Table of Content

05 April 2016, Volume 32 Issue 2

Previous Issue    Next Issue

Thermodynamic investigation for the accurate calcium addition during calcium treatment of molten steels

2016, 32(2):  1-8. 

Asbtract ( 185 )  

Related Articles | Metrics

In the current paper, the development of classical thermodynamic calculation for the accurate calcium addition during calcium treatment of molten steel were briefly summarized. The calculation of the accurate addition amount of calcium alloys was calculated using thermodynamic software of FactSage. The purpose of calcium treatment is to modify undeformable solid inclusions, especially  Al₂O₃ based ones into deformable liquid inclusions, especially liquid calciumaluminates. The steel composition, especially the content of the total oxygen, dissolved aluminum and sulfur, and the temperature have great influence on the addition amount of calcium. After addition of calcium into the molten steel, many kinds of inclusions are generated, such as CaO·6Al₂O₃, CaO·2Al₂O₃, 12CaO·7Al₂O₃, 3CaO·Al₂O₃, CaO and CaS. Only 12CaO·7Al₂O₃ and 3CaO·Al₂O₃ are liquid at the temperature of the molten steel. With the increasing addition of calcium, many liquid and solid inclusions may simultaneously generated. The addition of calcium must satisfy that all inclusions are liquid and no solid CaS inclusions generated. Hence, there is a "liquid window" with minimum and maximum addition of calcium alloys. The minimum addition is to satisfy the vanishing of the solid inclusions and the maximum addition is to assure no generation of pure solid CaS inclusions. In the current study, the accurate addition of CaAl alloy wires, with a 1∶1 mass ratio of  Ca∶Al, for the LF refining process of a steel plant was calculated. At 1 571 ℃, in order to assure that all inclusions are liquid and no pure CaS inclusions are generated, the calcium content in the molten steel should be (22.6－ 47.4)×10^-6. If the yield of the alloy is 30 %, the length of the added wire should be 197－334 m. In the current industrial production, the addition is 350 m, slightly longer than the calculated value. It revealed that inclusions in the steel samples contained a slightly high amount of CaO, which agree well with the calculation. The current model for the accurate calcium addition of calciumalloys can be used for the real production.

Parameter optimization of oxygen lance in 65 t converter

2016, 32(2):  9-11. 

Asbtract ( 193 )  

Related Articles | Metrics

The percentage of abnormal loss such as sticking steel and slag for 65 tconverter oxygen lance was larger in Tangshan Iron and Steel Company. The poor smelting slag and dry slag in smelt process was the main reason. Therefore, it was necessary to optimize the process parameter and oxygen nozzle life. The research results were shown as follows. For the optimized nozzle, the open blowing position of oxygen lance decreased by 100 mm and the process blowing position of oxygen lance decreased by 200 mm，the slag melting effect was good in smelting process. The variation of oxygen blowing time, smelting period and the oxygen consumption was little. The qualified rate of phosphorus content in steel increased 11.43 %. The FeO mass fraction in final slag decreased  0.25 %. The nozzle life was more than 200 heats, the steel consumption increased by 4.99 kg/t. The cold burden consumption increased 0.62 kg/t with the nozzle life increasing 50 heats. The FeO mass fraction in final slag increased 1.3  % with the nozzle life increasing 100 heats. Therefore, the economical nozzle life for 65 tconverter was 200 heats.

Effect of slag deoxidation modification process optimization on the total oxygen content of aluminium deoxidization silicon steel

2016, 32(2):  12-17. 

