The temperature you selected is in the sensitization range that should be avoided. A higher temperature (possibly that for solution treatment, about 1050C) would be preferable with rapid quenching, not for any influence on magnetism but to avoid precipitation of chromium carbides.
You need to get up to solution temperatures in order to remove the cold work which has induced the magnetism. The cooling rate has absolutely no effect upon demagnetization, but as Goahead suggests, your quench rate should be in function of the final application of your SS.
Sensitization will depend on your Carbon level.
You need to heat to solution anneal temp. If the parts have a clean microstructure then 1010C should be enough. If the structure has some secondary phases in it then 1050C should be the minimum.
The critical part of the cooling is how long it takes to get below aprox. 600C
You can find the cooling curves in many texts. If your C is low (say 0.030% or less) then the required cooling time is measured in hours, not minutes.
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Corrosion never sleeps, but it can be managed.
If the finished parts were to be demagnetised, will the high temperature heating and fast cooling cause distortion?As compared to 304 why should 316 shows effects of magnetism?
You could get some distortion.
Often, if such a treatment is required, people will do before the final machining. You may need to blast and pickle in order to clean the parts. This could effect finished surfaces.
In either 304 or 316 this will be a function of the chemistry and prior thermomechanical history. This effect is more pronounced in compositions that have lower Ni (or Ni equivalent).
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Corrosion never sleeps, but it can be managed.
