Steel Cleanliness and Clean Steel Technologies
Steel Cleanliness and Clean Steel Technologies
Steel cleanliness is an important factor which decides the quality of the steel. It can have a remarkable influence on steel properties, such as tensile strength, formability, toughness, weldability, cracking-resistance, corrosion resistance, and fatigue-resistance etc. The demand for better mechanical properties of steels from the steel consumers has urged the steel producers to improve the cleanliness of steel.
In the present environment, the improvement of steel cleanliness has become a necessity for the steel industry. It has guided the development of the secondary steelmaking processes. The driving force behind these developments has been to enable new steels which can tolerate highly demanding applications such as transmission components for the automotive industry, and construction parts and tubes for aggressive and corrosive environments.
Clean steels refer to the steels which are free from inclusions. Inclusions are non metallic particles embedded in the steel matrix. In addition to lowering of the non-metallic oxide inclusions and controlling their morphology, composition and size distribution, clean steels require lowering of other residual impurity elements such as phosphorus (P), sulphur (S), total oxygen (O2), nitrogen (N2), hydrogen (H2), also sometimes carbon (C), and trace elements such as arsenic (As), tin (Sn), antimony (Sb), selenium (Se), copper (Cu), lead (Pb), and bismuth (Bi). These impurity elements vary with different grades of steel. Some elements are harmful to certain steel grade, but can be less harmful or even useful to another steel grade. In other words, the control elements are different for different performance requirements needed from the steel.
For achieving a satisfactory cleanliness in steel, it is necessary to control and improve a wide range of operating practices throughout the steelmaking processes. These include (i) additions of deoxidizing agents and ferro-alloys, (ii) secondary metallurgical treatments, and (iii) shrouding systems and casting practice.
History of clean steel development
The term ‘clean steel’ was coined in the middle of the 20th century. This was the time when the steel production has started to increase globally. At that time, it was understood that quality of steel is also to be considered as a special and important issue. Advances in steelmaking since then have resulted in the development of many steel grades with very low level of impurities. In recent years, new ‘clean and ultra-clean’ steels have been developed and commercialized for addressing the current and future quality requirements needed by the steel consuming industry. Steel cleanliness has also significantly improved mechanical properties (such as fatigue strength and impact toughness) and corrosion resistance of the steels.
The concept of cleanliness was born initially from the observation under the optical microscope of non-metallic inclusions (NMIs) by the newborn discipline of metallography during the middle of the 20th century. Cleanliness was rated against standard images of microscopic fields, where geometry (shape and size) and distribution of the NMIs was distinguished against various image types. The trained observer had established that some shapes were acceptable in some steel grades and that smaller inclusions generally were more acceptable than the 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 S content and deoxidation history of the steel.
These methods developed in the 20th century were soon standardized. They pre-empted the general use of the secondary steelmaking processes and the continuous casting of the steels . The further development of the concept of cleanliness went on by exploring various issues in parallel, connected with the physical chemistry of steelmaking, development of new process reactors, and new, innovative solutions to control inclusions composition, shape, size and distribution which overtime became the routines of the steelmaking practice. A modern vision of cleanliness has emerged from this concept-building effort made during the initial 30 years to 40 years. Further, the subject of the steel cleanliness has reached some degree of maturity now, especially for the new secondary steelmaking processes and for those steels produced by the continuous casting process.
Non metallic inclusions
The NMIs are constituted by glass-ceramic phases embedded in the steel matrix. Presence of the NMIs in the steel is the main reason which affects the steel cleanliness. NMIs in steels come from many sources which include the following.
Deoxidation products – Example of such inclusion is the alumina (Al2O3) inclusions which cause the majority of indigenous inclusions in low C aluminum (Al) killed (LCAK) steel. These inclusions are generated by the reaction between the dissolved O2 and the added deoxidizing agent, such as Al. Al2O3 inclusions are dendritic when formed in a high O2 environment, or can result from the collision of smaller particles.
Reoxidation products – Example of such inclusion is the Al2O3 inclusion generated when (i) the Al remaining in the liquid steel is oxidized by FeO in the slag, or (ii) by the exposure of the liquid steel to the atmosphere.
Slag entrapment – Slag entrapment takes place when metallurgical fluxes are entrained during transfer between steelmaking vessels. The slag entrapment forms liquid inclusions which are generally spherical.
Exogenous inclusions – These inclusions are from other sources, such as loose dirt, broken refractory brickwork and refractory lining particles. They are generally large and irregular-shaped. They can act as sites for heterogeneous nucleation of Al2O3.
Chemical reaction inclusions – These inclusions are the products of inclusion modification when Ca (calcium) treatment is improperly performed.
The inclusion size distribution is very important since large inclusions are the most harmful to the mechanical properties of the steel. One kg of LCAK steel typically contains 10,000,000 to 1000,000,000 inclusions, including only 400 inclusions of size 80 microns to 130 microns, ten inclusions of size 130 microns to 200 microns, and less than one inclusion of size 200 microns to 270 microns. Obviously, detecting the rare large inclusions is very difficult. Though the large inclusions are far outnumbered by the small ones, their total volume fraction can be large. Sometimes a catastrophic defect is caused by just a single large inclusion in a whole steel heat. Thus, clean steel involves not only controlling the mean inclusion content in the steel but also on avoiding inclusions larger than the critical size harmful to the product.
