Showing papers in "Tetsu To Hagane-journal of The Iron and Steel Institute of Japan in 2013"

Simulation of the Center-Line Segregation Generated by the Formation of Bridging

Abstract: The macrosegregation model that heat transfer, solidification, liquid flow and solute movement were considered was developed to simulate the generation of the center-line segregation in the casting of steel. The classical model which considers only the liquid flow caused by solidification shrinkage leads the negative segregation which contradicts the fact. In order to explain the contradiction, the bulging of the cast slab has been claimed to be important factor to form the positive segregation at the center of the cast. However, some experimental data show that the bulging has not been necessarily formed during the generation of center-line segregation. In this case, the bridging with the solidification shrinkage has been found to be formed instead of the bulging. In this paper, the macrosegregation model is developed considering, thus, three driving forces of fluid flow; solute concentration, thermal expansion and solidification shrinkage. This simulation results show that the primary driving force which results in the center-line segregation is the solidification shrinkage with the bridge. In addition to that, the mechanism of generating the center-line segregation is discussed based on the simulation results.

Influence of Fe Oxidation on Selective Oxidation Behavior of Si and Mn Added in High Strength Sheet Steel

Abstract: In the process of hot dip galvanizing high tensile strength sheet steels containing Si and Mn, selective surface oxidation of Si and Mn causes coating defects. One promising method for overcoming this problem is an oxidation-reduction process. When the steel surface is exposed to an oxidizing atmosphere, it will react primarily by forming an Fe oxide, which can be reduced by hydrogen in a reduction process that follows. It has been explained that good wettability can be obtained due to the formation of pure iron. However, the mechanism of suppression of selective surface oxidation has not been clearly understood in detail yet. In order to reveal this mechanism, the present study focused on both Mn and Fe oxidation behavior during the oxidation-reduction process for a cold-rolled sheet steel containing 0.25mass%Si-1.8mass%Mn. Surface and cross-sectional analyses were performed by using secondary electron microscopy and transmission electron microscopy. Selective surface oxidation behavior was investigated by glow discharge optical emission spectroscopy. The main results obtained are as follows. First, selective surface oxidation of Mn was suppressed even if soaking was continued after completion of the reduction of the Fe oxide. Second, as the reduction process proceeded, Mn was trapped as an internal oxide under the Fe oxide layer. Moreover, depletion of solute Mn was observed in the matrix. From these results, depletion of solute Mn is supposed to suppress the outer diffusion of Mn during soaking. Therefore, selective surface oxidation of Mn is suppressed even after Fe oxide reduction is completed.

Internal reversible hydrogen embrittlement of austenitic stainless steels based on type 316 at low temperatures

Abstract: The internal reversible hydrogen embrittlement (IRHE) of austenitic Fe(10–20)Ni17Cr2Mo alloys based on type 316 stainless steels hydrogen-charged to around 40 mass ppm was investigated by performing tensile tests using the slow strain rate technique at temperatures from 80 to 300 K. The susceptibility to IRHE depended on the Ni content. IRHE occurred below a Ni content of 15% (Ni equivalent of 29%), increased with decreasing temperature, reached a maximum at 200 K and decreased with further decreasing temperature. Hydrogen-induced fracture due to IRHE occurred in brittle transgranular mode associated with the strain-induced α' martensite structure at temperatures from 200 to 300 K and occurred simultaneously with fracture along the prior annealed-twin boundary at 200 and 250 K, then changed to dimple rupture mode due to hydrogen localization at 150 K. IRHE was controlled by the amount of strain-induced α' martensite above 200 K, whereas it was controlled by hydrogen diffusion below 200 K.

