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ISIJ International
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Predecessor

Transactions of the Iron and Steel Institute of Japan

ISIJ International Vol. 56(2016) No. 9

Fundamentals of High Temperature Processes

Regular Article

Flow Field Characteristics of Coherent Jet with Preheating Oxygen under Various Ambient Temperatures

Fuhai Liu, Rong Zhu, Kai Dong, Shaoyan Hu

Released: September 24, 2016

[Advance Publication] Released: July 15, 2016

p.1519-1528

In the electric arc furnace steelmaking process, the coherent jet was adopted to improve the stirring effect of supersonic oxygen jet and reaction rate in molten bath. However, so little research had been done for the coherent jet behavior. In this study, the coherent jet flow field with preheating oxygen in hot (1700 K) and cold (298 K) condition were studied. The axial velocity and total temperature of main oxygen jet were measured by combustion experiment. Flow field characteristics of coherent jet were simulated by Fluent software. The detail chemical kinetic mechanism including 53 species and 325 reversible reactions was adopted to achieve a more accurate result of shrouding flame and main oxygen jet. The results showed that the low-density region surrounding the main oxygen jet were formed by the combustion, and thus suppressed the entrainment of the ambient gas into the main jet. The higher oxygen temperature increased the axial velocity of coherent jet, with the potential core reducing. Moreover, the higher ambient temperature could prolong the potential core of coherent jet and conventional jet.

Effect of Impurity Te on the Morphology of Alumina Particles in Molten Iron

Lichun Zheng, Annelies Malfliet, Patrick Wollants, Bart Blanpain, Muxing Guo

Released: September 24, 2016

[Advance Publication] Released: July 15, 2016

p.1529-1536

Surface active elements in liquid steel may affect the morphology of non-metallic particles during deoxidation. The effect of Te on the morphology of alumina particles was therefore studied by adding Te to molten iron before Al deoxidation at 1873 K. Dendritic, spherical, faceted, plate-like and clustered particles were identified in all samples. Te, however, considerably changes the relative frequencies of the morphologies, decreasing the amount of dendritic and spherical particles whereas increasing the amount of faceted and plate-like particles. This effect is closely related with the supersaturation degree, stirring, and Te content. The way Te influences the morphology of the alumina particles and the factors influencing Te effectiveness are discussed.

Three Dimensional Simulation of Lime Particle Penetration into Molten Iron Bath Using Smoothed Particle Hydrodynamics

Mieko Nakano, Kimihisa Ito

Released: September 24, 2016

[Advance Publication] Released: July 28, 2016

p.1537-1542

It is desirable to enhance the desulfurization and dephosphorization rates and their efficiencies in steelmaking. Injection of flux particles is one of the key technologies, and knowledge of the behavior of the particles when they penetrate into a metal bath is important to control the process. In this study, two-dimensional and three-dimensional simulation models were developed to study the penetration behavior when a solid body penetrates into a liquid, using the SPH method with a new pairwise potential. The three-dimensional simulation model could reproduce the experimental results in the water model satisfactorily, and was applied to a lime sphere-molten iron bath system. The penetration length and the residence time increased with the increase in initial velocity. The residence time of the sphere also increased as the wettability increased. The critical velocity for penetration, estimated by simulation, showed good agreement with that calculated using Ozawa’s equation.

Law of Mass Action Based Kinetic Approach for the Modelling of Parallel Mass Transfer Limited Reactions: Application to Metallurgical Systems

Mika Järvinen, Ville-Valtteri Visuri, Eetu-Pekka Heikkinen, Aki Kärnä, Petri Sulasalmi, Cataldo De Blasio, Timo Fabritius

