2016年, 第23卷, 第4期　刊出日期：2016-04-15

  

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Metallurgy and Metal Working
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  Electroslag Remelting Withdrawing Technology for Offshore Jack- up Platform Rack Steel Manufacturing Process

  Xi- min ZANG,Tian- yu QIU,Wan- ming LI,Xin DENG,Zhou- hua JIANG,,Hua SONG

  中国钢铁期刊网. 2016, 23(4): 297-304.

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  Offshore jack- up platform rack steel must exhibit high strength and toughness as well as excellent welding properties. A high- quality large ingot is a prerequisite for obtaining a high- performance rough part. The electroslag remelting withdrawing (ESRW) technology using a T- shaped mold and bifilar mode was introduced to replace casting technology. Numerical simulation of the ESRW process was performed to determine the distribution of the temperature and velocity fields and to determine the optimum process for producing rack steels. Several A514Q slab ingots with dimensions of 0. 32 m×1. 40 m×4. 00 m were produced using ESRW technology in an industrial plant. The industrial test indicated that slab ingots produced by the ESRW method exhibited uniform chemical compositions and compact macrostructures. A 115. 4 mm thick plate was produced from the rough ingot after 11 rolling passes. Samples were obtained from different positions in the steel plate to test the mechanical performance and examine the microstructure, and the results revealed that the properties of the steel plate satisfied ASTM standards. The ESRW process improved the tensile strength and toughness of the slab ingot, enabling significant improvements in the anisotropy and low temperature toughness, which are critical for the development of rack steel for offshore platforms.
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  Modeling of Liquid Level and Bubble Behavior in Vacuum Chamber of RH Process

  Yi- hong LI,,Yan- ping BAO,Rui WANG,Min WANG,Qing- xue HUANG,Yu- gui LI

  中国钢铁期刊网. 2016, 23(4): 305-313.

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  In the Ruhrstahl- Heraeus (RH) refining process, liquid steel flow pattern in a ladle is controlled by the fluid flow behavior in the vacuum chamber. Potassium chloride solution and NaOH solution saturated with CO2 were respectively used as a tracer to investigate the liquid and gas flow behaviors in the vacuum chamber. Principal component and comparative analysis were made to show the factors controlling mixing and circulation flow rate. The liquid level and bubble behavior in the vacuum chamber greatly affect fluid flow in RH process. Experiments were performed to investigate the effects of liquid steel level, gas flow rate, bubble residence time, and gas injection mode on mixing, decarburization, and void fraction. The results indicate that the mixing process can be divided into three regions: the flow rate- affected zone, the concentration gradient- affected zone, and their combination. The liquid steel level in the vacuum chamber of 300 mm is a critical point in the decarburization transition. For liquid level lower than 300 mm, liquid steel circulation controls decarburization, while for liquid level higher than 300 mm, bubble behavior is the main controlling factor. During the RH process, it is recommended to use the concentrated bubble injection mode for low gas flow rates and the uniform bubble injection mode for high gas flow rates.
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  Self- reduction Mechanism of Coal Composite Stainless Steel Dust Hot Briquette

  Sok- chol RI,,Man- sheng CHU,Shuang- yin CHEN,Zheng- gen LIU,Hun HONG

  中国钢铁期刊网. 2016, 23(4): 314-321.

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  To efficiently recycle valuable metals such as chromium and nickel in stainless steel dust, self- reduction experiments were carried out to study the reduction mechanism of metal oxides in coal composite stainless steel dust hot briquette, which is defined as a CCSB here. Self- reduction of CCSB is proceeded by volatile matter and fixed carbon contained within CCSB. Experiments were performed to study the effects of temperature and carbon to oxygen (C/OCoal) ratio on self- reduction of CCSB. At 1400 and 1450 ℃, volatile matter in coal used for experiment could take the place of about 40% of fixed carbon in coal. Under the present experimental conditions, reduction product of chromium appears as FeCr2O4, Cr2O3, Cr7C3, and ［Cr］ in turn during reduction. To evaluate the formation of metal nuggets in self- reduction process of CCSB, metal nuggets containing chromium and nickel were observed in outside of reduction products under various conditions, and thermodynamic equilibrium calculation was carried out for possible products and formation of molten metal by fixed carbon. SEM and EDS analyses were made for metal nugget and slag in reduced product. The results reveal that it is reasonable to achieve the metal nuggets at 1450 ℃, 0. 8 of C/OCoal ratio and 20 min of reduction time. The nugget formation can indicate one innovative process for comprehensive utilization of stainless steel dust.
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  Analysis of Crystallization Behavior of Mold Fluxes Containing TiO2 Using Single Hot Thermocouple Technique

