55Si7 -- standard Si spring steel. Rm 1200-1600 MPa in service. The classic flat spring steel for automotive leaf springs, disc springs and flat spring elements.
55Si7 (EN 1.0904) per EN 10089. Si (1.5-2.0%) solid-solution strengthening without alloying gives the required elastic limit in heat-treated condition (Rm 1200-1600 MPa). Hardenability limited (~20 mm). Standard for automotive leaf springs (traditional), Belleville disc springs, flat spring elements, retaining rings and railway fishplate springs. Si reduces decarburisation sensitivity vs plain C steels.
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