Self-locking inner nozzle system comprising: an inner nozzle comprising: (a) an upstream surface and a downstream surface joined to one another by a lateral surface of nozzle height, h, and comprising a bore extending along a longitudinal axis from the upstream surface to the downstream surface, (b) N protrusions, with N ≥ 2, distributed around a perimeter of the lateral surface, each protrusion comprising a downstream face and an upstream face separated from one another by a thickness, t, of the protrusion, the protrusions having an azimuthal width measured normal to the longitudinal axis an upper frame suitable for being rigidly fixed to a bottom surface of a metallurgic vessel, a locking ring, rigidly fixed to the upper frame and extending along the longitudinal axis from an upstream edge to a downstream edge, and defining an opening defined by an inner surface joining the upstream and downstream edges, characterized in that, the inner surface of the locking ring is provided with a number N of L-shaped channels, each L-shaped channel having: (a) a first channel portion extending along the longitudinal axis from the downstream edge to a first channel end of the first channel portion, and having a width larger than the width of the protrusions, allowing the translation along the longitudinal axis of the inner nozzle through the opening of the locking ring with the upstream surface engaged first on the downstream edge side of the locking ring, with the protrusions engaged in corresponding first channel portions until the protrusions abut against the corresponding first channel ends, where the inner nozzle is prevented from translating further along the longitudinal axis and (b) a second channel portion extending transverse to the longitudinal axis from the first channel end and having a width, W2, larger than the thickness, t, of the protrusions, allowing engagement of the protrusions into the corresponding second channel portions by the rotation of the inner nozzle about the longitudinal axis into a locking position, where the inner nozzle is prevented from being pulled out of the locking ring by the protrusions being engaged in the second channel portion. an inner nozzle comprising: (a) an upstream surface and a downstream surface joined to one another by a lateral surface of nozzle height, h, and comprising a bore extending along a longitudinal axis from the upstream surface to the downstream surface, (b) N protrusions, with N ≥ 2, distributed around a perimeter of the lateral surface, each protrusion comprising a downstream face and an upstream face separated from one another by a thickness, t, of the protrusion, the protrusions having an azimuthal width measured normal to the longitudinal axis (a) an upstream surface and a downstream surface joined to one another by a lateral surface of nozzle height, h, and comprising a bore extending along a longitudinal axis from the upstream surface to the downstream surface, (b) N protrusions, with N ≥ 2, distributed around a perimeter of the lateral surface, each protrusion comprising a downstream face and an upstream face separated from one another by a thickness, t, of the protrusion, the protrusions having an azimuthal width measured normal to the longitudinal axis an upper frame suitable for being rigidly fixed to a bottom surface of a metallurgic vessel, a locking ring, rigidly fixed to the upper frame and extending along the longitudinal axis from an upstream edge to a downstream edge, and defining an opening defined by an inner surface joining the upstream and downstream edges, (a) a first channel portion extending along the longitudinal axis from the downstream edge to a first channel end of the first channel portion, and having a width larger than the width of the protrusions, allowing the translation along the longitudinal axis of the inner nozzle through the opening of the locking ring with the upstream surface engaged first on the downstream edge side of the locking ring, with the protrusions engaged in corresponding first channel portions until the protrusions abut against the corresponding first channel ends, where the inner nozzle is prevented from translating further along the longitudinal axis and (b) a second channel portion extending transverse to the longitudinal axis from the first channel end and having a width, W2, larger than the thickness, t, of the protrusions, allowing engagement of the protrusions into the corresponding second channel portions by the rotation of the inner nozzle about the longitudinal axis into a locking position, where the inner nozzle is prevented from being pulled out of the locking ring by the protrusions being engaged in the second channel portion.