Grain refinement of magnesium alloy AZ31 was studied in multidirectional forging (MDF) under decreasing temperature conditions. MDF was carried out up to large cumulative strains of 5.6 with changing the loading direction during the decrease of temperature from pass to pass. MDF can accelerate the uniform development of very fine grain structures and increase the plastic workability at low temperatures. New grain structures with the minimal grain size of 0.23μm can be developed by continuous dynamic recrystallization at a final processing temperature of 403K. The MDF alloy showed higher strength, as well as high ductility, at room temperature and also superplastic elongation at 423K. The relationship between the yield stress or hardness at room temperature and the grain size can be represented by a Hall-Petch equation in strain-induced fine-grained magnesium alloy.

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Grain refinement processing by severe deformation, combining Equal-Channel Angular Extrusion (ECAE) processing and conventional tube extrusion, was applied to AZ31 magnesium alloy. By a combination of ECAE processing and tube extrusion, a fabricated tube, with outer and inner diameters of 2 mm and 1 mm, respectively, had fine, homogeneous, and equiaxed grain structure with an average grain size of 1.5μm. Tensile test results indicate that the fine-grained tubes exhibited a superplasticity potential m value = 0.55. The maximum elongation (688%) was obtained at a temperature of 673K and at a strain rate of 1.0×10^-4s^-1. In addition, compared with tubes only processed by extrusion, ECAE-extruded tubes showed remarkable ductility with an elongation of 245% at a high strain rate of 5.2×10^-2s^-1. From these results, the effectiveness of new grain refinement processing for fabricating fine-grained narrow tubes was demonstrated experimentally.

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There is high demand for the use of magnesium alloys in automotive and electric parts, because that they have the lowest density among metals for practical use, and have excellent properties such as high specific strength, high vibration damping ratio and high electromagnetic interference shielding. However, there is a little application for magnesium alloys as metal-forming products because of their poor plastic deformability in the normal temperature range and their high production cost. To promote products manufactured by metal forming, rolling technologies enabling the production of thin sheets with suitable mechanical properties and surface qualities are required. For this purpose, hot-rolled AZ31 sheets are experimentally rolled with or without lubrication under temperatures ranging from 293K to 573K, and warm-rolled sheets are annealed for 1h at 523K. Grain structures are observed and their mechanical properties such as tensile strength, elongation and r value are investigated by tensile tests. Finally, the effect of warm-rolling conditions on their grain structures and mechanical properties are discussed for both as-rolled and annealed sheets.

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The bendability and deformation behavior of a magnesium alloy tube during press bending is experimentally investigated to clarify the effect of forming temperature. Bending experiments using a mandrel of wires are carried out at room temperature and at 473K. A magnesium alloy AZ31 circular tube with a 25.4mm outer diameter and a 1.5mm wall thickness is used in the experiments. The unique deformation behavior of the AZ31 tube is discussed in detail. In the bending experiments, the AZ31 tube was able to be bent with the internal mandrel at an R₀/D₀ of 2 and 473K. The bending limit of the AZ31 tube is considered to be enhanced, for the tube having a low average flow stress and a low r-value at room temperature, and a high average flow stress and a relatively low r-value at an elevated temperature in the compression range.

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In this research, we examined the rolling and processing of AZ31Mg alloy at 623 K and 723 K using various strain rates to fabricate an AZ31Mg alloy cross-slot screw. We successfully made one without any defects in appearance. The hardness of the thread of the new cross-slot screw is higher than that of the central part, and the grains of the thread are fine. The different characteristics of these two parts are attributed to the refinement of composition due to dynamic recrystallization induced during the hot rolling.

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In an attempt to accommodate a smaller-diameter roll in hot rolling, a high-speed steel roll has been developed using tool-steel material on the outer surface, since tool steel has good abrasion resistance and mechanical strength. To employ such a roll in the existing hot-rolling process, it was necessary to develop a roll manufacturing technology to ensure a uniform structure for each large-size roll, and the same time, it was necessary to carry out an analysis of the properties required of rolls for hot rolling and to develop other supporting technology. We have optimized the chemical composition and size of the carbide, and improved the matrix structure, and thereby improved the roll's resistance to abrasion, roughened surfaces, and roll failure. At the same time, we developed a good lubricant suitable for the high-speed steel roll. As a result, we have successfully developed a high-speed steel roll with good abrasion resistance and mechanical strength.

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A high-speed steel roll has been developed for use in hot-rolling employing tool steel compositions, which exhibits good abrasion resistance and good resistance against roughened surfaces. To apply the high-speed steel roll in an existing hot strip mill, it was necessary to develop supportive technologies for preventing roll failure, for accommodating thermal expansion and for stabilizing the friction coefficient. We have improved the flow-detecting technology, and established a control technology to accommodate changes in shape and friction coefficient, and also quantified the aforementioned basic characteristics. The high-speed steel roll development has enabled not only the extension of the roll life, but also the improvement of productivity and quality, and the reduction of production cost.

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