Study On Cleanliness Control Technolohy Of Acid-resistant Pipeline Steel In Tisco

The acid-resistant pipeline steels have been widely used in the construction of long-distance oil and gas transportation systems.Because of severe environmental conditions,they were required to have a good combination of strength,toughness and weld ability;especially hydrogen induced cracking（HIC）resistance.The hydrogen induced cracking（HIC）susceptibility was particularly related to sulfide inclusions and oxide inclusions in the steel.At present,through the optimization of process and the adoption of super-low sulfur technique,the effect of the sulfide inclusions decreased significantly.However,the control of CaO-Al2P3 inclusions has always been a problem at home and abroad.TISCO is one of major suppliers of acid-resistant pipeline steels.In this dissertation,for the production line of acid-resistant pipeline steels in TISCO,the process parameters were analyzed systematically.First of all,the sulfur content,the calcium content and the inclusions shape in the pretreatment process and secondary refining process were analyzed.At the same time,before and after calcium treatment,the change of calcium content and the variation of inclusions were discussed.During the whole process of pipeline steel production,steel samples and slag samples were taken.Through optical microscope and scanning electron microscopy,the variation of inclusions were analyzed and discussed.The following results were obtained.During the low sulfur smelting process,there were some reactions between high-basicity reducing slag and steel.The initial inclusions were MnO and SiO2.After addition of aluminum,Al₂O₃ inclusions formed.Through calcium treatment,the inclusions turned into CaO-Al₂O₃-MgO.Finally,the inclusions were CaO-Al₂O₃-CaS.During the calcium treatment,with the raise of calcium content in the steel,the CaS and CaO content in the inclusions increased obviously.Nevertheless the Al₂O₃ content in the inclusions reduced.In the final slab,the CaO-Al₂O₃-CaS inclusions and CaS-CaO inclusions formed.On the premise of stable steel composition,the shape of inclusions was decided by the addition of calcium.Inadequate and excessive addition of calcium both affected the steel quality.There would be unqualified products.During LF refining process,RH refining process,calcium treatment process and continuous casting process,the inclusions transformed constantly.Discussing the variation of inclusions,a thermodynamic model was proposed.Combining with previous research and based on the relevant data,the flow field model in the ladle,the vaporization model of calcium and the variation model of inclusions were established.Through CFX fluid calculation software,the three models were calculated.The flow field in the ladle during soft blowing process,the change of calcium content during calcium treatment and the variation of inclusions were calculated respectively.The calculation results of calcium content and inclusions shape were in accordance with the measured results.According to the calculation results,the addition of calcium in TISCO was modified from 0.511 Kg/t to 0.767Kg/t.On the basis of above research,adjusting the addition of calcium in the actual production,the expected inclusions in the acid-resistant pipeline steels were acquired.Hydrogen induced cracking（HIC）were avoided effectively,and the quality of pipeline steels were improved.Meanwhile,this research method can be as a reference model for the improvement of stainless steel in TISCO,which promoted the competiveness of the products.