Steelmaking

Investigation on non-metallic inclusions in ultra-low oxygen special steels

2015, 31(6): 1.

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Systematic investigations were made on characteristics of the non-metallic inclusions in secondary refining and continuous casting of ultra-low oxygen special steel and following results were obtained: 1) It was hard to further lower the T.O content and eliminate the inclusions in LF refining when the steel T.O mass fraction was lowered to (13－15)×10-6. RH degassing had much stronger ability to eliminate the inclusions and lower the T.O to ultra-low content level. By elongating the RH degassing time to 33 minutes in the experiments, the T.O mass fraction and amount of inclusions larger than 1.5 m were decreased to 4.7×10-6 and 1.77 N/mm2 respectively. 2) In the secondary refining, the inclusions evolved from Al2O3 to MgO-Al2O3, CaO-MgO-Al2O3 and CaO-Al2O3 system inclusions progressively, in which the evolution of Al2O3 to MgO-Al2O3 inclusions was mainly due to the reaction [Mg]+ n/3Al2O3=MgO(n－1)/3Al2O3+2/3[Al] and the dissolved [Mg] in liquid steel was proved in the experiment mainly from the ladle refractories owing to the reaction 2[Al]+3(MgO)lining= 3[Mg]+Al2O3. 3) Large-sized inclusions of 100－330 m were found in specimens of cast bloom of the steel which contained 5.9×10-6 T.O. Most of the larger-sized inclusions above 100 m were CaO-MgO-Al2O3 clusters constituted by smaller particles which had very similar chemical compositions to the small inclusions found in liquid steel samples, indicating that the large-sized CaO-MgO-Al2O3 cluster inclusions in the bloom were formed due to the aggregation of the small inclusions in continuous casting. 4) Most of the inclusions had been transferred to liquid inclusions of CaO-Al2O3 system after RH degassing in the experiment. However, owing to reoxidation in continuous casting, the inclusions were changed to solid inclusions of CaO-Al2O3, MgO-Al2O3 or CaO-MgO-Al2O3 system, which were easier to aggregate to larger inclusions. Hence, reoxidation must be very strictly controlled in production of ultra-low oxygen special steels.

The dynamic prediction model of FeO mass fraction in slag in 300 t topbottom combined blowing converter

2015, 31(6): 13.

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A FeO mass fraction dynamic model of prediction in slag of converter was established combined with the production data of 300 ttopbottom combined blowing converter, using the oxygen balance of converter material balance. It could predict the mass fraction of FeO in slag and the relationship between the FeO mass fraction in slag and topblowing oxygen quantity, phosphorus distribution ratio LP. The results showed that the FeO mass fraction calculation of slag had a good agreement with the measured value, and the ratio of absolute error less than 5% and relative error less than 10% was more than 80%.

Mathematical model of thermalsolidification and shellstressin exit of mold of 301 stainless steel slab

2015, 31(6): 17.

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Aiming at the problem of high occurrence of surface crack defects in 301 stainless steel billet, a thermomechanical coupled model for the formation and stress and deformation of solidified shell in 301 stainless steel slab continuous casting mold was established. The model considered characteristic of high temperature phase change and solidification in mold of peritectic steel. The 3D transient heat conduction finite element, the element birth and death technology and the 3D thermoelasticplastic contact finite element method were used to solve the problem by ANSYS software. The solidification process of the 301 stainless steel slab mold was studied. The results indicate that the temperature is lowest on the center of width face of mold exit, the temperature is highest from the corner of 40 mm distance, the shell thickness is thinnest, and the shell temperature increases with the increasing of δ-ferrite transformation content at the exit, the shell thickness get thin. The displacement of slab wide face center is small, but largest in the corner; narrow surface displacement is greater than the width of face. With the increasing of δ-Fe transformation content, the level of stress decrease at the exit, the cracking index increase near the hot spot and occurrence of crack probability increases.

Simulation on the influence of continuous casting process parameter on solidification structure of large round billet

2015, 31(6): 23.

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A mathematical simulation was conducted using the ProCAST software for the solidification process of the large round billets. The effect of liquid steel superheat in tundish, casting speed and secondary cooling intensity on solidification of continuous casting billet were studied. The results showed that under the condition of mold electromagnetic stirring (EMES) with 300 A current and 3 Hz frequency, the equiaxial crystal ratio of casting billet increased significantly with the reduce of the degree of superheat, from 49 ℃ to 29 ℃, and the equiaxial crystal ratio increases by 4 % roughly for every 10 ℃ drop in the superheat. The equiaxial crystal ratio of casting billet increased insignificantly when casting speed increased for every 0.02 m/min up from 0.16 m/min to 0.20 m/min and secondary cooling intensity reduced for every 0.5 times from 1.5 times to 0.5 times of the standard water flow rate, only roughly 1.5 %. The grain sizes of solidification structures were refined significantly with decrease in superheat, but the changes of the casting speed and secondary cooling intensity had little effects on it. So reducing the superheat of the steel in tundish is the most effective way to improve the equiaxial crystal ratio and refine the grain size of continuous casting billet among the continuous casting process parameters.