Released: September 24, 2016

[Advance Publication] Released: August 23, 2016

p.1543-1552

The objective of this paper was to present a new law of mass action based rate expression for mass transfer limited reversible reactions. A simple reaction model was derived for parallel oxidation of silicon, chromium and carbon under conditions relevant to the argon-oxygen decarburization (AOD) process. Our hypothesis is that when the forward rate coefficients approach infinite values, the composition at the reaction surface approaches a constrained equilibrium. In numerical analysis, however, only finite numbers are allowed and therefore only finite values are accepted for rate coefficients. In order to circumvent this problem, additional residual affinity constraints were introduced. This assures that the affinities of all the reactions at the reaction front reach a pre-defined non-zero residual affinity and the rate coefficients remain finite. The calculated equilibrium composition is essentially the same as that obtained with the equilibrium coefficient method. In the case of effective gas side mass transfer, the component having the highest mole fraction or the highest mass transfer rate on the liquid side consumes most of the oxygen. When the gas side mass transfer rate is decreased, the mass transfer rate of oxygen begins to limit the overall rate and the partial pressure of oxygen at the reaction interface decreases. Then, the role of interfacial equilibrium becomes important as the species start to compete for the oxygen. The proposed method provides a transparent and direct solution of the mass transfer limited reaction rates and is thus suitable for process simulators and CFD software.

Ironmaking

Regular Article

Effect of Mineral Matter on the Reactivity of Coke and its Replication in a Coke Analogue

Raymond James Longbottom, Brian Joseph Monaghan, Azrin Akhter Chowdhury, Mark Henry Reid, Guangqing Zhang, Merrick Russel Mahoney, Kim Hockings

Released: September 24, 2016

[Advance Publication] Released: July 26, 2016

p.1553-1558

The reactivity of coke analogues doped with minerals to mimic the mineralogy of specific industrial cokes was compared with the reactivity of the industrial cokes. The reactivity was assessed in a pseudo-CRI type test. This involved reacting the carbonaceous materials (analogue and industrial coke) in CO₂ at 1100°C in a thermos-gravimetric system. In this comparison, the mineral matter added to the coke analogue was prepared from ashing of the industrial cokes. A distinct ranking of reactivity for the industrial cokes was determined to be coke A < coke B < coke C. The high reactivity of coke C was attributed to the high iron content in its ash. The higher reactivity of coke B over coke A was attributed to its higher porosity and lower rank (of the original coal carbon type) of the industrial cokes. The coke analogue replicated the increased reactivity of coke C over cokes A and B, indicating that the coke analogue is able to some extent to replicate the effect of coke mineralogy on coke reactivity in CO₂. The use of the coke analogue allowed assessment of the difference in the reactivities of cokes A and B. When porosity and carbon type were fixed by use of the analogue, the reactivities of the analogues of cokes A and B were found to be similar.

Effects of Operation Parameters on Cohesive Zone in COREX Melter Gasifier

Lihao Han, Weiqing Huang, Yanxia Liu

Released: September 24, 2016

p.1559-1562

The cohesive zone plays very important role in the operation of COREX melter gasifier. A two-dimensional 1/30 scale thermal dynamic model, in which paraffin and corn are used to simulate DRI, coke and lump coal respectively, has been established to study the cohesive zone of COREX melter gasifier. A set of different operation parameters, such as discharging rate (melting rate), DRI to lump coal and coke volume ratio, blast temperature as well as blast volume, were taken into account. The effects of these mentioned parameters on cohesive zone position and thickness in COREX melter gasifier have been analyzed in detail.

Influence of Lime Coating Coke on NOx Concentration in Sintering Process

Kazuaki Katayama, Shunji Kasama

Released: September 24, 2016

p.1563-1569

Decreasing NOx emission in sintering process is a key issue in steel industry. NOx emission in sintering process is decreased by coke combustion under high temperature. It has been investigated that coating layer of CaO–Fe₂O₃ composition on coke surface (CF coating method) is effective for decreasing NOx. It has been considered that CaO–Fe₂O₃ coating layer promotes high temperature combustion and functions as catalyst for reduction of nitrogen oxide.

