Kazuhiro Seto

Coarsening Behavior of Nanometer-Sized Carbides in Hot-Rolled High Strength Sheet Steel

This study deals with a relationship between strength and coiling temperature of high strength hot-rolled sheet steels consisting of ferrite and nanometer-sized carbides in order to evaluate the stability of the strength against the variation of the coiling temperature. Ti-Mo-bearing and Ti-bearing steels were prepared to form (Ti,Mo)C and TiC in ferrite matrix, respectively. Ti-Mo-bearing steel exhibited the high strength even under the high temperature coiling while the strength of Ti-bearing steel decreased significantly. Ti-bearing steel just after transforming at 923K had the same hardness as that at 898K. In addition, hardness of Ti-bearing steel coiled at 898K decreased significantly by holding at 923K for 8.64ks while Ti-Mo-bearing steel did not represent a large change in hardness. These results confirm that (Ti,Mo)C is not coarsened easily by Ostwald ripening at the high coiling temperature unlike TiC. Consequently the retardation of Ostwald ripening of (Ti,Mo)C is attributed to the small amount of titanium in solution in Ti-Mo-bearing steel.

Hardness of Oxide Scales on Fe-Si Alloys at Room- and High-Temperatures

Hardness of oxide scales on Fe-(0, 0.5, 1.5, 3.0)Si alloys was studied at room temperature after oxidation at 1273 K for 18 ks in oxygen, and at 1073 and 1273 K for 180 and 1080 ks in dry air, by micro-Vickers hardness measurements. After oxidation at 1273 K for 18 ks, high-temperature hardness of oxide scales on Fe-(0, 1.5, 3.0)Si alloys was also measured at 1273 K. Oxide scales on Fe-Si alloys were mainly Fe2O3, Fe3O4, FeO and Fe2SiO4. Hardness of Fe2O3, Fe3O4 and FeO on Fe was 6.7, 4.0 and 3.5 (GPa), respectively, and hardness of Fe2O3 on Fe-Si alloys slightly increased with increasing silicon content at room temperature. At 1273 K, hardness of Fe3O4 and FeO on Fe was 0.08 and 0.05 (GPa), respectively, and hardness of Fe2O3 on Fe-1.5Si alloy was 0.32 (GPa), and that of Fe2O3 and Fe2SiO4 on Fe-3.0Si alloy was 0.53 and 0.63 (GPa), respectively.

Strengthening of Ferritic Steel by Interface Precipitated Carbides in Rows

Precipitation-strengthening is widely applied to high strength steel sheet for automotive use since several strength grades are easily achieved by controlling amount of microalloyed component. Recently, finer carbide dispersion has been required to obtain higher strength by smaller addition of carbide formers like titanium and niobium. Here, interface precipitation, one of the carbide formation phenomena during γ→α transformation, can be the efficient method to promote very fine carbides by lowering precipitation temperature. This study deals with relationship between transformation temperatures and hardness of ferritic steel strengthened by carbides generated by the interface precipitation. Two kinds of 0.04%C steels containing Ti and Nb of the same amount as carbon content in atomic were hot-rolled, followed by the soaking at various temperatures for 600s. The rapid-cooled samples before the soaking for 600s exhibited higher hardness than slow-cooled samples. Large carbides generated by interface precipitation were observed in slow-cooled Ti-bearing steel with a transmission electron microscope. In slow-cooled Nb-bearing steel, large NbC precipitated in austenite before γ→α transformation. The results are suggesting that lowering transformation temperature and suppressing carbides precipitation in austenite are important to obtain high strength by interface precipitation.

Effect of scale microstructure on scale adhesion of low carbon sheet steel

For industrial purposes, the adhesion control of secondary scale on hot rolled steel sheet is important. A basic study was carried out to clarify the effect of scale microstructure on the scale adhesion of low carbon steel (0.03%C-0.2%Mn). When scale of FeO (about 8μm thickness) was generated at 800°C and transformed by continuous cooling from 250~600°C to 200°C, the scale transformed from 400°C showed good adhesion. The scale consisted of magnetite seam from the steel substrate, lamellar structure of magnetite and α-Fe, and magnetite layer from the scale surface. The orientation analysis by TEM showed the relationship {110}Fe // {100}Fe3O4, <110>Fe // <100>Fe3O4, and the lattice strain was calculated as 4%. On the other hand, FeO/Fe substrate showed the relationship {100}Fe // {110}FeO, <110>Fe // <110>FeO, and 25% lattice strain was calculated. It is considered that the adhesion of scale should be affected by the lattice strain, thus Fe3O4/Fe substrate showed better adhesion than FeO/Fe substrate. The temperature of FeO formation also affects the scale adhesion through the extent of Fe super saturation in FeO.

Effect of Ferrite Grain Boundary on Strain Aging Behavior in Nb-Bearing Ultra-Low-Carbon Steel Sheets

Effect of grain boundary on strain ageing behaviour of Nb-bearing ULC steel sheets has been studied at the aging temperature from 70 to 220°C, using 2% pre-strained specimens with different ferrite grain sizes of 9.5μm and 183μm. Two different hardening stages were exhibited in the fine-grain specimen, whereas only a single hardening stage was shown in the large-grain specimen. The increase in YP of the first hardening stage was around 30MPa; the activation energy of this stage was estimated to be from 83 to 86kJ/mol, which is close to that of body diffusion of carbon atoms in α-Fe. The increase in YP of the second hardening stage reached 90MPa; the activation energy was 135kJ/mol, which is close to that of body diffusion of Fe atoms in grain boundary and precipitation of η-carbide. From TEM observations and nanoindentation analyses, it was inferred that the dominant mechanism could be dislocation pinning by carbon atoms for the first hardening stage, and grain boundary hardening or hardening around it for the second.

Recrystallization Behavior Immediately after Hot-Rolling in Ferrite Region in Ultra Low Carbon Sheet Steels

One-pass hot rolling in the ferrite region was conducted at higher temperatures, using various rolling temperatures and rolling reductions, with two types of ULC steels, 0.016% Nb and 0.023% Ti, and recrystallization behaviors immediately after hot rolling were investigated. The γ-fiber strength reached its maximum at around 50% rolling reduction at 1273K with the Nb-added steel and 1323K with the Ti-added steel. On the other hand, in high temperature rolling of the Ti-added steel, the γ-fiber did not develop, independent of rolling reduction. These changes corresponded to the recrystallized fraction, in that the strength of the γ-fiber decreased when recrystallization occurred immediately after rolling. New recrystallized-like grains were produced in the domain where distortions were particularly concentrated. Recrystallization seemed to be the result of various mechanisms, as some recrystallized grains were formed by a bulging mechanism, whilst others were surrounded by high angle grain boundaries.