Abstract
Detailed studies on microstructure–property relationships of thermomechanically processed medium-Mn steels with various manganese contents were carried out. Microscopic techniques of different resolution (LM, SEM, TEM) and X-Ray diffraction methods were applied. Static tensile tests were performed to characterize mechanical properties of the investigated steels and to determine the tendency of retained austenite to strain-induced martensitic transformation. Obtained results allowed to characterize the microstructural aspects of strain-induced martensitic transformation and its effect on the mechanical properties. It was found that the mechanical stability of retained austenite depends significantly on the manganese content. An increase in manganese content from 3.3% to 4.7% has a significant impact on the microstructure, stability of γ phase and mechanical properties of the investigated steels. The initial amount of retained austenite was higher for the 3Mn-1.5Al steel in comparison to 5Mn-1.5%Al steel—17% and 11%, respectively. The mechanical stability of retained austenite is significantly affected by the morphology of this phase.
Highlights
A microstructure consisting of ferrite, bainite and retained austenite typical for the first generation AHSSs (Advanced High-Strength Steel) is characterized by a large difference in hardness between ferrite and martensite formed as a result of strain-induced transformation
Microstructural observations show that the Mn content affects the microstructure of the investigated steels (Figure 2)
The results showed that the increase in Mn content from 3.3% to 4.7% significantly affects the the microstructure and strengthening
Summary
A microstructure consisting of ferrite, bainite and retained austenite typical for the first generation AHSSs (Advanced High-Strength Steel) is characterized by a large difference in hardness between ferrite and martensite formed as a result of strain-induced transformation. The hardness difference can be reduced by ferrite strengthening, using thermomechanical treatment or precipitation strengthening [1,2]. An alternative solution is to replace the ferrite matrix by bainite [3]. This can be done by an increase in steel hardenability. The microstructure consisting of bainite and retained austenite allows to reduce the hardness difference between microstructural components, which results in much better edge formability, stretch flangeability and mechanical properties [5,6,7]
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