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The process of brittle fracture in steels can be divided into three distinct steps: (1) initiation of a microcrack in a brittle particle, (2) propagation of the microcrack into the surrounding matrix and, finally, (3) crack progression through the matrix. Depending on microstructure, temperature and loading rate, the critical step which controls cleavage fracture is subject to change. In this work the behaviour of different microalloyed steels is considered and the energies ypm and y&, which defme the stress necessary for the microcrack to surmount steps 2 and 3 have been experimentally determined. While the yp value remains constant around 7 J/mz, it has been observed that y-is dependent on temperature. At -196°C the value is lower than 50 J/mz and at room temperature it is higher than 200 J/m2. This increase in the matrix-matrix energy with temperature increases the probability of microcracks, generated in particles, arresting at grain boundaries. This is the reason why refinement of grain size has an important effect in improving the fracture toughness at room temperature.Keywords-Cleavage fracture; Ti micro-alloyed steels; Effective surface energy; Critical failure stress. NOMENCLATURE a = particle size D = grain size E = Young's modulus f. = ferrite volume fraction rn = Weibull modulus n = Hollomon hardening coefficient UTS = ultimate tensile strength fi =misorientation angle 4 = correction for ellipticity v = Poisson's ratio u-, = maximum and minimum particle size D , , D-=maximum and minimum first cleavage facet size fuTa = coarse TiN particle volume fraction y-, yp = effective surface energy matrix-matrix and particlematrix, respectively u,,,, =yield stress uF, OF* = critical failure stress, macroscopic and local, respectivelyu-= stress necessary for crack to cross matrix-matrix boundary up = stress necessary for crack to cross particle-matrix boundary

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Resumen Se analiza la influencia de los tratamientos termomecánicos, en primer lugar, llevados a cabo en laboratorio y, posteriormente, en un proceso industrial, sobre las propiedades mecánicas de aceros microaleados de forja. Se han variado las temperaturas de forja y las velocidades de enfriamiento tras la misma y se ha estudiado su influencia sobre la resistencia y la tenacidad. La optimización de los tratamientos termomecánicos se ha realizado a nivel de laboratorio, probándose posteriormente las secuencias optimizadas a escala industrial. Las propiedades mecánicas se pueden explicar en términos de las microestructuras obtenidas en los diversos tratamientos termomecánicos. Así, microestructuras gruesas, obtenidas por enfriamientos lentos tras la deformación a elevadas temperaturas, muestran un comportamiento frágil. Se pueden obtener buenos valores de tenacidad, manteniendo la resistencia por afino de la microestructura, bien deformando el material a menores temperaturas de forja (ferrita+perlita finas) o enfriando aceleradamente tras la forja convencional (ferrita acicular). Influence of the microstructure on the fracture toughness of forging microalloyed steels AbstractThe influence of thermomechanical treatments, first carried out at laboratory and afterwards on an industrial scale on the mechanical properties of microalloyed forging steels is analysed. The influence of hot working conditions and the cooling rate after hot working on both the strength and the toughness is analysed. Optimization of the thermomechanical treatments parameters has been carried out at laboratory and the optimized schedules have been carried out industrially. The actual experimental results of mechanical properties have been rationalized in terms of the microstructure developed during the different thermomechanical treatments. After slow cooling of steel bars hot rolled at low temperature (below 850 °C), the ferrite-pearlite is much refined and the toughness improved. Alternatively, accelerated cooling after hot working at high temperature produces a microstructure of acicular ferrite with improved toughness.