ISIJ International
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ONLINE ISSN: 1347-5460
PRINT ISSN: 0915-1559

ISIJ International Vol. 42 (2002), No. 12

  • Preface to the Special Issue on “Advanced Structural Steels”

    pp. 1325-1325

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    DOI:10.2355/isijinternational.42.1325

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    1. Microtexture of Thin Gauge Hot Rolled Steel Strip ISIJ International Vol.43(2003), No.3
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    3. The State of Hydrogen in Nb-based Nb-Mo Alloys Analyzed by the Channelling Method ISIJ International Vol.43(2003), No.4
  • Scanning Electrochemical Microscopic Study of Detecting Non-homogeneity in Surface Reactions of Metals

    pp. 1326-1333

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    DOI:10.2355/isijinternational.42.1326

    Scanning electrochemical microscopy (SECM) is powerful to investigate non-homogeneity of surface reaction in solution. It is useful for evaluating the distribution of electrochemical reactions taking place on the electrode surface such as corrosion reaction, redox reaction on passive film and so on. The heterogeneous electrochemical reaction on polycrystalline iron, titanium and single crystal magnetite electrode surfaces were discussed. SECM can be also employed for inducing locally a certain surface electrochemical reaction on the specimen by operating a probe electrode as liquid-phase ion gun (LPIG). The application of LPIG to the passive film formed on iron for inducing the local film breakdown has revealed the details of its local breakdown process.
  • Weldability of High Nitrogen Stainless Steel

    pp. 1334-1343

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    DOI:10.2355/isijinternational.42.1334

    The purpose of this paper is to give a short survey of nitrogen influence on stainless steel welds. The review are covers: the levels of nitrogen in weld metal, the influence of nitrogen on stainless steel weld metal characteristics such as weld defects, corrosion resistance and mechanical properties. High nitrogen steel welding must consider the risk of nitrogen escape from the weld pool. Avoiding nitrogen losses may be accomplished by controlling shielding gas, welding parameters and compositions of filler metal. The increase of nitrogen in the weld metal decreases in the δ ferrite content. The reduction of δ ferrite in austenitic weld metals will result in an increase in the solidification cracking susceptibility. However, the role of nitrogen in affecting the solidification cracking susceptibility of fully austenitic weld metals is unclear. Nitrogen addition increases the pitting corrosion resistance in weld metals whereas decreases resistance to stress corrosion cracking because of δ ferrite reduction. Nitrogen also improves mechanical properties in weld metals. However, the presence of nitrides may be detrimental to the mechanical properties in stainless steel welds.
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    3. Mill-balance Control Technique for Tandem Cold Mill ISIJ International Vol.42(2002), No.6
  • Inclusion Formation and Microstructure Evolution in Low Alloy Steel Welds

    pp. 1344-1353

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    DOI:10.2355/isijinternational.42.1344

    The paper presents an overview of research performed at Oak Ridge National Laboratory on inclusion-formation, weld-solidification, and solid-state transformations in low-alloy steel welds. The competition between oxide and nitride formation in Fe-C-Al-Mn self-shielded flux-cored arc steel welds was predicted using computational thermodynamics. Nonequilibrium austenite phase selection was monitored in a Fe-C-Al-Mn weld using an in situ time-resolved X-ray diffraction technique. The competition between acicular ferrite and bainite formation from austenite was evaluated in Fe-C-Mn steel welds containing small amounts of titanium.
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  • Present Status and Perspectives of European Research in the Field of Advanced Structural Steels

    pp. 1354-1363

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    DOI:10.2355/isijinternational.42.1354

    The status of steel research in Europe with particular emphasis to multipartner projects sponsored by the European Community for Steel and Coal (ECSC) is reviewed through specific examples in the field of high strength (HS) designed with various metallurgical options and made possible by different production routes. Modern HS sheets for car body and structural parts of the automotive as well as the new generation of very high strength pipes for high pressure gas lines are discussed in the light of their recent developments inside the European Community R&TD circuit. A rapid glance to HS steel wires for suspension bridges is also given with reference to the newly designed Messina Strait Bridge in Italy.
    Some reference to possibilities offered in properties enhancement by the new casting technologies (Thin Slab & Strip Casting) is rapidly commented.
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  • Microstructural Degradation of the HAZ in 11Cr-0.4Mo-2W-V-Nb-Cu Steel (P122) during Creep

    pp. 1364-1370

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    DOI:10.2355/isijinternational.42.1364

    The microstructural degradation of the heat affected zone (HAZ) in 11Cr-0.4Mo-2W-V-Nb-Cu steel (P122) during creep was investigated. Creep testing was conducted using two types of specimens at 650°C and 675°C, and ruptured in the fine grained HAZ, known as type IV failure. Weldments were known to be weaker in creep strength than base metal in this test condition. The coarse and fine grained microstructures were observed, and the average grain sizes were measured. The HAZ adjacent to the base metal was characterized by a fine grained microstructure consisting of subgrains with low dislocation density. Hardness of the intercritical area between HAZ and the base metal was the lowest after PWHT and during creep. Creep cavities tended to form at the grain boundaries in the fine grained HAZ due to creep. Small cracks gathered with cavities were observed in the fine grained HAZ after creep, and these corresponded to the fracture portion. M23C6, M7C3 and MX type carbides had already precipitated in the HAZ before the creep test. A Laves phase arose at the grain boundary of the coarse and fine grained zones of the HAZ during the test. It is presumed that Laves phase precipitation in the coarse grained HAZ is slower than in the fine grained HAZ and base metal during creep.
  • Improvement of Wear Resistance of Steels by Nitriding Using Supersonic Expanding Nitrogen Plasma Jets

    pp. 1371-1375

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    DOI:10.2355/isijinternational.42.1371

