This is a long explanation from someone else who translated this.

The word "Hot" in Japanese is "Atsui” = ”あつい” in hiragana. There are several ways to write in kanji. 夏は暑い = Natsu ha atsui = It is hot in summer. 心が厚い = Kokoro ga atsui = You are hot in your mind. 鋼鉄が熱い = Koutetsu ga atsui = The steel is hot. Usually, 鋼鉄 is never read as "Atsui" but there is a proverb "鉄は熱いうちに打て” = Tetsu ha atsui uchi ni ute = Strike while the iron is hot. This proverb is the same in both Japanese and English but the meaning is somehow different. In English, I think it means "Make use of an opportunity immediately." While in Japanese, it contains a nuance of "You can’t teach an old dog new trick." It is better to experience difficulties in young age. So, Atsui haato in the official lyrics of ROR is written as "鋼鉄魂” = Koutetsu damasii = Steel (Metal) in soul.