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Posted: May 10, 2010

Nano-engineered steels for structural applications

(Nanowerk Spotlight) Steel is one of the most widely used engineering materials in the world. Its pre-eminent position amongst the engineering materials arises due to the abundance and low cost of its main constituent, i.e. iron, and its amenability to produce a wide variety of engineered microstructures with superior properties, and recyclability.
Currently, there is a growing awareness about the potential benefits of nanotechnology in the modern engineering industry, and a number of leading R&D institutes and companies are pursuing research in the area of nanostructured steels. The focus of the ongoing efforts has been largely manipulation of microstructures at the nano-scale through innovative processing techniques and adoption of novel alloying strategies. This is being aided by employing advanced characterization methods like high resolution transmission electron microscopy (HRTEM), atom probe tomography (APT) etc. and computational design of materials.
Steel is synonymous with strength. The theoretical strength of steel is 27.30 GPa (in <111> direction). There are two ways of achieving ultra high strength in steels. The first one is to reduce the size of a crystal to such an extent that it is devoid of any defects, like in the case of a whisker. Brenner in 1956 could achieve a tensile strength of greater than 13 GPa in an iron whisker. The second alternative is to introduce a very large density of defects in a metal sample that act as an obstacle to the motion of dislocations. This has been illustrated by drawing high carbon pearlitic steel wire, which is subjected to intense plastic deformation, thereby, introducing dense dislocation substructure. The carbon steel wire is a remarkable example of nanostructured steel produced on a mass scale. The strengthening arises due to the presence of nanoscale cementite/ferrite lamellar structure. The ferrite phase in this structure contains very high dislocation density and supersaturated carbon atoms, and the cementite phase contains amorphous and nano-crystalline regions. The high carbon steel wire is an important engineering material used for reinforcing automobile tires, galvanized wires for suspension bridges and power cable wires. In fact, the suspension cables of the world’s largest suspension bridge, Akashi Strait Bridge built in Japan in the year 1998, were made of pearlitic steel wires of 1800 MPa strength. To inhibit softening during hot-dip galvanizing, high-Si and high Si-Cr steel wires have also been developed for high-strength galvanized suspension-bridge wires. Similarly, the pearlitic wire for automobile tyre cords exhibits strengths of about 4000 MPa.
The main challenge in realizing the immense potential of nano-engineered steels is to manufacture large components of bulk nanocrystalline steel having superior properties and at a reasonable cost. To meet this challenge, a number of innovative approaches are being developed to produce nanostructured steels, as shown in Fig. 1.
Routes to Produce Nanostructured Steels
Fig. 1: Routes to Produce Nanostructured Steels.
The different processing strategies and alloy development aspects being currently explored for the manufacture of nanostructured steels are briefly outlined below.
Severe Plastic Deformation (SPD) Processing
SPD processing is one of the promising routes for grain size refinement to nano-scale levels. Non-traditional processes, such as equal channel angular processing (ECAP), accumulative roll bonding, torsion under very high pressures, multiple compressions etc. have been developed for this purpose. The ultra-fine (<1µm) grain sizes lead to exceptionally high strengths in conventional steels; however, there is a drastic reduction in tensile ductility, especially uniform elongation in tension. Therefore, a number of processing routes are being developed for the improvement of ductility.
Thermomechanical Controlled Processing (TMCP)
TMCP is based on microstructure control during hot rolling and subsequent cooling. Many of the microstructural events are controlled at the micron level while other events like precipitation hardening are at the nano-scale level of control.
Nanostructured Steels with High Work Hardening Rate by Exploitation of TWIP Effect
The high-manganese TWIP steels are subjected to plastic straining to introduce thermally stable nanometre-scale mechanical twins in the structure. Subsequent recovery treatment results in an excellent combination of high yield and ultimate tensile strengths, and work hardening.
Phase-reversion Induced Nano-grained/Ultra-fine Grained Steels
Microstructures comprising an optimized combination of nano- and ultra-fine grains are obtained in austenitic stainless steels by controlled annealing of heavily cold-worked metastable austenite. Reversion annealing of strain-induced martensite in severely deformed metastable austenitic steels results in nano-grained/ultra-fine grained structures with excellent combination of strength and ductility.
Computational Designing of Steels
A new class of martensitic stainless steels are being developed by following a system’s design approach. This approach combines predictive control of the alloy chemistry, transformation temperatures, cryogenic treatment and multi-step aging to produce radically new high-strength, high toughness corrosion resistant stainless steels.
Devitrification of Glassy Ferrous Alloys
Metallic glasses based on the specialized formulation of ferrous alloys have been developed. These glasses are subjected to devitrification treatment by subsequent heating above crystallization temperature to obtain nanoscale microstructures. These amorphous steels can also be used in the form of powders to produce amorphous/nanocomposite thermally sprayed coatings to enhance the wear and corrosion resistance of engineering components.
Advanced ODS Ferritic and Martensitic Steels
Ferritic or martensitic alloy powders are ball milled with Y2O3 and subsequently compacted and hot extruded to obtain nano-structured ferrous alloys. These alloys contain a large number density of ultra-fine cluster of atoms containing predominantly Y, O and Ti, called nano-clusters, which resist coarsening and prevent grain growth following isothermal aging.
