Table of Content

05 June 2016, Volume 32 Issue 3

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Research progress on efficiency techniques for desulfurization flux in hot metal with KR process

2016, 32(3):  1. 

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KR process (also known as mechanical stirring method) has obvious advantages in the realization of deep desulphurization and stability of hot metal by compared with the injection process. However, due to the aggregation phenomenon of desulfurization flux during the stirring, there is a problem of low utilization ratio of lime desulfurization. The recent techniques for increasing the desulfurization efficiency the flux in hot metal by mechanical stirring were reviewed. Free CaO in desulfurization slag reacts with moisture or CO₂ in the air during cooling, causing CaO to expand and break into small particles. It is possible to reuse the slag by utilizing this phenomenon. But avoiding lime particle aggregation through flux addition method and the stirring dynamic improvement has more valuable than the slag recycling. In particular, according to the research results of chemical industry, the formation of the central vortex at the top of the stirring head is inhibited when two baffles are arranged symmetrically on the inner wall of a mechanically-stirred vessel, which results in the less aggregation of the particles, the better mixing and mass transfer between desulfurization flux and hot metal. It is expected that increasing the utilization ratio of the desulfurization flux achieves a breakthrough.

Hot Metal Pretreatment

Study on gush skimming technology of hot metal ladle

2016, 32(3):  8-11. 

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In order to solve hot metal loss of skimming after desulphurization, the properties of desulphurization slag and the causes of hot metal loss during slag skimming were analyzed. The feasibility of hot metal gush skimming technology was researched, through water model experiment, thermal state experiment in 500kg top-bottom combined blowing furnace, experiments of cross gas injection gun design and industrial production test in 100t hot metal ladle. It was concluded that the range of hot metal ladle tilting angle range could satisfy two ejection guns vertically inserting into the hot metal; the surface of hot metal was half naked and the other half was covered by "semicircle" desulphurization slag which accumulated at the skimming mouth for skimming easily, with the action of gush power of hot metal bottom-up. Compared with conventional skimming of 100t hot metal ladle, gush skimming time was shortened from 11min to 7min, and hot metal loss was reduced from 5.89t to 2.97t per ladle with no additional slag-conglomerating agent.

Secondary Refining

Study of mixing criterion and alloy adding position during argon bottom blowing refining process

2016, 32(3):  12-17. 

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In the hydraulic model experiments of bottom argon blowing refining process, the traditional mixing criterion was determining the mixing degree by monitoring the tracer concentration with the help of one or two conductivity electrodes which were set in the ladle model. Due to the difference between the local mixing time at different positions and the randomness in the choice of monitoring positions, the measured mixing time may not be realistic. In regard to this, the fluid flow and mixing process were simulated by numerical calculation. The result showed that with the increase of mixing degree, the standard deviation of tracer concentration decreased. When the standard deviation was lower than 0.02, the mixing degree was 100%. Based on this, it was raised that 9 conductivity electrodes should be set at three sections which were distributed alone the height. When the standard deviation of tracer concentration was lower than 0.02, the concentration could be believed to be uniform. The effect of adding position on mixing time was also studied in this paper. The result showed that the mixing time was about 77.5s when the tracer was added from the position which was located at a distance about 1/6 radius from the center. When the tracer was added from the edge which was close to the plume, the mixing time was about 137.3s. The former position was the ideal alloy adding position.

Influence of calcium treatment and protective castingon steel cleanliness during continuous casting process

2016, 32(3):  18-23. 

