Steel Mill Tour


 

Balcones Forge Tours CMC's Steel Mill

On the morning of August 25, 2007, about 30 Balcones Forge members assembled in the parking lot of CMC's steel mill near Sequin Texas. We were greeted by several CMC personnel including Mike Osteen of the maintenance department. After a brief introductory video, we donned hard hats and Tyvek jackets and set out into the plant in groups of 10.

This mill runs entirely on scrap. Near the mill itself is a huge shredder that can turn an entire automobile into small pieces of metal in a few minutes. Those of you who've been in Austin for decades may remember the car shredder at Newell Salvage where the CMC salvage yard is now on St. Elmo. The one at Seguin is considerably bigger than the old Newell shredder. It has an 8,000 hp drive motor.

The first stop on the tour was the Melt Shop:

 

This hellish monster is an 80 MVA (i.e. 80,000,000 VA) electric arc furnace. This thing was absolutely incredible. The sound was a deafening roar and the floor was vibrating intensely. Three 24" diameter carbon rods establish three arcs to the metal inside the furnace quickly turning it into a molten pool. Our tour group scurried past this beast during a moment of relatively quiet operation and crowded into a nearby control room that was cool and relatively quiet.


In the control room we talked with the two very knowledgeable operators on duty. They said the furnace was running at about 70 megawatts just then. That's about 30,000 times the typical power (100 amps, 25 volts) used when stick welding...! The control room was quite modern with sophisticated computer displays showing all the critical furnace parameters. While we were standing around asking trivial questions, great gouts of flame started belching out of the furnace outside. The operators paid no attention to this spectacle so I timidly asked what was going on. They explained that it was just a "scrap cave". Apparently, the big carbon electrodes melt their way down into the charge of loose scrap and create a pool of molten steel at the bottom of the furnace. This often leaves big clumps of scrap hanging on the walls of the furnace and, when they eventually cave in to the molten pool, you get lots of fireworks as the inevitable organic contaminants (oil, grease, paint, etc.) burst violently into flame.

Here's a close-up of one of the big carbon electrodes descending into the scrap charge with arc blazing.

After the scrap is melted in this furnace, the molten steel is taken to a lower power arc furnace where it is held while the composition of the steel is analyzed and adjusted to meet specification:

Here you see a brave soul tossing a bag of sand into this holding furnace, presumably to increase the silicon content of the steel. They employ electromagnetic stirring to blend these additions into the 100+ tons of molten steel. In the control room for this operation, the CRT monitors wavered ominously when this stirrer was operating. They also carefully adjust the temperature of the molten steel at this station so that things will work properly at the next station...the continuous caster.


At the continuous caster, the molten steel is poured into an insulated head vessel called a tundish. From there it drains out in several parallel streams into the continuous casters. The casters are simple in concept. Molten steel pours into the top of a tapered water-cooled copper jacket and barely solidified steel bar slides out the bottom. Sounds like a tricky balance, doesn't it?

The barely solidified bars, typically 5" or 6" square ooze straight down out of the casters and then bend 90° in a graceful curve until they are lying flat on a roller conveyor.

As the bars move inexorably along (remember, it's continuous casting), large cutting torches wait until the desired length/weight has passed by. Then they begin moving along with the bar while cutting across it to make a square cut.

Here's one of these big torches in action. The incandescent slag spraying out of the cut fell into a pit whose bottom was continuously washed with water. Twice while we were watching, a big slug hit the water below and exploded upwards in a shower of small sparks that went everywhere including onto us. Just like torch cutting at home....only more so.

Freshly cut ends emerge from the continuous casting operation. From here they go onto a cooling rack where they are rotated 1/4 turn periodically while cooling to a black heat. At that point one of the many overhead cranes in this facility picks up the finished bars and takes them to one of two places. If the rolling mill is operating and the caster is making the right feed stock, they will "hot charge" the rolling mill with freshly cast bars. If the rolling mill is down, as it was on the day of our tour, they stockpile the finished bars. Some of the steel bars they make at CMC are fed into their own rolling mill. The rest of the stuff they make is shipped directly to customers who have their own subsequent forming equipment (e.g. drop forging).

The warehouse for finished bars is quite a sight. These bars are at probably about 30 feet long. The smallest are 4" square and the largest are 6" square. Check out the bone pile in the foreground. Just in front of that pile was a fellow with a huge cutting torch manually cutting off too-bent ends from some of the bars. Several times during the tour, I saw "scraps" that looked ideal for various purposes around my shop. Each time I had to remind myself....there's really no such thing as "scrap" at CMC. Everything that doesn't ship out as finished product goes back into the first furnace again.

It is hard to appreciate the size of this pile of steel. If you look closely at the center post, you'll see a broom leaning up against it. That gives you some idea of the scale. The pile is about 20 feet tall and 30 feet wide and probably 30 feet deep. As you can see the packing fraction is high, probably about 80%. That's around 3500 tons of steel. Mike Osteen estimated that it had taken them 2 or 3 days to make this pile. It was still very warm, you could feel the heat from 10 feet away.

Here you can see both the raw feedstock and the finished product.

Unfortunately the rolling mill was shut down on the day of our tour. However, we did get to see it up close. There are 17 stations which shape and reduce the steel as it passes through. So much work is done on the steel mill that, despite water sprays at every station, it emerges a bit hotter than it entered. At CMC, they pioneered the usage of a slitter to make several small bars at once.

 

This photo shows the final rolls used in making rebar. These rolls "print" the familiar ribs on the bar. Most of the rolls in the mill are made of steel. However, some of the finishing rolls are solid tungsten carbide...about $60,000 a set.

The last part of the tour was the big machine shop where they are constantly rebuilding parts of this mill. They have an automatic submerged arc welder that builds up worn-out rollers so they can then be machined to original size.

CMC is a marvelous place. Despite some really hazardous operations, they have a very good safety record. In my opinion, a good part of that record is the result of the obvious cross-training that goes on there. Everybody we met seemed to know all about the mill operations. That's the kind of group awareness that results in quick and appropriate corrective action when something starts to go wrong.