NMIs constitute a cloud of phases dispersed in the matrix of steel and defined by a multi-dimensional set of parameters, including composition, shape, size and distribution. This full description is normally not readily available and one of the main issues related to assessing cleanliness is to observe representative samples to estimate these parameters with a reasonable accuracy and representativity. One difficulty is related to large inclusions (of size 100 microns or more), which are very rare and hence difficult to see, unless very large size samples are analyzed.
Another issue is due to the fact that the population of the NMIs depends on time (in the process timeline of the steel melting shop) and on temperature. Thus 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 have almost no connection, whatsoever, with the cleanliness of the solid steel. Hence, there is necessity to assess when a representative sample of liquid steel is to be taken in order to assess both steel composition and NMI cleanliness.
Types of non-metallic inclusions
Based on their size, the inclusions are either micro-inclusions (size 1 micron to 100 microns) or macro-inclusions (size more than 100 microns). Macro-inclusions are harmful. Micro-inclusions are beneficial as they restrict grain growth, increase yield strength and hardness. Micro-inclusions act as nuclei for precipitation of carbides and nitrides. Macro-inclusions are required to be removed. Micro-inclusions can be used to enhance strengthening by dispersing them uniformly in the matrix.
According to a traditional classification, there are two main types of NMIs a function of their origin. They are (i) endogenous inclusion, and (ii) exogenous inclusion (Fig 1).
Fig 1 Endogenous and exogenous inclusions
The endogenous is the micro-inclusion which is formed by the physical-chemical effects which occur during the melting and solidification process. The endogenous inclusion is formed by precipitation within the liquid phase due to the decrease of the solubility of the chemical species contained in the steels. It can also be formed from the O2 and S remaining after deoxidation and desulphurization process or through reoxidation (Fig 1a and 1b). This class of NMI cannot be completely eliminated from the steel but the decreasing of its volume fraction and of the average size has to be taken under strict control in order to avoid the activation of damaging phenomena.
On the contrary, the exogenous inclusion is macro-inclusion which is due to 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 casting (Fig 1c). The NMI belonging to this class can be featured by large size and its origin cannot be immediately recognizable, although its presence can strongly compromise the micro-structural soundness of the steels and the associated mechanical reliability.
NMIs have a strong influence on the quality and performance of steels. These inclusions are mainly chemical compounds of metals like iron (Fe), manganese (Mn), Al, silicon (Si), and Ca etc. with the non metals such as O2, S, N2, C, and H2.
Various types of the NMIs are (i) oxides such as FeO, Al2O3, SiO2, MnO, Al2O3.SiO2, FeO.Al2O3, MgO.Al2O3, and MnO.SiO2 etc. (ii) sulphides such as FeS, CaS, MnS, MgS, Ce2S3, (iii) nitirides such as TiN (titanium nitrides), AlN, VN (vanadium nitride),and BN (boron nitride) etc., (iv) oxysulphides such as MnS.MnO, and Al2O3.CaS etc., (v) carbonitrides such as carbonitrides of titanium (Ti), vanadium (V), and niobium (Nb) etc., and (vi) phosphides such as Fe3P, Fe2P, Mn5P2. The fundamental tool for the description of the chemical composition of the oxide NMIs is the ternary phase diagram (CaO-SiO2-Al2O3), because this is the main system ruling the formation of these non-metallic compounds. This class of NMIs is formed by the deoxidizing elements added to the steel melt for removing the O2 content. The nitride inclusions perform a detrimental effect worsened by the peculiar edged shape which increases the amplification of the stresses which are developed at the interface between the NMI and the metal matrix.
Based on the mineralogical content, O2 inclusions are classified as (i) free oxides such as FeO, MnO, Cr2O3 (chromium oxide), SiO2, and Al2O3 etc., (ii) spinels such as ferrites, chromites, and aluminates, and (iii) silicates such as SiO2 with a mixture of Fe, Mn, Cr (chromium), Al (aluminum), and W (tungsten) oxides as well as crystalline silicates.
Another classification of the NMIs is by stability. NMIs are rather stable or unstable. Unstable inclusions are sulphides of Fe and Mn as well as some free oxides.
As per the morphology of inclusions, the inclusions can have (i) globular shape, (ii) platelet shape, or (iii) polyhedral shape. Globular shape of the inclusions is desirable. Certain inclusions like MnS, oxy-sulphides, iron aluminates and silicates are globular. Platelet shape of the inclusions is undesirable. Al deoxidized steels contain MnS in the form of thin films located along the grain boundaries. Inclusions with polyhedral shape are not very harmful.
The chemical elements initially involved in cleanliness are mostly the non-metals of the Mendeleev periodic table, because they show higher solubility in liquid steel than in the solid. These are mainly C, N2, O2, P, S, Se, and H2. To this list, the metalloid neighbours in the table such as B (boron), As, Sb, and Te (tellurium) can be added. Some of these elements originate from primary raw materials (P, S, As, Sb) or from hot metal from the blast furnace, while most of the others are either due to contamination by the atmosphere (O2, N2, and H2) and the operating practice used in steelmaking, or are voluntarily added (C, Se, Te, and B).
