I'm looking for the viscosity of molten steel, preferably as a function of temperature. Any help appreciated.

Look for Smithell's Metals Reference book.

Also, look for papers by B G Thomas, et al. on simulations of steel casting, etc. As is frequently the case, someone else has done legwork that you can put to good use.

Since turbulence is likely in many situations of interest for molten steel processing, what imprecision you suffer in the viscosity won't be as influential as the uncertainty in your turbulence model, which will also serve to smother the difficulty you'll have in obtaining values for the thermal conductivity of molten steel, too.

Good luck.