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ISIJ International Vol. 51 (2011), No. 10

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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ISIJ International Vol. 51 (2011), No. 10

On the Ductile-Brittle Transition in Lath Martensitic Steel

John William Morris, Jr.

pp. 1569-1575

DOI:

10.2355/isijinternational.51.1569

Abstract

The inherent brittle mode in dislocated lath martensitic steel is cleavage on {100} planes in the microstructure. The transition to {100} cleavage fracture on cooling determines the minimum value of the ductiule-brittle transition temperature. A half-century of research on the microstructure and toughness of lath martensitic steels has produced a semi-quantitative understanding of the brittle transition to cleavage. The results identify the crystallographic “block” of lath martensite as the effective grain size that controls cleavage, and clarify why the internal structure of a block has the microstructure it adopts. The ductile-brittle transition temperature is strongly affected by the block size. Several effective metallurgical processes are now available to refine the block size without excessive strengthening, leading to martensitic structural steels that combine high strength with good low-temperature toughness.

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On the Ductile-Brittle Transition in Lath Martensitic Steel

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In situ Synthesis of Nano-sized ZrC and Its Formation Mechanism by Combustion Synthesis from Zr–C–Cu System

Mengxian Zhang, Qiaodan Hu, Bing Huang, Junzheng Li, Jianguo Li

pp. 1576-1579

DOI:

10.2355/isijinternational.51.1576

Abstract

Nano-sized ZrC was prepared by combustion synthesis (CS) from Cu–Zr–C elemental powder mixtures. The ZrC particle size greatly decreased from about 10 μm with an irregular shape in free Cu addition to nano-meter order with a nearly spherical shape in Cu addition ranging from 10 wt.% to 30 wt.%. The formation mechanism of the nano-sized ZrC was discussed. The low dissolubility of C into Cu melt may greatly inhibit the growth of product ZrC, resulting in formation of ZrC particles as fine as nano-sized C. Melting of metallic Cu plays an important role in the formation mechanism of the nano-sized ZrC.

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In situ Synthesis of Nano-sized ZrC and Its Formation Mechanism by Combustion Synthesis from Zr–C–Cu System

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Surface Tension of Liquid Iron as Functions of Oxygen Activity and Temperature

Keisuke Morohoshi, Masahito Uchikoshi, Minoru Isshiki, Hiroyuki Fukuyama

pp. 1580-1586

DOI:

10.2355/isijinternational.51.1580

Abstract

Surface tension of liquid iron is an important property for process simulations evaluating Marangoni flow in the melt. In this study, we succeeded to measure accurate surface tension of liquid iron under various oxygen activities and temperatures with overcoming experimental difficulties through the following attempts: (1) To prevent chemical contamination of sample, an oscillating droplet method using an electromagnetic levitator was employed. Surface oscillation frequencies were determined with taking into account sample rotation. (2) To control the oxygen activity of liquid iron, a gas-liquid equilibrium method using CO/CO2 containing gas mixtures was employed. This method enables precise control of oxygen activity with keeping carbon activity low, and therefore, oxygen activity dependence of surface tension was clarified without carbon effect. (3) To measure the surface tension for higher oxygen activities up to the Fe/FeO equilibrium, high-purity iron (99.9972 mass%) was used. Otherwise, minor reactive impurities in the melt such as aluminum are oxidized, which affects the surface tension measurement. Based on the experimental data, the surface tension of liquid iron was expressed as functions of oxygen activity and temperature using the Szyszkowski model. In addition, thermodynamic properties on the oxygen adsorption reaction and structure of the adsorbed oxygen layer on the melt surface were also discussed.

