Evolution Mechanism Of Inclusions Research Articles

Evolution mechanism of inclusions and microstructure in low-alloy cast steel with cerium addition

Rare earth (RE) elements not only modify inclusions but also refine the steel's microstructure. This study investigates the evolution mechanism of cerium on inclusions and microstructure in low-alloy cast steel through experimental and thermodynamic analyses. After cerium is added to the steel, strip-like inclusions transform into spherical ones, accompanied by a reduction in size. The evolution path of inclusions follows MnS + MnS–Al2O3→Ce2O2S–Ce2S3–MnS + CeAlO3–Ce2S3–MnS with cerium addition. Notably, Ce2S3 and Ce2O2S exhibit an orientation relationship, with [1‾ 22]Ce2S3//[01 1‾ 1]Ce2O2S and (02 2‾)Ce2S3//(1‾ 011)Ce2O2S. Compared to Ce-free steel, the Ce-0.06 steel shows a reduced grain size and the absence of needle-shaped Widmanstätten structures, which may be mainly attributed to rare earth inclusions as heterogeneous nucleation of δ-Fe. This work can provide a theoretical basis for improving the overall mechanical properties of low-alloy cast steel by adding cerium.

An effective strategy for manufacturing ultra-high purity AISI 316L stainless steel by vacuum C deoxidization pretreatment plus Mg–Ca composite treatment

Aiming at the special ultra-high purity (UHP) requirements of AISI 316L stainless steel used in semiconductor equipment, a strategy of vacuum C deoxidization pretreatment plus Mg–Ca composite treatment was proposed in this work. The effects of this strategy on cleanness and inclusion modification were systematically investigated by experimental observations and thermodynamic calculations. The results demonstrated that this strategy significantly reduced the O and S contents to 0.0005 wt% and 0.0007 wt%, respectively. The main inclusions after vacuum C deoxidization were large-sized Al2O3·SiO2 and MnS. After Mg treatment, the oxide inclusions were gradually modified into MgO·Al2O3 and MgO, while the sulfide inclusion was still MnS. After additional Ca treatment, the inclusions were further modified into finer CaO, MgO, CaO·MgO, and CaS. Moreover, this strategy remarkably decreased the number and size of the inclusions. The best level was that all inclusions were smaller than 2 μm, and the proportion of inclusions smaller than 1 μm was as high as 75.47%. Meanwhile, the evolution mechanism of inclusions during different treatments was clarified in this work. Finally, the optimum process for preparing UHP AISI 316L stainless steel was proposed as vacuum C deoxidation plus 0.002 wt% Mg and 0.0016 wt% Ca composite treatment.

Effects of Calcium Addition on Nonmetallic Inclusions in High‐Al Steel

To investigate the formation and evolution mechanism of inclusions in high‐Al steel, four lab‐scaled experiments are carried out with different calcium (Ca) contents. The Al2O3 inclusions are modified to (CaO–Al2O3)–CaS after Ca addition. With the Ca content increased, the types of CAx‐CaS inclusions are changed from CaS‐attached inclusions to CAx‐wrapped inclusions. When the total calcium content (T.Ca) reached 29 ppm, the CAx–CaS inclusions are primarily distributed within the 50% liquidus in CaO–Al2O3–CaS ternary diagram. Some Al2O3 inclusions are not completely modified with 7 ppm Ca addition. CaS and CaO–CaS inclusions are observed with a higher Ca addition. The modification of Al2O3 by CaS and [Ca] occurs in CaS‐attached inclusions with a lower Ca addition. To clarify the formation of CaS, a new modification index for high‐Al steel is given based on the T.Ca/T.O index.

Evolution mechanism of inclusions during refining and continuous casting process of 321H stainless steel

ABSTRACT The evolution of inclusions during the refining and casting process was investigated on an industrial scale to clarify the quantity and distribution of non-metallic inclusions for 321H stainless steel. The analyses showed that Al2O3-SiO2-MnO-CaO and Al2O3 as the main types of inclusions at the late of AOD and VOD, respectively. Pure titanium nitride and titanium nitride with oxide as the nuclei dominated the inclusions at the late of LF. The types of inclusions in mold and slab were similar to those obtained at the late of LF, while titanium nitride inclusions in slab were higher in numbers and larger in size. The titanium nitride inclusions precipitating with MgO-Al2O3-CaO-(TiO2) nuclei showed a higher number density and area fraction than other oxide inclusions. The calcium treatment of molten steel was optimized by FactSage software and it was necessary to accurately control the calcium content in molten steel before titanium addition.

