## 4 Discussion  
Steels containing acicular ferrite have a great potential to improve mechanical properties [37; 38] by grain refinement, higher dislocation density or controlled precipitation. What is more, the toughness provided by acicular ferrite is considerably higher than that of bainitic ferrite due to a higher density of high angle grain boundaries [15]. Results of previous studies indicate that the introduction of intragranular nucleation sites and the suppression of bainitic ferrite lath lengthening are identified as the two key requirements for the occurrence of acicular ferrite [15]. Austenite deformation is a critical factor as it introduces the high density of dislocations that act as nucleation sites. It is accepted and proved that the acicular ferrite transformation is a transitional state between the formation of globular ferrite and bainite. The optimal cooling rate to achieve a sufficient fraction of acicular ferrite depends strongly on the production process and chemical composition of given steel. As a result, the very wide range of cooling rates is described in the literature as being suitable for acicular ferrite (from 6 to 3000 \({}^{\circ}\)Cs\({}^{-1}\)) [32]. In the current case, an increased cooling rate (15 \({}^{\circ}\)Cs\({}^{-1}\)) was applied at the beginning of the ferritin transformation to enable the formation of acicular ferrite. However, in a later step, cooling was much slower to maximize a ferrite fraction and thus the austenite stabilization. Using combined cooling methods enabled both above requirements to be obtained. In previous studies [13] the same steel was heat treated in similar conditions but with elevated hot-working temperatures (finishing rolling at 850 \({}^{\circ}\)C, fast cooling to 700 \({}^{\circ}\)C and slow cooling  
Figure 5: The mechanical stability of RA expressed as a \(\mathrm{k}\gamma\) parameter; numbers denote the retained austenite fractions at interrupted strain levels. to 650 \({}^{\circ}\)C). The lower dislocation density caused by plastic deformation at the higher temperature and successive phase transformations performed at higher temperatures caused a globular ferrite formation and obtaining YS\(\sim\)50 MPa and UTS\(\sim\)35 MPa lower; however, with total elongation 4% higher, caused mainly by the fraction and carbon content in retained austenite. The application of the finish hot-working temperature of 750 \({}^{\circ}\)C (lower than the recrystallization temperature) introduces the needed dislocations and affects the elongation of the ferritic matrix grains in the direction of plastic flow for samples deformed both in the Gleeble simulator and in semi-industrial conditions. In conjunction with slow cooling, it allows the acicular ferrite to be obtained instead of the polygonal one. The selection of hot working conditions had to take into account the limitations resulting from the rolling of steel in the double-reversible system, which shows significant differences with regard to industrial hot strip rolling in a continuous system, especially in terms of the number of applied deformation steps, strain rate, time intervals between passes, etc. [39]. In conventional C-Mn-Si TRIP steels with a Si concentration of \(\sim\)1.5% or in C-Mn-Si-Al steels with silicon partially replaced by Al, the SIMT occurs rapidly in the initial stage of deformation, which usually affects lower steel plasticity [40; 41]. Even in the case of obtaining the total elongation up to 30% in the C-Mn-Si steels, it is most often at the expense of high silicon content [42; 43], and in the case of C-Mn-Si-Al steels, the strength properties are lowered [44; 45]. A characteristic feature of the investigated steel is a sudden decrease in the strain hardening rate in the initial stage of deformation, which corresponds to the generation of new dislocations and the avoidance of obstacles by dislocations as a result of transverse slip [3].