2018年, 第25卷, 第7期　刊出日期：2018-07-15

  

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  Modeling on impact zone volume generated by coherent supersonic jet and conventional supersonic jet

  Guang-sheng Wei, . Rong Zhu, . Ling-zhi Yang . Kai Dong, . Run-zao Liu

  . 2018, 25(7): 681-691.

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  The supersonic oxygen supply technology, including the coherent supersonic jet and the conventional supersonic jet, is now widely adopted in electric arc furnace steelmaking process to increase the bath stirring, reaction rates and energy efficiency. However, there has been limited study on the impact characteristics of the coherent supersonic jet and the conventional supersonic jet. Thus, integrating theoretical models and numerical simulations, an optimized theoretical model was developed to calculate the volume of the impact zone generated by coherent and conventional supersonic jets. The optimized theoretical model was validated by water model experiments. The results show that the jet impact zone volume with coherent supersonic jet is much larger than that with conventional supersonic jet at the same lance height. The kd value, a newly defined variable that is the product of the dimensionless quantity of velocity and free distance, reflects the velocity attenuation and the potential core length of the main supersonic jet, which is a key parameter of the optimized theoretical model. The optimized theoretical model can well predict the jet impact zone volumes of coherent and conventional supersonic jets with the error no more than 3.62 and 9.37%, respectively.
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  A comparison study of existence forms of Fe species in coke for solution loss reaction of carbon

  Qiang Ling . Hui-juan Zhao . Ling-yu Chen . Rui-lun Xie . Zhao Lei . Zhi-gang Zhao . Ping Cui

  . 2018, 25(7): 692-699.

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  Fe species were loaded by two different loading ways (absorption method and addition method) to investigate their effect on thermal properties of coke. The particulate coke reactivity of coke samples indicated that the added sample showed higher catalytic activity than the adsorbed sample at first, owing to the decreased structure and properties of coke and more catalytic active sites caused by the strong interaction between Fe species and coke. The presence of Fe species in the added sample weakened the microstructure of coke, and the Fe species were easier to be reduced than those in the absorbed sample due to its different existence form in coke. With further increased loading of Fe species, the different existence positions of Fe species caused more decrease in surface active sites in the added sample than in the adsorbed sample, leading to lower catalytic activity of added sample when the total iron content exceeded 1%. The catalytic mechanism implied that there may be a catalytic dominant factor change in the reaction between the catalytic effect of iron species and carbon surface active sites in coke; the catalytic effect of iron species is dominant in the reaction at first, but the catalytic effect of carbon surface active sites is dominant in the reaction with the further increased loading amount of Fe species.
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  Optimal design of hot rolling process for C-Mn steel by combining industrial data-driven model and multi-objective optimization algorithm

  Si-wei Wu . Xiao-guang Zhou . Jia-kuang Ren . Guang-ming Cao . Zhen-yu Liu . Nai-an Shi

  . 2018, 25(7): 700-705.

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  Combining with the multi-objective optimization algorithm, the optimal design of hot rolling process for hot rolled strips is carried out to solve the problem of time-consuming and material resources-wasting experimental work. A successful mechanical property prediction model is the core of the optimal design of hot rolling process. However, the original industrial data, usually unbalanced, are inevitably mixed with fluctuant and abnormal values. Models established on the basis of the data without data processing can cause misleading results, which cannot be used for the optimal design of hot rolling process. In current work, a method of industrial data processing of C-Mn steel was proposed based on the data analysis. The Bayesian neural network was employed to establish the reliable mechanical property prediction models for the optimal design of hot rolling process. By using the multi-objective optimization algorithm, considering the personal requirements of costumers and the constraints of the equipment, the optimal design of hot rolling process was successfully applied to rolling process design for Q345B steel with 0.017 % Nb and 0.046 % Ti content removed. The optimal process design results were in good agreement with the industrial trials results, which verify the effectiveness of the optimal design of hot rolling process.
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  Multi-class classification method for strip steel surface defects based on support vector machine with adjustable hyper-sphere

  Mao-xiang Chu . Xiao-ping Liu . Rong-fen Gong . Jie Zhao

  . 2018, 25(7): 706-716.

