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The static recrystallization kinetics of four steels microalloyed with Nb, Ti and Nb-Ti have been investigated, covering conventional and hot direct rolling conditions. The influence of the grain size on the recrystallization time has been studied for a wide range of grain sizes (20-1000 mm). For the same deformation conditions it has been observed that the quadratic dependence on grain size works well in the range of finer grain sizes (Ͻ160 mm). In contrast, the recrystallization times measured at coarse grain sizes are significantly lower than those predicted by this dependence. However, when the whole grain size range analysed is considered, a linear dependence on grain size gives the best fit to experimental data. The effect of strain on the recrystallization kinetics has been also analysed for different initial grain sizes. It has been observed that the time to reach a 50 % recrystallized fraction (t 0.5 ) shows a dependence on strain of the type t 0.5 ϰe Ϫp , the exponent being influenced by the initial grain size. A unique equation has been proposed to describe the static recrystallization kinetics of Nb, Ti and Nb-Ti microalloyed steels. The predictions of the equations have been compared to previously published data on similar steels.KEY WORDS: static recrystallization; hot direct rolling; Nb; Ti and Nb-Ti microalloyed steels.austenite are not yet completely understood. These differences require an extension of the valid intervals of application for the equations already obtained. The slower recrystallization of the very coarse austenite in C-Mn and microalloyed steels in hot direct rolling processes, allied to the high supersaturation of microalloying elements, can modify the exponent values of Eqs. (1) and (2).A study has been carried out to determine the effect of initial austenite grain size on the 50 % recrystallization time, by using a wide range of grain sizes (20-1 000 mm) in one Nb, two Ti and one Nb-Ti low carbon microalloyed steels. The influence of the applied strain on the static recrystallization kinetics of these microstructures has also been investigated. Material and Experimental ProcedureThe chemical compositions of the steels analysed in wt% are listed in Table 1. Three grades of steel have been considered. The first steel is a Nb microalloyed steel (Nb steel). The second steel (Nb-Ti) has a similar Nb content to the first, but also contains 0.067 wt% of Ti. The other two steels (Ti-a and Ti-b) have Ti additions with different N levels, their Cu contents being similar to those steels produced in an electric arc furnace with scrap.In order to study the static recrystallization kinetics, twopass torsion tests were carried out, under isothermal conditions, to examine the softening behaviour of the austenite between intervals of hot working. The geometry of torsion specimens was a reduced central gauge section, 15.5 mm in length and 7.5 mm in diameter. The specimens are reheated at different soaking temperatures in the range from 1 100 to 1 460°C for 12-15 min. and cooled to test temperat...