  Yun LEI,Bing XIE,Wen- hui MA

  中国钢铁期刊网. 2016, 23(4): 322-328.

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  The crystallization behavior of mold fluxes containing 0-8 mass% TiO2 was investigated using the single hot thermocouple technique (SHTT) and X- ray diffraction (XRD) to study the possible effects on the coordination of heat transfer control and strand lubrication for casting crack- sensitive peritectic steels. Time- temperature- transformation (TTT) and continuous- cooling- transformation (CCT) curves were plotted using the data obtained from SHTT to characterize the crystallization of the mold fluxes. The results showed that crystallization of the mold fluxes during isothermal and non- isothermal processes was suppressed with TiO2 addition. From the TTT curves, it could be seen that the incubation and growth time of crystallization increased significantly with TiO2 addition. The CCT curves showed that the crystallization temperature initially decreased, and then suddenly increased with increasing the TiO2 content. XRD analysis suggested the presence of cuspidine in the mold fluxes with lower TiO2 content (＜4 mass%), while both perovskite and cuspidine were detected in the mold fluxes when the TiO2 content was increased to 8 mass%. In addition, the growth mechanisms of the crystals changed during the isothermal crystallization process from interface- controlled growth to diffusion- controlled growth with increasing the TiO2 content.
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  Solidification Structure of Continuous Casting Large Round Billets under Mold Electromagnetic Stirring

  Tao SUN,,Feng YUE,,Hua- jie WU,,Chun GUO,,Ying LI,Zhong- cun MA

  中国钢铁期刊网. 2016, 23(4): 329-337.

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  The solidification structure of a continuous casting large round billet was analyzed by a cellular- automaton- finite- element coupling model using the ProCAST software. The actual and simulated solidification structures were compared under mold electromagnetic stirring (MEMS) conditions (current of 300 A and frequency of 3 Hz). Thereafter, the solidification structures of the large round billet were investigated under different superheats, casting speeds, and secondary cooling intensities. Finally, the effect of the MEMS current on the solidification structures was obtained under fixed superheat, casting speed, secondary cooling intensity, and MEMS frequency. The model accurately simulated the actual solidification structures of any steel, regardless of its size and the parameters used in the continuous casting process. The ratio of the central equiaxed grain zone was found to increase with decreasing superheat, increasing casting speed, decreasing secondary cooling intensity, and increasing MEMS current. The grain size obviously decreased with decreasing superheat and increasing MEMS current but was less sensitive to the casting speed and secondary cooling intensity.
Material
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  Precipitation and Solid Solution of Titanium Carbonitride Inclusions in Hypereutectoid Tire Cord Steel

  Chen- fan YU,Zheng- liang XUE,Wu- tao JIN

  中国钢铁期刊网. 2016, 23(4): 338-343.

