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Dive into the research topics where Sebastián F. Medina is active.

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Featured researches published by Sebastián F. Medina.


Acta Materialia | 1996

General expression of the Zener-Hollomon parameter as a function of the chemical composition of low alloy and microalloyed steels

Sebastián F. Medina; Carlos A. Hernández

Using torsion tests the Zener-Hollomon parameter has been determined in a selection of 18 steels with compositions appropriate for the study of the influence of each alloying element (C, Mn, Si, Mo, Ti, V, Nb) on the Zener-Hollomon parameter (ϵexp(QRT) = A(sinh ασp)n). It is demonstrated that all the alloying elements influence, to a greater or lesser extent, the activation energy (Q). A rise in the content of any alloy was found to increase the activation energy, except in the case of carbon which has the opposite effect. An expression is given for Q as a function of the content of each alloying element and the second side of the equation is completed by determining the optimum values of α and n for all the steels, giving 0.01187 MPa−1 and 4.458, respectively. It is demonstrated that A is not a constant but is a function of the activation energy. In this way it is possible to calculate the peak stress (σp) at any temperature and strain rate for any low alloy or microalloyed steel in the austenite phase. This study brings a new dimension to the Zener-Hollomon parameter and potentially improves its applications, for example in the calculation of stress-strain curves.


Acta Materialia | 1996

Modelling of the dynamic recrystallization of austenite in low alloy and microalloyed steels

Sebastián F. Medina; Carlos A. Hernández

Using torsional tests in the austenite phase, the dynamic recrystallization of a selection of low alloy and microalloyed steels was studied and Avrami`s equation was modelled. The model is based on the experimental determination of flow curves and their subsequent modelling, making it possible to calculate the recrystallized fraction (X{sub d}) as a function of all the variables which intervene in hot deformation: temperature, strain rate, austenite grain size and the chemical composition of the steel. The influence of the first three variables is quantified using the Zener-Hollomon parameter where the activation energy is expressed as a function of the content of each alloy. The start of dynamic recrystallization was determined by regression, finding a value of 0.95{epsilon}{sub p}. In testing conditions all the elements were found in solution in the austenite, except Ti which was found partially precipitated. The results of these tests indicate that C, Si (low content), Mn and V have hardly any influence on the start of recrystallization while Mo, Ti and especially Nb delay it. Finally, the dynamic recrystallization kinetics are illustrated through the study of the microstructure during deformation.


Acta Materialia | 1996

Modelling austenite flow curves in low alloy and microalloyed steels

Carlos A. Hernández; Sebastián F. Medina; J. Ruiz

Abstract A model has been developed to predict the austenite flow curves of low alloy and microalloyed steels. The model consists basically of two expressions for stress, as a function of strain, temperature, strain rate and the chemical composition of the steel. The first predicts the hardening and dynamic recovery region and the second predicts the softening caused by dynamic recrystallization, the algebraic sum of both expressions predicting the complete flow curve. The models different parameters are a function of the dimensionless parameter Z A of Sellars and Tegarts equation, which in turn has been modelled not only as a function of temperature and strain rate but also as a function of the chemical composition. In this way one single model may be applied to any hot deformed steel whose composition is the same as or similar to those studied.


Acta Materialia | 1996

The influence of chemical composition on peak strain of deformed austenite in low alloy and microalloyed steels

Sebastián F. Medina; Carlos A. Hernández

Abstract Using hot torsional tests, the influence of the most common elements in low alloy steels (C, Mn, Si, Mo) and in microalloyed steels (Ti, V, Nb) on peak strain has been studied. At the temperatures (900, 1000 and 1100°C) and strain rates (0.544, 1.451, 3.628 and 5.224 s −1 ) at which the tests were carried out all the elements remained in solution during deformation with the exception of titanium which was in part previously precipitated. In these conditions, the results indicate that C, Si and Mn have hardly any influence on the value of peak strain ( ϵ p ) while Mo, V, Ti and particularly Nb considerably increase its value. The study has been completed with the modelling of ϵ p as a function of the Zener-Hollomon parameter and the austenite grain size for all the steels studied, showing the quantitative influence of the different alloying elements.


Scripta Metallurgica Et Materialia | 1994

The influence of niobium on the static recrystallization of hot deformed austenite and on strain induced precipitation kinetics

Sebastián F. Medina; Juana E. Mancilla

This work presents a study of the influence of Ti on the kinetics of static recrystallization measured at different temperatures and on the kinetics of induced precipitation. The recrystallized fraction has been measured using the method known as back extrapolation. It has been found that using this method the softened fraction and the recrystallized fraction are approximately equal. The method used has also made it possible to draw, as will be seen later, precipitation-time-temperature (PTT) diagrams, until now determined in other Nb microalloyed steels using theoretical equations, or using transmission microscopy, and discovering in this way the kinetics of the precipitation induced by the strain. The recrystallized fraction will be modeled in accordance with Avramis equation, as its application in hot rolling is of great use.


