2014年, 第21卷, 第12期　刊出日期：2014-12-15

  

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Metallurgy and Metal Working
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  Growth Rate, Microstructure and Phase Composition of Oxide Scales for Three Typical Steels in Simulated Continuous Casting Process

  Nan WANG,Jian-hong DONG,Wei-jun HUANG,Bo LI,Min CHEN

  中国钢铁期刊网. 2014, 21(12): 1065-1072.

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  Growth rate, microstructure and phase composition of scale layer formed during oxidation in 56%H2O-9%O2-N2 and following continuous cooling in ambient air were experimentally investigated by means of optical microscopy, scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS) and X-ray diffraction (XRD) for 45, 20CrMnTi and TTS443M steels respectively, to examine the effects of strand surface temperature and steel composition on the scale formation in simulated continuous casting process. The growth rates were found to be approximately parabolic and the calculated activation energy of TTS443M steel is much higher than those of the two other steels. For 45 and 20CrMnTi steels, the scale layers were looser and a distinct gap formed at the scale-substrate interface at higher strand surface temperature. The dominant phases within the scale were iron oxides except for FeO·Cr2O3 phase simultaneously existing in the oxide scale of 20CrMnTi steel. On the other hand, the scale layer formed on TTS443M steel was compact and tightly attached to the steel surface. At both lower and higher strand surface temperature, iron oxide was main phase in external layer of the scale, while chromia was dominant in inner layer with an appreciable Cr enrichment.
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  Effect of Magnesium on Evolution of Oxide and Sulphide in Liquid Iron at 1873 K

  Tong-sheng ZHANG,De-yong WANG,Mao-fa JIANG

  中国钢铁期刊网. 2014, 21(12): 1073-1080.

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  To reveal the effects of magnesium on the evolution of oxide and sulphide inclusions in liquid iron, both thermodynamic calculations and deoxidization experiments were carried out. The samples extracted from the liquid iron were polished and analyzed by optical microscopy and scanning electron microscopy. The results showed that magnesium could modify oxide and sulphide inclusions simultaneously. Spherical MgO and irregular spinel inclusions were observed in the samples. The elongated MnS inclusions were replaced by small MgO·MgS or MgO·MgS·MnS complex inclusions, and the sulphides were distributed dispersively. The evolution mechanisms of inclusions were discussed comprehensively, and a proposed model for the formation of oxysulphide was set up.
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  Vortex Flow Pattern in a Slab Continuous Casting Mold with Argon Gas Injection

  Zhong-qiu LIU,Feng-sheng QI,Bao-kuan LI,Mao-fa JIANG

  中国钢铁期刊网. 2014, 21(12): 1081-1089.

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  An Eulerian-Eulerian two-fluid model was developed to study the vortex flow inside a slab continuous casting mold with argon gas injection. Interfacial momentum transfer that accommodated various interfacial forces including drag force, lift force, virtual mass force, and turbulent dispersion force was considered. Predicted results agree well with experimental measurements of the water model in two-phase flow pattern and vortex flow structures. Three typical flow patterns with different argon steel ratios (ASRs) have been obtained: “double roll”, “three roll”, and “single roll”. The flow pattern inside the mold alternates among the three types or it may attain some intermediate condition. With increasing ASR, the positions of vortices move from the submerged entry nozzle to the narrow face of the mold, and the sizes of vortices are reduced gradually. The rotating directions of vortices are all from high velocity area to low velocity area. Two mechanisms of vortex formation on the top surface have been suggested, i. e. , congruous shear flow and incongruous shear flow.
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  Mass Loss and Direct Reduction Characteristics of Iron Ore-coal Composite Pellets

  Yi MAN,Jun-xiao FENG,Yan-mei CHEN,Jing-zhi ZHOU

  中国钢铁期刊网. 2014, 21(12): 1090-1094.

