is it possible to get "yield stress at ambient temperature" AS the "von misses stress at very high temperature"? (ABAQUS)

I didn't do any thermal analysis but only doing structural analysis.  I am using measured temperature values to my structural model. I created temperature loading with time (Using amplitude method )Temperature dependent plastic perfect material model created.

I  used Nonlinear Static Analysis with Linear Perfect plastic material model. (fy=612 at Temperature=20°C)

The yield stress and elastic modulus values were input with respect to temperature (Using FIELD)

The temperature was input as a boundary condition with respect to time (Using Amplitude)

Then I use Nonlinear Static Analysis to run the file in a time frame (say up to Total time=107).

However, after some analysis time (say Total time= 75min) we had some numerical problem.

Therefore we used STABILZE and the analysis was completed successfully.

Near Failure, (At Total time = 107),

In the joist due to high temperature (T=320°C), the maximum possible fy is 560 at cold flange near failure (Total time=107).

However, I'm getting  von misses of 612. That's the problem.

Specially, I’m getting these values after the STABILIZE is effective. (I mean after Total time=75)

 If i go through the results file von-misses values increases with step time up to ambient yield stress (Fy=612) there after it keeps same

Could you please comment on this? Or please advise me how von misses stress created while we use temperature loading?

I  mean, is it possible to get von misses stress of 612 at very high temperatures?