Hiroyuki Kajioka

Analysis of solute distribution in dendrites of carbon steel with δ/γ transformation during solidification

Solute distribution in dendrites during solidification of carbon steel was analyzed by unidirectional solidification experiments and mathematical analysis. The characteristic of the mathematical analysis is that diffusion of solutes in solid and redistribution of solutes at solid/liquid andδ/γ interfaces are taken into consideration. Based on the observed and calculated results, it was found that phosphorus was redistributed fromγ-phase toδ-phase, and that manganese was slightly redistributed fromδ-phase toγ-phase. Therefore the concentrated region of phosphorus can be separated from that of manganese duringδ/γ transformation in the case of slow cooling. Moreover, it was concluded that rapid diffusion inδ-phase and the redistribution duringδ/γ transformation played an important role in the variation of the interdendritic concentrations of solutes with lower carbon concentration.

Precipitation behavior of MnS during δ/γ transformation in Fe−Si alloys

The precipitation behavior of MnS after solidification was analyzed with low-carbon Fe−Si alloys. This system was chosen since it has a wide temperature range for the δ/γ transformation. Experimental results showed that the amount of MnS precipitates increased drastically between 1300°C and 1100°C, and MnS precipitates were segregated almost entirely in the δ phase. This result was interpreted quantitatively by a mathematical model, taking into account the diffusion and the redistribution of solute elements and also the solubility product limit of Mn and S in both phases. Mathematical analysis shows that the precipitation of MnS starts first in the γ phase, but its growth will be very limited because of slow diffusion of Mn in the γ phase. The effects of some factors such as cooling rate and Si content of alloys on the rate of precipitation were discussed, and the degree of contributions of diffusion of Mn and the redistribution of S were estimated.

Application of Nb-O Affinity in Clean Steel

This report introduces a new deoxidation process that starts with a vacuum degassing followed by the addition of ferro-niobium in the same vacuum facility. The addition of strong deoxidation substance like aluminum and/or silicon is not adopted. Owing to the degassing in vacuum, which in reality reacts as deoxidation with C in molten steel, the solute oxygen in molten steel (O) is stabilized by niobium, which generates an ample number of fine oxide particles composed of (Nb, Mn)2O3-phase. Provided the carbon-degassing is conducted so as to reach the O prior to the FeNb addition would be around 100ppm and less, these oxide-inclusions do not agglomerate before and during solidification, because of the weak cohesivity of niobium-based oxides in molten steel, which would enable defect-free cast products as well as clogging-less casting operation. Also, the steel containing these fine (Nb, Mn)2O3-particles but no Al, exhibits so-called intra-granular acicular ferrite transformation and good toughness of weld heat affected zone (HAZ).

Studies on Core Zone in Capped Steel Ingots

In sequence of the previous report (Tetsu-to-Hagane, Vol. 45, 1959, 1139~1144) concerning surface conditions of the capped ingot, features of core zone in a capped ingot were examined and investigation was made about effect of ingot weight, degree of deoxidation and another factors upon segregation. The following results were obtained: 1. The segregation in core zone of a capped steel ingot was less pronounced than that of a rimmed steel ingot. 2. In a capped ingot, the degree of maximum segregation was increased with the amount of Al added in the ladle, but it was generally less pronounced and the maximum segregated zone was narrower than in a rimmed steel ingot.