Cobbles: steel mill mishaps where hot steel reels out like a nightmare light saber


Originally published at:


Safety rules are just one more example of government overreach. Back in my day we used to catch those babies with our teeth.


That looks like a place where faster reacting process control would be a good idea.


I guess part of the trouble is that the steel is under some sort of pressure/momentum?


The linked article is worth reading.

Apparently once it starts happening there is sod all one can do to stop it.

A cobble will occur when there is a roller malfunction, the line of steel deviates from the roller path or, as mentioned above, the end of the steel splits. All of a sudden the continuous roll of steel will come to an abrupt halt although the steel behind is still being pushed through the working rollers at speeds which can reach up to 30 mph. The steel at the front of the line has nowhere to go, the pressure builds very quickly, the material begins to coil up and then all of a sudden it will flick into the air creating enormous loops which have been likened to a “light sabre”.

Although it’s not clear to me whether that is just saying there is nothing one can do about it that doesn’t run the risk of scrapping the machinery, so we prefer to run the constant risk of “numerous accidents and unfortunately a steel cobble will pass through any part of the human body like a knife through butter.”

That would be a very hot knife through very soft butter rather than the sort of butter we got at breakfast when I was at school.


The very last video i had seen during a safety meeting here at work, pretty terrifying stuff




You need a wizard


I’m surprised I’m only hearing about this now. I’d expect a dozen elaborate and only slightly unrealistic Hollywood movies about this already.


Me too. Especially since BB has posted about before. Maybe you aren’t a REAL Boinger?

It’s an imposter! A mole! I bet he’s a dirty Farker!


…Huh. Must have been out that day. That one’s possibly even splashier than the others in this post:


It’s not just risk of damaging machinery. If you start enclosing more of the system you trade the risk of a cobble for pressure related risks. If you think a cobble looks dangerous imagine the same material + a steam explosion.


You’d think though, with modern process control and sensing, the moment a deviation from normal pops up, it could trigger the whole line to shut down and stop pushing the molten steel bar along.

Sure there’s momentum, but, wouldn’t stopping the whole line put a stop to that pretty quick?


Maybe, but then you have to figure out what to do with a potentially 2km long piece of steel inside a machine at 900 C. If you slow any part of it at a different speed, you create a cobble somewhere else. Most aren’t this dramatic. If it were an easy switch the industry would have done it, not out of the goodness of their heart, but they lose the entire run if they have a cobble, in addition to possible machine and human damage.




To complicate issues, it’s not even all moving at the same speed. It’s moving quite slowly at the input side, but as each rolling operations makes it longer, the speed increases with every step. Quite impressive to watch in person – from the safety of a raised viewing gallery.


As I pointed out in the comments to the other article, this is just one of many hazards in steel mills. I appreciate the memories I have of my brief stretches in these mills, but I was young and thought I was immortal, you couldn’t get me back inside one today for anything in the world.


This. I’ve had enough fun accidents with hot small pieces of steel out of the forge, onto the floor, once onto a foot (with shoe luckily). I’m glad that steel mills exist, and I find them fascinating. I’m also glad I don’t have to be working in one.


Yup. There’s a reason why Terminator movies always end with a steel mill. Dangerous even for killer robots from the future.