In parallel, a study of precipitate sizes was carried out using transmission electron microscopy (TEM) and scanning electron microscopy (SEM

Madrid, SpalnRecrystallization-precipitation-time-temperature (RPTT)

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 g®a transformation (Ds r ), and recrystallization-precipitation interaction. It was found that when the strain applied was 0.2 and the interpass times were less than 30 s, the values of T nr and Ds r were rather high. However, greater strains and interpass times brought about very small values of T nr and Ds r . In addition to this, new aspects about the definition of T nr are also discussed in this paper.KEY WORDS: no-recrystallization temperature; static recrystallization; precipitation; RPTT diagram; transmission electron microscopy.some references on recrystallization-precipitation interaction in Nb microalloyed steels. 23) However, among the abundant literature on Nb microalloyed steels, practically none refers to the influence of a low Nb content (Ͻ0.010%) on the three aforementioned aspects (T nr , SRCT, R-P interaction) or the residual stress accumulated in austenite prior to reaching the A r3 temperature. The work that is described here has involved the performance of rolling simulation tests in different conditions (interpass time Dt and strain applied in each pass e), determining the evolution versus these variables of the different critical rolling temperatures (T nr , A r3 , A r1 ) and the residual stress accumulated in the austenite moments before the g®a transformation (Ds r ). In this way it has been found that certain rolling conditions (Dt, e) allow such a low Nb addition to be effective on the hardening of the austenite. Furthermore, an analysis has also been made of the influence of Ds r on the value of A r3 .On the other hand, SRCT has been calculated and the recrystallization-precipitation interaction studied by the determination of RPTT diagrams for two different strains. The study of the influence that the precipitation derived from a very low Nb addition can have on recrystallization is completed by TEM analysis of the state of the precipitation existing in the austenite at deformation temperatures slightly above T nr . This microscopy study will help to gain a deeper knowledge of the relation between the start of inhibition of recrystallization marked by T nr and the start of strain-induced precipitation. Experimental ProcedureThe steel studied, whose composition is shown in Table 1, was manufactured by Electroslag Remelting (ESR) in a laboratory unit capable of producing 30 kg ingots. This technique avoids macrosegregation, both in alloying elements and impurities, and there is considerably less micr...

In this paper, the first study on NH4CN polymerization induced by microwave radiation is described, where a singular kinetic behaviour, especially when this reaction is conducted in the absence of air, is found. As a result, a complex conjugated N-heterocyclic polymer system is obtained, whose properties are very different, and even improved according to morphological features, characterized by their X-ray diffraction patterns and scanning electron microscopy analysis, with respect to those produced under conventional thermal treatment. In addition, a wide variety of relevant bioorganics have been identified, such as amino acids, nucleobases, co-factors, etc., from the synthetized NH4CN polymers. These particular families of polymers are of high interest in the fields of astrobiology and prebiotic chemistry and, more recently, in the development of smart multifunctional materials. From an astrobiological perspective, microwave-driven syntheses may simulate hydrothermal environments, which are considered ideal niches for increasing organic molecular complexity, and eventually as scenarios for an origin of life. From an industrial point of view and for potential applications, a microwave irradiation process leads to a notable decrease in the reaction times, and tune the properties of these new series macromolecular systems. The characteristics found for these materials encourage the development of further systematic research on this alternative HCN polymerization.

The hot rolling process of a low Nb-microalloyed steel under different interpass time conditions is simulated by means of hot torsion tests. Subsequent graphic representation of the Mean Flow Stress (MFS) versus the inverse of the absolute temperature for each pass allows us to know the critical rolling temperatures (T nr , A r3 , A r1 ) and to characterize the progressive strengthening of austenite due to incomplete recrystallization between T nr and A r3 , thanks to the measurement of a magnitude called accumulated stress (Ds). Optical and electron microscopy studies demonstrate that the evolution of the microstructure and the precipitation state-particularly the mean particle size-over the rolling schedule is strongly dependent on the interpass time. A review is made of the expressions that have been proposed to estimate the values of recrystallization driving (F R ) and pinning forces (F P ). Using these expressions and the experimental data from the hot rolling simulations performed, the evolution of F R and F P during rolling is studied. A comparative analysis of hypotheses concerning the interaction between precipitates and migrating grain boundaries is achieved and the methods for estimating the volume fraction of precipitates and the dislocation density are assessed. Though the selected criterion significantly influences the values obtained for both forces, it is found that F P always grows faster than F R as the rolling temperature drops, which helps to explain the start of inhibition of the static recrystallization of austenite at temperatures below T nr .