The aim of the present work was to get an overview of the steel cleanness in tool steel. Plant trial studies of three steel grades were done with different focuses. Firstly the change of the inclusion characteristics during the vacuum degassing in the ladle was looked upon. The top slag composition was altered and sampling was made before and after vacuum degassing and the results showed that the top slag composition has an effect on the inclusion composition. Thermodynamic calculations indicated that the oxygen activity for the steel/inclusion was twice as large as that of the steel/top slag before vacuum degassing. However, after vacuum degassing the oxygen activity values were close. In order to study the inclusion characteristics during vacuum degassing the process was interrupted at five pre-determined time points for sampling. During vacuum the number of the smaller inclusions was decreased. However, for the larger inclusions (&gt;11.2µm) the number of inclusions had increased. Throughout the degassing process the inclusion composition was found to approach the top slag composition and at the end of the process only one type of inclusion composition was found. The removal of hydrogen, nitrogen and sulphur was studied. Samples were taken before and after vacuum degassing. It was found that the removal kinetics of hydrogen and nitrogen can be described with first-order reactions. When 10 minutes of vacuum degassing have passed, the removal of hydrogen and nitrogen is more or less finished for the studied steel grades. Two steel grades with similar process route and with only a few differences in steel composition were studied. The sum of FeO and MnO was found to be a clear indicator for when reoxidation had taken place. The results from oxygen activity calculations indicate that calculations of oxygen activities with multivalence slag species such as Fe and Cr requires measurements for validations to ensure that reliable results are obtained. / Syftet med denna studie var att få en överblick av verktygsståls renhet. Studier på tre olika stålsorter utfördes. Inneslutningsbilden under vakuumbehandling undersöktes. Resultaten är baserade på industriförsök likväl som termodynamiska beräkningar. Försöken gjordes på två olika stålsorter där toppslaggens sammansättning varierades och stål- och slaggprover tog ut före och efter vakuumbehandling. Proverna analyserades för att bestämma den kemiska sammansättningen på stål och slagg, men inneslutningsbilden undersöktes också. Med hjälp av termodynamiska beräkningar beräknades syreaktiviteten för jämvikterna stål/inneslutning och stål/toppslag. Beräkningarna jämfördes sinsemellan samt med uppmätt syreaktivitet från stålsmältan. Resultaten visar att toppslaggens sammansättning har en påverkan på inneslutningarnas sammansättning. Specifikt så fanns att när CaO-halten i toppslaggen ökar syns en ökning av CaO i inneslutningssammansättningen. De termodynamiska beräkningarna visar att syreaktiviten för jämvikten stål/inneslutning är dubbelt så stor som den för jämvikten stål/topplagg före vakuumbehandling. Dock är syreaktiviteterna för de olika jämvikterna lika efter vakuumbehandlingen. För att studera hur inneslutningsbilden utvecklas under vakuumbehandling avbröts vakuumbehandlingen vid fem förutbestämda tidpunkter. Vid varje avbrott av vakuumbehandlingen togs stål- och slaggprover som analyserades för att bestämma totalsyre, inneslutningsmängd och inneslutningssammansättning. Före vakuumbehandlingen var antalet inneslutningar stort, medan antalet små inneslutningar minskade under vakuumbehandlingen. Däremot konstaterades att antalet stora inneslutningarna (&gt;11.2μm) ökade under avgasningen. Vad beträffar inneslutningssammansättningen syntes en variation i analys före vakuumbehandlingen som sedan förändrades till att likna toppslaggens sammansättning under vakuumbehandlingstiden. I slutet av processen märktes endast en typ av inneslutningssammansättning i stålsmältan. Vad som också noterades var att efter ungefär 10 minuter av vakuumbehandling så hade inneslutningsantalet nått sitt minimumvärde. För att erhålla rent stål är väte-, kväve- och svavelreningen också av stor vikt under vakuumbehandlingen. Dessa tre element studerades genom att prover togs vid olika tillfällen i processen; före, under (vid fem olika förutbestämda tidpunkter) och efter vakuumbehandling. De kemiska sammansättningarna för stål- och slaggproverna användes för att beräkna väte-, kväve- och svavelreningarna. Beräkningarna visade att för väte och kväve så kan kinetiken beskrivas med en förstagradsekvation. Specifikt så gäller detta för stålsorten i studien, samt under förutsättningen att svavelhalten är lägre än 0,003 