Two complementary techniques are mainly used for the detection and characterisation of the inclusions populations present in the steel bars in a large range of sizes: from 1 μm to 1 mm: the automatic scanning electron microscope associated with image analysis and the 80 MHz ultrasonic inspection. The emphasis has been put on the composition of the inclusions whose size is above the critical size (10 - 20 μm) for long life fatigue. The mechanism of formation of these harmful inclusions is determined through the process with the help of thermodynamics models. This approach makes it possible to improve the process: avoiding slag entrapment upon casting of bearing steels, adjusting closely the Ca addition to calcium treated steels.