ISIJ International
Online ISSN : 1347-5460
Print ISSN : 0915-1559
ISSN-L : 0915-1559
Volume 59 , Issue 10
Showing 1-31 articles out of 31 articles from the selected issue
Publication Data
Regular Articles
Fundamentals of High Temperature Processes
  • Kailai Chen, Deyong Wang, Dong Hou, Tianpeng Qu, Jun Tian, Huihua Wang
    Type: Regular Article
    2019 Volume 59 Issue 10 Pages 1735-1743
    Published: October 15, 2019
    Released: October 17, 2019
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    Contact angles of five common grades of molten steel on Al2O3, MgO and MgO·Al2O3 inclusion substrates have been measured precisely by a sessile drop technique at 1823/1873 K, and the wettability of each type of inclusions is discussed. Interfacial tension between the steels and the inclusions is calculated through the Young equation. Thus, coagulation coefficient, which is used to evaluate agglomeration tendency of inclusions, has been calculated based on the collision-coagulation theory combined with the interfacial tension data. The results of the coagulation coefficient show that the agglomeration tendency of the three typical oxide inclusions in molten steel is Al2O3 > MgO > MgO·Al2O3, and the relationship between coagulation coefficient αt of inclusions and contact angle θ between inclusions and molten steels can be expressed as αt = 4.923 × 10−5[μrI3(ρMε / μ)1/2 / (σIσMcosθ)]−0.242. Furthermore, in order to verify the validity of the results obtained, three deoxidation experiments in resistance furnace were carried out. The resulting samples were quenched and then analyzed by SEM-EDS and ASPEX. The results of inclusions observed in the quenched samples show good agreement with the results of inclusion agglomeration tendency. Overall, the effects of wettability and interfacial tension on inclusion agglomeration behaviors are proved to be remarkable.

  • Miyuki Hayashi, Kenya Horita, Rie Endo, Takashi Watanabe, Masahiro Sus ...
    Type: Regular Article
    2019 Volume 59 Issue 10 Pages 1744-1751
    Published: October 15, 2019
    Released: October 17, 2019
    [Advance publication] Released: July 26, 2019
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    Activities of FeO and FeO1.5 in Na2O–SiO2–FeO–Fe2O3 melts have been investigated in terms of the coordination structure of iron ions. The melts were placed in Pt containers at 1573 K and equilibrated at partial pressures of oxygen in the range between 10−9 atm and 10−6 atm, and the activities were derived from Fe concentrations in the Pt containers using the activity coefficient of Fe in Pt–Fe alloys reported as a function of molar fraction of Fe. At the same time, the percentages of Fe2+, Fe3+ in octahedral symmetry (Fe3+(oct)) and Fe3+ in tetrahedral symmetry (Fe3+(tetr)) were also measured by Mössbauer spectroscopy. It has been found that the activity coefficients of FeO (γFeO) are larger than those of FeO1.5 (γFeO1.5), suggesting that FeO is prone to liberate from the silicate network more than FeO1.5. It has also been found that the values of γFeO monotonically increase with increasing Fe2+/Fetotal ratio; in contrast, the values of γFeO1.5 seem relevant to neither Fe3+(oct)/Fetotal nor Fe3+(tetr)/Fetotal ratio. The activity coefficients have been discussed from the perspective of the coordination structure via the effective ionic radii of Fe2+, Fe3+(oct) and Fe3+(tetr). The magnitude of effective ionic radii is in the hierarchy of Fe2+ > Fe3+(oct) > Fe3+(tetr), and thereby the bond strength between iron ion and oxide ion is in the hierarchy of Fe3+(tetr) > Fe3+(oct) > Fe2+. This suggests that Fe2+ ions are more loosely bound to the silicate skeleton than Fe3+(tetr) and Fe3+(oct) ions, which situation would be reflected in the magnitude of the activity coefficients and their dependencies on Fe2+/Fetotal, Fe3+(oct)/Fetotal and Fe3+(tetr)/Fetotal ratios.

