Quantitative Characterization of the Microstructure of Hot Stamped Boron Steel

University essay from Luleå/Department of Engineering Sciences and Mathematics

Abstract: Hot stamping is an efficient thermo mechanical process widely used in the automotive industry. It allows steel sheets to be transformed into ultra-high strength steel parts with a tensile strength up to 1500MPa. This is achieved by in-die quenching, where the hot blanks undergo a solid-state phase transformation. If the cooling rate is controlled as in soft zones, a microstructure composed of up to five phases can be formed to intrinsically control the mechanical properties at a macroscopic scale. Thus, microstructural characterization is primordial to develop and model safe products which answer to the constant requirements of international standards regarding road security and air pollution. However, the similarities between phases bring another issue to the mission of phase quantification and microstructural characterizations. Nowadays, tremendous efforts are made to develop an efficient and reliable method to distinguish the different constituents of multiphase steel and several technics have been introduced. We propose in this work to test the efficiency of two promising tools to identify the composition of multiphase steel microstructures provided by Gestamp Hardtech, namely colour etching and electron backscatter diffusion (EBSD). Both methods were tested and the results were compared afterward. In this work, we report on the introduction of a new sequential method to fully distinguish the different phases composing the microstructure via colour etching, which showed very good results in term of phase distinction thanks to a suitable contrast created between the phases.
Lepera’s solution was first applied to distinguish ferrite, bainite and the austenite with martensite, followed by a second etching step with Klemm’s solution which helped effectively to separate austenite from martensite. Secondly, EBSD showed higher resolution. When combined to band contrast and the local misorientation criterion, this technic turned to be a very efficient approach for the identification of the different phases based on the intrinsic characteristics of each component. Efforts were also undertaken to analyse the same areas in order to evaluate the agreement between both technics. Excellent results were obtained, as phase content agreed within 7%. A large area (1488×465 μm²) was also characterised by colour etching for statistical investigation. Variations of only 3% were observed across and along the sheet plane. Since no microstructural gradient was present on the analysed samples, an additional investigation allowed to show that a small area of 372'279μm² can be sufficient to provide a proper evaluation of the entire microstructure. In conclusion, both techniques have advantages for the investigation of hot stamped multiphase steels. Colour etching and metallography can be exploited to provide a satisfactory approximation of the phase content within a short period of time but with limited resolution, while EBSD offers higher resolution and accurate analysis but is very time consuming to study the microstructure.