A new annealing route for industrial processing of dual phase steels to obtain improved mechanical properties

Abstract

Dual-phase (DP) steels offer high potential of weight reduction without sacrificing mechanical properties for their application in automotive industry. They derive their properties through the second hard phase (martensite/ bainite) in a ferrite matrix. DP steels are mostly produced through the continuous annealing process route in the industry because of the requirement of high production rates, uniformity in properties, and leaner chemistry design feasibility. Important mechanical properties which are desirable for the final components include absence of yield point, low yield point to ultimate tensile strength, high strain hardenability along with high ductility etc. The main objective of the present work was to improve the mechanical properties of a low carbon Si based ferrite-martensite DP steel by tailoring the second phase (martensite) morphology, distribution, and size in the ferrite matrix. An existing conventional continuous annealing process (CAL) route was modified to develop an improved annealing process route suitable for industrial usage. A custom designed annealing simulator (capable of simulating conditions similar to industrial continuous annealing lines) was used to simulate the various annealing processes. Several combinations of processing routes depending on the governing mechanisms such as, ferrite recrystallization, pearlite dissolution, and phase transformation etc. were investigated for their effect on the morphology, and distribution of the martensite phase and the resulting mechanical properties. The main focus of the current work was to study the effect of heating rates, isothermal annealing temperatures, and soaking time periods (with no changes in cooling regime of conventional CAL) on the morphology, and distribution of martensite phase. Further, the effect of combining thermal cycling as a pretreatment to conventional CAL processing was also investigated. It was observed that by varying the above stated annealing parameters, it was possible to trigger the ferrite recrystallization, pearlite dissolution, and phase transformation at various stages of the annealing process cycle. All such changes resulted in change of martensite morphology, distribution, and even grain size and thus affected the final mechanical properties of DP steel. This entire experimentation effort resulted in the development of a new processing route called “Continuous Heating Annealing Process (CHAP)” that gave strength levels of 625 MPa with ductility similar to that obtained with the conventional CAL process with a significant improvement in strain hardening exponent. Thus, the present research provided a new annealing route (without any major changes in the conventional CAL process) for processing of DP steels with improved strength-ductility combination