Effect of Deoxidizing and Alloying Routes on the Evolution of Non-Metallic Inclusions in 55SiCr Spring Steel

Compared to the conventional deoxidation process with Al or Si-Mn for 55SiCr spring steel production, the possibility of using a Si-Ca-Ba compounded deoxidizer to control the behavior of non-metallic inclusions in spring steel was explored in this study. The effect of the addition sequence of deoxidizer and alloys into molten steel on the morphology, size, and composition of the inclusions was emphatically studied at 1873 K (1600 °C) using a high-temperature electric resistance furnace. The results indicate that adding alloy first can form less harmful inclusions in steel, which are roughly spherical and smaller than 3 μm in diameter, and its inclusion evolution path is MnO-SiO2-Al2O3→CaO-SiO2-Al2O3 and CaO-Al2O3-MgO. While adding deoxidizer first, the inclusions in steel are harmful due to its mostly irregular geometry and relatively large size over 5 μm. The inclusion evolution path is Fe-O→CaO-Al2O3(-SiO2) and CaO-Al2O3-MgO. The formation and evolution mechanism of inclusions under different addition sequences were discussed. In addition, the solubility limits of MgO from refractory into steel were studied to inhibit its corrosion by molten steel.

Evolution mechanism of Ca-containing inclusions in 20Mn23AlV high-manganese non-magnetic steel during the refining process

ABSTRACT Industrial experiments and thermodynamic calculations were carried out to investigate the evolution mechanism of Ca-containing inclusions in 20Mn23AlV steel with the Al content up to 2 wt-%. After Al deoxidation in AOD, the main inclusions were spherical shape Ca–Al–Si–Mn–O inclusions. Spherical Ca–Alrich–Si–Mn–O and spherical Ca–Al–Mn–O became the main inclusions after the first Al alloying. After Ca treatment in LF, the typical inclusions were aggregate CaS–CaO inclusions. In tundish, the types of inclusions were CaS and CaSrich–CaO, and the size and quantity of inclusions were substantially decreased. Thermodynamic analysis indicated that with the Al content increase to about 2 wt-%, the inclusions containing high Al2O3 content formed, which were extremely easy to be removed in the steelmaking process. Moreover, after the second Al alloying, the decrease of oxygen content promoted the formation of CaS inclusions, and CaS or CaS–CaO inclusions were formed Lastly, the evolution mechanism of inclusions was summarized in detail.

Effect of Reoxidation on Inclusions Characteristic During Casting in Al‐Killed Steel Containing Rare Earth

To investigate the effect of reoxidation on inclusions characteristic during casting in Al‐killed steel containing rare earth, two industrial heats, including one of steady casting and the other of reoxidation, are performed with sampling from Rheinstahl Heratus, tundish, submerged entry nozzle, and slab. In the present study, air absorption occurs at the submerged entry nozzle and caused the reoxidation of molten steel. As a result, the number of inclusions increases significantly, and nearly spherical Ce2O2S inclusions are transformed to clustered‐like CeAlO3, which increases the average size of inclusions from 3.92 to 9.21 μm, thereby resulting in serious nozzle clogging and slab defects. The results of thermodynamic calculation show that Ce2O2S is transformed to CeAlO3 with total oxygen content increases under different Ce additions, and this transformation of inclusions can be inhibited by controlling the value of [Ce]/[O] above 2.12. Based on the experimental results and the theoretical analysis, the evolution mechanism of inclusions during reoxidation is clarified. This study can provide theoretical guidance to alleviate the harm of reoxidation to Al‐killed RE‐steel from the inclusion‐controlling perspective.

Temperature-dependent evolution of oxide inclusions during heat treatment of stainless steel with yttrium addition

The evolution of oxide inclusions during isothermal heating of 18Cr-8Ni stainless steel with yttrium addition at temperatures of 1273 to 1573 K was investigated systematically. Homogeneous spherical Al-Y-Si(-Mn-Cr) oxide inclusions were observed in as-cast steel. After heating, most of the homogeneous inclusions were transformed into heterogeneous inclusions with Y-rich and Al-rich parts, even though some homogeneous oxide particles were still observed at 1273 and 1573 K. With the increase in heating temperature, more large-sized inclusions were formed. The shape of the inclusions also changed from spherical to irregular. The maximum transformation temperature of inclusions was determined to be 1373 K. The evolution mechanism of inclusions during heating was proposed to be the combined effect of the (i) internal transformation of inclusions due to the crystallization of glassy oxide and (ii) interfacial reaction between inclusions and steel matrix. Meanwhile, the internal transformation of inclusions was considered to be the main factor at heating temperatures less than 1473 K.

