Removal of hydrogen in the vacuum treatment of steel

The degassing of 09Г2C steel produced in an arc furnace and treated in a ladle–furnace unit at AO Uralskaya Stal is analyzed. The vacuum-treatment parameters that determine the effectiveness of hydrogen removal from the steel are identified: the depth and duration of vacuum treatment; the argon flow rate; the steel temperature; the thickness of the slag layer; and the free board in the vacuum chamber. The hydrogen content changes most significantly when the degassing time is increased to 20 min. Longer treatment is not recommended. The greatest effect of the residual pressure in degassing is observed with simultaneous decrease in the minimum pressure to 2 mbar. Vacuum treatment of the steel is considerably impaired with increase in the residual pressure. Hydrogen removal is improved with increase in the steel temperature to 1600–1620°C, but slows considerably at higher temperatures. The influence of the vacuum-treatment parameters is established quantitatively, and a regression equation is derived for predicting the results of hydrogen removal and selecting the parameter values corresponding to specified hydrogen content in the steel. Vacuum-treatment parameters that permit the economical production of steel with 2.1 ppm are determined: steel heating before vacuum treatment by 100–110°C; vacuum treatment for 20 min at a pressure no higher than 1.5 mbar in the vacuum chamber; argon flow rate 0.05 m³/t. The temperature losses of the metal are determined by the total treatment time, consisting of the active degassing time and the auxiliary time (the preliminary evacuation time), which depends on the capabilities of the equipment and the organization of the process. The minimum residual hydrogen content in the steel for the given equipment (1.6 ppm) is ensured by vacuum treatment for 40 min at a pressure no higher than 1 mbar in the vacuum chamber, with preliminary heating of the steel by 120–125 °C and with an argon flow rate up to 0.072 m³/t.     

Impact of slag–refractory lining reactions on the formation of inclusions in steel

Abstract

The present study was carried out to investigate the impact of slag– refractory lining reactions on the formation of inclusions during ladle treatment of steel. The experiments were conducted on an industrial scale in the ladle at Uddeholm Tooling AB in Hagfors, Sweden. The inclusion chemistry and population during ladle treatment were studied along with the composition of the ladle glaze, taken from the ladle lining. The inclusions in the steel were classified into four groups according to the Swedish standard SS 111116. SEM/EDS analyses were carried out to identify the phases present in both the inclusions and the ladle glaze. The number of inclusions in the steel before deoxidation was found to increase with the ladle age, i.e. the number of times the ladle had been in use. A similar increase was also found after vacuum degassing and before casting. A great portion of inclusions before casting was found to be supplied by ladle glaze. This observation was further confirmed and explained by thermodynamic analysis. The present results show that ladle glaze is a major source of inclusions in the ladle at Uddeholm Tooling.     

Distribution of metal droplets in top slags during ladle treatment

Abstract

The investigation focused on the mixing of the metal and slag phases during ladle refining from the point of tapping the EAF to casting. Steel droplet distributions were determined for slag samples taken at different stages in the ladle refining process at two different steel plants in Sweden. The droplet distributions were determined using light optical microscopy and classification according to the standard SS111116. Sample analysis results showed the slag samples taken before vacuum degassing to contain the greatest concentration of steel droplets. The total interfacial area between the steel droplets and slag was determined to be 3–14 times larger than the projected flat interfacial area between the steel and slag. The effects of slag viscosity and reactions between steel and slag on metal droplet formation in slag were also considered.     

Effect of Top Slag Composition on Inclusion Characteristics during Vacuum Degassing of Tool Steel

The focus of the study was to investigate the effect of the chemical composition of the top slag on the inclusion chemical composition during vacuum treatment of a plastic mould tool steel. Sampling was done before and after vacuum degassing. The chemical composition of the inclusions was determined by using SEM combined with EDX. The results showed that several inclusion compositions were found before vacuum degassing, while only one main composition of inclusions was present after vacuum degassing. Furthermore, the composition of the top slag was found to have a great influence on the composition of the inclusions found in samples taken after vacuum degassing. The present study also shows that the vacuum degassing effectively reduces the number of inclusions in steel. Finally, the thermodynamic calculations of the activities using Wagner's equation were found to predict a lower oxygen activity value than the calculations made using the Thermo‐Calc software.     

