2017年, 第24卷, 第5期　刊出日期：2017-05-15

  

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  Effects of vertical electromagnetic stirring on grain refinement and macrosegregation control of bearing steel billet in continuous casting

  Yu Xu,,En-gang Wang,,*,Zhuang Li,,An-yuan Deng,

  中国钢铁期刊网. 2017, 24(5): 483-489.

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  The grain refinement and macrosegregation control of GCr15 bearing steel were investigated under a type of rarely-used electromagnetic stirring, vertical electromagnetic stirring (V-EMS), in continuous casting. V-EMS can create an upward electromagnetic force and generate longitudinal loop convection, which enables the better mixing of the upper part with the lower part of the liquid steel. The results showed that applying V-EMS can enlarge the region of the equiaxed grain, decrease the secondary dendrite arm spacing (SDAS) and reduce the segregation of both carbon and sulfur. After applying V-EMS, liquid steel with a high solute concentration is brought to the dendrite tips, making the dendrite arms partially melt. The length of the dendrite fragment is approximately 18 mm, 10 to 12 times the SDAS. Upon increasing the amount of cooling water from 20 to 35 m3/h, the dendrite fragments exhibit an obvious aggregation following V-EMS. Finally, a criterion for dendrite fragmentation under V-EMS was derived based on the dendrite fragmentation theory of Campanella et al.
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  Hydraulic experiment on mushroom head in bottom-blown smelting furnace

  Fu-yong Su*,Zhi Wen

  中国钢铁期刊网. 2017, 24(5): 490-494.

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  There are many bottom-blown smelting furnaces in metallurgical industry. When oxygen or air sprays from the jet nozzle into the bottom of the furnace, the melting phase will be frozen and a hemispherical porous zone with a mushroom head shape will be formed around the nozzle. The mushroom head can protect the jet nozzle and mitigate the liquid spray on the surface of melt. In order to analyze the formation process of a mushroom head in the bottom-blown smelting furnaces, a hydraulic experiment system was designed and the formation of the mushroom head was investigated by hydraulic experiment. The hydraulic experiment results show that the formation process is mainly divided into generating crushing generating process and stable mushroom head generation process. The formation of stable mushroom head requires certain thermodynamic condition and water splash is more intense without a mushroom head than with a mushroom head. The size, porosity and diameter of the mushroom head are affected by the flow rate, temperature and heat capacity of the bottom-blowing gas and the temperature of the superheat and the physical parameters of the melt.
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  Application of insulation padding in a heavy turbine guide vane casting

  Jin-wu Kang,*,Qing-xian Ma,Chi Zhang,Pei-chao Zhou,Zhong-ren Liu,Zu-liang Li,Hao Zhang

  中国钢铁期刊网. 2017, 24(5): 495-500.

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  To replace metal padding by insulation padding for castings can save the melt and reduce cleaning work of castings. The design of insulation padding was investigated. The equation of the modulus extension factor for insulation padding and the ratio of its thickness over the modulus of a casting were improved to determine the thickness of insulation padding. The insulation padding was designed for a turbine guide vane casting weighing 35 t. A sound casting was obtained with 750 kg steel saved. On the other side, the casting obviously expanded at the interface with the insulation padding, which is perhaps the reason that the use of insulation padding has been suspended for many years. To avoid the expansion of insulation padding, a shielding layer made of a kind of material of good fire resistance was adopted to prevent the insulation layer from touching the melt. The shielding layer serves as a cushion of heat and expansion during solidification process so as to resist the expansion of castings and guarantee the feeding effect at the same time. Furthermore, insulation padding can be placed by a certain offset into the mold cavity so as to counteract the expansion of castings.
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  Thermal analysis of walls in a new-type billet caster tundish with avacuum shell

  Jian-bo Xie,,Jian-an Zhou,,*,Lan-hua Zhou,Bao Wang,,Hua Zhang,

  中国钢铁期刊网. 2017, 24(5): 501-507.

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  To reduce thermal loss from molten steel in a tundish during continuous casting production, a new tundish fabricated by welding radiation-proof steel plates onto the steel plates of the exterior walls of a billet caster tundish was proposed. This new tundish was used to investigate the effect of pressures inside the vacuum chamber on the uniformity of the temperature of molten steel and the thermal conditions of the vacuum layer. The results show that the conversion radiation coefficient is not sensitive to pressure and its value at high temperatures is merely 15 times greater than that at low temperatures. Pressure is the key factor affecting additional factor of conversion convection. This factor is more than 100 times greater at 105 Pa than at 102 Pa, and the temperature at inner points at 102 Pa is, on average, 4 K higher than that at 105 Pa. Meanwhile, the local temperature difference of the inlet at 102 Pa is 1 K higher than that at 105 Pa. Thus, the proposed vacuum billet caster tundish can achieve low superheat teeming of steel because of the thermal preservation capability of the vacuum, which helps to reduce the tapping temperature and improve the uniformity of the temperature of steel.
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  Tailored properties of a novelly-designed press-hardened 22MnMoB steel

  Ming-bo Zhou,Jing-lin Tang,*,Jie Yang,Cun-yu Wang

  中国钢铁期刊网. 2017, 24(5): 508-512.

