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ISIJ International Vol. 59 (2019), No. 9

ISIJ International
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Grid List Abstracts
オンライン版ISSN: 1347-5460
冊子版ISSN: 0915-1559
発行機関: The Iron and Steel Institute of Japan

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キーワードランキング

03 Dec. (Last 30 Days)

ISIJ International Vol. 59 (2019), No. 9

Effects of Interfacial Oxygen Potential and Slag Phase Changing during Slag Formation Process on Dephosphorization Behavior

Yunjin Xia, Jie Li, Dingdong Fan, Guangda Hou

pp. 1519-1526

DOI:

10.2355/isijinternational.ISIJINT-2019-052

抄録

The effects of interfacial oxygen potential and slag phase changing during slag formation process on dephosphorization behavior when the decarburization slag is recycled are investigated by XRD and SEM. The research results show that the remove of phosphorus is mainly accomplished in the first 5 minutes for the condition 1(initial lump-sum addition of lime and Fe2O3), however the remove of phosphorus is mainly done from 5 to 10 minutes for the conditions 2(divided addition of lime and Fe2O3) and 3(initial lump-sum addition of lime and divided addition of Fe2O3). The dephosphorization rate of the condition 1 is about 78.9%, which is significantly higher than those of the conditions 2 and 3. The phase of decarburization slag is mainly made up of 2CaO·SiO2-3CaO·P2O5 and Ca3Fe2O5, the phosphorus mainly enriches in 2CaO·SiO2-3CaO·P2O5. After the recycling of decarburization slag, the slag phase with reactions under different conditions is basically the same, which is made up of FeO, 2CaO·SiO2, 2CaO·SiO2-3CaO·P2O5 and Ca3Fe2O5. However, the contents of 2CaO·SiO2 and 2CaO·SiO2-3CaO·P2O5 change seriously. The reason for the difference of dephosphorization behavior is that the interfacial oxygen potential and the slag phase changing during slag formation process are significantly different between different conditions.

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Causes of Particle Trajectory Fluctuation on the Rotating Chute in Circumferential Direction at Bell-less Top with Parallel Type Hoppers

Chengfeng Sun, Jian Xu, Zhehan Liao, Yang Xu, Mingyin Kou, Liangying Wen

pp. 1527-1533

DOI:

10.2355/isijinternational.ISIJINT-2018-862

抄録

Uneven burden distribution at bell-less top has a negative influence on the smooth operation of blast furnace with parallel type hoppers. Although previous works agreed that the initially oval-shaped particle trajectory on the chute causes the above-mentioned segregation, the subsequent particle trajectory fluctuation against the circumferential direction was still not fully understood. As a result, this work employs both the discrete element method (DEM) simulation and the theoretical model calculation, to quantitatively elucidate the causes of particle fluctuating behaviors on the rotating chute. The consistent results show that, on the one hand, a sine-like particle velocity distribution causes the Coriolis force to have a maximum magnitude around 120 deg while a minimum around 300 deg in the circumferential direction. On the other hand, the alternately sparse and intensive granular flow on the chute causes the particle mass flow rate to present a sine-like result with a maximum rate around 220 deg while a minimum around 60 deg. The superposition of two results contributes to the particle trajectory fluctuation on the rotating chute in the circumferential direction at bell-less top with parallel type hoppers.

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Online Prediction of Hot Metal Temperature Using Transient Model and Moving Horizon Estimation

Yoshinari Hashimoto, Yoshitaka Sawa, Manabu Kano

pp. 1534-1544

DOI:

10.2355/isijinternational.ISIJINT-2019-101

抄録

Precise control of hot metal temperature (HMT) is crucial for achieving stable operation of a blast furnace, but it is difficult due to the sluggish dynamics caused by the huge heat capacity. To cope with such difficulty, this work aims at developing a method that can predict future HMT by adopting moving horizon estimation (MHE) based on a one-dimensional transient model. MHE is useful to successively adjust model parameters so that the undesirable influence of past disturbances on the prediction is minimized. The real application result demonstrated that the root mean square error (RMSE) of HMT of eight-hour-ahead prediction was only 11.6°C. The high-performance prediction enables operators to realize the efficient operation of the blast furnace.