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  Mass loss and direct reduction characteristics of iron ore-coal composite pellets under different technological parameters were investigated. Meanwhile, changes of iron phase at different temperatures were analyzed by using X-ray diffraction (XRD), and characteristics of crushed products were studied by using a scanning electron microscope (SEM). The results showed that heating rate had little influence on the reduction, but the temperature played an important role in the reduction process. The mass loss rate increased rapidly from 800 to 1100 ℃. The reduction process can be divided into three steps which correspond to different temperature ranges. Fe2O3 began to transform into Fe3O4 below 500 ℃, and FeO was reduced into Fe from 900 ℃. At 900 ℃, the reduction product showed a clear porous structure, which promoted the reduction progress. At 1000 ℃, the metallic Fe dominated the sample, and the reduction reached a very high degree.
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  Effect of Electromagnetic Stirring on Molten Steel Flow and Solidification in Bloom Mold

  Zhen-guo YANG,,Bao WANG,,Xiao-feng ZHANG,Ya-tao WANG,Hong-biao DONG,Qing LIU

  中国钢铁期刊网. 2014, 21(12): 1095-1103.

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  The effect of electromagnetic stirring on molten steel flow and heat transfer in a 260 mm×300 mm bloom mold was investigated by using a method combining both finite element method and finite volume method. The simulation results related to magnetic fields were consistent with the onsite measured data. The magnetic flux density increased with increasing the current intensity but decreased with increasing the current frequency. Electromagnetic stirring caused molten steel to flow with rotation on a horizontal section and two sets of recirculation regions with opposite reflow directions in a longitudinal section formed. The maximum tangential velocity increased with increasing the current intensity and frequency. Furthermore, the superheat degree of the molten steel on the outlet cross section of the mold decreased with increasing the current intensity. Electromagnetic stirring caused the emergence of a zero growth zone of solidified shell in the effective stirring zone. For the 260 mm×300 mm bloom continuous caster of bearing steel, the appropriate values of current intensity and current frequency of electromagnetic stirring were found to be 300 A and 3 Hz, respectively.
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  3D Thermo-mechanical Coupled Simulation of Whole Rolling Process for 60 kg/m Heavy Rail

  Nuan-nuan PEI,Guo-ming ZHU,Bo LI,Gong-ming TAO,Yong-lin KANG

  中国钢铁期刊网. 2014, 21(12): 1104-1110.

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  3D thermo-mechanical coupled simulation of whole rolling process for 60 kg/m heavy rail was accomplished by FEM method. The finite element model, physical parameters of U75V and parameter setting of simulation were introduced in detail. The whole rolling process of 60 kg/m heavy rail was divided into 27 time cells to simulate respectively, and the model rebuilding and temperature inheritance method in intermediate pass were proceeded. Then, based on simulation results, the workpiece deformation result, metal flow, stress and strain of 60 kg/m heavy rail for typical passes were obtained. The temperature variation curves of whole rolling process for section key points of 60 kg/m heavy rail were plotted, and the temperature falling law of whole rolling process for 60 kg/m heavy rail was studied. In addition, temperature distribution of 60 kg/m heavy rail after whole rolling process was analyzed, and the results showed that temperature was highest at center of rail head and lowest at fringe of rail base. Moreover, the simulation results and measured results of rolling force for 60 kg/m heavy rail were compared, and the regularity was in good agreement.
Material
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  Effect of Strengthening Phase on Deformation Behaviour during Uniaxial Tension of Hot-rolled Dual Phase Steel

  Le-yu ZHOU,Bo JIANG,Tian-hao CUI,Dan ZHANG,Jian-zhong HE,Ya-zheng LIU

  中国钢铁期刊网. 2014, 21(12): 1111-1115.

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  Two kinds of C-Si-Mn-Cr series tested steels were designed to obtain dual phase microstructures of ferrite (F)+martensite (M) or ferrite (F)+bainite (B) with different mechanical properties. Effects of strengthening phase on yielding and fracture behaviours during uniaxial tension of dual phase steel were discussed. Compared with hot-rolled martensite dual phase steel, ferrite-bainite dual phase steel has high ratio of yield strength to tensile strength (YS/TS) and low elongation. During necking process of uniaxial tension, microvoids of ferrite-martensite steel are generated by fracture of ferrite/martensite boundary or martensite islands with irregular shape. But ferrite matrix elongated remarkably along deformation direction, and strengthening phase also coordinated with ferrite matrix. Compatible deformation between ferrite and bainite is distinct. Ferrite-bainite dual phase steel has fine and less microvoid, and phase boundary of ferrite and bainite is beneficial for restraining generation and extending of microvoid.
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  Flow Stress Behaviors and Microstructure Evolution of 300M High Strength Steel under Isothermal Compression

  Rong-sheng QI,Bao-feng GUO,Xin-gang LIU,Miao JIN

  中国钢铁期刊网. 2014, 21(12): 1116-1123.