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  The properties of titanium carbonitride Ti(CxN1-x) inclusions precipitated during solidification of tire cord steels and the thermodynamic conditions for their decomposition and solid solution during billet heating were investigated using a thermodynamics method. The solid solution of Ti(CxN1-x) inclusions during high- temperature heating was also studied experimentally. The results revealed that: (1) the higher the content of carbon in the tire cord steel is, the greater the value of x in the Ti(CxN1-x) inclusions is; (2) the higher the content of carbon in the tire cord steel is, the earlier the Ti(CxN1-x) inclusions precipitated during the solidification process and the lower the solidification front temperature is during precipitation; (3) when an 82A steel sample was heated to 1087 ℃, the Ti(CxN1-x) inclusions possess the thermodynamic conditions of decomposition and solid solution; and (4) when 82A samples were heated to 1150 and 1250 ℃, the total number of Ti(CxN1-x) inclusions larger than 5 μm in diameter decreased by 55. 0% and 70. 3%, respectively. In addition, although smaller inclusions with diameter less than 2 μm continued to decompose when the sample was heated at 1250 ℃ for 2 h and then cooled to 1000 ℃ in the furnace, the number of inclusions larger than 5 μm in diameter increased.
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  Effect of Deformation Temperature on Deformation Mechanism of Fe- 6. 5Si Alloys with Different Initial Microstructures

  Hua- dong FU,Yuan- ke MO,Long- chao ZHUO,Zhi- hao ZHANG,Jian- xin XIE

  中国钢铁期刊网. 2016, 23(4): 344-349.

  摘要 ( )   可视化   收藏

  Deformation behaviors and mechanisms under different temperatures for columnar- grained Fe- 6. 5Si (mass%) alloys fabricated by directional solidification and equiaxed- grained Fe- 6. 5Si alloy fabricated by forging were comparatively investigated. The results showed that, with increasing the deformation temperature from 300 ℃ to 500 ℃, the elongation increased from 2. 9% to 30. 1% for the equiaxed- grained Fe- 6. 5Si alloy, while from 6. 6% to about 51% for the columnar- grained Fe- 6. 5Si alloy. The deformation mode of equiaxed- grained Fe- 6. 5Si alloy transferred from nearly negligible plastic deformation to large plastic deformation dominated by dislocation slipping. Comparatively, the deformation mode of the columnar- grained alloy transferred from nearly negligible plastic deformation to plastic deformation dominated by the twining, and finally to plastic deformation dominated by dislocation slipping. Meanwhile, compared with the alloy with equiaxed grains, it was found that ultimate tensile strength and elongation could be increased simultaneously, which was ascribed for the twinning deformation in columnar- grained Fe- 6. 5Si alloy. This work would assist us to further understand the plastic deformation mechanism of Fe- 6. 5Si alloy and provide more clues for high- efficiency production of the alloy.
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  Strengthening Mechanisms for Ti- and Nb- Ti- micro- alloyed High- strength Steels

  Chuan- feng MENG,,Yi- de WANG,,Ying- hui WEI,Bin- qing SHI,Tian- xie CUI,Yu- tian WANG

  中国钢铁期刊网. 2016, 23(4): 350-356.

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  The strengthening mechanisms of hot- rolled steels micro- alloyed with Ti (ST- TQ500) and Nb- Ti (NT- TQ500) were investigated by examining the microstructures of steels using optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM). The results revealed almost no differences in the solute solution strengthening and fine- grained strengthening of the two steels, whereas the contributions of precipitation strengthening and dislocation strengthening were different for ST- TQ500 and NT- TQ500. The measured precipitation strengthening effect of ST- TQ500 was 88 MPa higher than that of NT- TQ500; this difference was primarily attributed to the stronger precipitation effect of the Ti- containing nanoscale particles. The dislocation strengthening effect of ST- TQ500 was approximately 80 MPa lower than that of NT- TQ500. This is thought to be related to differences in deformation behavior during the finishing rolling stage; the inhibition of dynamic recrystallization from Nb in NT- TQ500 (Nb- Ti) may lead to higher density of dislocations in the microstructure.
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  Effect of Solution Annealing Temperature on Pitting Behavior of Duplex Stainless Steel 2204 in Chloride Solutions

  Liang HE,,Yan- jun GUO,Xia- yu WU,Yi- ming JIANG,Jin LI

  中国钢铁期刊网. 2016, 23(4): 357-363.