The sulphides are often the consequence of the Ca treatment applied in order to modify the oxide inclusions, but the little and finely dispersed highly refractory CaS inclusions can be detrimental for the casting (nozzle clogging) and for the damaging effect. On the contrary the MnS NMIs (often modified by the combination with CaS) are useful for cutting tool workability during the machining of the steel.
Three main mechanisms have been recognized at the origin of the NMI. These mechanisms are related to the damaging effects played by the non-metallic phases against the metal matrix. These mechanisms consider the NMIs as (i) notching elements which amplify the stress field around the NMIs, (ii) pressurized tanks of gas which progressively migrates into the NMIs generating a stress field around the NMIs, and (iii) non-metallic phases which generate a residual stress due to the different thermal expansion coefficient associated to the metal phase and the glassy-ceramic ones.
Methods of evaluating steel cleanliness
Knowing accurately the realistic inclusion content in the steel is vital for both improving steel quality and also in predicting how a component made of the steel is going to perform. In order to study and control steel cleanliness, it is critical to have accurate methods for its evaluation. The amount, size distribution, shape and composition of inclusions are to 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.
There are several direct methods to evaluate steel cleanliness and they are summarized below.
Metallographical microscope observation (MMO) – This is the traditional method in which two-dimensional slices through steel samples, are examined with an optical microscope and quantified by the eye. In it, the results are evaluated using charts such as the JK reference scale. This technique is only suitable for qualifying inclusions between 2 microns to 15 microns and is limited to very small sample sizes. This method does not provide any data on the chemical composition of inclusions. Problems arise when interpreting slices through complex-shaped inclusions. Although there are some methods to relate two-dimensional results to three-dimensional reality, this is generally very problematic.
Image analysis – This is enhancement to MMO which improves on eye evaluation by using high-speed computer evaluation of video-scanned microscope images to distinguish dark and light regions based on a gray scale cutoff. This method can easily evaluate larger areas and greater inclusion numbers than MMO, but is subject to errors such as mistaking scratches, pitting, and stains for NMIs.
Sulphur print – This is a popular and inexpensive macrographic method which distinguishes macro-inclusions and cracks by etching sulphur-rich areas. It is subject to the same problems as other two dimensional methods.
Blue fracture testing – It is a historically a well-established technique used to reveal macro inclusions larger than 0.5 mm. It is performed on a bar cross-section area which has been hardened, fractured and then tempered blue to increase the visibility of defects.
Slime (electrolysis) method – This is an accurate method but time consuming. A relatively large (200 g to 2 kg) steel sample is completely dissolved in hydrochloric (HCl) acid and the NMIs which remain undissolved are collected for counting and further analysis. Alternatively, in order to protect FeO inclusions, most of the dissolution is accomplished by applying electric current through the steel sample immersed in a FeCl2 or FeSO4 solution. This method is used to reveal the individual, intact inclusions.
Electron beam (EB) melting – A steel sample is melted by an electron beam under vacuum. Inclusions float to the upper surface and form a raft on top of the molten sample. The usual EB index is the specific area of the inclusion raft. An enhanced method EB-EV (extreme value) has been developed to estimate the inclusion size distribution. This is done by measuring the maximum inclusion size in several fields of the raft and extrapolating the results over the entire raft, assuming an exponential inclusion size distribution.
Cold crucible (CC) melting – Inclusions are first concentrated at the surface of the melted sample as in the EB melting. After cooling, the sample surface is then dissolved, and the inclusions are filtered out of the solute. This method improves on slime extraction.
Scanning electron microscopy (SEM) – This method clearly reveals the three-dimensional morphology and the composition of each inclusion examined. Composition is measured with electron probe micro analyzer (EPMA). SEM is capable of assessing large areas and provides rich data on inclusion chemistry, morphology, and size.
Optical emission spectrometry (OES) with pulse discrimination analysis (PDA) – The OES method is conventionally used for analysis of dissolved elements in steel. This technique has been further improved to analyze the total O2 content, micro-inclusion size distribution and composition within 10 minutes of collecting the sample. To discriminate solid inclusions (OES-PDA), light logging is made at the frequency of the emission spark. Electrical characteristics are defined to optimize the light ratio between the background signal of the dissolved elements and the disturbance signal due to heterogeneities such as inclusions. The number of high intensity Al peaks spark is the PDA index.
Mannesmann inclusion detection by analysis surfboards (MIDAS) – Steel samples are first rolled to remove porosity and then ultrasonically scanned to detect solid inclusions and compound solid inclusions / gas pores. This method has been recently rediscovered as the ‘liquid sampling hot rolling (LSHP) method.
Laser diffraction particle size analyzer (LDPSA) – This laser technique can evaluate the size distribution of inclusions which have been extracted from a steel sample using another method such as slime.
Conventional ultrasonic scanning (CUS) – This method can obtain size distributions of inclusions larger than 20 microns in solidified steel samples.
Cone sample scanning – In this method, a cone-shaped volume of continuous cast steel is scanned with a spiraling detector, such as a solid ultrasonic system, which automatically detects surface inclusions at every location in the area of the sample, including from surface to centerline.
