Showing papers in "Tetsu To Hagane-journal of The Iron and Steel Institute of Japan in 1991"

Structure and Corrosion Resistance of Zinc Alloy Coated Steel Sheets Obtained by Continuous Vapor Deposition Apparatus

Abstract: Synopsis : New zinc alloy coated steel sheets have been developed by continuous vapor deposition apparatus. In a deposition chamber, zinc and alloying metal were evaporated individually by Electron-Beam (EB) irradiation, and were deposited on steel strip continuously. Some properties of Zn-Al, Zn-Cr, Zn-Mg, Zn-Ni and Zn-Ti alloy coated steel sheets were investigated. Crystal orientation of these coatings was affected by alloying metals. Depth profiles of alloy composition in Zn-Al and Zn-Mg coatings are almost uniform. The other coatings had double-layered structure, composed of an intermetallic compound layer and a zinc rich layer. The intermetallic compound phases were not the same as those shown in a equilibrium phase diagram for binary alloys. Corrosion resistance of Zn-Mg alloy coated steel without painting is excellent in salt spray test. Corrosion product layer of this coating consists of only ZnCl2•E4Zn (OH)2 without ZnO and c -axis of hcp structure of ZnCl2•E4Zn(OH)2 was parallel to the direction of the film growth. Mg ion dissolved from the coating during corrosion seemed to play a role to form this corrosion product layer.

Effects of Nitrogen Content and Tempering Treatment on Creep Rupture Properties of 9Cr-1Mo-V-Nb Steel with Simulated HAZ Heating

Abstract: Synopsis : Creep rupture properties of 9Cr-1Mo-V-Nb steel welded joints change with chemical compositions and heat treatments. Therefore, simulated HAZ heating below or over Act temperature of 830•Ž was conducted in order to investigate microstructural changes of welded joints of the steel. Effects of nitrogen contents and tempering treatments on hardness and creep rupture properties were discussed. The results are summarized as follows. ( 1 ) Hardness and creep rupture strength of the steel heated at 850•Ž ( simulated HAZ heating) decreased significantly after post-weld heating at 740•Ž for 5 h. It was confirmed that recovery and softening of martensitic structure were accelerated by partially ƒÁ-transformed heating just over Ac1 temperature. (2) On the above simulated HAZ heating, finely dispersed Vanadium Nitride ( VN) , which increased in amount by addition of nitrogen, was observed. These precipitates were confirmed to retard recovery, and to improve creep rupture strength effectively. (3) In case of relatively low temperature tempering at 620•Ž, excess free carbon and nitrogen precipitated as fine VN and M23C6 during simulated HAZ heating and post-weld heat treatment. These precipitates were confirmed to improve creep rupture strength.

High Temperature Oxidation of Fe-Si Alloys in Ar-H2O Atmospheres

Abstract: Synopsis : The oxidation of Fe(1 .5, 3%) Si alloys at 1 000-1 400 K has been investigated in Ar(1, 10%)H2O atmosphere using metallographic and electron probe microanalysis techniques. The scale layers developed consisted in general of the external, the internal and the subscale. Oxides phases in each scale layer were identified as the mixtures of FeO, Fe2O3 , and small amount of Fe3O4 the mixtures of FeO and Fe2SiO4 , and the mixtures of Fe2SiO4 and SiO2 respectively. With increase in Si content in the alloys the rate of the total oxidation decreased significantly at 1 2001 300 K. The oxidation kinetics of Fe-3%Si alloy and Fe-1 .5%Si alloy were irregular and regular with each temperature, respectively. The latter was obeyed a parabolic rate law. The apparent activation energies for the growths of the internal scale and the subscale of Fe-1 .5%Si alloy were estimated to be 123-132 kJ/mol and 119-156 kJ/mol, respectively. The rate determining diffusion element for these scale growths was considered to be Fe in FeO in the internal scale and /or in the external scale.

