[I'm assuming this is a Mekanism problem because all other mods that have steel are recognized by Tinkers' as far as I can tell]

Mekanism Steel Ingots are registered to the ore dictionary, but oddly enough Tinkers' Construct doesn't recognize it.

Using latest (CurseForge releases, not GitHub) Tinkers' Construct and Mekanism for 1.11.2, but according to this thread it is also a problem in 1.10.2. http://www.minecraftforum.net/forums/mapping-and-modding/minecraft-mods/mods-discussion/2800566-no-steel-in-tinkers-construct

@aidancbrady Although you are correct that steel is registered in the oredict, it is registered too late: you are registering them in init, whereas they are supposed to be registered in preInit. This is causing problems with other mods which depend on the oredicts, such as Tinkers Construct.

The same problem occurs with your copper, tin, and bronze.

(I discovered this problem early on when working on my addon mod PlusTiC and adding Mekanism integration, and had to manually register the metals as a workaround.)

Steel is definitely registered, make sure another mod isn't changing recipes.

By the look of @Landmaster 's comment, it's also responsible for #4288

Created pull request #4465 to fix this issue.