  • Sun-joong Kim, Masaaki Kageyama, Xu Gao, Shigeru Ueda, Shin-ya Kitamur ...
    Type: Regular Article
    2019 Volume 59 Issue 10 Pages 1752-1755
    Published: October 15, 2019
    Released: October 17, 2019
    [Advance publication] Released: June 22, 2019
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    The formation of CaS around CaO–Al2O3 causes pitting corrosion in ferritic stainless steel. To prevent the precipitation of CaS, the solubility of CaS in both the liquid and solid CaO–Al2O3 system has to be clarified. In this study, the sulfur content in the CaS-saturated solid CaO–Al2O3 system was measured. The results showed that, sulfur was soluble only in 12CaO·7Al2O3 at approximately 1.2 mass% while the sulfur content in the other solid compounds was very low. In addition, the sulfur content of 12CaO·7Al2O3 was independent of the heating temperature and was higher than that in the liquid oxide of the same composition. Therefore, 12CaO·7Al2O3 can dissolve sulfur in the solid state, preventing CaS formation.

Ironmaking
  • Koji Osuga, Takero Adachi, Kazuya Miyagawa, Toshihide Matsumura, Kenta ...
    Type: Regular Article
    2019 Volume 59 Issue 10 Pages 1756-1764
    Published: October 15, 2019
    Released: October 17, 2019
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    Kakogawa Works of Kobe Steel, Ltd. has a set of sinter and pellet plants for the production of blast furnace raw materials. Recently, the use of magnetite fine ore has attracted attention while deterioration of iron ore grade. We clarified the problem of granulation and firing in using large quantity of magnetite fine ore, and developed countermeasures.

    The most serious problem is forming un-granulated fine and huge agglomerates which causes decrease of permeability and deterioration of productivity. Since the strength of huge agglomerates is low, they collapse during transportation and sintering process. And then, they decrease permeability of packed bed and reduce productivity.

    Selective granulation of magnetite fine ore is effective to suppress un-granulated fine and huge agglomerate. We propose the parallel granulating process which produce the double layer mini-pellet of magnetite fine ore which consists of core and adhesive layer. Double layered structure prevents collapse effectively because of aggregate effect of core particle. In addition, as the adhering layer of magnetite fine ore become thinner, magnetite is fully oxidized. Using double layer mini-pellet improves productivity and reduces coke consumption.

  • Shengqiang Song, Petrus Christiaan Pistorius
    Type: Regular Article
    2019 Volume 59 Issue 10 Pages 1765-1769
    Published: October 15, 2019
    Released: October 17, 2019
    [Advance publication] Released: July 19, 2019
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    Iron ore pellets prepared from magnetite concentrate are strengthened by oxidation of magnetite to hematite during pellet firing. In the present work, hematite whiskers were observed to grow on the oxidized surface of magnetite concentrate particles, over the entire temperature range studied (800°C to 950°C). The whisker thickness increased from approximately 30 nm for oxidation at 800°C to 200 nm at 950°C. The whiskers likely act as bridges between concentrate particles during pellet firing, contributing to pellet strength.

    Hematite whiskers formed on magnetite particles during oxidation in air for 10 minutes at (a) 800°C, (b) 850°C, (c) 900°C, and (d) 950°C. Fullsize Image
  • Fan Xiaoyue, Jiao Kexin, Zhang Jianliang, Ma Hongxiu, Jiang Haibin, Ya ...
    Type: Regular Article
    2019 Volume 59 Issue 10 Pages 1770-1775
    Published: October 15, 2019
    Released: October 17, 2019
    [Advance publication] Released: July 20, 2019
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    Changes in porosity and graphitization degree of coke samples passing through different locations in the hearth deadman of a commercial blast furnace were investigated. SEM-EDS and Raman spectra were used to measure the coke microstructure and carbon structure. It indicates that the final slag migrate into the coke matrix and react with the minerals in the coke, resulting in the various conditions of the compositions of coke minerals. Al2O3 content in the coke matrix is higher compared with the final slag. With the deadman coke downward, the basicity of the slag increases firstly and then decreases. Si particles with diameter of about 30 um are deposited in the coke pores and different shaped Fe3Si are found in the deadman coke. The porosity of coke shows a liner relationship with coke descending. The porosity of coke above the taphole centerline is 27.74%. It is 33.72% near the taphole centerline and is with 42.47% below the taphole centerline. The structure as well as graphitization of deadman coke indicates higher ordering of coke below the taphole centerline regions than that of the other two samples. These results have an important practical significance for the dissolution reaction of coke in the blast furnace hearth.

