The easiest way to increase the strength of steel is to increase its carbon content. This traditional expedient, however, also deteriorates other desirable properties such as weldability, toughness and formability.
Modern science has found alter-native ways of enhancing the strength of steel while still preserving acceptable secondary properties. Microalloying with niobium, vanadium or titanium in amounts below 0.10wt% is a most cost-effective method of achieving a "balanced package of properties".
A number of factors influence the choice of steel composition. In low carbon HSLA steels (carbon less than 0.1wt%), niobium is a more effective strengthening element than vanadium or titanium and most modern HSLA steels fall into this "low carbon" category.
However, there are some circumstances where it is acceptable to use vanadium and/or titanium instead of niobium. For example, cheaper titanium is used in much interstitial-free steel for "unexposed" automobile parts. The poorer surface quality that results from the use of titanium is not important since the part is hidden from view.
There are also situations where one of the other microalloys may be a convenient technical choice such as in the use of vanadium in rebar, in modern high speed bar mills.
The best results, however, are usually achieved with a combined addition of microalloys to exploit synergistic benefits. An example of this is the use of niobium and titanium together in "exposed" interstitial-free steel automobile parts where the superior surface quality that results from this combined addition is obviously important.
Synergy is much more consequential when niobium and titanium are working together than when niobium and vanadium are both present in the steel.
Plates - Niobium microalloyed high strength steel plates are used in a variety of applications.
Large diameter linepipe for the transmission of gas (and oil) is the most important item produced from plate.
Gas transmission linepipe requires a high level of strength to contain the high-pressure gas as well as acceptable toughness to prevent propagation of a long fracture in the event that external forces (such as an earthquake) initiate a fracture. This is especially important where "rich" gas is involved. Good weldability is also needed to allow for easy fabrication of a transmission system. In addition to the virtually mandatory addition of niobium to linepipe steel, strict control of chemical composition is necessary, and the plate hot rolling process must be specially optimized to maximize strength and toughness and minimize cost. The Japanese achieve this optimization of the properties of their linepipe by using high power rolling mills and/or water spray quenching after rolling to minimize their reliance on niobium and other alloying elements.
In the US, less severe rolling regimes are used and much more reliance is placed on the use of niobium for strengthening and toughening rather than on the application of special rolling techniques.
European plate producers fall between these extremes. Some companies have invested in powerful rolling mills and water quenching equipment, while other companies tend to follow the US pattern.
As in most markets today, competition for linepipe orders is "global" Producers include Berg Pipe, Europipe, Nippon Steel, Oregon Steel, Stelco, Sumitomo, Usiminas, etc. Consumers include British Gas, Exxon, Gazprom, Norwegian Statoil, Petrobras, etc.
Each user has written his own linepipe purchase specification which prescribes composition and property ranges. Steelmakers have some flexibility in designing their steel to meet these specifications. Usually they are allowed to use niobium, vanadium and titanium, singly or in combination. Usually, a maximum limit is placed on all these three elements by the pipeline operator-to-be.
Plate for shipbuilding and offshore platforms constitute another important application for niobium microalloying. In this application, plate in excess of 50mm thickness is common. Many of the international steel companies that produce plate for linepipe also produce ship plate.
Important consumers of ship plate include Hitachi Zosen and Mitsubishi Heavy Industries (Japan) and Hyundai Heavy Industries and Samsung Heavy Industries (Korea). In Europe, important shipyards are in Germany and in Poland. Offshore platforms are constructed by engineering companies according to specifications prepared by oil companies.
Civil construction, is another typical application area for high strength niobium microalloyed plate products. They are used in the construction of bridges, viaducts , high rise buildings, etc. Final customers vary from governments to individuals. Heavy machinery, pressure vessels, etc. represent additional applications of microalloyed plates.
Hot Rolled Strip is produced for the fabrication of spirally welded pipes and for the manufacture of electric resistance
Hot rolled strip is produced in a continuous strip mill. This product is used for the production of linepipe with toughness and good weldability and for automotive steels where good formability is a necessity.
Welded (ERW) pipes (longitudinal electric resistance weld) - Final applications are similar to those for pipe plate produced in plate mills. ERW pipe is produced in smaller diameters, e.g. less than 550mm.
Hot rolled strip also finds application in the automotive industry. Typical uses are truck frames, wheels, and some structural members. In addition to pipe and the automotive industry, other examples of the use of niobium microalloyed hot strip steel include crane booms, railway wagons, containers and off-road construction vehicles. Strip thickness is in the range 2 - 20mm.
