High-strength sheet steel that bends

  • 22-May-2014 11:28 EDT
NanoSteel Structural Formation.png

Principal steps in the formation of unique nanostructures in NanoSteel AHSS sheet alloys. (Click the arrow at top right of image to view more images.)

Basic car structures have traditionally been stamped from sheet steel, but stricter fuel-economy standards have the auto industry turning increasingly to more costly alternative materials including thin-gauge advanced high-strength steel (AHSS) grades that unfortunately do not stamp well, or light-but-strong aluminum alloys and fiber-reinforced polymer composites, which entail costs not just from the materials themselves but from the need to retool assembly lines and reconfigure supply chains to use them.

Metallurgists in industry and academia are working overtime to develop so-called third-generation AHSS grades whose enhanced physical properties—high strength plus ready formability—aim to keep steel in the auto frame game. But those developmental high-performance steel alloys have yet to hit the market.

Now a dark-horse newcomer is trying to elbow its way into this budding market. NanoSteel, a family of nanostructured ferrous alloys, offers high elongation properties at ambient temperatures, which provides cold-forming capabilities not seen in current high-strength steels, which are brittle.

NanoSteel of Providence, RI, which developed the new AHSS alloys, is working to provide samples for trial by automakers such as General Motors as the new metals undergo production trials, said the inventor, Daniel Branagan, Chief Technology Officer and founder. The company hopes to license the technology to industrial users.

New class of steel sheet

“Today, when car designers are fighting to save every gram, steel mills have found that current AHSS alloys are difficult to form, which has opened opportunities for aluminum and composites,” Branagan said. “We’re trying to keep steel in cars with what we think is a game-changing technology with much better formability. We made NanoSteel sheet so that designers could use less steel to get lighter but still maintain safety.”

“This is a whole new class of sheet steel alloys with unique combinations of properties that we make using a pathway that differs from those of conventional steels,” he claimed.

The metals, he explained, exploit new mechanisms to provide high strength plus cold formability, which include using novel alloy chemistries, unique nanoscale grain and phase structures less than 100 nm in width, and specific structural formation pathways that have not been used before. In addition, “everything we develop is designed to be used on existing production equipment,” which is crucial to early adoption by industry.

The route that Branagan took to develop NanoSteel stretches almost two decades. It began, he recalled, in the 1990s at the Department of Energy’s Ames Laboratory in Iowa, where he was working on nanomagnetic materials such as neodymium-iron-boron magnets. The scientist designed nanomaterials for enhanced magnetic resistivity and energy density, often using devitrified metallic glasses (amorphous) and refining the grain sizes to small diameters to achieve better single magnetic domain behavior.

Eureka moment

“Then in 1996 at Idaho National Lab, I had my ‘Eureka!’ moment when I realized that I could apply the same techniques to steel,” Branagan said. “But rather designing for enhanced magnetic properties, I saw that if you could refine the grain size you could get much more strength. Right away I looked up the theoretical strength of iron’s atomic bonds and found that most steel is around 10% to 12% of that value, so it was clear that a lot could be done to harness more of the total potential strength of iron compounds.”

By 2002, Branagan and his colleagues had formed a company to commercialize his patented research, always with the ultimate aim of producing nanostructured steel sheet somewhere down the road.

“We’ve been following the same roadmap ever since—a kind of golden path—that we hoped could bring what was considered an old technology into the 21st century. Of course, nowadays high-strength steel is in the technical news everyday,” he said.

NanoSteel’s initial product offering was a micron-thick protective coating for oil mining that provided “basically the hardness of a ceramic like alumina and the wear-resistance of tungsten carbide,” he said.

In the next few years, the company introduced a thermal spray technology that produces a shield coating about a thousandth of an inch (25 microns) thick, and then a weld-overlay product that creates layers from 10 to 12 mm (0.39 to 0.47 in) thick.

Many rivers to cross

“But back in 2006 there were many tough challenges to solve,” Branagan continued. “Once we turned our focus toward cold-formable high-strength sheet steel, we were confronted with the fact that though we could get to strengths that were around 50% of theoretical maximum, we had essentially hit a wall. We could create nanostructures with sufficient strength, but developing the high ductility we needed turned out to be an even bigger challenge.

“We soon realized that all existing strategies were dead-ends for this because all the kinetics are wrong,” he said. “Steel alloy grains grow at 80% or 90% of its melting temperature.” How could a nanostructured metal “get through a large-scale industrial steel process where you have very high temperatures up to and above steel’s melting point and slow cooling rates? For example, you can heat-treat a coil of sheet and it can take 6 hours for its condition to homogenize.”

For the next few years NanoSteel’s R&D team searched for a mechanism that could provide ductility at high temperature. “Eventually, we found the kinetic and reaction pathways—the thermochemistry—to bring the nanostructure to where we wanted.”

As in all of its products, the company’s sheet steel solution begins with alloy compositions that differ greatly from the norm, with 10 to 20 atomic percent of P-group elements—the “boron-carbon-nitrogen-oxygen-fluorine” rank of the periodic table, which are much higher than standard levels.

“These ‘nontraditional’ alloying elements are actually widely used conventionally, but only sparingly because they make steel strong but brittle,” he said. “In our case, we had started out using very high atomic percentages of P-group elements. That’s how we achieved our high strengths.”

High-temperature shrinkage

Then the researchers found ways to enhance ductility via unique microconstituent nanostructures that, contrary to expectations, undergo static nanophase refinement at high temperatures rather than coarsening, producing grains and phases that are an order of magnitude smaller, together with the ability to strain-harden during cold deformation with the creation of nanoscale precipitates through a “dynamic nanophase-strengthening mechanism.”

The first process yields tiny matrix grains of dendritic (finger-like) austenite crystals with complex boride pinning phases that keep everything in place, while the second transforms the austenite matrix grains to ferrite with high fractions of nanoscale phase precipitates.

“This is a different kind of nanotechnology,” Branagan concluded. “People are more familiar with particulate efforts that focus on building nanomaterials one atom at a time or from small groups of atoms. We’re kind of doing the same thing but in bulk by using thermochemical-enabling mechanisms that allow the structures to change at high temperatures and still have all the atoms simultaneously form the nanostructures we need.”

(A recent SAE Technical Webcast covering “Lightweighting with Multi-Material Vehicles” included Craig Parsons, President of Automotive, NanoSteel, among other speakers. Go to https://event.webcasts.com/starthere.jsp?ei=1031713 to access the webcast.)

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