July 15, 2008
Advanced high strength steels spur stampers to think about metal forming in new ways.
Mention a certain material is predicted to overtake mild steel as the auto sector's most popular material in 10 years, and people take notice.
That's been Peter Ulintz's experience in recent years. As advanced product engineering manager for Cleveland-based stamper Anchor Manufacturing Group, Ulintz has toured the country giving seminars on what many believe eventually will become a preferred metal for automotive and other industries: advanced high-strength steels. At a recent seminar at the METALFORM tradeshow in Birmingham, Ala., Ulintz picked up a prop on his podium. "See? I'm waving a red flag. They're coming, and you'd better be ready for them."
AHSS has been gaining ground in an automotive industry hunting for all the weight savings it can while satisfying or surpassing safety standards. The material could be called the next step in steel's evolution toward exhibiting the high-strength, lightweight characteristics of more expensive materials, like aluminum and magnesium.
Challenges arise, however, from a formability standpoint—hence Ulintz's red flag. The varieties of AHSS do exhibit high formability, but in entirely different ways from legacy materials. According to sources, it's more than just high amounts of springback; instead, metal formers need to throw out the old rulebooks. AHSS has spurred stampers to think about metal forming in new, unconventional ways.
Daniel Schaeffler pointed to his wedding band. A metallurgist and president of Engineering Quality Solutions, a Southfield, Mich.-based consultancy, Schaeffler admitted the band was only 18 karat, not pure gold—but for good reason.
"Pure gold is like any pure metal," he said."It's extremely formable. It has no strength."
The same holds true for pure iron. But mix that formable element with"impurities" like carbon and manganese, and the story changes. The material, mild steel, has less formability but higher strength than pure iron. To further increase steel strength historically has required adding primarily carbon and manganese. This decreases formability and weldability to a point where, unfortunately, it isn't practical for forming complex parts calling for yield strengths greater than 275 to 345 MPa (40 or 50 KSI).
Above this strength level, steelmakers add very small amounts of elements like titanium, vanadium, and niobium (also called columbium) to the melt, and adjust the steel mill processing to use much less carbon and manganese to produce a steel of a given strength level. Compared to conventional CMn steels, these high-strength, low-alloy (HSLA) steels (also known as microalloyed steels) generally have increased strength, better formability, and better weldability.
Until recently the nature of the strength-formability trade-off has remained the same: Making a material stronger reduced formability. HSLA steels reduced that trade-off. But to make steel of still higher strength and lower weight—as the automotive industry is demanding—requires that trade-off to be reduced further, or even eliminated.
To accomplish this required a new approach to alloy development and, in turn, forming. This led in recent years to a new class of steels—AHSS. The various grades are grouped under the AHSS umbrella not because they share microstructures, but because they alter the relationship between ductility and strength. According to a report from a 2006 AHSS workshop in Arlington, Va.,"The typical trade-off between strength and ductility has been taken as gospel by generations of faculty, students, and engineers. The ability to increase both of these properties simultaneously with AHSS is indeed a technical paradigm shift of the highest order."
Because this fundamental trade-off changes, so do the rules for metal forming. Yes, the steels do exhibit high formability, allowing the stamping of high-strength materials with tensile strengths, in some cases, of 700 MPa and above, values so high that stampers wouldn't have bothered to put them through a press a decade ago. However, the steels respond to deformation in unconventional ways, and this is the root of the challenges associated with AHSS.
Metallurgists classify most AHSS into first and second generations. The first generation includes dual-phase (DP), transformation-induced plasticity (TRIP), complex-phase (CP), and martensitic steels. The second generation includes those with a corrosion-resistant austenitic matrix, such as twinning-induced plasticity (TWIP) and lighter-weight steels with induced plasticity (L-IP). Some even classify emerging multi- and nanophase materials into a third generation.
According to Schaeffler, DP steels—a family of high-strength steels with similar weldability and better formability as an HSLA steel of similar strength—are likely to continue dominating the AHSS market."In general, the amount of dual-phase applications is going to dwarf everything else combined," he said."With TRIPs and others, there are many more ingredients in the cake, and that means potential weldability issues and probably a higher price tag. But in the right application, [using TRIPs and other, more expensive steels] will be worth it."
Consider a DP blank formed in a drawing die. The blank is deformed into the desired product shape, but perhaps with some springback, side-wall curl, or edge cracking that wouldn't be expected with traditional HSLA or mild steel. What happened?