  • Qun Niu, Shusen Cheng, Wenxuan Xu, Weijun Niu
    Type: Regular Article
    2019 Volume 59 Issue 10 Pages 1776-1785
    Published: October 15, 2019
    Released: October 17, 2019
    [Advance publication] Released: June 25, 2019
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    In this paper, the residual thickness of carbon brick, residual carbon brick and skull of a Chinese 2800 m3 blast furnace hearth were studied in detail and the formation mechanism of skull and brittle layer were proposed. The results show that the remaining thickness of carbon brick is highly inhomogeneous in the height and circumferential direction. In the circumferential direction, the sidewall erosion in the range of 3.6 m under the taphole is more serious. In the height direction, the carbon brick at the distance of 1.0–2.0 m below the central line of the taphole is more obvious. The erosion of hearth bottom is “mumps face+ bowl” type erosion. The minerals of the hot surface of carbon brick used for more than nine years are mainly composed of KAlSiO4, KAlSi2O6, Zn2SiO4 and ZnO as well as a small amount of ZnS, KCl and ZnAl2O4. Micro cracks resulted from the KAlSiO4, KAlSi2O6, Zn2SiO4 and ZnAl2O4 are the inducement of formation of brittle layer. The main reason for the formation of macro cracks and brittle layer in carbon brick is the continuous accumulation of ZnO in micro cracks. The brittle layer mainly occurs in the region where the temperature of carbon brick is lower than 950°C. The skull above the central line of the taphole is mainly composed of Ca2Al2SiO7, Ca2MgSi2O7, CaTiO3 and KAlSiO4. The skull below the central line of the taphole is primarily comprised of Ca2Al2SiO7, Ca2MgSi2O7, CaS, Fe and Fe3Si. The blast furnace slag phase in the skull below the central line of taphole is derived from the blast furnace slag that penetrates into the deadman coke. The blast furnace slag can be present below the central line of the taphole and adhere to the hot surface of the carbon brick to isolate the direct contact between the molten iron and carbon brick.

  • Anton Andersson, Amanda Gullberg, Adeline Kullerstedt, Anita Wedholm, ...
    Type: Regular Article
    2019 Volume 59 Issue 10 Pages 1786-1795
    Published: October 15, 2019
    Released: October 17, 2019
    [Advance publication] Released: June 22, 2019
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    Ore-based steelmaking generates various residues including dusts, sludges, scales and slags. Recycling of these residues within the process or via other applications is essential for sustainable production of steel. In blast furnace (BF) ironmaking, the gas-cleaning equipment generally recovers the particles in the off-gas as dust and sludge. Traditionally, the dry dust is recycled via the sinter or, in the case of pellet-based BF operation, via cold-bonded briquettes and injection. As the BF sludge mainly consists of iron and carbon, this residue is of interest to recycle together with the BF dust. However, depending on how the BF is operated, these two residues are more or less the major outlet of zinc from the furnace. Thus, to limit the recycled load of zinc, both materials cannot be recycled without dezincing the sludge prior to recycling. Dezincing and recycling of the low-zinc fraction of BF sludge via sinter have been reported whereas recycling via cold-bonded briquettes has not been performed. In the present study, cold-bonded briquettes containing the low-zinc fraction of dezinced BF sludge were charged as basket samples to the LKAB Experimental Blast Furnace (EBF). The excavated basket samples from the quenched EBF suggested that additions of up to 20 wt.% of upgraded BF sludge was feasible in terms of reducibility and strength. Based on these results, BF sludge were added to cold-bonded briquettes and charged in industrial-scale trials. The trials indicated that the annual generation of BF sludge, after dezincing, could be recycled to the BF.

  • Zhiyu Chang, Kexin Jiao, Jianliang Zhang, Xiaojun Ning
    Type: Regular Article
    2019 Volume 59 Issue 10 Pages 1796-1800
    Published: October 15, 2019
    Released: October 17, 2019
    [Advance publication] Released: July 02, 2019
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    The accumulation behavior of harmful elements (K, Na, Zn) in the upper area of the cohesive zone was reported for the first time. The alkalis-bearing aluminosilicate minerals and the kalsilite were found in the coke, while a number of zinc oxide crystals mainly existing as hexagonal wurtzite habit and zinc-bearing minerals were observed in the mixture-like phase of slag-iron. The findings further deepen the understanding of degradation behavior of coke in the cohesive zone.

