Calcium in Steels
Calcium in Steels
Calcium (Ca) (atomic number 20 and atomic weight 40.08) has density of 1.54 gm/cc. Melting point of Ca is 842 deg C and boiling point is 1484 deg C.
Ca additions are made during steel making for refining, deoxidation, desulphurization, and control of shape, size and distribution of oxide and sulphide inclusions . Ca is not used as alloying element since its solubility in steel is very low. Further it has a high vapour pressure since it boiling point is lower than the temperature of the liquid steel. It has a high reactivity and hence special techniques are necessary for its introduction and retention of even a few parts per million in the liquid steel.
Advantages directly attributable to Ca treatment include greater fluidity, simplified continuous casting and improved cleanliness (including reduction in nozzle blockage), machinability, ductility and impact strength in the final product.
Ca is added to steel in the stabilized forms of calcium silicon (CaSi), calcium manganese silicon (CaMnSi), calcium silicon barium (CaSiBa) and calcium silicon barium aluminum (CaSiBaAl) alloys or as calcium carbide (CaC2). Elemental Ca is difficult and dangerous to add to liquid steel.
CaSi in steel sheath (also called cored wire) is the most commonly used addition agent for Ca addition. The cored wire is injected into the liquid steel with help of wire injection system. It has higher recovery of Ca in steel than the virgin Ca / CaSi lumps addition into the ladle. The CaSi cored wire contains 4.5 % of iron (Fe) and 55 % to 65 % of Si. Ca content is usually in three ranges of 28 % to 31 %, 30 % to 33 %, and 32 % to 34 %. It contains around 1 % carbon (C) and about 1.5 % aluminum (Al). It has a melting point in the range of 980 deg C to 1260 deg C and boiling point of around 1500 deg C. However, melting point range is not an overriding factor as Ca vapourizes so quickly that it can provide a beneficial agitation as it bubbles through the liquid steel.
The treatment with CaSi is normally made after trim additions and argon (Ar) rinsing in most of the steel grades. It is not uncommon practice to perform a preliminary deoxidation with Si and/or Al before adding Ca.
In the ladle, the low density and high reactivity of Ca addition agents make their efficient use difficult and this has led to the development of special addition techniques. One technique is to inject Ca deep into the liquid steel bath such that the ferrostatic pressure overcomes vapour pressure of Ca. In another method wire containing Ca components are injected at speeds of 80-300 m/min.
Unless the ferrostatic head is quite high (as at the bottom of the ladle), Ca exists only in vapour form at the steelmaking temperatures. This, plus the fact that solubility of Ca in steel being low at any temperature means that any reaction between Ca and the oxygen (O) and sulfur(S) it is intended to remove can only take place at the Ca vapour/liquid steel interface unless Ca is present as a component of the slag. The addition of Ca through the cored wire injection make the contact between Ca and liquid steel as intimate as possible for as long as practical limits allow.
It is important that all efforts be made to prevent reoxidation of the steel after the Ca addition. Use of protective slag blankets, inert gas or refractory shrouded nozzles and even submerged nozzles are necessary if Ca protection is to be maintained. There are two reasons for this precaution. The first reason is the reoxidation of the steel can cause S rejection from calcium sulphide (CaS) since CaO is more stable than CaS and proper conditions can even lead to the reformation of manganese sulphide (MnS). The second reason is that there are a number of thermodynamically possible calcium aluminates and excessive reoxidation favours the formation of high melting point calcium aluminates which are just as harmful, especially in terms of nozzle blockage, than the alumina(Al2O3)/silica(SiO2) precipitates which the Ca was intended to remove or modify.
Influence of calcium on steels
During calcium treatment, the Al2O3 and SiO2 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.