Study on solidification characteristic of Fe-25Mn-3Si-3Al-TWIP steel

2015, 31(6): 29.

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The as-cast Fe-25Mn-3Si-3Al TWIP steel produced by electroslag remelting has cracked in the process of forging at high temperature. The high temperature mechanical properties of Fe-25Mn-3Si-3Al TWIP steel have been investigated using the Gleeble 3500 machine. The results show that the brittleness temperature interval Ⅰ is from 1 250 ℃ to the melting point and the brittleness temperature interval Ⅲ is from 650 ℃ to 800 ℃. By means of the scanning electron microscope, the types of fractures under different tensile temperatures have been analyzed, and the structure morphologies have been observed by optical microscope. The microsegregation of specimens was investigated using electron probe micro analyser, and the phase composition was examined using electron backscatter diffraction detector to determine the fracture mechanisms. The results show that the ferrite eduction and microsgregation make the specimens brittle fracture in brittleness temperature interval Ⅲ. The determining fator is dynamic recrystallization to improve the high temperature plasticity of TWIP steel. The processes of electroslag remelting and hot working should be optimized to avoid cracks in the future.

Change in cleanliness of 60Si2MnA spring steel during steelmaking

2015, 31(6): 34.

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“BOF→LF→CC”process was adopted to produce aluminium killed 60Si2MnA spring steel, and high basicity refining slag was made to intensify refining, and then the cleanness of steel during steelmaking process was investigated and analyzed, moreover, the changes of the T.O and nitrogen contents as well as the number, size and type of nonmetal inclusions in steel during steelmaking process were analyzed by theory. Results showed that the T.O content decreased and the nitrogen content increased gradually during steelmaking process, as a result, the average mass fraction of T.O and nitrogen in wire rods were 14.5×10-6and 30.4×10-6, respectively. The change in type of inclusion during smelting process was Al2O3-SiO2 →Al2O3-SiO2-MgO-CaO quaternary inclusions →Al2O3-SiO2-MgO-CaO=CaS quinary inclusions. The aluminum mass fraction of steel should be about 0.03% and the w(Ca)/w(Al) should be 0.08－0.11after calcium treatment, thus the Al2O3 could get the full modification and form the the quaternary inclusions which were in the low melting point area, while the quinary inclusions containing too much CaS with high melting point deviated from the low melting point area.

Study on cleanliness of 8620RH microalloyed forging steel in production process

2015, 31(6): 40.

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In the current study, evolution characteristics of inclusions were evaluated using SEM with automatic scanning feature in 8620RH nonquenched and tempered steel samples during the “EAF→LF→VD→CC”production process. Cleanliness of the steel in producion process were evaluated, combined with the detection of total oxygen and nitrogen of the steel. The results show that type of oxide was Al2O3-CaO-MgO, mainly consisted of Al2O3. After adding sulfur operation, content of CaO in inclusions decreased, and the amount of sulfides and oxysulfides increased obviously. Content of MgO increased gradually during the production process, and number density of inclusions with high MgO content in the end of each production steps was all higher than that in the beginning. Few large spinel inclusions were tested in steel samples. High cleanliness was achieved in 8620RH microalloyed forging steel dioxidized by aluminium，and w(T.O)was 10×10-6 in slab. Number densities of oxides in billets and rolled products are lower than 10 per mm2. Most sulfides adhere to small oxides and grow around them. Most sulfides were adhered to oxides so that it contributed to improvement of machinability, but there are still some largesize sulfides in rolling samples. Proper increase on T.O content could be adopted so that sulfides could percipitate on the tiny oxides and improvement on machinability can be achieved.

Quality contrast of domestic and overseas spring steel wire rod for valve spring

2015, 31(6): 47.