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Surface Tension of Liquid Iron as Functions of Oxygen Activity and Temperature

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Controllability of Radiative Heat Flux across Mould Flux Films by Cuspidine Grain Size

Masahiro Susa, Atsushi Kushimoto, Rie Endo, Yoshinao Kobayashi

pp. 1587-1596

DOI:

10.2355/isijinternational.51.1587

Abstract

Apparent reflectivities and transmissivities have been measured as functions of cuspidine grain diameter for mould fluxes having constant degrees of crystallinity. Samples used were two types of synthesised mould flux with the basicity of 1, one of which samples contained 1 mass% of Fe2O3, and the grain diameter was varied in the range 1–3.5 μm. The optical measurements were carried out in the wavelength range 300–2600 nm at room temperature using a spectrophotometer with an integrating sphere. With increasing grain diameter, the apparent reflectivity tended to increase and the apparent transmissivity tended to decrease at higher wavelengths for iron oxide free mould fluxes: it seemed that the apparent reflectivity showed a maximum value and the apparent transmissivity showed a minimum value in the grain diameter range 2–3 μm. In contrast, there was less significant dependence on grain size for mould fluxes containing iron oxides. The total radiative heat flux which may reach the mould from the steel shell has been evaluated using apparent reflectivity and transmissivity data on the basis of an optical process model. It has been found that the total radiative heat flux would be smallest in iron oxide free mould fluxes having the highest apparent reflectivity and the lowest apparent transmissivity at higher wavelengths. Effects of grain size on the radiative heat flux are smaller for mould flux containing iron oxides. Comparison of the total radiative heat flux with the total heat flux including conductive contribution suggests that control of cuspidine grain diameter would lead to reduction of the total heat flux by 7–8% for iron oxide free mould fluxes. In addition, the air gap layer would affect the total heat flux more efficiently where the volume fraction of air in the layer exceeds 85%.

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Controllability of Radiative Heat Flux across Mould Flux Films by Cuspidine Grain Size

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Determination of Mixing Time in a Ladle-Refining Process Using Optical Image Processing

Teng-Yi Kuo, Jui-Chao Kuo

pp. 1597-1600

DOI:

10.2355/isijinternational.51.1597

Abstract

Water model experiments were performed to measure the mixing time and to investigate the effect of nozzle depth in a ladle-refining process. The depth of the submergence nozzle was varied by 6.0, 7.2, and 8.4 cm, which correspond to fractional depths of 0.5, 0.6, and 0.7, respectively. A new technique was proposed in the present study to measure the mixing time using optical image processing. The mixing time for fractional depths of 0.5, 0.6, and 0.7 determined is 26, 19, and 20 sec, respectively. The gas dispersions in the plume zone are distributed asymmetrically. An increase in the nozzle depth, which enhances recirculation speeds, leads to a decrease in mixing time.

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Determination of Mixing Time in a Ladle-Refining Process Using Optical Image Processing

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Catalyzing Carbothermic Reduction of Siderite Ore with High Content of Phosphorus by Adding Sodium Carbonate

Shaojun Bai, Shuming Wen, Dianwen Liu, Wenbin Zhang, Yongjun Xian

pp. 1601-1607

DOI:

10.2355/isijinternational.51.1601

Abstract

The characteristics of Huimin siderite ore with high content of phosphorus and the reduction of this ore bearing pulverized coal and sodium carbonate were investigated. The study focuses on the influence of Na2CO3 dosage on the carbothermic reduction process. Iron particle size in reduced ore is tiny and the fayalite is abundant without Na2CO3 additive. With ratio of Na2CO3 to ore at 1:20 or 1:10, iron particle size in reduced ore is coarse and the diffraction intensity of metallic iron increases obviously in the XRD pattern. The reduction of siderite can be catalyzed by adding an appropriate amount of Na2CO3. The catalyzing activity may be caused by the increase of the reducing reaction activity of FeO and the acceleration of the carbon gasification reaction rate. The phosphorus compounds were not reduced in the low temperature reduction process and remained as fluorapatite in the gangue phases. Ultrafine grinding-magnetic separating of magnetic minerals is an efficient way to obtain qualified iron concentrate.

