Effect Of Rare Earth On Microstructure And Property Of X80Pipeline Steel

With the increasing of clean steel production technology, rare earth (RE) existence and mechanism of action has been changed. It has been shown that RE micro-alloyed effect is more clear and stable in clean steels. However, so far, the fusion of RE in TMCP process could not be shown unequivocally, which has become one of the major obstacle to research and development of a unique RE micro-alloyed steel. In this paper, select the object of study of X80pipeline steel, RE existence and the effect of RE on microstructure and mechanical properties has been systematic studied. On this basis, design and development of RE micro-alloyed pipeline steel has been proposed, and under simulate process conditions, with good mechanical properties and low cost RE micro-alloyed pipeline has been prepared in the laboratory.In this experimental condition, the following main conclusions are obtained:1. RE segregation at the grain boundaries in pipeline steel, but the distribution of RE in austenite and hot-rolled microstructure is different. RE segregation is significantly increased in hot-rolled microstructure. RE can capture harmful elements such as S and Sb to form a stable complex intermetallic compounds, effective in improving the grain boundaries.2. Nb dissoled and precipitate behavior is changed with RE additions. Nb dissolved temperature is decreased, and Nb strain-induced precipitation is inhibited in austenite, but Nb precipitated in ferrite increases with RE additions.3. With RE additions, the austenite grain growth behavior is changed, both of reheated and deformation conditions. And the austenite grain size can be refined clearly. Using the recrystallization activation energy, combined with the traditional rolling model of C-Mn steel, the suitable model of RE alloyed pipeline steel is designed, and it can be used and provided the basis for guiding and optimizing the pipeline steel rolling process.4. RE significantly inhibited the eutectoid ferrite transformation, the stability of austenite is improved. Moreover, RE induced quasi-polygonal ferrite and MA island transformation.5. Effect of RE on mechanical properties of pipeline steel is closely related to the hot rolling process and cooling rate. Appropriate rolling conditions, RE can increase the pipeline steel strength and toughness significantly.6. Proposed three RE alloyed pipeline steel design and development concepts, considering the yield strength, tensile strength, impact toughness, and alloy and production costs, according to the overall design philosophy of alloying elements, RE alloyed pipeline steel is the best choice. Preparation of RE alloyed pipeline steel yield strength up to590MPa, tensile strength up to735MPa,-20℃impact absorbing energy can reach220J. In addition, the mechanical properties of RE alloyed pipeline steel to achieve a significant increase, compared with the existing X80pipeline steel with no RE additions, but the costs can be reduced167yuan/ton.