Macroscopic inclusions were intentionally produced by promoting air-oxidation of steel. Relation of macroscopic inclusions to cast structure and effect of the condition of solidification of steel on the occurrence tendency of macroscopic inclusions were studied. Consequently the following results were obtained. 1) Macroscopic inclusions produced by air-oxidation of steel were disposed preferentially inside interdendritic spaces, andthe occurrence tendency ofthem was influenced by the condition of solidification of steel. Therefore, macroscopic inclusions produced by air-oxidation of steel were nothing but secondary deoxidation products of oxygen (oxide) dissolved by air-oxidation having been precipitated during solidification of steel. 2) The growth mechanism of macroscopic inclusions which are very large in comparison with microscopic inclusions is thought to be as follows; When the segregation of oxygen which occurred in melt not by phase change of liquid steel to solid and the enrichment of oxygen to interdendritic spaces by phase change happened together overlapping, a great amount of oxygen capable of growing to macroscopic inclusions is enriched and precipitated in the form of macroscopic inclusion. In order to show that the foregoing presumption is probable interpretation for the growth mechanism of macroscopic inclusions, the phenomena in which oxygen or tin had already segregated in melt not by cell formation were shown. 3) It was shown that macroscopic inclusions produced by air-oxidation contained calcium. From the result, it is concluded that some calcium can dissolve in molten steel, and that exogenous oxide particles may be floatedout by simple mechanism according to Stokes' law qualitatively, and thatthere are, accordingly, only rare macroscopic inclusions in melt, and that exogenous oxide particles develop seldom into macroscopic inclusions.
The first section of this paper deals with the three dimensional observation on the macrostructure of string ghosts in steel ingot. The characteristic features of string ghost were examined by applying knowledge gained from previous studies upon the gravity segregation of Al-Cu alloy. The following conclusions were obtained: 1) The string ghost was observed to be formed in the transitional solidification zone, at which stage some amounts of dendritic crystals grow. The dendritic crystals in steel ingot were found to be plate-like. 2) The solute enriched liquid located at a relatively large spacing of dendrite arms was considered possibly to move upward due to the gravitational force and to remelt dendritic crystals in process of flow. Therefore, the ghost with stringlike configuration was formed in the transitional solidification zone. 3) Direction of the flow of enriched liquid through the dendrite arms was considered to be inclined towards the center of the ingot because of smaller resistance to its movement. In tfte second section attempts are made to reproduce the stringlike segregation line by model experiments of Al-Cu alloy. Consequently, it was found that the stringlike segregation line could be formed artificially or naturally in the transitional solidification zone of Al-Cu alloy, and thus the above-mentioned conclusions were verified.
Temperature of a bottom working roll of the first stand of a hot strip finishing mill during rolling was measured by means of thermocouples embedded in the surface layer of the roll. Instantaneous temperature change was detected and led to the recorder through slip rings. Maximum temperature rise measured was about 100 degrees C at about 1 mm from surface. Characteristics of periodical change in temperature in various zones in the roll were examined. In the zone II, 0.5 to 5mm from surface, temperature depended solely on the angle of rotation. In the zone II, 0.5 to 5mm from surface, temperature depended both on angle of rotation and on the number of rotations during one rolling. In the zone III, 5 to 30mm from it, temperature depended both on number of rotations and on the number of rollings. In the zone IV, 30mm to center, temperature depended solely on the number of rollings. Temperature at center approached a constant value after about 30 rollings. Dependence of temperature rise on rolling variables was examined. Amount of cooling water was found the most critical in determining the temperature in the roll and the reduction rate of plate was the second most so, while the interval of rolling was even less critical.
Individual and combined effects of additional elements Mo, Nb, Ti, N and B on creep rupture strength, deformation resistance, aging hardness and microstructure of 0.2C-18Cr-12Ni austenitic heat resisting steel were investigated, and the following results were obtained. 1) Creep rupture strength increased greatly by the additions of Mo, B and increased slightly by those of Ti, Nb in that order. The effect of N was little at 700°C but increased at 750°C. Moreover, there were two effective interactions on creep rupture strength under the simultaneous additions of Mo and B which suppressed the precipitation within the grains and at grain boundaries, and stabilized the structure, and under those of Mo and N which accelerated the fine and uniform precipitation within the grain sand gave good characteristics of age-hardening. Also, there was a useless interaction under the simultaneous additions of B and N. 2) Deformation resistance at high temperature was increased greatly by the addition of Mo, B and decreased slightly by Ti but was scarcely influenced by Nb. The addition of N increased the deformation resistance at 900°C, but scarcely influenced it at over that temperature. Especially, at 900°C under the simultaneous addition of Mo and B, Mo and N, increased the deformation resistance, but that of B and N, decreased it. 3) The hardness as solution-treated was increased by the addition of Mo, B, N, Nb in that order, but was decreased by Ti. In the simultaneous addition of those elements, the effects of single additions appeared cumulatively. 4) The characteristic of age-hardening was deteriorated by the addition of B, Nb, Ti but improved by N. Moreover, the simultaneous addition of Nb and N suppressed the age-hardening through the prevention of precipitation and that of Ti and N promoted the age-hardening through the acceleration of fine and uniform precipitation. 5) Finally, by the utilization of the effective interaction of added elements upon creep rupture strength, authors recommended two steels with the nominal compositions of 1) 0.2C-18Cr-12 Ni-3Mo-0.2B-0.2-0.6Ti and 2) 0.2C-18Cr-12Ni-3Mo-0.15N-0.02B-0.5-1.0Nb, which showed high creep rupture strength.