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ISIJ International Vol. 61 (2021), No. 10

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

21 Nov. (Last 30 Days)

ISIJ International Vol. 61 (2021), No. 10

Dissolution Mechanism of Modifying Agent in Fibrotic Process of Steel Slag

Aijun Deng, Haichuan Wang, Dingdong Fan

pp. 2475-2482

DOI:

10.2355/isijinternational.ISIJINT-2021-042

抄録

The kinetic behavior of SiO2 dissolving in molten steel slag at 1773–1923 K was studied using a combination of high-temperature confocal scanning laser microscopy and a synthetic physical property tester. The experimental results showed that the change in slag electrical conductivity can characterize the homogenization process of the modifying agent in molten steel slag, and the dissolution process of the modifying agent is controlled by solute diffusion in the slag. The concentration difference between the boundary layer and the bulk of the slag is the primary driving force of the dissolution process. By increasing the reaction temperature, the SiO2 particle dissolution rate in slag can be significantly increased. Moreover, the experimental data and model calculations revealed that the SiO2 dissolution rate was the fastest when the total Fe (T.Fe) content was 14% and the acidity coefficient (Mk) was 1.5. The dissolution factors were defined to quantitatively evaluate the dissolution mechanism of particle by studying the dissolution process of slag with different components.

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Nonisothermal Investigation of Reaction Kinetics between Electric Arc Furnace Dust and Calcium Chloride under Carbon-Containing Conditions

Ginji Iwase, Keiji Okumura

pp. 2483-2489

DOI:

10.2355/isijinternational.ISIJINT-2021-128

抄録

The volatilization of zinc in the electric arc furnace dust–CaCl2 and ZnFe2O4–ZnO–CaCl2–C reaction systems was investigated. Experiments were conducted under an N2 atmosphere in an infrared lamp heating furnace, and the activation energy of the reaction rate of zinc volatilization was determined by the Ozawa method. The activation energy in the dust–CaCl2 reaction system was 123 ± 27 kJ/mol; when the dust was decarburized, the activation energy was reduced to 84 ± 4 kJ/mol. Further, the simultaneous carbothermic reduction by the carbon contained in the dust and chloride volatilization of zinc improved the reaction rate. In the temperature range where carbothermic reduction and chlorination occur simultaneously, carbothermic reduction is favored. The reduction of metal oxides in the dust inhibits the chlorination and carbothermic reduction of zinc, reducing the reaction rate and activation energy.

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Microstructure and Viscosity of Dephosphorization Slag in New Double Slag Converter Steelmaking Process

Wenkui Yang, Jian Yang, Runhao Zhang, Han Sun

pp. 2490-2500

DOI:

10.2355/isijinternational.ISIJINT-2021-150

抄録

The structure and viscosity of the dephosphorization slag in new double slag converter steelmaking process with low basicity slag at the low temperature range of 1573 K–1723 K are studied with high-temperature laboratorial experiments. With increasing the end temperature of dephosphorization, the viscosities of different slags decrease rapidly at first, then decrease slowly, and the slag activation energies increase. The values of Non-Bridging Oxygen per Silicon (NBO/Si) decrease, indicating the increase in the degree of polymerization (DOP) of the dephosphorization slag and the viscous flow activation energy of the dephosphorization slag. The results of Raman and FTIR spectroscopy are consistent. The dephosphorization slag in the temperature range of 1573 K–1673 K is mainly composed of the P-rich region, Fe-rich region and liquid region. With increasing temperature from 1573 K to 1673 K, the proportion of the liquid region in slag increases, but the P-rich and Fe-rich regions correspondingly decrease.

