My answer used to be the early '90s when Columbus introduced the excellent Nivachrome steel tube sets. However there is still in intrest in and development of steel tubes. I also think at some point people who began riding on carbon will see the light and realize how much better their riding experience could be one a quality steel machine. I would not be surprised to see a steel renaissance in the next few years.
IMHO the big bike companies pushed carbon on everyone simply because it was cheap to produce.
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“One morning you wake up, the girl is gone, the bikes are gone, all that's left behind is a pair of old tires and a tube of tubular glue, all squeezed out"
Sugar "Kane" Kowalczyk