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Formation of Non-Metallic Inclusion and Acicular Ferrite in Ti–Zr Deoxidized Steel

Yongkun Yang, Dongping Zhan, Hong Lei, Guoxing Qiu, Zhouhua Jiang, Huishu Zhang

pp. 1545-1551

DOI:

10.2355/isijinternational.ISIJINT-2019-008

抄録

The formation of non-metallic inclusion and acicular ferrite (AF) during solidification is investigated by high temperature experiment, thermodynamic calculation and metallurgical analysis. The size distribution and composition of inclusions are carried out by optical microscope (OM), scanning electron microscope (SEM) equipped with energy-dispersive spectrometer (EDS) and image process software. The results indicate that adding Ti–Zr can modify the inclusions from Mn–Si–O–MnS to Ti–Zr–O–MnS, and the average size of inclusions can be refined obviously from 2.38 µm to 1.56 µm. The critical size of inclusion inducing AF is 0.21–0.37 µm, and the optimized size for AF nucleation induced by Ti–Zr–O–MnS inclusion is in range of 1.0–3.0 µm in this study. Moreover, for inducing AF nucleation, the inclusion larger than critical size is only necessary condition but not sufficient condition. A Mn-depletion zone resulted by MnS precipitating on Ti–Zr–O inclusion is observed adjacent to the Ti–Zr–O inclusion, which is believed to be one of the possible mechanisms to promote the nucleation of AF.

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Evolution Mechanism of Inclusions in H13 Steel with Rare Earth Magnesium Alloy Addition

Xing Li, Zhouhua Jiang, Xin Geng, Minjun Chen, Leizhen Peng

pp. 1552-1561

DOI:

10.2355/isijinternational.ISIJINT-2019-094

抄録

Rare earth metals have strong affinity with O and S in molten steel. However, due to high density of rare earth inclusions close to that of molten steel, rare earth inclusions are not easy to float up and it is not conducive to take away O and S. In this paper, the influence of rare earth magnesium alloy on the evolution mechanism of inclusions and deoxidization and desulfurization in H13 steel was analyzed, and the thermodynamic calculation was developed. The research results indicate that the composite inclusions of low-density MgO attaching or wrapping on the surface of high-density Ce inclusions are formed after adding Ce–Mg alloy to H13 steel. Compared to Ce inclusions alone, composite inclusions have lower average density and larger diameter, so that they have a faster floating rate according to Stokes law and are easier to float up. Therefore, rare earth magnesium treatment is beneficial to effectively remove the impurity elements such as O and S in the steel.

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Looper-gauge Integrated Control in Hot Strip Finishing Mill Using Inverse Linear Quadratic Theory

Jie Sun, Yunjian Hu, Fangchen Yin, Yaohui Hu, Wen Peng, Dianhua Zhang

pp. 1562-1572

DOI:

10.2355/isijinternational.ISIJINT-2018-721

抄録

Hot strip rolling is a significant process in the fields of manufacturing and processing. In order to further improve the control accuracy of looper angle, strip tension and gauge in hot strip finishing mill, an innovative looper-gauge integrated control scheme is developed in this paper. Based on the inverse linear quadratic (ILQ) theory, a proposed control scheme is designed for the looper-gauge integrated system. First, considering the numerous interactions between looper angle, strip tension and gauge, and the disturbances from several sources, a dynamic 6th order state space model is established and validated. Then, the control scheme based on ILQ theory is imported into gauge-looper integrated system. The desired poles are placed according to the dynamic characteristics requirements. The state feedback optimal control law is determined by an improved ILQ design method. Then the multivariable looper-gauge integrated control system is constructed. The proposed control scheme has the explicit ability to achieve desired looper and gauge control performances, with less external disturbances and no sensitivity of strip dimension changing. The effectiveness of the proposed looper-gauge integrated control scheme compared with traditional control strategies is shown in the simulation results. The interactions between looper control and guage control are also minimized.

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Practical Operation Guidance on Thermal Control of Blast Furnace

Yoshinari Hashimoto, Yuki Okamoto, Tatsuya Kaise, Yoshitaka Sawa, Manabu Kano

pp. 1573-1581

DOI:

10.2355/isijinternational.ISIJINT-2019-119

抄録

Blast furnaces are still manually operated in the steel industry. To realize an efficient and stable operation, the authors developed an operation guidance system for controlling hot metal temperature and applied it to actual furnaces. The system features a newly developed 2D transient model that describes the sluggish and complicated process dynamics. Based on the transient model, nonlinear model predictive control (NMPC) and moving horizon estimation (MHE) were adopted to provide operators with appropriate control actions while minimizing the undesirable influence of disturbances. The online validation results demonstrated that the developed operation guidance system successfully reduced the variance of HMT by 1.9°C.