Fractional thermal decomposition (FTD) – Inclusions of different oxides are selectively reduced at different temperatures, such as Al2O3-based oxides at 1400 deg C to 1600 deg C, or refractory inclusions at 1900 deg C. The total O2 content is the sum of the O2 contents measured at each heating step.
Laser microprobe mass spectrometry (LAMMS) – Individual particles are irradiated by a pulsed laser beam, and the lowest laser intensity above a threshold value of ionization is selected for its characteristic spectrum patterns due to their chemical states. Peaks in LAMMS spectra are associated with elements, based on comparison with reference sample results.
X-ray photoelectron spectroscopy (XPS) – This method use x-rays to map the chemical state of inclusions larger than 10 microns.
Auger electron spectroscopy (AES) – This method use electron beams to map the chemical state of photo scattering method. Photo-scattering signals of inclusions (which have been extracted from a steel sample using another method such as slime) are analyzed to evaluate the size distribution.
Liquid metal cleanliness analyzer (LIMCA) – This is an on-line sensor which detects inclusions directly in the liquid. Particles which flow into this sensor through its tiny hole are detected because they change the electric conductivity across a gap.
Coulter counter analysis – This method, which is similar to LIMCA, can be used to measure the size distribution of inclusions extracted by slime and suspended in water (inclusions larger than sub-micron).
Ultrasonic techniques for liquid system – This method captures the reflections from ultrasound pulses to detect on-line inclusions in the liquid steel.
Immersed ultrasonic testing method – It is used to test for larger inclusions and it produces impressive results. For testing inclusions with size above 120 microns, a single sample of 500,000 cum steel, milled plane parallel, and immersed in a water tank is scanned with a 10 MHz probe. This is the equivalent of 16,000 blue fracture tests. This test does not produce information about the chemical composition of the inclusions, but it is an important tool for the process. To test for smaller inclusions, it is possible to increase the frequency of the ultrasonic probe to 15 MHz, 25 MHz, 50MHz, or even higher. However, as the frequency and resolution is increased the size of the sampled volume decreases.
Because of the cost, time requirements, and sampling difficulties, steel cleanliness is normally measured in the steel industry using total O2, N2 pick-up, and other indirect methods.
Total O2 measurement -The total O2 in the steel is the sum of the free O2 (dissolved O2) and the O2 combined as NMIs. Free O2 or ‘active’ O2 can be measured relatively easily using the O2 sensors. It is controlled by equilibrium thermodynamics with deoxidation elements, such as Al. Since the free O2 does not vary much, the total O2 is a reasonable indirect measure of the total amount of the oxide inclusions in the steel. Due to the small population of large inclusions in the steel and the small sample size for total O2 measurement (normally 20 g), there are likely no large inclusions in the sample. Even if a sample has a large inclusion, it is likely to be discounted because of the anomalously high reading. Thus, total O2 content really represents the level of small oxide inclusions but not the larger ones. A low total O2 content, however, decreases the probability of large oxide inclusions. Thus total O2 is still a very important and common index of steel cleanliness. Total O2 in LCAK steel has steadily decreased with passing years, as new technology is getting implemented. For example, steel plants with the vacuum degassing achieve lower total O2 (10 ppm to 30 ppm) than the steel plants with only ladle gas-stirring (35 ppm to 45 ppm)). Total O2 generally drops after every processing step such as at the ladle 40 ppm, at the tundish 25 ppm, at the mould 20 ppm, and in the cast steel 15 ppm.
N2 pick-up – The difference in N2 content between steelmaking vessels (especially ladle and tundish) is an indicator of the air entrained during transfer operations. After deoxidation, the low dissolved O2 content of the steel enables rapid absorption of air. N2 pick-up thus serves as a crude indirect measure of total O2, steel cleanliness, and quality problems from reoxidation inclusions. With the implementation of new technology and improved operation, N2 pick-up has deceased over the years. Normally, N2 pick-up can be controlled at 1 ppm to 3 ppm from ladle to mould. With optimal transfer operations to lessen air entrainment, N2 pick-up can be lowered during steady state casting to less than 1 ppm. N2 level in LCAK steel is controlled generally at 30 ppm to 40 ppm level in most of the steel plants. It is controlled mainly by the steelmaking converter or electric furnace operation, but is affected by the refining and shrouding operations.
Dissolved Al loss measurement – For LCAK steels, Al loss also indicates that reoxidation has occurred. However, this is a less accurate measure than N2 pick-up since Al can also be reoxidized by the slag.
Slag composition measurement- Analysis of the slag composition evolution before and after operations can be interpreted to estimate inclusion absorption to the slag. Also, slag entrainment from a particular vessel can be determined by matching trace elements in the slag and inclusion compositions.
Submerged entry nozzle (SEN) clogging – Short SEN life due to clogging is generally an indicator of low level of steel cleanliness. Small Al2O3 inclusions in LCAK steel are known to cause nozzle clogging. Hence, SEN clogging frequency is another crude method to evaluate steel cleanliness.
Thus, it is seen that there is no single ideal technique to evaluate steel cleanliness. Some techniques are better for quality monitoring while others are better from the angle of problem investigation. Hence, it is necessary to combine several methods together to give a more accurate evaluation of steel cleanliness in a steel plant. Reliable quantification of inclusions has made it possible to develop a new generation of clean steels.