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Steel and slag samples during refining and casting of three heats of 510L beam steel produced by BOF→LF→CC process was taken and analyzed. The variation of total oxygen (T.O), nitrogen and dissolved content of Al, Mg and Ca content in steel were investigated. The evolution of number density, composition and size of inclusions in steel at different steps was discussed. Reoxidation during continuous casting could be controlled economically and effectively by covering the top of tundish with cotton and stoping blowing argon from stopper rod, by which the nitrogen absorbtion from air reduced from 22×10^-6 to 2×10^-6. Calcium treatment was used to modify high-melting point Al₂O₃-MgO inclusions to Al₂O₃-MgO-CaO complex inclusions with low-melting temperature. However, proper operations during calcium treatment and during subsequent continuous casting were needed, otherwise, severe reoxidation occurred during continuous casting destroyed the effect of successful calcium treatment during steel refining during the industrial trials of the current study. After calcium treatment, 3—7 μm inclusions in slab increased while above 7 μm inclusions decreased.

Solidification & continuous Casting

Effects of w(CaO)/w(Al2O3) on melting and flow behaviors of mould fluxes for high aluminum TRIP steel

2016, 32(3):  24-28. 

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For insuring the consumption of mold fluxes and lubricating the slab with CaO-Al2O3 mold fluxes during casting high aluminium TRIP steel，the effects of different w(CaO)/w(Al2O3) on melting and flow behaviors were studied. The results showed that the melting temperature increased and then decreased with the increase of w(CaO)/w(Al2O3), being a maximum at w(CaO)/w(Al2O3)=1.2. The trend of melting temperature change (with w(CaO)/w(Al2O3)) was contrary to the trend of liquidus temperature change of CaO-Al2O3 phase diagram. With the increase of w(CaO)/w(Al2O3), the viscosity firstly decreased and then increased. When w(CaO)/w(Al2O3)=0.5~2.3, the change of viscosity became mild, where the value of viscosity was 0.08~0.142 Pa?s. The break temperature varied with w(CaO)/w(Al2O3), being a minimum at w(CaO)/w(Al2O3)=2.0. Taking into account the melting temperature, viscosity, break temperature and crystallization behaviors of the mould fluxes, the optimum w(CaO)/w(Al2O3) was 1.5~2.0.

Position determination of the final-electromagnetic stirring of continuous billets

2016, 32(3):  29-35. 

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To improve the quality of continuous billets and reduce the central segregation of 72A tire cord steel, mathematical model of solidification and heat transfer was developed, then the model was revised based on the results of pin shooting tests and temperature measurements. By using the above revised model and numerical simulating, the appropriate position of final-electromagnetic stirring (F-EMS) was determined. The results showed that, the appropriate position of final-electromagnetic stirring (F-EMS) moved from 7.26—7.64 m back to 8.95—9.41m when the casting speed increased from 1.60 m/min to 1.90 m/min, it also moved from 7.95—8.35m back to 8.17—8.61m when the casting temperature increased from 1 485℃ to 1 498℃. Considering the influence of the temperature fluctuations, the optimal position of F-EMS was at distance from liquid meniscus 8.17—8.35m when the casting speed was 1.75m/min. The industrial tests showed that，when the casting speed was 1.75m/min, the center of F-EMS was at distance from liquid meniscus 8.25m, the center porosity grade and pipe grade both declined from 1.0 to less than 0.5, the center carbon segregation index also decreased from 1.15 to less than 1.05, the quality of continuous billets had improved.

Research on influence factors of sink vortex during ladle teeming

2016, 32(3):  35-40. 

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For the problem of large residual steel during ladle teeming in a steel factory, the physical simulation was carried out to investigate the effects of ladle long nozzle diameter, nozzle eccentricity and slag thickness on the critical starting height of vortex and penetrating height (the liquid height of air column penetrating through the nozzle) during liquid draining. The results showed that the critical height of votex increased with increasing nozzle diameter. The starting votex height and penetrating height rised from 89.6，25.6mm to 165.0 and 40.7mm respectively for the simulated ladle when the nozzle diameter varied from 30mm to 60mm. The penetrating height of drain sink votex varied slightly with different nozzle positions when nozzle eccentricity was within 0.7, however, its value dramatically rised when nozzle eccentricity increased from 0.7 to 0.8. The optimum nozzle position was at 0.7R from the center of the ladle bottom. The critical slag entrapment height rised with the increasing layer thickness non-linearly. The slag entrapment height increased by 10mm when model slag thickness rised from 40mm to 50mm under the condition of 50mm of nozzle diameter.