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Low Carbon Sinter Making: Use of Inert Gas to Improve Sinter RDI

Moni Sinha, Pradeep Chaudhary, Meghna Mondal, Dipankar Roy, Amit Singh, Surajit Sinha

pp. 2501-2506

DOI:

10.2355/isijinternational.ISIJINT-2020-366

抄録

Sinter making is an intermediate step in value-added chain for producing steel. Iron ore fines (−10 mm) is agglomerated to make it suitable blast furnace feed. In sintering process, iron-ore is partially reduced, and different fluxes are added to eliminate the undesirable elements in the slag. Solid fuel mainly coke breeze is used both as energy provider and reducing agent. During the sintering process, oxidised phase (Hematite) is converted to reduced phase (Magnetite). However, during post sintering process, the reduced phase again gets oxidised as it reacts with the atmosphere during cooling. Thus, higher the coke breeze input, higher is the generation of reduced phases and in turn higher % of reduced phases will be retained in the product sinter. With increase in low grade ores, consumption of fuel and fluxes will further increase. The challenge, therefore, is to produce sinter with good RDI without increasing the solid fuel. The knowledge on this domain is very limited and thus in this work an innovative approach on injecting an inert gas during cooling of sinter was suggested. The hypothesis is that the injection of inert gas reduces the availability of oxygen therefore inhibiting the re-oxidation of reduced phase to oxidised phase in sinter. Lab scale trials followed by a plant trial in TSL India sinter plant strengthened the hypothesis as the sinter RDI improved by 8 points. This technology has the potential to significantly decrease the coke breeze requirement and thereby reduce CO2 emissions, increased mine life, without affecting the sinter quality.

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Premature Failure of Copper Staves and Applied Results for New Designed Staves

Jungil Kim, Sang-Woo Choi

pp. 2507-2512

DOI:

10.2355/isijinternational.ISIJINT-2021-158

抄録

In a blast furnace employing a copper stave as a cooling system, premature stave wear is experienced in many steel mills. To confirm this failure mechanism, an online ultrasonic thickness measuring device was installed in the Pohang 4 blast furnace from blow-in to acquire data. Through this, the refractory damage in hot face and real-time wear data were obtained, and the correlation with the operational factors was analyzed. Through the CFD simulation, the effect of temperature due to the refractory was confirmed, and the possibility of acceleration of the copper body was confirmed in the absence of the refractory in hot face. To improve the life of the entire refractory as well as the stave body, a refractory stress test was conducted in the laboratory, and a stable groove structure was derived using the structure program DEFORM using the data obtained from cold tests. The new designed long-life copper stave was applied to the Pohang 4 blast furnace in November 2015, and it has been proved that the life of the stave has been significantly improved since wear has not been measured until now.

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Effects of Temperature and Carbon Content in Brick on MgO–C Reaction Behaviour

Yuta Hino, Katsunori Takahashi

pp. 2513-2523

DOI:

10.2355/isijinternational.ISIJINT-2021-108

抄録

The effects of the ambient temperature, gas flow rate and carbon content in the brick on the behaviour of the MgO–C reaction, which is an inherent phenomenon of MgO–C bricks, were investigated. As a result, the amount of the MgO–C reaction increased as the carbon content in the brick and the temperature increased, but was not significantly changed by increasing the gas flow rate. The apparent activation energies for the following reactions were determined from the results of this study and the results of previous reports.CO(g) + MgO(s) = Mg(g) + CO2(g) E = 348 kJ / molCO2(g) + C(s) = 2CO(g) E = 296 kJ / molA new MgO–C reaction model was developed based on the shrinkage core model in order to discuss the effects of the temperature and carbon content in the brick quantitatively. The reaction model in this study could explain not only the results of the present study but also the results reported previously by other researchers. In addition, the effect of the particle diameters of MgO and carbon on theMgO–C reaction is also discussed using the reaction model proposed in this study.