In this study, CaO coating of coke (Lime coating coke: LCC) has been studied as simple technology for decreasing NOx. As the results, LCC has been effective like a CF coating method and it has been understood that CF melt formation on coke surface is important for decreasing NOx. About coating CaO ratio, 10% was preferable. And decrease in mixing time of LCC with iron ores (Post-mixing) was also effective. By LCC post-mixing, 17.6% NOx decreased and sinter productivity increased.

Investigation of Coke Collapse in the Blast Furnace Using Mathematical Modeling and Small Scale Experiments

Tamoghna Mitra, Henrik Saxén

Released: September 24, 2016

[Advance Publication] Released: July 28, 2016

p.1570-1579

Burden distribution plays a key role in controlling the gas flow conditions inside a blast furnace. The distribution of ore and coke influences the gas permeability distribution in the lumpy zone and also in the cohesive zone, where the gas flows mainly through the coke slits. Charging an ore dump on coke can sometimes cause the coke layer to collapse under the force of the heavier ore particles. This is known as ‘coke collapse’ or ‘coke push’, which results in a higher coke fraction near the center of the furnace than expected. In this work coke collapse phenomena are evaluated on model charging programs using small scale experiments and Discrete Element Modeling (DEM). DEM simulations were used to study the extent of collapse for different charging programs, and experiments were undertaken to verify the results of the simulations. The slope stability method was used to classify the collapse conditions into no collapse, impact failure or gravity failure, depending on the stability of the coke layer. The findings were also compared with results from an in-house mathematical model, which was modified to consider the effect of the collapse on the underlying layer. The corrected mathematical model was found to show results in general agreement with results from the DEM simulation.

Factor of safety for the coke layers in the charging programs CP1–CP5 as the coke apex is moved towards the furnace center. The cross and the circle indicate the initial and the final factor of safety calculated using the correction scheme. Fullsize Image

Influence Factors on PM_2.5 and PM₁₀ Emissions in Iron Ore Sintering Process

Zhiyun Ji, Xiaohui Fan, Min Gan, Xuling Chen, Qiang Li, Ye Tian, Xiangning He, Yang Zhou, Tao Jiang

Released: September 24, 2016

[Advance Publication] Released: July 28, 2016

p.1580-1587

Influences of factors on PM_10/2.5 (particulate matters less than 10/2.5 µm in aerodynamic) emission in iron ore sintering process have been investigated using a lab-scale sinter pot. Research findings show that increasing moisture content contributes to decreasing the emission concentration of PM_10/2.5 due to the stronger scrubbing effect of over-wetted layer; increasing coke breeze rate would increase the generation of PM_10/2.5 since more trace elements are volatilized and conversed into particle phase. Prolonging granulation time can improve the mechanical strength of granules and also restrain the removal of volatile trace elements to particulate matter, which enables to reduce PM_10/2.5 emissions. The utilization of recycling materials remarkably enhances PM_10/2.5 emissions for more fine particles (<10/2.5 µm) are brought in and more volatile trace elements are entrained into particulate matters. Taking sintering indexes into consideration, applying recycling materials wisely or adding solid sorbents serve as more prospective approaches than merely regulating process parameters. This information is able to guide the development of in-bed controlling techniques for reducing PM_10/2.5 emissions.

Gas-Solid Flow and Shaft Injected Gas Penetration in an Oxygen Blast Furnace Analyzed Using a Three-Dimensional DEM-CFD Coupling Mathematical Model

