X80管线钢塑性变形行为与损伤断裂机理研究

发布时间：2018-05-05 02:11

本文选题：X80管线钢 + 应变硬化　；参考：《北京科技大学》2016年博士论文

【摘要】：强度与塑性、韧性等力学性能的合理匹配是钢铁材料研究的关键内容。X80管线钢是当前油气管线工程中应用较广泛的钢种,施工与服役条件较为恶劣,对其提出高强度的同时也提出了较高的塑性和韧性要求。本文以热轧X80管线钢为研究对象,研究其塑性变形行为及组织演变规律,从细观力学角度揭示其损伤断裂机制,为优化X80管线钢的综合性能提供理论支持和实践指导。借助于OM、SEM (EBSD)和TEM等技术研究X80管线钢板的微观组织特征：横向的有效晶粒尺寸在1μm以内的分布频率高于纵向和45°方向；拉伸断口在横向和450方向上的韧窝均匀分布,断面高低起伏,其形貌间接反映了断裂功的排序：450方向横向纵向；横向、纵向和45°方向上分别为铜型织构、立方织构和立方织构,{111}面的织构强度较高,其中{110}112为主织构,111//TD的晶粒占多数,造成其力学性能的各向异性。采用由刃型位错构建的倾侧晶界模型分析晶界强化机制,结果表明：晶界对位错的作用力FAB→C随晶界取向差θ的增加而增加；在取向差θ增加的初期,该作用力迅速上升；取向差θ增至150,该作用力增幅显著下降,基本稳定在最大值,与晶界能-取向差的关系吻合。X80管线钢焊接区域力学特性的研究表明：该区域为柱状晶组织,具有凝固组织和相变组织的综合特征,其冲击功排序：母材区热影响区焊缝区域。而焊缝裂纹形成过程为：在凝固冷却造成的拉伸应力作用下,裂纹沿柱状晶界扩展,裂纹内部存在非金属夹杂物,其断口大部分区域为解理断裂,近裂纹边缘为准解理断裂。X80管线钢拉伸过程的力学行为分析表明,横向应变硬化指数n最大,纵向次之,450方向最小；随着应变硬化指数的增加,均匀伸长率增加,应变强化系数K与n、σb正相关；其应变硬化行为呈现平台、缓慢下降和快速下降等三个阶段；基于C-J方法的应变硬化行为分析表明,其两个迅速下降的过程分别对应了屈服和颈缩两个不均匀变形阶段。X80管线钢在横向上的有效晶粒尺寸最小,45°方向最大,晶粒细化造成晶界数量增加,有利于塑性；小角度晶界有利于塑性变形而无益于强度；大角度晶界强化使晶界应力水平提高,促使邻近晶粒变形。其快冷组织的应变硬化性能分析表明,铁素体体积分数增加,均匀伸长率增加,屈强比与应变硬化指数负相关,其加工硬化行为经历迅速下降、缓慢下降和再次迅速下降等三个过程,其转折点对应了屈服强度和抗拉强度。当铁素体体积分数达到22.8%时,加工硬化率呈现前快、后慢的两个下降阶段,其中MA岛状组织参与变形,保证了应变硬化性能。X80管线钢的冲击断裂行为分析表明：室温条件下其冲击韧性呈现各向异性：横向最优,45°方向最差,主要原因在于细晶韧化机制,其核心要素为晶界的数量与取向差;温度愈低,阻碍裂纹扩展的能力愈弱,脆性断裂特征愈显著；夹杂物(MnS、Al2O3等)是韧窝形成的起点,裂纹在扩展过程中受到界面的阻碍,其路径较曲折,裂纹扩展区域有变形带。采用二维有限元模拟夏比冲击试验中断裂面上夹杂物等在损伤断裂机制中的作用,结果表明：夹杂物界面处的损伤及夹杂物本身的破裂均可能是冲击断裂的裂纹源;应力三维度Rσ值在圆形夹杂物界面的45°和135°处(方形夹杂物的两个尖角处)为最低水平,损伤最为严重,而在90°方向损伤最小,与塑性应变能密度分析结果一致；夹杂物界面的最大等效应变值、塑性应变能密度最大值与其尺寸大体呈线性正相关,方形夹杂物的指标更高；带尖角的方形夹杂物界面处的塑性应变能密度更大,其损伤区域更为集中,相对于圆形夹杂物产生的韧窝尺寸要小,断裂吸收的冲击能量要少；夹杂物周边基体中出现了损伤程度次高的区域,圆形夹杂物造成的损伤更低、更均匀；小尺寸的夹杂物导致界面处有较低的塑性应变能密度,而其周边区域产生的损伤相对均匀,分布面积相对更大,形成的韧窝尺寸更大,所吸收的能量更多。
[Abstract]:The rational matching of strength, plasticity, toughness and other mechanical properties is the key content of steel material research.X80 pipeline steel is widely used in the current oil and gas pipeline engineering. The construction and service conditions are worse, high strength is put forward at the same time, high plasticity and toughness requirements are also put forward. This paper takes hot rolling X80 pipeline steel as research. In order to provide theoretical support and practical guidance for optimizing the comprehensive properties of X80 pipeline steel, it provides theoretical support and practical guidance for optimizing the comprehensive properties of X80 pipeline steel. By means of OM, SEM (EBSD) and TEM, the microstructural characteristics of X80 pipeline steel are studied with the help of the technology of transverse effective grain size within 1 mu m. The distribution frequency is higher than that of the longitudinal and 45 degrees. The dimple of the tensile fracture is evenly distributed in the transverse and 450 directions, and the section height fluctuate. The morphology of the fracture reflects the sorting of the fracture work indirectly: the 450 direction is transverse longitudinal. The transverse, longitudinal and 45 degrees are respectively copper texture, cubic texture and cubic texture, and the texture strength of the {111} surface is higher. Among them, the main texture of {110}112 is the texture of the 111//TD, which makes the anisotropy of the mechanical properties. The grain boundary strengthening mechanism is analyzed by the dip grain boundary model constructed by the edge dislocation. The result shows that the force FAB to C of the grain boundary dislocation is added with the increase of the grain boundary orientation difference theta, and the action is fast in the initial stage of the orientation difference theta. The increase of the orientation difference of theta to 150, the growth of this force decreased significantly, basically stable at the maximum value, and the relationship with the grain boundary energy orientation difference coincides with the study of the mechanical properties of the.X80 pipeline steel welding area. It shows that the region is columnar crystal structure with the comprehensive characteristics of the solidification and phase transition, and the impact work is ordered: the heat effect of the parent material zone The formation process of the weld seam is that the crack formation process is that under the tensile stress caused by the solidification cooling, the