Effect of Si content on novel medium-Mn complex phase steels

Abstract

This investigation presents the development of third-generation advanced high-strength steels (AHSS) for automotive industries, focusing on materials with higher strength, plasticity, and crashworthiness. Three alloy steels Fe-6Mn-(1, 1.6, 2.0) Si were developed via a melting route using an arc melting furnace followed by homogenization at 1200 °C, hot rolling at 1100 °C, and then air cooling. The developed steels are characterized using FE-SEM, XRD, microhardness tester, and universal testing machine. The FE-SEM micrographs exhibit a complex phase microstructure containing pearlite, retained austenite, martensite, ferrite, and bainite. The microhardness values of novel alloys are measured as 423, 441, and 475 VHN. The tensile strengths of alloys were achieved at 1409 MPa, 1497 MPa, and 1438 MPa with elongations of 18, 15, and 14% respectively. Atom probe tomography results confirmed the presence of retained austenite of film thickness 25–27 nm. The fracture surface of alloy steel containing 1 wt% Si and 6 wt% Mn exhibits dimples, causing ductile fracture, while alloy steel containing 1.6–2.0 wt% Si and 6 wt% Mn combines dimples and facets, confirming intergranular fracture with a 14 % ductility limit. The average dimple size decreases from 0.82 µm to 0.64 µm with an increase in silicon from 1 wt% to 2 wt%.