Development and prospects of molten steel deoxidation in steelmaking process

In the long traditional process of steelmaking,excess oxygen is blown into the converter,and alloying elements are used for deoxidation.This inevitably results in excessive deoxidation of products remaining within the steel liquid,affecting the cleanliness of the steel.With the increasing requirements for steel performance,reducing the oxygen content in the steel liquid and ensuring its high cleanliness is necessary.After more than a hundred years of development,the total oxygen content in steel has been reduced from approximately 100×10^(-6)to approximately 10×10^(-6),and it can be controlled below 5×10^(-6)in some steel grades.A relatively stable and mature deoxidation technology has been formed,but further reducing the oxygen content in steel is no longer significant for improving steel quality.Our research team developed a deoxidation technology for bearing steel by optimizing the entire conventional process.The technology combines silicon–manganese predeoxidation,ladle furnace diffusion deoxidation,and vacuum final deoxidation.We successfully conducted industrial experiments and produced interstitial-free steel with natural decarbonization predeoxidation.Non-aluminum deoxidation was found to control the oxygen content in bearing steel to between 4×10^(-6) and 8×10^(-6),altering the type of inclusions,eliminating large particle Ds-type inclusions,improving the flowability of the steel liquid,and deriving a higher fatigue life.The natural decarbonization predeoxidation of interstitial-free steel reduced aluminum consumption and production costs and significantly improved the quality of cast billets.

New steelmaking process based on clean deoxidation technology

After the converter steelmaking process,a considerable number of ferroalloys are needed to remove dissolved oxygen from the molten steel,but it also forms a lot of oxide inclusions that cannot be completely removed.At the same time,it increases the carbon emis-sions in the steel production process.After years of research,our team have developed a series of clean deoxidation technologies,includ-ing carbon deoxidation,hydrogen deoxidation,and waste plastic deoxidation of molten steel to address the aforementioned issues.In this study,thermodynamic calculations and laboratory experiments were employed to verify that carbon and hydrogen can reduce the total oxygen content in the molten steel melt to below 5×10^(-6) and 10×10^(-6),respectively.An analysis of the deoxidation mechanisms and ef-fects of polyethylene and polypropylene was also conducted.In addition,the applications of carbon deoxidation technology in different steels with the hot-state experiment and industrial production were discussed carefully.The carbon deoxidation experimental results of different steels were as follows:(1)the oxygen content of bearing steel was effectively controlled at 6.3×10^(-6) and the inclusion number density was lowered by 74.73%compared to aluminum deoxidized bearing steel;(2)the oxygen content in gear steel was reduced to 7.7×10^(-6) and a 54.49%reduction of inclusion number density was achieved with almost no inclusions larger than 5μm from the average level of industry gear steels;(3)a total oxygen content of M2 high-speed steel was as low as 3.7×10^(-6).In industrial production practice,car-bon deoxidation technique was applied in the final deoxidation stage for non-aluminum deoxidized bearing steel,and it yielded excellent results that the oxygen content was reduced to below 8×10^(-6) and the oxide inclusions in the steel mainly consist of silicates,along with small amounts of spinel and calcium aluminate.

Thermodynamic model for deoxidation of liquid steel considering strong metal-oxygen interaction in the quasichemical model framework

Herein,a thermodynamic model aimed at describing deoxidation equilibria in liquid steel was developed.The model provides explicit forms of the activity coefficient of solutes in liquid steel,eliminating the need for the minimization of internal Gibbs energy preliminarily when solving deoxidation equilibria.The elimination of internal Gibbs energy minimization is particularly advantageous during the coupling of deoxidation equilibrium calculations with computationally intensive approaches,such as computational fluid dynamics.The model enables efficient calculations through direct embedment of the explicit forms of activity coefficient in the computing code.The proposed thermodynamic model was developed using a quasichemical approach with two key approximations:random mixing of metallic elements(Fe and oxidizing metal) and strong nonrandom pairing of metal and oxygen as nearest neighbors.Through these approximations,the quasichemical approach yielded the activity coefficients of solutes as explicit functions of composition and temperature without requiring the minimization of internal Gibbs energy or the coupling of separate programs.The model was successfully applied in the calculation of deoxidation equilibria of various elements(Al,B,C,Ca,Ce,Cr,La,Mg,Mn,Nb,Si,Ti,V,and Zr).The limitations of the model arising from these assumptions were also discussed.

