Microstructural characterization of a 1100 MPa complex-phase steel

Abstract

The increasing market demand for Advanced High Strength Steels (AHSS) is associated with their high mechanical strength and adequate toughness, as the automotive industry requires stronger materials with low aggregated cost. One suitable way to increase the mechanical properties of steels is to control their microstructure. The first step is to quantify the initial microstructure and then theorize how processing could further develop microstructure to achieve the desired mechanical properties. In the present work, a new industrially produced Complex-Phase (CP) steel with tensile strength of 1100 MPa was investigated and characterized in order to determine if the mechanical properties could be further improved. A Continuous Cooling Transformation (CCT) diagram was created using dilatometric treatments on cold-rolled samples removed from the industrial process prior to heat treatment. The industrial heat treatment was reproduced in the dilatometer to identify the phase transformations that occurred during processing. The industrially produced steel had its microstructure characterized using Light Optical Microscopy (LOM) in combination with Electron Backscatter Diffraction (EBSD). This allowed the quantification of the complex microstructure presented by this kind of steel. Scanning Electron Microscopy (SEM)/Energy Dispersive X-ray Spectroscopy (EDS) maps, and electron diffraction obtained by transmission electron microscopy (TEM) were used to corroborate the findings. The results showed a possibility to increase the mechanical properties using a new heat treatment that favors the formation of less ferrite and more bainite.

Keywords

• Multi-phase steel
• Complex phase steel
• Microstructural characterization