2015年, 第22卷, 第7期　刊出日期：2015-07-15

  

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Metallurgy and Metal Working
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  An Estimation of Component Activity of Vanadium-bearing Liquid Iron and Transition Temperature of Vanadium Oxidization Using MIVM

  Ya-yu LI,Zhen-nan LIU,Dong-ping TAO

  中国钢铁期刊网. 2015, 22(7): 557-565.

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  The component activity of Mn in Fe-C-Mn system as well as the component activities of C and Si in Fe-C-Si system was predicted by applying the pseudo-multicomponent approach of the molecular interaction volume model (MIVM) and the Wagner interaction parameter formalism (WIPF) respectively. The average relative errors between the predicted values of MIVM and the experimental data for the three components were 4. 5%, 17. 0% and 13. 0%, respectively, and those between the calculation results of the WIPF and the experimental data were 18. 0% for Mn, 9. 0% for C and 27. 0% for Si. The results indicated that the MIVM method could better predict the component activity of carbonaceous iron-based solution. Based on the data in an actual blowing process, the MIVM method was applied to predict the component activities of C and V as well as the transition temperature of vanadium oxidization (TTVO) in Fe-C-V-Si quaternary iron-based solution, and a comparative analysis of the predictions against the experimental data was carried out, with their average relative errors being 24. 0% for C, 7. 3% for V and 1. 0% for TTVO respectively. On that basis, the TTVO at Panzhihua Iron and Steel (Group) Co. , Ltd. was estimated by the MIVM method and an expression that the TTVO changed with composition and temperature of iron solutions was obtained by multiple linear regression method. The research results showed that the estimated values were in good agreement with the practical data.
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  Characterization and Thermodynamics of Al2O3-MnO-SiO2(-MnS) Inclusion Formation in Carbon Steel Billet

  Guo-cheng WANG,,,Sheng-li LI,,Xin-gang AI,,Chong-min ZHANG,,Chao-bin LAI

  中国钢铁期刊网. 2015, 22(7): 566-572.

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  A method to extract inclusion particles from solid steel by electrolysis with organic electrolyte solution was introduced; meanwhile, thermodynamics of inclusion formation was calculated using FactSage software. The results showed that there were two kinds of inclusions in the billet, i. e. Al2O3-MnO-SiO2-MnS (AMS-MnS) and Al2O3-MnO-SiO2 (AMS). Most of AMS-MnS inclusion particles, with diameter of 10-30 μm, showed three-layer structures: SiO2-rich core with a small quantity of Mn, intermediate AMS layer, and MnS outer layer containing small quantities of Al and O. Most AMS inclusion particles were 50-90 μm and exhibited homogeneous composition. Thermodynamic results indicated that SiO2-rich core could form firstly by Si reacting with O in molten steel at temperatures above 1923 K during Si-Fe alloy addition, and then, the SiO2-rich core could react with Mn and Al to form liquid AMS enveloping the SiO2-rich core at 1823-1873 K. MnS began to precipitate from AMS when temperature reached 1728 K. Liquid AMS could form by coupled reaction among Si, Mn, Al and O in molten steel.
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  Characteristics and Metallurgical Effects of Medium Basicity Refining Slag on Low Melting Temperature Inclusions

  Hui-xiang YU,,Xin-hua WANG,Jing ZHANG,Wan-jun WANG

  中国钢铁期刊网. 2015, 22(7): 573-581.

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  Recently, large sized CaO-Al2O3 inclusions with low melting temperature have been the main reason for lowering mechanical properties of high strength low alloy (HSLA) steel plates. New philosophy, i. e. refining by top slag with relatively low basicity and Al2O3 content, was proposed to control such kind of inclusions. Firstly, the characteristics of refining slag, such as component activities and sulphide capacity (CS) of CaO-Al2O3-SiO2-MgO slag, were studied through thermodynamic calculation. Then, slag-metal equilibrium experiments were carried out in laboratory to investigate the exact chemical composition of refining slag together with thermodynamic analysis. Finally, industrial trials were done to verify the desulphurization ability and inclusions control in steel refined by the new slag. Thermodynamic calculations indicated that the slags with basicity of 3. 5 and Al2O3 content of 20% and basicity of 5. 0 and Al2O3 content of 20% or 25% have high values of CaO activity, sulphide capacity and ratio of MgO activity to Al2O3 activity. Laboratory equilibrium experiments showed that the slag with basicity of about 4. 5 and Al2O3 content of about 20% is helpful for increasing the melting temperature of inclusions in steel. After introducing such kind of refining slag in industrial trials, the sulphur content in eight heats of steel is below 20×10-6, which meets the requirement of HSLA steel, and most inclusions distribute in relatively high melting zone (≥1773 K) of CaO-Al2O3-MgO(-SiO2) quasi-ternary diagram. New philosophy of top refining slag is feasible to control low melting point inclusions of CaO-Al2O3 system in HSLA steel.
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  Effect of Tempcore Processing on Mitigating Problems of Tramp Elements in Low C Steel Produced from Recycled Material

