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Clean Steels

• satyendra
• November 29, 2013
• 0 Comments
• clean steel, Non metallic inclusions, residual elements, Solute elements, tramp elements, Types of inclusions,

Clean Steels

Steel is a versatile material which is having very wide applications. It is of interest for several applications because of its several advantages such as high strength to weight ratio, durability, versatility, recyclability, and most importantly its economic viability in comparison to other engineering materials. Besides its common uses, it is also the material of choice for several industrial components used in critical applications. These critical applications demand very stringent requirements in terms of the steel properties. Such requirements vary in terms of their specific needs ranging from light weight, high strength, high toughness, ability to withstand high pressures, ability to withstand sub-zero temperatures, excellent weldability, good corrosion resistance, and more frequently than not a combination of such properties is needed.

Steel is the most important material for mankind in its present and future role. Pursuing towards lighter constructions, better combination of strength and toughness, or longer life of an engine, steels with higher and better controlled purity are needed to be developed. The progress during the last 50 years has been spectacular in this regards.

The versatility of steel allows the engineer to tailor its properties by modifying the chemistry and / or the microstructure. Despite the fact that several developments have taken place with regards to these two variables, another crucial aspect which determines the performance in service of steel is how free of impurities it is (sometimes called cleanliness). Steel cleanliness is determined by the non-metallic inclusions (or simply inclusions) which are embedded in it. In order to improve the performance of steels, inclusions are to be controlled since the inclusions are a critical problem of steels for structural applications and depending on their size, shape, and distribution, they can be very detrimental to the mechanical properties.

The chemical elements initially involved in cleanliness are mostly the non-metals of the Mendeleev periodic table, since they show higher solubility in liquid steel than in the solid. These are carbon, nitrogen, oxygen, phosphorous, sulphur, selenium and hydrogen. To this list, metalloid neighbours in the table, like boron, arsenic, antimony and tellurium can be added. Some of these elements originate from primary raw materials (phosphorus, sulphur, arsenic, and antimony) or from ironmaking (carbon), while most of the others are due either to contamination by the atmosphere (oxygen, nitrogen, and hydrogen), to the normal oxidizing practice (oxygen) used in steelmaking, to the electric arc in the electric arc furnace (nitrogen), or are voluntarily added (carbon, selenium, tellurium, and boron).

Clean steels are those steels which contain limited number of inclusions in terms of size, shape, composition, distribution, and frequency. As a result, clean steels are capable of outperforming other materials and excel in applied high stress conditions, such as those used in transportation equipment and other applications.

Clean steels were ‘invented’ in the middle of the 20th century, at a time when steels were started to be produced in large scale and when it was understood that quality is also to be addressed as a special and important issue, both in terms of the strategy of the steel industry and as a major study topic for the science and technology which accompanies the industry.

There are different definitions of clean steel. The term clean steel is also vague. Clean steels are normally those steels which have (i) low levels of the solute elements sulphur, phosphorus, nitrogen, oxygen and hydrogen, (ii) controlled levels of the residual elements copper, lead, zinc, nickel, chromium, bismuth, tin, antimony and magnesium, and (iii) low level of or oxide and other inclusions. The requirements vary with the steel grade and its end use. Kiessling has argued that the clean steel concept is debatable. He mentioned that clean steel is relative, and it is true to say ‘the cleanliness of steel, like beauty, is very much in the eye of the beholder’. He further mentioned that the concept leads to the impression that steel with fewer numbers of inclusions are superior in performance which is not always the case.

Clean steels used for one application can be frequently 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 frequently called ‘low residual steels’. Sometimes steels with a low frequency of product defects which 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 continuous cast steel 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 hence continually variable depending upon the application.

Historical construction of the concept of clean steels

The review of the construction of the concept of clean steels sheds light on the concept of modern steel materials.

Historical narrative – When iron and steel emerged in history, the metal was reduced in the solid phase in a bloomery, hence the iron bloom was a mixture of reduced iron and of the gangue of the ore, a true composite material. The gangue was removed by forging the bloom to expel the mineral elements out of the metal. The outcome was an iron very different from the present day steel. For example, the quantity of minerals in samples from the late ‘Iron Age’ was between 10 % and 2 % in volume, the latter being considered as a clean piece of material. In terms of total oxygen (T.O.) content, the spread was hence roughly between 14,000 ppm and 200 ppm. People had not talked explicitly about cleanliness then, although the quality issues which were raised (early fracture) were probably understood by the smiths of that time.

