Design of a modular alloying concept for HDG lowcarbon DP steel, its industrial implementation and experiences with OEM parts

The application of higher strength multiphase steels (AHSS) in automotive body construction can significantly contribute to an increased crash safety level and at the same time reduce bod y weight. However, frequently encountered difficulties during body manufacturing, due to reduced formability, enhanced crack formation at trim edges, increased spring-back or deteriorated fracture behavior of weld joints, are t ypically related to unfavorable microstructures. The strict limitation of the C content to below 0.1 wt.% across all strength classes ranging from 500 to 1000 MPa allowed the development of hot dip galvanized dual phase steels which avoided the mentioned manufacturing difficulties. Key to the successful development was a well-tuned addition of Nb for grain refinement, in combination with Mn and Cr bulk alloying on the one hand and adjusted cold rolling reductions, as well as annealing and cooling conditions on the other hand. This modular alloying concept not only brings about advantages for the steel user but it also simplifies sequencing during production in the steel mill when producing various strength grades. For instance, the transition between heats of two strength classes during continuous casting can be shortened and also coil welding between continuous processes such as pickling, tandem rolling and hot dip galvanizing becomes simpler and safer. Extended annealing simulations were utilized to optimize the combination of alloying elements during the design of the modular alloy concept. These were helpful to fine-tune the temperature profile with regard to the transformation behavior, which in turn is influenced by cold reduction and chemical composition. These simulations were performed under boundary conditions reflecting the process parameters in the industrial annealing lines. By that approach, swift industrial implementation became possible. The resulting trial coils rapidly satisfied general customer demands and in particular concerning surface quality with regard to roughness and zinc adhesion. Consequently, a quick production release by OEMs could be achieved. (AU) © 2015 Companhia Brasileira de Metalurgia e Mineração (CBMM)