To understand the generation mechanism of surface defects appeared on the final sheet product, the defects found in cast production slabs were investigated. Defects located in a cast production slab of ultra-low carbon steel were detected by using an ultrasonic test. After that, using laboratory mill, hot rolling, cold rolling and galvanized coating were applied to the samples to simulate actual production conditions. And cross section of the sample was observed at the location corresponding to the original location of defects in the cast slab. Some samples showed similar defect shapes like defects of scabs in the steel sheet. And foreign materials which consist of Al, Fe, and O were observed at the location of the scab-like defects. Various defects are shown by the diameter and depth from the surface to the top of defects in cast slabs with their reproduction. It is clearly divided by a boundary related to the diameter and depth into two regions. Based on the information on scabs observed in steel sheet product, the size and depth of defects from the surface existed in the cast slab were estimated. Boundary plotted by the diameter and depth of the defects estimated from the surface defects of sheet products also shows the similar tendency of the threshold of possibilities of the generation of defects in the reproduction experiment.
Nondestructive ultrasound imaging system is the most popular industrial evaluation technique for various bonding interfaces, such as a composite materials and IC products and so on. This method is widely known as a reliable evaluation technique even for the strength of the bonding. However recently, some reports pointed out that the conventional ultrasound imaging system may miss detect the special bonding interface so-called kissing bond which the bonding interfaces are contact but no joining. In this study, we would like to mix the subharmonic ultrasound mainly investigated as the advanced crack evaluation technique to the ultrasonic imaging system and to develop the new nonlinear ultrasound image system. The new imaging equipment in which all the RF waveforms were digitized for ultrasonic advanced imaging and the new designed high aspheric surface acoustic lens to obtain higher amplitude ultrasound were combined. Furthermore, the wavelet analysis for all the RF waveform could be used to obtain any frequency image. Thus both the fundamental frequency ultrasonic imaging and the subharmonic ultrasonic imaging could be obtained using the developed ultrasonic imaging system. As a sample of the bonding interface, industrial diffusion bonding structure was used and the diffusion bonding was controlled by nm order in thickness including the difficult bonding interface to detect by the conventional system, and the availability of the newly developed ultrasonic measurement system was investigated.
The influence of coke breeze positioning on the sintering behavior of pellets and raw material bed with embedded pellets was investigated. With increasing the coke breeze ratio in the green pellet made from ore and coke mixture fine of −125μm, the tensile strength of the green and dried pellets decreased. Based on the evaluations under the green and dried pellet equations concerning particle bond, it was presumed these results were caused by the decrease in the contact points between particles due to the reduction of mixture fine’s specific surface area. With increasing the coke breeze ratio in the nucleus pellet, the tensile strength of the fired pellets also decreased. From the change in the porosity up to 900°C, sintering was predominant to the tensile strength. In the temperature from 900°C to 1200°C, the increase of strength became small by adding coke breeze in the nucleus pellet. It was presumed this result was caused by remarkable increase of void after coke combustion. In the sintering of raw material bed with embedded the coke breeze coating pellet, the surrounding temperature of pellet increased and the large void was generated under the pellet. Based on the 2-dimensional numeric evaluation, it was found the heat amounts of the pellet and gas heated by coke combustion were transported to the area under the pellet where the gas flow rate was low. It was presumed this heat concentration caused the remarkable melting and large void formation.
In order to understand the behavior of scale deformation on hot-rolling, the characteristic of not only scale itself but also the interface between scale and material is important. For this reason, the following experimental methods were developed for measuring the quantitative strength of the interface. At first, after generating scale on the two pillar specimens at high temperature, the scale/scale interface between these was joined by pressurizing at high temperature. Next, by the tensile test of the joined specimen, the force and the stroke curve were obtained. In this experiment, there are some peaks on the force and the stroke curve by exfoliations of scale/scale and scale/material. To estimate the exfoliation stress, the exfoliated area of each interface was evaluated from the curve. By using this technique, the influence that a condition of the pressure or the interface roughness gave to exfoliation stress was studied. As a result, it was found that the exfoliation stress between a scale/material was affected by the joining stress. In addition, it was found that exfoliation stress becomes greater, with the rougher interface surface between a scale/material.
Effect of Mg and Ti addition on the solidification macrostructure of ferritic stainless steel was evaluated in order to obtain the fine equiaxed grains. Equiaxed solidification was promoted by adding Mg and Ti. Ti addition affects the amount of TiN as a nucleation agent of δ-Fe and the increase in constitutional undercooling. Mg addition produces Spinel oxide in the molten steel, which accelerate the TiN formation. [Al] content also have an influence on the equiaxed grain formation.
The effects of Cr and Mo on microstructure, retained austenite characteristics and tensile properties of a 0.2%C, 1.5%Si, 1.5%Mn, 0.05%Nb TRIP-aided steel with annealed martensite matrix, “TRIP-aided annealed martensitic steel; TAM steel”, were investigated to enhance the hardenability. Volume fraction and carbon concentration of retained austenite were decreased with increasing hardenability, although the volume fraction was increased in a 0.5% Cr bearing steel. Simultaneously, blocky second phase consisting of carbon-enriched martensite and retained austenite increased with increasing hardenability. These interesting characteristics were mainly caused by preferential nucleation of austenite on prior austenitic grain and packet boundaries on intercritical annealing and delayed bainitic transformation during austempering. Tensile strength of the steels was increased with increasing hardenability, accompanied with the decreased yield stress. Combination of tensile strength and total elongation decreased with increasing hardenability due to the increased blocky second phase.
In Japan, 15 million tons of steelmaking slag is produced annually as a by-product of the steelmaking process. It is known that steelmaking slag has specific properties to decrease hydrogen sulfide in seawater. To date, however, the specific mechanism is still unknown. This is the first report that explains the process of the slag reducing the hydrogen sulfide level. We added slag to the artificial seawater containing sulfide ion and obtained fine particles. We analyzed the reaction products with X-ray absorption fine structure (XAFS) and scanning electron microscopy-energy dispersive X-ray detector (SEM-EDX) and confirmed corresponding peaks of FeS, FeS2, sulfur and sulfate. We also measured the oxidation-reduction potential (ORP) and found that a shift to oxidizing atmospHere was promoted by adding slag, which oxidized sulfide ion to sulfuric acid ion. These data suggested that hydrogen sulfide level in seawater is reduced not only by the reaction of sulfide with iron contained in steelmaking slag but also by oxidation of sulfide in seawater with steelmaking slag. These results imply that effective utilization of steelmaking slag in coastal areas can significantly improve the surrounding marine environment.