Showing papers in "Isij International in 2020"

Crystallographic Orientation Control of 316L Austenitic Stainless Steel via Selective Laser Melting

Abstract: Metal additive manufacturing (AM) has attracted the most attention in recent years among various metal processing technologies. Conventionally, metal AM has been characterized by its flexible shaping ability and relatively high processing speed. Taking advantage of such features of AM, metal AM technology has been applied to the fabrication of products with hollow structures, three-dimensional complex porous bodies, and tailor-made products, among others.1,2) Meanwhile, in recent studies, metal AM has emerged as a methodology for controlling the crystallographic texture of metallic materials3–5) in addition to controlling the shape of products. Crystallographic texture, which is an important material parameter for metallic materials, is a major factor that affects the functionality of a product, independent of shape parameters. The mechanical properties of metallic materials are the most significant factor controlled by the crystallographic texture. Highly texturized materials with preferential crystallographic orientations can feature mechanical anisotropies, such as Young’s modulus,6,7) yield stress and elongation,8) fatigue resistance,9) and creep resistance.10) The single crystalline form, in which the anisotropy based on the atomic arrangement is manifested to the limit, exhibits Crystallographic Orientation Control of 316L Austenitic Stainless Steel via Selective Laser Melting

A Review on Prevention of Sticking during Fluidized Bed Reduction of Fine Iron Ore

Abstract: The fluidized bed ironmaking technology has attracted the attention of many researchers for decades as a direct reduction ironmaking method with many advantages. This process has been applied as a pre-treatment method in many non-blast furnace ironmaking processes. However, the sticking problem hindered its development greatly. Defining the essential cause of sticking, and fundamentally solving this problem are the key steps encountered by this process. The research works related to the prevention of sticking problem during fluidized bed reduction of fine iron ore are comprehensively summarized in this article. The causes of sticking, the influencing factors of sticking and the solution of sticking are firstly discussed, followed by the analysis on the possible development direction of future fluidized bed ironmaking technology.

Deoxidation of Electroslag Remelting (ESR) – A Review

Abstract: Electroslag remelting (ESR) is widely used to produce some varieties of special steels and alloys, mainly because of its ability to provide high cleanliness level and excellent homogeneity of solidified ingot structure (reducing segregation, shrink holes, etc.) simultaneously.1–3) Deoxidation of liquid steel and alloy is always an ongoing concern for eliminating the detriments of oxide inclusions to the processing and mechanical properties of the steel and alloy. Great efforts have been put forward to minimize the amount of oxide inclusions by decreasing the oxygen content of the steel during ESR process.4–9) Deoxidation of liquid steel during ESR is basically different from that in other steelmaking process operations. The iron oxide activity of the slag is a measure of its oxygen potential during ESR.4,5,9,10) The intention of deoxidizing agent addition in ESR process is to deoxidize the molten slag.4–9,11,12) In this process, the oxygen level of liquid steel is determined by the interactions of atmosphere-slag-metalinclusion. Deoxidation during ESR is therefore affected by multiple factors, such as alloy compositions, deoxidizing agents, absorption ability of slag to oxide inclusions, remelting atmosphere, slag compositions, and oxide inclusion evolution. Some of these factors could only be varied in a limited range in the ESR practical production. As for a specific ESR practice, the contribution ratio of each of these Deoxidation of Electroslag Remelting (ESR) – A Review

Structure and Viscosity of CaO–Al2O3–B2O3 Based Mould Fluxes with Varying CaO/Al2O3 Mass Ratios

Abstract: Recently, high Al steel (e.g. high Al-TRIP and TWIP steel) has received increasing attention as it has the perfect combination of strength to weight ratio and excellent ductility.1) Significant amounts of aluminum are added into high Al-TRIP and TWIP steel to improve properties of steel.2) However, high levels of Al in the steel may cause great process control problems during the continuous casting, it is due to the chemical reactions between SiO2 in mould flux and [Al] in steel. This reaction can be illustrated by following equation:3,4)

Multiphase Flow Behavior in a Single-Strand Continuous Casting Tundish during Ladle Change

