2014年, 第21卷, 第4期　刊出日期：2014-04-15

  

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Metallurgy and Metal Working
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  Mathematical Modeling of Multi-sized Argon Gas Bubbles Motion and Its Impact on Melt Flow in Continuous Casting Mold of Steel

  Chong-lin LIU,Zhi-guo LUO,Tao ZHANG,Shen DENG,Nan WANG,Zong-shu ZOU

  中国钢铁期刊网. 2014, 21(4): 403-407.

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  The 3D turbulence k-ε model flow of the steel melt (continuous phase) and the trajectories of individual gas bubbles (dispersed phase) in a continuous casting mold were simulated using an Eulerian-Lagrangian approach. In order to investigate the effect of bubble size distribution, the radii of bubbles are set with an initial value of 0. 1-2. 5 mm which follows the normal distribution. The presented results indicate that, in the submerged entry nozzle (SEN), the distribution of void fraction is only near the wall. Due to the fact that the bubbles motion is only limited to the wall, the deoxidization products have no access to contacting the wall, which prevents clogging. In the mold, the bubbles with a radius of 0. 25-2. 5 mm will move to the top surface. Larger bubbles issuing out of the ports will attack the meniscus and induce the fluid flows upwards in the top surface near the nozzle. It may induce mold powder entrapment into the mold. The bubbles with a radius of 0. 1-0. 25 mm will move to the zone near the narrow surface and the wide surface. These small bubbles will probably be trapped by the solidification front. Most of the bubbles moving to the narrow surface will flow with the ascending flow, while others will flow with the descending flow.
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  Characteristics of Calcined Magnesite and Its Application in Oxidized Pellet Production

  Qiang-jian GAO,Guo WEI,Xin JIANG,Hai-yan ZHENG,Feng-man SHEN

  中国钢铁期刊网. 2014, 21(4): 408-412.

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  Calcined magnesite is a binding additive and an MgO-bearing flux for pellets production. The effects of calcination temperature and time on the characteristics of calcined magnesite were investigated. Experimental results indicated that the best calcination condition was 850 ℃ and 1 h. Under this condition, the hydration activity of the calcined magnesite was 80. 56%, and the average diameter of crystal grain D, specific surface area S and the medium particle size D50 were 25. 4 nm, 45. 40 m2/g and 3. 41 μm, respectively. This kind of calcined magnesite was a good binding additive for pellets production. At the same proportion of calcined magnesite, the effects of activities of calcined magnesite on metallurgical properties of green pellet and indurated pellet showed that calcined magnesite with high activity could improve the dropping strength and compressive strength of green pellet and enhance the burst temperature of green pellet; however, the effects of activity on compressive strength, low-temperature reduction degradation index, reduction swelling index and reduction index of indurated pellet were not obvious.
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  Computational Insights into Interactions between Ca Species and α-Fe2O3 (0 0 1)

  Yan-qin SUN,Qing L,Xin-yu WAN,Shu-hui ZHANG,Jiong-ming ZHANG

  中国钢铁期刊网. 2014, 21(4): 413-418.

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  CaCl2 can be sprayed onto sinter surface, which can improve the low temperature reduction degradation index (RDI+3. 15) of sinter. This has been recognized; however, there are various opinions on the inhibition mechanism of it. At the same time, the corrosion of Cl element on equipment is very serious. First-principle calculations based on density functional theory were performed to investigate the binding mechanisms of calcium species on a α-Fe2O3 (0 0 1) surface. This is crucial in demonstrating the role of the CaCl2 on improving the low temperature reduction degradation index. It has been determined that Cl could greatly increase the adsorption of the vacuum layer for the Ca/Fe2O3 system and the relaxation produced by adsorption made bond length decrease, bond energies increase and structure compact. Those are the main reasons that inhibiting the reduction disintegration of sinter.
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  A New Simplified Zonal Method for Furnace Thermal Radiation Calculation Based on Imaginary Planes

  Wei-jun ZHANG,Zhi YI,Hai-geng CHEN

  中国钢铁期刊网. 2014, 21(4): 419-426.

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  When solving the complex radiative heat transfer problems in reheating furnaces, there are a number of difficulties with the traditional zonal methods. To circumvent these difficulties, a new simplified method was proposed, which employed imaginary planes, referred to as the imaginary plane model. With the new model, crown wall reduction process was simplified. Therefore, every model zone could be treated as a closed square cavity. It could also solve the problem of radiative blocking in industrial furnaces more effectively. Besides, the new imaginary plane based model may lead to a problem that the denominator was zero. This problem was solved by transforming the expressions of reflex heat flux in the model. The model was capable of dealing with the systems that included black surfaces. The model was validated by considering the heat transfer in a reheating furnace where the temperature fields in the furnace chamber (including the steel, wall and gas) were obtained. A detailed comparison was made between the simulation and the black box experiment. The results show that the new model developed was valid and accurate.
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  Carbothermic Reduction of Zinc and Iron Oxides in Electric Arc Furnace Dust

  Hui-ning ZHANG,Jian-li LI,An-jun XU,Qi-xing YANG,Dong-feng HE,Nai-yuan TIAN

  中国钢铁期刊网. 2014, 21(4): 427-432.