Controlling oxygen content in electro-slag remelting steel by optimizing slag-steel reaction process

The thermodynamic equilibrium of deoxidation reactions between molten slag and steel was calculated using a slag-steel coupling thermodynamic model and the mass conservation model based on the ion-molecular coexistence theory.The study focused on the effects of slag composition and deoxidizer type on the oxygen content of low alloy steel during the electroslag remelting(ESR)process.The measured and predicted values of the oxygen content in remelted ingots,and the contents of FeO and MnO in slags were compared and analyzed.Results show that the measured content of total oxygen has a certain correlation with the trend of dissolved oxygen predicted by the model when using Ca-Si alloys as deoxidizer,but it is not correlated with the trend of dissolved oxygen predicted by the model when using Al as deoxidizer.The deoxidation mechanisms of Ca-Si and Al are different.Ca-Si alloy directly reacts with FeO and MnO in slag to reduce the oxygen potential of slag,hence it can inhibit the transfer of oxygen from the slag to molten steel.While,when Al deoxidizer is used,the oxygen content in steel is mainly reduced through floating up the alumina inclusions.Compared to Al,utilizing Ca-Si alloy as a deoxidizer is more effective in reducing the oxygen content and the amount of inclusions in ESR ingot.

Calculation of phase diagram of Ti-Ni-O system and application to deoxidation of TiNi alloy

The Ti-Ni-O ternary system was assessed by means of Calphad method using ternary experimental data in previous study.Isothermal sections at 1 173 and 1 273 K were calculated.The result shows that the present calculated results are in good agreement with most of the experimental results.The consistent model parameter set determined in this work may provide theoretical guidance for the deoxidation of TiNi alloy.

Oxide Inclusions at Different Steps of Steel Production

The formation of oxide inclusions in one of the carbon steel productions of Mobarakeh Steel Complex of Isfahan has been evaluated. Several samples from different steps of steel production were taken, from arc furnace, ladle furnace, tundish, and continuous casting mold. Moreover, samples of slab and hot rolling products were prepared. The samples were investigated by optical and scanning electron microscopes equipped with the EDS system. According to the results, the number, composition, and kind of inclusions were directly influenced by the production variables. It was found that when the amount of dissolved oxygen was high （say more than 0. 002 5%）, the dissolved aluminum was able to reduce silicon oxide and react with the dissolved oxygen simultaneously, whilst, the dissolved aluminum could reduce the magnesium oxide only when the oxygen content was below 0. 000 5%. Based on this research, a mechanism for forming the complex inclusions was suggested. It was also found that if the aluminum is added to the melt as late as possible, a cleaner melt with fewer inclusions is prepared; this method will be more effective, especially in the case of complex inclusions.

Allelopathic effect of Acorus tatarinowii upon algae

Besides competing with algae for light and mineral nutrients (i. e. N, P, etc.), the root system of Acorus tatarinowii excretes some chemical substances, which injure and eliminate alga cells, to inhibit the growth of the algae. When the algae cells were treated in 'A. tatarinowii water', some of the chlorophyll a were destroyed and the photosynthetic rate of algae decreased markedly and the ability of alga cells to deoxidize triphenyltetrazolium chloride (TTC) reduced greatly. Then alga cells turned from bright red to bluish green under fluorescence microscope. These showed that the allelopathic effects of A. tatarinowii on algae were obvious and planting A. tatarinowii can control some green algae. The experiment on the extractions of the secretions of the root system showed that the inhibitory effect had a concentration effect. If the concentration of the root secretion was below 30 μl/disc, the inhibitory rate was negative; if it was over 45 μl/disc, the inhibitory rate was positive. This proved that the influence of the root secretion on the same acceptor was a kind of concentration effect. When the concentration of the root secretion was low, it promoted the growth of algae; when the concentration reached a definite threshold value, it restrained the growth of algae. In present case, the threshold value was between 30 μl/disc and 45 μl/disc.

