Steel casting processes represent one of the major areas of technological development within the steel industry. The advent of strip casting for carbon and alloy steels and near-netshape part production could radically change the conventional view of a steel plant from a large production facility of semi-finished material into a smaller, product-oriented concern. However, ingot casting continues to be the preferred method to produce steel for some uses, such as intermediate and large bar applications (e.g., power transmission) and high-performance bar and tubing applications (e.g., bearings and gears). Foundries and specialty producers also continue to use ingot casting and to produce large cross-sections or thick plates.
Current casting technologies include ingot, thick slab, thin slab, billet and bloom casting. Future developments will lead to ultra-thick slab casting for thick plates, direct strip casting for sheets 0.03 to 0.15 inches thick, continuous casting products with fewer inclusions, rod casting, rapid prototyping of complex geometries via droplet consolidation or laser/wire technologies, rheocasting, and direct part fabricating via computer-controlled casting/milling machines. Commercial strip casting of carbon and stainless steels at Nucor Crawfordsville by the end of 2001 represents a major new technology in flat rolled steel production.
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Generic Casting Issues
Trends and Drivers. The technological drivers for current operations are increased productivity, yield, and quality. New technologies are driven by:
Low capital and operating costs
Flexibility
Niche markets
Potential for new product development
In addition, casting developments will need to consider environmental factors and recyclabihty of waste products, such as wastewater, refractories, and slag.
All of the above requirements drive casting development towards more streamlined near-net-shape processes. This results in the cast surface becoming the finished part surface and leads to the necessity of defect-free castings that must be produced at geometrical tolerances defined by the end product. Strip cast products will undergo little or no rolling. Research on as-cast structure and properties is required. New alloys and alloying techniques may be needed for strip cast products to develop properties comparable to conventionally cast products. This progression results in a number of generic requirements in casting R&D.
Technological Challenges. Following technological challenges occurs:
Steel cleanliness: The presence of inclusions or clusters of inclusions greater than the minimum size for defect formation, or of the wrong chemistry, often leads to inconsistent product quality.
Nozzle clogging is a common operational occurrence that can result in quality and production problems when steels containing solid inclusions are cast.
There is a lack of economical materials for containment vessels and molds that are sufficiently inert, long-lived, and able to withstand large thermal gradients.
Active control of fluid flow and temperature is necessary to avoid the production of defects during casting and to ensure product consistency. Current gravity-fed flow systems only allow gross control of the above parameters.
Instead of having cast surface defects or imperfections removed by grinding, oxidizing, or treating, technology must be developed to eliminate casting or oscillation marks.
Operators do not always have the most up-to-date information when operating a caster. Sophisticated casting machines require a detailed knowledge of the science and engineering principles involved in the design and operation philosophy of the machine in order to solve any problems that occur.
New and Emerging Technologies. Strip casting is the most obvious new technology. Other technologies that are under development include:
Applications of electromagnetics in the area of fluid flow control, heating, and containment
Advanced vision systems for defect detection and identification
Advanced computer diagnostic controls
Liquid steel temperature control in the mold and tundish
Advanced ceramics for clean steel production
Future developments will focus on near-net-shape production of all castings and droplet consolidation technologies for rapid prototyping and rheocasting.
Slab, Billet, and Bloom Casting
Trends and Drivers. Issues in casting are related to productivity, operating cost, quality, and the energy content of the cast piece before rolling. Issues vary between thin and thick slab casters; however, there are a number of common issues, especially in the areas of control and quality.
Some specific trends include:
Increasing strand cleanliness Thinner cast slabs
Net Shape
Higher cast speeds
Technological Challenges. Following challenges occuried for the Slab, Billet, and Bloom Casting:
Steel cleanliness requires to have low oxygen content, and to be delivered to the caster, protected from exposure to the atmosphere, and exposed to minimal contamination from the refractories, tundish, and mold fluxes.
The materials systems used to contain and conduct the steel must be stable with respect to any steel grade and not add to the inclusion population. They also must be cost-effective and capable of long exposures associated with sequence casting.
