and with low strain rates ( < 10−2 s−1), showing very high elongations (>100%) without localized necking and mainly intergranular fractures. This behavior requires that the starting grain size is small (<10 μm) so the flow of matter can be non-homogeneous (sliding and rotating of the grain boundaries, accommodated by diffusion). This work presents the superplastic characteristic of shipbuilding steel deformed at 800 °C and a strain rate slower than 10−3 s−1. The fine grain size (5 μm) is obtained when using Nb as a microalloying element and manufactured by controlled rolling processes (three stages). After the superplastic deformation, the steel presents mixed fractures: by decohesion of the hard (pearlite and carbides) and ductile (ferrite) phases and by intergranular sliding of ferrite/ferrite and ferrite/pearlite, just as it happens in stage III of the creep behavior. This is confirmed through the Ashby–Verrall model, according to which the dislocation creep (power-law creep) and diffusion creep (linear-viscous creep) occur simultaneously.

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Mechanical behaviour of thermomechanically produced ultrafine grained dual-phase steels

2014 , Gonzalez-Ojeda, Roberto , García García, José Ovidio , Verdeja González, Luis Felipe , Quintana, María José , Verdeja González, José Ignacio

Dual-phase (DP) steels are an excellent alternative in the production of automotive parts that require high mechanical resistance, high impact strength and elevated elongation. These materials are produced using low alloy steels as a basis, reducing costs and resulting in a combination of martensite and ferrite structures with ultrafine grain size. These characteristics are achieved through strict control of rolling conditions, strain rate, cooling rate and coiling temperature. This work presents the results of tension testing of two types of DP steels, along with microstructural characterisation, in order to understand the effect of the advanced thermomechanical controlled rolling processes on the formation of the microstructure and resulting mechanical properties.

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Ultrafine Grained HSLA Steels for Cold Forming

2010 , Gonzalez-Ojeda, Roberto , García García, José Ovidio , Barbés Fernández, Miguel Ángel , Quintana, María José , Verdeja González, Luis Felipe , Verdeja González, José Ignacio

The industrial level production of ultrafine grained (or ultrafine ferrite) ferrous alloys was investigated through three examples of steels that complied with the EN 10149-2 Euronorm and were produced by advanced controlled hot rolling techniques. The steel samples were tension tested and chemically analyzed, and the microstructure was evaluated through quantitative metallographic techniques to determine parameters such as yield stress, amount of microalloying elements, strain hardening coefficient, grain size, and grain size distribution. These steels were micro-alloyed with Ti, Nb, and Mn with ASTM grain sizes of approximately 13–15. The careful control of chemical composition and deformation during production, giving a specific attention to the deformation sequences, austenite non-recrystallization temperatures and allotropic transformations during cooling, are indispensable to obtain steels with an adequate strain hardening coefficient that allows cold working operations such as bending, stretching or drawing.

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The Nodal Wear Model (NWM) as an Alternative to Understand the Mechanisms of Flow and Wear in the Blast Furnace Crucible

2010 , Gonzalez-Ojeda, Roberto , Barbés Fernández, Miguel Ángel , Verdeja González, Luis Felipe , Ruiz Bustinza, Íñigo Eloy , Mochón Muñoz, Javier , Duarte, Ramón Martín , Karbowniczek, Mirosław

The presence of thermocouples in the lining of crucibles has become a general practice in the new construction of blast furnaces. The Nodal Wear Model (NWM) has also emerged as an instrument that, while using experimental data, obtains nodal variables whose experimental measurement is not possible: global coefficient of pig-iron/refractory heat transfer h pig−iron/lining g−i and nodal temperature Ti . Starting from these nodal properties, the wear of the lining or the growth of scabs may be controlled, independently of the mechanisms responsible for them. In the same way, the properties and influence zone of the dead man in the hearth of the blast furnace may be calculated, along with those regions where the fluid is allowed to move without any other restrictions than the ones of the corresponding viscous flow (raceway hearth region).

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Effect of pressure in the microstructure of die cast Al-8.5Si-3.5cu alloys

2014 , Barbés Fernández, Miguel Ángel , Verdeja González, José Ignacio , Quintana, María José , Verdeja González, Luis Felipe , Gonzalez-Ojeda, Roberto

The microstructure of Pressure Die Casting of an Al-8.5 Si-3.5 Cu alloy used for clean room tiles shows amount of constituents (eutectic and Si phases) that do not correspond to the ones indicated by the phase diagram. Furthermore, there are differences in amount of constituents between the core and the surface zones of parts produced by this process. The work presents quantitative microstructural analysis of this type of industrially produced part, and as a possible cause for these differences, simulations of the effect of pressure on the eutectic (temperature and chemical composition) in the Al-Si system. As the production rate of these parts is very high, and solidification is very fast, microstructural heterogeneity may be related to local entrapment of gasses unable to escape during casting.

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