MASTER'S THESIS

Optimization of Argon Blowing duringVacuum Tank Degassing to Increase

Toughness in Steel Wear Plate

Jens Sörlin

Master of Science in Engineering TechnologyEngineering Physics

Luleå University of TechnologyDepartment of Engineering Sciences and Mathematics

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Abstract This work aims to find the optimal argon blowing procedure during vacuum tank degassing in order to achieve optimal toughness and steel cleanliness. During vacuum degassing three different blowing procedures were tested in full scale production of Hardox 450 DQ. Image analysis of the spout eyes during degassing was performed were the spout eyes area was measured every 10’th second and compared to total visible slag surface. Resulting mean values correlated with N and H removal and oxidization of Al, which was found to be an indicator of the degree of stirring. Inclusions in rolled plate, made from charges with variated blowing procedure, were classified according to SS 11 11 16 and it was found that rinse stirring during the latter half of degassing is beneficial for inclusion removal. Results from test charges have been compared to other vacuum degassed charges during 2010 and it was concluded that a high degree of gas stirring is beneficial for H, N and S removal. For the particular steel studied, toughness was not seen to depend on inclusions.

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Table of contents Abstract ................................................................................................................................................... 1

Acknowledgements ................................................................................................................................. 4

Introduction ............................................................................................................................................. 5

Background and theory ........................................................................................................................... 5

SSAB Oxelösund ............................................................................................................................... 6

Improving steel .................................................................................................................................... 7

Hydrogen and nitrogen removal ..................................................................................................... 7

Inclusion removal ............................................................................................................................ 8

Vacuum tank degassing station ....................................................................................................... 8

Problem description ........................................................................................................................ 9

Experimental procedure ........................................................................................................................ 10

Slag skimming ................................................................................................................................ 10

Experimental set up....................................................................................................................... 11

Steel sampling ............................................................................................................................... 12

Image analysis of degassing .......................................................................................................... 12

Sample preparation ....................................................................................................................... 13

Results ................................................................................................................................................... 14

Chemical composition ....................................................................................................................... 14

Test charges ................................................................................................................................... 14

Regular charges ............................................................................................................................. 17

Degree of stirring ............................................................................................................................... 20

Image analysis ............................................................................................................................... 20

Oxidized aluminium ....................................................................................................................... 25

Impact of stirring ............................................................................................................................... 26

Test charges ................................................................................................................................... 26

Regular charges ............................................................................................................................. 29

Statistical description ........................................................................................................................ 32

Hydrogen ....................................................................................................................................... 32

Nitrogen ......................................................................................................................................... 33

Sulphur .......................................................................................................................................... 34

Inclusions ........................................................................................................................................... 35

Mechanical testing ................................................................................................................................ 38

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Test charges ................................................................................................................................... 38

Regular charges ............................................................................................................................. 42

Discussion .............................................................................................................................................. 43

Conclusions ............................................................................................................................................ 46

Recommendations ............................................................................................................................ 46

Future work ........................................................................................................................................... 46

References ............................................................................................................................................. 47

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Acknowledgements I am very grateful for the support provided throughout the work by my supervisor Dr. Niklas Kojola, at SSAB Oxelösund. From the steel plant introduction, when I first arrived in Oxelösund, to the reviewing of a draft of this report he has always provided cheerful guidance and insights. I am grateful to senior metallurgist Tor-Björn Larsson for helping me to understand the toughness testing procedure at SSAB Oxelösund and how to obtain and evaluate the data from such tests. I am also grateful to metallographer Lena Eklund who taught me sample preparation and inclusion classification and also validated my classifications. The operators at the VTD station have my thanks for being forthcoming during my experiments and sharing their knowledge with me in the process. Finally I wish to thank my supervisor at LTU, lecturer Esa Vuorinen, for guiding me through various steps of my work.

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Introduction The deleterious influence of non-metallic inclusions and unwanted trace elements in steel has been known for a long time. Inclusions can provide crack nucleation sites, surface defects, corrosion sensitivity and thus reduced mechanical properties. Fatigue crack initiation often occur at large defects, such as inclusions, while fatigue crack propagation is dependent on defect size, volume fraction and density (1, 2, 3). When the removal of inclusions and tramp elements became popular in the steel industry the concept of clean steel was mounted. Kiessling reviewed the field of clean steel in 1980 (4), Lagneborg did it with focus on common unwanted elements in 1981 (5) and Zhang published a review focusing on state of the art in clean steel in 2002 (6). Evaluation of steel cleanliness is possible using a multitude of methods (6) depending on what information is needed. Large inclusions are, in general, the most harmful and hardest to detect but ways of evaluating them are available (7). The most common unwanted elements in steels are generally phosphorus P, sulphur S, oxygen O, nitrogen N and hydrogen H. Sulphur is, except for machineability, very detrimental to steel properties. Manganese sulphides are soft during hot rolling and become prolonged to needle shapes which act as stress raisers and crack initiation sites during cold forming (8, 5, 9, 10). Phosphor has an embrittling effect in steel, which reduces toughness (9). Oxygen has very low solubility in iron and readily forms oxides which contribute to easier crack growth and reduced toughness (5, 6, 9, 10). High nitrogen content reduces formability and weldability as the toughness of the weld decreases with higher N. Nitrogen also reduces hardenability by forming boron-nitrides. Hydrogen induced cracking (HIC) is a common problem in steels, lowering the H content is always favourable (9). Inclusions can act as starting points for HIC (11). During secondary steelmaking the molten steel is cleaned using inclusion absorbing slags, gas blowing and in some cases vacuum treatment. When gas is injected, at the bottom of a ladle, bubbles rise creating turbulent mixing of the steel and slag. The region where gas emerges is called the spout and if this area is uncovered by slag it is called a spout eye. Secondary steelmaking processes are often complex as they involve multiple phases, turbulent motion and many elements and compounds. Inclusion evolution (12, 13, 14), total oxygen evolution (15, 16) and sulphur removal by calcium addition (17, 18) have been studied in plant trials by sampling before, during and after degassing. The oxygen potential in the melt is of importance when removing nitrogen and sulphur. This is shown for sulphur removal using thermodynamics (19) and addition of deoxidisers on nitrogen removal (20, 21). The opacity and high temperatures of molten steel makes direct measuring of inclusion removal hard. However, cold model experiments have been made to increase understanding of the spout eye (22, 23), inclusion removal (24, 25, 26), slag entrainment (23) and skimming (27). Mathematical models have also been developed describing parts of secondary steelmaking such as the spout eyes (28), inclusion growth and removal (29, 30, 31), slag-steel interfaces (32) and nitrogen removal (33).

Background and theory In this chapter an introduction to the steel plant, at which the work has been carried out, and a background to the principles of steel improvement is presented. Also the theory behind hydrogen, nitrogen and inclusion removal is presented along with an introduction to the vacuum VTD station, at which the process variations have been done, and a problem description.

