Mohsen Ayaz - M.S. Student, Shahid Rajaee Teacher Training University/Department of Mechanical Engineering
Nasrollah Bani Mostafa Arab - Assistant Professor, Shahid Rajaee Teacher Training University/Department of Mechanical Engineering
Daavood Mirahmadi Khaki - Lecturer, Shahid Rajaee Teacher Training University/Department of Materials Engineering
Mohammadmorad Sheikhi - Assistant Professor, Shahid Rajaee Teacher Training University/Department of Mechanical Engineering

Hot rolling is a metalworking process performed above the recrystallization temperature of the material and sometime in non-recrystallization area. In sheet metalforming, strain hardening exponent plays a crucial role. The purpose of this paper is to determine the influence of major hot rolling process parameters of roughing,finishing and coiling temperatures on strain hardening exponent of Nb-microalloyed steel sheet and optimizingthese parameters to obtain maximum strain hardening exponent. For this purpose, four levels for the above temperatures were chosen and 192 experiments wereconducted based on general full factorial design and three replication of experiment. Analysis of variance was done to determine the most significant factors affectingthe response and develop a mathematical model relating the above process parameters to the strain hardeningexponent. Based on calculations, optimum values of the parameters were obtained. It is observed that increasing the coiling and finishing temperatures causes an increaseof the strain hardening exponent. Also, microstructure analysis indicates that decreasing the coiling andfinishing temperatures effectively causes more grainrefinement of the microstructure, and the morphology is changed from polygonal ferrite to an acicular one.Findings of this research show that the predicted values obtained are close to the experimental value indicating suitability of the model.

hot rolling, Nb-microalloyed steel, strain hardening exponent, factorial design, Optimization