Abstract: The three-phase flow behavior in a single-strand continuous casting tundish during ladle change was investigated using physical modeling. These phenomena observed from physical modeling were explained by employing the multiphase model volume of fluid, which can track the interface behavior between the liquid steel, slag, and air during this operation. The effects of the refilling time and lowest operating level on the slag entrainment and the steel exposure during ladle change were analyzed and discussed, respectively. Increasing the refilling time significantly decreased the amount of entrained oil and the exposed area in the impact zone during ladle change. However, the increase in the lowest level had little influence on reducing the slag entrainment. To reduce the slag entrainment and the steel exposure during ladle change, the refilling time in the prototype should be larger than 3 minutes. Furthermore, the use of the turbulence inhibitor has also been evaluated. By diminishing the turbulence intensity in the impact zone and the velocity magnitude at the steel-slag interface, the turbulence inhibitor reduced considerably the amount of entrained slag and the steel reoxidation. The results indicated that the emulsification phenomenon during ladle change could be eliminated using TI-2, and the maximum exposed area fractions in the impact zone for different refilling times and lowest levels were less than 13% and 23%, respectively. Therefore, the TI-2 was recommended to improve the steel cleanliness during ladle change.

Development of High Accuracy Segmentation Model for Microstructure of Steel by Deep Learning

Abstract: We studied on automation of segmentation using deep learning, which has been remarkably developed in recent years. For the microstructural image of ferrite-martensite dual phase steel, we tried to segment the ferrite phase, martensite phase, and ferrite grain boundary in different colors individually. We created two models, SegNet and U-Net that can perform segmentation with high accuracy and compared the accuracy with an existing method. As a result, we demonstrated that models using deep leaning is more accurate than the existing method. In particular, U-Net model shows highly accuracy of segmentation for material microstructures.

State of the Art in the Control of Inclusions in Spring Steel for Automobile - a Review

Abstract: Spring steel wires are extensively utilized in automotive engines valve and suspension applications. Owing to spring often work under high-frequency dynamic loads and periodic alternation stress, nondeformable inclusions often act as fatigue fracture origin of spring steel. The control of inclusions in spring steel for automobile are extensively reviewed. On the one hand, the new perspective on the problems left over the past decades, include the new viewpoint on whether FeO is an inherent constituent of inclusions, the new understanding of the origin of CaO-based inclusions, the new perspective of whether the VD treatment progress should be removed but RH retain, have been discussed. On the other hand, the new approaches, via, calcium treatment, alkali oxide treatment, rare earth treatment, new refractory application, have also been summarized. Finally, the unsolved problems, the source of CaO-based inclusions, the mechanism of alkali metals modified inclusions, the based thermodynamic data for reactions between rare earth yttrium (Y) and non-metallic inclusions, the operability of CaO-containing refractory in industry, that should be explored further are also been discussed.

Accuracy Evaluation of Phase-field Models for Grain Growth Simulation with Anisotropic Grain Boundary Properties

Abstract: The process of competitive grain growth under curvature-driven boundary migration is one of the most important microstructural evolution phenomena in the manufacturing of polycrystalline materials because it determines the final microstructures and resultant physical properties of materials.1–3) However, the experimental study of grain growth is severely limited by the high temperatures at which it occurs and the optical opacity of polycrystalline samples. Therefore, to systematically examine grain growth phenomena, numerical approaches have been developed using continuum-based grain growth models,4) including the Monte-Carlo,5–8) cellular automaton,8–11) vertex or front-tracking,12–17) surface evolver,18–20) and phase-field7,21–26) models. In particular, the phase-field method has become common because it can accurately address curvature-driven grain boundary migration without explicit boundary tracking. With the benefit of rapid developments in high-performance computing, recent phase-field simulations have enabled significant contributions toward elucidating various metallurgical processes, including grain Accuracy Evaluation of Phase-field Models for Grain Growth Simulation with Anisotropic Grain Boundary Properties

Influence of H2–H2O Content on the Reduction of Acid Iron Ore Pellets in a CO–CO2–N2 Reducing Atmosphere

