*4.1. As-Received Samples*

Figure 7 shows optical micrographs of the air-cooled plates. It can be seen for the base alloys L0 and H0 in Figure 7a,b that the microstructures consisted mainly of polygonal ferrite (PF), and carbon-enriched constituents including acicular ferrite and/or martensite-austenite (MA) microconstituents. When the carbon is increased from 0.03 wt.% to 0.06 wt.%, the fraction of these carbon-enriched constituents was increased, as shown in Figure 7b. For the same cooling rate, the addition of Nb to the base alloys results in a complex microstructure, as can be observed in Figure 7c through Figure 7f. These complex microstructures consist of mainly acicular ferrite, MA and/or bainite. Another observation is that polygonal ferrite in the high Nb steels is suppressed significantly.

**Figure 7.** Optical metallographic microstructures of air-cooled samples: (**a**) L0, (**b**) H0, (**c**) L4, (**d**) H4, (**e**) L8 and (**f**) H8.

A TEM micrograph for H4 is shown in Figure 8. It is clearly evident that the steel contains a complex microstructure consisting of MA microconstituents and acicular ferrite. Additionally, a coarse, titanium-rich precipitate can be seen in the air cooled condition.

**Figure 8.** (**a**) Bright field image of H4 showing ferrite and carbon-enriched constituents. (**b**) Titanium-rich precipitate and the corresponding (**c**) EDS spectrum of the precipitate.

The hardness values of microstructures taken in the three normal directions, namely, normal direction (ND), rolling direction (RD) and transverse direction (TD), are shown in Figure 9. As expected, in general, as the carbon and/or niobium content increases, the hardness value increases, although for different reasons. For example, the higher Vicker's hardness number (VHN) values observed with increasing the carbon content appeared to be due to the fraction of carbon-enriched constituents. Although there is no clear evidence of which direction gives the highest hardness values in the investigated samples, the TD sample of H8 shows the highest value. It can be inferred from this that pancaking of the low carbon austenite in the roughing mill can occur easily in this direction, meaning that there is a higher tendency to nucleate hard carbon-enriched microconstituents in this direction.

**Figure 9.** Vicker's hardness number (VHN) values of air-cooled samples in three orthonormal directions.

Another observation was made using EBSD mapping. Figure 10 depicts inverse pole figure data and misorientation angles of H0 and H8 taken from the transverse direction. For the same carbon level, there was a higher fraction of low angle grain boundaries (red boundaries) observed in H8 compared to H0, which is a typical characteristic of acicular ferrite.

**Figure 10.** Inverse pole figure data and misorientation mapping of H0 (**a**,**c**) and H8 (**b**,**d**).