  Ahmed RAMADAN,A. Y. SHASH,I. S. EL-MAHALLAWI,D. SENK,Taha MATTAR

  中国钢铁期刊网. 2015, 22(7): 582-589.

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  The effect of tramp elements in the steel was intensively studied. It was found that the solubility of tramp elements decreased as the temperature decreased under normal cooling conditions. The tramp elements (Cu, Pb, and Sn) diffused toward the grain boundaries, and intermetallic compounds or rich phases which have low melting points were formed, causing reduction in ductility and failure during the bending test. Rebars with Cu content which were left to air cooling after the last step showed drop in elongation, up to 32%. On contrast, the samples with high percentage of tramp elements (Cu, Pb, and Sn) in the billet, which were rolled and subjected to Tempcore process, did not show drop in elongation or failure in bending test (especially for rebar with diameter less than 32 mm); however, copper must be less than 0. 35 mass% to prevent the precipitation of Cu-rich zones of critical size in 32 mm. When quenching was applied, the tramp elements remained in the interstitial supersaturated solid solution positions inside the grains and would not have the chance to diffuse and form precipitates, hindering the copper precipitates from reaching the critical size necessary for impairing the properties. This would hinder the occurrence of the harmful effect of the tramp elements on the elongation or the hot shortness after rolling.
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  Optimization of Calcium Addition to High-strength Low-alloy Steels

  Gu-jun CHEN,Sheng-ping HE,Yin-tao GUO,Bo-yi SHEN,Shuo ZHAO,Qian WANG

  中国钢铁期刊网. 2015, 22(7): 590-597.

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  Nozzle blockage is a common problem during continuous casting of Al-killed steel, and calcium treatment is widely used to resolve it. In consideration of the production costs, the technology of nonmetallic inclusion control was studied to optimize the Ca consumption. The proposed process of slag washing was employed, and the refining slag composition, deoxidation conditions and alloying systems were optimized. Using these measures, the steel cleanliness before Ca addition was improved significantly, and the corresponding Ca consumption was reduced. Moreover, the continuous casting could be conducted smoothly.
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  Preparation of Sintered (Ce1-xNdx)30FebalCu0. 1B1 Magnets by Blending Powder Method

  Shu-lin HUANG,,,Hai-bo FENG,,Ming-gang ZHU,,An-hua LI,,Yue ZHANG,Wei LI,

  中国钢铁期刊网. 2015, 22(7): 598-601.

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  Magnets with nominal compositions of (Nd1-xCex)30FebalCu0. 1B1 (x=0, 0. 15, 0. 3 and 0. 4, mass%) have been fabricated by blending powder method. The remanence (Br), intrinsic coercivity (Hc) and maximum energy product (BH)max of the RE2Fe14B type magnets deteriorated when Nd was replaced by Ce. The chemical composition and crystal structure of magnet were investigated systemically. Backscattered electron (BSE) and energy dispersive spectroscopy (EDS) results revealed that Ce-rich and Ce-lean matrix grains coexisted in the magnets. The magnetic coupling mechanism among the double hard magnetic phases was discussed. Low melting point RE-Cu phase was in favor of the formation of uniform continuous grain boundary. Transmission electron microscopy (TEM) investigation showed the presence of fcc (Nd,Ce)Ox phase in the grain boundary. When the Ce content was 15% of the total amounts of all the rare earth, the maximum energy product of the sintered magnet was 359. 8 kJ/m3.
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  Verification of Interpretation of Dynamic Strain Aging for Intermediate Temperature Embrittlement in Ni-Bi Alloy

  Wu-qiang YANG,,Min XU,Ye MENG,Lei ZHENG,Xiao-dong MENG

  中国钢铁期刊网. 2015, 22(7): 602-606.