The major use of Cold Rolled Strip is in the production of thin gage car outer body panels. In this case, niobium is used as a strengthening element, as in hot rolled sheet.
Cold rolled strip is produced in a tandem mill and heat-treated by annealing after cold rolling.
Until the early 80's, microalloying was applied only in the high strength grades (e.g. minimum yield strength of 350 MPa), with somewhat limited formability. Since the early 80's it has become feasible to produce Interstitial Free (IF) steels with extra low carbon content (less than 0.005wt%), with excellent formability. They are called Interstitial Free because interstitial elements nitrogen and carbon are fixed by microalloys Ti and/or Nb. They are produced on modern continuous annealing lines.
The application of both hot rolled and cold rolled high strength microalloyed strip gained importance after the first oil crisis in 1973, when car manufacturers, especially in the USA, needed to reduce car weight to save fuel consumption.
The use of Interstitial Free steel utilizing niobium and titanium microalloying has made possible the production of large integrated sheet panels and complex parts, contributing to a reduction in the number of welds, reduction in the number of parts being formed and reduction in weight. Although developed in the US at Armco Steel in the late 60's, initial production on a large scale began in Japan in the early 80's. They are now also widely produced in North America, Europe and in some developing countries such as South Korea and Brazil.
Steel producers are flexible in defining the chemical composition of steels, provided they meet the property requirements demanded by the automobile companies.
More than 30 major steel companies in the world are engaged in a project to develop Ultra Light Steel Autobody (ULSAB). A demonstration prototype has been built by Porsche and if this project comes to fruition, a large percentage of steels used will contain niobium.
Long products are steel products such as bars, sections or wire rod. They are mainly produced in smaller steel companies called mini mills. All these products can be produced in higher strength grades using niobium.
Structural sections (e.g. angles, I beams) are widely used in civil construction, railway wagons, transmission towers, etc. Niobium has been competing with vanadium in this application.
Steel Reinforcing Bar. This product is used in large concrete structures to increase their resistance to tensile loads. Larger diameter high strength grades are produced with an addition of vanadium or niobium. Some modern steel mills use water cooling, which replaces microalloying as a means of increasing strength.
Engineering Bar. Widely applied in the production of forged components for the automobile industry (e.g. crankshafts, connecting rods) engineering bar can benefit from microalloying technology. It is possible to avoid expensive quenching and tempering heat treatments in the final forging, reducing processing cost.
Wire Rod. Wire rod is the initial raw material used to produce nuts and bolts, fasteners, springs, etc. Niobium has found application, along with vanadium, in some high strength fasteners used in the automobile industry, where the application of microalloying technology allows the elimination of intermediate processing (spheroidize annealing), quenching and tempering of the final part. Niobium, with vanadium, has also become a common addition in spring steels. The higher strength gained from niobium microalloying allows weight reduction in the finished part.
Rail Steels. Niobium has found some application in high strength and wear resistant rails for railroad tracks operating under high axle loads. An important producer is Nippon Steel Corporation.
Stainless and Heat Resistant Steels
Stainless steels, specially the ferritic grades (nickel free) consume about 10 % of the total world consumption of niobium. In Japan, about 25% of the demand for niobium is in stainless steel. The major application for ferritic grades containing niobium is in exhaust systems of automobiles. This use of stainless steel (which replaces carbon steel) is a consequence of higher working temperature, introduction of catalytic converters and the guarantee of a longer life for the component.
Heat resistant steels are used in the petrochemical industry and power plants. Frequently they are centrifugally cast. Pont-A-Mousson and Wisconsin are relevant producers for the petrochemical industry.
Other Iron and Steel Products
Other miscellaneous products using niobium include seamless pipe, tool steel, cast iron and steel castings.
Seamless pipe is produced from solid billet using one of several piercing and drawing processes. High strength niobium microalloyed grades are used in oil and gas well drilling operations (drill pipe and well casing). Usually, diameters are smaller than 430 mm and there is some competition with electric resistance welded pipe.
A variety of alternative cutting tool materials have been developed such as carbides. However, tool steels are still the most important machining devices. Usually these steels rely on a variety of alloying elements to develop cutting capability. Among these are the carbide-formers chromium, molybdenum, tungsten, vanadium and, more recently, niobium.
Tool steels are essentially hard metal carbides embedded in a tough matrix. New metallurgical concepts being used to upgrade the performance of tool steels include the addition of niobium to form hard niobium carbides. Manufacturers who have adopted this modification include Böhler, Cartech and Villares. Some tool steel compositions are being used to manufacture other items such as rolling mill rolls and hard-facing electrodes.
