Mould for the production of steelwork semi-finished products by casting a liquid metal within a crystallizer of said mould in which the mould includes a detection system provided with calculation and measuring means for detecting a position of a meniscus or level of said liquid metal within said crystallizer, wherein said detection system comprises at least one transmitter of a detection signal transmitted towards said crystallizer and at least one receiver of at least one received measuring signal dependent on said transmitted detection signal wherein said transmitter is at least one first ultrasonic element configured as a transmitter of a transmission ultrasonic elastic wave and said receiver is at least one second ultrasonic element configured as a receiver of a received ultrasonic elastic wave, said first ultrasonic element being oriented according to a direction of transmission towards a wall of said crystallizer containing said liquid metal, said first ultrasonic element is oriented according to a direction of transmission such that said at least one transmission ultrasonic elastic wave is inclined according to an angle of incidence Ai with respect to said wall of the crystallizer, characterised in that the ultrasonic elements (16, 20) are located in correspondence of a same side of the mould in such a way that said second ultrasonic element is placed in a spaced position with respect to the position of said first ultrasonic element according to a direction essentially parallel with respect to the longitudinal development of said wall of said crystallizer, and further characterised in that said second ultrasonic element is oriented in such a way as to receive at least one portion of said transmission ultrasonic elastic wave which has penetrated and has been transmitted within said wall of said crystallizer in the form of a transverse-mode wave transmitted in the crystallizer moving inside the wall of the crystallizer along a zigzag path defined by the following reflections, said second ultrasonic element being configured and structured to receive the reflected wave of said transverse-mode wave transmitted in the crystallizer, and further characterised in that the measuring means of the detection system are configured to measure the time elapsed between said phase of transmission of said at least one detection signal and said phase of reception of said at least one received measuring signal, the time elapsed between said phase of transmission and said phase of reception corresponding to a time interval required for the ultrasonic waves to travel across the crystallizer between the first ultrasonic element and the second ultrasonic element, the calculation means of the detection system being configured to calculate the position of the meniscus or level of said liquid metal within said crystallizer by means of a correlation relation between the position of a meniscus or level within the crystallizer and the time elapsed between said phase of transmission and said phase of reception, in which the time elapsed between said phase of transmission and said phase of reception depends on the temperature of the crystallizer between the first ultrasonic element and the second ultrasonic element.