高级别管线钢板的组织特征与析出强化

Microstructure Characteristics and Precipitation Strengthening of High Grade Pipeline Steels

利用扫描电镜、透射电镜对炉卷轧机生产的高级别X70,X80管线钢板的显微组织与析出相进行了观察与分析,并对管线钢板组织中碳氮化物析出相的成分、相结构以及粒度分布进行了定量分析,并采用Ashby-Orowan修正模型对析出粒子的强化贡献进行了理论计算。结果表明:X70,X80管线钢板的组织以针状铁素体为主,其高密度位错与交叉分布的大角度晶界特点保证了管线钢的高强韧性;X70,X80管线钢板中的析出相主要有两种类型,一种为尺寸较小的NbC析出相(1～30nm),另一种为尺寸较大的(Ti,Nb)(C,N)复合析出相(50～300nm),两种类型均为面心立方结构;生产工艺对析出相的粒度分布有明显影响,避开900～950℃碳氮化物的快速析出温度区间,以及适当增加冷却速度可以有效提高20nm以下的析出粒子体积分数,细化析出物尺寸,从而提高析出强化对屈服强度的贡献。

Microstructure and precipitates of high grade X70 and X80 pipeline steels produced by Steckel mill were observed and investigated using scanning electron microscope （SEM） and transmission electron microscope （TEM）. Chemical compositions, crystal structure and size distribution of carbonitride precipitation phase were quantified by way of physicochemical phase analysis, and the precipitation strengthening contribution was theoretically calculated by Ashby - Orowan correction model. The results show that microstructures of X70 and X80 pipeline steels mainly consist of refined acicular ferrites, which can guarantee the combination of both high strength and toughness due to its high density dislocations and interwoven high - angle boundaries. Two kinds of precipitates were observed in the steels, where the fine ones are mainly NbC about 1 - 30 nm and the coarse ones are mainly （ Ti, Nb） （ C, N ） combined precipitates about 50 - 300 nm. The two kinds of precipitates both belong to face - centered cubic structure. Production technology has a significant influence on the size distribution of precipitation. It is proved in practice that, avoiding deformation in the temperature region of 900 -950℃ and properly accelerating the cooling rate can dramatically improve the volume fraction of precipitate particles smaller than 20 nm and refine the precipitates size, thereby increasing the contribution to strength.