Fully Killed Steel means perfectly deoxidised steel by adding Si or Al in the molten steel to eliminitae all entrapped Oxygen from the molten steel so that there is no evolution of gas ( by rkn of C & O ) during ingot solidification . Gives through thickness and top-to-bottom compositional homogeneity and freedom from segregation. Vacuum degassed Steel on the other hand is to subject the molten metal to vacuum to remove out Hydrogen and Carbon . This process ensures steel to be free from hydrogen traps and flakes and also can produce low carbon quality depending on type of degassing procedures. This process is most popular in forging and railroad manufactureings. Now at first you have to check from ASME Sec. II Pt A the technical and ordering information/ supply condition of SA 516 -70N. As your steel is to be used in Sour service cleanliness of steel and at the same time freedom from hydrogen induced defect is a must but not at the cost of mechanical strength. If you have been supplied VAD steel you have to sample check for grain size, mechanical props to tally with code condition. regards As Vacuu>In our specifications we have asked for fully killed steel. >We have been offered vacuum degassed steel instead. Are >these two interchangeable or not similar at all? This steel >is SA516-70N and will be used in sour service.