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• The effect of IBT time on the microstructure and properties was studied. • The TRIP effect is jointly controlled by the content and stability of RA. • The elongation is 31.5%, while its strength meets the demand of TRIP-980 steel. Insufficient ductility is the main reason that limits the application of transformation-induced plasticity (TRIP) steel in the automotive industry. In this work, by designing a two-stage heat treatment method of intercritical annealing (IA) + isothermal bainitic transformation (IBT), the effect of IBT time on the microstructure and properties was studied. The results show that the volume fraction of retained austenite (RA) has almost no difference at different IBT times. When the IBT time is 4 min, the RA has the highest stability. Finally, the perfect combination of strength and ductility is achieved in this sample. While the strength meets the requirements of TRIP-980 steel, the total elongation reaches more than 30 %. [ABSTRACT FROM AUTHOR]

A multi-heterogeneous austenite morphologies consisting of equiaxed + coarse lath-like + fine lamellar austenite grains is constructed in 3Mn steel through warm rolling in the dual-phase region and tempering. Lower tempering temperature induces a more uniform distribution of multi-heterogeneous austenite morphologies and effectively preserves the stacking fault network formed during warm rolling, which catches exceptional mechanical properties (ultimate tensile strength of 1591 MPa and total elongation of 24.5 %) as well as suppresses plastic instability behaviors including yield plateau and stress serrations in 3Mn steel. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • The introduction and retention of austenite during warm rolling is beneficial to the improvement of mechanical properties. • Warm rolling in dual-phase region and tempering induces the formation of multi-heterogeneous austenite morphologies. • The formation of superlattice structures is attributed to the construction of stacking fault networks. Facing the current lack of clarity in temperature selection mechanisms for warm rolling in medium Mn steel, three warm rolling methods were conducted in 3Mn steel according to the rolling temperatures in the dual-phases region or ferrite region. Subsequently, the effect of heat treatment on the austenite morphologies and tensile properties of warm-rolling samples was studied. Rolling in dual-phase regions (750 °C) led to heterogeneous austenite morphology composed of equiaxed and lamellar grains while rolling in the ferrite region (600 °C) resulted in significant cementite precipitation and the absence of retained austenite. Warm rolling promoted the formation of dislocation cells in the lath-like grains, and they did not evolve into grain boundaries completely because low deformation temperature didn't trigger the complete dynamic recrystallization. Tempering after rolling provided a significant enhancement in plasticity through the enrichment of C and Mn in austenite and the formation of multiple heterogeneous austenite morphologies with equiaxed, coarse lath-like and fine lamellar grains. In addition, superlattice structures in the matrix are formed through the formation of stacking fault networks and the spinodal decomposition. Ultimately, the exceptional properties were achieved in 3Mn steel, with an ultimate tensile strength of 1409 MPa and a total elongation of 27.3 %. [ABSTRACT FROM AUTHOR]

This work explores the carbide evolution behavior and mechanical property evolution mechanisms of intercritical-annealed 3Mn steel during the tempering process by comparing different tempering times (5/10/20 min). The results showed that the elongation of 3Mn steel increased from 19.5% to 26.5% after tempering while maintaining a tensile strength above 1250 MPa. It is also found that carbide precipitation during tempering is first followed by dissolution. The mechanical properties exhibited abnormal degradation at tempering for 10 min, with a tensile strength of 1265 MPa and a total elongation of 16.0%. However, the optimal mechanical properties were achieved at tempering for 20 min, with a tensile strength of 1286 MPa and a total elongation of 26.5%. During the initial stage of tempering, a transformation occurs from M 3 C to stable M 7 C 3 , with the presence of numerous dislocations and grain boundaries promoting the precipitation and growth of abundant carbides. This leads to a reduction in the C and Mn content in the matrix, consequently decreasing the austenite content and stability, resulting in the transformation of austenite to the BCC phase during tempering instead of cooling. The decreased austenite content and stability lead to reduced ductility. Subsequently, dislocations were annihilated during tempering, suppressing carbide precipitation, and the over-precipitated carbides dissolved under thermodynamic driving force, leading to an increase in C and Mn content in the remaining austenite, thus enhancing its stability. The precipitation and dissolution of carbides during tempering, along with the partitioning of elements between carbides and austenite, strengthen austenite stability and improve mechanical properties. • During tempering, M 3 C was transformed into M 7 C 3 as well as the carbides precipitated first and then dissolved. • During tempering, dissolution of carbieds leads to the partitioning of C and Mn to retained ausetnite, which strengthens the stability of austenite. • Tempering promotes the product of strength and elongation from 26.7GPa∙% to 34.1 GPa∙%. [ABSTRACT FROM AUTHOR]

The present study on 3Mn steel demonstrates that the newly proposed "intercritical annealing-tempering (IAT)" process led to different performances of austenite stability and plastic instability by adopting different cooling methods (water quenching and air cooling) in the intercritical annealing process (IA). Air cooling promoted the partitioning of C and Mn to austenite and improved the stability of austenite, accompanied by the increase of M 7 C 3 precipitation during tempering. As two competing mechanisms affecting the elements partitioning during tempering, carbon partitioning and M 7 C 3 precipitation enhanced and weaken austenite stability, respectively, and more precipitation of M 7 C 3 in IA-AT (tempering after air cooling) resulted in more austenite decomposition than that in IA-WT (tempering after water quenching). In addition, compared to the appearance of the yield plateau and stress serration in IA-WT, the suppression of plastic instability behavior in IA-AT was attributed to the rapid proliferation of geometrically necessary dislocations (GNDs) caused by the stronger TRIP effect and a large amount of M 7 C 3 precipitation. The study of microstructure evolution and plastic instability behavior induced by different annealing cooling methods would help in further optimizing the heat treatment process to achieve better mechanical properties for medium Mn steel. • Air cooling promoted the partitioning of C and Mn into austenite in IA and retained more austenite than water quenching. • During tempering, the stability of austenite was increased for the partitioning of C and reduced for more M 7 C 3 precipitation. • Rapid TRIP effect and more M 7 C 3 particles promoted the proliferation of GNDs and suppressed the plastic instability behavior. [ABSTRACT FROM AUTHOR]