As an innovative measure to mitigating CO₂ emissions during ironmaking, the enhancement of carbon reactivity in blast furnaces is promising. It can reduce the temperature of the thermal reserve zone (TRZ), which is among the limiting factors to reaction efficiency in blast furnaces, thereby enabling operation under a low reducing agent rate (RAR). Therefore, reaction behaviors of two types of agglomerates with high carbon reactivity, composite agglomerates (CAs), and Ferro-coke, were evaluated using a softening-melting tester and via large-scale thermogravimetry. Process estimation of the blast furnace using them was also performed using a counter-current reaction simulator. CAs exhibited low-temperature gasification, efficiently promoting reduction by mixing with sintered ores. The carbon-consumption ratios of CAs and Ferro-coke were higher than that of coke. The reactive coke agglomerate, which is reinforced CAs with high carbon content toward reducing the RAR, exhibited the highest carbon reactivity, because of the coupling phenomena between the gas reduction of iron oxide and gasification of carbon. The addition of metallic iron to the CA increased the consumption of carbon and reduction of sintered ores, because of the catalytic effect. A combined use of the CA and Ferro-coke in the blast furnace successfully reduced the temperature of the TRZ by 150°C, offering the potential to decrease RAR by 35 kg/t-HM. Estimation of the distance between carbon and iron oxide or metallic iron in these agglomerates revealed that reducing the temperature of the TRZ by them was closely associated with shortening the distance.

Low-MgO sintered ores have developed into dominant burden materials for large blast furnaces operating under high pulverized coal injection in Japan, because of their low gangue content and high strength. Mixing MgO-bearing burdens with low-MgO sintered ores is an effective approach to satisfy the MgO requirement of blast furnaces. Therefore, a basket-evaluation test was performed in an experimental blast furnace (EBF) to investigate the reduction behavior of olivine pellets mixed with low-MgO sintered ores. The reduction behavior with lime-fluxed pellets was also evaluated as a reference. Softening-melting tests were also conducted under the same mixing conditions as those in the EBF tests. Olivine pellets exhibited smaller pores and contained finer hematite grains before reduction. These microstructural features influenced their reduction behavior, with low size disintegration observed in the lumpy zone in the EBF. Numerous cohesive masses with slag formed at the interface between sintered ores and lime-fluxed pellets in the EBF, facilitating their melting. In contrast, a small amount of slag was found at the interface between sintered ores and olivine pellets. The results of the softening-melting tests also revealed the superiority of olivine pellets during melting. Despite the low temperature of the initial melt formation during reduction, olivine pellets exhibited lower liquid ratios at high temperatures, resulting in a decrease in exuded slag when mixed with low-MgO sintered ores. This work proposes a general mechanism for the melting behaviors of mixed burden materials for blast furnaces.

The recrystallization behavior of a cold-rolled IF steel sheets, which had experienced recrystallization at higher temperature, was investigated at the active interface between IF steel and pure Al, heat-treated at 650ºC. In the region surrounded by the tongue-like η-Fe₂A_l5 phase, the recrystallized structure of the IF steel was characterized by equiaxed structure including subgrains, whereas elongated pancake shaped ferrite was majored in other region. The η phase grew preferentially to the c axis, and the growing η phase distorted the surrounding iron due to the difference in the molar volume; compressive deformation was expected both in the a-axis and b-axis directions, and tensile deformation was expected in the c-axis. The growth of η phase obviously changed the recrystallization behavior of the IF steel. The development of γ-fiber crystalline texture (<111>//ND), which was a typical recrystallization texture of IF steel, was not observed in the region surrounded by tongue-like η phase. The local misorientation in the vicinity of the two-phase interface could not explain the texture change in the region surrounded by the η phase. It was considered that the growth and restraint of the η phase also affected the nucleation of recrystallization.

