| This paper analyzed the actual working conditions of expandable tubular technology. Based on the analysis of large plastic deformation of the expandable tube during the downhole expansion, combined with the mechanical models of the bulge forming of thin-walled cylinders and the mechanical autofrettage technique of thick-walled cylinders, using the plasticity theories, a new mechanical model of the expandable tube’s plastic deformation during the downhole expansion was been established. The expansion force of the downhole expandable tube can be calculated as F= 2πtk()?n〔r0’(?)(1+-f/sin a)/(n+1)+fL〕Based on the mechanical model, the existing technological bottlenecks of the expandable tubular technology in the complex drilling and monobore technology were been analyzed. According to these, the new technical requirements of the expandable tube with low coast and large expansion rate were been established, i.e. lower yield strength, lower yield-tensile ratio, higher tensile strength, better plastic, better homogeneous deformation and work hardening ability.Following the technical requirements of the expandable tubular steel, under the guidance of the advanced steel material theory of alloying, structure/functional integration and microstructure controlling, the ferrite/martensite dual phase steel 07MnSi was designed and developed. This DP steel was low cost, low carbon and low alloying, and it could satisfy with the requirements of the mechanical properties during the expansion which was in the downhole condition. The alloying elements in this steel were mainly Mn and Si. It could be obtained by composition optimization, pure smelting, structure refinement, transformation controlling and strengthening and toughening match.Using the modern analytical test methods such as OM, XRD, SEM, EDS, TEM and universal testing machine, the microstructure and mechanical properties of 07MnSi F/M DP steel which had been intermediate quenching at different tempertures were investigated. The microstructure evolution process of these specimens was observed by the EBSD technology. These analyses showed that the martensite volume faction would increase with the intermediate heating temperature. And during that process, the morphology and texture orientation also changed. These changes made the strength of this steel increase. Expansion tests were also did to the sample tubes, and the test results could help to choose the best heat treatment process.Using some mechanical models, suach as Hollomon stress-strain model, Crussard-Jaoult analytical method and modified C-J analytical method, the work hardening behavior of 07MnSi F/M DP steel which had been intermediate quenching at different temperatures was been analyzed. The result showed that this steel had an obvious phenomenon of double yielding. And the strain hardening exponent would decrease with the increase of strain.Based on the the mesoscopic mechanics theory of plastic constitutive, according to the character of DP steel’s dual-phase distribution, the change of the martensite carbon content and the influence of transformation strengthening of ferrite coused by inner force of martensite transformation, the flow stress model of this steel was established. The stress-strain curves were calculated by the Eshelby equivalent inclusion model, the Mori-Tanaka mean-field theory and the Tomota fractional-step method.Based on the theoretical model, the relationship between the meso structure and properties of this quenched dual phase steel and the mechanical properties, such as yield strength, tensile strength, work hardening, elongation, was discussed. Through this research, the flow stress of this steel could be predicted and analyzed.Using the fixed expansion tool, the expansion tests were done to the sample tube (Φ 219 X 12mm) of 07MnSi F/M DP steel with the different expansion rates of 35% and 50%. The cone angle of the expansion cone was selected as 10°. The expansion tests proved that this steel had excellent expansion performance. |
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