Evolution of nonmetallic inclusions in 80-t 9CrMoCoB large-scale ingots during electroslag remelting process

In combination with theoretical calculations,experiments were conducted to investigate the evolution behavior of nonmetallic inclusions(NMIs)during the manufacture of large-scale heat-resistant steel ingots using 9CrMoCoB heat-resistant steel and CaF_(2)–CaO–Al_(2)O_(3)–SiO_(2)–B_(2)O_(3)electroslag remelting(ESR)-type slag in an 80-t industrial ESR furnace.The main types of NMI in the consumable electrode comprised pure alumina,a multiphase oxide consisting of an Al_(2)O_(3)core and liquid CaO–Al_(2)O_(3)–SiO_(2)–MnO shell,and M_(23)C_(6)carbides with an MnS core.The Al_(2)O_(3)and MnS inclusions had higher precipitation temperatures than the M_(23)C_(6)-type carbide under equilibrium and nonequilibrium solidification processes.Therefore,inclusions can act as nucleation sites for carbide layer precipitation.The ESR process completely removed the liquid CaO–Al_(2)O_(3)–SiO_(2)–MnO oxide and MnS inclusion with a carbide shell,and only the Al_(2)O_(3)inclusions and Al_(2)O_(3)core with a carbide shell occupied the remelted ingot.The M_(23)C_(6)-type carbides in steel were determined as Cr_(23)C_(6)based on the analysis of transmission electron microscopy results.The substitution of Cr with W,Fe,or/and Mo in the Cr_(23)C_(6)lattice caused slight changes in the lattice parameter of the Cr_(23)C_(6)carbide.Therefore,Cr_(21.34)Fe_(1.66)C_(6),(Cr_(19)W_(4)C_(6),Cr_(18.4)Mo_(4.6)C_(6),and Cr_(16)Fe_(5)Mo_(2)C_(6)can match the fraction pattern of Cr_(23)C_(6)carbide.The Al_(2)O_(3)inclusions in the remelted ingot formed due to the reduction of CaO,SiO_(2),and MnO components in the liquid inclusion.The increased Al content in liquid steel or the higher supersaturation degree of Al_(2)O_(3)precipitation in the remelted ingot than that in the electrode can be attributed to the evaporation of CaF_(2)and the increase in CaO content in the ESR-type slag.

Dynamic mass variation and multiphase interaction among steel, slag, lining refractory and nonmetallic inclusions: Laboratory experiments and mathematical prediction

The mass transfer among the multiphase interactions among the steel, slag, lining refractory, and nonmetallic inclusions during the refining process of a bearing steel was studied using laboratory experiments and numerical kinetic prediction. Experiments on the system with and without the slag phase were carried out to evaluate the influence of the refractory and the slag on the mass transfer. A mathematical model coupled the ion and molecule coexistence theory, coupled-reaction model, and the surface renewal theory was established to predict the dynamic mass transfer and composition transformation of the steel, the slag, and nonmetallic inclusions in the steel. During the refining process,Al_(2)O_(3) inclusions transformed into Mg O inclusions owing to the mass transfer of [Mg] at the steel/refractory interface and(Mg O) at the slag/refractory interface. Most of the aluminum involved in the transport entered the slag and a small part of the aluminum transferred to lining refractory, forming the Al_(2)O_(3) or Mg O·Al_(2)O_(3). The slag had a significant acceleration effect on the mass transfer. The mass transfer rate(or the reaction rate) of the system with the slag was approximately 5 times larger than that of the system without the slag. In the first 20 min of the refining, rates of magnesium mass transfer at the steel/inclusion interface, steel/refractory interface, and steel/slag interface were x, 1.1 x, and 2.2 x,respectively. The composition transformation of inclusions and the mass transfer of magnesium and aluminum in the steel were predicted with an acceptable accuracy using the established kinetic model.

Control of nonmetallic inclusions of non-oriented silicon steel sheets by the rare earth treatment

Based on the industrial production of non-oriented silicon steel, the rare earth （RE） treatment during the Ruhrstahl Heraeus （RH） refining process was studied. The morphology and the size distribution were observed for the steel specimens treated with different RE treatment conditions. Furthermore, the formation and change of the nonmetallic inclusion characteristics of finished steel sheets after the RE treatment were discussed. The results have shown that in the present work,the suitable RE metal additions are 0.6 -0.9 kg/t steel. After the suitable RE treatment,the formation of AIN and MnS inclusions were restrained, and the aggregation, flotation and removal of nonmetallic inclusions were efficiently promoted and the cleanliness of liquid steel was significantly increased. Meanwhile, the total oxygen concentration reached the minimum value and thle desulfurization efficiency was optimal ,and the type of main inclusions was approximately spherical or elliptical spherical RE radicle inclusions whose size was relatively large.

