The yield to tensile strength ratio is the important performance index of seamless steel tube for high strength structures. Studied the change law of mechanical properties of S770QL steel （mass fraction） /%： 0.15C，0.28Si，1.30Mn，0.009P，0.003S，0.20Cr，0.02Ni，0.47Mo，0.025Al，0.08V，after different quenching and tempering processes above Ac

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， and and the microstructure was analysed.The results show that after the quenching temperature of the steel pipe from 880 ℃ to 940 ℃， the grain size of austenite grain is relatively small， not exceeding 20 μm ， and all the microstructure are transformed into martensite. With the increase of the quenching temperature， the average grain size of austenite gradually increases. Under the condition of the same tempering process， the bending strength ratio decreases with the increase of the quenching temperature. After high temperature tempering， all the microstrucute are transformed into tempering chordite. With the increase of tempering temperature， under the combined action of precipitation enhancement and fine crystal reinforcement， the bending strength ratio increases first， and decreases again after the tempering temperature exceeds 660 ℃. In c

omprehensive consideration， 920 ℃ 60 min quenching （water cooling） + 600 ℃ -630 ℃ 120 min tempering （air cooling） process is adopted， and the yield strength ratio of steel pipe can be maintained between 0.89 and 0.90， and the comprehensive performance is optimal.