When producing seamless pipes, they are inserted into the inside of the pipe body and rolls and other tools are used to form the annular hole to form the pipe. Mandrel bars are used in continuous pipe rolling, tube diagonal rolling, tube rolling, pipe jacking, and tube cold rolling.

The mandrel is a very long round rod that participates in the deformation of the pipe in the deformation zone like the head. The difference is that the mandrel moves axially in the pipe as the pipe rotates while being rolled; In the case of periodic tube rolling and pipe jacking), the core rod does not rotate but also moves axially with the tube.

On floating mandrel and restricted mandrel continuous rolling mills (see continuous tube rolling mill rolling), mandrels are important tools. They require high strength and high wear resistance, but they also require High surface quality, such as grinding and heat treatment after turning. Floating mandrels are long (up to 30m) and heavy (up to 12t). The length of the mandrel is slightly shorter, but the material requirements are higher. The mandrel should be able to withstand a large jacking force on the mandrel used. The mandrel of the cyclic tube rolling machine is heated for a long time during operation. The mandrels of the diagonal rolling extension machine include tensile mandrels, floating mandrels, limited mandrels and retracted mandrels.

A tensile mandrel is a mandrel whose axial movement speed is greater than the axial movement speed of the pipe during operation (see Pipe Rolling Extension). The mandrel applies a tensile force to the inner surface of the pipe. The retractable mandrel is a mandrel whose axial movement direction of the mandrel is opposite to the axial movement direction of the pipe and which applies a rear tension. The requirements for mandrels for diagonal rolling stretchers are lower than those for longitudinal rolling stretchers.