Table of Content

05 October 2019, Volume 35 Issue 5

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Experimental research on predephosphorization of semi-steel with LF refining final slag

2019, 35(5):  1-4. 

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This paper mainly related to the feasibility and experimental research on predephosphorization of semi-steel with LF refining final slag. The experimental results showed that LF refining final slag could be used for predephosphorization of semi-steel，and the dephosphorization effect was good after adding appropriate amount of dolomite. The minimum，maximum，and average dephosphorization rates were 61.31%，71.76%，and 65.03%，respectively，which were higher than that of the contrast test with 55.36% dephosphorization rate by 10.75%，29.62% and 17.47%，respectively. The optimum process parameters in this work were 1 700 mm top lance height，18 500 m3/h oxygen flow rate，360 s oxygen blowing time，2.0 t LF refining final slag addition，1.0 t dolomite addition，and 300—400m3/h bottom blowing gas flow rate.

Improvement of refractory for liquid steel ladle in BOF steel-making plant

2019, 35(5):  5-9. 

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On the basis of site investigation，some problems in the production of BOF steelmaking were found. 1）The ladle cranes were running at a full capacity in the middle & later stages of converter lining service，there was a potential hidden danger. 2）When the weight of liquid steel in ladle reached 120t，the space height above liquid steel in the ladle was between 200mm and 300mm，that was not satisfied with the requirement of ladle furnace (400—600mm).3）The life of the ladle lining was low，the consumption of lining was high，the labor intensity of workers was high.4）In the later stages of ladle lining service，the surface temperature of ladle approached creep temperature，there was also a potential hidden danger. Therefore，the following works were carried out，and some good results were got.1）By adjusting the refining slag composition and the alloy kinds and proportion， enhancing the quality of lining and masonry，the eroding rate of ladle lining was decreased from 2.5mm to 1.1mm at a heat，the life of ladle lining was raised from 67heats to 157heats.2）In order to ensure the height space above liquid steel in the ladle be satisfied with the refining requirement when the weight of liquid steel in ladle reached 120t，the thickness of working layers was cut down 30mm，the life of ladle lining was adjusted from 157 to 127 heats.3）Because of reducing thickness of work layers and adjusting the material of permanent layers，the weight of ladle lining was reduced 3.8t，the weight of ladle with lining & liquid steel was decreased from 180t to 176.2t，ladle cranes were safer.4）Because of permanent layer with the good thermal insulation property，the surface temperature of steel ladle was decreased form 280—292℃ to 252—262℃，the steel ladles were safer.5）After reform of refractory for liquid steel ladle，the consumption of the lining，alloys，refining slag，electrical power and electrode was significantly reduced，the producing cost was reduced 12.16yuan/t .

Study on control technology of carbon-oxygen equilibrium atblowing end-point of300t BOFsteelmaking

2019, 35(5):  10-16. 

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In order to further improve the stability of end-point control of converter，the main factors affected the carbon-oxygen equilibrium of converter were analyzed from the point of view of improving kinetics，and the control technology of reducing the carbon-oxygen equilibrium of converter was determined by industrial test. The results showed that，the mixing time of molten steel was the shortest when the number of tuyereswas 4 and the layout of tuyereswas 0.50D on the circumference of the converter bottom；the proportion of particles size of magnesia 0—1mm in the fetting materialwas 20%，the high temperature flexural strength was increased from 4.2MPa to 5.3MPa，and the permeability was increased by 227%. The oxygen mass fraction at the end of the converter further decreasedfrom 482×10-6 to 425×10-6，reduced by 57×10-6by employingthe quick change technology of the tuyere for converter. Through the above measures，the visualization of the tuyerewas realizes in the whole campaign life，and the carbon-oxygen equilibrium wasreduced from 0.00238 to 0.00178 in the whole campaign life.

Research on optimization of steelmaking process for large converter in Hanbao steel

2019, 35(5):  16-22. 

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In order to solve the problems of low dephosphorization rate in the early stage of steelmaking，high oxygen content at the end-point of blowing，low tapping temperature and short life of oxygen lance in 260 t converter in Hanbao Steelwork, Handan Iron and Steel Company，a series of investigations were carried out during the steelmaking process through sampling，temperature measurement，recording and analyzing key operating parameters. According to the investigation results，the parameters of oxygen lance nozzle and the curve of lance position during blowing were optimized，and the maintenance of bottom tuyeres was strengthened. The pressure loss in the oxygen supply pipeline and the characteristics of the jet flow field of the nozzle were measured，and the water model test of the effect of the jet on the converter bath was carried out.Through the above process optimization measures，the dephosphorization result of the converter has been improved，the end-point carbon and oxygen content product in the molten steel has been reduced to 0.002 39，the average tapping temperature has reached 1645℃，and the average lance life has been doubled to 320 heats. The consumption of other materials such as ferrous charge and lime has also been improved significantly，the amount of scrap has been increased and the economic benefit achieved has reached 40.02 yuan/t. The converter steelmaking process reached the technical level required to produce low-carbon high-quality deep drawing steel.

