Following the investigation described in the former report (Tetsu-to-Hagane, 48 (1962) 2, p.117), further experiments were conducted with small test ingots and also 10t slab ingots concerning the effect of degree of deoxidation and ladle refractories upon black spots appearing on the sulphur prints along the skin layers of semi-killed steel ingots. The mechanism of occurrence of the black spots tentatively referred to in the former report was confirmed. The main results obtained are as follows: - (1) The number of these black spots can be reduced by intensifying the degree of deoxidation, the effect of which will be more noticeable when aluminium is used as deoxidizer than when silicon is used. Intensification of the degree of deoxidation, however, is not favorable because of the resultant large pipe cavities which are produced in semi-killed steel ingots. (2) At the same ratio of added silicon against added aluminum and at the same degree of deoxidation, the number of black spots becomes smaller when high-alumina bricks are used for ladle refractories than when silicious bricks. Roseki bricts. are used. (3) To prevent the appearance of the black spots when high-alumina ladle refractories are used, a little aluminum must be added together with silicon. (4) High-alumna bricks are, however, very expensive and liable to increase the ladle skull owing to their high thermal capacity. Moreover, it is difficult to remove this skull for lack of glassy layer between bricks and skull. Therefore, there are still many problems unsolved to put this brick into practical use. (5) Consequently, on the present stage of development, the use of aluminium as the main deoxidizer for the special semi-killed steel products that must be subject to severe cold processing, as mentioned in the former report, is the most desirable method for preventing appearance of these black spots, and hence, for preventing occurrence of surface defects.
In the previous report (Tetsu-to-Hagané, 45 (1959) 12, p. 1341), experiments were carried out on effects of the primary cooling, in other words cooling of billets in molds, and on the surface of billets. The importance of setting the primary cooling under the optimum conditions were confirmed. In this report, at first the mechanism of the primary cooling is considered, and then effects of some factors on the primary cooling are investigated experimentally. Experimental results are summarized as follows. Majority of the primary cooling is conducted by heat transfer from the surface of the billetto the mold cooling water through the mold wall. Main factors influencing the primary cooling are the casting speed of the billet and the time of the use of a mold. On the other hand, the casting temperature and the quality of steel only slightly influence the primary cooling. Basic data obtained from these experiments presumably have a great meaning in considering the ideal primary cooling later.
In order to improve the resistance to fire cracking and wear of steel rolls for primary hot-rolling mills, such as blooming, slabbing and roughing mills, without reduction of their strength and toughness, some characteristics of cast iron must be accompanied with the properties of steel rolls. It was, therefore, undertaken to apply nodular graphite steel to rolls for primary mills to realize the above idea. One of the authors has recently developed nodular graphite steel, with use of a special Fe-Ca alloy, this steel having free nodular graphite even in as-cast state. The characteristics of nodular graphite steel were first investigated mainly from the metallurgical point of view, and its promising applicability to rolls was clarified. The trial manufacture of actual rolls with nodular graphite steel was next performed for several millsThe results of service performance of these rolls were, as expected, superior to that of rolls, which hitherto had been used, with special reference to the following points: (1) Nodular graphite steel roll has a considerable strength and toughness. It has, therefore, almost no tendency to breakage in service which has been often experienced in cast iron rolls. (2) Nodular graphite steel roll has a great resistance to fire cracking. It has, therefore, further less occasions to be shortened its life by surface cracking than usual steel rolls. (3) Nodular graphite steel roll has a great resistance to wear. Combining with the resistance to fire cracking, it can give more rolling tonnage and finer rolled products with dimensions of higher precision than usual steel rolls.
