Table of Content

05 December 2016, Volume 32 Issue 6

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Optimization of insertion depth of stirring paddle of KR desulfuration

2016, 32(6):  1-4. 

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In order to explore the effect of insertion depth of stirring paddle on desulfurization efficiency and to reduce the sulfur content of the molten iron charged into converter , the flow field of hot metal and desulfurization efficiency with different insertion depth of stirring paddle were investigated by numerical simulation and industrial test. The results show that when the stirring paddle insertion depth and height of fluid level ratio is 0.287, the upper and lower circulation area is almost consistent. And when the insertion depth is 800 mm in the industrial test, the average final sulfur mass fraction is lower than 20×10-6. The result of numerical and that of industrial test concide with each other.

Research and progress on technology of assisted slag skimming from hot metal ladle

2016, 32(6):  5-10. 

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In this paper, the research and progress on technology of assisted slag skimming from hot metal ladle were reviewed. The main conclusions of this article are as follows：how to use the blowing gas pushes the molten iron liquid level movement driving the desulfurization slag to skimming area is overall technical ideas of new processes; because it can not only ensure the ladle  independence and integrity, but also can meet the needs of performance requirements under different ladle turnaround modes with low operating cost, top blowing assisted slag skimming technology has being the slag skimming process development trend; developing structure of top blowing lance nozzle is one of the top blowing assisted slag skimming technology development direction.

Experimental study of limestone and double slagging in BOF steelmaking process

2016, 32(6):  11-14. 

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In order to explore the process of converter using limestone instead of lime and doubleslag for dephosphorization, the test results of 100 t converter using limestone and double-slag in a steel plant were studied, and the advantages of limestone slagging for dephosphorization are analyed theoretically. The test results show that the dephosphorization rate of test furnace is 88.67 % on average. Compared with lime and double-slag process, the average CaO charging amount of limestone and double-slag process is reduced by about 29.45 %,which means the new process can further reduce the consumption of materials and slag amount. And the dephosphorization rate improves with the increase of limestone adding batches.

Study on dissolution mechanism of dolomite in LD slag

2016, 32(6):  15-18. 

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According to the actual needs of using dolomite instead of doloma slagging in converter steelmaking, the dissolution experiment of the dolomite and doloma in converter steelmaking slag was carried out. By scanning electron microscopy and energy spectrum analysis, the dissolution mechanism of dolomite and doloma were compared and analyzed, and the industrial test of slagging with dolomite instead of doloma is carried out. Results show that the dissolution mechanism of dolomite is different to doloma in LD slag. Dolomite firstly occurs pyrolysis reaction in steelmaking temperature, then it becomes MgO, CaO tiny particles and take dissolves reaction with liquid slag. Moreover,  because of the stirring effect of CO₂ bubbles, dolomite dissolve faster than doloma. Dolomite is found to decompose into CaO, MgO and CO₂ in one step at the converter steelmaking temperature. By using dolomite slagging, dephosphorization rate increases from 83.2 % to 84.1 % and sulfur mass fraction increment reduce from 78×10^-6 to 71×10^-6. Final slag properties also conducive to the maintenance of furnace lining, and the cost of steelmaking flux decrease 2.98 yuan/t.

The analysis about effect of ladle thermal status on liquid steel temperature drop and algorithm optimization

2016, 32(6):  19-26. 

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There are the problems of high temperature steel converter and low liquid steel temperature control precision in Q steel works. To study the effect of ladle thermal status on liquid steel temperature, the multi factor coupling value has been compared with the double factors coupling superposition values by building and solving the heat transfer computation model for ladle, the result and algorithm has also been analyzed and optimized. The results have shown that the difference between multi-factor coupling and the double-factors coupling superposition values is 0.4—1.024 ℃ when offline preheating before being put into use or having short ladle life, and the difference is 0.4—3.338 ℃ when offline preheating after being put into use. The difference shows that the multifactor coupling should be used in liquid temperature compensation, otherwise the doublefactors coupling superposition should be used. The results are applied to the study on influence factors of ladle thermal state, can reduce the amount of calculation. The results are applied in production can make the tapping temperature compensation system more reasonable.

Investigation on direct alloying using molybdenum oxide in EAF with high ratio of hot metal

2016, 32(6):  27-33. 

