2014年, 第21卷, 第10期　刊出日期：2014-10-15

  

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Metallurgy and Metal Working
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  Thermogravimetric Analysis of Coal Char Combustion Kinetics

  Guang-wei WANG,Jian-liang ZHANG,Jiu-gang SHAO,Hui SUN,Hai-bin ZUO

  中国钢铁期刊网. 2014, 21(10): 897-904.

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  Four chars prepared from pulverized coals were subjected to non-isothermal and isothermal combustion tests in a thermogravimetric analysis (TGA) device. Three different test methods, i.e., non-isothermal single heating rate (A), non-isothermal multiple heating rate (B), and isothermal test (C), were conducted to calculate the kinetic parameters of combustion of coal char. The results show that the combustion characteristics of bituminous coal char is better than that of anthracite char, and both increase of heating rate and increase of combustion temperature can obviously improve combustion characteristics of coal char. Activation energies of coal char combustion calculated by different methods are different, with activation energies calculated by methods A, B and C in the range of 103.12-153.77, 93.87-119.26, and 46.48-76.68 kJ/mol, respectively. By using different methods, activation energy of anthracite char is always higher than that of bituminous coal char. In non-isothermal tests, with increase of combustion temperature, the combustion process changed from kinetic control to diffusion control. For isothermal combustion, the combustion process was kinetically controlled at temperature lower than 580 ℃ for bituminous coal char and at temperature lower than 630 ℃ for anthracite char.
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  Structure Model of Granules for Sintering Mixtures

  Li-shun YUAN,Xiao-hui FAN,Min GAN,Gui-ming YANG,Yi WANG

  中国钢铁期刊网. 2014, 21(10): 905-909.

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  Structure model of granules, boundary value of nucleus and powder, and the relationship between granulation efficiency and boundary value were investigated. Granules of sintering mixtures are composed of adhesive powder and nucleus. In the mixtures, particles larger than 1.00 mm act as nucleus and particles smaller than 0.25 mm act as adhesive powder. Particles with size between 0.25-1.00 mm can be adhesive powder as well as nucleus depending on the granulation conditions. When the boundary value is close to 0.25 mm, the granulation efficiency is lower than 50%. When the boundary value is close to 1.00 mm, the granulation efficiency is above 90%. The boundary value is influenced by the iron ore type, granulation moisture, fineness of raw materials and burnt-lime activity. Good adhesive capability, suitable moisture content, appropriate particle size distribution and high burnt-lime activity make the boundary value move towards 1.00 mm and improve the granulation efficiency.
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  Experimental Study and Thermodynamic Optimization of the FeO-V2O3 System

  Li ZENG,Yu WANG,Li-kai FAN,Bing XIE

  中国钢铁期刊网. 2014, 21(10): 910-914.

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  Optimization of the phase diagram of FeO-V2O3 system is a part of an on-going research project to develop a self-consistent multi-component thermodynamic database for vanadium slag from hot metal. Due to the lack of experimental data for optimization, a novel experimental investigation has been carried out by thermal analysis (DSC) with a series of slags on different V2O3 contents (i.e. 3mass%-12mass%). All available thermodynamic and phase diagram data for the binary systems have been simultaneously optimized with CALPHAD (Calculation of Phase Diagrams) methods to give one set of model equations for the Gibbs free energy of the liquid slag as functions of composition and temperature. The modified quasi-chemical model was used to describe the binary slag system. It was demonstrated that the calculated phase diagram with the optimized parameters was in good agreement with the experimental data.
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  Effect of Actual Cooling Rate of Ladle Stream on Persistent Metallurgical Performance of a Given Tundish

  Hai-bo SUN,Jia-quan ZHANG

  中国钢铁期刊网. 2014, 21(10): 915-922.

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  To evaluate the effect of actual cooling rate of liquid steel in the ladle on the metallurgical performances of a tundish, a transient and coupled computational model was developed to reveal the flow fields, temperature fields, residence time distribution of the molten steel and the inclusion removal efficiency in a typical single-strand tundish with given geometry and process parameters. The results showed that, with the decrease of the ladle stream cooling rate, the temperature difference of bulk flow at the outlet of tundish over a normal casting period decreased from 11.3 to 2.6 K, and the dead volume fraction of the tundish decreased from 17.58% to 14.35%, while the inclusion removal efficiency was increased especially for the inclusions with the diameter less than 50 μm, whose removal ratio could be increased by 20.62%. When the cooling rate was less than 0.3 K·min-1, however, the variation rates of the three evaluation criterions above declined significantly, which suggested that a critical value existed for the effect of the cooling rate of ladle stream on the tundish performances. The establishment of the critical ladle stream cooling rate should be very important to achieve persistent metallurgical properties of tundish over the whole casting stage, together with the reasonable ladle insulation design.
Material
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  Ductile Fracture Prediction of 316LN Stainless Steel in Hot Deformation Process

  Jian-li HE,,Juan LIU,Zhen-shan CUI,Chao-zhong YANG,Fei CHEN

  中国钢铁期刊网. 2014, 21(10): 923-930.