Steelmaking
  • Do-Hyeong Kim, Seung-Yeon Won, Min-Kyu Paek, Hae-Sol Son, Jong-Jin Pak
    Type: Regular Article
    2019 Volume 59 Issue 10 Pages 1801-1805
    Published: October 15, 2019
    Released: October 17, 2019
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    Nickel-molybdenum alloys can be produced by the carbothermic reduction of spent catalysts containing nickel and molybdenum discarded from the petroleum refinery operations for desulfurization. However, sulfur can be picked up into alloy melts from the sulfur bearing spent catalysts during the smelting process. Thermodynamics of sulfur in this alloy melt is very important for producing low sulfur Ni–Mo alloys to be used in steel industry. In the present study, thermodynamic interaction coefficients of carbon and molybdenum on sulfur in carbon saturated liquid Ni–Mo alloys were measured using the slag/metal equilibration technique at 1773 and 1873 K. The equilibrium sulfur distribution was measured between a slag with a known sulfide capacity and carbon saturated liquid Ni–Mo–S alloys of various compositions. The carbon solubility in liquid Ni–Mo alloy was significantly changed with molybdenum content, and the specific effects of carbon and molybdenum on sulfur was determined using Wagner’s formalism as the first- and second-order interaction parameters.

  • Zhe Wang, Guanghua Wen, Ping Tang, Zibing Hou, Shaopeng Gu
    Type: Regular Article
    2019 Volume 59 Issue 10 Pages 1806-1810
    Published: October 15, 2019
    Released: October 17, 2019
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    To solve the problem of slow heating rate of traditional surface and interfacial tension test method of molten slag, a fast testing method of interface property is proposed in this paper, which bases on the single hot thermocouple technique (SHTT). The results are compared with those of the sessile drop method and literature. The results show that: 1) Using the capillary action of molten slag on the B-type thermocouple, the contact angle can be obtained quickly. The relative error between the contact angle of the slag and that of the sessile drop method is 0.10%, and the maximum fluctuation range of the repeated experiment is ±2.00°. 2) Substituting the contact angle and surface tension of the slag into Young’s equation, the interfacial tension between the slag and the B-type thermocouple can be obtained. 3) For the commonly used CaO–SiO2–Al2O3–MgO metallurgical slag system, without flux (CaF2, Na2O), the interfacial tension between slag and B-type thermocouple is less affected by the change of composition. The surface tension of the slag can be obtained by the SHTT method, and the relative error with the literature value is less than 2.94%.

Casting and Solidification
  • Hong Lei, Bin Yang, Qian Bi, Yuanyou Xiao, Shifu Chen, Changyou Ding
    Type: Regular Article
    2019 Volume 59 Issue 10 Pages 1811-1819
    Published: October 15, 2019
    Released: October 17, 2019
    [Advance publication] Released: July 26, 2019
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    Numerical simulation is an effective tool to analyze the inclusion behavior in the tundish with channel induction heating. And the inclusion mass/population conservation model is applied to predict and describe inclusion physical field. Due to the channel induction heating, Archimedes slipping velocity and Archimedes collision are applied to describe the inclusion behavior in the tundish with channel induction heating. The predicted values agree with the experimental data for the inclusion model. Numerical results show that Joule heat and electromagnetic force can prompt the inclusion removal rate. Compared with Joule heat, electromagnetic force is a more important factor to affect the inclusion’s movement and Archimedes collision is also one of the important collision mechanisms for inclusion coalescence. The inclusion removal rate in the channels is up to one third of the inclusion removal rate in the tundish, and the inclusion removal rate in the tundish increases from 21.4% to 35.05% if channel induction heating is applied.

