Cold model experiments were carried out to understand the behavior of bubbles and liquid flow both in the nozzle and the mold during continuous casting. Distributions of pinholes and inclusions on the surface of a slab were investigated to clarify the effect of Ar gas injection. In the cold model, bubbles were generated at the exits of holes located above the sliding gate. The mean diameter of bubbles changed in the downward and horizontal directions in the immersion nozzle due to coalescence. In the mold larger bubbles lifted up along the outer wall of the immersion nozzle through the effect of buoyancy forces acting on them, while smaller bubbles were carried in the entire mold by water issuing out of the port. The larger bubbles cause mold powder entrapment. On the other hand, the smaller bubbles are closely associated with pinholes observed in slabs.
Refining mechanism of solidified structure in which a static magnetic field and an alternating electrical current are simultaneously imposed on the local area of a metal or an alloy, has been experimentally examined using a Sn-10mass%Pb alloy. The refining period of the structure was examined by changing the imposing period of the electromagnetic vibration on the sample and it was confirmed that the refining period was the initial stage of solidification. The refining region was specified to be around the electrodes by inserting a stainless steel wire net in the sample while it was independent of the inserting position of the electrodes. Convection was induced by the electromagnetic vibration because temperature difference in the sample drastically decreased as soon as the vibration was excited in the sample. From these experimental results, the estimated mechanism in this process is that dendrite tips around the electrodes are cut off by the electromagnetic vibration in the initial stage of the solidification and it spread to the whole area of the sample by the convection induced by the electromagnetic vibration. Furthermore, nucleation is induced by an intense electromagnetic vibration.
The shape change of surface micro-defects in plate rolling was investigated using a rigid-plastic finite element analysis. Rolling of plate with rectangular-shaped scratches or plunges, which lay perpendicular to the rolling direction, was simulated. The influence of rolling conditions, such as reduction, friction coefficient, roll diameter and pass schedule on the shape of existing defect after rolling was evaluated. From these results, the relationship between the shapes of surface defect before and after rolling was obtained, and the optimum condition to eliminate defects was discussed. The proposed prediction method of surface defect can be utilized to make defect free products in rolling and forging processes.
A simple testing device for evaluating the formability of metal tubes is proposed. In this testing device a tubular specimen is subjected to internal hydraulic pressure to bulge freely with both ends sealed simply using O-rings; thus it bulges under a near uniaxial tension stress state in the circumferential direction. Formability tests using the hydraulic bulging device are performed for eight kinds of electric resistance welded tubes with different mechanical properties, thickness and manufacturing processes. The manufacturing processes of the tubes are classified into three groups: as-rolled, as-rolled and normalized and cold-drawn and normalized. It is found that the uniformity of initial thickness distribution of the tubes significantly depends on the manufacturing process and that the more uniform the thickness, the higher the formability (the limit strain in the circumferential direction) of the tube. The cold-drawn and normalized tubes exhibit anomalous bulging behavior; they do not bulge axisymmetrically and do not burst at the center of the tube. The anomalous behavior of the cold drawn and normalized tubes is successfully reproduced by a FEM analysis taking account of the inhomogeneous thickness and work hardening characteristics of the tube.
The effects of Nb and Mn on the microstructure and the toughness of simulated HAZ of 590MPa class low carbon bainitic steels were investigated. It was found that the toughness of the low carbon bainitic steel, of which hardenability is increased by adding Mn, is superior to the that increase gained by adding Nb. It was also revealed that the facets of Charpy impact surface in the Mn addition steel are more refined than those in the Nb addition steel. Therefore it was supposed that the coarse facets impaired the toughness of the Nb addition steel. The observations of the microstructures also clarified that the bainite blocks of the Mn addition steel are more refined than those of the Nb addition steel. The reason why adding Nb coarsened the bainite blocks may be explained by a decrease of the interfacial energy on austenite grain boundaries.
Size of bainite lath is sub-micron, corresponding to that of ultra-fine grained steel. Bainite has Kurdjumov-Sachs (K-S)crystal orientation relationship with austenite, in spite of the ultra-fine grained steel having no crystal orientation relationship. Parallelism of slip systems between variants was examined using stereographic analysis. The stereographic analysis clarified that the fraction of parallelism of slip systems in variants of bainite is greater than that of grains in ultra-fine grained steel without crystal orientation relationship. It could be suggested that 24 variants of bainite laths formed at random (intragranular bainite) are preferable to develop mechanical properties, for example, elongation and/or toughness.
Long-term isothermal aging behaviors up to 30000 h has been investigated in V modified 2.25Cr-1 Mo steels. When impurity elements were added, FATT in Charpy impact test vs. aging time curves showed basically of parabolic type, indicating the importance of segregation of impurities as the critical factor in controlling aging behavior. On the other hand, when the amount of impurities was small, FATT showed complex behavior with aging time. This suggests the importance of other factors in controlling aging behavior such as recovery of dislocations and enlargement of various carbide precipitates. When the degree of temper embrittlement after aging is compared in terms of the increase in FATT due to aging between the modified steels and the conventional 2.25Cr-1 Mo steel, it is found to be smaller in the modified steel at the same impurities level (J-factor). One of the reasons has been attributed to the increased Mo content in solid solution of the modified steel based on the chemical analysis of precipitates. The hardness generally increases first and then decreases with aging time. The change of hardness is more pronounced for the conventional steel. The increase in hardness is due to new precipitation and the decrease due to enlargement of precipitates and recovery of dislocation structure. Behavior of recovery was investigated with SEM-EBSP. Image quality (IQ) and grain average misorientation (GAM) were analyzed. Based on these results, it has been suggested that the more stable behavior in hardness in the modified steel can be attributed to the presence of stable MX type precipitates containing V.
Model investigation was carried out to treat wastewater including oil using a swirl motion of a cylindrical wastewater bath. A mixture of air and water was injected through a centered single-hole nozzle into a water bath covered with a top oil layer. An air-water two-phase jet was generated in the bath. The jet swirled around the vertical vessel axis together with the water in the bath under a specific condition. The basic characteristics of the swirl motion such as period, amplitude, starting time, and damping time were experimentally determined in addition to the generation condition of the swirl motion. Treatment of wastewater including oil was successfully achived using a swirl motion of an ozone-wastewater two-phase jet.
Stainless steels are corrosion resistant alloys which are widely used in consumer goods and industrial equipments. Stainless steels contain scarce and energy intensive elements such as Ni, Cr, Mo, etc. So, closed loop recycling is preferable for sustainable production of stainless steel. However, some parts of stainless steel are not separate from ordinal steel scraps in recycling processes and not recycled as "stainless steel". The objective of this study is to analyze the dynamic material flow of stainless steels in Japan and assess CO2 emissions reduction potentials by promotion of closed loop recycling of stainless steels in the future. First, the authors analyzed material balance of input elements, i.e. Fe, Ni and Cr, in the production of stainless steels in 2002 to figure out the material flow of stainless steel which can not be obtained from statistics. The amount of post-consumer stainless steel that would come into society in the future was estimated by PBM (Population Balance Model). It was found that only 2% of post-consumer stainless steel of Fe-Cr alloys was collected as stainless steel scraps and the rest was collected as ordinal steel scrap. On the other hand, approximately 95% of post-consumer stainless steel of Fe-Ni-Cr alloys was collected as stainless steel scraps. CO2 emissions reduction potential by the increase of 1% in closed loop recycling of stainless steel scraps of Fe-Cr alloys was estimated at 75000t/year by using LCA.