Evaluation of Permeability for Columnar Dendritic Structures by Three Dimensional Numerical Flow Analysis

Abstract: In order to evaluate the permeability of columnar dendritic structures, three-dimensional (3-D) flow simulations of interdendritic liquid were carried out. The 3-D columnar dendrite morphologies, created by means of a computer-aided design (CAD) software, were based on two-dimensional dendrite morphologies calculated by a phase-field method. The artificial 3-D columnar dendrites were regularly arranged, and six kinds of 3-D columnar dendritic structures were observed, each with different liquid volume fractions between 0.56 and 0.95. For these 3-D columnar dendritic structures, the flow parallel and normal to the primary arms were calculated using FLUENT, and the permeability of six 3-D columnar dendritic structures for both flow directions were determined using the Darcy law. The values of our simulated permeability were compared with those values of permeability obtained experimentally [K. Murakami, A. Shiraishi and T. Okamoto: Acta Metall., 31 (1983), p. 1417, 32 (1984), p. 1423, C. Y. Liu, K. Murakami and T. Okamoto: Mater. Sci. Tech., 5 (1989), p. 1148]. For both flow directions, our simulated permeability for high liquid volume fractions complemented their experimental permeability for low liquid volume fractions. Therefore, we confirmed the consistency of simulated reading with extrapolations of experimental values of low volume fractions to high volume fractions. In addition, we discussed the limitation of flow within the mushy region, and found that defining the limiting permeability of interdendritic flow, in order to evaluate the relationship between dendritic morphology and the solid volume fraction where interdendritic liquid flow ceases, was effective.

Development of Continuous Steelmaking Slag Solidification Process Suitable for Sensible Heat Recovery

Abstract: 1) Steel Research Laboratory, JFE Steel Corporation, 1, Kawasaki-cho, Chuo-ku, Chiba, 260-0835 Japan. 2) Formerly Steel Research Laboratory, JFE Steel Corporation. Now at Shinagawa Refractories Co., Ltd, 1, Mizushimakawasakidori, Kurashiki, 712-0874 Japan. 3) East Japan Works, JFE Steel Corporation, 1, Kawasaki-cho, Chuo-ku, Chiba, 260-0835 Japan. 4) Slag Business Planning & Control Dept., JFE Steel Corporation, 2-2-3, Uchisaiwaicho, Chiyoda-ku, Tokyo 1000011 Japan. 5) Formerly Technology Planning Dept., JFE Steel Corporation. Now at The Iron and Steel Institute of Japan, 3-2-10, Nihonbashi-Kayabacho, Chuo-ku, Tokyo, 103-0025 Japan. 6) Division of Materials and Manufacturing Science, Graduate School of Engineering Osaka University, 1-1, Yamadaoka, Suita, Osaka, 565-0871 Japan. 7) Department of Materials Engineering, Graduate School of Engineering, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo, 113-8656 Japan. 8) Department of Advanced Materials Science, Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5, Kashiwanoha, Kashiwa, Chiba, 277-8561 Japan.

Powder X-ray diffraction analysis of lime-phase solid solution in converter slag

Abstract: In steelmaking processes that refine pig iron into tough steel, calcined lime (CaO) is added to remove unwanted components, such as Si, P, and S, and steelmaking slag is produced as a by-product. Japan produces about fifteen million tons of steelmaking slag annually. This slag consists primarily of CaO and SiO2 and is effectively utilized as the base course material of roads, aggregates for asphalt concrete, and earthwork and ground improvement materials. However, part of the CaO in the slag remains in the unreacted form, i.e. as free lime, which expands to about double volume by hydration (CaO+H2O→Ca(OH)2). Therefore, fresh slags that contain free lime must undergo aging in order to prevent damage to the road or concrete. The aging treatments for slag stabilization are performed such that hydration by rain water is promoted by leaving the slag outdoors, or treating with steam or high-pressure steam in order to accelerate the reaction.1) In general, slags containing high amounts of free lime have higher expansibility than their counterparts, which have lower CaO contents. The expansibility, however, varies with the particle size, physical state of the free lime, and the corresponding chemical composition. Therefore, estimating the expansibility solely as a function of the free lime content is difficult. The free lime in converter slag is typically classified into two categories namely, crystallized lime and undissolved lime. The former dissolves once and Powder X-ray Diffraction Analysis of Lime-Phase Solid Solution in Converter Slag