Experimental research on improving internal quality of 55CrSiA spring steel billet

2016, 32(3):  41-45. 

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In view of the lower center equiaxed crystal ratio, more serious central carbon segregation, central porosity and shrinkage defected that affected internal quality of 55SiCrA spring steel billet, influence of M-EMS process parameters on magnetic field distribution characteristics was systematically studied in 150mm×150mm billet mould. Moreover, research of the effect of stirring electric current and frequency of mould electromagnetic stirrer（M-MES） on macro solidified structure and central carbon segregation was also carried out by industrial experiments. The experimental results proved that with stirring electric current of M-EMS increasing, central carbon segregation index decreased and center equiaxed crystal ratio rised. There was a non-linear relationship between stirring frequency of mould electromagnetic stirrer（M-MES）and center equiaxed crystal ratio besides central carbon segregation. Center equiaxed crystal ratio increased from 18.5% to 32.5%, central carbon segregation index decreased from 1.23 to 1.04 and the central porosity and corner cracks were eliminated when stirring electric current was 320A and frequency was 3.5Hz. Finally billet internal quality was evidently improved.improved.

Product Process & Quality Control

Experiment on production process optimization of Q345C steel by adding TiN nanoparticle in ladles

2016, 32(3):  46-49. 

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In order to optimizate Q345C steel production process, the experiment has been carried out on Q345C steel by adding TiN nanoparticle in Q345B ladles without changing the process of steelmaking and rolling. The effect of diffrent TiN nanoparticle additions on mechanical properties and microstructure of Q345 steel was researched. The results showed that the mechanical properties of Q345B test steel could meet the requirements of Q345C steel with 0.05% (mass fraction) TiN nanoparticle addition, and the efficiency of Q345C steel rolling production was increased by 20% with the test process. The grain size of the test steel was refined slightly and reached to between Class 8 and Class 10. The TiN nanoparticlethe added in the Q345 steel existed in the form of pure substance in the steel and play the role of heterogeneity nucleation and nail in steel.

Comparison between the results of oxide metallurgy with Ca deoxidation and Mg deoxidation

2016, 32(3):  50-54. 

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To improve the low-temperature toughness of heat-affected zone（HAZ） of heavy plates, the inclusions of deoxidation with aluminum calcium magnesium were studied on the basis of thermodynamic calculation by Inclusions Automatic Analyzer System. The results showed that the inclusions with aluminum deoxidation were large size and low density and the main types of inclusions were Al2O3 and calcium aluminate. The main types of inclusions with calcium deoxidation were calcium aluminate parcelled MnS which size was below 3μm. The main types of inclusions with magnesium deoxidation was MgO and MgO which surfaced precipitation of TiN or MnS, and the sizes were below 1μm. After simulating welding test with heat input of 400 kJ/cm, the impact energy value at -20 ℃ on HAZ of specimen with magnesium deoxidation and calcium deoxidation reached more than 180 J.

Influence of slag denaturalization on inclusions in IF steel

2016, 32(3):  55-59. 

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In order to reduce the flocculating flow or clogging of continuous casting and the content of inclusions in steel, a new technology of ladle top slag denaturalization was introduced in the production of IF steel with the process of “BOF→Argon station→RH→CC” at one company. To examine the effect of the use of the ladle slag modification technology, trial research on slag denaturalization was also carried. Molten steel samples were taken from RH refining end-point and tundish by using the barrel sampler. The corresponding slab samples were also taken. Inclusions were tested by using ASPEX Explore automatic Scanning Electron Microscopy(SEM). Contrast analysis of the quantity, type and size of the inclusions of RH→tundish→slab were carried on under the condition of denaturalization and non-denaturalization process. The results show that slag denaturalization effectively reduce slag oxidizability, improving steel cleanliness; the main types of inclusions under both process conditions are Al2O3，TiOx-Al2O3 and Oxides. The proportion of Oxides inclusions with denaturalization process is significantly higher than non-modification process; clusters of inclusion types in slab with the process of modification are single and the quantity and size are smaller than non-modification process.