The sulphidation of iron and steel at high temperatures is lately discussed in many literatures. The author has experimented on the sulphidation of cast iron at high temperatures. The following experimental results were obtained. (1) Al, Cr, Cu, Mn, Si V, Zr gave good effect on the sulphidation-resistance of cast iron when added in a small quantity, especially Al, Cr, Zr showed remarkable effect. The addition of Al, Mo, W enhanced the sulphidation-resistance of cast iron with increase of their additions, but the W raised it in a slight ratio. In the case of Cu and Zr, the addition in excess rather increased the sulphidation rate. Ti was found to give no good effect. (2) The alloying element seemed to concentrate on the inner or outer layer of sulphide coating and to make it compact, consequently to develop the sulphidation-resistance. In this experiment, the sulphide coating of alloy cast iron resembled that of ordinaly cast iron.
The effect of zirconium compounds on the austenitic grain size refining and grain-coarsening temperature was studied by adding ferrozirconium to the pure irons which had contained oxygen, nitrogen or carbon respectively. It was found that the grain size was refined and its coarsening temperature was enhanced by the presence of zirconium nitride or zirconium carbide; and that the nitride was more effective than the carbide, but zirconium in solid solution or in oxide gave no effect on them. The amount of zirconium nitride required for grain refining was about 0035%, but its effect was not so remarkable as that of aluminum nitride. In order to clarify the behaviour of zirconium nitride as a grain-growth inhibitor, some considerations were made on the experimental results obtained with the specimens which had been heat-treated at various temperatures.
In order to prolong the life and to strengthen mechanical properties of exhaust valves made of high-N-19Cr-8Ni valve steel which was widely used for automotive industry, the authors investigated the influence of C, Si, Ni, W, Mo, V, Nb, Cu and N on various properties of this valve steel. Some properties measured were hardness-change by solution treatment, aging and cold working, mechanical properties at room and elevated temperatures, rupture strength, magnetic permeability, oxidation resistance in the air at elevated temperature and corossion resistance to liquid PbO at 1000... The experimental results obtained were as follows: (1) The as-quenched and the as-aged hardnesses, the yield and the tensile strength at room temperature and the rupture strength were increased, but the impact strength were decreased with C contents. (2) With the alloy containing 037-319% Si, as the Si was increased, the hardness after solution treatment, the tensile strength at room temperature and the oxidation resistance were increased. Especially the oxidation resistance in air at elevated temperature was excellent with the alloy containing 2-3% Si. The as-aged hardness, the impact strength and the corossion resistance to liquid PbO were decreased with the alloy containing more than 1% Si. (3) The impact strength, the rupture strength and the tensile and yield strength at elevated temperature were increased by addition of Ni more than 8%. (4) When the carbide-forming elements such as W, Mo, and Nb were added, the hardness, the mechanical properties and the rupture strength were increased, but the impact strength was decreased. When the sum of W+Mo contained were more than 1% or when Nb added was more than 05%, the strength at room temperature was decreased, but the rupture strength was enhanced by addition of these elements. The hardness and the strength were decreased by V addition. (5) The hardness and the strength were decreased, but the rupture strength was definitely increased by Cu addition. (6) When about 0'3% N was added, the hardness as solution-treated and the strength at room and elevated temperatures were extremely improved.
The as-aged structures of an Fe-Co-Cr-Ni base heat-resisting alloy LCN-155 were examined by metallographic methods-both optical and electronic. Mechanical properties at room temperature were then measured and correlated with the as-aged microstructure. The experimental results obtained were as follows: In a lower-temperature or shorter-time aging, first of all, M23C6 carbide precipitated at the grain boundaries, resulting in a marked decrease in ductility and there followed formation, of very fine nuclei of M23C6 along the cleavage planes within matrix, resulting in a progressiv increase of strength and hardness. But in a higher temperature or longer time aging, the dispersion state of the intragranular precipitates which correlated with the mechanicat properties was affected with the transition of precipitates. With an alloy which only onephase (M23C6) precipitated, the intragranular precipitates grew along the cleavage planes. and it showed a continuous localized structure. However, with an alloy in which two more phases (M23C6 and X phase) precipitated and transfered from one phase to another (M23C6→X phase) during aging, the cleavage precipitates was redistributed due to the carbide reaction and it showed a continuous (homogeneous) structure. It was concluded that this redistribution of precipitates due to carbide reaction would cause excellent mechanical properties.