Abstract
Medium-Mn steels are characterized by ultrafine-grained (UFG) duplex microstructure consisting of ferrite and a large amount of retained austenite. Intercritical annealing is of great importance to achieving the UFG duplex microstructure and adjusting amount as well as stability of retained austenite. In the present work, the influence of intercritical annealing temperature on the microstructure and mechanical properties was investigated in a cold-rolled medium-Mn steel Fe-12Mn-3Al-0.05C. Particularly, the dependence of microstructural morphology on intercritical annealing temperature was emphasized to reveal the genesis of the microstructural morphology in medium-Mn steels. The ferrite-austenite duplex microstructure manifested an elongated morphology in the specimen annealed at 555 °C, which inherited the lath structure of the cold-rolled state. The medium-Mn steel exhibited a continuous yielding behavior and a relatively low strain-hardening rate. With an increase in intercritical annealing temperature up to 650 °C, the amount of retained austenite increased and microstructure was partially recrystallized, showing a mixture of elongated and equiaxed grain morphologies. When the intercritical annealing was applied at 700 °C, the medium-Mn steel mainly exhibited recrystallized microstructure with equiaxed morphology. The optimal balance between the amount and the stability of retained austenite led to an enhancement of strain hardening and ductility. With a further increase in the intercritical annealing temperature to 750 °C, the medium-Mn steel possessed pronounced strain-hardening behavior at the beginning of the tensile deformation with deteriorated ductility.
Highlights
The lightweight concept in the automotive industry promotes the progressive development of advanced high strength steels (AHSS) [1]
In addition to therelies on intercritical annealing as well.of retained austenite on intercritical annealing temperature, dependence of thetemperature fraction and stability we found in the present study thatwas the carried morphology of the cold-rolled medium-Mn closely relies
The intercritical annealing temperature has a great influence on the microstructure of the cold-rolled medium-Mn steel, in terms of amount and stability of retained austenite, as well as the microstructural morphology
Summary
The lightweight concept in the automotive industry promotes the progressive development of advanced high strength steels (AHSS) [1]. The first-generation AHSS with ferritic matrix possess limited combination of strength and ductility, yielding in a product of ultimate tensile strength (UTS) and total elongation (TEL) of about 20 GPa% [2,3], while the second-generation AHSS with austenitic matrix offer extraordinary high strength (UTS > 800 MPa) and superior ductility (TEL up to ~80%) [4,5,6]. The high alloying concentration and sophisticated metallurgy processes hinder the industrial production of the second-generation AHSS [6,7]. Medium-Mn steels containing 3–12 wt % Mn become one of the most promising candidates in the third-generation AHSS, showing great potential for Metals 2018, 8, 357; doi:10.3390/met8050357 www.mdpi.com/journal/metals. The most common alloy systems of medium-Mn steels include
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