    Plasma jets have been successfully used as heat sources of thermal plasma spraying process. However, since the plasma jet is accelerated to supersonic under a low pressure environment, the plasma jets can be used as low temperature plasmas with high chemical reactivity due to supersonic adiabatic expansion and frozen flow. From this viewpoint, nitriding of titanium plates using supersonic expanding nitrogen plasma jets under a low pressure environment was carried out in our previous study. As a result, it was proved that the plasma jets had enough reactivity to form a hard and thick titanium nitride layer on the surface of a titanium plate by only a few minutes of plasma jet irradiation at 30 Pa chamber pressure. In this study in order to develop a practical low temperature and high rate nitriding process, nitriding of nitriding steel, carbon steel and stainless steel using this process was carried out and wear resistance of these nitrided samples was investigated. Consequently, surface hardening was obviously promoted on the condition that hydrogen/ nitrogen mixture gas were used as working gas in the cases of all substrates. Especially, hard layers with over 1000 Hv in hardness were formed without any surface damages on the surfaces of the samples by only 5 min of operation in the cases of nitriding steel and stainless steel. Besides, according to the results of wear testing, wear resistance of these steels was dramatically improved.
  • Prevention of Chloride-induced Corrosion Damage to Bridges

    pp. 1376-1385

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    DOI:10.2355/isijinternational.42.1376

    The annual direct cost of bridge infrastructure corrosion to the U.S. economy is estimated at $8.3 billion, with indirect costs approximately 10 times higher. Of the approximately 600000 bridges in the U.S., between 15% and 20% are listed as “structurally deficient”, frequently due to corrosion damage. Five technologies are presented for reducing the cost of chloride-induced corrosion damage: (1) conductive coating anodes for cathodic protection of existing reinforce concrete bridges, (2) epoxy-coated rebar (ECR), (3) stainless steel rebar, and (4) high-performance concrete for extending the service life of new structures, and (5) metalizing to provide economical, long-term corrosion protection of steel bridges. Conductive coating anodes and stainless steel rebar represent ongoing work by the Oregon Department of Transportation with final verdicts not expected for years. The ECR and metalizing technology have longer track records and are better established in the bridge construction and protection industry. Application of these technologies is guided by a thorough understanding of their performance, of characteristics of the bridge and its environment, and of the results that are sought.
  • Effect of Argon Ion Bombardment on Diffusion Bonding of SUS304L Stainless Steel and Pure Iron

    pp. 1386-1390

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    DOI:10.2355/isijinternational.42.1386

    The surfaces of SUS304L stainless steel and pure iron specimens prepared by a lapping method were treated with argon ion bombardment, and then diffusion bonding was carried out. The effect of argon ion bombardment treatment on the properties of diffusion bonding joint was investigated by the tensile tests of joints and microstructure analyses of the fractured surfaces. The results showed that the argon ion bombardment treatment before bonding was effective to clean the bonding surface and reduce the inclusions at the bonded interface, so that the tensile strength of the bonded joints was improved and bonding temperature was lowered. The joint properties of pure iron were more excellent than those of SUS304L stainless steel. The amount of the inclusions at the joints with argon ion bombardment treatment depended on affinity of oxygen to metal.
  • Chromium Nitride Precipitation Behavior in Weld Heat-affected Zone of High Nitrogen Stainless Steel

    pp. 1391-1398

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    DOI:10.2355/isijinternational.42.1391

    High nitrogen stainless steels (HNS) containing about 1 mass% (%) nitrogen without adding Mn were manufactured by pressurized electro-slag remelting method. The chromium nitride precipitation behaviors at the weld heat-affected zone (HAZ) of HNS with different compositions were investigated. We also evaluated the localized corrosion resistance of the welded joints.
    The nitride precipitates were identified as Cr2N containing a small amount of Mo and Fe by TEM/EDS analyses. Time-temperature-precipitation curves of some HNS were obtained. It was found that for 23Cr-4Ni-2Mo-1N steel, aging for 2 s in the temperature range between 1173 K and 1373 K caused Cr2N precipitation and decreased the critical pitting corrosion temperature (CPT) in the 6% FeCl3 solution. Precipitation was delayed to 4 s by decreasing the nitrogen content from 1 to 0.8% and was further slightly delayed to over 5 s by adding the pre-deformation followed by the thermal treatment of 23Cr-4Ni-2Mo-0.8N steel.
    Precipitation at the HAZ was accelerated by a continuous thermal cycle test compared to the prediction based on the additivity rule, and that the existence of a ferritic phase affected acceleration was estimated.
    Joints welded by laser and minimum heat input conditioned plasma arc welding showed a 348 K CPT in a 6% FeCl3 solution and no crevice corrosion occurred at 308 K in artificial seawater. The CPT dropped notably against holding time above 1073 K at the HAZ. The CPT drop was slightly relieved by decreasing nitrogen content in the base metal from 1 to 0.8%.
  • In Situ Analysis of Pitting Corrosion in Artificial Crevice of Stainless Steel by X-ray Absorption Fine Structure

    pp. 1399-1403

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    DOI:10.2355/isijinternational.42.1399