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Catalyzing Carbothermic Reduction of Siderite Ore with High Content of Phosphorus by Adding Sodium Carbonate

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Permeation and Blockage of Fine Particles Transported by Updraft through a Packed Bed

Hiroshi Takahashi, Hideki Kawai, Tokihiro Kondo, Masataka Sugawara

pp. 1608-1616

DOI:

10.2355/isijinternational.51.1608

Abstract

The fines generated in iron making blast furnace bring about the unsteady or unstable gas/liquid permeability and solid motion. Fundamental experiment was carried out to investigate the inhomogeneous phenomena such as flow channel blockage by fine particles transferred by updraft through a fixed packed bed. The alumina sphere and coke were used for packed bed and fine particles transferred, respectively. Conclusion of this study is as follows. The principal factor controlling a criterion for the blockage is a particle diameter ratio of coke to alumina particle, and fines concentration is not a primary factor. Consideration on a hydraulic equivalent diameter of a regular packing model of spheres is effective to predict the critical diameter ratio and it resulted in good agreement with the experimentally determined. Locally blocked mass, that is, a cluster consisting of coarse alumina particles and fines through which gas is incapable of flowing is dispersed in the bed. The void fraction of packed bed being equivalent to the pressure drop, calculated from Ergun's equation, is considerably small compared with the initial value when the local blockage is in existence. Fraction of the local blockage calculated from the void fraction and static holdup of fines is 15–20% as far as the present experimental condition.

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Permeation and Blockage of Fine Particles Transported by Updraft through a Packed Bed

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Development of the Burden Distribution and Gas Flow Model in the Blast Furnace Shaft

Jong-In Park, Ui-Hyun Baek, Kyoung-Soo Jang, Han-Sang Oh, Jeong-Whan Han

pp. 1617-1623

DOI:

10.2355/isijinternational.51.1617

Abstract

It is important to control the burden distribution, which affects the gas flow pattern in the blast furnace. Therefore, a burden distribution analysis model is needed to predict the burden profile. In this study, the burden descent and gas flow models were developed to complete a blast furnace analysis model. A previous study reported two models based on the burden trajectory and stock model. The burden profile that is due to the burden trajectory was calculated using stock model in the upper part of shaft. The entire burden profile, which is classified into five burden types calculated using the descent model, was used for a gas flow calculation as the initial conditions in the gas flow model. The analysis models were developed using a visual basic based spread sheet, and compared with the 1/12 scaled model experiment. In addition, a GUI (Graphic User Interface) was added for the convenience of the operators.

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Development of the Burden Distribution and Gas Flow Model in the Blast Furnace Shaft

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Behavior of Vanadium and Niobium during Hot Metal Dephosphorization by CaO–SiO2–FetO Slag

Farshid Pahlevani, Hiroyuki Shibata, Nobuhiro Maruoka, Shin-ya Kitamura, Ryo Inoue

pp. 1624-1630

DOI:

10.2355/isijinternational.51.1624

Abstract

V and Nb are key elements for the production of high-grade steel. These elements are produced by few countries, and V has been nominated as a national stockpile element in Japan. Despite the importance of these elements, there are no strategies to obtain stable resources for them. Some types of iron ore that contain V and Nb can be used as sources of these elements. This study clarified the possibility of extracting V and Nb by means of a conventional hot metal dephosphorization process without using CaF2 as a flux. First, the influences of V and Nb oxides on the distribution ratio of P2O5 between the solid solution and liquid phases in slag were measured. Then, the behaviors of V and Nb in hot metal dephosphorization by CaO–SiO2–FetO slag were investigated. Finally, a previously described hot metal simulation model was modified and the behaviors of various elements, including V and Nb, during hot metal dephosphorization were simulated, and the results were compared with experimental ones. The results are summarized as follows: (1) The addition of V and Nb oxides slightly increases the distribution ratio of P2O5 between the solid solution and liquid phases in slag. (2) The decreasing rate of the elements by the flux addition can be expressed as Nb > Mn > P > V, and these decreasing rates increase as the basicity increases. (3) The simulation results for the behavior of P and V or Nb are in good agreement with the experimental results; that is, the decreasing rate for Nb is greater than that for P, and the separation of Nb leaving P in the hot metal is easier than the separation of V.

