The straight answer: no, you should not put a stainless steel container in the microwave. Metal reflects microwaves instead of letting them through, which stops your food heating and can cause sparking. This is the one honest limitation of steel, and we’d rather state it plainly than bury it.
Why not. A microwave heats by penetrating food with waves. Metal blocks them. At best your food stays cold; at worst, thin or damaged metal can arc against the oven wall.
What to do instead — it’s simple. Tip your food onto a plate or into a bowl and heat that. Ten seconds of habit. This is how most people reheat a steel-stored lunch and never think about it again.
Other ways to warm food without a microwave. A stovetop pan, an oven (some steel containers are oven-safe — check first), or letting a portion come to room temperature all work. If you specifically want a container you can heat in, glass is the material to choose for that job.
What steel is better at. Everything else: it won’t warp, won’t leach, won’t stain, keeps food colder for longer, and lasts for years. The microwave is the single thing it gives up.
So the real question isn’t "is steel microwave-safe" — it’s "is plating my food to reheat a fair price for a container that never wears out or leaches?" For most people, easily yes.
Food should be stored without doubt.