Table of Content

05 October 2016, Volume 32 Issue 5

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Technology of bath stirring in top and bottom combination blown converters and its application

2016, 32(5):  1-10. 

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Influence of different bottom tuyere number and configuration in top and bottom combination blown converters on bath mixing time was investigated in laboratory for improvement of BOF’s bath stirring effect. It is found from the investigation that the suitable arrange where the bottom tuyeres should be set is (0.40—0.65)D (D is bath diameter). Too many bottom tuyere number and symmetrical, dispersive and uniform configuration along the same circumference is detrimental for the whole bath mixing, while unsymmetrical and relatively concentrative bottom tuyere configuration can form the trend of horizontal mixing in the bath, which is benefit to the whole bath mixing and can decrease the mixing time by about 30%. It is proved from the application of the unsymmetrical and relatively concentrative bottom tuyere configuration in industrial converters of Sansteel, Fujian that the metallurgical results at converter blowing end can be improved. Mean product of [C] and [O] concentrations, [%C]•[%O], at end-point of blowing in No.1 and No.2 100 t BOFs were reduced from 0.003 1、0.003 3 to 0.002 6、0.002 7, respectively. In No.1 BOF, average [Mn] mass fraction at end point was increased from 0.183% to 0.212%, mean [P] mass fraction at blowing end was decreased from 0.016 7% to 0.014 8%, and average w(T.Fe) in slag at end-point of blowing was reduced from 15.23% to 13.36%. Mean [%C]•[%O] at end-point of blowing in 120 t BOF was 0.002 5.

Blowing process and effect in 260 t BOFs with exposed visible bottom tuyeres

2016, 32(5):  11-14. 

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Based on investigation of factors influencing bottom-blowing effect and tuyeres maintenance methods, the reasonable bottom-blowing intensity schemes were proposed for various steel grades, the suitable thickness of furnace bottom was determined and final slag composition and slag splashing process were optimized. The bottom tuyeres exchange technology was used to realize tuyeres exposing and visualization. Thus the tuyeres life was increased and molten pool stirring effect was improved. Now the bottom blowing system could guarantee carbon-oxygen product to be less than 0.002 7 during 4 000 heats in one service life, and combined blowing rate to 80% with 2 bottom tuyeres.

Bottom blowing technique and its application results in 100 t top and bottom combined blown converter

2016, 32(5):  15-19. 

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Based on the optimization of bottom tuyere configuration in water model experiment, asymmetric bottom tuyere configuration layout was used in the 100 t combined-blowing converter of Sanming Iron and Steel Co. Then, bottom blowing process and bottom tuyere maintenance were optimized for start-up, heating-up and whole campaign of the converter and maintenance modes of bottom tuyeres of the converter and bottom blowing intensity at different stages and different bottom thickness were determined. Influence of end-point temperature, converter life and bottom blowing intensity on [%C]•[%O] at end-point was investigated. The investigation results showed that [%C]•[%O] at end-point is between 0.002 63—0.002 72, on average 0.002 67, in the earlier stage of the converter campaign and at 1 620—1 635 ℃. Bottom lining thickness of the converter during whole campaign could be kept 600—750 mm, which ensured stirring effect of bottom gas and [%C]•[%O] at end-point was kept 0.002 6—0.003 0. [%C]•[%O] at end-point rose with increase in end-point temperature under the same bottom blowing, about 0.000 03 rising on each 10 ℃ up in temperature. Contents of carbon, phosphorus and sulfur decreased, while content of residual manganese increased, with increase in intensity of pure bottom post-stirring.

Practice of combined blowing of large converter in WISCO

2016, 32(5):  20-24. 