Investigation and control of sliver defects of 200-series stainless steel

2016, 32(3):  60-66. 

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In the current study, steel samples were taken at each step of steelmaking, LF refining and casting for the production of 200-series stainless steels. The content of total oxygen (T.O), the morphology, amount, composition, and size distribution of inclusions in steel samples were analyzed, which was in favor of the solution of the product defects and improvement of steel cleanliness. The total oxygen in steel samples during LF refining gradually deceased and it was 41×10-6 at the end of ladle refining, 16×10-6 lower than that at LF start. There was an increase in the T.O during transfer operations from the end of ladle refining to the start of casting due to the air entrainment, which was avoided through shortening the transport and transformation of the ladle. Inclusions in steel samples were mainly MnO-SiO2 and its average composition deviated from the low melting temperature region. Inclusion plasticization could be achieved by adjusting the basicity and strictly controlling the sources of Al in raw materials. From the end of LF refining to the slab, the total number density of inclusions increased to a maximum value of 473.6/mm2 in the steel sample at 30 min of casting in tundish inlet and then gradually decreased and it was 74.7/mm2 in the slab. For oxide inclusions, the variation of its maximum size was the same as the total number density of inclusions. The maximum value was 73.5 μm and it was 22 μm in the slab. Through three measures of lowering the slag basicity from 2.2 to 1.5~1.8 during LF refining, increasing the tundish capacity to 45 t and strictly controlling the air absorption during continuous casting, the sliver defects was decreased from 20% to 3.0%.

Control of transverse corner cracks on hypoperitectic steel slabs

2016, 32(3):  67-72. 

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In this article, various means were used to study the influence factors and control of transverse corner cracks on hypoperitectic steel slabs for improving the slab surface quaility. The microstructures implicated that proeutectoid ferrites along austenite grain boundaries could increase two times of the occurrence fraction of cracks. The carbon content makes an essential influence on the hot ductility of steel. Reduce carbon content to 0.07% could sharp reduce the cracks occurrence fraction from 44% to about 4% in the carbon cotent range less than 0.1%. A secondary cooling scheme of a weak cooling intensity before the bending segment and followed by an strong cooling intensity at the straightening segments was suggested and was proved to be effective in eliminating transverse corner cracks, both the cracks number and length were markly reduced.

Others

Research on the reactivity of lime by calcining limestone with different properties at high temperature

2016, 32(3):  73-78. 

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The relationship between physical and chemical properties of limestone and the activity of lime was studied by rapid calcining the limestone at the temperature, which was much higher than the typical calcination temperature of active lime. The lime activity was tested by the titration of hydrochloric acid, and the phase compositions of limestone and lime were tested by XRD. The grain size of CaCO3 and the microstructure variation of CaO grains were analyzed by SEM. The results showed that the smaller CaCO3 grain size of the limestone resulted in better sintered situation of CaO and higher lime activity after calcining the limestone rapidly at high temperature. When prolonging the calcination time to 15min at 1450℃, the CaO grains in the lime that obtained by calcining the limestone with different CaCO3 grain size had fused and grown. The fusion and growth of CaO grains led to the significant decrease of the lime activity. The lime activity was greatly influenced by the content of SiO2 in the limestone. When the content of SiO2 was higher, the lime activity after calcining decreased remarkably. Therefore, the smaller CaCO3 grain size and the lower content of SiO2 of the limestone lead to the higher activity of lime after calcining the limestone rapidly at high temperature.