## 2 Materials and Methods  
Material containing 0.24%C, 1.55%Mn, 0.87%Si, 0.40%Al, 0.004%S, 0.010%P, 0.034%Nb and 0.023%Ti (in wt.%) was used in this study. The steel was designed to obtain a ferritic-bainitic structure with retained austenite. It is characterized by an increased content of Mn, Si and Al. The carbon content is limited to 0.24% to ensure good weldability, and at the same time high strength properties and optimal enrichment of the retained austenite with this element. Manganese acts as an austenite stabilizer and lowers the temperature of cementite precipitation. The Si concentration was lowered--from the most used 1.5% in TRIP steels--to 0.87%, and the rest was replaced by adding 0.4% Al. The purpose of the Nb and Ti micro additions was to produce carbides, nitrides and carbonitrides during hot-working. Due to the precipitates, the possibility of producing fine-grained structures is expected. The 25 kg alloy was melted in a VSG-50 vacuum furnace (Balzers). The material was cast in argon atmosphere and forged to the form of 20 \(\times\) 220 mm flat bars. The forging temperature range was 900-1200 \({}^{\circ}\)C. Next, the steel was roughly rolled in four passes to the thickness of 4.5 mm in the temperature range 900-1200 \({}^{\circ}\)C.  
After the initial production process, the material was subjected to pilot, laboratory-scale thermomechanical processing simulating the final process. For this purpose, the Gleeble 3800 by DSI was used. This treatment was composed of two major steps:  
Hot compression simulating the rolling process (samples 15 \(\times\) 20 \(\times\) 35 mm)  
1 st compression. T--1050 \({}^{\circ}\)C, \(\epsilon\)--0.28,  
2 nd compression. T--900 \({}^{\circ}\)C, \(\epsilon\)--0.30,  
3 rd compression. T--750 \({}^{\circ}\)C, \(\epsilon\)--0.22. Controlled cooling with two isothermal holding steps:  
Fast cooling from 750 \({}^{\circ}\)C to 650 \({}^{\circ}\)C (15 \({}^{\circ}\)Cs\({}^{-1}\)), performed to start the ferritic transformation at a high cooling rate and relatively low temperature for the purpose of producing acicular ferrite [16,32]. Slow cooling to 580 \({}^{\circ}\)C (1.4 \({}^{\circ}\)Cs\({}^{-1}\))--1 st step of stabilization. The timing was chosen to maximize a ferrite fraction and thus C redistribution. Fast cooling to 450 \({}^{\circ}\)C (40 \({}^{\circ}\)Cs\({}^{-1}\))--to avoid pearlite transformation,  
Isothermal holding for 600 s--2 nd step of stabilization,  
Final cooling to room temperature (0.5 \({}^{\circ}\)Cs\({}^{-1}\)). To obtain strongly deformed pancake austenite, the finish temperature of the hotworking was set at 750 \({}^{\circ}\)C. The cooling parameters were selected based on CCT (continuous cooling transformation) diagrams. After conducting pilot tests at the laboratory scale, semi-industrial thermomechanical processing was applied based on the same process parameters (3 passes with \(\epsilon\): 0.28, 0.30, 0.22, respectively). The rolling was conducted in the semi-industrial line, the detailed characteristics of which is described in [33]. For the rolling, the D550 two-high reversing hot rolling mill with a roll diameter of 550 mm and a roll barrel length of 700 mm was used. The final material was subjected to an interrupted tensile test using Zwick \(Z/100\). The samples used were 2 mm thick, 12.5 mm wide, and the gauge length was 50 mm. The performed tensile tests were stopped at 5, 10, 15% strain, and finally the full tensile tests to the fracture of the sample were applied. The following mechanical characteristics were calculated:  
* Work hardening rate using the true stress and true strain [34]: \[\sigma=\frac{F}{S}=\sigma_{p}\big{(}1+\varepsilon_{p}\big{)},\ \text{MPa}\] (1) \[\varepsilon=\ln\big{(}1+\varepsilon_{p}\big{)}\] where: \(\sigma\)--true stress, \(\varepsilon\)--true strain, \(F\)--load, \(S\)--true cross-section of the sample, \(\sigma_{p}\)--engineering stress and \(\varepsilon_{p}\)--engineering strain.