Creation of high performance steels from the waste scrap is drawing much attention recently due to the economical and environmental reasons. Our group previously reported the refinement of prior austenite grains in high phospho-rous（P） steels by the retardation of the austenite formation due to the micro-segregation of P. An analytic model was proposed for the prediction of the prior austenite grain size (hereafter PAGS) based on the classical grain growth model.

In our previous report, systematic analysis on the segregation behavior of P was not made theoretically despite all its importance in the grain refinement mechanism of as-cast high P steels. Besides, the reduction of the liquid/solid interface energy by P addition was not appreciated at all although it has a substantial influence on the dendrite arm development and consequently on the initial PAGS.

In our recent work, to address the objectives, the dominant factors, which determine the final PAGS, such as the primary dendrite arm spacing (hereafter PDAS), the micro-segregation and the growth kinetics are considered.
Then, a theoretically-sound model (we dare say) for predicting PAGS of high P steels is suggested. The effect of P addition on the phase evolution of low carbon steels during liquid/δ-ferrite/austenite transformation is theoretically analyzed using a computational phase-transformation simulation technique.

The following points are noted in this research.

1. With increase in P content, both δ-ferrite and liquid phases are retained to lower temperatures and the completion of austenite formation is greatly delayed.
   
   
2. The increase of unit-segregation-zone size retards the completion of solidification and decreases the starting temperature of rapid grain growth greatly.
   
   
3. The effect of P addition on the starting temperature of rapid grain growth is not monotonous. The effects of micro-segregation and liquid/solid interfacial energy reduction caused by P addition must be evaluated comprehensively in the austenite grain growth modeling.
   
   
4. The inter-dendritic micro-segregation of phosphorous lowers the rapid growth temperature while the decrease of PDAS resulted from liquid/solid interfacial energy reduction raises it.
   
   

Using the information derived from the phase transformation simulation, a PAGS prediction model was derived based on the classical grain growth theory [1]. The predicted PAGS showed excellent agreement with the experimental data which were measured at a quarter thickness position of 100mm slab [2].
The present methodology may be applied to elucidate the effect of other elements and could be used in alloy design of other systems.

References:

1. Han S. Kim, Y. Kobayashi, and K. Nagai: “Prediction of Prior Austenite Grain Size of High-impurity Steels through Simulation of P Influence on the Phase Transformation”, International Organized Session in 150th ISIJ Meeting, CAMP-ISIJ, Vol.18 (2005), pp.865-868.
   
   
2. N. Yoshida, O. Umezawa and K. Nagai: “Analysis on Refinement of Columnarγ Grain by Phosphorus in Continuously Cast 0.1 mass% Carbon Steel”, ISIJ Int., Vol.44 (2004), pp.547-555.