2016年, 第23卷, 第2期　刊出日期：2016-02-15

  

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Metallurgy and Metal Working
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  Analysis of Non-uniform Mechanical Behavior for a Continuous Casting Mold Based on Heat Flux from Inverse Problem

  Feng-ming DU,Xu-dong WANG,Yu LIU,Tian-yi LI,Man YAO

  中国钢铁期刊网. 2016, 23(2): 83-91.

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  The distortion of mold plates plays an important role in the formation of surface cracks on continuously cast steel products. To investigate the non-uniform distortion of a mold, a full-scale stress model of the mold was developed. An inverse algorithm was applied to calculate the heat flux using the temperatures measured by the thermocouples buried inside the mold plates. Based on this, a full-scale, finite-element stress model, including four copper plates, a nickel layer and water slots in different depths, was built to determine the complex mechanical behavior of the continuous casting mold used to produce steel slabs. The heat flux calculated by the inverse algorithm was applied to the stress model to analyze the non-uniform mechanical behavior. The results showed that the stress and distortion distributions of the four copper plates were not symmetrical, which reflected the non-uniform distortion behaviors of copper plates, water slots, nickel layer and the corner region of the mold. The gap between the mold and the slab was increased because of the corner distortion, which was very important for the heat transfer of initial solidifying shell, and it may be a major reason for the slow cooling of the slab corner.
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  Flow Characteristics and Inclusion Removal in a Ten-strand Continuous Casting Tundish: Physical Modelling and Industrial Trials

  Shu-guo ZHENG,Miao-yong ZHU,Ye-lian ZHOU,Wang SU

  中国钢铁期刊网. 2016, 23(2): 92-97.

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  The flow characteristics and inclusion removal in a ten-strand continuous casting tundish were investigated with physical modelling and industrial trials. The results show that, among the strands, the strand with the minimum dimensionless time of the first appearance of tracer at the tundish exit appears to be the worst one for inclusion removal, while the strand with the maximum dimensionless mean residence time shows the best inclusion removal efficiency. The inclusion number decreases with increasing inclusion size for all strands. The inclusion number distribution among strands is the same for all inclusion sizes and the descending order of inclusion number is basically consistent with the ascending order of dimensionless mean residence time among individual strands. However, when the strand with the minimum dimensionless time of the first appearance of tracer at the tundish exit is not the same one with the minimum dimensionless mean residence time, the former seems to be inferior to the latter for inclusion removal.
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  Thermodynamic Analysis and Experimental Study on Reaction of CO2 Gas with Hot Metal

  Guo WEI,Zhi-tao LI,Zi-liang LI,Qiang-jian GAO,Feng-man SHEN

  中国钢铁期刊网. 2016, 23(2): 98-102.

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  The reaction of CO2 gas with hot metal was investigated based on the thermodynamic analysis and experimental results. It shows that both silicon and carbon in hot metal can be oxidized by CO2 gas in the temperature range of 1300-1500 ℃. When using graphite crucible, temperature has little influence on final mass percent of carbon w［C］ because of the carburization effect. Decarburization degree rises significantly with increasing gas injection rate and w［C］ can be reduced to 32% at most when using MgO crucible. Lower temperature or higher gas injection rate is propitious to promote desilication reaction, but only 5%-10% of desilication ratio could be obtained in 20 min. The final mass percent of silicon w［Si］ when using MgO crucible is lower than that when using graphite crucible. Experimental results also demonstrate that CO2 injection has no effect on the concentration of manganese, sulfur and phosphorus in hot metal. In view of the weak oxidation ability and temperature drop of hot metal, CO2 gas is suggested to be used as carrier gas in desilication process rather than oxidizing agent.
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  Influence of Na2CO3 as Additive on Direct Reduction of Boron-bearing Magnetite Concentrate

  Yong-li LI,,Jing-kui QU,,Guang-ye WEI,,Tao QI,

  中国钢铁期刊网. 2016, 23(2): 103-108.