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A Mill Set-up Model Using a Multi-output Regression Tree for a Tandem Cold Mill Producing Stainless Steel

Hyun-Seok Kang, Chi-Hyuck Jun

pp. 1582-1590

DOI:

10.2355/isijinternational.ISIJINT-2018-770

抄録

In a tandem cold mill for stainless steel, an optimum reduction rate is necessary for each stand. A conventional mill set-up uses a lookup-table to optimize the rolling schedule. However, to reflecting all the input conditions and manual interventions on a model is difficult. In this paper, we propose a mill set-up model that can efficiently predict the reduction rate for each stand by considering various input conditions. The proposed prediction model has a multi-output tree structure with a smaller time complexity for easy interpretation. The key contribution to the proposed algorithm is variable selection. According to the results of an analysis of the time-complexity, the proposed algorithm is less time consuming and is capable of learning datasets with a large number of variables more efficiently than the single-output CART (classification and regression trees). To evaluate the performance of the proposed algorithm, we applied it to the rolling reduction rate of a tandem cold mill in POSCO. The proposed algorithm achieves a similar level of R-squared in only 18% of the computing time required for an existing single-output CART algorithm.

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Characterization of Dislocation Evolution in Cyclically Loaded Austenitic and Ferritic Stainless Steels via XRD Line-profile Analysis

Masoud Moshtaghi, Shigeo Sato

pp. 1591-1598

DOI:

10.2355/isijinternational.ISIJINT-2019-077

抄録

Dislocations in austenitic and ferritic stainless steels (SSs) under cyclic loading were quantitatively evaluated via X-ray diffraction line-profile analysis to determine the relationship between the dislocation density and low-cycle fatigue (LCF) life in both SSs. The dislocation density of the austenitic and ferritic SSs varied linearly with respect to the LCF life in a double-logarithmic graph, with different slopes of the line. The dislocation density normalized by the maximum work hardening for both SSs exhibited a log–log linear relationship with the LCF life. The fraction of screw dislocations in the ferritic SS decreased with decreasing LCF life owing to the easy cross-slip of dislocations. Because of the difficulty of the cross-slip of dislocations in the austenitic SS, the fraction of screw dislocations remained almost constant throughout the LCF life. Analysis of the crystallite size and the dislocation arrangement with respect to the dislocation density under tensile and cyclic loading revealed that the dislocation arrangement for cyclic loading was smaller than that for tensile loading. Thus, the dislocation arrangement was related to the cyclic loading. In the plot of the dislocation evolution versus the number of cycles, two stages were observed in the variation of the dislocation characteristics for both SSs. In the first stage, the dislocation density increased, and the crystallite size decreased. The dislocation arrangement parameter of the ferritic and austenitic SS decreased and remained the same, respectively, in the first stage. In the second stage, the dislocation density, dislocation arrangement parameter, and crystallite size remained constant.

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Accurate Online Measurement of Iron Concentration in Galvannealed Coating Layers by Shift of X-ray Diffraction Peak

Tomohiro Aoyama, Hisato Noro

pp. 1599-1603

DOI:

10.2355/isijinternational.ISIJINT-2019-085

抄録

The authors have developed a new online measurement system for the Fe concentration in galvannealed (GA) coatings based on the proportionality between the peak angle of X-rays diffracted by the δ1 phase and the Fe concentration. The online system was realized by high-speed measurement of the peak angle of diffracted X-rays using a one-dimensional detector in conjunction with correction algorithm for the angular error caused by the displacement of the GA steel strip surface from the measurement position. The performance of the developed online system was demonstrated in a continuous galvanizing line with GA strips of Si-added and Si-free base steels. The developed system showed higher accuracy even with the Si-added base steel, with which enough accuracy could not be obtained with conventional online systems based on the diffraction intensity of the Γ phase. Since the δ1 phase is the major phase of GA coatings, the main advantage of the developed system is that its accuracy is less sensitive to the composition of the base steel.

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A New Data-driven Roll Force and Roll Torque Model Based on FEM and Hybrid PSO-ELM for Hot Strip Rolling

Zhenhua Wang, Dianhua Zhang, Dianyao Gong, Wen Peng

pp. 1604-1613

DOI:

10.2355/isijinternational.ISIJINT-2018-846

抄録

In this paper, a new Extreme Learning Machine (ELM) regression model of roll force and roll torque based on data-driven is proposed. The three-dimensional elastic-plastic finite element model (FEM) is established to solve the roll force and roll torque under different parameters (including rolling reduction rate, roll radius, rolling speed, average width of strip, entry temperature of strip). The regression model of ELM optimized by Particle Swarm Optimization (PSO) is established through using the datasets obtained by FEM. The PSO-ELM model prediction values of roll force and roll torque are compared with the single ELM and PSO-SVM model, and the error results of the prediction values are analyzed. The error results fully verify the feasibility and accuracy of the PSO-ELM model proposed. It is found that the new data-drive model of roll force and roll torque is simple in structure and it can make up for the deficiency of traditional mathematical mechanism model in dealing with nonlinear problems. The research result reveals that PSO-ELM method is suitable for parameters prediction and model optimization in strip rolling process.