Asbtract ( 185 )  

Related Articles | Metrics

To the question of the unsteadiness of the steel oxygen content at the end of RH vacuum treatment for the aluminium deoxidization nonoriented silicon steel refining, the oxygen potential of molten steel and slag was optimized during tapping and deoxidation alloying process. Results show that the oxygen mass fraction of nonoriented silicon steel is stably controlled below 650×10^-6 as it was (400－900)×10^-6 before optimization. Furthermore, T.O mass fraction is controlled below 450×10^-6 at the end of decarbonization in RH. With the optimization process, the mass fraction of T.Fe in slag is decreased from 10 %－25 % to less than 15 % at the beginning of RH, which is also decreased from 2 %－12 % to below 3 % at the end of RH, and the slag basicity is controlled above 3.0 during RH treatment process. T.O mass fraction at the end of RH is reduced from (15－30)×10^-6 to below 15×10^-6 using the optimization process, which indicated that the cleanliness of molten is significantly improved.

Process technology of long service life on copper mould tube in billet continuous

2016, 32(2):  18-24. 

Asbtract ( 183 )  

Related Articles | Metrics

In view of low steel passthrough amount phenomenon of mould copper tube in small billet continuouscasting, the factors influencing the service life of copper pipe from the mold assembly and process were analyzed. Through optimizing mould copper tube material, size, taper and water slot width, improving the cool water quality, water pressure and the cooling water flow rate and reducing liquid steel superheat in tundish and properly decreasing casting speed, average steel passthrough amount per mould copper tube has been increased to 7 820 t after the optimization, the highest one being 9 510 t per copper tube, and numbers of nonnormal casting stop per month has been decreased by 10 times averagely, which saved 180 min. As a result, the production costs has been reduced, the production stability has been ensured and the economic benefit of the enterprise has been increased.

Influence of online roll gap calibration accuracy on the internal quality of continuous casting slabs

2016, 32(2):  25-29. 

Asbtract ( 139 )  

Related Articles | Metrics

Aiming to increasing slab percentage of center segregation grade below 1.0 induced by relaxed displacement sensor calibration and poor roll gap accuracy, the accuracy of roll gap before the start of a sequence casts was measured through a multifunctional roll gap measurement instrument and the factors influencing online roll gap accuracy were analyzed. Considering the typical roll gap control system, incorrect offset values of displacement sensors upon cylinders on segments, especially on the solidification crater end zone, was the main reason of poor roll gap accuracy on the soft reduction region. Through strictly implement of offline calibration of displacement sensors, the online roll gap accuracy was highly improved with high precision between the left and right displacement sensors at the segments exit or entrance. Through the application of offline calibration of displacement sensor, the online roll gap deviation could be stably controlled within ±0.2 mm，resulting in the increasing percentage of center segregation grade below 1.0from 29.5 %to 92.0 % and eliminating triangular crack defect.

Study on crack tendency of the heavy reduction on liquid core of billet

2016, 32(2):  30-35. 

Asbtract ( 130 )  

Related Articles | Metrics

Heavy reduction is a important technology that can effectively improve the internal quality of the billets. However, effective prevention of cracks was very essential during the implementation of this mechanical reduction technology. In this study, formation mechanisms of cracks in the 72A steel ascast billets (180 mm×180 mm) with heavy reduction of 5～20 mm by a single pair of rollers were discussed. It was found that tendency of cracks was weaker during heavy reduction compared to soft reduction. With heavy reduction by 16 mm at a solid fraction of larger than 0.3by the single pair of rollers, no cracks were observed in the 72A steel billets. Based on the experimental result, formation mechanisms of cracks were proposed as following: compared to soft reduction, stress can be much more effectively transferred to the center of the billets. With the initiations of cracks during reduction, they tended to develop towards the center of the billets by single pair of rollers, nevertheless, the initiated cracks preferred to grow towards the shells of the billets during soft reduction by multi-pairs of rollers.

Comparison between permanent magnet stirring and final electromagnetic stirring during continuous casting bloom

2016, 32(2):  36-40. 

