This article discusses the vacuum degassing technology that improves quality of products and shortens processing cycles. This technology is becoming more popular thanks to the greater demand for better steels by customers in the automotive, construction, and rail markets. The future of vacuum-degassing systems is bright, in spite of their hefty price tag. Kvaerner Metals has built vacuum-degassing systems costing $25 million on the low end up to $55 million for its steel-making clients. Maintenance of the vacuum system is an ongoing challenge for the steel plant. To maintain high availability, routine searches and repair of air leaks is required. Special attention is needed for the refractory systems to maximize their life cycles. Despite those concerns, Kvaerner Metals’ Holmes remarked that vacuum-degassing systems are likely to become more widely used by major steel companies striving to keep their competitive edge in the niche applications now under assault by smaller competitors already equipped with the equipment.
Steel companies are increasingly adopting the practice called vacuum degassing-subjecting molten metal to a vacuum to remove hydrogen or carbon-to improve the quality of their products and shorten processing cycles. This technology is becoming more popular thanks to the greater demand for better steels by customers in the automotive, construction, and rail markets.
"Vacuum degassing causes hydrogen to diffuse and separate from the liquid steel so as to prevent hydrogen-induced defects," said John Speer, professor of l11.etallurgy at the Advanced Steel Processing and Product Research Center of the Colorado School of Mines in Golden. The most common defect is hydrogen flaking, which creates tiny flaws that reduce the integrity of the steel when the dissolved gas leaves the cooling metal and forms high-pressure pockets of hydrogen gas within the solid.
The traditional method of removing hydrogen involves letting steel cool slowly sometimes for more than 48 hours, depending on the hydrogen concentration and the thickness of the bar or rail, according to Speer, who develops steel-alloys for specific applications by vacuum degassing and other methods. This incurs handling costs and adds to production times. In the case of carbon removal, vacuum degassing is used to create new grades of steel that are more ductile, and thus more easily formed in stamping operations.
Whether removing hydrogen or carbon, vacuum degassing is not without costs, including capital expenditures of more than $50 million, and there are considerations of timing and scheduling to fit this new technique into existing steel processing lines. However, steel makers who have adapted vacuum degassing to suit their facilities- including Bar Technologies Inc. in Johnstown, Pa.; Pennsylvania Steel Technologies in Steelton, Pa.; and L TV Steel Co. in Cleveland-have reaped the benefits of improved quality, opening new markets for their products. At the same time, engineering firms such as EMC International (EMCI) and Kvaerner Metals, both based in Pittsburgh, have won multimillion-dollar contracts to design and install degassing systems.
Vacuum-degassing systems, which are all customized, fall into two categories. The first type, RH recirculating degassers invented by Heraeus-Rheinstahl in Germany, involves inserting two legs, or snorkels, of a vacuum chamber into a ladle of liquid steel. The metal is drawn into the chamber via one snorkel that injects argon to promote turbulence; it is then exposed to the vacuum to remove gases and recirculated back through the other snorkel. The other system, a tank degasser, is a vessel into which the ladle is sent and stirred by the injection of argon. The chamber is depressurized to remove gases, and finally the ladle is removed.
A ladle containing 150 tons of liquid steel (opposite page) is lowered into the tank degasser at Pennsylvania Steel Technologies to remove hydrogen from steel for harder rail heads. At Bethlehem Steel's plant in Burns Harbor, Ind., Fluent Inc.'s fluid-dynamics analysis software was used to model the vacuum-degassing process (this page).
Controlling Steel Chemistry
Bar Technologies installed its RH recirculating vacuum-degassing system in 1984 primarily to help the company compete in the automotive and cold-worked steel markets, according to Dennis W Thomas, a metallurgist who is manager of technology and development at the primary operations in Johnstown. Cold-worked steel is shaped into its final form without the benefit of heating, for critical end-use applications. The Johnstown plant makes 6:t4-inch-square semifinished billets that are sent to a sister facility in Buffalo, N.Y, which converts the billets into %- to 3 V4-inch rounds and hexagonal bars for auton1.otive, construction, and other industrial markets.