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Modeling of Effects of Temperature and Alloying Elements on Austenite Phase Growth Rate in Duplex Stainless Steel

Kazuhiro Ogawa, Akira Seki

pp. 1614-1620

DOI:

10.2355/isijinternational.ISIJINT-2018-869

抄録

The growth rate of austenite phase from overcooled ferrite phase in duplex stainless steel was investigated to clarify the quantitative effects of temperature and chemistry in isothermal heating process. The proper fraction of ferrite and austenite phase is very important to obtain the maximum performance such as toughness and corrosion resistance of the steels. However, the fraction in heat affected zone (HAZ) in weldments can be changed during welding process.A physical model of growth rate as function of temperature including the parameter regarding chemistries was suggested. The measurement of austenite phase conducted experimentally after isothermally heated at various temperature, employing 25%Cr or 22%Cr duplex stainless steels containing various level of nitrogen. Referring those data obtained, it was confirmed that the effects of temperature and chromium and nitrogen contents on the growth rate is explained by the physical model suggested in this work.

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Reduction of Contact Resistance on Titanium Sheet Surfaces by Formation of Titanium Carbide and Nitride, and its Stability in Sulfuric Acid Aqueous Solution

Kazuhiro Takahashi, Taku Kagawa, Kohki Tanaka, Hiroshi Kihira, Kohsaku Ushioda

pp. 1621-1631

DOI:

10.2355/isijinternational.ISIJINT-2018-787

抄録

Separators for solid polymer fuel cells must have a low contact resistance with the carbon paper and stability in a corrosive environment of sulfuric acid in the cell. The titanium surface is highly resistant to corrosion thanks to a passive film but has high contact resistance.In this study, titanium carbide or nitride as the electrical conductor was formed on the surface by annealing commercially pure titanium sheet. The contact resistances of these sheets were evaluated before and after a sulfuric acid aqueous solution exposure test, “with a pH of 4 at 80°C for 4 days”, briefly simulating the operating environment. In addition, the same evaluation test was conducted with a surface with TiC formed dipped in nitric acid to enhance the stability in a sulfuric acid solution.The initial contact resistance falls below 10 mΩ·cm2 by formation of TiC and TiN, Ti2N on sheet surface. However, the contact resistance rises to 100 or above after the exposure test because a large amount of TiO2 precipitates. This is probably because TiC and TiN are dissolved by sulfuric acid, generating TiO2.By contrast, dipping in nitric acid hardly raises the contact resistance from less than 10 even after the exposure test. It is considered from the results of surface analyses that Ti ion generated by partial dissolution of TiC is turned into TiO2 by the oxidizability of nitric acid, changing the surface structure covering TiC. It is considered that the newly formed TiO2 film enhanced stability in a sulfuric environment.

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Effects of Electrolysis Conditions on the Formation of Electrodeposited Invar Fe–Ni Alloys with Low Thermal Expansion

Keisuke Fukuda, Yuki Kashiwa, Satoshi Oue, Tomio Takasu, Hiroaki Nakano

pp. 1632-1641

DOI:

10.2355/isijinternational.ISIJINT-2019-023

抄録

To elucidate the effects of the electrolysis conditions on the formation of electrodeposited invar Fe–Ni alloys with low thermal expansion, Fe–Ni electrodeposition was performed at 10–5000 A·m−2 and 5 × 105 C·m−2 in an agitated solution containing NiSO4, NiCl2, FeSO4, H3BO3, C7H4NNaO3S, and C3H4O4 at 40°C–60°C. In the low-current-density region, the Ni content in the deposits significantly decreased with increasing current density, reached a minimum, and then increased after reaching the diffusion-limiting current density for Fe deposition. With the increasing concentration of FeSO4 in the solution, the Ni content in the deposits decreased in the lower-current-density region, reached a constant at a moderate current density, and then began to increase at higher current density. As a result, the current density range in which the Ni content in the deposits reached a minimum and remained constant became wider with increasing FeSO4 concentration. With the decreasing pH of the solution, since the partial polarization curve for H2 evolution and the total polarization curve shifted to a higher-current-density region, the curve of the Ni content in the deposits as a function of the current density shifted toward a higher current density. The change in the composition of the Fe–Ni alloy deposits because of the electrolysis conditions can be explained by the changes in the total polarization curve and the partial polarization curves for Fe and Ni deposition and H2 evolution.

