Steel Mill Question

[emzee6]

Hey guys. Im a new guy here and hope y'all can help out.I work at a steel mill where we cast a slab up to 8 feet wide and then roll it down to as low as 0.187". Then we coil it up into a coil that has a width of 96" and an 'eye' of 30". The OD varies by what the customer wants, and they order in tons. The maximum weight of our coil is 90,000 pounds or 45 tons.

After we coil it up, we set it in the coil yard for a minimum of 48 hours, as we can only further process it, i.e. uncoil it and temper pass, level the material and cut to length, resulting in stacks of flat plate, when the temp of the coil is 250 deg F or lower. The coil come off the hot mill at about 1100 F. So due to natural radiant losses, we lose about 850 F in 48 hours.

Currently, our coil yard is a slag/gravel mix that we can set the coils directly on the ground. We are constantly grading the coil yard due to potholes, etc., so we have decided to pave the yard. I did a bit of homework and see that concrete starts to erode and fine cracks start at about 550 F. So our choices are to insulate the ground where we place the coils or to keep the coils off the ground a certain distance so we dont expose the surface of the crete to 550 F. How do I figure out the distance we need to keep the above described coils off the ground so as not to ruin our new concrete surface?

many many thanks for any responses.

Mark

 

[emzee6]

Possibly wrong forum or not enough information?

 

[racookpe1978]

Does your process require the metal rolls be cooled as quickly as possible, or do you want them to be kept warm for as long as possible?

How are the rolls handled and moved?
How are they supported while waiting above the new concrete?

 

[IRstuff]

Is there air circulation over the concrete? What's the steel look like; is it shiny?

some back of the envelope calculation suggests a separation of 2 meters

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529

 

[emzee6]

The coils are removed from the upcoiler with a large forklift whaich doesn't have a fork but a long 'snout' which goes into the 'eye' of the coil. Again, they are about 1100 degrees at this time. Within 5 minutes, they are placed on the ground in a coil yard, where we allow them to cool to about 250 before we process them into plate. This takes about 48 hours, a little faster in the winter, a little longer in the summer. The coils are not shiny. They are dull grey in color. The only air circulation is whatever the wind is blowing, and we have found that it is not a good idea to lay hot coils next to each other as it slows the cooling rate way down. We would like for them to cool off as quickly as possible but that is not an option because physical characteristics regarding tensile and impact change. We would build coil racks for them to sit on and this is the reason for my request. We need to know how high to build them. And if 2 meters is a real number, we would probably investigate a layer of insulation (kaowool) and some secondary plate on the ground as an insulator to protect the concrete. This would certainly be more expensive, but multiple coils 6 or 7 feet off the ground may be a safety issue. i appreciate all the insight so many thanks.

mark

 

[crshears]

Different question: what's causing the potholes in the first place?

Coils being spun while in contact with the slag/gravel mix? Forklift operators must stop doing that... [I know it's possible to not do that; one of my summer jobs between secondary and post secondary education was forklift operator ]

Forklifts being turned on a dime and gouging out a divot? Again, forklift operators must stop doing that...or if they can't or won't, perhaps using an articulated forklift [the kind with a knuckle joint in the middle] might eliminate the problem...

The surface you already have may well be the best choice available; I don't know that I'd be in a big hurry to go with a solid surface of any kind due to the thermal issues involved...I'm no civil engineer, but I'm wondering if it might be possible to obtain a durable enough surface by adding limestone screenings to fill in the voids, then tamping...

I ditto your safety concerns about elevated racks to support loads that heavy.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[emzee6]

The potholes are caused by exactly the reasons you have described, and it is a constant source of conversation regarding how to minimize the problem and still meet production and shipping. That being said, we will be paving the yard one way or the other. The benefits of keeping the dust down and eliminating the problem of splashing a muddy/pebbly mess during rainstorms on the sides of the coil outweigh the need for constant grading. I am assuming that to determine the need for determining the distance off the ground to prevent the concrete going to hell in a handbasket is some sort of differential equation using the cooling rate of the steel and the ability of the concrete to absorb heat. IRstuff, did you come up with the 2 meters by using the 1100 degrees as a constant instead of it cooling down?

Again, thanks everyone for some insight....

 

[MatthewL]

How about this...

