# On the Effect of Filling Rate on Positive Macrosegregation Patterns in Large Size Cast Steel [PERSON]
## 1 Introduction  
Ingot casting is the only method for the production of large size mono-block high-strength steels used in the energy and transportation industries. Various forms of casting defects, however, may stem from the liquid metal filling stage, and then remain during the solidification process. Among them, macrosegregation, defined as chemical heterogeneity on the scale of the entire ingot, is one of the most important, particularly in high alloyed steels. Such compositional variation over large distances results in local changes in mechanical properties, which may lead to reduced ingot quality and sometimes even to ingot rejection [1, 2]. While the severity of macrosegregation could be significantly reduced in small to medium size ingots through subsequent homogenization and forging operations; the situation is more complex when it comes to large size ingots. Therefore, a better understanding of macrosegregation mechanisms, with the aim to control its extent in heavy castings, is an important scientific challenge with direct industrial implications. This is particularly true when it comes to high value-added products, such as large size casting of high strength steels used for turbine shaft applications. Macrosegregation patterns in heavy low-alloy steel ingots is often characterized by intense positive segregation in the upper part, negative segregation at the bottom, and A- and V-shaped channel segregations in the body [3]. The pioneering work of [PERSON] et al., developed in the 1960s [4; 5], set the theoretical bases for studying the mechanisms of macrosegregation. Since then, intensive research has been underway, and some critical process parameters have been identified and used as 'countermeasures' to produce sound ingots with improved quality and limited macrosegregation [6; 7; 8; 9]. Specially, stringent control of the superheat [7], accelerating the solidification process in the mold [8], and improving ingot dimensions and configurations [9] have been investigated experimentally and numerically by researchers. In recent years, the influence of filling process parameters has been studied by several researchers. [PERSON] et al. [10] numerically studied the velocity, stress, and temperature fields of pure metal during mold filling and the solidification process, showing the important effect of the filling process on fluid flow and heat transfer at the early stages of the casting process. [PERSON] et al. [11] modeled the effects of wall temperature and filling flow on solidification, and pointed out that the residual flow due to the filling stage had an important effect on flow characteristics and the liquid metal solidified more slowly in higher flow situations. [PERSON] et al. [12] presented a model to investigate the effect of mold filling on cold shuts formation and metal front migration, and incorporated inter-dendritic flow models to simulate the effect of solidification on the filling patterns and temperature fields. [PERSON] et al. [13] modeled the filling and solidification of a Pb-15 wt % Sn alloy in a small side-cooled cavity (50 \(\times\) 60 mm\({}^{2}\)), and investigated melt superheat and filling velocity on the evolution of mushy regions and macrosegregation. It was found that residual flow due to filling effects significantly affected the shape of the mushy zone and delayed the development of solutal convection, resulting in less intense macrosegregation near the cold wall and a farther distance of A-segregates from the cold wall. [PERSON] et al. [14] simulated a 6-ton Cr-Mo low alloy steel to study the effect of bottom pouring rate and mold dimensions on solidification behavior and crack susceptibility during subsequent hot forging. Results showed that pouring the melt at a constant rate with a lower mold slenderness ratio would improve the riser efficiency and thereby possibly reduce crack susceptibility during subsequent hot forging. However, the effect of filling velocity on macrosegregation in large size steel ingots has not yet been reported, either numerically or experimentally.