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  A novel 22MnMoB hot stamping steel was designed. The continuous cooling transformation (CCT) measurement of the 22MnMoB steel showed that the ferrite-bainite microstructure could be obtained at cooling rates lower than 25°C/s, and the complete martensite structure required the cooling rate higher than 30°C/s. The experiments with non-uniform die temperatures were carried out to obtain tailored properties. The results showed that strength of 1411 MPa and elongation of 6% could be obtained in the hard zone, and strength of 916 MPa and elongation of 9% could be obtained in the soft zone, which can be realized by controlling the die temperature at 400°C. The transition zone was found smooth and could be beneficial to reduce the stress concentration and therefore improve the performance of components.
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  Microstructural evolution and mechanical properties of a new Ni-based heat-resistant alloy during aging at 750°C

  Zhong-liang Tian,,Sen-bao Jiang,Zheng-zong Chen,Han-sheng Bao,Zheng-dong Liu,*

  中国钢铁期刊网. 2017, 24(5): 513-519.

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  Microstructural evolution and mechanical properties of a new candidate Ni-based heat-resistant alloy for advanced ultra-supercritical (A-USC) steam turbine rotors were investigated during aging at 750°C up to 10000 h. The evolutions of γ′ particles inside austenitic grain and M23C6 carbides along grain boundaries were characterized according to their morphologies, distributions, and growth kinetics. Mean radius of the γ′ spherical particles grew from 203 to 900 nm after aging for 10000 h, and the corresponding coarsening behavior was conformed to the law of Lifschitz-Slyosovd-Wagner (LSW). The weight fraction of γ′ particles slightly increased from 100 to 120 wt% after aging of long duration at 750°C. The Cr-rich M23C6 carbides discontinuously precipitated along grain boundaries, while other detrimental phases were not formed during the aging treatment, and hence the strength of grain boundary was enhanced by these discontinuously distributed carbides. The critical size of γ′ had a direct influence on the maximum hardness of this alloy. Moreover, this alloy presented a good impact toughness for the safety after long time aging at high temperature.
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  Effect of vanadium on modification of inclusions in Mn- and Si-deoxidized steel during heat treatment at 1473 K

  Cheng-song Liu,Xiao-qin Liu,*,Hong-wei Ni,Shu-feng Yang,Jing-she Li,Fei Ye

  中国钢铁期刊网. 2017, 24(5): 520-528.

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  The effect of vanadium (V) on the modification of oxide inclusions in steels with different concentrations of Mn, Si, and V was determined before and after heat treatment at 1473 K. Changes in the morphology, size, and composition of these inclusions were analyzed. Equilibrium relations between the inclusions and steels at 1873 K and 1473 K were calculated using available thermodynamic data to roughly approximate the stable oxide inclusions in the steels. The results revealed that the concentrations of V and Si in the steel are critical for controlling the modification of the inclusions during heat treatment at 1473 K. MnO-SiO2-type oxide inclusions gradually transformed into MnO-V2O3-type or Mn-SiO2- and MnO-V2O3-type inclusions in low-Si high-V steels heat-treated for 60 min. In addition, the morphology of the inclusions changed from spherical to irregular. A Si-accumulated zone and a V-depleted zone formed close to the interface in the steel matrix. The experimental and calculation results indicated that, during the heat treatment, an interface chemical reaction occurred between the Fe-Mn-Si-V steel and the MnO-SiO2-type oxide inclusion.
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  Effects of process parameters on fragment and refinement of millimeter-grade coarse grains for 316LN steel during hot cogging

  Da-shan Sui,*,Hai-ming Zhang,Hong-yang Zhu,Zhe Zhu,Zhen-shan Cui

  中国钢铁期刊网. 2017, 24(5): 529-535.

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  The heterogeneous mixed-grain microstructure is a common defect for the heavy forging of 316LN austenitic stainless steel. Isothermal compression experiments were performed on a Gleeble-3500 thermo-mechanical simulator to investigate the effect of process parameters on the fragment and refinement of millimeter-grade coarse grains (MCGs) during hot cogging. The experimental results indicate that the stress of MCG specimens is much larger than that of fine grain (FG) ones at 1150°C, while the stress difference between MCG and FG samples became smaller at 1200°C. Moreover, the MCGs can be well fragmented and refined under the condition of temperature of 1200°C, strain rate of 001 s-1, and reduction rate of 50%. Meanwhile, numerical simulations were conducted to study the influences of temperature, strain and strain rate on microstructure evolution. The results of experiments and simulations comprehensively demonstrate that the MCG results in the increase of deformation resistance and incompatibility of deformation, and it can be fragmented and refined at 1200°C so that the plastic deformation energy decreases remarkably with the increase of temperature from 1150 to 1200°C.
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  Effects of micro-alloying and production process on precipitation behaviors and mechanical properties of HRB600

  Hong-bo Pan,,*,Meng-jiao Zhang,Wei-ming Liu,,Jun Yan,,Hui-ting Wang,Chang-sheng Xie,Zhan Guo

  中国钢铁期刊网. 2017, 24(5): 536-543.