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SSAB Oxelösund The steel plant in Oxelösund was started in 1913 and it has since 1961 been Sweden’s only fully integrated steel plant. A coking plant, two blast furnaces, secondary refining metallurgy, two continuous casters and a rolling mill with features such as quenching, tempering and painting are all located at SSAB Oxelösund. At the site wear plate, construction plate, tool steel and ballistic protection plate, under the brand names Hardox, Weldox, Toolox and Armox, is produced. In the blast furnaces coke and iron ore is turned to pig iron, which is transported to the steel plant in torpedo cars. In total is 90-95% of the dissolved sulphur removed in the torpedo car by nitrogen gas-injected calcium-carbide powder. The pig iron is fed into a Linz-Donawitz (LD)-converter where the carbon content is lowered by injected oxygen. The steel is killed (deoxidized) by silicon and/or aluminium additions during the tapping from LD-converter to ladle. A synthetic slag is added and the ladle is moved to the Tyssen-Niederrhein (TN)-station where further deoxidation is performed and argon is injected through a submerged lance providing stirring which enhances sulphur removal. The sulphur rich slag is removed by skimming before the ladle arrives at either the Ladle Refining Furnace (LRF) or the Vacuum Tank Degasser (VTD). At the LRF and the VTD new slag is added, the steel heated and alloyed, and unwanted elements and particles are removed. After LRF or VTD the ladle is moved to a continuous caster where slabs are formed and cut. The slabs are rolled to plates, quenched and heat treated at the rolling mill. An overview of the production flow at SSAB Oxelösund is shown in figure 1.

Figure 1. Production flow at SSAB Oxelösund.

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Improving steel Today steel products are major parts of almost every section of society, thus the benefits of being able to improve steel properties are huge. Longer component lifetime in high wear environments, less moving weight by reduced component size and higher loading capacities are all possible if the component material is improved. In this study the aim is to improve the toughness of a particular type of steel which, for example, is used in high wear applications such as scrap containers, excavator buckets and dump truck boxes. However, high toughness and strength are two properties that are hard to combine. Strength, generally, is increased by limiting material deformation and toughness, generally, is increased by allowing increased material deformation. The large benefits are gained by finding solutions that improves one, or several, properties without the trade-off of decreasing other properties. One way of doing this is to reduce the content of undesired and harmful particles and elements. Depending on material, processing and application what is deemed harmful and undesired varies greatly. In steels the main toughness decreasing elements are S, P, O, N and H. Generally speaking, lowering the contents of those elements results in a higher quality of steel. (5, 6, 8, 9, 10, 11) Oxides and sulphides form inclusions in the melt which can act as crack nucleation sites in the finished steel component. Removal of inclusions is done by applying an oxide and sulphide absorbing slag on top of the melt. The slag also protects the melt from nitrogen, hydrogen and oxygen from the air. The main principle of inclusion removal is to maximize the steel-slag contact area. Removing old slag with high sulphur content and replacing it with fresh slag (dolomit, lime) further enhances the removal rate and efficiency. To achieve high slag-steel mixing the melt is stirred by injecting inert gas through porous plugs in the bottom of the ladle. Applying a vacuum above the slag surface improves the process even further, especially N and H removal. Several mechanisms of steel cleaning are active during gas injection.

Hydrogen and nitrogen removal Argon is, contrary to hydrogen and nitrogen, inert in molten steel and thus no injected argon remains in the melt. However, even though nitrogen and hydrogen dissolves in molten steel it is thermodynamically favourable for the two elements, to a certain degree and at a certain pressure, to be mixed in argon gas. This leads to hydrogen and nitrogen going from the melt to the argon bubbles which are rising to the surface. If a vacuum is applied above the surface of the melt there is a driving force towards a lowering of the hydrogen and nitrogen content in the steel. It is assumed in that hydrogen and nitrogen removal is not highly dependent on slag absorption of H and N. The efficiency of this process of nitrogen and hydrogen removal is depending on several factors. The lower the nitrogen content above the surface the higher the driving force will be in the direction of nitrogen removal, rather than pick-up. A low oxygen potential is favourable for nitrogen removal (20, 21), as is low contents of sulphur due to the high surface activity of the element (33). Too low injected argon gas purity and nitrogen will favour staying in the melt rather than to add to the argon-nitrogen injected gas. The argon gas bubbles have to come close enough to dissolved nitrogen and hydrogen, for a long enough time, otherwise diffusing particles can not reach the bubble before it has passed. The more bubbles rising through the melt, the more surface area and thus the closer the distance needed for diffusion from dissolved particles in the melt to the argon gas. The pressure of the liquid steel in the ladle is acting against gas formation in the melt.

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Inclusion removal The main principal of removing inclusions is to absorb them in the slag. To achieve this, the slag has to have a high affinity to sulphide and oxide inclusions in order to maintain a high driving force of inclusions going from steel to slag. To move inclusions from steel to slag the inclusions have to be brought into contact with the slag. In addition to being brought up to the slag, inclusions must also overcome an energy barrier to break the steel film before attaching to the slag. As a result only a fraction of inclusions being brought to the surface actually are trapped when passing through the slag (24). Injected gas in the bottom of the ladle creates circular motions of the steel melt which brings steel from the bottom of the ladle to the top and from the top down. Around the rising bubbles motion is turbulent which creates high degree of steel and slag mixing when they rise through the surface. The argon bubbles can also bring inclusions up with them through the melt in other ways. When a bubble rises through the melt it can hit an inclusion which may stick to the bubble surface, this is called bubble adhesion. This is due to the inclusion having a lower wetting angle to argon gas than to molten steel. Inclusions can also rise in the drag behind a bubble. Inclusions may also float up to the slag by themselves due to density differences between molten steel and inclusions. The larger the inclusion, the higher the buoyancy and thus the higher the removal rate by floating. Inclusions grow, when colliding with other inclusions, due to their low affinity to steel. Collisions may occur due to the turbulence created by bubbles rising through the melt, Stoke’s flotation collisions and due to Brownian motion. Collisions due to Brownian motion are considered of less importance than Stoke’s and turbulent collisions. Stoke’s flotation collisions occur when an inclusion floating up, due to buoyancy, is hitting another inclusion. (6, 25, 26, 29, 30, 31)

Vacuum tank degassing station The main reason for vacuum treatment is to get the contents of sulphides, oxides, nitrogen and hydrogen in the steel as low as possible. Several factors contribute to the efficiency of the VTD process such as steel conditions upon arrival, alloying procedure, slag properties, mixing dynamics and time. Upon arrival at the VTD station the ladle contains 180 – 200t molten steel and 2t sulphur rich slag from TN sulphur removal. The temperature of the steel in the ladle may vary between 1530-1600°C when it arrives to the VTD station depending on ladle status and delays. Steel composition naturally varies with steel type but also slightly within each type. To achieve good sulphide removal the slag needs to have a high sulphide affinity, a property that is more important the lower the sulphur content limit in the steel. This is why it is important to skim as much of the old sulphur rich slag as possible before a new slag is added during the pre-vacuum heating. The ladle to be skimmed is tilted forward to allow for a mechanical rake to reach in and drag slag over the ladle edge. However, the slag is partly or wholly fluid, depending on temperature, which makes complete slag removal impossible. It is the job of the rake operator to decide when the skimming procedure is complete, a decision based on the colour differences of slag and exposed steel and amount of smoke which starts to form when steel is exposed to air. After skimming the ladle is moved to a heating station. Graphite electrodes heat the melt while argon gas is injected through porous plugs in the bottom of the ladle. New slag is added and melted before alloying is performed, mainly using wire feeding. The gas stirring during heating is held at a low level to prevent the electrodes from heavy shaking and malfunction and also to minimize nitrogen pickup through exposed steel at the spout eyes.