The tundish fluxes must have the proper fusing temperature and fluidity and not be corrosive to the refractories, while providing protection from reoxidation and the ability to capture inclusions. Flux design for continuous caster operation continues to be a difficult issue because the tundish and mold flux requirements are complex and there is not a complete understanding of the exact design requirement for fluxes.
Nozzle clogging remains an issue that impacts productivity and quality in casting. A solution that allows the casting of steel grades containing solid second-phase particles will ensure consistent production of aluminum-killed steels.
Lack of refractories for use in the ladle, tundish, and molds that are sufficiently long-lasting, stable, and non-porous to allow cast quality to reach its full potential. Ultra-clean steel production is limited by refractory interactions with the steel.
Higher surface quality of castings would allow the cast surface to be used directly in all applications without modification. Mold friction, surface defects (including meniscus marks), sub-surface defects, and argon and other bubbles are all quality problems that require attention.
Strip Casting
Compared with conventional casting, the strip casting has very high speed
(about 20 times higher), tighter tolerance, higher cooling speed in the mold,
and suppressed inclusion size range.
Trends and Drivers. Because of its novelty, many fundamental phenomenon need to be studied. These issues are related to process control, consistency, productivity, and quality (including tolerances). Because of the high speed of strip casters, the control tolerances must be significantly tighter than in conventional casting processes.
Technological Challenges. The technological challenges associated with casting include process knowledge and control deficiencies. For example, the required cast tolerances for strip casting cannot be achieved without intimate knowledge of the variation of heat transfer with casting conditions. Process control at high casting velocities is not possible without good data on the thermal conditions in the growing shell and in the rotating roll, better knowledge of initial solidification phenomena, and control strategies for strip profile and gauge at high speeds. The need for control of fluid flow and temperature is even more severe in strip casting than in conventional technologies. There are problems with liquid steel control, process consistency, productivity, and ability to make different structures. The strategy for conversion of a strip-cast structure to a structure that allows equivalent or improved properties for all strip applications is at this time unknown for all but 304 stainless steels.
New and Emerging Technologies The advent of commercial production of strip-cast 300 series stainless steels by Nippon Steel in 1997 represents a major new technology. Commercial strip casting of carbon and stainless steels at Nucor Crawfordsville by the end of 2001 is an important new technology in flat rolled steel production.
Casting R&D Needs and Opportunities
Some generic casting R&D needs have been identified, as well as specific R&D needs for slab, billet, and bloom casting and strip casting.
Generic
The R&D needs and opportunity for the generic casting is listed as follows.
(1) The ability to produce liquid steels with strictly controlled inclusion contents. This is necessary to restrict inclusion size to less than 0.0002 inch (0.005 mm) in diameter and to minimize the total mass of inclusions. In this area an understanding of the interaction of fluid flow at the slag-metal interface must be developed and modeled to eliminate formation and facilitate the removal of inclusions during processing.
(2) Techniques to calculate stability diagrams for all grades of steel. This allows the exact stability of all possible inclusions to be calculated for a particular alloy to ensure that the designed inclusion chemistry is achieved.
(3) The ability to monitor and actively control fluid flow, temperature, and chemistry. This would improve casting performance. Given the high cost and reliability issues associated with wiring in the harsh environments common to the steel industry, sensors that employ wireless technology to communicate data and diagnostic information could be beneficial to the infrastructure of iron and steel manufacturing facilities. Fluid mixing control in which mixing of unlike grades can be either enhanced or minimized in the tundish or mold, is also a necessary development to allow seamless grade transition and order size that is better matched with optimum heat sizes.
(4) Ability to monitor the process to ensure consistent quality. Related needs include advanced process control strategies, vision systems for defect detection and identification, and the implementation of advanced computer diagnostic controls for identification of potential operation problems and the scheduling of maintenance.
(5) Improved surface quality through improved mold oscillation using hydraulic oscillation. Development of modeling techniques is required to gain a detailed knowledge of surface formation in castings, an area that has been somewhat ignored in conventional casting.