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Control of Heterogeneous Nucleation in Carbon Steel by Magnesium Vapor Injection during Continuous Casting

Hideo Mizukami, Mitsuhiro Numata

pp. 2524-2533

DOI:

10.2355/isijinternational.ISIJINT-2021-206

抄録

The behavior of magnesium vapor injected in the molten steel and the potential of magnesium vapor for the miniaturization of TiN were evaluated via continuous casting experiments. The position where magnesium vapor should be injected was determined through a non-steady state three-dimensional numerical simulation for molten steel flow in a tundish. The yield of magnesium in the molten steel was changed by the injection position. A high additive yield was achieved when magnesium vapor was injected between the immersion nozzle and the narrow wall of the tundish. The concentration of the soluble magnesium was maintained using a novel method that continuously added magnesium vapor into the molten steel in the tundish, which also ensured stable miniaturization of TiN.

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Prediction Model for Degree of Solid-shell Unevenness during Initial Solidification in the Mold

Sang-Hum Kwon, Young-Mok Won, Gu Seul Back, Hyeju Kim, Jae Sang Lee, Dong-Gyu Kim, Yoon Uk Heo, Chang Hee Yim

pp. 2534-2539

DOI:

10.2355/isijinternational.ISIJINT-2021-155

抄録

The unevenness of the solid-shell and heat flux (Φq) during solidification were measured for various steel grades with casting apparatus. Then the data were used to develop a model to predict the degree of solid-shell unevenness according to the steel composition. The surface of the solid-shell was the least uniform when the steel composition was near the peritectic point, at which Φq was the lowest. A model to calculate an index of solid-shell unevenness was developed under the following conditions. Solid-shell unevenness is formed at the initial solidification under the constant cooling rate, which occurs only where solid fraction (fs) is from 0.9 to 1.0. In addition, solid-shell unevenness is proportional to the amount of phase transformation and inversely proportional to the temperature range of ΔTPT; between the temperature of fs = 0.9 and the solidus temperature. This model shows about 87% of the prediction accuracy in the experimental results of this study and also gives reasonable explanations of solid-shell unevenness observed in previous researches on steels.

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Camber Prediction Based on Fusion Method with Mechanism Model and Machine Learning in Plate Rolling

Jing Guo Ding, Yang Hao Chen He, Ling Pu Kong, Wen Peng

pp. 2540-2551

DOI:

10.2355/isijinternational.ISIJINT-2020-357

抄録

In order to realize the high-accuracy prediction of steel plate camber and the accurate control of roll gap tilt for straightness, a fusion method with mechanism model and machine learning of the strip at the exit side is investigated. Based on the basic equation of transverse asymmetric rolling, a mathematical model of plate camber curvature radius of the exit side and entry side is derived. Meanwhile, a machine vision method for camber measuring is adopted in which the subpixel coordinates of the rolled piece edges can be obtained, and the size of the plan view of rolled piece can also be settled indirectly to carry out feedback control on the camber defect. Tilt value of the roll gap can be controlled in advance to avoid the occurrence of camber which predicted with high accuracy. Prediction model of camber synthesis leveling based on PSO-LSSVM algorithm is used, the relative error is within ±5% of both the training set and the testing set. Combine the mathematical model of roll gap tilt adjustment and PSO-LSSVM camber prediction, the roll tilt adjustment for different processes and product specification is calculated by predicting plate camber accurately to obtain good straightness for the final product, the relative error range of curvature value is within ±6% after being compensated by PSO-LSSVM algorithm. The research result reveals that this method is suitable for camber prediction and model optimization in plate rolling process.

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Decoupling Strategy and Mechanism-intelligent Model of Non Square Flatness Control System

Ming-ming Song, Hong-min Liu, Yang-huan Xu, Xin-cheng Gao, Dong-cheng Wang

pp. 2552-2563

DOI:

