2018年, 第25卷, 第8期　刊出日期：2018-08-15

  

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  Application of tellurium in free-cutting steels

  Ping Shen Qian-kun Yang Dong Zhang Yan-xin Wu, Jian-xun Fu

  . 2018, 25(8): 787-795.

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  Te is widely used in iron and steel industry. After adding a certain amount of Te in the steel, many physical and chemical properties can be improved. As a free-cutting element, a small amount of Te can significantly improve the machinability of steel. The existing form of Te in the steel, the modification law of MnS inclusion by Te and the influence rule and mechanism of Te on the machinability of steel are summarized and expounded in detail, providing a reference for further study and development of Te-containing free-cutting steels.
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  Inclusion control study in high aluminum steel

  Bo-wen Peng Fang-jie Li Shao-bo Zheng ? Hui-gai Li Qi-jie Zhai

  . 2018, 25(8): 796-802.

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  In order to reduce the negative effect of gross Al-rich inclusions on high aluminum steel, both thermodynamic calculations and designed deoxidization experiments were performed in condition of different silicon and aluminum deoxidization sequences. Thermodynamic calculations demonstrated that the complex inclusions with low melting point (harmless inclusions) are more favorable to be formed in condition that the melt was deoxidized using ferrosilicon first and then pure aluminum (Si/Al deoxidization) than that deoxidized using pure aluminum first and then ferrosilicon (Al/Si deoxidization). Al-rich inclusion decreases 83% from Al/Si deoxidization to Si/Al deoxidization. The experimental results showed that the total quantity of inclusions decreases by 24% and the proportion of harmless inclusions is doubled when comparing Si/Al deoxidization with Al/Si deoxidization. The morphology of complex inclusion tends to be spherical, and the mean size is less than 5 lm in condition of Si/Al deoxidization. The formation mechanism of harmless inclusions was discussed, and it is believed that the inclusions can be controlled reasonably in high aluminum steel using Si/Al deoxidization.
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  Analysis of precipitation behavior of MnS in sulfur-bearing steel system with finite-difference segregation model

  De-lin Hu Hui Liu Jian-bo Xie Juan Cheng Jian Li Jian-xun Fu

  . 2018, 25(8): 803-812.

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  To further reveal the influence of micro-segregation on the precipitation behavior of MnS in sulfur-bearing steel system, a coupled model of micro-segregation and MnS precipitation was established by the finite-difference method based on various calculation domains and the solid diffusion degrees, and a new controlled diffusion equation with more stable convergence was also used. The steels, 49MnVS3 and 1215, were used to analyze the influence of calculation domain, segregation model and S content on the precipitation behavior of MnS. The calculation results were verified by a high-temperature confocal laser scanning microscope (HT-CLSM). The results show that the domain has little effect on the precipitation temperature, precipitation solid fraction and precipitation amount of MnS, but affects the precipitation location and segregation of the solutes. For low- and medium- sulfur steels, the temperatures calculated by the diffusion control growth (DCG) model and the Lever model were nearly identical, whereas the temperature calculated by the Scheil model was smaller. However, for high-sulfur steels, the precipitation temperatures calculated by three segregation models were nearly same. The precipitation solid fraction is more reasonable to describe the precipitation behavior of MnS. The precipitation behavior of MnS, observed by the HT-CLSM, matches well with the DCG model.
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  Thermomechanical analysis of triangular zone cracks in vertical continuous casting slabs based on viscoelastic–plastic model

  Juan Cheng Yan-xin Wu ? Yang Wang Jian-xun Fu

  . 2018, 25(8): 813-820.

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  The triangular zone cracks in 2101 duplex stainless steel produced by the vertical continuous caster have troubled company A for a long time. To simulate the temperature and thermal stress distributions in the solidification process of 2101 duplex stainless steel produced by the vertical continuous caster, a two-dimensional viscoelastic–plastic thermomechanically coupled finite element model was established by the secondary development of the commercial nonlinear finite element analysis software MSC Marc. The results show that the thermal stress on the surface reaches a maximum at the exit of the mould, and the highest thermal stresses at the centre of the wide face and the narrow face are 75 and 115 MPa, respectively. Meanwhile, the internal temperature of slab is still higher than the solidus temperature, resulting in no thermal stress. The slab shows different high-temperature strengths and suffers from different stresses at different positions; thus, the risk of cracking also varies. At a location of 6–8 m from the meniscus, the temperature of the triangular zone is 1270–1360 C and the corresponding permissible high-temperature strength is about 10–30 MPa, while the thermal stress at this time is 60 MPa, which is higher than the high-temperature strength. As a result, triangular zone cracks form easily.
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  Relationship between secondary dendrite arm spacing and local solidification time of 30Cr2Ni4MoV alloy at slow cooling rates

  Jing Zhao Hong-gang Zhong Ke Han ? Ren-xing Li Zhi-shuai Xu Qi-jie Zhai

  . 2018, 25(8): 821-829.