The Johnstown degasser consists of a 9 Y2-foot-diameter vacuum chamber made of steel with an 18-inch-thick lining of refractory material. It has two 3-footlong refractory snorkels with a 12-inch interior diameter. After liquid steel is tapped from an electric arc furnace into a ladle, it is transported by crane to the degassing area. The degasser is lowered vertically on a •superstructure by a geared motion that positions the snorkels into the liquid steel.
The ladle is covered with a layer of slag that is penetrated approximately 18 inches deep by the snorkels. A thin plate is located over the tip of each snorkel to pre':' vent slag from being drawn into the degasser. Argon gas is injected into one snorkel to facilitate drawing the molten steel into the vacuum chamber. As the snorkels are inserted into the ladle, Nash Kinema steam ejectors create a vacuum of 2 to 21/2 torr in the vacuum chamber to draw the molten steel into the chamber. The lower partial pressure within the vacuum chamber removes both hydrogen and nitrogen gases from the steel (high nitrogen in steel makes it unsuitable for many cold-forming applications).
Both gases are vented from the degasser as the steel is continuously circulated through the degasser vacuum chamber. Once activated by an operator, the degasser automatically cycles up to 750 tons of steel, or one heat (the steel contained in one ladle), through five circulations in 20 minutes.
In addition to producing the desired higher grades of steel, vacuum degassing has improved Bar Technologies' steel-making chemistry. Computer analysis is performed on the liquid steel prior to tapping the heat, and a computer calculation determines the precise alloy additions at tap. "The computer automatically aims to the low side of the specifications," Thomas said, " so that we can take advantage of the stable ferroalloy recovery conditions in the degasser, where additions are made under vacuum. We can achieve chemical compositions as precise as our laboratory instruments can measure and meet very demanding chemical specifications."
Better control of the steel's cleanliness is also a prime benefit of degassing. Alumina, inherent in all steelmaking processes, is a major source of poor steel cleanliness. "By creating ladle slags that have an affinity for alumina and coupling this with tremendous mixing action and exposure of the molten steel to the slag during the degassing cycle, we are able to produce very clean steel products to meet our customers' require-ments," Thomas said.
Making Harder Railheads
A tank degasser was installed in 1994 at Pennsylvania Steel, a subsidiary of Bethlehem Steel Corp. in Bethlehem, Pa., as part of a general modernization that included the installation of a dc electric arc furnace and a ladle metallurgy furnace. The company turned to vacuum degassing specifically to support a new railhead-hardening technology at the Steelton plant. "We have also used the de gasser to increase our sales capacity, by reducing the controlled cooling cycle," said Robert Siddall, manager of prill1.ary operations at Steelton. The plant makes 1.1 million tons of steel rail, bars, and semifinished steel annually for the railroad, construction, and forging markets.
The degassing process at Steelton begins when a crane lifts a ladle holding 150 tons of liquid steel into the tank degasser thro ugh the top opening. The custom-designed vacuum-degassing equipment was supplied by EMCI. The vacuum tank is a two-piece assembly consisting of an upright cylindrical tank and a flanged tank cover that is moved on and off the tank by hydraulics.
The ladle rests within the tank on spreader beams supported at their ends by stub columns connected to the tank base plate. The stub columns and the tank interior are lined with protective refractories. The empty volume below the ladle can hold the entire 150 tons of liquid steel in case of a mishap.
Once the ladle is set in place, the tank is covered. The cover has a machined mating flange that compresses a silicone rubber a-ring to provide an airtight seal during de gassing. "When the cover is removed, water floods a trough that the a-ring sits in to protect it from the heat the cover picks up from the molten steel," said Jim Simmons, a metallurgist and vice president of sales at EMCI. When the cover is in place, the tank is approximately 35 feet tall with a diameter of 18Y2 feet.
When the tank is sealed, five series-connected steam-jet ejector stages create a vacuum of 1/2 torr or less. At the same time, argon purging gas is percolated through porous refractory material in the bottom of the ladle. This stirs the steel, promoting the removal of hydrogen, which is drawn off and vented with the other off-gases.