10.2355/isijinternational.ISIJINT-2020-662

抄録

Taking 1420 mm UCM six-high cold rolling mill as the research object, a non square flatness control system with five input and four output is decoupled into a square subsystem with two input and two output which controls the primary and cubic flatness and a non square subsystem with three input and two output which controls the quadratic and quartic flatness by using the relative gain theory. By decomposing the unstable poles of the generalized inverse matrix of the non square system, the method of the generalized inverse matrix decoupling control the quadratic and quartic flatness is proposed, which solves the unstable problem of decoupling of non-square system. According to the characteristics of intermediate roll shifting, the variable model of roll shifting influence coefficient and the control strategy of minimum roll shifting adjustment and threshold are proposed. The dynamic characteristics of the system are improved and the adjustment of intermediate roll shifting is reduced. In order to overcome the shortcomings of low accuracy and poor generalization ability of shallow neural network, a mechanism-intelligent influence matrix model based on big data and deep neural network is proposed. Simulation calculation and industrial application show that the control system runs stably, the adjustment speed is fast, the control precision is high, the change of intermediate roll shifting is small, and it is suitable for online control.

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An Improved CBR Model Using Time-series Data for Predicting the End-point of a Converter

Mao-qiang Gu, An-jun Xu, Fei Yuan, Xiao-meng He, Zhi-feng Cui

pp. 2564-2570

DOI:

10.2355/isijinternational.ISIJINT-2020-687

抄録

The end-point temperature is one of parameters for the end-point control in the converter. Accurate prediction of the end-point temperature is helpful to improve the hit rate of the end-point. An improved CBR model using time-series data (CBR_TM) was proposed to predict the end-point carbon content and temperature in the converter according to the data types of process parameters. The attributes of the cases in the model not only include the influencing factors of single-value type such as composition and temperature of hot metal, but also include the influencing factors of time-series type such as lance position and oxygen flow, in the case retrieval process, the single-value data similarity and time-series data similarity between the cases were calculated based on the Euclidean distance and the dynamic time warping algorithm, and then weighted to obtain the comprehensive similarity. Then the influence of the weight of the time-series data similarity on the prediction accuracy was studied based on the production data. Finally, the prediction accuracy of the established model was also compared to models based on SVR and BPNN. The results show that: The prediction accuracy of the model increases at first and then decreases with the increase of similarity weight of time series data. The prediction accuracy of the model was the highest when the weight of time-series data similarity was 0.4 and was better than the SVR and BPNN models. The established can meet the requirements of field production.

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Quick Recognition and Elimination of an Additional Signal Caused by Deflection of an Integral Roller Flatness Meter

Huaxin Yu, Tongyuan Zhang, Shuai Zhang, Dongcheng Wang, Hongmin Liu

pp. 2571-2579

DOI:

10.2355/isijinternational.ISIJINT-2021-006

抄録

Compared with the sectional roller flatness meter, the integral roller flatness meter is more suitable for products with higher requirements on surface quality, and thus becomes the trend of the contact-type flatness meter for cold rolling. Due to its structure design, some problems arise. Under the action of the dead weight or an external load, the deflection deformation of the roller changes the stress state of the sensor installed inside it and generates an additional signal. This paper studies the mechanism of the signal and the recognition and elimination method. First, according to the structure of the detection roller and the connection mode of the sensors, the ideal waveform characteristics of the flatness signal are analysed. Through an experiment on the 650 mm integral roller flatness meter platform, the ideal waveform and actual waveform characteristics are compared, and the influence of the additional signal on the detection signal is analysed. Then, according to the deflection deformation of the roller and its corresponding stress distribution characteristics, the generating mechanism of the additional signal is revealed. Finally, a minimum error method is proposed, which ensures the elimination of the deflection influence on all detection units in real time. During the process of flatness detection, the zigzag feature of the flatness distribution curve is eliminated; thus, during the process of flatness control, the bad influence of the roller deflection is removed. The industrial application shows that the work presented in this paper can obviously improve the products quality.

