The origin of sand marks is caused by the ingot defects, and those defects are not changed macroscopically by the heat treatment, but will be deformed by forging or rolling.
Generally, the origin of sand marks is considered as ingot defects, such as non-metallic inclusions, blow holes, pin holes, cavities and segregations. But the sand marks of the steel rod appear more or less (mostly) in all the steel rod and its surface anywhere, therefore. the origin of sand marks must be the defects that appear very frequently in ingots.
(A) Blow holes, pin holes and cavities
(a) By experimental results, generally, those defects (blow holes, pin holes) appear rarely in ingots.
(b) Many cavities appear in the center of long and small diameter ingots, but those cavities are welded by rolling. Therefore, it is considered that, when cavities, blow holes and pin holes in ingots have not oxides in the inside, they are welded by rolling or forging and do not appear as flaws (such as the sand marks) of steel rods.
(c) If the small blow holes of H2 are the origin of sand marks, the sand marks are to appear with more uniformity than they are.
(d) Surface blow holes, and pin holes of ingots, cut by about 4-5mm inside from the periphery, are seldom.
Therefore, the origin of sand marks is not caused by blow holes, pin holes and cavities of ingots.
The segregation sand mark is very rare, and it has origin only in high-C steel such as bearing steel.
(C) Non-metallic inclusions
(a) Non-metallic inclusions appear very often and every where in ingots.
(b) Non-metallic inclusions are deformed and elongated by rolling or forging.
(c) In parallel to increase of the forging ratio, non-metallic inclusions are elongated,