Production Technology of Cored Wire Used for Liquid Metal Treatment in Steel Plants: Part Three

Morphology of Cored Wire

Structurally the CW is made up of two components only i.e. a steel jacket & an additive core. The steel strip is used as a vehicle for the additive core to descend it into the molten metal safely. In steel industries often the term CW in conjunction with a prefix like alloy, mineral, metallurgical, refining etc because of its appearance, contents, applications, and also to differentiate it from CW used for other applications like in welding, optical fiber etc.

The CW is characterized by its chemical composition of additive primarily, and its physical properties like coil diameter, packing density etc also. It is produced with different additive contents & dimensions for different processing requirements. Such as the diameters (ф 6 mm to >20 mm), length (up to 5 km), sheath thickness (€ 0.25 mm to > 0.8 mm). Usually lower diameter CW (up to 9mm) is used for smaller ladles and higher diameter (13mm) for larger ladles. For making CW the strips and additives are often outsourced from steel mills and chemical plants respectively. The quality of these consumables is very important in the production of CW as discussed below.

Steel Strip Coil - Aluminum killed, non-aging, steel strip coils with good flatness and low surface roughness, without cracks, sliver, burr, twist, corrosion etc produced in modern cold rolling mills are apparently suitable. For precise thickness and dimensional controls, digital methods and longitudinal slitting lines respectively are used. The ID of the strip coils falls between 250 to 500 mm & an OD close to 1500 mm and conforms to IS-513 CRCA-EDD specification or its equivalent.

The term CRCA (cold rolled close annealed), is used for strip which has been cold rolled to a reduced thickness and annealed thereafter in a closed atmosphere of inert/ nitrogen, as per the proven annealing cycle. Since the annealing process intended to bring about a stress free state for cold rolled strip & the designed parameters is largely set to control the micro structure (grain size/ shape, crystallographic orientation etc) of steel, this treatment is essential.

The term EDD (extra deep drawing), means that the strip is ductile enough and suitable for cold working, even where extremely severe forming conditions are required, it is capable of retaining the shape after plastic deformation. Draw-ability of EDD steel strip is characterized by its ratio to surface area and thickness. Accordingly existing practice for forming CW between 8 to 10 mm & 11 to 16 mm diameters, the strip thickness of 0.35 mm and 0.40 mm (± 0.02 mm) respectively are used, similarly for coil diameters of 10 mm and 16 mm, the strip width of 45 mm (+0.3 & ─0 mm) and 65 mm (+0.5 &─0 mm) respectively are used.

As rule of thumb, an allowance of 15 mm, more than the corresponding coil circumference is necessary for seam lock formation, e.g. for 16 mm diameter CW the strip width, the requirement will be (16 x π = 50.24 + 15 mm) or 65.24 mm. However for actual strip width determination standard equations should be used.

The physical properties of CW components are described below:

Steel Strip (Physical Properties) - In the roll forming process, the stress-strain pattern under load appears from anisotropic plasticity (plastic properties depending upon direction, relative to draw axes) which is firmly related to the texture of the strip and any change in texture may exhibit change in strain pattern. To sustain the impact of work hardening during shape alteration enough strip ductility is required, so that it can resist the formation of cracks. For this reason EDD quality steel strip with higher ductility and formability is used in CW manufacturing.

The physical properties of EDD quality steel like hardness (resistance against the plastic deformation), yield strength (maximum. stress in strip that can be tolerated without causing plastic deformation), UTS (withstand the stresses during stretching), elongation (ductility for cold deformation), & formability (shapeable through plastic deformation) etc are shown in the Table 3. When the CW jacket is cold-formed from strip, the yield strength, and ultimate strength, is increased, particularly in the bend area and the values may appear depending on the thickness of the strip and degree of cold working.

The factors that can affect CW forming include die pressure, bending radius and the properties of the steel strip. During the cold working process, bending/ curving etc. take place, which should not create fracture and localized thinning of strip, for this requirement the formability (draw ability and stretch ability) values like (r-bar & η-value) are controlled.