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Behavior of Vanadium and Niobium during Hot Metal Dephosphorization by CaO–SiO2–FetO Slag

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Measurement and Control of NOx Emissions at Two AC Electric Arc Furnaces

Thomas Echterhof, Herbert Pfeifer

pp. 1631-1636

DOI:

10.2355/isijinternational.51.1631

Abstract

Nitrogen oxides (NOx) are important air polluting gas species. The emission of nitrogen oxides from industrial plants is strictly regulated in most industrial countries around the world and the emissions limits have been and will be lowered further in the future. Therefore it is important to know typical emissions values of processes like electrical steelmaking. In a second step it is also vital to investigate measures to control and reduce the NOx emissions of the electric steelmaking process. This will ensure the ability to compete with other steel production technologies and also with alternative production sites. In this paper the results of measurements and trials regarding the formation of NOx in the electric arc furnace (EAF) are reported. The measurements and trials have been conducted at two European industrial EAFs and their goal was to investigate the influence of current and modified operational practices on the emission of NOx at these EAFs. The results of the work performed have lead to control strategies to minimize NOx formation within the electric steelmaking process.

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Measurement and Control of NOx Emissions at Two AC Electric Arc Furnaces

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Mathematical Comparison of Two VOD Nozzle Jets

Zhili Song, Mikael Ersson, Pär G. Jönsson

pp. 1637-1646

DOI:

10.2355/isijinternational.51.1637

Abstract

Studies of physical phenomena in a jet caused by VOD (Vacuum Oxygen Decarburization) nozzles have been carried out. The VOD process is a metallurgical process where the steel-making route is controlled under vacuum environment with oxygen top blowing. In this work, two VOD nozzle models have been employed for an investigation based on two real De Laval geometries used in industry. Numerical modeling was used to study oxygen blowing states of the nozzles at different temperatures and ambient pressures. The nozzle models were numerically computed with two dimensional domains, where vacuum conditions and temperatures were specifically defined. The modeling results showed that one of the nozzles was more applicably proper for lower pressures, displaying a more stable flow pattern. Furthermore, it was found that a change in ambient pressure has a stronger effect on the jet force than a change in ambient temperature. In addition, it was proved that the profiles of the dynamic pressure at a certain blowing distance fit well to Multi-Gaussian curves.

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Mathematical Comparison of Two VOD Nozzle Jets

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Effect of Shape and Flow Control Devices on the Fluid Flow Characteristics in Three Different Industrial Six Strand Billet Caster Tundish

Anurag Tripathi, Satish Kumar Ajmani

pp. 1647-1656

DOI:

10.2355/isijinternational.51.1647

Abstract

Fluid flow phenomenon in a tundish depends upon the shape of tundish. The change in the shapes can bring drastic change in the flow pattern and thus effect the fluid flow characteristics in the tundish. The signifiacnt effect of flow control devices on fluid flow phenomenon is also noticed. In the present work, investigation of fluid flow phenomenon was performed for the symmetrical half of a six strand tundish. Fluid flow pattern gets altered by changing the shapes of tundish and pouring chamber. The impact of this change results in a development of various flow patterns. These flow patterns are analysed for capturing the better inclusion flotation. The mathematical model has been validated by the experimental results of Singh and Koria for a single strand bare tundish.

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Effect of Shape and Flow Control Devices on the Fluid Flow Characteristics in Three Different Industrial Six Strand Billet Caster Tundish

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Effect of Feeding Modes of Molten Steel on the Mould Metallurgical Behavior for Round Bloom Casting

Haibo Sun, Jiaquan Zhang

pp. 1657-1663

DOI:

10.2355/isijinternational.51.1657

Abstract

Three kinds of modes for molten steel delivery have been studied in the paper for better understanding of their mould metallurgical effects during round bloom casting. The melt flow, free surface fluctuation, temperature field and solidification behavior in the mould region have been numerically analyzed upon the respective adoption of conventional straight single SEN, quad-furcated SEN with outlets in radial direction and a new type of quad-furcated SEN with outlets at tangential direction. For the latter, a strong horizontal swirling flow has been observed along with the upper and lower recirculation region in the mould. It is shown that the horizontal swirling flow in the mould can reduce the impingement depth of molten steel remarkably, inhibit the mould level fluctuation, and create an active bulk flow below the meniscus, which can also move the hot spot upward and promote superheat dissipation of the molten steel. Meanwhile, the temperature of molten steel near the free surface can be increased by 2.6 K to 4.4 K as compared with the two other normal nozzles. Moreover, compared with quad-furcated SEN with outlets in radial direction, the jet impingement on shell is much weaker, while the thickness of solidified shell at the cross-section of the mould is more even. Furthermore, the shell thickness at exit of the mould under the new type of SEN can reach approximately 18.6 mm, while that under quad-furcated SEN with outlets at radial direction is about 17.4 mm, which leave much allowance for the speed enhancement of present round bloom castings.

