Older knives marked as 'Queen Steel' were 440C, according to a couple of collecting guides I've looked at. From very early on, in fact, going back to the mid-40s or so, maybe even earlier. The very earliest of them were actually marked as stainless, but buyers/users didn't like that, because stainless didn't have a good rep back then. Hence the re-labelling to 'Queen Steel', without actually changing the steel itself.
440C was 'Queen Steel' right up until the switch to D2 around 2002, I believe. The D2 knives were marked & boxed as having 'Tool Steel' blades from there on. There are some current-generation knives from Queen that are still marked as 'Queen Steel', and I believe those are in a 420-variety (either 420HC or 420J2; I've seen vendor descriptions listing either type). One of those knives is the fillet knife in Queen's very popular Amber Carved Stag Bone (ACSB) line. There might be another model or two, but I'm not sure.
The above pertains to the 'Queen Cutlery Co.' line, and not to the current line from the 'Queen City' brand, which are all carbon steel (1095).