Are you using cutting oil on the saw?

I've made many cuts similar to this, and I lube the saw liberally and
continuously with the heavy dark cutting oil I use on my mill for steel.

I also run the saw pretty slow, probably a lot slower than 160 rpm, and go
easy on the feed if you want the blade to last. I pull the blade up every
few seconds to clear out the swarf with an oil brush and re-lube it. I would
guess you're building up too much heat from the combination of no cutting
oil and spinning the saw too fast.

Its also possible that even the "better" bi-metal blade you bought is a
piece of junk from you know where. I only buy hole saws from name brand
suppliers, typically from MSC. I've never had a blade go dull yet, and I
don't use them every day but some of them have lots of cuts in steel on
them.

Good luck-

Paul T.