Effects of CGL-compatible thermal processing, starting microstructure, and Sn micro-alloying on the mechanical properties of a medium-Mn third generation advanced high strength steel

Two prototype medium-Mn third generation advanced high strength steels (Fe-0.2C-6Mn-1.5Si-0.5Al-0.5Cr-xSn wt%, x = 0 or 0.05) were annealed with continuous galvanizing line (CGL) compatible thermal processing parameters. It was found that the starting microstructure, intercritical annealing (IA) parameters and Sn micro-alloying had a significant effect on the volume fraction, stability, and deformation-induced transformation kinetics of the retained austenite and, therefore, the resultant mechanical properties of the steels. It was shown that Sn segregation at the carbide/matrix interface inhibited C diffusion and consequently, reduced carbide dissolution and precipitation kinetics during intercritical annealing. This resulted in lower volume fraction of retained austenite with lower stability after intercritical annealing of the as-received cold-rolled Sn-containing alloy. CGL-compatible IA treatments, therefore, were only able to produce target 3G-AHSS mechanical properties in the as-received cold-rolled alloy without Sn micro-alloying. However, the martensitic starting microstructures were more robust in achieving the 3G-AHSS target mechanical properties for both alloys. A martensitic starting microstructure resulted in higher volume fraction of stable retained austenite for 650 °C and 675 °C IA treatments. Gradual transformation of this stable retained austenite to martensite (TRIP effect) and mechanical twinning (TWIP effect) resulted in high sustained work hardening rates during plastic deformation. As a result, a favourable strength/ductility balance was achieved in the prototype steels. Moreover, Sn micro-alloying did not have a significant detrimental effect on the mechanical properties for the martensitic starting microstructure heat treatments which achieved target 3G-AHSS properties. This is promising in terms of developing a CGL-compatible processing route for these prototype steels.