*4.3. Rough Rolling Simulations*

The first series of samples was subjected to hot deformation of either two passes or three passes in the temperature range between 1150 and 1050 ◦C, where each pass was under 15%. Figure 13a–c shows samples subjected to two-pass deformation at 1150 ◦C and 1100 ◦C of H4, H8 and L8, respectively. Complete recrystallization is observed in all three samples under two passes and three passes. It is expected from solubility products that the fraction of NbC or NbCN in H8 during cooling is higher than those in H4 and L8. For the three pass microstructures, it can be seen in Figure 13d–f, that the average austenite grain sizes are reduced, and the highest reduction in grain size was observed in L8 samples during hot deformation. Figure 14 summarizes the average austenite grain sizes of steels H4, H8 and L8.

**Figure 13.** Two-pass reduction of 15% at 1150 and 1100 ◦C for (**a**) H4, (**b**) H8 and (**c**) L8; three pass reduction at 1150, 1100, and 1050 ◦C for (**d**) H4, (**e**) H8 and (**f**) L8.

**Figure 14.** Mean austenite grain size after two passes with 15% reduction/pass at the respective deformation temperature; each sample was quenched, and prior austenite grain size was analyzed.

The second series of samples was subjected to three passes under 25% reduction in each pass. The micrographs of these samples are shown in Figure 15. It is evident from these micrographs that pancaking of austenite did not occur, even at 1050 ◦C. However, a non-uniform austenite grain distribution can be observed at temperatures at/or below 1050 ◦C in the high Nb steels. This leads to a mixture of recrystallized and unrecrystallized austenite grains (i.e., duplex austenite grains) in

the roughing stage of plate steels. To demonstrate this idea, the L8 sample was reheated at 1200 ◦C for two minutes and subjected to three 25% passes, but the last pass was done at 1030 ◦C, which is near the end of the roughing passes. The interpass time was set to be 15 s after each pass in order to see whether complete static recrystallization could be observed. Figure 16 shows the prior austenite grain size of L8 subjected to the previously mentioned schedule, and the fraction of unrecrystallized grains was measured to be 15% based on the non-circularity of the grains. This may show that the recrystallization stop temperature is close to the range of 1030–1000 ◦C.

**Figure 15.** Three-pass reduction of 25% each at 1150, 1100 and 1050 ◦C for (**a**) H4, (**b**) H8 and (**c**) L8.

**Figure 16.** L8 subjected to three passes of 25% each at 1150, 1100 and 1030 ◦C; the interpass time was 15 s. The compression axis is vertical.

This also indicates that the NbC pinning force is large enough to retard recrystallization in the roughing stage. The resulting duplex microstructure at the end of the roughing passes and can deteriorate strength and toughness in the final product.

It is immediately apparent that higher pinning forces of NbC on austenite grain boundaries will increase the *T*5% to higher temperatures, which may lead to a duplex microstructure at the end of rough rolling. Thus, it is necessary to choose Nb levels perhaps well below 0.1 wt.% to have uniform austenite grain sizes at the end of roughing rolling. This is true for both plate mills and thin slab casting mills, where the few, if any, roughing passes or the early finishing passes acting as roughing passes occur at temperatures above 950 ◦C, thereby eliminating or reducing the possibility of grain refinement before experiencing the low temperature finishing passes.

While further investigation is needed to confirm the effects of C and Nb levels on grain coarsening and recrystallization behaviors based on precipitate analyses, this study draws attention to the necessity to keep Nb levels optimum to produce a uniformly refined austenite grain size during the rough rolling of plate steels. Additionally, the study of the interaction between precipitation and recrystallization in the finishing passes of hot rolled plates needs to be considered in future studies.
