厚规格X80管线钢微观组织控制及力学性能研究

发布时间：2018-04-03 20:10

本文选题：X80管线钢　切入点：厚规格　出处：《郑州大学》2017年硕士论文

【摘要】：随着世界经济的迅速发展,石油、天然气等能源资源的需求也在逐年提升,石油、天然气管道的敷设跨向极地、海洋和一些地质不稳定区域。为了提高石油、天然气的输送效率,管线钢逐渐向着高纲级、厚规格和大口径的方向发展,也使管线钢的设计要求更为严格。本研究主要对象为Φ1422mm中俄东线用X80管线钢钢板、钢管及焊接接头,采用金相显微镜和扫描电子显微镜对组织进行观察,通过夏比冲击试验、硬度试验、拉伸试验和落锤撕裂试验对力学性能进行研究。对27.2mm厚度规格针状铁素体组织X80管线钢钢板微观组织和落锤撕裂试验断口形貌进行观察,研究了X80管线钢的断裂行为与组织之间的关系。研究发现,钢板从表面到厚度中心,针状铁素体体积分数逐渐降低,并出现较多的粒状贝氏体、多边形和准多边形铁素体。具有较高针状铁素体体积分数的钢板,其落锤撕裂剪切面积也越高,多边形和准多边形铁素体以及大尺寸MA岛的存在能够导致解理断裂的产生,不利于钢板的低温断裂韧性。在裂纹扩展的过程中,强烈的内应力会使主裂纹附近组织出现变形,导致脆硬性的MA岛周围由于应力集中而出现微孔,微孔与微孔之间随着组织变形相互连接而形成微裂纹。二次裂纹通常在针状铁素体周围出现转折或停滞,说明针状铁素体有利于阻碍裂纹的扩展,提高钢板的断裂韧性。对25.7mm和30.8mm厚度规格多相混合组织X80管线钢的微观组织进行观察和力学性能进行测试,研究X80管线钢钢板厚度方向组织对力学性能的影响。研究发现,多相混合组织X80管线钢组织较为复杂,钢板表面位置主要为多边形铁素体和贝氏体铁素体,沿着厚度方向组织逐渐转变成多边形铁素体、准多边形铁素体、粒状贝氏体、贝氏体铁素体、MA岛等多相组织组成,且贝氏体铁素体能明显提高钢板硬度。但多相组织的结合使钢板整体强度和韧性均十分优异。落锤撕裂试验发现,钢板沿轧制方向的带状组织越严重,落锤断口处的分离沟的条数越多。多相混合组织使裂纹在扩展过程中更为曲折,降低裂纹的扩展能。对25.7mm和30.8mm厚度规格多相混合组织X80管线钢管体和焊接接头的组织和性能进行研究。研究结果发现,多相混合型组织具有优异的强韧性结合,管体DWTT断口的分离沟既存在韧性断裂,也存在脆性断裂。在焊接热循环的影响下焊缝、热影响区仍有良好的冲击韧性。受到外焊热循环的影响,Qg焊表面位置MA岛尺寸和体积分数均小于外焊。临界粗晶区MA岛在原奥氏体晶界上呈链状分布,且硬度明显高于粗晶区其他位置,但宽度较小,对焊接接头整体危害较弱。
[Abstract]:With the rapid development of the world economy, the demand for oil, natural gas and other energy resources is also increasing year by year.In order to improve the transportation efficiency of petroleum and natural gas, pipeline steel is gradually developing towards the direction of high grade, thick specification and large caliber, which also makes the design requirements of pipeline steel more strict.The main objects of this study are X80 pipeline steel plate, steel pipe and welded joint used in 桅 1422mm East Russia Line. The microstructure is observed by metallographic microscope and scanning electron microscope, and the hardness is tested by Charpy impact test.The mechanical properties were studied by tensile test and drop weight tear test.The microstructure of 27.2mm thickness gauge acicular ferrite steel X80 steel plate and the fracture morphology of drop weight tear test were observed. The relationship between fracture behavior and microstructure of X80 pipeline steel was studied.It is found that the volume fraction of acicular ferrite decreases gradually from the surface to the thickness center, and more granular bainite, polygon and quasi-polygon ferrite appear.The higher the falling weight tear shear area of steel plate with higher acicular ferrite volume fraction, the higher the shear area. The existence of polygonal, quasi-polygonal ferrite and large size MA island can lead to cleavage fracture, which is not conducive to the low temperature fracture toughness of steel plate.During the process of crack propagation, the microstructure near the main crack will be deformed by the strong internal stress, which will lead to micropores around the brittle and hard MA island due to the stress concentration. The microcracks between the micropores and micropores are connected with each other with the deformation of the microstructures.Secondary cracks usually turn or stagnate around acicular ferrite, which indicates that acicular ferrite can hinder the crack propagation and improve the fracture toughness of steel plate.The microstructure and mechanical properties of multiphase mixed structure X80 pipeline steel with 25.7mm and 30.8mm thickness specifications were observed and the effect of thickness direction on mechanical properties of X80 pipeline steel plate was studied.It is found that the structure of multiphase mixed structure X80 pipeline steel is more complex, the surface position of the steel plate is mainly polygonal ferrite and bainitic ferrite, and the structure is gradually transformed into polygonal ferrite and quasi-polygonal ferrite along the direction of thickness.The microstructure of granular bainite, bainite ferrite and MA island is multiphase, and bainitic ferrite can obviously improve the hardness of steel plate.But the combination of multiphase structure makes the whole strength and toughness of steel plate very excellent.It is found that the more serious the strip structure along the rolling direction is, the more the number of strips at the fracture surface of the drop hammer is.The multiphase mixed structure makes the crack propagation more zigzag and reduces the crack propagation energy.The structure and properties of multiphase mixed structure X80 pipe with 25.7mm and 30.8mm thickness specifications were studied.The results show that the multiphase mixed structure has excellent strength and toughness, and there are both ductile and brittle fracture in the separation ditch of DWTT fracture of tube body.Under the influence of welding thermal cycle, the HAZ still has good impact toughness.The size and volume fraction of MA island of QG welding surface are smaller than that of external welding.The MA island in the critical coarse grain zone is chained along the grain boundary of the original austenite, and the hardness is obviously higher than that in the other positions of the coarse grain zone, but the width is smaller, and the overall harm to the welded joint is weaker.
【学位授予单位】：郑州大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG142.1;TG407

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