HIGH STRENGTH LIGHT WEIGHT STEEL
A law of fuel consumption: The lighter a car, the lower its fuel consumption and the lower its CO2 emissions during driving. An old theory of automotive safety design was: the heavier the vehicle, the greater protection provided to its occupants, but this theory has been superseded by the introduction of high strength lightweight steels. Innovative high-strength steels allow auto components to be made thinner, and thereby lighter, without sacrificing safety. Despite their high strength, these materials are designed to be readily formable and can be processed with special equipment or processes at auto stamping plants.
The challenge with such materials is that high strength and good formability are usually mutually exclusive. Resolving this conflict are solutions such as special micro-alloying elements and targeted heat treatment. This results, for example, in ductile dual-phase steels that attain their ultimate strength during forming into automotive components; or bake-hardening steels that gain their strength from the heat of the paint-baking operation.
High-strength steels are a cost-efficient means of reducing the weight, not only of premium-segment cars, but also of mid-size and compact vehicles. Costs play a particularly important role for manufacturers and buyers in these segments, while at the same time their high production volumes provide a particularly effective lever for climate protection. More than 75 percent of the eight million cars built in Europe in 2011 were from these market segments. Depending on part and use, high-strength steels permit weight savings of up to 30 percent. In this way steel makes a key contribution to sustainable mobility.
LIGHTER WEIGHT AUTOMOBILES
What material is seeing the most rapid growth in automobiles? If you guessed aluminum or composites, you’d be wrong. It’s advanced high-strength steel (AHSS). This material comprised just a small fraction of cars and light trucks a few years ago, but it could grow to over 30% of vehicle weight within 10 years.
Although higher-priced, lower-volume vehicles like the Audi A8 have converted many of their parts to aluminum in response to fuel-economy pressures, the trend has been for moderately priced vehicles to stick with steel. These mainstream vehicles are using better manufacturing techniques — such as laser-welded blanks, hydroformed components, and better joining techniques — in addition to containing up to 30% AHSS.
When the Honda Insight was first launched, it had among the highest percentages of alternate materials of any vehicle on the road. However, the latest version of the Insight is one of the most AHSS-intensive vehicles. Likewise, BMW came out with an aluminum front end on its 5-Series a few years ago, but recently switched back to steel.
So what is AHSS, and what makes it so attractive to automakers? Grades of AHSS have strengths to 1,500 MPa but retain the formability of lower-strength steels. In general, elongation, the property that equates to formability, degrades as strength increases. AHSS is formulated for more elongation at equivalent strengths.
ALUMINUM VERSUS AHSS
Despite AHSS’ better strength, steel still has to compete with aluminum’s lower density. But aluminum may not be a panacea for automakers. First off, 30 times the amount of steel is produced compared to aluminum, so it can be hard to sustain production or keep prices down on an all-aluminum car.
Various research studies (see “High-strength history” sidebar) have shown that proper application of AHSS can cut a vehicle’s weight between 10 and 25%. When fuel economy is paramount, the 5% fuel-economy boost a 10% reduction in weight provides is a nice carrot. But designers should be aware of certain approaches that can take full advantage of AHSS’ capabilities.
Automotive bodies can be considered a combination of plates, beams, and joints — the intersections of two or more beams. If all the steel in a car was replaced with aluminum, and no design changes were made other than making the aluminum parts thick enough to achieve the same overall stiffness and strength of the steel vehicle, the distribution of strength and stiffness would still be somewhat different on the aluminum car versus the steel one.
For mechanical responses that are proportional to the specific modulus — the modulus of elasticity divided by the density — there is no advantage to switching from one metal to another from a weight perspective. For instance, the torsional stiffness of circular thin-walled tubes made from aluminum and steel of the same weight is the same. Looked at another way, two thin-walled tubes having the same torsional stiffness, one made out of steel and the other made out of aluminum, weigh the same.
However, the torsional stiffness of a plate is not directly proportional to specific modulus. A steel plate weighs twice as much as an aluminum plate of the same area (but not thickness) and the same torsional stiffness.
So, designers can improve structural efficiency and minimize the benefit of converting to aluminum in two ways. First, use higher strength steels to minimize the weight of plates in a vehicle. Second, get the vehicle’s internal beams to behave more like closed-section, circular tubes.
Advanced architectural elements such as laser-welded blanks or hydroformed beams can help a steel body or frame compete with aluminum in terms of weight. Laser welding two sheets of dissimilar-thickness or dissimilar-grade steel together puts strength where it is needed in a single steel stamping.
Hydroforming a continuous hollow tube to the desired contour in a die using water pressure eliminates spot-welded sheet-metal beams. Because the tubes are continuously fastened and of a thicker gage than the sheet-metal beams, they have higher strength and stiffness.
Joints are a third area designers should consider. The joints of an all-steel vehicle will usually have lower strength and stiffness than those of its all-aluminum counterpart because the joints have typically higher stresses and steel is thinner gage than in the aluminum vehicle. So, strengthening and stiffening the steel vehicle’s joints will reduce the weight benefit of aluminum.