    A new technique by XAFS (X-ray Absorption Fine Structures) and XANES (X-ray Absorption Near Edge Structures) has been developed for in situ observation of metal corrosion. XAFS and XANES spectra were obtained with a special electrochemical cell to elucidate pitting for stainless steel. A sheet of stainless steel was attached below a reservoir of a specific aqueous environment with thin films. The solution in the reservoir attached to the metal thorough the film, and the solution inside the film corresponds to an artificial pit. X-ray beams pass through the film containing the solution, and XAFS and XANES measurements were performed in a transparent geometry with keeping the specimen at a pitting potential. Measurements were performed for Cr-K, Mo-K, Cl-K and Br-K edges, and changes of concentrations and coordination states of ions were successfully obtained as a function of the distance (d(z)) from the metal/solution interface.
    Concentrations of chromium and bromide ions inside the artificial crevice of Fe-18Cr-12Ni-2Mo (mass%) alloy shows a linear dependence on the distance d(z). Structures of bromide ion were changed with positions; the distance between bromide ion and the nearest-neighboring ion at a position near the interface was shorter than that at a position far from the interface. This indicates the formation of hydrobromo-complex near the metal/solution interface.
    The state of molybdenum inside the artificial crevice of Fe-18Cr-20Ni-5Mo (mass%) alloy was investigated using two solutions: LiCl and LiBr. The formation of the [MoO4(H2O)2]2− octahedra was observed in both solutions, but there was observed a significant difference in networking of the octahedral. In LiCl solutions it was similar to that of molybdate ions (MoO42−), but in LiBr it was rather different form that of molybdate ions. This shows that the favorable effects of molybdenum can be attributed to the formation of MoO42− network near the interface.
  • Evolution of Size, Composition, and Morphology of Primary and Secondary Inclusions in Si/Mn and Si/Mn/Ti Deoxidized Steels

    pp. 1404-1411

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    DOI:10.2355/isijinternational.42.1404

    Primary and secondary inclusions in Si/Mn and Si/Mn/Ti deoxidized structural steels subjected to different thermal histories were investigated in view of evolution of size, composition, and morphology. Primary inclusions quenched from 1600°C contained very low levels of sulfur, and hence MnS precipitation on them was hardly found. The mean diameter of secondary inclusions lied in the range of 1-3 µm depending on the cooling rate and chemical compositions of steels. Both MnO and MnS content were higher in smaller secondary inclusions. MnS which precipitated on manganese silicate inclusions in Si/Mn deoxidized steels mostly grew into the inclusions. As inclusion size increased, the number of MnS precipitates on each inclusion was also increased. Titanium in steel had a tendency to reduce SiO2 content in inclusions and to associate with MnO in the inclusions to form a stoichiometric relationship of Mn/Ti ratio in the inclusions. If Ti content in Si/Mn/Ti deoxidized steels was low, the secondary inclusions were found to form with multiple phases; viz., manganese silicate phase, Mn-Ti oxide phase, and MnS phase. The MnS phase always precipitated in the manganese silicate phase. The proportion of manganese silicate phase in each inclusion decreased with a corresponding increase in Ti content in the steel, and eventually disappeared completely when the Ti content exceeded a certain level (70 ppm in the present steel compositions). In this case MnS was found to precipitate outside Mn-Ti oxide inclusions and grew into the steel matrix. In order to interpret and predict the behavior of inclusion precipitation and growth, a model has been developed which incorporates both thermodynamic and kinetic considerations.
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  • Automated Image Processing and Analysis of Fracture Surface Patterns Formed during Creep Crack Growth in Austenitic Heat-Resisting Steels with Different Microstructures

    pp. 1412-1418

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    DOI:10.2355/isijinternational.42.1412

    A computer program of automated image processing was developed for fractal analysis of creep fracture surface profiles in this study. Change in the creep fracture surface patterns during crack growth was examined by the computer-aided image processing and analysis. Fractal analysis was then made using software on the processed images. Creep crack growth experiments were carried out on the surface notched specimens of the austenitic 21Cr-4Ni-9Mn steel at 973 K. Creep crack growth rate was lower in the specimens with serrated grain boundaries than in those with straight grain boundaries. The fractal dimension of the grain-boundary fracture surface profiles was larger in the former specimens than in the latter ones. The fractal dimension was larger in the specimens tested under the lower stress, and decreased with increasing distance from notch root. Effects of grain-boundary microstructures and stress on the fracture patterns were correlated to the microstructure and stress dependence of the density of grain-boundary microcracks linked to the fracture surface. Quantitative evaluation of fracture surface patterns may give an important information about the fracture origin or crack growth direction.
  • The Application and the Problems of High Strength Steel on Penstock in Chinese Hydroelectric Station

    pp. 1419-1422

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    DOI:10.2355/isijinternational.42.1419

    The application of high strength steel has increasingly improved the technology on penstock manufacture and assembling, as well as management, with economy benefit being equally obtained. However, some misunderstandings on using high strength steel plate are still existed, which weaken their advantage. In this paper, the following problems are discussed.
  • Prediction of the Laves Phase Morphology in Fe-Cr-W-C Quaternary Steels with the Aid of System Free Energy Concept

    pp. 1423-1429

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    DOI:10.2355/isijinternational.42.1423

    In order to elucidate the microstructure evolution in advanced high Cr heat resisting ferritic steels, a time-composition-temperature diagram of Fe-10mass%Cr-W-C quaternary steels is calculated on the basis of the system free energy theory proposed by Koyama and Miyazaki. Microstructures of the steels are predictable using such a system free energy consisting of chemical free energy, interface energy and strain energy between the matrix and precipitates in the steels. In this study, our attention is focused on the Laves phase as the precipitate, because this phase is formed in recently developed heat resisting ferritic steels and affects their creep strength largely. The calculations conducted in the Fe-Cr-W-C quaternary steels lead to the following findings: (i) The Laves phase is not formed in those steels containing less than 0.4 mol% W (about 1.4 mass% W) even after a long term aging at 923K; (ii) Only granular Laves phase precipitates in the steel containing 0.4 mol% W to 1.05 mol% W (about 3.5 mass% W) even in the early stage of aging at 923 K; (iii) The morphological change of the Laves phase from the fine coherent precipitate to the granular one occurs in a regular aging sequence in the steel if it contains more than 1.05 mol%W; (iv) Coherent precipitation line for the Laves phase is determined in the quasi-binary phase diagram as a function of the W content. These findings obtained theoretically are consistent with experimental results.
  • Nanocrystallization in Fe-C Alloys by Ball Milling and Ball Drop Test

    pp. 1430-1437

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    DOI:10.2355/isijinternational.42.1430

