Table of Content

05 August 2017, Volume 33 Issue 4

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New tendency and process innovation for special steels

2017, 33(4):  1-11. 

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From the summary on domestic and international development in special steels, it can be seen that the focal point for quality improvement has been transferred from cleansteel refining technology to largesizeinclusion controltechnology, especially to the sulfide control. The maximum size of inclusions affects the fatigue property of special steels obviously, and when the total oxygen content w(T.O)≤5×10^-6，the influence of sulfide is most serious. The large size inclusions are mainly originated from MgO-Al₂O₃ and CaO-Al₂O₃  by slagmelt reaction, as well as from CaO-SiO₂-Al₂O₃ by involved slag. To control the large-size inclusions, the following process technologies must be improve: lower down the operating load of refining process, optimize deoxidation process, control slag-involving strictly, and promote inclusions floating upward during continuous casting.

The research on deep desulphurization process of bearing steel during LF refining

2017, 33(4):  12-18. 

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Through analyzing the industrial data and discussing the factors affecting the slag composition, basicity，C_S and w(Ca)/w(Al), the corresponding optimization measures of the smelting process was proposed from two aspects of desulfurization thermodynamics and kinetics in order to further reduce the sulfur content in bearing steel. The results showed that controlling w(SiO₂)≤5 %, refining slag basicity R≥10, w(FeO+MnO)≤0.5 % and w(CaO)/w(Al2O3)=1.6－1.7, desulphurization effect was the best, desulphurization rate of bearing steel could be reached 94 %, the mass fraction of sulfur in steel was no more than 20×10^-6.

Research on continuous casting grades transition process by hydraulic model of CSP 45 t single strand tundish

2017, 33(4):  19-22. 

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Aimed at the problem of complicated production organization and high cost of continuous casting, which is due to multiproduct and small batch of high performance steels in CSP plant of Wuhan Iron and Steel Co.,Ltd. Length of the transition slab during the CSP casting was investigated by hydraulic modelling and pilot trials. The used water model was in a proportion of 1∶2 to simulate the industrial tundish with a capacity of 45 t. During the simulation, the conditions satisfied the Froude number similarity principle. Important factors which affect the flow and mixing of liquid steel were taken into account, including the casting speed and the weight of residual molten steel in tundish. As a result, concentration curves along the casting direction were analyzed and obtained. It was found that the minimum length of the intermixing slab was about 22.4m when the residual molten steel and casting speed were about 10 t and 4.5m/min, respectively. Hereafter, industrial trials were carried out, which agreed with the obtained results in the cold model simulation. After application, the problem of complicated production organization, which is due to multiproduct and small batch of high, was be relieved. Productivity of high performance steels was increased and the cost was effectively reduced in CSP plant.

Study of liquid steel solidification shrinkage during continuous casting process

2017, 33(4):  23-30. 

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The analyses of the heat transfer, solidification and solidification shrinkage in the 130 mm×130 mm billet continuous casting process was carried out by including the solidification shrinkageheat transfer feedback regulation algorithm, which was very important for the taper design of continuous casting mold. The effect of the solidification shrinkage on the heat flux density and temperature at the billet corner could be given out by comparing with traditional calculation method which applied the assumed heat flux density for the taper design of continuous casting mold. For CB300V steel, crosssection 130 mm×130 mm billet continuous casting at 3 m/min casting speed, the calculated results showed that the temperature is 1 250 ℃ at the billet corner which is different from the temperature calculated by classical heat flux. The obvious solidification shrinkage at the face center begins at 0.4 m from the mold upper edge, its value is about 0.000 2 m, while the corner ones is about 0.001 575 m. Solidification shrinkage decreases nonlinearly with the increasing of the distance from the billet face.

Effect of submerged nozzle geometry on flow behavior for 180 mm×240 mm billet continuous casting mold

2017, 33(4):  31-38. 