Technologies and operational practices for clean steels
Secondary steelmaking has become an accepted tool to manage the steel cleanliness since it makes it possible not only to make additions to the liquid steel under controlled conditions, but also contributes to careful slag-metal stirring, slag reduction, temperature trimming, inclusion coalescence, elimination by flotation and entrapment in the slag and composition control, vacuum degassing, and sometimes C deoxidation etc. The functions needed for engineering steels thus has become available to the steel producers and a subset of them are being used for all grades of steel and this has made the distinction between commercial quality and the specialty steels a bit blurred.
One important feature of secondary steelmaking and the 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 be standardized, sometimes automated and better controlled. On the other hand, sources of contamination have multiplied but can also be better controlled. The ladle to tundish (ladle nozzle, sliding gate, and ladle stream gas protection etc.), tundish (powder, weirs, dams and baffles, and bubbling elements etc.), tundish to mould (nozzle, sliding gate or stopper rod, submerged nozzle and gas bubbling etc.), mould (mould powder, mould level control, and submerged nozzle geometry etc.), continuous casting itself (straight, curved mould, straight mould and curved, electromagnetic stirring, electromagnetic brake, and 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 and the continuous casting machine, especially its mould, also act as a metallurgical reactor, where the fate of the NMIs continues to be decided.
These large number of technologies and operational practices throughout the secondary steelmaking processes for improving cleanliness in the steel, include the time and location of the additions of the deoxidizing agents and ferro-alloys, the extent and sequence of secondary steelmaking processes, stirring and transfer operations, shrouding systems, tundish geometry and practices, the absorption capacity of the various metallurgical fluxes, and casting practices.
The formation and the control of the chemical composition of the NMIs involve the different steps of the production processes and the industrial systems through which they are performed. The production process has to be carefully implemented in each step in order to avoid problems related to (i) difficulties during the casting operation associated with the nozzle clogging between the tundish and the mould (continuous casting process) and between the ladle and the casting column (ingot casting process), and (ii) detrimental effect on the mechanical properties of the steel.
At the end of steelmaking in the BOF (basic oxygen furnace) or the EAF (electric arc furnace), O2 is at equilibrium with C, which means very high levels for low C steel grades (1250 ppm O2 for 0.02 % C). If steel is to simply solidify as such, eutectics of Fe, S, and O2 precipitate in the inter-dendritics, while a strong C deoxidation take place in the initial stages of solidification, thus producing rimming steels, full of blowholes near the surface. The resulting steel in addition to being porous is brittle during hot rolling or hot forging operation and subsequent use at room temperature.
To avoid precipitating O2 and S iron eutectics, deoxidation agents (C, especially under reduced pressure, Mn, Si, Al, Ca, and Ti etc.) and desulphurizing agents (Mn, and Ca) are introduced into the process in order to promote new equilibriums whereby third phases precipitate and rimming is avoided altogether. The third phases constitute the endogenous NMIs (oxides, nitrides, carbides, sulphides, and phosphorides etc.) which are initially created in the liquid steel, usually in the ladle. These equilibriums can be implemented by adding deoxidizing agents into liquid steel by bulk additions or wire injection or by ensuring that the liquid metal is in equilibrium with an active metallurgical slag of the proper composition.
The population NMIs changes all the time, since the existing inclusions coalesce, float out and get finally adsorbed in a slag or a simple covering powder or flux, by aggregation against refractory in the ladle, the tundish or inside nozzles that some of them (solid non-metallic inclusions, like Al2O3 or spinels) tend to clog. Steel and slag change as well, and inclusions entertain complex connection with them, at equilibrium, if time allows, or out of it. Gas evolution at the solidification front can still take place if N2 and H2 are not properly controlled. More inclusions appear, since temperature drops, which generally means more precipitation, or solidification starts, or O2 penetrates the system (reoxidation),from the slag, the refractories, from the atmosphere at refractory junctions (sliding gates, submerged nozzle mounting, and across the refractories etc.), or because the slag or the refractories generate new inclusions or release inclusions previously captured. The latter is known as the exogenous NMIs. Of course, the trend is generally towards improved cleanliness with all these mechanisms are being deeply looked into for finding counter-measures.
An important point regarding reoxidation is that the phenomenon does not take place at thermo-dynamic equilibrium, but rather generates oxides of whichever element happens to meet the incoming O2, most often generating Fe oxides. Out of the equilibrium in deoxidized liquid steel, these oxides later reverse back to equilibrium NMIs, if time permits.
The distinction between endogenous and exogenous NMIs is however somewhat ad-hoc, as deoxidation or reoxidation are actually an integral parts of the total process of the steelmaking and both result from the technology put in place to produce steel. As an example, deoxidation does not take place inside liquid steel, but at the interface of the deoxidizing agent injected.
NMIs are large enough to interact with the metal matrix as mechanical discontinuities, basically like holes. There are other third phases in steel of much smaller dimensions called precipitates, which interact with the matrix as the scale of dislocations or even at atomic scale. Precipitates, normally carbides or nitrides, constitute the key features of the micro-alloying of steels or of more substantial alloying like in tool steels or in stainless steels.
