## 2 The Effect of Alloying Elements on Microstructure
### Cr\({}_{\alpha}\)/Ni\({}_{\alpha}\)-Equivalent Ratio  
Chromium (_Cr_) is a stable element of ferrite, and the ability of chromium to promote ferrite formation can be represented by chromium equivalent (_Cr\({}_{\alpha}\)_), while nickel (_Ni_) is a key element in austenite formation. The ability of austenite formation can be quantified by nickel equivalent (_Ni\({}_{\alpha}\)_) [17]. The influence of chromium (_Cr_) and nickel (_Ni_) on AISI 321 stainless steel is relative and is usually expressed by the _Cr\({}_{\alpha}\)/Ni\({}_{\alpha}\)_-equivalent ratio, which is an important indicator for evaluating the solidification mode of austenitic stainless steel [18; 19]. The _Cr\({}_{\alpha}\)/Ni\({}_{\alpha}\)_-equivalent ratio formula is as follows:  
\[\frac{Cr_{\alpha}}{Ni_{\alpha}}=\frac{Cr(wt.)\%+Mo(wt.)\%+1.5(wt.)Si\%}{Ni (wt.)\%+30\text{C}(wt.)\%+0.5 Mn(wt.)\%} \tag{1}\]  
The formula _Cr\({}_{\alpha}\)/Ni\({}_{\alpha}\)_ = 1.48 defined the boundary between the ferrite-austenite (FA) mode and the austenite-ferrite (AF) mode. When the ratio was above 1.48, it was the FA mode; below 1.48, it was the AF mode.  
The phase diagram of AISI 321 stainless steel is simulated using JMatPro (Version 7.0), with the chromium (_Cr_) content fixed at 17 wt.%. The simulation outcomes reveal that, when the nickel (_Ni_) content reaches 11%, the solidification pattern adopts the FA  
Figure 2: The microstructure of AISI 321 steel after thermal deformation at 1000 \({}^{\circ}\)C, (**a**) strain rate is 0.01 s\({}^{-1}\); (**b**) strain rate 0.1 s\({}^{-1}\)[14].  
mode (Figure 3a), whereas at a nickel (_Ni_) content of 12 wt.%, it transitions to the AF mode (Figure 3b). Furthermore, the \(C_{\textit{rd}}\)/\(Ni_{\textit{qq}}\)-equivalent ratio undergoes a variation from 1.45 to 1.34. Despite a minor discrepancy with the formulation outlined in Equation (1), the overall alignment remains fundamentally consistent.  
The varying solidification modes are pivotal in influencing the types of dynamic recrystallization that take place. As illustrated in Figure 3, under the FA solidification mode, the highest proportion of high-temperature ferrite peaks at roughly 35%, whereas it declines to below 20% when the solidification mode shifts to AF. This decrease suggests a reduction in residual ferrite content during the final solidification phase. [PERSON], [PERSON] had also noted this phenomenon, where the ferrite content decreases as the solidification mode transitions from FA to AF [20]. The amount of these residual ferrites significantly impacts the dynamic recrystallization process. In the absence of ferrite, the austenite grain boundaries exhibit high energy storage. Upon stretching, large-angle grain boundaries promptly form and migrate along the bowed grain boundaries, a characteristic of discontinuous dynamic recrystallization. Conversely, when ferrite is present, the energy storage in its vicinity is relatively low. Due to the dynamic recovery effect, the grain size gradually increases from smaller to larger grains, a process termed continuous dynamic recrystallization [21]. Essentially, an increase in nickel (_Ni_) content from 11 wt.% to 12 wt.% facilitates the production of more continuous dynamic recrystallization, resulting in a more uniform and finer microstructure. This, in turn, contributes to higher strength properties.