Influence of cold forging and annealing on microstructure and mechanical properties of a high-Mn TWIP steel

Abstract

The influence of cold biaxial forging and annealing on the microstructure evolution and mechanical properties of Fe-18Mn-0.6C-0.1N TWIP steel was investigated. The microstructure after thermomechanical treatment was examined by means of scanning electron microscopy (SEM) with electron back-scatter diffraction (EBSD) analyzer and transmission electron microscopy (TEM). The microstructure containing a high density of deformation twins was evolved by cold forging. Then, the deformation microstructure was recovered or partially recrystallized during subsequent annealing for 1 h at 673 or 873 K, respectively. The cold biaxial forging followed by partial recrystallization resulted in a very attractive combination of mechanical properties. The yield strength and the ultimate tensile strength of the steel after cold biaxial forging were 1320 and 1540 MPa, respectively, with elongation of 6 %. On the other hand, the yield and ultimate tensile strengths of 770 and 1110 MPa, respectively, and elongation about 40 % were obtained after partial recrystallization. The as-forged and recovery annealed samples showed fatigue limit of 600 and 550 MPa, respectively, and the sample after partial recrystallization showed slightly lower fatigue limit of 500 MPa.K e y w o r d s : TWIP-steel, deformation twinning, microstructure, mechanical properties, fatigue

Microstructure and Mechanical Properties of a High-Mn TWIP Steel Subjected to Cold Rolling and Annealing

“…It is clearly seen that the dislocation strengthening can be expressed by Equation (4) with α = 0.6. The value of α has been reported varying from about 0.2 to 0.5 [14,15,24,[30][31][32]. Relatively large α = 0.6 obtained for the present work hardened steel samples may be attributed to an enhanced efficiency of dislocation strengthening in high-Mn austenitic steels with low SFE as well as to somewhat underestimated dislocation density by the KAM values, which are actually associated with excess dislocations of similar Burgers vectors rather than total dislocation density [33].…”

“…The mean grain size (D) and the kernel average misorientation (KAM) were obtained using OIM Analysis 6 software (EDAX Inc., version 6.2.0, Mahwah, NJ, USA). The dislocation density was evaluated by means of KAM as [14] …”

“…The range of annealing temperatures and the annealing time were selected to cover the operation of various restoration mechanisms including recovery and recrystallization and, therefore, to obtain variety of annealed microstructures-i.e., recovered, partially recrystallized, and fully recrystallized [13,14]. The effect of annealing temperature on the hardness and fractional softening of the present high-Mn steel subjected to cold rolling to various total strains is shown in Figure 3.…”

“…On the other hand, low SFE hampers any dislocation rearrangements and, thus, slows down the dislocation recovery processes. Therefore, high-Mn austenitic steels are hardly susceptible to dynamic recovery under warm rolling conditions, and the work hardened microstructures remain almost unchanged up to high temperatures sufficient for recrystallization development [13,14].…”

“…High-Mn austenitic steels exhibit significant strain hardening owing to increasing the dislocation density during cold-to-warm working [13][14][15]. The dislocation strengthening leads to remarkable increase in both the yield strength and the ultimate tensile strength [15].…”