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Stress and Adhesion of Protective Oxide Scales on Stainless Steels and RE Effects

Kazuyuki Kitamura, Yoshitaka Nishiyama, Shinji Fujimoto, Nobuo Otsuka

pp. 1642-1649

DOI:

10.2355/isijinternational.ISIJINT-2019-037

抄録

The stress and adhesion of a protective oxide scale formed on 25Cr-20Ni steel were characterized by in situ acoustic emission (AE) and Raman spectroscopy in order to examine breakaway oxidation during cyclic oxidation and the effects of various reactive elements (RE), such as Y, La, and Ce. Scale failure, such as spalling and/or cracking, was detected from AE measurements during cooling after isothermal oxidation at 1273 K. The internal stress of the scale during cooling was evaluated using in situ Raman spectroscopy to determine the stress increase that arises due to mismatch between the thermal expansion coefficients of the scale and the steel. Stress relaxation induced by scale failure or creep deformation in the steel substrate was also examined. Comparing the two measurements, we introduced the maximum permissible stress, σmax, as an index describing scale failure resistance. It is considered that scale failure occurs when the internal stress of the scale exceeds σmax during cooling. Scale failure makes the scale more susceptible to oxidation, resulting in increased Cr consumption from the substrate. The steel oxidizes drastically if the Cr content decreases below a critical concentration, exhibiting breakaway oxidation. The addition of RE improves scale adhesion and suppresses the oxidation rate, delaying breakaway oxidation.

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Influence of Selective Surface Oxidation of Si and Mn on Fe–Zn Alloying Reaction on Hot-rolled Steel Sheets

Masaki Koba, Yusuke Fushiwaki, Yasunobu Nagataki

pp. 1650-1658

DOI:

10.2355/isijinternational.ISIJINT-2019-067

抄録

The Fe–Zn alloying reaction and selective oxidation behavior of 0.7 mass% Si-1.15 mass% Mn added hot-rolled steel annealed at 600–800°C were investigated by comparison with those of cold-rolled steel. The Fe–Zn reactivity of the hot-rolled steel improved from 600°C to 700°C but deteriorated from 700°C to 800°C. Above 700°C, the amount of Fe–Si–Mn oxide on the steel surface increased with increasing temperature, and this oxide deteriorated Fe–Zn reactivity. Below 700°C, a thin layer of Fe oxide on the steel surface deteriorated Fe–Zn reactivity. This oxide layer was reduced by Si and Mn that diffused from the steel substrate. Therefore, as the temperature increased from 600°C to 700°C, Fe–Zn reactivity improved due to the formation of reduced iron on the steel surface. In the case of the cold-rolled steel, the same selective oxidation behavior and reduction mechanism of the Fe oxide were also confirmed, and as a result, the Fe–Zn reactivity of the cold-rolled steel showed behavior similar to that of the hot-rolled steel. However, the Fe–Zn reactivity of the cold-rolled steel improved at a lower temperature than that of the hot-rolled steel. This can be explained by the faster diffusion rates of Si and Mn in the cold-rolled steel than in the hot-rolled steel. That is, reduction of the surface Fe oxide layer by diffused Si and Mn proceeded at a lower temperature, and as a result, the Fe–Zn reactivity of the cold-rolled steel also improved at a lower temperature.

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Simultaneous Measurements of Corrosion Potential and Hydrogen Permeation Current in Atmospheric Corrosion of Steel

Saya Ajito, Eiji Tada, Azusa Ooi, Atsushi Nishikata

pp. 1659-1666

DOI:

10.2355/isijinternational.ISIJINT-2019-113

抄録

To investigate hydrogen absorption behavior into carbon steel during corrosion in an aqueous sodium chloride (NaCl) droplet, a simultaneous measurement system of the corrosion potential, Ecorr and hydrogen permeation current, iper was developed using the Kelvin probe (KP) technique and the Devanathan–Stachurski (DS) method, respectively. This system outputs the interrelation between corrosion and hydrogen absorption into steel throughout the drying process of an NaCl droplet. Our results showed that hydrogen absorption into the steel occurred when the Ecorr shifted in less noble direction under wet conditions, and ceased at a higher potential of Ecorr when the steel surface dried up. Based on the results of the transients of the iper, the amount of hydrogen absorbed during the drying of the NaCl droplet increased with NaCl concentration, which was attributed to the negative shift of the Ecorr. Furthermore, the amount of hydrogen absorbed within one wet-dry cycle changed with the number of cycles, due to the expansion of the corroded area and the formation of iron rust.