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  The compressive deformation behaviors of 300M high strength steel were investigated over a wide range of temperatures (850-1200 ℃) and strain rates (0. 001-10 s-1) on a Gleeble-3800 thermo-mechanical simulator. The measured flow stress was modified by the corrections of the friction and the temperature compensations, which nicely reflect negative effects of the friction and temperature on the flow stress. The corrected stress-strain curves were the dynamic recrystallization type on the conditions of higher deformation temperature and lower strain rate. Flow stress increases with the increase of strain rate at the same deformation temperature and strain. By contrast, flow stress decreases with the increase of temperature at the same strain rate and strain. Dependence of the peak stress on temperature and strain rate for 300M steel is described by means of the conventional hyperbolic sine equation. By regression analysis, the activation energy (Q) in the whole range of deformation temperature is determined to be 367. 562 kJ/mol. The effects of the temperature and the strain rate on microstructural evolution are obvious. With the increase of the deformation temperature and the decrease of the strain rate, the original austenite grain sizes of 300M steel increase. At the same time, the corrected flow stress curves more accurately determine the evolution of the microstructure.
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  Factors Influencing Dual Phase Steel Flanging Limit Punching

  Jian-qing QIAN, Yuan YUE

  中国钢铁期刊网. 2014, 21(12): 1124-1128.

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  Based on pre-punching to hot rolled martensitic and bainitic dual phase steels with similar elongation and work hardening index (n), as well as similar thickness and hole expansion test using taper die, analysis of the hole expansion rate indicated significant difference in their flanging performance. The results indicated that mechanical property was not the only major factor that influenced flanging. Further analysis of the hole border microstructure, work hardening condition, and fracture property showed that the fracture property of sheets was the major factor that influenced flanging performance.
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  Effects of Hot Bending Parameters on Microstructure and Mechanical Properties of Weld Metal for X80 Hot Bends

  Xu WANG,,Bo LIAO,Da-yong WU,Xiu-lin HAN,Yuan-sheng ZHANG,Fu-ren XIAO

  中国钢铁期刊网. 2014, 21(12): 1129-1135.

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  Mechanical properties of weld metal are the key factors affecting the quality of heavy-wall X80 hot induction bends. The effects of bending parameters on the mechanical properties of weld metal for hot bends were investigated by simulation conducted on a Gleeble-3500 thermal simulator. Continuous cooling transformation (CCT) diagrams of the weld metal were also constructed. The influences of hot bending parameters (such as reheating temperature, cooling rate, and tempering temperature) on the microstructure and mechanical properties of weld metal were also analyzed. Results show that the strength of all weld metal specimens is higher than the value indicated in the technical specification and increases with the increase of reheating temperature, cooling rate, and tempering temperature. The impact toughness is apparently related to the variation of reheating temperature, cooling rate, and tempering temperature.
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  Non-equilibrium Grain-boundary Segregation of Phosphorus in an Fe-17Cr Alloy

  Kai WANG,,Hong SI

  中国钢铁期刊网. 2014, 21(12): 1136-1139.

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  In order to confirm the segregation characteristic of phosphorus in an Fe-17Cr alloy at grain boundary, the phosphorus segregation was obtained by Auger electron spectroscopy. The results show that a maximum phosphorus segregation appeared at 450 ℃ for all specimens aged for 30 min at 350, 400, 450, 500, and 600 ℃ after being solution-treated at 1000 ℃ for 1 h. These results were analyzed by the characteristics of segregation peak temperature of non-equilibrium segregation.
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  Precipitation Strengthening by Nanometer-sized Carbides in Hot-rolled Ferritic Steels

  Xiao-pei WANG,,Ai-min ZHAO,Zheng-zhi ZHAO,Yao HUANG,Zhi-da GENG,Yang YU

  中国钢铁期刊网. 2014, 21(12): 1140-1146.

