*3.2. The Interfacial Microstructure of Hot-Rolled Sheets at Di*ff*erent Temperatures*

The interface SEM microstructure of TA1/Q235B hot rolled sheets at different temperatures (1003 K, 1053 K and 1103 K) is shown in Figure 6. The vortex region disappears after hot rolling, and the interfacial is straight linearity. The transition layer is obvious. With the increases of rolling temperature, the width of transition layer increases from 3 μm (1003 K) to 4.5 μm (1053 K), and finally reaches 5 μm at 1103 K.

**Figure 6.** Interface SEM microstructure of TA1/Q235B sheets hot-rolled at (**a**) 1003 K, (**b**) 1053 K, (**c**) 1103 K.

The TA1 layer and Q235B layer of the 1103 K hot-rolled composite sheet were separated by a mechanical method, and each of the two separated surfaces were tested by XRD. Figure 7 shows the XRD patterns of the two surfaces. It indicates that the interfacial compounds are primarily TiC and FeTi, and Ti mainly exists in the form of α-Ti rather than β-Ti, because the rolling temperature is not high enough to reach the phase transition temperature. The TiC and FeTi compounds are high-temperatures phases, which have relatively high hardness and low ductility. In addition, the FeTi distributes mainly in the carbon steel side, while TiC distributes on both sides. When the rolling temperature is lower than the phase transition temperature (1155 K) of titanium, the solid solubility of Fe in α-Ti is lower than that of Ti in Fe; therefore, Ti is easier to diffuse into Fe. During cooling after rolling, the FeTi intermetallic compound is formed in the Q235B layer. Due to the strong combining capacity of Ti and C, TiC compounds are easily formed at the interface.

**Figure 7.** XRD patterns of both separation surfaces of TA1/Q235B hot rolled sheet.

The Figure 8 shows the distribution of C element in the TA1/Q235B interface. The thickness of the C element gathering region decreases from 5 μm at 1003 K to 2 μm at 1103 K. With the increase of rolling temperature, the distribution of the C element gathering region shows a substantial decrease along with the transition layer. At 1003 K, the diffusion of Ti in Q235B is very slow, and the TiC is formed preferentially at the interface, which hinders the grain boundary diffusion of Ti element and delays the formation of FeTi intermetallic compounds. Due to the high temperature and inhibition effect on carbides TiC [19], the amount of FeTi increases and that of TiC decreases, when the rolling temperature rises to 1103 K.

**Figure 8.** Electron probe micro-analysis (EPMA) for C element distribution in the interface of TA1/Q235B hot-rolled sheets at the temperatures of (**a**) 1003 K (**b**) 1053 K (**c**) 1103 K.

Figure 9 shows the EPMA element distribution of Ti and Fe in the interface of hot-rolling TA1/Q235B sheets at different temperatures. It can be seen that the interface of the two metals indicates the element concentration gradient of Ti and Fe. From the Ti distribution maps in Figure 9b,e,h, it can be found that titanium diffuses to 3 μm, 4.5 μm, and 5 μm to the Q235B when the rolling temperature is 1003 K, 1053 K, and 1103 K, respectively, while in the Fe element distribution map from Figure 9c,f,i, Fe diffuses less than 3 μm to the titanium owing to the titanium diffusion coefficient in steel is much larger than that of Fe in Ti [28,29].

The composition of diffusion layers are two different interfacial compounds, the carbide TiC forms earlier, and FeTi forms later. The main factor affecting the composition of TA1/Q235B composite plate is the carbide TiC and the interface intermetallic FeTi. In addition, the growth rate of interfacial compounds varies from the rolling conditions. Figures 8 and 9 show that TiC layer is always thicker than the FeTi layer, which is attributed to the segregation of C element in the transition layer and it distributes linearly along the bonding interface of the Q235B. Atoms can spread along the grain interface, and the interface of Q235B provides a rapid diffusion channel for C and Ti atoms [30], so TiC generates more rapidly than FeTi.

The EBSD IPF mapping of TA1/Q235B sheets hot rolled at different temperatures is shown in Figure 10. The size of the grain gradually increases with the increasing rolling temperatures both in the TA1 and Q235B. In the Q235B, the grain is pulled into strips because of hot rolling, and the recrystallization of the grain occurs at a relatively high temperature [31]. While the grain grows from about 10 μm in 1003 K to about 30 μm in 1153 K in the TA1. In addition, there are some new grains generated due to the breakage and refinement of the original grains.
