Table of Content

05 August 2015, Volume 31 Issue 4

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Pretreatment of 200 t BOF steelmaking with less slag

2015, 31(4):  1-5. 

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Through thermodynamic calculation and material balance, the total amount of slag that needed for BOF dephosphorization was obtained. Meanwhile, combined with three key elements of BOF dephosphorization, the parameters and measures of BOF steelmaking with less slag were adjusted. The industrial test showed that the total amount of slag was reduced by 11.46 kg/t, oxygen consumption was decreased by 417 m3 per heat, the volume of gas recovery was increased by 57 m3 per heat, and the ferrous charge consumption was declined by 2.08 kg/t on condition that the phosphorus content of steel was kept stable.

Control of nitrogen content for GCr15 steelmaking process by EAF-converter

2015, 31(4):  6-10. 

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Based on the statistical production data，indicating a relatively high nitrogen content of bearing steel GCr15 produced by 90 t EAF-converter, the main effects on the nitrogen content of molten steel at such stages as tapping, LF refining and VD treatment of steelmaking were analyzed, and the optimized process parameters were proposed to control the nitrogen content. Taking the advantage of the measures for nitrogencontrolling, the average nitrogen content before LF refining decreased by 7.2×10-6, the nitrogen pickup during LF refining reduced by 32.2 %, while the nitrogen desorption rate of VD furnace increased from 20.2 % to 29.2 %, and the nitrogen content of molten steel in tundish could be decreased from 49.7×10-6 to 35.7×10-6.

Optimization of LF refining slag of spring steel and industrial tests

2015, 31(4):  11-15. 

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With total oxygen and nitrogen content analysis, X-ray fluorescence, scanning electronic microscope (SEM), and energy dispersive X-ray spectroscopy (EDS), the total oxygen mass fractions, components of LF refining slag, and shapes, sizes, and components of inclusions in billets of spring steel produced by “100 t EAF→LF→VD→CC” process in a domestic steel factory were analyzed. And based on this, thermodynamic calculation was also carried out by using FactSage software to optimize present refining slag. The results showed that the main composition of refining slag after optimization was below: w(CaO)=36 %－44 %，w(SiO2)=36 %－44 %，w(Al2O3)＜10 %，w(MgO)=9 %－11 % and basicity was around 0.8－1.2. Applying this new refining slag in industrial tests, inclusion plasticity was promoted and the percentage of inclusions which enter the lowmeltingpoint area increases from 12.5 % to 75 %. Besides, the average size of inclusions decreases to 1.48 μm and the inclusions whose sizes were over 2.5 μm had not been observed.

Technical study on deep decarburization process of 300 t RH in SGJT

2015, 31(4):  16-20. 

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In light of the problem of too long time in RH refining IF steel, the process status were investigated and analysised in Shougang Jingtang United Iron & Steel Co.,Ltd. It was concluded that low decarburization rate during early and last stage of decarburization was the controlling step. Therefore, some factors influencing decarburizationrate, such as vacuum pressure drop pattern, flow rate of blowing gas, scarp amount, initial carbon content in RH were optimazed. The results showed that prevacuum operation could eliminate the platform of pressure drop curve and enhance the early stage decarburization. Furthermore, gas flow rate was adjusted from 3 000 L/min to 3 666 L/min in the last stage of decarburization to increase the circulating steel flow rate as well as decarburization rate. It also showed that ending carbon content was sensitive to scarp addition. So the amount of scarp should be strictly controlled and added as late as possible. After taking these measures, the early and last stage of decarburization rate increase from 0.05 min-1 and 0.03 min-1 to 0.226 min-1 and 0.07 min-1, respectively. And the average ending carbon content and decarburization time decrease from 15.5×10-6 and 17.5 min to 10.2×10-6 and 14.4 min, respectively.

Effect of RH deoxidization process optimization on the cleanliness of W470 nonoriented silicon steel

2015, 31(4):  21-25. 

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The process of RH deoxidization of W470 non-oriented silicon steel was optimized in Shasteel. The results showed that the total oxygen content of liquid steel at RH end was reduced from (20－30)×10-6 to (15－20) ×10-6. Due to reducing the oxidation of silicon and manganese during the alloying process, the inclusions containing SiO2 and MnO reduced rapidly. The inclusion density of slab and the largest size of inclusion observed by SEM decreased by a large margin. As a result of the secondary oxidation of liquid steel caused by the ladle slag, there was still a large number of inclusions with small size. Besides, the yield of silicon and manganese increased evidently.

Causes and control of corner transversal crack on SS400B steel slab

2015, 31(4):  26-29. 

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Based on the serious corner transversal cracks on SS400B steel slabs in certain steel plant of Angang Steel, to prevent creating hot rolled steel coil defects, the cause of corner transversal crack was analyzed. By taking relevant countermeasures, such as improving the performance of mold power, reducing the strength of second cooling and adjusting the arrangement of caster rollers, the corner crack problem of SS400B steel slab had been effectively controlled. The crack ratio had been dropped from 87.6 % to 9.7 %, the slab without clearance angle processing could meet the requirements of hot rolling process and the number of hot rolled steel coil defects were less than 20, then the hot rolled steel could meet the users’need.