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  Solidification process of 231 t 30Cr2Ni4MoV ingot manufactured by slow cooling process was studied using experimental and numerical simulations, which tackled the problems of high cost and long period in large ingot studying. Based on the numerical results of large ingot, five characteristic locations under different temperature gradients and cooling rates chosen from the large ingot were simulated. The experiments were performed under the same temperature conditions as in numerical simulations with specialized instrument. The influences of temperature gradient in the solid–liquid interface and solidification rate on the size and morphology of solidification structure were analyzed at cooling rate ranging from 10-3 to 10-2 C s-1. Solidification macrostructure and microstructure showed that no columnar dendrite was found in any specimen. The grain size and secondary dendrite arm spacing decreased at larger cooling rate, and the relationship between secondary dendrite arm spacing and local solidification time or cooling rate was determined.
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  Microstructure and mechanical behavior of a low-density Fe–12Mn– 9Al–1.2C steel prepared using centrifugal casting under near-rapid solidification

  Wei He? Bi-lei Wang Yang Yang Yun-hu Zhang Lian Duan Zhi-ping Luo Chang-jiang Song Qi-jie Zhai

  . 2018, 25(8): 830-838.

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  It is vital for emission reduction and energy saving to lighten the weight of automobile. Low-density Fe-Mn-Al-C steels with high strength and excellent ductility have become a promising type of material in the automotive industry. In this work, a new approach was proposed by using centrifugal casting to produce the low-density Fe-12Mn-9Al-1.2C steel with high performance under near-rapid solidification in a near-net shape. The produced steel strips, with a thickness of 2.5 mm and a density of 6.89 g/cm3, were examined for their microstructures and mechanical properties. The results showed that mechanical properties of as-cast steel strip reached 1,182 MPa in ultimate tensile strength, and 28.1% in total elongation. Aging treatment at 400 oC or 600 oC for 3 h enhanced tensile strength of the steel strips, while aging at 800 oC dramatically decreased its elongation. Moreover, Young’s modulus of the steel strip improved with the increment of aging temperature. The relationship between the mechanical properties and the microstructure was discussed. This work demonstrated that advanced low-density steels with promising mechanical properties can be directly produced from liquid by this simple process.
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  Anomalous residual austenite formation in root of welding layers of 9% Cr/CrMoV dissimilar welded joint

  Kai Ding Hui-jun Ji Quan-liang Zhang Xia Liu ? Peng Wang Xiao-hong Li Ling Zhang Yu-lai Gao

  . 2018, 25(8): 839-846.

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  The morphology and distribution of residual austenite in the welded zone of 9% Cr/CrMoV dissimilar welded joint were observed by color metallographic method. Moreover, the details of the distribution, shape, length, length-to-width ratio, and the content of residual austenite in each welding layer were systematically characterized using Image-Pro Plus image analysis software. Different from the techniques such as X-ray diffraction, vibrating sample magnetometer, and electron backscatter diffraction that only involve the content of residual austenite, color metallographic method can offer much more details of the residual austenite, including the location, shape, size, and content, helping evaluate the overall characteristics of the residual austenite produced in the weld. It is interesting that a large amount of block-like and film-like residual austenite was found in the root of each welding layer, while rare at the top. The area fraction of residual austenite located in this zone was approximately 7.5% compared with only 0.5% at the top. In addition, the average area, length, and length-to-width ratio of dispersed residual austenite were quantitatively evaluated to be (4.7 ± 0.6) lm2, (5.1 ± 0.6) lm, and (1.1 ± 0.1) lm, respectively. This variety of residual austenite was resulted from the tempering effect by latter welding layer on the previous one and different cooling rates in these two regions during the welding process.
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  Prevention of carbon migration in 9% Cr/CrMoV dissimilar welded joint by adding tungsten inert gas overlaying layer

  Kai Ding Hui-jun Ji Xia Liu Peng Wang Quan-liang Zhang Xiao-hong Li Yu-lai Gao

  . 2018, 25(8): 847-853.

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  High chromium (9–12% Cr) steels with excellent heat resistance and CrMoV steels with good toughness were potential candidates for combined rotor for steam turbine operated over 620 C. Two welding techniques were used to fabricate 9% Cr and CrMoV dissimilar welded joint. The results show that the carbon migration only appears in the specimen using narrow gap submerged arc welding (NG-SAW) technique, yet it can be effectively prevented by adding tungsten inert gas (TIG) overlaying process before the NG-SAW. The carbon migration occurred in NG-SAW resulting from the sharp transition of the strong carbide-forming element Cr between the weld (* 2.7 wt%) and the base metal (* 9 wt%). On the contrary, the application of TIG overlaying layers can promote the diffusion of Cr element, and therefore result in its much smaller concentration gradient. That is to say, a gentle transition zone of Cr element can be created among the SAW weld, TIG overlaying layers and the base metal, which effectively prevents the carbon migration and therefore produces a decreased carbon concentration adjacent to the fusion line.
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  Influence of electromagnetic parameters on solidification structure of pure Al in the case of identical power

  Yong-yong Gong, Shu-min Cheng Yu-yi Zhong Xiang Wang Yun-hu Zhang, Qi-jie Zhai,, Hong-gang Zhong, Zhi-shuai Xu Rong Yue Ning Pei

  . 2018, 25(8): 854-861.

