Abstract: Microstructure and mechanical properties of a high strength oil casing steel under different heat treatment conditions as hot rolling, quenching at 860 ℃, and quenching at 860 ℃+quenching at two phase region of 800 ℃+tempering at 200 ℃ and 660 ℃(QTL), respectively, were analyzed, and the relationship between microstructure and mechanical properties of the steel after low temperature QLT and high temperature QLT treatment was studied by means of EBSD and TEM. The results show that the microstructure of the hot-rolled steel is granular bainite+pearlite+a small amount of polygonal ferrite. After quenching at 860 ℃, the microstructure consists of lath martensite and a small amount of bainite. After low temperature QLT treatment, the microstructure transforms into tempered martensite and ferrite, the yield strength is 936 MPa, the tensile strength is 1260 MPa, the yield ratio is 0.74, and the impact absorbed energy at 0 ℃ is 40 J, meanwhile, the proportion of LAGB is 34.9%, the dislocation tensity of GND is 10.2×10¹⁴ m^-2, and the equivalent grain size is 1.43 μm. After high temperature QLT treatment, the microstructure transforms into tempered sorbite and ferrite, yield plateau appears in the stress-strain curve, the yield strength is 833 MPa, the tensile strength is 912 MPa, the yield ratio is 0.91, the impact absorbed energy at 0 ℃ increases to 93 J, all of which show best strength and toughness matching, however, the proportion of LAGB decreases to 30.9%, the dislocation density of GND decreases to 9.6×10¹⁴ m^-2, the equivalent grain size decreases to 1.39 μm, which makes the strength of the tested steel decrease and the plasticity increase.

Key words: oil casing steel, QLT heat treatment, EBSD analysis, strength and toughness matching