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Vacuum degassing, and the addition of room temperature alloys, lowers the temperature of the molten steel. At SSAB Oxelösund a heat needs to fulfil a certain temperature criteria after treatment at the VTD station to be accepted to the continuous casting. Temperature control is achieved by heating with graphite electrodes or cooling by addition of metal scrap. Late addition of scrap is undesired as it increases the amount of unwanted elements in the steel. Operators at the VTD station aim to preheat the ladle enough before vacuum treatment to avoid a second heating session, but not as much as to require scrap addition before casting. When the desired temperature is reached the ladle is moved to the vacuum tank which is then sealed with a lid. The pressure is lowered to 0.4mbar and argon is injected through porous plugs. A camera is mounted in the vacuum tank lid providing a view of the liquid surface during degassing. The camera lens is cooled using nitrogen gas. Stirring is controlled by the operators by adjusting the argon flow based on visual observation of the liquid surface. Due to leakage in the argon supply and wear and clogging of the porous plugs the value of argon flow is not used to control the stirring, but rather the size of the spout eyes. Aluminium and silicon is added, to remove free oxygen from the steel, and alloying elements are added to get the desired composition. The vacuum degassing is done for 30 minutes and when completed the ladle is either moved directly to the continuous casting or, if needed, placed in the heating station again. Calcium and silicon is added before the ladle is moved to casting in order to avoid the formation of manganesesulphides. At the heating station the steel might get a final alloying addition, or just the extra needed heat before casting. During this final heating the argon stirring is at a low rate to avoid nitrogen pickup while homogenizing the melt.

Problem description To achieve a better product and production, in this case tougher plate steel and better removal of unwanted elements, it is essential to understand the consequences of parameter variation in the various steps of manufacture. Even though many studies and models of inclusion, H and N removal during gas stirring have been carried out, most of them contain assumptions that make direct application in industry impossible. Leakage in the argon supplying, clogging in the porous plugs, refractory wear, differences in composition and cleanliness are all unavoidable in practise but near impossible to take into account in modelling. Therefore it is important to do full scale trials of different argon stirring practises, along with microscopy studies of inclusion characteristics. Since argon flow rate can not be used directly as an indication of stirring it is important to validate the use of spout eyes size as a means to control the stirring in the ladle. This to achieve not only a higher quality product, but also a better understanding of the mechanisms involved during vacuum tank degassing.

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Experimental procedure Slag skimming analysis, experimental setup of blowing procedures, sampling, image analysis and sample preparation and mechanical testing procedure is presented.

Slag skimming The slag skimming procedure was either video recorded in whole or photographed when the skimming operator had finished skimming. A three level scale describing the degree of slag skimming was established based on slag free surface, smoke rising from exposed steel and perceived slag thickness. Figures 2-4 shows three ladles that have recently been skimmed, one from each grade of skimming.

Figure 2. Example of low amount of slag left after skimming.

Figure 3. Example of medium amount of slag left after skimming.

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Figure 4. Example of high amount of slag left after skimming. In figure 2 the grade low amount of slag left was based on the large exposed surface, the smoke and the fairly thin layer of remaining slag. The grade medium amount of slag left shown in figure 3 was based on the smaller amount of uncovered steel. Figure 4 shows a ladle with little uncovered steel and thick remaining layers of slag.

Experimental set up In each ladle two porous plugs, placed opposite each other on the diagonal, are used during the vacuum tank degassing process. Three different argon blowing procedures were tried in full scale production. Operation of the gas blowing during vacuum degassing is based on visual observation of the liquid surface in the ladle from a camera situated in the vacuum tank lid. The camera lens is cooled and protected by nitrogen gas. If the argon gas flow is too high the steel and slag will splash, from bubbles breaking the surface, hit the vacuum tank lid, solidify, and a need to manually remove the slag will arise. Too low gas flow and the steel-slag interface is small and, in theory, removal of unwanted elements inefficient. Two different kinds of blowing, high and low, from the porous plugs were used in this study to produce the different blowing procedures. High blowing from a plug was said to be achieved when the spout eye was as large as possible, without risking splashing of slag and steel onto the tank lid. Low blowing from a plug was said to be achieved when the gas flow was as low as possible, while still having a visible spout eye at all times. The three different blowing procedures were as follows:

High-Low (H-L). High blowing through one plug and low blowing through the other during the

whole 30 min vacuum degassing.

Low-low (L-L). Low blowing through both plugs during the whole 30 min vacuum degassing.

High-high, low-low. (H-H, L-L). High blowing through both plugs during the first 15 minutes of

the vacuum degassing followed by low blowing through both plugs during the remaining 15

minutes.

The level of gas flow was monitored and adjusted by an operator during the whole vacuum degassing step to fit the desired blowing procedure for each tested charge. The video signal from the camera in the vacuum tank lid was recorded during the vacuum degassing step to enable the option of image analysis of the steel surface in the ladle.

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Furthermore, standard operating procedure has been H-H at SSAB Oxelösund with the exception of a period of H-L blowing. Data from regular charges produced using H-H and H-L blowing is presented in this report and provides comparable data to experimental charges.

Steel sampling Lollipop samples are taken from the steel melt at several stages throughout the steelmaking process. The samples are analysed with Optical Emission Spectroscopy (OES) with spark excitation and Leco in the case of N and S. Only the last sample at TN, VTD and in the tundish has been used in this study, for test charges and regular charges.

Image analysis of degassing During vacuum degassing the operators can view the slag surface in the ladle from a camera mounted in the vacuum tank lid. This allows the operators to adjust argon flow rate based on the desired spout eyes to achieve effective degassing and inclusion removal without too much splashing of the tank lid. The video signal from the vacuum lid camera was recorded for analysis of the spout eyes during vacuum degassing. Using the software VLC media player, every 250’th frame - corresponding to every 10’th second, was captured from the 30 minutes of vacuum degassing videos of 7 charges. An average of 180 images were analysed per charge. Spout eye area, in pixels, was measured and divided by the total slag surface area, in pixels, to get the percentage of spout eye coverage of the slag surface every 10’th second. This was done using the software ImageJ. Solidified slag on the camera lenses prevented image analysis of some charges since the image of one spout eye was covered by slag stains. Video quality was still good enough for operators to achieve the desired stirring. An example of the image analysis is shown in figure 5. The large ellipse is the area of the slag surface and is used as a reference area for the spout eyes. It is measured once for every charge on the first image after the surface has gone down to a stable level after the initial rise due to applied vacuum and gas injection. Spout eye area is measured on every picture as shown in figure 5. The spout eye in the lower left corner is in this study considered as spout eye 1 and the spout eye in the upper right corner spout eye 2. If the two spout eyes have merged to one, the spout eye area is considered the area of one spout eye while the other spout eye is assigned 0 pixels area in that particular frame.

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Figure 5. Example of area selections in a H-L, the reference surface and spout eyes, in ImageJ.

