by Vaclav Smil
High-quality steels (advanced ferritic steel, austenitic steel, and nickel alloys) have always been essential for thermal electricity-generating plants: they are the only materials to use in supercritical steam cycles where high temperature strength must be combined with good resistance to oxidation (Alstom, 2013). Chromium steel is used for the rotors of massive steam turbogenerators, whose blades are made of stainless steel or titanium alloys, while abrasion- and corrosion-resistant steels are used in capturing particulate matter in electrostatic precipitators and to desulfurize hot flue gases. Electrical (lamination) steel is indispensable for transformers to change transmission and distribution voltages: their magnetic cores are made of stacked, cold-rolled strips of iron-silicon alloys that have high permeability and low core loss (small energy dissipation per cycle), reducing electricity losses by a third compared to older designs.
Energy infrastructure is unthinkable without steel, and transition from fossil fuels to renewable sources of energy will not eliminate that dependence. The most important way of renewable electricity generation has always been steel-intensive: large concrete dams require reinforcing steel, large-diameter penstocks leading water to turbines are made of plate steel, water turbines are cast and machined from high-quality steels, and steel dominates generators as well as towers for long-distance transmission links required to bring electricity from remote regions to major load centers. And the two leading new renewable ways of electricity generation—wind turbines and photovoltaics—are even more steel-intensive.
While hydrogeneration requires 20–30 t of steel per installed MW, the rate is close to 300 t/MW for PV (Walsh, 2011). And a typical 100-m tall tower, using bolted friction connections to support a 3.6-MW wind turbine will require about 335 t of steel (83 t/MW), and a 150-m tall tower for a 5-MW turbine will need about 875 t, or 175 t per installed MW (Gervásio et al., 2014). WSA (2012) estimates per MW for onshore turbines are 30 t for the foundation, 50 t of steel for nacelle and rotor, and 100 t for tower, and, respectively, 300 t, 50 t, and 100 t for offshore machines. Many countries have bold plans for electricity from wind, but even if it were to supply only 10% of the global demand by 2030 (forecast to be about 30 PWh), then (even when using a high average capacity factor of 35% and a large average turbine capacity of 5 MW) some 175 Mt of steel would be needed to produce that output (not counting steel needed for extensive new high-voltage transmission links). In any case, wind turbines can be dismantled at the end of their useful life (now about 20 years) and steel parts can be either remanufactured or recycled.
And although steel industry is a major source of CO2 emissions, German calculations show that new high-performance steels used in a wide range of energy converters (including central power plants, wind turbines, lighter cars and trucks, efficient electric motors) could potentially save about six times as much CO2 as is emitted during their production (Stahlinstitut VDEh, 2013). Steel’s central importance in the quest for new electricity generation is best summarized by the recent publication of a German steel group entitled Energiewende beginnt mit Stahl (Energy transition begins with steel), which also reminds all uncritical green enthusiasts that the process cannot succeed without competitive electricity prices (Wirtschaftsvereinigung Stahl, 2013).
Diffusion of new renewables, above all wind and solar PV generation, will change some specific forms of steel dependence: more steel will be needed for towers for wind turbines, less steel for large coal excavators. But as many sunny and windy regions (in the US windy Great Plains from northern Texas to North Dakota, sunny Southwest) are far away from major consumption centers (in the United States the eastern and western coasts), more steel will be required for new high-voltage transmission links (for tower foundations, towers, cables, and wires) in order to improve the reliability of supply and to bring electricity from large wind and solar plants to distant cities.
And although renewable conversions now receive most of the attention, nuclear fission is also expanding: according to the expectations of the early 1970s, by now it should be the dominant way of electricity generation. Obviously, this is not the case, as its Western progress became (for a variety of reasons) virtually arrested (Smil, 2010), but Asian plans call for scores of new reactors during the coming decades, mostly in China and India. Safety considerations make nuclear generation a highly steel-intensive endeavor. Zirconium steel alloy is used as the preferred cladding for fuel elements containing fissionable uranium; reactor pressure vessels, which contain the nuclear fuel encased in rods, are made of thick steel plates that are welded together in cylindrical shapes with hemispherical caps. Neutrons from the fuel in the reactor irradiate the vessel as the reactor is operated and this embrittles the metal (USNRC, 2014). Containment structures, usually dome-shaped, are made from thick (90–150 cm) steel-reinforced concrete, and radioactive wastes are temporarily stored on site in steel drums.
