ULSAB: Working High-Strength Steel into Automotive Design

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ULSAB...Working High-Strength Steel into Automotive Design

 

 

 

 

High-strength steels (HSS) represent a new frontier in automotive design and manufacturing. Their properties allow thinner gauges to be applied throughout an auto body, rendering a stronger, lighter vehicle without significant changes in cost structure. As HSS content rises, its positive impact on fuel economy, emissions and safety will give OEMs the vital breathing room needed to build cars and trucks that still appeal to consumers.

But like any other new material introduced into a process, high-strength steels have unique characteristics that require attention in design and manufacturing. Automotive engineers and the steel industry continue to work together to hike over that learning curve, gaining valuable experience in applying HSS in volume production. These industry leaders will help to create the designs and the processes to build the next generation of vehicles - vehicles that deliver what many other technologies only have promised.

Proof of Concept

Pressure to reduce fuel consumption in vehicle design has been the driving force behind HSS development for more than two decades now. The first fuel crunch in 1973 sent some engineers to the drawing board to build smaller cars. Other engineers went to the lab, examining materials and manufacturing, looking to lightweight instead of decontent.

By the 1980s, high-strength steel was being applied in key structural areas on vehicles: first safety-critical applications and then dent resistance, both reducing vehicle weight.

This year, the worldwide steel industry took HSS applications to the next level with its UltraLight Steel Auto Body project (ULSAB). More than 90% of the vehicle was composed of high-strength steel defined by the Auto/Steel Partnership (A/SP) as steel with an incoming yield strength of 210 MPa (30 ksi) or greater. The ULSAB body showed dramatic results in safety and structural testing, exceeding project benchmarks in some cases by significant margins.

High-strength and ultra-high-strength steels are used for more than 90 percent of the body structure (colored parts), allowing designers to reduce weight and increase strength. Image

The project also proved the concept that, despite some of the manufacturing issues that come with applying high-strength steel, almost every piece of a vehicle body can be effectively manufactured with grades above 210 MPa. Some auto manufacturers already have started to learn how to work around idiosyncrasies inherent in HSS, developing a much lighter, stronger auto body that fits one of the most popular vehicle segments today.

Adjusting to HSS Manufacturing

The appeal in applying high-strength steels comes first from its ability to carry a great deal of stress versus other materials, and peak faster than other materials during high energy loading like a car crash. This property allows engineers to scale back on the mass applied for a particular body part, cutting weight out of the entire structure.

That same property, however, has posed some unique challenges to the auto industry. The lower ductility of HSS means greater care needs to be applied at design and manufacturing time. As automotive experience and data continues to collect, engineers are overcoming some of these production issues, applying HSS in ever-increasing volume.

Engineers often rely on several key guidelines when working with HSS:

Stamping: Manufacturers need careful calculations on the hold-down geometry, precise control over the stroke and good definition of the strain. In development, start with the most critical pieces of a component or subassembly and then make changes to the less important parts. Key in on the functional requirements of a subassembly and work out details with your vendors. Flexibility can be a major asset in working with HSS. Take measures to compensate for changes that will occur in development and tryout.

Welding: Start by identifying weld gap tolerance requirements and main tolerances during control roll forming and be prepared to make accommodations in the process to allow for weld gap variability. Don't forget that different gas shields can affect materials and processes in a different way, and some parts may need cleaning prior to welding. And remember, there are a number of innovative welding technologies and weld designs that can help to optimize the overall performance of a structure.

Roll Forming: Look to use the minimum allowable bend radius for the material, positioning any holes away from the radius to achieve better tolerances. Use the appropriate lubrication in the process, and set up an aggressive maintenance schedule for the roll-forming line. Don't forget that higher-strength steels require more stands on the roll former. End-flare can occur during the roll forming because of the material strength. Make sure to have a sizer, bracket, plate or some other device to help take in the flare, bring the part back to specifications and hold while it is being welded. Also, keep an eye out for side-wall buckling while the part is being swept. Engineers also need to watch for localized stretching. This can result in an uneven distribution of the strain, putting areas of the part outside of the tolerance.

