Control of MgO·Al2O3 Spinel Inclusions during Protective Gas Electroslag Remelting of Die Steel

The effect of calcium treatment and/or aluminum-based deoxidant addition on the oxygen control and modification of MgO·Al₂O₃ spinel inclusions during protective gas electroslag remelting (P-ESR) of H13 die steel with low oxygen content was experimentally studied. It is found that all the inclusions in the consumable electrode are MgO·Al₂O₃ spinels, besides a few MgO·Al₂O₃ spinels surrounded by an outer (Ti,V)N or MnS layer. After P-ESR refining combined with proper calcium treatment, all the original MgO·Al₂O₃ spinels in the electrode (except for the original MgO·Al₂O₃ spinels having been removed in the P-ESR process) were modified to mainly CaO-MgO-Al₂O₃ and some CaO-Al₂O₃ inclusions, both of which have a low melting point and homogeneous compositions. In the case of only Al-based deoxidant addition, all the oxide inclusions remaining in ESR ingots are MgO·Al₂O₃ spinels. The operation of Al-based deoxidant addition and/or calcium treatment during P-ESR of electrode steel containing low oxygen content is invalid to further reduce the oxygen content and oxide inclusions amount compared with remelting only under protective gas atmosphere. All the original sulfide inclusions were removed after the P-ESR process. Most of the inclusions in ESR ingots are about 2 μm in size. The mechanisms of non-metallic inclusions evolution and modification of MgO·Al₂O₃ spinels by calcium treatment during the P-ESR process were proposed.     

Investigation of Oxide Inclusions and Primary Carbonitrides in Inconel 718 Superalloy Refined through Electroslag Remelting Process

The effect of remelting atmosphere and calcium treatment during electroslag remelting (ESR) of Inconel 718 superalloy on the oxide inclusions and primary carbonitrides was investigated. The results show that after ESR refining combined with calcium treatment, the original oxide inclusions in the electrode, mainly MgO·Al₂O₃ spinels and some MgO inclusions, were modified to CaO-Al₂O₃ system inclusions or the inclusions of MgO·Al₂O₃ spinel core surrounded by CaO-MgO-Al₂O₃ system inclusion layer. Without the calcium treatment in ESR process, all the oxide inclusions in superalloy ingots are MgO·Al₂O₃ spinels. All the oxide inclusions in ESR ingots act as the nucleation site for carbonitride (Nb,Ti)CN with two-layer structure precipitation, except for those with a single (Nb,Ti)CN layer containing a small amount of Ti and N in the ingot refined by a proper amount of calcium addition in ESR process. The carbonitrides (Nb,Ti)CN formed directly on the oxide inclusion have a small amount of Nb and C as well as a relatively fixed atomic ratio of Nb/Ti (about 0.6:1). There is a Nb-rich and C-rich (Nb,Ti)CN layer on the pre-existing (Nb,Ti)CN formed on the oxide inclusion. The size of the observed carbonitrides is in the range of 5 ??m to 15 ??m. The calcium treatment in the ESR process has a significant effect on the morphology of carbonitrides in superalloy ingot due to modification of oxide inclusions by Ca-treatment resulting in the change of precipitation and growth conditions for carbonitrides. The morphologies of carbonitrides were changed from clustered block or single octahedral to skeleton-like after calcium treatment.     

Flux Entrapment and Titanium Nitride Defects in Electroslag Remelting of INCOLOY Alloys 800 and 825

Electroslag remelted (ESR) ingots of INCOLOY alloys 800 and 825 are particularly prone to macroscale slag inclusions and microscale cleanliness issues. Formation of these structures near the ingot surface can cause significant production yield losses (~10 pct) due to the necessity of extensive surface grinding. Slag inclusions from near the outer radius of the toe end of alloy 800 and 825 ingots were found to be approximately 1 to 3 mm in size and have a multiphase microstructure consisting of CaF₂, CaTiO₃, MgAl₂O₄, MgO, and some combination of Ca₁₂Al₁₄O₃₂F₂ and/or Ca₁₂Al₁₄O₃₃. These inclusions were often surrounded by fields of 1- to 10-μm cuboidal TiN particles. A large number of TiN cuboids were observed in the ESR electrode with similar size and morphology to those observed surrounding slag inclusions in the ESR ingots, suggesting that the TiN particles are relics from the ESR electrode production process. Samples taken sequentially throughout the AOD processes showed that the TiN cuboidals that are found in ESR ingots form between tapping the AOD vessel into the AOD ladle and the casting of ESR electrodes.     

