【作者】 张凯；

【导师】 吴迪； 杜林秀；

【作者基本信息】 东北大学 ， 材料加工工程， 2011， 博士

【摘要】 随着管道工程的快速发展,高强、高韧、高塑性管线钢的研究与开发日益受到重视。目前,X100和X120作为超高强度管线钢由于具有高强韧性、抗氢致裂纹(HIC)性能、抗硫化氢应力腐蚀开裂(SSCC)性能以及抗应力腐蚀开裂(SCC)性能,可以保证管线安全、降低输送成本等优点,被预测为未来最有可能全面应用的超高性能管线钢。目前通过调整控轧控冷工艺(TMCP),利用低温大压下和微合金化途径生产高钢级管线钢存在着设备要求苛刻、微合金化成本过高、生产效率低下等问题。本文通过热模拟技术、控轧控冷工艺手段、力学性能测试及显微分析方法,对X100管线钢成分设计、轧制、冷却工艺及其组织性能进行系统研究；同时,对X120管线钢提出了“控轧+弛豫+快速冷却”、“控轧+间歇式快速冷却”以及热处理工艺路线,对其强韧性和组织的变化规律进行研究。论文主要工作及研究成果如下：(1)在MMS-200热模拟试验机上,研究了Mn-Nb-Mo-Ti系X100和Mn-Nb-Mo-B-Ti系X120实验钢高温奥氏体热变形行为及等温过程中静态再结晶,分别计算出实验钢的动态再结晶激活能和静态再结晶激活能,并确定了动态再结晶特征参数。分析研究结果表明,随变形温度的降低和应变速率的提高,实验钢动态再结晶与静态再结晶均难以发生；随变形量的增加,形变存储能增加,静态再结晶速率加快。利用最小二乘法确定X120管线钢的动态、静态再结晶激活能均高于X100。含B的X120无论是峰值应力、应变还是临界应力、应变也都比X100要高,对于没有明显峰值点的应力-应变曲线,也有可能发生动态再结晶,需通过应变硬化率和应力的关系来确定是否有极小值点,从而确定是否发生动态再结晶。(2)通过对奥氏体连续冷却相变行为的研究,测定了不同变形条件下X100和X120实验钢的奥氏体连续冷却相变动力学曲线(CCT曲线)。结果表明,变形量的增加与变形温度的降低均显著加速了相变,使针状铁素体和粒状贝氏体开始相变温度显著提高；合金元素B的添加,抑制先共析铁素体和中温转变组织的形核,促进下贝氏体相变,扩大下贝氏体相变区域,使CCT曲线变得更加扁平；B元素的添加还抑制下贝氏体板条束长大,起到细化晶粒的作用。(3)通过轧制工艺热模拟试验,研究加热温度、轧制和冷却工艺参数对组织及细化作用的影响。分析结果表明,对奥氏体、M/A组元和析出相等显微组织采用热加上过程的全过程控制的方法,即精确控制加热温度、粗轧、精轧和冷却速度等工艺参数,能获得最佳的组织细化效果。(4)在实验轧机上对Mn-Nb-Mo-Ti系及高Nb系X100实验钢进行了控轧控冷实验,通过TMCP工艺与HTP工艺的钢板的组织、性能比较。结果表明,与TMCP工艺相比,通过HTP工艺实验钢组织基体内部晶粒度小于TMCP工艺,强度、低温冲击韧性和延伸率方面都有不同程度的降低,但HTP工艺较显著地改善其屈强比。其中,Mn-Nb-Mo-Ti系X100实验钢最优工艺参数为：精轧开轧温度780℃～800℃,终轧温度760℃～780℃,终冷温度440℃～460℃,冷却速度30℃/s左右。此工艺钢板屈服强度达到751MPa,抗拉强度为894MPa,延伸率为20.6%,-20℃夏比冲击功达到247J,韧脆转变温度为-62℃。根据热变形工艺回归公式推算,屈服强度、抗拉强度和延伸率分别为769MPa,907MPa,19.7%,与实际值相吻合。(5)通过对X120超高强度管线钢弛豫阶段中温转变组织超细化原理分析,提出“控轧+弛豫+快速冷却”新型工艺路线。通过调整水冷开冷温度来控制软相针状铁素体的数量,在弛豫过程中,晶内通过变形位错重新排列,形成了位错胞状结构和亚晶,并通过随后的快速冷却将弛豫过程形成的亚晶界快速形核,生成硬相组织(接近马氏体组织的下贝氏体),这种针状铁素体+下贝氏体的复相组织不仅具有很高的强度,还具有良好的塑韧性,软相组织的加入优化屈强比和抗大变形性能。理想的工艺参数为：终轧温度800℃左右,弛豫至650℃附近,冷却速度稍高于50℃/s,终冷温度400℃附近。(6)采用“控轧+间歇式冷却”工艺,研究了轧制、冷却参数对X120超高强度管线钢组织性能的影响。通过降低终轧温度,使在较低温度下充分变形的奥氏体中产生高畸变积累,在高冷却速度下,迅速在奥氏体向铁素体相变的动态相变区前终止冷却,进入到弛豫阶段,得到针状铁素体或粒状贝氏体等中温转变组织,并通过随后的快速冷却至Ms以下,得到低温下贝氏体/马氏体组织。最优的工艺参数为：终轧温度850℃左右,冷却速度均为50℃/s以上冷却至560℃弛豫区间,弛豫时间为10s。这种硬化奥氏体不仅提供了弛豫过程中软相针状铁素体相变所需的能量,而且在其基体中形成大量的形变带,为最终相变组织提供足够的形核位置,该工艺板材力学性能优良。(7)对比研究了不同在线热处理工艺对X120级管线钢组织性能的影响。结果表明,在450℃保温40min的回火工艺下,采用直接淬火工艺实验钢回火后组织中大角度晶界百分含量略大于两段式淬火上艺。但采用两段式淬火工艺实验钢综合力学性能均优于直接淬火工艺,且在450℃～500℃回火后,各项指标均达到X120管线钢性能标准。从节能减排环保角度出发,采用轧后直接两段式淬火+回火工艺开发一种既有高强韧性,又有高稳定性的热处理钢板,可作为X120超高强度管线钢热处理工艺储备。更多还原

【Abstract】 With the development of pipeline transportation industry, the pipeline steels with high strength, toughness and deformability are paid more and more attention. X100and X120ultra-high strength pipeline steels with good match of strength and toughness, resistant HIC, SSCC, SCC are keys to ensure the security and reduce the operation cost, for which the X100and X120grade pipeline steel will be extensively applied in the future.This paper was based on the problems which include the rigorous requirements of equipments, increasing cost of alloying elements and production efficiency using low temperature large deformation and micro-alloying process by adjusting TMCP. Effect of rolling and cooling processes on microstructures and properties of high grade X100pipeline steel was systematically researched by means of heat simulation technique and mechanical properties test and microscopic analysis method. Meanwhile,"controlled