    Microstructural evolution and nanocrystallization in various carbon steels by ball milling and ball drop test has been studied. In ball milling, nanocrystallization was observed in all the carbon steels irrespective of the carbon content (up to 0.9 mass% C) or starting microstructure (ferrite, martensite, pearlite or spheroidite). In ball drop test, nanocrystallization was observed in high carbon steels or ultrafine grained low carbon steels. It is realized that high strength before ball drop test is required for the nanocrystallization. The nanocrystalline structure obtained by ball milling and ball drop test has similar microstructure with dark smooth contrast. The morphologies such as pearlite lamellar, spheroidite cementite, ferrite grain boundary disappeared by nanocrystallization. The boundary between the nanocrystalline and work-hardened regions is quite sharp. The hardness of the nanocrystalline region is about two times higher than that of work-hardened region. The annealing of nanocrystalline region shows substantially slow grain growth and re-precipitation of fine cementite. This annealing behavior is quite different from the work-hardened region which is characterized by recrystallization and fast grain growth.
    From the present study, it was confirmed that the nanocrystalline structure produced by ball milling and ball drop test has quite similar in structure, hardness and annealing behavior although the number of deformation applied is substantially different.
  • Fabrication of Fine-grained High Nitrogen Austenitic Steels through Mechanical Alloying Treatment

    pp. 1438-1443

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    DOI:10.2355/isijinternational.42.1438

    Mechanical alloying (MA) treatment was applied for the fabrication of fine-grained high nitrogen stainless steels. Chromium nitride (Cr2N) powder was mixed with Fe-Cr binary alloy powder to control its mean chemical composition to be Fe-23mass%Cr-1mass%N which is enough to stabilize austenitic structure at room temperature. The powder mixture was mechanically alloyed up to 360 ks in an argon gas atmosphere (MA powder). The MA powder was packed in a stainless steel tube in a vacuum and consolidated by warm rolling at 1 073 K. The consolidated materials were finally heated to various temperatures (1 173-1 473 K) for austenitizing and then quenched without holding at the temperatures. Although the materials heat-treated below 1 323 K had bcc (martensitic) matrix, those heat-treated above 1 373 K had stable austenitic structure with a small amount of Cr2N. The grain size of matrix was maintained to be fine due to dispersed oxide particles within matrix in all steels. For example, the materials heat-treated at 1 473 K had fine austenitic structure in which the grain size was 2.2 µm and the solute nitrogen concentration was 0.86 mass%. The steel had very high yield strength of 1.1 GPa and moderate elongation of 30%. Such a high strength of the steel was explained by the combined strengthening mechanism of nitrogen solid solution strengthening and grain refining strengthening.
  • Relation between Creep Rupture Strength and Substructure of Heat Resistant Steel

    pp. 1444-1451

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    DOI:10.2355/isijinternational.42.1444

    Creep rupture tests were performed at around 650°C on a low carbon steel, a W-bearing low carbon steel, a low carbon steel containing 0.1% of TaN and a 0.1%C-8%Cr-2%W-0.2%V-0.04%Ta steel (F-82H), structural material for fusion reactors. The equivalent obstacle spacing for mobile dislocations is calculated using only rupture data according to the creep theory developed by some of the authors. Thin films taken from the ruptured specimens were examined under a transmission electron microscope (TEM). The observed sub-grain size or the calculated inter-particle distance roughly coincides with the equivalent obstacle spacing. This indicates that the microscopic variable, i.e. the obstacle spacing for mobile dislocations, such as sub-grain size or inter-particle distance, can be directly calculated from only the macroscopic variable, i.e. time to rupture or creep rate.
  • Effects of Copper Addition on Mechanical Properties of 0.15C-1.5Mn-1.5Si TRIP-aided Multiphase Cold-rolled Steel Sheets

    pp. 1452-1456

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    DOI:10.2355/isijinternational.42.1452

    The main emphasis of the present study was placed on understanding the effects of copper addition on the mechanical properties and microstructures of low-carbon TRIP-aided multiphase cold-rolled steel sheets. These steel sheets were intercritically annealed at 790-800°C, and isothermally treated at 430°C for various times. Tensile tests were conducted, and the changes of retained austenite volume fractions as a function of tensile strain were measured using X-ray diffraction. The copper addition increased the volume fraction of retained austenite, although it did not affect the stability. However, the hardness of ferrite in Cu containing steel was higher than that of Cu free steel. As a result, the strain-induced transformation of retained austenite was sustained up to the high strain region, thereby leading to the simultaneous enhancement of strength and ductility. These findings indicated that when copper, a representative tramp element, was positively utilized in cold-rolled steel sheets, excellent mechanical properties could be achieved.
  • A New Surface Treatment by Pulsed Plasma Nitriding for Chromium Plated Austenitic Stainless Steel

    pp. 1457-1460

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    DOI:10.2355/isijinternational.42.1457

    The present paper deals with the methodology adopted for pulsed plasma nitriding of chromium plated type 316LN austenitic stainless steel. The influence of nitriding temperature, time and gas mixture ratio of nitrogen to hydrogen on the nitriding behaviour of chromium plated type 316 stainless steel has been investigated. The results indicated that the nitriding temperature plays a dominant role in obtaining hardness and case depth in this material. Plasma nitriding at a temperature of about 833 K produced a case depth of about 5 µm and surface hardness of about 550 HV. At temperatures more than 1 073 K, a large fraction of chromium has been found to be converted to chromium nitride with hardness exceeding 1 000 HV. Nitriding at an intermediate temperature of 913 K for 45 h has been found to produce nitrided layer of optimum properties. Calculation of diffusion co-efficients and activation energy for nitrogen diffusion are presented to demonstrate that the nitride layer growth in chromium plated stainless steel is a diffusion controlled process.
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  • Inhibition of Abnormal Grain Growth during Isothermal Holding after Heavy Deformation in Nb Steel

    pp. 1461-1467

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    DOI:10.2355/isijinternational.42.1461