Sample preparation To be able to measure steel cleanliness, by slag counting, samples from rolled plates were taken. For each charge a sample was cut from rolled plate made from that particular charge. The samples were grinded, then polished 4 minutes using 15µm diamonds, 4 minutes using 6µm diamonds and finally polished 3 minutes using 1µm diamonds. Care was taken to clean the sample thoroughly with alcohol to remove residual lubrication and dust that could otherwise be mistaken for inclusions. Samples were studied in an optical microscope and inclusions classified according to SS 11 11 16. A

total of 100 unique sights were scanned for inclusions on each sample, resulting in a scanned area of

50mm^2. Inclusions are classified based on their shape, size and quantity. Sulphides are prolonged in

the rolling direction while oxides are spherical.

The mechanical tests are carried out by SSAB Oxelösund’s Provhuset. Toughness is measured using Charpy-V impact testing. The resulting energy mean value (EMV) is the mean value of three impact tests carried out on identical rods of plate steel. Several sets of rods are cut and tested from various plates from most produced charges. Rods are cut orthogonal to the rolling direction and grinded to the desired dimensions. Test rods used in this study are exclusively B-type with a square area of 5*10mm^2. Impact testing is done at -40C. All toughness results presented are, contrary to OES and Leco results, based on testing on only the same internal steel grade as experimental charges. Hardness is measured in Brinell and each sample from rolled plate is measured three times with a resulting hardness mean value (HMV). Several hardness samples are taken from plates in each charge.

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Results In this chapter the results from all parts of the study are presented.

Chemical composition The results from Leco and OES analysis of lollipop samples taken at various stages during the steelmaking process are presented here.

Test charges

Figure 6. Nitrogen content after VTD vs N after TN for different blowing procedures. In figure 6 it can be seen that H-H,L-L argon blowing produces better results for the same nitrogen content after TN than the L-L blowing procedure. Blowing procedure H-L can be seen to achieve the same nitrogen content after VTD as L-L blowing procedure but for a higher nitrogen content after TN. The line represents NVTD = NTN.

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Figure 7. Nitrogen content in steel in tundish vs nitrogen content after degassing. There is little change in nitrogen content between the last sample at the VTD station and the last sample taken in the tundish, as can be seen in figure 7. The line represents a 1:1 ratio.

Figure 8. Hydrogen content after various blowing procedures. In figure 8 it can be seen that the blowing procedure L-L does not remove as much hydrogen as does the blowing procedures H-L and H-H,L-L. Lowest levels of hydrogen in the tundish was achieved in charges subjected to H-H,L-L argon blowing. Mean values are 1,8, 2,2 and 1,5ppm H for H-L, L-L and H-H,L-L blowing procedures respectively. If the lower limit in a two-sided Student’s t-test of 95% confidence is higher than 0 for the difference in means a significant difference can be seen, as in the case shown below for L-L charges having higher H content than H-H,L-L charges. Lower limit: 2,092 - 1,489 - 1,96*sqrt(0,460^2/7 + 0,140^2/8) = 0,249

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Figure 9. Sulphur content after various blowing procedures. As can be seen in figure 9, the sulphur content of the melt after degassing is dependent on the sulphur content in the melt before degassing if the sulphur content before degasing is high. L-L blowing procedure gave the lowest sulphur content after degassing out of the three experimental procedures. Lines show 1:1 ratio and a linear approximation of the sulphur content using all charges.

Figure 10. S content after VTD depending on S content before VTD for various degrees of skimming. The degree of skimming based on the three level scale Low, Medium and High slag amount can

partially explain the sulphur cleaning ability during degassing. It can be seen in figure 10 that none of

the charges rated with a High slag amount left after skimming reached Sulphur contents lower than

7ppm after degassing.

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Regular charges Standard operating blowing procedure at the VTD station is H-H. However, during a period a H-L blowing procedure was used. Data for charges treated at the VTD station between 1 January 2010 and 19 September 2010 are presented as comparison to the charges produced using experimental blowing procedures.

Figure 11. Nitrogen before and after degassing. H-H blowing.

Figure 12. Nitrogen before and after degassing. H-L blowing. It can be seen that there was no large difference in nitrogen removal between the blowing procedures of H-H and H-L by comparing figure 11 and 12. The line represents 1:1 ratio of NVTD and NTN. Nitrogen content is lowered during degassing for a majority of the charges, especially so for charges with high N after TN. Mean values of N content for regular charges during the period of 1 Jan to 19 Sep are presented in table 1.

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Table 1. Mean values of N content for regular charges.

Blowing Mean NVTD (ppm) Mean NTN (ppm) # charges

H-H 26,89 30,16 1201

H-L 27,00 28,38 217

Table 1 shows that H-H blowing reaches lower mean N levels after degassing than H-L blowing, even though the mean N content before degassing was higher during the H-H period.

Figure 13. S content after VTD vs S after TN. H-H blowing. Figure 13 shows S content in the steel before and after degassing during a period when H-H blowing procedure was used. A majority of charges have sulphur contents lowered to below 10ppm after degassing. It can also be seen in figure 13 that charges with an S content of 25ppm or higher, after TN, do not reach the lowest values of S after degassing.

Figure 14. S after VTD vs S after TN. H-L blowing.

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Figure 14 shows a similar relationship between sulphur contents before and after degassing when H-L blowing is used as when H-H blowing is used as shown in figure 13. In general, higher sulphur content after TN results in higher sulphur content after VTD. Mean values of sulphur contents are shown for regular charges in table 2. Table 2. Mean values of S content for regular charges.

Blowing Mean SVTD (ppm) Mean STN (ppm) # charges

H-H 6,69 21,81 1201

H-L 6,61 17,38 217

H-H blown charges can in table 2 be seen to have a higher mean final S content than H-L blown charges. However, the mean S content before degassing is also higher during the period of H-H blowing compared to the period of H-L blowing.

Figure 15. H in tundish for charges processed in VTD. When the H-L blowing procedure was introduced H levels in the tundish became slightly elevated compared to charges produced with H-H blowing, as can be seen in figure 15. However, after reinstating H-H as the standard blowing procedure H levels did not drop compared to the H-L blowing procedure. Charges produced after the summer stand still have higher H contents in the tundish than charges produced before, even though an H-H blowing procedure is to be used. Mean values are presented in table 3. Table 3. Mean values of H content for regular charges.

Blowing Mean H (ppm) # charges

H-H 1,55 1201

H-L 1,57 217

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Degree of stirring Results describing the degree and measurement of ladle stirring during degassing will be presented here. Results from image analysis of spout area are shown as are results from calculated contents of oxidized aluminium.

Image analysis The total spout eyes area divided by reference area, as shown in figure 5, was measured every 10’th second during vacuum degassing for some charges. Results are presented in figures 16-22.

Figure 16. Ratio of spout eyes to visible surface area during vacuum degassing, (H-L).

Figure 17. Ratio of spout eyes to visible surface area during vacuum degassing, (H-L).

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Spout evolution #58059

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Spout evolution #58382

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Figure 18. Ratio of spout eyes to visible surface area during vacuum degassing, (H-L).

Figure 19. Ratio of spout eyes to visible surface area during vacuum degassing, (L-L).

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Spout evolution #58386

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Spout evolution #58453

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Figure 20. Ratio of spout eyes to visible surface area during vacuum degassing, (L-L).