Steel refining and continuous casting operations have important effects on improving steel cleanliness. A systematic study of inclusion removal carried out in a steel plant has indicated that the ladle treatment drops inclusions by around 65 % to 75 %, the tundish removes inclusions by around 20 % to 25 %, although reoxidation can sometimes occur, and the mould removes inclusions around 5 % to 10 % of the total inclusions.
The tap O2 content is measured during tapping the liquid steel in the ladle or before the addition of the deoxidizing agents. The value is typically high. It varies in a wide range (250 ppm to 1200 ppm) depending on the primary steelmaking practice. Al additions when used to deoxidize the steel, create larger amounts of Al2O3. This suggests that a limitation on tap O2 content is to be imposed for clean steel grades. However, there is no correlation between furnace practice and steel cleanliness, since around 85 % of the Al2O3 clusters formed after large additions of Al, float out to the ladle slag, and that the remaining clusters are smaller than 30 microns. Naturally, the decision to ignore tap O2 depends on the time available to float inclusions and on the availability of ladle refining, which can remove most of the generated inclusions. However the tap O2 content strongly affects the decarburization rate for producing ultra low C steel.
FeO and MnO in slag – An important source of reoxidation is the carryover slag from the converter to the ladle, which contains a high content of FeO and MnO. These oxides react with the dissolved Al to generate Al2O3 in liquid steel, owing to the strong favourable thermodynamics of the reactions 3FeO (l) + 2Al = Al2O3 + 3Fe (l), and 3MnO + 2Al = Al2O3 + 3Mn (l). The higher is the FeO and MnO content in the ladle slag, the greater is the potential for reoxidation and the corresponding generation of the Al2O3 inclusions. Many slivers in the final product have been traced to reoxidation that originated from FeO in the ladle slag.
Many counter-measures can be adapted to lower FeO and MnO contamination. These counter-measures are (i) minimizing of slag carryover from converter to ladle during tapping, (ii) increasing aim turndown C, (iii)avoiding the reblows, thus minimizing the dissolved O2 content in the steel and reduce the amount of FeO in the furnace slag, (iv) use of a sub-lance in the BOF substantially reduces the frequency of reblows, (v) use of an efficient mechanical slag stopper, such as a slag ball (which floats in steel and sinks in slag), and (vi) using other sensors which are alternatively available. A thick ladle slag layer after tapping suggests high slag carryover problems. In some plants, the ladle slag for critical grades is mechanically skimmed at the ladle furnace to a thickness in the range of 25 mm to 40 mm.
Ladle slag reduction treatment – It has been found that minimizing slag carryover, together with adding a basic ladle slag and basic lining to lower the ladle slag to less than 1 % to 2 % of FeO + MnO, can reduce total O2 content to 10 ppm for LCAK steel. Another way to lower the FeO + MnO content of the ladle slag is to add a slag conditioner (i.e. slag reduction or deoxidation treatment), which is a mixture of Al and burnt lime or limestone. There is a drop in FeO + MnO content after ladle slag reduction treatment. On an average, this treatment lowers the FeO + MnO level to below 5 %. This results in sharp improvement of coil cleanliness.
Effect of vacuum treatment and ladle stirring – Vacuum treatment of liquid steel started with the production of engineering steels for the automotive, power, and the aircraft sectors with the purpose of increasing the reliability and life of the mechanical parts of vehicles or nuclear reactors. The major need is to control the H2 level in liquid steel (to less than 1 ppm in a C steel) in order to avoid its departure at solidification and its entrapment in the solid, which leads to serious integrity defects during the use of the steel part. The use of vacuum, which removes H2 straight forwardly, came into existence in the steelmaking shops, using various technologies like tank degassing, stream degassing, and DH and RH (Rheinstahl Heraeus) ladle degassing processes. The vacuum degassing besides reducing the non-metallic inclusions, also allows other benefits such as (i) C deoxidation, which has the major advantage of producing gaseous deoxidation products, (ii) intensive stirring with its several advantages, (iii) allows for the time management in the logistics of ladle flow, hence on the quality of temperature control of liquid steel, and (iv) reheating of the liquid steel by Al and O2 injections.
Ladle stirring and the ladle degassing processes greatly promote inclusion growth and removal. The effect of vacuum treatments on the cast steel inclusion levels shows the improvement of steel cleanliness over argon (Ar) stirring in the ladle. The pronounced benefit of Ca-based powder injection is due to its greater stirring power in addition to its primary effect of deoxidization and liquefying inclusions. The vacuum degassing and Ca treatment together can drop the total O2 to 15 ppm level.
However, excessive stirring is detrimental, since the upward circulation of steel onto the slag layer can expose an ‘eye’ region of the steel surface to reoxidation as well as due to the refractory erosion. Sufficient stirring time (more than 10 min) after the addition of ferro-alloys is also important, to allow the Al2O3 inclusions to circulate upto the slag and be removed. In some plants, the practice of first stirring vigorously to encourage the collision of small inclusions into large ones, followed by a ‘final stir’ which slowly re-circulates the steel to facilitate the removal of inclusions into the slag while minimizing the generation of more large inclusions via collisions.
