Here is our spec, sorry the formatting does not copy:
3.3.2 Annealing
Temperature, time, atmosphere and rate of cooling shall be as
specified in Sections 3.3.2.1 through 3.3.2.3.
3.3.2.1 Annealing Temperature and Time
The parts shall be heated to the temperature range in
the chart below and held at the selected temperature
for 2 to 4 hours.
Silicon content Annealing Temperature
Less than 1% 788-843°C (1450-1550°F)
1% or more 843-1000°C (1550-1830°F)
Note: Furnace control thermocouple(s) may be used
for the temperature and time measurements provided
sufficient allowance is made to ensure all the parts in
the load reach the selected temperature for the
specified time.
3.3.2.2 Atmosphere
A dry hydrogen or vacuum atmosphere shall be used
during the heating and soaking periods of the cycle.
If a vacuum atmosphere is used, of 1X10-3 millimeters
(1 micron) of mercury or less shall be maintained
throughout the soaking period.
3.3.2.3 Cooling Rate
Furnace cool to 425°C (800°F) at a rate not
exceeding 56°C (100°F) per hour. Below 425°C
(800°F) cooling rate is optional. Nitrogen with a dew
point of -50°C (-60°F) or lower can be used to
assist cooling. Below 180°C (356°F) air cooling is
permissible.
I recommend the dry hydrogen atmosphere if you want a bright, scale free finish. If you want maximum magnetic properties be sure to get "Solenoid Quality" material, not generic 430.