Non Metal Inclusions in Steels...

Non Metal Inclusions in Steels Non metallic inclusions are naturally occurring and typically undesired products that are formed into various types depending on their favourable thermodynamic conditions during the production of steel and in all manufacturing and treatment processes involving liquid steels. They are constituted by glass-ceramic phases embedded in steel metal matrix.All steels contain non metallic inclusions to a greater or lesser extent. The type and appearance of these non metallic inclusions depends on factors such as grade of steel, steel making process, secondary metallurgy treatments and casting of steel etc. Because of this, it is of particular significance to determine how pure the steel is. The term steel cleanness is relative one, since even steel with only 1 ppm each of oxygen and sulphide will still contains billion to trillion non metallic inclusions per ton. Various factors which influence the non metallic inclusions in steel are shown in Fig 1. Fig 1 Factors influencing the non metallic inclusions in steel  Non metallic inclusions are chemical compounds of metals (e.g. iron, manganese, aluminum, silicon, and calcium) with non metals (e.g. oxygen, sulphur, carbon, hydrogen, and nitrogen). Non metallic inclusions form separate phases. The non metallic phases containing more than one compound (e.g. different oxides, oxide + sulphide) are called complex non metallic inclusions (spinels, silicates, oxy-sulphides, carbonitrides). Despite the presence of non metallic inclusions in steels in small percentage (0.01 % to 0.02 %), they have a significant effect on the properties of steels. They are the cause for dangerous and serious material defects such as brittleness and a wide variety of crack formations. However, some of these inclusions can also have a beneficial effect on steels properties by nucleating acicular ferrite during the austenite to ferrite phase transformation especially in low carbon steels.  The...

Clean Steels

Clean Steels There are various definitions of clean steel. The term clean steel is also vague. Clean steels are generally those steels that have low levels of the solute elements sulfur, phosphorus, nitrogen, oxygen and hydrogen; controlled levels of the residual elements copper, lead, zinc, nickel, chromium, bismuth, tin, antimony and magnesium; and, a low level of non metallic or oxide inclusions. The requirements vary with the steel grade and its end use. Clean steels used for one application may be often not acceptable for a different use. Steels with low levels of solutes are sometimes termed as ‘high purity steels’ while steels with low percentage of tramp elements are often called ‘low residual steels’. Sometimes steels with a low frequency of product defects that can be related to the presence of oxide inclusions are called clean steels. Hence the definition of ‘clean’ is not absolute. Instead it is based upon the product formed from the casting and the in-service use or life of the product. In addition, the definition ‘clean’ is comparative since the cleanliness standard desired by the customer is continuously changing as a function of time and technological improvements. The term ‘clean steel’ is therefore continually variable depending upon the application. Effect of solute elements on steels The individual or combined effect of solute elements such as carbon (C), phosphorus (P), sulphur (S), nitrogen (N), hydrogen (H) and total oxygen (T.O.) is known to have a remarkable influence on the steel’s properties, such as tensile strength, formability, toughness, weldability, cracking resistance, corrosion resistance, and fatigue resistance etc. The extent of control of the solute elements needed in the steels depends on the performance expected from the steel. The influence of the solute elements on the properties of steels is given in...