Table of Content

05 February 2020, Volume 36 Issue 1

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Technological practice of desulfurization of molten iron in 60 t AOD furnace#br#

2020, 36(1):  1-3. 

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According to the process model of AOD furnace smelting stainless steel,the process of desulfurizing molten iron in AOD furnace was developed to meet the supply of desulfurizing molten iron after the failure of desulfurizing equipment.The practice shows that the sulfur mass fraction can be removed to below 0.002 0 % by desulfurization of molten iron in AOD furnace.In order to improve desulfurization efficiency,the end temperature of molten iron desulfurization was controlled at 1 350-1 400 ℃,and the shortage of temperature was made up by ferrosilicon.The slag quantity was 20-30 kg/t,the mass fraction of silicon after reduction was controlled at 0.2 %-0.4 %,and the side blowing agitation intensity was controlled at 0.4-0.6 m3/(t·min) in the reduction stage,and the stirring time was 5 min.


Research on the gas supply intensity and bottom slag layer thickness in 120 t top-bottom combined blowing converter

2020, 36(1):  4-7. 

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The bottom blowing gas supply intensity of 120 t top and bottom combined blowing converter was increased from 0.026 m3/(t·min) to 0.103 m3/(t·min), which provides dynamic conditions for molten steel. At the same time, the bottom slag layer thickness was reduced from 200-300 mm to 50-100 mm, reducing gas supply resistance, increasing the dynamic conditions of molten steel, strengthening the stirring of molten steel, promoting the carbon oxygen reaction and reducing the end point oxygen content of molten steel. The [%C][·[%O] in the end-point of converter was reduced from 0.003 1 to 0.002 3, and the FeO mass fraction in the end-point slag was reduced from 18.64 % to 18.12 %, which effectively improves the cleanliness and quality of molten steel.

Thermodynamic study on deoxidization and inclusion control of sulfur-containing gear steel

2020, 36(1):  8-14. 

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In order to study the deoxidation and inclusion control of gear steel, this paper combines the factory experiment and thermodynamic calculation to analyze the composite deoxidation products of sulfur-containing gear steel at different smelting moments. The results show that in the smelting process, with the deoxidation reaction and the movement of deoxygenation equilibrium, the dissolved oxygen content in the steel decreases continuously, and the inclusion composition is transformed from pure Al2O3 inclusions to Al2O3-MgO-CaO-Ti2O3-CaS composite inclusions. There is a big gap between the deoxidation thermodynamics of pure iron and the actual molten steel, and the actual production can not be guided by the deoxidation thermodynamics of pure molten iron.

Decarbonization practice of cover-type circulating machinery vacuum refining device

2020, 36(1):  15-20. 

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A new cover-type circulating mechanical vacuum refining equipment has been built in Jingye Iron and Steel Co.，Ltd. to produce ultra-low carbon steels，such as non-oriented silicon steel and industrial pure iron. In the control link，ultra-low carbon steels such as industrial pure iron of w(C)≤20×10-6 were smelted from aspects of carbon temperature coordination in converter tapping，development of decarbonization process of cover-type circulating mechanical vacuum refining device and control of carbon content of raw materials. The results showed that in the production process of ultra-low carbon steel，the carbon content of converter tapping should be controlled between 0.025% to 0.040%. Under the condition of vacuum oxygen blowing，the carbon content of converter tapping could be relaxed to less than 0.060%. Carbon and temperature should be coordinated at the end of converter tapping，and deoxidizing alloys containing aluminium or silicon should be strictly prohibited during tapping. Vacuum decarburization needed to control vacuum degree in decarburization period and gas flow driven by bottom blowing of steel ladle in stages. The carbon content of alloys and refractory materials used should be as low as possible.

Study on transformation of non-metallic inclusions in sulfurbearing gear steel during solidification and cooling process

2020, 36(1):  21-26. 