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The parameters of converter oxygen lance nozzle and the bottom blowing flow rate were optimized in No.3 Steelmaking Plant of WISCO. The splashing frequency decreased from 3.4% to 1.5% in blowing process, the slag T.Fe mass fraction reduced from 15.99% to 15.49% at the end point of blowing, good blowing effect was achieved. At the condition of monitoring the bottom thickness by laser measuring thickness, extending visible porous brick life up to 4 000, improving the bottom blowing gas flow to 1 100m3/h, using 8~16 visible porous bricks, [%C]•[%O] was in 0.001 5~0.001 8 at blowing end, which achieved better metallurgical effects. The parameters of oxygen lance nozzle and the lance profile in blowing process were optimized in No.4 Steelmaking Plant, the problem of scrap melting was solved. The stirring mode was optimized through the large gas flow stirring test at the end blowing. When using 4 visible porous bricks and stirring flow 500m3/h, stirring time for 3~4 minutes, the mass fraction of oxygen reduced 20.47×10-6 average per minute in molten steel, the mass fraction of carbon reduced 29.23×10-6 average per minute.

Effects of heating and holding process on the MnS inclusions in the bloom and rail of heavy rail steels

2016, 32(5):  25-29. 

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The effects of different heating and holding process on the MnS inclusions in the bloom and rail of heavy rail steels was investigated. It was found that the large elongated MnS inclusions in rail would split into small particles after heating and holding process. The amount of MnS inclusions larger than 80 μm decreased, while the amount of MnS particles smaller than 5 μm increased when the soaking time exceeded 3.5 hours. However, the phenomenon was not obvious for the MnS inclusions in bloom. The behavior of MnS inclusions followed the Ostwald ripening mechanism, that longer soaking time lead to larger MnS particles. The MnS inclusions were more likely to split at 850℃, at which sulfur was the diffusion controlled element. The three-dimensional morphologies of MnS inclusions in bloom included plate-like, strip-like and irregular, which were then elongated to strip-like along the rolling direction during the subsequent rolling process of bloom into rail. Pure MnS inclusions would start to precipitate only when the solid fraction reached 0.94 on the basis of calculation by Scheil model, the morphology was influenced by the pre-existed grain boundaries. The restrictive factors of the formation of MnS were concentration product of [%Mn]•[%S] and temperature. To reduce the accumulation of Mn and S in the front of solidification and further to reduce the precipitating amount and size of MnS inclusions, decreasing sulphur content and increasing cooling rate were proposed under the condition of constant Mn content in molten steel.

Analysis and research on the oxide and nitride in H13 steel

2016, 32(5):  30-35. 

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The oxide and nitride in H13 die steel were studied by scanning electron microscope (SEM) and electron microprobe analysis (EMPA), showing that most oxide inclusions were spherical Al2O3 with a size of 5μm and a few MgO•Al2O3. Besides of oxide, some complex nitrides (Tix,V1-x)N were found, some of which existing cores of 2—3μm MgO•Al2O3 and Al2O3 in the center. The formation possibility of Al2O3, Ti3O5 and SiO2 was calculated based on thermodynamics. It was found that Al2O3 might only be formed in liquid steel when temperature was below 1 823K, while the later two oxides could not be formed. Three nitrides including TiN, VN and AlN could not be formed in liquid and mushy zone under equilibrium. Alloying elements especially Ti could segregate obviously during non-equilibrium solidification process. Theoretical analysis indicated that the binary nitrides of TiN and VN could be generated at critical solid fraction of 0.699 and 0.987 in the solidification front, respectively. Together with the experiment results that Ti and V content having the same trend in nitrides, it was illustrated that the complex nitrides (Tix,V1-x)N might be formed in a way of solid solutions. Experiment showed that the molar fraction x ranged from 0.5 to 0.6, indicating that the complex nitrides could be formed in mushy zone when the solid fraction reached 0.7. The influence of oxide as a precipitation core of nitride was taken into account in the view of forming condition and lattice mismatch. Further research will be done on the influence of oxide on nitride nucleation.

Generation mechanism of heterogeneous (Ti,Nb,V)(C,N) in H13 tool steel modified by niobium

2016, 32(5):  36-40. 