自耗电极冶金质量对G20CrNi2Mo轴承钢电渣锭洁净度的影响

研究了G20CrNi2Mo轴承钢电渣重熔过程自耗电极对电渣锭洁净度的影响。结果表明,电渣锭洁净度与自耗电极的冶金质量有较大的相关性。随着自耗电极氧含量的升高,电渣锭氧含量呈升高趋势。通过扫描电镜-能谱仪分析发现,氧含量较高的自耗电极中低熔点CaO-MgO-Al₂O₃夹杂物数量比低氧含量自耗电极的要多。由于低熔点夹杂物与钢液的界面能较低,限制了其在电渣重熔过程中的去除效率,从而导致电渣锭氧含量较高。通过电弧炉出钢高拉碳操作,氧含量低于0. 002 0%的锭子数量占到总量的90%以上。     

Evolution of CaO–MgO–Al2O3–CaS–(SiO2) inclusions in H13 die steel during electroslag remelting process

The effect of Al–Mg alloy addition on the cleanliness and CaO–MgO–Al₂O₃–CaS–(SiO₂) inclusions during electroslag remelting of H13 die steel with low oxygen content was investigated by experimental study and thermodynamic calculation. The results show that the oxygen content of consumable electrode (15?×?10^?6) was invalid to be reduced after Al–Mg alloy addition during protective gas electroslag remelting (P-ESR) process. In the case of Al–Mg alloy addition during P-ESR process, the oxygen content pick-up and silicon loss of remelted ingot were avoided, and sulphur content was further decreased. CaS content of complex inclusions were decreased significantly after P-ESR process. The type of inclusions was not changed during P-ESR process with Al–Mg alloy addition, except the increase in the concentrations of CaS in individual inclusion. The original oxide inclusions inconsumable electrode transformed to liquid state during P-ESR, and original CaS inclusions were eliminated. The CaS portion in CaS-bearing complex inclusions is the inclusion that formed on the surface of oxide inclusion during the cooling of liquid steel process in liquid metal pool.     

Effect of protected electroslag remelting on cleanliness of G20CrNi2Mo bearing steel

To meet the high cleanliness requirements of bearing steel used in high-speed railway trains, a new production process combining vacuum induction melting and electroslag remelting (ESR) was used to produce G20CrNi2Mo bearing steel. To investigate the effect of remelting on the cleanliness of the steel, two kinds of G20CrNi2Mo steels were prepared using an ESR furnace with and without high-purity argon protection. The results show that the G20CrNi2Mo electrodes smelted using a vacuum induction furnace have very high cleanliness 0.010[P%]–0.004[S%]–0.0012[O%]–0.0041[N%]). Unprotected ESR leads to an increased oxygen content, while protected ESR prevents any increase in oxygen content. Both protected and unprotected ESR results in significant desulphurisation, with desulphurisation rates reaching over 50%. The protected ESR process removes Al₂O₃–SiO₂–MnO inclusions, and the remaining inclusions in the steel can be divided into two categories, Al₂O₃ and Al₂O₃–MnS.     

降低电渣重熔GCr15钢的氧含量

在420mm／250mm方形结晶器的工业炉上重熔1．2t锭,比较了含高氟的二元渣系、低氟和中氟的五元渣系、重熔气氛（Ar气保护）、自耗电极表面质量、渣中aFeO值和自耗电极含氧量对重熔钢氧含量的影响。结果表明,自耗电极氧含量为5．87×10-6和10×10-6．在氮气气氛、低氟五元渣系下重熔,电渣过程是一个增氧过程,重馆过程中降低渣中aFeO值,可使GCr15钢的氧含量降至15×10-6左右。     

