New high-strength, room-temperature-formable steels sought

  • 06-Aug-2013 08:43 EDT
auto-steel-project2.jpg

A Colorado School of Mines researcher heat-treats a machined tensile specimen to the desired microstructural state.

Many of the critical elements of a vehicle’s crash structure such as the B-pillars and frame rails are often made of a high-strength steel (HSS) called press-hardened boron steel. This specialized steel grade (with 0.002% to 0.005% boron) is especially good at absorbing large amounts of impact energy and offers strength levels that allow design engineers to use thinner sections and thus save weight. Although designers like to specify boron steels for complex, precision components that must be especially strong, they are both expensive and difficult to process.

Making vehicle components from boron steels requires, for example, that the blanks be heated red-hot—as much as 950°C (1742°F)—before they can be hot-stamped in huge presses at pressures between 800 and 1200 metric tons to achieve the desired geometries. But that’s not all: to achieve the steel’s prized physical properties the press must be equipped with water-circulating cooling passages to quickly cool, or quench, the hot-formed blank to about 200°C (392°F) in the die. This press-hardening portion of the hot-stamping process is how the microstructure is converted from a more ductile phase state (predominantly, the austenitic crystalline form) that was created by the high heat to a harder phase (mostly the martensitic crystalline form) that is produced by the rapid cooling.

Planners at the U.S. Department of Energy (DOE) hope to avoid the need to consume the huge amounts of energy and investment that are expended to produce hot-stamped auto parts, so they have funded the Colorado School of Mines (CSM) and the Los Alamos National Laboratory (LANL) to conduct a three-year, $1.2-million collaborative research effort to develop a new class of advanced high-strength lightweight steels that can be formed into parts at room temperature. CSM professor Emmanuel De Moor leads the project.

The use of new Q&P (quenched and partitioned) steels in the production of 10 million cars annually could save nearly 30 trillion Btu/year, which as natural gas could meet the energy needs of some 300,000 to 330,000 American homes for a year.

Several of the industrial members of CSM’s Advanced Steel Processing and Products Research Center, a National Science Foundation-supported Industry/University Cooperative Research Center, are also participating in and supporting the program. These corporate partners—steel producers AK Steel, Nucor Steel, Severstal, and US Steel, and automakers General Motors and Toyota—are providing more than $300,000 in cost-sharing funds.

Q&P processing route

The processing method that the CSM/LANL research team will pursue to achieve the desired material capabilities is called quenching and partitioning, explained Kester Clarke, R&D engineer at Los Alamos, who focuses on materials processing.

“It’s one of several metallurgical alloying and processing approaches that are being investigated for making what the industry calls ‘third-generation’ advanced high-strength steel grades, ones that could go into stronger, safer, and lighter weight automobiles that are still affordable,” he said.

In this approach and many of the others, metallurgists “are trying to develop an ‘organic’ composite that is made of intermixed harder and softer regions, which can give you a combination of strength and ductility that’s often difficult to achieve,” he explained.

Q&P is a new way of producing martensitic steels containing enhanced levels of retained austenite after heating. The steel is quenched from high temperature to produce a partially martensitic, partially austenitic microstructure. The following partitioning step aims at enriching the concentration of carbon in the austenite by partially depleting the carbon in the martensite and transporting it to the austenite. Consequently, carbon-stabilized austenite is retained in the microstructure after final quenching to room temperature.

When using Q&P techniques, Clarke said, “we’re trying to partition certain useful alloying elements to certain areas of the microstructure. Essentially we’re trying to take the carbon out of the martensite, the hard part, and move it (diffuse it) into the austenite, the more ductile part, to stabilize it. In doing so, you end up with metastable austenite, which results in a so-called TRIP effect, which means it now has what we call transformation-induced plasticity.”

Later, if the finished components are deformed in a road crash, the carbon-rich austenite will transform to martensite, which is much harder and resistant to plastic flow, physical properties that are crucial for ensuring safety.

“Although CSM professor John Speer started working on Q&P 10 years ago,” he said, “alloying strategies have not yet been developed to use Q&P in existing steel plants, which generally have limited heat-treating flexibility.”

Alloying additives and thermal processes

“Our thermodynamic databases on steels let us predict what certain alloying additives will do under equilibrium conditions, but getting the specific reaction kinetics right is difficult since there are competing reactions, which make it hard to put a finger on the exact mechanism over time,” Clarke said. “Through further study and experimentation, we’ll choose the best-performing, most cost-effective alloying strategy and then develop an ideal heat treatment to achieve the alloy’s best physical properties.”

The researchers will take advantage of specialized equipment at Los Alamos such as a quench dilatometer to measure the tiny dimensional changes that occur when the austenitic phase (which has a certain crystal unit volume) transforms into martensite (which has a different characteristic unit volume) during heating and cooling.

Advanced microstructural characterization techniques, including electron microscopy, neutron diffraction, and bulk thermal- and deformation-processing capabilities will be used to simulate and predict industrial-scale processing.

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