Figure 21. Ratio of spout eyes to visible surface area during vacuum degassing, (H-H,L-L).

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Spout evolution #58461

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Spout evolution #58697

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Figure 22. Ratio of spout eyes to visible surface area during vacuum degassing, (H-H,L-L). As can been seen in figures 16-22 spout eyes are highly dynamic over time. In figure 22, for charge 58790, an approximation of the L-L argon injection was made due to loss of video data. The approximation is equal to the mean value of the same period for charge 58697, seen in figure 21. Mean values of the total percent surface area in the ladle covered by spout eyes are presented in table 4. Table 4. Mean spout eyes area.

Charge # Injection mode Mean spout eyes area (%)

58059 H-L 8,53

58382 H-L 21,93

58386 H-L 15,55

58453 L-L 6,32

58461 L-L 9,50

58697 H-H, L-L 13,45

58790 H-H, L-L 11,7 - Estimated

Mean total spout eye area can be compared to the measured mean total argon flow as shown in figure 23.

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Spout eyes area (%)

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Spout evolution #58790

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Figure 23. Total mean spout eyes area vs total mean argon flow. No significant correlation can be seen in figure 23 between total spout eye area and mean total argon flow. A comparison between spout eyes and injected gas through the respective porous plug can be made and is shown in figure 24. However, in the case of merged spout eyes all the area was assigned to one eye, as described.

Figure 24. Spout eye vs argon flow from respective porous plug. Figure 24 shows a trend of increasing spout eye area with increased argon flow. Spout eye area and Argon flow are mean values over the 30 minutes degassing time.

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Spout eyes area (%)

Ar (Nl/min)

Spout eye vs Ar flow

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Spout eye area (%)

Ar (Nl/min)

Spout eye vs Ar flow

Eye 1

Eye 2

25

Oxidized aluminium Aluminium in the melt is oxidized by free oxygen and when coming in contact with oxides in the slag. The amount of oxidized Al (OxAl) is the difference between Al content after TN plus Al additions at the VTD station and the Al content after degassing.

Figure 25. Oxidized Al vs total spout eye area %. Figure 25 shows larger oxidized aluminium amounts during degassing for charges with larger total spout eye area. The amount of aluminium that was oxidized at the VTD station can thus be used as an indicator of the degree of stirring.

Figure 26. Al after VTD vs Al content after TN plus Al additions at the VTD. 1Jan – 19Sep. In figure 26 it can be seen that alloying towards different final Al contents is done with success for most charges. Table 5 shows the mean values of oxidized aluminium for H-H and H-L blowing periods.

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OxAl (ppm)

Spout eye area (%)

Al oxidation vs spout area

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Al oxidation in VTD

26

Table 5. Al oxidation 1 Jan – 19 Sep.

Mean OxAl (ppm) STDEV # charges

H-H 820 183,9 1201

H-L 797 155,4 217

If a Student’s t-test of 95% confidence is applied it can be seen that the lower limit of the difference

between the means in Table 5 is less than 0 and thus no significant difference can be observed.

Lower limit: 820-797 – 1,96*sqrt(183,9^2/1201 + 155,4^2/217) = -0,145

Impact of stirring Results from evaluation of the effect of stirring degree on VTD cleaning ability are presented here. Test charges are evaluated using both total spout area % and amount aluminium oxidized. Charges produced in the period 1 Jan – 19 Sep are evaluated using amount of oxidized aluminium.

Test charges Total spout eye area % and Al ox are used along with observed correlations to present results of the impact of stirring on VTD cleaning for test charges. Since it has been shown that the nitrogen content after degassing is not independent of the nitrogen content before degassing, figure 6, an adjustment of the nitrogen levels after degassing is needed to evaluate the spout eyes area % influence on nitrogen removal. An adjustment was made based on a linear fit of nitrogen after degassing as a function of nitrogen before degassing and resulted in the estimated Nadj, eq 1. Nadj = NVTD – 0,45*NTN (1)

Figure 27. Adjusted N content after VTD vs mean total spout eye area %.

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Nadj (ppm)

Spout eyes area (%)

Nitrogen removal

27

As can be seen in figure 27, a large total spout area results in good nitrogen removal during degassing. The charge with largest measured spout eye area, square in figure 27, was alloyed during reheating after degassing due to machine problems.

Figure 28. Hydrogen content in tundish vs mean total spout area %. A clear correlation between spout eye area and hydrogen content can be observed in figure 28. Low spout eye area % results in bad hydrogen removal during degassing. The charge with largest measured spout eye area, square in figure 28, was alloyed during reheating after degassing due to machine problems. Applying the same logic for sulphur as for nitrogen an estimated sulphur value (SAdj) based on final and starting sulphur content, shown in eq 2, was compared to the total spout eye area % as shown in figure 29. SAdj = SVTD – 0,19*STN (2)

Figure 29. Adjusted S content after VTD vs mean total spout eye area %.

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HTundish (ppm)

Spout eyes area (%)

Hydrogen removal

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SAdj (ppm)

Spout eyes area (%)

Sulphur removal

28

Figure 29 shows no large influences of spout eye area on the sulphur removing ability at the VTD.

Figure 30. Adjusted nitrogen content vs oxidized Al for different blowing procedures. Figure 30 shows adjusted nitrogen content, according to eq 1, vs oxidized Al for different blowing procedures. It can be seen that low amounts of oxidized Al are detrimental to the nitrogen removal ability of the VTD. L-L blowing can be seen to be the worst in terms of nitrogen removal efficiency. The seemingly out-of-place value at 20,4ppm NAdj, 1042ppm OxAl, was alloyed during reheating after degassing.

Figure 31. Hydrogen in tundish vs oxidized Al for different blowing procedures. Figure 31 shows final hydrogen levels in tundish for different blowing procedures and amount of oxidized aluminium. It can be seen that low hydrogen removal is connected to the amount of oxidized Al. Higher amount of oxidized aluminium corresponds to lower final hydrogen levels. It can also be seen that H-H,L-L blowing procedure results in slightly lower hydrogen contents than other options for the same OxAl levels.

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NAdj (ppm)

OxAl (ppm)

Nitrogen removal

H-L

L-L

H-H,L-L

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HTundish (ppm)

OxAl (ppm)

Hydrogen removal

H-L

L-L

H-H,L-L

29

Figure 32. Adjusted nitrogen content vs oxidized Al for different blowing procedures. No correlation between adjusted sulphur content, SAdj, and oxidized Al can be seen in figure 32.

Regular charges Amount of Al ox is used along with observed correlations to present results of the impact of stirring on VTD cleaning for charges produced during the period of 1 Jan to 19 Sep.

Figure 33. Nitrogen after VTD vs amount oxidized Al. 1 Jan – 19 Sep. Figure 33 shows a decrease in nitrogen content of the melt following high amounts of oxidized Al.

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SAdj (ppm)

OxAl (ppm)

Sulphur removal

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NVTD (ppm)

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Nitrogen removal

30

Figure 34. NTN/NVTD vs OxAl. 1 Jan – 19 Sep. Figure 34 shows no obvious trend of increasing NTN/NVTD ratio with increasing OxAl.

