In order to understand the effect of surface roughness of CC slabs such as oscillation mark on the surface cracking, hot deformation of some low carbon low alloy and austenitic stainless steels has been investigated by means of hot tensile tests using the specimens with round notch at temperatures from 800 to 1100°C at average strain rates from 10^-4 to 10^-1 s^-1. Although the total elongation of the parallel portion decreased with strengthening by the notches, the effects were markedly reduced by lowering either deformation temperature or the average strain rate or by Nb addition. This can be explained in terms of dynamic precipitation behavior of carbonitrides such as NbC and/or AlN, i.e., the precipitation can be suppressed by the increase of true strain rate which arises from local straining in the notched region, resulting in the ductility improvement. The ductility loss due to the notch is determined by the depth and does not depend on the initial sharpness, since the shape can easily change in the early stage of deformation. Therefore, control of the oscillation mark depth is the most important to prevent surface cracking of CC slabs.

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The properties of a newly developed oxidation inhibitor composed of refractory powder, SiO₂, aluminum, synthetic mica, colloidal silica and coking bond were clarified and the oxidation inhibition mechanism of this inhibitor was examined. This inhibitor is applicable to all steel grades and significantly improves their yields. At the same time, it completely eliminates surface defects due to scale by suppressing the intergranular oxidation and selective oxidation of alloying elements. During reheating, the metallic silicon in the film of the inhibitor decomposes mullite (3Al₂O₃• 2SiO₂) and SiO₂ to form metallic silicon. This metallic silicon is oxidized with a gradual formation of protective amorphous SiO₂, which then prevents the diffusion of O₂. After the decomposition of mullite and SiO₂, aluminum is oxidized and forms an α-Al₂O₃ film and an FeO•Al₂O₃ film, which are also highly protective. Furthermore, the formation of fayalite (2FeO•SiO₂) is suppressed in this inhibitor due to the formation of FeO•Al₂O₃. Therefore, the protective films exist stably even if steels are reheated at high temperatures for a long time. In addition, many pores are formed due to a decrease in volume resulting from the chemical changes of Al→Al₂O₃ and SiO₂→Si. These pores constitute barriers to the diffusion of Fe⁺⁺ and O^--. The excellent oxidation-preventive capacity of this inhibitor is due to the combined actions mentioned above.

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For application to thermomechanical processing (TMP) with acceleated cooling, effect of a combined addition of boron and a grain-refining element such as niobium, titanium or vanadium in very low carbon steels was studied in terms of mechanical properties and microstructures. It was found that, although boron as a single addition has little effect on the properties, the combined addition of niobium and boron improves the strength and toughness balance. A separate addition of boron produces coarse Fe₂₃(CB)₆ precipitates at austenite grain boundaries after rolling and boron is consumed to invalidate the effect of γ-α transformation suppression. Niobium addition to boron steel suppresses the precipitation of Fe₂₃(CB)₆ and strongly retards the γ-α transformation to form a finegrained bainitic structure. Titanium has the same effect as that of niobium, but vanadium does not. The strengthening and toughening mechanism by the combined addition of niobium and boron was also investigated by examining the effect of alloying elements on the minimum recrystallization temperature of austenite during rolling, and γ-α transformation behavior.

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Effects of chemical composition and variables in thermomechanical processing (TMP) with accelerated cooling were studied on the mechanical properties and microstructure of Nb-B steel. It was clarified that, in order to achieve a good strength and toughness balance by a combined addition of niobium and boron, it is necessary to reduce the carbon content, to dissolve niobium during slab-reheating and to roll with sufficient reduction in an unrecrystallization region of austenite. With these conditions, a fine-grained low-carbon bainitic structure is obtained. The changes in the mechanical properties and microstructure of Nb-B steel with composition and processing variables can be elucidated in terms of the distribution of boron and the amount of solute niobium of plates. The factors controlling strength and toughness of Nb-B steel were also discussed.

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In order to elucidate the formation of Goss grains after an intermediate annealing in grain oriented silicon steel, computer color mapping was performed by processing the orientation information of Kossel diffraction patterns taken from the recrystallized grains in advance.
The Goss grains forming colonies in the vicinity of steel surface after an intermediate annealing occupy an extremely large area inside the defined {hk0}<001> matrix bands. The size of each Goss grain in this region is comparable to or slightly larger than that of other matrix grains. The perfection o f the [001] alignment o f the colonized Goss grains is much better than that of (110) plane and the colonized Goss grains are formed with the small boundary width of within 10° in the crystallographic plane. It is considered that the formation areas of Goss grains inherited by the structure memory from the original hot rolled texture take place with the strong perfection of the [001] alignment inside the defined {hk0}<001> matrix bands.

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