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ISIJ International Vol. 58 (2018), No. 2

ISIJ International
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ONLINE ISSN: 1347-5460
PRINT ISSN: 0915-1559
Publisher: The Iron and Steel Institute of Japan

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  1. Vol. 63 (2023)

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ISIJ International Vol. 58 (2018), No. 2

Key Lubrication Concepts to Understand the Role of Flow, Heat Transfer and Solidification for Modelling Defect Formation during Continuous Casting

Pavel Ernesto Ramirez Lopez, Pooria Nazem Jalali, Ulf Sjöström, Pär Goran Jönsson, Kenneth C. Mills, Il Sohn

pp. 201-210

DOI:

10.2355/isijinternational.ISIJINT-2017-482

Abstract

Surface defects are recurrent problems during Continuous Casting of steel due to the introduction of new grades that are often difficult to cast, as well as the everlasting pursuit for higher quality and improved yield. Accordingly, numerical modelling has become a ubiquitous tool to analyse the formation mechanisms of such defects. However, industrial application of simulations is often hampered by oversimplifications and omissions of important process details such as variations in material properties, specific casting practices or shortcomings regarding fundamental metallurgical concepts. The present manuscript seeks to create awareness on these issues by visiting key notions such as slag infiltration, interfacial resistance and Lubrication Index. This is done from a conceptual point of view based on industrial observations and numerical modelling experiences. The latter allows a re-formulation of outdated concepts and misconceptions regarding the influence of fluid flow, heat transfer and solidification on lubrication and defect formation. Additionally, the manuscript addresses common challenges and constraints that occur during industrial implementation of numerical models such as the lack of high-temperature material data for slags. Finally, the manuscript provides examples of improvements on product quality and process stability that can be achieved through a holistic approach which combines modelling with laboratory tests, experiences from operators and direct plant measurements.

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Key Lubrication Concepts to Understand the Role of Flow, Heat Transfer and Solidification for Modelling Defect Formation during Continuous Casting

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Effect of Solidification and Cooling Methods on the Efficacy of Slag as a Feedstock for CO2 Mineralization

Corey Adam Myers, Takao Nakagaki

pp. 211-219

DOI:

10.2355/isijinternational.ISIJINT-2017-478

Abstract

Iron and steel making (ISM) slag is often utilized to partially offset CO2 emissions associated with metal production. Currently, the primary recycling method for slag is as β-Ca2SiO4 utilized in the cement industry, termed ground granulated blast furnace slag (ggbs). However, the cement market is not large enough to exploit the entirety of ISM slag as ggbs, relegating a large quantity of slag to reuse pathways with minor impacts on CO2 reduction. Recent years have seen an increase in research into mineralizing CO2 using the Ca and Mg content of ISM slags as a feedstock. Unfortunately, it has not been widely recognized that the solidification and cooling processes of slag dramatically effects its efficacy as a CO2 mineralizing feedstock via modification of mineralogy, crystallinity, grain size, and micromorphology. This paper clarifies the key properties determining mineralization effectiveness and elucidates how to control these properties during the solidification and cooling process. The effect of solidification and cooling method on net CO2 reduction is shown to be strongly dependent on solidification and cooling method along with the CO2 intensity of energy generation.

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Effect of Solidification and Cooling Methods on the Efficacy of Slag as a Feedstock for CO2 Mineralization

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Use of a Structural Model to Calculate the Viscosity of Liquid Silicate Systems

Josué López-Rodríguez, Antonio Romero-Serrano, Aurelio Hernández-Ramírez, Miguel Pérez-Labra, Alejandro Cruz-Ramírez, Enrique Rivera-Salinas

pp. 220-226

DOI:

10.2355/isijinternational.ISIJINT-2017-483

Abstract

A viscosity model for binary and ternary silicate melts is proposed in this article. The temperature dependence of viscosity is expressed using the Arrhenius equation and the composition dependence is made through the concentration of oxygen bridges (Si–O–Si) in the silica structure. A previous proposed structural thermodynamic model is used to calculate the content of oxygen bridges. The model requires only three parameters to obtain a good agreement between experimental and calculated data for the SiO2–CaO, SiO2–MgO, SiO2–MnO, and SiO2–Na2O binary systems. The viscosity of ternary systems is calculated with the model assuming a linear function of the parameters from binary systems; however, the content of oxygen bridges is calculated using the structural thermodynamic model for ternary systems. Comparison is made between the experimental and model results for the SiO2–CaO–MgO, SiO2–CaO–MnO, and SiO2–Na2O–MgO systems. The viscosity model can take into account the effect of substituting one metal oxide for another in the ternary systems.

