Abstract
Complex working condition of coiled tube (CT) steel demands high strength and low ratio of yield strength (YS) to tensile strength (TS). Reasonable microstructure control is a key problem of CT steel. Controlled rolling and controlled cooling experiments were carried out by two kinds of tested steels with different chemical compositions to develop the non-quenched and tempered CT80 steel. Mechanical properties of the tested steels at different processes are all in good agreement with the properties requirement of CT80. Increasing of Mo and Nb contents improves transformation of acicular ferrite and martensite–austenite (M–A) islands. 4 vol% of fine M/A islands increase TS and decreases ratio of YS/TS. Cooling rate increasing also improves acicular ferrite transformation and enhances TS, but has little effect on ratio of YS/TS. To meet the requirement of high strength and low ratio of YS/TS, optimized complex microstructure of the CT80 steel is composed of acicular ferrite, about 30 vol% of bainite and 4 vol% of M–A islands.
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