EMCI engineers considered a variety of process variables, including a desired throughput of 24 heats of steel per day, the time available for degassing, cleanliness ratings, and hydrogen reduction, when designing the degasser for Steelton with the help of AutoCAD software from Autodesk in San Rafael, Calif. Pennsylvania Steel added 3 feet to their ladles' height "to provide the freeboard needed to contain the boiling reactions caused when liquid steel and slag are exposed to vacuum and argon stirring of the molten mass in the ladle," said Ron Gray, a metallurgist and vice president of technology at EMCI.
According to Richard Reinbold, a mechanical engineer and senior engineer at Bethlehem Steel's research center, the steam-ejector patterns are automated through programn1able logic controllers, but the change in vacuum rate and argon-circulating rate can be done manually to control action in the ladle. This ensures sufficient time at low vacuum to remove the hydrogen. Hydrogen content is checked at the caster to ensure the steel meets required hydrogen levels.
"We degas about 90 percent of our product at Steelton today to better serve our rail, construction, and forging markets," said Siddall, who added that new products are being made at the Steelton facility by vacuum degassing. These are large bottom-pour ingots aimed at the forging industry, large-diameter rolls for hot-strip mills, and backup rolls for cold reduction and plate mills. End uses for these products include oil-field valve bodies and gears. "We could not reduce the hydrogen in these steels without degassing," he added.
Modeling the Ideal System
Experts generally agree that the basic vacuum-degassing equipment will not change drastically in the foreseeable future. " Instead, there will probably be more automation to improve the process incrementally," said Alan Cramb, a metallurgist and the POSCO Professor of Iron and Steel making at Carnegie Mellon University in Pittsburgh. Vacuum degassing, like other industrial processes, may be improved by computer modeling. For example, Bethlehem Steel is using Fluid Dynamics Analysis Package (F IDAP) software developed by Fluent In c. in Lebanon, N.H., to improve the continuous-circulation vacuum-degassing process used in its Burns Harbor, Ind., plant. Bulent Kocatulum, a mechanical engineer and research engineer at Bethlehem Steel, conducted a parametric study in 1995 and 1996 on what effect the argon injection rate has on the recirculating flow of steel to find alternative designs that could improve process efficiency. (His findings were published in a paper presented at ASME's 1996 Fluids Engineering Division Conference, held in San Diego.)
The FIDAP model of the degassing process consists of both the ladle, which holds the liquid steel, and the degasser that is dipped into it. Kocatulum used FIDAP to study the effects of several argon injection rates on the steel flow field. Argon injection was modeled using FIDAP's two-phase Lagrangian approach.
The steel-circulation rate predicted by FIDAP compared well with experience in the plant and with experimental results reported in literature on the subject, according to Kocatulum. Based on these results, Bethlehem Steel is planning further computational-flu id-dynamics modeling projects to design degassing loops that will reduce the production costs of vacuum degassing.
Companies striving to stay competitive in niche applications are more likely to use vacuum degassing.
Automakers Drive Demand
In addition to removing hydrogen from liquid steel, vacuum degassing is used to reduce the carbon content in sheet steel. "The ultralow-carbon steels, which have less than 50 parts per million of carbon, can only be made by vacuum degassing," Cramb said.
High carbon levels make steel less ductile and thus harder to work in applications that call for cold-worked steel, such as stamped parts for automotive end users. Cramb noted that automakers' demand for ultralow-carbon sheet steel to make lightweight but durable car panels for more-fuel efficient cars has been the major factor driving vacuum degassing's growing share of the sheet-steel market in the later 1980s and early 1990s.
"One plant has been making over 1.2 million tons of degassed sheet steel to supply the automotive market" out of an annual production of 6 million tons, according to R on Holmes, a metallurgical engineer and senior process consultant at Kvaerner Metals, a subsidiary of Kvaerner ASA in Oslo, Norway. Kvaerner Metals has designed and installed numerous vacuum-degassing systems for processing sheet steel for automotive end users.
"Although this is a relatively new story in the United States, dating from the late 1980s, Japanese steel makers were de gassing sheet metal for autos at least 10 years earlier," Holmes said. The engineer said that virtually all sheet steel degassing was performed by RH recirculation systems, which provide the greatest efficiency in removing carbon from steel for high-production shops.