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Use of a Structural Model to Calculate the Viscosity of Liquid Silicate Systems

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Fate of the Chlorine in Coal in the Heating Process

Naoto Tsubouchi, Yuuki Mochizuki, Yanhui Wang, Yasuo Ohtsuka

pp. 227-235

DOI:

10.2355/isijinternational.ISIJINT-2017-302

Abstract

Pyrolysis of 29 coals with carbon contents of 71–92 mass% on a dry, ash-free basis (daf) has been performed mainly in a temperature-programmed mode at 10°C/min up to 800°C with a flow-type fixed bed quartz reactor, and some factors controlling HCl formation have been examined. The rate profiles of HCl formation exhibit at least three distinct peaks at around 260–360, 470–510 and 580–630°C, and the lowest temperature peak is present for 8 coals alone, whereas the middle and highest temperature peaks are common with almost all of the coals. The HCl profile is also affected by the size of coal particles and the height of coal particles in the fixed bed. Yields of HCl and char-Cl at 800°C for 28 coals except an American bituminous coal are 44–95 and 4–54%, respectively, and tar-Cl is as low as ≤ 7% in all cases. The chlorine distribution is almost independent of the heating rate in the range of 2.5–400°C/min and has no distinct relationship with carbon or chlorine content in coal, but HCl tends to increase with increasing amount of (Na + 2Ca) in coal with a corresponding decrease in char-Cl. When an Indonesian sub-bituminous coal is injected into an O2-blown entrained bed gasifier under pressure, there is an almost 1:1 relationship between carbon and nitrogen conversions, whereas the sulfur and chlorine are enriched in the remaining char, and the degree of the enrichment is higher with chlorine. The method of evaluating coal-Cl forms quantitatively using model chlorine compounds is proposed.

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Fate of the Chlorine in Coal in the Heating Process

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Formation of Nitrogen Mono Oxide (NO) during Iron Ore Sintering Process

Wei Lv, Xiaohui Fan, Xiaobo Min, Min Gan, Xuling Chen, Zhiyun Ji

pp. 236-243

DOI:

10.2355/isijinternational.ISIJINT-2017-370

Abstract

The effects of mineral phases evolution and atmosphere transition on the transformation of fuel-N during sintering process were studied in this paper. With the temperature increasing, the conversion rate of fuel-N to NO decreased as the coke was burning in the granules, due to the effect of sintering materials. This presented the contrary law compared with the fuel burning alone. As coke burnt at temperature below 1000°C, it was mainly the oxidizing reaction between fuel-N and oxygen. Iron ores and fluxes getting in touch with coke could promote the oxidation of fuel-N that converts to NO. While the temperature increased to higher than 1000°C, the gasification reaction between carbon and CO2 occurred apparently and generated CO. There was a competition between fuel-N’s oxidation and NO’s reduction. Meanwhile, iron oxide and calcium ferrite (CF) had a catalytic effect on NO–CO reduction reaction. Particularly, the CF generated at high temperature had the most significant catalytic activity. As a consequence, the transformation of fuel-N was inhibited, making NO emission less at higher temperature.

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Formation of Nitrogen Mono Oxide (NO) during Iron Ore Sintering Process

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Detection Method for Pulverized Coal Injection and Particles in the Tuyere Raceway Using Image Processing

Ruixuan Zhang, Shusen Cheng, Changsheng Guo

pp. 244-252

DOI:

10.2355/isijinternational.ISIJINT-2017-433

Abstract

Accurate monitoring and characterization of the working condition of pulverized coal injection plays an important role in the operation of the blast furnace. Existing monitoring systems neglect particles in the tuyere raceway and lack robust quantitative analysis. This paper presents the image-based intelligent detection method to extract features of the pulverized coal injection and particles in the tuyere raceway simultaneously and real-timely. Intelligent circle and line detection based on Hough transform is applied to obtain the background template of the raceway image. The merged local segmentation algorithm is used to obtain the regions of pulverized coal and particles. The mass flow of PCI is characterized by the coal feature calculated by the linear weighted method. The support vector machine and the width of the minimum enclosing rectangle are finally employed to obtain the size distribution of particles and determine what these particles are mainly composed of. Massive raceway images captured from 15 raceways of a 2500 m3 blast furnace were used to evaluate the detection method. The results demonstrate that the method is effective to reflect the working condition of PCI and can obtain accurate size distribution of particles in the raceway image.

