I have broken four steel frames, but two of them don't really count, because they had been straightened following front-end bend-back-the-head-tube collisions.
1960 Capo Modell Campagnolo (db Reynolds 531): front of downtube, just behind the butting, where it had been bent and restraightened;
1962 Bianchi Corsa (plain carbon steel): same;
1971 Nishiki Competition (db Ishiwata CrMo): seat tube lug broke off of top of BB shell;
1973 Peugeot UO-8 (plain carbon steel): right chainstay cracked between the tyre clearance and chainring clearance dimples
My other five steel frames (see signature) are still going strong.
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"Far and away the best prize that life offers is the chance to work hard at work worth doing." --Theodore Roosevelt
Capo: 1959 Modell Campagnolo, S/N 40324; 1960 Sieger (2), S/N 42624, 42597
Carlton: 1962 Franco Suisse, S/N K7911
Peugeot: 1970 UO-8, S/N 0010468
Bianchi: 1982 Campione d'Italia, S/N 1.M9914
Schwinn: 1988 Project KOM-10, S/N F804069