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With the ultimate target of improving the deep drawability of a dual-stabilized 21%Cr ferritic stainless steel, the evolution of the flow stress, microstructure, texture, dislocation structures and precipitation during multi-pass hot deformation were studied. Plane strain compression in three passes with 0.4 -0.5 pass strains and 20 s inter-pass times was employed together with scanning electron microscopy combined with electron backscatter diffraction (SEM-EBSD) and transmission electron microscopy (TEM). The temperature of the final pass was varied between 1 223 K and 923 K and the final cooling took place either by water quenching to room temperature or water cooling to 923 K followed by cooling at 0.33 K/s to room temperature. At 1 223 K, static recrystallization was almost complete during the 20 s inter-pass times and this randomized the texture. When the deformation temperature was lowered to 1 073 K or 923 K, in-grain shear bands were formed in the grains belonging to the γ fibre. The deformation temperature of the third pass had only a minor effect on the deformation texture intensity maxima. The final dislocation structure was not changed by the cooling rate from 923 K, but slow cooling enabled precipitation to occur. The results indicate that although the deformation conditions affect the deformed microstructures and dislocation structures, the effect of the deformation temperature on the texture was insignificant.

A novel steel combining the “quenching and tempering (Q&T)” process was exploited that can achieve the enhancement of austenite by interface migration during tempering the martensitic matrix mixed with films of austenite. A high uniform elongation (12%) combined with high yield tensile strength (1500 MPa) was obtained, which showed distinct advantages over all the other advanced high strength steels under development for a lightweight car body. Furthermore, the effect of austenite on enhancement of ductility in “Q&T” steels with a martensite matrix was elucidated, which suggested that the ductility was promoted by enhancing boundary sliding and delaying work hardening of the martensitic matrix.

In the direct quenching and partitioning (DQ&P) process, tough ultra-high-strength steel is made by combining thermomechanical processing with quenching and partitioning to obtain martensite toughened by thin films of retained austenite. The hot rolling stage with deformation and recrystallization between the rolling passes affects the state of the austenite before quenching and partitioning. This paper describes the static recrystallization kinetics of two steels with compositions suited to DQ&P processing, viz. (in wt.%) 0.3C-1Si-2Mn-1Cr and 0.25C-1.5Si-3Mn. The stress relaxation technique on a Gleeble thermomechanical simulator provided recrystallization times over a wide range of temperature, strain, strain rate and initial grain size. The higher levels of Si and Mn made the recrystallization kinetics less sensitive to strain, strain rate and temperature. The equations derived to describe the recrystallization kinetics can be used in the design of the rough rolling part of thermomechanical processing.

The microstructure and mechanical properties of cold-rolled Fe-18Mn-3Al-3Si-0.03C transformation induced plasticity/twinning induced plasticity (TRIP/TWIP) steel in the temperature range of 25 to 600 • C were studied. The experimental steel exhibited a good combination of ultimate tensile strength (UTS) of 905 MPa and total elongation (TEL) of 55% at room temperature. With the increase of deformation temperature from 25 to 600 • C, the stacking fault energy (SFE) of the experimental steel increased from 14.5 to 98.8 mJm −2. The deformation mechanism of the experimental steel is controlled by both the strain induced martensite formation and strain induced deformation twinning at 25 • C. With the increase of deformation temperature from 25 to 600 • C, TRIP and TWIP effect were inhibited, and dislocation glide gradually became the main deformation mechanism. The UTS decreased monotonously from 905 to 325 MPa and the TEL decreased (from 55 to 36%, 25-400 • C) and then increased (from 36 to 64%, 400-600 • C). The change in mechanical properties is related to the thermal softening effect, TRIP effect, TWIP effect, DSA, and dislocation slip.