Alberto Quispe - Academia.edu (original) (raw)

Papers by Alberto Quispe

Steel Research, 1998

Resumen Para aplicar el proceso de electroescoria a la fabricación de aceros es necesario compren... more Resumen Para aplicar el proceso de electroescoria a la fabricación de aceros es necesario comprender los complejos procesos termodinámicos que tienen lugar, en los que intervienen los óxidos componentes de la escoria, los aleantes disueltos en el acero líquido y los elementos de las ferroaleaciones y desoxidantes que se adicionan. Calculando las actividades termodinámicas de los elementos en el metal fundido y de los óxidos en la escoria, se plantean los equilibrios teóricos de oxidación de los elementos con mayor afinidad por el oxígeno, así como sus equilibrios de intercambio mutuo. Comparando los equilibrios teóricos con los resultados experimentales se puede interpretar cómo evoluciona el sistema durante la refusión y ajustar la composición de la escoria y demás parámetros para fabricar un acero de composición distinta a la del electrodo. Palabras clave: Refusión por electroescoria. Actividad. Equilibrio termodinámico. Thermodynamics of exchange reactions in the manufacturing of microalloyed steels by the ESR process

steel research international, 2005

In microalloyed steels, static recrystallisation is temporarily inhibited by precipitation which ... more In microalloyed steels, static recrystallisation is temporarily inhibited by precipitation which is occurring at the same time. A high number of microalloyed steels containing various combinations of carbon, nitrogen and precipitate forming elements like V, Nb and Ti were recrystallised at different temperatures and strain rates. From these results recrystallisation-precipitation-time-temperature (RPTT) diagrams were established. The influence of grain size and strain rate on the RPTT diagrams was studied. The precipitation kinetics were mathematically described for isothermal conditions and converted to cooling conditions, which enables an application to hot rolling. Under cooling conditions, completion of recrystallisation is prevented, especially for Nb alloyed steels.

Revista de Metalurgia, 2015

Niobium and vanadium precipitates (nitrides and carbides) can inhibit the static recrystallizatio... more Niobium and vanadium precipitates (nitrides and carbides) can inhibit the static recrystallization of austenite but this does not happen for Ti, which form nitrides at high temperatures. RPTT diagrams show the interaction between recrystallization and precipitation allowing study the strain induced precipitation kinetics and precipitate coarsening. Based on Dutta and Sellars's expression for the start of strain-induced precipitation in microalloyed steels, a new model has been constructed which takes into account the influence of variables such as microalloying element percentages, strain, temperature, strain rate and grain size. Recrystallization-Precipitation-Time-Temperature (RPTT) diagrams have been plotted thanks to a new experimental study carried out by means of hot torsion tests on approximately twenty microalloyed steels with different Nb, V and Ti contents. Mathematical analysis of the results recommends the modification of some parameters such as the supersaturation ratio (k s) and constant B, which is no longer a constant but a function of k s. The expressions are now more consistent and predict the Precipitation-Time-Temperature (PTT) curves with remarkable accuracy. The model for strain-induced precipitation kinetics is completed by means of Avrami's equation. Finally, the model constructed in isothermal testing conditions, it has been converted to continuous cooling conditions in order to apply it in hot rolling.

Materials Science and Technology, 2001

El artículo seleccionado no se encuentra disponible por ahora a texto completo por no haber sido ... more El artículo seleccionado no se encuentra disponible por ahora a texto completo por no haber sido facilitado todavía por el investigador a cargo del archivo del mismo.

Materials Science and Technology, 2000

ABSTRACT A study has been made of the recrystallisation–precipitation interaction in three microa... more ABSTRACT A study has been made of the recrystallisation–precipitation interaction in three microalloyed steels, containing respectively V, Nb, and V–Ti, applying two different strain rates. Recrystallised fraction v.time curves were determined and used to draw recrystallisation–precipitation–time–temperature (RPTT) diagrams. The influence of strain rate has been shown to be similar in the three steels. On the basis of the results the value of }}SB0·19 has been found for the exponent of the strain rate, following Dutta and Sellars'model for the parameter t 0·05 , which differs from the value }}SB0·5 proposed by these authors. Simultaneously, the influence of strain rate on the static recrystallis ation critical temperature has been determined, it being observed that an increase in the former leads to a drop in the latter. Furthermore, strain rate is shown to have an influence on the recrystallisation–precipitation interaction, acting on those parameters that best contribute to defining RPTT diagrams In this sense, it was found that an increase in the strain rate led to a drop in the curve nose temperature T N and a reduction in the time necessary for precipitation to finalise t 0·95 , as well as an increase in the recrystallisation rate.