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Effects of Mn on Isothermal Transformation Microstructure and Tensile Properties in Medium- and High-carbon Steels

Hayato Ishigami, Nobuo Nakada, Takuya Kochi, Shigenobu Nanba

pp. 1667-1675

DOI:

10.2355/isijinternational.ISIJINT-2019-022

抄録

The effects of Mn addition on the microstructure formed through an isothermal transformation at 873 K and its tensile properties were investigated over a wide range of concentrations in medium- and high-carbon steels with 0.4–1.0 mass% C. The Mn addition changed the pearlite transformation mode from relatively slow non-partitioning pearlite with a small amount of proeutectoid ferrite to an extremely slow partitioning pearlite without any proeutectoid ferrite in the hypoeutectoid steel. The transformation rate of the partitioning pearlite associated with the Mn partitioning between ferrite and cementite in Fe–C–M alloy was more than three orders of magnitude lower than the pearlite transformation in Fe–C binary alloy at a given interlamellar spacing. The decrease in the transformation rate in the non-partitioning and partitioning pearlite transformation was caused by the decrease in the carbon flux controlling the pearlite transformation, which can be explained by the theory of local equilibrium at the austenite/ferrite and austenite/cementite interphases. The Mn addition increased the thermal stability of the lamellar cementite. Corresponding to the change in the transformation microstructure, the Mn addition improved the tensile properties in the pearlite steel, particularly the strength and local deformability balance, regardless of the difference in the transformation mode between the non-partitioning and the partitioning transformation, unless the proeutectoid cementite precipitated at prior austenite grain boundaries. The strength increase of the pearlite after the Mn addition was caused by the refinement of the interlamellar spacing and/or the increase of the lattice strain in the pearlitic ferrite.

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Role of Interface between Ferrite and Martensite in Hydrogen Embrittlement Behavior of Ultra-high Strength Dual-phase Steel Sheets

Katsutoshi Takashima, Takamasa Nishimura, Ken’ichi Yokoyama, Yoshimasa Funakawa

pp. 1676-1682

DOI:

10.2355/isijinternational.ISIJINT-2019-013

抄録

The effects of interfacial conditions between the ferrite and martensite phases of an ultra-high strength dual phase (DP) steel sheet on its hydrogen embrittlement behavior has been investigated by a sustained tensile-loading test using as-quenched DP steel and tempered DP steel specimens. The yield ratio (yield stress/tensile strength) of the as-quenched DP steel is lower than that of the tempered DP steel. In the hydrogen thermal desorption analysis, the second desorption peak disappeared and the amount of absorbed hydrogen is decreased by tempering. Under the same applied stress in the sustained tensile-loading test, the time to fracture shows no significant difference between the two steels, but the critical applied stress for fracture is increased by tempering. A quasi-cleavage fracture occurs at the fracture initiation site of both steels. On the cross section near the fracture surface, many cracks nucleate in blocks or packets in martensite and the interface between prior austenite grains, but no cracks is observed in ferrite grains. Under applied stress higher than the yield stress of the as-quenched DP steel, fracture occurs in a short time. A unique intergranular-like morphology is observed at the fracture initiation area, and the crack propagates in blocks or packets in martensite or along the interface between the ferrite and martensite phases while avoiding ferrite grains. Early fracture is inhibited by tempering. When excessive plastic deformation is applied before the sustained tensile-loading test, the time to fracture and critical applied stress of the as-quenched DP steel decreased slightly. The results of the present study indicate that the interfacial conditions between ferrite and martensite play important roles in crack propagation associated with hydrogen embrittlement of the DP steel.

