高等级厚规格含铌抗大变形管线钢组织与性能研究

【作者】 谭峰亮；

【导师】 雷霆； 刘清友；

【作者基本信息】 昆明理工大学 ， 钢铁冶金， 2013， 博士

【摘要】 抗大变形管线钢是一种针对基于应变设计而开发的新型管线钢,主要用于冻土、泥石流和地震频发等地质条件恶劣地带,要求具有高强、韧性的同时,还必须具有低屈强比、高加工硬化指数和高均匀延伸率等纵向变形性能。多边形铁素体-贝氏体(“PF-B”)双相管线钢具有良好的应变性能和组织可控性好成为抗大变形管线钢研发的焦点。对于“PF-B”双相管线钢中的多边形铁素体,既要保证一定数量,还要求晶粒细小,才能获得高强、韧、塑匹配性能。通过不同于传统管线钢的生产工艺结合成分优化,就解决了控制和细化铁素体加快铁素体相变动力学,“组织-力学性能”关系以及强、韧、塑机理,焊接性能,加工硬化和应变时效后性能变化等“PF-B”抗大变形管线钢生产、应用和推广过程中的重点和难点,并从可动位错的角度提出提高强、韧、塑的方法。根据“控制轧制-缓冷弛豫-加速冷却”工艺,采用铸坯再加热、热变形和控制冷却、实验室控轧控冷、工业试制、焊接热模拟、thermo-calc热力学计算、OM、 SEM.EDAX.EBSD.TEM.HV等系统地研究了不同Nb含量实验钢奥氏体细化控制,Nb含量、变形参数、弛豫制度和铁素体相变动力学之间的耦合关系,厚规格钢板“开冷温度-铁素体-力学性能”以及“板-管(时效前后)”性能关系,焊接热影响区软化、脆化及其改善措施。实验温度范围内,奥氏体晶粒随温度升高明显长大,Nb含量≤0.051%时,晶粒尺寸与温度符合Arrhenius方程-lnD=A-Q/RT,Nb含量提高到0.08%时,由于(Ti,Nb)(C,N)对奥氏体晶界强烈钉扎作用,导致二者不符合Arrhenius方程；晶粒尺寸与保温时间遵循指数关系一D=ktn,指数<0.1,超过20分钟时,晶粒尺寸增加不明显；提高Nb含量能有效抑制奥氏体晶粒长大,≥1150℃时,Nb含量提高到0.08%能将晶粒尺寸控制在100μm以内：当钢中钛含量约为0.01%时,未固溶Nb含量≥0.0047%时能将奥氏体晶粒尺寸控制在100μm以内。高温变形后保温,再结晶完全时晶粒尺寸达到最小,能够细化到40μm以内,最小尺寸不随变形温度和保温时间变化；提高Nb含量能有效细化晶粒并维持细化效果,Nb含量提高到0.08%时再结晶晶粒能够控制在30μm以内。奥氏体未再结晶区,变形温度≤800℃,变形量≥50%,能够加快铁素体析出动力学,细化晶粒尺寸；缓冷速率≤1.0℃·s-1、开冷温度≤690℃时,铁素体转变量和尺寸明显增加；Nb含量≤0.051%时,增加Nb含量对应变诱导铁素体转变量影响不大,Nb含量提高到0.08%时,明显抑制了应变诱导铁素体相变,其转变量不超过10%；弛豫过程中,增加Nb含量,有效抑制铁素体相变,其转变量及尺寸明显减小,Nb含量提高到0.08%时,铁素体转变量小于15%,但其尺寸有效控制在3μm以内。降低开冷温度,铁素体转变量增加,但是开冷温度过低,铁素体晶粒会明显粗化；Nb含量提高至0.080%时,铁素体相变动力学明显减慢,但能将铁素体晶粒控制在4μm以内。钢中铁素体含量和尺寸分别为30%-75%、5.5μm以内时能满足抗大变形性能要求：建立了26.4mm"PF-B"抗大变形管线钢板开冷温度-铁素体含量-力学性能关系及“板-管”性能变化关系经验关系式,随铁素体含量增加,钢板的抗拉强度变化不明显、屈服强度明显降低、均匀延伸率增加、冲击性能降低,铁素体内较高的可动位错密度是降低屈强比和增加均匀延伸率的关键,铁素体晶内业结构极少,依靠铁素体晶界提高冲击性能：铁素体含量越多,加工硬化和应变时效现象越明显,加工硬化会增加屈服强度、应变时效会损失均匀延伸率,“PF-B”抗大变形管线钢管性能对铁索体体积分数不敏感,提高其力学性能的关键在十细化铁素体晶粒尺寸。t8/3小于某一值时,粗晶区的冲击性能与母材基本一致,但是当t8/3延长至出现较多MA组元时,冲击性能迅速降低；峰值温度约800℃时,由于铁素体含量较多而出现软化现象,750℃时,由于Nb等微合金元素的二次析出和出现大量大尺寸MA组元,冲击性能最差；组织粗化和MA组元是降低焊接HAZ韧性的关键因素,提高Nb含量,仃利于减少MA组元和细化组织,提高焊接热输入量,明显改善热影响区软化现象和冲击性能。要进一步提高强、韧、塑性,将铁素体含量和尺寸分别控制在30%左右、5μm以内,增加铁素体/板条贝氏体两相的接触面积及铁素体内可动位错密度。在保证铁素体较快析出动力学的基础上细化晶粒,钢中适宜的Nb含量约为0.05%,合理的工艺参数如下：铸坯再加热至1150-1200℃保温2-4小时、奥氏体再结晶区变形温度1000～960℃、变形量≥25%、道次间隔时间≤30s,奥氏体未再结晶区变形温度800～760℃、变形量≥50%,开冷温度700～660℃。更多还原