The material's unique makeup—starkly different from earlier generations of steel—is to blame. DP's microstructure could be described as an ocean of ferrite (the first phase) with islands of dense martensite (the second phase); the ferrite ocean gives the elongation and stretchability, the martensite islands the strength. Those martensite islands cause unique work-hardening characteristics that are the principal challenge when designing for and manufacturing with AHSS. The crux of the matter is that the n value (also known as the work-hardening exponent or strain-hardening exponent), which describes a metal's ability to distribute strain to avoid strain localization, actually changes during forming.
To demonstrate n value in his seminars, Anchor Manufacturing's Ulintz said he borrows an example he saw from Stuart Keeler, Sc.D., who developed the forming limit diagram approach.
"He had about seven people stand in the front of the room, and he had them each extend their hands out with open palms. They placed their hands palm to palm. In the center, you push the guy and tell him he needs to keep his hands in contact with his neighbors as long as possible. You push him to a point, with his arms moving backward, where he can't stay in contact anymore. That's analogous to a material with a very low n value and low work-hardening capacity. That material failed quickly.
"Now, we go back to the same arrangement, and he pushes the center again, but as soon as he pushes that center person, he grasps the hands of his neighbors. That's work-hardening. As he continues to deform, he pulls the people with him. The person next to him work-hardens, so they grab the hands of the people next to them. You can extend that line and push it out forward a much greater distance. When n value goes up, you can deform deeper and stretch farther."
However, in dual-phase material, the n value actually increases very rapidly at low strain rates (see Figures 1 and 2)."As soon as you start forming, you create lots of n value," he said."After you reach about 10 percent strain, the n value actually drops, similar to HSLA material. This is where issues arise with simulation. Historically, when we test material for n value, we're looking at an n value of a material between 10 and 20 percent strain. Well, the dual-phase and HSLA material will have the same n value in that range. We can't just take n values and plug them into the conventional simulation formulas. We need to use the entire stress-strain curve as input data."
In traditional steels,"the stress-strain curve is reasonably approximated by what is known as the Holloman relationship," Schaeffler added,"which relates K [strength coefficient] and n values to every given stress and strain. The K and n values are key elements to formability simulation. The Holloman relationship assumes the n value is constant throughout the forming process. And though it works well for mild and high-strength CMn and HSLA steels, it's a very poor assumption for AHSS."
Why does the n value change with AHSS and not with HSLA and others? As Schaeffler explained,"HSLA steel has a uniform, high-strength microstructure. Each grain has precipitates within it, and this uniform precipitation-hardening throughout keeps the yield and tensile strength high." Compare this with dual-phase, which has a ferritic matrix (essentially pure iron, which is very ductile), with discrete islands of martensite (a hard phase with limited ductility)."As a part is being formed with dual-phase, the lower strength and higher n value offered from the ferrite matrix allows for greater formability. In dual-phase, there is a much wider gap between the yield strength and tensile strength compared to an HSLA steel of the same strength, meaning you can do more deformation on more complex parts before it breaks."
But within that gap, the martensite islands increasingly cause the microstructure (using the terms in Keeler's example) to"grab hold" and work-harden at faster rates—hence, the varying n value.
Though DP's n value does complicate matters, it does help avoid undesirable strain gradients by distributing the strain more evenly across the part. As Ulintz explained,"We get a greater amount of n value early in the forming process. Consider a sharp feature in a HSLA stamping that, right away, as soon as we make contact with the material, we begin putting a lot of localized strain in the panel, which means that the panel may soon fail at this point. But with dual-phase steel, because it has that increased n value early, the strain is distributed more broadly earlier in the stroke. This reduces the amount of localized straining, which enables you to get more stretch by delaying the onset of necking and failure."
AHSS material doesn't play by the traditional formability rules. The material curls easily; twisting is commonplace; edge cracking crops up frequently; thin material requires very robust tooling that may be PVD- or CVD-coated—all for a material that's supposed to have high formability.
Issues arise because controlling the way AHSS material flows requires a bit of counterintuitive thinking, which leads to some new thinking from a tooling standpoint.
For dual-phase,"we need to stretch the side walls to eliminate the side-wall curl that results when forming this material," Ulintz added."Because they're stronger, they have more springback, so we have to be able to remove that springback. One method of removing springback is by poststretching the material. After forming the material most of the way into a die cavity, we may choose to use active beads that rise up near the end of the punch stroke to lock out the material so it no longer draws into the cavity. So the last few millimeters of the stroke ends up being stretch. Stretching that material helps to balance the stresses and reduce springback."