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Influence of Stress Re-distribution on Hydrogen-induced Fatigue Crack Propagation

Daisuke Sasaki, Motomichi Koyama, Hiroshi Noguchi

pp. 1683-1690

DOI:

10.2355/isijinternational.ISIJINT-2019-012

抄録

In order to clarify influence of stress re-distribution effect on hydrogen-induced fatigue crack propagation, we investigated fatigue crack propagation rates and brittle-like fracture ratio. The experiments were conducted in nitrogen and hydrogen gas atmosphere with ferrite-pearlite steels having different pearlite ratio, respectively. The crack propagation rates and the brittle-like fracture ratio decreased as pearlite ratio increased. To explain the changes of crack propagation rates and fracture ratio, we proposed that the stress re-distribution effect causing stress and strain relaxation at a crack tip contributes to suppression of the hydrogen-induced fatigue crack propagation. As a verification, finite element methods were operated with models having different width of the hard phase and different distance between a crack tip and a hard phase in plane stress and strain conditions, respectively. The finite element method analysis showed that stress re-distribution effect was smaller in plane strain condition than that in plane stress condition, indicating that a large hardness difference is crucial in plane stress condition to suppress the hydrogen-induced fatigue crack propagation.

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Properties-to-microstructure-to-processing Inverse Analysis for Steels via Machine Learning

Zhi-Lei Wang, Toshio Ogawa, Yoshitaka Adachi

pp. 1691-1694

DOI:

10.2355/isijinternational.ISIJINT-2019-089

抄録

As traditional experimental science appears to be inefficient for designing novel materials with desired properties because of the complex combination of processing conditions and chemical compositions, data-driven materials science is becoming increasingly important for materials design. A properties-to-microstructure-to-processing inverse analysis for steels is attempted via a machine learning approach in this work, where a potential best balanced property of tensile strength (TS) and total elongation (tEL) TS × tEL and its corresponding microstructure and processing conditions are explored using a genetic algorithm, which is implemented by an independently developed machine learning tool called the Materials Genome Integration System Phase and Property Analysis (MIPHA). The results demonstrate that a property-to-microstructure/processing method is sufficient to identify a best model performance, potential TS × tEL, and a reasonable relationship description among the processing, microstructure and property. A microstructure with Widmanstatten ferrite, banite, and martensite is found to be beneficial to a good balanced property.

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Effect of Creep Degradation on Hardness Changes of Ni-based Alloys for A-USC Power Boiler

Nobuhiko Saito, Kazuki Ishiyama, Tomiko Yamaguchi, Fujimitsu Masuyama

pp. 1695-1704

DOI:

10.2355/isijinternational.ISIJINT-2018-878

抄録

A project to develop Advanced Ultra-Supercritical (A-USC) power plant which operate at 700°C has been under way in Japan with the intension to reduce the CO2 emission and improve the power generation efficiency. Ni-based alloys, UNS N06674 (ASME Code Case 2826, 47Ni-23Cr-23Fe-7W, HR6W) and UNS N06617 (52Ni-22Cr-13Co-9Mo, Alloy 617) are candidate materials used in the high temperature regions of the A-USC power boilers. In this study, effect of creep degradation on the hardness change of both Ni-based alloys was investigated in order to evaluate the potential of the hardness method for creep life assessment. The investigation results showed that the scattering of hardness values in the latter stage of the creep life was increased as a result of hardening and softening of the material due to the increase of dislocation density with creep deformation and coarsening of precipitates followed by the formation of creep cavities, respectively. This tendency shows a good correlation with the creep life fraction. Therefore, it could be an useful tool for creep life evaluation. In addition, it was found that the hardness variation of the HR6W can be expressed as a function of temperature, stress and creep life fraction and thus leads to postulate a regression equation with hardness by a multivariate analysis. The creep life fraction can be calculated by substituting the values of service temperature and stress, and measured hardness into the multivariate regression equation. Those findings suggested that the hardness method was useful for the creep life assessment of Ni-based alloys.

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Hydrogen Embrittlement Susceptibility Evaluation of Tempered Martensitic Steels Showing Different Fracture Surface Morphologies

Kippei Ogawa, Yu Matsumoto, Hiroshi Suzuki, Kenichi Takai

pp. 1705-1714

DOI:

10.2355/isijinternational.ISIJINT-2019-130

抄録

The effects of the crosshead speed, hydrogen content and temperature on fracture strength and fracture surface morphology were investigated using a tempered martensitic steel containing 1.67 mass% of Si (H-Si) and one containing 0.21 mass% of Si (L-Si). When L-Si specimens were charged with a small amount of hydrogen, fracture surfaces showed a transition from quasi-cleavage (QC) to intergranular-like (IG-like) to intergranular (IG) at room temperature. In contrast, when H-Si specimens were charged with a small amount of hydrogen, fracture surfaces showed a transition from QC to IG-like at room temperature. This transition in the fracture surface morphology can be explained by the magnitude relationship between intergranular and transgranular strengths under hydrogen charging. At a temperature of −196°C, hydrogen did not lower the fracture strength nor did it change the fracture surface morphology. Hence, hydrogen embrittlement at room temperature was presumably caused by hydrogen accumulation and lattice defect formation during stress application as well as by hydrogen trapped before stress was applied. Fracture strength decreased and converged to a constant value (lower critical stress) with decreasing crosshead speed. The crosshead speed for obtaining lower critical stress decreased as the fracture surface changed from IG to IG-like to QC. Therefore, the crosshead speed for obtaining lower critical stress should not be treated as a constant but should be determined experimentally for each type of fracture surface.

