noun
iron core; iron will
Literal meaning: the iron core of an electromagnet or transformer. Figurative meaning: an unyielding, determined spirit.
この変圧器の鉄心は高品質のケイ素鋼でできている。
The iron core of this transformer is made of high-quality silicon steel.
彼は鉄心の持ち主で、どんな困難にも負け
He has an iron will and never gives in to any difficulty.