Abstract
This article is focused on the mechanical behavior and its relationship with the microstructural changes observed in two high-manganese steels presenting twinning-induced plasticity (TWIP) and transformation-induced plasticity (TRIP), namely Steel B and Steel C, respectively. Chemical compositions were similar in manganese, but carbon content of Steel B approximately doubles Steel C, which directly impacted on the stacking fault energy (SFE), microstructure and mechanical response of each alloy. Characterization of as-cast condition by optical microscope revealed a fully austenitic microstructure in Steel B and a mixed microstructure in Steel C consisting of austenite grains and thermal-induced (εt) martensite platelets. Same phases were observed after the thermo-mechanical treatment and tensile tests, corroborated by means of X-Ray Diffraction (XRD), which confirms no phase transformation in Steel B and TRIP effect in Steel C, due to the strain-induced γFCC→εHCP transformation that results in an increase in the ε-martensite volume fraction. Higher values of ultimate tensile strength, yield stress, ductility and impact toughness were obtained for Steel B. Significant microstructural changes were revealed in tensile specimens as a consequence of the operating hardening mechanisms. Scanning Electron Microscopy (SEM) observations on the tensile and impact test specimens showed differences in fracture micro-mechanisms.
Highlights
The constant technological development that took place during the last decades in many areas of the metal-mechanical industry led to important challenges
Two 25 kg steel ingots of 90 mm thickness × 90 mm width × 400 mm length with different chemical compositions were cast by induction melting
The driving force for the ε-martensite transformation established by Hirth [17] and popularized later by Olson and Cohen [28]: stacking fault energy (SFE) = 2ρ∆G γ→ε + 2σγ/ε where ρ is the molar surface density, ∆G γ→ε is the total Gibbs free energy change for the γ→ε γFCC →εHCP transformation (∆G γ→ε ) which has a chemical contribution (∆Gchem ) and a γ→ε magnetic contribution (∆Gmag ) and σγ/ε is the interfacial energy between the austenite γ→ε and ε-martensite adopted as 10 mJ/m2 [34]
Summary
The constant technological development that took place during the last decades in many areas of the metal-mechanical industry led to important challenges. High-manganese twinning-induced and transformation-induced plasticity (TWIP/TRIP) belong to a new generation of AHSS with typical manganese contents between 15 and 30 wt%. Their exceptional combination of strength and ductility (~900–1000 MPa at ~50–60%) and excellent energy absorption in impact tests took the attention of the automotive industry [1,2,3,4,5,6,7]
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