Figure 35. NTN/NVTD vs SVTD. 1 Jan – 19 Sep. In figure 35 it can be seen that a net nitrogen pickup is more likely to occur the higher the S content.

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Nitrogen level change

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Nitrogen level change

31

Figure 36. Hydrogen in tundish vs amount oxidized Al. 1 Jan – 19 Sep. It can be seen in figure 36 that H contents are slightly higher for charges with lower amounts of oxidized Al. Most charges have a final hydrogen content of 1 - 2ppm.

Figure 37. Sulphur after VTD vs amount oxidized Al. 1 Jan – 19 Sep. The final sulphur content can be seen, in figure 37, to decrease slightly with increasing OxAl.

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Figure 38. STN/SVTD vs OxAl. 1 Jan – 19 Sep. Figure 38 shows that the higher the amount of oxidized Al the higher the minimum ratio of STN/SVTD.

Statistical description Results from MiniTAB regression analyses of hydrogen, nitrogen and sulphur removal during degassing in charges produced in the period 1 Jan to 19 Sep are presented. Charges with nitrogen content outside the boundaries shown in 3 are not considered in the evaluation. Mean(NVTD) - 2*StdDev(NVTD) < NVTD < Mean(NVTD) + 2*StdDev(NVTD) (3)

Hydrogen The regression equation is HTund = 1,39 - 0,000165*OxAl + 0,000030*Time + 0,0167*SVTD + 0,0115*NVTD - 0,00950*NTN (4) Predictor Coef SE Coef T P Constant 1,39391 0,07359 18,94 0,000

OxAl -0,00016464 0,00004834 -3,41 0,001 Time 0,00002952 0,00000560 5,27 0,000 SVTD 0,016662 0,002884 5,78 0,000 NVTD 0,011487 0,001845 6,23 0,000 NTN -0,009500 0,001188 -8,00 0,000 S = 0,290035 R-Sq = 13,0% R-Sq(adj) = 12,7% Analysis of Variance Source DF SS MS F P Regression 5 16,2796 3,2559 38,71 0,000 Residual Error 1296 109,0199 0,0841 Total 1301 125,2995

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STN/SVTD

OxAl (ppm)

Sulphur level change

33

Equation 4, obtained through regression analysis, shows a general trend of decreasing hydrogen levels in the tundish with increasing amounts of oxidized aluminium and better nitrogen removal during degassing. Higher sulphur content after degassing predicts higher H content in the tundish. Hydrogen pickup during reheating is predicted as H increases with time. The parameter “Time” is measured from vacuum start to the time of the last sampling before the ladle leaves for casting.

Nitrogen As was shown above hydrogen content in the tundish can be seen to correlate with nitrogen removal during degassing. However, since it is assumed that hydrogen and nitrogen removal are based on the same mechanism, and independently, hydrogen content is not used in the attempt to explain final nitrogen content after degassing. The regression equation is NVTD = 13,2 + 0,519*NTN - 0,000282*NTN*OxAl + 0,303*SVTD + 0,0528*STN + 0,000334*Time (5) Predictor Coef SE Coef T P Constant 13,1696 0,7057 18,66 0,000 NTN 0,51938 0,02255 23,04 0,000 NTN*OxAl -0,00028232 0,00002365 -11,94 0,000 SVTD 0,30312 0,04758 6,37 0,000 STN 0,05276 0,01308 4,03 0,000 Time 0,00033394 0,00008290 4,03 0,000 S = 4,39787 R-Sq = 34,8% R-Sq(adj) = 34,5% Analysis of Variance Source DF SS MS F P Regression 5 13745,3 2749,1 142,13 0,000 Residual Error 1332 25762,5 19,3 Total 1337 39507,9 The regression analysis of final nitrogen content after the VTD shows a strong correlation to nitrogen content before degassing. However, the more aluminium is oxidized the lower the dependence of starting N content. Higher sulphur content in the melt indicates higher final nitrogen content. Nitrogen pick-up during reheating is predicted.

34

Sulphur It is assumed that nitrogen content does not influence the sulphur removal, even though sulphur is harmful for nitrogen removal. The regression equation is SVTD = 2,88 + 0,226*STN - 0,000104*STN*OxAl + 0,000158*Time (6) Predictor Coef SE Coef T P Constant 2,8754 0,3022 9,52 0,000 STN 0,22556 0,01506 14,98 0,000 STN*OxAl -0,00010393 0,00001394 -7,46 0,000 Time 0,00015835 0,00004723 3,35 0,001 S = 2,51983 R-Sq = 24,3% R-Sq(adj) = 24,2% Analysis of Variance Source DF SS MS F P Regression 3 2722,03 907,34 142,90 0,000 Residual Error 1334 8470,29 6,35 Total 1337 11192,32 Final sulphur content after degassing can be seen to be highly dependent on sulphur content before degassing. However, the regression analysis suggests that the higher the amount of oxidized aluminium the lower the dependence of final sulphur content on starting sulphur content. The final sulphur content is also, in general, increasing with longer reheating times after degassing.

35

Inclusions Results from SS 11 11 16 slag inclusion classifications are presented here along with correlations with predicted indicators of stirring for experimental charges.

Figure 39. Total inclusion density vs OxAl for different blowing procedures. As can be seen in Figure 39, the number of inclusions is not directly proportional to the amount of oxidized aluminium. Moreover, in figure 40 below, it can be seen that the blowing procedures L-L and H-H,L-L tend to provide cleaner steel in terms of total number of inclusion than the H-L blowing procedure.

Figure 40. Total inclusion density for different blowing procedures.

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Inclusion vs OxAl

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36

Figure 41. Large inclusion density vs total other inclusion density. Inclusions classified as DM or DH using SS 11 11 16 are shown, in figure 41, to increase with the total number of other inclusions. Total mean density of inclusions are shown in table 6.

Figure 42. Total sulphide length vs total inclusion density. It can be seen in figure 42 that H-H,L-L blown charges have the least total sulphide length. Total sulphide length can also be seen to increase with increasing total inclusion density.

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Sulphide lenght vs Inclusion denisty

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Figure 43. Sulphide length vs SVTD for different blowing procedures. Figure 43 shows H-H,L-L charges having equal or lower total sulphide length than L-L charges. H-L charges can be seen to have a higher total sulphide length than L-L and H-H,L-L charges. However, H-L charges are also seen to have higher final sulphur content.

Figure 44. Sulphide density vs SVTD for different blowing procedures. In figure 44 it can be seen that H-H,L-L and H-L charges have equal sulphide density as L-L charges but with a higher SVTD. H-H,L-L charges have lower sulphide density than H-L charges. Table 6. Mean values of inclusion density for different blowing procedures.

Mean total incl [#/mm^2]

Mean large oxide incl [#/mm^2]

Mean total sulphide length [mm/mm^2]

Number of samples

H-L 2,03 0,15 0,0059 4

L-L 1,55 0,14 0,0027 7

H-H,L-L 1,20 0,09 0,0006 4

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38

Table 6 shows mean values of the total inclusion density, total large inclusion density and total sulphide density of measured samples. Samples from charges subjected to H-H,L-L argon blowing had the lowest mean total inclusion density and the lowest mean sulphide inclusion density along with the lowest sulphide to total inclusions ratio of the three experimental blowing procedures.

