If someone is described as having a "heart of steel," it means that he or she is not swayed by any emotion. This is an English idiom used to describe people who prefer to let their judgment be based on facts or other data rather than anything that might sway their hearts. The phrase also implies that the person who is described in this manner can be cold and tactless in his or her dealings with other people. "Heart of steel" is one of many idioms that use the heart's symbolic place as the location of all human feelings as the basis for its meaning.
There are times when a person may use a word or a short phrase with the understanding that what is said is meant to be taken symbolically, rather than literally. These words or phrases, also known as idioms, may actually have intended meanings that are far different from what their words might literally denote. After they are used in a certain culture for a period of time, their meaning evolves until it is understood by all within the culture. One of many idioms derived from the workings of the human heart is the phrase "heart of steel."
This particular idiomatic expression implies that the person in question is not about to be moved by any emotional pleas on a subject. By contrast, this person will instead disregard such pleas and instead use more objective means to make an important decision. As an example, someone might say, "Sometimes, when you're a boss, it's better to have a heart of steel and not get too wrapped up in how your employees feel."
Since it is likely that someone acting in this manner isn't likely to ever get too emotional, the phrase can often be used when someone is considered cold toward other people. When used in this manner, the phrase can even have a negative implication. It can be used in such a manner that the person in question might seem mean or nasty. For example, consider the sentence, "He must have a heart of steel if he wasn't moved at all by the plea of those poor people for leniency."
Many idioms are based on how the human heart is perceived as the figurative home of emotions and feelings. The material used to describe the heart is a good indicator of what each of those idioms means. In this case, steel is hard to dent or move. That immovability is transferred to anyone who has a "heart of steel."