Abstract
We investigated both the microstructures and tensile properties of 7Mn steel, which was either hot-rolled, warm-rolled or cold-rolled before intercritical annealing at 700 °C for 5 h. It can be concluded that the warm-rolled and annealed microstructures are a kind of mixture of hot rolled and cold rolled ones. They are composed of ferrite and retained austenite, the latter having a wide size distribution and two types of morphologies: equiaxed and lamellar. These retained austenite grains are expected to transform to martensite in a more sustainable way—the warm-rolled and annealed steel exhibits the best combination of ultimate tensile strength and total elongation among the three studied steels and a shorter yield point elongation than the cold-rolled one.
Highlights
A new class of transformation induced plasticity (TRIP) steels containing 5–10 wt %Mn have received a wide attention because they exhibit an excellent combination of ultimate tensile strength (UTS) and total elongation (TE) owing to the presence of stable austenite in the matrix.The usual manufacturing process of these steels is the cold rolling process followed by intercritical annealing (IA), during which a part of the initial martensite or ferrite grains shall transform to austenite grains so that they can be retained afterwards at ambient temperature for the TRIP-assisted effect during deformation [1,2]
It can be seen that the hot rolled microstructures were composed of the as-quenched martensite and the equiaxed retained austenite (RA) grains, the latter having the sub-micrometer size (Figure 3a1)
During the subsequent IA treatment, new austenite grains were nucleated at the martensite lath boundaries and they grew to a lath-like morphology together with the pre-existing RA grains; which led to most of ferrite and austenite grains having the lamellar morphology after IA (Figure 3a2)
Summary
A new class of transformation induced plasticity (TRIP) steels containing 5–10 wt %. The usual manufacturing process of these steels is the cold rolling process followed by intercritical annealing (IA), during which a part of the initial martensite or ferrite grains shall transform to austenite grains so that they can be retained afterwards at ambient temperature for the TRIP-assisted effect during deformation [1,2]. Developed a 0.2C–10Mn–3Si–3Al steel, which had a dual-phased microstructure (ferrite and austenite) and exhibited the UTS value of more than 900 MPa and TE of 23%. They found that both TE and UTS increased when a larger fraction of RA transformed to martensite during the tensile deformation. The initial microstructure before IA could affect the morphology and fraction of RA and the tensile properties of medium-Mn TRIP steels.
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