Metallurgy of plastic inclusions to improve fatigue life of engineering steels

This paper summarises an ECSC project (1997-2000) involving as partners Sidenor I + D, Saarstahl AG, voestalpine Stahl Donawitz GmbH, Haldex Garphyttan Wire AB, and Innotec GmbH. Modification of the inclusion type, aiming for deformable non-metallic inclusions, which are less hazardous for machining and fatigue behaviour of engineering steels, has been the focus of the research work. This leads to the use of a Si killed route and control of the secondary steelmaking process to place the inclusion composition on the zone of the low melting point of the quaternary thermodynamical equilibrium MgO-CaO-SiO₂-Al₂O₃. To verify the suitability of the process modifications (acid slags, low basicity, reduction of Al and Ti contents to be as low as possible) wire rod cleanness analysis and fatigue tests (Nakamura) were performed on a typical spring steel grade. Fatigue test results were promising, finding no inclusion as a cause of failure in several tested heats, which fulfils the usual cleanness and fatigue requirements. As a consequence, some general rules are provided to allow steelmakers to manufacture high quality engineering steels, guaranteeing high fatigue life and performance to the automotive industry. More improvements still should be developed to obtain a more competitive and reliable product.