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...

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 .

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a b s t r a c tA series of anisothermal hot torsion tests were carried out to simulate hot rolling on a high-strength low-carbon CMnNbMoTi microalloyed steel corresponding to an industrial X80 grade for pipeline construction. Mean Flow Stress was graphically represented against the inverse of temperature to characterize the evolution of austenite microstructure during rolling, which was also studied by optical microscopy and SEM on samples quenched from several temperatures. On the other hand, particles precipitated at different temperatures during rolling were analyzed by means of TEM using the carbon extraction replica technique and their size distribution and mean size were determined, as well as their morphology, nature and chemical composition. The effect of rolling temperature and austenite strengthening obtained at the end of thermomechanical processing on final microstructure and precipitation state was studied. Austenite strengthening was characterized by means of the parameter known as accumulated stress ( ). It was found that ferrite grains are finer and more equiaxed when the austenite is more severely deformed during finishing (higher values of ) but lower values of generate a higher density of acicular structures after cooling, which should improve the balance of mechanical properties. The increase in strength associated to acicular ferrite compared to polygonal ferrite is revealed by the higher values of Vickers microhardness measured on samples corresponding to low . On the other hand, (Ti, Nb)-rich carbonitrides can be found from reheating and their size keeps a constant value near 20-30 nm during thermomechanical processing. A second population of much finer (Nb, Mo)-rich carbonitrides whose size is close to 5 nm forms from lower temperatures, near 1000 • C. The accomplishment of two different levels of at the end of hot rolling schedule does not seem to introduce major differences in precipitation state before final cooling.

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).