Asbtract ( 148 )  

Related Articles | Metrics

As the demand for highcarbon 82A steel increase the improvement of internal quality such as center carbon segregation in rectangular bloom with the section size of 180 mm×240 mm, both final permanent magnetic stirring and final electromagnetic stirring (FEMS) were applied in a rectangular bloom caster. The results showed that the permanent magnet stirring provides an essential improvement in the internal quality of highcarbon rectangular blooms compared with the F-EMS. The mean center segregation degree of carbon was 1.06 with the permanent magnet stirring and 1.10with the F-EMS. Compared with the FEMS, the center segregation degree of carbon less than 1.05was increased by 40%, the grade of center porosity equals to 0.5was increased by 50 %and the grade of shrinkage cavity equals to 0 was increased by 20 %by adopting the permanent magnet stirring. Besides, the calculating results showed that the stirring velocity of the molten steel with permanent magnet stirring was 1.15 times that of F-EMS.

Study on the mathematical model of acicular ferrite nucleation induced by irregular inclusions

2016, 32(2):  41-43. 

Asbtract ( 124 )  

Related Articles | Metrics

In consideration of the influence of irregular inclusions to the nucleation of acicular ferrite, building the nucleation model whose base phase and precipitated phase are normal triple prism. Theoretical analysis reveals that not only spherical inclusions can induce acicular ferrite to nucleation, but also irregular inclusions with corner angle. The smaller of the wetting angle, the easier of the acicular ferrite to nucleation. When the side corner of the base phase is between 75°－120°, the irregular inclusions induce the acicular ferrite to nucleation easily.

EAF→LF→VD technological test of 2 100 MPa grade ultra-high strength low alloy steel

2016, 32(2):  44-47. 

Asbtract ( 104 )  

Related Articles | Metrics

EAF→LF→VD technological test was conducted to replace VIM-ESR in order to increase the production efficiency and to reduce process cost of over 2 000 MPa grade ultrahigh strength low alloy steel for mass production capacity. The test result shows that the total mass fraction of O, N and H in steel is smaller than 50×10-6；the nonmetallic inclusion of A, B and C class is evaluated as 0 grade and class D inclusion is less than 1.0 grade. The tensile strength of finished steel plate is more than 2 100 MPa after quenching and low tempering and its elongation is greater than 7 %, which is equivalent to the result of using VIMESR technology. By using the new technology, the cost is reduced by over 44 % and the production efficiency is increased. The new technology is suitable for large-scale production of ultra-high strength low alloy steel.

The influence of different deoxidization process on the inclusions in SG45VCM steel

2016, 32(2):  48-52. 

Asbtract ( 155 )  

Related Articles | Metrics

In order to eliminate clogging of nozzle that caused by the the accumulation of Al2O3 which was generated in the irrational process of deoxygenation during concasting SG45VCM steel, research on the influence of different deoxidization processes on nonmetallic inclusions in steel was studied, through thermodynamic calculation and experiments. Thermodynamic calculation shows that deoxidization by using aluminum and titanium can get low dissolved oxygen level, deoxidization ability of aluminum and titanium is better than that of silicon and manganese. At the end of two kinds of deoxidization process the oxygen mass fraction in steel were similar, the lowest was 7×10-6. Size of inclusions in the new deoxidization process was small. There were various types of inclusions, and they were distributed evenly in the steel. Phenomenon of clogging of nozzle caused by the pure Al2O3 which was generated in the original process of deoxygenation could be avoided. The experiment results show that the presence of aluminum in steel can against the formation of titanium oxide, and the inclusion distribution is uniform dispersion.

Purity of IF steel during continuous casting process

2016, 32(2):  53-59. 

