ANALYSIS AND CONTROL OF SURFACE DEFECTS OF 25CrMnMo STEEL
The causes of surface defects in 25CrMnMo steel pipe production were analyzed by means of macroscopic morphology observation,microstructure(SEM)and energy spectrum analysis(EDS).Through the analysis of the physical and chemical properties of 25CrMnMo steel,it is found that the macro-defects on the surface of the steel pipe exist the spalling phenomenon of mold powder,while the micro-analysis shows that there are many large-sized irregular inclusions of FeO-Na2O-Al2O3-CaO under the steel pipe skin.At the same time,the inclusion composition in the surface defect of steel tube is compared with that of long nozzle,submerged nozzle,slag composition and mold flux,it is found that Na and Al in the inclusions in steel are the representative elements of mold powder,and it is inferred that the mold powder which is involved and can not rise in time is captured by the solidified shell at the solidification front under the original technological conditions,surface and subcutaneous inclusion defects are the main causes of defects in 25CrMnMo steel pipe.By changing the insertion depth of the submerged nozzle from 80～130 mm to 90～130 mm,reducing the electromagnetic stirring current from 300 A to 200 A,and keeping the frequency 3.0 Hz constant,the slag entrapment index can be reduced,the problem of slag entrapment in mould of 25CrMnMo steel was effectively improved,and the quality of billet was improved.
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