Opposites of the verb phrase "turn on steam" can vary depending on the context. In a literal sense, antonyms for this phrase could include "turn off steam," "shut down steam," or "stop steam." However, if we think about "turn on steam" in a figurative manner, we can consider alternative phrases to convey an opposite meaning. For example, "calm down," "relax," or "cool off" could serve as antonyms for "turn on steam" in the sense of becoming upset, angry, or agitated. Choosing the appropriate antonym will depend on the context and intended meaning of the original phrase.