Brown, as in oxidation (there should be yellow/gold and blue as well in that case),
or brown coming from soot, deposits, dirt/debris?

If oxidation, consider your tank no longer stainless.
Is this a one-time event, or will it have an in-service temperature that high?

one-time event (firing, burning. Because of my misunderstaning in english, colors yellow/gold and blue are typical of firing or only oxidation ? Thks

Do you mean this type of fire?

Or was this a heat treatment?
If it was an uncontrolled fire then you have a lot of questions to answer.
This type of damage assessment is done by specialized professionals.
If it was a controlled heat treatment then you know the time and temp, so you know the impact on properties.
But the oxide scale has left the surfaces not corrosion resistant.
The only fix is to pickle with acid to remove it.
And you hire professionals for this also.

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P.E. Metallurgy, consulting work welcomed

Mint & Ed : yes, uncontrolled fire. So I will do as suggested by Ed. Thks

This will involve a bunch of physical measurements and cutting of coupons out of the tank.
The biggest risks with SS are sensitization and oxidation.
Sensitization happens when you expose the material to a temperature in the range of 500C-800C.
If you have formed grain boundary carbides it is scrap, the corrosion resistance is shot.
IF the only issue (unlikely) is surface oxidation then you can pickle it to clean it.
But it is difficult as you have to handle a mixture of Nitric and HF acids.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed