Abstract
This study investigated the microstructure–tensile properties relationship and the retained austenite room temperature stability of a Nb and Mo micro-alloyed medium manganese transformation induced plasticity (TRIP) steel. A number of findings were obtained. Most importantly, the steel after being processed by quenching and tempering (Q & T) exhibited excellent tensile properties, i.e., the strength of 878–1373 MPa, the ductility of 18–40% Mo, and Nb microalloying served to control the fraction of retained austenite and to improve tensile strength by fine grain strengthening. Excellent tensile properties were attributable to the large amount of retained austenite which produced a discontinuous TRIP effect. This effect led to the production a large amount of martensite which relieved the stress concentration, contributing to the coordinated deformation between the phases and thus improving the deformability of the steel. Additionally, the differences in Mn and C contents led to varying degrees of austenite stability and the length of the Lüders band decreased as the intercritical annealing temperature increased. The micro-alloyed medium manganese steel experimented on our study showed considerable improvement in tensile properties in comparison with the 5Mn-0.1C medium manganese steel in previous studies.
Highlights
Research on automobile plate steel has focused on improving fuel economy and reducing exhaust emissions by lessening vehicle weight in recent years [1]
Grain recovery occurred at 675–750 ◦ C in the cold rolled structure as the annealing temperature increased, the martensite plates were completely recovered at 750 ◦ C, and the grains were gradually coarsened with increase in annealing temperature (Figure 3d–f)
We have come to the following conclusions: (a) The steel subjected to Q & T and quenched from 650 ◦ C
Summary
Research on automobile plate steel has focused on improving fuel economy and reducing exhaust emissions by lessening vehicle weight in recent years [1]. As a candidate for automobile plate, medium-Mn steel has such attractive properties as excellent combination of high strength and considerably large ductility (>30 GPa%), light weight, high safety, and relatively lower alloying cost [3,4,5,6,7,8,9]. These properties result from the addition of a number of elements featuring Mn. Mn, an austenite stabilizer, triggers an extensive TRIP effect by enhancing the austenite volume fraction, which in turn increases steel strength and ductility.
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