Understanding and Controlling Transformations in the HAZ of 3G AHSS

Implementation of 3rd generation (3G) ultra-high strength steels (UHSS), such as quench and portioned (Q&P) steels, into autobody construction is needed to further improve vehicle fuel economy limiting production of greenhouse gas (GHG). However, the implementation of these materials is limited due to the unknown weld strength of welded parts. Currently, 1st generation UHSS such as dual-phase (DP), transformation induced plasticity (TRIP), and press-hardened steels (PHS) are being used. However, these steels are limited in their applications due to strength reduction near to the weld nugget area. 3G UHSS can potentially replace the 1G steels in some applications. Currently there are no known applications for 3rd generation UHSS due to the lack of satisfactory welding solutions. Hence, there is a research gap between the 1G and 3G steels. To overcome the gap, solutions must be found to enable resistance spot welding (RSW) of these materials while maintaining high post-welded properties.

Q&P steel is of particular interest due to the current offerings of 3G UHSS in the market. Q&P steels are made using a complex thermal cycle where a highly hardenable, high C steel is partially austentized, and then given a partial quench to between its Ms and Mf temperatures to form a martenite/austenite structure. This dual-phase structure is then heated below the Ac1 temperature so that C can partition from the martensite to the austenite phase. After the C partitioning, the material is quenched. This forms a multiphase structure containing: ferrite, martensite, tempered martensite and retained austenite. These steels have an excellent combination of strength and ductility, however their composition and structure leads to weldability concerns.

To develop the necessary properties, Q&P steels are alloyed with high amounts of C, Mn, Si, and Cr. Furthermore, these materials also can have microstructures containing high volume fractions of martensite. Both the material’s high hardenability and the high martensite volume fraction contribute to property heterogeneity across the weld zone by tempering of martensite. These changes will result in local softened or hardened zones within the weld area and will lead to an easy crack path. There are a limited number of reports were available on the Q&P steels. It is still unknown why the mechanical properties of the Q&P steels vary drastically, even though both the steels having martensite microstructure. This project will understand how welding affects the microstructure and post-welded properties of Q&P, which will be compared the changes in DP steels, a much better understood alloy system. With an understanding how the welding thermal cycle affects Q&P microstructure, welding parameters will be optimized to maximize joint properties and weldability of Q&P steels.

Industry Sponsor: ArcelorMittal Dofasco

Faculty: Elliot Biro (Waterloo)

Graduate Student: Dileep Ramachandran

Faculty Contact: Dr. Andrew Macwan