Study of the stretch-flangeability improvement of dual phase steel

Abstract

Two different types of dual phase steels DP780 were manufactured and investigated. Steel A contained higher carbon, the other one B had lower carbon with Nb added. The mechanical properties and microstructures were analyzed and compared. The hole expansion test were conducted to evaluate stretch-flangeability. An Ultra Micro-Indentation System (UMIS) were employed to carry out the nano-hardness test to check the nano-hardness of ferrite and martensite. The result showed that for DP steel A, higher over-aging temperature during manufacturing would result in lower ultimate tensile strength with martensite partly resolved, which would lead to higher stretch-flangeability. In comparison with A, steel B with lower carbon and Nb added exhibited higher stretch-flangeability, which was related to the refined grain size of ferrite. It could be concluded that for dual phase steel stretch-flangeability was mainly affected by the strength difference between ferrite and martensite. Generally the bigger the strength difference is, the lower the stretch-flangeability is. Either decreasing martensite strength by increasing over-aging temperature or increasing ferrite strength by adding Nb could improve the stretch-flangeability. The same parts formed from the two different dual phase steel were compared. There was crack in some edge area during forming for steel A, while the part formed from Steel B showed good quality without crack, which verified the stretch-flangeability improvement was very effective. However, increasing over-aging temperature during manufacturing is not easy to be implemented and it could result in lower ultimate tensile strength, which may not meet the strength requirement. Adding Nb is considered to be a more feasible and effective way to improve ferrite strength and stretch-flangeability for dual phase steel DP780.