A Hard Worker With a Soft Touch

BY MEANS of it, pianos produce music, jets produce sonic booms, watches tick, motors hum, skyscrapers reach the sky, and suspension bridges stay suspended. What is it?

It is steel. Steel is at the heart of large-scale construction. Colossal ships made of it traverse the seven seas. Pipelines made of it bring oil and gas hundreds of miles from distant wells. But this versatile substance is woven far more deeply into the fabric of daily living. Consider, for example, the steel-belted tires on the bus you take to work or the steel rope that lifts and lowers the elevator in your apartment building. What about the steel hinges of your eyeglasses and the steel spoon with which you stir your cup of tea? Thousands of uses exist for this durable yet delicate metal. How is it made, and what makes it so useful?

Carbon and Crystals

Steel is an alloy, or a mixture, of two unlikely collaborators—iron and carbon. Pure iron is soft, as metals go, and is therefore unsuited for harsh applications. Carbon is nonmetallic. Diamonds and chimney soot are simply different forms of this particular element. But if a small amount of carbon is mixed with molten iron, the result is a material very different from carbon and far stronger than iron.

The key to steelmaking is something called a crystal. Did you know that iron is composed of crystals? * Actually, all solid metals are, and it is this crystalline makeup that gives them workability, luster, and other traits. But iron crystals go a step further.

The Effect on Steel

When steel is being made, molten iron is mixed with other elements. As this mixture solidifies, iron dissolves the other materials, in effect absorbing them and holding them inside its crystal structures. Other metals behave the same way. What is so special about iron?

Iron is unusual because its crystal structure can be changed with heat while it is still a solid. This characteristic allows iron crystals to be changed from a relatively closed form to a more open form and then back again. Imagine a well-built house in which the walls move sideways and the floor up and down while you sit in the living room. Something like that happens inside iron crystals when the metal is brought to a high temperature without being melted and is then cooled.

If carbon is present when these changes occur, a hard alloy can become soft or a soft one hard. Steel producers take advantage of this and adjust the hardness of their product with heat treatments like quenching, tempering, and annealing. * But there is more.

When other elements—such as manganese, molybdenum, nickel, vanadium, silicon, lead, chromium, boron, tungsten, or sulfur—are mixed in, steel becomes not just hard or soft  but strong, tough, ductile, corrosion resistant, machinable, flexible, magnetic, nonmagnetic—and the list goes on. Just as a baker adjusts his ingredients and oven settings to make different kinds of bread, metal producers vary alloys and heat treatments to make thousands of different steels unmatched in versatility. Steel rails safely carry 12,000-ton freight trains, and yet steel bearings the size of a pinhead support a watch’s balance wheel.

Steelmaking—Old and New

Centuries ago metalworkers formed iron into utensils and weapons. They discovered that smelted iron (iron separated from mineral-bearing rocks called ore) had impurities that gave the metal strength and hardness. They also learned that quenching an iron tool in water made it even harder. Today huge furnaces have replaced the blacksmith’s hearth; and gigantic rolling mills, his hammer and anvil. But modern processors follow the same basic steps as did the brawny forgers of old. They (1) melt iron, (2) mix in alloying materials, (3) let the steel cool, and (4) form and heat-treat it.

Note the quantities in the adjacent box. Impressive as they are, a steel mill can devour all of that in a single day. The plant covers a vast area, on which stand mountains of the minerals that feed its insatiable appetite.

A Marvelous Metal Takes Many Forms

The usefulness of steel shows up in many out-of-the-way places. You will find some under the lid of a grand piano. The wire there, one of the strongest steels made, produces beautiful music. Hadfield manganese steel is used in making giant rock crushers, and the harder it works smashing boulders, the tougher the steel gets. Stainless steel is formed into surgeon’s scalpels, wine vats, and ice cream machines. Like the hairs on your head, the uses of steel are more than you care to count.

Each year, almost 800,000,000 tons of steel are produced throughout the world. Not an ounce of it would exist without iron, which just happens to be one of the most plentiful elements on earth. Since coal and limestone are also in good supply, it appears that steel will be available well into the future.

So the next time you sew with a metal needle or cast a hook with a rod and reel, or the next time you use an adjustable wrench or open the gate on a chain link fence, or the next time you travel in an automobile or plow straight furrows in a field, think of the extraordinary blend of iron and carbon that makes it possible.

[Footnotes]

[Box on page 23]

Materials required to make 10,000 tons of steel

6,500 tons of coal

13,000 tons of ore

2,000 tons of limestone

2,500 tons of steel scrap

400,000,000 gallons [1.5 billion L] of water

80,000 tons of air

 [Box/Pictures on page 24, 25]

How Steel Is Made

Some details are omitted for visual simplicity

Steelmaking requires heat. With a thermometer as our signpost, let’s follow the road to finished steel.

▪ 2500°F [1400°C]. Huge ovens are baking coal in airtight chambers, vaporizing undesired matter without consuming the pieces. The resulting sooty chunks are called coke, which supplies heat and carbon needed further along the line.

▪ 3000°F [1650°C]. Coke, iron ore, and limestone cascade into a blast furnace and meet a wall of flame and superheated air. The coke burns, and in the blistering heat, unwanted material in the ore combines with the limestone, forming a by-product called slag. The materials liquefy and settle to the bottom of the furnace. The slag, floating on the iron, is drawn off in a container for removal. The liquid iron flows into bottle cars that roll their scalding cargo to the next station.

▪ 3000°F [1650°C]. Ninety tons of carefully sorted scrap metal are dumped into a 30-foot [9 m]-tall pear-shaped vessel known as the basic oxygen furnace.  A huge ladle pours searing fluid iron onto the scrap metal, igniting a burst of sparks as a water-cooled tube called a lance is lowered into the vessel. From the lance roars a supersonic jet of pure oxygen, which soon has the metal boiling like soup on a hot stove. Chemical reactions take place. In less than an hour, the furnace has done its job, and a 300-ton batch of liquid steel, called a heat, pours into transport ladles. Alloys are added. The fiery flow surges into casting machines. The steel begins to take shape.

▪ 2200°F [1200°C]. Red-hot steel is squeezed ever tighter between rollers until the desired thickness is attained. This grueling workout makes the metal hard, so hard that it resists further forming.

▪ Room temperature. The steel has been cast, cut, hot-rolled, cold-rolled, and even pickled (cleaned in an acid bath). It has been reheated time and again. Finally, the thermometer drops for good. The liquid steel, or heat, has become stacks of sheet steel. A metal shop soon shapes it into ductwork for an office complex.

Since the major parts of a steel mill are made of that same metal, why don’t they melt while they are doing their work? The inside surfaces of furnaces, bottle cars and ladles are lined with bricks made of a refractory, or heat-resisting, material. A three-foot [1 m]-thick lining of this protects the basic oxygen furnace. But those bricks also suffer from the outrageous heat and must be replaced regularly.

[Diagram]

(For fully formatted text, see publication)

1. IRONMAKING

2500°F Coal → Coke ovens

↓

3000°F Limestone

Iron ore → BLAST FURNACE

↓

Molten iron

2. STEELMAKING ↓

3000°F Scrap

Lime and flux

Oxygen

↓

BASIC OXYGEN FURNACE

3. COOLING ↓

CONTINUOUS CASTING

Bloom

Billet

Slab

4. FINISHING ↓

2200°F Steel rolling (bars or beams)

Galvanizing

Cold rolling

Hot rolling

Room temperature

[Picture]

Note the size of people

[Picture Credit Line on page 23]

All photos on pages 23-5 except watch: Courtesy of Bethlehem Steel