Behaviour of Hydrogen in Industrial Scale Steel Melts

The behaviour of hydrogen during controlled industrial scale secondary steel making process has been examined in a variety of low alloy steels, sensitive to hydrogen flaking. The study examines the role played by the moisture in input raw materials such as the ferro-alloys, type of carbon additive and fluxes in enhancing the hydrogen content in the ladle furnace. Post alloying, the influence of vacuum degassing parameters such as the vacuum level, vacuum holding time, Ar flow rate, type of porous plug used, slag chemistry and the steel grade was examined. The vacuum degassing process was analysed using a kinetic model, which could justify the trends seen in the vacuum level, holding time and Ar gas flow rate. Finally, the hydrogen pick-up post vacuum degassing through slag cover and the casting tundish was found to be influenced by parameters such as the quality of the tundish spray mass, and casting sequence. The influence of steel grade in hydrogen removal was also examined.     

Vacuum degassing of chromium-nickel-molybdenum steel at the Magnitogorsk Metallurgical Combine

The Magnitogorsk Metallurgical Combine has conducted a study of the effect of technological factors on the hydrogen content of chromium-nickel-molybdenum steel after vacuum degassing. It was established that the most important factor is the hydrogen content of the steel before the degassing operation. The study also determined the effects of the circulation coefficient, the duration of the degassing operation, and the vacuum used in the treatment. __________ Translated from Metallurg, No. 7, pp. 68–69, July, 2006.     

Combined decrease of sulphur,nitrogen, hydrogen and total oxygen in only one secondary steelmaking operation

The development of a matured metallurgical process allows the reduction of the sulphur, nitrogen, hydrogen and total oxygen content to the lowest values in only one secondary metallurgical step during treatment in a ladle tank degassing unit. By careful adjustment of a lime-saturated ladle top slag at the beginning of the vacuum treatment, it is possible to obtain a desulphurization degree of more than 95%. The process of nitrogen removal by a stirring gas can be described with a dynamic model in excellent agreement with plant trials. The nitrogen content after vacuum treatment depends mainly on the nitrogen content before vacuum, the stirring gas quantity and the sulphur content. The achievable hydrogen contents mainly depend on the stirring gas quantity and dehydrogenisation can be described by the same model used for denitrogenisation. By a cleanliness stirring after the vacuum treatment under a nearly SiO₂-free-, lime-alumina slag, saturated with lime, the total-oxygen content can be lowered to less than 20 ppm.     

Change of Inclusion Characteristics during Vacuum Degassing of Tool Steel

The inclusion characteristics were studied during vacuum degassing by interrupting the operation at five different times after the start of operation. Slag and steel samples were collected and thereafter assessed with respect to steel and slag composition, total oxygen content, size distribution of inclusions and chemical composition of inclusions. The main conclusion is that the number of inclusions in the different size classes as well as the total oxygen content seem to reach a minimum value after around ten minutes of vacuum degassing. Furthermore, it seems to be a consistent trend that, during vacuum degassing, the top slag influences the inclusion composition.     

Ladle glaze: major source of oxide inclusions during ladle treatment of steel

Abstract

Samples of ladle lining covered by glaze were taken from industrial ladles of different ages at Uddeholm Tooling AB, Hagfors, Sweden. It was found in the samples taken at and below the slag line that a slag infiltrated layer was covered by an outer layer containing many MgO 'islands' of various sizes. The microstructure of the infiltrating slag was the same as the matrix of the outer layer. The slag was found to decompose into the compound 3CaO.Al₂O₃ and a liquid phase during the cooling process. The former phase along with tiny MgO particles from the ladle glaze was found to be one of the major sources of inclusions during the degassing and flotation periods of ladle treatment. Thermodynamic analysis indicated that the reaction between the ladle glaze and the slag from the electric arc furnace resulted in the formation of MgO–Al₂O₃ spinel and an oxide solution, which were also the main inclusions found at the initial stages of ladle treatment. Evidence of this reaction was found in the lining samples taken above the slag line.     