【Abstract】 High deformability pipeline steel is a new pipeline steel developed for strain-based design. It is applied mainly in geologies conditions scurviness areas with earthquakes and landslides et al., and requires having lower yield ratio, higher work hardening index and better uniform elongation et al. as well as high strength and toughness. Polygonal ferrite-bainite ("PF-B") dual phase pipeline steel has been the focus of research because of excellent strain capacity and better microstructures controllability. For "PF-B" dual phase pipeline steel, only should polygonal ferrite balance a certain volume fraction and fine size could get higher strength-toughness-ductility. The emphases and difficulties in production, adhibition and generalization as follows:How to control and refine ferrite grains and make the fine ferrite rapid precipitation, relationship of technical microstructures-mechanical properties, mechanism of strength-toughness-ductility, weldability, properties chang after work hardening and strain ageing all have been solved by technology different from traditional X80pipeline steel and composition optimization, and proposed solutions to improve strength, toughness and plasticity.According to the "controlled rolling-slow cooling relaxation-accelerate cooling" technology, ca-st billet reheat and hot deformation and controlled cooling simulation, pilot controlled rolling an-d cooling, industrial trial, welding thermal simulation, Thermo-Calc thermodynamic computing, optical metallographic, scanning electron microscope with EDAX and EBSD, transmission ele-ctron microscopy were employed to research austenitizing and recrystallization dynamics of dif-ferent Nb bearing experimental steels, relationship among Nb content-strain parameter-relaxati-on parameter-ferrite transformation dynamics, relationship among start cooling temperature-fer-rite-mechanical properties of steel plate and mechanical properties relationship of plate-pipe-pi-pe after strain aging, microstructur-es’ and the reasons of softening and embrittlement of HAZ and the solutions.At the tested temperature range, austenitic grains grow up as reheat temperature increases obvio-usly. When Nb content is less than0.051%, the austenitic grain size and reheat temperature foll-ow Arrhenius equation-In D=A-Q/RT, but it didn’t when Nb content is increased to0.080%be-cause of the pinning effect of (Ti, Nb)(C, N). The austenitic grain size and soaking time follow exponential relationship-D=ktn, and the index is less than0.1, its does not significantly inc- rease when the soaking time is more than20minutes. Increasing Nb content can repress growth of austenite grain in the reheating process. Reheat temperature>1150℃, the average size of aus-tenitic grain could be controlled within100μm when Nb content increased to0.08%. If the Ti element percent in the tested steels is around0.01%, the average size of austenitic grain can be repressed within100μm when undissolved Nb is more than about0.0047%. Deformation and holding process at high temperature, austenitic grain size reach minimum when happen comple-tely static recrystallization, it is controlled within40μm, the minimum size does not change wi-th the strain temperature and holding time. Increasing Nb content can reduce the austenite recry-stallization grain size, and maintain refining effect effectively, the austenite recrystallization gra-in can be controlled within30μm in0.08%-Nb steel.Deformation in austenite non-recrystallization region, as strain temperature<800℃and deform-ation>50%, it could speed up ferrite nucleation rate and refine polygonal