Table of Content

05 August 2016, Volume 32 Issue 4

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Several important scientific research points of non-metallic inclusions in steel

2016, 32(4):  1-16. 

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In the current paper, several projects on non-metallic inclusions in steel that are still not well solved and clarified are discussed, including the detection of the three-dimensional morphology of inclusions, thermodynamics of deoxidation using multiple deoxidizers, calcium treatment of the molten steel, the dependence of the deformation ability of inclusions on their melting temperature and composition, the effect of alloys and added materials on the composition of inclusions, the clogging mechanism of the submerged entry nozzle during continuous casting, the mechanism of the erosion reaction between molten steel and lining refractory, the kinetics of the size and composition of inclusions and coupled with the fluid flow in the metallurgical vessel during steel refining and casting process. The projects above are the key problems of inclusions in steel. If these projects are solved within next 5-10 years and used in steel production process, the quality of steel can be well improved.

Study on non-metallic inclusions in high quality saw wires

2016, 32(4):  17-22. 

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In present study, formation of inclusions in high quality saw wires were discussed in details by systematical samples in pilot trials. It was found that inclusions have readily located in the lower melting point regions of MnO-SiO₂-Al₂O₃ and CaO-SiO₂-Al₂O₃ ternary systems. The former type of inclusions was endogenous ones originated from the chemical reactions between liquid steel and the Si-Mn deoxidation products. While the latter was caused by the entrapment and the following emulsion the of top slag particles into molten steel. During the solidification of liquid steel in continuous casting, silica content in the formed MnO-SiO₂-Al₂O₃ in secondary refining showed an obvious increase, resulted in the precipitation of pure SiO2 phase in the inclusion matrix. Because of the differences in deformability, the newly produced dual-phased inclusions would be gradually separated from each other in the following multi-pass rolling. Therefore, SiO₂ based inclusions singularly existed in the steel can be frequently observed in the rolled coils. However, it was impressive that those SiO₂based inclusions was changed into very tiny dotshaped particles dispersed along the rolling direction. Particularly, the coil was continuously drawn to 29 602 km without fracture despite the existence of the SiO₂based inclusions. Therefore, it was concluded that there was no need to emphasize on the targeting of lower melting point of inclusions for saw wires. Instead, systematic optimization of the whole process and size minimization of inclusions were considered to be more important.

Precipitation and control of titanium inclusions in tire cord steel production

2016, 32(4):  23-32. 

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It has been studied that: (1) The law of precipitating titanium inclusions during the solidification of tire cord steel and titanium inclusions’ characteristics; (2) The effect of the composition (C、N、Ti) in tire cord steel and cooling rate of solidification on the titanium inclusions’ composition and its size; (3) The thermodynamic conditions and the factors influencing the decomposition, solid solution and reprecipitation of titanium inclusions during heating of billets at high temperature, by means of thermodynamics, dynamics and experimental detection. The results show that: (1) The inclusions of Ti（C_xN_1-x） start to precipitate in earlier stage of the solidification of molten tire cord steel, while the inclusion of TiN precipitates at the end period of the solidification. (2) The higher the carbon content of tire cord steel is, the lower the temperature of precipitating the titanium inclusions, the greater the supersaturation of solute elements, the earlier the precipitation of titanium inclusions at solidification front, the larger the value of x in Ti（C_xN_1-x）inclusions is. (3) The cooling rate has a remarkable influence on the size of titanium inclusion precipitated during molten steel solidification. The lower the initial nitrogen content of molten steel, the smaller the size of titanium inclusion. But the initial titanium content of molten steel has little influence on the size of titanium inclusion. (4) The titanium inclusions in billets can be decomposition and then solid solution during heating at high temperature before hot working. The carbon content in tire cord steel has a little influence on  the temperature of thermal decomposition of titanium inclusion, but the contents of nitrogen and titanium have great influence on  the temperature of thermal decomposition of titanium inclusion. The solute atoms originated from decomposition of titanium inclusion are dissolved in austenite by diffusion, which make the small inclusion dissolve and disappear, the larger inclusion become smaller and smaller gradually. (5) If the billets are slow cooling after heating at high temperature, the solute atoms in a supersaturation could reprecipitate to make the titanium inclusion grow up. During the quick cooling after hot rolling, the supersaturated solute atoms can reprecipitate to form titanium inclusion,but have no opportunity to grow up.

Effect of the LF refining time on the composition of inclusions in tire cord steels

2016, 32(4):  33-38. 

