Yes, but too much heat can cause damage. However, there are ways to minimize the damage to your curls. Before reaching for your flat iron, make sure to:
Shield your hair from the heat with a heat protectant spray
Start with a low heat setting and gradually increase it if necessary
Use a flat iron with ceramic or titanium plates, as these materials are less damaging to the hair
Add moisture back to your hair after flat ironing with a hair oil or serum
Avoid flat ironing your hair too often as it can cause damage to the cuticles and make the hair dry and brittle over time
After washing, your curly hair will return to it's naturally curly state. However, the curls may loosen over time with repeated use of too much heat.. Which can affect the texture over the long-term.