Steel cleanliness is the one unifying theme in all steel plants as problems in steel cleanliness can lead to internal rejects or customer dissatisfaction with steel products. Thus all steel plants are continually attempting to improve their practices to produce more consistent products. In addition to product consistency, it is now clear that fatigue resistance and tool and die life correlate strongly with steel cleanliness and that improvements in steel cleanliness will result in further improvements in steel performance.

The future of steelmaking and casting will be to continue to reduce the total oxide inclusion mass in liquid steels and to ensure that the remaining inclusion chemistry and size distribution is closely controlled. The purpose of this proposal is to determine the potential limiting factors in the production clean steels and to produce on the laboratory scale ultra clean steels beyond that currently available in bulk production. This project will lead to the development of processes or process strategies that will allow cleaner more consistent steels to be produced. This issue is extremely important in the production of bar grades where fatigue strength is vitally important.

This project has been developed by discussion with the member companies of the Center for Iron and Steelmaking at Carnegie Mellon University and the necessary industrial cost share for the project was approved by the board of the CISR. No additional steel industry funds are requested in the project. Industrial co-sponsors include the following AISI producer member companies: USS, Bethlehem Steel, AK Steel, LTV, National Steel, Ispat-Inland, Cleveland Cliffs, The Timken Company and North Star Steel and the following associate member companies: Air Liquide-America, Air Products, Foseco, Inc. and Praxair.

The term "clean steel" is commonly used to describe steels that have low levels of the solute elements sulfur, phosphorus, nitrogen, oxygen and hydrogen; controlled levels of the residual elements copper, lead, zinc, nickel, chromium, bismuth, tin, antimony and magnesium; and, a low frequency of product defects that can be related to the presence of oxides created during the act of steelmaking, ladle metallurgy, casting and rolling. This last definition causes extreme problems to the steel manufacturer as the definition of "clean" is not absolute. Instead it is based upon the product formed from the casting and the in-service use or life of the product. In addition, the definition "clean" is comparative as each customer of a steel product has the ability to buy steel from around the world and compare the performance level of a given product based upon the supplier. In this system, the cleanliness standard desired by the customer is continuously changing as a function of time and technological improvements. The term "clean steel" is therefore continually variable depending upon the application and the competition between steel suppliers. In this proposal the term "clean steel" will refer only to the oxide inclusion content and other chemistry related issues will be referred to as purity or residual issues.

?In Table 1, from the review of Marukawa , it can be seen that there are a number of high purity and cleanliness requirements by grade and application. The difference in the criterion for successful application between steels is due to ones ability to measure the defect during processing. In automotive sheet, small surface and internal defects do not affect any physical or mechanical properties of the sheet. After the automotive sheet is coated surface imperfections of less than 50 m m do not appear as readily measured defects during visual inspection in this application. During the etching of a sheet to form a small hole, small inclusions can result in measurable defects in the shadow masks used for a cathode ray tube (CRT) and this application is therefore more susceptible to the presence of any small inclusions. These two applications indicate that the problem in "clean steel" is measurement of defect incidence. Once the defect incidence can be measured the customer requirement for a specific application becomes stricter. In general the thinner the product, the smaller the permissible inclusion.

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?In long products, non-destructive measurement techniques are routinely used to characterize inclusion sizes above 40 microns and fatigue testing can give a very good idea of the frequency of larger defects in cast steels. Thus steels for application to bearing qualities are subjected to the most stringent quality control and are the applications where significant breakthroughs in steel quality have been made. It is well known that total inclusion content (as measured by total oxygen content) has traditionally correlated with bearing life and decreased total oxygen contents (below 10 ppm) have lead to significant increases in bearing life. In addition to total oxygen content, the total length of stringer inclusions after forging correlates well with bearing life and, at low total oxygen levels, efforts to reduce inclusion clustering leads to very long fatigue life for bearings.

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There is one constant in the world of clean steels and that is the continual drive to minimize the total mass of oxide inclusions and to control the chemistry, frequency and size distribution of the inclusions that are found in all steels. The purpose of this proposal will be outline a project that aims to determine the potential limits of steel cleanliness, to produce ultra-clean steels in the laboratory and to determine the benefits of ultra clean steels.

The key technical issue in clean steel manufacture is the control of inclusion quantity, size distribution and chemistry. As steels become cleaner it is becoming more difficult to remove the small mass of inclusions that remain within the steel and conventional process techniques may be limited in their ability to produce steels that are significantly cleaner than today’s steels without some radical changes in operating philosophy. In addition, processing time for cleaner steels may be longer than is currently available and a practical solution to clean steel manufacture must also fit within the processing cycle of today’s steel manufacturer.

The production of clean steel is not without severe technical problems. In order to remove inclusions it is necessary to precipitate the inclusions at liquid steel temperatures and thermodynamics then limits our ability to produce a clean steel. An example of this problem can be seen in current low carbon, aluminum killed steel manufacture where the limit of clean steel is thermodynamically defined at approximately 5 ppm oxygen due to the equilibrium between aluminum and alumina. Calcium and slag treatment can reduce this thermodynamic limit to approximately 1 ppm oxygen in aluminum killed steels by changing the activity of alumina; however, such a low level of oxygen is not reached practically in low carbon steels and rarely approached in vacuum treated higher carbon steels.