天津钢管公司100t RH的设备功能和冶金效果

国内自行设计制造的100 t RH真空循环脱气设备的真空度可达67 Pa以下,真空处理能力为500kg/h以上。生产实践表明,氩气流量越大,钢水循环速度越大,高真空脱气时间越长,钢水脱气效果越明显,所处理的管线钢的氢、氧、碳含量可分别降至1×10^-6、20×10^-6和20×10^-6以下。     

100t VD精炼脱气工艺实践

总结了安阳钢铁集团有限责任公司第一炼轧厂100tVD装置在热调试和试生产期间的精炼工艺实践及精炼效果,分析了真空处理时间、氩气搅拌流量、渣量、钢中硫和氧含量等对真空脱氢、脱氮的影响。同时分析了真空精炼对钢液脱氧及夹杂物控制的影响。     

超纯轴承钢的精炼工艺

通过控制电炉(供氧强度、渣中氧化铁比例、出钢挡渣率、出钢钢液的氧活度)、钢包炉(精炼渣系、脱氧剂、钢液温度、精炼时间、底吹氩压力、精炼钢包耐火材料的选择、铁合金种类的选择)、真空脱气(真空度、真空时间、底吹氩压力)的工艺参数以及真空后的软吹氩搅拌、并采用IPAS系统和控制钢液浇铸速度,使超纯轴承钢(SFGCr15)的w(S)、w(Ti)、w(O)分别达到0.003%、0.001 2%和0.000 7%以下,钢中非金属夹杂物也处于较好水平,满足了国际顶尖轴承厂家对轴承钢的超纯要求.     

CAS-OB refining: slag modification with B2O3–CaO and CaF2–CaO

Abstract

Evaluation of CAS-OB refining slags showed that the melting temperature and viscosity were very high and could further increase during the CAS-OB refining process, causing excessive slag to stick to the snorkel with resulting operational problems. To avoid this, B₂O₃–CaO (mass ratio 1 : 1) and CaF₂–CaO (mass ratio 1 : 1) were employed as modifiers added to the slag. The fusibility (melting temperature and viscosity) and desulphurising capacity of modified slag were investigated. Both B₂O₃–CaO and CaF₂–CaO can effectively lower the melting temperature and viscosity of slag. The results of experiments on sulphur partition equilibrium between metal and slag indicate that the sulphur content of metal can be further decreased by the modified slag.     

Thermodynamics of Interstitial Impurities Removal from Refractory Metals

Abstract

The applied thermodynamic aspects of removing hydrogen, nitrogen, oxygen, carbon, and silicon from vanadium, niobium and tantalum metals by pyrovacuum treatments are considered in this paper. Two major processes operate in refining by pyrovacuum treatment. One is distillation and the other is degassing. Distillation is mainly used to remove substitutional impurities that are either already present in the metal or have been added for removing any interstitial impurity. The success is determined by the partial pressure of the impurity element as well as by the difference in partial pressures of the impurity and the metal. The maximum rate of vaporization of impurity can then be estimated using the free evaporation equation. Interstitial impurities, particularly the gases hydrogen, nitrogen and oxygen, are removed by degassing. Thermodynamics of classical degassing, which is essentially the reverse of absorption, is described usefully by the pressure-composition isotherms. This is applicable mainly for the removal of hydrogen and nitrogen. The removal of oxygen, known as deoxidation, occurs by a more complex mechanism that involves the formation and evaporation of metal suboxides. Depending on the suboxide species responsible for deoxidation, the process is known as sacrificial deoxidation, carbon deoxidation, silicon deoxidation, or aluminium deoxidation. The applicability of these different processes to a particular metal, M is determined by the thermodynamic properties of the relevant M-Q, M-C-O, M-Si-O, and M-AI-0 system. It is shown in this paper that all the four processes are applicable to niobium and tantalum where as only aluminium deoxidation is useful for vanadium. For the removal of carbon and silicon also, the relevant deoxidation process can be used.     