    The microstructural evolution during isothermal holding at 590-750°C after heavy deformation was examined in Nb steel. The strain induced ferrite transformation of Nb steel was significantly retarded compared with that of plain C-Mn steel, when Nb was mostly dissolved. When the grain boundary ferrite was present before deformation and strain free ferrite was formed during the following isothermal holding, abnormal grain growth occurred at the regions near the deformed ferrite by strain-induced boundary migration (SIBM). The SIBM was caused by the energy unbalance at the boundaries between deformed and strain free ferrite grains transformed from the deformed austenite. This rapid growth was not inhibited by the strain-induced NbC precipitates, which means the driving force for the abnormal growth was greater than the pinning force by the precipitates. However, the abnormal grain growth could be prevented by isothermal heat treatment in either fully transformed or untransformed structure. The ultrafine and polygonal ferrite grains were obtained by the recrystallization of ferrite and the grain growth was inhibited by strain-induced NbC precipitates during isothermal annealing at 650°C after deformation.
  • Effects of Nitrogen in 9Cr-3W-3Co Ferritic Heat Resistant Steels Containing Boron

    pp. 1468-1476

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    DOI:10.2355/isijinternational.42.1468

    Many researchers revealed that the addition of small amount of boron (B) significantly improves the creep strength of ferritic heat resistant steels. However the mechanism for such an effect caused by B has not been clarified yet. In the present study, the effect of nitrogen (N) on the behavior of B in 9Cr-3W-3Co base steels was examined using Alpha-particle Track Etching (ATE), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and creep test. It was confirmed that creep lives have significantly increased by lowering the N content in boron containing steels. The prediction of creep life using Larson- Miller parameter suggests that high N-high B steel shows lower creep strength compared with high N-low B, low N-low B and low N-high B steels. SEM showed that B increases the precipitates stability against coarsening. TEM revealed that the stability of the matrix against recovery reaches maximum in low N-high B steels, while this stability is very weak in high N-high B steel. ATE showed that B segregating along packet and block boundaries and free B homogeneously distributing in the matrix are more effective on strengthening than B contained in M23C6 carbides. In addition, B in steels of a suitable content of N is contributable to the enhancement of creep strength through increasing the amount and the stability of fine precipitates such as VN. High content of N is expected to reduce such effects of B probably through the precipitation of BN, although no experimental evidence has been obtained.
  • Effect of Nitrogen Alloying on the Pitting of Type 310 Stainless Steel

    pp. 1477-1482

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    DOI:10.2355/isijinternational.42.1477

    The effect of nitrogen alloying on the pitting behavior of type 310 stainless steel has been investigated through measurements of pitting potential (Epit) as a function of temperature and concentration of NaCl (CNaCl). Nitrogen was effective to shift Epit to nobler direction especially at temperatures below critical one. The critical pitting temperature was defined as the temperature below which the usual linear relationship between Epit and logarithm of CNaCl did not stand. Alloying the stainless steel with nitrogen increased the critical pitting temperature. Below the critical temperature where Epit did not follow the usual dependency on CNaCl, pitting was retarded most effectively by nitrogen except when CNaCl was so high that Epit lay below ca. 400 mV. Although the whole mechanism of nitrogen is not still clear, nitrogen is most likely to suppress acidification of pitting site through formation of ammonium ion. Nitrogen in a metal matrix and nitrate in a solution seemed to have a common feature with respect to the potential dependency of inhibition efficiency. The fact that nobler potentials were more favorable for both nitrogen in metal matrix and nitrate in a solution for inhibition seemed to indicate that oxidation of nitrogen to nitrate might also be involved in the inhibition mechanism.
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  • Deformation Behavior of Low Carbon TRIP Sheet Steels at High Strain Rates

    pp. 1483-1489

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    DOI:10.2355/isijinternational.42.1483

    Two high strength transformation-induced plasticity (TRIP) sheet steels with 0.10 wt% and 0.14 wt% carbon were produced with retained austenite volume fractions varying from less than 3% up to 16%. These TRIP steels were tensile tested at strain rates ranging from 10−3 to 2.5×102 s−1 to determine the effects of strain rate and retained austenite volume fraction on tensile properties. Increasing the retained austenite volume fraction increases UTS, total elongation, uniform strain and total absorbed energy, but decreases yield strength and absorbed energy below 10% engineering strain. Increasing strain rate increases yield strength and UTS, and creates a better-defined yield point, but has little effect on strain hardening behavior for TRIP steels with 11% or less retained austenite. The TRIP steel with 16% retained austenite shows increasing strain hardening rate with strain rate at low strains.
  • Ultrafine Grained Low Carbon Steels Fabricated by Equal Channel Angular Pressing: Microstructures and Tensile Properties

    pp. 1490-1496

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    DOI:10.2355/isijinternational.42.1490

    Equal channel angular pressing (ECAP) was conducted on the two grades of low carbon steel, with or without vanadium, in order to produce an ultarfine grained structure. As a result, the ferrite grains were re-fined from 30 µm to 0.2–0.3 µm. The strength of the ECAPed steels increased remarkably, over twice of the strength of the steels before ECAP. A series of static annealing experiments showed that the increment of ECAP strain and the dilute addition of microalloying element such as vanadium were very effective on enhancing thermal stability of the ultrafine gained low carbon steels produced by ECAP in terms of microstructure and tensile properties. This enhanced thermal stability resulted from; (a) presence of excessive carbon content in the ferrite matrix by carbon dissolution from pearlitic cementite during ECAP; (b) preservation of high dislocation density due to addition of vanadium, providing the effective diffusion path for dissolved carbon atoms; (c) precipitation of excessive carbon as the form of nano-sized cementite particles during subsequent annealing and its effect on suppressing grain growth.
  • Microstructural Investigations on Type IV Cracking in a High Cr Steel

    pp. 1497-1504

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    DOI:10.2355/isijinternational.42.1497

