Lightweight steel is stronger than titanium

  • 11-Mar-2015 11:04 EDT
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The annealed microstructure of the new lightweight steel shows the FeAl-type B2 precipitates between B2 bands. (Hansoo Kim)

Owing to its strength, formability, joinability, and affordability, steel has been the structural material of choice for mass-produced motor vehicles since it replaced wood in the 1920s, but that doesn’t mean that automakers and their materials suppliers ever stopped searching for better alternatives. Witness the recent efforts to build lightweight cars from aluminum and carbon-fiber composites; however, these substitute substances are generally more costly than steel.

Recently, however, three materials scientists at the Graduate Institute of Ferrous Technology at Pohang University of Science and Technology in South Korea have come up with another potential option—lightweight steel. Professors Hansoo Kim and Nack J. Kim, together with doctoral student Sang-Heon Kim, have developed a low-density steel alloy that exhibits higher specific tensile strength and ductility than titanium alloys—the lightest and strongest metals known, but potentially at one-tenth the cost, according to a paper published in the February 5th issue of the journal Nature. (See http://www.nature.com/nature/journal/v518/n7537/full/nature14144.html)

“Because of its lightness, our steel may find many applications in automotive and aircraft manufacturing,” Hansoo Kim stated in an e-mail communication.

Probably the most surprising point about the new steel composition is that it gains its mass advantage through the addition of aluminum, a low-density alloying agent that had been tried many times before but had always yielded unsuitably brittle results. Decades ago metallurgists in Russia and elsewhere attempted to add aluminum to steel, and even though the resulting metal was very strong and lightweight, it invariably had little ductility—that is, when subjected to large forces, it would break rather than bend. Manufacturing products from a low-ductility metal is very difficult.

Photomicrography studies subsequently revealed that the experimental aluminum-rich steel alloys contained a very hard but very brittle cubic crystal of iron and aluminum called B2 that made them mostly unusable. B2 is an intermetallic compound—a crystalline material in which different elements replace other more typical elements in certain atomic sites. In the previous high-aluminum steel formulations, the B2 intermetallic compounds tended to arrange themselves into brittle bands at which the material would shear off when stressed.

Dispersion-strengthened steel

“My original idea was that if I could somehow induce the formation of these B2 crystals, I might be able to disperse them in the steel,” Kim said. He and his colleagues realized that if nanometer-scale B2 crystallites were uniformly distributed as a secondary phase throughout the steel’s ductile austenite (face-centered cubic crystal) primary alloy phase, they would strengthen the whole by halting microscopic crack propagation much like strong carbon fibers serve to reinforce the more flexible resin matrix in a polymer composite material.

After spending years researching the concept, the trio found that by adding nickel to the mix (which includes carbon and manganese besides iron and aluminum) and then specially annealing, or heat-treating, the solidified metal, B2 precipitates would evenly permeate the metal in nanometer-sized clusters rather than long bands. The small percentage of nickel, which reacts with the aluminum, offered greater control over B2 formation, as nickel made the crystals precipitate out at a much higher temperature.

Electron microscope images confirmed that Korean scientists had achieved their desired micromorphology, and tensile tests showed that the novel alloy, Fe-10%Al-15%Mn-0.8%C-5%Ni (weight percent), was strong and ductile.

“We developed a new type of flexible, ultra-strong, lightweight steel that is 13% less dense than normal steel and has a strength-to-weight ratio that matches even our best titanium alloys,” Kim said.

Production process tests

In their experiment, the researchers melted about 40 kg (88 lb) of the steel alloy in an induction furnace with a protective argon atmosphere and cast it into a rectangular ingot, Kim reported. Following a homogenization treatment—1150°C (2102°F) for 2 hours—the ingot was hot-rolled into strips 3 mm (0.12 in) in thickness. The hot-rolled strips were cold-rolled into 1-mm (0.04-in) -thick sheets that were next annealed at 870 to 900°C (1598 to 1652°F) for 2 to 60 minutes. The sheets were then immediately water-quenched or rapidly cooled to 25°C (77°F).

“All the steps except for the casting are very similar to the existing processes for industrial sheet steel production,” he noted.

Subsequent joining tests showed that “our steel can be welded by electrical resistance spot welding, laser welding, and argon TIG welding,” Kim said.

He stressed that the team’s B2-dispersion method is really more important than the new alloy: “Steel scientists all over the world can make many variants of our steel for their own applications based on the novel microstructure, which comprises a steel alloy matrix and intermetallic precipitates.”

The Pohang University researchers are now working with the South Korean company Posco, one of the world’s largest steel manufacturers, to scale up their technology.

“We are planning a mill trial production of our steel this year at Posco, not for direct commercialization but for checking possible difficulties that are frequently met during scale-up,” he said. “If everything goes smoothly, you may see our steel on the market in two to three years.”

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