Mechanical Alloying and Consolidation
Mechanical alloying via high energy ball milling of iron and carbon powders (or other alloying elements) is carried out and the powders are subsequently consolidated by various techniques such as spark plasma sintering, warm compaction, HIP etc. This approach can result in nano-crystalline/ultra-fine grained structures with excellent mechanical properties.
Combination of TRIP Effect with Maraging Treatment
This approach combines the TRIP mechanism with maraging treatment in a Fe-Mn base alloy system. These steels contain a low-carbon martensitic matrix with precipitates of intermetallic (Ni, Ti, Al, Mo) nano-particles.
Surface Nanocrystallization of Steels
A nanostructured surface layer can be fabricated by subjecting the steels to various surface treatment techniques, such as ultrasonic shot peening and surface mechanical attrition treatments (SMAT). SMAT provides a simple, flexible and low cost approach to enhance the bulk properties of steels, without any change in the chemical composition.
Advanced Bainitic Steels by Low Temperature Isothermal Transformation
New generation bainitic steels (e.g. Fe-0.98C-1.46Si-1.89Mn-0.26 Mo-1.26Cr- 0.09V) are designed using detailed phase transformation theory for the bainitic reaction. The bainitic transformation occurs at low temperatures (200–300oC), which avoids the diffusion of iron or any substitutional solutes. As a consequence, the plates of bainite are extremely slender, 20 – 40 nm thick, making the steel very strong.
TRIPLEX Steels
TRIPLEX steels are designed on the basis of Fe-Mn-C-Al with Al > 8%. Mn is usually > 19%. The alloy consists of an austenitic FCC matrix and about 8% ferrite and nano-size k-carbides regularly distributed in the FCC matrix in an orderly fashion. The TRIPLEX alloys exhibit low density, high strength level, excellent formability and high energy absorption capacity.
Applications of Nanostructured Steels
Applications of Nanostructured Steels
Key Players
Currently, interest in nanostructured steels is just beginning to gather momentum. However, with the entry of industrial giants like Nippon Steel, Sandvik, Arcelor Mittal, Exxon, JFE Steel and others, there is a good scope that broader industrial adoption could occur in the near future. Moreover, some of the new players such as QuesTek Innovations, Max Planck Institute for Steel Research, MMFX Technologies and Cambridge University are able to demonstrate significantly greater benefits in nanostructured steels at a reasonable cost with their innovative approaches and this is likely to change the scenario quickly. A few key players active in the field of nanostructured steels are listed below:
The NanoSteel Company, Inc., U.S.A.
Technology: Has developed nanostructured ferrous alloys by devitrification of metallic glass. The alloys are used in the form of thermal spray coatings or weld overlay to tackle the problems of wear, corrosion, erosion etc.
QuesTek Innovations LLC, U.S.A.
Technology: Has developed computationally designed high strength and environmentally friendly corrosion resistant steels.
Sandvik Materials Technology, Sweden
Technology: Has developed nanostructured ‘Nanoflex’ stainless steels.
JFE Steel Corp., Japan
Technology: Has developed hot rolled, high strength nanosize carbide precipitation strengthened ‘NANOHITEN’ steel for automobile industry.
Kawasaki Steel Corp., Japan
Technology: Has developed non-heat treated ultra-low carbon, Cu precipitation strengthened bainitic steels produced by thermo-mechanical precipitation control process (TPCP).
Kobelco Research Inc., (Kobe Steel), Japan
Technology: Has developed ODS 9Cr martensitic steel (12YWT) for fuel cladding tubes of nuclear reactor.
Exxon Mobil Upstream Research Co., U.S.A.
Technology: Has developed high strength pipeline steel for the transportation of natural gas in collaboration with Nippon Steel and Mitsui & Co. It is 20-50% stronger than the currently used pipeline steel. A mile-long section in the TransCanada Pipeline utilizes this nano-steel under -40oC temperature conditions.
MMFX Technology Corp., U.S.A.
Technology: Has developed the microcomposite Fe/Cr/Mn/C steels with superior combination of strength-toughness-corrosion properties for concrete members reinforced with high-strength rebars.
Nippon Steel Corp., Japan
Technology: Has developed nanostructured steels for various applications: Fatigue resistant steels containing Cu nano-precipitates for transportation and bridges; High strength steels with resistance to delayed fracture (by hydrogen trapping with nano-size precipitates) for bolts to be used in automobiles and high-rise buildings; High HAZ toughness steel ‘HTUFF’ using nano-size dispersion of oxides and /or sulfides; High strength steel wires for reinforcing automobile tires, galvanized wires for suspension bridges and power cable wires.
Intellectual Property Scenario
Since the technologies pertaining to nanostructured steels are mainly based on process innovations, they are relatively difficult to actually protect despite the legal cover that patents are intended to provide. Therefore, the technology developers are often inclined to maintain trade secrets rather than rely on patents for protection. This strategy helps them in avoiding IP conflicts and also protects their technologies from being exploited in other countries where IP protection is weak. Of course, this strategy makes the producers vulnerable if a competitor develops a similar process independently (Lux Research Inc., 2006). Notwithstanding the above, the following patents relating to nanostructured steels are pertinent to mention:

Title: Precipitation hardenable martensitic stainless steel

Patent Number: US 2008 / 0210344 A1 Filing / Publication Date: Dec.22, 2005 / Sep.4, 2008

Assignee: Sandvik Intellectual Property AB, Sweden

Inventor(s): Hikan Holmberg, Sweden

Key Features: A precipitation hardenable stainless Cr-Ni steel with high strength, high ductility and excellent formability. Exhibits very good corrosion resistance and finds applications as springs, surgical needles, dental instruments etc.