Evolution Mechanism of Inclusions in H13 Steel with Rare Earth Magnesium Alloy Addition

Evolution Mechanism of Inclusions in H13 Steel with Rare Earth Magnesium Alloy Addition

The Effect of Al‐Mg Deoxidation on the Cleanliness of Steel during the Electroslag Remelting Process

Herein, the effect on inclusions and oxygen content of steel with Al‐Mg deoxidant addition during the electroslag remelting (ESR) process is studied. Combined with thermodynamics, the evolution mechanism of inclusions during the ESR process is analyzed. The results indicate that the inclusions in the consumable electrode are SiO2‐Al2O3‐MnO complex inclusions, where most are located in the 1873 K low‐melting‐temperature region. The inclusions in remelted ingots are mainly Al2O3 and small amounts of MnS in the Al‐Mg deoxidant‐free trials, and the inclusions are in the size of 1–3 μm. In the Al‐Mg deoxidant addition trial, the main inclusions are Al2O3, MgO‐Al2O3, CaO‐Al2O3‐MgO, and CaO‐Al2O3. The proportions of CaO‐Al2O3‐MgO and CaO‐Al2O3 inclusions increase with the increase in the deoxidant.

Evolution Mechanism of Inclusions in Medium-Manganese Steel by Mg Treatment with Different Aluminum Contents

To investigate the effect of magnesium addition on the evolution of inclusions in medium-manganese steel with different aluminum contents, both thermodynamic calculation and high-temperature simulation experiments were carried out in the present work. The in situ observation experiments were used to clarify the related evolution process of inclusions. The samples taken from the melts were analyzed by scanning electron microscopy and energy dispersive spectroscopy. The compositions of steel were determined by inductively coupled plasma-optical emission spectrometer. The results show that the aluminum content in steel has a certain influence on the magnesium content, under the same conditions, magnesium content increases with increasing aluminum content. The inclusions transformed from MnO·Al2O3 to Al2O3 when the aluminum content was higher than 0.0076 mass pct. After magnesium treatment, the inclusions gradually transformed into MgO·Al2O3, and the MgO/Al2O3 mole ratio in inclusions decreased with the increase of aluminum content. The diameter of the inclusions decreased, and number density of inclusions increased in steel after magnesium addition. The phenomenon that large-sized cluster-like Al2O3 inclusions transform into finely dispersed Mg-containing inclusions was firstly observed in situ by confocal laser scanning microscope.

Industrial experiment study on inclusion evolution in 95CrMo Steel

In order to clarify the evolution mechanism of inclusions in 95CrMo, the industrial experiments were conducted. The composition, morphology and size of inclusions during refining and solidifying processes were analyzed by SEM-EDS detection and thermodynamic calculations. The inclusion evolution during refining process in molten 95CrMo steel can be concluded as: Al2O3 → Al2O3-MgO-CaO/CaS → Al2O3-MgO-SiO2-CaO-CaS, which is affected by slag-metal reaction and corrosion of refractory. The change of inclusion size during refining was analyzed based on their collision and floatation behavior. The composition of inclusions in billets indicate that there were mainly five types of inclusions, including spinel with sharp angle, spherical oxide inclusion composed of Al2O3, MgO, SiO2and CaO, oxide-sulfide inclusion with single phase, inclusion with duplex phase, and MnS inclusion. The type of inclusions affects their size. The thermodynamic results indicate that most complex oxides formed in molten steel and no spinel generated during cooling process. CaS formed in the molten steel after tundish metallurgy and there were still some CaS precipitated in solid steel. MnS precipitated after steel solidification. The calculated results based on FACTSAGE 7.1 are in agreement with experimental results on the whole.