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Design of Reduced Activation Ferritic/Martensitic Steels by Multiphase Optimization during the Entire Processing

Chenchong Wang, Qing Cui, Xiaojie Huo, Chi Zhang, Wei Xu

pp. 1715-1722

DOI:

10.2355/isijinternational.ISIJINT-2019-143

抄録

Improving the mechanical properties of reduced activation ferritic/martensitic (RAFM) steels has been a long-standing challenge that inhibits the construction of fusion engineering test reactors. To increase the toughness of RAFM steels without sacrificing too much strength, a modified RAFM steel was designed by optimizing multiple phases, including M23C6, MX, and martensite blocks; the combination of the composition and the entire thermomechanical processing scheme was also considered. To improve the composition, thermodynamic calculations were used to tailor the precipitation fraction. To optimize the thermomechanical treatment and heat treatment, a modified thermomechanical control process and intermediate heat treatment were employed to refine both the precipitation and martensite blocks. The experimental results for the microstructure and mechanical properties showed that, compared with that of CNA1 and EUROFER97, the newly designed RAFM steel had a more rational microstructure and greatly improved toughness with sufficient strength. The strengthening was quantitatively analyzed by Olson’s strengthening model to guide the additional development of modified RAFM steels.

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Mechanism of Pore Formation in Novel Porous Permeable Ceramics Prepared from Steel Slag and Bauxite Tailings

Yu Li, Yupeng Ren, Dejian Pei, Hongjian Sheng, Yaodong Yi, Daqiang Cang

pp. 1723-1731

DOI:

10.2355/isijinternational.ISIJINT-2018-782

抄録

Porous permeable ceramics (PPC) were prepared from composite ceramsites (CC) via a single firing process. CC were granulated with steel slag as a core and bauxite tailings in an outer-layer. XRD, SEM, EDS, mercury porosimetry and metallographic microscopy were used to study its properties and the pore formation mechanism. Results showed that during sintering process, gradual diffusion of cations from slag to tailings layers enhanced bonding among CC with formation of new crystals: anorthite and pyroxene. PPC had a wider distribution of pores from 0–300 µm sintered at 1160°C. With an increase in sintering temperature, ceramics were densified with disappearance of the small pores, which had an increasing threshold diameter values from 45 µm at 1180°C to 70 µm at 1190°C. Big pores larger than the threshold values would be remained and enlarged due to shrinkage of CC during the densification process. The decreasing amounts of pores and an increasing pore diameter had contrary effects on its permeable properties. PPC sintered at 1180°C with porosity of 27.5% and medium pore diameter of 92.7 µm had the optimum properties with bending strength of 10.92 MPa, water permeability of 0.039 cm/s and qualified leaching properties of harmful elements (Mn, Cr, V and Pb). This study would promote a more feasible and economic method for producing porous permeable ceramics and improving added value of steel slag and tailings.

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Erosion Behavior of Blast Furnace Tuyere

Rajeswar Chatterjee, Samik Nag, Subhashis Kundu, Ranjan Chakraborty, Uttam Singh, Sanjay Chandra

pp. 1732-1734

DOI:

10.2355/isijinternational.ISIJINT-2018-710

抄録

This study focuses on the study of erosion pattern of Blast furnace tuyere which is a copper casted component used for injecting hot air inside the furnace to help in combustion of pulverized coal. As tuyere is operating at high adverse thermal condition; it has provision of two cooling circuits, body and nose separated by solid copper. Identification of the most vulnerable zone in tuyere has been done by scanning used tuyere for which nose cooling circuit does not get failed. This is required to get the actual information regarding erosion pattern as tuyeres changed during shutdown are generally having burnt nose and therefore no use for this study. Scanned model is compared with three dimensional model of original tuyere to find out the erosion pattern. This exercise reveals that the top part of nose front is the weakest zone and hence the maximum material loss is observed at this zone. The reason behind why this particular zone becomes the weakest part is being explained with temperature profile observed in numerical analysis.

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