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To improve the internal quality of high carbon wire steel produced by a domestic steel factory, the fluid flow behavior in a 180 mm×240 mm billet continuous casting mould was investigated with 1∶1 water model, based on the similarity principle. The distribution of the molten steel, the fluctuation of the mold liquid surface and the state of the slag layer using singleport straight nozzle and fourport swirling nozzle were compared by the experiment. The results show that using swirling nozzle can effectively decrease the impact depth of fluid; the average fluctuation height of swirling nozzle is larger than the straight one at the same immersion depth and casting speed. In addition, casting speed and immersion depth affect the fluctuation of mold liquid surface of swirling nozzle more obviously, and the fluctuation of slag layer is more frequent for the swirling nozzle than for the straight one. Considering the fluctuation of the mold liquid surface, the state of the slag layer and cost, the swirling nozzle is more fit for casting the high quality billet in this section, and the optimum process is at the casting speed 1.0 m/min and the immersion depth 120 mm with the average wave height of free surface at 0.18 mm. In this case, the slag layer fluctuates properly, and no slag is entrapped and molten steel exposed. The billet of high carbon steel produced by using this nozzle combining with moldEMS and secondary cooling has stable quality; over 95 % of wire rods in diameter 5.5mm possess center carbon segregation level below 2.0 grade, and the porosity and shrinkage levels of all wire rods are below 0.5 grade.

Research on internal quality control of 37Mn5 steel round billet with high casting speed

2017, 33(4):  39-45. 

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According to quality defects such as internal crack,center shrinkage and center porosity of 37Mn5 steel round billet with high casting speed,the main reasons which lead to internal quality defects were high superheat,weak mold electromagnetic stirring intensity,unreasonable secondary cooling process system and arranging form of nozzles respectively based on analysis of production practice.Mathematical model of solidification and heat transfer verified by nailshooting experiments and surface temperature testing was applied in optimizing secondary cooling process system.Stirring parameters were optimized by measuring the electromagnetic field distribution with Gauss meter in the mold at different stirring parameters,in addition arranging form of nozzles in second cooling zone was also improved.The results showed that Φ200 mm 37Mn5 steel round billet casting speed increased from 1.4 m/min to 1.8 m/min,the internal quality significantly has been improved.Internal cracks disappeared and the average grade of center shrinkage and center porosity was 0.5 ,but when superheat was greater than 30 ℃，center porosity still existed.

Investigation on clogging mechanism of submerged entry nozzle for Ti-bearing IF steel during BOF→RH→CSP process

2017, 33(4):  46-51. 

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In order to investigate the clogging mechanism of submerged entry nozzle during casting Tibearing IF steel by BOF→RH→CSP process.The composition of clogs on submerged entry nozzle was studied by scanning election microscopy (SEM), energy dispersive spectroscopy (EDS).Finally, the clogging mechanisms of submerged entry nozzle were elucidated by combining with the characteristics of inclusion in tundish.The investigated results indicated that the clogging easily occurs because that wetting angle was decreased due to containing titanium in molten steel.The clogs close to the inner surface of submerged entry nozzle, which was composite oxide of Al2O3-CaO-SiO2-MgO but mainly consists of Al2O3, was resulted from reaction between refractory material of SEN and acidsoluble aluminum in molten steel.However, the clogs close to molten steel comes from the deposition of inclusions containing Al2O3 or spinel of MgO·Al2O3 and iron particles.The rate of clogging was increased because that the molten steel easily adhered to net sintered inclusions.The studied results will provide theoretical guidance for rising the continuous casting heats of tundish and improving the quality of products by means of optimizing the process parameters.

Optimization of continuous casting soft reduction process for S650MC high strength steel

2017, 33(4):  52-57. 

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For resolved the problem of the S650MC steel hot rolled coil has a certain percentage of lamination,one of the main reasons of the tensile lamination is the slab center segregation,the process optimization experiment was carried out.The result showed that:the amount of reduction was increased from 3.6mm to 4.4mm in the fs =20%－75% reduction zone，the total amount of reduction was increased from 4.5 mm to 5.5mm，this optimal process project could significantly improve the quality of center segregation.The average center segregation etching grade of S650MC steel was improved from B1.0 to C1.0.The rate of the tensile lamination was decreased from 4.20 % to0.78 %.Meanwhile,showed that the effect of improving S650MC steel center segregation quality by S-EMS was not obvious,but the equiaxed grain ratio of slab can increase about 2 %－3 %.

