Microstructural characterization and mechanical behavior of a low-carbon 17%Mn steel

Abstract

Steels containing high levels of Mn, Si and Al exhibit high plasticity when deformed, owing to twinning-induced plastitity (TWIP) and transformation-induced plasticity (TRIP) effects. In this study, we investigated the microstructural evolution of samples of samples of a 17%Mn steel subjected to war rolling at 700° and 800°C. We also studied the effects of the microstructure of the steel samples on their mechanical behavior. Using a mathematic model the stacking fault energy of the steel was estimated to be 14.5 mJ/m 2 . This value was indicative of a martensitic transformation. The presence of martensite and twinned austenite was verified using optical microscopy, scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) analyses. The presence of austenite and ε-and α'-martensites was confirmed using X-ray diffraction (XRD) analyses and dilatometry. Increasing the rate of cold reduction resulted in the formation of a α'-martensite phase and a decrease in the volume fraction of the ε-martensite phase. The volume fractions of the various phases were measured by integrating the areas under the XRD peaks. The sample subjected to a cold-rolling reduction of 81% and a subsequent annealing treatment did not contain a α'-martensite phase, indicating that it was fully recrystallized. The Vickers microhardness of the samples increased with an increase in the cold-rolling reduction rate. However, the microhardness values of the cold-rolled samples decreased after the annealing treatment.

Effect of the Cold Rolling Reduction on the Microstructural Characteristics and Mechanical Behavior of a 0.06%C–17%Mn TRIP/TWIP Steel

“…Usually, ϵ‐martensite transforms to austenite at about 200 °C and α‐martensite reversion occurs at 550–600 °C. In the present steel, the austenite starts to recrystallize at 700 °C after 100 s, which finishes after 1000 s of soaking …”

“…For the quantification of the austenite and the ϵ‐ and α′‐martensite, the direct comparison method was used, which involves the integration of the most intense peaks of each phase characterized by the (111), (200), (220), and (311) planes of austenite, the (110), (200), (211), and (220) planes of ferrite, and the (100), (002), (101), and (102) planes of ϵ‐martensite. The integration of the intensities was done with support of a graphic software (Origin), following the methodology described by Dafé and coworkers …”

“…The austenitic microstructure keeps certain stability during plastic deformation, and as the deformation proceeds, mechanical twins are formed inside the grain together with planar dislocation arrays. These twins have similar effect as grain boundaries; they restrain the dislocation movement and refine the microstructure which allows, despite their high strength, higher strain levels, and higher energy absorption capacity …”

“…Despite the fact that most of the austenitic steels (stainless steel and high Mn) have a low to moderate SFE, they tend to form stacking faults and twins. These crystal defects influence the texture during cold rolling; for a tensile strength of 700 MPa and a uniform strain of 50%, a structure preferably oriented and with fine grains is required, which is obtained during cold rolling and recrystallization …”

The effect of the austenite grain refinement on the tensile and impact properties of cast Hadfield steel

“…Additionally, FCC metals are known to feature low values for this coefficient, suggesting that the austenite grain refinement might not be very effective to increase the yield stress of the Hadfield steel. However, the Hadfield steels show intermediate values for the stackingfault energy (20 to 40 mJ/m 2 ), which promotes the plastic deformation by the twinning mechanism (see Figure 1), using the {111}<11-2> twinning system of the austenite [11][12][13] . The synergetic action of the twinning and slip mechanisms during the plastic deformation might increase the values of the work-hardening coefficient and the toughness of the Hadfield and TWIP (twinning-induced plasticity) steels [11][12][13][14][15][16] .…”