    In the present study, microstructural changes that lead to Type IV cracking in a high Cr ferritic steel (11Cr-0.5Mo-2WCuVNb) weld joint has been investigated. Microstructure of the heat affected zone (HAZ) of a weld joint made by gas tungsten arc welding (GTAW) process was systematically studied in the as-welded condition, after different post weld heat treatments (PWHT) and after creep test. In addition, HAZ microstructures were simulated both by heat treatment and by using a weld simulator and creep tests were conducted using specimens with simulated HAZ microstructures. Results showed that, undissolved precipitates present in the fine grained HAZ (FGHAZ) and intercritial HAZ (ICHAZ) accelerate the tempering of these zones during PWHT and microstructural deterioration during creep. Creep tests of HAZ simulated specimens indicated that the creep was minimum for the specimens in which peak temperature of simulation was close to Ac3. Results also suggested that differences in creep properties observed between an actual weld joint and that of a simulated Ac3 microstructure could be due to mechanical constraints present in the weld joint in which a narrow zone of lower creep strength (FGHAZ) is sandwiched between zones of higher creep strength (coarse grained HAZ (CGHAZ) and weld metal on one side and base metal on the other side).
  • Martensitic/Ferritic Super Heat-resistant 650°C Steels - Design and Testing of Model Alloys

    pp. 1505-1514

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    DOI:10.2355/isijinternational.42.1505

    In the recent two decades advanced martensitic/ferritic 9-12% Cr steels are recognized to be the most potential materials for 650°C Ultra Super Critical (USC) Power Plants. The critical issues are the improvement of long-term creep strength and corrosion resistance. The aim of the present research is to design new super heat-resistant 12% Cr martensitic/ferritic steels using basic principles and concepts of physical metallurgy, to test and optimize model alloys and to investigate and clarify their behavior under long-term creep conditions with emphasis on microstructural stability and corrosion resistance.
    Fine distributions of stable precipitates, which block the movement of subgrain boundaries (M23C6 carbides, Laves phase) and dislocations (MX carbonitrides) and delay coarsening of microstructure, are the key to high creep strength of this type of steels. Therefore, different carbide, nitride and Laves phase forming elements (Cr, W, Nb, V, Ta, Ti) have been used to provide precipitation hardening. Furthermore, the aim is to produce a sequence of precipitates with different kinetics, i.e. with precipitation of a new phase during coarsening of the prior one. Co has been used for obtaining 100% martensite initially and for slowing down diffusion processes and particle coarsening. The partial replacement of Co by Cu is also investigated to reduce costs.
    The first results of mechanical tests of the studied model alloys have shown positive effects of the addition of W and Ta as Laves phase and MX forming elements, respectively, as well as of the addition of B. Alloying with Co has also shown beneficial effects on the creep strength.
  • The European Efforts in Material Development for 650°C USC Power Plants - COST522

    pp. 1515-1519

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    DOI:10.2355/isijinternational.42.1515

    “Advanced Steam Power Plant” is one of three working groups within the frame of European COST 522 with the aim of developing and evaluating ferritic steels for steam conditions up to 650°C.
    Today's state-of-the-art large fossil-fired steam turbines comprise live steam conditions of up to 610°C/300 bar and re-heat temperatures of up to 630°C. These ultra super critical steam parameters significantly increase plant efficiency and reduce fuel consumption and emissions of CO2.
    Ferritic materials should be used for thick-walled components to maintain high operational flexibility of such large plants. Rotors, casings, bolts, tubes/pipes and waterwalls are the critical components under current investigation. The class of the 9-12% Cr steels offers the highest potential to meet the required property level for critical components. Therefore a significant effort to increase the application temperature of these steels was the focus of study within the European COST 501 programme and has led to improved materials for 600°C application of forged and cast components and for pipework. These 600°C materials are already being successfully utilised in a number of advanced European power plants. Further potential for improvement in creep strength seems possible after taking into account the oxidation resistance for T>600°C.
    A large number of new ferritic-martensitic compositions, which have been designed on the basis of the positive outcomes attained in previous studies as well as on the results obtained with advanced thermodynamic calculation tools are under investigation in the new COST 522 programme. Full-size cast and forged components have been manufactured from the most promising compositions and now are being evaluated by intensive mechanical testing.
  • Effects of Titanium Addition on Precipitate and Microstructural Control in C-Mn Microalloyed Steels

    pp. 1520-1526

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    DOI:10.2355/isijinternational.42.1520

    A study was carried out on the effects of Ti in a Ti-lean and a Ti-modified C-Mn microalloyed steels of otherwise comparable compositions. Analyses were carried out by SEM and TEM on steel microstructure, microalloying element precipitates and on non-metallic inclusions. A theoretical support to the experimental data was obtained by thermodynamic analyses aimed at stating phase stability and composition as a function of temperature for the steels investigated. Experimental results and theoretical predictions were combined to cast light on carbide, nitride and sulphide evolution during thermal cycles associated to fabrication route of the steels.
  • Fine-grained Structures Developed along Grain Boundaries in a Cold-rolled Austenitic Stainless Steel

    pp. 1527-1533

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    DOI:10.2355/isijinternational.42.1527

