Using a thermobalance, industrial hematite pellets were reacted with H2-CH4 mixtures at 800950°C to produce iron carbide in the pellets. H2S of low pressures unable to form FeS was added to the mixtures. First, reduction of iron oxides proceeded and meanwhile carbidization of metallic iron took place. The addition of traces of H2S into gas promoted iron carbides (Fe3C, Orthorhombic) rather than free carbon or metallic iron as final products with nearly complete carbide conversion. The higher the temperature, the larger the carbidization rates. The sulfur index in gas around is=0.05 provided high yields of iron carbides with low sulfur content of less than 0.05 mass% S. The tests without H2S gave lower iron carbide contents with much free carbon (soot) or metallic iron. Initial carbidization rates of a reduced iron pellet coincided with a reaction rate model proposed earlier by Grabke.
A new bell-less top was installed at Chiba No. 6 blast furnace in 1998 for the advanced burden distribution control. Main features of this bell-less top are a rotating chute with stabilizer and reverse/conventional tilting function. These were developed by series investigations by use of reduced scale model experiments. Furthermore, an accurate simulator for burden distribution control was developed. An Operational trial of reverse tilting charging was implemented based on simulation by using the simulator. The controllability of burden profile and gas distribution has been confirmed during the test operation.
Experimtents were carried out in 2×103 kg converter in order to quantitatively evaluate the contributions of condition of top and bottom blowing to critical carbon content, Cp and (T. Fe) in slag. Cp decreased and (T. Fe) in slag increased as increasing the bottom stirring power and ratio of cavity depth to bath depth, L/Lo. Effect of L/Lo could not be explained by conventional indices such as ISCO, BOC and I because they did not take top blowing condition into consideration precisely. Newly defined index for carbon oxidation, ICO, was introduced to attain good correlation between operational conditions and (T. Fe) in slag. ICO= 4.2×103·(Q/W)·(L/Lo)-0.4·ε-0.33·[C]-1
A vessel geometry plays an important role in a molten metal motion caused by a quasi-sinusoidal magnetic field. The characteristics of the motion have been studied by comparing the free surface motion of a molten metal in a rectangular vessel with those in a square and a cylindrical ones. The theoretical expressions of Lorentz force and a magnetic pressure, which are induced by imposing a quasi-sinusoidal magnetic field on a molten metal in a rectangular vessel, have been derived. It has been found in all the three vessels with different geometrical shapes, a periodical oscillation motion and an irregular motion predominate the surface motions in a low and a high frequency ranges, respectively.
By using 10CrMoVNbWCoBN steel which developed for steam turbine rotor material, the effect of the boron addition on γ grain behavior at approximately the quenching temperature during the heating austenitizing temperature was investigated. During the heating austenitizing temperature, at first γ phase is formed after α/γ transformation, and then granular grains are formed by the recrystallization process through the high dislocation density as driving force. The temperature of completing the recrystallization is 1025°C in boron free steel, and is 1075°C in 90 ppm boron containing steel. It shifts to the high temperature side by containing boron. From the microstructural observation, it was confirmed that the solid solution temperature of M23C6 and (Nb, V)C shifts to 50°C high temperature side by containing the boron, therefore, the recrystallization temperature also shifts to the 50°C high temperature side. In order to increase the creep strength of the boron containing steel, it was indicated that the quenching from higher temperature was important for dissolving the precipitates. If the similar effect is obtained in the martensite, it is estimated that the addition of boron contributes to the creep strengthening by increasing high-temperature long term stability of the carbonitride of Nb and/or V.
Martensitic transformation and γ→α transformation behavior were investigated in low carbon 13% chromium stainless steels containing 2% nickel or 3% copper. The main conclusions are as follows: (1) Hardness of 2% nickel added low carbon 13% chromium steel was independent of cooling rate after hot working at large reduction. Structure of the steel was martensitic even after being subjected to such large reduction of 75%. This result suggests that ferritic transformation was hard to occur under an usual cooling rate because austenite phase was sufficiently stablized by the addition of chromium and nickel. (2) Austenite to ferrite transformation occurred only for the low carbon 13% chromium 3% copper steel without nickel even at the small cooling rate, such as 0.01K/s. This result was mainly attributed to the unstabilization of austenite phase which caused by the precipitation of ε-Cu. Furthermore, austenite of the steel becomes easy to transform to ferrite due to heavy hot working. This phenomenon was seemed to be caused by the increase in the area of austenite grain boundary owing to recrystal-lization. Thus, it was considered that the nucleation of ε-Cu at the grain boundaries promoted ferrite formation.