ferrite grain. A lot of ferrite crystal nucleus can grow up when slow cooling rate<1℃·s-1and start cooling temperature≤690℃, the volume fraction and grain size of ferrite increased significantly. The ferrite transfor-mation can be restrained and refined when Nb content increases. The strain induced ferrite trans-formation is inhibited obviously when Nb increases to0.08%during the deformation process, its volume fraction is less than10%. And ferrite fraction and grain size decrease significantly when the Nb increased from0.031%to0.08%during the slow cooling relaxation process, the volume fraction only is15%, but the average grain size can be controlled within3μm.Reducing the start cooling temperature can improve ferrite transformation, but the ferrite grain would be excessively coarse as the start cooling temperature lowest. The ferrite grain can be refined under4μm, and ferrite transformation dynamics slow down evident when Nb increased to0.080%. The larger strain resistance capability performance requirement can be satisfied as the ferrite volume fraction is controlled in the range from30%to75%and its average size is repressed within5.5μm.The relational expression of start cooling temperature-ferrite fraction-mechanical properties, plate-pipe mechanical properties for26.4mm "PF-B" pipeline steel have been established. For steeel plate, the tensile strength changes is not obvious, the yield strength decreases significantly, the uniform elongation increases, and the impact toughness decreases as the volume fraction of ferrite increases, respectively. High movable dislocation density in ferrite grain is the key point to reduce yield ratio and increase uniform elongation, there is a little substructure in ferrite grain, and improving impact property depend on ferrite grain boundary. The more the ferrite fraction, the more obvious the work hardening and strain ageing phenomenons, the yield strength could be increased because of work hardening, and the uniform elongation could be lost caused by strain ageing, lead to properties of "PF-B" high deformability linepipe no sensitive to ferrite fraction. The key point to improve strength and toughness is refining ferrite grain.The impact toughness of coarse grain HAZ same as base metal when t8/3is less than some value, but the impact value will decrease sharply because of a lot of MA island as t8/3is too long. The softening phenomenon will appear because of a lot of fine ferrite as the peak temperature is about800℃. The impact toughness is the worst at750℃because of precipitation of alloying elements such as Nb and a lot of larger size MA components. The key factors to reduce impact toughness of HAZ include coarseness of microstructures and MA components. Increasing Nb can reduce MA island and refine microstructures, increase welding heat input, ease softening and improve impact toughness obviously of HAZ.For "PF-B" high strength and deformability pipeline steel, the ferrite fraction and average size could be controlled about30%and within5μm especially, and increasing the ferrite/bath bainite interfacial area to further improve the strength, toughness, ductility. To get finer ferrite grain as well as guarantee faster ferrite transformation dynamic, the suitable Nb content is about0.05%in steel and the appropriate technological parameters as follows:cast billet reheat to1150～1200℃for2-4hours; austenite recrystallization and strain temperature of1000～960℃, deformation≥25%, rolling pass interval time≥30s; austenite non-recrystallization region, strain temperature of800～760℃, deformation≥50%; start cooling temperature of700～660℃.更多还原