Koh Matsumura1 and Kazuo Tsuruoka1
1Research Laboratories, Kawasaki Steel Corporation, Chiba
In order to evaluate the influence of precipitates on grain boundary migration quantitatively, the rate of normal grain growth of 3% silicon-iron containing aluminum nitrides was investigated.
The 3% silicon-iron ingots containing different amounts of aluminum and nitrogen were prepared by vacuum melting. These ingots were forged and hot-rolled to plates 2 mm in thickness, then the plates were cold-rolled to 0.5 mm in thickness. After decarburizing the specimens at 700°C in wet hydrogen, the rate of normal grain growth during isothermal annealing at 800, 850 and 900°C were measured.
The size and distribution of aluminum nitrides in the specimens after annealing at 700 to 900°C depend on the concentration of solute nitrogen in the raw materials during heating before hot rolling. The rate of normal grain growth was controlled by the distribution of precipitates. When the precipitates were more dense, the rate of normal grain growth became slower. Values of activation energy Qg and activation free energy Δ Fa for grain boundary migration were the same in all specimens, which were 65 and 34 kcal/mol respectively. The difference in the rate of normal grain growth could be explained in terms of the difference in the driving free energy Δ F which depended on the distribution of precipitates. In order to modify the Zener's theory more quantitatively, a new equation was derived by considering the Ostwald growth of precipitates. The rate of normal grain growth measured coincided with the theoretical values calculated from the equation.
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