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.     

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.     

Behaviour of inclusions in RH vacuum degasser

Abstract

Vacuum degassing processes are now an integral part of secondary steelmaking operations and besides the more obvious benefits of steel chemistry control to precise specifications, they also reduce the number of non-metallic inclusions. Care has to be taken, however, that the change in steel composition through loss of C, O, Mn, etc. does not result in a deleterious change in the composition of the remaining inclusions. Attempts in the past to determine the effect of vacuum on inclusion compositions have been through the use of thermodynamic models, following the inclusion engineering approach. The calculated inclusion compositions do not, however, compare well with the inclusions as analysed in samples taken from the liquid steel after the degassing operation. Clearly, it is important to take into account time dependent effects during degassing and this has been achieved by the development of a combined fluid flow-thermodynamic model. Using the computational fluid dynamics model, CFX, to establish the temperature, flow and species contours in a two-dimensional steel ladle under the influence of natural convection, the results are transferred as start conditions in a three-dimensional RH degasser model. A body force is then applied to simulate the argon bubbles that are injected into the up-leg of the degasser and changes in flow, temperature and species concentrations are calculated. Allowing for additions made during the process, the composition of the top slag and any local inclusions within the steel is predicted. The influence on top slag and inclusion chemistry of any glaze on the snorkels of the degasser is also taken into account.     

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.     

Influence of calcium carbide slag additions on inclusion characteristics in steel

Abstract

An investigation into the cleanness of steel manufactured by a major Scandinavian producer of steel for railway wheels has been carried out. Samples taken before and after secondary refining in the ladle and after degassing were compared with samples taken similarly from a heat including the addition of a calcium carbide (CaC₂ ) slag modifier. The amount, size distribution, and composition of non-metallic inclusions, as well as the slag compositions of both heats, were determined. With the addition of CaC₂ , the total number of inclusions in the sample taken before casting was found to decrease, and the size distribution revealed the presence of mostly small inclusions (between 2·8 and 5·6 μm). The assessment of railway wheel specimens revealed that the inclusion content in CaC₂ treated steel was lower than that for standard steel.     

气泡搅拌和炉渣成分对低碳钢水洁净度的影响

在80t氧气转炉、钢包吹氩和RH真空处理工序进行净化钢水的试验,考察反应器内氩气流量、真空脱气操作和顶渣成分对低碳钢水全氧含量的影响以及夹杂物的尺寸分布和全氧的排除速率,改进操作的结果表明,二次精炼后低碳钢水全氧w[T.O]降低到0.001%。     

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.     

Relationship between Oxygen Chemical Potential and Steel Cleanliness

Control of the oxygen activity andits content inthe molten steel are the main factors in the produc-tion of clean,high quality and homogeneous steel[1].Deoxidation of the steel includes reducing both theoxygen activity and total oxygen in molten steel.Sin…     

Modelling of ladle glaze interactions

Abstract

A potential source of inclusions in liquid steel is ladle glaze, the slag/inclusion coating on the refractory lining of the ladle which is always present after casting and before reuse of the ladle. The composition and quantity of this glaze can vary considerably, depending on the range of steel qualities being made and, in order to model the effect of glaze on inclusion/top slag formation, a combined fluid flow-thermodynamic model has been developed. The behaviour of ladle glaze for various different steels has been studied under pilot plant conditions and the results incorporated into the mathematical model. The implications of the pilot plant and model results are discussed in the context of the production of clean steel.     

Hydrogen pick-up after vacuum degassing

Abstract

The present work aimed at determining the major source for hydrogen pick-up of the steel after vacuum degassing but before casting. Samples of slag and metal were taken at different stages during ladle treatment at SSAB. Hydrogen increase after vacuum treatment was observed. Moisture contents of the industrial slags were analysed and their water capacities were calculated. It could be seen that the hydrogen increase was correlated to the amount of moisture in the slag and the water capacity. The study showed that the slag containing most water was also the heat having the largest hydrogen increase. The slag with most water had the highest water capacity. It could be concluded that the major source for hydrogen increase after degassing was due to the slag–metal reaction.     