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In the current study, the effect of LF refining time on the composition of inclusions in tire cord steel was investigated to optimize the LF refining time of a domestic steel plant. Steel samples were taken during LF refining. The evolution of total oxygen and nitrogen contents, and the composition, number density, area fraction, and size of inclusions in steel samples were systematically analyzed to determine an appropriate refining time. It was found that the oxide inclusions were concentrated in the center and edge of the cross section of rolled products and the composition of part of inclusions were controlled in the region of low melting temperature. The total oxygen mass fraction gradually decreased during LF refining and then reached stable to 20×10^-6. Due to the operation of slagging-off after tapping of BOF, the molten steel was oxidized by air, resulting in the rapid increase of nitrogen content at the initial stage of LF refining. The nitrogen mass fraction was then reached a stable value of 22×10^-6during the refining process. When the calcium mass fraction in the molten steel was larger than 1.1×10^-6, the CaO in inclusions was gradually generated and the type of inclusions were transformed from SiO2-MnO to CaO-SiO₂-Al₂O₃. The Al₂O₃ in inclusions mainly came from raw materials such as alloy and deoxidizer. Besides, the MgO in inclusions came from ladle lining. Based on the variation of average composition, size fraction and area fraction of MgO-CaO-SiO₂-Al₂O₃-(MnO) inclusions, the soft blowing time during the current LF refining was suggested to shorten 20 min and the appropriate refining time was proposed as 96 min.

Effect of inclusions in Ti-bearing stainless steels on nozzle clogging of submersed nozzle

2016, 32(4):  39-43. 

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The formation mechanism of nozzle clogging in the submerged entry nozzle (SEN) used for AISI321 stainless steel was analyzed by means of scanning electron microscope (SEM) and FactSage software. The results showed that the deposit was mainly classified into three parts. The first part was composed of frozen steel at the surface of refractory. The formation of frozen steel was resulted from TiO₂ which was formed due to the oxidation of TiN or the reduction of SiO₂ in refractory by Ti in steel. For the second part, it was a dendritic materials, which was consisted of complex inclusions of CaO·TiO₂-MgO·Al₂O₃ containing rich CaO·TiO₂. The accumulation of the complex inclusions in steel resulted in the formation of second layer. There was massive frozen steel closed to molten steel at the third part, which was consisted of TiN with a core of MgO·Al₂O₃  and frozen steel. The precipitation of TiN induced by MgO·Al₂O₃  promoted the freeze of steel and formation of complex TiN inclusions. In order to solve the problem of nozzle clogging, the following measures were proposed: reducing the carbon and nitrogen content in steel and the additive amount of titanium alloy; reducing the SiO₂ content in the refractory of submersed nozzle; improving the cleanliness of steel, especially promoting the removal of CaO·TiO₂ inclusions; modifying the CaO·TiO₂ inclusions to liquid particles by aluminum control and calcium treatment during refining process.

Research on the manufacturing process of high-quality steel ingots

2016, 32(4):  44-48. 

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As raw product，steel ingots play an irreplaceable role on manufacturing key forgings in some fields of the equipment manufacturing industry. The manufacturing process of top-poured steel ingots of 30Cr2Ni4MoV and bottompoured steel ingots, covering 20MnMo, 38CrMoAl low alloy steel and H13 mould steel, were researched to improve the purity of molten steel, which would promote domestic development of technology in steel ingots. Among manufacturing process, EAF→LF→VD→VP process route was adopted by toppoured steel ingots; EAF/added with convert steelmaking process →LF→VD→IC process route was adopted by bottompoured steel ingots. Results showed that lower total oxygen content w(T.O)≤12×10-6 in 30Cr2Ni4MoV steel was obtained by adopted process of vacuum carbon deoxidization in the location of VD, VP in comparison with common aluminum deoxidization, and the special requirement of chemical composition on w(Si)≤0.03 %,w(Mn)≤0.05 %,w(Al)≤0.005 % also could be reached, and sensitivity coefficientJ of tempering embrittlement also could reach to 3.4;w(T.O)≤10×10-6，w(S)＜0.001%,lower content of phosphor and nitrogen were obtained in high-purified bottom-poured steel ingot, involving low alloy and H13 mould steel, by applied with converter process of electric furnace and control oxygen technology in whole operation.

Study of large size Ds type inclusions in bearing steel

2016, 32(4):  49-53. 