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In view of the feasibility of direct alloying using molybdenum oxide in high metal ratio electric arc furnace (EAF), industrial trials on direct alloying using molybdenum oxide under different molybdenum (Mo) contents, hot metal ratios and charging rates were carried out in a 100 t EAF in the current work. The feasibility of the reduction reaction was proved using thermodynamic calculation. Meanwhile, the effect of slag composition and temperature on reaction rate was predicted using kinetic calculation. Without any external reductive agents, with the addition of CaO, the alloy yield and the effect on quality of direct alloying using molybdenum oxide were investigated. It was found that the yield of Mo could be larger than 91 % for both high Mo steel grades and low Mo steel grades. Contrastively, the alloy yield in low Mo steel was larger than that in high Mo steel. And overall, the optimal conditions to reach the highest Mo yield was 760 kg/t steel of hot metal ratio and 9.0 t/min of charging rate of hot metal, under which the yield of Mo could be larger than 93 %, and at the same time high steel cleanliness was ensured, realizing the energy conservation and emissions reduction and valuable economic benefits.

Development and application of RH molten steel temperature continuous measuring system

2016, 32(6):  34-37. 

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In order to continuous on-line measure the temperature of liquid steel in RH vacuum tank, timely and accurately calculate the temperature drop rate at all stages of refining process, instructing the temperature adjustment operation，the temperature measuring device was designed and developed based on infrared thermal image and colorimetric temperature measurement technology. The industrial test of the device was carried on 120 t RH refining furnace and 298 heats’ data was obtained. The temperature value corresponding each stages of steel fluid circulation begin, end of decarburization, end of aluminum adding, end of powder injection and endpoint obtained by the device was contrasted with the manual measurement results measured by means of thermocouples. The comparison results show that the temperature difference between this device and manual measurement varies in the range of －5.0 ℃ to 6.3 ℃. Among them, the proportion of the difference fluctuation in range of ±3 ℃ is 92.1 %,within ± 5 ℃ up to 98.7 %. At present，the technology has been successfully online applied in 120 t RH refining furnace.

The end point prediction model for RH refining

2016, 32(6):  38-44. 

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In order to improve the hit rate and accuracy of temperature and composition of molten steel during RH refining process, the prediction model for the end point temperature and composition of molten steel was established based on integrating of metallurgical mechanism and NARX Neural network. The prediction model program that was able to calculate the amount of oxygen and alloy and to predict the end point was developed by utilizing of Visual Basic 6.0 and Matlab neural network toolbox. The model showed high prediction accuracy with over 85 % hit rate of temperature and composition at the same time. The rate of temperature errors less than 5 ℃ reached 90 % and all the carbon mass fraction prediction errors were within 5×10^-6. The rates of Si，Mn，P and Al_s prediction errors less than ±5 % exceeded 90 %. In addition, the rates of oxygen, lowcarbon ferrosilicon, iron phosphate, aluminum and microcarbon ferromanganese amount prediction error within －3 % to 7 % were 90 %，75 %，75 %，95 % and 70 %, respectively.

Effect of refining procedures on inclusion characteristics in SPA-H steel coils

2016, 32(6):  45-50. 

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To reduce the cost, one steel mill try to use CAS refining to replace traditional LF refining process in SPA-H steel. However, it is still in dispute whether this replacement may worsen the cleanliness of steel coils. The total oxygen content and automatic feature analysis (AFA) in ASPEX SEM were used to evaluate the cleanliness of SPA-H steels coils, and the effect of refining methods，such as CAS refining and LF refining process on them were compared. The result shows that the T.O mass fraction of coils were almost the same, ranging from 18×10^-6 to 19×10^-6, Four different kinds of micro inclusions were detected in steel coils: CaS-Al₂O₃，Al₂O₃,(Ca,Mn)S and TiN, and all the macro inclusions were pure MnS. Furthermore, (Ca,Mn)S and Al₂O₃ inclusions account for over 90 % of inclusions in CAS refining process while CaS-Al₂O₃dominates micro inclusions in steels coils for LF refining process, occupying 81.3 %.The number density of largesized inclusions for CAS refining process were much larger than that for LF refining process. Equilibrium inclusion composition calculated by Factsage agree well with the experimental results. Lower calcium yield ratio and smaller Ca/S ratio for CAS refining process due to the relatively higher TFe content in refining slag may account for the difference of the inclusions between the two different refining methods. Accordingly, Ca alloy should be more added and top slag should be reduced when refining SPA-H steels using CAS process.

Physical simulation of steel flow and inclusions behavior in SEN with adsorption rod

2016, 32(6):  51-56. 

