In 0.1 mass% carbon steels with phosphorus contents ranging from 0.01 to 0.2 mass%, slabs were continuously cast to a thickness of 100 mm with a laboratory scale caster. Their macro-and micro-structures were characterized, focusing on the effects of phosphorus addition on the structural evolution during solidification and subsequent cooling. Cast slabs of high phosphorus steels have a fine columnar-g-grain structure. The mean width of the columnar grain was approximately half of that in the cast slabs without the phosphorus addition. Dispersed globular a grains were observed in the a grain structure of high phosphorus steels. The globular grains evolved at the phosphorus-rich spots in the prior-g grain. The micro-segregation of phosphorus results in these structural evolutions. Since the phosphorus enrichment stabilizes bcc (d or a) phase locally at the inter-dendritic region, the phosphorus-rich spot makes d phase retained to lower temperature for the d/g transformation, and provides a predominant site for the g/a transformation. In the high phosphorus casts, therefore, the dispersed d phase are thought to pin the g grain growth more effectively in the dϩg region when the completion of the d/g transformation is remarkably suppressed by the phosphorus segregation.

Based on a grain growth model, this paper discusses the g grain refinement by phosphorus (P) in as cast 0.1 mass% C slabs. Two important factors, the starting temperature of the rapid grain growth (T rg ) and the grain growth rate, are especially focused in the model. Local Equilibrium Mapping (LEM), an analytical method with local equilibrium calculations in the micro-segregation maps determined by EPMA, is carried out to evaluate the local transformation temperature and the g phase fraction. The LEM analysis shows that the micro-segregation of P extends the d/g transformation temperature range to a lower temperature and retains the d phase at a lower temperature. The Classical Grain Growth Model (CGM) based on a theory by Burke (1949) and Turnbull (1951) is derived, which assumes a normal grain growth with a parabolic law. The CGM successfully evaluates the g grain growth curve in the as cast 0.1 mass% carbon steels considering the rapid growth after the d/g transformation and the change of the growth rate during cooling. The CGM analysis predicts the decrease of both T rg and growth rate by the P addition. The decrease of T rg was determined to be 160-170 K and agrees with the extension of the d/g transformation temperature range evaluated by the LEM analysis. Hence, the grain growth curves can be predicted by considering the extension of the d/g transformation temperature range due to the P addition and its micro-segregation.KEY WORDS: carbon steel; continuous casting; solidification; grain growth; phosphorous; segregation; phase transformation; austenite; delta ferrite. should emphasize two significant factors when determining the g grain size. The first factor is the temperature at which the rapid grain growth starts, and the second is the grain growth rate within the range of temperature for the rapid growth. The first factor, the starting temperature of rapid growth, is closely related to the d/g transformation temperature range. Since the micro-segregation changes the transformation temperature locally, a new analytical method, i.e. the local equilibrium mapping (LEM), is developed, in which the local equilibrium is determined based on the micro-segregation mapping data by EPMA, using ThermoCalc 7) with a multi-component database. This method can evaluate the local transformation temperature and the phase fraction as a function of temperature.The second factor, the grain growth rate during rapid growth, is evaluated from a grain growth model, i.e. the classical grain growth model (CGM). The basic grain growth equation in the CGM is based on the classical theory proposed by The analytical model is derived by integrating 3,[11][12][13][14][15][16] the basic equation over the duration of the rapid grain growth, considering the temperature dependency of the grain growth rate.9,10) The grain growth rate and the grain size will be predicted by the CGM.Consequently, we will discuss the mechanism of the g grain refinement by phosphorus by analyzing the results of the previous paper by coupling the LEM with the...

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Phosphorus, one of typical impurities in steel, has been traditionally tried to be removed to the refining slag in the conventional steel making process. On the other hand, the inverse-utilization of impurities in steel was introduced recently to provide a resource circulating society. In these processes, phosphorus can be and must be restored in the steel during the deoxidation and solidification. The usage of elements with high deoxidizing and low dephosphorizing abilities such as manganese and silicon will be beneficial for obtaining such kind of steel. However, the thermodynamic behavior of phosphorus in such oxide fluxes has not been established. Therefore, the phosphate capacity as the phosphorus containing ability for the MnO-SiO 2 -Fe t O system, one of the typical slags for deoxidation, has been investigated by measuring the phosphorus partition between the slag and solid or molten iron. The phosphate capacities for the present system were determined to be from 8.5ϫ1014 to 3.8ϫ10 18 at temperatures from 1 673 to 1 923 K. The present system has shown a much smaller phosphate capacity by several orders of magnitude compared to the conventional CaO bearing systems and provides the estimation of a very low phosphorus distribution ratio between the slag and the steel. In addition, the heats of the phosphate formation reaction were derived from the temperature dependence of phosphate capacities.

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