Abstract: The steel industry contributes to approximately 4–7% of man-made greenhouse gases.1) According to a statistical report published by the World Steel Association, blast furnace-converter route contributed to more than 70% of global crude steel production in recent years.2) Although blast furnaces operating in Europe are considered the most efficient with CO2 emissions of only 1 470 kg/tHM,3) due to tighter environmental regulations, constant efforts are made to reduce CO2 emissions further. Extensive research is dedicated to emission reduction, with a special focus on reducing iron ore with the help of hydrogen as a reducing agent, which is considered an innovative technology in CO2 emissions reduction.1) Several types of reactors have been used to study iron ore reduction. These reactors include fluidized bed reactors, packed bed reactors, and thermogravimetric analyzers (TGAs).4) Iron ore pellet reduction has been investigated in numerous studies in recent decades. Previous research found that at 700°C, there is a positive correlation between H2 content (up to 20%) and an improved reduction efficiency. H2 content also has a positive influence on the extent of Influence of H2–H2O Content on the Reduction of Acid Iron Ore Pellets in a CO–CO2–N2 Reducing Atmosphere

Development and Prospects of Refining Techniques in Steelmaking Process

Abstract: In 2014, the Iron and Steel Institute of Japan published a review entitled “Steelmaking Technology for the Last 100 Years: Toward Highly Efficient Mass-Production Systems for High-Quality Steels1)”, which presented an overview of the history of the development of iron and steelmaking technologies. This year, ISIJ International (formerly Transactions of ISIJ), which celebrates its 60th anniversary. During this period, ISIJ International provided information on the development of iron and steel production processes in Japan to interested parties in other countries and has also presented proposals related to technology. As part of this Special Issue commemorating its 60th anniversary, ISIJ International is publishing this review of the development of the refining process during the last 60 years, especially in Japanese integrated steel works. Table 1 shows an outline of the history of improvement and development of refining techniques since 1960. The individual techniques in each area are reviewed from Chapter 2.

Effect of B2O3 on Structure of CaO–Al2O3–SiO2–TiO2–B2O3 Glassy Systems

Abstract: Vanadium–titanium magnetite is an important ore resource, of which there are abundant reserves in China, especially in the Panxi area, which account for more than 90% of China’s reserves.1,2) During the ironmaking process, vanadium–titanium magnetite is commonly used as a raw material for extracting iron. After smelting in a blast furnace, most of the titanium is transferred to slag, referred to as Ti-bearing blast furnace slag. More than 3 million tons of Ti-bearing blast furnace slag is produced annually, with a total accumulation of more than 7 million tons in China.3–5) The accumulation of titanium slag not only wastes valuable titanium resources but also occupies a large amount of land and seriously pollutes the environment. Since the titanium components in blast furnace slag are mainly dispersed in the mineral phase, such as perovskite, carbon (nitrogen) titanium, and a small amount of spinel and diopside,6–8) this makes it considerably difficult to extract Ti from blast furnace slag. Presently, the utilization rate of titanium in Ti-bearing blast furnace slag is low. Therefore, new economical and efficient methods of treating Ti-bearing blast furnace slag are of great significance for the comprehensive utilization of titanium-bearing slag. Modifiers are normally added to blast furnace slag, which can affect the precipitation, growth, and morphology of crystals in molten blast furnace slag. Much research has been Effect of B2O3 on Structure of CaO–Al2O3–SiO2–TiO2–B2O3 Glassy Systems

Optimization of Thermal Soft Reduction on Continuous-Casting Billet

Abstract: Thermal soft reduction (TSR) is an effective technique to improve the inner quality of continuous-casting billet, but it may lead to undesired internal and surface cracks. In this work, the technologic parameters of TSR were optimized to ensure its effect and control the cracks of 82A tire cord steel billet. A heat transfer model with comprehensive thermo-physical parameters was established to simulate the thermal behavior of continuous-casting billet. The model was verified by comparing the measured surface temperatures and the calculated ones. According to the mechanism of TSR on billet, both the location and water flow rate were comparatively optimized. TSR was determined to locate at 6.96 m–8.46 m from meniscus, where the temperature of billet center dropped rapidly to liquid impenetrable temperature. The water flow rate of TSR was set to 2.2 m/h, which allowed the reheating rate and surface temperature in a reasonable range and prevented the formation of the cracks. Plant trials were conducted to verify the effect of the optimized TSR. The results showed that the central porosity, V segregation and central segregation of the billet were obviously improved by applying TSR. Meanwhile, the internal and surface cracks were well controlled in the billet.