Transportation
Transportation in general, and (given the large number of passenger vehicles and trucks) the vehicular sector in particular, now ranks as the leading consumer of finished steel products whose advantages include not only their strength and durability but also UV resistance and a very high (already >80% in the sector, aiming at >90% in the future) recycling rates. According to WSA (2012) all forms of transportation now claim about 17% of the global steel consumption, and road vehicles account for about 12%; as expected, automakers consume more steel, about 20% of annual production, in affluent countries (WSA, 2012). Other major consumers of transportation steel are shipyards (particularly those building massive oil and LNG tankers, container ships and bulk cargo carriers) and manufacturers of trains and airplanes.
In order to minimize energy consumption and maximize revenue-earning payload, the latter two industries now rely mostly on lighter materials (aluminum alloys, composites, plastics) to build railway cars and airplane fuselages, wings, and tails. But up to 25% of the mass of high-speed trains is steel, mainly in the heavy bogies (axes, wheels, bearings, and electric motors), while landing gears of jetliners are made from 300 M, a high-quality durable steel alloy containing chromium, manganese, molybdenum, nickel, vanadium, and silicon and providing an excellent combination of toughness, fatigue strength, and ductility (Metal Suppliers, 2015).
The mass of steel used in automobiles has been influenced by several trends that can be traced in detail in American data. First are the changes in average curb weight of cars, with a long secular increase culminating at 1692 kg in 1975, followed by a sudden decline of average mass (precipitated by oil price rises of the 1970s), a shift to smaller vehicles (the average was 1300 kg in 1985) and then yet another period of rising curb weight brought on by a spell of low oil prices and growing ownership of SUVs that lifted the average to 1470 kg in 2004. In 2013 the average light-duty vehicle (with nearly half of them being pick-ups, SUVs, and minivans) weighed 1820 kg, 1% more than a year before. In contrast to these oscillations, average steel content of American vehicles has been declining.
As a part of their pioneering analysis, Berry and Fels (1973) calculated that in 1967 91.5% (1471 kg) of an average US vehicle mass was iron and steel. That share declined to about 87% of the total mass in 1970, to 75% in 1990, and to 68% in the year 2000 (Sullivan, 2005). As Schnatterly (2008) pointed out, establishing the steel content of an average American car is not that easy: besides the direct steel shipments to automakers there are many indirect channels in the steel supply chain, imports and exports of steel components must be accounted for, and weighted curb weight average must be calculated for an ever-changing profusion of models. His detailed account established that in 2008, when average mass of all vehicles was about 1860 kg, 65% of that was steel. By coincidence, 65% of that total was steel directly shipped to automakers, and the top three finished products were, as expected, sheets and strips (in the order of galvanized and coated, hot-rolled, and cold-rolled), followed by hot-rolled bars and tube and pipe.
By 2015 about 62% of a typical US vehicle was steel, and about 70% of tha
t total was flat-rolled carbon steel for chassis and body panels (USDOE, 2013). Cullen et al. (2012) calculated that 61% of steel in a typical car is sheet metal but because of the relatively low yield of fabrication (only about 60%), 91 Mt of it, rather than 54 Mt, were needed in 2008 (Fig. 8.4). Steel’s declining share in average vehicle mass is indisputable, but as both the total and average mass of American vehicles have been increasing (from 98 million and about 1500 kg in 1970 to about 255 million and 1820 kg in 2013), steel stock contained in operating American automobiles rose from about 130 Mt in 1970 to nearly 290 Mt in the year 2013.
Figure 8.4 Coils of hot-rolled sheet steel ready for distribution at Pohang Iron and Steel Company in Pohang, South Korea. Corbis.