The evolution towards modern steelmaking, i.e. to an all through liquid production from hot metal to liquid steel (Bessemer and Martin-Siemens processes), had changed the picture in terms of cleanliness very significantly. The production of liquid hot metal in the early blast furnaces eliminated gangue inclusions, the liquid gangue being separated by density. However, new inclusions of a different kind, due to oxidation, were introduced during subsequent forging, a completely new genesis of these ternary phases. Most of these were eliminated when liquid steel was produced as crucible steel or puddled iron but new kinds of inclusions were created, due to the reoxidation of liquid steel along with contamination by refractory and liquid mineral phases (slags). Studies picturing this historical evolution in a quantitative way, i.e. a time evolution of cleanliness, measured for example by total oxygen content, are not available.

The concept of cleanliness was born initially from the observation under the optical microscope of inclusions by the newborn discipline of metallography. Cleanliness was rated against standard images of microscopic fields, where geometry (shape and size) and distribution of inclusions were distinguished against various image types. The trained observer had established that some shapes were acceptable in some steel grades and that smaller inclusions normally were more acceptable than larger ones. Although the composition of inclusions was not available by then, the observer had established a correspondence between grades and inclusion composition by families (sulphides, silicates, aluminates, alumina, and composite inclusions) based on the sulphur content and deoxidation history of the steel. These methods, developed in the 20th century and standardized after the Second World War, pre-empted the general use of continuous casting and of ladle metallurgy, and hence were invented in a process technology context fairly different from the present day context.

The further development of the concept of cleanliness went on by exploring various issues in parallel, based on laboratory work, basic studies into the physical chemistry of steelmaking, experimentation at the steel melting shop, development of new process reactors, and new, innovative solutions to control inclusions composition, shape, size and distribution to be eventually introduced in the routines of steelmaking practice.

The present day vision of cleanliness – The present day vision of steel cleanliness has emerged from this 30 year to 40 year concept building effort. Inclusions constitute a cloud of phases dispersed in the metal matrix and defined by a multi-dimensional set of parameters, including composition, shape, size and distribution. This full description is not readily available and one of the main issues related to assessing the cleanliness is to observe representative samples to estimate these parameters with a reasonable accuracy and representativity. One difficulty is related to large inclusions (100 micro meters or above), which are extremely rare and hence difficult to see, unless very large sized samples are analyzed.

Another issue is due to the fact that the population of inclusions depends on time (in the process timeline of the steel melting shop) and on temperature. Hence a ladle sample, collected and analyzed with care and finesse, can give a reasonably good estimate of the cleanliness there and then, but it can bear almost no connection, whatsoever, with the cleanliness of solid steel. There is hence a large number of studies are available which discusses when a representative sample of liquid steel is required to be taken in order to assess both steel composition and inclusion cleanliness.

The demand for clean steel increases every decade. In addition to lowering the oxides and sulphides inclusions, and controlling their composition and morphology, clean steel needs lowering the contents of residual impurities such as phosphorus, hydrogen, nitrogen and other trace elements in the steel.

The clean steel normally 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 impurity elements vary with different grades of steel. Some element is harmful to certain steel grade, but can be less harmful or even useful in another steel grade. In other words, the control elements are different for different performance demands of steel. For example, in IF (Interstitial Free) steel, the content of carbon, nitrogen, total oxygen and inclusions are to be as low as possible in order to gain good flexibility, high ‘r’ value, and perfect surface quality. The high quality pipeline steel needs ultra low sulphur, low phosphorus, low nitrogen, low total oxygen content and a certain ratio of calcium / sulphur (Ca/S).

The level of the cleanliness of steel can be defined by its content of harmful elements i.e. the oxygen, sulphur, nitrogen, carbon, phosphorus, and hydrogen contents. It has been reported that advances in steel manufacturing since the 1970s have resulted in new clean steel grades which contain less than 10 ppm (parts per million) of impurities and which have an excellent mechanical strength and an excellent corrosion resistance. The driving force behind these advances comes partly from the automotive sector and partly from the oil and gas industry. Specifically these sectors have demanded steel grades which can tolerate a high mechanical load or an aggressive and corrosive environment, respectively. In contrast to the excellent mechanical properties and / or corrosive resistance of ultra-clean steels, these steels are typically considered to be difficult to machine, which result in a reduced cutting tool life. Hence, the challenge of future steel technology is to produce high performance steel grades with an improved combined mechanical strength and machinability.

The cleanliness assessment in steel is having a problem. There are only very few large (macro) inclusions, which are difficult to detect for the reason that their number is small. In contrast, the number of very small (micro) inclusions is almost infinitesimal and their size makes them nearly undetectable. It appears that 5 micrometers (0.005 mm) represents the borderline between tolerable micro inclusions and potentially harmful macro inclusions. These sporadic large inclusions represent the foremost quality problem for steel plants in producing clean steel.