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Detection Method for Pulverized Coal Injection and Particles in the Tuyere Raceway Using Image Processing

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Influence Mechanism of Lignite and Lignite Semi-coke Addition on Drum Strength of Coke

Tengfei Song, Jianliang Zhang, Guangwei Wang, Haiyang Wang, Runsheng Xu, Qinghai Pang

pp. 253-258

DOI:

10.2355/isijinternational.ISIJINT-2017-447

Abstract

In order to clarify the effect of lignite and lignite semi-coke addition during coking process on the drum strength, I-type drum, thermogravimetric analyzer, SEM and XRD were used to investigate the properties of coke, such as drum strength, coking process, microstructure and crystallite structure. The result showed that because of the dramatically interaction of the volatile matter of lignite with coking coal in the initial coking stage resulting in the increase of uneven degree of macroscopic pores, the drum strength of coke decreased rapidly with the increase of lignite addition ratio. However, when the ratio of lignite semi-coke addition was less than 6%, the stacking height Lc value of coke increased and the evolution of macroscopic pores was restrained. By adding lignite semi-coke obtained from carbonization of lignite at 700°C, the drum strength of coke was improved obviously. Thus, it was promising measure to expand the application scope of lignite in the ironmaking system of blast furnace through carbonization.

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Influence Mechanism of Lignite and Lignite Semi-coke Addition on Drum Strength of Coke

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Thermodynamic Modeling of the SFCA Phase Ca2(Fe,Ca)6(Fe,Al,Si)6O20

Reiko Murao, Takayuki Harano, Masao Kimura, In-Ho Jung

pp. 259-266

DOI:

10.2355/isijinternational.ISIJINT-2017-459

Abstract

The thermodynamic model of a silico-ferrites of calcium and aluminum solution, SFCA phase () was newly developed in the framework of the Compound Energy Formalism (CEF). Preferred substitution of Al atoms to tetrahedral sites in the SFCA solution was verified by X-ray absorption near edge structure (XANES) analysis. On considering crystallographic information in particular the short-range-ordering nature in the SFCA solution, the structure was considered for modeling the SFCA solution. The optimized Gibbs energies of all end-members can successfully reproduce the experimental single phase region of the SFCA solution.

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Thermodynamic Modeling of the SFCA Phase Ca2(Fe,Ca)6(Fe,Al,Si)6O20

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Experimental Study of the Effects of Operation Conditions on Burden Distribution in the COREX Melter Gasifier

Yang You, Zhiguo Luo, Runyu Yang, Qinfu Hou, Zongshu Zou

pp. 267-273

DOI:

10.2355/isijinternational.ISIJINT-2017-474

Abstract

The melter gasifier (MG) is the core unit in the COREX process for the final melting and reduction of iron. This work performed an experimental study to investigate the effects of charging pattern, burden bed height and burden material type on the burden distribution in a MG. The ore and coal (coke) were discharged intermittently to a 7.5:1 scaled-down model of a typical COREX MG. After the burden surface reached to a steady state, the burden was analysed in terms of ore-to-coal (coke) ratio, voidage distribution and particle size segregation. With different charging patterns, the ore and coal were not distributed uniformly but significant variation. The ore-to-coal (coke) ratio reached a maximum at the radius position of 0.6R where the thickness of ore was significantly larger than that of coal. The voidage distribution along the radial direction shows a U-shape with a minimum at the middle region. In addition, particle size segregation was observed along the radial direction of the burden pile: the smaller particles tended to accumulate in the centre while the larger ones segregated more evidently near the wall. The results showed that the charging pattern was the major factor affecting the burden distribution, followed by burden material type while the burden bed height had a minimum effect.