Simulation research on monomer agglomeration of nonmetallic inclusions in steel with a diffusion limited aggregation model

The monomer agglomeration of nonmetallic inclusions was simulated with a diffusion limited aggregation （DLA） model of the fractal theory. The simulation study with a random two-dimensional diffusion was carried out. The results indicate that the DLA model can be used for the simulation of agglomeration behavior of the cluster-type inclusions. The morphology of clusters was observed with SEM and compared with the simulated agglomerates. The modelling procedure of the DLA model is applicable for the agglomeration process. The uncertainty of agglomeration process and the persuasive average agglomerative ratio was analyzed. The factors about the agglomerative ratio with the collision path distance and the size of particles or seed were discussed. The adherence of the nonmetallic inclusions on the dam, the weir and the walls of a tundish, and the absorption of inclusions by stopper or nozzle were also discussed.

Evolution of plasticized MnO–Al2O3–SiO2-based nonmetallic inclusion in 18wt%Cr-8wt% Ni stainless steel and its properties during soaking process

The properties of MnO–Al2O3–SiO2-based plasticized inclusion are likely to change during soaking process due to its low melting point. In this study, the evolution of the MnO–Al2O3–SiO2-based inclusion of 18 wt%Cr-8 wt%Ni stainless steel under isothermal soaking process at 1250°C for different times was investigated by laboratory-scale experiments and thermodynamic analysis. The results showed that the inclusion population density increased at the first stage and then decreased while their average size first decreased and then increased. In addition, almost no Cr2O3-concentrated regions existed within the inclusion before soaking, but more and more Cr2O3 precipitates were formed during soaking. Furthermore, the plasticity of the inclusion deteriorated due to a decrease in the amount of liquid phase and an increase in the high-melting-pointphase MnO–Cr2O3 spinel after the soaking process. In-situ observations by high-temperature confocal laser scanning microscopy(CLSM) confirmed that liquid phases were produced in the inclusions and the inclusions grew rather quickly during the soaking process. Both the experimental results and thermodynamic analysis conclude that there are three routes for inclusion evolution during the soaking process. In particular, Ostwald ripening plays an important role in the inclusion evolution, i.e., MnO–Al2O3–SiO2-based inclusions grow by absorbing the newly precipitated smaller-size MnO–Cr2O3 inclusions.

Nonmetallic inclusions in SUS304 strip produced by twin-roll strip casting

The shape, type, content, and dimension of nonmetallic inclusions in SUS304 strip produced by twin-roll strip casting were studied using scanning electron microscopy （SEM）. The results show that the inclusions are mainly spherical Al2O3 and complex oxides composed of MnO, Al2O3 , and SiO2. The percentage of fine oxides smaller than 3 μm reaches up to 51.8%. The theoretical calculations show that fine oxides have precipitated during solidification. Therefore, it is concluded that during twin-roll strip casting, because of high cooling rate, the size of inclusions precipitated during solidification decreases, and the amount increases.

EFFECT OF Zr ADDITION TO Ti-KILLED STEEL ON INCLUSION FORMATION AND MICROSTRUCTURAL EVOLUTION IN WELDING INDUCED COARSE-GRAINED HEAT AFFECTED ZONE

Effects of Zirconium on the chemical component and size distribution of Ti-bearing inclusions, favored the grain refinement of the welding reduced, coarse-grained heat affected zone （CGHAZ） with enhanced impact toughness in Ti-killed steels, which were examined based on experimental observations and thermodynamic calculations. It indicated that the chemical constituents of inclusions gradually varied from the TiO oxide to the Ti-O＋Zr-O compound oxide and a single phase of the ZrO2 oxide, as the Zr content increased from zero to 0.0100%. A trace of Zr （0.0030%-0.0080%, depending on the oxygen content in liquid steel） provided a large amount of nucleating core for Ti oxide because of the larger specific density of ZrO2 oxide, and produced a small size distribution of the inclusions favorable for acicular ferrite transformation with a high nucleation rate in the CGHAZ, and a high volume fraction of acicular ferrite was obtained in the CGHAZ, with enhanced impact toughness. Otherwise, a high content of Zr （-0.0100%） produced a single phase Zr02, which was impotent to nucleate acicular ferrite, and a microstructure composed of ferrite side plate and grain boundary ferrite developed in the CGHAZ. The experimental results were confirmed by thermodynamic calculations.

