Why banded armor (lorica segmentata)? Why not breastplates?

[Tarbicus]

Some claim that the Romans had no idea of steel and that gladii simply had steeled or carburized edges, and others claim that they had some idea of steel even if the process was poorly understood and almost accidental:

When Scipio captured the Spanish swordsmiths and forced them to teach Romans how to make gladii, they could very well have passed on the knowledge of steel. One bit I read was that the Spaniards would bury the iron in the ground and leave it. After time they would return and retrieve the iron after the earth and moisture had eaten away the poorer quality iron, leaving the best quality stuff for the gladii. Could have been steel they were retrieving? And if the Romans followed the practice then ....