Clean steel technology.

The development of techniques for producing and delivering consistently clean steel to the casting cavity has been identified as one of the key requirements for maintaining a competitive edge over foreign competition and for opening new markets for steel casting applications.

In the early 1980s, a high-level management task force representing the steel casting industry identified oxide macroinclusions as the major factor responsible for the lack of acceptance of steel castings by the design engineering community. This study augmented the already well-known requirements for cast steel to be free from tramp elements, gases and microinclusions. In shot, the mandate for 'clean steel' has been issued.

Following the release of this study, many research projects directed at solving these problems have been conducted, both in the U.S. and overseas. Projects have focused on the sources of oxide macroinclusions and techniques for minimizing their occurrence. Perhaps the most significant were the studies sponsored by the Steel Founders' Society of America and the U.S. Dept. of Commerce, which carefully delineated the role of reoxidation in the formation of these harmful inclusion defects. Again, emphasis was on practices for elimination of inclusions.

The topic of clean steel production was reviewed in the October 1988 issue of modem casting. This article will present the more promising new technologies for clean steel production and describe the future research plans of the AFS Clean Steel Technology Committee 9-E). Importance of Oxygen

Larry Heaslip, Advent Process Engineering, who is somewhat of a clean steel guru,' has often stressed that the secret" of producing clean steel is to minimize and control the metal's exposure to oxygen. The primary characteristic of clean steel is a low content of both macro- and microinclusions. Consequently, the technologies that have proved most beneficial have focused on the control of oxygen exposure.

In addition to control, it is also desirable to measure the oxygen content of the molten steel. This provides a baseline to evaluate the effectiveness of our clean steel efforts. A new portable oxygen probe system (Fig. 1) allows steel foundrymen to evaluate the oxidation of the metal before pouring, thus allowing the opportunity to correct deficiencies before scrap castings are produced.(1)

Melting and Refining

Since it is imperative that the steel must be in the best state of 'cleanliness' when it leaves the furnace, increased attention is being focused on metal control during the furnace operation itself. This essentially translates into greater application of basic arc melting to remove phosphorus and sulfur.

Steel foundries that were traditional 'acid shops" are converting to basic melting in order to meet more stringent customer requirements. In addition to offering the capability to remove sulfur and phosphorus, basic melting features more stable refractories, thereby reducing the contribution to inclusion problems from the furnace lining.

While sulfur and phosphorus can be effectively removed in the furnace by using the double-slag practice, this is not necessarily the most efficient procedure. Many foundries are following the lead of the basic steel industry and using external refining processes. This essentially involves melting and removing phosphorus in a basic arc furnace, transferring the metal to a second external vessel, removing sulfur and providing finishing operations in this secondary unit. This creates a duplex" process and allows each vessel to perform in the most efficient manner.

The most common external refining process for steel foundries has been. he AOD argon-oxygen-decarburization) process. Originally developed for the production of stainless steel, this process is extremely efficient in removing sulfur and excess gasses. The process has found acceptance in about 50 foundries worldwide and has stabilized at that level in recent years.1

More recent approaches to this concept involve the use of a ladle as the treatment vessel, hence the term "ladle metallurgy.' While this has been standard practice in the basic steel industry for many years, problems of scale with smaller foundry ladies have limited application in the foundry,(3) that is until now.

A novel application and refinement of DC arc furnace technology, developed at the University of Toronto, is being applied to a seven-ton ladle furnace to provide heating and electromet allurgical refining. This demonstration installation Fig. 2) is being funded by the EPRI Center for Materials Production as part of its overall foundry research program.

It is anticipated that very low levels of both oxygen and sulfur (<10 ppm) can be attained and, when combined with proper metal delivery systems, can produce bearing-grade steel castings. Since the vessel also is the pouring ladle, an additional metal transfer step is eliminated, further reducing the opportunity for reoxidation to occur.

A variation on this theme is the application of this same technology to the coreless induction furnace. Again, the low levels of sulfur and oxygen 4 Can be achieved, and the technology is easily retrofitted to existing furnaces (Fig. 3).

A somewhat different approach consists of placing a ladle with a non-conducting shell inside a fixed induction coil to provide heating and stirring. Powered-window equipment is an example of this idea. Although the slag is not heated by the induction field, supplemental heating by plasma torch or oxy-fuel burner can produce a highly reactive, fluid slag to facilitate refining.

Another approach used to minimize reoxidation during melting is the introduction of an inert gas, usually argon, to the furnace chamber at the beginning of melting and continuing additions throughout the melt cycle. Various industrial gas suppliers approach the application in different ways, but the two most common are introducing the argon as a liquid or covering the furnace opening with a laminar barrier flow from a manifold. Excellent results have been achieved with both methods in coreless induction furnaces.1.6

Calcium Treatment

One of the most promising technologies developed through recent research is the treatment of steel with calcium, either as a powder or a wire.(1) Although the mechanism is not completely understood, there is ample evidence that the proper addition of calcium aids in the agglomeration and flotation of oxide macro inclusions and provides some protection against reoxidation.