Use of Computational Thermodynamic Calculations in Studying the Slag/Steel Equilibrium during Vacuum Degassing

The effectiveness of a ladle refining process for production of tool steel with focus on the top slag is studied using computational thermodynamic calculations. The theoretical assessment was based on compositional data of steel and slag samples collected during trials in a plant equipped with a 65‐t EAF, a ladle furnace and a vacuum degasser. The calculation of slag‐metal equilibrium during ladle treatment was performed for top slag excluding (Case A) and including (Case B) slag carryover and deoxidation products. The predictions showed that slag carryover and deoxidation products in the ladle significantly affect the composition of the top slag. Thus, these conditions must be taken into consideration when predicting the actual top slag composition. It was also found that it is possible to predict the final aluminium and sulphur content in steel based on the same calculation approach.     

Origins of Non‐metallic Inclusions and their Chemical Development during Ladle Treatment

In the present work, steel, slag and refractory samples were taken from the ladle at OVAKO Steel in Hofors, Sweden. The steel samples were analysed in LOM, SEM and OES PDA. The chemical compositions of the inclusions were determined by EDS. According to the morphologies and compositions, the inclusions were classified into 5 different types, namely, (1) type‐1, alumina inclusions, (2) type‐2, calcium aluminate, (3) type‐3, spinel+calcium aluminate, (4) type‐4, calcium aluminate surrounded by a CaS shell, and (5) spinel+calcium aluminate surrounded by a CaS shell. Ladle glaze was found to be a major supplier of the inclusions, while the inclusions brought over from EAF could be another important source. The results of OES PDA showed that removal of inclusions took place mostly during the vacuum degassing period.     

电渣重熔工艺对高氮钢脱硫的影响

对电渣重熔制备高氮钢的脱硫过程进行了研究,分别采用不同渣系和熔炼速率对高氮钢进行制备。对电渣重熔前后夹杂物进行电镜及成分分析,分析结果表明:硫化物夹杂的平均直径和单位面积数量大大减少,夹杂物的主要类型为MnS+Al2O3复合型夹杂物;适度提高渣中CaO含量可提高硫分配比,是提高脱硫效率的有效手段。同时,实验结果表明重熔速率对电渣重熔中的脱硫率具有重要影响。通过脱硫动力学推导,发现重熔速率越低,脱硫效果越明显,但实验发现脱硫率随重熔速率的降低有时呈现先降低后升高的趋势,其原因在于渣池中发生硫化物富集,导致"回硫"现象发生,降低了脱硫率。     

260 t转炉-RH流程IF钢DC06顶渣改质剂优化生产实践

为保证精炼渣有较好的脱氧与吸附夹杂物的能力,采取5种顶渣改质剂对无间隙原子钢(IF钢)转炉出钢钢包顶渣进行改质工业试验.并采用电感耦合等离子体原子发射光谱法(ICP-AES)对渣成分进行检测,运用FactSage软件对精炼渣粘度进行计算.研究结果表明,5种改质剂改质后,真空循环脱气(RH)出站时,5炉精炼渣中TFe含量...     

The Effect of Ladle Treatment on Inclusion Composition in Tool Steel Production

An in‐depth SEM‐investigation of the inclusion composition change in the ladle refining process during tool steel production was done. Plant trials were carried out at Uddeholm Tooling AB in Hagfors, Sweden. A series of samples was taken during ladle refining, from deslagging through vacuum treatment. The goal was evaluating the effects of the top slag and other process parameters on the inclusion composition during ladle refining. The main conclusion was that normal variations in the process have a large influence on inclusion composition. Furthermore, it was concluded that the MgO content in the top slag had a large influence on the inclusion composition throughout the whole ladle refining process. Also, the SiO₂ content in the ladle slag originating from the EAF‐slag, had a noticeable effect on the inclusions. The inclusions belonging to the system Al₂O₃‐CaO‐MgO‐SiO₂ showed a continuous composition change throughout the ladle refining process, from high Al₂O₃, via MgO‐spinel to finally complex types rich in CaO and Al₂O₃.     