Addition of Ca to liquid steel containing O and S forms two phases namely oxide and sulphide. Oxide phase consists of the compounds in CaO?Al2O3 system (Fig 1). The different oxide compounds (3 CaO. Al2 O3, 12 CaO.7 Al2 O3, CaO. Al2 O3, and CaO. 2 Al2 O3) have different melting temperature which ranges from 1400 deg C to 1727 deg C.
Fig 1 CaO-Al2O3 binary system
12 CaO.7Al2O3 has the lowest melting point of 1400 deg C and has a Ca/Al ratio of 1.27 and hence it remains in the liquid form at steelmaking temperature. This suggests that Ca/Al ratio is to be adjusted at 1.27 (theoretically) in order to obtain liquid product.
The sulphide phase consists of CaS and MnS). The melting point of CaS is 2000 deg C and that of MnS 1610 deg C. CaO.Al2O3 liquid has some solubility for CaS.
The addition of Ca is made for Al killed steels in order to decrease the volume fraction of oxide and sulphide inclusions through deoxidation and desulphurization and to control the composition, morphology, and distribution of those remaining inclusions. A major advantage of this practice is that nozzle clogging during the continuous casting of liquid steel is eliminated since solid Al2O3 inclusions are transformed to liquid calcium aluminates which do not clog the nozzle.
Ca has two beneficial effects. The first is that reduces the total number of inclusions remaining in the steel (S, for example, can be brought down to 0.001 % – 0.003 % with a little extra care and down to 0.007 % in routine practice) and the second is that it modifies the shape of the remaining inclusions into one that is less detrimental to mechanical properties in the final product. Thus, Ca breaks up inter dendritic Al2O3 galaxies into fine Type III inclusions. These minute particles will remain in the steel through solidification but, unlike the original galaxies, have no tendency to clog continuous caster nozzles. In addition, their globular shape is retained even during hot rolling and the resulting absence of stringer or pancake inclusions gives the steel more uniform properties in all directions.
Other advantages of Ca treatment of steel are obtained in the mechanical properties of different grades of steel. For example, the ductility and toughness of high strength low alloy (HSLA) steel and high quality structural steel are improved as the volume fractions of sulphides and oxides are decreased. In free machining steel grades, hard Al2O3 inclusions cause excessive tool wear. Ca transforms these inclusions into softer calcium aluminates or calcioaluminosilicates.
Rolling and hot working
Ca improves the hot workability of steels. Even at low Mn levels, Ca treated steels are free from any trace of iron sulfide (FeS) and are therefore not be subject to hot shortness. High alloy steels, particularly high Ni steels (in which Ca is somewhat more soluble), are much improved by Ca treatment.
Since Ca modifies the composition, size, and structure of oxides, sulfides and silicates already present in the liquid steel, the resulting Ca bearing inclusions retains their globular shape, even when large, throughout hot working operations. Thus, Al2O3 and sulfide stringers are eliminated and directional anisotropy, particularly with regard to through thickness ductility, is greatly reduced.
Ca has no effect on the transformations occurring during heat treatment.
The demand for steels with high toughness and ductility, especially in the transverse direction, has prompted steelmakers to (i) lower the S content of their products and/or (ii) modify the shape of sulphide inclusions so that they are less damaging. Ca is beneficial in both respects, and one or another form of Ca treatments has been recognized and used since the 1930s.
Ca improves cleanliness, desulphurizes, and reduces directional anisotropy in steels. Ca treatment is applied wherever these properties are required.
The low S levels brought about by Ca treatment improve corrosion resistance and reduce susceptibility to stress corrosion cracking in specific media.
Additions of Ca are made to high alloy steels to increase fluidity, cleanliness and surface quality. Typical examples include AISI 52100, grade 430 stainless steel used for automotive trim and the titanium (Ti) modified grade 321 austenitic stainless steels used in welded or sensitization prone structures.
Ca is used to improve the fluidity and cleanliness of cast irons and cast steels. The violent agitation that accompanies Ca addition to liquid metal also reduces the gas content of the metal. This leads to sounder and less porous cast structures.