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Experimental Study of the Effects of Operation Conditions on Burden Distribution in the COREX Melter Gasifier

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Conditions for Minimizing Direct Reduction in Smelting Reduction Iron Making

Yang-Sub Shim, Sung-Mo Jung

pp. 274-281

DOI:

10.2355/isijinternational.ISIJINT-2017-479

Abstract

In a 2 stage-smelting reduction process, it is favorable to lower the reduction degree of iron ore in a pre-reducing unit by lowering its temperature to avoid any troubles due to stickiness of high reduced iron ore. However, less pre-reduced iron ore can induce direct reduction in a melter-gasifier, which can increase coal ratio. In this case, the direct reduction ratio is determined by the reducibility of Pre-Reduced Iron ore (PRI) and the reactivity of coal with CO2. PRI was made into pellets and its reducibility was measured in various temperatures and H2 contents. Coal was carbonized in melter-gasifier condition and then its reactivity with CO2 was measured in various temperatures and CO2 contents. The possibility that direct reduction takes place in a melter-gasifier was high because the PRI pellet was reduced more slowly than unreduced iron ore and the char reacted more actively than coke. The direct reduction rate in a melter-gasifier was roughly drawn as the product of the CO2 content in the ascending gas and the reaction rate constant of coal with CO2 and the way to minimize the direct reduction ratio was discussed with that diagram.

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Conditions for Minimizing Direct Reduction in Smelting Reduction Iron Making

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Detailed Modeling of Melt Dripping in Coke Bed by DEM – SPH

Shungo Natsui, Ko-ichiro Ohno, Sohei Sukenaga, Tatsuya Kikuchi, Ryosuke O. Suzuki

pp. 282-291

DOI:

10.2355/isijinternational.ISIJINT-2017-496

Abstract

A high-resolution numerical model was performed for a detailed understanding of packed-bed structures constructed by actual scale cokes, and the molten slag (SiO2–CaO–Al2O3) trickle flow characteristics in the lower part of an actual blast furnace. Smoothed particle hydrodynamics (SPH) simulations can track the motion of liquids containing dispersed droplets, and the discrete element method (DEM) with a multisphere approach makes possible to simulate non-spherical solid-particle motion. We carried out high-resolution large-scale trickle flow simulations using more than 10 million particles, carried out case studies of statistical processing, and evaluated the effects of physical properties varied by the composition or temperature of slag samples. We clarified that there is a limitation to predict the holdup accurately based on the capillary number, which is a widely used approach. We analyzed the influence of melt viscosity on trickle flow, and clarified that an increase in viscosity increases holdup because limiting the effective flow path and suppressing the dispersion of the droplets promoted the enlargement of each stagnant droplet. This detailed direct dynamic model could explain the mechanism underlying different holdup tendencies in conventional research.

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Detailed Modeling of Melt Dripping in Coke Bed by DEM – SPH

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TiN Particles and Clusters during Ladle Treatments of Ni-based Alloy 825 using Different Stirring Modes

Hans Erik Olof Kellner, Andrey Vladimirovich Karasev, Olle Sundqvist, Pär Göran Jönsson

pp. 292-298

DOI:

10.2355/isijinternational.ISIJINT-2017-355

Abstract

Today, titanium is often used in steelmaking not only for deoxidation but also for micro-alloying and alloying for a wide range of steel grades. Therefore, many studies are focused on investigations on the formation and behavior of Ti-containing non-metallic inclusions (such as oxides, nitrides and carbides) during production of different Ti-containing steels and their effect on final steel properties. This study has examined the behavior of TiN clusters and particles in the melt during the ladle treatment of Alloy 825 containing up to 1.2 wt% of Ti. The industrial trials were performed at the end of the ladle treatment by using argon gas in combination with electromagnetic stirring using an upwards or a downwards stirring direction. Metal samples were taken before and after ladle treatment to enable three-dimensional investigations of non-metallic inclusions and clusters. The composition, size and number of particles and clusters were determined after electrolytic extraction of the metal samples by using SEM in combination with EDS. It was found that agglomerations of TiN clusters and particles in the melt are faster during an upwards stirring in comparison to a downwards stirring. However, the removal of clusters from the melt is more effective when using a downwards stirring direction compared to when using an upwards stirring in combination with gas stirring. It was also found that the Turbulent collision is the dominant factor for the agglomeration of TiN particles in the melt.