Effect of Slag on Inclusions During Electroslag Remelting Process of Die Steel

Many factors influence the non-metallic inclusions in electroslag steel including furnace atmosphere and inclusions’ content in the consumable electrode, slag amount and its composition, power input, melting rate, filling ratio, and so on. Fluoride containing slag, which influences the non-metallic inclusions to a great extent, has been widely used for the electroslag remelting process. The current paper focuses on the effect of fluoride containing slag on the inclusions in electroslag ingots based on the interaction of the slag-metal interface and electroslag remelting process. In this work, die steel of CR-5A and several slags have been employed for investigating the effect of slag on inclusions in an electrical resistance furnace under argon atmosphere in order to eliminate the effect of ambient oxygen. Specimens were taken at different times for analyzing the content, dimensions, and type of non-metallic inclusions. Results of quantitative metallographic analysis indicate that a multi-component slag has better capacity for controlling the amount of inclusions; especially protective gas atmosphere has also been adopted. The findings of inclusions in electroslag steel by SEM–EDS analysis reveal that most non-metallic inclusions in electroslag steel are MgO-Al₂O₃ inclusions for multi-component slags, but it is Al₂O₃ inclusions when remelting using conventional 70 wt pct CaF₂-30 wt pct Al₂O₃ slag. The maximal inclusions’ size using multi-component slags is less than that using conventional binary slag. Small filling ratio as well as protective gas atmosphere is favorable for controlling the non-metallic inclusions in electroslag steel. All the results obtained will be compared to the original state inclusions in steel, which contribute to choice of slag for electroslag remelting.     

Effect of the Slag Composition and a Protective Atmosphere on Chemical Reactions and Non‐Metallic Inclusions during Electro‐Slag Remelting of a Hot‐Work Tool Steel

The remelting behavior of the hot‐work tool steel X37CrMoV5‐1 is investigated with several experimental melts on a lab‐scale ESR‐plant. The investigated parameters comprise a variation of the slag compositions and the use of a protective nitrogen atmosphere. Variations of the slag composition include slags with different contents of CaF₂, CaO, and Al₂O₃ as well as a variation of the SiO₂‐content in the slag. The remelted ingots are forged and analyzed regarding their chemical composition. The distribution and composition of the non‐metallic inclusions (NMI) is studied by an automated SEM‐EDX method. Additionally, the chemical composition of the slag after remelting is analyzed. The results show clearly an equilibrium reaction between Si and Al in the steel with SiO₂ and Al₂O₃ in the slag as well as the effect of oxygen in open ESR operation. A protective atmosphere reduces the Si‐losses during remelting, but has no major effect on the number or composition of NMI compared to open remelting. The content of NMI, especially the larger ones, is reduced significantly in all remelting experiments. The majority of the NMI are MA‐spinel type except for the CaO‐free slag, where alumina inclusions prevail. In general, remelting leads to an almost complete removal of sulfides, a reduction of oxisulfides, and a slight increase of oxides.     

电渣重熔对FeCrAl不锈钢中夹杂物的影响

应用扫描电镜(SEM)及X射线能谱分析仪(EDS)对真空感应炉熔炼(VIM)和电渣重熔炉熔炼(ESR)的FeCrAl不锈钢中夹杂物的成分、形貌、数量、大小进行统计分析。结果表明,AlN是2个钢锭的主要夹杂物,电渣重熔过程中产生的高熔点稀土化合物可作为AlN析出的异质核心。电渣重熔使自耗电极中尺寸大于10μm的夹杂物基本消失,ESR锭中尺寸小于5μm的夹杂物达到86.7%。电渣重熔减小了AlN的平均尺寸,去除了大尺寸的AlN,使AlN的总面积明显减小。热力学计算结果表明,VIM锭中AlN可以直接在液相中析出;ESR锭中氮含量的降低造成AlN不能直接在液相中析出,随着凝固的进行,[Al]和[N]在残余液相中富集,当凝固分率大于0.615后,AlN才能在凝固前沿的残余液相中析出。     

