高强耐蚀管钢点状偏析及带状缺陷的特征与演变

Characteristics and Evolution of the Spot Segregations and Banded Defects in High Strength Corrosion Resistant Tube Steel

基于钢坯源头对高强耐蚀管钢热轧材与调质材中发现的带状偏析缺陷进行了实验研究。利用OM、SEM、EDS、EPMA等手段,揭示了高强石油套管钢圆坯中存在的各种点状偏析特征及其与热轧管带状组织缺陷的联系,并分析了热轧管和调质管带状缺陷的成因及差异。结果指出,铸坯中存在斑块型和疏松型2类点状偏析,其中均存在较为严重的C、Cr、Mo、Mn等合金元素正偏析,前者偏析区域内还往往存在细小的枝晶亚结构。统计计算表明,铸坯中不同偏析类型在轧材中造成的带状缺陷宽度也明显不同,点状偏析是管材中难以消除的大尺度带状缺陷的源头。热轧管和调质管中带状缺陷组织中均存在明显的合金元素偏析,但前者为珠光体加贝氏体(P+B)组织带状,后者为颗粒型碳化物带状,两者对管材硬度均匀性有不同程度影响。

C110 casing tube is one of the high strength corrosion resistant steel products for deep well oil exploration. Due to the co-existence of acidic media such as H2 S and the high pressure, there are frequently sulfide stress corrosion cracking(SSC) failures produced in the tubes, which are supposed to be closely connected with their banded segregation defects. The relationship between the as-cast spot segregation and the following as-rolled banded defects, together with the impacts of quenching and tempering(QT) treatment have been revealed. The banded defects in high strength corrosion resistant oil tube have been studied experimentally from its very beginning of as-cast state. With aids of OM, SEM,EDS and EPMA observation and analysis, the various spot like segregations in round casting were revealed along with their following banded structure in both as-rolled and QT tubes. The mechanism and appearance of the segregation induced banded defects were investigated comparatively of the both tubes. It is pointed out that there are normally two kinds of spot like segregations in steel castings, speckle type and porosity type, respectively. There are not only severe positive segregations of solutes, such as C, Cr, Mo and Mn etc., in the macro-etched spot like areas, a finer dendritic sub-structure has also been observed in the speckle type spot segregation zones. It has been found that the width of the banded defects in the as-rolled tubes is closely related to the types of segregations, and the severe banded defects, which are difficult to remove by heat treatment, are recognized to originate directly from the spot like segregations. Solute segregations are found in the microstructure of banded defects of the both asrolled and QT tubes but with different existences. A kind of pearlite plus bainite banded structure is present in the former tube, while the banded defect of latter is composed of concentrated granular carbides,which explains the difference of their hardness behavior.