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  Focusing on strip steel surface defects classification, a novel support vector machine with adjustable hyper-sphere (AHSVM) is formulated. Meanwhile, a new multi-class classification method is proposed. Originated from support vector data description, AHSVM adopts hyper-sphere to solve classification problem. AHSVM can obey two principles: the margin maximization and inner-class dispersion minimization. Moreover, the hyper-sphere of AHSVM is adjustable, which makes the final classification hyper-sphere optimal for training dataset. On the other hand, AHSVM is combined with binary tree to solve multi-class classification for steel surface defects. A scheme of samples pruning in mapped feature space is provided, which can reduce the number of training samples under the premise of classification accuracy, resulting in the improvements of classification speed. Finally, some testing experiments are done for eight types of strip steel surface defects. Experimental results show that multi-class AHSVM classifier exhibits satisfactory results in classification accuracy and efficiency.
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  Prediction of alloy composition and microhardness by random forest in maraging stainless steels based on a cluster formula

  Zhen Li . Dong-hui Wen . Yue Ma . Qing Wang . Guo-qing Chen . Rui-qian Zhang . Rui Tang . Huan He

  . 2018, 25(7): 717-723.

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  Fe–Ni–Cr-based super-high-strength maraging stainless steels were generally realized by multiple-element alloying under a given heat treatment processing. A series of alloy compositions were designed with a uniform cluster formula of [Ni16Fe192](Cr32(Ni16–x–y–z–m–nMoxTiyNbzAlmVn)) (at.%) that was developed out of a unique alloy design tool, a cluster-plus-glue-atom model. Alloy rods with a diameter of 6 mm were prepared by copper-mold suction-cast processing under the argon atmosphere. These alloy samples were solid-solutioned at 1273 K for 1 h, followed by water-quenching, and then aged at 783 K for 3 h. The effect of the valence electron concentration, characterized with the number of valence electrons per unit cluster (VE/uc) formula of 16 atoms, on microhardness of these designed maraging stainless steels at both solid-solutioned and aged states was investigated. The relationship between alloy compositions and microhardness in maraging stainless steels was firstly established by the random forest (RF, a kind of machine learning methods) based on the experimental results. It was found that not only the microhardness of any given composition alloy within the frame of cluster formula, but also the alloy composition with a maximum microhardness for any given VE/uc, could be predicted in good agreement with the guidance of the relationship by RF. The contributions of minor-alloying elements to the microhardness of the aged alloys were also discussed.
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  Effect of Ostwald ripening of carbide particles on mechanical properties of SCM435 steel during subcritical annealing

  Cheng Ji . Jun-lu Yao . Miao-yong Zhu

  . 2018, 25(7): 724-731.

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  Ostwald ripening of carbide occurs during the process of subcritical annealing, and its extent of maturation influences the microstructure and mechanical properties of SCM435 steel. In this paper, to research the regularity of Ostwald ripening, we simulated different soaking times at 680 °C with SCM435 steel. The following conclusions were obtained by analysing the experimental results: the cementite particles became coarser according to the law of Ostwald ripening, and when the soaking time was extended, the spheroidization ratio and plasticity of carbide improved until the soaking time reached 6 h. A formula for the Ostwald ripening of carbide is presented and is applicable to SCM435 steel in the process of subcritical annealing with a relative error less than 8.02%.
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  Interfacial microstructure and mechanical properties of stainless steel clad plate prepared by vacuum hot rolling

  Jun Jiang,. Hua Ding . Zong-an Luo . Guang-ming Xie

  . 2018, 25(7): 732-738.

