Microstructure and properties control of non-quenched and tempered CT80 grade coiled tubing steel

The non-quenched and tempered CT80 grade coiled tubing steels with different composition were designed.Effects of cooling rate,coiling temperature and Mo and Nb content on microstructure and mechanical properties of the non-quenched and tempered CT80 steels were investigated.The results show that when cooling rate increases from 52 ℃/s to 69 ℃/s,acicular ferrite is obtained,and yield strength and tensile strength of the tested steel increase by 25 MPa and 30 MPa,respectively.When the coiling temperature is 530 ℃,pearlite is observed in the microstructure of the tested steel,and its tensile strength decreases by 20 MPa and yield strength increases to 690 MPa.The increase of Mo content is benefit of the formation of acicular ferrite and hardenability of tested steel,and martensite-austenite(M/A) islands increase in the microstructure.As the content of element Nb increases,it is favourable of grain refining sterngthening and precipitation strengthening of the steel.