Steel Strikes Back Against Alternative Materials
One of the things that the steel industry has done over the past several years is to aggressively position itself vis-à-vis competitive materials. Not only is this being done on a company-by-company basis, as new materials are developed, but also through coordinated efforts, specifically those of the Automotive Applications Committee of the American Iron and Steel Institute (www.autosteel.org). The objective is not to simply react to what other material suppliers are doing, but to take a proactive approach in proving to vehicle and component manufacturers that, although the proffered approach is different materials for different applications, chances are the material they’re most interested in seeing being deployed happens to be ferrous. And more specifically, of the steel variety, as opposed to iron.
Consider Cranks. Consider, for example, that the move toward cast iron crankshafts starting in the 1970s raised some considerable concern because the material that was losing out in this switch happened to be steel. Yet as Dave Anderson, director, Bar and Rod Programs, AISI, points out, GM’s Buick Div., which is looking for performance and improved NVH for its vehicles, is using engines fitted with forged steel crankshafts under the hoods of its latest vehicles, the Lucerne and the LaCrosse. Specifically, the 275-hp 4.6-liter Northstar V8 in the Lucerne and the 255-hp, 3.6-liter V6 in the LaCrosse both have forged steel crankshafts. Because of the rigidity of these cranks, there’s little vibration, which helps Buick achieve the sort of “QuietTuning” that it is seeking. In addition to which, there is the performance aspect of the steel cranks, too. So a great example here is found under the hoods of two Cadillac performance models, the STS-V and the XLR-V, both how which have the 4.4-liter supercharged Northstar V8, which produces 443-hp for the XLR-V and 469-hp for the STS-V.
And as an example where the steel crankshaft really performs is under the hood of the Z06 Corvette, which uses the 7-liter, 505-hp LS7 hand-built V8 engine. 4140 steel is used.
Anderson points out that if diesel engines grow in popularity for light vehicles, then the number of forged steel crankshafts will grow accordingly, as the higher compression ratios of that type of engine necessitates the use of steel cranks.
Strength & Durability. Another material that steel is staring down under the hood is powdered metal, which, Anderson admits, has the larger proportion of connecting rod applications under the hoods of cars built in the U.S. However, he points out that the preponderance of engines built for European and Asian applications have forged steel con rods, primarily because, Anderson says, “forged components are stronger and more durable.” Those engines tend to operate at comparatively higher rpms, which puts stress on the con rods, which means that strength and durability are paramount. While he acknowledges that there may be a weight advantage to using powdered metal, “If the con rods are redesigned”—for forged steel—“then they can be lower in weight than powdered metal.” He also explains that while there is also a cost advantage for powdered metal, “the cost differences are coming down because of near-net shape forging.” The use of a material like C-70, which is a crackable steel (which allows the cap to be separated from the rod without requiring machining of the mating surfaces for subsequent reattachment) has been shown to allow weight and manufacturing cost reductions for connecting rods. (It should be noted that the aforementioned LS7 engine doesn’t use steel for its connecting rods—they’re forged titanium.)
Trapping Emissions. Another area where steel has lost significant ground that it is working to recover is in the production of fuel tanks. Here the alternative material is plastic, which is light and easily formable. So the AISI organized the Strategic Alliance for Steel Fuel Tanks (SASFT) initiative, which examined the ways and means to regain some of what has been lost to blow-molded products. One of the concerns with steel has been that of corrosion. So SASFT undertook a two-year corrosion test program to assess both internal and external corrosion potential. According to the results of the study, the steel tanks are good for “up to 20 years,” which means that they meet the California Air Resource Board (CARB) 15-year durability requirement. Chuck Potter, AISI senior automotive consultant, also points out that when it comes to evaporative emissions—a key concern for vehicle manufacturers that are looking to meet CARB’s Partial Zero Emission Vehicle (PZEV) requirements—a single layer of steel can get the job done versus multiple layers of plastic. And while on the environmental front, there is another issue related to gas tanks: What happens at the end of life? Potter says, “Steel is 100% recyclable. Plastic tanks get thrown into land fills.” Certainly that is a consideration for the environmentally minded.
Although plastic had previously had advantages as regards to both weight and formability for fuel tanks, Potter says that advanced high-strength steels are competitive on both counts.
Another application that steel once had 100% of the market for is wheels. Now steel accounts for less than 50%, with aluminum being the material of choice when it comes to styled wheels. Steel hasn’t disappeared, but has been pretty much relegated to the low trim levels for vehicles. However, what some wheel manufacturers are doing is using steel for creating a base and then adding trim (including plastic), plating, or painting, thereby producing comparatively lower-cost wheels with a higher-cost appearance. Applications of these wheels range from the Ford F-150 to the Lincoln Navigator.
Body Building. So far as the body-in-white goes, steel is strong. Yes, Potter and Anderson both admit that vehicle manufacturers are always looking at alternatives like aluminum and composites, but that steel continues to hold the advantage. Not wanting to lose and then regain as steel has in the aforementioned applications, the steel industry is working hard at creating new materials—Anderson says, with tongue perhaps only partially in cheek, “Steel is old and has been around for a long time. We’re looking to change its name—ferro-alloy.” Something that sounds sexier. But from the technical point of view, the materials that are being promulgated by the industry are advanced high strength steels (AHSS). These are materials with a yield strength greater than 550 MPa. Included in this are materials with improved formability—dual phase and transformation induced plasticity (TRIP) steels—and those with increased tensile strength, crush resistance, and energy absorption—complex phase and martenstic steels. One of the advantages that Potter cites for steel versus, say, aluminum, is that there is a greater familiarity in the auto industry as regards steel, especially when it comes to manufacturing with the material. He admits that the higher mechanical properties and the microstructures of the AHSS materials are different than what has historically been the norm means there must be some adjustments and precautions when it is used in place of the traditional HSS materials, “it doesn’t take wholesale manufacturing equipment changes.” Upgrades to dies (springback is greater for the AHSS materials) and different welding parameters are likely to be necessary, but those are comparatively minor.
One vehicle that makes the steel people exceedingly happy is the Pontiac Solstice. Although the vehicle “borrows” the hydroformed frame rails from the Corvette (i.e., it has the same underlying architecture as the ‘Vette, not the same rails), it doesn’t borrow the plastic body panels. It has a steel body—even though one might think that with clam shell hood plastic would be a natural. (Sheet hydroforming is used instead of plastic molding.) “Plastic used to assume the low-volume advantage,” Potter says. “It’s interesting that we can now say that there’s a low-volume scenario with steel.” The Solstice and platform partner Saturn Sky are going to account for about 20,000 units per year (a number which is likely to go up with demand).
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