Development Of 1000MPa Low Carbon Bainitic Steel For Sucker Rod

With the rapid development of national economy, crude oil demand increases rapidly. After years of exploration, the depth of oil well and the corrosion caused by H2S, CO2, and NaCl in oilfield are on the rise. The sucker rod is the key lift component used for the exploitation of crude oil. Improving combination properties including the strength and toughness, corrosion resistance, and corrosion fatigue resistance and reducing the cost of production are the important development direction of the sucker rod steel.Foreign H grade steel for sucker rod is expensive Ni-Cr-Mo alloy steel. Domestic sucker rod steel was cheaper Si-Mn-Mo bainitic steel containing high Mn content, but its mechanical properties was not stable. Based on the strengthening and toughening mechanism of granular bainitc, considering the hardenability and corrosion resistance, a low-cost Nb-microalloyed low carbon high Cr, trace Mo Si-Mn-Cr-Mo-B bainitic steel was designed.Experimental researches were carried out in the laboratory. And a bainitic steel used for sucker rod was developed successfully, which had excellent and stable comprehensive mechanical properties. The tensile strength, the elongation rate of A200, and the impact energy Aku2 of the commercial product were greater than 1000MPa,10%, and 60J, respectively, which satisfied the H grade sucker rod steel standards. The main research aspects are as follows.By Gleeble-2000 thermal simulator, the austenite grain recrystallization behavior after deformation of Nb-containing low carbon bainitic steel, the growth behavior of austenite grain before bainitic transition, the influence of cooling speed on bainitic transition behavior were studied systematically. Studies have shown that no strain-induced precipitation of Nb(C.N) was observed during rolling; reducing finishing temperature can refine austenite grain before bainitic transition, and in the process of air cooling after rolling. Nb strain-induced precipitation suppresses the bainitic structure growth in the process of bainitic transition.By thermal simulation experiment, the medium temperature phase transition microstructure of the deformed austenite during continuous cooling in low carbon bainitic steel was investigated. The results show that granular bainite was obtained in the test steel at a wide range of cooling rate, with stable performance. According to the results of thermal simulation experiment, the rolling process test was performed, and the scheme of the optimized rolling process was put forward.The corrosion fatigue properties of low carbon bainite steel was investigated, and research showed that the corrosion fatigue crack propagation mechanism of the test steel is hydrogen induced cracking mechanism, and crack propagation involves the diffusion of the hydrogen to the crack tip. Low carbon bainitic steel containing high silicon content showed superior corrosion fatigue resistance, which is attributed to more residual austenite film between bainite ferrite laths reducing the propagation rate of hydrogen induced cracking.The influence of the tempering temperature and the stability of retained austenite of low carbon bainitic steel on toughening mechanism was investigated. The result indicated that with increasing the tempering temperature, the residual austenite size decreases, but the stability of retained austenite increases, thereby increasing the energy required for the crack propagation and improving the ductility and toughness; however, when the tempering temperature is above 500℃, all of the residual austenite is decomposed, and ductility and toughness are deteriorated.Finally, the corrosion resistance of low carbon bainitic steel was discussed by the comparative trial with 20CrMoA steel on coupon corrosion test.Based on the above research,1400 tons of 15Cr2SiMnMoNbA sucker rod steels have been produced and supplied to market. Products are successfully used in making polished H grade sucker rods which have been well running for many years in good condition. 15Cr2SiMnMoNbA steel has been included in the product catalog of Chinese standard GB/T26075-2010.