Influence of crucible material on inclusions in 95Cr saw-wire steel deoxidized by Si–Mn

To investigate the interaction mechanism between 95 Cr saw-wire steel and different refractories,we conducted laboratory experiments at 1873 K.Five crucible materials(SiO2,Al2 O3,MgO·Al2 O3,MgO,and MgO-CaO)were used.The results indicate that SiO2,Al2 O3,and MgO·Al2 O3 are not suitable for smelting low-oxygen,low-[Al]s 95 Cr saw-wire steel,mainly because they react with the elements in the molten steel and pollute the steel samples.By contrast,MgO-CaO is an ideal choice to produce 95 Cr saw-wire steel.It offers three advantages:(ⅰ)It does not decompose by itself at the steelmaking temperature of 1873 K because it exhibits good thermal stability;(ⅱ)[C],[Si],and[Mn]in molten steel cannot react with it to increase the[O]content;and(ⅲ)it not only desulfurizes and dephosphorizes but also removes Al2 O3 inclusions from the steel simultaneously.As a result,the contents of the main elements([C],[Si],[Mn],[Cr],N,T.O(total oxygen))in the steel are not affected and the content of impurity elements([Al]s,P,and S)can be perfectly controlled within the target range.Furthermore,the number and size of inclusions in the steel samples decrease sharply when the MgO-CaO crucible is used.

Influence of calcium content on solid ratio of inclusions in Ca-treated liquid steel

It has been found that Ca-treated liquid steel can be cast at a lower superheat. To ascertain the reason for the improved castability of Ca-treated liquid steel and to find the optimal range of calcium content, the behavior of calcium in liquid steel was studied in terms of the relationship between the calcium content and solid ratio of inclusions. The relationship between the calcium content and solid ratio of inclusions was obtained by means of the classification of nonmetallic inclusions in solid and liquid steels at casting temperature according to the Al2O3-CaO-SiO2 phase diagram. The optimum calcium content should be 17-23 ppm.

Inclusion control of automotive spring steel in the basic oxygen furnace and ingot casting process

The process of automotive spring steel in the basic oxygen furnace and ingot casting （BOF-IC） of Baosteel is described in this paper. High quality spring steel can be obtained through designing reasonable deoxidization and alloying processes ,enhancing steel purity and reducing inclusions in steel.

Study of porosity tendency in the Cr-Ni austenitic weld based on inclusion’s characteristic in the droplet

The effect of nonmetallic inclusions in the droplet of the stainless steel covered electrode on the porosity was researched.The result shows that the nonmetallic inclusions in the droplet are spherical,their composition is different from the one of slag and the inclusions have the character of “inner formation”.When the ratio of rutile to ilmenite in the coated material is increased, the droplet becomes coarse, the content of nonmetallic inclusion in the droplet decreases,and the porosity sensitivity in the weld metal also decreases.When the ratio of fledspar to ilmenite in the coated material is increased, the droplet becomes fine,the content of nonmetallic inclusion in the droplet increases, and the porosity sensitivity in the weld metal increases. When the ratio of Fe_2O_3 to ilmenite in the coated material is increased, the droplet becomes fine, the content of nonmetallic inclusion decreases, while the porosity sensitivity does not reduce.

Distribution of nonmetallic inclusions in molten steel under hot-top pulsed magneto-oscillation treatment

To control the distribution of nonmetallic inclusions in molten developed method, hot-top pulsed magneto-oscillation （HPMO）, steel is of highly commercial importance. A newly was employed to control nonmetallic inclusions in a medium-carbon steel ingot. The experimental results present that the position of nonmetallic inclusions of spinel （Al-Mg-O） and Mn-S inclusions is significantly influenced by HPMO. The number of nonmetallic inclusions gradually increased from the lateral wall to the center of ingot along the radial direction under the impact of HPMO treatment, whereas this distribution trend cannot be observed in the reference sample without HPMO treatment. In addition, the number of inclusions along vertical direction is proportional to the height of ingot, especially for the inclusions with the size of larger than 10 pro. It indicates that the application of HPMO can push away inclusions from lateral to center, and then the expelled inclusions aggregate and rise up to the top of ingot.