Effects of silicon content on the total oxygen and inclusions of Ti-bearing stainless steel

2019, 35(5):  23-29. 

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The total oxygen content and inclusions in Ti-bearing austenitic stainless steel with different silicon content wereinvestigated through the coupling of thermodynamics calculation and experimental results. The results showed that the total oxygen content，the inclusion number and size decreased throughout the process. The types of inclusions at the end of AOD stagewere mainly CaO-SiO2-Al2O3-MgO，and the types of inclusions before Ti addition in LF refining processwere mainly CaO-Al2O3-MgO-SiO2.The inclusions after Ti addition in LF refining processwere mainly CaO-Al2O3-MgO-TiOx.The results calculated by FactSage software were in good agreement with those obtained by experiment.The heats with higher silicon content had lower total oxygen contentand lower titanium content in CaO-Al2O3-MgO-TiOx inclusions compared with the heats with lower silicon content when the other conditions were almost same.

Effect of deoxidation method on the total oxygen content and inclusion evolution during LF refining process

2019, 35(5):  30-36. 

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In recent years，various industries have higher requirements on the service performance of steel materials，so continuously improving the cleanliness of steel and reasonably controlling inclusions in steel have become important tasks for various steel enterprises，and the excessive oxide inclusions in steel are related to the deoxidization process. In this paper，through industrial test，X-ray fluorescence analyzer and scanning electron microscope (SEM)，the effect of deoxidation methods on the total oxygen content，inclusion evolution and slag oxidizability of No.70 steel has been systematically studied. The experimental results showed that different deoxidization methods had little effect on the types and sizes of inclusions in No.70 steel. The deoxidation method of adding 1.2kg/t Si-Ca compound deoxidizer in two stages and three batches per stage was to the benefit of reducing slag oxidizability and further removal of oxygen content in steel，so as to control the slab of No.70 steel with lower total oxygen content and amount of oxide inclusions. Using this deoxidation method，the total oxygen content could be controlled between 0.001 3% and 0.001 5%，and number of oxide inclusions could be controlled between 10.2--11.3/mm2. The results of this study can provide theoretical basis and technical guidance for the high-purity control of high carbon steel used for high quality rope grade fine steel wire.

Quality analysis of silicon-calcium wire and optimization of calcium treatment process for pipeline steel

2019, 35(5):  37-42. 

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In view of the low qualified flaw detection rate caused by X80M inclusions，the steelmaking process was investigated and found that the effective calcium content in the silicon-calcium wires used for calcium treatment was low. By changing the types of silicon-calcium wires and optimizing the process of calcium treatment，such as controlling w(S) below 25×10-6，increasing the feeding speed of silicon-calcium wires from 1.8 m/s to 2.5 m/s，reducing the bottom blowing flow rate from 180~200 L/min to 120~150 L/min，w(Ca) in molten steel was controlled at (13~25)×10-6，Al2O3 inclusions couuld be modified into calcium aluminate inclusions in the low melting point zone of CaO-Al2O3-CaS ternary Phase Diagram，and their sizes were all below 50μm. The qualification rate of flaw detection for X80M steel increased from 94.60% to 99.69% on average，which ensures the quality of products.

Water model test and application for optimizing flow control device of single-flow tundish

2019, 35(5):  43-49. 

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Based on the similarity principle，a 1:2.5 water model was established for orthogonal test，and the range analysis and variance analysis of the test results were carried out. The results showed that the main factors affecting the optimization of flow field characteristic parameters were the height of the lower retaining wall and the distance between the upper retaining wall and the bottom of the upper retaining wall. The optimal combination of flow control devices was proposed: the distance between upper and lower retaining walls was 867 mm，the distance between upper and lower retaining walls was 225 mm，the distance between upper and lower retaining walls was 200 mm，and the height of lower retaining walls was 350 mm.The average total oxygen content and the occurrence rate of large inclusions were reduced by 16% and 57% respectively by using the optimal flow control device.

Improving internal quality of C610L continuous casting slab with soft reduction

2019, 35(5):  50-53. 

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In order to solve the central quality problems in the continuous casting production process of C610L slab，a two-dimensional solidification and transient heat transfermathematical model of 200mm×1500mm continuous casting slabfor simulated solidification heat transfer process was established.The solidification temperature field was reasonable displayed by calculated results andthe end point of solidification was 17.54m from the meniscus of the mould. Accuracy of the modelwas verified through the nail-shooting experimentsand the calculation results could guide the formulation of soft reduction.4millimeter reductions was applied in the final stage of solidification where central solid fraction ranged from 0.3 to 0.7 corresponding sector 6~7. According to macroscopic examination results，remarkable resultswasachieved from reduction programin improving center quality，and pushed the online defect rate from 3.34% down to 0.89%.

