Tetsu-to-Hagane

Steelmaking

Regular Article

Control of Formation of Spinel Inclusion in Type 304 Stainless Steel by Slag Composition

Yasuhiro EHARA, Seiji YOKOYAMA, Masahiro KAWAKAMI

2007 Volume 93 Issue 7 Pages 475-482
Published: July 01, 2007
Released on J-STAGE: February 13, 2009

DOIhttps://doi.org/10.2355/tetsutohagane.93.475

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The inclusion of the spinel, MgO · Al₂O₃, in type 304 stainless steel slab causes defect of surface, deterioration of mechanical property and so on. Thus, it is required to reduce the spinel inclusion. In this study, inclusions in molten type 304 stainless steel at each successive reactor, which were AOD, LF and tundish, were investigated in order to control the formation of the spinel in type 304 stainless steel slab. The formation of the spinel inclusions in the slab was discussed from the viewpoint of the effect of slag composition.
The Al₂O₃ content in inclusion increased in the course of production process from AOD to slab. Moreover, the Al₂O₃ content in inclusion in the slab increased with the increase of Al₂O₃ content in slag and %CaO/%SiO₂ ratio of slag. The following formation mechanism of inclusion is proposed based on these results. The origin of inclusion is formed by incorporation between the suspended AOD slag in molten steel and the deoxidation products that are SiO₂ and Al₂O₃. When the temperature of molten steel falls, the original inclusion works as a nucleation site, namely, Al₂O₃ as a secondary deoxidation product precipitates at the original inclusion. Accordingly, the Al₂O₃ content in the inclusion increases. Based on the formation mechanism of inclusion, the semi-empirical model was constructed. This model revealed that reduction of Al₂O₃ content in slag and %CaO/%SiO₂ ratio of slag was effective for eliminating the formation of harmful spinel inclusion.

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Casting and Solidification

Technical Report

Improvement of Inline Cutting Device for Continuous-cast Slab in High-Ni Alloys

Yoshiharu MIYAZAKI, Kenji MIZUNO, Atsuya HONGO, Teruaki ISHII, Hidekaz ...

2007 Volume 93 Issue 7 Pages 518-523
Published: July 01, 2007
Released on J-STAGE: February 13, 2009

DOIhttps://doi.org/10.2355/tetsutohagane.93.518

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An improvement has been carried out to minimize the time for cutting continuously cast slabs in high-Ni alloy by means of a torch-cut equipped with Fe and Al powder supply. Two major reasons why it was so tough to cut the slabs in high-Ni alloy were considered. Firstly, heat generation by torch-cut is insufficient to keep burning a slab for high-Ni alloy because Ni is a metal nobler than Fe or Cr. Secondly, fluidity of the flown melt formed by torch-cut of slab is lower for high-Ni alloy than for low-Ni stainless steel. The formed flown melt consists of a mixture of molten alloy and oxides. The flown melt for high-Ni alloy includes higher ratio of solid oxides which lowers the fluidity. In order to cut the slab against low fluidity of the flown melt, the cutting device was improved by increasing ability to supply combustion gas and powder. In addition, the powder ratio of Al to Fe was optimized as 25 mass%. Consequently, the improvement succeeded in shortening the time for cutting a slab to half for high-Ni alloy.

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Chemical and Physical Analysis

Regular Article

Inspection of the Defects of Steel by Using Laser Induced Breakdown Spectroscopy

Michihiro AIMOTO, Hiroyuki KONDO, Hideaki YAMAMURA, Takehiko TOH

2007 Volume 93 Issue 7 Pages 483-488
Published: July 01, 2007
Released on J-STAGE: February 13, 2009

DOIhttps://doi.org/10.2355/tetsutohagane.93.483

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Quick evaluation technique of the defects of steel materials by Laser-Induced Breakdown Spectroscopy (LIBS) was established. A Q-switched Nd-YAG laser with high pulse energy was used for laser breakdown, and the emission from breakdown plasma was transmitted and introduced into a spectrometer via optical fiber. A Paschen-Runge mounting type spectrometer was used for multi elemental analysis. As a result of measurement, LIBS could detect specific elements in defect parts due to alumina inclusions (Al) and contaminations from continuous casting mold flux (Al, Ca, Mg, Si, Na) as well as slag (Al, Ca, Mg), which caused surface defects. The proposed method made it possible to inspect the steel defects within 30 min including the sample preparation at the production site. Elemental mapping (C, Si, S) of central segregation of slabs was also shown. The results demonstrated the possibility of the fast mapping technique for the purpose of screening of the slabs.

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Surface Treatment and Corrosion

Regular Article

Effect of Mn Content in Steels on the Selective Surface Oxidation Behavior of 1 mass% Si Bearing Low C Steels in Recrystallization Annealing

Yoshitsugu SUZUKI, Yoshiharu SUGIMOTO, Sakae FUJITA

2007 Volume 93 Issue 7 Pages 489-497
Published: July 01, 2007
Released on J-STAGE: February 13, 2009

DOIhttps://doi.org/10.2355/tetsutohagane.93.489

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Effect of Mn content and dew point of recrystallization annealing atmosphere were investigated, on the selective surface oxidation and the internal oxidation behavior of 1 mass% Si bearing steels at 800°C, by glow discharged spectroscopy, X-ray photoelectron spectroscopy, secondary electron microscopy, transmission electron microscopy, auger electron spectroscopy, and IR absorption spectroscopy. For the steel of which Mn content was 1 mass% or less, the amount of the selective surface oxides of Si and Mn on the surface after annealing, were increased with the increase in the Mn content in steels, due to suppression of protective SiO₂ formation. For the steel of which Mn content was more than 1 mass%, both the selective surface oxidation and the internal oxidation behavior were remarkably affected by dew point of recrystallization annealing atmosphere. In case the dew point was -40°C, the amount of Si selective surface oxidation was almost constant and the internal oxidation was not occurred regardless of Mn content in steel. While in case the dew point was -30°C, the amount of Si selective surface oxidation was decreased and the amount of internal oxidation was increased with the increase in the Mn content in the steel. Those differences were explained by lowering thermodynamical oxidation potential of Si, Mn bearing steels with increase in the Mn content. At higher dew point, non-protective Si, Mn-oxides were formed at lower dew point of annealing atmosphere, which accelerated internal oxidation of Si, Mn bearing steels.