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  A stainless steel clad plate composed of stainless steel and carbon steel was prepared by vacuum hot rolling process, and its microstructure, especially the bonding interface, was evaluated using an optical microscope, a scanning electron microscope and a transmission electron microscope (TEM). The corresponding mechanical properties were also assessed by means of hardness and shear tests. The results showed a bonding interface formed between stainless steel and carbon steel, which was relatively straight in macroscope but serrated in microscope. Decarburization layer and carbon-enriched layer were distinguished at the side of carbon steel and stainless steel near the interface, respectively, which should be related to diffusion of carbon and alloying elements. The carbon-enriched layer could also be identified as a recombination region, whose microstructure was mainly recognized as martensite by TEM. Consequently, the hardness was the highest at this region. Furthermore, the result of shear test at the bonding interface showed that the shear strength was 395 MPa and the fracture mode was dominated as ductile fracture, indicating the bonding interface with good quality.
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  Interlayer engineering for titanium clad steel by hot roll bonding

  Xi-yang Chai, . Tao Pan . Feng Chai . Xiao-bing Luo . Hang Su . Zhi-gang Yang . Cai-fu Yang

  . 2018, 25(7): 739-745.

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  Hot roll bonding was carried out between commercially pure titanium TA2 and high-strength low-alloy steel Q390 using pure Nb or Mo interlayer at 950 °C with a total reduction ratio of 86.7%. Interfacial microstructure and bonding properties of titanium clad steel plates were investigated by electron microscopy and mechanical tests. The results showed β-Ti, TiC and Fe2Ti reaction phases were generated at Ti/steel interface for the clad plates with no interlayer. Inserting Nb or Mo interlayer can effectively suppress the formation of brittle phases, while the weak bonding joint transferred to Nb/steel or Mo/steel interface. And some micro-voids were found at the interface of Nb/steel and Mo/steel. The improved shear strength of clad plates with Nb interlayer might be attributed to the elimination of brittle phases at bonding interface. The small size and little quantities of the micro-voids at Nb/steel interface had a relatively weak effect on shear strength. However, the large number and big size of micro-voids were responsible for the degradation of shear strength for the clad plates with Mo interlayer.
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  Effect of milling duration on hydrogen storage thermodynamics and kinetics of ball-milled Ce–Mg–Ni-based alloy powders

  Dian-chen Feng, . Hao Sun . Xi-tao Wang . Yang-huan Zhang,

  . 2018, 25(7): 746-754.

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  To improve the hydrogen storage performance of CeMg12-type alloys, substituting the partial Mg with Ni in the alloy was conducted. The way to synthesize the aimed alloy powders was the mechanical milling method, by which the CeMg11Ni + x wt.% Ni (x = 100, 200) alloy powders with nanoctystalline and amorphous structure were obtained. The influence of the milling time and Ni content on the hydrogen storage properties of the alloys was discussed. The XRD and HRTEM were used to investigate the microstructures of the ball milling alloys. The hydrogenation/dehydrogenation dynamics were studied by Sievert instrument, and DSC which was link a H2 detector. The hydrogen desorption activation energies of the alloy hydrides were evaluated by Arrhenius and Kissinger means. From the results point of views, there is a little decline in the thermodynamic parameters (ΔH and ΔS) with the increase of Ni content. However, the alloys desorption and absorption dynamics are improved distinctly. What’s more, the variation of milling time results in a dramatic influence on the hydrogen storage performances of alloys. There are some maximum values of the hydrogen capacities correspond with the different milling time, which are 5.805 wt.% and 6.016 wt.% for the CeMg11Ni + x wt.% Ni (x = 100, 200) alloys, respectively. The kinetics tests suggest that the hydrogen absorption rates increase firstly, and then decrease with grinding time prolonged. The improvement of the gaseous hydrogen storage kinetics results from the decrease in the activation energy caused by the increase of Ni content and grinding time.
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  Long-term thermal-aging stability of oxide-dispersion-strengthened ferritic steels at 753 K

  Shuai Xu . Zhang-jian Zhou . Shao-fu Li . Hao-dong Jia

  . 2018, 25(7): 753-784.