电渣重熔过程电源频率对电渣锭洁净度的影响

电渣重熔采用低频供电可以提高功率因数、降低电耗,并实现电力系统的三相平衡。然而,其对电渣锭冶金质量特别是洁净度的影响还缺乏足够的数据支撑。为了研究电源频率特别是低频操作对电渣重熔锭洁净度的影响,采用实验室小型低频电渣重熔炉,以304奥氏体不锈钢、GCr15轴承钢为研究对象,详细分析了不同的电源频率对电渣锭化学成分、气体含量、夹杂物分布的影响规律。研究结果发现,与工频电渣重熔相比,不论是不锈钢还是轴承钢,当采用低频电源(2、1、0.4、0.1 Hz)电渣重熔后(在其他工艺参数如渣系、渣量、电流、电压、气氛等完全相同的情况下),电渣锭中的氧质量分数(0.010%～0.013%)大幅增加,对氮含量影响很小。电渣锭中的铝含量明显增加,而其他化学成分变化很小。与此相对应,低频电渣重熔锭的夹杂物数量也明显增加,且增加的夹杂物主要以氧化铝为主,但是夹杂物主要以小于10μm的细小夹杂为主,大颗粒夹杂物略有增加,但是数量较少。氧含量增加的主要原因是低频电源的直流倾向增大,使重熔渣池中的氧化铝发生了电解(30%Al₂O₃+70%CaF₂渣系...     

Mathematical modelling of electroslag remelting P91 hollow ingots process with multi-electrodes

Abstract

Electroslag remelting (ESR) hollow ingot process with T-shape current supplying mould is a new metallurgical technology. A mathematical model was developed to describe the interaction of multiple physical fields of this process for studying the process technology. Maxwell, Navier-Stokes and heat transfer equations have been adopted in the model to analyse the electromagnetic field, magnetic driven fluid flow, buoyancy driven flow and heat transfer using finite element software ANSYS. Moreover, the model has been verified through the metal pool depth measurements, which were obtained during remelting of 10 electrodes into Φ900/500 mm hollow ingots of P91 steel, with a slag composition of 50–60 wt-% CaF₂, 10–20 wt-% CaO, 20–30 wt-% Al₂O₃, ≤8 wt-% SiO₂. There was a good agreement between the calculated results and the measured results. The calculated results show that the distribution of current density, magnetic induction intensity, electromagnetic force, Joule heating, fluid flow and temperature are symmetric but not uniform due to the multi-electrode arrangement in two symmetric groups. Simulation of the ESR hollow ingot process will help to understand the new technology process and optimise operating parameters.     

Control of Carbide Precipitation During Electroslag Remelting-Continuous Rapid Solidification of GCr15 Steel

The effect of the electroslag remelting-continuous rapid solidification (ESR-CRS) process on element segregation and carbide precipitation in GCr15 bearing steel was investigated. The results showed that a microstructure with fewer primary carbides and less segregation can be achieved via the ESR-CRS process. In the specimen subjected to ESR, the morphology of primary carbide changed from angular to lumplike. After the ESR-CRS process, the dimension of primary carbide and the mass fraction of Cr in primary carbide decreased. With the increase of cooling intensity during the ESR-CRS process, the microstructure of ESR ingot became more refined and uniform and the size of large primary carbides in ESR ingot gradually decreased. Suppressing the formation of grain boundary cementite and primary carbide during ESR of GCr15 steel is beneficial to inhibiting the presence of large secondary carbide formation after annealing. With the increase of cooling intensity, the mass fraction of carbides in ESR ingot decreased and the mass fraction of Cr in carbides increased, whereas the types of carbides did not change; all the M₃C, M₃C₂, and M₇C₃ exist before and after ESR-CRS. Al₂O₃ inclusions promoted the formation of Ti(N,C) by serving as preferred heterogeneous nucleation sites, whereas the formation of Ti(N,C) was suppressed through the refinement of Al₂O₃ inclusions by increasing the cooling intensity.     