This technology is being adopted by many steel foundries and has resulted in significant reductions of machining costs in several applications.

Metal Delivery Systems

In the old days, we called them pouring and gating systems; today's term is Metal Delivery Systems.' The important thing is that the role these processes play in the formation of reoxidation products has finally been recognized, with research into better system under way.

When one visualizes the turbulent flow conditions that are encountered by the steel while traveling from the furnace, into the ladle and through the gang system to the mold cavity, it becomes obvious that the potential for reoxidation is enormous. Water modeling work by the University of Toronto and others has clearly established the sad fact that we just do not know how to gate very well. Further work is certainly indicated in this area.

That is not to say that no improvements have been made. Most steel foundries use bottom-pour ladles, and the development of the Cruciform 

In addition to control, it is also desirable to measure the oxygen content of the molten steel. This provides a baseline to evaluate the effectiveness of our clean steel efforts. A new portable oxygen probe system (Fig. 1) allows steel foundrymen to evaluate the oxidation of the metal before pouring, thus allowing the opportunity to correct deficiencies before scrap castings are produced.'

Melting and Refining

Since it is imperative that the steel must be in the best state of "cleanliness" when it leaves the furnace, increased attention is being focused on metal control during the furnace operation itself. This essentially translates into greater application of basic arc melting to remove phosphorus and sulfur.

Steel foundries that were traditional "acid shops" are converting to basic melting in order to meet more stringent customer requirements. In addition to offering the capability to remove sulfur and phosphorus, basic melting features more stable refractories, thereby reducing the contribution to inclusion problems from the furnace lining.

While sulfur and phosphorus can be effectively removed in the furnace by using the doubleslag practice. this is not necessarily the most efficient procedure. Many foundries are following the lead of the basic steel industry and using external refining processes. This essentially involves melting and removing phosphorus in a basic arc furnace, transferring the metal to a second external vessel, removing sulfur and providing finishing operations in this secondary unit. This creates a "duplex" process and allows each vessel to perform in the most efficient manner.

The most common external refining process for steel foundries has been the AOD (argon-oxygen-decarburization) process. Originally developed for the production of stainless steel, this process is extremely efficient in removing sulfur and excess gasses. The process has found acceptance in about 50 foundries worldwide and has stabilized at that level in recent years.

More recent approaches to this concept involve the use of a ladle as the treatment vessel, hence the term "ladle metallurgy." While this has been standard practice in the basic steel industry for many years, problems of scale with smaller foundry ladles have limited application in the foundry, that is until now,

A novel application and refinement of DC arc furnace technology, developed at the University of Toronto, is being applied to a seven-ton ladle furnace to provide heating and electrometallurgical refining. This demonstration installation (Fig. 2) is being funded by the EPRI Center for Materials Production as part of its overall foundry research program.

It is anticipated that very low levels of both oxygen and sulfur (<10 ppm) can be attained and, when combined with proper metal delivery systems, can produce bearing-grade steel castings. Since the vessel also is the pouring ladle, an additional metal transferstep is eliminated, further reducing the opportunity for reoxidation to occur.

A variation on this theme is the application of this same technology to the coreless induction furnace. Again, the low levels of sulfur and oxygen' can be achieved, and the technology is easily retrofitted to existing furnaces (Fig. 3).

A somewhat different approach consists of placing a ladle with a non-conducting shell inside a fixed induction coil to provide heating and stirring. Powered-window equipment is an example of this idea. Although the slag is not heated by the induction field, supplemental heating by plasma torch or oxy-fuel burner can produce a highly reactive, fluid slag to facilitate refining.

Another approach used to minimize reoxidation during melting is the introduction of an inert gas, usually argon, to the furnace chamber at the beginning of melting and continuing additions throughout the melt cycle. Various industrial gas suppliers approach the application in different ways, but the two most common are introducing the argon as a liquid or covering the furnace opening with a laminar barrier flow from a manifold. Excellent results have been achieved with both methods in coreless induction furnaces.(5,6)

Calcium Treatment

One of the most promising technolgies developed through recent research is the treatment of steel with calcium, either as a powder or a wire.(7) Although the mechanism is not completely understood, there is ample evidence that the proper addition of calcium aids in the agglorneration and flotation of oxide macro inclusions and provides some protection against reoxidation.

This technology is being adopted by many steel foundries and has resulted in significant reductions of machining costs in several applications.

Metal Delivery Systems

In the old days, we called them pouring and gating systems; today's term is "Metal Delivery Systems." The important thing is that the role these processes play in the formation of reoxidation products has finally been recognized, with research into better system under way.

When one visualizes the turbulent flow conditions that are encountered by the steel while traveling from the furnace, into the ladle and through the gating system to the mold cavity, it becomes obvious that the potential for reoxidation is enormous. Water modeling work by the University of Toronto and others has clearly established the sad fact that we just do not know how to gate very well. Further work is certainly indicated in this area.

