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.KEY WORDS: austenite; static recrystallization; low alloy steels; microalloyed steels.compositions are shown in Table 1. The twenty-six steels include different combinations of C, N and metallic precipitate-forming elements such as V and Nb. Given that the nitrides, carbides or carbonitrides of Nb are less soluble in austenite than those of V, the limit imposed on C and N contents has been that the solubility temperature should not exceed 1 300°C. In this sense, some compositions, such as steel N9, with a very low Nb content and high C content, are not currently standard compositions, but the interest in studying them lies in ascertaining the influence of low Nb contents on recrystallization.Torsion specimens were prepared with a gauge length of 50 and 6 mm diameter. The reheating temperature prior to torsion deformation was different depending on whether the steel was microalloyed with V or with Nb (Table 1), as the solubility temperature of the precipitates depends on their nature and on the precipitate-forming element content. For steels containing vanadium, designated by the letter V, the reheating temperature was 1 230°C for steels V1, V2 and V3 and 1 200°C for the rest, which is sufficient to dissolve vanadium nitrides and carbides. In the case of niobium steels, designated by the letter N, the reheating temperature depended on the C, Nb and N contents, but was always above the solubility temperature of niobium carbonitrides. 17)All the C-Mn/Si steels (C1, C2, C3, M1, M2, S1, S2), C-Mo steels (Mo1, Mo2) and the microalloyed steels V1, V2, V3, N1 and N2 have been studied previously 14,15) in relation with static recrystallization, but their results are taken into account in order to achieve greater universalization of the model proposed in this work.To ensure that the testing temperatures corresponded to the austenitic phase, critical transformation temperatures were measured by d...

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...

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 in¯uence 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).