Along that same vein, die designers must pay careful attention to die-entry-radius-over-thickness (r/t) ratios. Large die-entry radii can result in dimensional issues, while tight radii work-harden the material rapidly and drive up binder tonnage requirements. These concerns, along with the high tensile strengths of the AHSS grades, often necessitate robust tooling with PVD or CVD coating."Across industry, we often overengineer our dies," explained Ulintz."However, even these overengineering practices will prove not to be nearly robust enough for AHSS processes."
According to Ulintz, many maintain a kind of "tunnel vision" when tackling an AHSS project. It's not just about building a die, he said, but building the entire process around the new material."Most of all, we overestimate the capability of the pressroom equipment to handle the material. We think the material is rolled into the steel coil, and we feed it like any other job. The rated capacity of most straighteners is based on processing relatively mild steel with yield strengths below 340 MPa. AHSS materials, due to their higher yield strengths, have a greater tendency to retain their coil set. This requires greater horsepower to straighten the material within an acceptable level of flatness. It's not reasonable to expect higher-strength materials to run through the same straightener as conventional mild steels for a given material width and thickness. AHSS materials require larger-diameter rolls and wider roll spacing in order to work the material effectively."
Consider an AHSS material 2 mm thick but three to four times stronger than low-carbon steel."You have to envision that material being up to four times thicker, so now you need to have material handling equipment, tooling, and press equipment to handle 8-mm-thick material because of the forces that will be generated," Ulintz explained."And we don't intuitively see that, because we just see 2-mm-thick material.
"You also have greater snap-through forces," he continued."The higher tensile strength requires more force from the ram to cut through the material. The ram puts all this pressure on the material and then all of a sudden it is released when the punches cut through. The ram wants to continue to go downward, while the crankshaft is trying to reverse its direction for the upstroke. So the machine is literally trying to pull itself apart. In addition, you also have an increase in die mass [weight] because the tools have to be thicker and more robust to withstand the higher forces, which adds to the snap-through energy being released.
"Many older presses can withstand about 10 percent reverse-energy tonnage. [But with AHSS], we're hearing reports of upward of 30 percent," he added."We're destroying pressroom equipment because we're not aware of the reaction forces going on when we're running these materials."
The higher-strength material also requires more restraining force, meaning more blank holder pressure."The result of going to an AHSS is you can make the material thinner," Ulintz said."But the thinner material is more likely to buckle, which means you need even more blank holder pressure. And in some instances, you cannot do it with nitrogen cylinders. You may need to be in a hydraulic or toggle press," depending on how large and thin the part is.
Ulintz added that edge condition becomes extremely important when edge stretching is involved."You need to have clean edges in order to be able to stretch the flanges. AHSS material has reduced edge stretchability. In a low-carbon or HSLA application, we might not need to pay that much attention to the condition of the sheared edge, because there's enough ductility in the material to form a flange or expand a hole, or whatever else it is we're doing. But because the advanced high-strength steels have a high sensitivity to edge stretching, the edge condition is critical." All the stress will locate at any imperfection, be it a hot spot from laser cutting or a burr from a trim die.
AHSS also changes things for the die maintenance department. Ulintz mentioned a recent study cited in the International Steel and Iron Institute's AHSS Design Guidelines which showed that burr growth doesn't happen in the conventional way with AHSS, because AHSS appears to fracture differently and is less ductile than HSLA."What ends up being the criteria for die maintenance is punch wear," Ulintz said,"rather than burr generation."
Put another way: The tools wear without significant burr growth on the part, which means die maintenance personnel have to play by new rules.
Ulintz concluded that most AHSS development challenges boil down to strong and light, two adjectives that have remained a dichotomy throughout most of metal forming's history.
"One of the advantages of going to the advanced high-strength steels is that automakers can down-gauge. Now you complicate the issue. I used to have a 2-mm-thick HSLA material, and I'm going to down-gauge to a 1.8-mm-thick advanced high-strength steel. The material is thinner, but the die has to be designed and built as though it were three to four times thicker."
Thinner material requires more robust dies and presses. How's that for counterintuitive thinking?