Nanostructured Bainitic Bearing Steel

Abstract: Bearings are the most important component for nearly all mechanical equipment, playing a key role in the equipment manufacturing industry. Therefore, improving the properties of bearing steels is the ambition of material researchers and expectation of manufacturing industry. The conventional bearing steel is martensitic, but still contains a small amount of retained austenite and cementite. It exhibits high hardness, excellent wear resistance, and rolling contact fatigue resistance. Therefore, this kind of bearing steel has been used widely since it was developed in 1905.1,2) However, disadvantages of the martensitic microstructure such as low toughness, susceptibility to hydrogen embrittlement can lead to a reduced service life when the component is subjected to impact-loading and adverse environments.3) Since 1954, research has been carried out on austempered high-carbon chromium bearing steel based on a lower bainitic microstructure. The hardness of the lower bainitic microstructure is slightly lower than that of the tempered martensite; however, its toughness is nearly thrice than that of the tempered martensite. The lower bainitic bearing exhibits better wear resistance and fatigue performance. The residual stress on the surface of lower bainitic bearing is compressive, which can retard fatigue crack propagation. Moreover, the austempering treatment eliminates microcrack formation during quenching.3–7) Therefore, the lower bainitic bearings are used in rolling mills and train rails. Nanostructured Bainitic Bearing Steel

A CFD Study on Refractory Wear in RH Degassing Process

Abstract: The Ruhrstahl-Heraeus (RH) degasser is widely used for the production of the ultra-clean steel, since it could efficiently reduce carbon and sulfur contents in molten steel.1–3) It is well known that the complicated flow of the molten steel in the RH degasser, caused by the vacuum and the lifting gas, would create a significant wear and tear of the lining refractory.4–6) Besides, the slag impregnation, depended on the chemical composition and the smelting temperature, also induces the degradation of the lining refractory.7–9) In order to improve the service life of the lining, it is valuable to gain a deeper understanding of the refractory wear behavior during the RH degassing process. Running experiments on a real device is the most intuitive approach for studying the refractory wear in the RH degasser. However, in the majority of cases, the measurement limitation and the harsh process environment result in the information provided by the experiments being inadequate.10) The critical phenomena such as the flow pattern of the molten steel, the floating motion of the gas bubble, and the fluctuaA CFD Study on Refractory Wear in RH Degassing Process

Numerical Analysis of Effect of Operation Conditions on Bubble Distribution in Steel Continuous Casting Mold with Advanced Bubble Break-up and Coalescence Models

Abstract: Continuous casting has been widely applied in the steel industry. As the last step of steelmaking, continuous casting plays an important role in controlling the quality of steel. Argon gas is usually injected to prevent nozzle clogging, encourage mixing molten steel and remove inclusions in the process of steel continue casting.1) The bubble size distribution plays an important role in the gas-liquid flow process. Large bubbles tend to float up to liquid steel surface and may cause the exposed eye of steel around the SEN,2) resulting in slag entrapment at the same time. Small bubbles can be carried to the sidewall and may be captured by solidifying shell, causing defects in the final products, such as pencil pipe, sliver and so on. The operation conditions play a vital role in production efficiency and product quality. Besides, in the turbulent gas-liquid system, the behavior of bubble coalescence and break-up determine the bubble size distribution. Therefore, the research about the Numerical Analysis of Effect of Operation Conditions on Bubble Distribution in Steel Continuous Casting Mold with Advanced Bubble Break-up and Coalescence Models