Mechanical testing Results from mechanical testing, such as Charpy-V toughness tests and Brinell hardness tests, are presented here for test and regular charges. Results are evaluated with respect to N, inclusions and blowing procedure.

Test charges

Figure 45. Toughness vs nitrogen content after degassing for different blowing procedures. In the case of several toughness measurements on several plates from the same charge, all were included in figure 45 and 46. As can be seen in figure 45, toughness of rolled plate is highly dependent on the nitrogen content of the steel. Lowering the nitrogen content increases the toughness. Highest toughness was measured in H-H,L-L subjected charges. However, in order to investigate the effect of different blowing procedures conditions should be equal for all reference material. Equation 5, obtained through regression analysis, is rewritten with the nitrogen indicator (NInd) replacing the constant term, as shown in equation 7. NVTD – (0,569*NTN - 0,000377*NTN*OxAl + 0,0748*SVTD + 0,0703*STN + 0,00388*OxAl + 0,000181*Time) = NInd (7) Since NVTD, NTN, OxAl, STN, SVTD and Time are all known parameters for each charge, NInd can be used, instead of NVTD, to compare charges with respect to nitrogen content with different chemical composition before degassing.

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39

Figure 46. Toughness vs NInd for different blowing procedures. As can be seen in figure 46 toughness increases with better nitrogen removal, i.e. lower NInd. NInd for L-L produced charges can be seen to be higher than for H-L and H-H,L-L produced charges. No significant difference in toughness can be noted in figure 45 between H-L and H-H,L-L produced charges with equal NInd. However, H-H,L-L produced charges generally obtained lower NInd values than H-L produced charges. In order to evaluate possible effects of inclusions on toughness an adjusted value (EMV BAdj) was used which was derived from a linear fit of mean EMV B dependence on N content after degassing, eq 8. EMV BAdj = EMV B + 0,34*NVTD (8)

Figure 47. Adjusted toughness vs total inclusion density for different blowing procedures. Figure 47 shows no correlation between total inclusion density and the adjusted toughness value.

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Toughness vs NInd

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Toughness vs inclusion density

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40

Figure 48. Adjusted toughness vs total sulphide length for different blowing procedures. Figure 48 shows no correlation between total sulphide length and the adjusted toughness value.

Figure 49. Adjusted toughness vs total large inclusion density for different blowing procedures. Figure 49 shows no correlation between large inclusions density and the adjusted toughness value. Hardness and toughness are generally not measured on pieces from the same plate of steel from each charge. In this case the mean toughness for each charge is set compared to all hardness measurements of the same charge, as shown in figure 50.

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Toughness vs sulphide length

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Large inclusion density (#/mm^2)

Toughness vs DMDH

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41

Figure 50. Hardness vs NVTD for different blowing procedures. Figure 50 shows hardness measurements vs NVTD for charges produced using different blowing procedures. It can be observed that hardness increases slightly with increasing nitrogen content. Statistical data of hardness measurements are presented in table 7. Table 7. Hardness values for different blowing procedures.

Mean hardness (Brinell)

StdDev #

H-L 449,1 8,280 182

L-L 447,6 7,124 243

H-H,L-L 446,0 6,645 234

As can be seen in table 7 hardness is higher for charges produced using H-L blowing than for charges produced using H-H,L-L blowing.

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430

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475

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Hardness (Brinell)

NVTD (ppm)

Hardness vs NVTD

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42

Regular charges

Figure 51. Toughness, B pieces, for internal steel grades 959 and 956. 1 Jan – 19 Sep 2010. As can be seen in figure 51 results from samples of H-L produced charges did not drop below 20J in EMV B and are less scattered than results from H-H produced samples. Mean values of impact toughness on type B pieces of internal steel grades 959 and 956 are presented for the period of H-L and H-H in table 8. Table 8. Mean toughness values.

Mean EMV B (J)

StdDev EMV B

Mean NVTD (ppm)

Mean NTN (ppm)

Samples (#)

H-L 22,2 1,58 26,1 26,6 25

H-H 21,9 2,82 25,3 26,1 209

As can be seen in table 8, no difference in mean toughness between H-H produced and H-L produced charges can be observed in EMV B samples.

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EMV B (J)

Order from 1 Jan - 19 Sep

Toughness grades 959 & 956

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43

Discussion Optimization of the VTD station performance includes, but is not limited to, ensuring that the N, H and S removing ability is as high and consistent as possible by using the best process operation. Lowest nitrogen and hydrogen levels were achieved in H-H,L-L charges, as seen in figure 6 and 8, while S levels were lowest in L-L charges, as seen in figure 7. However, as shown in figure 13 and 14, for N and S respectively in regular charges, final contents depend on contents before degassing. In the case of nitrogen this may indicate that no equilibrium is reached between dissolved nitrogen and atmosphere above the surface, since nitrogen levels in the atmosphere can be assumed to be constant due to the applied vacuum. However, the final N content after degassing is seldom 20ppm or lower, figure 33. H-H,L-L charges had the lowest N content after degassing, but also the lowest N content before degassing. If N levels before degassing is taken into consideration, eq 1, nitrogen removal is seen to depend on the spout eye area, figure 27, and the amount of oxidized Al, figure 30. This is confirmed in figure 33 for regular charges. Even though H-H charges had higher mean NTN than H-L charges the final mean NVTD reached equal levels, table 1. Ono-Nakazato et al. (20) concluded that, based on experiments of Al additions to molten iron, nitrogen removal rate is increased by the lowering of the oxygen partial pressure following Al additions. Fan et al. (21) reported, based on static atmosphere experiments, very low slag absorption of nitrogen for CaO-Al2O3 slags compared to slags with TiOx. This validates the assumption made in this study that the N removal due to slag absorption is negligible compared to the degassing removal. Regression analysis of nitrogen suggests that the better the stirring and the lower oxygen potential, indicated by a higher amount of oxidized Al, the lower the dependence of the nitrogen content before degassing. It also shows nitrogen pick-up during reheating and a detrimental effect of sulphur in the melt on nitrogen removal, which can also be seen in figure 35. This is consistent with theoretical calculations by Goldstein et al. (33) showing that nitrogen removal rate is dependent on sulphur content. In (12) a table of steel composition data for heats before and after argon stirred vacuum degassing is presented by Steneholm et al. which, if analysed, shows a trend of better N removal with higher amounts of oxidized Al and lower S content, similar to results in this study. The importance of alloying during degassing is made evident by the high NAdj value compared to spout eye area for the one charge when alloying was carried out during reheating, figure 27. Hydrogen content is considerably higher in some L-L charges than in other experimental charges, figure 8, while H-H,L-L is seen to reach the lowest H levels after degassing. Hydrogen content can be seen to correlate with spout eye area, figure 28, and can also be linked to amount of oxidized Al, figure 31. The trend of lower H content with higher amounts of oxidized Al can also be seen in figure 36 for regular charges. However, when comparing H-L regular charges to H-H regular charges, figure 15 and table 3, no significant difference is observed. Regression analysis predicts lower H contents with higher amounts of oxidized Al, along with predictions of H pick-up during reheating, higher H content with higher S in the melt and lower H content with better N removal. Even though regression analysis of H has a low coefficient of determination it shows that the assumptions of H and N removal mechanisms being similar to be true. Sulphur is removed from the melt through absorption in the slag. This implies that sulphur removal will be lower the higher the sulphur content already dissolved in the slag. The three level slag skimming scale confirms that if a high amount of sulphur rich slag is left before degassing final sulphur levels are not as low as they could have been with more skimming, figure 10. Grading the skimming was often difficult when only a final image was available, video material are preferred. The adjusted S level of experiment charges shows no correlation with spout eye area, figure 29, or oxidized Al, figure 32. The one L-L charge that had a negative SAdj value shows that a linear fit of SVTD