(6) Ability to predict cast shape, inclusion, or bubble distribution and structure. The details of those areas in three dimensions are currently unknown. Models to predict on-line the details of micro- and macro-structure solidification and the inclusion or bubble distribution in the casting is required.
(7) Advanced heat transfer and fluid flow models. The models that include the free surface of liquid/liquid boundaries, the prediction of slag emulsification; the final position and shape of the cast surface; and a detailed prediction of cast structure, inclusion, or bubble distribution and segregation patterns are necessary. In addition, a detailed understanding of the interactions between steel shell, the flux, and the interface of the mold is necessary.
(8) Techniques to minimize scaling or develop scales that are easily removed in post processing. This is to maximize yield and eliminate scale-related defects. This problem will increase in severity as castings become thinner and closer to a product dimension
(8, 10, 13) Processing techniques need to be developed to improve quality and production rates. For example, techniques are needed to either minimize scaling or develop scales that are easily removed in post processing to maximize yield and eliminate scale-related defects. This problem will increase in severity as castings become thinner and closer to a product dimension.
(9) Enhanced education on the science and engineering principles involved in the design and operation of casters. A strong foundation in traditional engineering disciplines and metallurgy will be a necessary requirement for caster operation.
(10) Direct rod or wire casting at high rates and production directly from liquid steel, possibly through droplet consolidation or rheocasting.
(11) On-line control systems.
(12) Development of products associated with thin slab rolling in the 2 phase (alpha & gamma) field
(13) Development of a heavy rod casting process with in-line rolling to variable diameters.
Slab, Billet, and Bloom Casting
Areas of development needed in slab, billet, and bloom casting are summarized as follows:
- Methods of reducing total inclusion content. This includes appropriate refractory systems that are less prone to clogging, and optimized fluid flow and heat transfer within steel pouring systems.
- Improved bulk fluid flow and meniscus control, optimized mold flux design and heat flux control in the meniscus area. The production of a smooth cast surface without meniscus marks caused either by electromagnetics or the development of a "hot-top" mold may lead to the elimination of certain subsurface defects, especially at high casting speeds. The elimination or complete removal of argon and other bubbles from cast steels must also be developed to produce ultra-clean steels that are beyond the quality levels currently produced.
- Flux design improvements. Work is needed on flux crystallization phenomena, flux physical and chemical properties, and flux compatibility with liquid steel, refractories, and other surroundings.
- Nozzle development to allow a very controlled, stable fluid flow into the mold. The fluid flow into the mold should not encourage mold slag emulsification and decreases the tendency of nozzle clogging.
- Fluid flow control and stream shrouding techniques for small-section billet casters
- Mold designs to control billet shape and to allow for increased cast speed and improved quality. Mold designs that incorporate instantaneously controllable taper and temperature profile are needed to control shape and allow further increases in casting speed. Technologies must be developed to improve surface quality so casting can be used for all applications without surface grinding or treatment. Also, soft reduction needs to be investigated as a potential technique to produce sound structures in near-as cast state.
Strip Casting
Strip casting R&D needs include gains in process and technical knowledge as well as control systems and techniques. They are summarized as follows:
Knowledge of the variation of heat transfer with casting conditions and alloy chemistry. The details of the initiation of solidification and the effect of mold coating and texture on this phenomenon must be known to improve product quality.
New models, sensors, and control systems. They are needed for process control at high cast velocities. The development of comprehensive heat transfer, fluid flow, and solidification models will also allow the thermal conditions in the growing shell and in the rotating roll to be defined and enable in situ compensation or correction for roll distortions.
Novel techniques of liquid flow control. Techniques to control fluid turbulence within the pool of a twin roll caster will result in improved process consistency. In addition, development of rheocasting or a superheat removal technology for use in the entry nozzle of a strip caster will lead to enhanced productivity and novel strip cast structures.
Applications of strip casting for conventional and novel alloys.
Post-processing steps necessary for strip-cast material to have better mechanical properties than conventionally processed materials. For example, techniques can be developed to achieve texture control in strip cast materials without significant reduction in thickness of the strip.
Determined inclusion engineering requirements.