The reason that the lower limit in practical steelmaking is significantly higher than the thermodynamic limit is due to the kinetics of inclusion separation from liquid steel and to environmental conditions. Very low oxygen content steels must have low oxygen activities. To prevent reoxidation during steel production it is necessary to ensure that the steel containers and covering slags are in equilibrium with the liquid steel. If this is not achieved the inclusion content in liquid steel will be determined by kinetic factors where the rate of change of inclusion content (J_o) in grams per second will be determined by the flux of inclusions created by reaction with the slag (J_slag ), refractories (J_refractory ) and air (J_air ) and the flux of inclusions that separate to the slag (J*_slag) and to refractories (J*_refractory) :

Achieved cleanliness is a combination of the kinetic factors governing the creation and elimination of inclusions. As is always true of general equations, their very simplicity hides enormous complexity. For example J*_slag is a function of starting oxygen content, inclusion size distribution, slag chemistry and associated physical properties, slag volume and the details of particle trajectory in the bath (fluid flow), thus its functional relationship to time must include all of these factors. All of the factors are similarly complex and quickly the problem of practical steelmaking becomes clear - to improve steel cleanliness one must control and minimize the fluxes responsible for inclusion creation while maximizing the fluxes responsible for inclusion elimination. Thus thermodynamics creates the potential for clean steel formation and a knowledge of kinetics allows this potential to be realized. It is also clear from equation 1 that if J_slag + J_refractory + J_air > J*_slag - J*_refractory then the steel inclusion content increases.

If the external sources of inclusion formation can be eliminated, then inclusion separation kinetics can be maximized; however, as steel becomes cleaner the average inclusion size decreases and it becomes more difficult to remove inclusions as their flotation velocity decreases resulting in longer treatment times. One mechanism to overcome this kinetic limit would be to increase inclusion agglomeration rates to increase the overall inclusion diameter; however, as larger inclusion diameters are more problematic in the product, complete separation of agglomerated inclusions must be achieved for this type of clean steel production to be successful.

Even if kinetic factors allow inclusion contents to be reduced, one can end up with a low mass of inclusions with a low frequency of very large inclusions. In many applications the presence of any larger inclusions is a reason for part failure. Therefore, the attainment of a low mass of inclusions does not necessarily mean that the steel will be clean for all applications. A good example of this problem can be found in bearing quality steels where the presence of any inclusions greater than 40 microns in diameter can cause a premature part failure. Even though the total oxygen content in bearing steels can be low (< 5ppm), the steels are non-destructively tested to ensure that there are no agglomerated inclusions in parts sent to the customer.

The key to improving our ability to reduce the mass and size of inclusions in steels lies in our understanding of two phenomena - (1) why do large inclusions form ? and (2) what prevents large inclusions from being removed from liquid steels? Both of these questions rely on observation of phenomena under steelmaking conditions, an observation that was not possible until recently.

A Confocal Scanning Laser Microscope equipped with a gold image high temperature furnace makes it possible to image liquid steel surfaces and this technique was first developed in Prof. Emi's lab in Tohoku University in Japan in 1996 [,]. The first CSLM for steelmaking studies was installed at CMU during 1999 and applied for the first time to the study of slag metal interfaces [] and the dissolution of particles in slags. Both of these approaches have been successful. This technique of observation makes it possible to study the formation and elimination of both liquid and solid inclusions from liquid steels. This ability is unique to CMU. For example, in figure 1, a large liquid inclusion od 100 micron in diameter can be seen sitting at a slag-metal interface. This was the first documentation of the fact that inclusion separation at a slag-metal interface is not instantaneous and that even 100 micron diameter inclusions can sit at slag-metal interfaces for significant time frames. In Figure 2 alumina inclusions are seen to agglomerate at a slag metal interface - a phenomena that was unknown at slag-metal interfaces before this work. Dissolution of an alumina particle in liquid steel can be seen in figure 3. This is again a unique observation from the CSLM at CMU. This technique also allows inclusion dissolution rates to be determined.

It is clear that this new observational technique gives us the ability to clearly define and quantify phenomena in slags and at slag-gas and slag-metal interfaces. This technique is the key to developing an understanding of inclusion formation and elimination in steel production.

This research will be aimed at elucidating the factors that control inclusion formation and removal of both solid and liquid inclusions from liquid steels. The findings of this research will be used to determine the types of slags and interfacial conditions that are necessary to ensure complete separation of inclusions from steels at a slag metal interface and to determine the conditions that can lead to the formation of larger inclusions in steels. In this area direct observation of phenomena at high temperatures is necessary and Confocal Scanning Microscopy will be used to elucidate the mechanisms responsible for inclusion agglomeration and removal. The findings will be compared to the limits for inclusion removal set by thermodynamics of the steel and slag.

This program will lead to a better understanding of the practices and mechanisms responsible for the formation of large inclusions in cast steels. This will lead to practices that will result in the ability to produce cleaner steels, increase productivity and minimize product defects. Thus this work will aid in reducing the overall energy consumption of the steel industry by aiding in increased productivity and decreased defect ratio, both key components of successful hot charging.

• To determine the methodology necessary to produce ultra-clean steels beyond that currently possible with today’s technology

• To determine the kinetic factors governing inclusion removal from liquid steels at a slag metal interface

The sponsoring companies of the CISR will monitor the project and detailed project planning will occur at the biannual CISR meeting that will also serve as the official meetings of the project oversight committee. The plan for the project follows the objectives outlined above:

1. Determine the factors controlling solid inclusion separation at a metal-gas interface

The project is budgeted to last three years. The project start date is August 1, 1999. There will be monthly updates posted on a web page for the project and official quarterly and annual reports to the DOE. In addition there will be biannual project meetings with the projects advisory board held in conjunction with the CISR meetings. A Gantt chart outlining project timing is attached.

The total project budget is broken down in the official university budget attached to this proposal. Details are given in Table 1. The cost share of 30% will be supplied by the industrial co-sponsors. The following AISI producer member companies will be part of this project: USS, Bethlehem Steel, AK Steel, LTV, National Steel, Ispat-Inland, Cleveland Cliffs, The Timken Company and North Star Steel and the following associate member companies: Air Liquide-America, Air Products, Foseco, Inc. and Praxair.