Instrumentation, Control and System Engineering
  • Hiroaki Ono, Akihiro Ogawa, Takahiro Yamasaki, Takahiro Koshihara, Tos ...
    Type: Regular Article
    2019 Volume 59 Issue 10 Pages 1820-1827
    Published: October 15, 2019
    Released: October 17, 2019
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    In optical surface inspection of steel products under hot conditions, overdetection caused by signals from harmless surface such as scale texture pattern is usually a problem. Therefore, the authors proposed a new inspection technique called the “twin-illumination and subtraction technique,” which is able to remove only harmless signals, based on the finding that most harmful defects on these products have concave shapes, whereas most harmless features that might be overdetected have flat or convex shapes. In this technique, an image in which the concave and convex parts are emphasized could be acquired from the difference between two images of the same position illuminated from two opposite directions. As an application of this technique, after laboratory tests to confirm its effectiveness, we conducted inline test for hot pipes which have poor surface. Although harmless signals from flat surfaces could be removed as expected, harmless signals from convex surfaces such as peeled scale and micro-surfaces which cause specular reflection conditions remained in addition to harmful signals from concave defects. To improve detectability, we introduced a discrimination method based on the characteristic bright/dark patterns of concave defects, a detection and decision tree judgment function using features from images. Based on the above, we constructed a prototype system which can obtain two images with a slight time difference by using strobes and dual CCD cameras. As a result, we have confirmed that the system has sufficient performance for prevention of massive rejects and is an effective technique for detection with poor surface.

    Twin-illumination and subtraction technique. (Online version in color.) Fullsize Image
Chemical and Physical Analysis
  • Hiroshi Okano, Shusaku Takagi
    Type: Regular Article
    2019 Volume 59 Issue 10 Pages 1828-1837
    Published: October 15, 2019
    Released: October 17, 2019
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    In view of the heightened risk of hydrogen embrittlement in ultra-high strength steels, it is necessary to clarify the mechanism of hydrogen embrittlement and develop steels with superior hydrogen embrittlement resistance. Hydrogen is trapped at various types of trapping sites in steels, and the influence of hydrogen on hydrogen embrittlement depends on the trapping site. Therefore, it is important to identify the kinds of hydrogen trapping sites in steels. The purpose of this study is to identify the hydrogen trapping sites that exist in ultra-high strength steel sheets. In this study, 1180 MPa grade dual-phase steel was used. Various levels of strain were applied to the samples by rolling, which was followed by cathodic electrolytic hydrogen charging. The hydrogen desorption rate was measured from −50°C by using a Thermal Desorption Analysis (TDA) device which enables evaluation of each hydrogen trapping site. The TDA results were analyzed with a Gaussian function to identify each hydrogen trapping site. Four types of hydrogen trapping sites were identified in the DP steel. The analysis showed that the type of hydrogen trapping site detected as a peak at 35°C was dislocations, the type at 54°C was carbides in the martensite structure of the DP steel, that at 75°C was various interfaces in the martensite and the ferrite-martensite interface in the DP steel, and that at 110°C was vacancy clusters.

  • Tetsuhiko Matsuda, Kazuaki Wagatsuma
    Type: Regular Article
    2019 Volume 59 Issue 10 Pages 1838-1845
    Published: October 15, 2019
    Released: October 17, 2019
    [Advance publication] Released: July 20, 2019
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    This paper described how the lateral resolution of an elemental mapping was estimated in laser-induced breakdown optical emission spectrometry (LIBS), when the focus point of a high-frequency Q-switched Nd:YAG laser was moved on a sample surface, along with measuring the emission signal from the resultant plasma. Several measuring parameters were optimized to improve the lateral resolution; namely, they were an averaged laser power of 1 mJ/pulse, a laser repetition frequency of 1 kHz, a scanning rate of the laser beam of 0.5 mm/s, and an atmospheric gas pressure of He 1000 Pa. Using these optimal parameters, a lateral resolution was obtained to be ca. 20 µm in the one-dimensional direction of laser scan. Furthermore, two model samples, in which regularly-aligned copper circles were deposited on a nickel plate, were irradiated by a scanning laser beam to determine actual resolving abilities both in a line direction along travelling the laser and in a two-dimensional direction over a certain sample area. The sample having an interval of 85 µm between the copper circles could give an emission image which was appropriately resolved in the two-dimensional as well as the one-dimensional direction; however, in the other sample having the 25-µm interval, the two-dimensional resolution became degraded compared to the resolution of the line scan, probably because the ablation grooves, which were left on the sample surface, had a width of more than 100 µm and were overlapped with each other in the observed area.