vikts-%. Dessutom så visade resultaten att väte- och kvävereningen är i stort sett klar för den studerade stålsorten efter 10 minuters vakuumbehandling. När det gäller svavel så kan en förstagradsekvation inte användas vid beräkningarna Istället uppvisar svavel en tendens att följa jämviktshalten av svavel genom hela avgasningsprocessen. Genom att kontrollera processen finns stora möjligheter att erhålla en bra renhet på stålet. Två stålsorter studerades i denna specifika studie, där de hade en liknande processväg samt endast ix några små skillnader i stålanalysen. Stål- och slaggprover samlades in och analyserades. Summan av FeO och MnO visade sig vara en klar indikation på när reoxidation har skett, men mängden "carry-over" slagg från ljusbågsugnen kunde inte predikteras med hjälp av någon specifik indikator. Dessutom så beräknades syreaktiviten och beräkningarna jämfördes sedan med uppmätt syre i stålet. Resultaten indikerar att beräkningar av syreaktivitet med toppslagger som innehåller multivalenselement, såsom Fe och Cr, kräver valideringar med mätningar för att trovärdiga predikteringar ska erhållas. / <p>QC 20160523</p>

Há uma demanda por competitividade entre as indústrias de aços, a fim de alcançar a excelência definida pelo termo clean steel. O processo de desgaseificação a vácuo (VD) ocorre durante o Refino Secundário de aços especiais. Sua principal função é remover gases indesejáveis, especialmente o hidrogênio. Entretanto, durante este processo, o fenômeno de flotação e a absorção de inclusões são reportados. O objetivo do presente trabalho foi estudar a viscosidade de escórias e a capacidade da estação de desgaseificação a vácuo do tipo tanque na limpeza de aços sob uma perspectiva industrial. Para realizar este objetivo, foram coletadas amostras de escória e aço antes e após a etapa de vácuo. Os resultados em limpeza de aço foram relacionados à energia de agitação durante o tratamento a vácuo (associada a um parâmetro cinético - βs) e ao efeito da viscosidade de escórias. É possível observar um decréscimo expressivo na população de inclusões entre as condições antes e após tratamento de desgaseificação a vácuo. A remoção de inclusões durante o vácuo atinge 64, 75 e 78% para as faixas de diâmetro de 2,5-5, 5-15 e ≥ 15 μm, respectivamente. Após o processo de desgaseificação, a composição das inclusões não-metálicas aproxima-se da composição química da escória. O processo de agitação na estação de desgaseificação a vácuo promove uma diminuição significativa na densidade de inclusões na faixa de diâmetro de 2,5-15 μm. Além disso, ao aumentar a intensidade do parâmetro cinético, a composição química de inclusões não-metálicas foi afetada e o teor de enxofre presente no aço líquido foi reduzido. Quanto às viscosidades efetiva das escórias, conclui-se que, para valores mais baixos (0,20 Pa.s) aumenta-se a capacidade da escória na remoção de inclusões, enquanto que valores mais altos (> 0,40 Pa.s) aparentaram ser prejudiciais à limpeza do aço. / There is a demand in competitiveness within the steel industry towards achieving excellence defined by clean steel term. The process of vacuum degassing (VD) occurs during the secondary refining of special steels. Its main function is to remove undesirable gases, primarily hydrogen. However, during this process, flotation phenomenon and inclusions absorption are reported. The aim of the present work was to study the slag viscosity and vacuum degassing (tank type) capacity in steel cleanliness from an industry perspective. To achieve this objective, slag and steel samples were taken before and after vacuum stage. The results in steel cleanliness were related to the stirring energy of the vacuum station (associated to a kinetic parameter – βs) and to the effect of slag viscosity. It is possible to observe an expressive decrease in the population of inclusions between the conditions before and after vacuum degassing treatment. The removal of inclusions during the vacuum stage reaches 64, 75 and 78% in the diameter ranges of 2,5-5, 5-15 and ≥ 15 μm, respectively. After the degassing process, the composition of non-metallic inclusions seemed to approach the slags’ chemical compositions. The stirring process in the vacuum degassing station promotes a significant decrease in the inclusion density with 2,5-15 μm diameter range. Also, by increasing the kinetic parameter intensity, the composition of non-metallic inclusions was affected. The sulfur content present in liquid steel was reduced. Regarding the effective viscosities of slags, it was concluded that lower values (0,20 Pa.s) increased slag capacity in inclusion removal, whereas higher values (> 0,40 Pa.s) was detrimental to steel cleanliness.

