Clean Steel: Part One

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Steel cleanliness is an important factor of steel quality and the demand for cleaner steels increases every year. The so-called clean steel generally is the steel in which the content of impurity elements, such as phosphorus, sulphur, total oxygen, nitrogen, hydrogen (including carbon sometimes) and inclusions are very low. The improvement of steel cleanliness has therefore become a more and more important subject in the development of ferrous metallurgical technology, and also an important task for the iron and steel producers.

The demand for better mechanical properties of steels was urging steel producers to improve cleanliness of their final products. In order to obtain the satisfactory cleanliness of steel it is necessary to control and improve a wide range of operating practices throughout the steelmaking processes like deoxidant- and alloy additions, secondary metallurgy treatments, shrouding systems and casting practice.

Due to the vague nature of the term "clean steel", some authors imply that it is more precise to refer to:

• steels with low levels of solutes as "high purity steels"
• steels with low levels of impurities that originate from the re-melting scrap as "low residual steels" steels with a low frequency of product defects that can be related to the presence oxides as "clean steels".

It has been well known that the individual or combined effect of carbon [C], phosphorus [P], sulphur [S], nitrogen [N], hydrogen [H] and total oxygen (T.O.) in steel can have a remarkable influence on steel properties, such as tensile strength, formability, toughness, weldability, cracking-resistance, corrosion-resistance, fatigue-resistance, etc. Also, clean steel requires control of non-metallic oxide inclusions and controlling their size distribution, morphology and composition.

The control of the elements mentioned above is different for different performance demands. Those impurity elements also vary with different grades of steel. Table 1 lists the influence of common steel impurities on steel mechanical properties which means that some element is harmful to certain steel grades, but may be less harmful or even useful to another steel grades.

For examples for IF steels, the content of carbon, nitrogen, total oxygen and inclusions should be as low as possible in order to get good flexibility, high "r" value, perfect surface quality etc. In other hands the high quality pipeline steel requires ultra low sulphure, low phosphorus, low nitrogen, low total oxygen content and a certain ratio of Ca/S.

 

Element	Form	Mechanical Properties Affected
S, O	Sulfide and oxide inclusions	Ductility, Charpy impact value, anisotropy Formability (elongation, reduction of area and bendability) Cold forgeability, drawability Low temperature toughness Fatigue strength
C, N	Solid solution	Solid solubility (enhanced), hardenability
	Settled dislocation	Strain aging (enhanced), ductility and toughness (lowered)
	Pearlite and cementite	Dispersion (enhanced), ductility and toughness (lowered)
	Carbide and nitride precipitates	Precipitation, grain refining (enhanced), toughness (enhanced) Embrittlement by intergranular precipitation
P	Solid solution	Solid solubility (enhanced), hardenability (enhanced) Temper brittleness Separation, secondary work embrittlement

 

Table 1: Influence of typical impurities on mechanical properties

As we mentioned before, steel cleanliness depends on the amount, morphology and size distribution of non-metallic inclusions. The inclusions generate many defects and many applications restrict the maximum size of inclusions so the size distribution of inclusions in steel products is also important. For certain applications where stringent mechanical properties are required the internal cleanliness of steel is very important. Table 2 shows the cleanliness requirements for various steel grades.

 

Steel product	Maximum allowed impurity fraction	Maximum allowed inclusion size
IF steels	[C]≤30 ppm, [N]≤40 ppm, T.O.≤40 ppm [C]≤10 ppm, [N]≤50 ppm
Automotive and deep-drawing Sheets	[C]≤30 ppm, [N]≤30 ppm	100 µm
Drawn and Ironed cans	[C]≤30 ppm, [N]≤40 ppm, T.O.≤20 ppm	20 µm
Alloy steel for Pressure vessels	[P]≤70 ppm
Alloy steel bars	[H]≤2 ppm, [N]≤20 ppm, T.O.≤10 ppm
HIC resistant steel sour gas tubes	[P]≤50 ppm, [S] ≤10 ppm
Line pipes	[S]≤30 ppm, [N]≤50 ppm, T.O.≤30 ppm	100 µm
Sheets for continuous annealing	[N]≤20 ppm
Plates for welding	[H]≤1.5 ppm
Bearings	T.O.≤10 ppm	15 µm
Tire cord	[H]≤2 ppm, [N]≤40 ppm, T.O.≤15 ppm	10 µm
Non-grain-orientated Magnetic Sheets	[N]≤30 ppm
Heavy plate steels	[H]≤2 ppm, [N]=30-40 ppm, T.O.≤20 ppm	Single inclusion 13 µm Cluster 200 µm
Wires	[N]≤60 ppm, T.O.≤30 ppm	20 µm

 

Table 2: Cleanliness requirements for various steel grades

As Table 2 shows for sheets used for car body, carbon [C], nitrogen [N], and total oxygen (T.O.) are each required to be very low. For sheets for tin plate application, total oxygen is not only needed below 20 ppm, but the size of the non-metallic inclusions in steel has to be less than 20 µm.

For steel cord used in tires, the size of non-metallic inclusions in steel has to be less than 10 μm and even smaller (5 µm) for TV shadow masks. For ball bearings, in order to improve their fatigue-resistance properties, T.O. in steel has to be below 10 ppm and the size of non-metallic inclusions has to be less than 15 µm. For meeting the specification of increasingly improved toughness for petroleum pipeline and of Hydrogen Induced Cracking (HIC) resistance for the transport of sour natural gas, the sulphur [S] content in steel has to be extremely low, less than 10 ppm.

Steel cleanliness is controlled by a wide range operating practices throughout the steelmaking processes. These include the time and location of deoxidant and alloy additions, the extent and sequence of secondary metallurgy treatments, stirring and transfer operations, shrouding systems, tundish geometry and practices, the absorption capacity of the various metallurgical fluxes, and casting practices.

A one of the steelmaking process routes for the production of clean steels is outlined in Figure 1.

 

Figure 1: The process route for the production of clean steels