Decarburizing and chromium keeping process optimization of AOD refining 430 stainless steel using BOF dephosphorized hot metal

2017, 33(4):  58-62. 

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The decarburizing and chromium keeping process optimization in the early of decarburization of 180 t AOD refining 430 stainless steel using BOF dephosphorized hot metal was studied by the calculation methods of thermodynamic,material and heat balance.The results show that the reasonable temperature control of decarburizing and chromium keeping process is key technology for improving the economic and technical indicators of AOD refining 430 stainless.The hot metal temperature into AOD increased from 1 520 ℃ to 1 580 ℃ and further improving the molten steel temperature in the early of VOD decarburization were guaranteed.Through the implementation of the process optimization the economic and technical indicators of AOD refining 430 stainless using BOF dephosphorized hot metal can be significantly improved,in which chrome recovery increased from 92.3 %to 94.63 %,refining time reduced by 9.2min and reduced silicon consumption decreased 4.9 kg/t.

Practice of improving the yield rate of tungsten in GX12CrMoWVNbN10-1-1 steel

2017, 33(4):  63-66. 

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For the low yield rate of ferrotungsten in smelting GX12CrMoWVNbN10-1-1 steel， the ferrotungsten alloying process was optimizatied in Shanghai Electric Shangzhong Casting and Forging Co.,Ltd. Results shows that ferrotungsten alloying in electric furnace has oxidation loss，simplely control temperature factors cant improve the yield rate. Make the alloying process of ferrotungsten carried out in LF，take such as strengthen the deoxidation operation in LF， increasing the alloying temperature and prolonging the reduction time measures，can make the yield rate of ferrotungsten from 50 % to 70 %。

Causes and countermeasures of drawing fracture for flux-cored welding wire steels

2017, 33(4):  67-72. 

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To solve frequent broken for flux-cored wire during the drawing process in a steel company,the morphology and chemistry of the fracture was analyzed.And the broken mechanism and its countermeasures was put forward based on the production process.The distortion and lamination were detected with SEM for fracture analysis.The energy dispersive spectrometry show the mold powder account for that lamination.A comparison between the defective heats and normal heats show that the percentage of level fluctuation smaller than 3 mm of fractureinduced heats was smaller than the normal ones.Thus the mechanism of the broken wire can be summarized as follows:mold powder entrapments due to large level fluctuation occurs and it may be captured by the solidifying shell,persist in the slabs until to the cold rolled sheets.[JP2]The lamination occur because of the ductility difference between inclusions and matrix.The wire break when the pulling stress exceed the affordable stress of this defective wire.Several countermeasures were proposed to prevent the mold power entrapments,including controlling the argon gas flow rates changing the SEN bottom shapes,and so on.After applying these countermeasures,the wire fracture ratio decreased from 8.7 % to 0.5 %.

Study on cleanliness and inclusions for 51CrV4 steel during refining by CSP process

2017, 33(4):  73-78. 

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Aiming at the problems of nozzle clogging and surface quality of high carbon steel produced by CSP process, the change of the cleanliness of molten steel and the morphology, the size and the compositions of inclusions in 51CrV4 steel during refining were studied by industrial experiments in “BOF-LF-thin slab casting” process. The results showed that mass fraction of T.O and [N] of molten steel in tundish were about 20×10-6 and 45×10-6 respectively with high basicity refining slag and process of soft argon blowing. The density of inclusion was 16.62/mm2，and the proportion with less than 10 μm was more than 97 %. The solid inclusions of Al2O3-MgO、SiO2-CaO-Al2O3 were modified into liquid compound inclusions of CaO-SiO2/MgO-Al2O3、CaO-MgO/SiO2-Al2O3-CaS by Ca treatment. The inclusion modification conditions with Ca treatment were analyzed by theory. When the mass fraction of [Al] was about 0.025 %, the mass fraction of [Ca] and [S] should be 0.001 % and less than 0.001 % respectively. And the inclusion of Al2O3 can be modified into liquid calcium aluminates.