Study Of Macroscopic Inclusions In Thick Slab Continuous Casting Slab Of Pipeline Steel

Pipeline steel is mainly used for long-distance transportation of oil and natural gas.As China's energy consumption increases year by year,the pressure on pipeline steel to transport natural gas is also increasing.Pipeline steel requires higher performance to meet these requirements.Bubble and non-metallic inclusion defects are the key to restricting the quality of pipeline steel,especially large-scale macro inclusions(including bubbles,size?50 ?m).Inclusions not only direc tly affect the quality and performance of the steel,but also react with the mixed acidic impurities in the transport medium to produce hydrogen-induced cracking.Hydrogen induced cracking would cause a tube burst.Therefore,it is necessary to study the distribution and source of macroscopic inclusions in pipeline steel in depth,and provide guidance for the improvement of pipeline steel quality.In this paper,the X70 pipeline steel thick slab continuous casting billet is taken as the research object,and the large section slab planing layer detection platform is established independently.The main equipment of the detection platform includes X5036 milling machine and 500X electron microscope.By anatomizing the continuous casting billet layer by layer,the inclusion morphology,size and three-dimensional distribution of the thick slab continuous casting billet are obtained.Scanning electron microscopy and energy spectrum analysis were used to detect the morphology and composition of typical inclusions.The accuracy of the detection method of the planing layer was verifi ed,and the source of various inclusions was judged according to the chemical composition of the inclusions.The research results are as follows:(1)Morphological characteristics of inclusions.The morphology of macro inclusions can be divided into three types:near spherical,irregular and cluster.Among them,the size of the near-spherical inclusions is large and the number is large,the number of irregular inclusions is second,and the number of cluster-like inclusions is small.The morphology of the inclusions is related to the position in the slab:as the distance between the slab and the nozzle increases,the percentage of near-spherical inclusions decreases gradually,and decreases rapidly in the narrow side of the slab.As the distance between the slab and the nozzle increases,the percentage of irregular inclusions gradually increases.The percentage and number of cluster-like inclusions in the narrow-side slab area are significantly increased compared with other positions,and irregular inclusions and cluster-like inclusions are more easily captured at the narrow-edge solidification front.(2)Size distribution of inclusions.Count the number and size of macro inclusions in the entire slab sample and find the law:The inclusions with a particle size of 50-100 ?m accounted for 53.67%of the total;the inclusions with a particle size of 100-150 ?m accounted for 33.4%.The inclusions with a particle size of 150-200?m accounted for 7.94%.The number of inclusions larger than 150 um is less,accounting for 4.99%.(3)Spatial distribution of inclusions.The three-dimensional spatial distribution model of inclusions shows that the inclusions are asymmetrically distributed on the inner and outer arc sides.In the narrow side area,the outer arc side inclusions are more than the inner arc side.In the near-nozzle area,the inner arc side inclusions are more than the outer arc side.In the direction of the pulling speed,there is a significant fluctuation in the distribution of inclusions.There are a lot of inclusions in the 15 mm area under the skin of the casting blank,accounting for nearly 75%of the total inclusions.With as the thickness of the solidified shell at the boundary between the vertical section and the curved section of the continuous casting machine,79%of the inclusions in the cast strand have been captured in the vertical section of the continuous casting machine.(4)Analysis of inclusion sources.The composition of the inclusions indicates that the macroscopic inclusions in the pipeline steel are mainly composite inclusions,accounting for 60.24%of the total inclusions.The number of single-phase inclusions is small,accounting for 5.15%of the percentage of inclusions.The slag inclusions accounted for 31.4%of the total inclusions.Other inclusions such as bubbles accounted for 3.21%.