No, it is not recommended to heat a stainless steel lunch box in a microwave oven. Stainless steel is a metal, and metal objects should not be placed inside microwave ovens. When exposed to microwaves, metal can cause sparks and potentially damage the microwave oven. Additionally, stainless steel does not heat up as effectively as microwave-safe materials such as glass or ceramics, so it may not heat the food evenly or as quickly. It is best to use microwave-safe containers specifically designed for heating food in the microwave.