rolling&relaxation&fast cooling" process and "controlled rolling&intermittent fast cooling" process were proposed, and the relationship was studied between strength-toughness combinations and microstructure of high grade X120pipeline steel. The main works involved as follows:(1) The hot deformation behavior and static recrystallization for two tested steels, Mn-Nb-Mo-Ti micro-alloyed X100pipeline steel and Mn-Nb-Mo-B-Ti micro-alloyed X120pipeline steel were studied by using MMS-200thermal simulator. The dynamic recrystallization activation energy and the static recrystallizational activation energy were determined, respectively. Dynamic recrystallization characteristic parameters were also determined. Results show deformation temperature was the main factor that influences the dynamic static recrystallization behavior and the recrystallization behavior, and decreasing deformation temperature inhibits recrystallization. Deformation storage energy increased with increasing strain and static recrystallization rate could be accelerated. Least square method was used to confirm the static recrystallization activation energy and dynamic recrystallization activation energy of X120were higher than that of X100. Peak characteristic parameters and critical characteristic parameters of X120pipeline steels containing boron were also higher than that of X100. For no obvious peak value point strain-stress curves, the existence of dynamic recrystallization could be determined by analyzing the relationship between strain-harden rate and strain.(2) Microstructure and transformation behavior were investigated by the continuous cooling experiments, and the continuous cooling phase transition kinetics for XI00and XI20pipeline steels have been constructed. The results show that the acicular ferrite phase transformation and granular bainite phase transformation were accelerated observably, and the starting temperature of phase transformation were raised notablely by increasing deformation and decreasing deformation temperature. The addition of B could refine lower bainite grains and accelerate lower bainite transformation, thus retarded the formation of ferrite and intermediate transformation microstructures, meanwhile, CCT curves became more flat.(3) The influence of heating temperature, rolling and cooling parameters on microstructure and refinement had been studied by rolling processes simulation test. The results show that the best effect of controlling and refinement for austenite, martensite and retain austenite (M/A) and precipitates could be achieved by all controlling hot working processes, namely, accurate controlling the parameters of reheat temperature, rough rolling, finish rolling and cooling rate.(4) The effects of TMCP and HTP on microstructures and properties of Mn-Nb-Mo-Ti micro-alloyed X100pipeline steel and high Nb pipeline steel were studied, respectively. Compared with the grains obtained by TMCP, the grains by HTP were larger, the strength, elongation and CVN toughness were not improved, while yield ratio dropped sharply. For Mn-Nb-Mo-Ti micro-alloyed XI00pipeline steel, when the started rolling temperature, finished rolling temperature, finished cooling temperature and cooling rate was780℃-800℃,760℃-780℃,440℃-460℃and about30℃/s, respectively, and the yield strength, tensile strength, elongation, CVN toughness at-20℃, ductile-brittle transition temperature (DBTT) was751MPa,894MPa,20.6%,247J, and-62℃, respectively, the comprehensive mechanical properties were better in laboratory. According to the regression formula, the mechanical properties of the tested steel were predicted σs;=769MPa, σb=907MPa, δ5=19.7%, which were well correspondence to actual ones.