Page 87 - MetalForming Magazine June/July 2022 80th Anniversary Issue

Metal Matters By Daniel J. Schaeffler, Ph.D.
Decarbonization of Sheet Steel and Aluminum
Metal formers use a substantial amount of steel and alu- minum. Unfortunately, steel and aluminum production contribute significantly to the rising levels of atmospheric carbon dioxide, although global efforts seek to dramatically reduce emissions.
The goal of decarbonization is not to create alloys with lower carbon con- tent in their composition. Rather, decarbonization focuses on using less carbon and emitting lower amounts of carbon dioxide during the conversion of raw-material feedstock to iron and subsequently steel, or aluminum and subsequently aluminum alloys.
New approaches to decarbonization have essentially no effect on the quality or properties of the resultant green steel or aluminum. Users should see no change in forming or joining prac- tices, just more environmentally friend- ly materials.
Conventional Approaches
Steel does not come out of the ground as steel; it exists as taconite, a combination of iron ore and other ele- ments collectively described as impu- rities. Similarly, aluminum exists as bauxite in nature, a combination of alumina and impurities.
Dr. Danny Schaeffler, with 30 years of materi- als and applications experience, is president of Engineering Quality Solutions (EQS) and chief content officer of 4M Partners. EQS provides product-applications assistance to materials and manufacturing com-
panies; 4M teaches fundamentals and practical details of material properties, forming technolo- gies, processes and troubleshooting needed to form high-quality components. Schaeffler is the metallurgy and forming technical editor of the AHSS Application Guidelines available from Worl- dAutoSteel at AHSSinsights.org.
Danny Schaeffler
248/66-STEEL • www.EQSgroup.com
E-mail ds@eqsgroup.com or Danny@learning4m.com
After removing the impurities using different but environmentally costly processes, the remaining product is iron ore or alumina. Iron ore consists of iron and oxygen, while alumina is a combination of aluminum and oxygen. The extraction of oxygen from iron oxide or aluminum oxide is a necessary precursor to producing elemental iron or aluminum.
Under many conditions, oxygen rapidly and tightly bonds with ele- mental iron and aluminum. Rusting occurs when iron and oxygen com- bine. And, the nanometer-thick alu- minum-oxide layer residing on top of aluminum alloys is why we often say that aluminum doesn’t corrode. How- ever, aluminum actually does corrode, but in a different mechanism and under different conditions.
Breaking these bonds requires ener- gy. A blast furnace combines carbon and iron oxide, providing the conditions for oxygen to preferentially combine with carbon rather than iron. This process yields liquid iron and carbon dioxide. The electrolytic process, com- monly used to produce aluminum from aluminum oxide, uses consumable car- bon anodes, with the reaction producing molten aluminum and carbon dioxide.
Green Aluminum Production
The electrolytic process commonly used to convert alumina to aluminum requires significant electrical energy. Emissions from power production account for more than 60 percent of the total emissions from aluminum production.
Historically, the industry relied on coal-fired power plants, but newer ones use renewable energy such as that pro- duced by hydroelectric plants. The con- centration of aluminum-production facilities around Lake Ontario and nearby waterways in part relates to access to hydroelectric power.
A new type of nonconsumable inert
anode nearing commercialization negates the need for carbon anodes and emits oxygen as a byproduct of the reac- tion rather than carbon dioxide. Use of renewable-energy sources takes on heightened importance as this approach may require greater energy input.
Starting with recycled aluminum bypasses the significant energy input required to first convert bauxite to alu- mina and then reduce alumina to alu- minum. Known as secondary alu- minum, this is the most common pathway for aluminum production in the United States. The term secondary does not connote lower quality; its use distinguishes this route from the pri- mary path starting with bauxite.
Remelting scrap comprised of differ- ent aluminum-alloy families results in a loss in properties, and an associated loss in value. While beverage-can alu- minum has high recycling rates, this content is kept out of scrap used in auto- motive-aluminum production. Even the scrap from grades applied to automobile structural members will negatively impact the properties of skin panels, and vice versa. Scrap segregation, which avoids mixing different grades, is a crit- ical strategy to retain scrap value.
Automakers increasingly employ closed-loop recycling strategies for cost savings as well as emissions reductions. Here, the stamping plant collects scrap aluminum from specific parts, all made from a particular grade ordered from a specific aluminum supplier, and returns the scrap to the supplier for remelting. Aluminum producers each have specific recipes for a given grade tailored to their equipment and man- ufacturing infrastructure, and this returned scrap flows seamlessly back into their process.
Key to scrap-based production: scrap availability. Consumers recycle less than half of all beverage cans, so the production of new cans obviously requires the higher-emissions primary
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