随着管道向低温地区的延伸,对输送管线的低温性能提出了更高的要求,突破寒冷地区用高强度管线钢强韧性配合的瓶颈需要对现有管线钢材料的组织结构设计和TMCP工艺进行优化。为研究TMCP关键参数和复杂组织之间的关系规律从而指导实际轧制过程,采用Gleeble热模拟试验机通过改变冷却速度、终轧温度、终冷温度和驰豫时间,观察得到的不同组织并分析变化规律。结果表明,随冷却速度提高,多边形(准多边形)铁素体体积分数下降,贝氏体铁素体体积分数增加;提高终轧温度,晶粒粗化,但针状铁素体组织比例基本不变;提高终冷温度到550 ℃时,组织严重粗化,并伴随大量恶化低温韧性的大尺寸尖角状MA岛;增加驰豫时间,多边形铁素体晶粒尺寸及体积分数逐渐增大。结合性能研究结果,设计出X80低温管线钢组织为细小的准多边形铁素体＋粒状贝氏体＋少量贝氏体铁素体(QF+GB占90%以上)的组织,其中大角度晶界占比高于50%。最终工业化TMCP参数设定为终轧温度750 ℃+终冷温度480 ℃+冷速20 ℃/s,得到的产品具有优异的低温冲击韧性,满足了X80低温管线钢的综合性能要求。
Abstract
As the pipeline extends to low temperature areas,higher requirements are imposed on the low temperature performance of the pipeline. In order to break through the bottleneck of the strength and toughness balance of high strength pipeline steel laid in the low-temperature region,it is necessary to optimize the microstructure and the thermal mechanical controlled process (TMCP) of the commercial available pipeline steel. In order to study the relationship between the TMCP parameters and the complex microstructure to guide the actual rolling process,the Gleeble thermal simulator was used to observe the different microstructures by changing the cooling rate,finishing rolling temperature,finishing cooling temperature and relaxed time,and then,the change rule in the different microstructures was analyzed. The results showed that with increase of the cooling rate,the volume percent of polygonal (quasi-polygonal) ferrite decreased but that of the bainitic ferrite increased. With increase of the finishing rolling temperature,the grains were coarsening but the fraction of acicular ferrite was basically unchanged. When the finishing cooling temperature was raised to 550 ℃,severely coarsened microstructure was emerged and a large number of MA islands with sharp-angled shape which are harmful to the low temperature toughness appeared. When the relaxed time was prolonged,the grain size and the volume percent of polygonal ferrite were both gradually increased. Based on the results on properties,a microstructure composed of fine quasi-polygonal ferrite (PF),granular bainite (GB) and a small amount of bainitic ferrite (BF) (QF+GB accounted for more than 90%) was designed for the X80 pipeline steel used in extremely-cold region. In this microstructure,the percentage of high angle grain boundaries was more than 50%. Finally,the industrial TMCP parameters such as finishing rolling temperature,finishing cooling temperature and cooling rate were optimized to be 750 ℃,480 ℃ and 20 ℃/s,respectively,which made the X80 pipeline steel with excellent low temperature impact toughness,satisfying the requirements of comprehensive properties,especially in the extremely-cold region.
关键词
热模拟 /
组织 /
TMCP /
低温韧性 /
大角度晶界
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Key words
thermal simulation /
microstructure /
TMCP /
low temperature toughness /
large angle grain boundary
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脚注
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基金
国家重点研发计划资助项目(2017YFB0304901)
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