| 其他摘要 | The requirement of oil and natural gas has markedly increased for the rapid development of economy in our country, which accelerates the development of industry of oil and natural gas pipeline. The major reserves of oil and gas are in the western region of China, which are far away from markets. Application of ultra-high strength pipeline steel is a way to reduce the cost of long distance transmission for gas and oil. At the same time, the demand for oil casing steel is more rigorous for bad exploitation condition in the western region. However, research and development on ultra-high strength pipeline steel and oil casing steel in our country are not satisfied with these requirements. Study on the ultra-high strength pipeline steel just started and the mechanism for strengthening and toughening has not been clarified in our country, which is far behind the developed countries such as Japan and America. As for the oil casing steel, low grade steels such as JK55 can be produced in our country at present, while the high grade such as N80 can not be well produced because of the serious galling problem.
Mechanical properties of materials are the presentational behavior of microstructure and microstructure is the main essence of mechanical properties. Therefore the optimized of mechanical properties of steels always come from the optimization of their microstructure. The optimum chemical design and TMCP processing design are the precondition of attain the optimum microstructure. According to the mechanism for strengthening and toughening of microstructure, the chemical composition design and TMCP design were made in the present work through adjusting the factors for strengthening and toughening, which is a necessary way to attain the optimized combination of strength and toughness for steels. Therefore, the microstructure optimization for pipeline steels and the study of mechanism on strengthening and toughening can not only improve the properties of pipeline steels, but also the provide experience for the development of new micro-alloying steels.
Under the support by a Key Technologies R&D Project of Liaoning Province (No. 2005221002), both the ultra-high strength pipeline steel and the oil casing steel were studied in this work. At first, the chemical composition design was optimized and then the TMCP process was studied. The target microstructure was attained based on the above work. The principle of chemical composition design and microstructure optimization was discussed and the mechanism of strengthening and toughening in micro-alloying steels was analyzed.
The transformation characteristic, microstructure and micro-hardness of ultra high-strength pipeline steels with different ratios of Mo/B were investigated. Combined additions of Mo and B were found to more effectively increase the hardenability of the steel than the individual additions, by increase of the volume fraction of lower bainite in microstructure and enhancement of the hardness of the steel. The experimental results showed that there existed an optimum value of R Mo/B to achieve the optimum microstructure and the highest micro-hardness of the steel. The mechanism for the effect of combined addition Mo-B on ultra high-strength pipeline steels was also discussed.
The continuous cooling transformation of ultra-high strength pipeline steel and oil casing steel were studied, and the TMCP process was designed according to the above results. The experimental results showed that the target microstructure could be attained through the designed TMCP. There existed a wide process window for ultra-high strength pipeline steel to achieve the objected microstructure of LB when the final rolling temperature was under Tnr, i.e., cooling rate≥30℃/s and 150℃≤Tc≤460℃. As for N80 grade ERW oil casing steel, the (AF+M) dual phase microstructure could be attained by two stages controlled cooling.
The experimental results in this work showed that the ultra-high strength pipeline steel with LB microstructure had both high strength and high toughness, and its toughness was similar to the steel with AF microstructure. N80 grade ERW oil casing steel with (AF+M) dual phase microstructure had much higher strength, hardness and work-hardening exponent than those of the steel with AF microstructure, while the toughness was not obviously decreased. Furthermore, the strength, hardness and work-hardening exponent were increased with increase of the M volume fraction in the dual phase.
The microstructure optimization and the mechanism of strengthening & toughening for the ultra-high strength pipeline steel and ERW anti-galling oil casing steel were studied in this work, and conclusions were made as follows:
I. In the microstructure of ultra-high strength pipeline steel, the domain size can be used to describe the effective grain size, where the adjacent domains are characterized by the orientation difference at least 10°. The domain size of LB is smaller than that of AF because of its transformation at low temperature. Therefore, AF is replaced by LB to get high strength and good toughness in ultra-high strength pipeline steels.
II. The plastic zone size at crack tips has some relation with the yield strength, which is decreased with increase of the strength, so does the toughness. When the effective grain size is decreased, loss of the toughness can be compensated by the higher strength.
III. (AF+M) dual phase microstructure has much higher strength, hardness and work-hardening exponent than those of AF microstructure, while the toughness is not obviously decreased. The main reasons can be explained that the deformation of AF is restricted by M, which increases the work-hardening ability. The hard phase in the microstructure can improve both the hardness and strength. Transformation of AF can divide the prior austenite grains, which can refine the packet size of M to be transformed. The AF itself has good toughness. The carbon content is low in the M phases, which is prone to harmonize the deformation.
IV. Distribution of dual phases has much effect on toughness of the steels. When the hard phase exists in the soft phase like islands, toughness of the steel is not affected obviously. While the hard phase surrounds the soft phase, toughness of the steel can be decreased. |
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