Effects of Reoxidation of Liquid Steel and Slag Composition on the Chemistry Evolution of Inclusions During Electroslag Remelting

Electroslag remelting (ESR) is increasingly used to produce some varieties of special steels and alloys, mainly because of its ability to provide extreme cleanliness and an excellent solidification structure simultaneously. In the present study, the combined effects of varying SiO₂ contents in slag and reoxidation of liquid steel on the chemistry evolution of inclusions and the alloying element content in steel during ESR were investigated. The inclusions in the steel before ESR refining were found to be oxysulfides of patch-type (Ca,Mn)S adhering to a CaO-Al₂O₃-SiO₂-MgO inclusion. The oxide inclusions in both the liquid metal pool and remelted ingots are CaO-Al₂O₃-MgO and MgAl₂O₄ together with CaO-Al₂O₃-SiO₂-MgO inclusions (slightly less than 30 pct of the total inclusions), which were confirmed to originate from the reduction of SiO₂ from the original oxide inclusions by dissolved Al in liquid steel during ESR. CaO-Al₂O₃-MgO and MgAl₂O₄ are newly formed inclusions resulting from the reactions taking place inside liquid steel in the liquid metal pool caused by reoxidation of liquid steel during ESR. Increasing the SiO₂ content in slag not only considerably reduced aluminum pickup in parallel with silicon loss during ESR, but also suppressed the decrease in SiO₂ content in oxide inclusions. (Ca,Mn)S inclusions were fully removed before liquid metal droplets collected in the liquid metal pool.     

电渣重熔空心钢锭技术的开发

 首先总结和评价了大型筒体的制造方法,以及不同方法生产空心钢锭的优缺点,重点分析了传统电渣重熔法生产空心钢锭存在的主要问题。在此基础上,与乌克兰Elmet-Roll公司合作开发了基于双电源、T型导电结晶器和电极交换的电渣重熔空心钢锭的新技术。成功地试制了多个钢种（35CrMo、P91、TP347和Mn18Cr18N等）和不同规格（[?]900/[?]500 mm、[?]900/[?]200 mm、[?]650/[?]450 mm,最长锭尺寸为6 000 mm）的电渣空心钢锭。工业试验表明,生产的空心锭表面质量和内部质量优异,结晶组织致密,洁净度高,是生产高端厚壁管和筒体锻件的理想材料。     

Electrode polarization in the DC electroslag melting of pure iron

It is evident from the known ionic properties of the slags used in electroslag melting, that the dc melting process must be accompanied by Faradaic reactions on the slag/ingot and slag/electrode interfaces. The present work has determined the magnitude of the overpotentials resulting from concentration polarization at these interfaces, in the case of pure iron/CaF₂+Al₂O₃, CaF₂+CaO slags using a galvanostatic pulsing technique in an electrolytic cell. The polarization overpotential existing on an electrode in an operating ESR unit has been measured by the same technique. It is found that the potentials observed on the ESR electrode agree well with the results from the electrolytic cell. The primary anodic process is postulated to be the corrosion of iron, leading to an Fe²⁺-saturated layer on the anode surface at sufficiently high current densities. The cathodic process is suggested to be the Faradaic reduction of Al³⁺ or Ca²⁺, to give a concentration of [Al]_Fe or (Ca)_slag in the cathode interface region. This observation is supported by the fact that the cathodic potentials with respect to a C/CO reference electrode are close to those predicted from the reactions: (Al₂O₃)+3C=3CO(g)+2Al(l) or (CaO)+C=CO(g)+Ca(g) At very high current densities both the anodic and cathodic processes may convert to arcs, leading to process instability. The chemical and thermal effects of the overpotentials are briefly discussed and compared with the present results on ESR ingots of pure iron.     

17-7PH沉淀硬化不锈钢电渣重熔过程洁净度的变化

采用35 t电弧炉-AOD脱碳-LF精炼-模铸工艺制备了17-7PH沉淀硬化不锈钢自耗电极,并通过气体保护电渣炉重熔得到了2 t重的电渣锭。利用ASPEX扫描电镜分析了电渣重熔前后17-7PH钢中夹杂物数量、尺寸、成分的变化规律,并采用SEM-EDS进一步观察夹杂物的形貌及组成。研究结果发现,电渣重熔后,O含量由6.6×10^-6降至5.7×10^-6,N含量由200×10^-6降至180×10^-6。重熔前后夹杂物的类型没有变化,重熔后总的夹杂物数量大幅减少,特别是大颗粒夹杂物的数量明显减少、尺寸减小。电渣锭中总的夹杂物以AlN夹杂物为主,其尺寸较大、数量最多。为了提高17-7PH钢电渣锭的洁净度,应尽可能减少自耗电极中的N含量,以减少电渣重熔过程AlN夹杂物的生成量。     