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The precipitation of carbonitride in H13 hot working die steel would be detrimental to the strength, hardness, toughness and fatigue resistance properties of steel. The characteristics of heterogeneous precipitates (Ti,Nb,V)(C,N) with the size larger than 1 μm in a Nb containing H13 forging bar pre-produced by electro-slag remelting (ESR) process were studied. The generation mechanism was analyzed through calculation by Thermo-Calc software. Most of the precipitates observed consisted of three-layer structures. The center was an oxide core which was mainly MgO-Al2O3 spinel or Al2O3, the middle layer was a Ti-V-rich (Ti,Nb,V)(C,N) phase and the outer layer was a Nb-rich (Ti,Nb,V)(C,N) layer. The normalized compositional distributions of Ti, Nb and V in the Ti-V-rich and Nb-rich layers were concentrated. The average composition of Ti-V-rich layer was (Ti0.477V0.337Nb0.187)CxNy and that of Nb-rich layer was (Ti0.127V0.255Nb0.617)CxNy. The size of the heterogeneous precipitates could be up to ten microns. The generation mechanism of the precipitates could be well speculated through Thermo-Calc calculation. The spinel and alumina have already existed in the liquid steel before solidification. During solidification the Ti-rich phase will firstly start to precipitate on the oxide when the solid fraction reaches 0.823. Then the V-rich phase and Nb-rich phase will precipitate successively. During the subsequent heat treatment process, the V-rich phase will be dissolved in Ti-rich phase, resulting in the appearance of large precipitate with three-layer structure.

Dynamics of carbonitrides inclusions precipitation during solidification in heavy rail steels

2016, 32(5):  41-47. 

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In the current work，the segregation of elements was studied on basis of segregation model and influence of cooling rate on precipitating and growing up of nitride inclusions was investigated during solidification. The calculating results showed that concentration of Ti，V，C and N elements were 4.68，2.34，4.55，and 3.3 times than initial concentration near the liquid-solid interfaces. It could postpone the precipitation of TiN inclusions and decrease the size of them by reducing the content of Ti and N in molten steel and increasing cooling rate from 0.27 K/s to 37.96 K/s. According to regular solution sublattice model，themodynamic calculation of carbonitrides inclusions formation mechanism was performed in heavy rail steel.

Research of optimization model of steel ladles interaction in steelmaking works

2016, 32(5):  48-53. 

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Steel ladles of Kunsteel Steelmaking Works were taken as the object of research, for the further optimization of steel ladle interaction among continuous caster in steelmaking works. The relationship among cast number, the turnover number, turnover times and last turnover number are further analyzed, which is build on the basis of revolving quantity calculation model of single cast steel ladle. Then, the simulation of steel ladle turnover rules was taken on the 2 continuous casters with Gantt chart. After that, the relationships of turnover number and last turnover number and non-turnover number are researched deeply. Combining with the Gantt chart, the expressions of start casting time, empty ladle ending time and heavy ladle starting time were put forward. Finally, the precondition of steel ladle interaction was obtained, which meant the steel ladle no longer undertake the transport task at first stop continuous caster and the exchange ladle’s turnover ending time was prior to ladle’s turnover starting time of last start casting continuous caster. Studies have shown that the steel ladles of steelmaking works can be reduced from 13 to 10.

Erosion analysis of MgO-C refractory brick at the slag line of ladle

2016, 32(5):  54-61. 

