Two complementary techniques are mainly used for the detectionand characterisation of the inclusions populations present in thesteel bars in a large range of sizes: from 1 μm to 1 mm: the automaticscanning electron microscope associated with image analysisand the 80 MHz ultrasonic inspection. The emphasis has been puton the composition of the inclusions whose size is above the criticalsize (10 - 20 μm) for long life fatigue. The mechanism of formation ofthese harmful inclusions is determined through the process with thehelp of thermodynamics models. This approach makes it possible toimprove the process: avoiding slag entrapment upon casting of bearingsteels, adjusting closely the Ca addition to calcium treatedsteels.