Abstract: 34CrMo4 seamless steel pipe with wall thickness of 0.9 mm was prepared by cold drawing process, and the changes of residual stress, microstructure and properties of the cold-drawn pipe during vacuum annealing were studied by X-ray stress tester, scanning electron microscope (SEM), electron backscatter diffraction (EBSD) technology and universal tensile testing machine. The results show that the vacuum annealing can effectively reduce the residual stress of the tested steel after cold drawing, where the strength decreases, the plasticity increases, and the structure of the cold drawing zone recovers, but the streamline microstructure of the cold drawing deformation is still maintained. The cold drawing leads to increase in the density of small angle grain boundaries, while after vacuum annealing, small angle grain boundaries are gradually replaced by large angle grain boundaries. When vacuum annealed at 650 ℃, the effective ferrite grain size of the tested steel is 3.62 μm, the residual stress is -13.1 MPa, the tensile strength is 693 MPa, the elongation is 26.1%, which reduces the residual stress while obtains a good strength plasticity matching.

Key words: vacuum annealing, thin-walled pipe, residual stress, microstructure, mechanical properties