Although Cast Iron is very useful and is quite tough this also makes it a more brittle form of Iron. As well as having a fairly low Modulus of elasticity (or Young's Modulus), and therefore it does not have the required capabilities that would make it suitable for a car spring
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I think steel Because it is versatile Building material
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Using my rational, if a metal can be welded they probably make a wire for it. Cast Iron is the only one I would question.
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