With cleanliness requirements more stringent and the development of new steelmaking grades, understanding the inclusion formation and evolution process and developing methods for improving their removal from the liquid steel is important. Inclusion removal is favoured not only by a large inclusion size but also by a high interfacial energy between the inclusion and the steel and large contact angles between the inclusion and steel in a steel-inclusion-gas system.

Although the present day high-cleanliness steels have excellent mechanical properties and / or corrosion resistance, these advances in functional properties have come at the expense of more difficult chip breaking and in some cases a considerably reduced tool life in machining operations.

Types of inclusions

Inclusions are chemical compounds consisting of a combination of a metallic element (iron, manganese, silicon, aluminum, and calcium etc.) and a non-metallic element (oxygen, sulphur, nitrogen, and carbon etc.). The most common inclusions include oxides, sulphides, oxy-sulphides, phosphates, nitrides, carbides and carbo-nitrides.

According to a traditional classification, the inclusions can be distinguished in two main classes as a function of their origin. These classes are (i) indigenous inclusions, and (ii) exogenous inclusions. The indigenous inclusions form by precipitation within the liquid phase due to the decrease of the solubility of the chemical species contained in the steels. This class of inclusions cannot be completely eliminated from the steel but the decreasing of their volume fraction and of the average size has to be taken under strict control in order to avoid the activation of the damaging phenomena.

On the contrary, the exogenous inclusions are the consequence of trapping of non-metallic materials coming from slag, refractory fragments, or from rising and covering powders used for protecting the steel and avoiding sticking during the steel casting. The inclusions belonging to this class can be featured by large sizes and their origin cannot be immediately recognizable, although their presence can strongly compromise the micro-structural soundness of the steels and the associated mechanical reliability. Since the exogenous inclusions are always process-related, they can be eliminated by implementing suitable processing procedures. Fig 1 shows types of inclusions in steels.

Fig 1 Types of inclusions in steels

In terms of size, inclusions can be either micro inclusions or macro inclusions. The threshold value which has been used to distinguish between micro inclusions and macro inclusions is normally assumed to be 100 micrometers. The inclusions can be of globular shape, platelet shape, dendrite shape, and polyhedral shape. In terms of their shape the most desirable is the globular shape because of their isotropic nature with regard to their effect on the mechanical properties.

The inclusions are produced in liquid steel during refining at high temperatures and or from precipitation during solidification. Inclusions which are produced during steel refining at high temperatures are known as primary inclusions and inclusions which are produced during solidification are known as secondary inclusions. Once inclusions are formed in steel, the characteristics of the inclusions such as size, quantity, composition, and morphology remain the same or change / evolve due to the physico-chemical reactions in the liquid steel, between the liquid steel and surrounding slag and ladle refractory, and from deformation. Depending on their final characteristics, they can be harmful to the casting process, reduce the steel mechanical properties, and decrease the surface and overall quality of the steel product. Inclusions, the presence of which defines purity of steel, are classified by chemical and mineralogical content, by stability, and by origin.

Effect of solute elements on steels

Inclusions are non metallic particles which are trapped in the matrix of steel. They are undesirable components of steels. They play an important role with respect to their effect on the steel properties. Among various types of inclusions, oxide and sulphide inclusions have been thought to be harmful for common steels. Inclusions in steel normally have a negative contribution to the mechanical properties of steel, since they can initiate ductile and brittle facture.

Harmful effects of inclusions are highly dependent on their chemical compositions, volume fractions, dispersions, and morphologies. Normally large and unbreakable inclusions with high melting points are the most unwanted ones. However compared to these inclusions, small and breakable ones or those with lower melting points are more preferred. The reason for these preferences is that inclusions which have lower melting points or are breakable are likely to be deformed, crushed to smaller inclusions, or disappear in following hot or cold forming processes (effect of forming process and reduction ratio) or heat treatments which the steels undergo after casting and solidification process.

The mechanical behaviour of steel is controlled to a large degree by the volume fraction, size, distribution, composition, and morphology of inclusions and precipitates, which act as stress raisers. The inclusion size distribution is particularly important, since large macro-inclusions are the most harmful to the mechanical properties. Sometimes a catastrophic defect is caused by just a single large inclusion in a whole steel heat. Though the large inclusions are far outnumbered by the small ones, their total volume fraction can be larger.

The individual or combined effect of solute elements such as carbon, phosphorus, sulphur, nitrogen, hydrogen, and total oxygen is known to have a remarkable influence on the properties of steel, 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 Tab 1. This table shows that some elements are harmful for certain steels but can be less harmful or even useful to some other steel grades.