Zeshang Dong, Qingguo Xue, Haibin Zuo, Xuefeng She, Jun Li, Jingsong Wang

Released: September 24, 2016

[Advance Publication] Released: July 28, 2016

p.1588-1597

Decreasing coke rate by establishing an oxygen blast furnace ironmaking process is an attractive method to reduce energy consumption in the iron-steel industry. An important feature of this process is shaft gas injection. In this work, a three-dimensional coupling model of DEM-CFD about OBF was performed to analyze the gas-solid flow in an OBF. The presence of melting zone and raceway were taken into consideration. The particles number is closer to the actual blast furnace by improving the coupling method. Three shaft tuyere configuration modes were proposed for studying the penetration behavior of the shaft injected gas (SIG). The solids distribution in furnace is discussed. Next, the effect of the SIG on gas pressure and the velocity vector was investigated. Finally, the SIG penetration behavior in both the radial and vertical cross-sections under different cases are investigated. The results showed that in conventional charging the solid phase volume fraction at the edge was larger than at the center in the shaft, which is conducive to center gas development. The effect of the SIG on the gas pressure and velocity vector was only observed in the vicinity of the shaft tuyere outlet. The rapid pressure drop and change of gas flow direction were observed above the melting zone. The shaft tuyere configuration mode where the number of shaft tuyeres and hearth tuyeres is equal and where the shaft tuyere was located between the hearth tuyeres is regarded as the optimum mode for a more reasonable heat distribution in the OBF.

Optimization of the Dehydration Temperature of Goethite to Control Pore Morphology

Genki Saito, Takahiro Nomura, Norihito Sakaguchi, Tomohiro Akiyama

Released: September 24, 2016

p.1598-1605

This study optimizes the dehydration temperature of goethite to control pore morphology. The pore morphology was characterized by using transmission electron microscopy and the nitrogen adsorption method. When the goethite was dehydrated at 200–250°C, slit-like pores with a width lesser than 2 nm were formed along the [010] direction. These slit-like pores changed to spherical micropores (300–500°C), and eventually disappeared (600–800°C). Compared to the synthetic goethite, natural goethite has a lower crystallinity and smaller primary particle size of under 100 nm. The natural goethite before dehydration contained 4 nm pores as cracks that remained even after heating to 800°C. In the case of natural goethite, the optimum dehydration temperature for higher surface area and pore volume was 350°C, which was higher than that of 250°C for the synthetic goethite.

Fullsize Image

Casting and Solidification

Review

Thermo-fluid Mathematical Modeling of Steel Slab Caster: Progress in 21st Century

Vikas Singh, Suchandan Kumar Das

Released: September 24, 2016

[Advance Publication] Released: August 04, 2016

p.1509-1518

Interaction of solidified shell and fluid flow inside the mold cavity governs the quality in the continuous slab casting of steel. A careful mathematical model can reveal the phenomena at play to take corrective action for better quality and productivity of the caster. These phenomena include turbulent flow in the nozzle and mold, heat transfer and solidification, the transport of bubbles and inclusion particles, multi-phase flow phenomena, the effect of electromagnetic forces, interfacial phenomena, interactions between the steel surface and the slag layers, the transport of solute elements and segregation etc. Substantial development has been done across the world in this direction during past few years. This paper reviews recent progress in modeling the thermo-fluid aspect of a steel caster.

Regular Article

Microstructure of As-cast Ferritic-pearlitic Nodular Cast Irons

Jacques Lacaze, Jon Sertucha, Lena Magnusson Åberg

Released: September 24, 2016

p.1606-1615

A review of past works on the formation of ferrite and pearlite in nodular cast iron is proposed. The effects of cooling rate after solidification and of nodule count on the formation of both constituents are stressed, though much emphasis is put on alloying elements and impurities.

Influence of Liquid Core Reduction on Stress-strain Distribution and Strand Deformation in a Thin Slab Caster

Hong Xiang Li, Wen Zhen Zhu, Demian Ruvalcaba, Dag Mortensen, Dirk van der Plas, Hallvard Gustav Fjaer, Lin Zhong Zhuang

