Three distinct layers are present in commercially produced tinplated steels: the top tin layer, the middle Fe-Sn alloy layer, and the bottom steel substrate. The brightness of these steels is inversely proportional to the roughness of the top layer. Substrate steels with recrystallized structure, sharper texture, and cleaner matrix result in flatter Fe-Sn interlayer and subsequently smoother top Sn layer.
Link: https://link.springer.com/article/10.1007%2Fs11661-019-05129-9