$Research paper thumbnail of Characterization by Electron Diffraction of Two Thermodynamical Phases of Precipitation in Nb-Microalloyed Steels$

Materials Science Forum, 2005

Excellent mechanical properties (high strength and toughness) of microalloyed steels are mainly c... more Excellent mechanical properties (high strength and toughness) of microalloyed steels are mainly caused by induced precipitation during thermomechanical treatment (TMT) and grain refinement. It has been recently found that TMT of Nb-microalloyed steels can give rise to two different kinds of precipitates, manifested by the double plateau in the statically recrystallised fraction (Xa) against time curves. This work presents an electron diffraction study performed in a transmission electron microscope, equipped with an EDS analytical system. Lattice parameters of a great deal of particles, smaller than 200 nm and with face cubic centred structure, have been measured. Frequency distribution of the values of lattice parameters shows that these are grouped in two sets whose mean values are close. Comparison of these values with those found in the literature for carbides, nitrides and carbonitrides usually present in microalloyed steels demonstrates that they are Nb carbonitrides with slig...

Boletín de la Sociedad Española de Cerámica y Vidrio, 2004

Para conseguir una disolución completa de los nitruros de titanio (TiN) es necesario alcanzar tem... more Para conseguir una disolución completa de los nitruros de titanio (TiN) es necesario alcanzar temperaturas próximas a la temperatura de fusión o incluso temperaturas superiores correspondientes al estado líquido. Esto hace que los TiN sean buenos controladores del tamaño de grano austenítico. Esta característica es aprovechada en los aceros estructurales para controlar la evolución de la microestructura en todo el proceso de laminación en caliente y en procesos de aporte de calor como la soldadura en la zona afectada térmicamente (ZAT). Sin embargo, es prácticamente desconocida la influencia cuantitativa que ejercen dichas partículas sobre la resistencia en caliente y sobre la cinética de la recristalización estática, responsable esta última de la evolución microestructural durante la laminación en caliente. En el presente trabajo se ha estudiado la influencia de diferentes composiciones de Ti y N sobre la resistencia en caliente, determinando la tensión pico de las curvas de fluencia y la energía de activación. La influencia sobre la recristalización estática se ha determinado mediante las curvas de fracción recristalizada frente al tiempo y con la energía de activación. Para comprender la influencia de la relación Ti/N de los aceros se realizó un análisis de los precipitados mediante Microscopía Electrónica de Barrido y de Transmisión. Palabras clave: Relación Ti/N, tensión máxima; recristalización estática; estado de precipitación Influence of the Ti/N ratio on the strength in the hot deformation and on the static recrystallisation in structural steels To obtain a complete dissolution of the nitrides of titanium (TiN) is necessary to reach temperatures close to the melting temperature or even higher temperatures corresponding to the liquid state. For this reason, TiN are good controllers of the austenitic grain size. This characteristic is used in structural steels to control the microestructural evolution in the hot rolling and in heat processes like in the welding in the zone affected thermically (ZAT). However, the quantitative influence of TiN particles on the hot deformation strength and on the static recrystallisation, this latter responsible for the microestructural evolution, is practically unknown. In the present work, the influence of different Ti and N compositions on the hot deformation strength have been studied determining the peak stress of the flow curves and the activation energy. The influence on the static recrystallization have been determined by means of the static recrystallisation fraction versus time and with the activation energy. A precipitate study by means of SEM and TEM was done in order to better understand the influence of the Ti/N ratio of the steels.

Revista de Metalurgia, 2005

Resumen Mediante ensayos de torsión y usando el método hack extrapolation se ha determinado la ci... more Resumen Mediante ensayos de torsión y usando el método hack extrapolation se ha determinado la cinética de recristalización estática de aceros microaleados con vanadio (V), niobio (Nb) y titanio (Ti) y, a partir de las mismas, ha sido posible dibujar los diagramas recristalizaciónprecipitación-tiempo'temperatura (RPTT), que muestran gráficamente la interacción recristalización-precipitación. Estos diagramas muestran que el efecto de la deformación en la cinética de precipitación depende del contenido de microaleante. En este sentido, se propone una nueva expresión para relacionar la influencia de la deformación y del contenido de microaleante sobre el periodo mínimo de incubación de la precipitación inducida. Palabras clave Deformación. Acero microaleado. Diagrama RPTT Cinética de precipitación. Periodo de incubación. Influence mutual of the deformation and chemical composition on induced precipitation in microalloyed steels Abstract By means of torsion tests and applying the "back extrapolation" method, the static recrystallization kinetics in microalloyed steels with vanadium (V), niobium (Nb) and titanium (Ti) has been determined and, recrystallization-precipitation-time-temperature (RPTT) diagrams have been plotted also graphically, which show the Recrystallization-Precipitation interaction. These diagrams show that the effect of the deformation on the precipitation kinetics depends of the microalloy content. In this sense, a new expression is proposed to relate the influence of the deformation and the chemical composition on the minimum incubation of the precipitation kinetics.

