Critical Aspects Concerning Large-scale Production of a Batch-annealed Medium-Mn 780 MPa Grade for Automotive Applications

Abstract

AbstractThis article presents the results of the development of a medium-Mn780 grade at large scale production via conventional LD-converter route at the voestalpine steel plant in Linz. It highlights the property profile of the newly developed grade—being a well-balanced global-formability type for demanding deep drawing operations and crash applications. Furthermore, it offers the possibility of weight reduction by replacing grades with lower strength levels.The article highlights critical aspects of the application of the material that had to be overcome. The first aspect addresses the often-observed presence of extensive yield point elongation (YPE) in medium-Mn steels. This contribution clearly shows that YPE can be avoided by a two-step heat treatment. The second aspect concerns the robustness of the manufacturing process and refers to the sensitivity of mechanical properties to the intercritical annealing temperature, which is a concern for industrial scale production. Based on the large-scale material, this contribution can emphasize that, with a precise temperature control during a batch annealing cycle, stable mechanical properties throughout an entire coil can be achieved. The third aspect addresses the necessary suitability of the material for the resistance spot welding process. In our first investigations, the material revealed rather low cross tension strength (CTS) after welding with standard parameters. Therefore, the chemical composition was adjusted and a double-pulse regime was established to vastly increase CTS.Considering all the above-mentioned aspects, this contribution represents a compilation of critical points and possible solutions towards the large-scale implementation and subsequent use of the present material by the automotive industry.