Optimisation of CAL annealing parameters for achieving r-bar greater than 2
Optimisation of continuous annealing process parameters to achieve ř values above 2.0 in full hard cold rolled IFHS Steel using annealing Simulator
The present research aims to address the effect of chemical composition, in particular effect of stabilizing interstitial elements, on industrial manufacturing processes of interstitial free high strength (IFHS) steel. Four different interstitial free high strength steels stabilized with elements; a) Ti, b)Ti-Nb stabilized, and c) Al addition to Ti-Nb stabilized were used in this study. These steels were manufactured in the steel industry to a finished cold rolled thickness of 0.8mm. Cold rolled steels were subjected to annealing simulations using a custom designed annealing simulator. Annealing simulation parameters used were similar to industrial batch (BAF) and continuous annealing line (CAL) processes. Processed steel was evaluated for formation of Lüders bands, critical for surface finish for automobile panel manufacturing, during tensile deformation. The results of the present work show that chemical composition of stabilizing elements (Ti, Nb) imposes restriction on selection of batch or continuous annealing line manufacturing process. This is because formation/absence of Lüders bands strongly depends on the precipitation characteristics of steel during annealing process. This has been verified with the data of industrially produced IF and IFHS steel produced through BAF, CAL and Continuous Galvanizing Line (CGL). This study could be a very useful guide for selection of appropriate annealing process and chemical composition for industrial manufacturing of IF and IFHS steels.
The present research work is an attempt to understand the influence of commonly stabilizing elements, Ti and Nb, on selection of industrial manufacturing BAF or CAL annealing process routes for the production of IFHS steel. Some of the main conclusions of the present work are as follows:
- Selection of manufacturing processes BAF or CAL industrial processes depends on chemical composition of the steel.
- The formation/absence of Lüders bands was found tobe strongly dependent on the precipitation behaviour of steel. The deficiency in concentration of stabilizing elements was considered mainly responsible for the Lüders elongation.
- Use of Al in steel chemistry as stabilizing element showed a profound effect on precipitation behavior and mechanical properties of the processed IFHS steels. High Al content resulted in preferential formation of AlN precipitates over TiN precipitates when annealed through BAF process. Therefore, in BAF process the availability of Ti is increased in steel for stabilizing of interstitial Carbon.
In CAL process at higher annealing temperatures as well as higher cooling rate compared to BAF process, increased soluble C by way of loss of Ti in tying up with N and by dissociation of NbC at high temperature. To fix this dissociated C additional microalloying is necessary for effective locking of dislocation during deformation.
In the present work, effect of BAF and CAL process, which is typically followed in the steel industry, on four different IFHS steel compositions were looked into. Emphaisis was laid primarily on effect of annealing process on the efficacy of stabilising elements in eliminating undesirable Lüders bands formation. By using a custom desinged annealing simulator it was possible to simulate process parameters identical to industrial manufacturing processes. Finally, an attempt was made to understand the effect of annealing process on composition for stabilising the interstitial elements. Few industrially produced IF and IFHS grades chemistry has been varified for the stability.
Fig.1: Stress-strain curves for Ti-Nb stabilized IFHS steel annealed after (a) batch annealing (BAF) and (b) continuous annealing (CAL) process showing absence of Lüders elongation