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Non-metallic inclusions in steel have a great influence on the properties of steel. In the continuous casting process, as the temperature decreases, the molten steel cools and solidifies, and the thermodynamic equilibrium between the steel and the inclusions moves, resulting in a change in the average composition of the inclusions. Therefore, it is meaningful to study the transformation of inclusions during solidification and cooling of molten steel. In this paper, 20CrMnTiH sulfur-containing gear steel was studied, and the evolution of inclusions during solidification and cooling of molten steel was studied by thermodynamics and factory experiments. The results show that the inclusions in the tundish steel and the continuous casting billet are mainly Al2O3-Ti2O3-MgO-CaO-CaS inclusions, but the CaO in the inclusions in the continuous casting billet is significantly lower than the CaS content. This is consistent with the thermodynamic calculation of the solidification cooling process and the conversion law of CaO inclusions. However, due to the limit of thermodynamic and kinetic conditions, the calculated average composition of inclusions and the actual detection values still have some differences.

Simulation study on influence factors of flow field in high-speed slab mold

2020, 36(1):  27-33. 

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The mutual verification of 1∶1 physical simulation and numerical simulation was used to analyze the effects of casting speed, submerged depth of SEN and width of mold section on the flow field of the mold.The results show that when the section is 230 mm×1 490 mm and the submerged depth of nozzle is 150 mm, the casting speed increases from 1.3 m/min to 1.7 m/min, the maximum surface velocity increases from 0.21 m/s to 0.32 m/s, and the average increase of liquid level fluctuation 21.1%. When the section is 230 mm×1 490 mm and the casting speed is 1.7 m/min, the submerged depth of nozzle is reduced from 190 mm to 150 mm, the maximum surface velocity is increased from 0.26 m/s to 0.32 m/s，and the average increase of liquid level fluctuation is 30.6%. The section width of the mold is increased from 940 mm to 1 490 mm, and the liquid surface flow velocity and liquid level fluctuation are increased due to the increase in the amount of steel passing through. When the factors change, the increase of the angular velocity is much smaller than that of other parts, so the corner is a dead zone in the flow field.According to the water model data analysis, the most important influence on the liquid surface velocity is the change of the casting speed. The maximum influence of the liquid surface fluctuation is the section width of the mold, followed by the insertion depth of SEN, and the smallest is the pulling speed.

Optimization of mold flow field and SEN port shape in continuous casting of automobile exposed panel

2020, 36(1):  34-42. 

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In this paper, the velocimetric rod deflection method was used to measure the velocities near the mold surface, and the calculated results from mathematical model were compared and verified by the measured results. The flow fields in the mold with square nozzle, rectangular nozzle and runway-shape nozzle at different casting speeds and nozzle submerged depths were analyzed by the mathematical model. The results show that with three different submerged entry nozzles(SEN), the velocities near the mold surface increase as the casting speeds are increased, and decrease as the SEN submerged depths are increased. Under the same continuous casting process parameters, the velocities of the mold surface with the square nozzle is greater than those with the rectangular nozzle and the runway-shape nozzle.The velocities near the mold surface with the rectangle nozzle is comparable to those with the runway-shape nozzle. With three different submerged entry nozzles, the surface turbulent kinetic energy increase as the casting speeds are increased, and decrease as the SEN submerged depths are increased. Under the same continuous casting process parameters, the surface turbulent kinetic energy with the runway-shaped nozzle is minimal.

Physical simulation on optimization of submerged entry nozzle for non-equilibrium beam blank casting mold

2020, 36(1):  43-49. 

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A physical model of liquid steel flowing in beam blank casting mold was set at 0.7∶1 geometric similarity ratio, and the influence of response time at different position and liquid surface fluctuation was studied. It was showed that the flow field distribution in the existing through nozzle mould was extremely asymmetric and the response time of each position of the mould was quite different. The percentage of liquid level fluctuation kept within ±3 mm was 96.3 %. The optimal improvement scheme of nozzle (three side hole nozzle) effectively solves the problem of long and inconsistent response time at different positions of the mould, and the percentage of liquid level fluctuation kept within ±3 mm is 99.7 % and within ± 5 mm is 100 %.

