An apparatus (1, 1 a) for thermally treating a metallurgical semi-finished product having extended elongated shape according to a respective first longitudinal axis and having indefinite length, comprising: - an outer casing, - a passageway defining a crossing direction to allow the passage of said semi-finished product during operation, said respective first longitudinal axis being coincident with said crossing direction during said passage of said semi-finished product through said passageway, - a drum comprising a plurality of treatment lines arranged along a perimeter region of said drum, each of said treatment lines comprising a plurality of units (8, 8a, 8b, 8c, 8d) for the treatment of said semi-finished product, said units (8, 8a, 8b, 8c, 8d) comprising respective cavities (9, 9a, 9b, 9c, 9d) which are coaxial and parallel to said crossing direction, each of said units (8, 8a, 8b, 8c, 8d) comprising a base which a respective cover is hinged to, each of said cavities (9, 9a, 9b, 9c, 9d) comprising a first semi-cavity in said base and a second semi-cavity in said respective cover, each cover being movable between an open position wherein said semi-cavities (29a, b) are separate from each other and a closed position wherein said semi-cavities (29a, b) are near to each other, said drum being accommodated in said casing and rotatable about a second rotation axis parallel to the crossing direction to move each of said treatment lines between an operating position wherein said cavities (9, 9a, 9b, 9c, 9d) are aligned with said passageway to be crossed by said semi-finished product, and at least a resting position wherein said cavities (9, 9a, 9b, 9c, 9d) are separated from said passageway, characterized in that said outer casing is shaped in such a way as to touch and hold said respective cover in said closed position, when each treatment line is in said resting position. - an outer casing, - a passageway defining a crossing direction to allow the passage of said semi-finished product during operation, said respective first longitudinal axis being coincident with said crossing direction during said passage of said semi-finished product through said passageway, - a drum comprising a plurality of treatment lines arranged along a perimeter region of said drum, each of said treatment lines comprising a plurality of units (8, 8a, 8b, 8c, 8d) for the treatment of said semi-finished product, said units (8, 8a, 8b, 8c, 8d) comprising respective cavities (9, 9a, 9b, 9c, 9d) which are coaxial and parallel to said crossing direction, each of said units (8, 8a, 8b, 8c, 8d) comprising a base which a respective cover is hinged to, each of said cavities (9, 9a, 9b, 9c, 9d) comprising a first semi-cavity in said base and a second semi-cavity in said respective cover, each cover being movable between an open position wherein said semi-cavities (29a, b) are separate from each other and a closed position wherein said semi-cavities (29a, b) are near to each other, said drum being accommodated in said casing and rotatable about a second rotation axis parallel to the crossing direction to move each of said treatment lines between an operating position wherein said cavities (9, 9a, 9b, 9c, 9d) are aligned with said passageway to be crossed by said semi-finished product, and at least a resting position wherein said cavities (9, 9a, 9b, 9c, 9d) are separated from said passageway, characterized in that said outer casing is shaped in such a way as to touch and hold said respective cover in said closed position, when each treatment line is in said resting position.