The most common parameter which relates to steel cleanliness is inclusions, especially their composition, size, and distribution. All steel contains some level of inclusions. However, not all are equally harmful. Small inclusions (less than 4 micro meters) evenly distributed throughout the steel typically do not pose a problem. However, inclusions which agglomerate to form clusters detrimentally affect the steel quality and performance. The level of cleanliness needed for a specific operation depends on the application of the steel and the expectations of the customer. It also depends on the quantity, morphology, and size distribution of the inclusions in steel. The definition of ‘clean steel’ varies with steel grade and its end use. The inclusions generate several defects while several applications restrict the maximum size of inclusions. Hence the size distribution of inclusions is important in the steel. Tab 2 shows steel cleanliness requirements of different steel grades.

Control of non metallic inclusions during steel production

The increasing demand in recent years for high quality steels has led to the continuous improvement of steelmaking practices. There is a special interest in the control of inclusions due to their harmful effect on the subsequent stages and their great influence on the properties of the final steel product. The quality of the final product is controlled through the control of the quantity, size and chemical composition of the inclusions. The control of the formation of inclusions and the identification of their constituent phases are of extreme importance for achieving of cleanliness in the  steels.

A control of oxide inclusions and controlling their size distribution, morphology and composition is needed during the production of clean steels since the cleanliness of steel depends on these factors. Control of inclusions in steel is closely connected with the concept of ‘clean steel’. The aim during steel production is to eliminate undesirable inclusions and control the nature and distribution of the remainder to optimize the properties of the final steel product.

The cleanliness in steel is achieved through a wide range of operating practices which include the additions of deoxidizing agents and ferro alloys, 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 etc.

The presence of inclusions in steel is inseparable from the steelmaking processes. Their presence is frequently regarded as harmful, but sometimes equally advantageous. Whichever the effect, their presence in steel is part of the steelmaking process and is to be exploited to the advantage of the final steel properties.

The chemical composition of the inclusions and their volume fraction are determined by the management of the different steps involved in the production process of steel namely (i) primary steelmaking, (ii) secondary steelmaking, and (iii) continuous casting operation. Hence, the population of the inclusions depends on the relation existing between the applied operative parameters and the features of the steel grades being produced.

One important feature of the secondary steelmaking and continuous casting is that the metallurgical functions are spread out in space along the equipment line, deployed as along a time scale, and hence they can become standardized, sometimes automated and better controlled. On the other hand, sources of contamination have multiplied but can also be better controlled: ladle to tundish (ladle nozzle, sliding gate, and ladle stream gas protection), tundish (powder, weirs, dams and baffles, bubbling elements, etc.), tundish to mould (nozzle, sliding gate or stopper rod, submerged entry nozzle and gas bubbling, etc.), mould (mould powder, mould level control, submerged entry nozzle geometry, etc.), continuous casting itself (straight mould, curved mould, straight mould and curved, electromagnetic stirring, electromagnetic brake, transversally-shaped moulds of thin slab casters, etc.), all have become part of the process chain and turn into true metallurgical reactors. The expression ‘tundish metallurgy’ has become common knowledge. Fig 2 shows the phenomena taking place in the continuous casting tundish in connection with steel cleanliness.

Fig 2 Phenomena taking place in the continuous casting tundish in connection with steel cleanliness

The continuous casting machine, especially its mould, also acts as a metallurgical reactor where the fate of inclusions continues to be decided. Based on the origin, inclusions can be oxidation particle, refractory fragment, top slag entrainment, and reoxidation product etc. A variety of methods are applied to remove the inclusions, such as ladle stirring, slag refining, tundish operation, and continuous casting mould. The removal of inclusions in the continuous casting mould is difficult since the liquid steel becomes solid and inclusions have less opportunity to float out. The removal of inclusions and the final distribution of inclusions in the steel product highly depend on the properties of inclusions, transport of inclusions in the liquid steel and the interaction between inclusions and solidifying shell. Hence, the understanding of the entrapment of inclusions and their final distribution in the steel product are important for the control of cleanliness and the quality of the steel product. Fig 3 shows the phenomena taking place in the continuous casting mould in connection with steel cleanliness.

Fig 3 Phenomena taking place in the continuous casting mould in connection with steel cleanliness

The production of clean steel is based on technologies to control and / or to remove inclusions in steel. There are different steel grades produced for different purposes. The cleanliness level of the steel for each purpose depends on the inclusion number, morphology, composition, and size distribution of each steel grade. For example, in free machining or resulphurized steel, the idea is not to completely remove the inclusions but to modify them to improve machinability. Hence, a balanced opinion regarding permissible level of inclusion or cleanliness for each steel grade is really of great technical and economic importance both to the steelmaker and the user. To a large extent, clean steel are to meet the specifications of the customer and requirements for an application with regard to the characteristics of the inclusion.