Importance of Kinetic Models in the Analysis of Steelmaking Reactions

In this paper, examples of the use of a kinetic model in the analysis of a steelmaking process will be discussed. In decarburization by BOF, the relation between C and O contents is different from that obtained by equilibrium calculations. By the use of kinetic models, it was clarified that the O content in the metal is controlled not only by the C content but also by the FeO activity in the slag. In the vacuum degassing process, the partial pressure calculated on the basis of the relation between C and O contents is much higher than the operation pressure. The kinetic model which considers the circulation between the molten steel in the vacuum vessel and that in the ladle is well known; furthermore, various decarburization mechanisms were proposed. Hot metal dephosphorization occurs under non‐equilibrium conditions because the oxygen potential of the slag and that of the hot metal are different. Process analysis is performed by considering the reaction kinetics based on the coupled reaction model. Recently, a new reaction model has been proposed; this model considers the solid slag, liquid slag, and liquid metal phases and the reaction between the solid and liquid slag, in addition to the reaction between the liquid slag and liquid metal.     

Oxygen Activity Calculations of Molten Steel: Comparison With Measured Results

This study sets out to find some model/s that could calculate the closest oxygen activity of molten steel to the measured oxygen activity in an ASEA‐SKF ladle furnace. Ten steel heats grade SAE 52100 were chosen, the oxygen activities of the molten steel after vacuum degassing process were measured, then by means of two different equations and one computer programs the theoretical oxygen activities were calculated and compared to the measured results. In order to calculate the activities of Al₂O₃ in the top slag four different models and two different constants were applied. Both Wagner and Turkdogan's equations were found to be useful. It was seen that increasing the wt% Al from 0 up to 0.05 in molten steel, increasing CaO/Al₂O₃ ratio in top slag, and reducing equilibrium temperature of slag‐steel could contribute to reduction of oxygen activity of molten steel. The scanning electron microscope observations revealed that the main types of observed non‐metallic inclusions in these samples were spinels and calcium aluminates and by increasing the CaO content of the inclusions their equivalent circle diameters grew.     

VD真空精炼脱氢和脱氮过程的三维数值模拟

基于质量、动量守恒计算了真空脱气装置内流场,并结合反应热力学和动力学理论建立了真空精炼过程中脱氢、脱氮与流场耦合的三维数学模型,研究了不同工艺参数对VD脱氢和脱氮过程的影响。结果表明:钢包内流场对脱气过程中氢和氮浓度空间分布具有明显影响;脱氢和脱氮速率随底吹气量的增加而增大,终点氢和氮浓度随底吹气量的增加而减小;在实际生产中,应在避免发生卷渣的前提下合理选择吹气量;不同底吹方式对脱氢和脱氮过程影响明显,其中偏心底吹效果优于中心底吹,双底吹效果最好。     

EH36厚规格高强度船板的冶炼工艺实践

通过对济钢三炼钢厂采用KR铁水预脱硫处理、120 t BOF副枪终点控制及脱氧合金化、LF造白渣泡沫渣埋弧操作、VD真空脱气处理后喂硅钙线、CCM全程保护浇铸的工艺流程生产50 mm厚规格EH36高强度船板进行了系统分析和研究,确定了各工序关键参数和操作控制要点。     

Degassing Behavior of Nanostructured Al and Its Composites

The synthesis of bulk ultrafine-grained (UFG) and nanostructured Al via cryomilling can frequently require a degassing step prior to consolidation, partly due to the large surface area of the as-milled powders. The objective of this study is to investigate the effects associated with cryomilling with stearic acid additions (as a process-control agent) on the degassing behavior of Al powders. This objective was accomplished by completing select experiments with Al-7.5Mg, Al-6.4 wt pct Al₈₅Ni₁₀La₅, and Al-14.3 wt pct B₄C. The interaction between Al and stearic acid was determined using thermal analysis combined with Fourier transform infrared spectroscopy (FTIR). The degassing experiments were carried out under high vacuum (10⁻⁴ to ~10⁻⁶ torr) in a range from room temperature to 400 °C, with the pressure of the released gases monitored using a digital vacuum gage. The results showed that the liberation of chemisorbed water was suppressed in cryomilled Al powders and both the chemisorbed water and stearic acid were primarily released in the form of hydrogen. It was also demonstrated that under certain conditions, a nanostructure (grain size ~100 nm) can be retained following the hot vacuum degassing of cryomilled Al.