Study On Organization And Defect Of 26CrMoVTiB Petroleum Casing Steel Billets Produced By Continuous Casting

In recent years,China’s output of oil casing steel has more than half of global output,and maintained a trend of steady growth,even during the global financial crisis,in stark contrast with the fall of the world’s production.Because of the economic crisis,Tube industry of China will enter a deep adjustment period,competition becoming more intense,capacity distribution and industrial structure changing significantly.Over the years,developed countries have a very high quality and performance requirements for petroleum casing.The present paper took a large domestic steel production of oil casing steel slab as research objects,carried out a detailed analysis of its organization and defects.It promotes the optimization of the production process of continuous casting machine,and provides the basis for the heat treatment and rolling technology of the round billet for the following continuous casting.In the analysis of organization,the main detection methods are macroscopic examination,metallographic analysis and scanning electron microscopy(SEM)analysis,respectively researching slab structure from the macroscopic and microscopic state.In the process of the defect analysis of slabs,main used technological means are macroscopic examination,chemical analysis,metallographic analysis,scanning electron microscopy and electron probe microanalysis(EPMA),I analyzed macro segregation and micro segregation with these means.Solidification structures of the slab are edges fine equiaxed crystals,dendrites;cross dendrites and centers equiaxed from outside to inside.The rate of equiaxed crystals is about 40%.The microstructures are mainly lower bainite and lath martensite,and grain size level of original austenite is 10,so the oil casing steel is fine grain steel.In addition,there are mixed crystal in slab organizations,and grain size is not very well-proportioned.There is no obvious shrinkage cavity in slabs,its level of shrinkage porosity for level 1.Center shrinkage area is about 10 millimetres long,and center shrinkage points are all 0.5～0.6 mm in diameter,then shrinkage region accounts for about 0.3%of the total area.There are a small amount of microscopic cracks near the center.Three different kinds of inclusions in slabs are as follows:black spherical inclusions under such as aluminum oxide and calcium sulfide inclusions;yellow square inclusions under metallurgical microscope,such as carbide and nitride of microalloying elements vanadium,titanium and niobium;and complex inclusions with square yellow outside spherical black inside.The center segregation of continuous casting round billets is B class segregation,and the grade is 1.There are negative segregations in the center and edges about carbon,positive segregation at 1/2 radius,and carbon content of the 1/4 radius is close to the average content of whole slabs.The distribution of molybdenum is symmetric,and negative segregation appears in the borders,like carbon,content of molybdenum in the center and 1/2 radius close to the average content.Lower temperature,lower casting speed and appropriate electromagnetic stirring parameters can decrease macro segregation.Distribution of different microstructure and defects of slabs both can cause elements segregation,and crystal boundary segregation of each solute element is higher than that of coring segregation.