Asbtract ( 100 )  

Related Articles | Metrics

Purity of steel is vital important to the surface quality of cold rolled sheet. Samples of tundish steel and slab were analyzed by automatic feature analysis equipped on Aspex and LECO TCH600 in present paper to solve the problem of high surface defects occurring rate. T.O mass fraction of IF steel in tundish is in the range of 17.5×10-6 to 35.7×10-6, which means high purity. But about 60 t of steel become unqualified because of the reoxidation at the start of casting. Macro inclusions in the slab mainly are cluster Al2O3 inclusions and “bubble + Al2O3 inclusion” structures. The quantity density of inclusions over 50 〖WT〗μ〖WT9.5B4〗m is in the range of 0.29/cm2 to 0.63/cm2. Moreover, a new gathering area which is different from the one just blow the surface of macro “bubble + Al2O3inclusion” structures is found 20.0mm away from the surface. Measures such as lowering the sulphur content of steel, prudently lowering the argon amount from the stopper, and optimize the flow condition in the mold, are proposed to reduce the gathering phenomenon.

Characterization of large inclusions along the thickness direction in the ultra-low carbon steel slab

2016, 32(2):  60-63. 

Asbtract ( 119 )  

Related Articles | Metrics

To decrease the surface defects on cold rolling sheets caused by inclusions, large inclusions in ultra-low carbon steel slab produced by bending vertical caster were characterized in details along the whole thickness direction. Metallographic samples with large section dimension of 70 mm×80 mm were specially prepared, and the inclusions were inspected by the automatic secondary electron microscopy ASPEX explorer. It was found many large inclusions still exist even T.O mass fraction in slab was reduced to 0.001 2 %. Most majority of them were alumina clusters within 20－200 μm and the others were the “bubble+Al2O3” type with big sizes in the range of 30－100 μm and some were larger than 100 μm. While the wellknown moldflux entrapped inclusions were not detected. These large inclusions were not only in the surface/subsurface but also located deeply in the slab, indicating an obvious quarterband accumulation especially for the “bubble+Al2O3” inclusions.

Study on application of stainless steel dust in the process of foaming EAF slag

2016, 32(2):  64-68. 

Asbtract ( 159 )  

Related Articles | Metrics

Regarding the enviroment problem during stainless steel production, a new mode for stainless steel EAF slagging used recycled ferritic stainless steel dust was issued. The slagging agent were produced by the recycled ferritic stainless steel dust, and the EAF slaging process was developped. Application at the site, more than 5 t per furnace stainless steel dust are reclaimed，most metal iron in EAF slag are reclaimed and 3.85 % chromium losses are reduced in recycled ferritic stainless steel dust.

Study of improving of chromium recovery in stainless steel dust

2016, 32(2):  69-73. 

Asbtract ( 177 )  

Related Articles | Metrics

For exploring a efficient way to separate and recycle the chromium metallic resources in stainless steel dust，laboratory research by adding carbon, ferrosilicon and aluminum as reductants to stainless steel dust pellets were carried out. The results show that when only using carbon as reductant, the separation of alloy and slag in the reduction products is good at nC∶nO=1.0, and chromium content in slag develops to its minimum at slag basicity of 1.17. The recovery ratio of chromium reach as high as 96.39 % and residual mass fraction of chromium in the slag was reduced to 0.48 % by adding ferrosilicon on the basis of the dust with carbon. The recovery ratio of chromium was enhanced to 93.36 % when adding carbon and aluminum to dust.

Study on cementitious properties of steel slag

2016, 32(2):  74-78. 

Asbtract ( 140 )  

Related Articles | Metrics

In order to promote the application of steel slag in cement concrete, the cementitious properties of steel slag were investigated by XRD, nonevaporable water content determination of hydration products, strength test and particle size analysis. The influence of fineness on cementitious properties of steel slag was studied. The results show that the pure steel slag paste can harden slowly, however its cementitious properties is weak. The hydration process of cement and steel slag is similar. In early age the hydration rate of steel slag is much lower than that of cement, whereas in later age the hydration rate of steel slag is higher than that of cement, especially after 90 d. The hydration rate of steel slag is improved to a certain extent by increasing the specific surface area before 90 d in contrast with a reduced rate after 90 d. The hydration degree of different fineness steel slag reaches to a similar extent after 180 d.