No, steel tiffins should not be used in a microwave, even with adapters. Microwaves use electromagnetic radiation that reflects off metal, potentially damaging the microwave and creating sparks.
Microwave ovens work by emitting microwaves, a form of electromagnetic radiation, which excites water molecules in food, causing them to heat up. Metal, like steel, reflects these microwaves. When microwaves are reflected, they can create electrical arcs (sparks) and potentially damage the microwave's magnetron, the component that generates the microwaves. Furthermore, the metal container itself can become extremely hot, posing a burn risk. Adapters do not change the fundamental interaction between microwaves and metal; they simply provide a different physical interface. Therefore, using a steel tiffin in a microwave, even with an adapter, is unsafe and can damage your appliance. Always use microwave-safe containers made of glass, ceramic, or microwave-safe plastic.
Before using any container in the microwave, check for the microwave-safe symbol, which usually looks like a microwave oven with wavy lines inside. If you're unsure, err on the side of caution and use a container you know is microwave-safe.