Title: Nanocarbide precipitation strengthened ultra-high strength, corrosion resistant structural steels

Patent Number: WO 03 / 018856 A2 Filing / Publication Date: Feb.11, 2002 / Mar.6, 2003

Assignee: Questek Innovations LLC, USA

Inventor(s): Kuehmann, Charless, J., Olson, Gregory, B., Jou, Heing-Jeng, USA

Key Features: Ultra-high strength (UTS > 1930 MPa) precipitation strengthened structural steel possesses out-standing combination of corrosion resistance and strength. The alloy is strengthened by nano-scale M2C carbides. Potential applications include aircraft landing gear, machinery and tools used in hostile environment.

Title: Nano-composite martensitic steels

Patent Number: EP 1 461 466 B1 Filing / Publication Date: Dec.12, 2002 / July 23, 2008

Assignee: MMFX Technologies Corp., USA

Inventor(s): Ku Sinski, Gregorz, J., Pollack, David, Thomas, Gareth

Key Features: Steel alloys with high strength, toughness and cold formability. Unique microcomposite micro structure comprising of nano sheets of austenite between laths of dislocated martensite. Highly corrosion resistant steels resulting in extended service life of rebar in corrosive environments.

Title: High strength hot rolled steel sheet and method for manufacturing the same

Patent Number: US 7527700 B2 Filing / Publication Date: April 21, 2004 / May 5, 2009

Assignee: JFE Steel Corp., Japan

Inventor(s): Nobusuke Kariya, Shusaku Takagi, Tetsuo Shimizu, Tetsuya Mega, Kei Sakata, Hiroshi Takahashi

Key Features: High strength (780MPa) hot rolled steel sheet with low carbon (0.04 to 0.15% C) having excellent elongation and stretch flangeability. Microstructure comprising nano-scale (20nm) Ti-Mo carbide precipitates within ferrite matrix. The steel sheet is suitable for reinforcing members of automobile cabin and crash worthiness member of automobile.

Title: Process for forming a Nano-crystalline steel sheet

Patent Number: US 7449074 B2 Filing / Publication Date: April 28, 2005 / Nov. 11, 2008

Assignee: The Nano Company, Inc., USA

Inventor(s): Daniel James Branagan

Key Features: Special iron based metallic glass forming alloys are formed into a nano-crystalline steel sheet by rapid solidification of molten alloy using counter-rotating casting rolls. The resulting alloy can show tensile strength between 3.16 to 6.12 GPa.

Title: Ultra-high strength, weblable steels with excellent ultra-low temperature toughness

Patent Number: US 6264760 B1 Filing / Publication Date: July 28, 1998 / July 24, 2001

Assignee: Exxon Mobil Upstream Research Co., USA and Nippon Steel Corp., Japan

Inventor(s): Hiroshi Temehiro, Hitoshi Asahi, Takuya Hara, Yoshio Terada, Japan and Michael J. Luton, Jayoung Koo, Narasimha Rao V. Bangaru, Clifford W. Petersen, USA

Key Features: The invention relates to ultra-high strength, weldable steel plate with superior toughness, and to fabricated linepipe from this steel. The steels contain nano-precipitates of carbides or carbonitrides of V, Nb and Mo which resist HAZ softening and minimize the localized loss of strength.

Title: Nano structured steel alloy

Patent Number: US 5589011 Filing / Publication Date: Feb. 15, 1995 / Dec. 31, 1996

Assignee: The University of Connecticut, USA

Inventor(s): Kenneth E. Gonsalves, USA

Key Features: The invention relates to nanostructured M50 type steel synthesized by chemical methods, which has improved mechanical and physical properties such as hardness, strength and durability. The steel finds particular utility in the manufacture of cutting tools and bearings.

Challenges
The nanostructured steels (particularly, manufactured by SPD processing) exhibit extraordinary strength levels. However, their ductility is inadequate, and therefore, makes them unsuitable for certain applications. This drawback is a major hurdle in bringing nanostructured steels from laboratory to commercialization. In view of this, it is of paramount importance that innovative approaches are developed to improve the ductility of nanostructured steels. Consequently, nanostructured steels require non-traditional processing methods and specialized machinery, which calls for significant investments and application development to make them commercially viable.
References
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By Y. R. Mahajan, CKMNT. This article has appeared in the inaugural issue of Nanotech Insights.

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