Revista de Metalurgia, 1998

Materials Science and Technology, 1999

ABSTRACT Using torsion tests and applying the ‘back extrapolation’ method in isothermal condition... more ABSTRACT Using torsion tests and applying the ‘back extrapolation’ method in isothermal conditions, recrystallisation–precipitation–time– temperature (RPTT) diagrams have been determined for a microalloyed steel with 0·35%C, 0·033%V, and 121 ppm N. The RPTT diagrams provide abundant information about the recrystallisation–precipitation interaction. Data such as the minimum incubation time for precipitates and the corresponding recrystallised fraction, the temperature for the minimum incubation time, and the time necessary for recrystallisation to be completed are deduced from the RPTT diagrams. The present study is completed with the determination of the activation energy for recrystallisation before and after precipitation, arriving at a new concept of the phenomenon that establishes discontinuous variation both in the derived function t0·5 against the inverse of the temperature and in the function itself, where t0·5 is the time corresponding to a 50% recrystallised volume fraction.

Materials Science Forum, 2003

Hot torsion tests were used to simulate hot rolling of a Nb-microalloyed steel. Subsequent graphi... more Hot torsion tests were used to simulate hot rolling of a Nb-microalloyed steel. Subsequent graphic representation of Mean Flow Stress (MFS) versus the inverse of absolute temperature for each pass allowed to know the critical rolling temperatures (T nr , A r3 , A r1) and residual stress accumulated in austenite just before austenite to ferrite phase transformation. It has been found that, as successive rolling passes are applied at temperatures below T nr , mean precipitate size decreases as a result of deformation applied and hardening by incomplete recrystallization of austenite.

Materials Science Forum, 2012

Starting from the expression of Dutta and Sellars for the beginning of strain induced precipitati... more Starting from the expression of Dutta and Sellars for the beginning of strain induced precipitation in microalloyed steels, the influence of temperature on t0.05 parameter has been studied. Although the equation given by these authors reproduces well the typical “C” shape of the curve of precipitation start time Ps, the expression is not reliable for all cases. The precipitation-time-temperature (PTT) diagrams have been plotted thanks to a new experimental study carried out by means of hot torsion tests on approximately twenty microalloyed steels having different contents of Nb, V and Ti. Mathematical analysis of results recommends the modification of some parameters such as supersaturation ratio (Ks) and constant B, which is no longer a constant but a function of Ks when the latter is calculated at the nose temperature (TN) of curve Ps. The value of parameter B is deduced from the minimum point or nose of the Ps curve, where ∂t0.05/∂T is equal to zero, and it can be demonstrated th...

Materials Science Forum, 2013

Under certain conditions of temperature, time and deformation, static recrystallization of austen... more Under certain conditions of temperature, time and deformation, static recrystallization of austenite in microalloyed steels can be temporarily inhibited by means of the strain-induced precipitation of nanoparticles that cause a pinning effect on austenite grain boundaries in motion. This inhibition can be seen by the formation of a “plateau” in the curves of static recrystallization of austenite obtained from double-deformation tests carried out under isothermal conditions. In this work, several microalloyed steels with different compositions are studied by hot torsion tests in order to characterize the kinetics of recrystallization and its inhibition. The precipitation state in austenite is studied in several samples by means of transmission electron microscopy. The influence of the type of microalloying element (Al, Nb, V) and the mean size of the precipitates on the duration time of the plateau is studied and relationships between these variables can be obtained. Particularly, it...

