Table of Content

05 February 2016, Volume 32 Issue 1

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Hot Metal Pretreatment

Simulation on optimization of flow field in KR hot metal desulfurization ladle

2016, 32(1):  1-5. 

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In order to improve the utilization of lime and reduce the aggregation behavior of desulfurization flux during the mechanical stirring of hot metal, the flow behavior in the vessel with different numbers of baffles was investigated by physical simulation and numerical simulation. Results showed that the vortex was inclined, the increased height of fluid level was decreased and the tangential movement of fluid was changed after the addition of baffle in the vessel. The aggregation of flux was reduced effectively and the number of particles entrained into the water increased by 10 times during stirring in the vessel with two baffles compared with that in the normal vessel. The velocity and turbulent kinetic energy in the hot metal could be ensured in the vessel with two baffles. It indicated that there was a good kinetics condition of desulfurization in hot metal.

BOF & EAF Steelmaking

Study on slag forming route of dephosphorization stage during conventer slag-remaning and double slag process

2016, 32(1):  6-11. 

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In order to improve the dephosphorization effect during slag-remaning and double slag process，the influence of slag basicity and FeO，MgO and MnO content on lg LP（LP is the equilibrium distribution ratio of phosphorus in slag- steel）were calculated by thermodynamic empirical formula. The results indicate that the theoretical composition range of slag during dephosphorization process is that slag basicity is about 1.5~2.5， w（FeO）=15%~25%，w（MgO）＜8%，w（MnO）＜10%. Liquidus diagram of CaO-SiO2-FeO-MnO-MgO slag systems was calculated by thermodynamic software Factsage. Theoretical calculate slag forming route was designed for dephosphorization stage during slag-remaining and double slag process with the comprehensive consideration of empirical formula and liquidus diagram calculate results. The characteristics of the slag forming route is that FeO content is high and the slag basicity changes from high to low and then increases slowly. The production practice results indicate that the consumption of slag material decreased by 22.7% and dephosphorization rate reached 92.2% at the end of converter steelmaking with this slag forming route. The experiment achieved the dual goals of decreasing slag material consumption and efficient dephosphorization.

Optimization of charge structure and blowing process in converter

2016, 32(1):  12-14. 

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In order to reduce the production cost, the raw material structure was optimized on 100t converter in some plant. The influence of raw material structure change on end point control of converter was avoided through adjusting the hot metal scrap ratio, optimization of slagging and smelting process. The mass fractions of carbon and phosphorus and temperature at end point were controlled, and the oxidization of molten steel was reduced. The ferrous charge consumption in the converter was decreased by 4.87 kg/t, and the carbon-oxygen equilibrium of molten steel at end point was decreased to 0.0027 or so, and the consumption of aluminum was decreased by 1.31 kg/t.

Study on final slag composition control for highly efficient dephosphorization of 80 t converter in CISC

2016, 32(1):  15-19. 

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Together with the condition that the average phosphorus mass fraction is 0.16% in molten iron of Chongqing Iron & Steel （Group） Co.，Ltd., average dephosphorization rate couldn’t exceed 90% with single slag method in 80t converter. Considering that (FeO) can transfer oxygen at steel-slag interface, the effect of slag basicity and w(FeO) on the C-P selective oxidation temperature was studied through calculation. Based on dephosphorization thermodynamics and the meltability of slag which represents kinetics at the later blowing stage of BOF, the effects of two parameters on the dephosphorization rate were studied. The first is difference between the end-point temperature of BOF molten steel and C-P selective oxidation temperature, and the second is superheat of slag. In addition, the suitable slag basicity and w(FeO) were determined when the end-point temperature of BOF molten steel was given. The results show that with the temperature difference decrease a 10℃, the dephosphorization rate increase by 1.2%. When the end-point temperature of BOF molten steel is about 1640℃, basicity is controlled in the range of 4.2 to 4.55, and w(FeO) of end steel slag is between 23% and 27%, the dephosphorization rate reaches over 90%.

Practice of improving the using effect of tapping hole of converter

2016, 32(1):  20-22. 

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In order to solve the problems of 120 t converter tapping hole sleeve brick such as low service life, long tapping time and large temperature drop in the early stage, serious diffuser and secondary oxidation in the later period, poor slag-damming effect and serious rephosphorization in Maanshan Iron & Steel Co., Ltd., new material, new technology and new process have been adopted in the replacement, operation and maintenance of tapping hole brick. The results indicated that the service life of tapping hole brick was extended, the ratio of the optimist tapping time between 5min50s and 4 min was 73.6%, the temperature dropped reduced by 12℃, times of slag rephosphorization accident dropped from seven times to under three times per month.

Study on low-cost deoxidation technology for plain carbon steel

2016, 32(1):  23-26. 