TMCP (thermo-mechanical controlled process) linepipes have been long used for severe sour environment, but recently sulfide stress cracking (SSC) caused by local hard zones has become a concern. In order to clarify the hardness threshold that leads to SSC, four-point bend (4PB) SSC tests as per NACE TM0316 were conducted under several H₂S partial pressure conditions. For 1 bar and higher H₂S partial pressure conditions, the surface hardness threshold (at 0.25 mm from surface) observing 4PB SSC specimens without SSC cracking was approximately correlated to a maximum acceptable hardness level of 250 HV0.1. By suppressing the hard lath bainite (LB) and obtaining the soft granular bainite (GB) microstructure, stable low surface hardness of 250 or less HV0.1 was achieved, resulting in superior SSC-resistant property. On the other hand, it was found that SSC crack propagated when the surface hardness increased with increasing the volume fraction of LB microstructure. In the case of 16 bar H₂S partial pressure condition, the crack growth rate increased in the sour environment, and hydrogen embrittlement by H₂S was promoted. However, in the 4PB SSC test at 16 bar, since the shape of localized corrosion is semicircular due to low localized corrosivity, it was considered that the stress concentration and transition to crack were suppressed. This may be the reason why the SSC susceptibility was similar to 1 bar condition, especially in the 4PB SSC test using the samples with lower surface hardness level of 250 or less HV0.1.

Influence of the initial crystal orientation and carbon content on rolling texture was investigated using quasi-single crystals in 3.2 mass% Si steel. These specimens had {110}<001> and {110}<113> crystal orientation which were known for the near surface texture of the hot-rolled band.

In the case of the ultra low-carbon specimens, initial {110}<001> rotated to {111}<112> after 66% reduction cold rolling and initial {110}<113> rotated to near {211}<124>. It was thought that the crystal rotation from {110}<113> to near {211}<124> caused by an activation of {110} slip system which had the second largest schmid factor. {211}<124> was not known for the stable rolling texture, however {211}<124> intensity in present experiment was extremely strong. In addition, {211}<124> has geometric character that if it rotates by an activation of one slip system, it will revert to the initial crystal orientation {211}<124> by an activation of another slip system.

In the case of the specimens containing carbon, {110}<001> rotated to {111}<112> and {100}<011> caused by deformation twinning. On the other hand, {110}<113> rotated to {211}<113>-{111}<112> during the cold rolling. The deformation twinning was also observed. It was thought that the crystal orientation in the deformation twinning rotated to near {111}<112> by an activation of {110} slip system.

Super invar cast steel, Fe–32%Ni–5%Co by mass %, with excellent low coefficient of thermal expansion has disadvantages in the both of Young's modulus and strength, because of coarse columnar solidification structure having <100> texture. To simultaneously overcome these disadvantages, the variations of microstructure and mechanical properties through the novel heat treatment referred to as cryo-annealing, which is consisting of subzero treatment and subsequent annealing, were investigated in a super invar cast steel. The cryo-annealing promoted fcc-bcc martensitic transformation and then bcc-fcc martensitic reversion. The bidirectional martensitic transformations led to the formation of duplex austenitic structure consisting of untransformed and reversed austenite with a coarse-grained structure similar to solidification structure. Furthermore, it is found that the austenitic structure was varied depending on the annealing temperature of the cryo-annealing; reversed austenite was remained at lower annealing temperature, while it recrystallized to fine-grained structure as increasing annealing temperature. The high-density dislocations in reversed austenite and the randomized orientation of recrystallized austenite contributed to the development of strength and Young’s modulus, respectively. Therefore, the simultaneous development of rigidity and strength is not achieved by single cryo-annealing, but can be achieved by two-cycle cryo-annealing. Increasing the first annealing temperature and lowering the second annealing temperature in the two-cycle cryo-annealing are appropriate to randomize crystal orientation through austenite recrystallization and to make volume fraction of reversed austenite higher, respectively. As a result, Young’s modulus and 0.2% strength were simultaneously optimized.