Abstract: A working mandrel is composed of several similar sections, each having a working surface on the body thereof. The leading end of each section has a socket, while the trailing one has a projecting portion of a similar configuration for jointing the sections during the rolling step. Rolling is effected on the working mandrel assembled in a head-to-tail fashion and after the rolling cycle is terminated the leading section is transferred to the mandrel cooling and lubricating line and then, cooled and suitably lubricated, is positioned in front of the first roll stand, while the other sections remain between the first and the last roll stands. Put into position in front of the multistand mill may be a previously prepared section instead of that transferred to the mandrel cooling and lubricating line.