Microstructure and Mechanical Properties of Home-Made CT80 Grade Coiled Tubing

The C-Si-Mn-Cr-Mo-Nb alloy steel was designed.The high strength low carbon microalloy CT80 grade hot rolled coil,whose main microstructure was granular bainite,was developed.Using electric resistance weld(ERW) pipe-making and heat treatment technology,the CT80 grade coiled tubing with specification of  31.75 mm×3175 mm was developed.The microstructure and mechanical properties of the tubing were analyzed.The results indicate that the microstructure of the pipe body was mainly composed of polygonal ferrite with little acicular ferrite,fine pearlite and M-A islands.The yield strength in the longitudinal direction was 583 MPa,the tensile strength was 703 MPa and the elongation was 29.3%.The fatigue life reached 634 cycles when the internal pressure of the pipe body was 34.47 MPa.Both the pipe body and weld had good HIC and SSCC resistance performance.The performance data of the coiled tubing fully conformed to the requirements of API 5C7 and API 5ST specification.