The most popular definition of clean steel is in reference to steel with low inclusion content. This article will review some of the techniques that are used to produce steel with low inclusion content and how these practices are used in steelmaking.
|Fig. 1. Types of inclusions found in steel|
What is an Inclusion?
Inclusions are non-metallic particles that are trapped in the solid steel matrix of a forging or rolled product. Exogenous inclusions are those that come from sources outside of the steel, such as refractory bricks or flux used in molds and casters. Exogenous inclusions are typically large (>1 mm in size) because they originate as “crumbs” of these outside sources that become entrapped in the steel while it is being processed in liquid form. Indigenous inclusions are those that are formed from chemical reactions inside the liquid steel as it is processing, such as when manganese combines with sulfur in the liquid steel to form small manganese-sulfide inclusions. Indigenous inclusions are typically on the micro scale (0.001-1.000 mm in size). When striving for clean steel, it is the indigenous inclusions that we are trying to control.
Indigenous inclusions can further be separated into two main categories for steel: oxides and sulfides (Fig. 1). Oxides are generated for the most part by the addition of aluminum or silicon during secondary steelmaking. Sulfides are generated during solidification when sulfur combines with a sulfide former, most commonly manganese.
|Fig. 2. Clean-steel practices begin in the ladle furnace.|
Oxygen is Introduced in the EAF, Removed in the LF
Electric steelmaking starts with primary steelmaking in the electric arc furnace (EAF). It is the job of the EAF to turn solid scrap into raw liquid steel. Oxygen is blown into the EAF throughout primary steelmaking to accelerate the melting process through the addition of chemical energy. The oxygen also combines with carbon and phosphorus to form oxides that are removed from the bath. Carbon leaves the bath in the form of CO and CO2 gas, while phosphorus leaves the bath in the form of P2O5, which becomes part of the slag floating on top of the liquid steel.
After primary steelmaking in the EAF, the steel is tapped into a ladle and moved to the ladle furnace (LF) for secondary steelmaking. In secondary steelmaking (aka ladle refining), alloys are added, and the temperature of the liquid steel is adjusted by heating with carbon electrodes. One of the main goals in secondary steelmaking is to remove all of the oxygen that was introduced in the EAF. This is done mostly by the addition of (what else) a deoxidant. The most common deoxidants are aluminum and silicon. Both of these elements have a strong affinity for oxygen, so once they are introduced, they begin to combine with free oxygen dissolved in the steel to form aluminum oxides or silicon oxides. The removal of oxygen by adding an element with a strong affinity for oxygen is known as “killing.” This is why you might hear steel referred to as “aluminum killed” or “silicon killed.” After killing the steel, you are left with a multitude of either SiO2 or Al2O3 particles in the liquid steel.
Now that we have generated all of these oxides, the idea is to remove them by moving them to the slag. This can be accomplished given enough time and stirring. By stirring the bath via magnetic induction stirring and/or inert gas bubbling through the bottom of the ladle, the oxide particles are brought into contact with each other. When the oxides run into each other, they tend to agglomerate and make bigger oxides. As these alumina and silica particles agglomerate and form larger oxides, they have more buoyancy and have a greater tendency to float to the slag on top of the liquid-steel bath. To accomplish this removal of oxygen by deoxidant addition, you must have effective stirring to bring the oxides in contact with each other and sufficient time for the agglomeration and floating to take place. By the end of the processing time in the ladle furnace, most of the oxides should be removed from the liquid steel and trapped in the liquid slag on top of the steel.
Sulfur Removal in the LF
While oxygen is being removed with the help of aluminum and silicon, other processes are at work in the LF to remove sulfur. As mentioned earlier, sulfide inclusions are generated during solidification when sulfur combines with manganese to form MnS. The most effective way to reduce the number of MnS inclusions in the final product is to reduce the sulfur content of the steel. The reason we have sulfur in the steel in the first place is because it is an inherent element in iron ore itself and, therefore, is typically found in steel scrap as well. Sulfur is also added to the system as a tramp element contained in some ferro alloys such as ferro chrome. In order to remove sulfur, we need to produce a compound similar to the oxides generated when we killed the steel. For steelmaking purposes, this compound is CaS.
Sulfur will combine with calcium through the following simple reaction:
CaO + S → CaS + O
However, the more complete picture of what is actually happening in an aluminum-killed steel is given by this reaction:
3(CaO) + 2Al +3S → 3(CaS) + Al2O3
The amount of oxygen in the steel is directly related to how much deoxidant (Al) is in the steel. The more deoxidant, the less oxygen there is dissolved in the steel. So adding deoxidant will drive the above reaction to the right. In other words, deoxidizing aids in sulfur removal. Another way to drive this reaction to the right is to make sure there is plenty of CaO in the slag. A slag that is diluted with CaO has a higher capacity to absorb CaS. Again, increasing the reactants on the left side of the equation drives the reaction to the right and aids in sulfur removal.
|Fig. 3. In the ladle furnace, sulfur and oxygen are removed from the steel and are pushed to the slag through various chemical reactions and kinetic processes.|
Achieving the Lowest Possible Inclusion Content
So far we have discussed the mechanisms by which oxide and sulfide inclusions are removed from the liquid steel. It is impossible to achieve perfect steel cleanliness by these methods, however, and there will always be some inclusions that remain in the steel. The goal is to reduce the inclusion content to as low of a level as possible.
As mentioned previously, the most important processing variables for inclusion removal are time and adequate mixing. It takes time for the oxide particles to meet up with each other in the liquid steel and agglomerate, and the agglomeration of the oxide particles is necessary for the oxides to achieve adequate buoyancy to float up to the slag on top of the liquid bath. Adequate mixing also facilitates agglomeration of the oxide particles. The more the steel is stirred and mixed, the more likely the oxide particles are to come in contact with each other. Time and stirring also aid in sulfur removal. Proper stirring of the steel will increase the slag-steel interaction in the ladle. Increased surface-area interaction between the steel and the slag promotes the kinetics of the calcium-sulfur reaction. As mentioned earlier, it is also important to have a high CaO content in the slag to increase its capacity to absorb sulfur. Hence, the keys to sulfur removal are often shortened to the brief couplet “Lime and Time.” IH
For more information: Chuck Fryman, metallurgist, Ellwood Quality Steels, 700 Moravia St., New Castle, PA 16101; tel: 724-658-6527; fax: 724-658-6802; e-mail: firstname.lastname@example.org; web: www.ellwoodgroup.com
SIDEBAR: Processing Aerospace Materials
Although low-density metals such as titanium and aluminum alloys receive most of the attention in the aerospace industry, steel remains the material of choice for many airplane components. However, not just any steel will do when it comes to building planes. Steels that are used in aerospace applications such as landing gear and bulkheads need to have good fatigue properties to survive the cyclic loading that these components receive. In order to have good fatigue properties, the steel must be high strength and have a low inclusion content. Inclusions, or any other imperfections in the steel matrix, act as stress concentrators. These stress concentrators are then likely to act as fatigue-crack initiation sites. Once a fatigue crack is initiated, it does not require as much energy to propagate the crack. Given enough cycles of sufficient stress, a fatigue crack will grow and eventually lead to failure.
To produce clean steels such as those required for aerospace applications, clean-steel practices like the ones outlined in this article must be employed. In addition to these clean-steel practices, many aerospace steels are also required to undergo secondary melting operations such as vacuum-arc remelting (VAR) or electro-slag remelting (ESR). These remelting processes take an air-melted ingot and remelt it under controlled conditions to refine the steel further, resulting in steel with even fewer inclusions.