# Strengthening Mechanisms in a Medium-Carbon Steel Subjected to Thermo-Mechanical Processing
## 3 Results  
### Microstructure  
Some of the microstructural features such as the texture, the distance between HAB, the transverse lath/subgrain size after tempering and tempforming at 600 \({}^{\circ}\)C and 650 \({}^{\circ}\)C with a total strain of 1.4 were described in previous papers [6; 11]. The main structural parameters of the steel in four different conditions are summarized in Table 1. Typical OIM maps and TEM images of the steel after tempering and tempforming at different temperatures are shown in Figures 1-3. After tempering at 600 or 650 \({}^{\circ}\)C, the tempered martensite lath structure (TMLS) with an average distance between HAB of 1.1 \(\upmu\)m evolved(Figures 1a and 2a). The dislocations were mainly collected in dislocation sub-boundaries (Figures 1b and 2b). The average KAM values were 0.46 and 0.56 after tempering at 600 and 650 \({}^{\circ}\)C, respectively. Tempforming led to the formation of a lamellar microstructure consisting of LABs and HABs aligned in the rolling direction (RD) (Figures 1c,d and 2c,d). It is obvious (Table 1) that the prior austenite grains and packets tended to deform as a whole billet. As a result, the distances between HABs in normal direction (ND) decreased by factors of \(\sim\)3 and \(\sim\)5 during tempforming at 600 and 650 \({}^{\circ}\)C, respectively. Warm rolling after tempering induced dynamic recovery at 600 \({}^{\circ}\)C and continuous dynamic recrystallization (cDRX) at 650 \({}^{\circ}\)C. LABs also aligned with RD and lamellar microstructure consisted of longitudinal LABs and HABs (Figure 2a,b). The spacing between LABs decreased and the density of free dislocations within lamellae increased during warm rolling (Table 1, Figure 3f). As a result, the average KAM values increased to 0.92\({}^{\circ}\) and 0.85\({}^{\circ}\) after tempforming at 600 and 650 \({}^{\circ}\)C, respectively (Figures 1c and 2c, Table 1). Figure 1: Typical EBSD (**a,c**) and KAM (**b,d**) maps of medium-carbon low-alloy steel after tempering at 600 \({}^{\circ}\)C (**a,b**) and tempforming at 600 \({}^{\circ}\)C (**c,d**) with true strain of 1.4. Black and white lines in EBSD maps (**a,c**) indicate HAB and LAB, respectively. Figure 2: Typical EBSD (**a**,**c**) and KAM (**b**,**d**) maps of medium-carbon low-alloy steel after tempering at 650 \({}^{\circ}\)C (**a**,**b**) and tempforming at 650 \({}^{\circ}\)C (**c**,**d**) with true strain of 1.4. Black and white lines in EBSD maps (**a**,**c**) indicate HAB and LAB, respectively. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Processing** & **QT600** & **QT650** & **TF600** & **TF650** \\ \hline Distance between HAB, \(\mu\)m & 1.1 \(\pm\) 0.2 & 1.3 \(\pm\) 0.3 & 0.280\(\pm\) 0.008 & 0.195\(\pm\) 0.005 \\ Average KAM, degree & 0.46 \(\pm\) 0.2 & 0.56 \(\pm\) 0.2 & 0.92 \(\pm\) 0.2 & 0.85 \(\pm\) 0.2 \\ \(\rho_{\rm KAM}\), 10\({}^{15}\) m\({}^{-2}\) & 2.2 \(\pm\) 0.9 & 2.6 \(\pm\) 0.9 & 4.3 \(\pm\) 0.9 & 4.0 \(\pm\) 0.9 \\ Taylor factor (M) & 3.16 & 3.06 & 3.17 & 2.98 \\ Transverse lath/subgrain size, nm & 200 \(\pm\) 12 & 199 \(\pm\) 18 & 72 \(\pm\) 29 & 127 \(\pm\) 65 \\ Dislocation density in subgrain interiors, 10\({}^{14}\) m\({}^{-2}\) & 5.9 \(\pm\) 1.3 & 3.4 \(\pm\) 0.9 & 11.0 \(\pm\) 6.5 & 10.0 \(\pm\) 3.8 \\ M\({}_{25}\)C\({}_{6}\) particle size/volume fraction, nm/\% & (49 \(\pm\) 17)/7.2 & (48 \(\pm\) 3)/7.3 & (41 \(\pm\) 4)/7.2 & (61 \(\pm\) 4)/7.3 \\ Mo\({}_{6}\)C particle size/volume fraction, nm/\% & (208 \(\pm\) 7)/0.6 & (208 \(\pm\) 10)/0.6 & (208 \(\pm\) 7)/0.6 & (190 \(\pm\) 49)/0.6 \\ (V,Nb)C carbides particle size/volume fraction, nm/\% & (46 \(\pm\) 5)/0.41 & (37 \(\pm\) 11)/0.37 & (46 \(\pm\) 5)/0.41 & (46 \(\pm\) 5)/0.37 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Structural parameters of the medium-carbon UHS steel after tempering and tempforming. TMLS is characterized by high internal stresses as suggested by bend contrast at TEM images in Figure 3a,b. These stresses are exerted by lath boundaries, which are an irregular dislocation network [12].