3rd-generation advanced high-strength steel

Dr. Neil Canter, Contributing Editor | TLT Tech Beat February 2013

Researchers develop high-performance steels exhibiting high tensile strength and higher fracture toughness.

KEY CONCEPTS
• The steel industry has developed two generations of advanced high-strength steels that exhibit higher tensile strengths but lack fracture toughness.
• A third-generation advanced high-strength steel achieves the desired fracture toughness through a duplex microstructure.
• Further testing of the third-generation product will evaluate its machinability, welding capability and susceptibility to corrosion.

THE PUSH TO INCREASE THE FUEL ECONOMY OF AUTOMOBILES is continuing to lead to an increasing demand for lighter vehicles. In the U.S., the Corporate Fuel Economy (CAFE) has been set for 34 mpg by 2016 and will be significantly increased in a second phase to 54.5 mpg by 2025.

As a consequence, automotive manufacturers have long sought to increase the use of lighter metals such as aluminum at the expense of steel. In response, the steel industry is developing advanced high-strength alloys that provide superior strength and toughness while saving vehicular weight.

In a previous TLT feature article, a detailed look was provided on the development of advanced high-strength steels (AHSS), as well as information about the challenges faced in recommending the right lubricant type to be used in metal-forming operations (1). Lubrication selection is determined by factoring in such parameters as the workpiece, tool surfaces used and the forces and temperatures involved.

Susil Putatunda, professor of chemical engineering and materials science at Wayne State University in Detroit, says, “Conventional steels exhibit high strength but do not display sufficient toughness. They are prepared by heating iron to a high enough temperature to form an austenitic crystal structure, which is then quenched to form the desired martensite structure. Other metal alloys are also often added to improve the steel’s mechanical properties.”

From this process, steel has a tensile strength in the range between 500 and 700 megapascals (MPas). To improve the properties of steel, two generations of AHSS have been developed to date.

In the early 1980s, a first-generation AHSS was developed in the traditional way that steel is produced. Putatunda says, “This first-generation steels have a combination of a ferrite, martensite structure but does not exhibit sufficiently high tensile strength (only up to between 700 and 800 MPa) and fracture toughness to be effective in reducing significant weight in automobiles.”

A second-generation AHSS was developed in the 1990s to try to improve both the performance and enable significant weight reduction in automobiles to become feasible. Putatunda says, “This steel contains a metastable austenite that is stabilized through the use of a high percentage (20%-25%) of manganese. This enables the unstable austenite phase, which typically exists above 700 C, to become stable at room temperature.” The austenite phase transforms into a martensitic structure on deformation and contributes to enhance strength and toughness in these steels.

An example of a second-generation AHSS is Twin Induced Plasticity steel. While the tensile strength is increased to between 800 and 900 MPa, Putatunda maintains that this is still not sufficient to achieve significant weight reduction. Cost is also a problem due to the high treat cost for using manganese.

These first two generations of AHSS may have higher tensile strengths, but the structures have suffered from having reduced fracture toughness. One added problem is the high level of austenite present also adds to the difficulty in processing these steels and consequently their cost.

There is a need for a higher performing AHSS that contributes significantly better mechanical properties without a high percentage of alloying elements that is cost efficient to produce. Now a potential third-generation AHSS has been produced that addresses these issues.

DUPLEX MICROSTRUCTURE
Putatunda and his researchers have developed a third-generation AHSS that exhibits high tensile strength and higher fracture toughness. He says, “We have combined ultrafine grain ferrite and austenite into a duplex microstructure that displays superior performance.”

The steel is initially forged and annealed at 900 C for one hour. This is followed by a two-hour, austempering heat treatment process at 927 C that produces a high bainitic steel with the austenite, ferrite duplex microstructure. Figure 2 shows an optical microscopy image of the microstructure.


Figure 2. This optical microscopy image shows a third-generation advanced high-strength steel. This steel exhibits superior physical properties, making it a good candidate to improve the fuel economy of automobiles. (Courtesy of Wayne State University)

Other elements were incorporated into the steel to improve its physical properties. Putatunda says, “We added a small percentage of silicon to minimize the formation of cementite (iron carbide) that could adversely affect the properties of the steel. Small quantities of chromium, manganese and molybdenum were also added to maintain the hardenability of the steel during the austempering process.”

The tensile strength for this third-generation AHSS was 1388 MPa, which represents a significant increase over the second generation. Fracture toughness was much improved and found to reach a level of 105 MPa.

Putatunda says, “Our next goal is to improve the ductility of the steel. Besides helping with weight reduction in automobiles, we also believe that this steel can be used in armor plating.”

Further testing of the third-generation AHSS will be done to assess its machinability, welding capability and its susceptibility to corrosion. Putatunda adds, “No machinability tests have been done on this steel to date, but we believe that there should not be any problems.” He cites the presence of a high concentration of ferrite that is soft and ductile.

Additional information on this work can be found in a recent reference (2) or by contacting Lori Simoes at the Wayne State Technology Commercialization Team at ttoinfo@wayne.edu.

REFERENCES
1. Rensselar, J. (2011), “The Riddle of Steel: A-UHSS,” TLT, 67 (7), pp. 38-46.
2. Martis, C., Putatunda, S. and Boileau, J (2013), “Processing of a New High Strength High Toughness Steel with Duplex Microstructure (Ferrite + Austenite),” Materials and Design, 46, pp. 168-174.

Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.