I did have one steel bike frame break -- a Sears Free Spirit 3-speed, waaay back in 1974. Bike was 'only' six years old, but I rode it daily year 'round, including Cleveland winters and lots of road salt. You've seen cars of the 70s rust out in only four years in the Rustbelt, right? Well, rust got this one, too. I was diligent in giving it a detailed total rebuild at least once every winter, and poured motor oil through the frame to coat the inside - but it still rusted. Enough that when I hit a snowbank, the toptube separated from the seatpost. The tubing had rusted so thin from the inside that there just wasn't enough strength there. I had it welded, but it failed again just outside of the welded joint just three weeks later.