”My heart steeled and softened at the same time.” is an unusual use of ”steel”. I have never encountered this use before and I suggest you don't use it in a similar way. As a verb ”steel” is normally used with the meaning ”to mentally prepare (oneself) to do or face something difficult: ‘his team were steeling themselves for disappointment’ ‘she steeled herself to remain calm’ source: https://en.oxforddictionaries.com/definition/steel ”My heart steeled hard as coal.” sounds very odd to me, because you are comparing a heart to steel and then comparing steel to coal in the same sentence.