The box should be made of steel and be securely attached to the framing which has to be fairly robust.

You need a mounting device rated to handle that much weight. An ordinary junction box isn't sufficient, even a steel one.

I recommend a ceiling fan brace. It will include both a sturdy fan-rated box and a supplemental brace that goes between two ceiling joists. It's fairly easy to install in an existing ceiling.

Your local electrical supply shop can provide one. See the example below.

Here is a link that might be useful: Ceiling Fan Brace

Thanks for the info. I'll go with a brace.