Electrode reactions in dc electroslag remelting of steel rod

Abstract

The non-metallic inclusion content increased significantly when a steel rod of Fe-Ni was remelted by dc electroslag remelting. The silicon content increased slightly. The manganese and sulphur contents did not change. The total aluminium content in the ingot was max. 0·7%, while that in the electrode was only 10 ppm. The aluminium cations Al³+ in the slag are reduced to metallic aluminium at the slag/electrode interface, while O² - anions are oxidised to dissolved O in the metal pool. This Al and O subsequently recombine to form alumina inclusions in the metal pool. The inclusion content was dependent on the alumina content in the slag. When a rod of plain carbon steel was remelted, however, the increase in nonmetallic inclusion content was as little as one-tenth of that for the remelted Fe-Ni rod. The non-metallic inclusion content was independent of the polarity of the electrode.     

Simultaneous Modification of Alumina and MgO·Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Inclusions by Calcium Treatment During Electroslag Remelting of Stainless Tool Steel

Calcium modification of both alumina and MgO·Al₂O₃ inclusions during protective gas electroslag remelting (P-ESR) of 8Cr17MoV stainless steel and its effect on nitrides and primary carbides were studied by analyzing the transient evolution of oxide and sulfide inclusions in the P-ESR process. The oxide inclusions that were not removed during P-ESR without calcium treatment were found to retain their original state until in as-cast ingot. Calcium treatment modified all MgO·Al₂O₃ and alumina inclusions that had not been removed in the P-ESR process to liquid/partially liquid CaO-Al₂O₃-(MgO) with uniformly distributed elements, in addition to a small proportion of partially modified inclusions of a CaO-MgO-Al₂O₃ core surrounded by a liquid CaO-Al₂O₃. The modification of low-MgO-containing MgO·Al₂O₃ inclusions involves the preferential reduction of MgO from the MgO·Al₂O₃ inclusion by calcium and the reaction of calcium with Al₂O₃ in the inclusion. It is the incomplete/complete reduction of MgO from the spinel by calcium that contributes to the modification of spinels. Alumina inclusions were liquefied by direct reaction with calcium. Calcium treatment during P-ESR refining also provided an effective approach to prevent the formation of nitrides and primary carbides in stainless steel through modifying their preferred nucleation sites (alumina and MgO·Al₂O₃ inclusions) to calcium aluminates, which made no contribution to improving the steel cleanliness.     

Effect of Sulfur in Steel on Transient Evolution of Inclusions During Calcium Treatment

In the current study, the effect of S content in the molten steel on inclusions during calcium treatment was studied using an induction furnace. The calcium in steel decreased from 48 to 2 ppm, and the sulfur in steel changed a little with time. When sulfur content in steel was as low as 25 ppm during calcium treatment, inclusions shifted from CaO-Al₂O₃-CaS to Al₂O₃-CaO with about 35 pct CaO. When the sulfur increased over 90 ppm, more CaS-CaO formed just after the addition of calcium, and then the CaS content decreased from over 45 pct to lower than 15 pct and inclusions were mostly Al₂O₃-CaO-CaS and Al₂O₃-CaO with a high Al₂O₃ content. Thermodynamic calculation predicted the variation of the composition of inclusions, indicating good agreement with the measurement, while a certain deviation existed, especially for heats with 90 and 180 ppm sulfur. A reaction model was proposed for the formation of CaO and CaS, which considered the reaction between calcium vapor bubbles in the zone and the dissolved oxygen and sulfur in the molten steel, as described by a Langmuir-type adsorption isotherm with a reaction occurring on the remaining vacant sites. The variation of transient CaS inclusions was discussed based on the thermodynamic calculation and the morphology evolution of typical inclusions containing CaS.