steel research international, 2014

Metallurgical and Materials Transactions A, 2013

Based on Dutta and Sellars's expression for the start of strain-induced precipitation in microall... more Based on Dutta and Sellars's expression for the start of strain-induced precipitation in microalloyed steels, a new model has been constructed which takes into account the influence of variables such as microalloying element percentages, strain, temperature, strain rate and grain size. Although the equation given by these authors reproduces well the typical "C" shape of the precipitation start time (P s) curve, the expression is not reliable for all cases. Recrystallization-precipitation-time-temperature (RPTT) diagrams have been plotted thanks to a new experimental study carried out by means of hot torsion tests on approximately twenty microalloyed steels with different Nb, V and Ti contents. Mathematical analysis of the results recommends the modification of some parameters such as the supersaturation ratio (K s) and constant B, which is no longer a constant but a function of K s when the latter is calculated at the nose temperature (T N) of the P s curve. The value of parameter B is deduced from the minimum point or nose of the P s curve, where t 0.05 /T is equal to zero, and it can be demonstrated that B cannot be a constant. The new expressions for these parameters derive from the latest studies undertaken by the authors and this work represents an attempt to improve the model. The expressions are now more consistent and predict the precipitation-time-temperature (PTT) curves with remarkable accuracy. The model for strain induced precipitation kinetics is completed by means of Avrami's equation.

Materials Science and Engineering: A, 2007

By means of torsion tests using small specimens, the influence of austenite grain size on strain ... more By means of torsion tests using small specimens, the influence of austenite grain size on strain induced precipitation kinetics has been determined in a vanadium microalloyed steel. Determination of recrystallisation-precipitation-time-temperature (RPTT) diagrams for two austenite grain sizes allows values of the aforementioned magnitudes to be determined. An ample discussion is made of the quantitative influence found and its relation with nucleation and growth mechanisms of precipitates. The results are compared with the quantitative influence exerted by the other variables, reaching the conclusion that the austenite grain size has a notable influence on strain induced precipitation kinetics which should not be underestimated. Finally, the influence of austenite grain size is included in a strain induced precipitation model constructed by the authors of this work and which also takes into account the other aforementioned variables.

ISIJ International, 2002

By simulation of hot rolling using torsion tests and subsequent graphic representation of mean fl... more By simulation of hot rolling using torsion tests and subsequent graphic representation of mean flow stress (MFS) versus the inverse of temperature for each pass, no-recrystallization temperature (T nr) was determined for a low niobium microalloyed steel at different interpass times and two strains of 0.20 and 0.35. Recrystallized fraction (X a) against time curves and RPTT diagrams were also determined for these two strains. This work has allowed evaluation of the influence of a very low Nb content on three aspects: T nr , the residual stress accumulated in the austenite just before the γ→α transformation (Δσ r), and recrystallization-precipitation interaction. It was found that when the strain applied was 0.2 and the interpass times were less than 30 seconds, the values of T nr and Δσ r were rather high. However, greater strains and interpass times brought about very small values of T nr and Δσ r. In addition to this, new aspects about the definition of T nr are also discussed in this paper.

ISIJ International, 2001

Using torsion tests a improved model has been constructed to predict the static recrystallization... more Using torsion tests a improved model has been constructed to predict the static recrystallization kinetics of deformed austenite in low alloy and microalloyed steels. The model quantifies the influence of the most common elements (C, Si, Mn, Mo) in low alloy steels and the typical elements (V, Nb) in microalloyed steels, when they are in solution. Activation energy (Q) is the parameter sensitive to the content and nature of each alloying element, and an expression for Q is shown as a function of the percentage of each one. Nb is the element that contributes most to increasing the value of Q, and thus that which most delays recrystallization kinetics. C is seen to be the only alloying element that contributes to lowering the value of Q, and thus to accelerating recrystallization kinetics. Extrapolation of the expression of Q to pure iron in the austenitic phase gives a value of 148 637 J mol Ϫ1 , which is similar to other values found in the literature for the grain boundary self-diffusion energy of pure Fe g. Static recrystallization kinetics follow Avrami's law and expressions are given for the parameter t 0.5 and the exponent n.

ISIJ International, 1999

Recrystallization-precipitation-time-temperature (RPTT) diagrams for strains of 0.20 and O.35 hav... more Recrystallization-precipitation-time-temperature (RPTT) diagrams for strains of 0.20 and O.35 have been determined for two microalloyed steels with niobium percentages of 0.024 and 0,058, respectively, and equal percentages of carbon and nitrogen. The method knownas "back extrapolation" has been used for the determination of static recrystallization kinetics and also for the plotting of the diagrams, While a single plateau was observed on the recrystallized fraction against time curves for the first steel, as a consequence of sttain induced precipitation, these curves for the second steel showed the formation of a double plateau. whose interpretation, confirmed by calorimetric analysis, supposes the formation of two types of preci pitates, The work uses transmission microscopy to show the precipitates which are formed in both steels, as well as the size most probably capable of inhibiting recrystallization. Final]y, an analysis is made of the RPTT diagrams and of the large amount of information which they offer for designing a more appropriate rolling schedule in order to obtain finer precipitates and a better austenitic microstructure before the austenite+ferrite transformation.