David Darling, director of operations of MBtech Autodie, Grand Rapids, Mich., proudly pointed to some galvanized AHSS parts on display at the company's booth during the METALFORM tradeshow in April. "These were all formed with one hit on bottom, without lubrication, in place, one time, without having to recut [the tools] over and over." Darling has reason to be proud. Those AHSS structural body panels represent quite an accomplishment for a team that experienced some tough times in the tool and die world. The predecessor company of what is now called MBtech Autodie LLC had, earlier this decade, been involved with an advanced high-strength steel project that went awry.
"The whole thing started with a project that went horribly wrong," Darling said.
The company, then a tooling house called Autodie International Inc., like many others dumped money into tooling development that didn't lead in the right direction, this at a time when toolmakers everywhere seemed to be closing up shop. While Darling kept mum on details, he did say the investment wasn't wasted entirely.
In fact, it laid the foundation for some pioneering work in AHSS, and thanks to this MBtech Autodie now seems to be in the right place at the right time. MBtech Autodie's parent, global engineering and consulting company MBtech Group, which purchased Autodie in 2006, has become a principal participant in the Lightweight Body Project. Other players include Daimler, Chrysler, and the American Iron and Steel Institute.
Although MBtech Autodie's shop floor may look like a parts production environment, it's not. The 300-employee firm has kept its focus on process development, tooling engineering, and die construction, devoting its 31 presses exclusively to die tryout. In fact, the company has plans to up its finite-element simulation efforts to reduce tryouts for most materials and free up more press time for its most challenging projects, AHSS jobs among them.
Jon Brouwer knows these challenges firsthand. Like everyone else who delves into AHSS, the CAE leader at MBtech Autodie went through a difficult learning curve. "The biggest challenge for us was to understand how the material truly flows during the process."
Knowing the material properties, Brouwer said the company's AHSS team decided to take a unique approach. "Many try to focus on 'anticipate and compensate'—anticipate springback and accommodate for it," he said. "Instead, we control it or eliminate it. Trying to win this battle exclusively by anticipating and overcompensating is where we feel most companies struggle or fail, whereas learning to control it or eliminate it is really where success lies." He added MBtech Autodie does still anticipate springback through finite-element simulation as well as through conventional compensation of tooling. However, the company uses various die and forming control measures to control and minimize springback as much as it can.
Darling added that the company focuses on material properties and specific tool and part geometries. "Without giving away too much—it's like our formula to Coca Cola—we can say that tool geometries and forming processes play a critical role. With that, we hope to eliminate or minimize the potential of springback occurring."
By maximizing material flow in some of the strongest alloys, the company has now come up with solutions for stamping unitized autobody parts out of various AHSS materials. The research has evolved from simple hat geometries to rails, pillars and cowl-side posts. Today the company's R&D efforts—which also include aluminum—have allowed it to form entire door ring inners from AHSS in one hit under the press.
"We've formed a door ring for a sedan out of TRIP 700 material," Darling said. The door ring combines A and B pillars, cowl side, sill, and a roof rail in one part and offers a reduction of assembly/weld time and costs.
Will the company's investment pay off? OEMs and Tier 1s may need to make use of higher-tonnage presses to form AHSS parts, so there might be some significant capital outlay decisions to make for many. But MBtech Autodie managers are forecasting that the industry will make those investments, considering recent studies projecting AHSS's future growth, improved crash worthiness, and, most important, the cost of fuel.
Success has come from some serious die-building expertise; the company employs die engineers and die-makers that have an average of 18 years' experience. But success also came about through new thinking. As Darling put it: "We've had success from concentrating on the specific forming challenges at hand, and sometimes perusing counterintuitive solutions to conventional diemaking methodologies."
AHSS materials could be compared to where HSLA steels were in the '70s. Material production standards haven't evolved, so variability abounds. Two materials labeled as identical grades coming from different mills may well have different welding properties even though tensile properties may be similar.
"It is rare to find two companies taking the same alloy-development approach," said consultant Daniel Schaeffler, "because each mill has different equipment and practices that affect the balance of alloying elements that must be used to get a given set of tensile properties. The implication is that different mills will use different chemical recipes to produce the same grade. Although the sheet strength and elongation may be similar, the carbon equivalent will be different." That, in turn, means different weldability.
He added that this, along with the different flavors of the same grade also being produced—optimized for yield strength, bendability, or hole expansion—means that the "end user must be extra diligent when it comes to spot buys or resourcing issues."
Nevertheless, he said that market forces in time will remedy the issue. As demand rises and production volumes increase, AHSS will become less variable as mills standardize operations. Until then stampers use tensile tests to get real data about the actual material in question, because as yet no reliable data bank exists.
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