Stress State During The Forming Process
After the strip is processed into a spiral tube blank, the stress generated by the tube wall is mainly affected by the forming force during the strip forming process. According to the plastic deformation theory, when the relative bending radius of the strip is equal to 100, the plastic deformation will be very obvious. Therefore, the bending deformation of the formed strip is considered to be linear pure plastic bending.
Usually, the work done by the external force on the metal will be converted into heat energy during the deformation process, causing the temperature of the metal to rise. Only a small part of the work will stay in the metal, and residual stress will be formed inside the metal. Among them, most of the energy storage in the metal is lattice distortion stress, which is mainly manifested by the deformation hardening of the pipe wall, and the tensile strength of the steel pipe increases. At the same time, the inner metal and the tens of layers of metal are subjected to pressure and tensile force, respectively, which will cause reverse plastic deformation, and then the Bauschinger effect will occur, resulting in a decrease in the yield strength of the strip.
The yield strength of the strip will have a significant effect on its work hardening ability and Bauschinger effect. As the yield strength of the strip increases, the work hardening ability decreases and the Bauschinger effect increases.
When the yield strength of the strip exceeds 450 MPa, the change in the Bauschinger effect will significantly exceed the work hardening ability, at which point the yield strength of the strip will decrease. In addition to plastic deformation, it may occur in elastic bending deformation proportional to the bending moment. When the bending moment is reduced, if a load is applied to the steel pipe, there is a linear relationship between the bending moment and the bending curvature.