Ultrahigh cycle fatigue fracture mechanism of high-quality bearing steel obtained through different deoxidation methods

The mechanism of oxide inclusions in fatigue crack initiation in the very-high cycle fatigue(VHCF)regime was clarified by subjecting bearing steels deoxidized by Al(Al-deoxidized steel)and Si(Si-deoxidized steel)to ultrasonic tension-compression fatigue tests(stress ratio,R=−1)and analyzing the characteristics of the detected inclusions.Results show that the main types of inclusions in Si-and Al-deoxidized steels are silicate and calcium aluminate,respectively.The content of calcium aluminate inclusions larger than 15μm in Si-deoxidized steel is lower than that in Al-deoxidized steel,and the difference observed may be attributed to different inclusion generation processes during melting.Despite differences in their cleanliness and total oxygen contents,the Si-and Al-deoxidized steels show similar VHCF lives.The factors causing fatigue failure in these steels reveal distinct differences.Calcium aluminate inclusions are responsible for the cracks in Al-deoxidized steel.By comparison,most fatigue cracks in Si-deoxidized steel are triggered by the inhomogeneity of a steel matrix,which indicates that the damage mechanisms of the steel matrix can be a critical issue for this type of steel.A minor portion of the cracks in Si-deoxidized steel could be attributed to different types of inclusions.The mechanisms of fatigue fracture caused by calcium aluminate and silicate inclusions were further analyzed.Calcium aluminate inclusions first separate from the steel matrix and then trigger crack generation.Silicate inclusions and the steel matrix are closely combined in a fatigue process;thus,these inclusions have mild effects on the fatigue life of bearing steels.Si/Mn deoxidation is an effective method to produce high-quality bearing steel with a long fatigue life and good liquid steel fluidity.

Mathematical Simulation of Flow Phenomena in CAS-OB Refining Ladle

The deoxidation behaviors of alloys bearing barium in pipe steel were researched with MgO crucible under argon atmosphere in MoSi 2 furnace at 1 873 K. The total oxygen contents of molten steel, the distribution, size and morphology of deoxidation products in the steel were surveyed. The metamorphic mechanism for deoxidation products of alloy bearing barium was also discussed. The results show that applying alloy bearing barium to the pipe steel, very low total oxygen contents can be obtained, and deoxidation products, which easily float up from molten steel, can be changed into globular shape and uniformly distributed in steel. The equilibrium time of total oxygen is about 25 min, and the terminal total oxygen contents range from 0 002 0 % to 0 002 2 % after treating with SiCa wire. The best deoxidizers are SiAlBaCa and SiAlBaCaSr.

Deoxidation Behavior of Alloys Bearing Barium in Molten Steel

The deoxidation behaviors of alloys bearing barium in pipe steel were researched with MgO crucible under argon atmosphere in MoSi2 furnace at 1 873 K.The total oxygen contents of molten steel,the distribution,size and morphology of deoxidation products in the steel were surveyed.The metamorphic mechanism for deoxidation products of alloy bearing barium was also discussed.The results show that applying alloy bearing barium to the pipe steel,very low total oxygen contents can be obtained,and deoxidation products,which easily float up from molten steel,can be changed into globular shape and uniformly distributed in steel.The equilibrium time of total oxygen is about 25 min,and the terminal total oxygen contents range from 0.002 0%to 0.002 2% after treating with SiCa wire.The best deoxidizers are SiAlBaCa and SiAlBaCaSr.