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Effect of Feeding Modes of Molten Steel on the Mould Metallurgical Behavior for Round Bloom Casting

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Estimation of Solid-liquid Interfacial Energy from Gibbs-Thomson Effect: A Molecular Dynamics Study

Ryo Hashimoto, Yasushi Shibuta, Toshio Suzuki

pp. 1664-1667

DOI:

10.2355/isijinternational.51.1664

Abstract

Solid-liquid interfacial energies of chromium and nickel are estimated from a Gibbs-Thomson relation. Molecular dynamics simulation shows that there is a critical temperature dividing shrinking or growing of a freestanding spherical crystal in the undercooled melt and the critical temperature is negatively correlated with inverse of crystal radius, which is regarded as the Gibbs-Thomson effect. The solid-liquid interfacial energies are then estimated from the proportional coefficient, to be 0.304 Jm–2 for chromium and 0.256 Jm–2 for nickel, respectively.

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Estimation of Solid-liquid Interfacial Energy from Gibbs-Thomson Effect: A Molecular Dynamics Study

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Identification of the Optimal Control Center for Blast Furnace Thermal State Based on the Fuzzy C-means Clustering

Shihua Luo, Jian Huang, Jiusun Zeng, Qiansheng Zhang

pp. 1668-1673

DOI:

10.2355/isijinternational.51.1668

Abstract

It is required to maintain silicon content in hot metal ([Si]) at a stable level to ensure smooth operation of the blast furnace ironmaking process. However, current blast furnace control strategy always leads to frequent fluctuation of silicon content in hot metal. To stabilize blast furnace operation, this article attempts to identify the optimum control centre of silicon content through exploring the operational data of blast furnace ironmaking process. A quantitative analysis of the impact of thermal state on the smelting efficiency and intensity is presented by combining wavelet denoising and fuzzy c-means (FCM) clustering. Simulation results show that the commonly adopted mean value of historical data is not necessarily the optimum state of blast furnace operation. There exists some optimum state lower than the mean value, under which higher smelting efficiency and intensity can be achieved. It is also proved that the “low silica smelting practice” attempt in the steel industry is feasible and meaningful.

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Identification of the Optimal Control Center for Blast Furnace Thermal State Based on the Fuzzy C-means Clustering

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Molten Steel Level Measurement in Tundish with Heat Transfer Analysis

Zhenwei Hu, Ying Ci, Zhi Xie

pp. 1674-1681

DOI:

10.2355/isijinternational.51.1674

Abstract

Temperature variation rate is different between “molten steel – tundish cover flux(TCF) – air” layers in tundish because heat transfer between these three materials with different thermal conductivities. Numerical calculation proves that temperature gradient extremums exist at the interfaces of “molten steel – TCF – air” layers in tundish. At the same time the change of initial temperature, thermal conductivity, convection coefficient have no effect on the interfaces locating. Thus temperature gradient extremums can be used for measuring molten steel level in tundish. In the process of measurement, a CCD camera is used to capture the thermal image of measuring sensor which is inserted into tundish to get the temperature distribution. By calculating temperature gradients of the measuring sensor in the thermal images, the interfaces of “molten steel – TCF – air” can be located, finally the molten steel level is obtained. The influence of measuring sensor adhering by TCF is also solved by gray image projection algorithm. Steel metallurgical field experiment shows that the method of molten steel level measurement is authentic and measuring error is less than 5 mm.

