A study on formability issues in advanced high strength steel

In a world where simply light-weighting or strengthening a vehicle isn’t enough, advanced high strength steel (AHSS) is of interest as it can do both of those things as well as reduce emissions. Compared to aluminium, it is known to decrease emissions throughout the entire material life-cycle, provide comparable weight savings and offer higher structural safety through strength and crash-absorption capabilities. For these reasons, it is believed that there are major benefits in more widely incorporating these steels into the mainstream consumer automotive market. There currently exists a major issue within this steel to be formed complexly and so this greatly inhibits its ability to be utilised by the automotive industry. This project aims to study the formability issues in AHSS to achieve a greater understanding of why the issues are present. This aim is met by utilising existing methodologies on a lesser studied grade of steel (SSAB Dual Phase 1000) followed by comparison to the literature. In order to study these issues, an extensive literature review followed by an exploration into current methodologies was conducted. The chosen methodology includes: 1) A tensile test, 2) Hole expansion ratio (HER) test with 2 different sample sets, 3) scanning electron microscope (SEM) observation of microstructure as well as failure mechanism of a deformed sample and lastly, 4) an optical microscopy (LOM) observation of microstructure. The tensile test found that the DP1000 steel had a tensile strength of 1054MPa, yield strength of 820MPa, elongation of 27% and percentage reduction of area of 10%. The literature presented a comparable grade of Dual Phase 1180 (DP1180) that yielded similar results. These results were compared and it became understood that DP1000 contains more martensite within its structure than DP1180. This understanding was further supported by subsequent SEM and LOM visual analysis. The HER test was then discussed and found a HER of 54.2% for water cut initial holes and 51.5% for drilled holes. This result was conflicting because the DP1180 has a HER of 25.6%. It was found that the method used to create the initial hole made approximately 10% difference in HER%. The rest of the difference was understood to potentially be the lack of sufficient clamping force on the sample during HER testing