Since the metal easily flows in the plane of sheet and resist thinning in the thickness direction the variation in plastic behavior with direction, is assessed by the Lankford Parameter or anisotropy coefficient, and is measured at 90° relative to the direction of rolling (also expressed as r factor which is equal to, true strain in width direction divided by true strain in thickness direction). The higher the r factor the higher draw ability (for Al killed LC steel strip r _{90 –} is 1.6 min).

Similarly the resistance of metal against localized thinning and complex non uniform deformation is expressed by Holloman’s equation, (relationship between the stress and the amount of plastic strain) or work hardening exponent, or η-value or η ₉₀. Higher r value (for Al killed LC steel strips η₉₀ -0.2 min) indicates lower localized thinning of the strip during roll forming. The steel strips from primary steel plants producing strips as per the above standards, are suitable for roll forming.

Table 3: Mechanical Properties of Steel Strip

(*Refer-ASTM E2218 – 02/2008. Standard Test Method for Determining Forming Limit Curves ASTM E-646 Test Method for Tensile Strain-Hardening Exponents -Values of Metallic Sheet Materials & ASTM E-517Test Method for Plastic Strain Ratio r for Sheet Metal).

Steel Strip (Chemical Properties) - the elemental concentration limits given in specifications (C-0.08, Mn-0.40, P-0.03, S-0.030 each % max) are meant for guidance, however the actual values are more stringent (C-0.06, Mn-0.35, Si-0.03, P-0.015, S-0.020, Al-0.05, Cr +Ni + Cu ≤ 0.03, O₂ -0.002, N₂ -0.003, H₂ -0.0005, each % max). The significances of various elements & their control may be attributed to the following metallurgical consequences.

1) Carbon has low solubility in alpha ferrite (0.006%) at room temperature and form carbide (Fe₃C), which reduces the ductility, hence low carbon steel is used. 2) Manganese forms high m.p. globular MnS preventing low m.p. FeS formation improves harden-ability, strength, toughness & promotes grain refinement. 3) Silicon is a stronger deoxidizer than Mn, but more efficient when both are present. 4) Aluminium improves draw ability of strip; it is a scavenger for nitrogen as AlN and a strong deoxidizer. 5) S & P are considered as impurities and need to be controlled to a minimum practical level.

Additives Core (Classifications) - Additives are used as recipes (consisting ingredients which help in achieving a desired end product) which modify/alter the characteristics of LM with respect to metal cleanliness, chemistry, texture and provide necessary perfections before casting. Additives are metal/alloys & other chemical substances, deliberately added (in specific percentages) into the LM for specific purpose in both primary steel & foundry metals, for purposes such as:

Desulphurising - is the process of removal of sulphur from the melt, the additive combines with sulphur and the sulphide formed is transferred into the basic slag and removed.
Deoxdising - is the process of removal of oxygen from the melt; additive combines with oxygen present in the molten metal & form oxides and removed with slag.
Shape modifier - the additive combines with the residual oxides/sulphides and alters its structure or properties compounds.
Alloying - (for high, low, & micro alloy steels) usually master alloys of iron are used instead of metallic elements for faster dissolution in melt.
Machinability improver - hard metals intrinsically have poor machinable properties so for developing the ease with which a metal or alloy can be machined is obtained by introducing additives like S, Pb etc.in the metal matrix.
Nitriding - nitrogen in iron improves the tensile strength, and it is introduced in the LM by the nitrogenised master alloys additives.
Trimming agents - for the exact chemistry and temperature control, achieved by addition of additives.

In foundry industries CW is used for desulfurizing, dephosphorizing, desiliconizing, denitriding, inoculation & nodularization of cast iron, two applications are stated below:

Inoculating agents - in cast iron production these additives are used for grain refinement, by improving the microstructure, where the inoculating agent creates a large number of sites for the dissolved carbon to precipitate out as graphite instead carbide.

Nodularizing agents - used in making ductile cast iron, to promote the formation of spheroid graphite to enhance the metal ductility, where free graphite is induced by adding Ce or Mg etc in the LM. The CW additives are categorized in the groups given in Table 4.

Table 4: Classification of additives used in CW for LM treatments

From above table it may be seen that for each and every category of treatment, several additives have been mentioned, among these some are readily available but for some reactive metals, minor metals and noble Ferro alloys which are very specific, they are outsourced by the CW producers.

By Dr Pradeep K. Maitra
dipalyconsultants@hotmail.com