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Guide Vane Opening Prediction for Constant Speed Axial Blowers in Blast Furnace Ironmaking with Variation Information

Xiao Fu, Junyi Han, Michael Castle, Ying Peng, Kuo Cao

pp. 2580-2586

DOI:

10.2355/isijinternational.ISIJINT-2021-201

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Determining appropriate guide vane openings (GVOs) of axial blowers under varying industrial conditions is vital for smooth operations in blast furnace ironmaking. This work analyses the influence GVO variations have on outlet air flow rate and pressure by using data taken from operating, industrial blast furnaces, based on which a support vector machine (SVM)-based GVO prediction model is developed. Outcomes reveal that the change status of GVOs, i.e., whether the GVO angle increases or decreases, is critical in determining the relationship between air flow and pressure. By introducing the change status and removing the transition outliers, predictions for the optimal GVOs required to meet the desired air flow rate and pressure in real time can be more accurately determined. The measured values of GVOs range from 0% to 100%, and the SVM-based model developed in this work predicted the GVOs with an RMSE of 0.2480%, significantly improving upon the baseline model which had an RMSE of 0.6047%. The resulting method can provide insights into the operation of complex ironmaking processes, enabling a more efficient adjustment of GVOs.

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Wide-area Operation Monitoring of Conveyors Using a Panoramic Vibration Camera

Kohei Shimasaki, Zulhaj Muhammad Aliansyah, Taku Senoo, Idaku Ishii, Tomohiko Ito

pp. 2587-2596

DOI:

10.2355/isijinternational.ISIJINT-2021-182

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In this study, we propose a novel non-contact vision sensing method for wide-area monitoring of the operation of conveyors in ironworks by using a panoramic vibration camera in real time. This method can capture magnified images, including vibrations from a single camera with mirror-driven viewpoint switching. The rotation of multiple rollers with conveyor belts was detected using a function of full-pixel vibration spectrum imaging, which can calculate peak frequencies from time frequency responses. Through experiments in different situations, such as loading and unloading, we evaluated the efficiency of this method, which can monitor the operation of multiple rollers and conveyors, when the camera is located 15 m away, or more, from the conveyors to be monitored.

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Effect of MC Type Carbides on Wear Resistance of High Wear Resistant Cast Iron Rolls Developed for Work Rolls of Hot Strip Mills

Kazunori Kamimiyada, Shinya Ishikawa, Hirofumi Miyahara, Yuji Konno

pp. 2597-2604

DOI:

10.2355/isijinternational.ISIJINT-2021-127

抄録

High-speed steel cast-iron rolls were developed around 1990 and have been widely used for the earlier stand of hot strip mills. However, for the later stand of hot strip mills, the use of high-speed steel cast-iron rolls has been limited due to the insufficient crack resistance. Therefore, in order to improve the wear resistance of the later stand, enhanced indefinite chilled rolls in which MC-type carbides of high hardness are crystallized in a conventional indefinite chilled roll has been developed. However, since the wear resistance of enhanced indefinite chilled rolls is significantly inferior to that of high-speed steel cast-iron roll, the development of a new cast iron roll with superior wear resistance applicable to the later stand of hot strip mills was studied. The present development roll has improved wear resistance by increased amount of the high hardness MC-type carbide-forming elements. In addition, the reduction of the carbon equivalent for less amount of eutectic carbide resulted in the reduction of the residual stress down to the same level as the indefinite chilled roll, which improved the crack resistance. As a result, it was confirmed that the wear resistance was improved about three times compared with the conventional indefinite chilled roll. In addition, the results suggest that the wear resistance of work rolls in hot strip mills is greatly controlled by the amounts of MC-type carbides, despite the roll hardness being the same.

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Effect of Al2O3 on Evolution of Oxide Inclusions in Tire Cord Steel during Hot Rolling

Kaijun Niu, Alberto N. Conejo

pp. 2605-2612

DOI:

10.2355/isijinternational.ISIJINT-2021-156

抄録

The evolution of oxide inclusions with different concentrations of Al2O3 in tire cord steel during hot rolling was investigated by industrial trials using an automatic scanning electron microscope. Changes in chemical composition, particle size, and number density of oxide inclusions in the entire hot rolling process were studied with systematical samplings. The maximum diameter of most inclusions is less than 5 µm, a value independent of the Al2O3 concentration; the number density of oxide inclusions increases with the increase of Al2O3, when this concentration is lower than 50%. The average chemical composition of oxide inclusions changed little during the hot rolling process when the concentration of Al2O3 was approximately 35%. Deformation and fracture of oxide inclusions occur simultaneously during the entire hot rolling process causing changes in the average diameter, number density and area fraction. Higher deformation in oxide inclusions was observed when the concentration of Al2O3 was in the range of 20%–25%. This behavior was explained due to a decrease in their melting point.