Released: September 24, 2016

p.1616-1624

A new numerical model called SteelSim, which is based on the Finite Element Method (FEM), has been developed in order to investigate the Liquid Core Reduction (LCR) section in a thin slab continuous caster. As a SteelSim application example, the influence of roll alignment on temperature distribution, solidification, stress-strain distribution and strand deformation has been studied. The thermo-elasto-viscoplastic constitutive equations and the Arbitrary Lagrangian Eulerian (ALE) kinematic description for the conservation equations were calculated in the SteelSim model. From the results, it can be seen that the roll configuration: (a) symmetrical alignment of rolls with LCR, (b) asymmetrical alignment of rolls with LCR, and (c) symmetrical alignment without LCR, has an influence on the local stress-strain distribution and thus local strand deformation, which in turn affects the evolving shape profile of the narrow face. Eventually, the SteelSim model can be utilized for determining optimal roll configurations and casting conditions for specific steel grades in order to avoid the formation of critical casting defects, such as: excessive bulging, macrosegregation, cracking etc., which may lead to poor product quality or even total failure of the casting process due to a breakout. The results presented here may allow us to further analyze and validate the model with actual plant casting data.

Reliability of Inclusion Statistics in Steel by Stereological Methods

Tao Li, Shin-ichi Shimasaki, Shoji Taniguchi, Shunsuke Narita

Released: September 24, 2016

p.1625-1633

In this paper, we investigate the reliability of estimating of inclusion size distribution and number density in steel by using stereological methods. The magnitude of the inclusion concentration in steel is evaluated by the total oxygen and the assumed average inclusion sizes. The principles of Schwartz-Saltykov (SS) and modified SS (MSS) methods are introduced. A simulation model is developed to disperse particles with a predefined particle size distribution (PSD) randomly into a three dimensional (3D) space. A series of test planes are generate to measure the two dimensional (2D) PSD and particle number density (PND) on the cross-sections (CS). The SS and MSS methods are applied to investigate the reliability of the translation between the 3D and 2D information of the system, such as the 2D and 3D PSD and PND. The influence of predefined 3D PSD on the reliability of the stereological methods are studied, such as mono sized, lognormal and normal distributions. The effect of the representative group diameters in the discretized groups for SS and MSS methods is investigated as well.

Instrumentation, Control and System Engineering

Regular Article

A Catalytic Combustion-type Carbon Monoxide Gas Sensor Incorporating an Apatite-type Oxide

Ayaka Hosoya, Shinji Tamura, Nobuhito Imanaka

Released: September 24, 2016

p.1634-1637

A novel catalytic combustion-type CO gas sensor was devised using a catalyst composed of 11 wt% Pt supported on the apatite-like Mn-doped oxide La_9.0Si_5.8Mn_0.2O_27−δ. Its CO gas sensing performance was also investigated. Since the 11 wt% Pt/La_9.0Si_5.8Mn_0.2O_27−δ catalyst oxidized CO at a temperature over 60°C, the sensor also responded to CO at temperatures above 60°C, showing superior CO detection at 130°C with high sensitivity and a rapid response time of 10–20 s.

Surface Treatment and Corrosion

Regular Article

Effect of Si Concentration on Carbon Concentration in Surface Layer after Gas Carburizing

Tatsuya Koyama, Manabu Kubota, Suguru Yoshida

Released: September 24, 2016

p.1638-1645

The mechanisms for the decrease in the carbon concentration in the surface layer after gas carburizing that occurs with increasing the Si concentration was investigated using three kinds of steel having compositions based on JIS SCr420 with varying Si concentrations (0.25%Si steel, 1%Si steel, 2%Si steel). The carbon concentration in the surface layer after gas carburizing decreased with the increase in the Si concentration, and the carbon concentration was particularly low in 2%Si steel. The carbon concentration in the surface layer after gas carburizing of 1%Si steel was nearly equal to the thermodynamic calculation value for a simulated gas carburizing reaction, whereas that of 2%Si steel was much lower. This substantial change seems to be due to the oxide formation on the surface, with 2%Si steel showing the different oxide formation on the steel surface compared to the other steels. The oxides layer of the surface of 2%Si steel densely covered the surface. By removing the oxide layer, the carbon concentration in the surface layer of 2%Si steel increased to the nearly calculated value. These results reveal that the mechanisms for the decrease in the carbon concentration in the surface layer change depending on the Si concentration and are the thermodynamic interaction between Si and carbon for 1%Si steel and 2%Si steel and the oxide layer’s inhibitory effect on the carburizing reaction for 2%Si steel.