Leave a section of the gravelled area unpaved (call it 10' deep by XXX' long). Drive the forktrucks on the paved areas and park the rolls on the unpaved areas. If necessary, do this in strips, alternating paved with unpaved areas, making sure that the paved areas have enough room for the forktruck to turn in and spear the rolls. Alternately, you can have a smaller unpaved footprint by angling the rolls, dropping maybe 3' off the width (if you use a 45 degree angle) of the unpaved sections. Also, those trucks are going to be HEAVY with the roll and counterweight, make sure you check with the paving firm for how long you need to stay off of the concrete, it's usually at least one week (60% of maximum strength) but you may need to wait 4 weeks (~90% of max strength). Also, paint some "parking" stalls on the surface to try and enforce spacing and to reduce clutter in the yard.

Regards,

Matt

Quality, quantity, cost. Pick two.

 

[crshears]

I'm not a civil engineer, but further to MatthewL's suggestion: I've heard that as well as having concrete thick enough, cutting stress-relief grooves, and perhaps adding re-bar, it can be very, very important to keep the surface of the concrete somewhere on the damp to moist to almost wet continuum for as long as possible, since true 'curing' ceases once the surface of the concrete dries.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[FredRosse]

This transient heat transfer problem is very very difficult without a proper analysis tool. The GOTHIC Code would be what I would use, available from EPRI.

 

[racookpe1978]

Don't place the 90,000 lb rolls directly on the ground, but rather, raise then up off the ground (the new concrete) on the cheaper of salvaged railroad rail, W6 or W8 laid on the concrete. (W6 will be likely be least expensive.)

If I understand your description, then the length (axial distance) of each coil is 8 feet, right? Use either three or four rails per coil, with the fork lift approaching each set of 3x rails from the side - perpendicular to the set of rails.

Intent is to raise the coil up off the concrete so there is no conduction of heat from the coil to the concrete, and so the coil is not damaged (scarred, scratched, dented or dirty, contaminated) by laying on the rock as it is now, but only limited areas of each coil touches the top of the 3 or 4 rails. (Check your collapse or crushing load, you only need enough rails to avoid physically damaging the outside coil layer.)

If the coil is raised, conduction is stopped (except coil-to-rail, but that has little surface area) and convection is increased under and around the coil into the air. (Assume each coil is at least 6 inches from its neighbor in the row of coils)

So, that leaves radiation as the only possible damaging heat transfer from coil outside layers to the concrete. Since the rails are supporting the 9potential) 45 tons of steel, you don't need any fancy (expensive!) barrier to stop the radiation transfer, merely a LOOSE and LIGHTWEIGHT reflective surface between the coil and the concrete. Loose is essential so the reflector can be cooled by air flow, lightweight means there is little heat transfer conductive contact between the reflective surface and the concrete below.

It won't last, but imagine a folded sheet of reflective sheetmetal between each rail, above the concrete and below the coil. The top layer of sheetmetal reflects back the heat, the bottom layer of sheetmetal separates the top layer from the concrete, the air space between the two allows cooling air to flow and cool both. Over time, the upper sheetmetal will get dirty and corroded 9darker) and so become less effective, but that may be acceptable, or may require changing every three years.

Once raised up, other forced air cooling methods may be used as well to speed up the coil cooldown rate.

 

[corus]

As a rough calculation: If you think of the ground as being perfectly insulated, ie. it has no heat loss anywhere, then if all it 'sees' is the coil then the temperature of the ground will be that of the coil in steady state. If the coil is a distance away, however, then the ground will 'see' the coil and also the ambient surroundings. The temperature of the ground will therefore be proportionate to what it 'sees' of the coil, and what it sees of the surroundings assuming they have the same emissivity. If you only consider the patch of ground under the coil and consider the shape of the coil that the ground sees as rectangular then the view factor that the ground 'sees' of the coil can be calculated from expressions on this site

http://webserver.dmt.upm.es/~isidoro/tc3/Radiation View factors.pdf

If at a certain distance away, the view factor to the coil is 0.5, say, then the view factor to the surroundings will be (1-0.5)= 0.5 and so the temperature of the ground will be 0.5*1100F+0.5*68F = 584 F. You can calculate the actual distance required from the expressions on the web site given to give a more definitive answer.
Note that this excludes convection to air circulating around the coil and transient effects and probably half a dozen other things, but may give some kind of idea on how far to raise the coils.

 

[chicopee]

See my attachment for an analysis to your problem. It will be a trial and error by using a set of chart and or a calculator when you open the following link:

http://www.thermalradiation.net/indexCat.html.

Theitem #74 in section C should be applicable to your situation. Certain assumption were made as explained in the attachment.

 

[chicopee]

Oops wrong attachment. See the correct one below.