    Fine-grained structures developed by cold-rolling in a 310S austenitic stainless steel have been investigated by using transmission electron microscopy (TEM). Particular emphasis is laid on the role of initial grain boundaries in the transmutation process of twin-matrix (T-M) lamellae into fine-grained structures.
    Microstructural evolution in a cold-rolled 310S steel is characterized by the development of dense T-M lamellae and their following collapse caused by shear band formation: shear bands destroy the T-M lamellae, and a fine-grained structure develops with increasing the area of shear bands. In the process of this grain refinement, such fine-grained structures have been found not only in shear bands but also in the area along initial grain boundaries. The fine-grained structure preferentially appeared around the intersections of grain boundaries with shear bands, and it expanded along the grain boundaries which were almost parallel to the rolling direction. In selected area diffraction patterns obtained from these fine-grained areas, large scatterings of crystal orientations were observed. On the basis of these results, the role of initial grain boundaries on the grain refinement is discussed.
  • Characterization of Nanostructure of Rusts Formed on Weathering Steel

    pp. 1534-1540

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    DOI:10.2355/isijinternational.42.1534

    Nano-scale structures of rusts formed on the weathering steel surface were investigated. It has been shown that the key structure is an Fe(O, OH)6 network, which is different from crystalline FeOOH. Atomic structures were analyzed quantitatively by a combination of X-ray absorption fine structure (XAFS) analysis including in situ observation under wet conditions, X-ray diffraction (XRD), and transmission electron microscopy (TEM). It has been shown that the Fe(O, OH)6 network structure evolves in the process of corrosion and a small amount of alloying elements such as chromium modifies the evolution process and the final morphology of rust.
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  • Interaction between Dislocation and Copper Particles in Fe-Cu Alloys

    pp. 1541-1545

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    DOI:10.2355/isijinternational.42.1541

    The strengthening mechanism due to copper (Cu) particles was discussed in terms of the interaction between dislocation and Cu particles in aged Fe-Cu alloys. Since Cu particles are softer than the iron matrix, its interaction with dislocation is different from that with the Orowan mechanism. The moving dislocations can cut the soft Cu particles and pass through them when the bowing angle reaches some critical value (θc), and the precipitation strengthening due to Cu particles is expressed as a function of mean particle spacing (λ) and the θc (π/2 gives Orowan stress). The θc increased with increasing the size of Cu particles and reached π/2 when the Cu particle size became 70 nm. This means that the precipitation strengthening due to Cu particles is dependent on not only λ but also the Cu particle size, and 70 nm is the minimum Cu particle size for obtaining the Orowan stress.
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  • In-situ Observation of Dislocation Motion and Its Mobility in Fe-Mo and Fe-W Solid Solutions at High Temperatures

    pp. 1546-1552

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    DOI:10.2355/isijinternational.42.1546

    The mobilities of the edge dislocations in Fe-W and Fe-Mo solid solution alloys at high temperatures were investigated with in-situ TEM observation in order to compare the effect of W and Mo on the solid solution hardening of ferrite. The dislocation behaviors were recorded continuously with VTR. TEM observations showed that the dislocations werfe moving viscously and that the dislocation velocites were constant in both Fe-W and Fe-Mo alloys. These results show that dislocations dragged solute atmosphere. The mobilities were determined to be 5.7×10−15 m/(Pa·s) at 993 K in the Fe-W alloy and 4.3×10−15 m/(Pa·s) at 1 011 K in the Fe-Mo alloy. It was found that the mobility in the Fe-W alloy is similar to that in the Fe-Mo alloy. Mobilities were estimated by simulation using the interaction between an edge dislocation and solute atoms. The results show that the simulated value was similar to the measured value in the Fe-Mo alloy while the simulated value is different from the measured in the Fe-W alloy. The simulated mobility of the dislocation in Fe-W is one-tenth as large as that in Fe-Mo. The simulated results for Fe-W did not agree with the experimental results. It is considered that the difference between the experimental values and simulated values chiefly came from the shape of the dislocations using for measurement and the diffusion constant which was used in the simulation. Experimental results showed that the mobility in Fe-W is as large as that in Fe-Mo. Therefore, it is suggested that the effect of W on the solid solution hardening of Fe is similar to that of Mo.
  • Refining of Intermediate Transformation Microstructure by Relaxation Processing

    pp. 1553-1559

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    DOI:10.2355/isijinternational.42.1553

    The influence of RPC (relaxation-precipitation-controlling phase transformation) processing parameters on the microstructure was studied by thermo-simulation for a low carbon Nb and Ti containing microalloyed steel. The semi-quantitative statistic work of the packet size was carried out by the aid of optical microscope. TEM, EBSD and SEM were applied to investigate the microstructure refinement, precipitation, the evolution of crystal defects configuration and their interaction during the relaxation. The results demonstrate that the steel is composed of ultra-fine bainite/martensite composite microstructure, and the microstructure could be refined markedly by RPC processing. The best thermo-simulation process for refinement in this experiment is deformation by 30% at 850°C, and then relaxing at this temperature for 60 to 200 s. Increasing the reduction ratio from 30 to 60% or decreasing the deformation temperature to 800°C would cause the optimized relaxation time to become shorter, and increasing the deformation temperature to 900°C would cause the refinement effect to be weaker. It is also indicated that the nucleation, growing and coarsening of precipitates, and the dislocations polygonizing occur during the relaxation. Both two processes could be helpful to refine the intermediate transformation microstructure, and when these two processes corporate and promote each other, the optimized processing is obtained.
  • Delayed Fracture Properties of 1 500 MPa Bainite/Martensite Dual-phase High Strength Steel and Its Hydrogen Traps

    pp. 1560-1564

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    DOI:10.2355/isijinternational.42.1560