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  A ductile fracture criterion of 316LN stainless steel, combined with the plastic deformation capacity of material and the stress state dependent damages, was proposed to predict ductile fracture during hot deformation. To the end, tensile tests at high temperatures were first performed to investigate the fracture behavior of 316LN stainless steel. The experimental results show the variation of the critical fracture strain as a function of temperature and strain rate. Second, the criterion was calibrated by using the upsetting tests and the corresponding numerical simulations. Finally, the proposed fracture criterion was validated by the designed tests and the corresponding finite element (FE) simulation. The results show that the criterion can successfully predict the onset of ductile fracture at elevated temperatures.
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  Interface-correlated Characteristics of Stainless Steel/Carbon Steel Plate Fabricated by AAWIV and Hot Rolling

  Qing-xue HUANG,Xiao-rong YANG,Li-feng MA,,Cun-long ZHOU,Guang-ming LIU,Hai-bing LI

  中国钢铁期刊网. 2014, 21(10): 931-937.

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  Stainless steel (SS)/carbon steel (CS) clad plates were generated by means of the all-around weld of interface and vacuuming (AAWIV) followed by hot rolling, wherein AAWIV was utilized for controlling the interface oxidation during hot rolling. The structure near the interface was analyzed by optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The mechanical properties of SS/CS clad plates were investigated by tensile and shear as well as bend tests. The SS/CS interface is relatively flat and no visible separation appears. Line scanning analysis shows that diffusion of Cr and Ni from SS to CS and C from CS to SS occurred during bonding. Higher dislocation densities are observed in both layers of parent plates adhering to the interface. The SS/CS clad plates reveal higher yield, tensile and shear strengths. Both macroscopic delamination at the interface and fracture of base CS as well as intergranular fracture appearance of flyer plate are observed in the tensile test. The shear specimen fails in a ductile manner and the bend specimen tested shows no visible crack at the interface. Taking both interfacial structure and mechanical properties into account, the SS/CS clad plates exhibit sound bonding by the process of AAWIV and hot rolling.
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  Shear Fracture of Advanced High Strength Steels

  Sheng HUANG,Yi-xi ZHAO,Chun-feng HE

  中国钢铁期刊网. 2014, 21(10): 938-944.

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  Failure experiments were carried out through a stretch-bending test system for advanced high strength steels, i.e. dual-phase (DP) steels and martensitic steels (MS). The die radius in this system was designed from 1 to 15 mm to investigate the failure mode under different geometries. Two failure modes were observed during the experiments. As a result, critical relative radii (the ratio of inner bending radius R to sheet thickness t) for DP590 and DP780 steels were obtained. The stretch-bending tests of DP980 display some trends unlike DP590 and DP780 steels, and curve of DP980 in different thicknesses does not coincide well. High blank holder force exhibits more possibility of shear fracture tendency than low blank holder force. The unique character of high strength martensitic steel (1500MS) is that no shear fracture is found especially over small bending radius (R=2 mm) under the same experimental conditions. Microstructure analysis indicates that there are obviously elongated grains on shear fracture surface. It shows smaller diameter and shallower depth of the dimples than the necking failure.
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  Characteristics of Precipitation in an As-cast Nb- and V-containing High Nitrogen CrMn Austenitic Steel

  Zhong-ping SHI,,Zhen-hua WANG,,Shu-hua SUN,Ze-an ZHOU,Ying-ping GUAN,Wan-tang FU

  中国钢铁期刊网. 2014, 21(10): 945-950.

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  The phase diagram of an Nb- and V-containing 18Mn18Cr0.5N steel was investigated using thermodynamic calculation. The compositions and characteristics of precipitates were investigated by scanning electron microscopy, energy dispersive spectrometry, X-ray diffraction, and differential scanning calorimetry. Under equilibrium conditions, MX precipitates at 1350 ℃. MX is a substitutional solid solution of Nb4N3.38, and its composition (at.%) is approximately: Nb 40, N 40, V 8, and Fe, Cr, and Mn balance. The precipitates, with shapes including irregular bulk, rod, and ellipsoid, are primarily distributed at grain boundaries and triple junctions, and their size is approximately 5-10 μm. Finally, the microalloying of high nitrogen CrMn austenite steels, control of precipitate size, and the effect of precipitate on hot forming were discussed.
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  Hydrogen Absorption and Desorption during Heat Treatment of AISI 4140 Steel

  Ming-da ZHANG,,Mao-qiu WANG,Han DONG

  中国钢铁期刊网. 2014, 21(10): 951-955.

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  Hydrogen plays an important role in the formation of quench cracks of structural steels. To clarify hydrogen absorption and desorption during heat treatment of AISI 4140 steel, thermal desorption spectrometry (TDS) analysis was carried out for the specimens in the as-rolled, as-quenched, and quenched and tempered conditions. Results show that hydrogen content increased from 0.127×10-6 in the as-rolled specimen to 0.316×10-6 in the as-oil-quenched specimen. After tempering at 200 ℃, the hydrogen content in the oil-quenched specimen decreased to 0.155×10-6, and the peak temperature of hydrogen desorption increased from 200 to 360 ℃. From the dependence of hydrogen content in the as-quenched specimens on austenitizing time, it can be deduced that hydrogen absorption occurs during austenitizing. The simulation of hydrogen absorption contributes to a better understanding on the distribution of hydrogen during the heat treatment in structural steels.
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  High Temperature Oxidation Resistance of 9Ni Steel

  Yi-zhi LIU,Cai-fu YANG,Feng CHAI,Tao PAN,Hang SU

  中国钢铁期刊网. 2014, 21(10): 956-963.

