Metallurgy of two types of precipitation hardened high strength flat-rolled products for automobile applications

Two types of high strength flat-rolled products, which were originally developed by JFE, are presented in this paper. One is a high strength cold-rolled sheet for automobile body panels aiming to achieve a strength of 440 MPa with excellent formability, sufficient galvannealing (GA) applicability and anti-secondary work embrittlement. The other is a hot-rolled strip for under-body or chassis parts aiming at a strength of 780 MPa and having an excellent balance between elongation and stretch-flangeability, less scatter of mechanical properties and sufficient thermal stability of strength for GA application. From the metallurgical viewpoint, both products have unique features. The former contains around 50-60 ppm carbon, and Nb at the stoichiometric level in relation to carbon, which forms fine NbC precipitates in a ferrite matrix and subsequently promotes the formation of a PFZ (Precipitation Free Zone). Since the PFZ acts as a micro-yielding site, a low yield to ultimate strength ratio is attained with a fine grain structure. The latter steel contains Ti and Mo, which form MC type complex carbides by interface precipitation during γ/α transformation after hot-rolling. Since the matrix structure is composed of a ferritic single phase, excellent stretch flangeability is achieved compared to multi-phase high strength steel types with the same strength. (AU) Copyright © 2014 Companhia Brasileira de Metalurgia e Mineração (CBMM) All rights reserved.