Deoxidization of CuCr25 alloys prepared by vacuum induction melting

The influence of crucible and vacuum on oxygen content of CuCr25 prepared by vacuum induction melting (VIM) has been investigated. The experimental results show that the selection of crucible is very important. Alkaline oxide MgO crucible can result in increment of oxygen content and segregation of Cr in the CuCr25 alloys prepared. Neutral oxide Al 2O 3 crucible has no contribution to oxygen in CuCr25. The results also indicate that some kinds of deoxidant, such as Al, are further beneficial to deoxidization of the alloys. [

Effects of deoxidizing degree on the pitting corrosion behavior of carbon and manganese steels

Nine steels with different deoxidizing degrees and two comparative steels were selected. Their pitting initiation susceptibility was compared by means of potentiodynamic polarization tests in 3wt% NaCl solution. The pit propagation rate was evaluated in artificial sea water and 3wt% sea salt solution by simulating occluded corrosion cell （SOCC） test and hanging plate test, respectively. The composition of inclusions and corrosive feature were studied by scanning electron microscopy （SEM）, electron probe micro-analysis （EPMA）, and optical microscopy （OM）. The results indicate that sulfide inclusions in steel are the sites for pit nucleation. The sulphide inclusions vary in shape from short spindle-like to long strip-like with increasing deoxidizing degree. Under the same conditions, the lower the deoxidizing degree gets, the lower the pitting initiation susceptibility becomes, and the stronger the resistance to pit propagation exhibits. For steels with different deoxidizing degrees, their pitting initiation susceptibility is mainly influenced by thermodynamic stability, while the pit propagation rate is primarily subject to the characteristics of inclusions in steel.

Thermodynamic calculations and experiments on inclusions to be nucleation sites for intragranular ferrite in Si-Mn-Ti deoxidized steel

Microstructures and inclusions in the Si-Mn-Ti deoxidized steels after cooling in the furnace were investigated. The composition and morphology of the inclusions were analyzed using a field emission scanning electron microscope （FE-SEM） with energy dispersive X-ray spectrometry （EDS）. The kind and composition of the inclusions calculated from the thermodynamic database were in good agreement with the experimental results. There were two main kinds of inclusions formed in the Si-Mn-Ti deoxidized steels. One kind of inclusion was the manganese titanium oxide （Mn-Ti oxide）. Another kind of inclusion was the MnS inclusion with segregation points containing Ti and N. According to the thermodynamic calculation, those segregation points were TiN precipitates. The formation of intragranular ferrite （IGF） microstructures refined the grain size during the austenite-ferrite transformation. The mechanisms of IGF formation were discussed. Mn-Ti oxide inclusions with Mn-depleted zone （MDZ） were effective to be nucleation sites for IGF formation, because the MDZ increased the austenite-ferrite transformation temperature. TiN had the low misfit ratio with IGF, so the TiN precipitated on the MnS surface also promoted the formation of IGF because of decreasing interfacial energies.

Al Preparation from Solid Al_2O_3 by Direct Electrochemical Deoxidation in Molten CaCl_2-NaCl at 550℃

Al was prepared by a new method in molten salt at low temperature. Sintered alumina pellets were used as cathode; graphite rod was employed as anode; and the molten CaCl2-NaCl was the electrolyte. A constant 3.2 V voltage was applied in this experiment, and oxygen in solid alumina cathode was reduced by direct electrochemical deoxidation at 550℃. In this process, the current gradually decreased with increasing time and the alumina pellets became grey and porous. The metallic particles were obtained and characterized by XRD （X-ray diffraction） and SEM （scanning electron microscopy）.

Mechanisms of reactive element Y on the purification of K4169 superalloy during vacuum induction melting

The effects of rare earth element Y on the purification of K4169 superalloy during vacuum induction melting were investigated at different superheating temperatures. The corresponding interaction mechanisms were also clarified. Results showed that the addition of Y remarkably promoted the purification effect on the K4169 melt. The contents of O and S in the K4169 as-cast alloy ingots after purification were 3–4 and 8–10 ppm, respectively. The degrees of deoxidation and desulfurization increased to 50% and 57%, respectively, upon the addition of 0.1 wt% Y. The yttrium-rich phase that precipitated at the grain boundary blocked the diffusion of C and the accumulation of S, thereby contributing to the purification of the alloy.

