Hot forming: Strong meets light
Caught between NHTSA and CAFE
Cosma International applies its own hot-forming technique to stamping automotive metal components such as A and B pillars, roof headers, roof rails, rockers, door intrusion beams, and bumpers, to meet new CAFE standards for weight, NHTSA requirements for strength, and to counter problems with springback in UHSS components.
|One of the challenges of meeting the NHTSA requirements for increased part strength while also meeting the CAFE requirements for decreased weight is that cold-stamped UHSS parts have a tendency to spring back. To meet this challenge, Cosma experimented with hot forming complex parts. The steel blank shown has been reddened by being heated to 900 degrees C.|
Cosma International Inc., Troy, Mich., a division of Magna International, designs, develops, and manufactures automotive systems, assemblies, modules, and components. Like many fabricators supplying the automotive segment, Cosma sought ways to meet the National Highway Traffic Safety Administration's (NHTSA) structural requirements for increased vehicle safety. To meet those requirements, the components' material has to be stronger, according to Swamy Kotagiri, director of research and development for Cosma. In an apparent contradiction, the components also have to be engineered to be as light as possible to meet increasingly stringent corporate average fuel economy (CAFE) standards for lower emissions, he said.
"So the objective is to get the required structural performance by adding as little weight as possible" Kotagiri said.
In today's market, components designed to withstand a large load with minimal intrusion into the passenger compartment to meet the safety requirements are manufactured of high-strength steels (HSS) and ultrahigh-strength steels (UHSS), Kotagiri said.
Kotagiri said that in the 2008 to 2009 time frame, the requirement to reduce the roof intrusion into the compartment area in a rollover will be even more demanding. In rollover accidents, roof crush is a factor in nearly 7,000 serious injuries or deaths each year, according to the NHTSA, CBS News reported.
"The roof crush standards are based on gross vehicle weight [GVW]," Kotagiri said. "Basically, the standard is saying that if a vehicle's GVW is 4,000 pounds, it has to withstand a force of up to 1.5 times that 4,000 pounds., or 6,000 pounds., in a roof crush test intended to simulate a rollover incident. In the future, parts that currently must meet the 1.5 times the GVW standard will have to be 2.25 to 2.5 times the GVW."
Kotagiri said that as the company addressed these challenges, the engineering department examined the challenges inherent in stamping advanced high-strength steels. One of the challenges is that cold-stamped UHSS parts have a tendency to spring back, Kotagiri said. This interferes with fit-up and causes difficulties in the welding and the assembly departments.
"For a simple part, you won't see much of a problem," Kotagiri said. "But it becomes really difficult to cold-stamp complex, angular parts or parts with a 3-D twist when they are made with very high-strength materials—150 to 200 kilopounds per square inch [KSI] in strength."
To address the challenge of meeting the high strength requirement with minimal weight penalty, Cosma experimented with hot forming. "Boron steel, in its original state, has tensile strength of around 50 KSI," Kotagiri said. "You heat the material to about 900 degrees—to an austenite temperature. Once it gets there, you transfer it quickly to a press, and you form the part when the material is hot. And then the part is quenched— cooled by water—in the die," Kotagiri explained. "So when you hot-form, you are forming it in a hot state and then you are cooling it in the die."
"So the problem you're solving with hot stamping is you're getting the advantages of advanced-high-strength material's properties without the manufacturing limitations of springback in very high-strength steels."
Kotagiri said that one of the most important aspects of successfully applying hot-forming technology is being able to understand how the material is affected as it goes through the heating process in the furnace, and to understand the technology for quenching in the die. Cosma has done extensive research to develop ways to make the hot-forming process work best, he said.
"How can I make parts consistently over a period of time and know exactly how quickly they cooled, how much they have cooled, and control the part temperature?" Kotagiri said. "What we have done is found the applications and ways to mass-produce components using this process. We have a very controlled, consistent cooling process when the part is in the die."
Different materials require different considerations, Kotagiri said, such as heating rates. The materials that can be hot-formed—uncoated boron steel and aluminum silicon-coated boron steel—require different heating configurations. The uncoated material requires a nitrogen-rich (or other inert gas) atmosphere in the furnace to prevent scaling. Otherwise, once it is exposed to the atmosphere, the uncoated material develops a scale on it that must be removed by shot peening or sandblasting, Kotagiri said. The aluminized steel does not require the nitrogen-rich atmosphere in the furnace, so shot peening and sand blasting are not required. As the aluminized material is heated, the iron—the parent steel—migrates into the aluminum silicon coating and bonds to it, giving it the properties that inhibit scale when it is exposed, Kotagiri said.
Time, Cost, Capability Considerations
In evaluating the suitability of hot forming for its automotive components, Cosma considered the per-piece process time, tonnage requirements, and overall capabilities.
Slower Forming. The hot-forming process is slower than cold stamping, Kotagiri said. From stroke to stroke, it takes about 15 to 20 seconds.