Evolution Mechanism of Inclusions in Al-killed, Ti-bearing 11Cr Stainless Steel with Ca Treatment

Evolution Mechanism of Inclusions in Al-killed, Ti-bearing 11Cr Stainless Steel with Ca Treatment

Inclusions modification in heat resistant steel containing rare earth elements

Based on the thermodynamic assessment of inclusions evolution, the modification experiments of inclusions containing rare earths were carried out in the Si–Mo furnace from 1600°C to 1500°C. The pure Si and Si–Al alloy were chosen as deoxidants under low and high initial oxygen content, respectively. The field emission scanning electron microscope (FE-SEM) and energy dispersive spectrometer (EDS) were employed to analyse the morphology and chemical compositions of inclusions in the samples. The spherical liquid Ce–Si–Al–O (Ce4Si5Al5O23.5) inclusions were observed in the steel, which was deoxidised by Si–Al alloy. The evolution mechanisms of inclusions were comprehensively discussed, and models for the formation of oxide inclusions were set up.

Effect of Mg on Behavior and Particle Size of Inclusions in Al-Ti Deoxidized Molten Steels

To investigate the effect of magnesium addition on the evolution of inclusions in Al-Ti deoxidized melts, both thermodynamic calculations and deoxidized experiments were carried out in the present work. The samples took from the melts were polished and analyzed by field scanning electron microscopy (FE-SEM) and energy dispersive spectroscopy (EDX). The results showed that the addition of magnesium significantly influenced the size of oxide inclusions in Al-Ti deoxidized melts. Superfluous MgO raised the melting point of these complex inclusions, and then degraded the steel castability. Therefore, the amount of magnesium addition should be carefully controlled. Based on thermodynamic calculations and experimental results, the recommended range of [Mg] is 1~5 ppm. All inclusions observed in the sample were nearly spherical except for solid titanium aluminate inclusions. Oxide inclusions were modified quickly after magnesium addition. Meanwhile, magnesium could also modify solid titanium aluminate inclusions in the melts similarly, but there were unreacted cores in the inclusions at the initial stage. The evolution mechanisms of inclusions were comprehensively discussed, and models for the formation of oxide inclusions were set up. As for the production practice, the addition amount of magnesium and titanium should be considered simultaneously to liquefy inclusions in molten steel.

Evolution of Al2O3 inclusions by magnesium treatment in H13 hot work die steel

To investigate the evolution mechanism of Al2O3 inclusions after Mg treatment in H13 die steel, the determination of inclusions characteristics and thermodynamic calculation were carried out in the present study. The results showed that irregular and cluster Al2O3 inclusions in H13 die steel were modified to MgO·Al2O3 and MgO after Mg treatment. Two types of MgO·Al2O3 were detected after Mg treatment. The long-range gravity between the MgO-containing inclusions is weaker than that of the MgO-free inclusions. The long-range gravity between the inclusions decreases with the increasing of Mg content of the steel. The evolution mechanisms of inclusion in low Mg-containing and high Mg-containing H13 die steel were comprehensive discussed. The different reaction mechanisms result in the different of the core of the MgO·Al2O3 inclusion.

Effect of Magnesium on Evolution of Oxide and Sulphide in Liquid Iron at 1873 K

To reveal the effects of magnesium on the evolution of oxide and sulphide inclusions in liquid iron, both thermodynamic calculations and deoxidization experiments were carried out. The samples extracted from the liquid iron were polished and analyzed by optical microscopy and scanning electron microscopy. The results showed that magnesium could modify oxide and sulphide inclusions simultaneously. Spherical MgO and irregular spinel inclusions were observed in the samples. The elongated MnS inclusions were replaced by small MgO • MgS or MgO • MgS • MnS complex inclusions, and the sulphides were distributed dispersively. The evolution mechanisms of inclusions were discussed comprehensively, and a proposed model for the formation of oxysulphide was set up.

Effect of magnesium addition on evolution of inclusions in Mn–Si–Al deoxidised molten steels

To reveal the effects of magnesium on the evolution of inclusions in Mn–Si–Al deoxidised steels, both thermodynamic calculations and deoxidised experiments were carried out in the present work. The samples extracted from the melts were polished and analysed by optical microscope and scanning electron microscopy (SEM). The results showed that magnesium can control the size of oxide inclusions in Mn–Si–Al deoxidised melts well. Superfluous MgO raise the melting point of the complex inclusions, and then worsen the steel castability. Therefore, the amount of Mg addition should be under control. The recommend value of [Mg] in present work is 1–3 ppm, based on thermodynamic calculations and experiments. All inclusions observed in the sample were nearly spherical. The evolution mechanisms of inclusions were discussed comprehensively, and proper models for the formation of oxide inclusions were set up. In practice, to achieve the full liquid inclusions in molten steel, the addition amount of magnesium and aluminium should be considered simultaneously.