As expected, the global mean of automotive steel content is somewhat smaller: WSA (2012) put it at 960 kg (steel and iron) per vehicle, with roughly a third in the body, panels, doors, and trunk, about a quarter in the drivetrain (engine, gears), and 12% in the suspension, with the rest in the wheels, fuel tank, steering, and brakes. Mild steel (tensile strength of up to 370 MPa) is used for interiors and on some exposed panels, while high strength steel (HSS) (up to 550 MPa) goes for some structural parts, including doors; steels with the highest tensile strengths form chassis parts and absorption barriers. Ultra-high steels, used for side collision panels, combine high tensile strength (980 MPa and 1180 MPa) with large elongation (20–50% at 980 MPa, 15–45% at 1180 MPa) and considerable bendability (Takahashi et al., 2012). Even higher strength (up to 2000 MPa) is achieved by hot stamping (heating the steel to austenitic temperature and quenching it by forming dies).
During the early 1950s the US car industry consumed about 20% of all steel shipments, and the shares were as high as 30% for bar steel and 48% for sheets. In 1950 a typical Ford four-door sedan contained about 1.6 t of steel (nearly 60% of it was heavy sheet metal); it had more color metals (Cu, Pb, Zn) than aluminum and hardly any plastic parts. Aluminum die castings made rapid inroads during the 1950s as they began to replace cast iron and sheet metal and then were used to make parts of transmissions and engines. By the early 1960s aluminum was used to cast entire engine blocks—but by 1958 average American cars still contained only about 17 kg of plastics (Hogan, 1971). That mass had nearly tripled by 1970, and the two rounds of oil price rises (1973–1974, 1979–1980) led to the introduction of smaller and lighter cars containing less steel and more aluminum and plastics.
Steel use in modern cars faces two contradictory challenges: lighter materials and lower total mass are the key steps toward making them more fuel-efficient and lowering their CO2 emissions—but reducing car weight might compromise the quest for higher driver and passenger safety, and enhanced crashworthiness may need heavier material and additional measures (side-bags, anti-lock brakes) that will add to overall weight (Galán et al., 2012; Takahashi et al., 2012). Studies have shown that reducing vehicle mass by 10% improves fuel economy by 6–8% (USDOE, 2013)—but they also show that mass reduction is associated with increase in fatalities and serious injuries. Lightweighting has been an essential component of the quest for the already legislated higher automotive energy efficiency—the US CAFE rates for light vehicles are to reach 54.5 mpg in 2025, double the rate in 2010—and it will be even more important for large future cuts that might be necessary to reduce CO2 emissions.
Lightweighting has also been applied to marine transportation, to the construction of large tankers, bulk carriers, and container ships, and to a rapidly expanding fleet of cruise ships. In terms of total numbers, dry bulk fleet (carrying ores, coal, fertilizers, grain, and other bulk loose cargoes) is composed mostly of vessels smaller than 55,000 deadweight tons (dwt), but the three larger categories—Panamax vessels of 60,000–80,000 dwt, capesize vessels of 80,000–200,000 dwt, and very large bulk carriers (>200,000 dwt) account for most of the fleet’s carrying capacity.
Even the largest container vessels are smaller than tankers or large bulk carriers, but their capacities have grown tremendously during their rather short history (Smil, 2010). Small container ships with capacities of less than 1000 TEU (20-foot equivalent units) correspond to bulk carriers of just 14,000 dwt, and the largest ships (carrying 18,000 TEU, 400 m long and 59 m wide) are equivalent to 165,000 dtw. Global container ship fleet numbered just over 5100 vessels in 2014, compared to nearly 17,000 bulk carriers (Fig. 8.5).
Figure 8.5 Triple steel: steel ships carrying steel containers unloaded by steel cranes in the port of Hamburg. Corbis.
Cruising has been among the fastest growing classes of tourist activities: in 2014 nearly 22 million passengers boarded the global fleet of 410 vessels, with 24 new ships to be added in 2015 (CLIA, 2014). The size of the largest of these increasingly massive vessels is rivaling the gross rate tonnage of tankers—ships of the Oasis class displace 225,282 t, have length of 360 m and height of 72 m and 20 stories—and a hull of such a vessel requires about 45,000 t steel covering 525,000 m2 (Ship Cruise, 2015). Thermo-mechanical control process (TMCP) high-strength steels have been the best choice for hull construction: they require no preheating for welding and are easily formed, bent, and edged while reducing plate thickness (bottom place of the largest cruise ships are just 2 cm thick). In late 2014 ArcelorMittal signed a new contract to supply 116,000 t of steel for the hulls and decks of three new giant cruise ships, one of the Oasis class and two of the new 315-m long Vista class, to be built by STX France (ArcelorMittal, 2014).
