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Thermo-Mechanical Control Process (TMCP) has been well known to improve both weldability and toughness of steel plates. This is because TMCP enables to decrease carbon content for obtaining target strength and to fine grain and/or packet size. Here in penstock application, welding environment is so sever due to high humidity and less room for proceeding in tunnel. And reliability of construction is very important because it would be fatal if accident occurs. So it is considered to be profitable to apply TMCP to steel plates for penstock. At first TMCP has been developed for 500MPa class tensile strength steel. But nowadays application of TMCP has spread to higher tensile strength steel up to HT950 even in penstock. TMCP is classified into several types of processing such as Thermo-Mechanical Rolling (TMR), accelerated cooling (Acc) process, and Direct Quench followed by temper (DQ-T) after ASTM A841. Microscopic point of view acicular ferrite and/or bainite is utilized for 600 MPa and below class tensile strength steel, and bainite/martensite for 700 MPa and above class. Scale of penstock is defined by water head (H) times diameter (D). And HxD has been increased year by year. This tendency resulted in demand of higher strength steel because of depressing of increasing plate thickness required. Thicker plates cause more fabrication costs, transportation costs and fabrication period. This is why higher strength steel, which can reduce the plate thickness, has been developed by applying TMCP to penstock construction. In addition, thickness of sickle plate (inner reinforcement) for bifurcation should be thicker in larger scale of penstock. This sickle plate is required high quality because it receives high water pressure. So manufacturing sickle plate requires combination of high technology on continuous casting, plate rolling, heat treatment and so on. Nippon Steel & Sumitomo Metal Corporation (NSSMC) has many supply record not only for penstock but also bifurcation plate including sickle.