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  Intermediate temperature embrittlement (ITE) is a general phenomenon in Ni alloys and recently was interpreted by dynamic strain aging (DSA). The relationship between ITE and DSA was studied by a binary Ni-Bi alloy. The experimental alloy of well-controlled purity was produced by vacuum induction melting and then heat-treated properly. Tensile tests were performed at various tensile temperatures, and the elongation at fracture was used to indicate the ductility. In order to identify the mechanisms of fracture and ITE, fracture morphologies of the samples of low ductility were observed by scanning electron microscopy. According to the tensile ductility, Ni-Bi alloy shows an obvious embrittlement behavior in the intermediate temperature range (700-750 ℃). However, the stress-strain curves of Ni-Bi alloy and the fracture morphologies indicate that DSA does not exist over the whole temperature range. Based on the experimental results and literatures, the interpretation of DSA was then discussed and proved to be invalid for elucidating the general feature of ITE in Ni-Bi alloy and Ni-based superalloys.
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  Effect of Aging on Hardening Behavior of 15-5 PH Stainless Steel

  Xin-yuan PENG,,,Xian-liang ZHOU,,,Xiao-zhen HUA,,Zhen-wei WEI,Hua-ying LIU

  中国钢铁期刊网. 2015, 22(7): 607-614.

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  Microstructure transformation and aging hardening behavior of 15-5 PH stainless steel were studied by optical microscopy (OM), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The results showed that the 15-5 PH stainless steel consists of NbC precipitates and lath matensite with a high dislocation density after solution treatment. With increasing aging temperature and aging time, the martensitic laths were resolved gradually. Meanwhile, the nanometric-sized Cu precipitates gradually coarsened and lost their coherency with the martensite matrix, which exhibited an elliptical shape finally. Fine Cu precipitates can lead to significant dispersion hardening effect, while the coarsened Cu precipitates have no contribution to strengthening. The reversed austenite was observed in the specimens aged at 550 ℃ and above; moreover, the amount of reversed austenite increased as aging temperature increased. The precipitation hardening behavior of 15-5 PH stainless steel may depend on the balance between the softening caused by the formation of reversed austenite and the hardening caused by the precipitation of copper.
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  Hot Deformation Behavior of Vanadium-microalloyed Medium-carbon Steel for Fracture Splitting Connecting Rod

  Wei-jun HUI,Si-lian CHEN,Cheng-wei SHAO,Yong-jian ZHANG,Han DONG

  中国钢铁期刊网. 2015, 22(7): 615-621.

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  Single compression tests were carried out with a Gleeble-3800 thermal simulator to investigate hot deformation behavior of two vanadium-microalloyed medium-carbon steels for fracture splitting connecting rod. The tests were performed to a total true strain of 0. 92 at true strain rates ranging from 10-2 to 10 s-1 and deformation temperature of 900-1150 ℃. The results show that hot deformation behavior of the tested steels is similar to that of conventional medium-carbon microalloyed steels and dynamic recrystallization is easier to occur at higher deformation temperature and lower strain rate. The austenite deformation resistance and activation energy of deformation increase with increasing vanadium content from 0. 15% to 0. 28% and thus the starting time of dynamic recrystallization was delayed. Finer recrystallized austenite grain could be obtained at higher strain rate, lower deformation temperature and higher vanadium content. TEM observation of the specimens quenched just before and after deformation reveals that vanadium is mainly in dissolved solute condition in austenite and thus affects the dynamic recrystallization behavior of the tested steels mainly through solute-drag effect.
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  Effect of Annealing Process on Microstructure and Mechanical Property of 1100 MPa Grade TRIP Steel

  Hong-xiang YIN,,Zheng-zhi ZHAO,,Ai-min ZHAO,,Xiao LI,,Han-jiang HU,,Jiang-tao LIANG,

  中国钢铁期刊网. 2015, 22(7): 622-629.