Different gas shields can affect materials and processes in a different way, and some parts may need cleaning prior to welding. And remember, there are a number of innovative welding technologies and weld designs that can help to optimize the overall performance of a structure. Image

Increasing Uniformity

Working high-strength steel can be a more detailed, sensitive process. And variances in the steel often can throw off the balance of the process, resulting in less uniform parts. This problem is constantly addressed by the steel industry and high-strength steel uniformity has improved over the years, according to James R. Fekete, staff development engineer with General Motors Corp.'s Metal Fabricating Division. But he says both the steel industry and the automobile industry need to work on improving the process.

"There are two issues you need to worry about," says Fekete. "First is the formability of steel. Can you actually elongate it and stretch it. Here you can predict whether or not a part has formability. Now with all of the computer tools and analysis, we can say 'we can make that part or we can't make that part."

The second issue, pulling uniform parts from the die each time, is a less-quantified process. "As an industry, we don't have the computer models yet to predict the shape of the part coming out. We have a lot of experience in designing with high-strength steel, but we don't have enough experience in predicting differences in uniformity," says Fekete.

Of course if the steel coming in is more uniform, then the automakers can do a better job at producing the part. But Fekete says it is a two-way street. "I wouldn't lay it all at the feet of the steel company. A significant and measurable component of the variability in a high-strength steel part is steel itself. There is a lot of pressure placed by the automotive industry on the steel industry to reduce the variability in the mechanical properties. And the steel industry has responded to that. The steels being made today are tremendously better than they were 20 to 25 years ago," he says.

Springback Moving Forward

Springback is one of the more notorious problems in working with HSS. The magnitude of springback depends on the steel's yield strength and sheet thickness. Of course most grades of automotive steel exhibit springback to some degree. And engineers often plan for the springback by overbending parts.

Again, the automotive industry is still developing the computer models and the experience to compensate for high-strength steel springback. "We can do a better job predicting springback with mild strength steel because we have much more experience and a lot of good design lines. With high-strength steel we don't have as much experience in computer modeling," says Fekete. "It is not adequate yet and so the models we have aren't enough to predict with computers, but we have a lot of development experience. And this is something that we are actively working on within the company."

EVI Cuts Time, Costs

Manufacturers may find that more vehicle development time is necessary in working with high-strength steels, but only if they go it alone. Early vendor involvement (EVI) programs can bring a wealth of knowledge into the development process well before any of the constraints are fixed, helping manufacturers build in significant weight savings. And with steel suppliers on the job from day one, lead time can be significantly reduced, spilling into the manufacturing and tooling setup as well.

EVI also can cut costs. Automakers can plan for specific dies to take particular steel grades, instead of building for an anticipated grade, and then spending dollars to upgrade for a HSS application. Manufacturers also can reduce the number of operations and presses to develop a part, cutting process time.

And EVI can even cut the number of parts and the total material manufacturers used to build a vehicle. ULSAB showed a significant mass reduction - meaning less material cost for the OEM. It also significantly reduced the number of parts necessary to build the body, through advanced manufacturing and design techniques.

Fekete says support from the steel industry is nothing new, and that an increasing alliance is building between OEMs and steel vendors. "There has been quite a level of technical support that has improved. More and more people working at steel companies that understand the auto manufacturing process. And conversely there are people coming out of the steel companies to the automakers. The more each side knows about the other side the better the chances of coming together to work out a product," he says.

Still the Best Game In Town

"But ultimately we must use high-strength steel in order to get the vehicle within the performance guidelines that are imposed by the public, by the government, by our own internal goals," says Fekete.

He believes that the auto industry still recognizes steel as the preeminent material in the industry. "I don't think the auto industry as a whole sees steel as an outmoded material. As long as you are not talking about cost, a lot of the alternative materials look good. But part of engineering is cost and steel is still the most cost effective and really the dominant material in terms of cost and manufacturability."