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Molten Steel Level Measurement in Tundish with Heat Transfer Analysis

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Acoustic Emission during Hydrogen Charging of a Pipeline Steel

Dmitry Merson, Sergey Dement'ev, Andrey Ioffe, Pavel Suvorov, Alexei Vinogradov

pp. 1682-1687

DOI:

10.2355/isijinternational.51.1682

Abstract

Acoustic emission (AE) during hydrogen-induced cracking (HIC) of low-carbon pipeline steel immersed into H2S containing media was investigated aiming at discriminating between damage mechanisms and getting a better insight on the kinetics of damage. Two kinds of steel samples - sensitive and resistant to hydrogen induced cracking - were tested. Three different kinds of acoustic emission signals were discriminated by a cluster analysis involving either spectral shape recognition or parametric c-means classifier. It is demonstrated that AE is associated with three primary mechanisms involving hydrogen bubbles evolution, sulfide film formation and fracture due to HIC. Hence, AE is shown to be very efficient for quantitative description of hydrogen induced damage accumulation in steels.

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Acoustic Emission during Hydrogen Charging of a Pipeline Steel

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Investigation on High Strength Hot-rolled Plates by Quenching-partitioning-tempering Process Suitable for Engineering

Shu Zhou, Ke Zhang, Nailu Chen, Jianfeng Gu, Yonghua Rong

pp. 1688-1695

DOI:

10.2355/isijinternational.51.1688

Abstract

Designed Fe-0.25C-1.5Mn-1.2Si-1.5Ni-0.05Nb (mass%) hot-rolled plates with different thicknesses are subjected to a novel quenching-partitioning-tempering (Q-P-T) process suitable for engineering with water quenching followed by salt bath tempering or water quenching followed by air cooling. A passing underwater quenching equipment (PUQE) is specially designed for realization of the Q-P-T process suitable for engineering. The tensile results show that the Q-P-T samples exhibit both high yield strength of over 900 MPa and elongation of over 15%. Microstructural characterization indicates that the high strength is attributed to the dislocation-type lath martensite or bainite and the dispersively distributed NbC-carbides in the martensite matrix, and the adequate elongation results from the flake-like retained austenite between martensite laths. In addition, effects of the retained austenite in Q-P-T steels on mechanical properties and the feasibility of Q-P-T processes for engineering industry applications are discussed.

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Investigation on High Strength Hot-rolled Plates by Quenching-partitioning-tempering Process Suitable for Engineering

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Blistering Behavior during Oxide Scale Formation on Steel Surface

Yasumitsu Kondo, Hiroshi Tanei, Noriyuki Suzuki, Kohsaku Ushioda, Muneyuki Maeda

pp. 1696-1702

DOI:

10.2355/isijinternational.51.1696

Abstract

Blistering occurs when oxide scale is swollen during oxidation. Blistered scale causes surface defect problems when it is rolled. Present study investigated the nucleation and growth behavior of blistering when steel is oxidized at high temperature. The following conclusions are drawn. Blistering phenomenon has the nucleation and growth process. At the nucleation stage scale is delaminated at the scale/metal interface. The gas compositions inside blisters at this stage are CO, CO2, and N2. The steel surface inside blisters is oxidized while the stage changes from nucleation to growth. At the growth stage, the separated steel surface from the scale is not oxidized.

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Blistering Behavior during Oxide Scale Formation on Steel Surface

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Tailoring Model Surface and Wetting Experiment for a Fundamental Understanding of Hot-dip Galvanizing

Takashi Kawano, Frank Uwe Renner

pp. 1703-1709

DOI:

10.2355/isijinternational.51.1703

Abstract

The wettability of molten zinc-aluminum on a steel substrate is an important issue in the hot-dip galvanizing process for automotive applications. Especially, the effects of surface oxides have been current topics during recent years. However wetting behavior under the effects of oxide has not been well illustrated due to the complexity of the surface of actually used steel. In this work, wetting experiments using molten zinc-aluminum were applied to tailored model surfaces of iron to investigate its fundamental behavior. Well-defined aluminum oxide islands were successfully patterned on iron surfaces by physical vapor deposition with masks for this purpose. The sessile drop method with zinc-aluminum was conducted in a laboratory-made apparatus. This apparatus included a unique spin-off technique that allowed us to make an interface analysis at the early stage of wetting. The initial contact angles of the patterned samples were revealed to have followed the Cassie equation. This indicates that the wetting of zinc-aluminum on an oxide-iron system can be treated as if it is a static wetting in its initial stage, even though this system is originally considered as a reactive wetting. The molten zinc-aluminum on a patterned sample diffused into the interface between the oxide islands and the substrate in the stage after initial wetting. It is suggested that this manner of zinc-aluminum diffusion plays a key role in the reaction and formation of the interface during the hot-dip galvanizing process.

