| Cr-Ni-Mo pressure vessel steel has high requirements on ablation resistance,strength and toughness.The cleanliness of Cr-Ni-Mo medium-carbon low alloy steel is the main factor affecting these performance indexes,and the content,type and distribution of inclusions in steel are important factors affecting the cleanliness of steel.Studying the types,sources and distribution of inclusions in molten steel plays an important role in improving the cleanliness of molten steel and optimizing the properties of pressure vessel steel.In this paper,the Cr-Ni-Mo medium-carbon low alloy steel is studied,and the inclusion state and the formation of inclusions in the steel are analyzed.The morphology and composition elements of inclusions in different parts of electrode ingot and electroslag ingot are analyzed by scanning electron microscope(SEM)and energy spectrum analysis system(EDS).The Zeiss microscope was used to enlarge each sample by 200 times,and the inclusion size and inclusion index of electrode ingot and electroslag ingot were calculated by Image software.The results show that the inclusions in the steel are mainly calcium silicate,aluminosilicate and alumina-magnesia spinel inclusions composed of calcium,aluminum,sulfur and silicon.Among them,the source of Al element is aluminum deoxidization adopted in the initial stage of electric furnace smelting,which is not well discharged during smelting.Ca comes from [C] in molten steel and participates in the reduction slag reaction,which leads to the rapid increase of [Ca] content in molten steel.Si inclusions come from ferrosilicon added in electric furnace smelting,ferrosilicon and calcium silicon added in LF refining,which react with oxygen in steel to form silicate or aluminosilicate.Oxygen comes from scale produced by oxidation on the surface of the consumer and oxygen in the air.S-containing inclusions are well removed in LF and VD refining process.The statistical results of inclusions show that the size of inclusions in steel ingots is concentrated in 5–10 ? m,accounting for about 60% of the total inclusions.The inclusion removal index is high at the head of electrode ingot and the center of electroslag ingot,and the proportion of inclusions with large particle size is also high,which is the place with the largest density and total number of inclusions,indicating that the large-particle inclusions floated up but could not be completely discharged during refining.The distribution of inclusions in the electrode ingot is characterized by increasing from edge,center to center and decreasing from head to tail.The inclusions in the head of electroslag ingot gradually decrease from the middle to the edge. |
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