The first time I encountered any mention of Stelco’s coilbox process was when prof. Dilewijns asked me to investigate the literature for a process of that name. Now this was way before the days of Google and even the Internet, and the best you could to was to go through the annual summaries of the iron & steel magazines we had in our library. I must admit that I was unsuccessful, but prof. Dilewijns informed me at a later stage that he had managed to unearth some info on the topic.
At the time I think there were only three coilboxes installed in the whole world (from what I read the demand has since picked up, mainly in Western Europe) : one at Stelco, one at Port Talbot, and I can’t remember where the third one was (it may have been a second Stelco plant). That’s as far as the topic went – that is, until I returned to the UK and joined Tinplate R&D in Port Talbot of all places.
At British Steel Tinplate we were only indirectly exposed to the coilbox process, since it made it easier to control scale in the finishing train of the hot strip mill. That’s why at times Llanwern was excluded as a supplier of hot rolled coil to Ebbw Vale, since their Type-D scale was linked with the occurrence of pinholes in the final tinplate material, a problem we never had with feedstock from Port Talbot.
There was again a gap of several years until I started extending use of the traffic light system to Port Talbot’s hot mill. Although I didn’t personally work on the coilbox, I became aware of some of the background to its installation in Port Talbot. In short it was lack of space – I don’t know the full story behind it, but presumably it all started with the charging of longer slabs when the reheat furnaces were last rebuilt. This led to the problem that when the slab had been rolled to intermediate bar in the roughing mill, its length was greater than the distance between the roughing mill and the finishing mill. To counter this problem, the roughing mill was converted from a multi-stand to a single stand reversing mill, and a coilbox was built to receive the semi-finished bar prior to final rolling.
Whether quality entered the picture when making a decision I don’t know, but because the coiled bar showed a more uniform temperature throughout, and there was less of a heat loss when the coiled bar resided in the coilbox, the rolling forces in the finishing train were reduced. On top of that, control of scale build-up in the coil shape was superior to that of a straight bar waiting to enter the first finishing stand, so even if this was not part of the initial reason to install the coilbox, it was a nice to have.
Still, if the process is that good, why isn’t it more common ? After all, if it produced a noticeably superior product, you’d have thought that every mill would be converting to have a coilbox. All I know is what I heard on the grapevine, which is that if the coilbox behaves like a dog, the whole mill behaves like one too.
Whatever the case may be, during the recent rebuild of the hot strip mill there were no plans to get rid of the coilbox. Meaning that whatever the reasons were for installing it in the first place must still hold true today.
For some background reading on the subject, have a look at this : Coilbox technology improves steel quality