Originally Posted by Roland_M
As far as I know, the only monosteel in the past up to the 19. ct was wootz. Because they never reached the temperatures of ~1600°C. Wootz was made and forged at much lower temperatures. I'm quite sure, that medieval european blacksmiths never made blades from wootz, because the forging of wootz (forged at ~750°C) is totally different compered to laminated (refined) or pattern welded steel (~1300°C).
The forging of wootz is different not because it is wootz, but because the carbon content is so high. If you made ultra high carbon steel in a bloomery, or by decarburising cast iron, you'd have to forge it in the same way as wootz.
Given the carbon content of the high-carbon Ulfberht swords, and the fact that these swords were forged, the methods for forging ultra high carbon steel were known where the Ulfberht swords were made. Whoever made them could have made them from crucible steel.
They had ultra high carbon steel available. If made in Europe, the technology was lost. If imported, then it stopped being imported. We know there was extensive trade with the East, e.g., through the Khazar Khanate, which also traded with Central Asia, which exported crucible steel. We know those trade links were disrupted. So imported crucible steel (probably Central Asian rather than Indian) as the ultra high carbon steel used for the high-carbon Ulfberht swords appears quite plausible. In my opinion, more plausible than an unknown and then lost local technology.