Synopsis Mold powder technology for continuous casting of aluminum-killed steel was developed to produce the clean and crack free steel slabs. The following items were investigated: 1) Mass balance of A120~ in the molten powder pool. 2) A1203 absorption capacity o f the molten powder. 3) Uniformization of heat removal through the mold during continuous casting. 4) Change in powder viscosity with absorption of A1203. 5) Design of optimum powder viscosity by consideration of A1203 absorption during continuous casting. The results obtained were as follows: (1) The A1203 content of the molten powder pool increases with the elapse of casting time in accordance with the A1203 absorption capacity of the powder, but after some time, reaches a constant value that matches the A1203 absorption capacity of the powder. (2) The A1203 absorption rate of the molten powder increases as the basicity index B; of the powder increases. (3) The viscosity of the molten powder generally increases with increasing A1203 content. (4) To uniformize heat removal through the mold during casting, it is effective to control the parameter iV over the range of 1.0 to 3.5, where 2 is the viscosity of the molten powder in poise at 1300 °C and V is the casting speed of the machine in m/min. (5) A method for designing the optimum powder viscosity that allows for the absorption of A1203 during casting has been established by the combination of the results mentioned above. (6) Powders designed by the above method have been applied to continuous slab casters to prevent the longitudinal surface cracking of mediumcarbon aluminum-killed steel slabs and to realize the hot direct rolling of unconditioned slabs.

Brazing of Al to Cu using Al-Si-Mg-Bi brazing alloy has been carried out in a vacuum furnace. In the brazed interlayer, there were two kinds of intermetallic compounds. One of these intermetallic compounds was θ phase and the other was δ(Cu3Al2). Tensile strength of the joint was only about 15MPa. Deformation behavior of Al/Cu brazing joint was brittle without deformation of the base metal. The specimen was fractured in the intermetallic compound which was mainly θ phase. In order to improve the tensile strength of Al/Cu dissimilar joint, Cu cladding Ag (thickness: 0.1mm) substituted for Cu. As the result, tensile strength of the joint was about 70MPa and the specimen was fractured in an Al base metal. In this joint, plate-like intermetallic compound, δ (Ag2Al) was formed in the brazed interlayer. The shape of δ(Ag2Al) was quite different from θ phase found in Al/Cu joint. It was considered that the shape of reaction layer remarkably affected to the strength of the Al/Ag-Cu clad dissimilar joint.

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Electron beam welding melts and solidifies steel plate without using any welding material, unlike the conventional welding. Therefore, the toughness at the weld metal can decrease, depending on the chemical composition of the steel plate. Toughness at the electron beam weld can be increased by turning the microstructure from upper bainite into lower bainite and making the effective grain size finer. The microstructure can be controlled by the addition of alloy elements and optimization of impurity elements. In case the chemical compositions cannot be varied, largely because of the specification for their ranges, and the weld metal microstructure remains as upper bainite even after the application of microstructure control, methods to improve the toughness of electron beam weld itself, regardless of steel grades, becomes necessary. Phenomena peculiar to the electron beam weld are segregation during solidification and intergranular segregation over the dendrite surface. The fracture initiation is accelerated by the microcracks caused by the segregations during solidification. The fracture propagation is promoted by intergranular cracking caused by the intergranular segregation. By reducing these segregations, the fracture initiation and propagation are restrained and toughness increases despite the upper bainite microstructure. This can be achieved by the higher purification of steel. Through the foregoing investigations, ASTM A533 Type B Class 2 steel plate of 100 mm in thickness for electron beam welds has been developed for pressure vessels. Various welding tests as pressure vessels have been conducted, and it becomes clear that the developed steel plate has excellent toughness at the weld superior to those obtainable by conventional welding. The use of this steel greatly reduces the welding period compared to the conventional welding method.

Installation and replacement of many PWR-steam generators are planned inside and outside Japan. The steel plates for steam generators are heavy in thickness, and increase the number of welding passes and prolong the welding time. Electron beam welding (EBW) can greatly reduce the welding period compared with conventional welding methods (narrow-gap gas metal arc welding (GMAW) and submerged arc welding (SAW)). The problems in applying EBW are t o prevent weld defects and to improve the toughness of the weld metal. Defect-free welding procedures were successfully established even in thick steel plates. The factors that deteriorate weld-metal (WM) toughness of EBW were investigated. The manufacturing process, which utilizes a new secondary refining process at steelmaking and a high-torque mill at plate mill in actual massproduction, were established. EBW base metal and WM have better properties including fracture toughness than those of conventional welding processes. As a result, an application of EBW to the fabrication of PWRsteam generators has become possible. Large amounts of ASTM A533 Gr B C12 (JIS SQV2B) steel plates in actual PWR-steam generators have come to be produced (more than 1,500 ton) by appling EBW.