# High-Temperature Mechanical Properties of 4.5%Al \(\delta\)-TRIP Steel
## 1 Introduction  
Transformation-induced plasticity (TRIP) steel refers to ultrahigh-strength steel that is ductility-elevated due to gradual martensitic transformation processes in steel structures. When there is a high content of Al in alloying elements (the Al content is higher than 3%), it is found that, during the solidification process, the \(\delta\)-ferrite does not participate in the reaction and remains in the whole temperature range, so it is named \(\delta\)-TRIP steel. Recently, \(\delta\)-TRIP steel, due to its low density, a product of ultimate tensile strength (UTS) and total elongation (TE) (up to 23 GPa\(\%\)) similar to the third generation of advanced high-strength steels (AHSS), and excellent welding properties, is expected to become the materials of automobile safety parts and structural parts [1; 2]. [PERSON] et al. [3] found an alloy (its precise composition is Fe-0.4C-0.5 Mn-0.2 Si-0.5 Cr-3.5 Al wt \(\%\)), having a density of \(7.455\times 10^{3}\) kg/m\({}^{3}\), is approximately 5% lighter than that without the aluminum addition, which has a density of \(7.8145\times 10^{3}\) kg/m\({}^{3}\). [PERSON] et al. [4] found a duplex low-density steel with an exact chemical composition of Fe-17.5 Mn-8.3 Al-0.74C-0.14 Si (wt \(\%\)), having a density of \(6.8\times 10^{3}\) kg/m\({}^{3}\). The measurement shows that the density of a 4.5% Al-containing \(\delta\)-TRIP steel is \(6.8325\times 10^{3}\) kg/m\({}^{3}\), which isapproximately 8% lighter than that with a 3.5 wt % aluminium addition. It is approximately 12.6% lighter than that without the aluminum addition, which has a density of \(7.8145\times 10^{3}\) kg/m\({}^{3}\). Many studies have been reported on mechanical deformation properties [5; 6], microstructures [7; 8], and properties of \(\delta\)-TRIP steel [9; 10]. However, there is an insufficient understanding of the high-temperature crack sensitivity of the \(\delta\)-TRIP steel. The influence of high Al content on it is still lacking, so it is necessary to understand its high-temperature mechanical properties further. There are many researches, which are regarding high-temperature mechanical properties of Nb-containing TRIP steel [11], P-containing TRIP steel [12], twinning-induced plasticity (TWIP) steel [13; 14; 15], dual-phase (DP) steel [16], and FeCrAl [17; 18]. A Gleeble thermal-mechanical testing machine is a common method for studying high-temperature mechanical properties [19]. He et al. [18] studied the hot ductility of FeCrAl alloys by using a Gleeble thermal-mechanical testing machine and thought the hot ductility of the alloys mostly depends on the temperature rather than the content of Al. The ferritic grain coarsening and the precipitation of carbides result in decreased ductility between 800 and 900 \({}^{\circ}\)C. [PERSON] et al. [18] found high-Cr-content ferrite stainless steel suffers from embrittlement when subjected to temperatures up to around 475 \({}^{\circ}\)C, due to the separation of the \(\alpha\) (Fe-rich) ferrite and the \(\alpha^{\prime}\) (Cr-rich) ferrite and the transformation of the Fe-rich \(\alpha\) phase and the Cr-rich \(\alpha^{\prime}\) phase to the \(\alpha\)-(Fe,Cr) ferrite phase by using differential scanning calorimetry (DSC). [PERSON] et al. [20] found the solid-state transformation of a \(\delta\) dendrite core to a single \(\gamma\) phase without a peritectic reaction at 1358 \({}^{\circ}\)C. Some researches [15; 21; 22] show that the increase of Al content leads to a worse hot ductility and high Al content in steel can easily lead to deterioration of surface quality of continuous casting slabs [23]. However, [PERSON] et al. [12] found that the increase of Al content in TRIP steel (0.03-0.87% Al) led to an overall increase of a reduction of area (RA) and better hot ductility. It was indicated that the effect of high Al content on the high-temperature mechanical properties of steel is a complicated change.