Important phenomena which are taking place in the tundish are shown the Fig 2. The factors which are affecting the steel cleanliness are (i) casting transitions, (ii) tundish refractory lining, (iii) tundish flux, (iv) gas stirring, and (v) tundish flow control.
Fig 2 Important phenomena taking place in the continuous casting tundish
Casting transitions – Casting transitions occur at the start of casting, during ladle exchanges and SEN (submerged entry nozzle) changes, and at the end of the casting sequence. Inclusions are often generated during transitions and can continue for a long time, hence contaminate a large quantity of steel. During these unsteady casting periods, slag entrainment and air absorption are more likely, which induce reoxidation problems
During the first casting heat, the entrainment of air and slag in the tundish pour box due to the turbulence during ladle open is accompanied by an initial maximum in total O2 content in the tundish (including both slag and Al2O3 inclusions). Open pouring at the start of the casting causes total O2 in tundish to increase to twice normal levels for more than an entire heat. Several minutes of filling are needed before tundish flux can be added. Eventually, during steady casting, the total O2 decreases to lower levels, consisting mainly of Al2O3.
One improvement during ladle transitions is to stop the flow of liquid into the mould until the tundish is filled and to bubbling gas through the stopper to promote inclusion flotation. Another improvement effect is to open new ladles with submerged shrouding. With this measure, the total O2 is decreased with more consistent quality throughout the sequence. Near the end of a ladle, ladle slag can enter the tundish, due in part to the vortex formed in the liquid steel near the ladle exit. This phenomenon needs some steel to be kept in the ladle upon closing (example 5 ton ‘heel’). In addition, the tundish depth drops after ladle close, which disrupts normal tundish flow and can produce slag vortexing, slag entrainment, and increased total O2 in the mould.
Tundish refractory lining – Dissolved Al in the liquid steel reacts with an O2 source in the lining refractory. The extent of this reaction can be quantified by monitoring the Si content of the liquid steel. The O2 for the reaction can come from CO (carbon monoxide) when C in the refractory reacts with binders and impurities or from SiO2 refractory decomposition. SiO2 based tundish linings are worse than MgO based sprayed linings.
Tundish flux – The tundish flux is to carry out many functions. Firstly, it is to insulate the liquid steel both thermally (to prevent excessive heat loss) and chemically (to prevent air entrainment and reoxidation). Further, the tundish flux with lower SiO2 content can decrease N2 pick-up from the ladle to the mould substantially. Secondly, in ideal circumstances, the flux is also to absorb inclusions to provide additional steel refining. A normal tundish flux is burnt rice husk, which is inexpensive, a good insulator, and provides good coverage without crusting. However, rice husk is high in SiO2 (around 80 %), which can be reduced to form a source of inclusions. Also, rice husk is very dusty and with their high C content, (C around 10 %), can contaminate ultra low C steel.
Basic flux (CaO-Al2O3-SiO2 based) is theoretically better than burnt rice husk at refining LCAK steels, and has been correlated with lower O2 in the tundish. Use of basic tundish flux (CaO-40 %, Al2O3-24 %, MgO-18 %, SiO2-5 %, Fe2O3-0.5 %, and C-8 %), together with baffles, significantly lowers the total O2 fluctuation, as compared to the flux (CaO-3 %, Al2O3-10 % to 15 %, MgO-3 %, SiO2- 65 % to 75 %, and Fe2O3-2 % to 3 %). The basic flux, however, show similar results for other parameters as compared to rice husk, may be because the basic flux also contains a high content of SiO2. The basic flux is thus ineffective since it easily forms a crust at the surface, owing to its faster melting rate and high crystallization temperature. Also, basic flux normally has lower viscosity, and hence it is more easily entrained. To avoid these issues, some steel plants use a two-layer flux, with a low-melting point basic flux at the bottom to absorb the inclusions, and a top layer of rice husk to provide insulation, which lowers the total O2.
Tundish stirring – Injecting inert gas into the tundish from its bottom improves mixing of the liquid steel, and promotes the collision and removal of inclusions. This technology lowers the total O2 in the tundish. The danger with this technology is that any inclusions-laden bubbles which escape the tundish and become entrapped in product result into severe defects in the product.
Tundish flow control -The tundish flow pattern is to be designed to increase the liquid steel residence time, prevent the ‘short circuiting’ and promote inclusions removal. Tundish flow is controlled by its geometry, level, inlet (shroud) design, and flow control devices such as impact pads, weirs, dams, baffles, and filters. The tundish impact pad is an inexpensive flow control device which suppresses turbulence and prevents erosion of the tundish bottom where the liquid steel stream from the ladle impinges the tundish. The incoming stream momentum is diffused and allows the naturally buoyancy of the warm incoming steel to avoid short circuiting, particularly at startup. Together with weir and dam, the impact pad improves steel cleanliness, especially during ladle exchanges.
One of the most important sources of O2 pick-up is atmospheric reoxidation of steel during the transferring from ladle to tundish or from tundish to mould. This generates inclusions which cause production problems such as nozzle clogging, in addition to defects in the final product. Optimization of shrouding system is very important to prevent this phenomenon. Using a shroud lowers the N2 pick-up relative to open pouring. Replacing the tundish pour box with a ladle shroud and dams also lowers the N2 pick-up (ladle to tundish) and also lowers the slag entrainment during transitions.