Transformations and Microstructures

Regular Article

Effect of Lanthanum on the Precipitation of NbC in Ferritic Steels

Haiyan Wang, Xueyun Gao, Weimin Mao, Guohui Zhu, Shuming Chen, Huiping Ren

Released: September 24, 2016

[Advance Publication] Released: August 02, 2016

p.1646-1651

The effect of lanthanum on the precipitation of NbC in ferritic steel has been studied within the experimental and theory methods. The diffusion coefficients of Niobium resulted from the diffusion couple experiment shows that Nb diffuses slightly faster in the presence of La than that in pure α-Fe. Using the first-principles calculations based on the density functional theory, we studied the interactions between any two defects of La, Nb and vacancy, as well as the variation of diffusion activation energies caused by the presence of La. The interactions between La and Nb are calculated to be repulsive in both the first and the second nearest neighbor configurations, and the vacancy is expected to form in the La-Nb clustering area, which leads to the decrease of the diffusion activation energy of Nb. To elucidate the effect of La on the precipitation of NbC in ferritic steel, the above diffusion coefficients were substituted into the precipitation kinetics model. Agree with the experimental data, the simulated evolutions of the fractions and radius of NbC precipitate reveal that the addition of La leads to the faster precipitation kinetics of NbC in ferritic steels.

Selection of Cementite Variants Precipitated under Magnetic Field in Ferrite Matrix of an Fe–C Alloy

Hui-jin Choe, Tomoyuki Terai, Izuru Miyazaki, Sukeyoshi Yamamoto, Mitsuharu Yonemura, Takashi Fukuda, Tomoyuki Kakeshita

Released: September 24, 2016

p.1652-1655

We have studied the influence of magnetic field on precipitation of cementite (Fe₃C) using an Fe-0.01C (mass%) alloy. Among twelve variants of cementite formed at 473 K in the ferrite (α-phase) matrix, the fraction of variants with the highest magnetocrystalline anisotropy energy decreases by the application of a magnetic field of 7 T. In addition, the total number of particles of cementite decreases after the application of a magnetic field. These results implies that microstructure of Fe–C alloys can be controlled by magnetic field.

Mechanical Properties

Regular Article

Suppression Mechanism of Strain-age-hardening in Carbon Steel Associated with Hydrogen Uptake

Takuro Ogawa, Motomichi Koyama, Hiroshi Noguchi

Released: September 24, 2016

p.1656-1661

We investigated the suppression mechanism of strain-age-hardening in a ferritic carbon steel associated with hydrogen uptake. We considered hydrogen-related three factors suppressing the strain aging: 1) solution softening, for instance, arising from a reduction in Peierls potential of screw dislocations and a change in Young’s modulus, 2) suppression of dislocation-carbon/nitrogen interaction through hydrogen/carbon and nitrogen site competition, and 3) change in plastic strain evolution behavior by hydrogen-enhanced localized plasticity (HELP). According to the present experiments under in-situ hydrogen charging, it was concluded that the solution softening (factor1) and the site competition (factor2) by hydrogen did not significantly suppress the strain aging but the change in the pre-straining behavior (factor3) did.

Effect of Temperature on Mechanical Properties of 9%Cr Ferritic Steel

Felix Peñalba, Xabier Gómez-Mitxelena, José Antonio Jiménez, Manuel Carsí, Oscar Antonio Ruano

Released: September 24, 2016

[Advance Publication] Released: July 28, 2016

p.1662-1667

Mechanical properties of a 9%Cr-ferritic steel grade P92 experimental alloy are studied. The effect of cooling rate on the hardenability was determined by means of continuous cooling diagrams and data provided by hardness measurements and microstructure observations. A fully martensitic microstructure after the solubilization treatment over a wide range of cooling rates was revealed. As this grade of steels is mostly supplied in tempered condition, tensile tests to determine the variation of the strength and ductility at temperatures ranging from 20 to 650°C were carried out after a treatment of 3 h at 760°C. In addition, Charpy V-notch tests were conducted to characterize the impact toughness of the steel and the ductile-brittle transition temperature. Finally, the creep strength was determined from creep tests in the range 550 to 650°C.

