How Is Steel Made?

An Overview —
The manufacture of steel involves many processes and stages.  The basic raw materials are coal, iron ore, limestone and various chemicals.  Coal is transformed into coke.  The coke is combined with limestone and ore in the blast furnace to form molten cast iron via feeding the fire a blast of pressurized oxygen.  The molten iron then moves on to the open hearth where it is transformed into steel by the addition of chemicals, elements and heat.  The steel from the open hearth is poured into forms yielding ingots, blooms and billets.  These rough forms are processed into slabs.  The slabs are then processed (rolled) into the end product ordered by the customer.  That end product is plate, bar, rail, coil, etc.

The Coke Plant —
Turning coal into coke is necessary in making steel because coal does not burn hot enough to melt iron ore.  Coal in its original state is mostly carbon, however there are impurities in coal such as tar, ammonia etc.  These impurities keep coal from burning hot enough to reduce and melt iron ore, therefore they must be removed through the coking process.

Typical Coke Oven size - 43 feet long, 13 to 15 inches wide (tapered), 11 feet tall.

Charging - The coal is dumped into the ovens from a larry car on top of the battery.

Cycle - The coal will bake at 2,400 degrees F. for 18 hours.

Quenching - The coke is pushed out of the oven and quenched with 4,000 gal of water. 1,000 gals. will be vaporized in the process.

Capacity - 14 tons of coal fed into the battery will yield 11 tons of coke.

By-products - The volatile gasses driven off in the process are cooled and refined into tar, fertilizer, oils etc.  Products made from coal products include plastics, perfume, aspirin, nylon, cosmetics etc.

The Blast Furnace —
Blast furnaces represent old technology.  A blast furnace makes cast iron from iron ore, coke and limestone.  Its functions are both reducing the iron ore or removing the oxygen and melting it into molten cast iron.

BF raw materials consumption (per furnace) - 3,750 Tons of Iron ore and 1,100 Tons of coke.

Iron Production - 2,300 tons per furnace, per day.

Summary - Air is heated to 1,800 Degrees F. and blown into the bottom of the furnace at 500 m.p.h. (the blast).  The hot air helps the coke to burn which reduces and melts the iron ore.  Temperatures in the BF reach 3,800 degrees.

The iron ore and limestone melt and drips into the trough.  The limestone makes slag which floats on top of the iron and is later skimmed off.  The trough is drilled every 2.5 hours and the iron runs through ditches to the ladles.

The Cupola Plasma Furnace
A cupola furnace functions similarly to a blast furnace.  It utilizes different raw materials in the process and generally has a smaller capacity.  The cupola furnace makes molten cast iron from scrap steel or a scrap substitute.  The fuel for this furnace is coke and 18 megawatts of plasma electricity.

Cupola raw materials consumption -

Iron Production - While a cupola may produce as much as 2,300 tons of iron per day, the average is approximately 1,500 tons..

Summary - The cupola furnace is charged with coke and scrap steel or a scrap substitute.  Heated air is boosted in temperature with a plasma torch and blown into the bottom of the cupola.  The coke is burned creating more heat which melts the iron.  The iron then exits the furnace and runs through a trough to an iron ladle.

The Basic Oxygen Furnace (BOF) —

Capacity - An average heat size is 240 tons.

Temperature - Steel is approximately 3,000 degrees when it is tapped.

Alloys - Alloy additions include carbon, aluminum, manganese, silicon, molybdenum, titanium, chrome, nickel, etc. Steel producers are capable of making over 2,000 different kinds of steel.

Cycle - BOF's are capable of running 24 hours a day.  A cycle takes approximately 35 minutes, including charging and tapping.

Environmental Info - During the process, oxygen is used as the fuel to remove carbon from the iron.  The oxygen turns into carbon monoxide.  After being quenched and scrubbed to remove the particulate, the carbon monoxide is flared at the top of the stack turning it into carbon dioxide.

The secondary emission control system consists of a baghouse which will filter the particulate from the off gasses when the vessel is not in blow.

Summary - The Basic Oxygen Furnace is new technology.  The BOF process turns iron into steel.  Cast iron from the blast furnace is approximately 4 percent carbon.  In the BOF, the carbon content is taken down to .02 percent. (One percent of carbon content or less is what differentiates steel from cast iron.)

Scrap is charged into the vessel first (approx. 40 tons).  Two ladles pour 100 tons of hot metal into the vessel with the scrap.  The vessel is turned into an upright position and oxygen is blown into the bath through 12 tuyeres (holes in the bottom of the vessel). After approximately 18 minutes in blow, the vessel is turned on its side and a sample is taken to verify the chemistry is accurate.

If the test results are correct, the vessel will be turned 180 degrees to pour the steel out through a small tap hole into the steel ladle. The ladle is then transported by carrier to Ladle Metallurgy for further chemistry refinement.

The Electric Arc Furnace (The Basics - very basic) —

 An electric arc furnace is essentially a big recycling machine.  Simply put, it's a large pot into which scrap steel, like old cars, bicycles, and refrigerators, are dumped.  The furnace operator loads (called charging the furnace) carefully screened and selected scrap steel into this big pot.  Then, the operator swings the furnace lid back over the pot and lowers it into place.

The electrodes are part of the furnace roof structure.  Electrodes can be small, 75 millimeters (3 inches) in diameter, or quite large, up to 750 millimeters (30 inches) in diameter and as much as 2800 millimeters (9 ft. +) long.  The largest weigh more than two metric tons (4,400 lbs

The electrodes are assembled into columns, usually 3 to a column.  Graphite connecting pins, tapered and threaded on each end, screw two electrodes together.  Some furnace operators assemble the columns on the shop floor and lift them into place, whole.  Others add electrodes one at a time to the top of the column while it is still on the furnace.  Each electrode column has a large steel arm which moves it up and down.

