## 3 Results and Discussion  
### Structure and Morphology of NM450 TP Steel  
According to the CCT curves of NM450 TP steel, decomposition of undercooling austenite would not occur during the first cooling stage because of fast cooling rate, small amounts of ferrite phases may form during air cooling, and the formation of martensite may happen in the third cooling stage. The related studies showed that the volume fractions of residual austenite in steel is related to the diffusion of carbon during the cooling process, the carbon element in supersaturated martensite will diffuse to the austenite when cooling rate is slow, this led the stability of austenite being improved and the residual austenite being retained at room temperature [20]. NM450 TP steel contains relatively higher contents of carbon and manganese, and if the cooling rate is not fast enough during the third stage, the diffusion of carbon elements will not be inhibited, and residual austenite would possibly remain in NM450 TP steel.  
The XRD phase analysis shows that only the diffraction peak of body-centered cubic structures was found in the specimen, no austenite diffraction peak was detected (Figure 3). This result indicates that during the third cooling stage, the cooling rate was fast for obstructing the diffusion of carbon elements, undercooling austenite became unstable and completely transformed into martensite.  
During the controlled rolling and cooling process, austenite grains will undergo complex dynamic deformation and recrystallization, their transformation procedure and morphologies will be affected by these factors. Figure 4 displays that the microstructure at three typical locations consists of soft ferrite and hard martensite matrix. Ferrite grains are mainly distributed around martensite grains, and a number of small ferrite islands are dispersed in the martensite matrix. Meanwhile, the martensite and ferrite grains are found to exhibit directional features at the longitudinal cross-section of the plate (Figure 4d-f), this means that the spatial morphology of martensite and ferrite grains are supposed to be platy grained. The observed results prove that the prior austenite grains after finishing rolling have a certain degree of deformation due to rolling occurring in the temperature range of incompletely dynamic recrystallization, and austenite grain size or defectiveness have certain effects on the formation of ferrite and martensite. Usually, the grain boundary is in an unstable state because of chemical composition and structure, and ferrite is easily nucleated at the grain boundary. From the observation of the morphologies of these grains  
Figure 3: Phase analysis on the RD-ND plane by XRD.  
in Figure 4, ferrite grain was formed around the martensite grain with the quasi-polygonal equiaxed state during the air-cooling stage. Their sizes and shape in different sites vary greatly. The rest of the austenite is finally transformed into lath martensite during the third stage of water cooling.  
It is suggested that because austenite grains are elongated along the rolling direction and their dynamic recrystallization is prohibited to some extent during the rolling process, the transformed ferrite and martensite grains keep some deformation characteristics of deformed austenite. In addition, it is also observed in Figure 4 that the amounts of martensite on both surfaces are less than that of martensite at 1/2 position, i.e., ferrite contents at 1/2 position are less than that at both surfaces. This distribution of martensite and ferrite is in contrast to the distribution of martensite in the steel upon quenching [21]; this result indicates that rolling within incompletely dynamic recrystallization and multi-stage cooling will affect transformation procedures and decomposition morphologies of austenite. In order to obtain deep understanding on the microstructures of the steel, a series of detailed microstructure characterizations were conducted based on the EBSD analysis.