Can I use a laminated silicon steel transformer core as a low frequency boost converter core?
No, for at least two reasons.  The reasons that I know are:
Eddy Current
You mention that the core is designed to operate at mains frequencies.  Inductors that use laminations of steel are subject to a phenomenon called &quot;eddy current&quot;; basically, the thickness of the steel needs to be sized to the intended frequency of operation.  If you look at old audio transformers, you'll see that their laminations are thinner than those seen in power transformers.
Core Saturation
A boost converter works by storing energy in its inductor during part of the cycle, and then releasing it into the output during another part of the cycle.  A transformer can be designed so that the core doesn't need to store energy -- it just makes sure that energy going into the primary comes out the secondary.  Trying to use a transformer for your inductor core will saturate the core (&quot;magnitize it too much&quot;).
Inductors for boost converters are either designed with outright air gaps (which, if you do the math, is where most of the energy is stored, not in the iron) that prevent this saturation, or they're designed with low-permittivity materials that are much less prone to saturation.  Either way, they're lower inductance for their size than if they used all-steel cores, yet they can store more energy.