SynopsisIn order to produce stainless steel with low phosphorous content below 0.01 %, experiments were carried out to find optimum conditions for the dephosphorization by metallic Ca and CaC2. The results are summarized as follows :(1) Dephosphorization by metallic Ca should be done at temperatures below the boiling point of Ca and the composition of the steel has to be determined not to precipitate CaC2.(2) On the other hand, dephosphorization by CaC2 is effective at high temperatures, where the composition of the steel has to be selected to keep the activity of carbon in the range 0.020.3(at 1 600 °C).(3) Based upon the experiments, a new process of mixing two melts, one of which is Cr free steel dephosphorized by conventional oxidizing method and the other is Cr containing steel dephosphorized by Ca or CaC2, has been developed to attain P content less than 0.01 %. I. IntroductionThough low phosphorous content of stainless steel below 0.01 % is known to be effective to stress corrosion cracking,l~ weld hot cracking2~ and so on, it is difficult to decrease the content by conventional oxidizing refining because of preferential oxidation of chromium. Therefore, special dephosphorization methods have been studied and these can be classified into following two methods. One is soft oxidizing dephosphorization by the slag containing Li2CO3,3~ Na2C034> or Ba05~ as principal elements which are more basic than Ca0 and have low melting point, and the other is reducing dephosphorization by CaCaF26~ and CaC2-CaF2.7~ Each of them, however, has demerits. The former needs a high flux ratio and can only achieve high degree of dephosphorization in high chromium steels below 20 % Cr. On the other hand, the latter requires high cost and leads to low productivity because of the necessity to use of ESR in the case of Ca-CaF2, and shows low efficiency of the reaction due to the errosion of refractories in the case of CaC2-CaF2.In the course of the experimental study to settle these demerits, it was made clear that reducing dephosphorization could be effectively achieved by adding metallic Ca at lower temperatures below the boiling point and sole CaC2 at higher temperatures to the molten high chromium steel. However, it was found to be difficult to reduce phosphorous content industrially below 0.01 % by these methods in case of 18 % Cr steel. Then a new process was developed to achieve low phosphorous level, where two melts were mixed. One was chromium free melt dephosphorized by conventional oxidizing method and the other was
In order to reduce the susceptibility to embrittlement of steels such as a rotorshaft for power plant, which are to be used at high temperatures, experiments were carried out on the optimum conditions for the removal of tramp elements like Sn, Sb and As by the addition of CaC2. Industrial scale tests were alsocarried out, based upon the experimental results obtained. The results obtained can be summarized as follows.(1) These impurities can be removed by reduction, and the reduction was accelerated by the rapid decomposition of CaC2 at low initial carbon content and high temperatures.(2) The amount of impurities removed was found to be proportional to the increase in carbon content during reaction and the initial impurity content. The proportional constant increased in the order of Sn, P, Sb and As.(3) It was found from the industrial scale tests that the impurities could be favorably removed by using Ar stirring from the bottom of the ladle and by using CaC2 injection increasing the reaction surface area.
The Mitsubishi Group and Japanese PWR utilities have been engaged in a continual process of main control board design improvement. Advanced main control boards with fully digital instrumentation and touch operation technology have been adopted in newly constructed plants. Moreover, instrumentation and control systems have been fully digitalized and applied not only to non-safety systems but to safety systems as well. Such a system in general is referred to as a fully digital system. From this time forward, the application of this fully digital system will become mainstream with the modernization of existing plants. Yet, unlike newly constructed plants, restrictions unique to existing plants will need to be taken into consideration. In the construction of Ikata Units 1 and 2 of Shikoku electric power co., measures such as those described below were implemented so as to achieve standardization with future construction in mind: (1) A thorough interface check was conducted to ensure compatibility with sections outside the scope of modernization. Additionally, to minimize the affect modernization had on the operating system, the system was verified by operators and verification results were reflected in the board layout, system specifications, and the like. (2) Due to limited space for board installation, a plan was devised for housing a processor and determining the remote input/output board location. (3) Onsite tests was minimized in order to complete construction within a short period of time, i.e., within the regularly scheduled inspection period, and the degree of completion of the product was enhanced prior to shipment by a full combination test conducted at the plant. In addition, the local construction period was shortened by devising a plan for installing the board and conducting interface tests during the previous regularly scheduled inspection and the current regularly scheduled inspection.
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