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TiN Particles and Clusters during Ladle Treatments of Ni-based Alloy 825 using Different Stirring Modes

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Transient Thermo-fluid and Solidification Behaviors in Continuous Casting Mold: Evolution Phenomena

Jie Yang, Zhaozhen Cai, Miaoyong Zhu

pp. 299-308

DOI:

10.2355/isijinternational.ISIJINT-2017-445

Abstract

A mathematical model coupling fluid flow with heat transfer as well as solidification in continuous casting mold is presented. The model features the formations of meniscus and slag films, including the growth of slag rim. Furthermore, the model describes the evolution of heat flux and thicknesses of shell and slag films from cast-start to steady-state in combination with actual operating conditions. The predictions in the developed model are in good agreement with plant measurements. The results show that a large amount of liquid slag infiltrates into the gap as the shell is withdrawn at a casting speed of 0.3 m/min, which creates the initial meniscus topography. The meniscus profile tends to bulge up at a higher casting speed, while the size of slag rim decreases. Large fluctuations of heat flux are found before forming a steady structure of slag films throughout the mold. Increasing in casting speed leads to thinner slag films and higher heat flux. This model provides a fundamental understanding on the influence of meniscus profile and slag films related to the casting speed on slab solidification, especially at the initial stage of casting process.

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Transient Thermo-fluid and Solidification Behaviors in Continuous Casting Mold: Evolution Phenomena

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An Analysis of the Metal Transverse Flow in the Roll Gap for Ultra-thin Strip Rolling Using the Energy Method

Zhongkai Ren, Hong Xiao, Xiao Liu, Zhichao Yan

pp. 309-315

DOI:

10.2355/isijinternational.ISIJINT-2017-519

Abstract

Strip shape control theory has been developed into a relatively accurate system. However, for ultra-thin strip rolling, strip shape control is still a bottleneck affecting production. The most important problem is that the transverse flow mechanism of the metal is not accurate. In this study, based on the theory of ultra-thin strip rolling proposed by Fleck, a new metal transverse displacement model is developed using the minimum energy principle. To verify the accuracy of the new model, experiments and finite element analyses were carried out. For the transverse flow of a thin strip, grids with a line thickness of 10 µm and clear boundaries were successfully manufactured on the strip surface using lithography. The transverse displacement for different working conditions was measured after rolling. For ultra-thin strip rolling, the distribution of the transverse flow is analyzed using FEA. Finally, a comparison shows that the calculations from the new model are more consistent with the measured value and the simulation results, verifying the accuracy of the new model.

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An Analysis of the Metal Transverse Flow in the Roll Gap for Ultra-thin Strip Rolling Using the Energy Method

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Corrosion Process of Inorganic Zinc-Rich Painted Steel Exposed to a High-Chloride Atmospheric Environment

Makoto Nagasawa, Nobuhiro Okada, Nobuo Otsuka, Toshiaki Ohtsuka

pp. 316-322

DOI:

10.2355/isijinternational.ISIJINT-2017-548

Abstract

The corrosion protection behavior of an inorganic zinc-rich paint (IZRP) layer on steel after exposure for five years in Okinawa, relatively high-chloride atmosphere, was studied. Zinc particles were uniformly dissolved in the IZRP-coated steel, independent of the distance from the IZRP surface. From XRD, FT-IR, Raman spectroscopy and EPMA analyses, Zn5(OH)8Cl2H2O and ZnO were generated in the entire corrosion layer. Mg and Ca were observed in the outer area. We proposed the corrosion process of IZRP-coated steel from a detailed analysis of the corrosion products.

