Casting and Constitutive Hot Flow Behavior of Medium-Mn Automotive Steel with Nb as Microalloying

Abstract

A novel medium-Mn steel microstructure with 0.1 wt.% Nb was designed using Thermo-Calc and JMatPro thermodynamic simulation software. The pseudo-binary equilibrium phase diagram and time–temperature transformation (TTT) and continuous cooling transformation (CCT) diagrams were simulated in order to analyze the evolution of equilibrium phases during solidification and homogenization heat treatment. Subsequently, the steel was cast in a vacuum induction furnace with the composition selected from simulations. The specimens were heat-treated at 1200 °C and water-quenched. The results of the simulations were compared to the experimental results. The microstructure was characterized using optical microscopy (OM) and scanning electron microscopy (SEM). We found that the as-cast microstructure consisted mainly of a mixture of martensite, ferrite, and a low amount of austenite, while the microstructure in the homogenization condition corresponded to martensite and retained austenite, which was verified by X-ray diffraction tests. In order to design further production stages of the steel, the homogenized samples were subjected to hot compression testing to determine their plastic flow behavior, employing deformation rates of 0.083 and 0.83 s−1, and temperatures of 800 and 950 °C.

Thermomechanical Processing of Steels

“…The present Special Issue on Thermomechanical Processing of Steels includes eleven research papers [1][2][3][4][5][6][7][8][9][10][11]. A wide range of steel grades are covered in these papers.…”

“…A wide range of steel grades are covered in these papers. Although most of the papers deal with low carbon microalloyed grades [1,3,6,9,10], several papers study ferritic stainless steels [2,5,7] and others focus on grades such as Fe-Al-Cr alloys [4], medium-Mn Nb microalloyed steels [8], and medium carbon V microalloyed grades [11].…”

“…While some papers cover specific performance problems and optimization alternatives for industrial processing conditions [3,7,8], most of the papers deal with more fundamental analyses of physical metallurgy mechanisms and microstructural evolution [1][2][3][4][5][6][7][9][10][11].…”

“…Cerda Vázquez et al [8] propose a novel medium-Mn steel (6.5 wt.% Mn) microstructure with 0.1 wt.% Nb designed using Thermo-Calc and JMatPro thermodynamic simulation software. The as-cast microstructure consists mainly of a mixture of martensite, ferrite, and a low amount of austenite, while the microstructure in the homogenization condition corresponds to martensite and retained austenite, which is verified by X-ray diffraction tests.…”

A scale up study on chemical segregation and the effects on tensile properties in two medium Mn steel castings

“…The second problem is the retention of δ-ferrite to room temperature. δ-ferrite is the first phase to form during the solidification of medium Mn steels but is expected to transform to austenite at typical hot rolling and slab reheat temperatures (< 1280 • C) [12,13]. However, δ-ferrite can be stabilised to room temperature when there are excess Al or Si additions [14,15].…”