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  Effects of micro-alloying elements and production process on microstructure, mechanical properties and precipitates of 600 MPa grade rebars were studied by using pilot test, metallographic observation, tensile test, thermodynamic calculation and transmission electron microscopy. The results show that the tested steels are composed of ferrite and pearlite, in which the content range of pearlite is 33%-45%. For vanadium micro-alloyed steel, interphase precipitation strengthening effect of V can be promoted and the yield strength of tested steels can be increased with increasing V content and decreasing finishing rolling temperature. The temperature of terminated cooling should be more than 700°C when the water cooling is used. When niobium is added to the steel, more coarse (Nb,V)C,N precipitates are generated at high temperature, so that the solid solubility of precipitated phases of vanadium is reduced and the precipitation strengthening effect of vanadium is weakened.
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  Multi-objective optimization of gas metal arc welding parameters and sequences for low-carbon steel (Q345D) T-joints

  Qing Shao*,Tao Xu,Tatsuo Yoshino,Nan Song

  中国钢铁期刊网. 2017, 24(5): 544-555.

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  Q345D high-quality low-carbon steel has been extensively employed in structures with stringent welding quality requirements. A multi-objective optimization of welding stress and deformation was presented to design reasonable values of gas metal arc welding parameters and sequences of Q345D T-joints. The optimized factors included continuous variables (welding current (I), welding voltage (U) and welding speed (v)) and discrete variables (welding sequence (S) and welding direction (D)). The concepts of the pointer and stack in Visual Basic (VB) and the interpolation method were introduced to optimize the variables. The optimization objectives included the different combinations of the angular distortion and transverse welding stress along the transverse and longitudinal distributions. Based on the design of experiments (DOE) and the polynomial regression (PR) model, the finite element (FE) results of the T-joint were used to establish the mathematical models. The Pareto front and the compromise solutions were obtained by using a multi-objective particle swarm optimization (MOPSO) algorithm. The optimal results were validated by the corresponding results of the FE method, and the error between the FE results and the two-objective results as well as that between the FE results and the three-objective optimization results were less than 172% and 215%, respectively. The influence and setting regularity of different factors were discussed according to the compromise solutions.
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  Heat transfer performance of porous titanium

  Shi-feng Liu,,*,An Li,Yao-jia Ren,Dong-feng Li,Zhao-hui Zhang

  中国钢铁期刊网. 2017, 24(5): 556-560.

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  Porous titanium fibre materials with different structural parameters were prepared by vacuum sintering method. The thickness, porosity and wire diameter of prepared materials were investigated to understand the effects of structural parameters on pool heat transmission performance of titanium fibre porous material. As a result, better heat transfer performance is obtained when overheating is less than 10°C. In addition, when the wire diameter is smaller, the heat transfer is better. However, when superheating is above 10°C, heat transfer performance can be improved by increasing the wire diameter. Moreover, thickness influences the superficial area of the prepared material and affects the thermal resistance when bubbles move inside the material; superficial area and thermal resistance are the two key factors that jointly impact the heat transfer in relation to the thickness of the materials. Experimental results also show that the materials of 3 mm in thickness exhibit the best performance for heat transmission. Furthermore, changes in porosity affect the nucleation site density and the resistance to bubble detachment; however, the nucleation site density and the resistance to bubble detachment conflict with each other. In summary, the titanium fibre porous material with a 50% porosity exhibits suitable heat transfer performance.
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  Effect of Si on high temperature oxidation of 30Cr13 stainless steel

  Hong-huan Mao,Xing Qi,Jing Cao,Li-cong An,Yi-tao Yang*

  中国钢铁期刊网. 2017, 24(5): 561-566.

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  The effect of Si on the high temperature oxidation behavior of the 30Cr13 martensitic stainless steels was investigated. The underlying mechanism was further discussed by oxidation kinetics curves, scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis. It was observed that the addition of Si can significantly reduce the mass gain per unit area and the oxidation rate of the experimental steels. With the increase of Si content, the film surface composed of the wheat-like oxides turned into the small granular oxides after oxidation for 120 h. The SiO2 film formed on the metal substrate inhibited the outward diffusion of the metal cation and the inward diffusion of the oxygen anion; thus, the high temperature oxidation resistance was enhanced.