Causes analysis and control measures of steel-heaping in HRB400E bar steel

2019, 35(5):  54-56. 

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Macro-morphology of steel heap sample during HRB400E bar rolling was observed，and morphology and structure of cross section of stacked steel sample were examined by optical microscope. It is considered that，the reason for causing steel heap accident is that the middle crack of billet is exposed at the end，the crack is oxidized on both sides during heating furnace，the head is split during rolling，and the rolling mill cannot bite in normally. Based on the analysis results above，the secondary cooling intensity of continuous casting was improved，the internal quality of slab was improved，and the steel heap accident caused by the internal quality of slab was eliminated.

Development of stacking and slow-cooling technology for thick high alloy slab

2019, 35(5):  57-62. 

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In view of the fact that the crack rate of high alloy steel in a factory is relatively high but there is no slow-cooling pit or other slow-cooling measures，the temperature finite element model of 738H plastic mould steel slab produced by slab continuous caster in this factory with the size of 400 mm×2 000 mm×3 000 mm was calculated by using the finite element software ANSYS. A 96h stacking and slow-cooling temperature measurement experiment was carried out on the plastic mould steel slab of the plant，and the results of numerical simulation were checked by the field-measured temperature. Based on the checked finite element model of stacking cooling temperature field，a stacking slow-cooling process suitable for the factory for continuous casting slab was developed，which could solve the problem well of no delay facilities for continuous casting slab，cracks and slow cracks of high alloy steel slab had been prevented effectively，and the qualified rate of crack control of high alloy steel continuous casting slab could be reached 100%.

Effect of inclusions on edge cracks of 20Mn23AlV medium plates

2019, 35(5):  63-67. 

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The 20Mn23AlV billet（/%：0.04~0.12C，≤0.50Si，21.50~25.00Mn≤0.030P，≤0.030S，1.50~2.50Al，0.04~0.10V）produced by continuous casting process is prone to edge cracks during hot rolling, which seriously affects the normal production. Through the analysis of chemical composition and scanning electron microscopy, it is confirmed that the edge cracking of plate is caused by AlN inclusion and mold slag entrapment. In the third brittle temperature range, AlN precipitates at the austenite grain boundary, which decreases the binding energy of austenite grain boundary, and then reduces the plasticity of steel; because of the high manganese content in steel, the columnar crystal structure is serious during solidification, and transgranular structure and cracks are easy to form; Slag entrapment occurs in the continuous casting process, and the inclusions involved in the billet have different ductility with the matrix, and cracks occur with the rolling process. Therefore, the process parameters of steelmaking and continuous casting are optimized, which reduces the number of AlN inclusions in steel and the frequency of flux entrainment in molten steel, and the edge cracking rate of plate is reduced from 28% to less than 3%.

Distribution of inclusions in non-quenched and tempered steel billets

2019, 35(5):  68-74. 

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Samples were taken from different locations of non-quenched and tempered steel billets,andinclusionsofthesesampleswere analyzedbySEM-EDSandAspex.Precipitation ofinclusion had been calculated usingFactsagesoftware. The results showed that,the average sizeofinclusions increasedgraduallyandinclusiondensity decreasedgradually from edge to center ofthe billet. Besides,oxides didn't change much in size and sulfide size increased gradually. The inclusions types were main sulfide and oxide. The precipitation temperature of oxide was 1400-1600 ℃, and composition of the oxide were in solid area of the ternary phase diagram at steel making temperature,so they were not easy to aggregate. The quantity of MnS was increasing at the temperature of 1450 ℃,and a substantial precipitation of MnS begins at 1350 ℃.Duringthe solidification process,MnSwasprecipitatedonoxide as the core.

Influence of Al2O3modification on enrichment behavior of chromium in stainless steel slag

2019, 35(5):  75-78. 

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The influence of Al2O3 modification on the enrichment behavior of chromium was analyzed，meanwhile，the enrichment mechanism was deeply investigated. The samples were heated to 1600°C and holding 30min for melting completely before quenching in water. The microstructure and occurrence behavior of chromium were analyzed by X-ray powder diffraction (XRD) and scanning electron microscope-energy dispersive spectroscopy (SEM-EDS). The precipitation behavior of spinel phase was also conducted. The results showed that the main mineral phase in stainless steel slag with 4% and 8% Al2O3 content were spinel and merwinite phase，large amount of chromium was still existed in liquid and merwinitephase. As Al2O3 content increases to 12%，merwinite phase was gradually disappeared and transformed to liquid phase. Chromium in silicate phases gradually migrated to the spinel phase.Addition of Al2O3was conducive to the precipitation of the spinel phase and gradually replacing chromium in spinel phase. Thus，the activity of MgCr2O4 in Mg(Cr，Al)2O4 decreased through Al2O3 modification treatment，which provided thermodynamic driving force for enrichment of chromium in spinel.