Important Translation Notes:
Also, just fyi, 鉄の心 is mistranslated as "Mind of Steel", but more accurately, it's "Heart of Iron".
鉄 is Iron, not Steel. Likewise, 鋼 is Steel, not Iron.
"心" is Heart (though you could argue otherwise, but, for the most part, it means "Heart" in both the literal and metaphorical sense in Japanese, similar to how it is in English).
Going along with the mistranslation, though,
鉄の心, 鋼の意志 = Mind of Steel, Heart of Iron
But more accurately,
鉄の心, 鋼の意志 = Heart of Iron, Will of Steel
(Also, though a person's will is more strongly associated with their mind in English, note that in Japanese, a person's will is more strongly associated with their heart.)
The word for "will" (意志) in Japanese, literally has the word for "heart" (心) as it's bottom half in both characters.
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So, the True Title is more like:
Heart of Iron, Will of Steel (鉄の心, 鋼の意志)
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Symbolically, it's also a very fitting title, since he forges his Will of Steel out of his Heart of Iron, creating his Steel Will (Sword) out of his Iron Heart (Unlimited Blade Works, his Internal World and his Heart's blacksmith's forge).