Designers can add internal stiffeners to boost joint strength. More manufacturers are also converting spot welds into continuous welds or adding continuous adhesives in the seams of the joint.
These strategies allow automotive designers to maintain or reduce vehicle weight in the face of increasing safety and crashworthiness requirements. One example is recent change to the National Highway Transportation Safety Administration’s roof-crush requirements. Since 1994, vehicle roofs had to withstand a load of 1.5 times the gross vehicle weight (GVW). In 2011, this grew to 2.5 to 3 times GVW, necessitating thicker or stronger roofs.
MICRO-ALLOYING AND MICROSTRUCTURES MAKE IT HAPPEN
The high-strength, high-ductility characteristics of AHSS originate in the metals’ unique microstructures. Where most steels, such as ferrite, have primarily one microstructural phase, AHSS typically has a combination of martensite, bainite, and ferrite phases.
Each microstructural phase has a different crystalline or molecular structure. Ferrite, for example, has a body-centered-cubic (BCC) structure, and martensite has a body-centered-tetragonal structure. Each structure has its own set of physical properties due to the forces within the crystals and the densities with which atoms are packed in a crystal cell.
Phases form during annealing, a process in which steel is heated to 850°C so that it becomes pure austenite. From there, the steel is cooled in a controlled manner that determines the final phase mixture.
For instance, if the steel is cooled at a very slow rate, the austenitic phase will transform into ferrite without transitioning into any other phase. Conversely, very rapid cooling or quenching produces 100% martensitic steel (MART). A slow cool followed by a rapid quench produces dual-phase (DP), complex-phase (CP), or transformation-induced plasticity (TRIP) steels, all of which have AHSS properties.
Micro-alloying elements and elements like silicon, manganese, chromium, niobium, vanadium, titanium, phosphorus, and molybdenum shift the phase of the transformation, making it possible to form each desired phase mix with a different cooling cycle.
Coal ash is created primarily from the burning of coal for heat in the production of electricity, and for other industrial processes. This residual material has be stored in ash piles, and in numerous large landfills, throughout the United States. Coal ash is composed primarily of the oxides of silicon, aluminum, iron, calcium, magnesium, titanium, sodium, potassium, vanadium, molybdenum, boron, fluoride, arsenic, mercury, and sulfur, plus small quantities of various rare earth elements and uranium and thorium. Fly ash (ash caught in flue gas) is primarily composed of non-combustible silicon compounds (glass) melted during combustion. Tiny glass spheres form the bulk of the fly ash.
Many of the various components of coal ash can be utilized as the most important components in making high strength lightweight steels. Many of the best micro-alloying elements are very expensive, and if processing coal ash were economical, then the large amounts of micro-alloying elements in coal ash could lower the market cost of these elements.
One of the reasons why coal ash is not processed into its individual components now is because of its thorium content. Under current federal government regulations, thorium must be handled and treated as a low level nuclear waste, and any material extracted from coal ash other than thorium and uranium and their decay isotopes means a higher ratio of radioactive components in coal ash. The uranium found in coal ash has a well established market as a nuclear fuel and other industrial uses, Unfortunately, thorium has no such well established market – yet.
Trace quantities of uranium in coal range from less than 1 part per million (ppm) in some samples to around 10 ppm in others. Generally, the amount of thorium contained in coal is about 2.5 times greater than the amount of uranium. For a large number of coal samples, according to Environmental Protection Agency figures released in 1984, average values of uranium and thorium content have been determined to be 1.3 ppm and 3.2 ppm, respectively. Using these values along with reported consumption and projected consumption of coal by utilities provides a means of calculating the amounts of potentially recoverable breedable and fissionable elements. The concentration of fissionable uranium-235 (the fuel for current nuclear power plants) has been established to be 0.71% of uranium content.
Researcher Alex Gabbard, of Oak Ridge National Laboratories, estimates that a 1GW coal power plant produces 5.2 tons uranium per year and 12.8 tons of thorium. He further estimates the uranium and thorium reserves contained in coal ash piles scattered across the United States contain in excess of 150,000 tons of uranium and 350,000 tons of thorium. One ton of thorium, by itself, if used as a fuel in a molten salt reactor (MSR) can produce the equivalent of a 1GW coal fired power plant. This calculates to over 12GW equivalent of energy stored in the annual coal ash produced by a single 1GW coal fired power plant, over 12 times the amount of energy produced originally by the coal when it was burned.
As long as there is no market for the element thorium there will not be any large commercial scale extraction of strategic materials from coal ash. Coal ash will continue to be an environmental problem.
Commercializing MSR (Molten Salt Reactor) technology with a reactor design that utilizes thorium as its fuel will allow for the environmental clean up of more than 150 years worth of coal ash piles, the production of prodigious quantities of low cost micro-alloying elements that will allow us to make lighter steels to make lighter vehicles that use less fuel, allow the United States to produce economically its own synthetic liquid transportation fuels from coal and trash via low cost energy provided by MSRs, and for America to be much more energy independent.