Abstract
Low-carbon capped-steel sheets having excellent drawability and ductility have been obtained by a continuous annealing process. Materials suitable for the purpose are low manganese steels (<0.25% Mn) with K values of 0-0.15, where the K value is defined as K=Mn(%)-55/16·O(%)-55/32·S(%)
These steels are hot-coiled at high temperatures such as 670°C or above, and thencold-rolled. The time required for annealing at 700°C is about 1 min, which is extremely short compared with that required for the conventional batch annealing. As the optimum conditions for improving the ductility by controlling the distribution of carbides as well as the content of solute carbon, the cooling rate after continuous annealing and the subsequent over-aging temperature have been found to be 5-30°C/sec and 350°C-450°C, respectively. It should be noted that the ductility is much reduced when stresses exceeding some critical value (about 13kg/mm2 in the present case) are applied to the sheets during the over-aging treatment. Under the stress whose magnitude is not far less than the critical value, the over-aging treatment can be finished up in a short time. It is necessary, therefore, to install hearth rolls with large radii for the over-aging zone in a continuous furnace.
