Cold-drawn steel deformation is determined by the deformation characteristics. Because there is no mandrel involved in the processing of the pipe wall during the drawing process, and the thickness of the pipe wall does not change much after the deformation, the great difference between the deformation conditions and deformation conditions of the inner and the outer layers of the seamless pipe is caused, resulting in uneven deformation.
In the process of deformation, the metal in the vicinity of the inner surface of the seamless pipe first enters the plastic deformation state, and the metal flows in the axial direction without being affected by the friction caused by friction. As a result, the inhomogeneity of the axial deformation in the thickness direction appears, as a result, the natural extension of the outer layer of seamless pipe is obviously lower than that of the inner layer. In this way, the axial and tangential additional tensile deformation and additional tensile stress in the outer layer of the seamless pipe reach the maximum; on the contrary, the axial and tangential additional compressive deformation and compressive stress in the inner layer of the seamless pipe when the maximum stress is reached, residual stress remains in the tube after deformation.
To sum up, due to the non-uniform deformation of seamless pipe after air-gap, so that there is a large residual stress in the seamless pipe, it is necessary to find an effective way to reduce the residual stress of the empty pipe to reduce the vertical split the harm.