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Nitride Precipitation during Isothermal Heating after Rapidly Cooled from High Temperature in Duplex Stainless Steel

Kazuhiro Ogawa, Akira Seki

pp. 2613-2619

DOI:

10.2355/isijinternational.ISIJINT-2021-113

抄録

Nitride precipitation during isothermal heating after rapidly cooled from the ferrite phase enriched temperature in duplex stainless steel was investigated to clarify the effect of nitrogen in steel as an alloying element and austenite phase growth. Nitride precipitation is frequently harmful for the properties such as toughness or corrosion resistance in duplex stainless steels. The heat affected zone (HAZ) in the vicinity of fusion boundary in weldments tends to have more remarkable nitride precipitation because of rapid cooling from the ferrite phase enriched temperature. During cooling process the growth of austenite phase occur resulting in the prevention of nitride precipitation.Employing 25%Cr duplex stainless steels containing various level of nitrogen of 0.1% to 0.3%, the measurement and observation of nitride precipitation were conducted in the specimens isothermally heated at 873 K to 1073 K after rapidly cooled from 1653 K. As results the low nitrogen containing steel had relatively much nitride precipitation than the higher nitrogen containing one because of the less increase in the fraction of austenite phase growth with larger nitrogen solubility. This result can be explained by the decease of oversaturation of nitrogen in ferrite phase by contribution of austenite phase growth during thermal cycle.

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Cut-Edge Corrosion Behavior of Prepainted 55% Al–Zn Steel with Chromate-free Primers in Various Atmospheric Environments

Takahiro Tsujita, Azusa Ooi, Eiji Tada, Atsushi Nishikata

pp. 2620-2628

DOI:

10.2355/isijinternational.ISIJINT-2021-148

抄録

Outdoor exposure tests of prepainted 55% Al–Zn steel sheets are performed for 24 months at five different exposure sites: Tokyo, Kagoshima, and Okinawa in Japan, and Arizona and Florida in the United States. The anticorrosive pigments in the primer with topcoat are strontium chromate (Cr), magnesium vanadate + zinc phosphate (MgV/ZnP), calcium silicate (Si), and magnesium vanadate (MgV). The edge creep is evaluated by measuring the creep width every six months. The electrochemical impedance and polarization curves of the non-coated 55% Al–Zn steel and carbon steel (CS) are measured in a 3% NaCl solution suspended with the anticorrosive pigments. The corrosion products at the cut edge are analyzed by scanning electron microscopy and energy dispersive X-ray spectrometry.The edge creep for all the specimens did not progress at all in Arizona (dry desert), although red rust appears on the CS surface at the cut edge. At Kagoshima and Okinawa (marine atmospheres), the progress of the edge creep is fast for all the specimens. It is especially significant in Okinawa where temperature and humidity are high. When comparing the effects of the pigments on the edge creep properties, the results show that Si is poor at all exposure sites except Arizona, and MgV/ZnP shows edge creep properties comparable to Cr. From the anodic polarization characteristics of 55% Al–Zn steel and the cathodic polarization curve of CS, the effect of the pigments on edge creep is successfully described.