Analysis and control of galvanized sheet surface shell defect

2015, 31(4):  30-33. 

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Shell defects of ultralow carbon galvanized sheet surface were detected and analyzed using SEM.The results showed that the large particles flawed Al2O3 inclusions were discovered between the surface shell and the substrate distinct layers,and the size of Al2O3 inclusions were 5-100 μm.Looked up the slab casting process of the defect coil,the SEN nozzle clogging was found and the stopper rised.Combined with a comprehensive analysis of the clogging substance of SEN,the clogging substance was caused of Al2O3 inclusions.Through process optimizing,improving the ladle slag effect to increase the w(CaO)/w(Al2O3) to 2.3, and reducing the steel oxygen at the end of RH decarburization from 367×10-6 to 315×10-6,the surface shell defects rate of ultralow carbon galvanized sheet was reduced from 0.10 % to 0.01 %,and controlled effectively.

Optimal choice of the first turndown temperature of BOFdouble slag dephosphorization process

2015, 31(4):  34-37. 

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The first turndown temperature of BOF double slag melting process was discussed, from the point of the selective oxidation of phosphorus and carbon, the equilibrium of the dephosphorization reaction, the distribution ratio of phosphorus in BOF bath, and the melting point of the dephosphorization slag. The temperature region from 1 350 ℃ to 1 400 ℃ was proposed as the optimal choice in this article, and trial experiments were also taken into action, the results of the first turndown show that, the average dephosphorization ratio of 63.11 % and the average P2O5 mass fraction of the dephosphorization slag of 3.1 % were got, while the first turndown temperature were located in the region from 1 350 ℃ to 1 400 ℃. And further investigation about the influence made by the slag basicity and oxidbillity were also made under the circumstance of the first turndown temperature was controlled in the region from 1 350 ℃ to 1 400 ℃, which revealed that the basicity of 1.8to 2.0 and the oxidbillity of 14 % to 17 % make more contribution to the acquisition of the optimal dephosphorization results of the first turndown point.

Technology study on applying CO2 in BOF

2015, 31(4):  38-41. 

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Through the thermodynamic analysis of top blowing CO2+O2 mixture gas in BOF, combined with laboratory thermal simulation result, the feasibility applying CO2+O2 in top blowing converter for decarburizingrestraining oxygen and decarbonizationpreservation of chromium were discussed. It was concluded that carbon could be reduced by blowing pure CO2, but the loss of temperature was relative large. When the intensity of supplying CO2 was 3.0 m3/(t·min), the velocity of temperature dropping could reach 15.1 ℃/min; to achieve temperature balance, the mode of top blowing mixture gas CO2+O2 was taken. The heat from exothermal reaction O2-Fe could make up for the loss from endothermic reaction CO2-C, and the whole heating effest was zero when decarburized by O2 and CO2, the maximum theoretical volume fraction of CO2 in mixture gas CO2+O2 could be increased to 79.1 %. When the mixture gas was φ(CO2)︰φ(O2)=1︰1, the \[%C\]·\[%O\] could be controlled between 0.0025－0.0032 in top blowing converter, almost equalled to the effect in combined converter. When the mixture gas was φ(CO2)︰φ(O2)=2︰1, the effect of decarbonization and preservation of chromium by top blowing CO2+O2 occurred.

Optimizing and designing the technology parameters of the F-EMS for the bloom continuous casting

2015, 31(4):  42-46. 

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Based on plant trials and the 2D slice solidification and heat transfer model validated by pin shooting test, the influence of the FEMS implemented positions and technology parameters on the strand internal quality has been compared and analyzed. Under the casting speed ranged from 0.36 m/min to 0.45 m/min, the strand center carbon segregation degree of the solute element, C, and the degree of center porosity and shrinkage cavity for the casting longitudinal section are firstly decreased and then increased with the increasing of casting speed. The increasing running current is beneficial to the improvement of casting internal quality in the range of the rated operation current. The optimal casting speed, working current and frequency of the FEMS for the GCr15 steel casting with the section size of 380 mm×490 mm under the present caster equipment are 0.4 m/min, 700 A and 4 Hz respectively.

Numerical analysis of heat transfer in 45 steel bloom solidification

2015, 31(4):  47-53. 

