Dynamic Strain Aging Phenomena and Tensile Response of Medium-Mn TRIP Steel

Abstract

Dynamic strain aging (DSA) and rapid work hardening are typical behaviors observed in medium-Mn transformation-induced plasticity (TRIP) steel. Three alloys with manganese ranging from 10.2 to 13.8 wt pct with calculated room temperature stacking fault energies varying from À 2.1 to 0.7 mJ/m 2 were investigated. Significant serrations were observed in the stress-strain behavior for two of the steels and the addition of 4.6 wt pct chromium was effective in significantly reducing the occurrence of DSA. Addition of chromium to the alloy reduced DSA by precipitation of M 23 (C,N) 6 during batch annealing at 873 K (600°C) for 20 hours. Three distinct DSA mechanisms were identified: one related to manganese ordering in stacking faults associated with e-martensite and austenite interface, with activation energies for the onset and termination of DSA being 145 and 277 kJ/mol. A second mechanism was associated with carbon diffusion in c-austenite where Mn-C bonding added to the total binding energy, and activation energies of 88 and 155 kJ/mol were measured for the onset and termination of DSA. A third mechanism was attributed to dislocation pinning and unpinning by nitrogen in a-ferrite with activation energies of 64 and 123 kJ/mol being identified. Tensile behaviors of the three medium manganese steels were studied in both the hot band and batch annealed after cold working conditions. Ultimate tensile strengths ranged from 1310 to 1404 MPa with total elongation of 24.1 to 34.1 pct. X-ray diffraction (XRD) was used to determine the transformation response of the steels using interrupted tensile tests at room temperature. All three of the processed steels showed evidence of two-stage TRIP where c-austenite first transformed to e-martensite, and subsequently transformed to a-martensite.

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

“…That is why the PLC effect is so pronounced in TWIP steels. However, recent studies proved that it also takes place in medium-Mn steels [15,24,25]. This process usually takes place in a range of specific temperatures and strain rates.…”

“…So far, in the literature, there are only a few reports available concerning the PLC effect in medium manganese steels [14][15][16][17]21,24]. Most of them concern cold rolled-steels.…”

“…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.…”

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

“…Presence of carbides or complex carbonitrides also reduces the probability of DSA. It was reported [39] that an addition of 4.6 wt % Cr to the medium-Mn 0.17C-12Mn-4.5Cr-1.2Si steel significantly reduced the DSA by precipitation of M 23 (C,N) 6 occurring during batch annealing (600 • C) within 20 h. It is related to a reduction in the carbon content in the solid solution.…”

“…The occurrence of plastic instability phenomenon in medium-Mn steels is also related to a grain size. It was reported [6,39] that the dislocation density in austenite is influenced, not only by the level of deformation, but also by the size of the austenite grain. If the austenite grain is less than the critical value, sufficient dislocations cannot be generated during deformation.…”