Effect of hot rolling temperature on the grain size of aluminium sheets with various purities

Effects of hot rolling temperatures on recrystallized grain size of aluminium sheets have already been studied by many investigators, and it is generally known that the higher the hot rolling temperature rises, the finer does the grain size grow. However, these investigations were carried out on aluminium of commercial purity, such as 99.0-99.5% aluminium, and few or no systematic studies on aluminium of a wide range of purity.Therefore, this work is undertaken to determine the effect of the hot rolling temperature on the recrystallized grain size of 99.1-99.99% pure aluminium sheets, hot rolled at the starting temperature of 400°C (finishing 340 °C), 500°C(finishing 400°C) and 600°C (finishing 450°C) respectively and cold rolled without interannealing.The results obtained are as follows:1) A high hot rolling temperature gives a fine recrystallized grain size in aluminium of lower purity than 99.9%, whereas it has a little effect on a grain size in 99.9% aluminium as well as in 99.99% aluminium having a cold reduction lower than approximately 85%, As regards 99.99% aluminium with a cold reduction higher than approximately 85%, when hot rolling temperature is high, the recrystallized grain size becoms large in case of a rapid heating rate in annealing, while the grain size becomes fine in case of a slow heating rate.2) With 99.99% aluminium hot rolled at a high temperature, a recrystallized grain size increases sharply at a cold reduction over 85%, in case of a rapid heating rate in annealing, and this phenomenon shifts to a high cold reduction in the case of a slow heating rate.3) As regards 99.99% aluminium with a cold reduction in excess of 85%, a rapid heating rate gives a large recrystallized grain size at a high hot rolling temperature, but a grain size of sheet heated at a very rapid rate as treated in a salt bath is comparable to that of sheet at a very slow heating rate.Whereas, a rapid heating rate gives a fine recrystallized grain size at a low hot rolling temperature.4) A hot rolling temperature also has an influence upon a recrystallization temperature. In the case of 99.99% aluminium, a low hot rolling temperature always goes with a low recrystallization temperature. This tendency becomes less pronounced as the purity of aluminium decreases. However, it again becomes marked with 99.1% aluminium. This apparently is attributable, unlike 99.99% aluminium, to a difference in the amount of cold work due to a difference in the degree of recrystallization on a hot rolled sheet.