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Deformation-Induced Martensitic Transformation Behavior of Retained Austenite during Rolling Contact in Carburized SAE4320 Steel

Kohei Kanetani, Taku Moronaga, Toru Hara, Kohsaku Ushioda

pp. 2629-2635

DOI:

10.2355/isijinternational.ISIJINT-2021-107

抄録

The deformation-induced martensitic transformation is a phenomenon that significantly improves the mechanical properties of steels, and is well known to be beneficial for the rolling contact fatigue (RCF) of bearings. In the present study, the characteristics of the deformation-induced martensitic transformation in the RCF of carburized, quenched and tempered SAE4320 steel were investigated in detail using scanning electron microscopy with electron backscattering diffraction and transmission electron microscopy with automated crystal orientation mapping. These analyses clarified that different variants of the extremely fine deformation-induced martensites as small as several tens of nm were formed within an austenite grain with RCF, and the martensites were speculated to have the Kurdumov-Sachs or the Nishiyama-Wasserman relationship with the retained austenite. Furthermore, the deformation-induced martensites were preferentially formed within the retained austenite grains rather than at the interface between the tempered martensite and retained austenite. This suggests that the deformation-induced martensites were formed from some localized regions that were plastically introduced within the retained austenite grains.

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Effect of Solute Ce on Mechanical Properties and Electronic Structure of γ-Fe: Insights from a First-principles Study

Xiangjun Liu, Jichun Yang, Fang Zhang, Changqiao Yang

pp. 2636-2643

DOI:

10.2355/isijinternational.ISIJINT-2021-139

抄録

In this work, first-principles calculations were used to investigate the effects of the common elements in steel on the solid solution of rare earth Ce in steel, and the effects of Ce on the mechanical properties and electronic structure of the doped system were analyzed. The calculated results of the formation enthalpy show that Ce can be solubilized in γ-Fe, and Cr, Ni, Cu, Nb, Mo, and W have negative effects on Ce solubility, while Si, V, Ti, Al, and Mn promote Ce solubility with the strongest effects from Si and the weakest from Mn. The elastic modulus calculated results show that Ce doping reduces the incompressibility, rigidity and hardness of the system, but the toughness and machinability are improved. Density of states shows that the interaction between Fe–Ce and Si–Ce is strong, while there are almost no interactions in Mn–Ce. A combination of Bader charge and differential charge density analysis shows that the strength of metallic bond of Fe–Ce system is weaker than that of the pure Fe system, which is the main reason for the decrease in incompressibility, rigidity, and hardness of the doped system; the higher electron cloud density of the doped system is the main reason for its increase in toughness. Furthermore, with the strong interaction between Si and Ce, and Si can effectively reduce the lattice distortion caused by the solid solution of Ce, which are the two main reasons why Si significantly increases the solid solution of Ce.

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Effect of Hydrogen on Spot-Welded Tensile Properties in Automotive Ultrahigh-Strength TRIP-Aided Martensitic Steel Sheet

Akihiko Nagasaka, Tomohiko Hojo, Katsuya Aoki, Hirofumi Koyama, Akihiro Shimizu, Zulhafiz Bin Zolkepeli, Yuki Shibayama, Eiji Akiyama

pp. 2644-2653

DOI:

10.2355/isijinternational.ISIJINT-2021-210

抄録

Effect of hydrogen on spot-welded tensile properties in ultrahigh-strength TRIP-aided martensitic steel (TM steel) sheet was investigated for automotive applications. Tensile test was performed on a tensile testing machine at a crosshead speed of 1 mm/min (strain rate of 2.8×10−4/s), using base metal and spot-welded specimens with or without hydrogen charging.The results are as follows.(1) The difference between the tensile strength (TS) of 1532 MPa for base metal specimen without hydrogen charging and the maximum stress (TS-H) of 1126 MPa for the base metal specimen with hydrogen charging (ΔTS-H=TS−TS-H) in the TM steel was smaller than that of hot stampted steel (HS1 steel) and superior to that of HS1 steel. On the other hand, the TS-H of 725 MPa for the base metal specimen with hydrogen charging was halved in comparison with the TS of 1438 MPa for base metal specimen without hydrogen charging in the HS1 steel. It is considered that this was because the retained austenite suppressed the strength reduction due to the hydrogen embrittlement of the TM steel.(2) The amount of hydrogen decreased in the order of the HS1 steel, the TM steel, and the tempered martensitic steel (HS7 steel), and the HS1 steel was the highest. This is thought to be due to the high dislocation density of the HS1 steel.(3) The difference between the maximum stress (TS-W) of the spot-welded specimen without hydrogen charging and the maximum stress (TS-WH) of the spot-welded specimen with hydrogen charging (ΔTS-WH=TS-W−TS-WH) in the TM steel and that of the HS1 steel were similar. It was considered that this is partly due to the effect of the stress concentration on heat affected zone (HAZ) softening of the hardness distribution of the spot-weld.