Determination of inclusions in liquid steel after calcium treatment

Nozzle blocking was eliminated by calcium-treated liquid steel through changing the chemical and phase composition of alumina inclusions in aluminium-killed steel. Three different methods were applied to determine the composition of inclusions in liquid steel： total oxygen content samples, sampling spoon samples, and ＂lollipop＂ steel samples. The results show that calcium modification of liquid steel influences the inclusion composition varying from 2wt% to 14wt% depending on the method used. The composition of inclusions contains mainly Al2O3-CaO only, or is associated with SiO2 or MgO depending on the initial input. The methods used in this study are indicators of the inclusion composition but can be improved to quantify the inclusion size.

Modification of Inclusions in ESR Bearing Steel

Electroslag remelting (ESR) process using consumable electrode deoxidized with Ca-Si and Fe-Si instead of Al, and acid slag (CaF_2SiO_2Al_2O_3CaO) instead of universal slag ANF_6(CaF_2. 70 %+A1_2O_3.30 %) could change brittle inclusion (alumina) to ductile inclusion (silicate) in remelted steel. Fatigue life of bearing steel could be increased significantly in this way.

Effect of magnesium addition on inclusions in H13 die steel

The effect of magnesium addition on the number, morphology, composition, size, and density of inclusions in H13 die steel was studied. The results show that the total oxygen content in the steel can be significantly decreased to 0.0008wt%. Al2O3 and Mn S inclusions are changed into nearly spherical Mg O·Al2O3 spinel and spherical Mg O·Mg S inclusions, respectively. The number of inclusions larger than 1 μm decreases and the number of inclusions smaller than 1 μm increases with increasing magnesium content. V（N,C） precipitates around Mg O·Al2O3 and Mg O·Mg S inclusions during solidification of liquid steel. The densities of Mg O·Al2O3 spinel inclusions are lower than that of alumina inclusions. With increasing magnesium content in the Mg-containing inclusions, the density of inclusions decreases, leading to the improvement of inclusion removal efficiency.

Spark Plasma Sintering Properties of Ultrafine Ti(C,N)-based Cermet

Ultrafine Ti(C,N)-based cermet was sintered by SPS from 1050℃ to 1450℃ and its sintering properties,such as porosity,mechanical properties and phase transformation,were investigated by optical microscopy (OM),scanning electron microscopy (SEM),X-ray diffraction (XRD),and differential scanning calorimeter (DSC).It is found that the spark plasma sintering properties of Ti(C,N)-based cermet differ from those of conventional vacuum sintering.The liquid phase appearance is at least lower by 150℃ than that in vacuum sintering.The porosity decreases sharply below 1 200℃ and reaches minimum at 1 200℃,and afterwards it almost keeps invariable and no longer increases.SPS remarkably accelerates the phase transformation of Ti(C,N)-based cermet and it has a powerful ability to remove oxides in Ti(C,N)-based cermets.Above 1 350℃,denitrification occurred.Fresh graphite phase formed above 1 430℃.Both the porosity and graphite are responsible for the poor TRS.

Preparation of Ultrafine Tungsten Powder by Sol-Gel Method

Sol-gel method was employed for the preparation of nanoscale tungsten powder. The effects of different preparation conditions on particle size were discussed and the optimum preparation condition was found. The products were characterized by X-ray diffraction, scan electron microscopy and so on. The results show that the intermediate is monoclinic WO3, its particle shape is approximately spherical, and the particle size distribution is narrow. The average particle size is about 60 nm. After deoxidization, WO3 turns into cubic tungsten powder with small particle size （average particle size about 120 nm） and narrow size distribution.