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.
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.
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.
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.
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.
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.
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.