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Crack Initiation and Propagation Behavior of Hydrogen-induced Quasi-cleavage Fracture in X80 Pipeline Steel with Stress Concentration

Tomoka Homma, Seiya Anata, Shoma Onuki, Kenichi Takai

pp. 2654-2665

DOI:

10.2355/isijinternational.ISIJINT-2021-228

抄録

The processes leading to hydrogen-related fracture in X80 pipeline steel with a stress concentration have been investigated comprehensively through observations of fracture surfaces and subsidiary cracks, stress analyses, crack initiation and propagation analyses and crystallographic analyses of fracture surfaces. Fracture morphology showed quasi-cleavage (QC) fracture under various amounts of hydrogen. It was found that QC cracks initiated in hydrogen-charged specimens in an area ranging from the notch tip to 100 µm inside based on interrupted tensile tests until just before fracture strength. A fracture surface topography analysis (FRASTA) revealed that QC cracks initiated at the notch tip. A finite element analysis indicated that the equivalent plastic strain was maximum at the crack initiation site at the notch tip. A backscattered electron image showed that nanovoids of 50–250 nm in diameter were present near the initiation site. Regarding the crack propagation process, field emission scanning electron microscopy (FE-SEM), electron backscattered diffraction (EBSD) and FRASTA results indicated that some microcracks in ferrite grains coalesced in a stepwise manner and propagated. Trace analyses using EBSD revealed that the QC fracture surface consisted of {011} slip planes, {001} cleavage planes and non-specific index planes. These findings indicate that QC fracture initiated at the notch tip due to the interaction between dislocations and hydrogen associated with local plastic deformation, and propagated in a stepwise manner by coalescence through vacancies, nanovoids and microcracks on various planes associated with/without plastic deformation in ferrite grains.

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Effects of Microstructural Anisotropy on the Dwell Fatigue Life of Ti-6Al-4V Bar

Kenichi Mori, Shohtaroh Hashimoto, Mitsuo Miyahara

pp. 2666-2676

DOI:

10.2355/isijinternational.ISIJINT-2021-245

抄録

Cyclic fatigue, dwell fatigue and crack growth properties were evaluated in the axial direction (L) and transversal direction (T) of Ti-6Al-4V forged round bar. In the SN curve where the stress is normalized by 0.2% proof stress, the cyclic fatigue life in the L/T direction is almost the same, whereas the dwell fatigue life in the T direction is as short as 1/5. In dwell fatigue, ductile fracture occurred when the maximum stress was higher than 95% of 0.2% proof stress. At stresses below 870 MPa, the inelastic strain range and the strain increase rate in the T direction gradually decreased with decreasing stress, and the fracture mode transitioned to that with fatigue crack growth. The gradual change must have been caused by the mixture of anisotropic microtexture regions. At stresses below 825 MPa, the fracture mode transitioned rapidly in the L direction, where the soft oriented microtexture regions were dominant. In the low ΔK region (≤15 MPa√m), the crack growth rate in the axial direction was about twice that in the radial direction of the bar. The shorter dwell fatigue life in the T direction under stress conditions showing fatigue crack growth was explained by the significantly earlier crack initiation compared to that in cyclic fatigue and the faster crack growth along the microtexture in the axial direction of the bar.

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