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Physical and chemical analysis，SEM-EDS detection，Factsage calculation，wettability analysis of the MgO-C refractory bricks at the slag line of ladle in LF refining before and after service were analyzed，and the erosion mechanism of MgO-C refractory brick was studied. The results showed that，the different temperature of the area where the MgO-C brick locates and the different structure of the MgO-C brick led to the different wear mechanism of refractory. In the hotter area closing to the molten steel，the MgO could react with C and formed a decarburization layer. The wettability between molten slag and MgO-C refractory brick was much better，and the solubility of MgO in the molten slag was much greater. Compared with the cooler area closing to the air，the erosion of MgO-C refractory brick was much serious in the hotter area. There were some differences of the impurity composition and structure between each MgO aggregate. The MgO aggregate with more impurity and small scale of MgO grain would be eroded more easily by molten slag for the good wettability between molten slag and MgO component. The molten slag would penetrate into the MgO aggregate through the grain boundaries，causing the disaggregation of MgO aggregate and the generation of Mg2SiO4 by further reaction. Besides，the redox reaction between molten slag and MgO-C refractory brick was also the main cause of the wear of refractory.

Analysis of temperature variation of molten steel in refining process ultra-low sulphur steel

2016, 32(5):  62-66. 

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In order to solve the problem of big fluctuation of molten steel temperature in ultra-low sulphur steel refining process，a temperature prediction model of molten steel from tapping to casting was developed using the MATLAB language based on an investigation of molten steel temperature variation of the total procedure converter → LF refining → RH refining→ wire feeding → casting. The molten steel temperature was predicted by the model，and the ratio of the average error within ±8℃ was 87.5%. The results showed that the molten steel temperature of LF-in could be about 1 580℃ if the following conditions were satisfied: maintenance time of ladle less than 1 hour，fast baking 10 minutes，temperature of tapping not lower than 1 650℃ and transport time less than 25 minutes. Molten steel heating rate was 2.0℃/min in LF early power supply and 3.0—3.5℃/min in LF late power supply. Moreover，RH vacuum chamber inner liner was more than 1 250—1 300℃ to reduce molten steel temperature drop. It could meet the demand of refining time of LF no longer than casting time under the premise of heavy desulfurization task-performing to make LF refining and casting match.

Optimization and improvement of the heat preservation effect of 8.4 t steel ingot mould riser

2016, 32(5):  67-71. 

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Numerical simulation methods were adopted in order to improve ultrasonic quality rate of rolled product of medium-carbon alloy steel by analyzing flow field during the filling and solidification process of solid phase fractions of 8.4 t steel ingot mould riser, the internal defects of shrinkage and porosity of ingot was also forecasted . The calculation results indicated that the filling process was stabilized, but shrinkage and porosity were generated in the ingot solidification process at the middle and inner ingot, which further extended upward and to mould riser for lacking enough heat insulation, the rate of porosity was 1.6% to 2.0%. The internal ultrasonic quality rate of ingot improved by taking following measures coupled with other technology：altering material quality of thermal insulation layer of the 8.4 t of steel ingot mould riser，specially speaking, the density of thermal insulation layer being decreased from1.07 g/cm3 to 0.67 g/cm3, the thickness being increased from 45mm to 60mm. Others technology were also adopted such as adding heat generation agent(1.0kg/t) and thermal insulation agent, strengthening heat insulation effect from axial and radial side of riser，guaranteeing enough liquid steel during solidification process for ingot. Finally, the ultrasonic quality rate for 250 mm×250 mm and 280 mm×280 mm rolled product increased by 1.19% and 6.73% respectively.

Technology research and application-practice about novel welded metallurgical slag pot

2016, 32(5):  72-78. 

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A new type of welded slag pot was innovatively developed and applied through the research of finite-element simulations，material performance evaluation，welding technology and manufacturing processes，because the traditional cast slag pot was liable to crack，hard to be welded and repaired，and short service life. Temperature field，distribution for stress and distortion of welded slag pot in different conditions were calculated by finite-element simulations and the results were basically in line with the field data，providing reliable guidance to structure design，improvement and application of welded metallurgical slag pot. Eight-years application of welding metallurgy slag pot demonstrated extensive application prospects for its better weld ability，great remediation potential，longer service life which was 2—3 times of the traditional cast slag pot，lower producing price which was 60%~65% percent of the traditional cast slag pot and green manufacturing concept.