Development Of HIC-resistance X65Pipeline Steel

Pipeline transport is a most economical and reasonable way to transport oil or natural gasover long distances. The construction of oil and gas pipe network can greatly reduce the pressureof the rail transport and good for the safety of oil and gas supply in the market. Laying ofpipelines often through many wet acidic environment and some oil or natural gas also contains alot of acid gases such as H2S or SO2, Therefore, the acid resistance of pipeline steels has beenattracted more and more attention. After continuous development, the strength of the pipelinesteel has been increased to X120level; however the development of acid resistance pipelineremains at a lower level. The hydrogen-induced cracking （HIC） is an important form of pipelinesteel in an acidic environment. To study the HIC resistance performance of pipeline steelsbecome one of the key topics in steel and pipeline industry.Inclusions in the investigated X65pipeline steels were extracted by means of non-aqueoussolution electrolysis method. Inclusions and microstructures were observed and analyzed forX65pipeline steels produced by two different deoxidizing methods using optical microscope,scanning electron microscope and energy dispersive X-ray spectroscopy in this paper. The HICtest results show that center segregation was the main factor for hydrogen-induced cracking. Thesize and type of inclusions were efficiently controlled in the Ti-Zr deoxidized steel, in whichMnS was spheroidzed and evenly dispersed in the matrix. However, large inclusions andelongated MnS particles were observed in the conventional Al deoxidized steel. Compared withthe conventional Al deoxidized steel, the Ti-Zr deoxidized X65pipeline steel had betterHIC-resistance property.The titanium microalloyed low-carbon low-alloy X65HIC resistance pipeline steel wasdesigned and produced by hot rolling and air cooling. The microstructure, inclusions andprecipitation of the investigated steels were observed and analyzed by optical microscopy,scanning electron microscopy and X-ray spectroscopy. The tensile properties were tested at roomtemperature and the Charpy-Votch impact was tested at0°C and-40°C. Results show that themicrostructure was ferrite+pearlite, and the grain size were refined, and the area fraction ofdegenerated pearlite increased when Ti content increased from0.05%to0.11%. Themicrostructure was changed from ferrite+pearlite to acicular ferrite when the plate thicknesswas changed from8mm to4mm. The inclusions in Ti-microalloyed steel were mostly Al₂O₃and TiN. There was a large umber of Nb-Ti carbonitrides precipitated in the matrix. The numbersof precipitates were increased with increasing Ti content and with increasing plate thickness. TheTi microalloyed pipeline steel obtained excellent combined strength and toughness when the Ti content was0.11%. The room temperature tensile properties and Charpy-Votch impact absorbedenergy at0°C were all met the requirements of "API Spec5L-2007".The HIC property of the Ti microalloyed pipeline steel was tested by "NACETM0284-2003" standard. The level of banded structure and sulphide clusters of the investigatedsteels were very low. Most of sulphur were combined with Ti and formed TiS sulphide. Theshape and size of sulphides were thus effectively modified. Therefore, no crack or hydrogenblisters were found in Ti microalloyed X65pipeline steel. The investigated steel obtainedexcellent HIC resistance performance.