Acta Metallurgica Et Materialia | 1994

Static recrystallization of austenite and strain induced precipitation kinetics in titanium microalloyed steels

Sebastián F. Medina; Juana E. Mancilla

Abstract Using torsion tests and applying the back extrapolation method the recrystallized fraction of austenite was measured at two strains (0.20 and 0.35) and several temperature makes it possible steels. Graphical representation of the activation energy vs the inverse of the temperature makes it possible to determine the static recrystallization critical temperature (SRCT) of each steel as a function of the strain. With the results obtained, a model was constructed for the recrystallized fraction both at temperatures above and below the SRCT, and this, together with other equations already published, permits calculation of the residual strain and austenite grain size which the steel is expected to have at the end of rolling and before the γ → α transformation. Graphs of the recrystallized fraction vs time show the kinetics of strain induced precipitation and PTT diagrams (precipitation-time-temperature) were constructed for the three steels. It is noted that the degree to which the strain affects the precipitation kinetics decreases as the titanium content of the steel is increased.


Materials Science and Technology | 2003

Strain induced precipitation effect on austenite static recrystallisation in microalloyed steels

Sebastián F. Medina; Alberto Quispe; Manuel Gómez

Abstract Using torsion tests and applying the back extrapolation method, the strain induced precipitation effect on austenite static recrystallisation in vanadium and niobium microalloyed steels has been studied and a model has been constructed. This model takes account of precipitation and its influence on recrystallisation kinetics, in particular on the activation energy, which is increased. The model is applied at temperatures below the temperature at which inhibition of recrystallisation commences owing to the induced precipitation. The new values of activation energy can be three times higher than the activation energy before precipitation has started, depending on the contents of elements responsible for the precipitation (Nb, V, N, C).


Journal of Materials Science | 1997

Determination of precipitation–time–temperature (PTT) diagrams for Nb, Ti or V micro-alloyed steels

Sebastián F. Medina

A method is described, developed in CENIM-CSIC, for studying the kinetics of strain induced precipitation in micro-alloyed steels. Using torsion tests, the statically recrystallized fraction has been determined for three Nb, V and Ti micro-alloyed steels at different temperatures and strains. When precipitation starts, the recrystallized fraction deviates from Avrami’s equation, making it possible to identify the moment at which precipitation starts (Ps) and finishes (Pf). In this way precipitation–time–temperature(PTT) curves can be drawn, showing the precipitation kinetics in graph form.


Materials Science and Technology | 2001

Model for static recrystallisation critical temperature in microalloyed steels

Sebastián F. Medina; Alberto Quispe; Manuel Gómez

Abstract By means of hot torsion tests, the static recrystallisation critical temperature (SRCT) has been determined for 18 microalloyed steels classified into two groups. In one group the metallic microalloying element is vanadium, and in the other it is niobium. In both groups the microalloying element, carbon, and nitrogen contents vary from one steel to another. Tests have been carried out at various strains and strain rates, and recrystallisation–precipitation–time–temperature (RPTT) diagrams have been drawn for each steel in each condition. The SRCT is the asymptote of strain induced precipitation start P s and end P f curves, and its determination has permitted the construction of a model that quantifies the effects of all the external variables implicit in hot working such as strain and strain rate, and the internal variables such as austenite grain size and chemical composition of the steel. Hence, the influence of each of these variables has been quantified, and the models prediction, comparing experimental values with calculated values, gives a correlation index of ∼0.9.


Materials Science and Technology | 1999

Effect of strain on recrystallisation–precipitation interaction in low vanadium microalloyed steel

Alberto Quispe; Sebastián F. Medina; J. M. Cabrera; J. M. Prado

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.

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Manuel Gómez

Spanish National Research Council

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Alberto Quispe

Spanish National Research Council

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Lucía Rancel

Spanish National Research Council

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P. Valles

Instituto Nacional de Técnica Aeroespacial

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M. Chapa

Spanish National Research Council

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María Isabel Vega

Spanish National Research Council

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Juana E. Mancilla

Benemérita Universidad Autónoma de Puebla

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Pedro Gómez

Spanish National Research Council

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Carlos A. Hernández

National Autonomous University of Mexico

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P. Valles

Instituto Nacional de Técnica Aeroespacial

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