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Tailoring Model Surface and Wetting Experiment for a Fundamental Understanding of Hot-dip Galvanizing

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A Molecular Dynamics Study of Bidirectional Phase Transformation between bcc and fcc Iron

Shinji Tateyama, Yasushi Shibuta, Tomohisa Kumagai, Toshio Suzuki

pp. 1710-1716

DOI:

10.2355/isijinternational.51.1710

Abstract

Bidirectional phase transformation between bcc and fcc iron is investigated by molecular dynamics simulation using the Finnis-Sinclair potential with a cutoff function in the atomic charge density. It is confirmed that the influence distance (i.e., cutoff distance) of the atomic charge density affects the relative stability between the bcc and fcc phases at high temperature: the bcc is stable at a long cutoff distance and the fcc is stable at a short cutoff distance. Hence, the bidirectional phase transformation across the A3 point comes true by changing the cutoff distance at the A3 point. The propagation of an fcc-bcc heterointerface with a Nishiyama-Wassermann orientation relationship is then examined by relaxation of an fcc-bcc biphasic system at various temperatures. The fcc-to-bcc phase transformation is observed below the A3 point, whereas the heterointerface does not move to any direction at the A3 point. On the other hand, the bcc-to-fcc phase transformation is observed above the A3 point, which has not been successful in previous studies using the original Finnis-Sinclair potential.

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A Molecular Dynamics Study of Bidirectional Phase Transformation between bcc and fcc Iron

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Multi-phase-field Simulations of Dynamic Recrystallization during Transient Deformation

Tomohiro Takaki, Akinori Yamanaka, Yoshihiro Tomita

pp. 1717-1723

DOI:

10.2355/isijinternational.51.1717

Abstract

To develop a multiscale hot-working model that enables us to simulate macroscopic mechanical behaviors depending on microstructural evolution by the coupling the finite element (FE) method and phase-field (PF) method, the multi-phase-field-dynamic recrystallization (MPF-DRX) model has been applied to transient deformation simulation, where strain rate and temperature rapidly change during deformation. As a result, it has been confirmed that the variations in the macroscopic stress-strain curve and average grain size during deformation are in good agreement with the experimental observation qualitatively.

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Multi-phase-field Simulations of Dynamic Recrystallization during Transient Deformation

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Microstructure – Properties Relationships in Carbide-free Bainitic Steels

Jean-Christophe Hell, Moukrane Dehmas, Sébastien Allain, Juscelino Mendes Prado, Alain Hazotte, Jean-Philippe Chateau

pp. 1724-1732

DOI:

10.2355/isijinternational.51.1724

Abstract

We elaborated two carbide-free bainitic steels with different microstructures through specific alloy design and austempering process. Microstructural characterizations were performed by means of EBSD analysis and in-situ high energy synchrotron diffraction in order to evaluate the phase fractions and the carbon content in the retained austenite, as well as the microtextures. These microstructural features were correlated to the tensile properties. Both steels exhibited an excellent compromise between high strength (above 1250 MPa), good ductility (uniform elongation up to 14%) and high fracture strain (reduction of area up to 46%). The volume fraction of MA blocks (blocks of retained austenite partially transformed into fresh martensite during the final cooling at room temperature) was a key relevant parameter that strongly influenced the work-hardening at the expense of the damaging processes at high strain.

