According to Indian standard specifications, SG 400/15 means

S. G. Iron (Spheroidal Graphite Iron) is a type of cast iron that has been treated while molten with an element such as magnesium or cerium to induce the formation of free graphite as nodules or spherulites. This imparts a measurable degree of ductility (easily manipulated) to the cast metal.
According to Indian standard specifications, SG 400/15 means Spheroidal graphite cast iron with minimum tensile strength 400 MPa and 15 percent elongation.