Evolution and control of nonmetallic inclusions in FeSiB alloy

Inclusions have a great infuence on the quality of FeSiB amorphous ribbon.The evolution behavior of inclusions was analyzed.The results show that the spherical and elliptical inclusions,consisting of Al2O3 and SiO2,mainly come from industrial pure iron.Ellipsoidal inclusions of 20-30μm in pure iron were removed by foatation during the smelting process,and spherical inclusions of 1-3μm combined with deoxidized products in FeSiB melt form Al2O3·SiO2 and CaO·Al2O3·SiO2 inclusions.Some inclusions accumulated on the inner wall of the nozzle during the spraying process,and others fowed out of the nozzle and remained in FeSiB amorphous ribbon.By controlling the total oxygen content in industrial pure iron to 31×10^-6,clogging on the inner wall of the nozzle can be reduced and the free surface smoothness of the amorphous ribbon can be improved.

Quantitative Study on Nonmetallic Inclusion Particles in Steels by Automatic Image Analysis With Extreme Values Method

The presence of nonmetallic inclusion particles which appear during steelmaking process is harmful to the properties of steels, which is mainly as a function of some aspects such as size, volume fraction, shape, and distribution of these particles. The automatic image analysis technique is one of the most important tools for the quantitative determination of these parameters. The classical Student approach and the Extreme Values Method （EVM） were used for the inclusion size and shape determination and the evaluation of distance between the inclusion particles. The results thus obtained indicated that there were significant differences in the characteristics of the inclusion particles in the analyzed products. Both methods achieved results with some differences, indicating that EVM could be used as a faster and more reliable statistical methodology.

Influence of GdCl_3 addition on purifying effectiveness and properties of Mg-10Gd-3Y-0.5Zr alloy

In order to improve the purifying efficiency of RJ6 flux,5%(mass fraction) GdCl3 was introduced into the flux for refining Mg-10Gd-3Y-0.5Zr(GW103K) alloy.The results show that the RJ6 flux containing 5%GdCl3 exhibits better adsorption ability to nonmetallic inclusions than the one without GdCl3.Moreover,the mechanical,corrosion properties and fluidity of the alloy refined with RJ6 flux and RJ6 flux containing 5%GdCl3 were investigated,respectively.It is found that these properties are improved to a certain degree due to the removal of nonmetallic inclusions in the alloy.Thermodynamic analysis and surface tension experiments indicate that the main reason can be ascribed to the decrease of the surface tension of the flux with 5%GdCl3,which promotes the combination of flux and nonmetallic inclusions.

Electromagnetic Separation of Nonmetallic Inclusions From Liquid Steel by High Frequency Magnetic Field

The currernt work is to investigate the removal of nonmetallic inclusionsfrom liquid steel using electromagnetic separation.In experiments,a certain amount of aluminum was added to the molten steel in a crucible to generate aluminum inclusions.A layer of aluminum clusters were observed close to the wall of the crucible.Three dimensional electromagnetic field and fluid flow simulation indicated that particles moved to the wall of the crucible by the EM force and were entrapped at the inside wall of the crucible.

Effect of MgO-Cr_(2)O_(3) and Mg0-MgAl_(2)0_(4)-based refractories on refractory-steel interface reaction and cleanliness of pipeline steel

