1180MPa Quenching Partitioning Steels with High Formability and Good Stretch-flangeability

摘要

Advanced high-strength sheet steels (AHSS) have received more attention from car makers due to the increasing requirements of energy cost,environmental protection and safety.Recently,a new type of AHSS called QP1180 (Quenching & Partitioning steel with tensile strength of 1180MPa) has been developed by Baosteel,exhibiting excellent combination of an ultra-high tensile strength over 1.2GPa,a sound elongation over 15％ and a hole expansion ratio over 30％,together with good phosphatability.The strengthening mechanism and surface characteristics of the developed steels were investigated by mechanical properties tests,SEM observation,EBSD and XRD quantitative analysis.As a result,a multiphase structure containing microstructure composed of ferrite,tempered lath martensite and retained austenite and the TRIP effect of retained austenite during deformation lead to the excellent comprehensive properties of QP1180 steels.Meanwhile,the issues of selective surface oxidation caused by the high contents of Si and Mn in QP steels have been well solved by tuning the atmosphere in the annealing furnace,in order to achieve good phosphatability by transiting the external oxidation of Si and Mn into internal mode.