Fundamental Properties of Long-term Creep Strength for Ferritic Heat Resistant Steels

Abstract: Synopsis : In view of inverse sigmoidal form of stress vs. time to rupture curve, long-term creep strength of ferritic heat resistant steels has been investigated using a large number of long-term creep data in the NRIM Creep Data Sheets. It was observed that the inverse sigmoidal form of stress vs. time to rupture curve was caused not by the change in mechanisms of strengthening or deformation, but by the decrease in creep strength and by the approach to steady internal stress due to microstructural change. At lower stresses, the complex form of creep rate vs. time curve which had two minima in creep rate was found and attributed to the inverse sigmoidal form of stress vs. time to rupture curve. Comparing the creep rupture strength for the different kind of steels, the large difference in creep rupture life of about four orders of magnitudes was found at lower temperatures and higher stresses. At higher temperatures and lower stresses, however, the creep rupture strength of all ferritic steels investigated was almost the same. It was concluded that the creep strength of the ferritic heat resistant steels was reduced by the microstructural change and approached to the inherent values which were nearly the same for all ferritic steels investigated.

Temperature Measurement during Rapid Solidification of 18Cr-8Ni Stainless Steel and Its Initial Solidification Structure

Abstract: Synopsis: A new measurement system using a silicon photo-diode was developed to investigate the precise thermal history during the initial solidification of stainless steel. From the measured cooling curve, it was confirmed there existed large undercooling before nucleation. Maximum undercooling obtained was 260 K. The undercooling increased with the cooling rate of a sample and has a strong linear dependence on the cooling rate. In the vicinity of the sample surface, the initial solidification layer was formed, which had cellular structure of metastable austenitic phase. Effect of undercooling on the micro and macro structure was also investigated quantitatively.

Formation Process of Colored Films on SUS304 Stainless Steel with the Square Wave Potential Pulse Method

Abstract: Synopsis : A thick film with interference color could be obtained on the surface of SUS304 stainless steel with the square wave potential pulse method in 5 kmol •E m-3 H2SO4 solution of 323 K with no addition of chromium ions. The higher potential, EH, and the lower potential, EL, of the square wave potential pulse were controlled in the range between the active peak potential and the transpassive potential of chromium. A. thick colored film was obtained at the condition that EL is kept in the transient region from the active peak to the passive range of chromium and EH in the passive range of iron and chromium. The deposited hydroxide film including Fe2+ and Cr3 + formed at EL losses protons and grows under the high field provided by EH. The colored film thus formed has many diffusion paths, which permit the further growth of the film without any decrease in the growth rate. Therefore, the thickness of the film, i. e. the color, could be easily controlled by the polarization time and / or potential.

Fluid Flow in Ladle and Its Effect on Decarburization Rate in RH Degasser

Abstract: Theoretical and experimental studies were carried out to quantitatively understand the effect of fluid flow on the decarburization reaction rate in an RH degasser. Hydrodynamic calculations on fluid flow in the ladle were proved reasonable by a cold model experiment. The decarburization model in conjunction with the fluid flow by hydrodynamic calculations in the ladle made it clear that the decarburization rate obtained from this model was faster than that for perfectly mixed flow, and slower than that for plug flow when the same parameters were used for the calculations. The decarburization model enabled us to predict the concentration distribution of carbon in the ladle during an RH treatment. The maximum carbon content, which was found in the recirculating zone in the ladle, reached about twice the minimum value obtained in the region just below the down-leg. No dead zone existed which would disturb the decarburization rate in the process, and it was also made clear that the shape of ladle had little effect on the decarburization rate.

Effect of Coating Structure on Powdering Resistance of Galvannealed Steel Sheet

Abstract: Synopsis : Effect of coating structure on powdering resistance of galvannealed steel sheet was investigated and the followings are clarified. 1)Higher galvannealing temperature deteriorates powdering resistance. But slower cooling from higher temperature (500•`550•Ž) improves powdering resistance. 2)In the case of lower galvannealing temperature (= 440•Ž) , the powdering resistance is rather good, although the cooling rate doesn't affect powdering resistance. 3)Exfoliation of galvannealed steel sheet after tensile test correlates with amount of powdering evaluated from draw bead test. 4)T1-phase (Fe5Zn21) is formed along the interface of coating galvannealed at lower temperature (•...500•Ž) or cooled slowly from high galvannealing temperature. 5)The decrease of T1 ratio [ T1 / (T + T1 ) ] deteriorates powdering resistance.