The hot ductility behavior of commercial low-carbon microalloyed Nb-V-Ti-steel was studied in both wrought and cast conditions by means of hot tensile tests over a wide range of temperature (750-1200 8C) and at three strain rates (0.01, 0.002 and, 0.0005 s À1 ). To replicate the straightening operation undergone by the two materials during hot drawing of plates and continuous casting process, respectively, two kinds of thermal cycles, namely solutionizing and precipitation treatments, were adopted to compare their respective influence on the formation of surface cracking. After deformation, the lowest ductility values were found at 900 8C in the single g-domain for the as rolled material in the former treatment, and at 800 8C in the two-phase domain for the as cast product in the second one. By contrast, the largest values were observed at 750 8C and above 1000 8C for the two examined materials in both types of treatment. On the whole, surface cracking was found as a result of grain boundary sliding which was acknowledged as the main fracture mechanism. An attempt was also made to determine a relationship between the synergistic effect of the various intervening micromechanisms and the resulting embrittlement.

In this study hot ductility has been determined from tensile tests for two states of a microalloyed steel: after casting and after rolling processes. Hot deformations were carried out at speeds varying from IO-~S-' to 10-~s-l and temperatures from 750°C to 110O0C. Two heat treatments were chosen before hot deformation. A ferrite precipitation is observed at austenitic grain boundaries in the intercritical temperature range, causing intergranular embrittlement. Ductility trough is deeper in the as-cast samples due to the growth of large grain size. Also, precipitation makes the hot ductility curve wider and deeper around 900°C. The results show a decrease in hot ductility. Minimum values of hot ductility are determined for (ITC) treatment at 9W°C and for (DTC) treatment at 800°C. For this second treatment another decrease in hot ductility was observed at 900°C. We can explain hot ductility losses by the presence of precipitates in the austenitic region and the presence of the two-phase structure in the intercritical region.

The flow behavior of CMn (Nb-Ti-V) micro alloyed steel was studied by hot compression tests in a wide range of temperatures (700 °C to 1050 °C, Step 50 °C), strain rates (0.000734 s-1 , 0.0029 s-1 , and 0.0146 s-1) and true strain of 0 to 0.8. Based on the experimental true stress-plastic strain data, the artificial neural network (ANN) methods were employed to predict the flow stress of CMn (Nb-Ti-V). The ANN model was trained with Levenberg-Marquardt (LM) algorithm. The optimal LM neural network model with two hidden layer network with ten neurons in the first and ten neurons in the second gives the best predictions is developed. It is demonstrated that the LV neural network model has better performance in predicting the flow stress. The results can be further used in mathematical simulation of hot metal forming processes.

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