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Taking 45 steel bloom as object for the study, a solidification heat transfer model has been built. The pair-wise mixture model was used to calculate the thermal parameters of 45 steel. Besides, actual temperatures and shell thicknesses of narrow face were measured by pin shooting method and infrared pyrometer to verify the correctness of heat transfer model and the results indicate the model could simulate the heat transfer process of 45 steel well. Simulation results have showed that appropriate position of soft reduction was 21.8－23.1 m from the meniscus at the condition that superheat was 35 ℃ and casting speed was 0.5m/min. And the amplitude of rising temperature on the surface of bloom after going out of the mould was huge, which easily leads to surface cracks. Steadying the casting speed was the key factor for improving quality of 45 steel. After the application of those results, centre porosity grades have decreased from 1.5to not more than 1.0; centre shrinkage grades were not more than 0.5;the largest carbon segregation was not more than 1.12；rates of surface cracks has dropped from 3.25 % to 0.5 %. As for the accelerated solidification at the end of solidification of bloom, that’s because of the rapid growth of equiaxed grains in the slab center with a great undercooling for liquid steel.

Mathematical simulation for tundish with the channel type induction heating

2015, 31(4):  54-58. 

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Tundish with the channel type induction heating is a powerful tool to control the superheat degree of molten steel in the mold precisely, so commercial software ANSYS and CFX were applied to analyze the electromagnetic field, velocity field and temperature field of molten steel in order to describe the process of electromagnetic metallurgy in the tundish. Numerical results indicated that, the induced current density in the channel was greater than that in the receiving chamber and the distributing chamber. In the channel, the nonuniform spatial distribution of magnetic field and electromagnetic force causes the molten steel to rotate, and there were two vortices at the cross section of the channel. But in the tundish with the channel type induction heating, the temperature of the molten steel was uniform, and the maximum temperature difference was only 1 K at the cross section of outlet. Without induction heating, the temperature drop between the inlet and the outlets was 7 K. With channel induction heater, the temperature drop could be made up.

Study on the static soft reduction fomed by the pulling straightening roller in continuous cast high carbon steel billet

2015, 31(4):  59-62. 

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During the 150 mm×150 mm billet continuous casting practice，static soft reduction at the solidification end of the high carbon steel billet had been implemented by adjusting the current pulling straightening roller pressure，drawing speed and the secondary cooling. The results showed that certain reduction formed by appropriate pressure matching with the appropriate casting speed could make the pulling straightening roller pressure be passed to the more suitable soft reduction zone. After the process optimization, the low power grading results indicate that the highest shrinkage defects rating decreased from grade 3.0 to grade 2.0, the highest center porosity rating decreased from grade 2.5 to grade 1.5, and the highest solidification center carbon segregation index decreased from 1.35 to 1.08. The whole shrinkage size and the center porosity areas had been improved to some extent under the static soft reduction formed by the pulling straightening roller.

Effect of soft reduction on inner quality of 82B steel rectangular bloom

2015, 31(4):  63-67. 

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To determine the soft reduction zone in 82B steel continuouscasting bloom with section size of 180 mm×240 mm, a mathematical heat transfer model was developed based on the finite element commercial software ProCAST. The adequacy of the model was compared with pinshooting results and surface temperature measurement. The results showed that the grade of center porosity equals to 1.0 was increased from 55 % to 77 %and the percentage of shrinkage cavity less than level 0.5 was increased from 45 % to 90 % with the soft reduction. Besides, soft reduction process was used successfully for suppressing Vsegregation. The mean center carbon segregation index was reduced from 1.17 to 1.07 and the maximum center carbon segregation index decreases from 1.27to 1.15.

Numerical simulation of grain distribution in 12Cr2Mo1R steel water-cooled ingot along Z-direction during solidification process

2015, 31(4):  68-71. 

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CAFE model was applied to investigate grain distribution in 12Cr2Mo1R steel water-cooled ingot along Z-direction, and cooling intensity and heterogeneous nucleation parameters were specially considered. The results showed that with the increase of cooling intensity, the width of columnar crystal was enlarged when the cooling intensity was between 50 and 800 W/(m2K). In contrast, the degree of the width increase of columnar crystal by raising cooling intensity was weakened, when cooling intensity was greater than 800 W/(m2K). However, the cooling intensity has little effect on grain distribution in central equiaxed zone. The growth of columnar crystal was restrained and the grain in central equiaxed zone was refined by adding nucleating agent.

Analysis of reasons for surface cracks of high strengthweathering steel YQ450NQR1

2015, 31(4):  72-78. 

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For crack problem on legs of high strength weathering steel YQ450NQR1 used for train center sill, the morphology of surface cracks was analyzed by the use of optical microscopy, scanning electron microscopy (SEM) and energy disperse spectroscopy (EDS). The conditions for decarburization of crack sides were investigated by the design of decarburization simulation experiments to determine formation process of cracks. The quality defects of YQ450NQR1 bloom, corresponding to cracks on legs, were identified by macroscopic examination and the causes for subcutaneous net cracks were analyzed based on the test results. The results showed that the crack defects on legs of high strength weathering steel YQ450NQR1 were caused by serious subcutaneous net cracks at the edge of bloom and uneven cooling of molten steel in mold, high level of residual elements as well as big fluctuations of nitrogen content contributed to the formation of subcutaneous net cracks. The occurrence ratio of cracks on legs of YQ450NQR1 steel, which fluctuated between 0.5 %～3.5 % originally, was stabilized below 1 % after some measures were taken.