No description available.

Há uma demanda por competitividade entre as indústrias de aços, a fim de alcançar a excelência definida pelo termo clean steel. O processo de desgaseificação a vácuo (VD) ocorre durante o Refino Secundário de aços especiais. Sua principal função é remover gases indesejáveis, especialmente o hidrogênio. Entretanto, durante este processo, o fenômeno de flotação e a absorção de inclusões são reportados. O objetivo do presente trabalho foi estudar a viscosidade de escórias e a capacidade da estação de desgaseificação a vácuo do tipo tanque na limpeza de aços sob uma perspectiva industrial. Para realizar este objetivo, foram coletadas amostras de escória e aço antes e após a etapa de vácuo. Os resultados em limpeza de aço foram relacionados à energia de agitação durante o tratamento a vácuo (associada a um parâmetro cinético - βs) e ao efeito da viscosidade de escórias. É possível observar um decréscimo expressivo na população de inclusões entre as condições antes e após tratamento de desgaseificação a vácuo. A remoção de inclusões durante o vácuo atinge 64, 75 e 78% para as faixas de diâmetro de 2,5-5, 5-15 e ≥ 15 μm, respectivamente. Após o processo de desgaseificação, a composição das inclusões não-metálicas aproxima-se da composição química da escória. O processo de agitação na estação de desgaseificação a vácuo promove uma diminuição significativa na densidade de inclusões na faixa de diâmetro de 2,5-15 μm. Além disso, ao aumentar a intensidade do parâmetro cinético, a composição química de inclusões não-metálicas foi afetada e o teor de enxofre presente no aço líquido foi reduzido. Quanto às viscosidades efetiva das escórias, conclui-se que, para valores mais baixos (0,20 Pa.s) aumenta-se a capacidade da escória na remoção de inclusões, enquanto que valores mais altos (> 0,40 Pa.s) aparentaram ser prejudiciais à limpeza do aço. / There is a demand in competitiveness within the steel industry towards achieving excellence defined by clean steel term. The process of vacuum degassing (VD) occurs during the secondary refining of special steels. Its main function is to remove undesirable gases, primarily hydrogen. However, during this process, flotation phenomenon and inclusions absorption are reported. The aim of the present work was to study the slag viscosity and vacuum degassing (tank type) capacity in steel cleanliness from an industry perspective. To achieve this objective, slag and steel samples were taken before and after vacuum stage. The results in steel cleanliness were related to the stirring energy of the vacuum station (associated to a kinetic parameter – βs) and to the effect of slag viscosity. It is possible to observe an expressive decrease in the population of inclusions between the conditions before and after vacuum degassing treatment. The removal of inclusions during the vacuum stage reaches 64, 75 and 78% in the diameter ranges of 2,5-5, 5-15 and ≥ 15 μm, respectively. After the degassing process, the composition of non-metallic inclusions seemed to approach the slags’ chemical compositions. The stirring process in the vacuum degassing station promotes a significant decrease in the inclusion density with 2,5-15 μm diameter range. Also, by increasing the kinetic parameter intensity, the composition of non-metallic inclusions was affected. The sulfur content present in liquid steel was reduced. Regarding the effective viscosities of slags, it was concluded that lower values (0,20 Pa.s) increased slag capacity in inclusion removal, whereas higher values (> 0,40 Pa.s) was detrimental to steel cleanliness.

