| Along with the development of deep sea oil field resources,the existence of chlorine ion / hydrogen sulfide gas in the energy causes the corrosion of the pipeline steel which is induced by inclusions.The inclusions have been shown to be seriously harmful to the safety and stability of the pipeline.In order to investigate the effect of the Ti-deoxidization and Mg/Ca treatment on the Characteristics of inclusions,and on the corrosion behavior（HIC and pitting corrosion）compared to that in the Al-deoxidization and Ca/Mg treatment,experiments were conducted in a 25 kg medium frequency vacuum induction furnace.The steels were rolled into sheets by thermo-mechanical control processing（TMCP）.The morphology,composition,deformation and distribution of inclusions in the as-casted and rolled steel were characterized.The HIC susceptibility was evaluated by NACE Standard TM0284-2011 method.The pitting corrosion was evaluated by the experiment of natural corrosion and electrochemical polarization.The results are as follows.（1）Compared to the Al-deoxidized steels,the number density of the inclusions with sphere or ellipsoid morphology in the Ti-deoxidized steels was relatively large,the size was small and the ratio of inclusions in the interval of small deformation aspect ratio（1-3）were large,respectively.（2）Compared to the as-casted steels,the composition of inclusion did not obviously changed,but the morphology,distribution,number and deformation interval greatly changed.The morphology of inclusions in as-casted steels were sphere,while their morphology turned into ellipsoid or elongated in the rolled steels.The average size of inclusions and the number of large inclusion increased,the number density of the inclusions decreased owing to the deformation of inclusions in rolled steels.（3）No hydrogen blisters were found in steels with Mg treatment compared to Ca treatment in which one or two hydrogen blisters were found.It is attributing to the fact that some inclusions with large size or deformation located beneath the surface of tested specimens with Ca treatment in the HIC test.All the tested steels were not susceptible to the HIC by NACE Standard TM 0284-2011 method and there was no difference between the Ti-deoxidized steels and the Al-deoxidized steels in thisresearch conditions due to MnS and Al2O3 inclusion’s modification by Al-Ti complex deoxidized and Mg/Ca treatment,but Ti-deoxidization and Mg treatment process had much better effect of inclusion modification with less deformation of inclusions by adding the content of TiOx and MgO in complex oxide core.No cracks induced by the single MnS or inclusions with MnS precipitated on their surfaces and Al2O3 are found in the steels.All the tested steels for HIC susceptibility（Table 3）has nothing to do with the number density of fine,dispersed inclusions with average size about 1um from 400 number/mm2 to 800 number/mm2.（4）Compared with Al-deoxidized steels,the natural corrosion rate of Ti-deoxidized steels were small.In the terms of the four steels.The pitting potential in Ti-deoxidized steels were higher than that in Al-deoxidized steels.That is to say,the pitting corrosion resistance of Ti-deoxidized steels was powerful than Al-deoxidized steels.The pitting corrosion resistance of Mg-deoxidized steels was powerful than Ca-deoxidized steels.The inclusion was the induced source of pitting corrosion of steels.The corrosion firstly occured in the interfacial surface area between the inclusion and steel matrix and the larger the inclsion was,the larger the corrosion steel matrix around the inclusion and the steel would be seriously damaged. |
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