Effect of Bottom Inclination Vessel on Particle Entrainment Behavior with Mechanical Stirring

Abstract: 近年,厚板,ラインパイプ材に代表されるような鋼材品 質要求の厳格化に伴い,製鋼工程での脱硫負荷が増大しつ つあり,溶銑脱硫工程の重要性が高まっている。例えば, Shimodaら 1)は,CaO系フラックスの脱硫能を評価してお り,また,Nakanishiら 2)は,溶銑脱硫反応に及ぼす溶銑温 度の影響について報告している。溶銑脱硫処理プロセスの 一つである機械撹拌式脱硫法(KR法)3)においても,脱硫 反応効率向上を目的として種々の研究開発が行われてい る。機械撹拌式脱硫法では,撹拌羽根(インペラー)の回 転により脱硫剤を溶銑浴中へ巻き込ませて脱硫を行うこと から,浴中への脱硫剤分散挙動の把握が重要であり,Nakai ら 4)は水モデル実験により,インペラー撹拌による粒子の 分散挙動を調査し,粒子の分散条件やインペラー深さ,回 転数などの撹拌条件と浴面形状の関係を定量的に評価し, 更にホットモデル実験において脱硫剤分散挙動と溶銑脱硫 反応の関係を詳細に調査している。また,邪魔板やインペ ラー偏心により粒子の分散を強化する研究開発も実施され ている。Nomura and Iguchi5)は,水モデル実験によりイン ペラー偏心時の影響を調査し,偏心によって傾斜渦が生成 し粒子の浴内分散が強化されることを明らかにしている。 Kuroyanagiら 6)は,水モデル実験により邪魔板を浴内に装 入した際の影響について調査し,邪魔板を設置することで 粒子の分散,混合が強化され,粒子の浴内滞留時間が増加 することを明らかにしている。 このように,粒子の分散強化のための研究開発がいくつ か行われているものの,例えば既存の溶銑脱硫処理設備に おいてインペラーを偏心させるには設備制約があること や,邪魔板を設置した場合のインペラーシャフトにかかる トルクの増加や邪魔板の耐用性といった問題があることか ら,十分な工業化が図られていないのが実状である。 論 文

Effects of Temperature and Strain Rate on Tensile Properties in a Lean Duplex Stainless Steel

Abstract: 1) Graduate School of Engineering, University of Hyogo, 2167 Shosha, Himeji, 671-2280 Japan. 2) Formerly Graduate Student of University of Hyogo. Now at KOBELCO Research Institute Inc., 1-5-5, Takatsukadai, Nishi-ku, Kobe, 657-2271 Japan. 3) Coil & Sheet Products, Automotive Products Research & Development Division, Nippon Steel & Sumikin Stainless Steel Corporation, 3434 Shimata, Hikari-city, Yamaguchi, 743-8550 Japan.

Evaluation of Sinter Quality for Improvement in Gas Permeability of Blast Furnace

Abstract: In the recent operation of blast furnace, it is supposed that high gas permeability of burden is important for low RAR and high PCR operation. In this work, sinter quality for improvement in gas permeability of blast furnace was investigated with reduction degradation and under-load-reduction tests. As the results, the reduction degradation of sinter is deteriorated by increasing H2 concentration in the reduction gas under the condition of below 3.8 vol% H2. However, over 3.8 vol% H2, increase of H2 has no effect on the reduction degradation because the diffusion of reduction gas in the sinter is limited. On the other hand, from the under-load-reduction test, there is possibility that increase in H2 concentration of reduction gas and decrease in slag ratio in sinter are effective to improve gas permeability of lower part of blast furnace rather than reducibility of sinter. Due to adoption of these experimental results to a 2-dimentional mathematical simulation model, the precision of pressure drop calculation of blast furnace was improved. It is considered from the evaluation by this model calculation that the RDI, a slag ratio and the slag viscosity as the sinter properties are greatly influence on the permeability of blast furnace.