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Optimization of deoxidization process and choice of new kinds of deoxidizers were studied systematically to reduce production cost of plain carbon steel. Deoxidation ability of common deoxidizing material was analysed by thermodynamic analysis, and the costs of different deoxidizers were evaluated, and appropriate deoxidization processes have been formulated. Concretely, cheap carburant was added to molten steel during tapping process for pre-deoxidization, and then silicon iron and silicon-manganese alloy were added to molten steel for deoxidization and alloying treatment. Lastly, aluminum wire was fed for deep deoxidization in the argon bubbling station. With the application of the new technology, the production system run smoothly, and the components and total oxygen content of the product could be controlled in the required range, and furthermore, the FeO content became more stable. On this basis, the cost of deoxidizer alloys reduced by 6.68 yuan/t.

Secondary Refining

Industrial practice of ladle feeding pure-calcium corded wire process

2016, 32(1):  27-29. 

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Pure-calcium corded wire feeding process in 180t ladle was developed by theory calculation and industrial experiments. The calculation results indicated that the wire feeding speed should be more than 1.48m/s with the corded wire of the diameter of 9mm and the core skin thickness of 0.75mm. Industrial experimental results showed that the calcium yield increased with the decrease of the feeding speed at the range of 2-3m/s, and maximum calcium yield was 26.9% when the feeding speed was 2m/s. In comparison with ferro-calcium corded wire feeding process, the calcium yield increased by 16.9%, and the wire feeding cost decreased by 6.4 yuan using pure-calcium corded wire feeding process.

Slag-forming model of the aluminium killed steel based on the deoxidization process

2016, 32(1):  30-34. 

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In order to solve the problem of the instability of slag quantity and end slag composition in the aluminium killed steel production process with LF in some factory, a deoxidation slagging model was proposed based on mass balance in SPHC production and aluminum deoxidization process. The model was written as offline test software by VB6.0, and it was run to test the result with input the field smelting data of SPHC. The results showed that, the stability of materials quantity and end slag composition was promoted greatly, and the stable refining process and good molten steel quality were well. The mass of slag calculated kept stable since the mean square deviation was decreased to 0.93 from 2.95. The mass of slag formed per ton of steel could reduce 2.41 kg in the calculation when compared with the factory.

Cleanliness study of tin plate steel during LF process

2016, 32(1):  35-41. 

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In order to understand the cleanliness of MRT-2.5 tin plate steel by a BOF-LF-CC route, the system sampling was performed to analysis the content of nitrogen and total oxygen in liquid steel during LF refining, and variation of the types of inclusions, number and size of inclusions were studied during LF process. Transformation mechanism of inclusions was also studied at different stages. The experimental results show that, w（T.O）=34.8×10^-6 and w（N）=21.1×10^-6 at the end of LF refining. The main types of oxide inclusions are MgO-Al2O3 after slag forming, Al2O3 after aluminum addition, and CaS-CaO-Al2O3 at the end of calcium treatment. The number density of inclusions which decreases during LF refining is 3.9/mm2 at the end of LF refining and the diameter of inclusion particle size is gradually concentrated in the range of 0-5μm. Under the conditions in a company, w（Ca） content in liquid steel should be controlled in （2—20）×10^-6, and w（Mg） content in liquid steel should be controlled in （0.45—8）×10-6, and a（O） should be controlled in （2—6）×10^-6 in order to contr^ol inclusions in low melting point and w（S） content in liquid steel should be controlled in less than 0.0042% in order to avoid CaS inclusions with precipitation.

Blowing nitrogen alloying process test of LF

2016, 32(1):  42-46. 

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The process test of nitrogen alloying by 80t LF gas injection was made in CISC based on theoretical analysis of thermodynamics and dynamics. The technological measures of reasonable control on steel composition (sulfur, oxygen content), nitrogen flow and nitrogen blowing time were proposed. The results showed that，the treatment of feeding 36kg aluminium wire per heat before LF for precipitation deoxidation was conducive to nitrogen content increasing quickly. The higher the molten steel temperature was before large blowing, the better the nitrogen absorption effect was. The nitrogen absorption rate was stabled over 7×10^-6/min when molten steel temperature was over 1580 ℃. In order to satisfy w（N）≤200×10^-6, the time of nitrogen alloying by LF gas injection should not exceed 60 min. The yield strength and tensile strength of steel were increased, and the elongation was decreased slightly, but all of these met the national standard.

Selection strategy for RH ladle lifting hydraulic system

2016, 32(1):  47-51. 

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To reduce the cost of RH ladle lifting hydraulic system and improve the speed control function, taking the system pressure and the maximum speed as the target, the parameter optimization was discussed basing on the design datum and experiences of past typical projects. By studying the system designed by institutes at home and broad, the common ladle lifting hydraulic systems were analyzed and compared.

Solidification & continuous Casting

Development and application of high basicity-high lubrication capacity mold fluxes

2016, 32(1):  52-54. 