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  The reduction of zinc and iron oxides from electric arc furnace dust (EAFD) by carbon was investigated at temperatures between 800 and 1300 ℃. The analytic technique employed includes chemical analysis, X-ray fluorescence spectroscopy (XRF), X-ray powder diffraction (XRD), scanning electron microscopy (SEM) equipped with X-ray energy dispersive spectrometry (EDS), and thermodynamic database FactSage 6. 2. It was found that the reduction of zinc and iron oxides depends largely on Boudouad reaction. At 900 ℃, zinc exists in tested samples as ZnO, which is reduced in the temperature range of 1000-1100 ℃. At 1100 ℃, 99. 11% of the zinc is evaporated. The metallization ratio of Fe is 79. 19% at 1300 ℃, as the content of Fe2+ is still 9. 40%. A higher temperature is thus required for a higher reduction degree of Fe oxides by solid or gaseous carbon.
Material
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  Influence of Finishing Cooling Temperature and Holding Time on Nanometer-size Carbide of Nb-Ti Microalloyed Steel

  Hai-long YI,Yang XU,Ming-xue SUN,Zhen-yu LIU,Guo-dong WANG

  中国钢铁期刊网. 2014, 21(4): 433-438.

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  The nanometer-size carbides formed in ferrite matrix of Nb-Ti microalloyed steel at different finishing cooling temperatures and holding time have been investigated. The characteristics of nanometer-size carbides in ferrite were observed by transmission electron microscopy, and mechanical properties of ferrite were detected by a nano-hardness tester. The results showed that interphase precipitation and diffusion precipitation were observed at different finishing cooling temperatures, and the interphase precipitation was planar and curved. Sheet spacing of interphase precipitation increased with the increase of finishing cooling temperature and changed a little when holding for 50-1000 s. Interphase precipitation shows higher nano-hardness at 640 ℃ compared with diffusion precipitation at 600 ℃, and the contribution of interphase precipitation to the mechanical properties of ferrite was larger than that of diffusion precipitation.
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  Strain Hardening of a Layered and Nanostructured AISI 304 Stainless Steel

  Yan-quan JIA,Yong-li LIANG,Ai-ying CHEN,Deng PAN

  中国钢铁期刊网. 2014, 21(4): 439-443.

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  In order to improve the low ductility of nanostructured materials, a layered and nanostructured (LN) 304 SS (stainless steel) is prepared from warm co-rolled 304 SS pre-treated by surface mechanical attrition treatment. The microstructure and mechanical properties, as well as strain hardening, are analyzed in details. The LN steels exhibit both high strength and large ductility resulting from good strain hardening behaviors. The strain hardening can be subdivided into two stages, which involves a multiple cracking along interlaminar at the first stage and a strain-induced martensite (SIM) transformation at the second stage. The SIM transformation of nanocrystallines and ultrafine grains induces a larger work hardening exponent by the formation of nanoscaled martensite phase. The effect of grain size on the transformation dynamics is discussed.
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  Effects of Sulphate-reducing Bacteria on Corrosion Behaviour of 2205 Duplex Stainless Steel

  Cheng-hao LIANG,Hua WANG,Nai-bao HUANG

  中国钢铁期刊网. 2014, 21(4): 444-450.

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  The effects of sulphate-reducing bacteria (SRB) on the 2205 duplex stainless steel in the sea water and oil industry environments were studied by electrochemical impedance spectroscopy (EIS), potentiodynamic polarization and microorganism analysis. The results showed that the detected SRB was the non-spore bacillus of about 0. 8 μm×(1. 8-2. 2) μm in size and the content of S was about 7. 59% in the corrosion products. SRB contributed to the corrosion evolution which caused the corrosion failure of 2205 duplex stainless steel pipe in the liquid hydrocarbon cooler. During the corrosion process, the produced H2S could significantly influence the anodic process and finally accelerate the corrosion. SEM observation indicated that the distribution of SRB on the surface of 2205 duplex stainless steel was nonuniform. X-ray photoelectron spectroscopy analysis revealed that the surface film was composed of Cr2O3, MoO3, FeS, FeS2, Fe(OH)2 and FeO after immersing the sample in the SRB medium for 14 d.
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  Hot Ductility of Ti-V Bearing Microalloyed Steel in Continuous Casting

  Yan-hui SUN,Ya-nan ZENG,Kai-ke CAI

  中国钢铁期刊网. 2014, 21(4): 451-458.

