Path Cleared
Originally published in Modern Metals
By Corinna Petry
Zinc and steel producers close in on the next technological leap for producing lightweight auto body material
The steel and the galvanizing industries must keep up with and supersede advances made by competing materials—such as the adoption of aluminum in the 2015 Ford F-150 pickup truck—while remaining the cost leader.
These are the goals of the Galvanized Autobody Partnership (GAP), formed in 1999 under the aegis of the International Zinc Association in Durham, North Carolina. Frank Goodwin, director of technology and market development for the association, leads GAP’s research.
GAP is a cooperative program between steel and zinc producers “to advance and defend zinc-coated advanced high-strength steels,” Goodwin says. Steel has been used to make automobiles since the early 1900s but corrosion resistance features became standard in vehicles beginning in the early 1980s when Japanese cars gained entrance to the U.S. market.
North American-made vehicles were having “real rust problems” that couldn’t be fought merely with heavier, and more expensive, paint. By the mid-1980s, “you really had to galvanize the whole car if you wanted to issue warranties.” By 1987, what became standard was a 10-year perforation and five-year cosmetic warranty from all the North American, European and Japanese automakers for vehicles sold in the North American market.
“That really put the burden on the steelmakers to ask, how are we going to galvanize cars? The first process adopted was electrogalvanizing,” says Goodwin. So large-volume electroplating lines were built and they processed 20 million tons of steel per year through early 1990s.
However, the industry found this process expensive because of the amount of electricity required and because the speed of the lines was constrained. At the time, however, EG was the only process approved for automotive quality.
GAP had begun working on various development projects by then. “Everyone saw the Holy Grail was to hot dip galvanize automotive steels. It was already done for building panels and appliances. Getting that to be automotive quality was the key,” says Goodwin, and that happened by the mid-1990s.
Fuel economy push
Once GAP participants realized that goal, they sought to keep pace with the demands of increasing fuel economy standards, which “began ramping up to the goals we have now: The metric is 54.5 mpg by 2025 in the United States,” while the European Union has similar goals using different measures. Automakers selling into the EU must reduce emissions from 160 grams of carbon dioxide per kilometer to 95 grams by 2020.
The primary concern was, and is, “how to galvanize all these new steels, performing pre-treatments, for example, before dipping. You have to use a very smooth substrate,” steels with high yield strength and tensile properties.They must be as strong as the old steels but thinner and feature pristine, coatable surfaces.
As new generations of steels have been developed and commercialized, GAP members have “been able to figure out processes that answer the technical questions. Everybody works together on it,” Goodwin says.
The third generation of steel sheet for which GAP is working to develop the right coating process has a yield stress of 1,270 MPa (184,200 psi), a tensile stress of 1,420 MPa (206,000 psi), a uniform elongation of 9.3 percent and a total elongation of 17 percent.
The material is getting down to 0.5 millimeter thick from 0.9 to 1.0 millimeter today, “and you actually have to be able to galvanize it at high production volumes,” says Goodwin.
GAP is performing initial laboratory tests and “cooperating with steel mills on hot dip galvanized 3G pilot trialsright now,” he says. “We are not yet to full mill trials.” However, there are early indications that the mechanical properties achieved with third-generation steel grades “will enable us to meet 2025 goal,” which will allow drivers to reduce CO2 emissions throughout the vehicle’s life.
“Until now, the most widely used steels are dual-phase steels. That only gets us so far.” The third generation must provide the combination of strength and elongation on thin-gauge sections to reduce weight.
“If you can get those properties in a steel-based car to meet the requirements—and we’ve shown you can galvanize these steels—you have to have the weight, the mechanical properties and corrosion warranties right.” The value of the knowledge sharing between all the partners in GAP “is to get all the vital measures together at the right time.”
Annealing breakthrough
Steel producers have, with each development in automotive sheet manufacturing, increased the power exerted by the rolling mills in order to guarantee the required material strength. Recently, GAP studies found that steelmakers can now anneal the steel at a lower temperature to 640 degrees Celsius. That is a breakthrough, according to Goodwin.
Why is a low annealing temperature important? “This gets into the technical meat. These steel grades have alloys like silicon and manganese for strengthening. These alloys are inexpensive compared with chromium and molybdenum. But the trouble with silicon and manganese is that they oxidize more at high temperatures. These alloys diffuse to the surface and make oxides.
“If you have a steel sheet you are trying to galvanize and it’s covered in silicon oxide, which is basically sand, you cannot coat it,” he continues. “So lowering the annealing temperature prevents oxidation and keeps the surface nice and clean. Immediately after cold rolling, you can put this material in an annealing furnace to achieve the third-generation microstructures without having the alloys come to the surface, which would make coatability difficult.”
Big picture
Ninety percent or more of a vehicle’s body-in-white structure is coated today, Goodwin notes. That includes “the skin and structures that you hang the assemblies on,” including the frame, doors, hood and truck lids, and everything below the top of the wheel down into the undercarriage. “If you are selling into the North American market, you have to do it otherwise you cannot issue a warranty.”
The International Zinc Association is not looking to increase that proportion. “We have gotten saturation [of coated automotive parts] in the EU, North America, Japan and South Korea. We are playing a defensive strategy now, against aluminum. What the steel industry and IZA support is developing high-grade, lightweight steels that are meant to ensure we have steel-based vehicles to match the CAFE standard, which is a very ambitious goal. We have to do it or aluminum will take a big chunk” of the business.
Asked if IZA members have an annual zinc consumption goal by 2025, Goodwin says, “We would like to continue following the market growth. We are seeing growth in vehicle consumption globally. If steel consumption declines, it means other materials are eating our lunch.” He cites the F-150, noting, “We’re losing coated steel volume with that material switch. Everybody is watching that and wondering, ‘will it be successful?’”
“It is a huge leap for Ford,” he continues. “If you look at other cars built with aluminum, you’ve got the Audi A8, but we’re talking about 20,000 to 30,000 units a year, compared with the Ford F-150, which is 700,000 units.”
Another development IZA is monitoring is the growing use of composites by airplane manufacturers. “What is the future for aluminum-based airplanes vs. composites? As aircraft become more composite based, then aluminum rolling capacity is going to be looking for something to do, looking for a market. So rolling mills will go after automotive.”
Meanwhile, however, IZA members are focused on the advances possible in automotive steel, tracing the metallurgy. “We are partway through the journey. We will be able to do more with steel and get higher performance than even this third-generation step. There is a long upward run for development,” says Goodwin.
“I really wonder how far aluminum will be able to take this. They are using 6000 series grades in the Ford F-150, with yield strengths of 30,000 to 40,000 psi. We will realize 210,000 psi, many times higher. How much greater strength will automotive aluminum be able to realize?”
Aluminum is light, he concedes, “a third of the weight of steel. But will aluminum producers be able to improve low mass with higher strength? Can the metallurgy for aluminum automotive applications develop much further? And how expensive will it be?”
Global aspirations
The annual consumption of zinc for autobodies today is roughly 120,000 metric tons, according to IZA. Says Goodwin, “There is almost no galvanized steel on Chinese-made vehicles except for exports from China by Volkswagen AG, General Motors Co. and others. That’s 20 million cars in China not using galvanized steel.”
India, too, is a huge and growing market for passenger vehicles, but galvanized steel is not used in auto manufacturing there. “So it’s the emerging market economies that could bring us to 600,000 metric tons of zinc consumption per year. We are playing offense in emerging markets, working with governments that don’t yet have standards for corrosion protection.” MM