Há uma demanda por competitividade entre as indústrias de aços, a fim de alcançar a excelência definida pelo termo clean steel. O processo de desgaseificação a vácuo (VD) ocorre durante o Refino Secundário de aços especiais. Sua principal função é remover gases indesejáveis, especialmente o hidrogênio. Entretanto, durante este processo, o fenômeno de flotação e a absorção de inclusões são reportados. O objetivo do presente trabalho foi estudar a viscosidade de escórias e a capacidade da estação de desgaseificação a vácuo do tipo tanque na limpeza de aços sob uma perspectiva industrial. Para realizar este objetivo, foram coletadas amostras de escória e aço antes e após a etapa de vácuo. Os resultados em limpeza de aço foram relacionados à energia de agitação durante o tratamento a vácuo (associada a um parâmetro cinético - βs) e ao efeito da viscosidade de escórias. É possível observar um decréscimo expressivo na população de inclusões entre as condições antes e após tratamento de desgaseificação a vácuo. A remoção de inclusões durante o vácuo atinge 64, 75 e 78% para as faixas de diâmetro de 2,5-5, 5-15 e ≥ 15 μm, respectivamente. Após o processo de desgaseificação, a composição das inclusões não-metálicas aproxima-se da composição química da escória. O processo de agitação na estação de desgaseificação a vácuo promove uma diminuição significativa na densidade de inclusões na faixa de diâmetro de 2,5-15 μm. Além disso, ao aumentar a intensidade do parâmetro cinético, a composição química de inclusões não-metálicas foi afetada e o teor de enxofre presente no aço líquido foi reduzido. Quanto às viscosidades efetiva das escórias, conclui-se que, para valores mais baixos (0,20 Pa.s) aumenta-se a capacidade da escória na remoção de inclusões, enquanto que valores mais altos (> 0,40 Pa.s) aparentaram ser prejudiciais à limpeza do aço. / There is a demand in competitiveness within the steel industry towards achieving excellence defined by clean steel term. The process of vacuum degassing (VD) occurs during the secondary refining of special steels. Its main function is to remove undesirable gases, primarily hydrogen. However, during this process, flotation phenomenon and inclusions absorption are reported. The aim of the present work was to study the slag viscosity and vacuum degassing (tank type) capacity in steel cleanliness from an industry perspective. To achieve this objective, slag and steel samples were taken before and after vacuum stage. The results in steel cleanliness were related to the stirring energy of the vacuum station (associated to a kinetic parameter – βs) and to the effect of slag viscosity. It is possible to observe an expressive decrease in the population of inclusions between the conditions before and after vacuum degassing treatment. The removal of inclusions during the vacuum stage reaches 64, 75 and 78% in the diameter ranges of 2,5-5, 5-15 and ≥ 15 μm, respectively. After the degassing process, the composition of non-metallic inclusions seemed to approach the slags’ chemical compositions. The stirring process in the vacuum degassing station promotes a significant decrease in the inclusion density with 2,5-15 μm diameter range. Also, by increasing the kinetic parameter intensity, the composition of non-metallic inclusions was affected. The sulfur content present in liquid steel was reduced. Regarding the effective viscosities of slags, it was concluded that lower values (0,20 Pa.s) increased slag capacity in inclusion removal, whereas higher values (> 0,40 Pa.s) was detrimental to steel cleanliness.