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  Boron-bearing magnetite concentrate is typically characterized by low grade of iron and boron (wTFe=51%-54%, wB2O3=6%-8%), as well as the close intergrowth of ascharite phase and magnetite phase. A promising technology was proposed to separate iron and boron by coupling the direct reduction of iron oxides and Na activation of boron minerals together. The influence of Na2CO3 as additive on the direct reduction of boron-bearing magnetite was studied by chemical analysis, kinetic analysis, XRD analysis and SEM analysis. The results showed that the addition of Na2CO3 not only activated boron minerals, but also reduced the activation energy of the reaction and promoted the reduction of iron oxides. Besides, the addition of Na2CO3 changed the composition and melting point of non-ferrous phase, and then promoted the growth and aggregation of iron grains, which was conducive to the subsequent magnetic separation. Thus, the coupling of the two processes is advantageous.
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  Application of Wood Char in Processing Oolitic High-phosphorus Hematite for Phosphorus Removal

  Hui-qing TANG,Yan-qi QIN,Teng-fei QI,Zhi-lei DONG,Qing-guo XUE

  中国钢铁期刊网. 2016, 23(2): 109-115.

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  Phosphorus removal from oolitic high-phosphorus hematite using direct reduction followed by melting separation was investigated. At the direct reduction stage, highly volatile wood char was prepared by carbonizing jujube wood at 673 K for 2 h and was used as reducing agent. The results of the direct reduction tests show that at a temperature of 1373 K, a char mixing ratio of 08, and a reduction time of 10-25 min, the briquettes reached a metallization degree of 80%-84% and a residual carbon content of 013-198 mass%. Phosphorus remained in the gangue as calcium phosphate after reduction. The results of the melting separation tests show that residual carbon in reduced briquette negatively affects the phosphorus content (w［P］) in hot metal. When the reduced briquettes obtained under the aforementioned conditions were used for melting separation, hot metal suitable for basic oxygen steelmaking (w［P］＜04 mass%) could not be obtained from metallic briquettes with a residual carbon content more than 10 mass%. In contrast, it could be obtained from metallic briquettes with residual carbon content less than 035 mass% by mixing with 2%-4% Na2CO3.
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  Fuzzy C-means Rule Generation for Fuzzy Entry Temperature Prediction in a Hot Strip Mill

  José Angel BARRIOS,César VILLANUEVA,Alberto CAVAZOS,Rafael COLS

  中国钢铁期刊网. 2016, 23(2): 116-123.

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  Variable estimation for finishing mill set-up in hot rolling is greatly affected by measurement uncertainties, variations in the incoming bar conditions and product changes. The fuzzy C-means algorithm was evaluated for rule-base generation for fuzzy and fuzzy grey-box temperature estimation. Experimental data were collected from a real-life mill and three different sets were randomly drawn. The first set was used for rule-generation, the second set was used for training those systems with learning capabilities, while the third one was used for validation. The performance of the developed systems was evaluated by five performance measures applied over the prediction error with the validation set and was compared with that of the empirical rule-base fuzzy systems and the physical model used in plant. The results show that the fuzzy C-means generated rule-bases improve temperature estimation; however, the best results are obtained when fuzzy C-means algorithm, grey-box modeling and learning functions are combined. Application of fuzzy C-means rule generation brings improvement on performance of up to 72%.
Material
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  Distribution Characteristics and Reinforcing Behavior of (Ti,Nb)C Reinforced Particle in the Coating Fabricated by Laser Rapid Cladding

  Qing-tang LI,Yong-ping LEI,Han-guang FU,Zhong-wei WU,Jian LIN

  中国钢铁期刊网. 2016, 23(2): 124-129.

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  (Ti,Nb)C reinforced Fe-based laser coatings were prepared with normal and high scanning velocities of the laser beam. The distribution characteristics of reinforced particles in the coatings were investigated. The mechanical properties of coatings were tested. The results showed that the morphologies of the microstructure and the reinforced particle changed dramatically at high solidification rate due to rapid laser processing compared with that prepared by normal processing. Two kinds of particles were observed in the coating. One was (Ti,Nb)C multiple carbide particle with the size of micron and sub-micron scales, in which a mass of dislocations were found. Another was nano-sized particle including α-Fe and (Ti,Nb)C obtained by rapid solidification. The microstructure of the coatings was highly refined and a large number of twin crystals were found in matrix. The results of mechanical properties test revealed that the wear resistance of the coating was improved by rapid laser processing, compared with that of the coating prepared with normal speeds. The above-mentioned conclusion indicated that rapid laser cladding can promote not only the processing efficiency but also the mechanical properties of the coating.
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  Work Hardening Behavior and Stability of Retained Austenite for Quenched and Partitioned Steels

  Cun-yu WANG,,Ying CHANG,Jie YANG,Wen-quan CAO,Han DONG,Yi-de WANG

  中国钢铁期刊网. 2016, 23(2): 130-137.