Modeling Manganese Silicate Inclusion Composition Changes during Ladle Treatment Using FactSage Macros

This work investigated the use of FactSage macros to simulate steel–slag and steel–inclusion reaction kinetics in silicon-manganese killed steels, and predict oxide inclusion composition changes during ladle treatment. These changes were assessed experimentally using an induction furnace to simulate deoxidation and slag addition. The average steel mass transfer coefficient for the experimental setup was calculated from the analyzed aluminum pick-up by steel. Average oxide inclusion composition was measured using scanning electron microscopy and energy-dispersive X-ray spectroscopy. Confocal laser scanning microscopy was used to assess the physical state (solid or liquid) of oxide inclusions in selected samples. The changes in the chemical compositions of the oxide inclusions and the steel agreed with the FactSage macro simulations.     

Investigation on the removal efficiency of inclusions in Al-killed liquid steel in different refining processes

Industry trials were carried out to study the removal efficiency of inclusions in Al-killed liquid steel in the processes of BOF–LF–RH–CC and BOF–RH–CC. It was found that the removal efficiency of inclusions has a high dependence on inclusion types. Solid inclusions are more easily to be removed than liquid inclusions. The removal efficiency of solid Al₂O₃ inclusions is higher than that of solid CaO–Al₂O₃–MgO inclusions. As liquid CaO–Al₂O₃–MgO inclusions coexisted with solid CaO–Al₂O₃–MgO inclusions in the liquid steel, the low removal efficiency of inclusions in RH degassing process was found in BOF–LF–RH–CC process. However, high removal efficiency and ultra-low total oxygen (T.O) content were obtained in BOF–RH–CC process because the inclusions were mainly composed of solid Al₂O₃ although initial T.O content before RH degassing was relatively high. This is due to the fact that solid Al₂O₃ tends to form cluster-shaped inclusions which have both a higher contact angle and a lower work of adhesion with steel than calcium aluminate, resulting in easier removal by RH degassing. Therefore, it is proposed to weaken steel–slag reaction and calcium treatment before RH degassing to retain solid Al₂O₃ inclusions in the steel.     

Effect of ladle slag of previous heat on number of non-metallic inclusions in ladle treatment of tool steel

Abstract

Nine series of industrial trials were carried out using the same ladle in each series to examine the effect of ladle slag on the number of non-metallic inclusions in the next heat. Steel and slag samples were taken after ladle vacuum treatment for chemical composition analysis. Samples of the final steel product were examined to determine the number of non-metallic inclusions. It was found that the number of inclusions increased with SiO₂ content of the ladle slag in the previous heat. No clear trends were found for the effects of viscosity and MgO activity of the previous slag on the number of inclusions.Theoretical analysis based on the experimental results suggested that the formation of 2CaO.SiO₂ followed, but the dusting of the compound made the refractory more porous, which was reasonable for the number of non-metallic inclusions.     

管线钢冶炼过程夹杂物行为研究

对管线钢从精炼开始到轧材整个生产过程中夹杂物的行为进行了研究。采用金相、化学等各种分析手段,分析了管线钢钢水、连铸坯及热轧卷的洁净度的变化。试验结果表明：现有的生产工艺在稳态浇注时钢水的洁净度满足产品质量要求,LF精炼后钢中夹杂物的含量明显降低。钢水和稳定态铸坯没有发现大于20μm的夹杂物和聚集的夹杂物。非稳态的头坯热轧卷2m长度内氧化铝夹杂超标,主要位于铸坯的内孤表面;头坯热轧卷的夹杂物检测中发现了K2O等成分,表明开浇过程由于拉速变化引起结晶器卷渣。     

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。