Lower Tonnage Requirement. Kotagiri said because the tonnage requirement to hot-form a high-strength steel part is substantially lower than is needed to cold-form it, more parts potentially can be stamped in the same press hit. Four to 12 parts per minute can be formed, depending on the size and complexity of the part (one, two, three, or four parts can be formed per stroke).
For example, hot forming the draw portion of a B pillar requires only 30 percent to 40 percent of the tonnage needed to cold-form it using a similar-strength material, he said. So the part, which requires 1,000 tons to be cold-stamped, needs only 300 to 400 tons to be hot-formed, he said.
Complex Part Features. Kotagiri said that hot forming can achieve very complex geometries at high strengths. "Honestly, there is not one other process today where you can form a part like an A pillar at the same strength, because of springback and because of being unable to meet the tonnage and strength requirements," Kotagiri said.
One-step Capability. Hot stamping is a one-step process—in fact, multiple processes cannot be performed—because the material is transformed during the cooling process, Kotagiri said. "For example, if you were to cold-stamp a B pillar, you would do a draw, and maybe then a redraw, or a flanging operation. With hot forming, once the transformation is done, you have this very high-strength material that cannot be redrawn or put through a flanging operation," Kotagiri said.
Limited to Uncoated, Aluminized Steel. This hot-forming process cannot be applied to galvanized or prepainted steel, Kotagiri said. Only boron steels, coated or uncoated, can be hot-stamped. The difference between materials that can be hot-stamped and materials that cannot be hot-stamped is the boron content in the steel, according to Kotagiri. In layman's terms, boron is the element that gives the material the capacity to change from a regular material to a martensite material when it cools. The boron metal is 50 KSI—not a high-strength material—before it is hot-formed, but is in the 200-KSI range after it is hot-formed.
Adding It Up
The biggest challenge for Cosma was calculating the weight and cost differences, because a direct comparison of a cold-formed part to a hot-formed part would not be accurate, Kotagiri said.
"For example, you cannot take a cold-formed B pillar and compare it to a hot-stamped B pillar and ask, "What's the cost difference?'" Kotagiri said. "You cannot make a direct comparison, because you're getting more out of your part in the hot-forming process than you are in a cold-stamping process. We were comparing the same performance, one to the other," Kotagiri said. "They don't say, "Increase the performance, and then tell us how much you saved in mass.' That's not the question. It's "We need this performance; now give us your design with hot stamping and tell us how much you saved in mass.'
"In the past, to beef up the safety performance of side-impact beams, the solution has been to add parts—to add more metal into the body side so it performs better. In many cases, with hot stamping we're able to eliminate the need for additional parts because the base part itself performs much better," Kotagiri said.
"Let's say you have a body side B pillar inner, B pillar reinforcement, and a B pillar outer. If I use hot-formed components in that assembly, now I'm able to eliminate the reinforcements. If you compare the B pillar inners alone, the hot-formed cost might be higher. But you got rid of a couple of reinforcements, and now you don't need to assemble them. So when you look at the whole, you have an advantage in hot forming them. Just looking at the roof crush case studies where conventional stampings and reinforcements have been substituted with hot-stamped components: the mass is reduced by, roughly, 30 to 40 percent," he said.
Flexibility to Become Design Source
All things considered—hot forming's slower per-piece rate, lower tonnage requirements, capability to form highly complex parts, and ability to reduce the number of parts needed—Cosma concluded that hot forming is well-suited to form parts that need to withstand high force with minimal deflection. "So typically good applications for hot-stamped components are A and B pillars, the roof headers, the roof rails, rockers, door intrusion beams, and bumpers," Kotagiri said.
Hot forming has empowered Cosma with the flexibility to respond to its clients' various needs optimally, said Gene Schilling, director of marketing and planning. "The OEMs generally will come to us with a problem and say, "Here's the performance we have to meet.' They're looking to their supply base to come up with the most cost-effective solution.
"One of Cosma's unique selling points is that we have multiple tools in the toolkit. So it's not as if we're out there pushing hot forming as a solution for everything. Hot forming is an additional tool in the toolkit to form high-strength steels. We also form other materials, and we cold-stamp some high-strength materials, but now we are able to offer hot forming of complex-shaped parts made of very high-strength materials when needed," Schilling said.
"In a way, we have become a resource to our customers for product design solutions," Schilling added. "We come up with the right solution to meet the OEMs' structural design requirements."
Kotagiri anticipates that the rising demands for high strength-to-weight ratios will heighten demand for hot-formed components. "We think it will increase until about 8 to 12 percent of body structures will be martensite material, and then it will plateau at that point."
Cosma International, 1807 E. Maple Road, Troy, MI 48083, 248-524-5300, www.cosma.com
The FABRICATOR is North America's leading magazine for the metal forming and fabricating industry. The magazine delivers the news, technical articles, and case histories that enable fabricators to do their jobs more efficiently. The FABRICATOR has served the industry since 1971.