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Corrosion Process of Inorganic Zinc-Rich Painted Steel Exposed to a High-Chloride Atmospheric Environment

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Reconstruction of the Three-dimensional Ferrite–austenite Microstructure and Crystallographic Analysis in the Early Stage of Reverse Phase Transformation in an Fe–Mn–C Alloy

Kengo Hata, Kazuki Fujiwara, Kaori Kawano, Masaaki Sugiyama, Takashi Fukuda, Tomoyuki Kakeshita

pp. 323-332

DOI:

10.2355/isijinternational.ISIJINT-2017-441

Abstract

We have investigated the crystallographic orientation relationship between ferrite and austenite and the interfacial planes between them of a low-carbon steel formed by the transformation from ferrite to austenite upon heating. Three-dimensional investigation using EBSD measurement with FIB serial sectioning technique is carried out on samples quenched from an early stage of the reverse transformation. The prior austenite orientation of martensite in the quenched microstructure is determined based on an analysis of the variants in the Kurdjumov–Sachs (K–S) relationship and the three-dimensional ferrite–austenite microstructure is successfully reconstructed. The crystallographic analysis on the three-dimensional microstructure has revealed that the nucleus of reverse-transformed austenite maintains the K–S relationship or close to it with two or three adjacent ferrite grains. The significance for reverse-transformed austenite to select these specific orientation relationships was estimated through a statistical assessment. In addition, the influence of crystallographic relationship between ferrite and austenite to nucleation and growth of reverse transformation is discussed.

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Reconstruction of the Three-dimensional Ferrite–austenite Microstructure and Crystallographic Analysis in the Early Stage of Reverse Phase Transformation in an Fe–Mn–C Alloy

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Austenite Grain Growth Kinetics after Isothermal Deformation in Microalloyed Steels with Varying Nb Concentrations

Kofi Ahomkah Annan, Charles Witness Siyasiya, Waldo Edmund Stumpf

pp. 333-339

DOI:

10.2355/isijinternational.ISIJINT-2017-464

Abstract

Grain growth equation constants n, Q and A for Nb bearing steels with the Nb varying from 0.002 wt% to 0.1 wt%, were experimentally determined under reheating and high temperature hot rolling roughing conditions. The constants from these treatments were then used to develop constitutive equations that incorporate the initial grain size Do and a Nb-effect for grain growth predictions in these steels. Comparative analysis of the results showed that the values of the constants generated under rough rolling deformation conditions were slightly higher than those generated under reheating conditions. The activation energy for grain boundary migration Q was found to be in the range of 256 to 572 kJ/mol, the exponential constant n ranged from 2.6 to 6.5 and the material and processing condition’s constant A was found to range from 5.23 × 1011 to 4.96 × 1028 in all cases as a function of the Nb content. Analysis of the influence of the initial grain size Do showed that any contribution of Do can be neglected unless it is equal or more than 70 percent of the average size of the measured austenite grain size D. A logical degree of precision in predicting austenite grain growth in microalloyed steels with different Nb contents, has been achieved in the current work.

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Austenite Grain Growth Kinetics after Isothermal Deformation in Microalloyed Steels with Varying Nb Concentrations

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Anisotropy in Hydrogen Embrittlement Resistance of Drawn Pearlitic Steel Investigated by in-situ Microbending Test during Cathodic Hydrogen Charging

Kota Tomatsu, Takafumi Amino, Tetsushi Chida, Shunya Uji, Makoto Okonogi, Hikaru Kawata, Tomohiko Omura, Naoki Maruyama, Yoshitaka Nishiyama

pp. 340-348

DOI:

10.2355/isijinternational.ISIJINT-2017-461

Abstract

To investigate causes of superior hydrogen embrittlement resistance of drawn pearlitic steel, notched microcantilevers with different notch orientations with respect to the lamellar interface were fabricated by focused ion beam, and microbending tests were conducted in air and during cathodic hydrogen charging by electrochemical nanoindentation. In air, indentation load monotonically increased with increase in indentation displacement, and no crack appeared for any notch orientations. During hydrogen charging, indentation load declined, and a crack appeared. The load reduction with respect to the displacement was larger, and the crack was deeper for the notch parallel to the lamellar interface than that normal to the lamellar interface. Furthermore, stationary cracks in the microcantilevers were observed by scanning electron microscopy and scanning transmission electron microscopy. For the notch parallel to the lamellar interface, a sharp long crack was identified along the lamellar interface. The crack stopped at the position where the cementite lamellae are disconnected. In lattice images, cementite was identified in one side of the crack, and ferrite in another side of the same crack. On the other hand, for the notch normal to the lamellar interface, a blunt short crack was identified. Thus, it was concluded that the ferrite-cementite interface is a preferential crack path, and hydrogen embrittlement resistance in the direction parallel to the lamellar interface is superior to that normal to the lamellar interface. The present results also indicate that directional lamellar alignment of the drawn pearlitic steel suppresses crack propagation in the radial direction of the drawn wire, improving the hydrogen embrittlement resistance in the drawing direction.

