literally made a batch of stainless steel yesterday. If you right click a stainless steel ingot in NEI and cycle through you’ll see the recipe under combination smelting. It takes 4 wrought iron ingots, 3 invar ingots, 1 chromium ingot and 1 manganese ingot.
Do make sure you are using wrought iron, iron heating to 2011K, and as Greg mentioned use pre-made invar (2 wrought iron and 1 nickel) otherwise you’ll end up with nichrome as the molten nickel will mix with the molten chromium before the iron has time to melt.