The interaction of MgO-MgAl_(2)O_(4)-based and MgO-Cr_(2)O_(3)-based refractories with X70 molten steel was studied by immersion experiments at 1560℃.The effects of immersion time(30 and 60 min)on the contents of total oxygen(TO),Al,Nb,Si,Mn,and Cr as well as the composition,number density,and size distribution of inclusions in the molten steel were investigated.The influence of the penetration and erosion degree of the molten steel to the refractory on the steel-refractory interface layer was analyzed.The results show that,at 1560℃,the MgO-MgAl_(2)O_(4)-based refractory can better control the contents of TO and the composition of molten steel compared with the MgO-Cr_(2)O_(3)-based refractory.The TO content is only 16×10^(-4) wt.%in the molten steel after reacted with the Mg0-MgAl_(2)O_(4)-based refractory at the end point of refining,4 accounting for 11.5%of that reacted with the MgO-Cr_(2)O_(3)-based refractory(139×10^(-4) wt.%).The number density of inclusions is only 14 mm^(-2),and the average size ofinclusions is only 1.31μm,with thelargest proportion of inclusions in 1-2μm(70%).The Al_(2)O_(3)-MnS-CaO complex inclusions in the original steel changed to complex inclusions dominated by Cr-Nb-Mn-S-O and MgO.Al_(2)O_(3),corresponding to the MgO-Cr_(2)O_(3)-based and MgO-MgAl_(2)O_(4)-based refractories,respectively.The MgO.Al_(2)O_(3) layer was formed at the reaction interface between MgO-MgAl_(2)O_(4)-based refractory and molten steel,which is helpful to restrict the erosion of refractories and the pollution of molten steel.The damage mechanism of the MgO-Cr_(2)O_(3)-based refractory is mainly permeation and chemical reaction,while the damage of the MgO-MgAl_(2)O_(4)-based refractory is mainlyscouring erosion.

Effects of rare earth on inclusions and corrosion resistance of 10PCuRE weathering steel

The types, morphologies and distributions of nonmetallic inclusions in Cu-P weathering steels with and without rare earth were analyzed through a quantitative image analyzer, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) attached to SEM. Solid-soluble content of rare earth in the steels was analyzed by non-aqua electroanalysis and ICP. The results showed that rare earth modified the types and the morphologies of inclusions in the weathering steels. The small spherical rare earth oxysulfides and rare earth sulphides replaced the elongated MnS inclusions in the RE weathering steels. The rare earth inclusions dispersedly distributed and most inclusions were smaller than 2 μm in size. The optimum content of RE was 0.0065%-0.016% for 10PCuRE weathering steels containing about 0.002% oxygen and 0.004% sulfur. Solid-soluble content of rare earth in steels was (14-20)×10-6, which can act as a micro-alloying element. The corrosion resistance of 10PCuRE weathering steels and Q235 were studied by dry-wet cyclic immersion test. Their corrosion rates were obtained respectively. The polarization curves and pitting corrosion behaviors of weathering steels with and without rare earth were measured by electrochemical methods. The corrosion resistance of Cu-P weathering steels was improved by adding an appropriate amount of rare earth. Less and fewer rare earth inclusions largely decreased pitting susceptibility and rate of pit propagation. The pitting potential and the resistance against pitting corrosion of the RE weathering steel were significantly improved due to the modification of rare earth to inclusions.

Inclusion engineering and metallurgy of calcium treatment

Although a great deal of research has been done in the area of inclusion control during steelmaking, a complete description of the process of inclusion modification and shape control cannot be found in literatures. As a result, the terms ＂inclusion modification＂ and ＂shape control＂ are often used loosely in the technical community. To utilize the full potential offered by a calcium treatment, it is important to understand why an inclusion whose composition has been modified may not necessarily have its shape modified. With the purpose of decoupling the process of inclusion phase change from inclusion shape control, and to provide a complete description of the metallurgy of calcium treatment, a detailed literature survey has been performed on the subject. By coupling the survey with a new concept developed to tailor the cleanliness of steels to the intended applications, the hope is to provide the reader with valuable information on calcium treatment for inclusion modification and shape control.

Estimation of maximum inclusion by statistics of extreme values method in bearing steel

A statistic method, statistics of extreme values （SEV）, was described in detail, which can esti mate the size of maximum inclusion in steel. The characteristic size of the maximum inclusion in a high clean bearing steel （GCrl5） was evaluated by this method, and the morphology and corn position of large inclusions found were analyzed by scanning electron microscopy （SEM）. When standard inspection area （S0） is 280 mm2, the characteristic size of the biggest inclusion found in 30 standard inspection area is 23.93 μm, and it has a 99.9% probability of the characteristic size of maximum inclusion predicted being no larger than 36.85μm in the experimental steel. SEM result shows that large inclusions found are mainly composed of CaS, calcium-aluminate and MgO. Compositing widely exists in large inclusions in high clean bearing steel. Compared with traditional evaluation method, SEV method mainly focuses on inclusion size, and the esti- mation result is not affected by inclusion types. SEV method is suitable for the inclusion eval uation of high clean bearing steel.