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Anisotropy in Hydrogen Embrittlement Resistance of Drawn Pearlitic Steel Investigated by in-situ Microbending Test during Cathodic Hydrogen Charging

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Hydrogen Embrittlement and Local Characterization at Crack Initiation Associated with Phase Transformation of High-strength Steel Containing Retained Austenite

Taku Nagase, Takuya Ito, Yoshiro Nishimura, Hiroshi Suzuki, Kenichi Takai

pp. 349-358

DOI:

10.2355/isijinternational.ISIJINT-2017-512

Abstract

States of hydrogen present in high-strength steels for use as bearing steel SUJ2 and hydrogen embrittlement susceptibility were examined using thermal desorption analysis (TDA) and tensile tests. SUJ2 specimens containing the retained austenite phase (γR) in the martensitic phase exhibited three hydrogen desorption peaks in the TDA profile. Two of the peaks desorbed at higher temperatures decreased with a decreasing amount of γR, indicating they corresponded to desorption associated with γR. Fracture strength in the presence of hydrogen increased with a decreasing amount of γR and with an increasing strain rate. For the specimens containing γR and hydrogen, a flat facet at the crack initiation site and a quasi-cleavage (QC) fracture in the initial crack propagation area were observed on the fracture surface. Local characterization using electron back-scattered diffraction (EBSD) revealed that the flat facet on the fracture surface corresponded not to γR but to strain-induced martensite. In addition, the facet was on the {112} plane of martensite, which is the slip plane or deformation twin plane of body-centered-cubic metals. The reason for high hydrogen embrittlement susceptibility of the specimens containing γR was attributed to the strain-induced phase transformation at the crack initiation site of the flat facet and in the initial crack propagation area of the QC fracture. Furthermore, the strain rate dependency of hydrogen embrittlement susceptibility is presumably ascribable to local plastic deformation, i.e., the interaction between dislocations and hydrogen.

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Hydrogen Embrittlement and Local Characterization at Crack Initiation Associated with Phase Transformation of High-strength Steel Containing Retained Austenite

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Analysis of Subsurface Fatigue Crack Generation in Ti–Fe–O Alloy at Low Temperature

Weibo Li, Osamu Umezawa, Norimitsu Koga

pp. 359-363

DOI:

10.2355/isijinternational.ISIJINT-2017-514

Abstract

Subsurface microcracks developed in the Ti–Fe–O cross-rolled material were characterized in order to clarify the subsurface fatigue crack initiation. A considerable amount of microcracks in β platelets were detected. The {0001} transgraular microcracks in α grains predominantly generated at the β grain boundary between recovered α grain and recrystallized α grain, and grew into the recrystallized α grain along the basal plane, although the microcracks at the twist boundary of α grains and at the α-β interface were occasionally detected. Stress concentration around the microcrack in β platelets may assist the microcrack initiation on basal plane (transgranular facet) in neighboring α grain due to a combination of the shear stress and tensile stress normal to the basal plane at α-β boundary. The {0001} transgraular microcracks growth and/or coalescence occurred with the assistance of shear stress on prismatic plane such as {1010} steps.

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Analysis of Subsurface Fatigue Crack Generation in Ti–Fe–O Alloy at Low Temperature

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Separation of Rare Earth and Fluorite Phases from Bayan Obo Ore at Low Temperature by Super-Gravity

Jintao Gao, Xi Lan, Fuqiang Wang, Zhancheng Guo

pp. 364-366

DOI:

10.2355/isijinternational.ISIJINT-2017-034

Abstract

The iron-slag separation of gaseous reduced Bayan Obo ore was conducted at a low temperature by super-gravity for the aim of separating rare earth and fluorite phases. Driven by the super-gravity, the slag melt went through the filter, whereas the iron grains were effectively intercepted by the filter. Moreover, the fluorite remained in solid particles and migrated to the lower slag, while all REEs precipitated into Ca3(Ce,La,Pr,Nd)2[(Si,P)O4]3F crystals and concentrated in the bottom slag at 1373 K–1473 K.