The greatest challenge that Kvaerner engineers face is retrofitting their RH systems to existing steel facilities. "We have to accommodate whatever space is available," Holmes said. " Fortunately, we' have engineers who have the skills and experience to meet the dimensional constraints as well as the process objectives." For example, the company has developed equipment to exchange refractory- lined vessels arid to deliver additives to the station.
Engineers at LTV Steel's Cleveland Works neatly sidestepped the limitations of designing a degassing system to fit into existing structures by erecting a new building that is attached to an existing basic oxygen furnace. The company installed a vacuum-degassing system along with a three- electrode arc furnace in 1991 in response to increased quality demands from its automotive, appliance, and electrical-motor customers.
An operator directs the steel car that transports the ladle of molten steel to the degassing area and operates the hoist that lowers the circulating degasser, made by MAN GmbH in Oberhausen, Germany, into the liquid steel. The degasser uses dual snorkels and argon injection, with a vacuum created by steam ejectors, to draw the molten steel into the vacuum recirculating chamber. There, off gases are removed to be flared in a stack, and the treated steel is returned into the ladle.
The entire degassing process is computer-controlled, with programmable logic controllers extending that control to the shop floor. Two years ago, L TV engineers retrofitted the vacuum circulating chamber with a Kawasaki Top Blow oxygen lance to cut processing time by 20 percent. These improvements have enabled L TV Cleveland to vacuum-degas up to 125,000 tons of steel in a single month.
LTV Cleveland degasses one-third of all the steel made in one of its basic- oxygen-furnace shops, and hopes to improve opportunities for their product by modifying the alloys to give the degassed steels the characteristics that its customers desire, especially in automotive markets. " Thinner steel body panels can reduce a car 's weight and, unlike plastics, are 100-percent recyclable," said Dale Heinz, manager of steel-producing operations at LTV Cleveland.
The future of vacuum-degassing systems is bright, in spite of their hefty price tag. Kvaerner Metals has built vacuum-degassing systems costing $25 million on the low end up to $55 million for its steel-making clients. Maintenance of the vacuum system is an ongoing challenge for the steel plant. To n1.aintain high availability, routine searches and repair of air leaks is required. Special attention is needed for the refractory systems to maximize their life cycles. Despite those concerns, Kvaerner Metals' Holmes remarked that vacuum- degassing systems are likely to become more widely used by major steel companies striving to keep their competitive edge in the niche applications now under assault by smaller competitors already equipped with the equipment.
Saving Steel Quality
MANUFACTURERS OF ROLLED and sheet steel rely on visual inspection of their product to detect and correct folds, scale, and rolled-in debris that can damage the finished product. Metallurgists at Matra Systemes & Information in Velizy-Villacoublay, France, designed their Streamlined Automatic Vision Equipment (SAVE) to provide 100-percent real-time surface inspection of both the top and bottom surfaces of rolled and sheet steel, enabling operators to halt production and fix the problem. The system provides resolution from 0.004 to 1 inch, and can inspect steel strip moving up to 66 feet per second.
Matra's new inspection system replaces visual inspections, with their attendant human error. SAVE consists of lighting to illuminate the surfaces and close-coupled-device (CCD) solid-state cameras that are placed above and below the product web in areas where defects can occur, such as tensioning loops or feed stations. The images captured by the CCD cameras are fed to a central processing unit. Flexible software that includes a dynamic product quality database transmits real-time images to video screens at operator consoles and can report in a variety of formats and graphs. The software also trains operators and enables management to review the system operators' performance, using stored images plus a detailed log with graphic output.
Operators can apply SAVE in any of three modes-automatic, manual, or semiautomatic-to detect imperfections. Automatic mode gathers and classifies all defects and can be programmed to alert the operator, while manual mode, used for training operators, confirms or rejects information. Semiautomatic mode enables the operator to choose which defects fall into automatic processing and which require manual intervention. In addition to correcting defects, steel strip inspected by the SAVE system can be graded and marked before leaving the line, ensuring its quality.
SAVE is currently used by Sollac, Europe's leading producer of sheet steel, at its French facilities in Dunquerque, Florange, Metz, and Montataire. Each plant's annual capacity ranges between 400 million and 600 million kilograms. According to Matra, the cost reductions realized by using SAVE will provide payback in less than 18 months.