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The behavior of inclusions in the submerged entry nozzle has direct effect on the cleanliness of steel. The installation of an inclusion adsorption rod, i.e. an extension rod on the stopper end was proposed. The effects of the rod length and groove structure on fluid flow and inclusions behavior in the submerged entry nozzle were investigated via physical model experiments with plastic particles to simulate inclusions. The results show that rectangular groove is benefit for inclusions assembling, but the inclusions out of the groove have a little chance to enter it, and the maximum impact area ratio was 33 %. For parallelogram-shaped grooves, the inclusions are able to enter the 45 groove and the maximum impact area ratio was over 48 %, but it is not benefit for inclusions assembling.

Formation reasons and control measures of abnormal oscillation marks for high carbon steel billets

2016, 32(6):  57-61. 

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In order to explore the formation process of the abnormal oscillation marks for the high carbon steel in this paper, the surface and longitudinal section morphology of the abnormal oscillation marks was investigated by optical microscope analysis. The results indicated that insufficient lubrication and large frictional resistance between the billet and the mold were the main reasons to make the oscillation mark bended, protruding and even the formation of double skin. With the oscillation frequency and amplitude set as 180 strike/min and 4.2－4.5 mm, the viscosity and melting temperature of mold powder set as 0.18 Pa·s and 1 030 ℃, the powder consumption was increased from 0.18kg/t to 0.31 kg/t. At last, the surface quality of the billet was improved obviously, the oscillation marks had returned to normal, and the rejection ratio decreased from 3 %－4 % to less than 1 %.

Effect of oxidizing slag on the cleanliness of IF steel

2016, 32(6):  62-67. 

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Interstitialfree(IF) steel was produced by KR→BOF→RH→CC process. Different amounts modifier were added to ladle slag after tapping. The effect of top slag conditioning was compared. The composition of molten steel and the morphology, composition, size, number of inclusions during RH refining were studied. In addition, total oxygen and the characteristic of inclusions in the surface layer of continuously-cast slab were analyzed. The results show that enough lime must be added to top slag to ensure the Al₂O₃ absorption capacity by using Al-based modifier. The loss rate of soluble aluminum in steel declines 30 %－40 % and the total oxygen at the end of RH refining is below 20×10^-6 when the mass fraction of FeO in slag decreases from 13 %－16 % to 0.77%, then the change of inclusion number in slab is significant from 5.66 cm^-2 to 2.96 cm^-2.

Research on dephosphorization converter utilizing hot returned steel slag of decarburization converter

2016, 32(6):  68-72. 

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In order to achieve less slag of “Full-Tri-de” smelting process, new technology which was study on dephosphorization converter using recycling of hot steel slag of decarburization converter was employed to further reduce material consumption of semi steel smelting process  by Shougang Jingtang. The dephosphorizing ability and recyclability of hot steel slag of decarburization converter were analysised. Simultaneously, the effect of hot steel slag on slag forming path and application effects in dephosphorization converter were investigated. The results show that average consumption of 1.6－2.4 t lime was saved through recycling hot steel slag 10－15 t per ladle. If silicon mass fraction in molten iron is lower than 0.15 %, lime could not be added to dephosphorization furnace. The consumption of 2.4 kg/t iron and steel raw material was correspondingly cut. Meanwhile, lightburned dolomite and fluorite as burden materials were eliminated in dephosphorization furnace. The endpoint phosphrous content was almost the same with the normal operation after adopting recycling of hot steel slag of decarburization converter. At the same time, the heat loss could be reduced by using sensible heat of converter hot slag.

Migrant and enrichment behavior of chromium in stainless steel slag at different basicities

2016, 32(6):  73-78. 

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The existential state of chromium has a great influence on stability of chromium in stainless steel slag, and spinel phase is considered to be the optimal stable phase for preventing the chromium from leaching. In order to effectively stabilize chromium element in stainless steel slag and reduce the leaching of chromium to decrease the risk of pollution, in the present work, the effect of slag basicity on the migration behavior and existential state of chromium was investigated in the experiment. The samples were heated for 30 min at 1 600 ℃ for completely melting and soaking for 30 min at 1 600 ℃and 1 300 ℃ before quickly quenched in water. The microstructure and existential state of chromium were determined by scanning electron microscopeenergy dispersive spectroscopy (SEMEDS) and Xray powder diffraction (XRD). The results showed that the basicity significantly influenced the migrant and enrichment behavior of chromium in stainless steel slag. It was beneficial for chromium enriching in spinel phase with w(CaO)/w(SiO₂) ratio of 1.5, especially at 1 300 ℃, the enrichment degree of chromium was nearly up to 100 %. When the basicity was 1.0, the solubility of chromium in the liquid phase was relatively high, and only minor amount of spinel phase could be formed at 1 300 ℃. As the basicity increased to 2.0, the chromiumbearing periclase phase was precipitated.