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Hot Corrosion of the Ni-20%Cr Alloy and Waspaloy Immersed in Na₂SO₄

Kiyoshi KUSABIRAKI, Masashi MORITA, Xiping GUO, Hiroshi ANADA

2007 Volume 93 Issue 7 Pages 498-505
Published: July 01, 2007
Released on J-STAGE: February 13, 2009

DOIhttps://doi.org/10.2355/tetsutohagane.93.498

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The nickel-based superalloy of Waspaloy and the Ni-20%Cr alloy were hot-corroded in the molten Na₂SO₄ under the flowing air for up to 720 ks at 1273 to 1473K to clarify the sulfidation/oxidation behavior by means of XRD, OM, SEM, EPMA and TEM. They formed a subscale which was composed of intergranular and intragranular compounds in the alloy interior together with an external scale on the alloy. The kinetic of the subscale formations was followed by a parabolic rate law. The external scale and subscale formed on the Ni-20%Cr alloy consisted of Cr₂O₃ and Cr₂S₃, Cr₅S₆ and a small amount of Cr₂O₃, respectively. Those formed on Waspaloy consisted of (Cr · Co · Ti)₂O₃ and TiS₂, Cr₂S₃, Cr₅S₆, Al₂O₃ and (Cr · Co · Ti)₂O₃, respectively. In order to observe directly the three dimensional morphology of the subscale, a metal matrix was dissolved using an organic solution that does not dissolve the metallic sulfides or oxides which constitute the subscale. The organic solution developed in this investigation consisted of bromine, cetylpyridinium bromide and acetonitrile. The SEM images of the three-dimensional subscale extracted from the corroded alloys with the organic solution showed that the intergranular compounds formed thin walls of sulfides and oxides along grain boundaries and an intragranular sulfide formed isolated needles in the alloy grain interior near the external scale.

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Transformations and Microstructures

Regular Article

Segregation and Precipitation of Carbon in the Vicinity of Dislocations during Strain Aging in Low Carbon Steels

Naoki MARUYAMA, Manabu TAKAHASHI

2007 Volume 93 Issue 7 Pages 506-512
Published: July 01, 2007
Released on J-STAGE: February 13, 2009

DOIhttps://doi.org/10.2355/tetsutohagane.93.506

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Segregation and precipitation behaviour of solute C in the vicinity of dislocations during strain aging was investigated by internal friction (IF) measurements, 3-dimensional atom probe (3D-AP) analysis, and TEM observations. IF measurement has indicated that approximately 3-6 C atoms per atom plane segregated along dislocation after straining. On the other hand, smaller amount of C segregation, 1.3 atoms per atom plane, along a dislocation after aging at 170°C was found by 3D-AP analysis. The distribution of C atoms after aging appears to show C atoms has segregated to dislocation core with strong interaction, whereas C atoms interact weakly with dislocations before aging. It was confirmed that iron carbide plays an important role on strain aging behaviour in low carbon (LC) steels containing more than several wt. ppm of initial solute C. During a strain aging at 170°C, the nucleation kinetics and distribution of iron carbides were observed to be affected by an initial dislocation density, due to the preferential precipitation of the carbides on dislocations. The reason for the pre-strain dependence on strain aging behaviour in the LC steels seems to be well explained as the effect of dislocations on nucleation kinetics and distribution of iron carbides.

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Effect of Duplex-grained Structure on Yield Stress of IF Steels

Riichirou MATOBA, Nobuo NAKADA, Yuichi FUTAMURA, Toshihiro TSUCHIYAMA, ...

2007 Volume 93 Issue 7 Pages 513-517
Published: July 01, 2007
Released on J-STAGE: February 13, 2009

DOIhttps://doi.org/10.2355/tetsutohagane.93.513

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The “nominal grain size” (average grain size) is generally applied to Hall-Petch relationship to evaluate grain refinement strengthening in polycrystalline materials. However, the steels with wide grain size distribution (duplex-grained structure) may not deform uniformly but yield preferentially from larger grains to finer ones. This phenomenon is called “micro-yielding”. In this study, the effect of duplex-grained structure on the yield stress was investigated by using some IF steels with different grain size distribution. As a result of tensile testing, the yield stress of duplex-grained steels could be conventionally plotted on the Hall-Petch relationship as a function of (nominal grain size)^-1/2 in the range from 100 to 10 μm, even though the micro-yielding phenomenon occurred within the coarse grains at a lower stress than the macroscopic yield stress. When the volume fraction of grains with identical size is summed from larger-sized ones, the summated volume fraction (defined as the integrated volume fraction) always reaches 70-80 vol% at the nominal grain size irrespective of the difference in grain size distribution. These results suggest that polycrystalline materials including duplex-grained structure materials cause the macroscopic yielding when the grains of 70-80 vol% are micro-yielded.

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