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Microstructure – Properties Relationships in Carbide-free Bainitic Steels

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Interphase Precipitation of VC and Resultant Hardening in V-added Medium Carbon Steels

Goro Miyamoto, Ryota Hori, Behrang Poorganji, Tadashi Furuhara

pp. 1733-1739

DOI:

10.2355/isijinternational.51.1733

Abstract

Interphase precipitation of vanadium carbide (VC) accompanying ferrite and pearlite transformations and its effect on hardness have been examined by using medium carbon steels containing 0.1, 0.3 and 0.5 mass%V. Specimens transformed in a temperature range between 873 and 973 K consist of pearlite and small amount of proeutectoid ferrite. Ferrite fraction increases with raising transformation temperature or with increasing the V content. In addition to proeutectoid ferrite and pearlite, bainite is formed below 853 K, whose fraction is increased by the V addition. Hardening is significant in the V-added alloy between 873 K and 973 K and becomes larger by increasing V content in this temperature range. Meanwhile the alloying effect of V on the hardness remarkably decreases at 823 K where bainite transformation takes place partly. TEM characterization has revealed that VC are precipitated in both of proeutectoid and pearlitic ferrite with holding Baker-Nutting (B-N) orientation relationship with ferrite in the manner of fine rows parallel to the austenite / ferrite interphase boundary. Single variant of VC, whose habit plane is closer to ferrite / austenite boundary than the other two B-N variants, tends to be formed. The size of VC decreases and its number density increases by lowering transformation temperature, corresponding to the larger hardness increase.

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Interphase Precipitation of VC and Resultant Hardening in V-added Medium Carbon Steels

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Effect of Texture on r-value of Ferritic Stainless Steel Sheets

Jun-ichi Hamada, Naoto Ono, Hirofumi Inoue

pp. 1740-1748

DOI:

10.2355/isijinternational.51.1740

Abstract

The effects of cold rolling texture on the formation of recrystallization texture and the relationship between recrystallization texture and r-value were investigated for sheets of titanium-stabilized extra-high purity Type 436L and Type 409L ferritic stainless steels. The recrystallization texture and average r-value were affected by the cold rolling texture. Annealed sheets with a sharp γ-fiber texture and very high average r-values were obtained from cold-rolled sheets with a strong γ-fiber texture (Type A). On the other hand, the {h,1,1}<1/h,1,2> fiber texture developed from cold-rolled sheets with a strong α-fiber texture (Type C). In terms of the relationship between {111} intensity and the average r-value, chromium content had less effect than the cold rolling texture. The planar anisotropies of r-values and the average r-values of Type 436L and 409L sheets with Type A cold rolling textures agreed well with those calculated from the textures in the center layers by using the relaxed-constraints model and the CRSS ratio τc{211}/τc{110} of 1.1. On the other hand, for the sheets with Type C cold rolling textures and a marked texture gradient, it was necessary to consider the texture gradient in the thickness direction. The texture inhomogeneity by the orientations other than {111} texture affected the relationship between {111} intensity and average r-value.

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Effect of Texture on r-value of Ferritic Stainless Steel Sheets

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Hydrogen Embrittlement of a Manganese–aluminum High-strength Bainitic Steel for Railway Crossings

Chunlei Zheng, Bo Lv, Cheng Chen, Zhigang Yan, Fucheng Zhang, Lihe Qian

pp. 1749-1753

DOI:

10.2355/isijinternational.51.1749

Abstract

The hydrogen embrittlement characteristics of a Mn–Al bainitic steel for railway crossings were studied by means of the slow strain rate test (SSRT), the delayed fracture test and X-ray diffraction (XRD) analysis. The binding energies and hydrogen diffusion barriers of an iron unit cell were tested separately with Al, Si and H, and were evaluated by the first principle calculation. The results showed that the hydrogen embrittlement decreased greatly with increased Al content. Microstructure examinations indicated that the content of retained austenite increased with increased Al content, which was as irreversible hydrogen traps and was not sensitive to hydrogen embrittlement. It is found that the characteristics of hydrogen embrittlement identified by SSRT and the delayed fracture test were different. From the first principle calculation, the binding energy of the iron lattice containing aluminum decreased less as compared to those of the cell containing silicon, but the diffusion barriers increased significantly.

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Hydrogen Embrittlement of a Manganese–aluminum High-strength Bainitic Steel for Railway Crossings

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