Furnaces using 3-phase AC (alternating current) electricity have 3 electrode columns.  DC (direct current) furnaces only need one column of electrodes, but these are generally larger in diameter.

Once the furnace lid is in place, the electrodes are lowered until the tip of the electrode column almost touches the top of the scrap steel.  Huge amounts of electricity--large furnaces use enough electricity for a town of 40,000 people--flow through large water-cooled cables into the electrodes.  At the bottom of the electrode column, the electricity jumps (or arcs) from the electrode tip to the nearest piece of scrap steel, hence the name of the electric arc furnace.

The intense heat of this electric arc melts the scrap steel.  The tip of the electrode will reach 3,000 º C / 5,000 º F, half the temperature of the surface of the sun.  Electrodes are made of graphite because only graphite can withstand this incredible temperature.

Eventually all the scrap is melted and rendered into a liquid form.  The furnace operator turns off the power and raises the electrode columns.  The furnace is then tipped on its side to pour the molten steel into giant buckets called ladles.  The ladles quickly carry the molten steel to the steel mill's caster which is the next step in making new and useful products from recycled scrap steel.

The Continuous Caster —
The continuous caster represents a major innovation in the manufacture of steel.  In this process, molten steel is turned into a solid slab.  A continuous caster, when built in line with a rolling mill, help reduces the energy cost of re-heating steel.

Slab sizes - Slab thickness can range from inches to over a foot.
Temperatures - Liquid steel going into the caster is approximately 2,800 degrees. Solid steel coming out of the caster is approximately 1,800 degrees.
Cast Speed - The speed of the caster depends on the size of the slab made. The average speed is 50" per minute.

Summary - The steel is brought to the continuous caster from the BOF via crane, rail or rubber tired transporter.  The steel is taken to the ladle metallurgy furnace (LMF) where the temperature and chemistry are adjusted.  The steel is then lifted onto the turret and poured into a tundish.  From the tundish the steel flows into the mold of the caster.  At this point the outer edges of the steel begins to solidify.  After passing through the mold, the steel travels through the various segments of the caster, all the while being sprayed with a water and air mixture.  When the steel exits the segments the steel is solid.  The slab is then cut by an oxygen torch that travels with the steel.  The average slab size is between 20 to 30 tons.

The Rolling Mills —
Rolling mills are of two (2) distinct types; roughing and finishing. —

A Roughing Mill takes a billet or ingot and transforms it into an intermediate product.  This would be a slab. Subsequent to this step, slabs may be further refined into steel plate or rolled to produce a finished product.  The largest roughing mill in the world is the U.S.S. - Gary Works 210" Plate Mill. (This is where I was employed in the electrical maintenance department for a few years.)  Far more common are plate mills with a 160" width capacity.

Heating capacity - Steel slabs are heated in either soaking pits or re-heat furnaces.  Induction furnaces take slabs directly out of the caster, bump the temperature up to 2,400 degrees and send them directly to the rolling mill.

Coilbox - Where the intermediate product is a coil, a coilbox coils the steel after the roughing process, and before the finishing process, to keep the steel at an even temperature and prevent heat loss.  The steel is then uncoiled and rolled through the finishing stands.

Finishing Mills —
Customers use steel for many different applications.  To accommodate these demands, there exist numerous finishing mills within the industry.  These facilities process many different kinds of steel.  The end product could be a bar, coil, pipe, (tin) plate, (train) rail, tube or (railroad) wheel.

Finishing Stands - Generally they are multiple stands, called tandem.  These mills can finish either plate steel or coil products. The steel comes into the finishing stands at approximately 1 inch thick and 2,000 degrees F. The finishing stands roll the steel to the customers specifications, varying in thickness from 1" thick to .070".

Cooling Beds and Down Coilers - The steel travels through a laminar flow cooling bed to the coilers where the steel is coiled for the final time.  Large coils will weigh over 30 tons.

Coil Products - Some of the steel is sold in a coil.  These coils are called hot-bands, coils and strip. Many of these coils will be processed further by the customer.  These processes include pickling and oiling, cold rolling, galvanizing, annealing and tin-plating.

Shear Line - This line handles coiled plate products.  The steel is allowed to cool and is unrolled, side trimmed, cut to length and then leveled.  The shear line can uncoil steel up to 1" thick.

Heavy Gauge - The Heavy Gauge line processes plate steel from .134" to .5" thick and 72" wide. This line also handles diamond floor plate.

Sheet Mill - The sheet mill processes strip products into sheet steel.  This line handles light gauge steel between .059" to .134" thick, and up to 72" wide. The steel is uncoiled, side trimmed, leveled and cut to length.

Temper Mill - Tempering steel means to work the steel in a rolling process without reducing the thickness.  The steel is uncoiled, worked through a rolling process, the sides are sheared and the steel is coiled.

Pipe Mill - The pipe mill takes coils and forms them into small diameter pipe products ranging from 8" in diameter to 16".  The steel is uncoiled, leveled, welded to the end of the previous coil, side trimmed, run through the forming rolls to make it into a pipe and electric resistance welded (ERW).  The pipe is then cut to length and tested.

Shipping - Finished product, in the form of slabs, plate (steel or tin), coil, tube, rail, pipe or specialty leave the mill by various means of transport.  The most common are rail and flat-bed truck.  The product is loaded with the aid of overhead cranes.

 

An excellent resource on all aspects of the steel making process, and on U.S. Steel facilities, is:  

U. S. Steel Corp. has also put together an excellent video, 6 min. 46 sec. in length, which describes   the modern (BOF) steel making process from beginning to end.  SEE IT HERE

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Created by GDY — Wednesday, 12 February 1997 06:02:48 Hrs.

© 2008, G. David Yaros.  All rights reserved.