SynopsisA method of mathematically analyzing interdendritic microsegregation was established using finite difference method and taking into consideration the diffusion of the solute in the solid and liquid phases. The cross-sectional shape of dendrites and the fact that the enrichment of the solute in the liquid phase at the solid-liquid interface restrains the advancement speed of the solid-liquid interface were considered. Directional solidification tests to examine interdendritic segregation were made to verify the mathematical analysis method established. The advantages of the new method over other methods were discussed. Then, spot-like segregation were mathematically analyzed applying the same method, and the results were in good agreement with the observations in continuously-cast slabs.

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A new reiation among the time to creep rupture, the stress and the temperature has been developed.

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Precipitation behavior of VN during isothermal tempering at 740-800°C of 7%Cr-0.4%V-0.09%N steel (% denotes mass%, hereinafter) has been studied. Initially, rapid softening takes place accompanied by the precipitation of VN and, after that, the quasi-steady state in a hardness vs. tempering time diagram is continuing for a while. After the quasi-steady state, re-dissolution of VN particles rapidly occurs followed by final precipitation of VN. Just before the peak time of the re-dissolution of VN particles, both the temporal decrease in hardness and the temporal increase in the integral breadth of an X-ray diffraction peak take place. The similar precipitation phenomenon is confirmed in 0.14%C-9%Cr-1%Mo-0. 2%V-0.09%Nb steel. In both steels the re-dissolution of VN or NbC (hereinafter MX) accompanies the decomposition of martensite. The following reactions are suggested as a mechanism for the re-dissolution of MX type particles: local stresses induced by the recovery of martensite unlock the pinning of dislocations by the MX type particles and the consequent isolated particles, which become energetically unstable, are re-dissolving into the matrix.KEY WORDS: precipitation; hardness; X-ray integral breadth; Nb; VN.VN system as observed in the NbC system. And, therefore, the precipitation behavior of NbC during only tempering has been also re-investigated. Experimental ProcedureA 50 kg ingot was hot-rolled to 12 mm plate. The chemical composition of the experimental steel is 0.0032%C-0.15%Si-0.51%Mn-0.002%P-0.001%S-7.2%Cr-0.3%Mo -0.41%V-0.091%N and the balance is Fe. The Nb content is less than 0.0005 %. The hot rolled plate was normalized at 1 200°C followed by tempering at 740, 780 and 800°C for maximum 500 h. It was confirmed that neither any precipitate composed of V nor delta ferrite was observed after normalizing at 1 200°C. 13)A heat-treated specimen was dissolved in 6 N-HCl solution and the residues were separated using a filter with 0.2 mm pores. The dissolved V in the filtered solution is directly measured using an inductively coupled plasma atomic emission spectroscope (ICP). The detailed experimental procedure and the accuracy of the analyzing method are reported in the previous paper.13) Vickers hardness number was determined under 10 kg load and optical microstructure and scanning electron microscopy were observed after etching using the Vilella's reagent. Integral breadth of a ͕110͖ diffraction peak was also measured using Cu Ka 1 characteristic X-ray. An extracted carbon replica was observed using a transmission electron microscope of a field emission type (FE-TEM) operating at 200 kV and the chemical compositions of some particles were analyzed using an energy dispersive spectroscope (EDS). Experimental Results Dissolved VThe measured values of the dissolved V in the matrix of the tempered state are shown in Fig. 1. The dissolved V decreases with increasing tempering time up to about 10 h and the decreasing rate is fast when tempering temperature is high. This decrease corresponds to the precipitati...

Dissolved Nb in both austetinic and ferritic phases of 9%Cr1%Mo0.2%VNb steel was measured using an inductively coupled plasma atomic emission spectroscope (ICP), and the microstructure has been characterized using a field emission scanning electron microscope of a ultra high resolution type and an analytical electron microscope. The dissolved Nb in austenitic phase measured by ICP is in agreement with that predicted by a phase equilibrium calculation system. However, the measured values of dissolved Nb in the specimens which are heat-treated for 2 h just below A C1 are much higher than the calculated values. It is newly confirmed by both chemical and physical analyses that the fine NbC particles which are preformed at 950°C and stable at this temperature are on one occasion re-dissolved into the matrix by the subsequent heat treatment at 800°C for 2 h. This fact suggests that high temperature tempering is recommended in order to improve creep resistance of high Cr heat resisting steels which contain strong carbo-nitride formers.

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