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  The mechanical properties of the hot-rolled plates of Ti steel and Ti-Mo steel after isothermal transformation in a temperature range of 600-700 ℃ for 60 min have been tested, and the microstructures of the matrix and the characteristics of precipitated nanometer-sized carbides have also been examined by scanning electron microscopy and transmission electron microscopy. The precipitation regularity of nanometer-sized carbides has been studied by thermodynamic method and the contributions of corresponding strengthening mechanisms to the total yield strength have been calculated. The tensile strength of hot-rolled Ti-Mo ferritic steel can achieve 780 MPa with an elongation of 20. 0% after being isothermally treated at 600 ℃ for 60 min, and the tensile strength of Ti steel is 605 MPa with an elongation of 22. 7%, according to the results of tensile tests. The critical nucleation size of (Ti,Mo)C is smaller than that of TiC at a given isothermal temperature, but the nucleation rate of (Ti,Mo)C is larger than that of TiC. The grain-refinement strengthening and precipitation strengthening contribute the main amount of the total yield strength. The major increase in yield strength with the decrease of isothermal temperature results from the contribution of precipitation strengthening. The contribution of precipitation strengthening to the yield strength of the steels has been estimated. The ferrite phase can be strengthened by about 400 MPa through precipitation strengthening in Ti-Mo steel isothermally treated at 600 ℃ for 60 min, which is about 200 MPa higher than that of Ti steel under the same conditions.
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  Metallurgical Studies of Austenitic Stainless Steel 304 under Warm Deep Drawing

  Jayahari Lade,Balu Naik Banoth,Amit Kumar Gupta,Swadesh Kumar Singh

  中国钢铁期刊网. 2014, 21(12): 1147-1151.

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  Austenitic stainless steel 304 was deep drawn with different blank diameters under warm conditions using 20 t hydraulic press. A number of deep drawing experiments both at room temperature and at 150 ℃ were conducted to study the metallography. Also, tensile test experiments were conducted on a universal testing machine up to 700 ℃ and the broken specimens were used to study the fractography of the material using scanning electron microscopy in various regions. The microstructure changes were observed at limiting draw ratio (LDR) when the cup is drawn at different temperatures. In austenitic stainless steel, martensite formation takes place that is not only affected by temperature, but also influenced by the rate at which the material is deformed. In austenitic stainless steel 304, dynamic strain regime appears above 300 ℃ and it decreases the formability of material due to brittle fracture as studied in its fractography. From the metallographic studies, the maximum LDR of the material is observed at 150 ℃ before dynamic strain regime. It is also observed that at 150 ℃, grains are coarse in the drawn cups at LDR.
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  Microstructure and Properties of an Advanced Nickel-base PM Superalloy

  Chao-jie WU,Yu TAO,Jian JIA

  中国钢铁期刊网. 2014, 21(12): 1152-1157.

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  The need for nickel-base powder metallurgy (PM) superalloy turbine discs is becoming increasingly evident. With the eventual aim of improving thrust-to-weight ratio of aeroengines for power generation, well integration of significantly high strength, high damage tolerance and high-temperature capability would be reasonably required. An advanced PM superalloy, which was designed for applications up to 815-850 ℃, was experimentally investigated. Emphasis was primarily put on microstructure and mechanical properties. The results indicated the measured phases in the sample were composed of γ, γ′, MC, and M3B2. With uniform coarse grain microstructure (ASTM 5-6), the sample appeared to exhibit overwhelming superiority over the prior art materials FGH95, FGH96, FGH97 and FGH98. The dominant embodiments consisted of high tensile strength (Rm=1000 MPa and Rp0. 2=800 MPa at 850 ℃), strong creep resistance (ξp=0. 12% at 815 ℃/400 MPa/50 h), and considerable stress-rupture life (τ=457. 4 h at 815 ℃/450 MPa). The technical practicability of applications up to 815-850 ℃ of this alloy was conclusively proved.