Why is SG Iron so strong?

Cast iron is an alloy of iron, carbon and silicon with carbon levels of about 3.2% and silicon at about 2.2%. If molten cast iron is allowed to cool normally the carbon forms flakes of graphite which run through the iron matrix, hence the term flake graphite iron. These flakes are at the microscopic level, the ends of which form stress points in the cast iron. If cast iron is subject to a compressive load these stress points are not particularly detrimental and flake graphite cast iron is excellent under compressive load. However, tensile loading above the natural tensile strength of the cast iron can cause rapid tensile failure as cracks propagate rapidly out from these stress points.

The result of this is that cast iron has virtually no elongation,
is a brittle material and is therefore limited in its use in tensile
and shock loading applications. For years foundrymen and
metallurgists tried to develop a new type of cast iron that
would withstand bending and shock loading and would have
the characteristics more of malleable cast iron but could be
produced at the lower cost of grey cast iron.