Benefits and Contributions of NB Microalloying to the Advanced High Strength Steels and Hot Stamping Steels for the Automotive Application

by Dr. Bian Jian

Publisher - CBMM, Brazil

Category - General Novel

In order to improve the fuel-efficiency and to reduce the CO2 emission the automotive industry worldwide is pursuing the lightweight technology. With application of advanced high strength steels like DP, TRIP or hot stamping steels a weight reduction about 20% can be achieved for the crash relevant components without safety compromise and cost increase. This paper presents some of the latest developments with regard to the material concepts of some selected 2011car models from Europe, America and Asia. The focus of the paper is to demonstrate how to use the Nb microalloying to further improve these high strength steels for the automotive application. The strengthening mechanism of advanced high strength steels as well as hot stamping steels is based on higher carbon content and transformation hardening compared to conventional high strength steels. Consequently this leads to on one hand the poor toughness from material side and poor weldability from processing side and on the other hand to the problem of the poor formability due to the inhomogeneity of the mixed microstructure of soft and hard phases. The key to the improvement lies in the grain refinement effect of Nb alloying, because the grain refinement is the only strengthening mechanism which can increase the strength and toughness at the same time. For the medium or high carbon steels like hot stamping steels a better toughness will ensure a better component performance in the crash situation. With the extra strength increase from grain refinement and precipitation hardening due to Nb alloying it is possible to reduce the carbon content in the steel which brings in return a better weldability. Finally the grain refinement can homogenize the microstructure of multiphase steels and reduce or eliminate the banded structure of hard phases like perlite and martensite, which usually causes problems during forming processes especially by bending and hole expansion operations. Finally an innovative alloying design for hot stamping steels with the aim to change the old Manganese -Boron concept will be introduced. Microalloying with Nb can not only upgrade the steel quality but also provide the automotive industry with the most reliable and most economic materials for the lightweight achievement.

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