  • Francis Gyakwaa, Matti Aula, Tuomas Alatarvas, Tero Vuolio, Marko Hutt ...
    Type: Regular Article
    2019 Volume 59 Issue 10 Pages 1846-1852
    Published: October 15, 2019
    Released: October 17, 2019
    [Advance publication] Released: July 23, 2019
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    Calcium aluminate (CaO–Al2O3) phases play a critical role in the study of non-metallic inclusions in aluminium killed, and calcium treated steels. In this study, the Raman spectroscopy technique, a versatile and non-destructive approach, was used to characterise binary calcium aluminate phases qualitatively and quantitatively. Calcium aluminate samples with varying CaO/Al2O3 ratios were synthesised to produce a binary phase samples mixture of C12A7–C3A and C12A7–CA. Quantitative estimation was based on plotting a linear regression calibration model between the ratio of Raman band intensities and the phase fraction in the samples. With the linear regression, the phase fraction of C12A7–C3A and C12A7–CA was estimated with average absolute errors of 2.97 and 2.55 percentage points. This work demonstrates the potential suitability of using Raman spectroscopy technique for evaluating whether calcium aluminate phases in oxide inclusions fall within the liquidus region at steelmaking temperatures.

Welding and Joining
  • Yasuhiro Aoki, Ryosuke Kuroiwa, Hidetoshi Fujii, Gen Murayama, Masanor ...
    Type: Regular Article
    2019 Volume 59 Issue 10 Pages 1853-1859
    Published: October 15, 2019
    Released: October 17, 2019
    JOURNALS OPEN ACCESS FULL-TEXT HTML

    Linear Friction Welding (LFW) is a solid-state joining process in which a joint is obtained through the relative motion of two components under a high contact load. The most important factor of this conventional method is to obtain a fresh surface at the interface by expelling the weld interface as flash. In this study, medium carbon steel was welded by LFW at a low frequency, low amplitude and high applied pressure. As a result of the temperature measurements and microstructure observations, the maximum temperature of the weld plane was confirmed to be below the A1 transformation temperature, and martensitic transformation was suppressed at the weld interface. The key concept of this method is applying a large strain deformation to the interfaces to recrystallize at a lower temperature which is different from the conventional LFW.

  • Tetsuro Hidaka, Nao-Aki Noda, Yoshikazu Sano, Nobuhiro Kai, Hiroyoshi ...
    Type: Regular Article
    2019 Volume 59 Issue 10 Pages 1860-1869
    Published: October 15, 2019
    Released: October 17, 2019
    [Advance publication] Released: July 30, 2019
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    In this study fatigue experiments are conducted for ductile cast iron (DCI) to compare with the fatigue strength of cruciform welded joints. Here, several DCI specimens are prepared to have nearly the same fatigue strength in smooth specimens before welding and to have similar cruciform shapes in the welded joints. It is found that the fatigue strength of DCI specimen is about three times larger than that of the welded joint specimens. The fatigue strength improvement can be explained in terms of the small stress concentration factor, notch insensitivity and compressive residual stress generated by shot blasting for DCI joints.

Surface Treatment and Corrosion
  • Jun Maki
    Type: Regular Article
    2019 Volume 59 Issue 10 Pages 1870-1877
    Published: October 15, 2019
    Released: October 17, 2019
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    Aluminized steel sheets are very resistant to corrosion in the outdoor exposure environment. We evaluated the corrosion behavior of aluminized steel sheets with a Type 1 coating containing approximately 10% Si and a Type 2 coating not containing Si in a 50-year outdoor exposure test. Both specimens had strong perforation resistance, but those with Type 2 coating exhibited excellent perforation resistance. Type 2 aluminized steel sheets are known as a superior corrosion resistant materials compared to Type 1 aluminized steel sheets due to their thicker intermediate layer and higher coating weight. The Type 2 aluminized steel sheets in this exposure test included two sublayers composed of Fe2Al5 and FeAl2 as the intermediate layer between the aluminized layer and the steel substrate. The FeAl2 phase has less noble potential than the steel substrate and the Fe2Al5 phase in an artificial rain environment. As a result, this layer provided sacrificial corrosion protection for the steel substrate, and was the reason why the specimens with Type 2 coating exhibited better perforation resistance than those with Type 1 coating.

