The Synergistic Influence of Chemical Composition and Industrial Annealing Processes on Lüders Bands Formation in Interstitial Free High Strength Steels

Abstract

The manufacturing of automobile components such as door inners, floor panels, bonnet linings etc. demand high formability and drawability. A type of low carbon steels called interstitial free steels (IF-steel) provide higher formability and drawability, which makes components manufacturing of these components easy. These steels consist of very low contents of carbon and nitrogen, typically < 0.0030 wt.% C and < 0.0040 wt.% N. In IF-steel, all the interstitial elements i.e. carbon (C) and nitrogen (N) etc. are removed from solid solution by forming precipitates with stabilizing elements viz. titanium (Ti), and niobium (Nb) etc. However, IF-steels have low yield strength. This limitation is removed by addition of solid solution strengtening elements like phosphorous (P), manganese (Mn) etc. to IFsteels. This new type of steel is known as Interstitial free high strength steel (IFHS steel). The current research is mainly related to the industrial processing of interstitial free high strength steels. Literature reports that IFHS steels generally do not show Lüders bands formation due to absence of interstitial atoms in the solid solution. But, there were inputs from industry providing evidence regarding Lüders bands formation in case of IFHS steels also. This present work mainly investigates with the presence/absence of Lüders bands formation in typically four different types of IFHS steel compositions named as C1 (Tistabilized), C2 (Ti-Nb stabilized), C3 (low Ti-low Nb) and C4 (high Ti-low Nb). C4 composition (the high Ti-low Nb) also contained higher aluminium (Al) content than the other compositions. These steel compositions were subjected to the typical industrially followed batch as well as continuous annealing processes. This work presents the effect of chemical composition and annealing processing parameters on the formation of Lüders bands formation in IFHS steels. Lüders bands formation is an undesirable phenomenon, which tends to deteriorate the surface finish of material. Hence, the steel composition which shows Lüders band formation cannot be used for the manufacturing of automobile panels. Hence, the elimination of Lüders bands is a critical issue. This research relates the formation of Lüders bands with the precipitation behaviour of specific IFHS steel composition subjected to a specific industrial annealing cycle. The results of the present work show that amount of stabilizing elements such as Ti and Nb in the chemical composition imposes restriction on the selection of batch or continuous annealing parameters. This is mainly due to the fact that the precipitation behaviour of these steels change with change in annealing parameters. Thus, inappropriate chemical composition and (v) annealing process parameters result in incomplete stabilization of carbon element. As a result carbon remains free in the solid solution and subsequently gives rise to Lüders bands formation. Hence, this study could be a very useful guide for the selection of appropriate chemical composition and annealing process parameters for industrial manufacturing of interstitial free high strength steels. C4 composition used in this research is considered as a good chemical composition for batch annealing cycles in the range of 680–730℃ due to absence of Lüders bands formation. Also the best combination of properties (YS= 190 MPa; -value= 1.57; Δr-value= 0.25) obtained from this composition after BAF process at 710 ℃ with heating rate of 60 ℃/h and cooling by furnace.