Cast irons - the use of niobium in cast iron is a relatively new technology. The most significant applications are in automotive cylinder heads, piston rings and truck brakes. The formation of very hard carbides (good for wear resistance) and the modification of graphite cell size are two of niobium's attributes in this application. COFAP supplies the Brazilian automakers and also exports its product to Europe. Mercedes-Benz, in Europe and in Brazil, produces for its own use.
Steel castings use niobium as a microalloy, for good combination of strength and toughness. Several new applications have been developed, such as ingot moulds, slag pots, rolling mill back-up rolls, nodes for offshore platforms and machinery components. This is a fertile area for the use of microalloying technology. In the USA, Blaw Knox Rolls and Whemco are important producers.
The so-called superalloys are materials designed to function for extended periods of time in highly oxidizing and corrosive atmospheres at temperature above 650°C. Superalloys represent the second largest use of niobium outside the steel industry.
There are literally scores of different superalloys used in a variety of high temperature or corrosive environments. However, the single most important member of the class is Inconel 718, a nickel-based alloy containing 5.3-5.5 wt% niobium. This alloy forms the backbone of commercial and military jet engine manufacture. The most common jet engine in service today, the CFM56 made by the GE/Snecma joint venture, contains about 300 kilos of niobium. Most of this niobium presently comes from CBMM's Araxá mine.
Other industrially important nickel-based alloys containing niobium are Inconel 706 (3 wt% Nb) and Inconel 625 (3.5 wt% Nb).
Alloy 718 was initially developed as a disk material for aircraft gas turbines even though its uses have expanded over recent years to include other engine parts such as bolts, fasteners and rotor shafts. Further uses for this remarkable alloy have also been found in other industries such as nuclear, cryogenics and petrochemicals. Land based turbines for electricity generation are becoming increasingly important as the efficiencies of these machines are being increased to acceptable levels (56-58%) by increasing operating temperatures. To solve the problems associated with the exposure of materials to high temperatures, some engine manufacturers are using Inconel 718 and Inconel 706 superalloys as solutions. There are some still trying to design better cooling systems to avoid using superalloys.
The main demand for 718, however, still comes from aircraft engine builders.
Consensus of opinion in the aerospace industry predicts that more than 6,000 large commercial jets will be delivered during the next ten years, which should assure continued strong demand for high purity '718' niobium units.
Although the ultimate end-users of superalloys are Boeing and Airbus, the decision as to which engine manufacturer supplies the engines on any given plane is made by the airline or the buyer of the airplane.
Historically, fuel consumption and noise have heavily influenced the choice of engine. Rapid improvements have been made in fuel efficiency through increases in rotation speed (and hence operating temperatures) and increased bypass ratios. These improvements were achieved by improving the manufacturing technology of the nickel-based superalloys, especially Alloy 718.
Today the rate of improvement is slowing down as the industry and its technology become increasingly mature. For example, the three engines available for the Boeing 777 from General Electric, Pratt & Whitney and Rolls Royce, all perform similarly. In these circumstances the purchaser will discriminate on the basis of weight, reliability, quality of customer support and, most importantly, the financial package on offer.
Higher temperature capability and lower density remain the twin prerequisites in potential engine building materials. Alloy 718 is now operating at a temperature equivalent to 85% of its melting point. It is clear, therefore, that the melting point of nickel imposes a natural ceiling on the potential for improvement of this alloy. Niobium-containing gamma titanium aluminides are rapidly emerging as practical engineering materials with higher temperature capability than 718 (GE: Ti47Al2Cr2Nb at%) but other titanium alloys that do not contain niobium are also being developed as well as other intermetallic materials and ceramic composites. The supply of cobalt is no longer dependent on politically unstable Africa and cobalt-based Waspalloy alloys could threaten 718's dominance.
Other alloys may be developed in the future. For instance, the use of pure niobium and niobium-based alloys has long been a goal of engine builders because niobium is the lowest density refractory metal. Niobium could dramatically increase the operating temperature capability of an engine if a solution could be found for niobium's poor oxidation resistance.
Alloying niobium with other elements such as titanium, zirconium, hafnium, tantalum, tungsten and other metals produces materials with highly desirable engineering properties.
Niobium itself has long being known to exhibit superconductivity (the loss of all electrical resistivity, below a critical temperature near absolute zero). Although pure niobium finds application in microwave cavities used in particle accelerators, the most important superconductor materials are niobium-titanium and niobium-tin.