(5)"Controlled rolling&relaxation&fast cooling" process was proposed by an investigation on refining intermediate transformation microstructures of XI20ultra high strength pipeline steel. The number of acicular ferrite as softer phases of the tested plate rolled by "controlled rolling&relaxation&fast cooling" process was controlled by adjusting start cooling temperature. Dislocation cells and substructure were formed by dislocation interaction during relaxation, then being rapid cooled became harder phases (lower bainite). The resisting large deformation property and yield ratio were optimized by acicular ferrite+lower bainite dual-phase microstructure, and the strength and toughness were improved. The ideal process scheme for producing X120grade pipeline steel was that the tested steel was deformed at final rolling temperature of800℃, cooled to about650℃in air, and cooled at a cooling velocity of above50℃/s with UFC process after about400℃.(6) The effects of rolling and cooling parameters on microstructures and mechanical properties of XI20ultra high strength pipeline steel were studied by means of laboratory hot rolling experiments adopting "controlled rolling&intermittent fast cooling" process. Distortional strain energy accumulate was formed by heavy deformation by lower finished rolling temperature, then was remained at the transition region of γ→a by fast cooling, in which acicular ferrite and granular bainite were obtained by means of relaxation. Subsequently, lower bainite and martensite transformation were occurred by cooling to below martensite transformation start (Ms). The most superior process parameter was that finished rolling temperature was about850℃, relaxation was560℃for10seconds, and cooling velocity was over50℃/s. A lot of deformation bands were retained in the harden austenite, which provided nucleation sites and the energy required for the transformation, leading to improvement of mechanical properties of the plates.(7) Effects of on-line heat treatment processes on microstructure and properties of X120pipeline steel were studied. The results show that amount of large angle grain boundaries of the direct quenched-tempered steel plates tempered at450℃for40min was slightly more than that of the two stage quenched-tempered steel plates. When tempering temperature range was450℃to500℃, the tempered steel which had good mechanical properties and mechanical stability could be obtained by two stage quenched-tempered process.In view of pollution reduction and protect the environment, the heat treatment steels with a high strength and toughness and high mechanical stability were obtained by the two stage quenched-tempered process, which were also used to technique reserve of heat treatment.更多还原