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Separation of Rare Earth and Fluorite Phases from Bayan Obo Ore at Low Temperature by Super-Gravity

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Viscosity Measurements of CaO–SiO2–CrO Slag

Tuo Wu, Fang Yuan, Yanling Zhang

pp. 367-369

DOI:

10.2355/isijinternational.ISIJINT-2017-558

Abstract

The viscosities of CrO-containing slags obtained using high temperature equilibrium technique were measured by the rotating cylinder method at 1823 to 1948 K (1550 to 1675°C). The slag viscosity decreased with increasing CrO contents. CrO also exhibits a strong basic character in slags, and can act as a network modifier to depolymerize the slag network structure in CaO–SiO2-based slags. The dependence of the slag degree of polymerization (DOP) on the CrO content further validates these observations.

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Viscosity Measurements of CaO–SiO2–CrO Slag

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A Theoretical Model for Assessing the Influence of N2 on Gaseous Reduction of Iron Ores with CO or H2

Lei Shao, Ko-ichiro Ohno, Ying-xia Qu, Zong-shu Zou

pp. 370-372

DOI:

10.2355/isijinternational.ISIJINT-2017-436

Abstract

The influence of N2 on gaseous reduction of iron ores has yet to be investigated more rigorously. A generalized theoretical model for iron ore reduction that is featured with equimolar counterdiffusion of gaseous reactant and product in the presence of an inert component is derived in the current paper, where the influence of N2 is quantitatively assessed in terms of a normalized overall reduction rate. The results show that N2 impacts CO reduction of iron ores mainly via the prevailing mechanism of dilution effect and the sole use of DAB in the corresponding ternary system brings merely minor errors. In contrast, especially under conditions of high N2 fraction and reduction degree, the effect of mass diffusion must be borne in mind for H2 reduction and the sole use of DAB can lead to marked errors. The main novelty of the present work is the derivations of the theoretical equations fully following the Maxwell-Stefan relation for multicomponent diffusion and the well-known concept of topochemical reaction for gaseous reduction of iron ores. It is hoped that the brief discussion will stimulate further application of the theoretical equations to the development of more versatile models and to studies of the engineering type.

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A Theoretical Model for Assessing the Influence of N2 on Gaseous Reduction of Iron Ores with CO or H2

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Grain Boundary Pop-in, Yield Point Phenomenon and Carbon Segregation in Aged Low Carbon Steel

Zhengyan Shen, Bilei Wang, Gaofei Liang, Yunhu Zhang, Ke Han, Changjiang Song

pp. 373-375

DOI:

10.2355/isijinternational.ISIJINT-2017-426

Abstract

Nanoindentation measurements, tensile tests, and carbon concentration analyses were conducted to study yield behaviors in as-received, aged, and prestrained low carbon steel. In aged sample, steel showed both yield point phenomenon (YPP) and grain boundary (GB) pop-ins besides initial pop-in, while steels in other two states showed no YPP and only the initial pop-in. 3-dimensional atom probe (3DAP) analysis in both as-received and aged samples showed that carbon content in the matrix decreased significantly after aging treatment, which is believed to contribute to the occurrence of both YPP and GB pop-in.

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Grain Boundary Pop-in, Yield Point Phenomenon and Carbon Segregation in Aged Low Carbon Steel

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Effect of the Surface Layer Strained by Mechanical Grinding on X-ray Diffraction Analysis

Fulin Jiang, Kentaro Hirata, Takuro Masumura, Toshihiro Tsuchiyama, Setsuo Takaki

pp. 376-378

DOI:

10.2355/isijinternational.ISIJINT-2017-578

Abstract

X-ray diffraction is a powerful tool for characterizing the microstructure of steels. However, the strained surface by mechanical grinding could cause some errors in X-ray diffraction analysis. In this work, the strained layer was found to affect peak intensity, peak positions and enlarge the full width at half maximum. A quantitative relation between the depth of damaged layer and particle size of sander papers was established.

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Effect of the Surface Layer Strained by Mechanical Grinding on X-ray Diffraction Analysis

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