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The complete comparison and analysis of quality between domestic high quality 55SiCrA spring steel, SAE9254V spring steel of POSCO and 65Si2CrV spring steel of Nippon Steel for valve spring were studied. Results show the main differences between domestic spring steel and overseas spring steel: the Als mass fraction of domestic spring steel is up to 40×10-6 and the Al2O3 mass fraction of inclusions in domestic spring steel is up to 42 %, so the aspect ratios of inclusions are all less than 3 and the width size of inclusions is large. In addition, the large and negative inclusions with high content of MgO or Al2O3 were found in domestic spring steel by nonaqueous solution electrolysis. In order to solve this problem, the Als mass fraction of steel must be reduced to less than 20×10-6, so the alloy with low content of aluminum should be used and the steel should be refined by the slag of low basicity and low Al2O3 content, moreover, the magnesia refractory containing Al2O3 should be avoided using during the smelting process, the Al2O3 mass fraction of inclusions should be reduced to 10 %-20 %, thus the plasticity of inclusions can be improved. Besides, there are much proeutectoid ferrite in microstructure and bad decarburized layer in domestic spring steel.

Production practice of 80 t converter with smelting high-phosphorus steel

2015, 31(6): 53.

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In order to solve the problems of low phosphorus and high cost in traditional smelting method of high phosphorus steels, based on the analysis of the basic theory of converter dephosphorization, experiments of smelting high phosphorus steels were carried out. Some key controlling parameters such as model of steel scrap, slagging schedule and oxygen lance control which influence the effect of reducing carburization and keeping phosphorus were established. The results of productions showed that, after the optimization of smelting technology, the average phosphorus mass fraction of high phosphorus steels at smelting endpoint reached 0.042 %, and the smelting cost reduced 22.71Yuan per ton of steel.

Process optimization of improving acid-soluble aluminiumqualified rate in liquid seel into LF

2015, 31(6): 57.

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Aimed to the problem of low qualified rate of acidsoluble aluminum in liquid steel from converter into ladle furnace, the statistical analysis was carried out based on unqualified samples. The results showed that the low qualified rate was mainly affected by the smelting process control in the converter; through further study for the process control, three main influencing factors for low qualified rate were slag in tapping, inconsistent carbon and temperature, and unstable aluminum yield. Then detailed analyses were performed for above three influencing factors, and according process management system and improvement measures were proposed based on the analysis conclusions, thus the qualified rate of acid-soluble aluminum from converter into LF furnace was increased from 81 % to 88 %.

Experimental study on alloying by blowing nitrogen withthe MnCr/CrMo steel in ladle furnace

2015, 31(6): 62.

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Increasing nitrogen by blowing nitrogen at the bottom of the furnace is an effective nitrogen alloying method which can save the nitrogen alloy and reduce the cost. The factors of the solubility of nitrogen in steel were analyzed by theoretical calculation, and the technological conditions of the time and pressure of the ladle bottom blowing were determined, then the experiments of industrialization have been carried out. The results showed that the mass fraction of nitrogen increased with the increase of pressure and flow rate, the nitrogen pressure increased by 0.2MPa and the solubility of nitrogen increased by 25×10-6, but the impact of the flow is far from the pressure. The steel whose nitrogen mass fraction was （100-150)×10-6 could be producted by means of bottom blowing nitrogen without adding nitrogen nitride.

Study on surface sliver defect of cold-rolled sheet

2015, 31(6): 67.

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Sliver defects appeared on surface of coldrolled sheet were studied by scanning electron microscope(SEM),and qualitatively analysis of dispersive spectrum(EDS) in Shougang Jingtang. The results show that the defect was caused mainly by Al2O3inclusions, mould powder and FeO. This kind of defect was effectively controlled by the modification of ladle slag to reduce the top slag oxidation, reasonable pure cycle time of RH and increasing the sedation time of ladle ensuring inclusion floating, optimizing performance of mould powder and structure of nozzle, control speed, decreasing argon blowing to reducing the liquid level fluctuation，reduce the tapping temperature and water descaling system reform measures, cold rolled steel strip surface defects by 0.59 %to below 0.25 %.

Characteristics of limestone calcined under different conditions

2015, 31(6): 73.

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In order to compare lime in the traditional process of “limestone calcination-BOF steelmaking” with that in the novel process of “limestone addition for BOF fluxing”, the thermal experience of limestone are described as “low temperature for long time+cooling+high temperature for short time” and “high temperature for short time”, respectively. Limestone lump can be regarded as countless layers. Flake samples were subjected to different thermal experience and the measurement of shrinkage, specific surface area, pore volume and diameter, crystal structure and hydration reactivity to simulate a single layer of lump. The results showed samples calcined at “high temperature for short time” are characterized by low shrinkage, high specific surface area and pore volume, small pore diameter, fine crystal grain size, high crystal distortion ratio and extraordinary high hydration reactivity compared with samples calcined at both “low temperature for long time” and “low temperature for long time+cooling+high temperature for short time”. It can be deduced that the newly generated lime at high temperature in the novel process should be with higher reactivity, which is favorable for the dissolution in slag.

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