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  Hot ductility of Ti-V bearing steel was studied by theoretical calculation and thermal simulation experiment. Meanwhile, microsegregation and precipitates were analyzed. The results showed that elements S, Ti and O were enriched at the grain boundaries, while the hot ductility was deteriorated by inclusions of (Fe,Mn,Si,Al)(S,O) in the interdendritic region. At the temperature of 1100 ℃, large TiN particles had little effect on hot ductility. In the temperature range from 975 to 925 ℃, reduction of area (R. A) declined rapidly from 81. 88% to 63. 16% with the size of particles decreasing from 40 to 20 nm and quantity increasing from 1. 4 to 11. 9 μm-2, respectively. In the temperature range from 875 to 825 ℃, R. A reduced unobviously from 35. 14% to 28. 6% with ferric films thickening gradually. The critical stress, 88. 79 MPa, was equal to tensile strength at 825 ℃. Intergranular fracture was easy to occur with higher critical stress below 825 ℃.
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  Effects of Offline Relaxation Quenching on Microstructure and Mechanical Properties of a Newly Designed High Performance Steel

  Wen-zhen XIA,Xian-ming ZHAO,Xiao-ming ZHANG,Di WU

  中国钢铁期刊网. 2014, 21(4): 459-463.

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  The newly designed high performance tested steel was prepared by the process of vacuum induction furnace smelting and forging. According to the ideas of online relaxation and quenching-intercritical quenching-tempering (Q-L-T) process, effects of offline relaxation process on microstructure and mechanical properties of the tested steel were investigated. Offline relaxation process was simulated using the MMS-300 thermal simulation testing apparatus and heat treating furnace. The results show that the microstructures are composed of bainite and martensite when the specimens are quenched in the austenite region. Once the specimens are air-cooled into the dual phase region, ferrite and granular bainite start to form. In the relaxation process, fine Nb(C,N) carbonitrides are precipitated first and then grow and coarsen. The microstructure is affected by process and determines mechanical properties. The Vickers hardness and yield strength (YS) first rise then fall when the relaxation temperature drops in austenite region, and then decline dramatically in dual phase region. When the relaxation time is 20 s, the tensile strength reaches the peak (1034 MPa), and at that time YS is 872 MPa, elongation is 17. 7% and Charpy V-notch impact energy is 171 J at -20 ℃. When the relaxation time increases to 100 s, the yield and tensile strengths are 750 and 934 MPa respectively, elongation is 18. 6% and Charpy V-notch impact energy is 165 J at -20 ℃.
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  Constitutive Model for F45V Microalloyed Forging Steel at High Temperature

  Jin WANG,Wen-long MA,Zhong CHU,Qi ZHANG

  中国钢铁期刊网. 2014, 21(4): 464-468.

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  Isothermal hot compression experiments of F45V, a microalloyed steel, were performed on a Gleeble-1500 thermo-mechanical simulator at temperatures of 950-1200 ℃ and strain rate of 0. 01-10 s-1. Based on the experimental flow stress curves, a constitutive model that was expressed by the hyperbolic laws in an Arrhenius-type equation was established, and the material parameters of the model were expressed as 6th order polynomial form of strain. Standard statistical parameters such as correlation coefficient and average absolute relative error were employed to quantify the predictability of the model. They were found to be 0. 995 and 4. 34% respectively. The results show that the established constitutive model can predict the magnitude and tendency of flow stress with the increase of deformation accurately, and can be used for the numerical simulation of hot forging process of the F45V steel.
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  Analysis of Fishscaling Resistance of Low Carbon Heavy Plate Steels

  Ai-wen ZHANG,Zheng-yi JIANG,Dong-bin WEI,Si-hai JIAO,Chun XU

  中国钢铁期刊网. 2014, 21(4): 469-475.

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  The precipitates and hydrogen permeation behavior in three kinds of hot rolled low carbon heavy plate steels for enameling were analyzed; then, both sides of the steels were enameled. The experimental results show that a large amount of coarse Ti4C2S2 and fine Ti(C,N) particles exist in the optimized Ti-bearing steel, quite a lot of fine Ti(C,N) particles exist in the optimized carbon steel, but only a little bit fine Ti(C,N) particles exist in the carbon steel. The fishscaling resistance of the steels can be correlated to the effective hydrogen diffusion coefficient, and a model of correlation between the effective hydrogen diffusion coefficient and the volume fraction of the precipitates was established and verified. The effective hydrogen diffusion coefficient should be lower than 3. 96×10-6 cm2/s to avoid fishscaling in heavy plate steels.
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  Evolution of Surface Transversal Crack in Intermediate Slabs during Hot Rolling Deformation

  Yan-zhao LUO,Jiong-ming ZHANG,Qiang LIU,Mao-kang LI,Yi-tong HAN,Yi-fei CAO

  中国钢铁期刊网. 2014, 21(4): 476-481.

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  In the continuous casting of steels, surface transversal cracks are often found. These defects may severely influence the final quality of the products. The evolution of transversal V-shaped cracks with different depth on the surface of a continuously cast steel slab during hot rolling was studied. The artificial V-shaped cracks were made on the surface. The rolling process parameters from an industrial rolling mill have been used as a reference. The specimens of rolled workpiece in intermediate slabs were obtained after different rolling passes. The morphology of surface crack and microstructure evolution in the rolling process were investigated by optical microscopy. The results show that the depth of surface transversal crack gradually decreased with the increase of rolling passes. The grain size of ferrite and pearlite on the sample surface also gradually reduced. The microstructures around cracks with the different depth are almost identical, without direct correlation with the initial crack depth.