That is not to say that no improvements have been made. Most steel foundries use bottom-pour ladles, and the development of the Cruciform nozzle in 1988 has provided a tool to aid in minimizing reoxidation. The development of slide-gate systems with small exterior dimensions offers foundrymen the opportunity to fit these useful devices to the smaller, foundry-size ladles. More advanced work on electromagnetic nozzles being conducted in Germany suggests the exciting capability of a flow control system with no refractory components whatsoever. Certainly, this is work to be followed.

Following the logic that steel cleanliness can be improved by eliminating contact with air, several schemes have been proposed to "shroud" the stream and protect it from the foundry atmosphere. Various types of gaseous and mechanical shrouds have been developed and tried over the years. Most are takeoffs of shrouds used with continuous casting machines in steel mills. Major differences in the application, however, have prevented these approaches from being completely successful.

In a mill, the ladle and shroud are stationary, and adequate time for alignment and adjustment is available before the start of pouring. The opposite situation exists in a foundry with short periods of flow and multiple placements over the pouring cup. Work continues, and it is my opinion that some sort of shroud combining mechanical and inert gas protection will be developed in the near future.

Perhaps a more important observation can be made from recent research. We may have been pouring our molds upside down" all along. Counter-gravity systems that introduce the metal from underneath the mold in a smooth, controlled manner offer an enormous potential for the reduction of reoxidation defects.

Although this technology is commonplace in high-integrity aluminum casting facilities, it has only recently been rediscovered for conventional steel foundries. New applications, dating back to the Pressure Pouring Process introduced by Lebanon Steel Foundry in 1960, often combine induction heated pouring/refining ladies with the counter-gravity pouring scheme.

Examples are the SCRATA system, the Griffin Wheel Process, the Reynold's Method, the Hitchiner Process and the Mercury Marine Vacuum-lift FoamFilled Process. In all of these variations on the same theme, the metal is introduced into the mold cavity in a smooth, non-turbulent fashion, resulting in substantially improved casting quality.

Recent advances in high-powered electromagnetic pumps from Germany and France offer the potential of providing precise flow conditions, controlled by microprocessors for each casting configuration. Newly developed numerical simulation software can precisely calculate the flow in a mold cavity so that the pouring control program can be optimized.(1) Combining these technologies would give the steel foundryman, for the first time, the opportunity to optimize the flow for each casting-an exciting concept indeed.

Liquid Filtration

Work continues in the application of ceramic filters in the production of steel castings. While routinely used in the production of iron and nonferrous castings, filters have received mixed reviews from steel foundrymen. Though considerable success has been realized with stainless steels, the twin problems of filter priming and durability (breakage) have limited the applications with carbon and low-alloy steels.

An interesting concept-based on water modeling and high-speed photography work-is that much of the benefit observed with filters is derived from the fact that they act more as flow control devices or flow modifiers than as mechanical and physical filters.

Work continues, and there is little doubt that these problems will be solved and filters will soon find their place as tools for clean steel production.

Future Directions

There is no question that major advances in clean steel production have been made in recent years. Reflecting on the state of quality in the steel foundry industry today, we quickly realize that our job is not finished. Although we have a good starting point, much remains to be learned with regard to the technologies developed in the above studies.

Further, a more fundamental requirement exists: the development of a method to quantify the cleanliness of a cast steel, and the determination of the relationship between cleanliness and the material properties and performance.

The AFS Clean Steel Committee's goal is to organize and administer a project that will not only develop the cleanliness test procedures and cleanliness/performance relationships mentioned above, but also complete the studies of specific processes that have been started by other agencies. A research proposal to accomplish these tasks has been prepared and submitted to AFS for funding. For a free copy of this article circle No. 313 on the Reader Action Card.

References

1 . G.R. Fitterer, Active Oxygen Control in Steel," AFS

Transactions, p. 215 (1981).

2. D.E. Dutcher, "AOD Refining Practices," Proceedings

1st International Foundry Congress, SFSA, p. 98

(1895).

3. J.M. Svoboda, Powder Injection Ladle Metallurgy

for Foundry Applications," Electric Furnace Proceedings,

ISS, Vol. 47, p. 337 (1989).

4. I.D. Sommerville, et al, Materials Processing in

Plasma Furnaces Equipped with Graphite Elec - trodes," Plasma Technology in Metalturgical Pro - cessing, J. Feinman, Ed. , ISS, p. 89 (1987).

5. S. Hornby-andersori, J. Foss, R. Nagan and B. Jhala,

SPAL for the Investment Caster," AFS Transactions

(1989).

6. S.K. Sharma and M.S. Nowotarski, Laminar Barrier

Inerting for Induction Melting,' Investment Casting 

Institute 37th Annual Technical Meeting, Los Angeles 

(1989).

7. F.J. Squire and R.G. Shepherd, "Improved Steel

Pourability and Cleanliness with Calcium Wire at

Harrison Steel Casting Company,' Electric Furnace

Proceedings, p. 265 1987).

8. J.M. Svoboda, Clean Steel Technology,' modem

casting, p. 43 (Oct 1988).

9. P.N. Hansen, G. Hartmann and J.C. Sturm, Elimination

of Shrinkage Defects Through Use of Computer

Simulation,' AFS Transactions (to be published

1991).