"Strong as steel" is a commonly used phrase to describe something that is physically or emotionally resilient. However, there are numerous other synonyms that can be used to achieve the same effect. Some alternatives include "firm as rock," "unbreakable," "sturdy," and "powerful." Similarly, you could use "tough as nails," "durable," "resistant," or "unyielding." Each of these phrases conveys a sense of unwavering strength and toughness, making them relevant in a variety of situations. Whether you're describing a person's character or the durability of an object, using these synonyms can add depth and nuance to your language.