Study on the properties of mold fluxes for the high speed casting of medium carbon steel

2020, 36(1):  50-56. 

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High speed casting is attracted much attention of people for its efficient production.In which,compact strip production (CSP) is the typical casting technology, which casting speed of 3～6 m/min,even up to 9 m/min.The requirements of the mold fluxes become quite strict because that many parameters in the mold have been changed with the improvement of the casting speed.In this paper,the mold flux of medium carbon steel for CSP was studied and its properties were compared with the mold flux for traditional low speed casting. The results show that the mold flux for high speed casting of medium carbon steel needed better lubrication,and the viscosity at 1 300 ℃ should be lower than 0.086 Pa·s (casting speed: 3.5 m/min).The horizontal heat flux from solidified shell to the mold wall has increased greatly due to the high speed casting. In order to avoid the bad quality of the casting billet caused the fast heat transfer due to the high speed casting, the mold flux which had more uniform and stronger ability to control the heat should be applied.

Analysis and optimization of factors affecting internal quality of casting billets for high-strength steel bars

2020, 36(1):  57-61. 

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In order to reduce the internal quality problems of high-strength steel bars billets produced by HBIS Cheng Steel，the single factor method was used to analyze the effects of molten steel superheat，casting speed, secondary cooling intensity and whether the mould powder is baked on the central segregation, center porosity, corner crack and intermediate crack of billets by macrostructure analysis and statistics of casting billets. The results show that the superheat of molten steel is controlled within the range of 16-25 ℃，the casting speed is 1.70 m/min，and the secondary cooling intensity is 1.0 L/kg and under the condition of baking mould powder, the internal quality of the billets can be effectively improved.

Research and application of continuous casting slab fixed-weight system based on multimodel control

2020, 36(1):  62-66. 

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In order to improve the accuracy of slab weight determination, a multi-model system was established to cover the influencing factors of continuous casting process, reduce the influence of molten steel composition, mould wear, superheat and casting speed on slab quality. Feedback mode combining feed-forward and feed-back was adopted to improve the accuracy and stability of slab weight determination system. The field implementation shows that the precision of slab fixed-weight has been greatly improved, and the proportion of continuous casting slab whose quality deviation is less than 8 kg has increased from 42.4 % to 81.2 %. It is advantageous to improve the yield of rolling.

Practice on control of oxygen and inclusions for 50Cr5MoV backup rolls

2020, 36(1):  67-72. 

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Combined with production practice, the carbon content reasonably was controlled in scrapped charge process and during arc furnace to decrease peroxidation of molten steel, final slag composition was aimed during LF process to remove oxygen and sulphur deeply, vacuum degassing time and efficient vacuum degree was carried out during VD process, gas flow was controlled during back to LF process to reduce slag entrapment, the refining ladle was adopted during the whole steelmaking process and blowing argon around the ladle nozzle was carried out during VC process. Results shows final oxygen mass fraction of backup roll were all below 20×10-6 and forging inclusions in microscope was qualified, ultrasonic examination was qualified and inclusions exposure were effectively reduced during subsequent machining process.

Practice of precise and specialized production for ultra-low carbon IF steel

2020, 36(1):  73-78. 

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Based on the control of time rhythm，the goals of low cost, high efficient, high quality and energy saving can be achieved in steelmaking system. Aiming at the indefinite furnacecaster corresponding relation in BOF steelmaking plant of Bensteel, seven specialized production lines were established. Such as producing ultralow carbon IF steel, the matching analysis of operation time among BOF, RH and CCM was carried out by applying GANTT diagram. The bottleneck of the previous flow running was the BOF and RH. By increasing the oxygen supply intensity from 2.86 m3/(t·min) to 3.19 m3/(t·min), enlarging the dimension of tapping hole, taking a pre-vacuum operation, rapidly reaching the limit vacuum and increasing the lifting gas amount at later stage of decarburization, BOF and RH process cycles were shortened. Consequently, production rhythm was precisely lowered to (32±2) min from averagely 45 min by employing the technology of constant casting period which was first put forward. The precise and specialized steelmaking was realized.