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  Oxidation rates and scale/steel interface configuration of 9Ni steels were investigated at 1000-1250 ℃ in air. The results revealed that Cu addition caused high temperature oxidation resistance to deteriorate. High temperature oxidation rates increased and scale/steel interface configuration became complicated due to Cu addition. Scale/steel interface appeared to be network above certain temperature. Temperature required to form network scale/steel interface dropped more than 100 ℃ for 1.5% Cu-containing steel. (Fe,Ni,Cu)xOy in inner oxidation layer dissociated to Fe-Ni-Cu phase and released active oxygen which can react with base steel easily. So the inner oxidation layer became the second source of oxidizing agent besides atmosphere. Internal stress at austenite grain boundary caused local oxide to fragment. So the scale/steel interface appeared to be network. Liquid Si-rich phase formed at sufficiently high temperature. Penetration of the liquid Si-rich phase along austenite grain enhanced austenite grain oxidizing.
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  Growth Kinetics of Proeutectoid Ferrite in an Fe-0.09C-1.5Mn-0.2Si Steel

  Lin CHENG,Kai-ming WU,Xiang-liang WAN,Guo-hong ZHANG

  中国钢铁期刊网. 2014, 21(10): 964-968.

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  Growth kinetics of proeutectoid ferrite, including grain boundary face nucleated ferrite, grain boundary edge nucleated ferrite allotriomorph and intragranular ferrite idiomorph, were experimentally measured in an Fe-0.09C-1.5Mn-0.2Si steel and compared with theoretical calculation in local equilibrium and paraequilibrium modes. Grain boundary edge nucleated ferrite exhibited larger growth rate than grain boundary face nucleated ferrite and intragranular ferrite idiomorph. Experimental kinetics of proeutectoid ferrite was within the window defined by the local equilibrium and paraequilibrium limits. A transition of growth kinetics from paraequilibrium to local equilibrium was observed in the temperature range of 650-750 ℃, which can be explained in terms of solute drag.
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  Wear Resistance and Bond Strength of Plasma Sprayed Fe/Mo Amorphous Coatings

  Chao-ping JIANG,,Ya-zhe XING,Feng-ying ZHANG,Jian-min HAO,Xu-ding SONG

  中国钢铁期刊网. 2014, 21(10): 969-974.

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  Fe-based and Fe/Mo composite amorphous coatings were deposited on the surface of plain carbon steel substrates by atmospheric plasma spraying (APS). With increasing the Mo alloy content, the microstructure of the coatings revealed more dense structure. The porosities of composite coating were all less than those of Fe-based coating due to Mo alloy self-bonding performance. The ML-10 friction and wear tester was employed to investigate the wear behaviors of the coatings under dry sliding conditions. It was found that the mass loss of the resultant coatings decreased with increasing Mo-based powders into the feedstock. This was attributed to the reduction of the delaminations resulting from improved intersplat bond with Mo addition.
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  Optimization of Hot Workability in Superaustenitic Stainless Steel 654SMO

  En-xiang PU,,Wen-jie ZHENG,Zhi-gang SONG,Jin-zhong XIANG,Xian-ping WEI

  中国钢铁期刊网. 2014, 21(10): 975-982.

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  Hot compression tests were conducted on a Gleeble-3800 machine in a temperature range of 950 to 1200 ℃ and a strain rate range of 0.001 to 10 s-1 in order to study the hot deformation behaviour of superaustenitic stainless steel 654SMO. The results show that peak stress increases with decreasing temperature and increasing strain rate, and the apparent activation energy of this alloy was determined to be about 494 kJ/mol. The constitutive equation which can be used to relate the peak stress to the absolute temperature and strain rate was obtained. The processing maps for hot working developed on the basis of flow stress data and the dynamic materials model were adopted to optimize the hot workability. It is found that the features of the maps obtained in the strain range of 0.2 to 1.0 are fundamentally similar, indicating that the strain does not have a substantial influence on processing map. The combination of processing map and microstructural observations indicates that the favorable hot deformation conditions are located in two domains of processing map. The first domain occurs in the temperature range of 980 to 1035 ℃ and strain rate range of 0.001 to 0.01 s-1 with a peak efficiency of 55%. The second domain appears in the temperature range of 1120 to 1180 ℃ and strain rate range of 0.3 to 3 s-1 with peak efficiency of 35%. Compared to other stable domains, the specimens deformed in these two domains exhibit full dynamic recrystallization grains with finer and more uniform sizes. An instability domain occurs at temperatures below 1100 ℃ and strain rate above 0.1 s-1, and flow instability is manifested in the form of flow localization.