The ever-increasing demands for high quality have made the steel producers to pay high attention to the ‘cleanliness’ requirements of the steel products being produced by them. Different steel grades are being produced by the steel producer for meeting various requirements expected from the steel products.

Steel cleanliness is an important factor of steel quality and the demand for cleaner steels increases every year. However, the term ‘clean steel’ is used with caution by the metal­lurgists. This is because of (i) the varying cleanliness demands for steels for differ­ent applications, (ii) varying cleanliness in steels produced in different operations, and (iii) the normal understanding of the term ‘clean steel’, which some liter­ally interpret as meaning the absence of inclusions in the steel. Steel cleanliness has implica­tions from both operational and product performance points of view.

Inclusions influence several properties of steels relevant to their performance in mechanical and structural applications. Some of the harmful effects that presented by the inclusions in the cast steel can be reduced with hot working as this process can induce orientation changes and a break up of inclusions. The inclusion requirements of the clean steel vary depending on the steel grade and application and the objective of inclusion engineering is to reduce the inclusions which are harmful and promote the formation of those inclusions which have beneficial effects.

Advances in steelmaking during the last several decades have resulted in steel grades with very low level of impurities. In recent years, new ‘clean’ and ‘ultra-clean’ steels have been developed and commercialized by steel producers around the world, thereby responding to the current and future market demands of steel having considerably improved mechanical properties (e.g. fatigue strength and impact toughness) and an improved corrosion resistance. These steels can have an extremely low content of oxygen (less than 10 ppm) and sulphur (less than 10 ppm). The driving force behind these advances has been to development of new steels which can tolerate highly demanding applications e.g. transmission components for the automotive industry, and construction parts and tubes for aggressive and corrosive environments.

Although the present day high cleanliness steels have excellent mechanical properties and / or corrosion resistance, these advances in functional properties have come at the expense of more difficult chip breaking and in some cases a considerably reduced tool life in machining operations.

Machining of steels with high-cleanliness is, in general, associated with high energy consumption, an increased cutting tool wear, and high production costs. It has been estimated that more than 40 % of the total production cost to produce an automotive component comes from different machining operations. Hence, the main issue is assessed as to optimize the present day steel grades with respect to the combined machinability and performance requirements. Hence the inclusions are to some extent necessary for a proper machinability performance. However, the content and the characteristics of the inclusions are still to ensure that high performance properties of the steel can be obtained.

The definition of ‘clean’ is not absolute, but depends on the individual steel production process and its in-service use of the final product. The term ‘clean steel’ is hence variable depending on the steel producer and steel application. Due to the variable nature of the term ‘clean steel’, it is sometimes proposed to talk more accurately of (i) high purity steel as steel in the case of low levels of solutes (sulphur, phosphorus, nitrogen, oxygen, and hydrogen) and (ii) low residual steel as steel with low level of impurities (copper, lead, zinc, nickel, and chromium to name just a few) mostly originated from scrap.

Steel cleanliness has implications both from operational and product performance points of view. The term ‘clean steel’ is normally used to describe steel which has (i) low level of solute elements, (ii) controlled level of residual elements, and (iii) low frequency of oxides created during steel making, ladle metallurgy, casting, and rolling.

Clean steels are steels with a low frequency of product defects which can be correlated to oxide inclusions. In addition, clean steel is increasingly understood as steel for which the composition is under tight control of alloying elements to improve product properties and property consistency. There is one constant in producing high purity, low residual and clean steel , which is the continual drive to reduce solute elements and residuals in the steel and control frequency, distribution, and size of inclusions.

Clean steel, in addition to lowering the oxides and sulphides inclusions, and controlling their composition and morphology, needs lowering of other residual impurities such as phosphorus, hydrogen, and nitrogen content and other trace elements in steel. Sometimes the concept of clean steel is argued as a debatable concept. It is since the term clean steel is relative. Further, as per the argument, the concept leads to the impression that steel with fewer numbers of inclusions are superior in performance which is not always the case.

In order to study and control steel cleanliness, it is critical to have accurate methods for its evaluation. The quantity, size distribution, shape and composition of inclusions should be measured at all stages in steel production. Measurement techniques range from direct methods, which are accurate but costly, to indirect methods, which are fast and inexpensive, but only reliable as relative indicators.