There are many specialty steel variants, and no doubt at least some of them are patented by Australian firms and therefore cannot be sourced from other places.
Also, the exact composition of the raw materials (any source will leave its trace contaminants) can have an effect on the resulting alloy, and Australian steel mills are going to source their raw materials from other sources than those in other countries, especially they're going to have a larger proportion of raw materials from Australian sources.
So yes, it's quite likely that Australia produces some steel alloys that are unique to its steel mills.
And it's likely that at least some of those alloys are important to US industry in certain applications, else that US industry wouldn't be importing them which no doubt is more expensive than sourcing steel from say China or India, or from the US itself.
Of course you can't prove a negative. It's impossible to prove that none of those patents are being infringed somewhere, anywhere, and leading to another steel mill in say Vietnam producing an alloy that would have identical physical and chemical properties to the point it would be considered the same alloy.
Mind that an alloy is not just defined by its chemical composition but also by its physical characteristics, especially the crystal lattice of the steel (yes, metals have a crystal lattice as well), all of which help define things like tensile strength, brittleness, melting point, electrical and heat conductivity, resistance to corrosion, etc. etc. etc.