Effect of Deformation Temperature on the Portevin-Le Chatelier Effect in Medium-Mn Steel

Abstract

Experimental investigations of the plastic instability phenomenon in a hot-rolled medium manganese steel were performed. The effects of tensile deformation in a temperature range of 20-140 • C on the microstructure, mechanical properties, and flow stress serrations were analyzed. The Portevin-Le Chatelier (PLC) phenomenon was observed for the specimens deformed at 60 • C, 100 • C, and 140 • C. It was found that the deformation temperature substantially affects the type and intensity of serrations. The type of serration was changed at different deformation temperatures. The phenomenon was not observed at room temperature. The plastic instability occurring for the steel deformed at 60 • C was detected for lower strain levels than for the specimens deformed at 100 • C and 140 • C. The increase of the deformation temperature to 100 • C and 140 • C results in shifting the PLC effect to a post uniform deformation range. The complex issues related to the interaction of work hardening, the transformation induced plasticity (TRIP) effect, and the PLC effect stimulated by the deformation temperature were addressed. stabilize the retained austenite through its carbon enrichment [8]. An alternative approach is to apply thermomechanical processing (TMP) as direct one-step cooling following the hot rolling. The TMP has the potential for energy savings and high productivity due to the elimination of the need for successive heating [9][10][11]. Chemical composition and processing parameters are designed to obtain the optimal TRIP (Transformation Induced Plasticity) effect, which ensures superior mechanical properties [12,13].Besides the many advantages of this group of steels, some problems during their processing can occur. Medium manganese steels [14-17] and high manganese steels [18][19][20] are prone to plastic instabilities phenomena associated with a serrated flow behavior-Portevin-Le Chatelier (PLC) effect and the appearance of Lüders bands. This is especially the case for cold-rolled steel sheets [21]. Moreover, it critically depends on the carbon level. With a higher C content, a pronounced effect is more visible. The heterogeneous deformation related to the increase in flow stress can lead to numerous cracks during the forming of sheets. Moreover, delayed cracking after deep drawing can appear. Delayed cracking is mostly observed in high-Mn content steels of the high austenite volume fraction. With the increased volume fraction of (retained) austenite, hydrogen embrittlement may be a more important concern. TWIP steels in the last decades have displayed important delayed cracking, which was industrially solved by decreasing the C content, Al alloying, and/or metallurgical processing [22,23]. Medium Mn steels have a less pronounced problem of delayed cracking compared to high-Mn steels [24,25].The PLC effect is commonly known as characteristic serrations which can be observed on a strain-stress curve during a tensile test. The PLC effect has not been fully characterized yet. However, some correlations between the TRIP a...

Explanation of the PLC Effect in Advanced High-Strength Medium-Mn Steels. A Review

“…Min et al [10] reported that a value of critical strain in 0.2C-2Mn-1.4Si QP steel increases with decreasing temperature. However, in works [30,46] concerning the plastic instability phenomenon in medium-Mn steels, the opposite trend was observed. Grzegorczyk et al [30] reported that a value of critical strain in 0.16C-4.7Mn-1.6Al-0.22Si-0.20Mo steel is higher at deformation temperatures of 100 and 140 • C than at 60 • C (Figure 3).…”

“…However, in works [30,46] concerning the plastic instability phenomenon in medium-Mn steels, the opposite trend was observed. Grzegorczyk et al [30] reported that a value of critical strain in 0.16C-4.7Mn-1.6Al-0.22Si-0.20Mo steel is higher at deformation temperatures of 100 and 140 • C than at 60 • C (Figure 3). The temperature drop in steels with a higher manganese content causes longer reorientation time of the C-Mn complex, which suppresses the DSA.…”

“…Due to a lower Mn content (5%-12%), medium-Mn steels are less prone to an occurrence of plastic instability phenomenon [46]. It was found [30] that an increase in a Mn content from 3% to 5% in C-Mn-Al medium-manganese steels results in the appearance of the PLC effect. It is related to higher amount of C-Mn pairs which interact dynamically with dislocations during deformation.…”

“…The PLC effect in medium-manganese steels manufactured by cold rolling has sparsely been investigated [30,46,48]. In contrast to cold-rolled medium-Mn steels, hot-rolled grades do not show a clear yield point as well as the next Lüders elongation (Figure 3).…”

“…In contrast to cold-rolled medium-Mn steels, hot-rolled grades do not show a clear yield point as well as the next Lüders elongation (Figure 3). The continuous yielding behavior is the result of mobile dislocations occurring during thermomechanical rolling [30,46,48]. The disappearance of the PLC effect in such steels can be also related to cementite precipitation which reduces a carbon content in solid solution, so the dynamic interaction with dislocations can be eliminated [48].…”