Fresh from posting on the winter of the steel industry in China, I see that Richard Jones has written a nice top-level view of the role of steel in the world economy, explaining broad indicators like per-capita steel demand, and steel intensity, or steel use per unit of GDP:

The dominant uses for steel now are in construction and infrastructure. 42% of steel output goes into buildings, the biggest fraction of which (44%) is in the form of rebar, and 14% in other infrastructure (again, mostly as rebar, but including some 6% as train tracks).

Steel, then, is the fundamental raw material of urbanisation. We can now understand the two periods of fast growth of steel output, in broad terms, as corresponding to two great waves of urbanisation–the first, between 1950 and 1980, in the USA and Europe, and the second, from 2000 and still continuing, as the rapid urbanisation of China. …

Steel is more important than ever as the foundation of our industrialised, urban economies. But more than a century of remarkable (and widely unappreciated) technological progress means that it is relatively less important in terms of its contribution to GDP, because we’ve learnt to make it so much more cheaply and efficiently.

But why exactly is steel the “fundamental raw material of urbanization”? To understand this we have to understand something of the history and physical properties of steel. I also recently stumbled across this nice account of the invention of the modern process of steelmaking in the American Scientist which explains the basics well:

Broadly speaking, steel is just iron with a bit of carbon in it. But that definition doesn’t capture the stunning metamorphosis that occurs when the iron and carbon merge in the correct way. The secret behind steel is that it isn’t just one substance like most metals, but a mixture. On the microscopic scale, steel turns into two different substances that stack up like a layer cake. One of the layers is rich in carbon and strong. This layer is a chemical compound called cementite, and with the right amount of force, it snaps like brittle chalk. The other layer has little carbon (around 0.2 percent) and is malleable (flexible and easily bent). This chemical compound is called ferrite, and with enough force, it can be pulled like taffy. These layers complement each other with strength and malleability. Most metals, being monolithic, have only one property or the other. The characteristics of strength and malleability usually are more like two ends of a seesaw; as one goes up, the other goes down. But in steel, its layers allow both properties to exist, and that makes steel versatile and suitable to build many things, from trains to tools to cars to cans.

So steel is useful for many things, but why is it particularly useful for buildings? I did not fully grasp this until I read Mark Miodownik’s wonderful book Stuff Matters: Exploring the Marvelous Materials That Shape Our Man-Made World. A delightful introduction to materials science, it includes chapters on both steel and concrete. And it turns out that steel by itself is not the “fundamental raw material of urbanization”; that title should properly belong to steel-reinforced concrete. The unusual interaction between the physical properties of concrete and steel makes this an incredibly useful combination:

Concrete is essentially a simulacrum of stone: it is derived from it and is similar in appearance, composition, and properties. Concrete reinforced with steel is fundamentally different: there is no naturally occurring material like it. When concrete reinforced with steel comes under bending stresses, the inner skeleton of steel soaks up the stress and protects it from the formation of large cracks. It is two materials in one, and it transforms concrete from a specialist material to the most multipurpose building material of all time. …

Most materials expand when they get warmer and contract when they get cooler. Our buildings, roads, and bridges expand and contract like this, observing day and night temperature cycles, as if they are breathing. It is this expansion and contraction that causes a lot of the cracks in roads and buildings, and if it is not taken into account in their design, then the stresses that build up can destroy the structure. Any engineer…might have assumed that concrete and steel, being so different, would expand and contract at such different rates that they would tear each other apart… But, as luck would have it, steel and concrete have almost identical coefficients of expansion. In other words, they expand and contract at almost the same rate. This is a minor miracle.

There’s a lot more fun stuff in all three of these readings.