Steel cleanliness as a function of stirring practice during vacuum degassing treatment have been investigated in industrial studies at the steel plants of SSAB Special Steels in Oxelösund and Uddeholm AB in Hagfors. The cleanliness was examined with regards to the contents of sulphur, nitrogen and large inclusions (&gt;10µm). The stirring practice during the vacuum degassing treatment has been studied with respect to time, namely by shortening the vacuum degassing treatment time from 24 to 15 minutes. Furthermore, the effect of the stirring intensity was studied by measuring the open eye zone using camera recordings. The focus has been to study the effect of a shortened vacuum degassing time as well as a controlled stirring intensity on the steel cleanliness. Moreover, study the effect of subsequent induction stirring, after vacuum degassing, on the amount of inclusions. By taking steel and slag samples taken before and after the vacuum degassing treatment, the different stirring practices could be investigated. It should be noted that all heats reached the desired composition regardless of the treatment time and stirring practice. The results from the trials at SSAB Special Steels showed that the stirring practice during vacuum degassing have an effect on the total number of inclusions. The strong argon stirring during vacuum degassing with visible open eyes resulted in an average increase of 400% of the total amount of inclusions &gt;10µm. However, no difference between a vacuum degassing time of 24 or 15 minutes was seen with respect to the increased amount of inclusions, denitrogenization or desulphurization. During the subsequent induction stirring, which was softer with no open eyes, the effect of the stirring practice was unclear due to overlapping confidence intervals. An average decrease of 65% of the total amount of inclusions &gt;10 µm was seen for the heats  with non-overlapping confidence intervals. Based on these results it can be suggested that the use of a soft induction stirring after the vacuum degassing treatment as a possible process change to reach lower amounts of large inclusions. The stirring intensity during the vacuum degassing treatment was measured as the average ladle eye size during the trials at Uddeholm AB. This, due to that stronger stirring results in larger ladle eye zone. The results show that the amount of smaller inclusions (&lt;11.2 μm) decrease by up to 90% compared to the original amount, regardless of the stirring intensity. A stronger stirring showed tendency to increase the amount of large inclusion (&gt;22.4 μm). Moreover, the denitrogenization and desulphurization was not affected by the changed stirring intensity during vacuum degassing. It was also found that the average ladle eye size did not correlate to the logged average argon flow. / <p>QC 20150522</p>

QC 20101221

This study explores the possibility of prediction and modification of some of the physicochemical properties of non-metallic inclusions by considering top slag-steel-ladle equilibria in an ASEA-SKF ladle furnace. To test the reliability of an available computational thermodynamic computer program, the first sub project was done. It was concluded that LiMeS, an interface for Thermo-Calc, is a useful tool for slag-steel equilibrium calculations. The second sub project was set out to find some model/s that could calculate the most accurate oxygen activity of molten steel compared to the measured one. This study concluded that both Wagner’s and Turkdogan's equations are useful. It was further seen that increasing the Al contents in the molten steel, increasing the CaO/Al2O3 ratio in the top slag, and reducing the temperature, resulted in reduction of the oxygen activity of the molten steel. In the third sub project a comparison was made between measured CaO and Al2O3 (normalised to CaO-Al2O3) in top slag, calcium aluminate inclusions, and the results of theoretical calculations. The average contents of CaO and Al2O3 in all inclusions were close to the composition of the phase Ca12Al14O33 and the contents of CaO and Al2O3 in the slags were close to the composition of the phase Ca3Al2O6 in the binary phase diagram of CaO-Al2O3. The forth sub project set out to study the effect of vacuum degassing time on non-metallic inclusions. It was concluded that during the vacuum degassing process the share of calcium aluminates compared to spinels, Ca content of the oxides, and the average equivalent circle diameters of the oxides were increasing, and oxides tended to form spherical shapes. Finally, based on the preceding four sub projects, the fifth sub project aimed to optimize the steel treatment in an ASEA-SKF ladle furnace. The final results showed that by adding 200 kg fluorite to the top slag of 1200 kg, it was possible to achieve a sulphur content of less than 10 ppm in the steel and a sulphur ratio between slag and steel of 1570, and at the same time reduce the oxygen activity of the molten steel and the degassing time. / <p>QC 20120917</p>