    Corrosion potential of intermetallic compound, steel substrate, and aluminized steel sheets exposed to artificial rain. Fullsize Image
  • Yuki Suzuki, Atsushi Morishita
    Type: Regular Article
    2019 Volume 59 Issue 10 Pages 1878-1885
    Published: October 15, 2019
    Released: October 17, 2019
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    Zinc-coated steel is widely used in the areas of home appliances, construction materials, and automobiles. Chromate-free chemical conversion coatings formed on zinc-coated layer can prevent zinc from corrosion. In this study, with the purpose of establishing guidelines for enhancing corrosion resistance in areas on zinc-coated steels where the underlying steel is exposed to the environment, we analyzed the effect of phosphate compounds used as a corrosion inhibitor on corrosion resistance in areas of exposed steel substrate and examined the mechanism behind the compounds. We demonstrated that adding phosphoric acid to a chemical conversion coating enhances corrosion resistance in scratches that expose the steel substrate, inhibiting zinc corrosion in the coated layer within the scratches. Since compounds consisting of Zn2+ and PO43− covered the steel substrate exposed in the scratches, we think these compounds acted as a barrier against salt water and oxygen. PO43− was eluted from the chemical conversion coating around the scratches by salt water in the salt spray test, and the progress of corrosion was inhibited. We therefore demonstrated that allowing a minimum specific amount or more of PO43− to be eluted from a chemical conversion coating can effectively inhibit the corrosion of zinc-coating in scratches.

  • Tatsuhiko Aizawa, Tomoaki Yoshino, Tomomi Shiratori, Sho-Ichiro Yoshih ...
    Type: Regular Article
    2019 Volume 59 Issue 10 Pages 1886-1892
    Published: October 15, 2019
    Released: October 17, 2019
    [Advance publication] Released: July 20, 2019
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    Coarse and fine grained AISI316 substrates were prepared to describe the grain size effect on the inner nitriding behavior at 623 K by using the high density plasma nitriding without precipitation of nitrides. In case of coarse grained AISI316, the nitriding process advanced homogeneously in one part of nitrided layer with high nitrogen content, and, heterogeneously in its other part. In the former, γα’ two-phase, fine microstructure was uniformly formed by the phase transformation and plastic straining with the nitrogen supersaturation. In the latter, the nitrogen super-saturation localized to selectively modify the coarse grains to form the transformed α’-phase zones with the plastically strained γ-phase ones, even below the nitriding front end of 30 µm. In case of fine-grained AISI316, the nitriding took place homogeneously to form fine, two-phase microstructure down to the nitriding front end of 40 µm. This difference in the inner nitriding behavior came from the synergetic relationship between the nitrogen diffusion and super-saturation processes.

    EBSD analysis of fine grained AISI316 substrate after plasma nitriding at 623 K for 14.4 ks. (a) Phase mapping, (b) KAM distribution, and (c) Inverse pole figure in the normal direction. Fullsize Image
Transformations and Microstructures
  • Yahui Han, Changsheng Li, Jinyi Ren, Chunlin Qiu, Yongqiang Zhang, Jia ...
    Type: Regular Article
    2019 Volume 59 Issue 10 Pages 1893-1900
    Published: October 15, 2019
    Released: October 17, 2019
    [Advance publication] Released: July 31, 2019
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    The microstructural evolution and microhardness changes of the H13 ingot during homogenization process were investigated to characterize the dendrite segregation changes by optical microscopy (OM), scanning electron microscopy (SEM), electronic probe micro-analyzer (EPMA), X-Ray diffraction (XRD) and microhardness testing. The results showed that the severe dendrite segregation existed in H13 ingot, and there were a large amount of coarse non-equilibrium eutectic carbides (MC and M7C3) in dendrite region. After the soaking time of 15 h at 1200°C, the dendrites almost disappeared, the contents of eutectic carbides decreased sharply. The values of segregation ratio SR were 1.20 for Cr, 1.26 for Mo and 1.41 for V, which were the critical values of SR that marked the end of homogenization. After 20 h, a small amount of MC carbides still distributed at the grain boundaries. The SR values almost remained constant with the values of 1.10, 1.14 and 1.14 for Cr, Mo and V element respectively. The homogenization kinetics was established and it matched well with the experimental data. The microhardess in different soaking time reflected well the dendrite segregation changes, it decreased from 817.8 HV to 645.7 HV and then increases to 752.5 HV, finally it decreases gradually.

