Clean Steel: Part Three

The increasing demand in recent years for high-quality steel products has led to the continuous improvement of steelmaking practices. There is a special interest in the control of non-metallic inclusions due to their harmful effect on the subsequent stages and their great influence on the properties of the final product. Through the control of the amount, size and chemical composition of the inclusions it is possible to obtain a final product of good quality. The control of the formation of non-metallic inclusions and the identification of their constituent phases are of extreme importance for the obtaining of clean steels.

The presence of non-metallic oxide inclusions is a major cause of incompatibility between the attainable and desirable level of cleanliness in many grades of commercial steel. Generally, inclusions degrade the mechanical properties of the steel and thereby reduce the ductility of the cast metal and increase the risk for mechanical and/or corrosion failure of the final product.

Oxide inclusions originate from two sources:

• residual products resulting from intentionally added alloying elements to deoxidize the molten steel after oxygen treatment (endogenous or micro inclusions);
• products resulting from reactions between the melt and atmosphere, slag, or refractory (exogenous or macro inclusions).

Among various types of nonmetallic inclusions, oxide and sulphide inclusions have been thought harmful for common steels.

Alumina inclusions occur as deoxidation products in the aluminum-based deoxidation of steel. Pure alumina has a melting point above 2000°C, i.e., these alumina inclusions are present in a solid state in liquid steel. The addition of calcium to steel which contains such inclusions changes the composition of these inclusions from pure alumina to CaO-containing calcium aluminates.

As it can be see from Figure 1, the, melting point of the calcium aluminates will decrease as the CaO content increases, until liquid oxide phases occur at about 22% of CaO, i.e., when the CaO.2Al₂O₃ compound is first exceeded at 1600°C. The liquid phase content continues to increase as CaO content rises further and is 100% at 35% of CaO. The minimum melting temperature for the liquid calcium aluminates is around 1400°C, i.e., such liquid calcium aluminates may be present in liquid form until, or even after, the steel solidifies.

Most grades of steel are treated with calcium using either a Ca-Si alloy or a Ca-Fe(Ni) mixture, depending on the silicon specification. This treatment is made after trim additions and argon rinsing.

In most melt shops the cored wire containing Ca-Si or Ca-Fe(Ni) injection system is used in the calcium treatment of steel. The melting and boiling points of calcium are 839°C and 1500°C respectively. During calcium treatment, the alumina and silica inclusions are converted to molten calcium aluminates and silicate which are globular in shape because of the surface tension effect. The change in inclusion composition and shape is known as the inclusion morphology control.

Figure 1: Binary system CaO-Al₂O₃

The calcium aluminates inclusions retained in liquid steel suppress the formation of MnS stringers during solidification of steel. This change in the composition and mode of precipitation of sulphide inclusion during solidification of steel is known as sulphide morphology or sulphide shape control.

Several metallurgical advantages are brought about with the modification of composition and morphology of oxide and sulphide inclusions by calcium treatment of steel, as for instance:

• To improve steel castability in continuous casting, i.e. minimize nozzle blockage
• To minimize inclusion related surface defects in billet, bloom and slab castings
• To improve steel machinability at high cutting speeds and prolong the carbide tool life
• To minimize the susceptibility of steel to re-heat cracking, as in the heat-affected zones (HAZ) of welds
• To prevent lamellar tearing in large restrained welded structures
• To minimize the susceptibility of high-strength low alloy (HSLA) linepipe steels to hydrogen-induced cracking (HIC) in sour gas or sour oil environments. The Ca content in the final product can be controlled within the range of 15 to 20 ppm
• To increase both tensile ductility and impact energy in the transverse and through-thickness directions in steels with tensile strengths below 1400 MPa

When calcium is injected deep into the melt, the following series of reactions are expected to occur to varying extents in Al-killed steels containing alumina inclusions:

Ca + O = CaO       (1)

Ca + S = CaS       (2)

Ca + (x+1/3)Al₂O₃ = CaO·x Al₂O₃ + 2/3[Al]       (3)

Depending on the steel composition, the manner of calcium adding in steel bath and other process variables, there will be variations in the conversion of alumina inclusions to aluminates inclusions, the smaller inclusions will be converted to molten calcium aluminates more readily than the larger inclusions.

Thermodynamically, if sulfur or oxygen is dissolved in the steel at moderate levels, or if Al₂O₃ inclusions are present in steel, calcium will react with oxygen or sulfur until the contents of reactants are very low (< 2ppm). One of the critical questions is whether or not calcium added to steel will react with sulfur by reaction (2) and form CaS or modify Al₂O₃ to liquid calcium aluminates by reaction (3).

The formation of calcium sulfide can occur if calcium and sulfur contents are sufficiently high. Since calcium has higher affinity for oxygen than for sulfur, the addition of calcium initially results in a more or less pronounced conversion of the alumina into calcium aluminates until the formation of calcium sulfides starts as the addition of calcium continues.

Calcium sulfides are solid at steelmaking temperatures and result in nozzle clogging similar to that caused by alumina. As can be observed from the Figure 2, the conversion of alumina into calcium aluminates occurs until all the inclusions in the steel are present only in liquid form.

Figure 2: Change of inclusions composition during calcium additions

To prevent nozzle clogging in continuous casting by solid inclusions, calcium is added to steel to modify inclusions and desulfurize the steel. Calcium will convert solid alumina (Al₂O₃) inclusions into lower melting point calcium aluminates, which will help prevent the clogging of the casting nozzles. However, when calcium is added to steel, it will also react with oxygen and sulfur and modify the sulfide inclusions. If the sulfur content of the steel is high, calcium will react with sulfur forming solid CaS, which could clog up the continuous casting nozzle.

The Figure 3 shows influence of calcium treatment on the type of inclusions formed and its relationship with nozzle clogging.

Figure 3: Influence of calcium treatment on the type of inclusions formed and its relationship with nozzle clogging

Calcium treatment cannot be applied to all kinds of steel. For those with high requirement on formability, such as automobile sheet, calcium treatment is not suitable, because this treatment causes the formation of calcium aluminates inclusion which is hard. Therefore, for those kinds of steel, the method of improving molten steel´s purity is usually taken to optimize castability. Through controlling carry-over slag from melting furnace, deformation treatment of ladle slag, metallurgy in tundish, protective casting and other measures, purity of steel is guaranteed and total oxygen content in molten steel decreases.