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  The effect of solution annealing temperature ranging from 950 to 1200 ℃ on the microstructure and corrosion performance of duplex stainless steel (DSS) 2204 were investigated. The proportion of the ferrite phase increased while the austenite phase decreased and the ferrite stabilizing elements diluted in the ferrite phase with the increase of annealing temperature. The critical pitting temperature (CPT) of specimens annealed at 1000 ℃ was higher than those annealed at 950 ℃, whereas further increasing the annealing temperature to 1200 ℃ decreased the CPT. The pitting initiation sites were observed in the austenite phase, at the boundary of ferrite/austenite phase and inside the ferrite phase for specimens annealed at 950, 1000 ℃ and exceeding 1100 ℃, respectively. The evolution trend of the CPT and the pit initiation site were analyzed by the pitting resistance equivalent number.
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  A Physically Based Dynamic Recrystallization Model Considering Orientation Effects for a Nitrogen Alloyed Ultralow Carbon Stainless Steel during Hot Forging

  Gan- lin XIE,An HE,Hai- long ZHANG,Gen- qi WANG,Xi- tao WANG

  中国钢铁期刊网. 2016, 23(4): 364-371.

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  The nitrogen alloyed ultralow carbon stainless steel is a good candidate material for primary loop pipes of AP1000 nuclear power plant. These pipes are manufactured by hot forging, during which dynamic recrystallization acts as the most important microstructural evolution mechanism. A physically based model was proposed to describe and predict the microstructural evolution in the hot forging process of those pipes. In this model, the coupled effects of dislocation density change, dynamic recovery, dynamic recrystallization and grain orientation function were considered. Besides, physically based simulation experiments were conducted on a Gleeble- 3500 thermo- mechanical simulator, and the specimens after deformation were observed by optical metallography (OM) and electron back- scattered diffraction (EBSD) method. The results confirm that dynamic recrystallization is easy to occur with increasing deformation temperature or strain rate. The grains become much finer after full dynamic recrystallization. The model shows a good agreement with experimental results obtained by OM and EBSD in terms of stress- strain curves, grain size, and recrystallization kinetics. Besides, this model obtains an acceptable accuracy and a wide applying scope for engineering calculation.
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  A Flow Stress Model for High Strength Steels with Low Carbon Bainite Structure

  Bao- sheng XIE,Qing- wu CAI,Wei YU,Shi- xin XU,Ban WANG

  中国钢铁期刊网. 2016, 23(4): 372-379.

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  Two kinds of steels (YP960 and YP690) with low carbon bainite structure were designed, and their flow stress and strain hardening exponents were studied. The results showed that, when Hollomon relation was applied to describe the flow stress, there were significant errors between the experimental and calculated points in specimens tempered below 400 ℃, while a high precision was observed in samples tempered above 400 ℃. Whereas, the modified Voce relation could effectively predict the flow stress as well as the strain hardening exponent at different tempering temperatures, which was verified by unbiased estimators such as maximum relative error (MRXE) and average absolute relative error (AARE). Besides, the modified Voce relation was also applied to estimate the maximum uniform strain, and the correlation coefficients (R) between the experimental data and calculated maximum uniform strain were more than 0. 91. The high correlation coefficients indicated that the modified Voce relation could effectively predict the uniform deformation ability of high strength steels with low carbon bainite structure at different tempering temperatures.
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  Surface Modification by Nitrogen Plasma Immersion Ion Implantation on Austenitic AISI 304 Stainless Steel

  Miguel CASTRO- COLIN,William DURRER,Jorge A. LPEZ,Enrique RAMIREZ- HOMS

  中国钢铁期刊网. 2016, 23(4): 380-384.

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  Surfaces of AISI 304 austenitic stainless steel plates nitrided by plasma immersion ion implantation (PIII) technology were studied by means of Auger electron spectroscopy (AES) and X- ray photoelectron spectroscopy (XPS) to determine the effect of the nitriding process on the surface and subjacent layers. Elemental compositions obtained by AES and XPS at varying depths indicate that the saturation of N is relatively constant as a function of depth, indicating the reliability of PIII technology for subsurface saturation. It is concluded that the concentrations of both Cr and O increase with depth, the subjacent oxide is driven by the Ar+ sputtering process used to access the lower layers, and then N is bound to Cr.
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  Strain- induced Precipitation in Ti Micro- alloyed Interstitial- free Steel

  Ya- jun HUI,Yang YU,Lin WANG,Chang WANG,Wen- yuan LI,Bin CHEN

  中国钢铁期刊网. 2016, 23(4): 385-392.