The present thesis was to study non-metallic inclusions during vacuum degassing in ladle treatment. Emphasize was mostly given to liquid CaO-Al2O3 inclusions. A series of industrial experiments were carried out at Uddeholm Tooling AB, Hagfors, Sweden. To gain an insight into the industrial findings, laboratory investigations were also performed. Large number of steel samples were collected and examined. Liquid calcium aluminate inclusions with low SiO2 and high SiO2 were often found with spinel inclusions before vacuum degassing. Laboratory experiments showed that spinel would react with the dissolved Ca in the liquid steel forming calcium aluminate inclusions. This laboratory results agreed with the industrial observation that spinel phase was quite often found in the center of the calcium aluminate phase. After vacuum degassing, most of the inclusions were calcium aluminate liquid inclusions. When dissolved Al level was low, 2 types of liquid calcium aluminate inclusions with considerably different SiO2 contents were found to coexist even at the end of the process. In view of the lack of the thermodynamic data for SiO2 activities in the low silica region, thermodynamic measurements were conducted in the CaO-Al2O3-SiO2-MgO system. The experimental results could reasonably explain the coexistence of the two types of the liquid oxide inclusions. While the total number of inclusions decreased during vacuum degassing, the number of bigger inclusions (&gt;11.3 μm) increased generally in used ladles. This finding was in accordance with the previous studies, wherein, ladle glaze was found to be responsible for the supply of bigger inclusions. The behaviors of several types of inclusions in liquid steel were examined using a laser scanning confocal microscope (LSCM). While alumina inclusions tended to impact on each other, agglomerate and grow very quickly, none of the other types of inclusions, such as spinel and calcium aluminate was observed to agglomerate. The results of LSCM study agreed well with the industrial observation. Examination on a huge number of inclusions did not show any indication of impact and physical growth of the inclusions, except the alumina inclusions. The removal of inclusions around open-eye in a gas-stirred ladle was experimentally studied by a cold model with oil and water. Most of the simulated inclusions were brought up to the oil phase by gas-water plume. Inclusion removal into oil layer took place when the inclusions passed through the sphere-bed of the oil layer around the open-eye. A calculation showed that the contribution of metal-gas plume in inclusion removal was much larger than that of buoyancy mechanism. The results of the industrial experiments revealed that the inclusions distribution strongly depended on stirring condition. When a ladle was stirred by both gas and induction, inclusion removal near slag layer was significant. / QC 20100803

The control of non-metallic inclusions is very important for the improvement of performance during the application of tool steel. This present study was performed to see the effect of changing of some process parameters during the vacuum degassing of the melt and how these changing parameters affects the characteristics of inclusions in tool steel. The main parameters that were changed during the vacuum degassing were the change of induction stirring, argon flow rate from both the plug 1 and 2 and different ladle ages for different heat. Electrolytic extraction method was used to observe the morphology and characteristics of inclusions as a 3 dimensional view in tool steel. Four lollipop samples from four different heats were used for the experiment and all the samples were after vacuum (AV) degassing. In this study four different types of inclusions were found and they are classified as type 1, 2, 3 and 4. Of them type 1 inclusion was the major one with mostly spherical shaped. This study shows that among the three parameters, induction stirring has the biggest effect for the total number of inclusions per volume in the sample than the other two parameters Heat 4A showed the lowest number of inclusions per volume comparing with the other heats. The main reason behind this can be said that the induction stirring was the lowest comparing with the other heats with moderate argon flow and ladle age of 12. Extreme value analysis was used in this study to predict the probability of getting largest size inclusions in a certain volume of the metal. For the prediction of the largest inclusion size, both the electrolytic extraction (3D) and cross-sectional (2D) method was used. Later in this study comparison was done to determine the accuracy of both the methods and it is concluded that for the type 1 inclusions electrolytic extraction method shows almost similar trend with cross-sectional method and electrolytic extraction method shows better accuracy for the prediction of largest size inclusions than the cross-sectional method. Electrolytic Extraction method is also applicable for the prediction of largest size inclusions for multiple types of inclusions.