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  Oxide-dispersion strengthened (ODS) ferritic steels are promising candidates for structural applications in the future nuclear reactors. The higher chromium contents of ODS ferritic steels, the better the corrosion resistance will be, which can meet the harsh corrosion environment of the advanced reactors. But the increasing of the Cr content may also lead to the brittleness of the ODS steels when service at high temperatures. In this study, the ODS ferritic steels with different Cr content (12 wt.%, 16 wt.% and 18 wt.%Cr respectively) were fabricated by mechanical alloying, hot isostatic pressing and forging. Mechanical properties and microstructure evolution of the ODS ferritic steels after aging at 753 K for 2000 h were investigated. It is found that both Vickers hardness and yield strength of 18%Cr ODS ferritic steel were strongly increased and the impact energy was decreased after aging at 753 K. In order to explore the reasons for changes in the mechanical properties, the fracture surfaces were characterized by SEM, and microstructures after aging were observed by TEM. The impact fracture of 18%Cr ODS ferritic steel belongs to quasi-cleavage facture, which is consistent with its very low impact energy. The grain size and dispersed oxide particles of different ODS steels are very stable. M23C6 carbide and M2C carbide have been found in 12%Cr ODS steel and 16%Cr ODS steels respectively.
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  Corrosion behavior of low-carbon Cr micro-alloyed steel for grounding grids in simulated acidic soil

  Jian Li . Hang Su . Feng Chai . Dong-mei Xue . Li Li . Xiang-yang Li . Hui-min Meng

  . 2018, 25(7): 755-765.

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  To improve the corrosion resistance of steels for grounding grids, a low-carbon Cr micro-alloyed steel was developed (C1 steel), and corrosion behavior of Q235 steel and newly developed C1 steel in simulated acidic soil was investigated. The corrosion rate was evaluated with the mass loss measurements, while the corrosion morphology of surface and cross section of rust layer was observed by scanning electron microscopy. The corrosion products were analyzed by energy-dispersive X-ray spectrometry, X-ray diffraction and X-ray photoelectron spectroscopy, and the polarization curve was measured using potentiodynamic polarization method. Results indicated that C1 steel displayed good corrosion resistance in the simulated acidic soil, of which the corrosion rate was only 30% of that of Q235 steel after corrosion for 360 h. The analysis of rust layer showed that lower carbon content in steel could reduce the tendency of micro cell corrosion and appropriate amount of chromium could improve the corrosion potential of metal matrix. Moreover, the analysis of X-ray photoelectron spectroscopy revealed that the chromium enriched in inner rust layer of C1 steel existed mainly in the form of Fe2CrO4, which facilitated the formation of Cr-goethite and improved the protection of corrosion products.
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  Effect of strain rate and temperature on the serration behavior of SA508-III RPV steel in the dynamic strain aging process

  Xue Bai,,. Su-jun Wu, . Li-jun Wei, . Shuai Luo . Xie Xie . Peter K. Liaw

  . 2018, 25(7): 767-752.

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  Dynamic strain aging (DSA) effect on SA508-III reactor pressure vessel (RPV) steel was investigated. The SA508-III RPV steel was subjected to tension tests at different strain rates (1.1 × 10-5 s-1 and 6.6 × 10-5 s-1) and different temperatures (500 and 550 °C) to evaluate the influence of strain rate and temperature on the serrated flow behavior, which is the repetitive and discontinuous yielding phenomenon on the stress–strain curves. The higher temperature leads to the higher density of precipitates, M23C6 carbides and needle-like Mo2C carbides. It was found that the samples under tension test of 6.6 × 10-5 s-1 and 500 °C possess superior mechanical properties and mainly show A-type serrations on the tension test curves. Then, the local regress method was used to filter the DSA curves, thus to show the real trend of the curves. It has been found that the less time of interaction between dislocations and precipitates under higher strain rates leads to a higher strength of the sample. The more tiny-stress drops on the 550 °C serration curve can be attributed to the hardening phase, M23C6 carbides and needle-like Mo2C carbides. The higher percentage of the small stress drops on the serration curves represents the higher mechanical strength.