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Based on features of peritectic steel continuous casting，a new technical route to ease the contradiction between heat transfer and lubrication was presented. In this work, a high basicity (1.5~2.0) mold flux with high lubrication ability was developed. More than 60 t of new mold fluxes for thick and wide peritectic steel slabs continuous casting were consumed. All the results reveal that the property of this high basicity mould flux is very stable, and no cracks and quality problems were found on slab surface, subsurface or on final product coils, the mould flux consumption is reasonable. The total defect rate of slabs decreased from more than 20% to less than 2%, which proves that the new mold flux has excellent results on easing the contradiction between heat transfer and lubrication.

Cleanliness of the first billet of SWRCH22A steel

2016, 32(1):  55-61. 

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The variation of cold heading steel cleanliness from LF end to continuous casting was investigated. The results showed that，Due to severe reoxidation from LF end to casting starts, the mass fraction of T.O and \[N\] in steel increased by 39×10^-6 and 21×10^-6 respectively, and the w(CaO)/w(Al2O3) of inclusions decreased. At steady casting period, the T.O and \[N\] content decreased and stabilized at 30×10^-6 and 65×10^-6 respectively, while the ratio of w(CaO)/w(Al2O3) of inclusions fluctuated and located in the low melting point area of CaO-Al2O3-MgO-CaS diagram. Thermodynamic calculation indicated that with current steel composition, a 3.5×10^-6 nitrogen pickup would generate solid inclusions. The first three meters of the head billet was suggested to be cut off to assure a good steel cleanliness.

Formation mechanism and control of internal cracking breakout for billets

2016, 32(1):  62-65. 

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In order to explore the characteristics of the breakout shell and its evolution process for the thread steel, it was investigated by optical microscope and scanning electron microscope analysis. The results indicate that the intergranular segregation especially sulphur plays an important role in the high temperature mechanical property of the steel and the crack propagation path. In the mold, the inhomogeneous growth of shell would cause the occurrence of longitudinal depression and then induce the formation of internal crack. The weak positions of the billet would subject to stress concentration at the exit of mold, cracks tend to propagate and the breakout may happen in severe cases. By controlling the content of sulphur in steel, adjusting the appropriate process parameters, the breakout ratio of the thread steel decreased from 6.32% to 1.10%.

Mechanical properties and solidification structures of large round bloom

2016, 32(1):  66-69. 

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The difference of room temperature mechanical properties between the columnar crystal and the center equiaxial crystal of large round bloom solidification structures obtained by the different current intensity of mold electromagnetic stirring (MEMS) was investigated. Moreover, the difference of porosity degree，tensile strength and tensile elongation between the columnar crystal and the center equiaxial crystal was studied by original position analysis (OPA) method and tensile tests at room temperature. And then the reason for the results was analyzed qualitatively. The results show that the porosity degrees of columnar crystal and center equiaxial crystal are both reduced, the tensile strengths are increased, and the tensile elongations are roughly decreased with the increase of the current of MEMS. The porosity degree of columnar crystal is better than center equiaxial crystal under the same current of MEMS. The tensile strengths along different directions of center equiaxial crystal are almost same, but in columnar crystal the tensile strengths along the radial direction of columnar crystal are higher than the vertical to radial direction. Besides, the tensile strengths of center equiaxial crystal are obviously higher than those of columnar crystal. The difference between tensile elongations along different directions of center equiaxial crystal is smaller than that of columnar crystal.

Mechanism analysis on macro-etching crack of high strength steel slab containing titanium and niobium

2016, 32(1):  70-73. 

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In order to explore the forming mechanism of crack on high strength slab containing titanium and niobium, the morphology of crack was revealed by macro-etching, the microstructure of slab and crack fractograph were examined by metallographic analysis and the characteristic of continuous cooling transformation was tested based on the thermal expansion method via GLEEBLE-2000. Consequently forming reason of macro-etching crack on high strength steel slab containing titanium and niobium was elucidated from aspects of temperature variation and phase transformation. The results showed that cooling rate of the slab could reach as high as 5℃/s within the range of 30mm from torch cutting plane after torch cutting; high cooling rate made Bainitic and Martensitic phase transformation taking place on the slab; and the combination of stress caused by the high cooling rate and the phase transformation led to the severe macro-etching cracks on the high strength slab bearing titanium and niobium.

Study on hydration characteristics of converter steel slag

2016, 32(1):  74-78. 

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The hydration of converter steel slag was investigated by the activity test, the amount of chemically combined water, the mechanical test and X-ray diffraction analysis (XRD). The results showed that steel slag had a low activity at early ages. When partly replacing the cement in concrete, the degree of hydration cement decreased at early ages and then the hydration process of cement inhibited. The bigger the proportion of steel slag is, the greater inhibition effect is. With a low water binder ratio, the concrete blended with steel slag had a slow development of strength at early ages, and the inhibition effect of steel slag on the concrete strength was weakened as the curing time increased. The main mineral phases of hardened slurry blended with steel slag include Ca(OH)2, C-S-H gel, inert components (C2F, Fe3O4，RO phase), and incomplete hydrated gelling components (C2S, C3S, Ca2Al2Si3O12 and C12A7).