Effect of alloying elements and heating rates on ferrite recrystallization in dual phase steels

Abstract

The Dual Phase Steels (DP Steels) have attracted interest of steel manufacturers, especially automotive industry, because of a good combination of high strength and ductility. The final DP steel microstructures are formed during continuous heating to intercritical temperatures, then isothermal holding and quenching to form a mix of ferrite and martensite. Final martensite content, morphology and spatial distribution depends on the austenite formed during the isothermal holding. The austenite formation is affected by the progress of ferrite recrystallization during continuous heating. Therefore, it is important to understand the process of ferrite recrystallization.
This thesis aims to study the effect of Si and Mn, which are common alloying elements in steels, on the ferrite recrystallization. The present study extensively uses in-situ 2D X-ray Diffraction (XRD) to investigate the ferrite recrystallization kinetics. This technique is faster and records more data than the other conventional methods used to study the kinetics of recrystallization until now. This study introduces a method to quantify the recrystallized grains using Scanning Electron Microscope (SEM) and Backscattered Electron Detector (BED). This method results in faster analysis of microstructure. The microstructure analysis helped in quantifying the grain growth, its dimensions and type of nucleation. Afterwards, those have been used as input parameters for modified non-isothermal JMAK model. This model is used to obtain the kinetic parameters from the experimental in-situ 2D XRD results by model fitting. The parameters obtained are: rate constant, activation energy and Avrami exponent. In all the cases, the microstructure analysis show site-saturated nucleation and predominantly 2D grain growth. The model fitting reveals that most nucleation cases show site-saturation. The density of nucleation sites is dependent on the pearlite content of the alloys. The boundaries between deformed pearlite and ferrite being the preferred nucleation sites. In some cases, the recrystallization nuclei are also seen at the boundaries between the ferrite grains which correspond to areas with micro-segregation of Mn.
Presence of Mn is leading to an increase recrystallization start temperatures. The solute drag effect of Mn is quite high. Similarly, Si is also retarding the recrystallization. The retardation effect of Si is not as significant as Mn. The solute drag effect seems to be dependent on the velocity of the grain boundaries and consequently, on progress of recrystallization. In presence of Mn, the Si is able to interact with the moving grain boundaries. This points to co-segregation effect between Mn and Si. This effect was found to be strongest for 1:1 Si to Mn atomic fractions, where the lowest growth rates were observed.
KEYWORDS: DP steels, ferrite recrystallization, solute drag effect, in-situ 2D XRD, recrystallization kinetics, SEM, JMAK model, activation energy, co-segregation.