You MUST use as special plastic shot cup for steel shot or as mentioned, the barrel will get scored full length...regular modern shot cups won't provide the protection...use shot cups made for steel at places like Ballistics Products, etc.
Since steel is much lighter than lead, you have to use 2 sizes larger steel pellets to get the same weight to carry the distance and get the same penetration...ie: #4 steel in place of #6 lead. Then the problem is that whatever you wanted to use #6's for, the pellet count just dropped significantly by going to the physically larger #4's.
Based on my experience with steel, my recommendation would be to find some Bismuth or Hevi-Shot...you'll still have to use the shot cup but they weigh almost the same as lead so you can usually use the same size shot that you would have used with lead, or worse case only change one size and keep up a better pellet count.
One of the newest non-toxic substitutes is called EcoTungsten/(Niceshot)...it's an exact replacement for lead and can be used in bare bores without any shot cup at all, can be used in old original barrels, etc...the downside is its very expensive, about $28 a pound...but all waterfowl shot shells have become so pricey in the past few years that it’s hardly much different.
For example, if you use 1.5oz shot charges of EcoTungsten for a load, that would be about 10 loads per 16oz pound of EcoTungsten...about $3/per shot...basically about the same as today’s modern high performance turkey or goose loads.
The good news is since its an exact replacement for regular lead shot, you can do all your pattern testing with regular lead, then a couple of shots with EcoTungsten to verify, then go hunting. I bought some EcoTungsten #6's and it actually patterned better than magnum lead #6's in my GM .62cal smoothbore.