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  Stress relaxation method was carried out on a Ti micro- alloyed interstitial- free (IF) steel at the temperature ranging from 800 to 1000 ℃. The results show that the softening kinetics curves of deformed austenite can be divided into three stages. At the first stage, the stress has a sharp drop due to the onset of recrystallization. At the second stage, a plateau appears on the relaxation curves indicating the start and finish of strain- induced precipitation. At the third stage, the stress curves begin to descend again because of coarsening of precipitates. Precipitation- time- temperature (PTT) diagram exhibited a “C” shape, and the nose point of the PTT diagram is located at 900 ℃ and the start precipitation time of 10 s. The theoretical calculation shows that the strain- induced precipitates were confirmed as almost pure TiC particles. The TiC precipitates were heterogeneously distributed in either a chain- like or cell- like manner observed by transmission electron microscopy (TEM), which indicates the precipitates nucleated on dislocations or dislocation substructures. In addition, a thermodynamic analytical model was presented to describe the precipitation in Ti micro- alloyed IF steel, which shows a good agreement between the experimental observation and the predictions of the model.
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  Effects of Solution Depletion and Segregation Oxidation on Morphology of Modified 310 Austenitic Stainless Steel

  Hong- ying SUN,Qiang HE,Zhang- jian ZHOU,Man WANG,Guang- ming ZHANG,Shao- fu LI

  中国钢铁期刊网. 2016, 23(4): 393-400.

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  The oxidation morphologies of modified 310 steel exposed in 900 and 1100 ℃ air were investigated. A double layer morphology consisting of a (Cr,Mn)- rich outer layer and a fine Cr- rich inner layer was formed at 900 ℃. It was related to the breakaway oxidation induced by the Cr- depletion and the Mn- segregation in inner layer. Some Cr- rich oxides with amorphous state were formed along grain boundaries. And some new finer oxide grains, voids and Cr- rich precipitates were observed in spallation areas at 1100 ℃. Correspondingly, the oxidation kinetic curve dropped with the spallation of scale and increased with the formation of some new oxide grains. It was caused by segregation of Cr and the transformation of oxides from Cr2O3 to the volatile oxides at elevated temperature. XRD analysis showed that the precipitates were mainly composed of CrO3. Segregation and depletion for solutions were also discussed by oxidation diffusion mechanisms.
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  Microstructure Evolution and Precipitation Behavior of 0Cr16Ni5Mo Martensitic Stainless Steel during Tempering Process

  Wu- hua YUAN,Xue- hui GONG,,Yong- qing SUN,Jian- xiong LIANG

  中国钢铁期刊网. 2016, 23(4): 401-408.

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  The microstructure, morphology of precipitates and retained austenite and the volume fraction of retained austenite in 0Cr16Ni5Mo stainless steel during the tempering process were analyzed using optical microscope (OM), transmission electron microscope (TEM), X- ray diffraction (XRD) and scanning transmission electron microscope (STEM). The results show that the microstructure of the tempered steel is mainly composed of tempered martensite, retained austenite, and delta ferrite. In the case of samples tempered from 500 to 700 ℃, the precipitates are mainly M23C6, which precipitate along the lath martensite boundaries. The precipitate content increases with the tempering temperature. During the tempering process, the content of retained austenite initially increases and then decreases, the maximum content of retained austenite being 29 vol. % upon tempering at 600 ℃. TEM analysis of the tested steel reveals two morphology types of retained austenite. One is thin film- like retained austenite that exists along the martensite lath boundary. The other is blocky austenite located on packet at the boundary and the original austenite grain boundary. To further understand the stability of reversed austenite, the Ni content in reversed austenite was measured using STEM. Results show a significant difference in nickel concentrations between reversed austenite and martensite.