Flat iron steak can be stored in the refrigerator for up to four days. Once cooled to room temperature, wrap your flat iron steak tightly in plastic wrap, or store it in an airtight container. If you want to store it longer than that, you can freeze flat iron steak for up to three months. Once ready to use, thaw it in the refrigerator overnight or by using the microwave - just be sure to cook it right away if you defrost it in the microwave.