crack propagates along the columnar grain boundary, and the non-metallic inclusions exist inside the crack, and the most part of the fracture is cleavage fracture. The mechanical behavior analysis of the tensile process of.X80 pipeline steel near the edge of the crack indicates that the transverse strain is transversely strain. The hardening exponent n is the largest, the longitudinal direction is the smallest, the 450 direction is the smallest; as the strain hardening exponent increases, the uniform elongation increases, the strain hardening coefficient K is positively related to N and sigma B; the strain hardening behavior presents the platform, the slow descent and the rapid descent, and the strain hardening behavior analysis based on the C-J method shows that the two rapidly decline. The process corresponds to the two non uniform deformation stages of yield and necking. The effective grain size of.X80 pipeline steel is the smallest in the transverse direction, the direction of 45 degree is the largest, grain refinement leads to the increase of grain boundary, which is beneficial to the plasticity; the small angle grain boundary is beneficial to the plastic deformation and is not beneficial to the strength; the grain boundary stress level is raised by the large angle grain boundary strengthening, and the grain boundary stress level is increased. The strain hardening behavior of the adjacent grains shows that the volume fraction of ferrite increases, the uniformity of elongation increases, the ratio of flexion intensity is negatively correlated with the strain hardening exponent, and its working hardening behavior goes through three processes, such as rapid decline, slow descent and rapid descent, and the turning point corresponding to the yield strength and tensile strength. When the volume fraction of ferrite reaches 22.8%, the working hardening rate presents fast and slow two decline stages, in which the MA island like tissue participates in the deformation. The analysis of the impact fracture behavior of the strain hardening.X80 pipeline steel shows that the impact toughness is anisotropic under the condition of room temperature: the transverse optimum, the worst in the direction of 45 degree, the main reason. The core element is the mechanism of fine grain toughening, its core element is the poor quantity and orientation of grain boundary; the lower the temperature, the weaker the ability to impede the crack growth, the more prominent the brittle fracture characteristics; the inclusion (MnS, Al2O3, etc.) is the starting point of the dimple formation, the crack is hindered by the interface during the expansion process, its path is tortuous, and the crack expansion zone has a deformation zone. The results show that the damage at the interface of the inclusion and the breakage of the inclusion itself may be the crack source of the impact fracture, and the three-dimensional R sigma of the stress is at 45 and 135 degrees of the circular inclusion interface (two tips of the square inclusions. " The maximum damage in the direction of the 90 degree is the least, and the minimum damage in the direction is the same as the result of the plastic strain energy density analysis. The maximum equivalent strain value of the inclusion interface, the maximum plastic strain energy density is linearly and positively correlated with its size, the index of the square inclusion is higher, and the plastic of the square inclusions with the sharp angle is plastic. The density of the strain energy is larger, the damage area is more concentrated, the size of the dimple produced by the circular inclusion is smaller, the impact energy of the fracture absorption is less, the area with the higher damage degree appears in the periphery of the inclusion, the damage of the circular inclusions is lower and more uniform, and the small size inclusions lead to the lower interface. The plastic strain energy density, and the damage in the surrounding area is relatively uniform, the distribution area is relatively larger, the dimple size is larger, and the energy absorbed is more.

【学位授予单位】：北京科技大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TG142.1

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