    It is very imperative to improve delayed fracture properties of high strength steel, which may enlarge its usage. The published literature shows that the susceptibility to hydrogen embrittlement of a novel 1 500 MPa bainite/martensite dual-phase high strength steel is inferior to that of conventional quench-tempered high strength steel. The stress corrosion cracking (SCC) in a 3.5% NaCl solution for novel 1 500 MPa bainite/ martensite dual-phase high strength steel was investigated in this paper by using modified wedge-opening-loading (WOL) specimens. The experimental results show that KISCC for novel 1 500 MPa bainite/martensite dual-phase high strength steel is larger than 50 MPa·m1/2, exceeding conventional high strength steel. Its crack growth rate (da/dt)II is about 1×10−5 mm/s, which is less than that of conventional high strength steel. Hydrogen trapping phenomena in the steel were investigated by electrochemical permeation technique. The lath boundaries and stable retained austenite are beneficial hydrogen trap, slowing down the segregation of hydrogen on the crack tip, hence KISCC increases and crack growth rate decreases.
  • Thermal and Mechanical Stability of Retained Austenite in Aluminum-containing Multiphase TRIP Steels

    pp. 1565-1570

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    DOI:10.2355/isijinternational.42.1565

    Stability of retained austenite is the key issue to understand transformation-induced plasticity (TRIP) effect. In this work, both thermal stability and mechanical stability are investigated by thermo-magnetic as well as in situ conventional X-ray diffraction and micro synchrotron radiation diffraction measurements. The thermal stability in a 0.20C-1.52Mn-0.25Si-0.96Al (wt%) TRIP steel is studied in the temperature range between 5 and 300 K under a constant magnetic field of 5 T. It is found that almost all austenite transforms thermally to martensite upon cooling to 5 K and MS and Mf temperatures are analyzed to be 355 and 115 K. Transformation kinetics on the fraction versus temperature relation are well described by a model based on thermodynamics. From the in situ conventional X-ray and synchrotron diffraction measurements in a 0.17C-1.46Mn-0.26Si-1.81Al (wt%) steel, the volume fraction of retained austenite is found to decrease as the strain increases according to Ludwigson and Berger relation. The diffraction measurements also show that the mechanical stability depends on the orientation of the grain with respect to the direction of the applied stress, and the austenite grains at an angle of 45° or 60° were found to be more stable than those at lower or higher angles. Both thermal and diffraction experiments show an increase in the average carbon concentration of the remaining austenite with lowering temperature or increasing stress. Thermal and mechanical stability of retained austenite is therefore attributed to the carbon distribution over different austenite grains.
  • Effect of Microstructure on the Yield Ratio and Low Temperature Toughness of Linepipe Steels

    pp. 1571-1577

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    DOI:10.2355/isijinternational.42.1571

    The present study aims at elucidating the effects of microstructural features on the yield ratio and toughness of high strength linepipe steels. The main emphasis has been placed on understanding the effects of constituents on the properties. Several alloy systems with different constituents, i.e. ferrite-pearlite steels, ferrite steels with acicular ferrite as second phase, acicular ferrite steels with ferrite as second phase, and bainite steels, have been investigated. Experimental results show that while the refinement of ferrite grain size improves both yield strength and low temperature toughness of ferrite-base steels, it increases the yield ratio. Modification of matrix from ferrite to acicular ferrite or bainite results in improvements in both yield strength and yield ratio. However, bainite steels have worse low temperature toughness (i.e., higher DBTT) than the other types of steels. It has been shown that the low temperature toughness of acicular ferrite steels can be improved by the introduction of polygonal ferrite as a second phase. This is mainly due to the refinement of effective grain size by the introduction of second phases. The relationship between the yield ratio and work hardening exponent has also been established using the Swift equation. Based on the results, the optimum microstructure for a better combination of strength, toughness and yield ratio is suggested to be the one having second phase of polygonal ferrite in an acicular ferrite or bainite matrix.
  • Analysis of Degradation of Creep Strength in Heat-affected Zone of Weldment of High Cr Heat-resisting Steels Based on Void Observation

    pp. 1578-1584

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    DOI:10.2355/isijinternational.42.1578

    The deterioration of creep rupture strength in welded joints of high Cr ferritic heat-resisting steels was investigated based on creep tests of internal pressure specimens and single pass welded specimens. Results showed that at high temperature and low stress cracks occurred in the fine-grained heat-affected zone (FGHAZ) and was identified to be Type IV cracking. It was found that the peak weld temperature between app. Ac1 and app. Ac3 led to the Type IV cracking and many creep voids were observed in the FGHAZ. Further, effects of creep time on specimen necking, void distribution and precipitate coarsening were investigated. It was found that the creep void occurrence and specimen diameter reduction increased at an accelerating rate during creep. Precipitate observation showed that after long time precipitate coarsening was more in the FGHAZ than in the base metal (BM) and in the coarse-grained HAZ (CGHAZ). Auger Electron Spectroscopy (AES) point analysis showed that many precipitates existed in creep voids. Based on this observation, it was suggested that large precipitates were preferential sites for void nucleation. Finally, simulations using a welded joint model and a matrix/precipitate model were performed to investigate creep void occurrence. Results showed that creep deterioration easily occurred in the FGHAZ and large precipitates present in this zone acted as nucleation sites for voids.
  • Ultrafine Grained Ferrite Formed by Interrupted Hot Torsion Deformation of Plain Carbon Steel

    pp. 1585-1590

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    DOI:10.2355/isijinternational.42.1585

    A plain carbon steel was deformed using a hot torsion deformation simulator. A schedule known to produce strain-induced ferrite was used with the strain interrupted for increasing intervals of time to determine the effect of an isothermal hold on the final microstructure. Microscopy and electron back-scattered diffraction (EBSD) were used to analyse the changes that occurred in the partially transformed microstructure during the hold and the subsequent applied strain. The strain-induced ferrite coarsened during the hold and this coarsened ferrite was refined during the second deformation. These results were compared to those obtained for a different plain carbon steel deformed in single pass rolling close to the Ar3 temperature.
  • Compositional Change of Refractory Elements in Solution during Aging in High Cr Heat Resistant Ferritic Steels

    pp. 1591-1593

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    DOI:10.2355/isijinternational.42.1591

  • High-speed Deformation for an Ultrafine-grained Ferrite-Pearlite Steel

    pp. 1594-1596

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    DOI:10.2355/isijinternational.42.1594

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