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  Both microstructure and mechanical properties of low alloy steels treated by quenching and partitioning (Q&P) process were examined. The mixed microstructure of martensite and large-fractioned retained austenite (about 273%) was characterized and analyzed, excellent combinations of total elongation of 19% and tensile strength of 1835 MPa were obtained, and three-stage work hardening behavior was demonstrated during tensile test. The enhanced mechanical properties and work hardening behavior were explained based on the transformation-induced plasticity effect of large-fractioned austenite.
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  Microstructure Characterization and Mechanical Properties of TRIP-aided Steel under Rapid Heating for Different Holding Time

  De-chao XU,Yan-dong LIU,Jun LI,,Qing-ge MENG,Pei LI

  中国钢铁期刊网. 2016, 23(2): 138-144.

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  In order to develop a comprehensive understanding about the effect of different holding time under rapid heating on the microstructural evolution and mechanical properties of transformation-induced plasticity (TRIP) steel, continuous annealing process simulations were performed using a thermal system with resistance heating method. The morphology and distribution of all phases present in the microstructure and the mechanical properties of TRIP steel were revealed. It appeared that the final tensile strength of the TRIP steel increased and retained austenite carbon content decreased with increasing holding time. An overlap between ferrite recrystallization and austenitization was observed during intercritical holding. In addition, the work hardening of the samples was evaluated by calculating the instantaneous n value as a function of the true strain. The difference in work hardening behavior corresponds to the rate of the retained austenite transformation during straining, which can be attributed to the carbon content and the morphology of the retained austenite.
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  Effect of Cyclic Annealing on Microstructure and Mechanical Properties of Medium Carbon Steel

  Zhi-qing L,Hui-fang ZHANG,Qing MENG,Zhen-hua WANG,Wan-tang FU

  中国钢铁期刊网. 2016, 23(2): 145-150.

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  The microstructure and mechanical properties of medium carbon steel after cyclic heat treatment were investigated. The effects of cyclic numbers and long time annealing on the microstructure and mechanical properties of the experimental steel were compared. A short-duration (5 min) holding at 1023 K (above A1 temperature) and a short-duration (3 min) holding at 893 K are adopted in each cyclic heat treatment. The spheroidization is accelerated during cyclic heat treatment, and the spheroidizing ratio grows with cyclic numbers. After 12-cycle heat treatments, there are few incompletely spheroidized regions in the specimens, and cementite lamellae mostly change into cementite particles. The morphological character of cementite for 12 cycles is similar to that undergoing annealing for 10 h at 973 K. The strength of the experimental steel after 5-cycle heat treatment is the lowest in the following cyclic heat treatment, but it is still higher than that of specimens with subcritical annealing over a long period (10 h). After 12-cycle heat treatment, the strength of the experimental steel is close to that of the normalized steel, and the plasticity is the best in all heat-treated specimens.
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  Metadynamic Recrystallization Behavior of As-cast 904L Superaustenitic Stainless Steel

  Wei ZHANG,Jing ZHANG,Ying HAN,,Rong LIU,De-ning ZOU,Guan-jun QIAO

  中国钢铁期刊网. 2016, 23(2): 151-159.

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  The metadynamic recrystallization (MDRX) behavior of as-cast 904L superaustenitic stainless steel was investigated by double pass isothermal compression tests at temperatures of 950-1150 ℃, strain rates of 005-5 s-1 and interval of 1-100 s. The effects of working parameters (deformation temperature, strain rate, pre-strain and interval time) on the flow curves and microstructural evolution were discussed. The MDRX fraction increased obviously with the increase of deformation temperature, strain rate and interval time. The MDRX softening was controlled by the migration of grain boundary, annihilation of dislocation and dynamic recrystallization. Moreover, the kinetic model was established for the prediction of MDRX behavior of as-cast 904L superaustenitic stainless steel based on the experimental data. A good agreement between the predicted and the experimental values was achieved (correlation coefficient R2=098), indicating a satisfactory accuracy.
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  Microstructure and Mechanical Properties of Semi-continuous Equal-channel Angular Extruded Interstitial-free Steel

  Bo YAN,,Si-hai JIAO,Dian-hua ZHANG

  中国钢铁期刊网. 2016, 23(2): 160-165.