Hardening Optimization of High Chromium-manganese Austenitic Steel

Silvia Barella, Carlo Cena, Andrea Gruttadauria, Carlo Mapelli, Davide Mombelli, Dario Ripamonti, Giovanni Fantini, Daniele Dioni

Released: September 24, 2016

p.1668-1674

The study is focused on the assessment of the best thermal range for plastic deformation of Cr–Mn austenitic steel, to obtain a correct hardening and mechanical properties at room temperature. This steel grade is featured by a fully austenitic microstructure deriving from the high concentration of Mn and N, and is mainly used for the retaining rings bearing of power generation shafts. These components should not have magnetic permeability and thus, the mechanical strengthening can be performed by strain hardening and activation of twinning systems during rolling and forging at high temperature. Different specimens were tensile tested at different temperatures and different strains without arriving at the fracture point. Once the strained specimens were cooled, they have been tested by complete tensile tests at room temperature to determine the final mechanical properties. The best combination of the final mechanical properties have been obtained for plastic deformation performed between 250°C and 350°C, but the formation of martensite at 250°C narrows the useful thermal range between 300°C and 350°C. The metallographic observations indicated that the best hardening conditions can be obtained through the exploitation of the twinning plasticity effect and when the deformation temperature avoids any recovery that can reduce the dislocation density maintained after the cooling at room temperature. The performed experimental trials have also allowed stating the most favorable thermal range for the strain hardening of Cr–Mn steels through forging process to maximize the strengthening effect without the detrimental chromium carbide precipitation.

Change in Dislocation Characteristics with Cold Working in Ultralow-carbon Martensitic Steel

Daichi Akama, Toshihiro Tsuchiyama, Setsuo Takaki

Released: September 24, 2016

[Advance Publication] Released: July 26, 2016

p.1675-1680

In a previous study, the authors used X-ray analysis with the classical Williamson–Hall (CWH) method to suggest that charging a small amount of cold working markedly decreases the dislocation density of ultralow-carbon martensitic steel, although this heightens the 0.2% proof stress. However, this method does not consider the dislocation arrangement. In the present study, a modified Williamson–Hall/Warren–Averbach (MWH/WA) method was applied to ultralow-carbon martensitic steel (Fe–18%Ni alloy) in order to evaluate not only the dislocation density but also the dislocation arrangement. Their effects on the yielding behavior were examined. With the MWH/WA method, the dislocation density did not change up to 40% cold rolling. On the other hand, the dislocation arrangement parameter M was high (M > 1) in the as-quenched state and became smaller (M < 1) when a small plastic strain was charged. This means that the dislocation distribution is random in as-quenched martensite but changes the cell structure with cold working. Owing to such a dislocation arrangement, the CWH method tends to overestimate the dislocation density of as-quenched martensite compared to the MWH/WA method. Tensile testing revealed that the elastic limit was very low in as-quenched martensite and high in cold-rolled martensite. In the case of a tangled dislocation structure, a higher stress should be required because of the stable dislocation structure. On the other hand, the random dislocations introduced by martensitic transformation can easily move at a low stress level owing to their unstable distribution, which leads to the low elastic limit in as-quenched martensite.

Change in dislocation density with cold rolling in ultralow-carbon martensitic steel. Fullsize Image

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Volume And Issue List ISIJ International

Predecessor

ISIJ International Vol. 56(2016) No. 9

Fundamentals of High Temperature Processes

Regular Article

Ironmaking

Regular Article

Casting and Solidification

Review

Regular Article

Instrumentation, Control and System Engineering

Regular Article

Surface Treatment and Corrosion

Regular Article

Transformations and Microstructures

Regular Article

Mechanical Properties

Regular Article

Recommended Article

System Maintenance

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ISIJ International