成型操作对测量钢管屈服应力的影响

Influence of the Forming Operationson the Yield Stress Measured on Pipe

测量钢管屈服应力的目的是为了评估钢管对内部流体压力的抵抗能力。遗憾的是,不可能在环向取得一段直的试样,因此在进行试验之前,必须将钢管压扁。当前研究中,对不同材料以拉伸-压缩模式进行试验以便为随动硬化模型提供数据。以此试验数据为基础,建立一个模型,并将一些材料行为特征(屈服点伸长率的存在,应变硬化等)和工艺路线(直缝或螺旋缝焊接,扩径机,水压)等均考虑在内。钢管生产也在不同工艺段采样(母材、矫平后、成型后及水压后)。试验程序包括拉伸试验和环胀试验,结果显示该模型所给出的预测值与试验结果有良好对应。该模型还推出一些试验事实,例如屈服点伸长率在母材的存在及其在扁平管试样的不存在。最后,该模型与工业生产数据库相比,含有不同的钢级(从B级到X80级)和不同壁厚直径(壁厚/外径)比的数据值。该数据库所预测的钢卷和钢管的屈服应力差为20 MPa。

The yield stress of pipes is measured among purposes to assess the resistance of the pipe to the internal pressure of the fluid. Unfortunately, it is not possible to sample a straight specimen in the hoop direction, and therefore the pipe has to be flattened prior to testing. In the present investigation, different materials were tested in tensile-compression mode in order to provide data for a kinematic hardening model. Based on this experimental data set, a model was built to take into account several features of the material behavior （presence of yield point elongation, strain hardening, etc.） and the processing route （longitudinal or spiral seam weld, expander, hydrotest,ete.）. Pipe production was also sampled at different moments（base material, after leveling, after pipe forming, after hydrotest）. The testing program included tensile testing and ring expansion tests. The results show that the model gives a prediction in good correspondence with the experimental results. The model also reproduces several experimental facts, like for example the presence of a yield point elongation on the base material and its absence on the flattened pipe sample. Finally, the model is compared with an industrial database containing different steel grades（from grade B to X80）and different ratios of wall thickness over diameter（t/OD） ratios. The difference of yield stress between coil and pipe is predicted on this database with an accuracy of 20 MPa.