Effects of Iron Ore Elements on Steel Making

Inclusion of very small amounts of some elements can have profound effects on the behavioral characteristics of a batch of iron or the operation of a smelter. These effects can be both good and bad, some catastrophically bad.

Some chemicals are deliberately added such as flux which makes a blast furnace more efficient. Others are added because they make the iron more fluid, harder, or give it some other desirable quality. The choice of ore, fuel, and flux determine how the slag behaves and the operational characteristics of the iron produced. Typically, iron ore contains a host of elements the effect of which in Steel making are described below.

Silica (SiO2) is almost always present in iron ore & is one of the principal deoxidizers used in steelmaking. Most of it is slagged off during the smelting process. At temperatures above 1300 °C some will be reduced and form an alloy with the Iron. More hot the furnace, the more silicon is present in the Iron. The major effect of silicon is to promote the formation of gray iron. Gray iron is less brittle and easier to finish than white iron. It is preferred for casting purposes. In Low-Carbon Steels, Silicon is generally detrimental to surface quality.

Phosphorous increases strength and hardness and decreases ductility and notch impact toughness of Steel. The adverse effects on ductility and toughness are greater in quenched and tempered Higher-Carbon Steels. Phosphorous levels are normally controlled to low levels. Higher Phosphorus is specified in Low-Carbon free-machining Steels to improve machinability. Phosphorous is a deleterious contaminant because it makes steel brittle, even at concentrations of as low as 0.6%. It is not easily removed by fluxing or smelting, so iron ores must generally be low in Phosphorus to begin with.

Sulfur decreases ductility and notch impact toughness especially in the transverse direction. Weldability decreases with increasing sulfur content. Sulfur is found primarily in the form of sulfide inclusions. Sulfur levels are normally controlled to low levels. The only exception is free-machining steels, where sulfur is added to improve machinability. Sulfur can be removed from ores by roasting and washing. Roasting oxidizes sulfur to form sulfur dioxide which either escapes into the atmosphere or can be washed out. In warm climates it is possible to leave pyritic ore out in the rain. The combined action of rain, bacteria, and heat oxidize the sulfides to sulfates, which are water soluble.

Aluminum is widely used as a deoxidizer. Aluminum can control austenite grain growth in reheated steels and is therefore added to control grain size. Aluminum is the most effective alloy in controlling grain growth prior to quenching. Titanium, zirconium, and vanadium are also valuable grain growth inhibitors, but there carbides are difficult to dissolve into solution in austenite. However, as per few experts, it does increase the viscosity of the slag. This will have a number of adverse effects on furnace operation. The thicker slag will slow the descent of the charge, prolonging the process. High aluminum will also make it more difficult to tap off the liquid slag.

Copper in significant amounts is detrimental to hot-working steels. Copper negatively affects forge welding, but does not seriously affect arc or oxyacetylene welding. Copper can be detrimental to surface quality. Copper is beneficial to atmospheric corrosion resistance when present in amounts exceeding 0.20%. Weathering steels are sold having greater than 0.20% Copper.

Lead is virtually insoluble in liquid or solid steel. However, lead is sometimes added to carbon and alloy steels by means of mechanical dispersion during pouring to improve the machinability.

Nickel is a ferrite strengthener. Nickel does not form carbides in steel. It remains in solution in ferrite, strengthening and toughening the ferrite phase. Nickel increases the hardenability and impact strength of steels.

Titanium is used to retard grain growth and thus improve toughness. Titanium is also used to achieve improvements in inclusion characteristics. Titanium causes sulfide inclusions to be globular rather than elongated thus improving toughness and ductility in transverse bending.

Vanadium increases the yield strength and the tensile strength of carbon steel. The addition of small amounts of Vanadium can significantly increase the strength of steels. Vanadium is one of the primary contributors to precipitation strengthening in micro alloyed steels. When thermo mechanical processing is properly controlled the ferrite grain size is refined and there is a corresponding increase in toughness. The impact transition temperature also increases when vanadium is added.

Mitra S.K. has the capability, infrastructure and expertise to analyse all the above elements in Iron ore samples.