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Bearing steel is mainly used for manufacturing the rolling element, rings and other components of bearings. Control of number and size of Ds type inclusions is an effective way to prolong bearing life. Effects of the slags on the total oxygen contents and the formation of Ds type inclusions in bearing steel were investigated by vacuum induction furnace experiments. The optimal middle basicity slags for VD treatment was selected in the range the CaO to SiO2 mass fraction ratio of 3－4, and the CaO to Al2O3 ratio of 1.6－1.8,resulting in lower than 10×10-6 total oxygen mass fraction and less Ds inclusions. Evolution of inclusions in bearing steel during the practical LFVD treatment was observed. It was found that more calcium aluminates Ds inclusions appeared after VD treatment due to the intensive mixing and reacting of steel and slag, resulting in the increase of calcium content in molten steel and enhancement of CaO content in inclusions. The Ds inclusions observed in the finished products were classified into three categories: those that derived from the endogenous inclusions without silica in the center, those that were wrapped by MgO from the tundish covering flux and those that came from ladle slag with silica. Combining the experiments results and the observation of inclusions, the Ds inclusions could be controlled less by controlling the refining slag, depressing the entrapment of inclusions aggregated under the covering agent and ladle slag.

Effect of RE on inclusions in highly clean bearing steel

2016, 32(4):  54-59. 

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Effect of different RE contents on inclusions in highly clean bearing steel was studied via adding highly pure rare earth elements. Inclusions in bearing steel were characterized and analyzed with metallographic microscopy, scanning electron microscopy, energy disperse spectroscopy and ImagePro Plus software. The results showed that a suitable amount of RE can not only make inclusions more dispersed and smaller, but also modify irregular shaped Al2O3 and strip MnS into spherical or ellipsoidal rare earth inclusions, both of which improve the quality of highly clean bearing steel. With an increase in the RE amount, inclusions gradually increase in size and their morphology becomes more and more irregular, so the metallurgical quality of highly clean bearing steel has a downward trend. In addition, increasing the RE amount causes the evoluation of main growth mechanism of inclusions from precipitationgrown to aggregationgrown. Considering inclusions modification and bearing steel performance optimization, RE mass fraction in highly clean bearing steel should be controlled around 0.02 % .

The mesoscale phase between atoms and inclusion particles as well as inclusion nucleation with two-step in molten steel

2016, 32(4):  60-72. 

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Many experiments and industrial practices show that it is hard to achieve the thermodynamic equilibrium during metal deoxidization reaction to form solid inclusions in liquid steel. The deoxidizing products cannot transform completely into the most stable structure of crystal or solid inclusions, possibly to form the metastable phase whose stability is lower than that of solid crystal. The metastable phase may be mesoscale phase, the size of which is between atoms and macroscopic particles, while the structure is in the evolution state from liquid (including amorphous state) to stable solid (including crystalline state). The computation by first principles shows that the mesoscale metastable phase in deoxidation system includes oxide clusters that formed with deoxidizer atoms and oxygen and aggregates before nucleation, critical nucleus and nanosized oxide inclusions after nucleation. The computation of equilibrium thermodynamics shows that the metastable phase is in equilibrium with deoxidizer and oxygen in the deoxidation system. The nucleation of inclusions during deoxidation is done in two steps. The first step is the reaction between deoxidizer atoms and oxygen to form clusters, which determines the dissolved oxygen content. In this step, the thermodynamic tendency is big and the reaction rate is high. The second step is the aggregation of clusters into nucleus, which is a limit step to determine the nucleation rate of inclusions that includes the diffusion of clusters and the transformation behavior of liquidlike into solid or crystal. Therefore, it is important to study the related factors affecting the second step and the control methods to control the size distribution of inclusions.

Study on distribution and source of large inclusions in the surface layer of IF steel slab

2016, 32(4):  73-80. 

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Aiming at the problem of large inclusions in the surface of IF steel slab, the influence of electromagnetic braking and casting speed on the amount and distribution of these inclusions was studied by slims method. Besides, the type and source of large inclusions were also discussed by scanning electron microscopy and numerical simulation. The results showed that large inclusions could accumulate in the edge of slab when the electromagnetic braking was kept off. And the accumulation of large inclusions would be weakened with the increase of casting speed. The degree of this accumulation was decreased by 17 % and amount of inclusions was also reduced by 5.3 % with the casting speed increased from 1.2 m/min to 1.5 m/min. The amount of inclusions decreased by 52% with a homogeneous distribution when the electromagnetic braking was kept on in the condition of 1.2 m/min. Due to low surface tension, the SiO2based complex inclusions was the main inclusion type of IF steel, and MgObased, Al2O3based, TiO2based were also found in IF steel. Meanwhile, refractory corrosion, reoxidation oxide and nozzle clogs were the main source for the large inclusions.