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  An innovative method called semi-continuous equal-channel angular extrusion (SC-ECAE) has been developed to produce ultrafine grained steel by inducing severe plastic deformation. In contrast to the external forces that are exerted on specimens in traditional ECAE, the driving forces are applied on the dies in the novel SC-EACE process. Commercial interstitial-free steel sheets with width of 160 mm and thickness of 2 mm were processed repeatedly to various passes at room temperature using this method. The microstructural evolution was characterized using high-resolution electron backscatter diffraction (EBSD), and the mechanical properties were investigated by tensile testing. The EBSD images indicated that the fraction of high-angle boundaries (HABs) began to increase gradually after four passes; after six passes, elongated HAB structures with nearly submicron-scale average spacings were formed. The tensile testing results showed that strengthening was accompanied by a decrease in tensile ductility, but no significant anisotropy was observed. After 10 passes, a final HAB fraction of about 90% and an overall grain size of 055 μm, yield strength of 6387 MPa, an ultimate tensile strength (UTS) of 7103 MPa, and a total elongation of 120% were obtained.
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  Evaluation of Microstructure and Magnetic Properties in Non-oriented Electrical Steel Strained by Tension

  J. SALINAS B.,A. SALINAS R.

  中国钢铁期刊网. 2016, 23(2): 166-170.

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  The effects of deformation on the microstructure and magnetic properties of non-oriented electrical steels were investigated. Box-annealed electrical steel sheets were deformed by tension at four different strains: 3%, 8%, 12% and 25%. The internal grain misorientation caused by tensile deformation was measured by electron backscattering diffraction (EBSD) with grain orientation spread (GOS) as an indicator of the lattice distortion. The experimental results showed that the average GOS value increases with the strain. The microstructure and crystallographic texture of deformed samples did not show a significant change in samples strained below 25%. However, the magnetic properties were strongly affected: coercivity was directly proportional to the square root of the GOS value and energy losses increased as the strain level was increased.
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  Effects of Alloy Element and Microstructure on Corrosion Resistant Property of Deposited Metals of Weathering Steel

  Xiao-ming XIAO,Yun PENG,Cheng-yong MA,Zhi-ling TIAN

  中国钢铁期刊网. 2016, 23(2): 171-177.

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  Alloy element and microstructure are key factors that dominate mechanical and corrosion resistant properties of weathering steel. The effect of Mo on microstructure, mechanical properties and corrosion resistant property of deposited metal was investigated. Experimental results show that with the increase of Mo content in deposited metals, the phase transformation temperature decreases, and the ferrite zone in CCT diagram moves rightward, resulting in enlarged bainite zone and reduced ferrite and pearlite zone. The addition of 024 mass% Mo in deposited metal results in the increase of tensile strength, more M-A constituent and less high angle grain which reduce the low temperature toughness. It is found that Mo can raise the weathering resistance of deposited metal in industrial atmosphere. Analysis indicates that Mo may enrich in the inner rust layer, produce MoO3, enhance the formation of compact rust film and impede the anode dissolution reaction. Granular bainite in deposited metals displays better corrosion resistance than acicular ferrite during the initial corrosion stage, but its long-term influence on the corrosion resistance is limited.
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  Constitutive Modeling for Flow Behaviors of Superaustenitic Stainless Steel S32654 during Hot Deformation

  En-xiang PU,,Han FENG,Min LIU,Wen-jie ZHENG,Han DONG,Zhi-gang SONG

  中国钢铁期刊网. 2016, 23(2): 178-184.

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  Hot deformation behavior of superaustenitic stainless steel S32654 was investigated with hot compression tests at temperatures of 950-1250 ℃ and strain rates of 0001-10 s-1. Above 1150 ℃, with strain rate lower than 01 s-1, the flow curves exhibit nearly steady-state behavior, while at higher strain rate, continuous flow softening occurs. To provide a precise prediction of flow behavior for the alloy, the constitutive modeling considering effect of strain was derived on the basis of the obtained experimental data and constitutive relationship which incorporated Arrhenius term and hyperbolic-sine type equation. The material constants α, n, Q and lnA are found to be functions of the strain and can be fitted employing eighth-order polynomial. The developed constitutive model can be employed to describe the deformation behavior of superaustenitic stainless steel S32654.