Abstract
The present work focuses on the investigation of both microstructure and resulting mechanical properties of different lean medium Mn Quenching and Partitioning (Q&P) steels with 0.2 wt.% C, 1.5 wt.% Si, and 3–4 wt.% Mn. By means of dilatometry, a significant influence of the Mn-content on their transformation behavior was observed. Light optical and scanning electron microscopy (LOM, SEM) was used to characterize the microstructure consisting of tempered martensite (α’’), retained austenite (RA), partially bainitic ferrite (αB), and final martensite (α’final) formed during final cooling to room temperature (RT). Using the saturation magnetization measurements (SMM), a beneficial impact of the increasing Mn-content on the volume fraction of RA could be found. This remarkably determined the mechanical properties of the investigated steels, since the larger amount of RA with its lower chemical stabilization against the strain-induced martensite transformation (SIMT) highly influenced their overall stress-strain behavior. With increasing Mn-content the ultimate tensile strength (UTS) rose without considerable deterioration in total elongation (TE), leading to an enhanced combination of strength and ductility with UTS × TE exceeding 22,500 MPa%. However, for the steel grades containing an elevated Mn-content, a narrower process window was observed due to the tendency to form α’final.
Highlights
Increasing requirements of the automotive industry related to lightweight construction and increased passenger safety drive the development of advanced high strength steels (AHSS) [1,2].Currently, research focuses on the development of the third generation AHSS, including the concepts of medium Mn and Quenching and Partitioning (Q&P) steels
These steel grades offer a promising combination of strength and ductility achieved by a microstructure having a substantial amount of retained austenite (RA) which transforms into strain-induced martensite (α’) due to the transformation induced plasticity (TRIP) effect [3,4,5]
Medium Mn steels with a typical chemical composition of 0.05–0.2 wt.% C and 3–10 wt.% Mn have a microstructure consisting of an ultrafine-grained ferritic (α) matrix and volume fractions of retained austenite (RA) up to 40 vol.%
Summary
Research focuses on the development of the third generation AHSS, including the concepts of medium Mn and Quenching and Partitioning (Q&P) steels These steel grades offer a promising combination of strength and ductility achieved by a microstructure having a substantial amount of retained austenite (RA) which transforms into strain-induced martensite (α’) due to the transformation induced plasticity (TRIP) effect [3,4,5]. Medium Mn steels with a typical chemical composition of 0.05–0.2 wt.% C and 3–10 wt.% Mn have a microstructure consisting of an ultrafine-grained ferritic (α) matrix and volume fractions of retained austenite (RA) up to 40 vol.% They are characterized by an excellent combination. After heating in order to obtain a fully austenitic microstructure, the steel is initially quenched to a specific quenching temperature (TQ ) in the MS -Mf temperature range, where austenite partially transforms into primary martensite (α’prim )
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