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  To develop super-high strength TRIP steel for automobile application with good combination of strength and plasticity, an ultra-high-strength TRIP steel 0. 2%C-2. 0%Si-1. 8%Mn was investigated at different intercritical annealing temperatures and bainitic isothermal temperatures. Microstructures were observed by scanning electron microscopy, transmission electron microscopy, electron backscatter diffraction, and X-ray diffraction, and mechanical properties were tested under a uniaxial tensile testing machine. It shows that the best comprehensive mechanical properties were obtained for the experimental steel particularly annealed at 820 ℃ and austempered at 410 ℃. Yield strength of the tested steel mainly depends on ferrite, while the high strength of the tested steel is related to martensite which forms through blocky austenite transformation during cooling and subsequent tensile test attributed to the TRIP effect. Hence, it could be concluded that the grain size of the retained austenite and carbon content in retained austenite are the main reasons for high elongation of the tested steel.
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  Ultrasonic Fatigue Damage Behavior of 304L Austenitic Stainless Steel Based on Micro-plasticity and Heat Dissipation

  Chao HE,Ren-hui TIAN,Yong-jie LIU,Jiu-kai LI,Qing-yuan WANG

  中国钢铁期刊网. 2015, 22(7): 638-644.

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  Very high cycle fatigue behavior (107-109 cycles) of 304L austenitic stainless steel was studied with ultrasonic fatigue testing system (20 kHz). The characteristics of fatigue crack initiation and propagation were discussed based on the observation of surface plastic deformation and heat dissipation. It was found that micro-plasticity (slip markings) could be observed on the specimen surface even at very low stress amplitudes. The persistent slip markings increased clearly along with a remarkable process of heat dissipation just before the fatigue failure. By detailed investigation using a scanning electron microscope and an infrared camera, slip markings appeared at the large grains where the fatigue crack initiation site was located. The surface temperature around the fatigue crack tip and the slip markings close to the fracture surface increased prominently with the propagation of fatigue crack. Finally, the coupling relationship among the fatigue crack propagation, appearance of surface slip markings and heat dissipation was analyzed for a better understanding of ultrasonic fatigue damage behavior.
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  Effect of Martensite Fine Structure on Mechanical Properties of an 1100 MPa Grade Ultra-high Strength Steel

  Fan ZHEN,Kuan ZHANG,Zhi-long GUO,Jin-bo QU

  中国钢铁期刊网. 2015, 22(7): 645-651.

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  An 1100 MPa grade ultra-high strength steel with different martensite fine structures, characterized by prior austenite grain size, martensite packet size, block width and lath width, was studied by various heat treatment processes. The result shows that with decreasing prior austenite grain size, both the packet size and block width decrease, while the lath width has virtually no change. Accordingly, both strength and toughness increase, while total elongation decreases. The yield strength has a Hall-Petch type relationship with the prior austenite grain size, packet size and block width, and the block width may be regarded as a key factor influencing strength. On the other hand, the ductile to brittle transition temperature (DBTT) is found to be more related to the packet size, which may be considered as a dominant factor influencing toughness.
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  Structure and Performance Changes of Ni-Co-Al Shape Memory Alloys in Relation to Co/Al Atomic Ratio

  Jia JU,Feng XUE,Liu-xia SUN

  中国钢铁期刊网. 2015, 22(7): 652-656.

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  As a potential ferromagnetic shape memory alloy, Ni-Co-Al has excellent mechanical properties, large magentic-field-induced strain and high martensitic transformation temperature. The relationship between microstructure and performance (mechanical and magnetic properties) of Ni-Co-Al with different Co/Al atomic ratios (RCo/Al) was investigated. Samples exhibit β and γ dual-phase structure. The γ phase grows coarse and the volume fraction of γ phase increases with the rise of RCo/Al. Besides, sample with high amount of γ phase content has smaller β grains owing to the pinning effect of γ phase. The martensite, transformed from β phase, is tetragonal L10 structure with a (111) twinning plane. The martensitic transformation temperature of samples ascends with increasing RCo/Al owing to more Co embedded into the cell, which makes the valence electron concentration (e/a) of system rise. The saturation magnetization (Ms) of samples increases as RCo/Al rises because Co-rich γ phase has excellent magnetic property. Meanwhile, both compressive and micro-hardness tests reveal that the samples containing more γ phase have excellent ductility due to the intrinsic good ductility nature of γ phase.