High tonnage consumption of niobium for superconducting applications is still strongly dependent on government funded projects, usually costing billions of dollars. While the approval of such projects is difficult, a large particle accelerator based on niobium-titanium superconducting magnets was recently approved in Europe. Niobium-titanium billet deliveries to cable manufacturers started in 1998 and will continue for three to four years. Other large projects with potential to becoming reality in the future are energy-related applications such as energy generation via atomic fusion and energy storage.
These government-managed projects purchase superconducting magnets from manufacturers such as General Dynamics and General Electric, who in turn buy superconducting cable from cable manufacturers such as Alsthom, Furukawa, Hitachi, IGC, Kobe, LMI, Outokumpu, Oxford, Sumitomo, Supercon, and Vacuumschmelze. The cable manufacturers buy niobium alloy products in such finished forms as billets, rods and sheets.
Magnetic Resonance Imaging (MRI) used in medical diagnostics, and Nuclear Magnetic Ressonance used in spectrographic (analytical) applications, are the two commercial applications for niobium as a superconductor material. General Electric has the lion's share of the MRI worldwide market, with Philips as the second major player. Siemens and Toshiba are smaller competitors. These companies buy their superconducting cables from the traditional cable manufacturers already listed above.
Niobium-based alloys are also used as refractory materials for aerospace applications since they have excellent high temperature strength above 1,300°C and readily accept coatings to protect against oxidation. The most important alloy in this case is called C-103 (a niobium-hafnium-titanium alloy) used mainly in rocket thrusters and rocket nozzles. Consumption of this alloy has recently increased due to booming satellite launching activity. C-103 is always used as a skirt on the Pratt & Whitney F100 engine, a high performance power plant used on fighters such as the F15 and F16.
The supply chain in this case will put companies such as General Electric, Kaiser Marquardt, Lockheed Martin, Loral and Pratt Whitney, as end users.
Niobium-1%zirconium alloy is used as a precision support member in high-efficiency and high-intensity sodium vapor street lamps. These tiny components require a material with high hot strength and superior formability which must be resistant to corrosion from sodium vapor. The manufacturers of these lamps are are General Electric, Osram and Sylvania.
Niobium is also used in heavy water nuclear reactors of the CANDU (Canada Deuterium Uranium) type in a Zirconium-2.5% niobium alloy. This alloy's high strength permits the use of thin wall sections, allowing better neutron economy. Another application is in nuclear reactors for US Navy submarines.
Other applications of niobium metal and its alloys include:
Platinized niobium anode wires for cathodic protection (corrosion protection) of large offshore platforms and reinforced concrete structures;
Pure niobium foil used in the production of synthetic diamonds;
Niobium-titanium non-sparking components used by mining companies (especially gold mining operations);
Niobium metal for sputtering targets used in the architectural glass industry, for razor blades and in the electronic industry;
Niobium-titanium alloys recently developed for use in surgical implants.
This is not a complete list of applications of niobium and niobium-based alloys. Companies always keep new developments secret for as long as they can in order to establish a competitive edge.
High purity niobium oxide is being used in the manufacture of fine ceramics. These special materials are generally classified as being either functional or structural (engineering) materials. The former category includes ceramic capacitors for electronics and optical lenses. The latter group consists of heat resistant and abrasion resistant materials, tools, engine parts and other structural articles. Industrial applications of functional materials have advanced far ahead of structural materials.
The estimated world demand for niobium oxide in functional ceramics is around 500 tonnes per year. Japan is responsible for almost 2/3 of this demand. The majority is 99.9% Nb2O5 for optical lenses and ceramic condensers and actuators. The balance is very high purity 99.99% Nb2O5 which is used to produce lithium niobate single crystals for application in surface acoustic wave (SAW) devices for TV receivers. Crystal technology in California is an important manufacturer of lithium niobate single crystals.
Hoya, Minolta, Nikon and Ohara (Japan), Corning (France) and Shott (Germany) are the main producers of optical lenses containing niobium, for ophthalmics, microscopes and video camera lens manufacture. NEC, Matsushita Electric, Murata Manufacturing, TDK and Trans-Tech (USA) are the main producers of niobium containing ceramic condensers and actuators.
Bayer, Sumitomo Chemical and Union Carbide are using small amounts of niobium compounds in catalyst systems.
* Copyright © 1999 by Companhia Brasileira de Metalurgia e Mineração
Authors: Harry Stuart, Klaus Hulka, Pascoal Bordignon, Solon Y. Tagusagawa, Tadeu Carneiro