The nitriding process is one of the common methods for surface hardening, and consists of heat treatment in a furnace for many hours. The nitriding behavior and strengthening mechanism of Ti added steels in the nitriding process, which is applicable to a high temperature and rapid process such as the continuos annealing of steel strip, were investigated. Cold rolled Ti added steel sheets were annealed for recrystallization and nitriding in electric furnaces. Then the hardness distributions in the cross section were measured. The sheets were hardened only near the surface. The maximum hardness depended on the Ti content, and the thickness of hardened layer depended on the nitriding time and the flow rate of NH3. The optimum temperature was 750°C in this experiment. Observation by means of electron microscopy showed contrasts due to fine particles with a size of 1 to 2 nm which were considered to be Ti nitrides or Ti-N clusters. These contrasts were observed only near the surface. This suggests that the hardening is caused by the small particles. The diffusion model of N that considered the precipitation of TiN was used for the simulation of the nitriding behavior. The result showed that N entering into steel immediately precipitates as TiN, then supersaturated N diffuses to the inside. The simulation result agrees with the experiment. The estimation of the amount of strengthening was carried out. It indicated that the strengthening mechanism is mainly the precipitation hardening of TiN that could be Ti nitrides or Ti-N clusters.
The influences of the applied stress and the stress concentration factor (Kt) on hydrogen embrittlement were investigated in the commercial JIS SCM440 steel with the tensile strength of 1403 MPa. The applied stress was changed from 0.33 times to 0.72 times the tensile strength of the notched specimen, while the Kt was altered from 2.1 to 6.9. The hydrogen embrittlement property was evaluated with the resistance to the diffusible hydrogen in the steel. The diffusible hydrogen concentrations were analyzed by thermal desorption analysis. The results are as follows. (1) The hydrogen embrittlement occurs under the lower diffusible hydrogen concentrations with increasing Kt. However, the HD-t (diffusible hydrogen concentration-time to failure) curves become almost the same when the Kt and the applied stress are high. (2) The criterion of the hydrogen embrittlement cannot be determined by the combination of the maximum local axial stress in the specimen and the maximum diffusible hydrogen concentration locally accumulated in the specimen. In the discussion part, it was proposed that it is very important to consider the stress distribution in the specimen for the evaluation of hydrogen embrittlement.
An attempt has been made to develop new alloys with high thermal fatigue resistance and interpret the thermal fatigue behavior based on the relation between plastic strain range and related elongation in tension test at elevated temperatures. The thermal stresses that lead to thermal fatigue failures are generally induced by rapid change in temperature due to start and stop of plants, so that the total number of cycles involved is relatively small. Resistance to thermal fatigue depends on the material properties such as a coefficient of thermal expansion, heat conductivity and resistance to the strains incidental to temperature change. At present work, three types of modified NCF625 alloys were candidated to control the material properties by changing the contents of elements such as aluminum, titanium and iron, and were cast as ingots for the test. Thermal fatigue behaviors of examined alloys were evaluated at elevated temperatures between 773K and 1073K, and between 773K and 1073K. As a result, it was derived that the modified NCF625 alloy containing 1% titanium, 1% aluminum, and 1% iron showed the highest thermal fatigue strength. It was found, form the thermal fatigue tests, that this alloy had thermal fatigue resistance about two times that of NCF625. The alloy thus selected was gas atomized to powder and PTA welded on a carbon steel plate, and it was confirmed that there was no problem found on the metallurgical structure and hardness distribution.
In order to clarify the difference of iron loss of single sheet and toroidal core and domain structure of TiN-coated very thin silicon steel sheets, TiN coating by the hollow cathode discharge (HCD) method was performed on polished very thin (0.10 mm) silicon steel sheets. Iron loss of single sheets and toroidal cores after domain refinig was measured. Also, domain structures of (011)  single crystals of silicon steel, which were chemical polished, TiN-coated and then domain refined by scribing with a knife at 4 mm widths perpendicular to the rolling direction, were observed by electron microscopy under (a) straight, (b) convex and (c) concave states. 0.10 mm TiN-coated very thin silicon steel showed ultra-low iron loss of W13/50=0.12 W/kg with a single sheet measurement after the domain refining, whreas similar iron loss with a toroidal core showed minimun value of W13/50=0.12 W/kg after 700°C annealing, being good value of building factor (B. F.) of 1.0. The domain structures with (a) straight state obtained by TiN coating and inducement of a small local strain by scribing produced a drastic domain refinement, whereas that with (b) convex and (c) concave states showed further domain refinement. Also, the domain structure of (c) concave state could be observed after annealing above 873K.