A study of the Change of Composition, Size, Amount and Distribution of Non-metallic Inclusions in Maraging Steel Grades during VIM

摘要

Non-metallic inclusions, such as oxides, sulphides or nitrides formed during the solidification of steel may change the metallurgical properties of various steel grades. Vacuum metallurgy is often used to achieve low levels of impurities in the material. The VIM-VAR process route is strongly recommended, particularly for maraging steel grades with their unique combination of high strength, good ductility, toughness and resistance to crack propagation and their high content of elements with high oxygen affinity e.g. titanium and aluminium. The amount of non-metallic inclusions which can be removed by VAR is limited. It is therefore of great importance to reduce the amount of non-metallic inclusions in the previous stage, i.e. in the electrode produced via VIM. To know more about the total number, size and distribution of inclusions and their behaviour in the VIM during production, six VIM heats of two maraging steel grades -AMS 6514 (Grade 300) and AMS 6512 (Grade 250) - were observed in this study. To obtain sufficient material (～10kg) from the VIM furnace, a new sampling device was created. After the sample was forged to shape, the chemistry of the samples was analysed and compared with the normal "lollipop" samples. The area content, size and distribution of the inclusions were investigated using automated SEM/EDX analysis and light microscopy (ASTM E45 Methods A and D). Additionally, the amount and size distribution of Ti(C,N) measured by automated SEM/EDX analysis was compared with the results of automated evaluation under light microscopy. A summary sheet was created to show the area contents and the mean values of all important inclusion classes of each sample. Between 5,000 and 10,000 inclusions in each sample were identified by the automated SEM/EDX-analysis system. The investigated area was about 100 mm{sup}2. Manganese-ironcalcium-sulphides, titanium sulphides, carbo-nitrides, carbonitrides with sulphur and oxi-sulphides (endogenous oxides with sulphides) smaller than 10μm were identified as the dominant inclusion types in these heats. To understand and describe the amount, type and size of the non-metallic inclusions observed, additional thermodynamic calculations were performed and compared with the results of the trials. The changes in the several inclusion types during the VIM-process in particular and the impact on the current VIM melting procedures as well as the impact on the quality of the finished product is discussed. To get an idea of the importance of the cleanliness of the VIM melts, the degree of cleanliness of the investigated melts was compared with the average degree of cleanliness of the finished product after VAR and rolling.