## 1 Introduction  
The most important structural constituent of multiphase TRIP (TRansformation Induced Plasticity) [1] steels is retained austenite (RA). Its metastability enables the occurrence of strain-induced martensitic transformation (SIMT) [2], which increases strain hardening rate and strongly influences the steel plasticity [3]. The stability of RA depends on many factors, including phase morphology [4, 5], localization [6], size [7, 8] and carbon content [9, 10, 11]. Numerous studies [10, 12] have proved that a full description of the SIMT [13] requires taking into account the amount and morphology of other structural constituents [14]. Initially, attention was drawn to the possibility of the partial replacement of ferrite with a polygonal morphology by an acicular morphology. For high strength low-alloy steels, an acicular ferrite microstructure ensures a very good balance of strength and toughness [15]. [PERSON] et al. [16] found that the modification of the cooling method resulting in obtaining acicular ferrite promotes an increase in the proportion of retained austenite in 0.22 C-1.55 Mn-1.55 Si-Nb steel from 9.5 to 12.3%. The formation of acicular ferrite causes greater fragmentation of supercooled austenite grains and promotes the geometric stabilization of this phase. Similar results of the effect of acicular ferrite on increasing a fraction of retained austenite were obtained in steels with Nb micro additions by [PERSON] et al. [17] and [PERSON] et al. [18]. Bainite morphology also has a significant influence on the stability of the retained austenite [19]. [PERSON] et al. [20] and [PERSON] and [PERSON] [21] showed that the optimal combination of mechanical properties is obtained when bainite has a granular morphology. Fine islands of austenite between the plates or grains of bainitic ferrite have optimal conditions for carbon enrichment while holding the steel in the bainite range. This is caused by the short carbon diffusion path and the small dimensions of the \(\gamma\) phase areas, which favor the stabilization of austenite. Equally important is the vicinity of the harder bainitic ferrite plates, which limit the transfer of deformation to the austenite. In the preliminary stage the strain is located in the ferrite. For this reason, the martensitic transformation first takes place in grains located in the \(\gamma\) phase matrix, and the deformation of austenite located in the granular bainite takes place at a later stage of deformation, ensuring a gradual martensitic transformation [12; 22; 23]. The development of multi-phase steels in recent years is associated with the partial or complete replacement of silicon [24] by Al or P [25; 26; 27]. The study of their impact on a fraction of retained austenite obtained under different heat treatment conditions and the mechanical stability of the \(\gamma\) phase during cold deformation is also of scientific interest. Both the solutions related to the use of aluminum and phosphorus [28] have a negative impact on the factors related to the stability of the retained austenite or the mechanical properties of steel [29; 30]. The complete replacement of silicon with aluminum causes a significant reduction in the solid solution hardening and a reduction in the strength properties of the steel. Moreover, the high content of aluminum raises the M\({}_{\rm s}\) (martensitic start) [31] temperature of the retained austenite (RA), which may cause a partial transformation of this phase into martensite during the cooling of the steel from the isothermal temperature in the bainite range (fresh martensite is formed). Increasing the phosphorus concentration to about 0.1% has a positive effect on increasing the strength properties and mechanical stabilization of the retained austenite, but is usually associated with an increase in the brittleness of the steel.