Industrial Equipment and Consumer Products
Both of these categories are very diverse, but one attribute they share is high shares of steel in the mass of their final products. Many pieces of common industrial and commercial equipment—ranging from filing cabinets to stainless steel wine tanks, and from distillation columns to massive shipyard cranes—are nothing but steel, and steel is also the only material used to make shipping containers. Introduction (during the late 1950s) and rapid adoption (starting a decade later) of standardized steel containers transformed the worldwide delivery of nonbulk products and revolutionized not only marine transportation but also distribution of goods by trains and trucks.
Ubiquitous containers are sized to fit trucks after they get offloaded from ships and trains, and hence their width varies only between 2.55 and 2.85 m, and their most common lengths are 6.09 m (20 ft) and 12.18 m (40 ft); their empty weight is, respectively, 2.2 and 3.7 t, and their maximum loads are 21.7 and 26.8 t (Smil, 2010). There are even longer steel shipping boxes of 45, 48, and 53 ft, and hence the world total of containers is counted in terms of TEUs. In 2015 there were about 34 million TEUs (total steel mass of about 75 Mt) in service (WSC, 2015). Their importance is perhaps best attested by the fact that all but a tiny share of all the clothes we wear, all household gadgets we use, and all electronic devices we carry reached us packed in steel containers.
Much more steel is embedded in a huge range of industrial machinery, ranging from simple but extremely powerful presses (used to stamp metals) and smaller presses to extract oils to modern, and now usually computer-controlled, machining tools including lathes, drills, gear shapers, and milling, honing, hobbing, planning, and grinding machines whose mass is typically more than 95% steel. Steel is also present in large quantities in internal infrastructures of industrial enterprises, where it forms many walkways, stairs, partitions, overhead cranes and hoists, pipes, towers, supports, and above- and underground storage tanks.
Steel that most people encounter daily in their houses and apartments has been used in making a still increasing array of kitchenware, tools, and household appliances. As already noted, manufacturing of steel cutlery predates the availability of cheap Bessemer steel, and now knives, forks, spoons, and many kinds of kitchenware are made from stainless steel varieties (Team Stainless, 2014). Post-1950 mechanization of household work in high-income countries has brought mass ownership of small appliances; the most common ones in North America have been toasters, microwave ovens, grills, mixers, fryers, pressure cookers, food processors, and juicers. But substi
tutions in this sector have been common, with plastics (including high-temperature-resistant and nonstick surfaces), aluminum, and tempered glass made into items ranging from cheap cutlery to baking sheets and mixing bowls.
Numbers of major household appliances (white goods) are much larger than the total numbers of road vehicles in operation: an average US household now owns two cars (the nationwide mean of car registrations is actually 1.95, slightly off its peak reached a few years ago) but it has half a dozen major appliances: refrigerator, range (gas or electric), dishwasher, washing machine, clothes dryer, and air conditioner (and many families also have a freezer and a range-like barbecue). And the United States is not alone: after decades of acquisition, ownership of nearly all of these appliances (clothes dryers, still uncommon outside the United States and Canada, are the greatest exception) has reached the saturation point in nearly all affluent countries.
Typically, washing machines, refrigerators, and ranges are owned by more than 90% of all households, and the share of families that have several air conditioner units has been increasing rapidly in such countries as Malaysia, Brazil, and China and also in the cities of India. Most people underestimate the steel content of common household appliances, which averages 56% for refrigerators, 53% for washing machines, and about 32% for air conditioners (Kubo et al., 2012). But because even major (and in the United States increasingly larger) appliances are much smaller and lighter than cars and because washing machines, clothes dryers, dishwashers, refrigerators, freezers, air conditioners, and electric and natural gas stoves are mostly made of thin sheet steel—electro-galvanized sheets with a coating weight of 20 g/m2 are the norm—the share of steel production claimed by their manufacturing is surprisingly small: they are ubiquitous, but making them consumes annually only about 2% of the global steel production.