Alberto Quispe

Strain induced precipitation effect on austenite static recrystallisation in microalloyed steels

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).

Model for static recrystallisation critical temperature in microalloyed steels

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.

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

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.

Model for Strain-Induced Precipitation Kinetics in Microalloyed Steels

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 the typical “C” shape of the precipitation start time (Ps) curve well, the expression is not reliable for all cases. Recrystallization–precipitation–time–temperature 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 (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 the Ps curve. 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 that B cannot be a constant. The new expressions for these parameters are derived 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 curves with remarkable accuracy. The model for strain-induced precipitation kinetics is completed by means of Avrami’s equation.

Characterization by Electron Diffraction of Two Thermodynamical Phases of Precipitation in Nb-Microalloyed Steels

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 slight stoichiometric differences (NbCxNy).

Influence of temperature on strain induced precipitation kinetics in microalloyed steels

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 that B cannot be a constant. The new expressions for these parameters derive from the latest studies developed by the authors and this work supposes an attempt to improve the model. The expressions are now more consistent and predict with remarkable accuracy the PTT curves.

Erratum to: Model for Strain-Induced Precipitation Kinetics in Microalloyed Steels

SEBASTIAN F. MEDINA, Professor, and MANUEL GOMEZ, Tenured Scientist, are with the National Centre for Metallurgical Research (CENIM-CSIC), Av. Gregorio del Amo 8; 28040 Madrid, Spain. Contact e-mail: smedina@cenim.csic.es ALBERTO QUISPE, Professor, is with the National University Jorge Basadre (UNJBG), Av. Miraflores s/n University City, Tacna, Peru. The online version of the original article can be found under doi: 10.1007/s11661-013-2068-1. Article published online April 3, 2014

Influence of (Al, Nb, V) Precipitates on the Recrystallization Inhibition in Microalloyed Steels

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 is seen that Al-alloyed steels present a much coarser particle size and a considerably shorter plateau compared to Nb and V-microalloyed steels.

Evolution of Austenite Microstructure and Precipitation State during Hot Rolling of a Nb-Microalloyed Steel

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 (Tnr, Ar3, Ar1) 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 Tnr, mean precipitate size decreases as a result of deformation applied and hardening by incomplete recrystallization of austenite. Introduction Most hot rolled microalloyed steels contain Niobium, since this element presents the advantage over vanadium of being less soluble in austenite at the temperatures usual in rolling. This low solubility leads to greater hardening of the austenite in the final stage of rolling, which ultimately implies a refining of the final ferrite/pearlite microstructure and consequently a generalized improvement of mechanical properties, especially toughness. No-recrystallization temperature (Tnr) represents the start of the inhibition of static recrystallization of austenite during hot rolling. The most common method for determining Tnr consists of simulating successive rolling passes and then graphically representing the mean flow stress (MFS) versus the inverse of the absolute temperature for each of the simulated passes. The inhibition of recrystallization indicated by Tnr appears as a rise in the slope of the MFS curve and in steel studied is mainly caused by strain-induced precipitation of niobium carbonitrides. This method also offers the possibility of obtaining the Ar1 and Ar3 phase transformation temperatures when the austenite is being deformed in conditions similar to rolling [1,2] as well as the residual stress accumulated in the austenite instants before the γ→α transformation [3]. In order to observe the microstructural changes and study the strain induced precipitation state during hot rolling, several samples were quenched from different temperatures along a hot rolling simulation schedule. Experimental procedure The steel studied, whose composition is shown in Table 1, was manufactured by Electroslag Remelting (ESR). Rolling simulations tests were carried out in a computer-controlled hot torsion machine, on specimens with a gauge length of 50 mm and a diameter of 6 mm. Prior to the simulation tests the specimens were austenitized at a temperature of 1250oC for 10 min. This reheating conditions were enough to dissolve completely the Niobium precipitates, as solubility temperatures calculated for carbonitrides, nitrides and carbides were equal to 1145 oC, 1112 oC and 1103 oC, respectively [4]. The temperature was then lowered to that corresponding to the first pass, which was 1150oC. The simulation consisted of the performance of 20 passes, with a temperature step of 25oC between passes and an interpass time of 100 s, the last pass being carried out at 675oC. The strain applied in each pass was of 0.20 and the strain rate was equal to 3.63 s. To study the microstructure and precipitation state during rolling, four samples were water quenched from different temperatures along rolling schedule. In every sample a last deformation step was performed and then temperature was lowered 25 oC for an interpass time of 100 s to reach the quench temperature. Microstructures were observed on a longitudinal surface and the characteristics of the precipitates were determined by TEM, using the carbon extraction replica technique. Table 1. Chemical composition of steel studied [mass %]. Results and Discussion Hot rolling simulation. The torsion test gives the values of torque applied versus the number of turns made on the specimen, which are transformed respectively into equivalent stress and strain using Von Mises criterion.[5] Fig. 1 shows the simulation of 20 rolling passes for the steel studied. At first deformations, stress raises as temperature decreases, after which there is a change in the slope with a growth in the stress, which means a greater tendency to strengthening. Later, stress drops and grows again at final passes. The meaning of these zones is better explained by observing Fig. 2 which shows the graphic representation of mean flow stress (MFS) versus the inverse of the absolute temperature. MFS is determined in each step by dividing the area below the stress-strain curve by the strain applied. In Fig. 2 it is possible to see four different zones. In the first zone (I), which corresponds to deformations at high temperatures, MFS grows as the temperature decreases. Austenite recrystallizes completely between passes and there is no accumulated stress. The increase in stress is due only to the decrease in temperature. In the second zone of the curve (II) there is a change in the slope, which indicates a greater tendency towards hardening. Here the stress accumulates in the austenite, whose recrystallization between passes is partially inhibited. The third phase (III), characterized by a drop in MFS as the temperature decreases, corresponds to the austenite→ferrite partial transformation. In the fourth and final region (IV), where the stress again rises as the temperature drops, the austenite→ferrite transformation finishes and the eutectoid transformation takes place. 0 1 2 3 4 0 50 100 150 200 250 300 Austenitization: 1250 oC×10 min First pass: 1150 oC Last pass: 675 oC Interpass time: 100 s Pass strain: 0.20 Strain rate: 3.63 s Eq ui va le nt S tre ss (M Pa )