Ladle opening – Ladle self open is a heat in which the ladle nozzle does not have to be lanced open, but opens on its own. When the nozzle is to be lanced open, then the shroud is to be removed. The cast is unshrouded from ladle to tundish during the first 600 mm to 1200 mm of the cast, and hence the reoxidation by air occurs. Hence, the total O2 level for the self-open ladle is lower than the lanced-opened ladle. Careful packing ladle opening sand is helpful to realize ladle self open.
Argon protection – Argon protection is used to prevent the liquid steel from air reoxidation. When adding the tundish flux too early, the flux can be entrapped into liquid steel and cast into the steel, thus normally there is no protective cover for the first few minutes of a cast. Also at the period of ladle opening, air is very easy to reach liquid steel. The effects of these two factors can last upto 15 minutes into the cast for a tundish of 60 ton capacity. For countering this problem, purging the tundish with inert gases (to displace the air) prior to opening the ladle into the tundish is adopted in some steel plants. Another measure to improve shrouding system for lowering of total O2 is to incorporate an appropriate gas injection.
Sealing issues –For decreasing the N2 pick-up during continuous casting, the factors normally considered are sealing of shroud from ladle to tundish, and SEN from tundish to the mould.
Nozzle clogging – In addition to interfering with production, the clogging of tundish nozzle and SEN is detrimental to steel cleanliness for three reasons. Firstly, dislodged clogs either become trapped in the steel, or they change the flux composition, leading to defects in either case. Secondly, clogs change the nozzle flow pattern and jet characteristics leaving the nozzle, which disrupt flow in the mould, leading to slag entrapment and surface defects. Thirdly, clogging interferes with mould level control, as the flow control device tries to compensate for the clog. Several practices can used to minimize clogging. In addition to taking general measures to minimize inclusions, clogging via refractory erosion can be countered by controlling nozzle refractory composition, (example avoiding of Na, K, and Si impurities), or coating the nozzle walls with pure Al2O3, BN, or other resistant materials.
Mould and continuous casting machine (CCM) operation
The casting of liquid steel in a continuous casting machine (CCM) involves many phenomena, shown in Fig 3, which have far reaching consequences on the strand quality. Inclusions carried into the mould through the nozzle include deoxidation products, nozzle clogs, and entrained of tundish/ladle slag (reoxidation by SiO2, FeO, and MnO in the slag), and reoxidation products from air absorption from nozzle leaks. Mould slag can be entrained by excessive top surface velocities or level fluctuations. New inclusions can precipitate as the superheat drops, such as TiO2 inclusions in Ti-treated steels. On the other hand, inclusions can be removed into the slag/steel interface by buoyancy flotation, fluid flow transport, and attachment to the bubble surfaces.
Fig 3 Schematics of the phenomena in the mould region of a CCM
The mould is the last refining equipment where inclusions are either safely removed into the top slag layer or get entrapped into the solidifying shell to form permanent defects in the cast product. Important insight into inclusion entrapment has been obtained in the past through collecting statistical data and conducting trials on the operating CCMs. It has been noticed that increasing steel flow rate increases the level of pencil blisters (from Ar bubble entrapment) considerably, while it reduces the level of slivers (from slag entrapment). While measuring the inclusion and bubble distribution in the cast steel, it has been observed that individual 1 mm bubbles are often coated with inclusion clusters, and can be carried from far upstream, even if no gas is injected into the tundish nozzle. It has been also observed that the inclusion entrapment varies from side to side, which suggests a link with variations in the transient flow structure of the lower recirculation zone.
Defects are frequently found associated with transients in the process, such as changes in casting speed, tundish changes, or clogged nozzles. Pencil pipe defects occur intermittently and are rare, relative to the quantity of injected gas. The conclusions made in one of the study are that 80 % the particle are eventually removed to meniscus (20 % entrapped in cast product), and a given particle circulate for upto 300 seconds before being removed or entrapped.
In a CCM with curved-mould, inclusions are preferentially trapped 1 m to 3 m below the meniscus. Thus, inclusions concentrate at one-eighth to one-quarter of the thickness from the top of the inside radius surface, in addition to the surfaces. It has also been reported that the electromagnetic stirring can improve the steel cleanliness by lowering the total O2 content in the cast product. CCM with curved mould machines are known to entrap many more particles than the CCM with straight (vertical) mould, since the inclusion spiral upwards the inside radius, where they collect at a specific distance through the thickness , corresponding to 2 m to 3 m below the meniscus.
It has been reported that the cast speed has its effect on the slivers. High speeds and high variation in casting speed result in a higher rate of slivers. Adequate stable casting speeds can be obtained with the use of a stopper. With a stopper, the speed is no longer determined by the level of steel in the tundish, but by the level of steel in the mould. It is better to control mould level control in the range +/- 3 mm. A beneficial tool for the optimizing of the fluid flow and hence improving the quality of the cast product is the electromagnetic brake (EMBR), which bends the jet and shortens its impingement depth, inclusions thus move more upwards, tend to top powder or be captured by the solidified shell at the surface of the cast product. After the use of EMBR, the inclusions distribution shows that there is a shift to the surface of the cast product.