Taking CP780 steel as the research object,the treatments of pre-stretch deformation and stress relief annealing was conducted on it.The microstructures,hardness,tensile properties and tensile fracture morphologies of the CP780 steel in different states were systematically studied by means of the optical microscopy(OM),scanning electron microscope(SEM),Vickers hardness tester and tensile testing machine.The results show that the microstructure of the CP780 steel in different states are mainly the bainite and ferrite.After stress relief annealing at 400-550 ℃,the bainite banded structure at the center of the specimen wall thickness is the main reason for the delamination of the tensile fracture surface.With the increase of annealing temperature,the plasticity reduction of the CP780 steel due to pre-stretch deformation is improved,and the strength firstly decreases,then increases and then reduces.The pre-stretch deformed CP780 steel stress relief annealed at 600 ℃ obtains the highest strength-plasticity combination,with product of strength and elongation reaching 16.5 GPa·％.
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