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  Pulsed magneto oscillation（PMO）a s a new electromagnetic technology to generate the grain refinement in metals and alloys was significantly developed in the recent years. This paper investigated the influence of electromagnetic parameters variation on the grain size reduction in pure Al under the application of PMO with a constant inputting power. The experimental results show that the grain refinement degree can be improved with increasing the current intensity in the case of the identical power. Moreover, through analyzing the electromagnetic force and resistance force applied on the nucleus, a criterion of the nucleus detached from the mould wall is constructed. Based on the criterion, the detachment of nuclei with finer radius can cause better effect of the grain refinement, when applying a higher current peak. The movement of the detached grains is considered by analyzing the influence of electromagnetic force and forced flow.
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  Influence of pulsed magneto-oscillation on microstructure and mechanical property of rectangular 65Mn steel ingot

  Jie Sun? Cheng Sheng Ding-pu Wang ? Yun-hu Zhang Hong-gang Zhong Zhi-shuai Xu Li-juan Li Qi-jie Zhai

  . 2018, 25(8): 862-866.

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  The pulsed magneto-oscillation (PMO) technique has the potential to be applied in the production of heavy steel ingot. In order to confirm it and achieve more insights, the solidification of rectangular 65Mn steel ingot with the size of 220 mm 9 220 mm 9 1000 mm was investigated under the impact of PMO. Experimental results present that PMO treatment can remarkably refine the solidified microstructure of 65Mn steel ingot in comparison with the reference ingot without PMO. The application of PMO not only significantly reduces the grain size, but also promotes the morphology transition of equiaxed grains from well-developed dendritic structures to globular structures. And the resulted globular morphology is mainly due to the fact that the PMO-induced forced flow enhances the stability of crystal growth. As a consequence, the average tensile strength of as-cast samples is enhanced from 643.4 to 762.9 MPa under the application of PMO.
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  Distribution of nonmetallic inclusions in molten steel under hot-top pulsed magneto-oscillation treatment

  Hui-cheng Li, Zhen Liu, Ren-xing Li, Yong-yong Gong Zhi-shuai Xu, Qi-jie Zhai,,

  . 2018, 25(8): 867-876.

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  To control the distribution of nonmetallic inclusions in molten steel is of highly commercial importance. A newly developed method, hot-top pulsed magneto-oscillation (HPMO), was employed to control nonmetallic inclusions in a medium-carbon steel ingot. The experimental results present that the position of nonmetallic inclusions of spinel (Al–Mg– O) and Mn–S inclusions is significantly influenced by HPMO. The number of nonmetallic inclusions gradually increased from the lateral wall to the center of ingot along the radial direction under the impact of HPMO treatment, whereas this distribution trend cannot be observed in the reference sample without HPMO treatment. In addition, the number of inclusions along vertical direction is proportional to the height of ingot, especially for the inclusions with the size of larger than 10 lm. It indicates that the application of HPMO can push away inclusions from lateral to center, and then the expelled inclusions aggregate and rise up to the top of ingot.
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  Phase modulation of bcc-structured Fe35Mn25Al15Cr10Ni15 high-entropy alloy by interstitial carbon

  Cong-hui Hu Jian-lei Zhang Yun-hu Zhang Ke Han Chun-ming Li Chang-jiang Song Qi-jie Zhai

  . 2018, 25(8): 877-882.

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  High-entropy alloys (HEAs) usually contain more than five alloying elements. The ductility of a body-centered cubic (bcc) type HEA typically is lower than their face-centered cubic (fcc) counterpart. And low ductility restricts engineering applications of the bcc-structured HEAs. In engineering materials, improvement in ductility usually results in deduction of mechanical strength. The application of the bcc type HEAs is restricted to the low ductility. A method to improve both mechanical strength and ductility in a bcc-structured HEA was proposed by adding interstitial carbon. Experimental results showed that replacement of 5 at.% Cr with 5 at.% C in a bcc-structured Fe35Mn25Al15Cr10Ni15 HEA resulted in an increase of fcc phase from 0.3 vol.% to 93.7 vol.%. Strength and ductility increased at the same time. The transition of bcc-structure to fcc-structure along with a remaining small amount of bcc phase improved mechanical properties. This work indicates that interstitial carbon can be employed to modulate the fraction of constituent phases in a bcc-structured HEA to enhance engineering mechanical properties.