44

depending on STN is not perfect. Figure 37 shows a clear correlation between oxidized Al and final S content for regular charges. Sulphur dependence on oxidized Al is seen in figure 38 where the minimum ratio of STN/SVTD is increasing with increasing amounts of oxidized Al. In table 2 it can be seen that even though H-H charges has a higher mean STN than H-L charges the mean value of SVTD does not differ much between the two blowing procedures. Regression analysis of sulphur suggests that increased amounts of oxidized Al correspond to decreased dependence of starting sulphur content on the final sulphur content. Image analysis of the spout eyes proved to be more difficult than expected as the lens protecting the camera in one of the vacuum tank lids was stained and blocked the spout eyes for some charges. Image analysis was impossible for all charges degassed under the other vacuum lid due to bad lens focus. Even so, the image analysis performed provides useful data adding to the understanding of the degassing process. Figures 16-22 shows that the spout eyes are highly dynamic over time. When comparing individual spout eyes to the measured argon flow through respective porous plugs, figure 24, the expected trend of larger spout eyes with higher argon flow is seen. This trend is also reported by Thunman et al. (23) in a laboratory study of the slag entrainment spout eyes dependence. Yonezawa et al. (22) describes laboratory experiments of spout eye dependence on geometry and gas flow with good precision but when the same description is used to explain spout eyes in a 350t ladle only a trend of increasing eye area with gas flow can be seen. However, during image analysis merged spout eyes were assigned to one spout eye 1. Measured values of eye 2 can thus be too low compared to the argon flow through plug 2 if the spout eyes were often merged during degassing, with spout eye area of eye 1 in those cases being too high compared to argon flow through plug 1. Total spout eye area is seen to correlate well with the amount of oxidized Al, figure 25. This proves that the amount of oxidized Al can be used as an indicator of the degree of stirring during degassing. A higher degree of stirring results in more Al coming in contact with the slag where it can be oxidized. This becomes evident when looking at figures 26, 34 and 38. Operators at the VTD station manage to reach the desired levels of Al before casting regardless of the stirring and amount of oxidized Al. With higher amounts of oxidized Al final nitrogen levels are lower and the minimum sulphur level change higher. No difference in the amount of oxidized Al is observed between H-L and H-H produced regular charges, table 5, and the small difference in final nitrogen content and toughness, table 8, can be explained by the difference in NTN. This leads to the conclusion that as long as the degree of stirring is high enough, and the amount of free oxygen low enough, optimum performance in the sense of N, H and S removal can be achieved with less than H-H blowing for the entire duration. Inclusion density is lowest in H-H,L-L charges with L-L charges having lower inclusion density than H-L charges, figure 40. This indicates that a high degree of turbulence causes inclusion collision and growth while during rinse stirring inclusion removal due to flotation is higher than the rate of inclusion growth due to collisions. Only a few inclusions of class DM or DH were found in each sample, ranging from 2 to 22 per sample (50mm^2) with all but two samples having 8 or less. Miki et al. (14) reported increased inclusion removal rate with increased gas flow by theoretical calculations verified in a plant RH-degasser. Zheng et al. (25) performed physical model experiments with various gas flows and reported fastest inclusion removal rates for low gas flows with less than 10 minutes gas stirring, and similar final inclusion levels, regardless of gas flow rate, after 16 minutes of gas stirring. Theoretical calculations of inclusion growth (29, 30, 31) all indicate that inclusion collisions due to turbulence is the major source of inclusions above 5µm. Steneholm et al. (12) reported inclusion numbers to reach a plateau after 10 min vacuum degassing of 65t ladles but that DH class inclusions did not diminish in numbers during treatment. H-H,L-L blowing procedure having the lowest number of inclusions can thus be attributed to a high gas flow for more than 10 minutes which effectively reduces the number of inclusions and a rinse stir which lowers the collision growth rate resulting in a net inclusion removal due to flotation.

45

Total sulphide length is lowest in H-H,L-L charges and highest in H-L charges. However, sulphur content after degassing is generally higher in the H-L charges than in H-H,L-L and L-L charges, which may explain the higher total sulphide length. Toughness is strongly dependent on final nitrogen content, figure 45, which in turn is depending on several other parameters, as shown throughout the work. In order to evaluate the effect of blowing procedure, on final nitrogen content and toughness, NInd was introduced, eq 7. A low NInd value corresponds to good nitrogen removal, given the operating conditions for each charge. H-H,L-L blowing produced the charges with lowest NInd value and highest toughness in the experimental set of charges, figure 46. After adjusting for the toughness reducing effect of nitrogen no correlation between inclusions and toughness is seen, figure 47 - 49. This is probably due to the low amount of DH class inclusions in all samples. Lagneborg (5) presented results of decreased impact toughness with increased inclusion size and numbers, a result that is not found in this study. No difference in toughness, table 8, can be seen between regular charges produced with H-H and H-L blowing when taking into account the different mean values of N. However, test charges with H-L blowing have a mean EMV B of 20,0J with mean 27,5ppm NVTD, while regular H-L charges have a mean EMV B of 22,2J with mean 27,0ppm NVTD. Highest toughness was measured in H-H,L-L charges which had a mean EMV B of 22,3J. H-H,L-L charges, however, also had a mean N content of 21,3ppm after VTD. Mean hardness was found to be lower in H-H,L-L charges than in L-L and H-L charges. All H-H,L-L charges had relatively good starting conditions before degassing in terms of low sulphur and nitrogen content. To be able to compare the H-H,L-L blowing procedure to H-H charges conditions should be similar. A limit of the highest nitrogen and sulphur contents before degassing found in H-H,L-L charges is set, NTN

46

Conclusions H-H stirring during 30 min is excessive. Rinse stirring during the last minutes of degassing reduces the number of inclusions. Amount of oxidized Al indicates the degree of stirring and the dependence on TN conditions. Visual control of spout eyes is sufficient for evaluation and control of stirring. Poor stirring results in bad S and H removal and N pick-up. Highest toughness and lowest hardness with H-H,L-L blowing. N and S content after TN has a large impact on final N and S content. Poor slag skimming results in higher final sulphur content. Inclusions are not seen to affect the toughness of rolled plate.

Recommendations Use full argon blowing with shorter total degassing time, rinse stir during the final minutes of degassing if inclusion density is desired to be as low as possible.

Future work Investigate the rinsing time needed for desired degree of inclusion removal. Switch nitrogen cooling of camera to argon. Test high Al additions. Investigate and improve porous plug conditions. Investigate and improve N and S contents at TN. Investigate N absorption in slag. Investigate slag skimming with more charges.

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