Mechanical Properties
  • Zoha Ghorani, Afshin Yousefi, Goroh Itoh
    Type: Regular Article
    2019 Volume 59 Issue 10 Pages 1901-1907
    Published: October 15, 2019
    Released: October 17, 2019
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    Hydrogen distribution in an electrolytically hydrogen-charged duplex stainless steel (DSS) was investigated by means of hydrogen microprint technique (HMPT), and quantitative analysis was made in terms of the sites where hydrogen atoms were desorbed. The results obtained were discussed with respect to the trapping site and diffusion path. The DSS (JISSUS329J4L) specimens with phase volume fraction of about 50:50 were examined as a function of hydrogen-charging and keeping time. The HMPT was performed on the charged side of the 1.5 and 24 h charged specimens with two keeping times in the ambient air for 0.5 (unkept) and 300 h. In the unkept specimen, hydrogen atoms (silver particles) were mostly detected on the interphase boundary at the shortest charging time, while they were also in the ferrite matrix for the longer charging time. The relative fraction of hydrogen atoms on the boundary against ferrite matrix was increased by increasing keeping time both in the cases of 1.5 and 24 h charging. Thus, the phase boundary was regarded as the preferential stable site for hydrogen in the DSS. Observation on the distribution of hydrogen in the cross section (middle thickness) of the unkept specimen for the charging time of 24 h revealed that relative ratio of silver particles inside the ferrite phase against the phase boundary was larger than that on the surface. This confirmed that some of the charged hydrogen atoms diffuse toward the other side, leaving the interphase boundary into the ferrite matrix because gamma phase is isolated.

    SEM images of HMPT from the cross section of specimens after 24 h hydrogen charging (regional volume fraction is 52% alpha and 48% gamma). The specimens were emulsion-covered as soon as possible after charging (holding time, 0.5 h). Fullsize Image
Social and Environmental Engineering
  • Zuoqiao Zhu, Xu Gao, Shigeru Ueda, Shin-ya Kitamura
    Type: Regular Article
    2019 Volume 59 Issue 10 Pages 1908-1916
    Published: October 15, 2019
    Released: October 17, 2019
    [Advance publication] Released: July 26, 2019
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    To suppress alkaline elution from steelmaking slag through microstructure control, in this study, several mineralogical phases identified in industrial steelmaking slag were synthesized, and their dissolution behaviors were investigated. The results indicated that in addition to free CaO and free MgO, 2CaO·SiO2-3CaO·P2O5 (C2S–C3P) is another reason for alkaline elution from steelmaking. To suppress the amount of C2S–C3P in steelmaking slag, reducing the slag basicity by the compositional modification of slag at the hot stage was considered. To determine the optimum composition, the contribution of newly formed primary crystalline phases must be clarified. Some primary crystalline phases in the CaO–SiO2–FeOx–MgO–Al2O3 system were synthesized, and their dissolution behaviors were evaluated. The results indicated that the formation of α-CaO·SiO2 (α-CS), β-CaO·SiO2 (β-CS), 3CaO·2SiO2 (C3S2), 2CaO·MgO·2SiO2 (C2MS2), and 3CaO·MgO·2SiO2 (C3MS2) must be restrained in slag modification. Based on the results, a slag with multiple components and low basicity was synthesized, and the suppression of alkaline elution by reduction of slag basicity and microstructure control was confirmed. Moreover, a method to predict Ca dissolution and pH change during the dissolution of slag by the combination of each crystal phase was proposed.

  • Yasutaka Ta, Takeru Hoshino, Hiroyuki Tobo, Keiji Watanabe
    Type: Regular Article
    2019 Volume 59 Issue 10 Pages 1917-1925
    Published: October 15, 2019
    Released: October 17, 2019
    [Advance publication] Released: June 29, 2019
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    A continuous blast furnace slag solidification process was developed to promote the use of air-cooled slag coarse aggregate for concrete. In this process, molten slag can be solidified in only 120 seconds, and the thickness of the slag is about 25 mm. After crushing the slag, the water absorption ratio is much lower than that achieved in the past because gas generation is suppressed. With this apparatus, most of the slag is crystalline, but part of the slag has a glassy surface. Therefore, EPMA and XRD were used to study the glass transition phenomenon. It found that the thickness of the glass layer is about 2 mm. To discuss the glass transition and crystallization phenomena, the thermal history was simulated by heat transfer analysis. The results clarified the fact that all the slag on the mold has a glassy surface layer of about 2 mm, and good agreement between the calculation and experimental data concerning the layer was obtained. It was also shown that most of the slag crystallizes in the slag pit because the temperature inside the piled slags rises to more than 1173 K. The measured slag temperature and calculated temperature were also in good agreement.

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